Consumption of coffee, tea, wine, curry, and soybeans has been linked to a lower risk of cancer in epidemiological studies. Several cell-based and animal studies have shown that dietary polyphenols like chlorogenic acid, curcumin, epigallocatechin-3-O-gallate, genistein, quercetin and resveratrol play a major role in these anticancer effects. Several mechanisms have been proposed to explain the anticancer effects of polyphenols. Depending on the cellular microenvironment, these polyphenols can exert double-faced actions as either an antioxidant or a prooxidant, and one of the representative anticancer mechanisms is a reactive oxygen species (ROS)-mediated mechanism. These polyphenols can also influence microRNA (miR) expression. In general, they can modulate the expression/activity of the constituent molecules in ROS-mediated anticancer pathways by increasing the expression of tumor-suppressive miRs and decreasing the expression of oncogenic miRs.
1. Introduction
Human epidemiological studies have shown that diets high in plant polyphenols have beneficial effects on various diseases including cancer
[1][2]. Researchers have discussed the anticancer effects of coffee, tea, wine, and curry based on recent evidence from human studies, in which chlorogenic acid (CGA), (-)-epigallocatechin gallate (EGCG), resveratrol (RES), and curcumin (CUR), respectively, are believed to be major contributors to the activity
[3] (
Figure 1 and
Table 1).
Figure 1. Chemical structures of CGA, EGCG, RES, CUR, QUE, and GEN.
Quercetin (QUE) is a flavonol found in a variety of fruits and vegetables including apples, grapes, broccoli, green tea, and onions
[4][5] (
Figure 1), and several human studies have shown that QUE-rich diets have anticancer effects
[5][6][7][8]. For example, Ekström et al.
[7] discovered that QUE intake had a strong inverse association with the risk of noncardia gastric adenocarcinoma, with an adjusted odds ratio (OR) of 0.57 (95% confidence interval [CI] = 0.40–0.83) when the highest quintile (≥11.9 mg/day) was compared to the lowest quintile (<4 mg).
Epidemiologic studies have also shown that a soy-rich diet reduces the risk of various diseases, including cancer, and one of the main contributors is thought to be genistein (GEN), a phenolic compound
[9][10][11] (
Figure 1). Wang et al.
[12] discovered a lower risk of papillary macrocarcinomas in women who consumed 1860–3110 μg/day of GEN (OR = 0.26, CI = 0.08–0.85) compared to women who consumed <760 μg/day in a population-based case-control study in Connecticut from 2010 to 2011. A meta-analysis conducted by Applegate et al.
[13] revealed that the pooled relative risk for GEN in the risk of prostate cancer was 0.90 (CI: 0.84–0.97).
Many epidemiological studies, on the other hand, have found that these foods have no anticancer effects
[1][14]. The inconsistent results could be due to a number of confounding factors, including the quantity and quality of plant foods consumed, as well as residual pesticides and acrylamide formed during preparation, cigarette smoking, alcohol consumption, differences in ingredients, hormonal activities, microbiota, and genetic background
[1][14][15]. Human intervention studies that are well-designed could provide significant evidence for the anticancer effects of dietary foods containing these polyphenols.
The anticancer properties of these polyphenols have been demonstrated in a large number of cell-based and animal studies, and their possible anticancer mechanisms have been proposed. Of them, one involving reactive oxygen species (ROS) appears to be the most likely, in which these polyphenols can act as both an ROS-generator and an ROS-scavenger
[16].
2. Anticancer Mechanism of Tumor Suppressor miRs Upregulated by Polyphenols
Table 2 summarizes the available data for tumor-suppressor miRs that are commonly upregulated by at least three different polyphenols in cancer cells. Figure 2 shows that several molecules involved in the anticancer mechanism are found in ROS-mediated pathways. Table 2 also provides information on the modulatory effects of miRs upregulated by these polyphenols on these molecules.
2.1. miR-16
CUR, EGCG, QUE, and RES have been shown to have anticancer properties
[3][57][58]. These polyphenols have been shown to increase the expression of the tumor suppressor miR-16. miR-16 has the ability to reduce the expression of the target Bcl-2
[18]. Claudin-2 expression is decreased by QUE-induced miR-16, which may downregulate Bcl-2
[19]. Bcl-2 is an anti-apoptotic protein, and its inhibition would result in an anticancer effect. QUE may increase miR-16 expression to decrease Homeobox A10 expression, which is involved in cancer proliferation, migration, and invasion
[20]. RES increased the expression and activity of Argonaute2, a central RNA interference component, which resulted in anticancer effects by increasing the expression of several tumor-suppressor miRs including miR-16
[21].
