Resveratrol’s Anti-Cancer Effects through the Modulation of Tumor Glucose Metabolism

Simple Summary The prevention and treatment of cancer is an ongoing medical challenge. In the context of personalized medicine, the well-studied polyphenol resveratrol could complement classical tumor therapy. It may affect key processes such as inflammation, angiogenesis, proliferation, metastasis, glucose metabolism, and apoptosis in various cancers because resveratrol acts as a multi-targeting agent by modulating multiple signal transduction pathways. This review article focuses on resveratrol’s ability to modify tumor glucose metabolism and its associated therapeutic capacity. Resveratrol reduces glucose uptake and glycolysis by affecting Glut1, PFK1, HIF-1α, ROS, PDH, and the CamKKB/AMPK pathway. It also inhibits cell growth, invasion, and proliferation by targeting NF-kB, Sirt1, Sirt3, LDH, PI-3K, mTOR, PKM2, R5P, G6PD, TKT, talin, and PGAM. In addition, resveratrol induces apoptosis by targeting integrin, p53, LDH, and FAK. In conclusion, resveratrol has many potentials to intervene in tumor processes if bioavailability can be increased and this natural compound can be used selectively. Abstract Tumor cells develop several metabolic reprogramming strategies, such as increased glucose uptake and utilization via aerobic glycolysis and fermentation of glucose to lactate; these lead to a low pH environment in which the cancer cells thrive and evade apoptosis. These characteristics of tumor cells are known as the Warburg effect. Adaptive metabolic alterations in cancer cells can be attributed to mutations in key metabolic enzymes and transcription factors. The features of the Warburg phenotype may serve as promising markers for the early detection and treatment of tumors. Besides, the glycolytic process of tumors is reversible and could represent a therapeutic target. So-called mono-target therapies are often unsafe and ineffective, and have a high prevalence of recurrence. Their success is hindered by the ability of tumor cells to simultaneously develop multiple chemoresistance pathways. Therefore, agents that modify several cellular targets, such as energy restriction to target tumor cells specifically, have therapeutic potential. Resveratrol, a natural active polyphenol found in grapes and red wine and used in many traditional medicines, is known for its ability to target multiple components of signaling pathways in tumors, leading to the suppression of cell proliferation, activation of apoptosis, and regression in tumor growth. Here, we describe current knowledge on the various mechanisms by which resveratrol modulates glucose metabolism, its potential as an imitator of caloric restriction, and its therapeutic capacity in tumors.


Introduction
Cancer cells depend heavily on their active metabolism for survival and proliferation. Most tumor cells have an adaptive and altered metabolism characterized by increased aerobic glycolysis and lactate production, leading to a significant pH gradient difference between cancerous and healthy tissues [1]. This well-known feature of cancer metabolism is referred to as the Warburg effect [2]. Indeed, in 1924, the German physiologist and Nobel laureate Otto Warburg observed that tumor cells metabolize glucose differently from healthy cells [2,3]. He reported that unlike typical mammalian tissues, most cancer cells solely "ferment" glucose to lactate, even when sufficient oxygen is present to support mitochondrial oxidative phosphorylation. Furthermore, this metabolic process enables tumor cells to produce sufficient energy to survive and proliferate despite limited resources.
Glycolysis is the primary degradation mechanism that enables mammalian cells to produce energy in the form of ATP through the oxidation of carbon bonds. The final product of glycolysis is either lactate or, after complete oxidation of glucose via the mitochondrial electron transport chain, ATP and CO 2 [4]. In notable contrast, glucose uptake increases dramatically in tumors and other rapidly proliferating cells, even in the presence of oxygen and active mitochondria, and thus leads to lactate production. Altered carbohydrate, protein, and lipid metabolism are implicated in cancer cell proliferation and growth, apoptotic resistance, therapeutic resistance, epithelial-mesenchymal transition (EMT), metastasis, invasion, and tumor recurrence [5]. As the hyperactive glycolytic process of tumor cells is reversible, it is a potential target for treatment. Therefore, the use of drugs that mimic energy restriction for selective tumor cells that are "dependent on glycolysis" could be an auspicious anti-cancer approach. In recent years, the so-called mono-target therapies were proven unsafe, ineffective, and expensive. Notably, phytopharmaceuticals derived from fruits and vegetables exhibit promising therapeutic potential against many chronic diseases, including cancer. The plant polyphenol resveratrol (3,5,4 trihydroxy-transstilbene) was firstly isolated from the root of the white hellebore (Veratrum grandiflorum O. Loes) by Takaoka in 1939. Resveratrol and its biological analogs (piceatannol and pterostilbene) were found in peanuts (Arachis spp.), various berry species (Vaccinium sp.) and red wine [6]. Resveratrol is produced as a phytoalexin by plants in response to stress induced by fungi, microbes, or ultraviolet (UV) irradiation [7].
Resveratrol was first shown in 1997 to affect tumor initiation, promotion, and progression [8]. A large number of reports have since demonstrated its broad preventive and therapeutic effects against various cancer types, including gastrointestinal, breast, lung, prostate, and liver tumors. Resveratrol's therapeutic potential is further underscored by its significant chemopreventive effects in combination with other cytostatic drugs [8].
In traditional medicine, resveratrol has long been used as an herbal remedy. In modern medicine, resveratrol is of great interest as a "multitargeting agent" because of its antioxidant, anti-inflammatory, anti-obesity, anti-diabetic, anti-bacterial, anti-carcinogenic, cardio-protective, and immunomodulating properties [7][8][9][10] (Figure 1). Epidemiologically, there is an inverse association between the consumption of red wine and the incidence of cardiovascular diseases in the French population despite its high intake of saturated fats; this phenomenon is called the "French Paradox" [7,[11][12][13]. Resveratrol exerts a broad spectrum of molecular effects associated with the control of cancer development. These include the reduction of glucose uptake and lactate synthesis, and consequent caloric restriction that inhibits proliferation and metastasis, and induces apoptosis [14][15][16][17][18]. Moreover, resveratrol can directly influence and modulate various metabolic enzymes and signal transduction pathways involved in oxidative glycolysis. Indeed, there is a clinical relationship between glucose resorption, diagnosis, and cancer prognosis [19,20]. This review focuses on the intracellular targets of resveratrol, and their roles in the regulation of cellular glucose metabolism and tumor growth, in the interest of improving cancer prevention and treatment.

