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Nutrients 2013, 5(2), 359-387; doi:10.3390/nu5020359

Review
Plant Sterols as Anticancer Nutrients: Evidence for Their Role in Breast Cancer
Bruce J. Grattan Jr.
Department of Family Medicine, Stony Brook University Hospital Medical Center, Stony Brook, New York, NY 11794, USA; E-Mail: bruce.grattan@stonybrookmedicine.edu; Tel.: +1-631-444-8245; Fax: +1-631-444-7552
Received: 25 September 2012; in revised form: 30 November 2012 / Accepted: 24 January 2013 /
Published: 31 January 2013

Abstract

: While many factors are involved in the etiology of cancer, it has been clearly established that diet significantly impacts one’s risk for this disease. More recently, specific food components have been identified which are uniquely beneficial in mitigating the risk of specific cancer subtypes. Plant sterols are well known for their effects on blood cholesterol levels, however research into their potential role in mitigating cancer risk remains in its infancy. As outlined in this review, the cholesterol modulating actions of plant sterols may overlap with their anti-cancer actions. Breast cancer is the most common malignancy affecting women and there remains a need for effective adjuvant therapies for this disease, for which plant sterols may play a distinctive role.
Keywords:
plant sterols; cholesterol; cancer; breast cancer; beta-sitosterol; AMPK; bisphosphonates; statins

1. Introduction

While many factors are involved in the etiology of cancer, it has been clearly established that diet significantly impacts risk of this disease [1,2,3]. The age-adjusted incidence of cancer in the US is 3 times higher than that in Asian countries, with immigrants to the US having increased risk for this condition [4,5]. This suggests critical roles for dietary and lifestyle factors. Despite the results of recent studies failing to demonstrate a large inverse association between produce consumption and overall cancer risk [6], the benefits of key nutrition components unique to plant foods, may still prove beneficial in reducing individual risk and may mitigate the risk of specific cancer subtypes.

There is compelling evidence that increased produce consumption may be associated with a reduction in breast cancer risk [7,8,9]. Specific food components such as sulfurophane [10], indole-3-carbinol [11], and lignans [12] have been investigated in both epidemiological and in vitro studies with regards to their effects on breast cancer. Studies such as by Torres-Sanchez have found an inverse correlation between consumption of phytochemicals and risk of breast cancer among postmenopausal women [13] suggesting a potential role for plant sterols in mitigating risk. Plant sterols may be one such nutritional component which may likewise play a role in attenuating breast cancer risk or perhaps serve a chemotherapeutic role.

2. Plant Sterols

Plant sterols (PS) are C28 and C29 carbon steroid alcohols [14] that are integral components of plant cell membranes, have been shown to be key components of plant plasma membrane microdomains [15], and may exert similar functions in human cells. These compounds cannot be synthesized by humans and are introduced through the diet where they are found concentrated in plant foods, especially those with are lipid rich [16]. In the American diet, vegetable oils and nuts are particularly significant contributors to plant sterol intake [17,18]. While a variety of PS exist, campesterol, stigmasterol and β-sitosterol are the most abundant PS in the diet, with the prevalence of β-sitosterol being particularly noteworthy [19]. Globally, dietary intake of these compounds has been estimated to be between 200 and 400 mg daily [20], making their intake similar in quantity to cholesterol. PS exist in both the sterol and stanol forms, with the bioavailability of sterols [21] and their dietary prevalence exceeding that of plant stanols [22].

The absorption of PS has been estimated to be 0.4%–3.5% [23] whereas the absorption of phytostanols ranges between 0.02% and 0.3% [24]. In general, the absorption of dietary sterols of plant or animal origin varies significantly. While between 45% and 55% of dietary cholesterol is absorbed, less than 20% and 7% of campesterol and β-sitosterol, respectively, are absorbed [25]. Among healthy subjects, supplementation with 2–3 g of PS has been shown to elevate serum levels of sitosterol and campesterol by 30% and 70%, respectively [26,27]. This limited absorption of PS is the result of their subsequent efflux from enterocytes, a process mediated by ATP binding cassette transports, ABCG5 and ABCG8 [28,29,30] resulting in plasma PS levels of <1 mg/dL [31]. Despite the relatively low circulating quantities of these compounds, they are still capable of exerting important biological effects. The typical American diet has been estimated to provide approximately 80 mg of PS daily, a value significantly lower than that of the Japanese (400 mg) or that seen among vegetarians (345 mg) [32], whose dietary patterns are consistent with a reduction in their breast cancer risk [33].

3. Plant Sterols and Cancer

Recent meta-analyses have concluded that doses of plant sterol or stanols of 1–2 g daily can effectively lower LDL-cholesterol levels 8%–12% [34,35,36]. However, despite the relatively strong evidence for a beneficial effect of these compounds on cardiovascular disease risk, these compounds have received comparably little attention with regards to their potential role in cancer etiology. Notwithstanding, the studies to date of PS role in cancer have been promising. The increasing evidence of the biochemical and molecular effects of PS may make them strong candidates for cancer therapy.

With regards to the effects of PS on prostate cancer epidemiological data has failed to find a correlation despite promising in vitro data [37,38]. In animal models of colon cancer, PS have been shown to exert beneficial effects [39]. In a case-control study by Mendilaharsu a 50% reduction (95% CI 0.31–0.70) in the risk of lung cancer was seen among those with the highest quartile intake of phytosterols [40]. Similarly, case-control work by De Stefani [41] assessing the effects of PS on stomach cancer risk showed an odds ratio of 0.33 (95% CI 0.17–0.65) among those with the highest PS intake. Populations such as Seventh Day Adventists, who have a lower cancer risk that that of the general population, have likewise been found to have greater intakes of PS [42]. While limited, epidemiological data suggests a correlation between plant sterol intake and a reduction in cancer risk. It has been estimated that phytochemical intake may be related to a reduction in cancer risk upwards of 20% [43]. Despite the promising evidence for PS in other cancer subtypes, the effects of these compounds on the etiology of breast cancer is less well known.

4. Plant Sterols and Breast Cancer

Breast cancer is the most common malignancy affecting women, with an incidence of approximately 1/4 of all cancer cases and 14% of all cancer deaths [44]. There are approximately 1 million newly diagnosed cases of breast cancer annually [45]. Even among patients diagnosed with node negative breast cancer, the 10 year estimated risk of recurrence ranges upwards of 50 percent [46]. Indicating a role of environmental exposure and nutritional status among other potential contributors, breast cancer incidence among women in Western countries is 6-fold greater than among women in Asia [47,48]. The dietary contribution of PS may be one nutritional factor affecting the distribution of this disease.

5. Estrogen and Breast Cancer

Estrogen is a significant and well recognized mediator of breast cell growth [49,50,51] and as such this hormone plays an important role in the etiology of breast cancer with elevated levels being recognized as a potentially modifiable risk factor for breast cancer [2,52]. A prophylactic effect tamoxifen treatment in those at high risk for developing the disease has been demonstrated in large clinical trials [53]. In those with mutations in the BRCA1 and BRCA2 genes, who are therefore at high risk, prophylactic oophorectomy has been shown to help mitigate risk [54]. At the cellular level, estrogen and estrogen metabolites are directly carcinogenic to breast cells [55,56]. However, recent epidemiological data from the Women’s Health Initiative (WHI) has called into question some of the previous thinking with regards to the effects of estrogen therapy on breast cancer risk suggesting that estrogen monotherapy and the timing of its administration may be important mediators of estrogen’s risk for breast cancer [57]. Notwithstanding, while the WHI trial concluded that estrogen alone was associated with a decreased risk of breast cancer, there are numerous methodological concerns with these data [58,59,60,61,62].

There are two divergent isoforms of the estrogen receptors (ER); alpha and beta, and the receptor to which estradiol binds is an important determinant of this hormone’s cellular effects. ER-α and β are the receptors for estrogen and function as ligand-dependent transcription factors, with the β receptor being considered inhibitory in its effects on breast cell growth [40,63] and the expression of the α receptor being suggested by some to be inversely correlated with breast cancer risk [64]. Epidemiological evidence for this is supported in a 2011 study by Goss et al. [65] which demonstrated a significant reduction in the risk of breast cancer in postmenopausal women at increased risk for this disease when treated with the aromatase inhibitor exemestane. Opposing effects are observed between ER-α and the ER-β form, with the alpha form of the ER being the predominant form involved in the proliferative effects of estrogen on cancer growth [66]. Induction of pathways through ER-β on the contrary induces apoptosis/growth arrest and partly underlies the chemopreventative effects of phytoestrogens [67,68].

Plant Sterols and Estrogen

A number of studies have shown that dietary components can influence ER status [69,70,71,72]. The ER-α is necessary for the proliferative effects of estradiol in breast cancer cells and is overexpressed in the transformed state [73,74,75,76].

It has been shown that intake of β-sitosterol is associated with a greater likelihood of estrogen receptor positive (ER+) than estrogen receptor negative (ER−) tumors (OR 0.49; 95% CI 0.18–0.98) [77]. From the standpoint that the presence of the ER maintains cell responsiveness to endocrine therapy, such as with the selective ER modulator (SERM), tamoxifen, ER+ breast cancer itself represents a more treatable condition than the ER− phenotype. ER− breast cancer is not susceptible to such treatment [78]. This fact underlies the current rationale for ER+ breast cancer treatments, which are aimed at minimizing the utility of this hormone and its resulting stimulatory effects on cell growth and division. However, despite the causal role of estrogen in the progression of ER+ breast cancer, nearly 30%–40% of breast cancers do not exhibit ER+ status [79]. Therefore, newer treatments and/or adjuvant therapies, which do not solely rely upon the ER, are of great importance.

β-Sitosterol has been demonstrated to competitively bind with equivalent affinity to both the α and β-isoforms of the ER and with an affinity comparable to that of coumestrol [80], which itself has been found to moderately stimulate growth of the ER+ cell line, MCF-7 [81]. Despite exerting an affinity for the ER, in rat models of PS exposure, β-sitosterol failed to increase uterine weight, a marker of estrogenic activity [82]. Likewise, plant stanols and stanol esters failed to stimulate estrogen responsive growth in MCF-7 cells [83]. There is, however some evidence for the estrogenic effects of PS [84], with evidence from reporter gene array studies in human breast cancer cell lines suggesting a role for PS as weak SERMs [80]. Additionally, PS may affect levels of sterol 27 hydroxylase (CYP27A1), an endogenous SERM [85], as β-sitosterol has been shown to inhibit the activity of sterol 27 hydroxylase upwards of 50% [86]. Despite the potential for PS to exert some estrogenic effects, these compounds may still exert beneficial effects on breast cancer, considering that tamoxifen, for instance is also known to be a SERM [87].

While PS may bind the ER or even act as SERMs, there is also the potential for PS to attenuate de novo steroid synthesis through reductions in cholesterol levels. To this end, some evidence exists for a reduction in androgens as a result of statin treatment [88], however direct evidence of PS exerting this modality has, to date, not been demonstrated. As will be discussed, PS may indirectly affect estrogen levels through means other than ER binding.

6. Plant Sterols and the Liver X Receptor

PS and stanols have been demonstrated to activate both the α (NR1H3) and β (NR1H2) isoforms of the liver X receptor (LXR) [89], whose classical agonists have been oxysterols and their derivatives. LXR-β is ubiquitously expressed [90] whereas expression of the alpha isoform is tissue specific [91,92]. This may be an additional mechanism by which PS may affect estrogen levels, cell division and breast cancer risk. Accumulating evidence has not only demonstrated the expression of LXR in both ER+ and ER− breast cancer cells but suggests that LXR agonists profoundly inhibit cell proliferation [93,94,95]. Mechanistically, LXR activation has been shown to down-regulate the ER while also increasing protein levels of P53 [93]. LXR activation affects multiple regulators of the cell cycle ultimately leading to arrest at G1 [93,95]. Such activation, which may similarly result from PS administration, also results in augmented hepatic cholesterol catabolism [96,97], which may in turn diminish cell proliferation through limiting the availability of the cholesterol needed for cell membrane production.

In addition, the hepatic LXR has been shown to regulate estrogen sulfotransferase, a mechanism through which LXR agonists (such as PS) may induce estrogen deprivation, as sulfonated estrogen is incapable of binding to the ER and activating gene transcription [98]. Furthermore, in xenograft models of invasive ER+ breast cancer (MCF-7/VEGF), LXR activation with a synthetic agonist resulted in a loss of estrogen induced tumorigenicity [99]. While these effects were not observed in ER-MDA-MB-231 cells, LXR is expressed in both cell lines. This suggests the importance of hepatic LXR in regulating systemic estrogen levels and being of particular importance to ER+ breast cancer. The influence of PS on LXR and subsequent estrogen metabolism is of great clinical important given that medications such as tamoxifen are limited in effectiveness with a relapse recurrence rate of approximately 50 percent [100].

7. Plant Sterols, the Immune System, and Inflammation

The immune system plays a vital role in cancer etiology with chronic inflammation being recognized as a fundamental aspect of the disease [101,102,103]. The immune system has a pivotal role in cancer prevention and prognosis [104,105,106] and it has been shown that regulatory T cells are both elevated in cancer patients and negatively associated with survival [107,108]. The Th1 axis has an established role in tumor suppression and in patients with HIV a sterol/sterolin mixture was found to increase secretion of Th1 axis cytokines in vivo [109].

Likewise, it is recognized that immune system dysregulation plays an important role in cancer metastasis. Interleukin 2 (IL-2) and interferon-γ (IFN-γ) have been demonstrated in animal models of breast cancer to be important in preventing metastasis [110]. PS have been shown to regulate cytokine secretion leading to increased secretion of both IL-2 and IFN-γ [111,112]. Similarly, liposomal delivery of β-sitosterol in a murine model of melanoma was shown to attenuate metastasis [113]. This occurred despite a lack of phytosterol distribution in the blood. This suggests that PS may stimulate the immune system through improving gut surveillance, as IL-2 levels and NK cell activity were noted to be elevated following liposome administration. Stimulatory effects of PS on cytokine production may thus be a means through which this phytosterol exerts preventive effects on cancer metastasis.