2.2. miR-22
CUR, EGCG, and QUE have been shown to upregulate miR-22, which may downregulate specificity protein 1 (Sp1)
, estrogen receptor 1 (ESR1)
[23], erythoblastic leukemia viral oncogene homolog 3 (Erbb3)
[24], and nuclear receptor coactivator 1 (NCoA1)
[25]. Sun et al.
[23] discovered that CUR increased miR-22 expression in PxBC-3 pancreatic cancer cells using oligonucleotide microarray analysis. Transfection with miR-22 mimetics reduced expression of the target genes Sp1 and ESR1, whereas antisense inhibition of miR-22 increased Sp1 and ESR1 expression. Sp1 is overexpressed in various cancers and has the potential to be a chemotherapeutic drug target
[59]. Sp1 can upregulate VEGF to promote cancer cell growth, angiogenesis, and metastasis
[60][61], downregulation of miR-22 upregulated by these polyphenols may contribute to the anticancer effects of these polyphenols.
In malignant T cells, transfection of recombinant miR-22 resulted in the inhibition of its targets including NCoA1, HDAC6, MAX, MYCBP, and PTEN
[25]. As PTEN is known to be tumor suppressing
[62], its downregulation by CUR does not appear to be consistent with CUR’s anticancer properties. Downregulation of other cancer-promoting molecules such as HDAC6, required for efficient oncogenic tumorigenesis
[63], and NCoA1, whose overexpression increases the number of circulating cancer cells and the metastasis
[64], may overwhelm PTEN’s efficacy in this case.
Zhang et al.
[27] showed that overexpression of miR-22 increased cancer cell apoptosis by targeting WNT1, and that the miR-22/WNT1/β-catenin axis is the downstream pathway for QUE to exert an antitumor effect in oral squamous cell carcinoma.
2.3. miR-34a
CUR upregulation of miR-34 resulted in Bcl-2 downregulation, cell cycle arrest, and/or c-Myc downregulation
[28][29][30]. RES increased apoptosis and miR-34a expression in ovarian cancer cells
[41]. miR-34a inhibition experiments revealed that miR-34a downregulates Bcl-2, upregulates Bax, and activates caspase-3.
EGCG has been shown to exert anticancer effects by upregulating tumor-suppressing miRs including miR-34a and downregulating oncogenic miRs such as miR-92, miR-93, and miR-106b
[33].
In an experiment with HNC-TICs cells from head and neck cancer, GEN inhibited their proliferation, downregulated epithelial–mesenchymal transition (EMT), and induced upregulation of miR-34a, which resulted in ROS production
[36]. Caspase-3 activation induced by overexpression of miR-34a was inhibited by
N-acetylcysteine, indicating that ROS are involved in the anticancer effects of GEN.
In, GEN induced apoptosis in prostate cancer PC3 and DU145 cells, increased miR-34a expression levels, and reduced those of oncogenic HOX transcript antisense RNA (HOTAIR), a target of miR-34a
[37]. HOTAIR is a non-coding RNA that has been shown to induce cell cycle arrest in the G
2/M phase
[65]. The GEN-mediated upregulation of miR-34a in pancreatic cancer cells also inhibited the Notch-1 signaling pathway
[38], whose activation promotes cancer cell growth and metastasis
[66][67]. Inhibition of Notch-1 would result in down regulation of NF-κB, leading to cancer suppression
[68].
RES increased the expression of tumor suppressor miR-34a, 424, and 503 in breast cancer cells
[40]. HNRNPA1, a heterogeneous nuclear ribonucleoprotein associated with tumorigenesis and progression, was directly downregulated by miR-424 and miR-503, but indirectly by miR-34a
[40]. According to Kumazaki et al.
[39], RES upregulates miR-34a, which causes downregulation of the target gene E2F3 and its downstream SIRT1, leading to inhibition of colon cancer cell growth.
Thus, polyphenols appear to upregulate miR-34 in general, but Subrama-niam et al.
[31] found that CUR decreased expression of miR-34a in esophageal cancer TE-7 cells. One possible explanation for the difference is that the p53 status of different cell lines differs, as TE-7 cells are p53-deficient and p53 is an upstream regulator of miR-34a.
2.4. miR-141
CUR upregulated the expression of EMT-suppressing miRs such as miR-34a, 101, 141, 200c, and 429 in 5-fluorouracil (5FU)-resistant HCT116 cells, but not in 5FU-resistant SW480 cells
[42]. EMT is a crucial step in the generation of cancer stem cells and the progression of cancer. The extent to which miR-141 contributes to EMT suppression is not known.