Goal of the Review
This review deals with resveratrol's tumor control potential, particularly its ability to suppress cancer cell glucose metabolism. We discuss the mechanisms of glucose uptake, metabolism, and degradation, as well as the molecular pathways that directly regulate tumor cell metabolism. Our review summarizes experimental studies on the anti-tumor effects of resveratrol through the modulation of glycolytic processes. Given the well-documented preclinical efficacy of resveratrol against tumor metabolism, we stress the need for targeted clinical research on the effects of resveratrol on cellular metabolic repair.

Source of the Data
Data were collected from the biomedical literature by using "resveratrol" and "cancer" or "glucose metabolism" or "glucose uptake" or "glucose transporter" or "Warburg effect" or "tumor microenvironment" or "polyphenols" or "apoptosis" as keywords or medical terms (MeSH) when searching the PubMed database.

Glucose Metabolism (Glycolysis) in Tumors and the Warburg Effect
High glucose absorption is necessary for cancer cell metabolism; this process is well regulated and involves several elements, such as growth factors [21] and interactions with the extracellular matrix [22,23]. To fulfill their glucose demands, tumor cells undergo oncogenic alterations to become independent of the processes that ordinarily regulate glucose absorption [24,25]. Essential glycolytic enzymes, glucose transporters, and transcription factors are often dysregulated during tumorigenesis [26].
In healthy cells, glucose uptake is facilitated by specific cell membrane transporters. Hexokinase phosphorylates intracellular glucose to form glucose-6-phosphate, which is subsequently converted to 3-carbon pyruvate in a process that yields NADH and ATP. In the presence of oxygen (aerobic glycolysis), healthy cells convert the intermediate pyruvate into acetyl-CoA and synthesize ATP and CO2 efficiently through oxidative phosphorylation (rather than glucose fermentation) [4]. In normal differentiated cells, large amounts of lactate are synthesized from pyruvate only if the oxygen supply is insufficient (anaerobic glycolysis). This review focuses on the intracellular targets of resveratrol, and their roles in the regulation of cellular glucose metabolism and tumor growth, in the interest of improving cancer prevention and treatment.

Goal of the Review
This review deals with resveratrol's tumor control potential, particularly its ability to suppress cancer cell glucose metabolism. We discuss the mechanisms of glucose uptake, metabolism, and degradation, as well as the molecular pathways that directly regulate tumor cell metabolism. Our review summarizes experimental studies on the anti-tumor effects of resveratrol through the modulation of glycolytic processes. Given the welldocumented preclinical efficacy of resveratrol against tumor metabolism, we stress the need for targeted clinical research on the effects of resveratrol on cellular metabolic repair.

Source of the Data
Data were collected from the biomedical literature by using "resveratrol" and "cancer" or "glucose metabolism" or "glucose uptake" or "glucose transporter" or "Warburg effect" or "tumor microenvironment" or "polyphenols" or "apoptosis" as keywords or medical terms (MeSH) when searching the PubMed database.

Glucose Metabolism (Glycolysis) in Tumors and the Warburg Effect
High glucose absorption is necessary for cancer cell metabolism; this process is well regulated and involves several elements, such as growth factors [21] and interactions with the extracellular matrix [22,23]. To fulfill their glucose demands, tumor cells undergo oncogenic alterations to become independent of the processes that ordinarily regulate glucose absorption [24,25]. Essential glycolytic enzymes, glucose transporters, and transcription factors are often dysregulated during tumorigenesis [26].
In healthy cells, glucose uptake is facilitated by specific cell membrane transporters. Hexokinase phosphorylates intracellular glucose to form glucose-6-phosphate, which is subsequently converted to 3-carbon pyruvate in a process that yields NADH and ATP. In the presence of oxygen (aerobic glycolysis), healthy cells convert the intermediate pyruvate into acetyl-CoA and synthesize ATP and CO 2 efficiently through oxidative phosphorylation (rather than glucose fermentation) [4]. In normal differentiated cells, large amounts of lactate are synthesized from pyruvate only if the oxygen supply is insufficient (anaerobic glycolysis). In contrast, tumor cells are highly dependent on glucose degradation (known as fermentation) [3,27], even under aerobic conditions, to meet their high energy requirements; this is known as the Warburg Effect [28] (Figure 2, light green area). Interestingly, tumor cells can activate both the Warburg effect and mitochondrial oxidative phosphorylation simultaneously [29]. Most tumor cells synthesize large amounts of lactate independent of oxygen availability, which is why their metabolism is often referred to as "aerobic glycolysis". Otto Warburg initially assumed that tumor cells have defective mitochondria and thus no aerobic respiratory chain [2]. This hypothesis was rejected by later research, as mitochondrial function is not impaired in most cancer cells [30][31][32].
Cancers 2021, 13, x 4 of 36 In contrast, tumor cells are highly dependent on glucose degradation (known as fermentation) [3,27], even under aerobic conditions, to meet their high energy requirements; this is known as the Warburg Effect [28] (Figure 2, light green area). Interestingly, tumor cells can activate both the Warburg effect and mitochondrial oxidative phosphorylation simultaneously [29]. Most tumor cells synthesize large amounts of lactate independent of oxygen availability, which is why their metabolism is often referred to as "aerobic glycolysis". Otto Warburg initially assumed that tumor cells have defective mitochondria and thus no aerobic respiratory chain [2]. This hypothesis was rejected by later research, as mitochondrial function is not impaired in most cancer cells [30-32].