Signal Transducer and Activator of Transcription-3 (STAT3) is often constitutively activated in cancer cells and is a causative agent in the transformation to a cancerous phenotype [114]. The downstream targets of STAT3 signaling are well known to be mediators of cancer initiation and progression such as Cyclin D1/D2, c-Myc, Bcl-xl, and Mcl-1 [115,116]. In multiple myeloma, inhibition of STAT 3 both in vitro and in vivo has been demonstrated to enhance the expression of pro-apoptotic proteins such as Bax while augmenting sensitivity to chemotherapy induced apoptosis [117]. In breast cancer in particular STATs 1, 3, and 5 are all constitutively activated [118,119]. In response to proinflammatory cytokines, STAT1 is activated leading to the upregulation of the innate immune response [120,121,122]. In addition, such activation is linked with the EGFR [123]. Signaling through the EGFR has become an increasing important target for breast cancer therapy. MAPK and PI3K pathways are both downstream targets of EGFR activation [124], and in relation to its link to breast cancer, over expression of the EGFR in MCF-7 cells directly leads to a cancerous phenotype and one that is estrogen independent [125]. Furthermore, the expression of the EGFR is twice as likely in breast cancers which are “double negative”, lacking both the ER and the progesterone receptor (PR) [126]. Indeed tamoxifen resistant MCF-7 cells exhibit upregulation of the EGFR, supporting the hypothesis that upregulation of the EGFR and its related signaling events offer a means of escaping the limits of estrogen-mediated growth [127]. Both EGFR and Her2 are members of the ErbB family of receptors. In highly aggressive breast cancers, Her2 is constitutively activated [128], where it shares tyrosine kinase activity as a binding partner with the EGFR [129]. In fact, it has been estimated that Her2 is amplified in 25%–30% of all breast cancer cases [130,131]. With regards to its immunologic effects, Her-2 overexpression is known to downregulate the major histocompatability complex class I (MHC-I) thereby reducing immune surveillance of breast cancer cells [132,133]. Plant sterols have been shown to activate AMPK in a manner similar to metformin, and may thus act as metformin mimetics. To this end, metformin has been shown to rescue MHC-I from downregulation by Her-2 overexpression in breast cancer cells [134].

Recently, β-sitosterol has been shown to decrease the nuclear translocation of nuclear factor kappa B (NF-κB) [135] which promotes inflammation, is constitutively activated in this disease and leads to a more aggressive, hormone independent phenotype [136,137]. In addition, downregulation of NF-κB in vivo has been shown to increase cancer cell susceptibility to the apoptotic effects of tumor necrosis factor alpha (TNF-α) [138] while also minimizing cell metastatic capability through modulation of growth factors and cytokines. Downregulation of NF-κB inhibits the production of vascular endothelial growth factor, interleukin-8, interleukin-6, and matrix metalloproteinase-9 (MMP9) [139,140] each of which are implicated in breast cancer.

8. The Effect of Plant Sterols on Cholesterol: Implications for Cancer

The most well recognized clinical outcome of PS intake is their hypocholesterolemic effects. Indeed in cultured enterocytes, sitosterol has been shown to decrease the expression of the Niemann-Pick C1-Like 1 transporter [141]. However the effects of PS on cholesterol may, in turn, mediate their potential anticancer effects. Awad and colleagues [142], have shown the capability of β-sitosterol to suppress growth and to induce apoptosis in MDA-MB-231 cells, suggesting a potential role for dietary constituents as adjuvant therapy for this disease. Following β-sitosterol treatment, a 66% reduction in cell growth was noted. Upwards of an 87% reduction in breast cancer cell growth was noted in another study by Awad utilizing the estrogen responsive MCF-7 cell line [143]. The fact that these effects persisted despite the differences in estrogen responsiveness between these two cell lines, suggests additional, non-hormonal effects of PS. Similar work by this group demonstrated that β-sitosterol in comparison with campesterol, induced a substantial reduction in the cholesterol fraction of total cellular sterols with β-sitosterol accounting for 75% of total sterols following treatment [144]. This may be an additional mechanism through which PS may affect cell growth and be of utility to breast cancer treatment or prevention.

8.1. Cholesterol and Cancer

The effect of PS on cholesterol levels has been recognized since the 1950’s [145] and the effect of PS on cholesterol may, in part, underlie their effects on cancer risk. A historic study by Hinds, found a positive correlation between dietary cholesterol intake and the risk of lung cancer, using a case control design. This finding extended across a variety of ethnic groups and remained even after controlling for age and occupational exposure to lung carcinogens [146]. A recent, large, prospective, study [147] demonstrated a positive correlation between total cholesterol and cancer risk with a hazards ratio of 1.17 being determined for breast cancer in particular (95% CI 1.03–1.33). Others have shown that every 10 mg/dL decrement in LDL is associated with a 15% (95% CI 12%–18%) reduction in cancer risk (p < 0.001) [148]. The ability of PS to reduce LDL cholesterol has been shown to occur with an average reduction of 8.8% [149]. To this end, statins, which lower LDL cholesterol an average of 1.8 mmol/L [150], have been suggested to lower cancer risk upwards of 50% [151,152]. The effect of PS is specific to LDL with no effect on HDL levels [153], however, through their LDL effects, PS consumption increases the relative levels of HDL. Consistent with these effects, an inverse relationship has been demonstrated between HDL cholesterol and cancer risk, with every 10 mg/dL increase being associated with a 36 percent reduction in overt risk (95% CI 24%–47%) [148]. Similarly, other studies have corroborated the association between increased HDL and diminished cancer risk [154].

Cholesterol is an integral component of cellular membranes and thus, demand for cholesterol is augmented during periods of rapid cellular proliferation [155]. Among many potential mechanisms, cholesterol has been shown to reduce levels of MMP-1 [156], the serum levels of which are negatively associated with survival among breast cancer patients [157].

Likewise, it has been recognized for some time that depletion of cholesterol has inhibitory effects on cellular growth [158,159,160]. Inhibition of HMG-CoA-reductase by lovastatin has been found to consistently induce G1 arrest to the degree that such treatment has been suggested as an experimental means of cell cycle synchronization [161]. Statins have been suggested to be of use in breast cancer therapy [162,163] and treatment of breast cancer cells with lovastatin led to an overexpression of PTEN and a resulting decrease in AKT/PKB signaling [164]. Cellular cholesterol levels however are tightly regulated. Such regulation occurs through a balance of uptake and synthesis via sensor mechanisms and feedback loops consisting of sterol regulatory element binding protein (SREBP) and SREBP cleavage activating protein (SCAP) [165]. Cholesterol regulates genes necessary for lipid metabolism which contain sterol regulatory elements (SREs) in their promoter regions. Reductions in intracellular cholesterol leads to an activation of endoplasmic reticulum (ER) bound SCAP protein to begin processing of SREBP-1a,-1c,-2. In turn, these factors migrate to the nucleus and influence the transcription of genes through the binding of SRE promoter regions [166,167]. In addition to the statin mediated effect on G1, in human acute promyelocytic leukemia cells (HL-60), and acute lymphoblastic leukemia cells (MOLT-4), there is a specific role for cholesterol in modulating the activity of the p34cdc2 kinase which regulates the G2-M transition [168]. Thus, there are multiple modalities for an effect of cholesterol on cell cycle regulation and overall cellular proliferation for which PS may modulate.

Recently, a number of studies have observed a reduction in breast cancer risk with bisphosphonate use as well as the potential for use of these medications as adjuvant therapies for this disease [169,170]. Likewise, evidence supports a role for bisphosphonates in reducing breast cancer metastasis to bone [171]. Bisphosphonates have a known role in the regulation of cholesterol synthesis. It was first demonstrated by Amin [172] that some bisphosphonates reduce de novo cholesterol synthesis through inhibition of squalene synthase, with alendronate and pamidronate being found to inhibit mevalonate as well as squalene synthase. Overlapping with the effects of PS, this work illustrates the role of these medications in the modulation of cellular cholesterol levels. Given the growing evidence for the role of bisphosphonates as a means of breast cancer treatment, or prevention, and the known role of cellular cholesterol metabolism in cell division and growth, the potential exists that these compounds may be exerting their chemotherapeutic effects via mechanisms influencing cellular cholesterol levels. Such a mechanism may be mimicked through intake of PS.

While operating at different points in the mevalonate pathway, treatment with either statins or bisphosphonates leads to a reduction in farnesyl pyrophosphate and geranylgeranyl-pyrphosphate, both of which are required for protein prenylation. Such prenylation has important roles in the generation of lipidated protein domains that enable protein-protein interactions and subsequent cell signaling. The prevention of protein prenylation by either statins or bisphosphonates also leads to endoplasmic reticulum stress as a result of a reduction in prenylated rab proteins [173,174]. Such ER stress results in the initiation of the unfolded protein response [175] and subsequent autophagy [176]. While significant advances have been made in understanding the effects of PS on cholesterol metabolism [177], much remains to be investigated with regards to their effects on breast cancer etiology. While the effects of PS are milder than statins, they may exert similar effects with a far more efficacious safety profile.

8.2. Plant Sterols and Oxidative Stress

In addition to their roles in hormone production, cell signaling and cell membrane organization, plant sterols may impart a cellular antioxidant effect. In comparison with noncancerous growths such as fibroadenomas, breast cancer cells have been found to exhibit greater oxidative stress [178]. Furthermore, breast cancer cells have been shown to possess lower levels of coenzyme Q-10, a potent antioxidant, than juxtaposed non cancerous cells, as well as higher malonyldialdehyde levels, indicating increased oxidative damage to lipids [179]. Additionally, breast cancer patients have been found to have significantly lower levels of glutathione and reduced total antioxidant capacity in comparison with healthy controls [180]. However, β-sitosterol has been shown in cell culture studies to modulate levels of both glutathione peroxidase as well as superoxide dismutase (SOD) [181]. Treatment of thymocytes derived from BALB/c mice with β-sitosterol was found to prevent radiation induced nuclear strand tears as well as stimulate antioxidant enzyme systems in these cells including catalase, SOD and glutathione peroxidase, while inhibiting cytochrome c release [182]. β-Sitosterol has also been shown to induce antioxidant defense systems in the pancreas of streptozotocin treated rats [183] and has been shown to protect against the depletion of antioxidants seen in models of chemically induced cancer, while also augmenting tissue levels of nonenzymatic antioxidants [39]. Furthermore, as HMG-CoA reductase inhibitors [184], PS may affect ROS generation in addition to cellular cholesterol content. Both statins [185] and PS [135] have been demonstrated to reduce NF-κB activation and this may be a mechanism underlying the antioxidant effects of PS [186].

9. Plant Sterols and Glucose Metabolism

Given the alterations in cell metabolism of some cancers towards glycolytic pathways, as first described by Warburg [187], interventions aimed at modulating glucose signaling, may prove salutary in cancer therapy. Metformin and associated dietary mimetics of this medication, including PS, may be one such therapy.

The biguanide anti-diabetes medication, metformin, has been shown to selectively induce apoptosis among cancer cells [188]. As well, this medication has been shown to reverse the loss of immune system surveillance by way of recovering MHC-I expression [134]. The downregulation of MHC-I is a process that is intertwined with the Warburg effect [189] and thus metformin and metformin mimetics may exert therapeutic effects through both pathways.

Mechanistically, metformin works in part through activation of adenine monophosphate kinase (AMPK) which is a component of the LKB1/AMPK/mTOR/IRS/Akt pathway [190,191,192,193]. AMPK also increases the AMP/ATP ratio resulting in reduced hepatic glucose output [194], with AMPK being allosterically regulated through binding of AMP to its alpha and gamma subunits [195]. The effects of metformin on the AMP/ATP ratio make this pharmaceutical a mimetic of dietary energy restriction and it has been suggested that metformin may be of clinical utility in breast cancer prevention [196].

It has been observed that plasma levels of β-sitosterol are lower among type 2 diabetics [197], with hypoglycemic effects of β-sitosterol being observed in other studies [198]. β-Sitosterol has been shown to be an AMPK agonist, with the beneficial effects of β-sitosterol on glucose metabolism being mediated, in part, through this mechanism [199]. The agonistic effects of PS on AMPK and the pathways activated by metformin may be another means through which these compounds may exert anticancer effects. Membrane cholesterol content is known to regulate GLUT4, with recent evidence demonstrating that AMPK induced insulin sensitization is in part a result of depletion of membrane cholesterol content [200]. As PS activate AMPK while also displacing membrane cholesterol, PS may have important effects on glucose metabolism and subsequent cell growth. Synthesized variants of PS such as disodium ascorbyl phytostanol phosphate have been shown to reduce body weight as well as cholesterol levels in animal studies [201]. Given the association between energy intake and cancer risk [202,203], such compounds may be of benefit in reducing the risk of both cancer as well as cardiovascular disease. In addition to these mechanisms, AMPK also phosphorylates and inactivates HMG-CoA reductase [204].

In addition to being part of antiapoptotic pathways, AMPK regulates a vital glucose-dependent cell cycle checkpoint at G1/S [205]. The sensitivity of this checkpoint for glucose is such that continued activation of mTOR and amino acid availability are not sufficient for overriding cell arrest at this junction. This finding has important mechanistic implications for PS insomuch as the phosphorylation of P53 by AMPK is required for arrest at this checkpoint and indeed persistent AMPK activation results in cellular senescence [205].

P53 is a well known apoptotic mediator that also plays an important role in stimulating cell cycle arrest induced by DNA damage [206]. Mutations in the phosphoprotein P53 have long been recognized to be prevalent in malignancies. In fact, it is estimated that P53 gene (TP53) mutations occur in nearly 50% of tumors [207]. Furthermore, mutant P53 also has been shown to upregulate the mevalonate pathway [208].

