Effects and Mechanisms of Curcumin for the Prevention and Management of Cancers: An Updated Review

Cancer is the leading cause of death in the world. Curcumin is the main ingredient in turmeric (Curcuma longa L.), and is widely used in the food industry. It shows anticancer properties on different types of cancers, and the underlying mechanisms of action include inhibiting cell proliferation, suppressing invasion and migration, promoting cell apoptosis, inducing autophagy, decreasing cancer stemness, increasing reactive oxygen species production, reducing inflammation, triggering ferroptosis, regulating gut microbiota, and adjuvant therapy. In addition, the anticancer action of curcumin is demonstrated in clinical trials. Moreover, the poor water solubility and low bioavailability of curcumin can be improved by a variety of nanotechnologies, which will promote its clinical effects. Furthermore, although curcumin shows some adverse effects, such as diarrhea and nausea, it is generally safe and tolerable. This paper is an updated review of the prevention and management of cancers by curcumin with a special attention to its mechanisms of action.


Introduction
Cancer is the leading cause of death worldwide, with nearly 10 million deaths, and an estimated 19.3 million new cases in 2020, which is expected to reach 28.4 million new cases in 2040, an increase of 47% [1]. The cancer mortality burden is high in lowand middle-income countries [2]. At present, the most effective cancer therapies include immunotherapy, chemotherapy, radiotherapy and surgery. However, these therapeutic strategies have limited efficacies and potential side effects including fatigue, anorexia, liver and kidney damage, anxiety and depression, etc. [3][4][5][6]. On the other hand, some natural products, including fruits, vegetables, tea and spices have shown potential for the prevention and management of cancers, which have attracted wide attention from researchers [7][8][9][10][11][12][13][14][15][16].

Inhibiting Cancer Cell Proliferation
Uncontrolled cell proliferation is a hallmark of cancer, and anti-proliferation is an important therapeutic intervention [95][96][97]. Many studies have found that curcumin could inhibit cancer cell proliferation. For example, a study showed that curcumin could reduce the viability of triple-negative breast cancer MDA-MB-231 and MDA-MB-468 cells, and it could also inhibit colony proliferation via inhibiting the Hedgehog pathway and the downstream target gene expression of PTCH1, SMO, Gli1 and Gli2 [27]. Furthermore, curcumin showed inhibition effects on the proliferation of prostate cancer PC-3 and DU145 cells through significantly increasing the expression of miR-34a [76]. Meanwhile, the cell cycle, a highly regulated process, is involved in enabling cell growth, cell division and duplication of genetic material [98]. Cyclin is often overactive in cancer cells, leading to uncontrolled proliferation of cancer cells, and targeting the cell cycle is considered as one of the targets of cancer therapy [99]. The cell cycle is composed of four phases: G1 (where cells decide to grow and divide or enter the G0 phase (enter quiescence)), S (DNA synthesis), G2 (preparation for mitosis), and M (mitosis) [100,101]. Cell cycle proteins are aberrantly activated in human cancers, which plays a pathogenic role in the development of most tumors [98]. A study found that curcumin could induce subG1 population accumulation and trigger G2/M arrest in breast cancer MCF-7, MDA-MB-453 and MDA-MB-231 cells, and upregulate the expression levels of p21 by targeting NF-κB signaling [36]. In addition, similar effects of curcumin on inducing G2 phase cell accumulation was observed in head and neck cancer SCC-9 cells, which indicated that curcumin could induce G2/M cell cycle arrest through inhibiting phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of the rapamycin (mTOR) pathway [64].
Some in vivo studies have found that curcumin can inhibit tumor growth. For example, curcumin could reduce lung tumor volume and weight in the BALB/c nude mice xenograft model by inhibiting circ-PRKCA [38]. Moreover, curcumin suppressed ovarian cancer growth in xenograft models by up-regulating circ-PLEKHM3 [86]. Curcumin could also reduce a transformative phenotype and tumor formation in the 4-nitroquinoline-1oxide-induced head and neck cancer model, and tumor volume was significantly reduced after curcumin treatment [63]. Another study found that curcumin significantly reduced tumor weight and tumor size in BALB/c nude mice with SGC-7901 gastric cancer cells' subcutaneous xenografts by promoting miR-34a expression [68]. In addition, the liver tumor volume and weight were significantly decreased by curcumin in a HepG2 xenograft mouse model [79].