Figure 2.
Glucose degradation and metabolism in tumor cells. The uptake of glucose by cell membrane glucose transporters (Glut) is followed by its glycolytic degradation to pyruvate. The first step is the phosphorylation of glucose by hexokinase (HK) and, under anaerobic glycolysis and aerobic glycolysis (Warburg effect, light green area), pyruvate is converted to lactate, thereby regenerating NAD+ to supply the glycolytic processes. This pathway represents an energy source for tumor cells and provides intermediates such as ribose-5-phosphate and NADPH, which are essential for cell proliferation. Pyruvate then undergoes oxidative phosphorylation in the mitochondria, which leads to the formation of ATP molecules from the tricarboxylic acid cycle (TCA). Abbreviations: ATP: adenosine triphosphate; ADP: adenosine diphosphate; NAD + : nicotinamide adenine dinucleotide (oxidized form); NADH, nicotinamide adenine dinucleotide (reduced form); NADPH: nicotinamide adenine dinucleotide phosphate (reduced form); TME: tumor microenvironment; TCA: tricarboxylic acid; LDH: lactate dehydrogenase.
After glucose is uptaken by membrane glucose transporters (Glut) that are overexpressed in tumor cells [33], it is converted into glucose-6-phosphate by hexokinase II (HK2). It is important to note that HK2 expression is often upregulated in malignant tumor cells [34][35][36][37], leading to increased glycolysis [38,39]. The next enzyme associated with aerobic glycolysis is phosphofructokinase (PFK), which catalyzes the phosphorylation of fructose-6-phosphate into fructose-1,6-bisphosphate; PFK is upregulated in various breast Glucose degradation and metabolism in tumor cells. The uptake of glucose by cell membrane glucose transporters (Glut) is followed by its glycolytic degradation to pyruvate. The first step is the phosphorylation of glucose by hexokinase (HK) and, under anaerobic glycolysis and aerobic glycolysis (Warburg effect, light green area), pyruvate is converted to lactate, thereby regenerating NAD+ to supply the glycolytic processes. This pathway represents an energy source for tumor cells and provides intermediates such as ribose-5-phosphate and NADPH, which are essential for cell proliferation. Pyruvate then undergoes oxidative phosphorylation in the mitochondria, which leads to the formation of ATP molecules from the tricarboxylic acid cycle (TCA). Abbreviations: ATP: adenosine triphosphate; ADP: adenosine diphosphate; NAD + : nicotinamide adenine dinucleotide (oxidized form); NADH, nicotinamide adenine dinucleotide (reduced form); NADPH: nicotinamide adenine dinucleotide phosphate (reduced form); TME: tumor microenvironment; TCA: tricarboxylic acid; LDH: lactate dehydrogenase.
GAPDH overexpression is considered an essential parameter of many tumor types [43-45] and a potential target for the treatment of malignant tumors [46]. In addition, phosphoglycerate mutase 1 (PGAM1) catalyzes the reversible conversion of 3phosphoglycerate and 2-phosphoglycerate during glycolysis. PGAM1 is overexpressed in various cancer tissues and plays an essential role in cancer progression and metastasis [47]. Moreover, enolase is a key glycolytic enzyme that converts 2-bisphosphate glycerate into phosphoenolpyruvate. Enolase is overexpressed in pancreatic ductal adenocarcinoma (PDAC) tissue; its expression is correlated with metastasis and poor prognosis in PDAC patients [48]. The enzyme pyruvate kinase M2 (PKM2) catalyzes the irreversible phosphoryl group transfer from phosphoenolpyruvate to pyruvate, from which ATP is formed. In fact, tumor cells often overexpress PKM2 [49][50][51][52]. Tumor cells switch to and depend on aerobic glycolysis for survival. Therefore, lactate dehydrogenase (LDH), which catalyzes the conversion of pyruvate to lactate, is the key enzyme for determining the glycolytic phenotype of tumor cells; as such, it could be utilized as a therapeutic target. In fact, LDH inhibition suppresses the progression of lymphomas and pancreatic cancer xenografts [53]. Interestingly, Shim and colleagues (1998) reported that apoptosis could be induced by glucose deficiency in tumor cells [54]; this underscores the functional and survival importance of the Warburg effect. In summary, these findings indicate that the specific suppression of critical glycolytic enzymes could be a fundamental approach to the treatment of malignant tumors.

Glycolysis in Tumor Cells and the Pentose Phosphate Pathway (PPP)
Rapidly dividing tumor cells require a large amount of energy to drive their hyperactive proliferation, and a stable and continuous supply of nucleotides for DNA synthesis. These raw materials are supplied by the PPP. Interestingly, numerous PPP enzymes are highly dysregulated in tumors. Glucose-6-phosphate-dehydrogenase (G6PDH), a critical enzyme that determines the growth rate of tumor cells, catalyzes the first step in the PPP, thus producing. Many malignant tumors exhibit elevated G6PDH expression and PPP activity [55][56][57][58]. G6PDH knockdown significantly reduced cell proliferation; therefore, the specific inhibition of G6PDH may be an effective avenue for the treatment of glycolytic tumors [59]. The next enzyme in the PPP signaling pathway, 6-phosphogluconatedehydrogenase (6PGDH), converts 6-phosphogluconate to ribulose-5-phosphate, and reduces NADP to NADPH. 6GPDH is an essential enzyme for lung carcinogenesis, and its specific suppression could be a new method for the treatment of glycolytic lung tumors [60]. The next enzyme in the PPP that converts ribulose-5-phosphate to ribose-5-phosphate is ribulose-5-phosphate isomerase. This enzyme is linked to tumor development [60]. The tumor cells activate de novo nucleotide synthesis to support their rapid proliferation and require ribose-5-phosphate for this purpose. Overall, the PPP, through its regulation of glycolysis, is essential for the survival and proliferation of tumor cells. More interestingly, it can yield promising diagnostic markers for the early detection and treatment of tumors.