Additionally, through activating AMPK, PS may not only inhibit cell proliferation but enhance cellular antioxidant capacity via FOXO transcription factors such as DAF-16 which is known to possess a sterol sensing domain [209]. In so doing, the subsequent upregulation of catalase, IGFBP1, and MnSOD may limit oxidative damage and stymie aberrant cell growth. Notwithstanding, insulin/IGF signaling, which is upregulated under conditions of insulin resistance, inhibits and subsequently suppresses SKN-1 which is involved in intestinal phase II detoxification [210]. Plant sterols may upregulate FOXO transcription factors through their effects on AMPK [211] and both FOXO1 and FOXO3 have been shown to promote apoptosis during the unfolded protein response (UPR). As discussed, the UPR may be induced following cholesterol depletion, by such means as PS, and FOXOs have been shown to promote apoptosis during endoplasmic reticulum stress through inhibiting the normal increase in NF-κB, which itself exerts anti-apoptotic functions [212].

Insulin resistance is becoming increasingly common and one of the potential mechanisms through which insulin resistance affects cancer risk is through an increase in the bioavailability of IGF-1. Insulin resistance has been linked with an increased incidence of a variety of cancers, including breast cancer [213]. Hyperinsulinemia increases hepatic IGF-1 production while concurrently diminishing IGFBGs [214]. Likewise, insulin reduces sex hormone binding globulin (SHBG) and thus increases the levels of bioavailable estrogen [215]. The inflammatory cytokine profile seen among those who are insulin resistance may also contribute to the transformed state [216]. The AMPK activating effects of PS may improve insulin resistance, thereby reducing IGF-1 levels.

IGFs play an important role in breast cancer etiology having been shown to exert mitogenic, transforming, and antiapoptopic properties, especially when coupled with other growth factors [217]. In addition, it has been demonstrated that overexpression of the IGF-1 receptor (IGF-1R) results in the transformation of non-cancerous cells to ones possessing a malignant phenotype [218]. Further evidence suggest that not only is IGF-1 responsible for induction of MMPs but there is a reciprocal effect between IGF-1 and MMPs such that MMPs function in part to maintain the IGF-1R [218]. Cellular cholesterol depletion, such as by plant sterols, disrupts the antiapoptotic effects of IGF-1 signaling through reducing the levels of phosphoinositol 3 kinase (PI3K) [219].

10. Plant Sterols, Membrane Organization, and Cell Signaling

There are several lines of evidence implicating PS in cell membrane organization including sphingolipid and ceramide metabolism and alterations to caveolae.

10.1. Beta Sitosterol and Ceramide Metabolism

Lipids rafts have important effects on cell signaling in breast cancer [220,221] and these moieties are affected by the levels of cellular ceramide. Ceramide is a sphingolipid which is believed to function as a tumor suppressing lipid [222], and has been shown to diminish the cholesterol content of lipid rafts [223]. In response to ceramide availability, PTEN is increased in caveolae enriched microdomains [224] and is known to negatively regulate insulin signaling. Through removing the 39-UTR phosphate of PIP3, PTEN antagonizes PI3K [225]. In vitro evidence supports a role for β-sitosterol treatment in inducing a reduction in sphingomyelin via activation of sphingomyelinase, as well as an increase in ceramide [143]. These alterations to the components of lipid rafts have a known role in apoptosis initiation [226,227]. β-Sitosterol has been suggested to operate in part, via modulation of the sphingomyelin cycle, and through alterations in phospholipase A2 [228].

Although ceramide may have antitumor effects, an association has been noted between increased levels of glycoslyated ceramide, glucosylceramide, and resistance to cancer treatment [229,230,231]. Levels of glucosylceramide are regulated by the activity of glucosylceramide synthase and targeted inhibition of this enzyme has been demonstrated to restore cancer cell sensitivity to therapeutics [232,233]. Lucci et al. [231] demonstrated elevated levels of glucosylceramide in tumor specimens from patients with breast cancer and melanoma whom were resistant to chemotherapy. Indeed, the increased glycosylation of ceramide may be a mechanism utilized by cancer cells to become drug resistant [234]. Glycosylation of ceramide may provide a means of escaping the growth inhibitory effects of ceramide. Awad demonstrated that combined treatment of tamoxifen with β-sitosterol potentiates the effects of this medication on the growth of MCF-7 cells and MDA-MB-231 cells. β-Sitosterol was found to be a potent activator of serine palmitoyltransferase, the rate limiting enzyme in ceramide synthesis [143]. Additionally, this adjuvant therapy was found to inhibit glucosylceramide synthase, and thus the combination of β-sitosterol and tamoxifen may lower glucosylceramide levels while increasing the relative quantities of nascent ceramide with its subsequent inhibition of breast cancer cell proliferation. Importantly, β-sitosterol was found to be more effective in inhibiting the growth of MDA-MB-231 cells, an ER− cell line with a more aggressive phenotype.

Caveolae are types of lipid rafts which function as platforms for organizing a variety of cell signaling pathways. Caveolae are known to be upregulated in multidrug resistant tumors [235]. The caveolae scaffolding protein Cav-1 contains a 20 amino acid microdomain which enables its binding to signaling molecules [236] and both Cav-1 and P53 have been shown to work in synergy [237]. Caveolin is a marker for caveolae and also functions directly to modulate the actions of a variety of signaling cascades including the ER [238], EGFR [239], src [240] and the insulin receptor (IR) [241]. The effects of IR localization in caveolae and their association with caveolin have been suggested to influence insulin’s downstream mitogenic and metabolic effects [242,243]. Likewise, the IGF-1R has been shown to localize to caveolae [244].

Cholesterol is a major component of caveolae [156]. Lipid raft domains are affected by cholesterol depletion and the signaling moieties associated with them are presumably altered in sequence. The localization and activity of the breast cancer resistance protein (BCRP/ABCG2) has been associated with both cellular cholesterol content as well as proximity to lipid rafts. Similarly, cholesterol depletion been shown to reduce the activity of BCRP by 40% [245]. Changes to the sterol content of caveolae may be a mechanism by which PS may affect signaling pathways involved in both cell metabolism and division

10.2. Beta Sitosterol and Apoptosis

β-Sitosterol has been demonstrated to activate Fas. In both MCF-7 and MDA-MB-231 cells, both the expression of Fas protein and the activity of caspase 8, were selectively increased by the addition of β-sitosterol [246]. Fas is a cell surface death receptor whose activation constitutes the extrinsic apoptotic pathway. Fas activation results in the recruitment of intracellular adapter proteins including FADD (Fas Associated Death Domain) and TRADD (TNF receptor associated death domain). Together, these molecular pathways induce caspase-mediated apoptosis [247]. Furthermore, PS may initiate apoptosis through their effects on tumor necrosis factor related apoptosis inducing ligand (TRAIL). TRAIL has been demonstrated to be a potent apoptotic mediator among a variety of cancer phenotypes, while demonstrating minimal effects to normal cells both in vitro and in vivo [248,249]. TRAIL may exert unique therapeutic effects against breast cancer stem cells [250]. TRAIL mediates its apoptotic effects through caspase 8 activation, with this caspase subsequently inducing a number of effector caspases with protease activity [251,252,253,254]. A study by Awad et al. [255] found that β-sitosterol induced apoptotic effect in MDA-MB-231 cells through upregulating caspases 3, 8, and 9. Likewise, β-sitosterol augmented the bax:bcl-2 ratio. Together, low dose β-sitosterol and TRAIL were found to synergistically stimulate apoptosis in MDA-MB-231 cells [256]. In murine xenograft models, β-sitosterol treatment has similarly been found effective in reducing growth of MDA-MB-231 cells [257]. In other studies, the addition of PS to the diet significantly reduced tumor size in several studies in which athymic mice were injected with human breast cancer cells. This process was found to be independent of estrogen signaling [257,258].

11. Conclusion

It has been suggested that approximately 35% of cancer deaths are attributable to modifiable risk factors including dietary intake [259] with inconsistencies remaining about the degree to which different nutritional factors and dietary patterns affect this condition. To effectively assess the potential of these compounds, given the potential efficacy of plant sterols as outlined in this review, further research, in particular clinical trials, is warranted.

Conflicts of Interest

The author declares no conflicts of interest.