Chiyomaru et al.
[44] discovered that treatment of renal carcinoma cells with GEN increased miR-141 expression and decreased HOTAIR, which is known to promote malignancy. HOTAIR expression was reduced in cells transfected with pre-miR-141. By increasing the expression of a number of tumor-suppressive miRs, including miR-16, 141, 143, and 200c, RES reduced the viability of breast cancer cells and inhibited cancer stem-like cell characteristics
[21]. The miR-141 inhibitor reduced the efficacy of RES’s inhibitory effect against cancer invasion, implying that miR-141 plays a role in RES’ anticancer effect.
Gordon et al.
[43] reported that treatment of multiple myeloma, MM1.s cells, with the carcinogen benzo[a]pyrene upregulated the expression of miR-15a, 16, 25, 92, 125b, 141, and 200a, all of which are p53 targets. EGCG inhibited the expression of tumor-suppressive miR-141 which upregulates p53. The finding appears inconsistent with EGCG’s anticancer activity. It is possible that EGCG’s downregulation of oncogenic miR-25 may be more effective in the anticancer effect than downregulation of miR-141 in these cells.
2.5. miR-145
Curcumin encapsulated in a non-toxic nanocarrier inhibited the proliferation of glioblastoma U-87 MG cells, increased miR-145 expression, and decreased the expression of transcription factors Oct4, SOX-2, and Nanog, all of which are upregulated and result in increased metastasis, invasion, and recurrence
[45][69].
CUR inhibited the proliferation, invasion, and tumorigenicity of prostate cancer stem cells HuPCaSCs (CD44
+/CD133
+ subpopulation isolated from prostate cancer cell lines Du145 and 22RV1) by increasing the expression of miR-145, which prevents cell proliferation by decreasing Oct4 expression
[46]. In colorectal cancer cells, EGCG increased apoptosis and cell cycle arrest, and upregulated miR-145
[34].
In GEN-treated retinoblastoma Y79 cells, miR-145 was found to be significantly upregulated
[47]. The siRNA downregulated miR-145 and the target of miR-145 has been identified as ABCE1 which has oncogene-like properties. By increasing the expression of miR-145, QUE was found to induce apoptosis in human ovarian carcinoma cells. The increased expression levels of cleaved caspase-3 induced by QUE were further increased by overexpression of miR-145
[48].
2.6. miR-146a
CUR upregulated miR-146a in human U-87 MG glioblastoma cells, and overexpression of miR-146a increased apoptosis and decreased NF-κB activation in cells treated with the anticancer drug temozolomide
[50]. miR-146a expression is lower in pancreatic cancer cells compared to normal human pancreatic duct epithelial cells. GEN treatment increased miR-146a expression with decreasing EGFR and NF-κB expression in these cancer cells. Transfection of miR-146a inhibited these cells’ invasive ability by downregulating EGFR and NF-κB, implying that upregulation of miR-146a is involved in the anticancer effect of GEN
[52]. The results of experiments with or without transfection of miR-146a mimic or anti-miR-146a revealed that QUE increased miR-146a, leading to apoptosis induction through downregulation of EGFR and activation of caspase-3 in a study of QUE’s anticancer effect
[53].
2.7. miR-200c
Experiments on overexpression or silencing of miR-200c in pancreatic cancer MiaPaCa-2 cells showed that a CUR analog upregulated PTEN expression, increased levels of MT1-MMP, and reduced tumor cell aggressiveness through upregulation of miR-200c
[54]. Toden et al.
[42] discovered that CUR improved the efficacy of 5-FU in suppressing tumor growth and EMT in 5FU-resistant colorectal cancer cells. miR-200c, a key EMT-suppressing miR, was upregulated by CUR, and miR-200c was found to downregulate BMI1, SUZ12, and EZH2 in a transfection experiment.
Upregulation of miR-200c was also observed in RES-treated nasopharyngeal carcinoma cancer stem cells
[55], EGCG-treated 5FU-resistant colorectal cancer cells
[34], and RES-treated breast cancer cells
[21]. Dermani et al.
[56] discovered that RES increased the expression of miR-200c and decreased the viability of colorectal cancer cells. Transfection with anti-miR-200c increased vimentin and ZEB1 expression, while decreasing E-cadherin expression and apoptosis. These changes were reversed by RES, indicating that RES induces apoptosis and inhibits EMT in colorectal cancer by regulating miR-200c.