The Influence of the Tumor Microenvironment on the Warburg Effect
Metabolic alterations associated with increased glycolytic degradation in tumor cells are influenced by both intracellular changes and extracellular factors in the tumor microenvironment [61]. Interestingly, a low pH (due to the Warburg effect) is the most important and consistent feature of the tumor microenvironment [62]. There is a characteristic change in the pH gradient between tumor and normal tissue [1]. Shamim and co-workers demonstrated that a low pH tumor microenvironment, associated with several factors such as reduced vascularization, nutrient deprivation, and hypoxia in the context of the Warburg Cancers 2021, 13, 188 6 of 35 effect, weakens tumor cells and supports successful anti-tumor therapy [63]. Fermentation is an essential metabolic pathway in tumor cells that maintain lower pH values (some as low as 6.0) due to lactic acid production and increased CO 2 content [64]. The tumor-specific acidic milieu may be an important prerequisite for the effective development and action of many cancer drugs [65][66][67][68]. Notably, low pH in normal mammalian cells can cause inter-nucleosomal DNA fragmentation and apoptosis [69].
Moreover, specific influences from the tumor microenvironment can modulate DNA methylation, histone modifications, and miRNA expression, which in turn influence the metabolic processes of the tumor. All glycolytic proteins and enzymes are posttranscriptionally regulated by miRNAs. There is a relationship between the deregulation of miRNAs (such as miRNA-150, miRNA-522-3p, and miRNA-10a) and Glut1 activity [70][71][72]. Furthermore, essential enzymes such as HK2 and PKM2 are upregulated during aerobic glycolysis in tumors. It has been reported that the identification of hexokinase 2 (HK2) as a direct target of miR-143, and show that reintroduction of miR-143 in the colon cancer cell line DLD-1 leads to decreased lactate secretion. They hypothesized that loss of miR-143-mediated suppression of HK2 may promote glucose metabolism in cancer cells, contributing to the shift toward aerobic glycolysis observed in many tumors [73]. In contrast, the activation of miRNA-155 is associated with the upregulation of HK2 in lung cancer cells [74]. The expression of PKM2 is downregulated in thyroid cancer cells by miRNA-148a and miRNA-326 [75], and in cervical cancer cells by miRNA-let-7a [76]; this consequently inhibits proliferation.
Preventing and treating tumors is an ongoing medical challenge. Mono-target therapies are insufficient because they cannot meet the challenges posed by the complex proinflammatory tumor microenvironment, which includes numerous interactions, crosstalk's, and regulatory mechanisms. Therefore, the identification of novel multi-targeting agents is necessary to affect both tumor cells and the multicellular tumor microenvironment. The multi-targeting activities of natural polyphenols were extensively investigated over the past twenty years. In this review, we focus on the well-studied polyphenol resveratrol. Resveratrol's strength lies in its ability to influence several vital stages of cancer, namely tumor initiation and progression; it also exerts chemopreventive effects [8]. Specifically, it acts as a pluri-targeting agent by modulating signal transduction pathways that affect cell cycle progression, inflammation, proliferation, apoptosis, metastasis, and angiogenesis ( Figure 1) in a wide range of cancer types (Table 1). Table 1. The influence of resveratrol on glucose transporters and glycolytic enzymes in tumors.

Type of Tumor Tumor Cells Mode of Action Reference
Breast cancer MCF-7 breast cancer cells. Suppresses PFK activity, disrupting glucose metabolism and reducing viability in cancer cells. [15] T47D cells, BALB/c-ν mice.
Suppresses tumor growth by interfering with glucose fermentation and thereby promoting oxidative respiration. [101] Ovarian cancer SKOV3 and CaOV3 Ovarian cancer cells.
Reduces glucose uptake and lactate production via suppression of the Akt/mTOR pathway. [18] Ovarian cancer cells. Inhibits glucose uptake and induces apoptosis by impairing the Akt/Glut1 axis. [102] A2780 and SKOV3 ovarian cancer cells.
Anti-proliferative, pro-apoptotic effects through the inhibition of glycolysis and targeting of the AMPK/mTOR pathway. [103] Preclinical mouse model of ovarian cancer.
Inhibits glucose uptake with antineoplastic effects; suppresses tumor regrowth after therapy with cisplatin. [104] Leukemia B-CLL and HCL lymphocytic leukemia.

Resveratrol, Inflammation, and Tumors
Inflammation plays an essential and fundamental role in the development of chronic diseases. Resveratrol exerts anti-inflammatory effects through its influence on various inflammatory signaling cascades. Inflammation is a physiological response aimed to reestablish homeostasis after tissue damage caused by exogenous or endogenous factors [113]. During the inflammatory response, metabolism shifts from anabolism to catabolism ranging from the determination of the activity of adenosine monophosphate (AMP) and nicotinamide adenine dinucleotide (NAD + ) by AMP-activated protein kinase (AMPK) and sirtuins. Thus, AMPK signaling and sirtuins functionally couple inflammation and metabolism with gene expression and transcription factors [114].
In addition, the evolutionarily conserved pro-inflammatory transcription factor nuclear factor kappa-light-chain-enhancer of activated B-cells (NF-κB) is activated by a variety of stimuli, including inflammatory cytokines and growth factors, and is significantly upregulated in many cancer cells [115,116]. NF-κB activation and NF-κB-promoting gene products are involved in tumor cell survival, proliferation, and invasion [117]. The resveratrol-Sirt1 signaling pathway significantly downregulates cancer cell migration, viability, clonogenicity, and growth by suppressing NF-κB phosphorylation [118][119][120][121], underlining the maintenance of homeostasis provides the energy metabolism balance with the inflammatory reaction [122]. Resveratrol activates the target subcellular histone deacetylase Sirt1 in various human tissues, including tumors [123,124].
Resveratrol is a phytochemical agent that can be used as a multi-targeted drug to supplement chemotherapy. To avoid treatment errors, co-treatment with a non-toxic, dietary, natural cancer drug that can chemosensitize and treat resistant tumors has potential [136]. Moreover, resveratrol can significantly increase the sensitivity of various cancer cells to cytostatic drugs, and improve these drugs' action by inhibiting and/or modulating different signaling cascades, including the metabolic pathways ( Figure 3) [118,[137][138][139][140][141][142].

Breast Cancer
As mentioned above, tumor cells depend primarily on glycolysis to provide the en ergy and intermediates required for cell growth and proliferation. The enzyme 6-phos phofructo-1-kinase (PFK) is a key glycolytic enzyme; its activity is directly associated with cellular glucose utilization. Resveratrol directly inhibits PFK activity, decreases Glut1-me diated glucose uptake, and inhibits intracellular ROS, which suppresses HIF-1α accumu lation and thereby disrupts glucose metabolism and reduces the viability of breast cance cells [15,17,98,126] (Table 1).