References

  1. Willett, W.C. Diet, nutrition, and avoidable cancer. Environ. Health Perspect. 1995, 103, 165–170. [Google Scholar]
  2. Hankinson, S.E.; Colditz, G.A.; Willett, W.C. Towards an integrated model for breast cancer etiology: The lifelong interplay of genes, lifestyle, and hormones. Breast Cancer Res. 2004, 6, 213–218. [Google Scholar] [CrossRef]
  3. Greenlee, H.; White, E.; Patterson, R.E.; Kristal, A.R. Supplement use among cancer survivors in the Vitamins and Lifestyle (VITAL) study cohort. J. Altern. Complement. Med. 2004, 10, 660–666. [Google Scholar]
  4. Messina, M.; Hilakivi-Clarke, L. Early intake appears to be the key to the proposed protective effects of soy intake against breast cancer. Nutr. Cancer 2009, 61, 792–798. [Google Scholar]
  5. Shu, X.O.; Zheng, Y.; Cai, H.; Gu, K.; Chen, Z.; Zheng, W.; Lu, W. Soy food intake and breast cancer survival. JAMA 2009, 302, 2437–2443. [Google Scholar]
  6. Boffetta, P.; Couto, E.; Wichmann, J.; Ferrari, P.; Trichopoulos, D.; Bueno-de-Mesquita, H.B.; van Duijnhoven, F.J.; Buchner, F.L.; Key, T.; Boeing, H.; et al. Fruit and vegetable intake and overall cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). J. Natl. Cancer Inst. 2010, 102, 529–537. [Google Scholar] [CrossRef]
  7. Gandini, S.; Merzenich, H.; Robertson, C.; Boyle, P. Meta-analysis of studies on breast cancer risk and diet: The role of fruit and vegetable consumption and the intake of associated micronutrients. Eur. J. Cancer 2000, 36, 636–646. [Google Scholar] [CrossRef]
  8. Aune, D.; Chan, D.S.; Vieira, A.R.; Rosenblatt, D.A.; Vieira, R.; Greenwood, D.C.; Norat, T. Fruits, vegetables and breast cancer risk: A systematic review and meta-analysis of prospective studies. Breast Cancer Res. Treat. 2012, 134, 479–493. [Google Scholar]
  9. Bao, P.P.; Shu, X.O.; Zheng, Y.; Cai, H.; Ruan, Z.X.; Gu, K.; Su, Y.; Gao, Y.T.; Zheng, W.; Lu, W. Fruit, vegetable, and animal food intake and breast cancer risk by hormone receptor status. Nutr. Cancer 2012, 64, 806–819. [Google Scholar] [CrossRef]
  10. Kanematsu, S.; Yoshizawa, K.; Uehara, N.; Miki, H.; Sasaki, T.; Kuro, M.; Lai, Y.C.; Kimura, A.; Yuri, T.; Tsubura, A. Sulforaphane inhibits the growth of KPL-1 human breast cancer cells in vitro and suppresses the growth and metastasis of orthotopically transplanted KPL-1 cells in female athymic mice. Oncol. Rep. 2011, 26, 603–608. [Google Scholar]
  11. Wu, Y.; Feng, X.; Jin, Y.; Wu, Z.; Hankey, W.; Paisie, C.; Li, L.; Liu, F.; Barsky, S.H.; Zhang, W.; et al. A novel mechanism of indole-3-carbinol effects on breast carcinogenesis involves induction of Cdc25A degradation. Cancer Prev. Res. (Phila.) 2010, 3, 818–828. [Google Scholar] [CrossRef]
  12. Zaineddin, A.K.; Vrieling, A.; Buck, K.; Becker, S.; Linseisen, J.; Flesch-Janys, D.; Kaaks, R.; Chang-Claude, J. Serum enterolactone and postmenopausal breast cancer risk by estrogen, progesterone and herceptin 2 receptor status. Int. J. Cancer 2011, 130, 1401–1410. [Google Scholar]
  13. Torres-Sanchez, L.; Galvan-Portillo, M.; Wolff, M.S.; Lopez-Carrillo, L. Dietary consumption of phytochemicals and breast cancer risk in Mexican women. Public Health Nutr. 2009, 12, 825–831. [Google Scholar]
  14. Otaegui-Arrazola, A.; Menendez-Carreno, M.; Ansorena, D.; Astiasaran, I. Oxysterols: A world to explore. Food Chem. Toxicol. 2010, 48, 3289–3303. [Google Scholar]
  15. Roche, Y.; Gerbeau-Pissot, P.; Buhot, B.; Thomas, D.; Bonneau, L.; Gresti, J.; Mongrand, S.; Perrier-Cornet, J.M.; Simon-Plas, F. Depletion of phytosterols from the plant plasma membrane provides evidence for disruption of lipid rafts. FASEB J. 2008, 22, 3980–3991. [Google Scholar]
  16. Weihrauch, J.L.; Gardner, J.M. Sterol content of foods of plant origin. J. Am. Diet. Assoc. 1978, 73, 39–47. [Google Scholar]
  17. Ito, T.; Tamura, T.; Matsumoto, T. Sterol composition of 19 vegetable oils. J. Am. Oil Chem. Soc. 1973, 50, 122–125. [Google Scholar] [CrossRef]
  18. Amaral, J.S.; Casal, S.; Pereira, J.A.; Seabra, R.M.; Oliveira, B.P. Determination of sterol and fatty acid compositions, oxidative stability, and nutritional value of six walnut (Juglans regia L.) cultivars grown in Portugal. J. Agric. Food Chem. 2003, 51, 7698–7702. [Google Scholar] [CrossRef]
  19. Dietary Guidelines for Americans; US Department of Health and Human Services: Washington, DC, USA, 2005.
  20. Ostlund, R.E., Jr. Phytosterols in human nutrition. Annu. Rev. Nutr. 2002, 22, 533–549. [Google Scholar] [CrossRef]
  21. Heinemann, T.; Axtmann, G.; von Bergmann, K. Comparison of intestinal absorption of cholesterol with different plant sterols in man. Eur. J. Clin. Invest. 1993, 23, 827–831. [Google Scholar] [CrossRef]
  22. Ostlund, R.E., Jr. Phytosterols and cholesterol metabolism. Curr. Opin. Lipidol. 2004, 15, 37–41. [Google Scholar] [CrossRef]
  23. Salen, G.; Ahrens, E.H., Jr.; Grundy, S.M. Metabolism of beta-sitosterol in man. J. Clin. Invest. 1970, 49, 952–967. [Google Scholar] [CrossRef]
  24. Ostlund, R.E., Jr.; McGill, J.B.; Zeng, C.M.; Covey, D.F.; Stearns, J.; Stenson, W.F.; Spilburg, C.A. Gastrointestinal absorption and plasma kinetics of soy Delta(5)-phytosterols and phytostanols in humans. Am. J. Physiol. Endocrinol. Metab. 2002, 282, E911–E916. [Google Scholar]
  25. Bradford, P.G.; Awad, A.B. Phytosterols as anticancer compounds. Mol. Nutr. Food Res. 2007, 51, 161–170. [Google Scholar] [CrossRef]
  26. Hallikainen, M.A.; Sarkkinen, E.S.; Gylling, H.; Erkkila, A.T.; Uusitupa, M.I. Comparison of the effects of plant sterol ester and plant stanol ester-enriched margarines in lowering serum cholesterol concentrations in hypercholesterolaemic subjects on a low-fat diet. Eur. J. Clin. Nutr. 2000, 54, 715–725. [Google Scholar] [CrossRef]
  27. Weststrate, J.A.; Meijer, G.W. Plant sterol-enriched margarines and reduction of plasma total- and LDL-cholesterol concentrations in normocholesterolaemic and mildly hypercholesterolaemic subjects. Eur. J. Clin. Nutr. 1998, 52, 334–343. [Google Scholar]
  28. Graf, G.A.; Li, W.P.; Gerard, R.D.; Gelissen, I.; White, A.; Cohen, J.C.; Hobbs, H.H. Coexpression of ATP-binding cassette proteins ABCG5 and ABCG8 permits their transport to the apical surface. J. Clin. Invest. 2002, 110, 659–669. [Google Scholar]
  29. Yu, L.; Hammer, R.E.; Li-Hawkins, J.; von Bergmann, K.; Lutjohann, D.; Cohen, J.C.; Hobbs, H.H. Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion. Proc. Natl. Acad. Sci. USA 2002, 99, 16237–16242. [Google Scholar]
  30. Yu, L.; Li-Hawkins, J.; Hammer, R.E.; Berge, K.E.; Horton, J.D.; Cohen, J.C.; Hobbs, H.H. Overexpression of ABCG5 and ABCG8 promotes biliary cholesterol secretion and reduces fractional absorption of dietary cholesterol. J. Clin. Invest. 2002, 110, 671–680. [Google Scholar]
  31. Chan, Y.M.; Varady, K.A.; Lin, Y.; Trautwein, E.; Mensink, R.P.; Plat, J.; Jones, P.J. Plasma concentrations of plant sterols: Physiology and relationship with coronary heart disease. Nutr. Rev. 2006, 64, 385–402. [Google Scholar]
  32. Messina, M.; Barnes, S. The role of soy products in reducing risk of cancer. J. Natl. Cancer Inst. 1991, 83, 541–546. [Google Scholar]
  33. Cui, X.; Dai, Q.; Tseng, M.; Shu, X.O.; Gao, Y.T.; Zheng, W. Dietary patterns and breast cancer risk in the shanghai breast cancer study. Cancer Epidemiol. Biomarkers Prev. 2007, 16, 1443–1448. [Google Scholar]
  34. Law, M. Plant sterol and stanol margarines and health. BMJ 2000, 320, 861–864. [Google Scholar]
  35. Jones, P.J.; Raeini-Sarjaz, M. Plant sterols and their derivatives: The current spread of results. Nutr. Rev. 2001, 59, 21–24. [Google Scholar]
  36. Katan, M.B.; Grundy, S.M.; Jones, P.; Law, M.; Miettinen, T.; Paoletti, R. Efficacy and safety of plant stanols and sterols in the management of blood cholesterol levels. Mayo Clin. Proc. 2003, 78, 965–978. [Google Scholar]
  37. Normen, A.L.; Brants, H.A.; Voorrips, L.E.; Andersson, H.A.; van den Brandt, P.A.; Goldbohm, R.A. Plant sterol intakes and colorectal cancer risk in the Netherlands Cohort Study on Diet and Cancer. Am. J. Clin. Nutr. 2001, 74, 141–148. [Google Scholar]
  38. Awad, A.B.; Fink, C.S.; Williams, H.; Kim, U. In vitro and in vivo (SCID mice) effects of phytosterols on the growth and dissemination of human prostate cancer PC-3 cells. Eur. J. Cancer Prev. 2001, 10, 507–513. [Google Scholar] [CrossRef]
  39. Baskar, A.A.; Al Numair, K.S.; Gabriel Paulraj, M.; Alsaif, M.A.; Muamar, M.A.; Ignacimuthu, S. Beta-sitosterol prevents lipid peroxidation and improves antioxidant status and histoarchitecture in rats with 1,2-dimethylhydrazine-induced colon cancer. J. Med. Food 2012, 15, 335–343. [Google Scholar]
  40. Mendilaharsu, M.; de Stefani, E.; Deneo-Pellegrini, H.; Carzoglio, J.; Ronco, A. Phytosterols and risk of lung cancer: A case-control study in Uruguay. Lung Cancer 1998, 21, 37–45. [Google Scholar] [CrossRef]
  41. De Stefani, E.; Boffetta, P.; Ronco, A.L.; Brennan, P.; Deneo-Pellegrini, H.; Carzoglio, J.C.; Mendilaharsu, M. Plant sterols and risk of stomach cancer: A case-control study in Uruguay. Nutr. Cancer 2000, 37, 140–144. [Google Scholar]
  42. Nair, P.P.; Turjman, N.; Kessie, G.; Calkins, B.; Goodman, G.T.; Davidovitz, H.; Nimmagadda, G. Diet, nutrition intake, and metabolism in populations at high and low risk for colon cancer. Dietary cholesterol, beta-sitosterol, and stigmasterol. Am. J. Clin. Nutr. 1984, 40, 927–930. [Google Scholar]
  43. The American Institute for Cancer Research (AICR). Plant Compound Continue to Challenge Science; AICR: Washington, DC, USA, 2006.
  44. Jemal, A.; Bray, F.; Center, M.M.; Ferlay, J.; Ward, E.; Forman, D. Global cancer statistics. CA Cancer J. Clin. 2011, 61, 69–90. [Google Scholar] [CrossRef]
  45. Bray, F.; McCarron, P.; Parkin, D.M. The changing global patterns of female breast cancer incidence and mortality. Breast Cancer Res. 2004, 6, 229–239. [Google Scholar]
  46. Kataja, V.; Castiglione, M. Primary breast cancer: ESMO clinical recommendations for diagnosis, treatment and follow-up. Ann. Oncol. 2009, 20, 10–14. [Google Scholar]
  47. Ganry, O. Phytoestrogen and breast cancer prevention. Eur. J. Cancer Prev. 2002, 11, 519–522. [Google Scholar] [CrossRef]
  48. Signorelli, P.; Ghidoni, R. Resveratrol as an anticancer nutrient: molecular basis, open questions and promises. J. Nutr. Biochem. 2005, 16, 449–466. [Google Scholar] [CrossRef]
  49. McGuire, W.L. Steroid receptors in human breast cancer. Cancer Res. 1978, 38, 4289–4291. [Google Scholar]
  50. Jordan, V.C. Progress in the prevention of breast cancer: Concept to reality. J. Steroid Biochem. Mol. Biol. 2000, 74, 269–277. [Google Scholar] [CrossRef]
  51. Begg, L.; Kuller, L.H.; Gutai, J.P.; Caggiula, A.G.; Wolmark, N.; Watson, C.G. Endogenous sex hormone levels and breast cancer risk. Genet. Epidemiol. 1987, 4, 233–247. [Google Scholar] [CrossRef]
  52. Rossouw, J.E.; Anderson, G.L.; Prentice, R.L.; LaCroix, A.Z.; Kooperberg, C.; Stefanick, M.L.; Jackson, R.D.; Beresford, S.A.; Howard, B.V.; Johnson, K.C.; et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: Principal results From the Women’s Health Initiative randomized controlled trial. JAMA 2002, 288, 321–333. [Google Scholar]
  53. Vogel, V.G.; Costantino, J.P.; Wickerham, D.L.; Cronin, W.M.; Cecchini, R.S.; Atkins, J.N.; Bevers, T.B.; Fehrenbacher, L.; Pajon, E.R., Jr.; Wade, J.L., III; et al. Effects of tamoxifen vs. raloxifene on the risk of developing invasive breast cancer and other disease outcomes: The NSABP Study of Tamoxifen and Raloxifene (STAR) P-2 trial. JAMA 2006, 295, 2727–2741. [Google Scholar]
  54. Rebbeck, T.R.; Lynch, H.T.; Neuhausen, S.L.; Narod, S.A.; Van’t Veer, L.; Garber, J.E.; Evans, G.; Isaacs, C.; Daly, M.B.; Matloff, E.; et al. Prophylactic oophorectomy in carriers of BRCA1 or BRCA2 mutations. N. Engl. J. Med. 2002, 346, 1616–1622. [Google Scholar]