Inhibiting Invasion and Migration
Cancer cells have the ability to migrate and invade extensively, and cancer invasion and metastasis are landmark events in the transformation of locally grown tumors into systemic, metastatic, and life-threatening cancers [102,103]. Activation of the epithelialmesenchymal transition (EMT) program may be a potential mechanism of cancer migration and invasion [104], conferring metastatic properties to cancer cells through raising invasiveness, mobility and resistance to apoptotic stimuli [105]. Inhibition of cancer cell migration and invasion may be one of the most essential anticancer mechanisms of curcumin. A study found that curcumin reduced breast cancer MCF-7 cell migration, as shown in the wound healing assay. At the same time, the results of the Transwell invasion assay also showed that curcumin significantly reduced MCF-7 cell invasion. The potential mechanisms might be attenuating lncRNA H19 [29]. Another study suggested that the migration and invasion of papillary thyroid cancer TPC-1 and BCPAP-R cells were suppressed by curcumin through up-regulation of miR-301a-3p [78]. Furthermore, curcumin significantly inhibited wound closure and invasion of pancreatic cancer Patu8988 and Panc-1 cells, which was mediated by inhibiting neural precursor cell expressed developmentally down-regulated protein 4 (NEDD4)/Akt/mTOR pathway [90]. Additionally, curcumin supplementation significantly reduced N-cadherin, twist, snail and vimentin, and increased E-cadherin in colorectal cancer SW480 cells, indicating that curcumin could suppress the EMT process by suppressing caudal type homeobox 2 (CDX2)/Wnt family member 3a (Wnt3a)/β-catenin pathway [55]. Moreover, curcumin decreased EMT of cervical cancer SiHa cells via pirindependent mechanism, enhanced the expression of E-cadherin and reduced the expression of N-cadherin, vimentin, slug and Zinc finger E-box binding homeobox 1 (Zeb1) through decreasing the levels of Pirin, which was further verified after Pirin knockdown [92].

Inducing Cell Apoptosis
Apoptosis is a kind of programmed cell death that occurs in an ordered and coordinated manner under pathological and physiological conditions and plays a crucial role in organism development and tissue homeostasis [106]. Apoptosis is associated with TNF-α, ROS and the activation of cysteine-protease and caspases [107]. During normal conditions, apoptosis is necessary for homeostasis but, in cancer, cells lose the ability to undergo apoptosis-induced death, leading to uncontrolled cell proliferation, which further leads to tumor survival, therapeutic resistance and cancer recurrence [108,109]. It was found that selectively inducing apoptosis in cancer cells has been considered as a promising treatment for many cancers [110]. A study found that the apoptotic ratios of breast cancer MDA-MB-231 and MDA-MB-468 cells were increased after treatment of curcumin, which was mediated by increasing the level of cysteinyl aspartate specific proteinase 9 (Caspase-9), and reducing the level of B-cell lymphoma-2 (Bcl-2) [32]. Another study pointed out that curcumin promoted prostate cancer PC-3 and DU145 cells apoptosis via enhancing the expression of miR-30a-5p and downregulating PCNA clamp associated factor (PCLAF) expression to increase the levels of Bcl-2 associated X protein (Bax) and Cleaved-cysteinyl aspartate specific proteinase 3 (Caspase-3), and to decrease the expression of Bcl-2 and Caspase-3 [73]. Furthermore, curcumin exerted a pro-apoptotic effect in cervical cancer Siha cells through increasing the expression levels of Cleaved-poly (ADP-ribose) polymerase (PARP) and Cleaved-caspase-3 [91]. Curcumin could also effectively promote the numbers of apoptotic tongue cancer CAL 27 cells, and decrease the expression of Bcl-2, increase the expressions of Bax and Cleaved-caspase-3 by regulating oxygen-related signaling pathways [93].