Lung Cancer
Resveratrol downregulates glucose metabolism, mainly by inhibiting HK2; this i mediated by the Akt signaling pathway and leads to glycolytic suppression and ulti mately apoptosis in lung cancer cells [93]. Moreover, resveratrol reduces glycolytic flux and Glut1 expression by targeting ROS-mediated HIF-1α activation in Lewis lung carci noma tumor-bearing mice [17]. Dasari and colleagues showed that resveratrol induce autophagy in A549 lung cancer cells by upregulating glucosylceramidase beta1 (GBA1) the gene associated with Gaucher disease that codes for glucocerebrosidase, which me tabolizes glucosylceramide to ceramide and glucose.
Interestingly, the expression and activity of glucocerebrosidase were significantly in creased and simultaneously associated with elevated intracellular ceramide levels; both of these correlated with the occurrence of the unique death features [94]. Gu et al. furthe reported that resveratrol and arsenic trioxide (ATO) are involved in ROS-mediated ER stress, mitochondrial dysfunction, and apoptosis in A549 human lung adenocarcinoma cells, providing new insights into the molecular mechanisms of resveratrol-mediated ATO

Breast Cancer
As mentioned above, tumor cells depend primarily on glycolysis to provide the energy and intermediates required for cell growth and proliferation. The enzyme 6-phosphofructo-1kinase (PFK) is a key glycolytic enzyme; its activity is directly associated with cellular glucose utilization. Resveratrol directly inhibits PFK activity, decreases Glut1-mediated glucose uptake, and inhibits intracellular ROS, which suppresses HIF-1α accumulation and thereby disrupts glucose metabolism and reduces the viability of breast cancer cells [15,17,98,126] (Table 1).

Lung Cancer
Resveratrol downregulates glucose metabolism, mainly by inhibiting HK2; this is mediated by the Akt signaling pathway and leads to glycolytic suppression and ultimately apoptosis in lung cancer cells [93]. Moreover, resveratrol reduces glycolytic flux and Glut1 expression by targeting ROS-mediated HIF-1α activation in Lewis lung carcinoma tumor-bearing mice [17]. Dasari and colleagues showed that resveratrol induces autophagy in A549 lung cancer cells by upregulating glucosylceramidase beta1 (GBA1), the gene associated with Gaucher disease that codes for glucocerebrosidase, which metabolizes glucosylceramide to ceramide and glucose.
Interestingly, the expression and activity of glucocerebrosidase were significantly increased and simultaneously associated with elevated intracellular ceramide levels; both of these correlated with the occurrence of the unique death features [94]. Gu et al. further reported that resveratrol and arsenic trioxide (ATO) are involved in ROS-mediated ER stress, mitochondrial dysfunction, and apoptosis in A549 human lung adenocarcinoma cells, providing new insights into the molecular mechanisms of resveratrol-mediated ATO sensitization. This synergistic effect was combined with the upregulation of ER stress mark-ers, including 78 kDa glucose-regulated protein (GRP-78), caspase 12, CCAAT/enhancerbinding protein-homologous protein (CHOP), cytochrome c release, and changes in Bax and Bcl-2 expression [143]. Moreover, Mollerup and colleagues demonstrated that resveratrol exerts chemopreventive effects on lung cancer through the modulation of genes involved in the metabolism of polycyclic aromatic hydrocarbons. Specifically, the inhibition of cytochrome P450 1A1 (CYP1A1) and 1B1 (CYP1B1), and upregulation of microsomal epoxide hydrolase (mEH), resulted in the modified formation of carcinogenic benzo[a]pyrene metabolites in human bronchial epithelial cells [95] (Table 1).

Colorectal Cancer (CRC)
In colorectal cancer cells, resveratrol modulates the lipidomic activity profile, increases oxidative activity, reduces glycolysis, and decreases pentose phosphate activity; it thus reverses the Warburg effect by targeting the pyruvate dehydrogenase complex. Moreover, resveratrol improves the oxidative capacity of colorectal cancer cells via the CamKKB/AMPK signaling pathway [98], suppresses glucose metabolism and tumor growth in vitro and in vivo [97], induces apoptosis by targeting the pentose phosphate and talin-FAK signaling pathways [99], and suppresses glucose uptake by targeting ROSmediated HIF-1α activation [17]. Furthermore, treatment of HT29 human CRC cells with resveratrol induces several ER stress markers (phosphorylation of initiation factor-2alpha (eIF-2alpha), ER stress-specific XBP1 splicing, and CHOP) and decreases glycolytic enzymes (pyruvate kinase and LDH) in Caco2 and HCT-116 cells. Simultaneously, resveratrol stimulates GRP-78, and decreases glucose uptake, Akt phosphorylation, and p-mTOR and p-p70S6K levels; these suggest the induction of ER stress. Finally, resveratrol-induced ER-stress leads to apoptosis of CRC cells [96,144,145] (Table 1).

Prostate Cancer
Resveratrol's anti-tumor effects (on cell growth, hydrogen peroxide production, and mitochondrial network properties) explicitly depend on the predominant oxygen (hypoxic conditions) and glucose levels; this precludes an increased dependence on oxidative phosphorylation. Resveratrol increases ROS production and the expression of the apoptotic biomarkers Bax, p53, and HIF-1α, and inhibits the anti-apoptotic protein Bcl2, thereby promoting cell death. Besides, resveratrol induces apoptosis in prostate cancer cells via the HIF-1α/ROS/p53 signaling pathway [146]. Resveratrol specifically suppresses the nuclear β-catenin protein by inhibiting HIF-1α, possibly in a proteasome-independent manner. It thereby downregulates the β-catenin-mediated transcriptional activity of androgen receptor (AR) signaling. Resveratrol thus suppresses tumor growth induces apoptosis in CRPC [147] (Table 1).
In summary, a large body of evidence shows that resveratrol inhibits the Warburg effect, reduces cancer drug resistance, and sensitizes tumor cells to chemotherapy by targeting and modulating glucose transporters and glycolytic enzymes ( Table 1).

Resveratrol: Its Impact on Intracellular Molecular Signaling Targets Related to Glucose Metabolism in Tumors
Aberrant metabolism and elevated glycolytic rates in cancer cells are linked to various oncogenic processes such as proliferation, evasion of apoptosis, angiogenesis, and reprogramming of the tumor microenvironment [148]. Metabolomic studies reveal that cancers exhibit diverse metabolic phenotypes [148]. Malignant tumor cells utilize aerobic glycolysis to meet their increased glucose requirements in support of their rapid growth and proliferation; to this end, they overexpress glucose transporters (e.g., Glut1) [33, [149][150][151][152]. Predominantly glycolytic tumors are characterized by the altered expression of glycolytic enzymes and transporters; therefore, these proteins represent potential targets in anticancer treatment [148].
Through multiple molecular targets, resveratrol suppresses growth, proliferation, and migration, and induces apoptosis, in tumor cells [153,154]. Resveratrol treatment significantly reduces glucose resorption, lactate production, and cell survival in several human ovarian cancer cell lines in a dose-and time-dependent manner [18,131,155,156]. Interestingly, resveratrol interrupts energy production by stimulating autophagy in tumor cells. Besides, resveratrol blocks glucose uptake in various tumor cells by inhibiting the cell membrane transport of Glut1 via the Akt/mTOR-dependent signaling pathway [16] ( Table 2). The Akt/mTOR signaling pathway plays an essential role in the targeting of metabolism by resveratrol in tumor therapy [18,102,155]. Resveratrol also targets "classical" tumor-promoting pathways, such as PI3K/Akt, STAT3/5, and MAPK, which support glycolysis through the upregulation of glycolytic enzymes and glucose transporters [14,148,[157][158][159][160] (Table 3). Table 2. Resveratrol suppresses glucose absorption and metabolism in tumors.