  55. Russo, J.; Hasan Lareef, M.; Balogh, G.; Guo, S.; Russo, I.H. Estrogen and its metabolites are carcinogenic agents in human breast epithelial cells. J. Steroid Biochem. Mol. Biol. 2003, 87, 1–25. [Google Scholar]
  56. Fernandez, S.V.; Russo, I.H.; Russo, J. Estradiol and its metabolites 4-hydroxyestradiol and 2-hydroxyestradiol induce mutations in human breast epithelial cells. Int. J. Cancer 2006, 118, 1862–1868. [Google Scholar]
  57. Mackey, R.H.; Fanelli, T.J.; Modugno, F.; Cauley, J.A.; McTigue, K.M.; Brooks, M.M.; Chlebowski, R.T.; Manson, J.E.; Klug, T.L.; Kip, K.E.; et al. Hormone therapy, estrogen metabolism, and risk of breast cancer in the women’s health initiative hormone therapy trial. Cancer Epidemiol. Biomarkers Prev. 2012, 21, 2022–2032. [Google Scholar]
  58. Clark, J.H. A critique of Women’s Health Initiative Studies (2002-2006). Nucl. Recept. Signal. 2006, 4, e023. [Google Scholar]
  59. Shapiro, S. Risks of estrogen plus progestin therapy: A sensitivity analysis of findings in the Women’s Health Initiative randomized controlled trial. Climacteric 2003, 6, 302–310. [Google Scholar]
  60. Machens, K.; Schmidt-Gollwitzer, K. Issues to debate on the Women’s Health Initiative (WHI) study. Hormone replacement therapy: An epidemiological dilemma? Hum. Reprod. 2003, 18, 1992–1999. [Google Scholar]
  61. McDonough, P.G. The randomized world is not without its imperfections: Reflections on the Women’s Health Initiative Study. Fertil. Steril. 2002, 78, 951–956. [Google Scholar]
  62. Goodman, N.; Goldzieher, J.W.; Ayala, C. Critique of the report from the writing group of the WHI. Menopausal Med. 2003, 10, 1–4. [Google Scholar]
  63. Strom, A.; Hartman, J.; Foster, J.S.; Kietz, S.; Wimalasena, J.; Gustafsson, J.A. Estrogen receptor beta inhibits 17beta-estradiol-stimulated proliferation of the breast cancer cell line T47D. Proc. Natl. Acad. Sci. USA 2004, 101, 1566–1571. [Google Scholar]
  64. Jeffy, B.D.; Hockings, J.K.; Kemp, M.Q.; Morgan, S.S.; Hager, J.A.; Beliakoff, J.; Whitesell, L.J.; Bowden, G.T.; Romagnolo, D.F. An estrogen receptor-alpha/p300 complex activates the BRCA-1 promoter at an AP-1 site that binds Jun/Fos transcription factors: Repressive effects of p53 on BRCA-1 transcription. Neoplasia 2005, 7, 873–882. [Google Scholar] [CrossRef]
  65. Goss, P.E.; Ingle, J.N.; Ales-Martinez, J.E.; Cheung, A.M.; Chlebowski, R.T.; Wactawski-Wende, J.; McTiernan, A.; Robbins, J.; Johnson, K.C.; Martin, L.W.; et al. Exemestane for breast-cancer prevention in postmenopausal women. N. Engl. J. Med. 2011, 364, 2381–2391. [Google Scholar]
  66. Eckert, R.L.; Mullick, A.; Rorke, E.A.; Katzenellenbogen, B.S. Estrogen receptor synthesis and turnover in MCF-7 breast cancer cells measured by a density shift technique. Endocrinology 1984, 114, 629–637. [Google Scholar] [CrossRef]
  67. Ingram, D.; Sanders, K.; Kolybaba, M.; Lopez, D. Case-control study of phyto-oestrogens and breast cancer. Lancet 1997, 350, 990–994. [Google Scholar]
  68. Paruthiyil, S.; Parmar, H.; Kerekatte, V.; Cunha, G.R.; Firestone, G.L.; Leitman, D.C. Estrogen receptor beta inhibits human breast cancer cell proliferation and tumor formation by causing a G2 cell cycle arrest. Cancer Res. 2004, 64, 423–428. [Google Scholar] [CrossRef]
  69. Rock, C.L.; Saxe, G.A.; Ruffin, M.T., IV; August, D.A.; Schottenfeld, D. Carotenoids, vitamin A, and estrogen receptor status in breast cancer. Nutr. Cancer 1996, 25, 281–296. [Google Scholar]
  70. Ingram, D.M.; Roberts, A.; Nottage, E.M. Host factors and breast cancer growth characteristics. Eur. J. Cancer 1992, 28A, 1153–1161. [Google Scholar]
  71. Harlan, L.C.; Coates, R.J.; Block, G.; Greenberg, R.S.; Ershow, A.; Forman, M.; Austin, D.F.; Chen, V.; Heymsfield, S.B. Estrogen receptor status and dietary intakes in breast cancer patients. Epidemiology 1993, 4, 25–31. [Google Scholar]
  72. Saxe, G.A.; Rock, C.L.; Wicha, M.S.; Schottenfeld, D. Diet and risk for breast cancer recurrence and survival. Breast Cancer Res. Treat. 1999, 53, 241–253. [Google Scholar]
  73. Ariazi, E.A.; Kraus, R.J.; Farrell, M.L.; Jordan, V.C.; Mertz, J.E. Estrogen-related receptor alpha1 transcriptional activities are regulated in part via the ErbB2/HER2 signaling pathway. Mol. Cancer Res. 2007, 5, 71–85. [Google Scholar]
  74. Herynk, M.H.; Fuqua, S.A. Estrogen receptor mutations in human disease. Endocr. Rev. 2004, 25, 869–898. [Google Scholar] [CrossRef]
  75. Oesterreich, S.; Zhang, P.; Guler, R.L.; Sun, X.; Curran, E.M.; Welshons, W.V.; Osborne, C.K.; Lee, A.V. Re-expression of estrogen receptor alpha in estrogen receptor alpha-negative MCF-7 cells restores both estrogen and insulin-like growth factor-mediated signaling and growth. Cancer Res. 2001, 61, 5771–5777. [Google Scholar]
  76. Krege, J.H.; Hodgin, J.B.; Couse, J.F.; Enmark, E.; Warner, M.; Mahler, J.F.; Sar, M.; Korach, K.S.; Gustafsson, J.A.; Smithies, O. Generation and reproductive phenotypes of mice lacking estrogen receptor beta. Proc. Natl. Acad. Sci. USA 1998, 95, 15677–15682. [Google Scholar]
  77. Touillaud, M.S.; Pillow, P.C.; Jakovljevic, J.; Bondy, M.L.; Singletary, S.E.; Li, D.; Chang, S. Effect of dietary intake of phytoestrogens on estrogen receptor status in premenopausal women with breast cancer. Nutr. Cancer 2005, 51, 162–169. [Google Scholar]
  78. Levi, J.A.; Wheeler, H.R. Current status of treatment for breast cancer. Ann. Acad. Med. Singap. 1990, 19, 281–285. [Google Scholar]
  79. Johnston, S.R.; Head, J.; Pancholi, S.; Detre, S.; Martin, L.A.; Smith, I.E.; Dowsett, M. Integration of signal transduction inhibitors with endocrine therapy: An approach to overcoming hormone resistance in breast cancer. Clin. Cancer Res. 2003, 9, 524S–532S. [Google Scholar]
  80. Gutendorf, B.; Westendorf, J. Comparison of an array of in vitro assays for the assessment of the estrogenic potential of natural and synthetic estrogens, phytoestrogens and xenoestrogens. Toxicology 2001, 166, 79–89. [Google Scholar]
  81. Sakamoto, T.; Horiguchi, H.; Oguma, E.; Kayama, F. Effects of diverse dietary phytoestrogens on cell growth, cell cycle and apoptosis in estrogen-receptor-positive breast cancer cells. J. Nutr. Biochem. 2009, 21, 856–864. [Google Scholar]
  82. Baker, V.A.; Hepburn, P.A.; Kennedy, S.J.; Jones, P.A.; Lea, L.J.; Sumpter, J.P.; Ashby, J. Safety evaluation of phytosterol esters. Part 1. Assessment of oestrogenicity using a combination of in vivo and in vitro assays. Food Chem. Toxicol. 1999, 37, 13–22. [Google Scholar]
  83. Turnbull, D.; Frankos, V.H.; Leeman, W.R.; Jonker, D. Short-term tests of estrogenic potential of plant stanols and plant stanol esters. Regul. Toxicol. Pharmacol. 1999, 29, 211–215. [Google Scholar] [CrossRef]
  84. Malini, T.; Vanithakumari, G. Effect of beta-sitosterol on uterine biochemistry: A comparative study with estradiol and progesterone. Biochem. Mol. Biol. Int. 1993, 31, 659–668. [Google Scholar]
  85. Umetani, M.; Shaul, P.W. 27-Hydroxycholesterol: the first identified endogenous SERM. Trends Endocrinol. Metab. 2011, 22, 130–135. [Google Scholar] [CrossRef]
  86. Nguyen, L.B.; Shefer, S.; Salen, G.; Tint, S.G.; Batta, A.K. Competitive inhibition of hepatic sterol 27-hydroxylase by sitosterol: Decreased activity in sitosterolemia. Proc. Assoc. Am. Physicians 1998, 110, 32–39. [Google Scholar]
  87. Frasor, J.; Stossi, F.; Danes, J.M.; Komm, B.; Lyttle, C.R.; Katzenellenbogen, B.S. Selective estrogen receptor modulators: Discrimination of agonistic versus antagonistic activities by gene expression profiling in breast cancer cells. Cancer Res. 2004, 64, 1522–1533. [Google Scholar]
  88. Dobs, A.S.; Schrott, H.; Davidson, M.H.; Bays, H.; Stein, E.A.; Kush, D.; Wu, M.; Mitchel, Y.; Illingworth, R.D. Effects of high-dose simvastatin on adrenal and gonadal steroidogenesis in men with hypercholesterolemia. Metabolism 2000, 49, 1234–1238. [Google Scholar]
  89. Plat, J.; Nichols, J.A.; Mensink, R.P. Plant sterols and stanols: Effects on mixed micellar composition and LXR (target gene) activation. J. Lipid Res. 2005, 46, 2468–2476. [Google Scholar]
  90. Song, C.; Kokontis, J.M.; Hiipakka, R.A.; Liao, S. Ubiquitous receptor: A receptor that modulates gene activation by retinoic acid and thyroid hormone receptors. Proc. Natl. Acad. Sci. USA 1994, 91, 10809–10813. [Google Scholar] [CrossRef]
  91. Seol, W.; Choi, H.S.; Moore, D.D. Isolation of proteins that interact specifically with the retinoid X receptor: Two novel orphan receptors. Mol. Endocrinol. 1995, 9, 72–85. [Google Scholar] [CrossRef]
  92. Zelcer, N.; Tontonoz, P. Liver X receptors as integrators of metabolic and inflammatory signaling. J. Clin. Invest. 2006, 116, 607–614. [Google Scholar]
  93. Vedin, L.L.; Lewandowski, S.A.; Parini, P.; Gustafsson, J.A.; Steffensen, K.R. The oxysterol receptor LXR inhibits proliferation of human breast cancer cells. Carcinogenesis 2009, 30, 575–579. [Google Scholar] [CrossRef]
  94. Vigushin, D.M.; Dong, Y.; Inman, L.; Peyvandi, N.; Alao, J.P.; Sun, C.; Ali, S.; Niesor, E.J.; Bentzen, C.L.; Coombes, R.C. The nuclear oxysterol receptor LXRalpha is expressed in the normal human breast and in breast cancer. Med. Oncol. 2004, 21, 123–131. [Google Scholar]
  95. Fukuchi, J.; Kokontis, J.M.; Hiipakka, R.A.; Chuu, C.P.; Liao, S. Antiproliferative effect of liver X receptor agonists on LNCaP human prostate cancer cells. Cancer Res. 2004, 64, 7686–7689. [Google Scholar] [CrossRef]
  96. Tontonoz, P.; Mangelsdorf, D.J. Liver X receptor signaling pathways in cardiovascular disease. Mol. Endocrinol. 2003, 17, 985–993. [Google Scholar] [CrossRef]
  97. Chawla, A.; Repa, J.J.; Evans, R.M.; Mangelsdorf, D.J. Nuclear receptors and lipid physiology: Opening the X-files. Science 2001, 294, 1866–1870. [Google Scholar] [CrossRef]
  98. Song, W.C. Biochemistry and reproductive endocrinology of estrogen sulfotransferase. Ann. N. Y. Acad. Sci. 2001, 948, 43–50. [Google Scholar] [CrossRef]
  99. Gong, H.; Guo, P.; Zhai, Y.; Zhou, J.; Uppal, H.; Jarzynka, M.J.; Song, W.C.; Cheng, S.Y.; Xie, W. Estrogen deprivation and inhibition of breast cancer growth in vivo through activation of the orphan nuclear receptor liver X receptor. Mol. Endocrinol. 2007, 21, 1781–1790. [Google Scholar] [CrossRef]
  100. Holm, C.; Kok, M.; Michalides, R.; Fles, R.; Koornstra, R.H.; Wesseling, J.; Hauptmann, M.; Neefjes, J.; Peterse, J.L.; Stal, O.; et al. Phosphorylation of the oestrogen receptor alpha at serine 305 and prediction of tamoxifen resistance in breast cancer. J. Pathol. 2009, 217, 372–379. [Google Scholar] [CrossRef]
  101. Kuraishy, A.; Karin, M.; Grivennikov, S.I. Tumor promotion via injury- and death-induced inflammation. Immunity 2011, 35, 467–477. [Google Scholar] [CrossRef]
  102. Lin, W.W.; Karin, M. A cytokine-mediated link between innate immunity, inflammation, and cancer. J. Clin. Invest. 2007, 117, 1175–1183. [Google Scholar] [CrossRef]
  103. Mantovani, A.; Allavena, P.; Sica, A.; Balkwill, F. Cancer-related inflammation. Nature 2008, 454, 436–444. [Google Scholar]
  104. Shankaran, V.; Ikeda, H.; Bruce, A.T.; White, J.M.; Swanson, P.E.; Old, L.J.; Schreiber, R.D. IFNgamma and lymphocytes prevent primary tumour development and shape tumour immunogenicity. Nature 2001, 410, 1107–1111. [Google Scholar]
  105. Schreiber, R.D.; Old, L.J.; Smyth, M.J. Cancer immunoediting: integrating immunity’s roles in cancer suppression and promotion. Science 2011, 331, 1565–1570. [Google Scholar]
  106. Mahmoud, S.M.; Paish, E.C.; Powe, D.G.; Macmillan, R.D.; Grainge, M.J.; Lee, A.H.; Ellis, I.O.; Green, A.R. Tumor-infiltrating CD8+ lymphocytes predict clinical outcome in breast cancer. J. Clin. Oncol. 2011, 29, 1949–1955. [Google Scholar]