Inducing Autophagy
Autophagy is another kind of programmed cell death, which is essential for maintaining cellular homeostasis in stressful conditions [111]. Dysregulation of autophagy has implications in disease [111,112]. Enhanced autophagy could enhance anticancer immune responses, therefore targeting autophagy is a potential approach to improve the efficacy of current cancer treatments [113]. Curcumin-induced autophagy in cancers is one of the main concerns of many research projects. A study pointed out that curcumin could induce the formation of autophagic vesicle by suppressing AKT/mTOR/p70S6K pathway in ovarian cancer A2780 cells, and enhancing the expression of microtubule-associated protein light chain 3B I/II (LC3B-I/II), autophagy-related 3 (Atg3) and Beclin1 [85]. In another study, curcumin inhibited LC3I expression, and enhanced LC3II, Beclin1, Atg3 and autophagy related 5 (Atg5) expression in gastric cancer SGC-7901 and BGC-823 cells. The potential mechanisms might be inhibiting PI3K/Akt/mTOR pathway and activating P53 signaling pathway [69]. Meanwhile, curcumin was found to induce autophagy through suppressing PI3K/Akt/mTOR pathway, decreasing p62 expression, and increasing the expression of Beclin1 and LC3-II in lung cancer A549 cells [47]. Besides, curcumin could downregulate the expression of p62, and increase autolysosome and the expression of Beclin1 and LC3-II, thereby inducing autophagy [41].

Suppressing Cancer Cell Stemness
Cancer stem cells have self-renewal ability, which may lead to therapeutic resistance, tumor progression and relapse [114,115]. Cancer cell stemness refers to the stem cell-like phenotype of cancer cells [116]. Therefore, targeting cancer cell stemness may provide more specific treatments and exert better efficacy, and curcumin targeting cancer cell stemness has been shown to be one of the mechanisms of cancer treatment. CD44 and CD133 are wellknown markers of cancer stem cells. In a study, curcumin supplementation significantly reduced the expression of CD44 and the number and size of tumor sphere formation of colon cancer HCT-116 and HCT-8 cells, which indicated that curcumin could inhibit the stem-cell like characteristics in colon cancer cells [62].

Increasing ROS Production
ROS is inextricably linked to cancer progression and therapy, which may be associated with complex ROS homeostasis in cancer cells and the tumor microenvironment [117]. ROS may exert cytotoxic effects on cancer cells, leading to malignant cell death, thereby limiting cancer progression [118,119]. A high level of ROS may provide avenues for cancer therapy by activating various cell death pathways, such as necrosis, apoptosis, autophagy and ferroptosis; therefore, increasing ROS is one of the main anticancer strategies [120,121]. Some studies revealed that curcumin could induce excessive ROS generation, then induce oxidative stress in cancer cells. A study showed that curcumin promoted ROS production in cervical cancer Siha cells [91]. In another study, the ROS levels were elevated in gastric cancer MGC-803 cells after treatment with curcumin, suggesting that curcumin had a pro-oxidative effect [66]. Treatment with curcumin also increased ROS production in colorectal cancer SW480 cells [52]. Additionally, curcumin treatment could enhance ROS levels in breast cancer MDA-MB-231 cells [24]. Curcumin-induced ROS upregulation also triggered endoplasmic reticulum stress in prostate cancer-associated fibroblasts via the PERK-eIF2α-ATF4 axis, ultimately leading to apoptosis [74].