Experimental Model Study Type Finding Reference
Nuclear magnetic resonance spectroscopy identified glycolysis as the primary glucose catabolic pathway.
In vitro/LY18 human diffuse large B-cell lymphoma cells.
Inhibits PI-3K signaling and glucose metabolism; induces cell cycle arrest. [14] Effects of resveratrol on PKM2 expression, and effects on cancer metabolism.
In vitro/multiple human ovarian carcinoma cells.
In vitro/human ovarian cancer cells.
In vitro/in vivo CT26 colon cancer cells; tumor bearing mice.
Resveratrol-nanoparticles (NP) increase apoptosis and reduce 18 F FDG uptake and ROS. [97] The effects of resveratrol on glucose metabolism.
In vitro/in vivo human ovarian cancer cells; murine xenograft model.
Inhibits glycolysis and glucose uptake by activating AMPK/mTOR; inhibits growth and metastasis. [103] The effects of resveratrol on glucose uptake and accumulation, and Glut1.
Resveratrol blocks Glut1-mediated hexose uptake. [134] Mice were treated with cisplatin, resveratrol, or vehicle alone. The effect of resveratrol on glucose uptake was determined using micro-positron emission.
In vivo/murine xenograft model of ovarian cancer.
Inhibits glucose uptake, with antineoplastic effects; suppresses tumor regrowth after cisplatin therapy.
[135]  Beyond the glycolytic enzymes and protein signaling molecules, glycolysis can also be regulated by miRNAs in cancers [148]. Onco-miRNAs are highly expressed in most cancers [163,164]; resveratrol administration in the Panc-1 pancreatic cancer cell line suppressed miRNA-21, leading to the inhibition of ROS-induced activation, invasion, and glycolysis [112]. PKM2, overexpressed in several cancers, was inhibited by resveratrol via the overexpression of miRNA-326 [165]. Resveratrol inhibited pancreatic cancer cell invasion and migration by suppressing ROS/miRNA-21-mediated activation and glycolysis [112]. These results provide further evidence for the association between the metabolic action of resveratrol and its anti-tumor properties (Figure 3).

Sirtuins as Major Intracellular Targets for Resveratrol in Modulating Tumor Glucose Metabolism
As previously discussed, resveratrol can alter glucose/carbohydrate metabolism in various cancers. Resveratrol modulates several signaling pathways, and thereby regulates gene expression transcription factor activity [166]. Resveratrol targets Glut1, inhibits cancer cell glucose uptake, and alters glucose utilization. Resveratrol reverses the Warburg effect and specifically targets the pyruvate dehydrogenase (PDH) complex, an important mitochondrial gatekeeper enzyme of energy metabolism, leading to increased PDH activity, inhibiting HK and PFK, and downregulating PKM2 activity [98]. Thus, it suppresses cancer cell proliferation, viability, growth, invasion, EMT, metastasis, and angiogenesis, and activates apoptotic cell death, thereby overcoming multi-drug and radioresistance [15][16][17][18]33,98,167]. One of the most important metabolic regulatory pathways stimulated by resveratrol is the molecular signaling pathway dependent on sirtuins (histone deacetylases) [168]. Resveratrol upregulates sirtuin 1 (Sirt1); Sirt1-dependent extensions of the lifespan were initially reported in yeast, worms, and flies [169,170]. Therefore, sirtuins may represent major intracellular targets for resveratrol-mediated modulation of glucose/carbohydrate metabolism in tumors. The seven highly conserved mammalian sirtuin (Sirt1-7) proteins (homologous to the yeast Silent Information Regulator 2; Sir2) are primarily a family of NAD + -dependent histone deacetylases. They modulate various cellular functions, including metabolism, longevity, energy homeostasis, mitochondrial function, and biogenesis, in physiological and pathological conditions [171][172][173]. The SIRTs, although localized in the nucleus (Sirt1, Sirt6, and Sirt7), cytoplasm (Sirt2), and mitochondria (Sirt3, Sirt4, and Sirt5) can reportedly translocate depending on various conditions related to the cell cycle, tissue type, developmental stage, stress, and metabolic status; this suggests that SIRT localization can regulate several metabolic pathways [174] (Table 4). Resveratrol is a specific activator of Sirt1. Resveratrol promotes the deacetylation of many metabolic transcriptional regulators via Sirt1 in vivo [175]; this is associated with diabetes treatment [176,177], apoptosis [178], inflammation, and neuroprotection [179]. Besides, Sirt1 has significant effects on caloric restriction and life extension in cells [180,181]. Interestingly, several intracellular metabolic pathways modulated by Sirt1 are also altered during tumor development. It should be emphasized that Sirt1 can control many different oncogenic proteins and drugs and thus many cellular metabolic problems [182], such as the proliferator-activated receptor-gamma-coactivator-1 (PGC-1). Indeed, PGC-1 is activated after deacetylation by Sirt1 and regulates mitochondrial gene expression [183,184]. Sirt1 also targets other transcription factors, such as NF-κB [120], PTP1B [185] and the FOXO (Forkhead O box) family [186]. FOXO1 regulates insulin secretion, insulin resistance, and insulin signaling pathways; in turn, it can inhibit cellular glucose uptake and metabolism [185]. Furthermore, Sirt1 activation regulates the expression of many genes that control metabolism, such as pyruvate dehydrogenase lipoamide kinase 4 (PDK4) and PDH. Notably, resveratrol modulates metabolism via the Sirt1-FOXO1 signaling pathway [187].
The anti-aging effects of resveratrol could be mediated through its anti-oxidant, anticyclooxygenase, and anti-free radical activities, its effect on the cell cycle in vitro and in vivo, and its stimulation of Sirt1 [169,170,188,189]. Resveratrol treatment mimics the protective effect of caloric restriction against cancer by inducting Sirt1 [160,190]. In estrogen receptor-positive breast cancer cells, resveratrol elevated NAD + /NADH, subsequently activated Sirt1, and in turn activated the AMP-activated kinase (AMPK), a key sensor of cellular energy levels [160]. AMPK activation further inhibits the mTOR pathway and protein translation by inhibiting 4E-BP1, thereby inhibiting cancer cell proliferation [160]. Resveratrol treatment in human NSCLC cell lines upregulated Sirt1; this correlated with the loss of NF-κB function and gene expression, and rendered the cells susceptible to TNFα-induced apoptosis [191]. Resveratrol treatment mediated a dose-dependent increase in micro-RNA (miRNA-27b) correlated with the Sirt1-dependent improvement in mitochondrial function in C2C12 myoblasts and skeletal muscle [192]. Sirt6, a key regulator of glucose homeostasis and modulator of Glut1, aldolase, PDK1, and PFK1, was induced by resveratrol in FaDu hypopharyngeal carcinoma cells [193,194]. Moreover, Sirt1 plays an essential role in epigenetic modifications of the chromatin pattern and DNA repair by deacetylation [173,195]. In addition, Sirt1 controls the cellular stress response. Therefore, the state and activity of Sirt1 in cancer may play an essential role in cellular responses to epigenetic conditions and treatments.