  107. Liyanage, U.K.; Moore, T.T.; Joo, H.G.; Tanaka, Y.; Herrmann, V.; Doherty, G.; Drebin, J.A.; Strasberg, S.M.; Eberlein, T.J.; Goedegebuure, P.S.; Linehan, D.C. Prevalence of regulatory T cells is increased in peripheral blood and tumor microenvironment of patients with pancreas or breast adenocarcinoma. J. Immunol. 2002, 169, 2756–2761. [Google Scholar]
  108. Beyer, M.; Schultze, J.L. Regulatory T cells in cancer. Blood 2006, 108, 804–811. [Google Scholar] [CrossRef]
  109. Breytenbach, U.; Clark, A.; Lamprecht, J.; Bouic, P. Flow cytometric analysis of the Th1-Th2 balance in healthy individuals and patients infected with the human immunodeficiency virus (HIV) receiving a plant sterol/sterolin mixture. Cell Biol. Int. 2001, 25, 43–49. [Google Scholar] [CrossRef]
  110. Qin, L.; Jin, L.; Lu, L.; Lu, X.; Zhang, C.; Zhang, F.; Liang, W. Naringenin reduces lung metastasis in a breast cancer resection model. Protein Cell 2011, 2, 507–516. [Google Scholar] [CrossRef]
  111. Calpe-Berdiel, L.; Escola-Gil, J.C.; Benitez, S.; Bancells, C.; Gonzalez-Sastre, F.; Palomer, X.; Blanco-Vaca, F. Dietary phytosterols modulate T-helper immune response but do not induce apparent anti-inflammatory effects in a mouse model of acute, aseptic inflammation. Life Sci. 2007, 80, 1951–1956. [Google Scholar] [CrossRef]
  112. Bouic, P.J.; Etsebeth, S.; Liebenberg, R.W.; Albrecht, C.F.; Pegel, K.; van Jaarsveld, P.P. Beta-sitosterol and beta-sitosterol glucoside stimulate human peripheral blood lymphocyte proliferation: Implications for their use as an immunomodulatory vitamin combination. Int. J. Immunopharmacol. 1996, 18, 693–700. [Google Scholar] [CrossRef]
  113. Imanaka, H.; Koide, H.; Shimizu, K.; Asai, T.; Kinouchi Shimizu, N.; Ishikado, A.; Makino, T.; Oku, N. Chemoprevention of tumor metastasis by liposomal beta-sitosterol intake. Biol. Pharm. Bull. 2008, 31, 400–404. [Google Scholar] [CrossRef]
  114. Turkson, J.; Bowman, T.; Garcia, R.; Caldenhoven, E.; de Groot, R.P.; Jove, R. Stat3 activation by Src induces specific gene regulation and is required for cell transformation. Mol. Cell. Biol. 1998, 18, 2545–2552. [Google Scholar]
  115. Bromberg, J.F.; Horvath, C.M.; Besser, D.; Lathem, W.W.; Darnell, J.E., Jr. Stat3 activation is required for cellular transformation by v-src. Mol. Cell. Biol. 1998, 18, 2553–2558. [Google Scholar]
  116. Bromberg, J.F.; Wrzeszczynska, M.H.; Devgan, G.; Zhao, Y.; Pestell, R.G.; Albanese, C.; Darnell, J.E., Jr. Stat3 as an oncogene. Cell 1999, 98, 295–303. [Google Scholar] [CrossRef]
  117. Bhardwaj, A.; Sethi, G.; Vadhan-Raj, S.; Bueso-Ramos, C.; Takada, Y.; Gaur, U.; Nair, A.S.; Shishodia, S.; Aggarwal, B.B. Resveratrol inhibits proliferation, induces apoptosis, and overcomes chemoresistance through down-regulation of STAT3 and nuclear factor-kappaB-regulated antiapoptotic and cell survival gene products in human multiple myeloma cells. Blood 2007, 109, 2293–2302. [Google Scholar] [CrossRef]
  118. Garcia, R.; Yu, C.L.; Hudnall, A.; Catlett, R.; Nelson, K.L.; Smithgall, T.; Fujita, D.J.; Ethier, S.P.; Jove, R. Constitutive activation of Stat3 in fibroblasts transformed by diverse oncoproteins and in breast carcinoma cells. Cell Growth Differ. 1997, 8, 1267–1276. [Google Scholar]
  119. Turkson, J.; Jove, R. STAT proteins: Novel molecular targets for cancer drug discovery. Oncogene 2000, 19, 6613–6626. [Google Scholar] [CrossRef]
  120. Hu, X.; Chen, J.; Wang, L.; Ivashkiv, L.B. Crosstalk among Jak-STAT, Toll-like receptor, and ITAM-dependent pathways in macrophage activation. J. Leukoc. Biol. 2007, 82, 237–243. [Google Scholar] [CrossRef]
  121. Miettinen, M.; Lehtonen, A.; Julkunen, I.; Matikainen, S. Lactobacilli and Streptococci activate NF-kappa B and STAT signaling pathways in human macrophages. J. Immunol. 2000, 164, 3733–3740. [Google Scholar]
  122. Sareila, O.; Hamalainen, M.; Nissinen, E.; Kankaanranta, H.; Moilanen, E. Orazipone inhibits activation of inflammatory transcription factors nuclear factor-kappa B and signal transducer and activator of transcription 1 and decreases inducible nitric-oxide synthase expression and nitric oxide production in response to inflammatory stimuli. J. Pharmacol. Exp. Ther. 2008, 324, 858–866. [Google Scholar]
  123. Lo, H.W.; Hsu, S.C.; Xia, W.; Cao, X.; Shih, J.Y.; Wei, Y.; Abbruzzese, J.L.; Hortobagyi, G.N.; Hung, M.C. Epidermal growth factor receptor cooperates with signal transducer and activator of transcription 3 to induce epithelial-mesenchymal transition in cancer cells via up-regulation of TWIST gene expression. Cancer Res. 2007, 67, 9066–9076. [Google Scholar]
  124. Chan, S.K.; Hill, M.E.; Gullick, W.J. The role of the epidermal growth factor receptor in breast cancer. J. Mammary Gland. Biol. Neoplasia 2006, 11, 3–11. [Google Scholar] [CrossRef]
  125. Nicholson, R.I.; Gee, J.M.; Harper, M.E. EGFR and cancer prognosis. Eur. J. Cancer 2001, 37, S9–S15. [Google Scholar]
  126. Klijn, J.G.; Berns, P.M.; Schmitz, P.I.; Foekens, J.A. The clinical significance of epidermal growth factor receptor (EGF-R) in human breast cancer: A review on 5232 patients. Endocr. Rev. 1992, 13, 3–17. [Google Scholar]
  127. Levin, E.R. Bidirectional signaling between the estrogen receptor and the epidermal growth factor receptor. Mol. Endocrinol. 2003, 17, 309–317. [Google Scholar] [CrossRef]
  128. Kari, C.; Chan, T.O.; Rocha de Quadros, M.; Rodeck, U. Targeting the epidermal growth factor receptor in cancer: Apoptosis takes center stage. Cancer Res. 2003, 63, 1–5. [Google Scholar]
  129. Xia, W.; Lau, Y.K.; Zhang, H.Z.; Xiao, F.Y.; Johnston, D.A.; Liu, A.R.; Li, L.; Katz, R.L.; Hung, M.C. Combination of EGFR, HER-2/neu, and HER-3 is a stronger predictor for the outcome of oral squamous cell carcinoma than any individual family members. Clin. Cancer Res. 1999, 5, 4164–4174. [Google Scholar]
  130. Gonzalez-Angulo, A.M.; Hortobagyi, G.N.; Esteva, F.J. Adjuvant therapy with trastuzumab for HER-2/neu-positive breast cancer. Oncologist 2006, 11, 857–867. [Google Scholar] [CrossRef]
  131. Srinivasan, V.; Spence, D.W.; Pandi-Perumal, S.R.; Trakht, I.; Esquifino, A.I.; Cardinali, D.P.; Maestroni, G.J. Melatonin, environmental light, and breast cancer. Breast Cancer Res. Treat. 2008, 108, 339–350. [Google Scholar]
  132. Lollini, P.L.; Nicoletti, G.; Landuzzi, L.; de Giovanni, C.; Rossi, I.; di Carlo, E.; Musiani, P.; Muller, W.J.; Nanni, P. Down regulation of major histocompatibility complex class I expression in mammary carcinoma of HER-2/neu transgenic mice. Int. J. Cancer 1998, 77, 937–941. [Google Scholar] [CrossRef]
  133. Herrmann, F.; Lehr, H.A.; Drexler, I.; Sutter, G.; Hengstler, J.; Wollscheid, U.; Seliger, B. HER-2/neu-mediated regulation of components of the MHC class I antigen-processing pathway. Cancer Res. 2004, 64, 215–220. [Google Scholar]
  134. Oliveras-Ferraros, C.; Cufi, S.; Vazquez-Martin, A.; Menendez, O.J.; Bosch-Barrera, J.; Martin-Castillo, B.; Joven, J.; Menendez, J.A. Metformin rescues cell surface major histocompatibility complex class I (MHC-I) deficiency caused by oncogenic transformation. Cell Cycle 2012, 11, 865–870. [Google Scholar] [CrossRef]
  135. Valerio, M.; Awad, A.B. Beta-Sitosterol down-regulates some pro-inflammatory signal transduction pathways by increasing the activity of tyrosine phosphatase SHP-1 in J774A.1 murine macrophages. Int. Immunopharmacol. 2011, 11, 1012–1017. [Google Scholar] [CrossRef]
  136. Nakshatri, H.; Bhat-Nakshatri, P.; Martin, D.A.; Goulet, R.J., Jr.; Sledge, G.W., Jr. Constitutive activation of NF-kappaB during progression of breast cancer to hormone-independent growth. Mol. Cell. Biol. 1997, 17, 3629–3639. [Google Scholar]
  137. Uzzo, R.G.; Leavis, P.; Hatch, W.; Gabai, V.L.; Dulin, N.; Zvartau, N.; Kolenko, V.M. Zinc inhibits nuclear factor-kappa B activation and sensitizes prostate cancer cells to cytotoxic agents. Clin. Cancer Res. 2002, 8, 3579–3583. [Google Scholar]
  138. Muenchen, H.J.; Lin, D.L.; Walsh, M.A.; Keller, E.T.; Pienta, K.J. Tumor necrosis factor-alpha-induced apoptosis in prostate cancer cells through inhibition of nuclear factor-kappaB by an IkappaBalpha “super-repressor”. Clin. Cancer Res. 2000, 6, 1969–1977. [Google Scholar]
  139. Huang, S.; Pettaway, C.A.; Uehara, H.; Bucana, C.D.; Fidler, I.J. Blockade of NF-kappaB activity in human prostate cancer cells is associated with suppression of angiogenesis, invasion, and metastasis. Oncogene 2001, 20, 4188–4197. [Google Scholar] [CrossRef]
  140. Uzzo, R.G.; Crispen, P.L.; Golovine, K.; Makhov, P.; Horwitz, E.M.; Kolenko, V.M. Diverse effects of zinc on NF-kappaB and AP-1 transcription factors: implications for prostate cancer progression. Carcinogenesis 2006, 27, 1980–1990. [Google Scholar] [CrossRef]
  141. Jesch, E.D.; Seo, J.M.; Carr, T.P.; Lee, J.Y. Sitosterol reduces messenger RNA and protein expression levels of Niemann-Pick C1-like 1 in FHs 74 Int cells. Nutr. Res. 2009, 29, 859–866. [Google Scholar] [CrossRef]
  142. Awad, A.B.; Downie, A.C.; Fink, C.S. Inhibition of growth and stimulation of apoptosis by beta-sitosterol treatment of MDA-MB-231 human breast cancer cells in culture. Int. J. Mol. Med. 2000, 5, 541–545. [Google Scholar]
  143. Awad, A.B.; Barta, S.L.; Fink, C.S.; Bradford, P.G. beta-Sitosterol enhances tamoxifen effectiveness on breast cancer cells by affecting ceramide metabolism. Mol. Nutr. Food Res. 2008, 52, 419–426. [Google Scholar] [CrossRef]
  144. Awad, A.B.; Williams, H.; Fink, C.S. Effect of phytosterols on cholesterol metabolism and MAP kinase in MDA-MB-231 human breast cancer cells. J. Nutr. Biochem. 2003, 14, 111–119. [Google Scholar] [CrossRef]
  145. Pollak, O.J. Reduction of blood cholesterol in man. Circulation 1953, 7, 702–706. [Google Scholar] [CrossRef]
  146. Hinds, M.W.; Kolonel, L.N.; Lee, J.; Hankin, J.H. Dietary cholesterol and lung cancer risk among men in Hawaii. Am. J. Clin. Nutr. 1983, 37, 192–193. [Google Scholar]
  147. Kitahara, C.M.; Berrington de Gonzalez, A.; Freedman, N.D.; Huxley, R.; Mok, Y.; Jee, S.H.; Samet, J.M. Total cholesterol and cancer risk in a large prospective study in Korea. J. Clin. Oncol. 2011, 29, 1592–1598. [Google Scholar]
  148. Jafri, H.; Alsheikh-Ali, A.A.; Karas, R.H. Baseline and on-treatment high-density lipoprotein cholesterol and the risk of cancer in randomized controlled trials of lipid-altering therapy. J. Am. Coll. Cardiol. 2010, 55, 2846–2854. [Google Scholar] [CrossRef]
  149. Demonty, I.; Ras, R.T.; van der Knaap, H.C.; Duchateau, G.S.; Meijer, L.; Zock, P.L.; Geleijnse, J.M.; Trautwein, E.A. Continuous dose-response relationship of the LDL-cholesterol-lowering effect of phytosterol intake. J. Nutr. 2009, 139, 271–284. [Google Scholar]
  150. Law, M.R.; Wald, N.J.; Rudnicka, A.R. Quantifying effect of statins on low density lipoprotein cholesterol, ischaemic heart disease, and stroke: Systematic review and meta-analysis. BMJ 2003, 326, 1423. [Google Scholar] [CrossRef]
  151. Graaf, M.R.; Beiderbeck, A.B.; Egberts, A.C.; Richel, D.J.; Guchelaar, H.J. The risk of cancer in users of statins. J. Clin. Oncol. 2004, 22, 2388–2394. [Google Scholar] [CrossRef]
  152. Poynter, J.N.; Gruber, S.B.; Higgins, P.D.; Almog, R.; Bonner, J.D.; Rennert, H.S.; Low, M.; Greenson, J.K.; Rennert, G. Statins and the risk of colorectal cancer. N. Engl. J. Med. 2005, 352, 2184–2192. [Google Scholar]
  153. Lau, V.W.; Journoud, M.; Jones, P.J. Plant sterols are efficacious in lowering plasma LDL and non-HDL cholesterol in hypercholesterolemic type 2 diabetic and nondiabetic persons. Am. J. Clin. Nutr. 2005, 81, 1351–1358. [Google Scholar]
  154. Ahn, J.; Lim, U.; Weinstein, S.J.; Schatzkin, A.; Hayes, R.B.; Virtamo, J.; Albanes, D. Prediagnostic total and high-density lipoprotein cholesterol and risk of cancer. Cancer Epidemiol. Biomarkers Prev. 2009, 18, 2814–2821. [Google Scholar] [CrossRef]