Effects on Gut Microbiota
Gut microbiota could play a vital role in health and diseases [122]. Gut dysbiosis may lead to cancer development, such as colon, gastric and breast cancers [123,124]. There are several strategies that can be used to target gut microbiota to prevent or treat cancer, such as dietary interventions, fecal microbiome transplant and targeted antibiotic approaches [125].
The studies also showed that some natural products could be anticancer, via targeting gut microbiota [126]. Curcumin significantly altered the gut microbiota composition in the H22 mice xenograft liver tumor model, and the abundances of Bifidobacterium and Lactobacillus were elevated. The oral bioavailability of curcumin was enhanced by increasing abundance of Escherichia_shigella [81]. Zinc complexes of curcumin attenuated degradation of intestinal mucus barrier and gut dysbiosis in a rat hepatocellular carcinoma model, and enhanced chemosensitizer for doxorubicin via gut microbiota. The ratio of Firmicutes/Bacteroidetes was reduced [82]. Moreover, curcumin could reduce the tumor burden in AOM-treated Il10 −/− mice through increasing the relative abundance of Lactobacillales and decreasing the relative abundance of Coriobacterales [127]. In the future, more studies are necessary to evaluate the effect of curcumin on various cancers via targeting gut microbiota.

Adjuvant Therapy for Cancers
The biggest obstacle in targeting cancer therapy is the inevitable emergence of drug resistance in the early or late stages of drug treatment, which is a major clinical problem [128]. Clinical resistance can lead to treatment failure and eventual patient death [129]. Therefore, curcumin has been used as a promising adjuvant to improve the efficacy of many chemotherapeutic drugs. For example, incubation of curcumin with anticancer drugs such as cisplatin, doxorubicin or methotrexate, respectively, significantly reduced the IC 50 of anticancer drugs and sensitized liver cancer HepG2 cells to anticancer drugs [80]. In addition, the combination of curcumin and metformin may have a synergistic effect, inhibiting the proliferation, migration and invasion of gastric cancer AGS cells [130]. It has also been reported that the combination of curcumin and 3 ,4 -didemethylnobiletin induced cell apoptosis and cell cycle arrest of colon cancer HCT-116 cells more effectively than individual compounds [131]. In another study, in vitro and in vivo experiments demonstrated that curcumin reduced oxaliplatin resistance in colorectal cancer by inhibiting transforming growth factor beta (TGF-β)/SMAD family member 2/3 (Smad2/3) signaling [59]. In addition, curcumin combined with photodynamic therapy has better anticancer activity for several cancers, such as oral, kidney, breast, prostate, bladder and cervical cancer, and the possible mechanism is through increasing ROS generation and inducing apoptosis [132].

Results from Clinical Trials
Several clinical trials have been conducted to assess the effects of curcumin on cancers (Table 2). For instance, a quasi-experimental design recruited 40 cervical carcinoma stage IIB-IIIB patients to ingest curcumin (4 g/day, 20 persons) or placebo (20 persons) for 7 days, who also received radiation therapy simultaneously. The results revealed that intake of curcumin decreased the level of the anti-apoptotic protein survivin in 15 patients (75%), and increased the level of survivin in five (25%). On the other hand, eight patients (40%) in the placebo group decreased the level of survivin, and 12 patients (60%) increased the level of survivin. The result indicated that curcumin was an effective ra-diosensitizer in the treatment of cervical cancer patients [133]. Moreover, 150 women participants with advanced and metastatic breast cancer received intravenous administration of curcumin (300 mg/week) + paclitaxel (80 mg/m 2 body surface area) or placebo + paclitaxel (80 mg/m 2 body surface area) for 12 weeks. The result showed that curcumin improved objective response rates and patient self-assessed performance status, and meanwhile reduced fatigue and did not decrease quality of life [134]. Besides, in 97 prostate cancer patients daily ingested with 1.44 g curcumin for 6-36 months, the elevation of prostate-specific antigen was suppressed during the curcumin administration period [135]. However, curcumin showed no significant effect in some cases. For example, a randomized controlled trial showed that no significant efficacy was observed with nanocurcumin supplementation (120 mg/day) in prostate cancer patients treated with radiation [136]. Additionally, treatment with curcumin (6 g/d) for 6 weeks had no significant benefits in metastatic castration-resistant prostate cancer [137]. The inconsistent results could be due to the intricate factors involved in clinical trials, and further research is necessary.