Pharmacokinetics of Resveratrol: Challenges and Future Perspectives
Resveratrol occurs as two isomers in plants: transand cis-resveratrol [156]. The transisomer predominates in Nature [345]; it is biologically more active and more frequently studied [346] due to its higher stability [347].
Although resveratrol is extensively studied in preclinical research, its mechanisms of action under different conditions and at different doses remain elusive, as many effects demonstrated in vitro cannot be reproduced in vivo [348]. Several factors contribute to this non-reproducibility and restrict resveratrol's clinical applicability [349]. One such factor is the compound's pharmacokinetic profile [348]. Moreover, resveratrol has low systemic bioavailability, which may reduce its efficacy [156,350]. The bioavailability of orally administered resveratrol is less than 1% due to its rapid and extensive metabolism in the intestine and liver [345,346].

Absorption
Resveratrol's appreciable solubility in alcohols and low solubility in water affects its absorption [347]. 75% of resveratrol is absorbed at the gastrointestinal level after oral administration in humans [345,346]. In the intestine, resveratrol is absorbed through passive diffusion or forms complexes with membrane transporters, such as integrins.
Within the systemic circulation, resveratrol can be found in its free form or as its metabolites (glucuronide, sulfate). Free resveratrol can bind to lipoproteins or albumin; this facilitates its membrane transport and entry into cells via lipoprotein and albumin receptors [347]. The hydroxyl groups in resveratrol's structure also enable it to associate with proteins and other nutrients. Therefore, resveratrol complexes retain their solubility and can be absorbed in the small intestine [347]. However, plasma resveratrol concentrations are not affected by protein content [351].

Metabolism
Resveratrol metabolism occurs via two main pathways [352]. First, the UDP-glucuronosyltransferase (UGT) enzyme family mediates the glucuronidation of resveratrol by catalyzing its conjugation to a glucuronic acid residue (either at the 3 or 4 -hydroxyl group); this alters its biological properties and facilitates its elimination from the body [352]. Critical glucuronide conjugates of resveratrol include resveratrol-4 -O-glucuronide and resveratrol 3-O-glucuronide [156,350,353]. However, the human liver microsomes contain high concentrations of UGT enzymes that preferentially form resveratrol-3-O-glucuronide [352].

Bioavailability and Tissue Distribution
The bioavailability and tissue distribution of resveratrol are limited. Nevertheless, resveratrol is effective in vivo; this can be explained by the reconversion of resveratrol metabolites into resveratrol in target tissues [347]. The deconjugation of enzymes such as β-glucuronidase and sulfatase and specific tissue accumulation may enhance resveratrol's efficacy at target sites. After the stable sulfate-conjugated form of resveratrol is delivered to the target tissue, the starting compound can be regenerated to produce beneficial effects in vivo [346]. The enterohepatic recirculation of resveratrol metabolites may also explain resveratrol's efficacy despite its low bioavailability and rapid metabolism [347].
Furthermore, there is little information on the biological activity of resveratrol metabolites [352], and preclinical research suggests that the bioactivity of glucuronated and sulfated resveratrol metabolites is weaker than that of their parent form [354]. The bioactivity of sulfated conjugates appears to decrease with increasing degrees of sulfation [352]. To the contrary, resveratrol's beneficial effects can also be associated with its metabolites [347], as they have strong pharmacological activities [355,356]. In addition to its rapid metabolism, 75% of all resveratrol consumed is rapidly excreted. The remaining resveratrol is metabolized, and the maximum observed concentration of free resveratrol is between 1.7% and 1.9% of the initial level [357].
Interestingly, after ingestion, higher concentrations of resveratrol and its metabolites were observed in the right side of the colon than the left side [358]. Lee et al. determined that resveratrol degradation is affected by small intestinal digestion, and that in vitro human digestion decreases resveratrol's free radical scavenging activity [359]. Nevertheless, a daily dose of 0.5 or 1 g of resveratrol produced sufficient concentrations for anti-carcinogenic effects in the human gastrointestinal tract [358].

Improving the Biological Effectiveness of Resveratrol
The pharmacokinetics of trans-resveratrol can be affected by administration routes, dosages, and treatment regimens [352]; the plasma concentration of resveratrol is associated with the ingested dose [353]. Orally administered resveratrol, in the form of 500 mg tablets, was well absorbed; the plasma concentrations of trans-resveratrol and its metabolites were within the reported ranges of in vitro efficacy [346]. Moreover, after repeated high dose administration in healthy volunteers, a micromolar concentration of resveratrol and much higher concentrations of its glucuronide and sulfate conjugates were observed in the plasma [360]. However, the plasma concentration of trans-resveratrol remained low despite high doses and a short dosing interval; nevertheless, trans-resveratrol's pharmacokinetics revealed circadian variations, with higher bioavailability after morning administration [361].