  155. Chen, H.W. Role of cholesterol metabolism in cell growth. Fed. Proc. 1984, 43, 126–130. [Google Scholar]
  156. Kim, S.; Han, J.; Lee, D.H.; Cho, K.H.; Kim, K.H.; Chung, J.H. Cholesterol, a major component of caveolae, down-regulates matrix metalloproteinase-1 expression through ERK/JNK pathway in cultured human dermal fibroblasts. Ann. Dermatol. 2010, 22, 379–388. [Google Scholar] [CrossRef]
  157. Kulic, A.; Dedic Plavetic, N.; Vrbanec, J.; Sirotkovic-Skerlev, M. Low serum MMP-1 in breast cancer: A negative prognostic factor? Biomarkers 2012, 17, 416–421. [Google Scholar] [CrossRef]
  158. Brown, M.S.; Goldstein, J.L. Suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and inhibition of growth of human fibroblasts by 7-ketocholesterol. J. Biol. Chem. 1974, 249, 7306–7314. [Google Scholar]
  159. Chen, H.W.; Kandutsch, A.A.; Waymouth, C. Inhibition of cell growth by oxygenated derivatives of cholesterol. Nature 1974, 251, 419–421. [Google Scholar] [CrossRef]
  160. Chen, H.W.; Heiniger, H.J.; Kandutsch, A.A. Relationship between sterol synthesis and DNA synthesis in phytohemagglutinin-stimulated mouse lymphocytes. Proc. Natl. Acad. Sci. USA 1975, 72, 1950–1954. [Google Scholar]
  161. Keyomarsi, K.; Sandoval, L.; Band, V.; Pardee, A.B. Synchronization of tumor and normal cells from G1 to multiple cell cycles by lovastatin. Cancer Res. 1991, 51, 3602–3609. [Google Scholar]
  162. Campbell, M.J.; Esserman, L.J.; Zhou, Y.; Shoemaker, M.; Lobo, M.; Borman, E.; Baehner, F.; Kumar, A.S.; Adduci, K.; Marx, C.; et al. Breast cancer growth prevention by statins. Cancer Res. 2006, 66, 8707–8714. [Google Scholar]
  163. Kumar, A.S.; Benz, C.C.; Shim, V.; Minami, C.A.; Moore, D.H.; Esserman, L.J. Estrogen receptor-negative breast cancer is less likely to arise among lipophilic statin users. Cancer Epidemiol. Biomarkers Prev. 2008, 17, 1028–1033. [Google Scholar] [CrossRef]
  164. Klawitter, J.; Shokati, T.; Moll, V.; Christians, U. Effects of lovastatin on breast cancer cells: A proteo-metabonomic study. Breast Cancer Res. 2010, 12, R16. [Google Scholar]
  165. Brown, M.S.; Goldstein, J.L. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor. Cell 1997, 89, 331–340. [Google Scholar] [CrossRef]
  166. Brown, A.J.; Sun, L.; Feramisco, J.D.; Brown, M.S.; Goldstein, J.L. Cholesterol addition to ER membranes alters conformation of SCAP, the SREBP escort protein that regulates cholesterol metabolism. Mol. Cell 2002, 10, 237–245. [Google Scholar] [CrossRef]
  167. Eberle, D.; Hegarty, B.; Bossard, P.; Ferre, P.; Foufelle, F. SREBP transcription factors: Master regulators of lipid homeostasis. Biochimie 2004, 86, 839–848. [Google Scholar] [CrossRef]
  168. Martinez-Botas, J.; Suarez, Y.; Ferruelo, A.J.; Gomez-Coronado, D.; Lasuncion, M.A. Cholesterol starvation decreases p34(cdc2) kinase activity and arrests the cell cycle at G2. FASEB J. 1999, 13, 1359–1370. [Google Scholar]
  169. Newcomb, P.A.; Trentham-Dietz, A.; Hampton, J.M. Bisphosphonates for osteoporosis treatment are associated with reduced breast cancer risk. Br. J. Cancer 2010, 102, 799–802. [Google Scholar] [CrossRef]
  170. Gnant, M.; Mlineritsch, B.; Schippinger, W.; Luschin-Ebengreuth, G.; Postlberger, S.; Menzel, C.; Jakesz, R.; Seifert, M.; Hubalek, M.; Bjelic-Radisic, V.; et al. Endocrine therapy plus zoledronic acid in premenopausal breast cancer. N. Engl. J. Med. 2009, 360, 679–691. [Google Scholar]
  171. Diel, I.J.; Solomayer, E.F.; Costa, S.D.; Gollan, C.; Goerner, R.; Wallwiener, D.; Kaufmann, M.; Bastert, G. Reduction in new metastases in breast cancer with adjuvant clodronate treatment. N. Engl. J. Med. 1998, 339, 357–363. [Google Scholar] [CrossRef]
  172. Amin, D.; Cornell, S.A.; Gustafson, S.K.; Needle, S.J.; Ullrich, J.W.; Bilder, G.E.; Perrone, M.H. Bisphosphonates used for the treatment of bone disorders inhibit squalene synthase and cholesterol biosynthesis. J. Lipid Res. 1992, 33, 1657–1663. [Google Scholar]
  173. Wu, Y.W.; Oesterlin, L.K.; Tan, K.T.; Waldmann, H.; Alexandrov, K.; Goody, R.S. Membrane targeting mechanism of Rab GTPases elucidated by semisynthetic protein probes. Nat. Chem. Biol. 2010, 6, 534–540. [Google Scholar] [CrossRef]
  174. Zerial, M.; McBride, H. Rab proteins as membrane organizers. Nat. Rev. Mol. Cell. Biol. 2001, 2, 107–117. [Google Scholar] [CrossRef]
  175. Schroder, M.; Kaufman, R.J. The mammalian unfolded protein response. Annu. Rev. Biochem. 2005, 74, 739–789. [Google Scholar] [CrossRef]
  176. Kim, I.; Xu, W.; Reed, J.C. Cell death and endoplasmic reticulum stress: Disease relevance and therapeutic opportunities. Nat. Rev. Drug Discov. 2008, 7, 1013–1030. [Google Scholar] [CrossRef]
  177. Marinangeli, C.P.; Varady, K.A.; Jones, P.J. Plant sterols combined with exercise for the treatment of hypercholesterolemia: Overview of independent and synergistic mechanisms of action. J. Nutr. Biochem. 2006, 17, 217–224. [Google Scholar] [CrossRef]
  178. Tas, F.; Hansel, H.; Belce, A.; Ilvan, S.; Argon, A.; Camlica, H.; Topuz, E. Oxidative stress in breast cancer. Med. Oncol. 2005, 22, 11–15. [Google Scholar] [CrossRef]
  179. Portakal, O.; Ozkaya, O.; Erden Inal, M.; Bozan, B.; Kosan, M.; Sayek, I. Coenzyme Q10 concentrations and antioxidant status in tissues of breast cancer patients. Clin. Biochem. 2000, 33, 279–284. [Google Scholar] [CrossRef]
  180. Nagulu, M. Oxidative Stress and Anti-Oxidant Status in Breast Cancer Patients. J. Pharm. Res. 2009, 2, 62–65. [Google Scholar]
  181. Vivancos, M.; Moreno, J.J. Beta-Sitosterol modulates antioxidant enzyme response in RAW 264.7 macrophages. Free Radic. Biol. Med. 2005, 39, 91–97. [Google Scholar] [CrossRef]
  182. Li, C.R.; Zhou, Z.; Lin, R.X.; Zhu, D.; Sun, Y.N.; Tian, L.L.; Li, L.; Gao, Y.; Wang, S.Q. Beta-sitosterol decreases irradiation-induced thymocyte early damage by regulation of the intracellular redox balance and maintenance of mitochondrial membrane stability. J. Cell. Biochem. 2007, 102, 748–758. [Google Scholar] [CrossRef]
  183. Gupta, R.; Sharma, A.K.; Dobhal, M.P.; Sharma, M.C.; Gupta, R.S. Antidiabetic and antioxidant potential of beta-sitosterol in streptozotocin-induced experimental hyperglycemia. J. Diabetes 2011, 3, 29–37. [Google Scholar] [CrossRef]
  184. Field, F.J.; Born, E.; Mathur, S.N. Effect of micellar beta-sitosterol on cholesterol metabolism in CaCo-2 cells. J. Lipid Res. 1997, 38, 348–360. [Google Scholar]
  185. Piechota-Polanczyk, A.; Goraca, A.; Demyanets, S.; Mittlboeck, M.; Domenig, C.; Neumayer, C.; Wojta, J.; Nanobachvili, J.; Huk, I.; Klinger, M. Simvastatin decreases free radicals formation in the human abdominal aortic aneurysm wall via NF-kappaB. Eur. J. Vasc. Endovasc. Surg. 2012, 44, 133–137. [Google Scholar]
  186. Bu, D.X.; Erl, W.; de Martin, R.; Hansson, G.K.; Yan, Z.Q. IKKbeta-dependent NF-kappaB pathway controls vascular inflammation and intimal hyperplasia. FASEB J. 2005, 19, 1293–1295. [Google Scholar]
  187. Warburg, O. On the origin of cancer cells. Science 1956, 123, 309–314. [Google Scholar]
  188. Menendez, J.A. Metformin is synthetically lethal with glucose withdrawal in cancer cells. Cell Cycle 2012, 11, 2782–2792. [Google Scholar] [CrossRef]
  189. Charni, S.; de Bettignies, G.; Rathore, M.G.; Aguilo, J.I.; van den Elsen, P.J.; Haouzi, D.; Hipskind, R.A.; Enriquez, J.A.; Sanchez-Beato, M.; Pardo, J.; et al. Oxidative phosphorylation induces de novo expression of the MHC class I in tumor cells through the ERK5 pathway. J. Immunol. 2010, 185, 3498–3503. [Google Scholar] [CrossRef]
  190. Hardie, D.G. AMPK: A key regulator of energy balance in the single cell and the whole organism. Int. J. Obes. (Lond.) 2008, 32, S7–S12. [Google Scholar] [CrossRef]
  191. Hardie, D.G. New roles for the LKB1→AMPK pathway. Curr. Opin. Cell. Biol. 2005, 17, 167–173. [Google Scholar] [CrossRef]
  192. Hardie, D.G. The AMP-activated protein kinase pathway—New players upstream and downstream. J. Cell Sci. 2004, 117, 5479–5487. [Google Scholar] [CrossRef]
  193. Towler, M.C.; Hardie, D.G. AMP-activated protein kinase in metabolic control and insulin signaling. Circ. Res. 2007, 100, 328–341. [Google Scholar] [CrossRef]
  194. Miller, R.A.; Birnbaum, M.J. An energetic tale of AMPK-independent effects of metformin. J. Clin. Invest. 2010, 120, 2267–2270. [Google Scholar] [CrossRef]
  195. Adams, J.; Chen, Z.P.; van Denderen, B.J.; Morton, C.J.; Parker, M.W.; Witters, L.A.; Stapleton, D.; Kemp, B.E. Intrasteric control of AMPK via the gamma1 subunit AMP allosteric regulatory site. Protein Sci. 2004, 13, 155–165. [Google Scholar] [CrossRef]
  196. Zhu, Z.; Jiang, W.; Thompson, M.D.; McGinley, J.N.; Thompson, H.J. Metformin as an energy restriction mimetic agent for breast cancer prevention. J. Carcinog. 2011, 10, 17. [Google Scholar] [CrossRef]
  197. Sutherland, W.H.; Scott, R.S.; Lintott, C.J.; Robertson, M.C.; Stapely, S.A.; Cox, C. Plasma non-cholesterol sterols in patients with non-insulin dependent diabetes mellitus. Horm. Metab. Res. 1992, 24, 172–175. [Google Scholar] [CrossRef]
  198. Ivorra, M.D.; D’Ocon, M.P.; Paya, M.; Villar, A. Antihyperglycemic and insulin-releasing effects of beta-sitosterol 3-beta-D-glucoside and its aglycone, beta-sitosterol. Arch. Int. Pharmacodyn. Ther. 1988, 296, 224–231. [Google Scholar]
  199. Hwang, S.L.; Kim, H.N.; Jung, H.H.; Kim, J.E.; Choi, D.K.; Hur, J.M.; Lee, J.Y.; Song, H.; Song, K.S.; Huh, T.L. Beneficial effects of beta-sitosterol on glucose and lipid metabolism in L6 myotube cells are mediated by AMP-activated protein kinase. Biochem. Biophys. Res. Commun. 2008, 377, 1253–1258. [Google Scholar] [CrossRef]
  200. Habegger, K.M.; Penque, B.A.; Sealls, W.; Tackett, L.; Bell, L.N.; Blue, E.K.; Gallagher, P.J.; Sturek, M.; Alloosh, M.A.; Steinberg, H.O.; et al. Fat-induced membrane cholesterol accrual provokes cortical filamentous actin destabilisation and glucose transport dysfunction in skeletal muscle. Diabetologia 2011, 55, 457–467. [Google Scholar]
  201. Looije, N.A.; Risovic, V.; Stewart, D.J.; Debeyer, D.; Kutney, J.; Wasan, K.M. Disodium Ascorbyl Phytostanyl Phosphates (FM-VP4) reduces plasma cholesterol concentration, body weight and abdominal fat gain within a dietary-induced obese mouse model. J. Pharm. Pharm. Sci. 2005, 8, 400–408. [Google Scholar]
  202. Kritchevsky, D. Caloric restriction and cancer. J. Nutr. Sci. Vitaminol. (Tokyo) 2001, 47, 13–19. [Google Scholar] [CrossRef]
  203. Thompson, H.J.; Zhu, Z.; Jiang, W. Dietary energy restriction in breast cancer prevention. J. Mammary Gland. Biol. Neoplasia 2003, 8, 133–142. [Google Scholar] [CrossRef]
  204. Carling, D.; Zammit, V.A.; Hardie, D.G. A common bicyclic protein kinase cascade inactivates the regulatory enzymes of fatty acid and cholesterol biosynthesis. FEBS Lett. 1987, 223, 217–222. [Google Scholar]
  205. Jones, R.G.; Plas, D.R.; Kubek, S.; Buzzai, M.; Mu, J.; Xu, Y.; Birnbaum, M.J.; Thompson, C.B. AMP-activated protein kinase induces a p53-dependent metabolic checkpoint. Mol. Cell 2005, 18, 283–293. [Google Scholar] [CrossRef]
  206. Levine, A.J. p53, the cellular gatekeeper for growth and division. Cell 1997, 88, 323–331. [Google Scholar] [CrossRef]
  207. Hollstein, M.; Sidransky, D.; Vogelstein, B.; Harris, C.C. p53 mutations in human cancers. Science 1991, 253, 49–53. [Google Scholar]
  208. Freed-Pastor, W.A.; Mizuno, H.; Zhao, X.; Langerod, A.; Moon, S.H.; Rodriguez-Barrueco, R.; Barsotti, A.; Chicas, A.; Li, W.; Polotskaia, A.; et al. Mutant p53 disrupts mammary tissue architecture via the mevalonate pathway. Cell 2012, 148, 244–258. [Google Scholar]
  209. Perens, E.A.; Shaham, S. C. elegans daf-6 encodes a patched-related protein required for lumen formation. Dev. Cell 2005, 8, 893–906. [Google Scholar] [CrossRef]