Enhancing Curcumin Bioavailability
Curcumin has shown anticancer activities. However, some limiting factors, such as its poor water solubility and extremely low oral bioavailability, could reduce its therapeutic effects [143]. Many techniques have been developed and applied to overcome this limitation [144]. For instance, protein/polysaccharide-decorated folate as a targeted nanocarrier of curcumin (fCs-Alg@CCasNPs) prolonged the sustained release of curcumin, and improved the bioavailability of curcumin, and in vivo and in vitro experiments demonstrated that fCs-Alg@CCasNPs had a higher therapeutic effect than treatment with free curcumin on pancreatic cancer and Ehrlich carcinoma [145]. Besides, a novel nano-system MSN_CurNQ was formed by loading curcumin and naphthoquinone (NQ) into the pores of mesoporous silica nanoparticles (MSN), aiming to increase the drug delivery of CurNQ via the enhanced permeation and retention effect and sustained release. The results of cellular experiments showed that MSN_CurNQ had tumor-specific toxicity and reduced the viability of cancer cells to a greater extent compared to healthy fibroblast cell lines [146]. Curcumin-loaded Gemini surfactant nanoparticles also significantly enhanced the solubility, uptake and cytotoxicity of curcumin, and inhibited breast cancer MCF-7, SkBr-3 and MDA-MB-231 cell proliferation by inducing apoptosis after effective delivery of curcumin [147]. Moreover, hydrophilic hyaluronic acid (HA) conjugated with hydrophobic curcumin form amphiphilic HA-ADH-CUR conjugates, and then subsequently self-assembled in aqueous solution to form nanoparticles HA@CUR NPs, effectively accumulated at the tumor site through endocytosis and attained a superior therapeutic effect of tumor growth inhibition [148]. Furthermore, loading curcumin onto the non-spherical delivery system zinc oxide-β cyclodextrin 3-mercaptopropionic acid (ZnO-βCD-MPA) conjugated folic acid to generate a ZnO-βCD-MPA-FA-curcumin formulation for aqueous delivery of curcumin, which allowed for sustained release of curcumin to enhance its targeting, bioavailability and release profile. Compared to free CUR, this formulation had a stronger anticancer effect on the breast cancer MDA-MB-231 cells via inducing apoptosis and had no cytotoxic effect on HEK293 normal cells [149]. In addition, curcumin-cyclodextrin/cellulose nanocrystal nano complexes were more soluble in water than free curcumin and had stronger cytotoxic activity against prostate cancer PC-3 and DU145 cells and colon cancer HT29 cells [150].

Safety of Curcumin
Curcumin has been permitted by the U.S. Food and Drug Administration as "generally regarded as safe", and 180 mg/day of curcumin supplementation is reasonable [151,152]. Some studies revealed that curcumin showed no toxic effects in humans, and was safe and tolerable [153]. However, some adverse effects of curcumin have been observed. For example, a phase I clinical trial of oral curcumin found that curcumin was well tolerated, but diarrhea was observed in some patients [154]. Another study showed that curcumin was a safe and tolerable adjunct, but nausea was observed in some patients [138]. In addition, curcumin patients group had urinary frequency [135].

Conclusions and Perspectives
Cancer is a serious public health problem. Many studies have reported the effectiveness of curcumin in the prevention and management of various cancers, such as thyroid, breast, gastric, colorectal, liver, pancreatic, prostate and lung cancers. The potential mechanisms include inhibiting cancer cell proliferation, suppressing invasion and migration, promoting cell apoptosis, inducing autophagy, decreasing cancer stemness, increasing reactive oxygen species production, reducing inflammation, triggering ferroptosis, regulating gut microbiota, and adjuvant therapy. Meanwhile, several nanomaterials have been developed to prolong the release or targeted delivery of curcumin to cancer tissues, and further enhance the bioavailability and anticancer activities of curcumin. Moreover, the studies have shown that curcumin is generally safe and well tolerated, although some side effects have been observed, such as diarrhea and nausea. In the future, the anticancer activities of curcumin on more cancers should be evaluated, and the relative mechanisms should be explored. In addition, more methods should be studied to improve the bioavailability of curcumin in order to increase its anticancer activities. Furthermore, more clinical trials should be carried out to assess the anticancer effects of curcumin on human beings. This paper will be helpful for research and development of the third-generation function food containing curcumin.