Delivery System Study Details Results Reference
Self-emulsifying drug delivery systems of resveratrol. In vitro, in vivo (rats). Increased solubility, reduced metabolism, and improved its pharmacokinetic profile. [362] Micellar solubilization of resveratrol.
Layer-by-layer nanoparticles and resveratrol nanocores enhanced systemic exposure compared to free resveratrol. [364] Oat protein-shellac nanoparticle delivery system.
In vitro, in vivo (rat model).
Protected resveratrol in gastric fluid and controlled its release into the small intestine. Improved cell uptake and transport compared to free resveratrol. Increased bioavailability. [365] Encapsulation of resveratrol in casein nanoparticles. In vitro, in vivo (rats).
Oral administration in rats: remained in the gut and reached intestinal epithelium. Produced high plasma levels of resveratrol (sustained for at least 8 h) and similar results for its metabolites. Oral bioavailability was 10 times higher compared to an oral solution of resveratrol. [366] Trans-resveratrol nanocrystals.
Improved bioavailability of resveratrol. Elevated concentrations in the main organs of rats compared to pure resveratrol. Highest concentrations were observed in the liver (promising liver-targeted delivery system). [368] Trans-resveratrol-loaded mixed micelles.
In vivo (nude mice; intraperitoneal injection). Improved dispersal and water solubility. Inhibited carcinoma growth in nude mice bearing human primary ovarian tumors. [370] Besides, the combination of resveratrol with other compounds could improve its bioavailability. Piperine, an alkaloid derived from black pepper, can inhibit glucuronidation; it may therefore increase resveratrol's bioavailability by slowing its metabolism. As shown by Johnson et al., piperine may improve resveratrol's bioavailability in mice [371]. Furthermore, De Santi et al. showed that quercetin could inhibit sulfotransferase 1A1 (SULT1A1) enzyme activity and thereby decrease the sulfate conjugate of resveratrol. However, this did not significantly increase resveratrol's bioavailability [372]. Various synthetic resveratrol derivatives, such as hydroxylated, methoxylated, and halogenated derivatives, have also received attention due to their improved pharmacokinetics and biological activity compared to resveratrol [373,374].

Clinical Trials with Resveratrol
Promising preclinical findings on resveratrol's anti-cancer effects have led to investigations into its clinical effects [375]. A study conducted on forty-two healthy volunteers demonstrated resveratrol's capability to modulate enzymes involved in carcinogen activation and detoxification and thereby prevent carcinogenesis [376]. Resveratrol also exerted chemopreventive effects in women at increased risk of breast cancer by decreasing the methylation of the tumor suppressor RASSF-1α [377]. Similarly, in postmenopausal women with high BMI (BMI ≥ 25 kg/m 2 ), resveratrol exerted favorable effects on hormone-related breast cancer risk factors (sex steroid hormone-binding globulin and estrogen metabolites) [378]. However, clinical evidence on the metabolic effects of resveratrol remains scarce. An in-depth search of clinical trial records yielded only indirect evidence of resveratrol's possible effects on tumor metabolism. High concentrations of insulin-like growth factor-1 (IGF-1) and insulin promote carcinogenesis and early tumor growth through antiapoptotic signaling and PI3K-Akt-mTORC1-mediated metabolic reprogramming. Diabetes and obesity are mainly associated with an increased risk of cancer with the Warburg phenotype [379]. A Warburg-like effect with elevated glycolysis is also present in tumors characterized by chronic treatment with insulin analogs with high affinity for the IGF-1 receptor (IGF1 and X10) [380].
Repeated administration of high-dose resveratrol in forty healthy volunteers reduced circulating IGF-I and IGF-binding protein-3 (IGFBP-3), suggesting that this mechanism is involved in resveratrol's chemopreventive efficacy [360]. Resveratrol also reduced fasting insulin levels in patients with polycystic ovary syndrome [381]. The Wnt signaling pathway is associated with obesity and diabetes through its effects on cell metabolism, and may also be involved in metabolic reprogramming for cancer [382]. However, resveratrol can modulate the Wnt pathway [383]. Nevertheless, a phase I pilot study on colorectal cancer patients showed that resveratrol-containing freeze-dried grape powder did not inhibit the Wnt signaling pathway in colorectal cancer, but significantly inhibited Wnt expression in normal colon mucosal cells [384]. Importantly, resveratrol is safe and well-tolerated in numerous clinical studies [361,385]. However, resveratrol may be associated with mild to moderate gastrointestinal symptoms (at daily doses of 2.5-5 g) [360] or diarrhea (at twicedaily doses of 2 g) [386].
Targeting cancer cell metabolism with resveratrol could be a promising oncologic approach. However, further clinical research into the effects of resveratrol on tumor metabolism is necessary.

Conclusions and Outlook
The increasing incidence of cancer, as well as unfavorable prognoses in the event latestage diagnosis and/or complications posed by treatment inefficacy and chemoresistance necessitate the identification of novel oncologic compounds. While metabolic and cellular signaling changes are relatively well-known processes in cancer development, the targeted manipulation of tumor metabolism can promote rapid progress in cancer treatment [387].
To the contrary, the clinical anti-cancer efficacy of resveratrol through the regulation of tumor metabolism is not sufficiently investigated. Existing clinical trial results only indirectly indicate relationships between resveratrol and cancer metabolism [360,381,384]. Resveratrol's application in living organisms is hindered by its low bioavailability, rapid metabolism, and pharmacokinetics [156,345,346,386]. Nevertheless, rapid progress is being made in new delivery systems to increase the efficacy of resveratrol [362][363][364][365][366][367][368][369]; this could contribute to the application of resveratrol in cancer treatment in the context of personalized medicine.
Author Contributions: A.B., P.K., D.B., M.S. designed the study and took the lead in writing the manuscript with critical input from S.S., A.L., E.V., S.M.S. and K.Z., who contributed to the design and helped with the interpretation of literature data. M.S. provided expert assistance and supervised the preparation of the manuscript. All authors provided feedback and contributed to the final manuscript. All authors have read and agreed to the published version of the manuscript. Acknowledgments: We thank Constanze Buhrmann, Sabine Miech, and Andreas Eimannsberger for skilled and excellent technical assistance. We note, that the research was conducted in part for the doctoral thesis of Aranka Brockmueller to be submitted to Fachbereich Humanmedizin, Ludwig-Maximilians-University Munich, Germany.

Conflicts of Interest:
The authors declare no conflict of interest.