  210. Tullet, J.M.; Hertweck, M.; An, J.H.; Baker, J.; Hwang, J.Y.; Liu, S.; Oliveira, R.P.; Baumeister, R.; Blackwell, T.K. Direct inhibition of the longevity-promoting factor SKN-1 by insulin-like signaling in C. elegans. Cell 2008, 132, 1025–1038. [Google Scholar] [CrossRef]
  211. Greer, E.L.; Banko, M.R.; Brunet, A. AMP-activated protein kinase and FoxO transcription factors in dietary restriction-induced longevity. Ann. N. Y. Acad. Sci. 2009, 1170, 688–692. [Google Scholar] [CrossRef]
  212. Mauro, C.; Crescenzi, E.; de Mattia, R.; Pacifico, F.; Mellone, S.; Salzano, S.; de Luca, C.; D’Adamio, L.; Palumbo, G.; Formisano, S.; et al. Central role of the scaffold protein tumor necrosis factor receptor-associated factor 2 in regulating endoplasmic reticulum stress-induced apoptosis. J. Biol. Chem. 2006, 281, 2631–2638. [Google Scholar]
  213. Cowey, S.; Hardy, R.W. The metabolic syndrome: A high-risk state for cancer? Am. J. Pathol. 2006, 169, 1505–1522. [Google Scholar] [CrossRef]
  214. Kaaks, R.; Lukanova, A. Energy balance and cancer: The role of insulin and insulin-like growth factor-I. Proc. Nutr. Soc. 2001, 60, 91–106. [Google Scholar] [CrossRef]
  215. Golden, S.H.; Dobs, A.S.; Vaidya, D.; Szklo, M.; Gapstur, S.; Kopp, P.; Liu, K.; Ouyang, P. Endogenous sex hormones and glucose tolerance status in postmenopausal women. J. Clin. Endocrinol. Metab. 2007, 92, 1289–1295. [Google Scholar] [CrossRef]
  216. Giovannucci, E.; Harlan, D.M.; Archer, M.C.; Bergenstal, R.M.; Gapstur, S.M.; Habel, L.A.; Pollak, M.; Regensteiner, J.G.; Yee, D. Diabetes and cancer: A consensus report. Diabetes Care 2010, 33, 1674–1685. [Google Scholar]
  217. Dupont, J.; Karas, M.; LeRoith, D. The potentiation of estrogen on insulin-like growth factor I action in MCF-7 human breast cancer cells includes cell cycle components. J. Biol. Chem. 2000, 275, 35893–35901. [Google Scholar]
  218. Brodt, P.; Samani, A.; Navab, R. Inhibition of the type I insulin-like growth factor receptor expression and signaling: Novel strategies for antimetastatic therapy. Biochem. Pharmacol. 2000, 60, 1101–1107. [Google Scholar] [CrossRef]
  219. Peres, C.; Yart, A.; Perret, B.; Salles, J.P.; Raynal, P. Modulation of phosphoinositide 3-kinase activation by cholesterol level suggests a novel positive role for lipid rafts in lysophosphatidic acid signalling. FEBS Lett. 2003, 534, 164–168. [Google Scholar] [CrossRef]
  220. Irwin, M.E.; Bohin, N.; Boerner, J.L. Src family kinases mediate epidermal growth factor receptor signaling from lipid rafts in breast cancer cells. Cancer Biol. Ther. 2011, 12, 718–726. [Google Scholar] [CrossRef]
  221. Brown, D.A. Lipid rafts, detergent-resistant membranes, and raft targeting signals. Physiology (Bethesda) 2006, 21, 430–439. [Google Scholar] [CrossRef]
  222. Ogretmen, B.; Hannun, Y.A. Biologically active sphingolipids in cancer pathogenesis and treatment. Nat. Rev. Cancer 2004, 4, 604–616. [Google Scholar] [CrossRef]
  223. Yu, C.; Alterman, M.; Dobrowsky, R.T. Ceramide displaces cholesterol from lipid rafts and decreases the association of the cholesterol binding protein caveolin-1. J. Lipid Res. 2005, 46, 1678–1691. [Google Scholar] [CrossRef]
  224. Hajduch, E.; Turban, S.; le Liepvre, X.; le Lay, S.; Lipina, C.; Dimopoulos, N.; Dugail, I.; Hundal, H.S. Targeting of PKCzeta and PKB to caveolin-enriched microdomains represents a crucial step underpinning the disruption in PKB-directed signalling by ceramide. Biochem. J. 2008, 410, 369–379. [Google Scholar] [CrossRef]
  225. Jiang, B.H.; Liu, L.Z. PI3K/PTEN signaling in tumorigenesis and angiogenesis. Biochim. Biophys. Acta 2008, 1784, 150–158. [Google Scholar] [CrossRef]
  226. Haimovitz-Friedman, A.; Kan, C.C.; Ehleiter, D.; Persaud, R.S.; McLoughlin, M.; Fuks, Z.; Kolesnick, R.N. Ionizing radiation acts on cellular membranes to generate ceramide and initiate apoptosis. J. Exp. Med. 1994, 180, 525–535. [Google Scholar] [CrossRef]
  227. Von Haefen, C.; Wieder, T.; Gillissen, B.; Starck, L.; Graupner, V.; Dorken, B.; Daniel, P.T. Ceramide induces mitochondrial activation and apoptosis via a Bax-dependent pathway in human carcinoma cells. Oncogene 2002, 21, 4009–4019. [Google Scholar]
  228. Awad, A.B.; von Holtz, R.L.; Cone, J.P.; Fink, C.S.; Chen, Y.C. Beta-sitosterol inhibits growth of HT-29 human colon cancer cells by activating the sphingomyelin cycle. Anticancer Res. 1998, 18, 471–473. [Google Scholar]
  229. Senchenkov, A.; Litvak, D.A.; Cabot, M.C. Targeting ceramide metabolism—A strategy for overcoming drug resistance. J. Natl. Cancer Inst. 2001, 93, 347–357. [Google Scholar] [CrossRef]
  230. Liu, Y.Y.; Han, T.Y.; Giuliano, A.E.; Cabot, M.C. Expression of glucosylceramide synthase, converting ceramide to glucosylceramide, confers adriamycin resistance in human breast cancer cells. J. Biol. Chem. 1999, 274, 1140–1146. [Google Scholar]
  231. Lucci, A.; Cho, W.I.; Han, T.Y.; Giuliano, A.E.; Morton, D.L.; Cabot, M.C. Glucosylceramide: A marker for multiple-drug resistant cancers. Anticancer Res. 1998, 18, 475–480. [Google Scholar]
  232. Zhang, X.; Li, J.; Qiu, Z.; Gao, P.; Wu, X.; Zhou, G. Co-suppression of MDR1 (multidrug resistance 1) and GCS (glucosylceramide synthase) restores sensitivity to multidrug resistance breast cancer cells by RNA interference (RNAi). Cancer Biol. Ther. 2009, 8, 1117–1121. [Google Scholar]
  233. Zhang, X.; Wu, X.; Li, J.; Sun, Y.; Gao, P.; Zhang, C.; Zhang, H.; Zhou, G. MDR1 (multidrug resistence 1) can regulate GCS (glucosylceramide synthase) in breast cancer cells. J. Surg. Oncol. 2011, 104, 466–471. [Google Scholar] [CrossRef]
  234. Lavie, Y.; Cao, H.; Volner, A.; Lucci, A.; Han, T.Y.; Geffen, V.; Giuliano, A.E.; Cabot, M.C. Agents that reverse multidrug resistance, tamoxifen, verapamil, and cyclosporin A, block glycosphingolipid metabolism by inhibiting ceramide glycosylation in human cancer cells. J. Biol. Chem. 1997, 272, 1682–1687. [Google Scholar]
  235. Lavie, Y.; Fiucci, G.; Liscovitch, M. Up-regulation of caveolae and caveolar constituents in multidrug-resistant cancer cells. J. Biol. Chem. 1998, 273, 32380–32383. [Google Scholar]
  236. Daniel, E.E.; El-Yazbi, A.; Cho, W.J. Caveolae and calcium handling, a review and a hypothesis. J. Cell. Mol. Med. 2006, 10, 529–544. [Google Scholar]
  237. Galbiati, F.; Volonte, D.; Liu, J.; Capozza, F.; Frank, P.G.; Zhu, L.; Pestell, R.G.; Lisanti, M.P. Caveolin-1 expression negatively regulates cell cycle progression by inducing G(0)/G(1) arrest via a p53/p21(WAF1/Cip1)-dependent mechanism. Mol. Biol. Cell. 2001, 12, 2229–2244. [Google Scholar]
  238. Schlegel, A.; Wang, C.; Pestell, R.G.; Lisanti, M.P. Ligand-independent activation of oestrogen receptor alpha by caveolin-1. Biochem. J. 2001, 359, 203–210. [Google Scholar] [CrossRef]
  239. Mineo, C.; James, G.L.; Smart, E.J.; Anderson, R.G. Localization of epidermal growth factor-stimulated Ras/Raf-1 interaction to caveolae membrane. J. Biol. Chem. 1996, 271, 11930–11935. [Google Scholar]
  240. Li, S.; Couet, J.; Lisanti, M.P. Src tyrosine kinases, Galpha subunits, and H-Ras share a common membrane-anchored scaffolding protein, caveolin. Caveolin binding negatively regulates the auto-activation of Src tyrosine kinases. J. Biol. Chem. 1996, 271, 29182–29190. [Google Scholar] [CrossRef]
  241. Gustavsson, J.; Parpal, S.; Karlsson, M.; Ramsing, C.; Thorn, H.; Borg, M.; Lindroth, M.; Peterson, K.H.; Magnusson, K.E.; Stralfors, P. Localization of the insulin receptor in caveolae of adipocyte plasma membrane. FASEB J. 1999, 13, 1961–1971. [Google Scholar]
  242. Parpal, S.; Karlsson, M.; Thorn, H.; Stralfors, P. Cholesterol depletion disrupts caveolae and insulin receptor signaling for metabolic control via insulin receptor substrate-1, but not for mitogen-activated protein kinase control. J. Biol. Chem. 2001, 276, 9670–9678. [Google Scholar]
  243. Yamamoto, M.; Toya, Y.; Schwencke, C.; Lisanti, M.P.; Myers, M.G., Jr.; Ishikawa, Y. Caveolin is an activator of insulin receptor signaling. J. Biol. Chem. 1998, 273, 26962–26968. [Google Scholar]
  244. Matthews, L.C.; Taggart, M.J.; Westwood, M. Effect of cholesterol depletion on mitogenesis and survival: the role of caveolar and noncaveolar domains in insulin-like growth factor-mediated cellular function. Endocrinology 2005, 146, 5463–5473. [Google Scholar] [CrossRef]
  245. Storch, C.H.; Ehehalt, R.; Haefeli, W.E.; Weiss, J. Localization of the human breast cancer resistance protein (BCRP/ABCG2) in lipid rafts/caveolae and modulation of its activity by cholesterol in vitro. J. Pharmacol. Exp. Ther. 2007, 323, 257–264. [Google Scholar] [CrossRef]
  246. Awad, A.B.; Chinnam, M.; Fink, C.S.; Bradford, P.G. Beta-sitosterol activates Fas signaling in human breast cancer cells. Phytomedicine 2007, 14, 747–754. [Google Scholar]
  247. Daniel, P.T.; Wieder, T.; Sturm, I.; Schulze-Osthoff, K. The kiss of death: Promises and failures of death receptors and ligands in cancer therapy. Leukemia 2001, 15, 1022–1032. [Google Scholar] [CrossRef]
  248. Ashkenazi, A.; Pai, R.C.; Fong, S.; Leung, S.; Lawrence, D.A.; Marsters, S.A.; Blackie, C.; Chang, L.; McMurtrey, A.E.; Hebert, A.; et al. Safety and antitumor activity of recombinant soluble Apo2 ligand. J. Clin. Invest. 1999, 104, 155–162. [Google Scholar]
  249. Herbst, R.S.; Eckhardt, S.G.; Kurzrock, R.; Ebbinghaus, S.; O’Dwyer, P.J.; Gordon, M.S.; Novotny, W.; Goldwasser, M.A.; Tohnya, T.M.; Lum, B.L.; et al. Phase I dose-escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J. Clin. Oncol. 2010, 28, 2839–2846. [Google Scholar]
  250. Yin, S.; Xu, L.; Bandyopadhyay, S.; Sethi, S.; Reddy, K.B. Cisplatin and TRAIL enhance breast cancer stem cell death. Int. J. Oncol. 2011, 39, 891–898. [Google Scholar]
  251. Muntane, J. Harnessing tumor necrosis factor receptors to enhance anti-tumor activities of drugs. Chem. Res. Toxicol. 2011, 24, 1610–1616. [Google Scholar]
  252. Srivastava, R.K. TRAIL/Apo-2L: Mechanisms and clinical applications in cancer. Neoplasia 2001, 3, 535–546. [Google Scholar] [CrossRef]
  253. Pan, G.; O’Rourke, K.; Chinnaiyan, A.M.; Gentz, R.; Ebner, R.; Ni, J.; Dixit, V.M. The receptor for the cytotoxic ligand TRAIL. Science 1997, 276, 111–113. [Google Scholar]
  254. Van Raam, B.J.; Salvesen, G.S. Proliferative versus apoptotic functions of caspase-8 Hetero or homo: The caspase-8 dimer controls cell fate. Biochim. Biophys. Acta 2011, 1824, 113–122. [Google Scholar]
  255. Awad, A.B.; Roy, R.; Fink, C.S. Beta-sitosterol, a plant sterol, induces apoptosis and activates key caspases in MDA-MB-231 human breast cancer cells. Oncol. Rep. 2003, 10, 497–500. [Google Scholar]
  256. Park, C.; Moon, D.O.; Ryu, C.H.; Choi, B.; Lee, W.; Kim, G.Y.; Choi, Y. Beta-sitosterol sensitizes MDA-MB-231 cells to TRAIL-induced apoptosis. Acta Pharmacol. Sin. 2008, 29, 341–348. [Google Scholar] [CrossRef]
  257. Awad, A.B.; Downie, A.; Fink, C.S.; Kim, U. Dietary phytosterol inhibits the growth and metastasis of MDA-MB-231 human breast cancer cells grown in SCID mice. Anticancer Res. 2000, 20, 821–824. [Google Scholar]
  258. Ju, Y.H.; Clausen, L.M.; Allred, K.F.; Almada, A.L.; Helferich, W.G. Beta-sitosterol, beta-Sitosterol Glucoside, and a Mixture of beta-sitosterol and beta-sitosterol glucoside modulate the growth of estrogen-responsive breast cancer cells in vitro and in ovariectomized athymic mice. J. Nutr. 2004, 134, 1145–1151. [Google Scholar]
  259. Danaei, G.; Vander Hoorn, S.; Lopez, A.D.; Murray, C.J.; Ezzati, M. Causes of cancer in the world: Comparative risk assessment of nine behavioural and environmental risk factors. Lancet 2005, 366, 1784–1793. [Google Scholar]
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