Targeting Epigenetics for Cancer Prevention by Dietary Cancer Preventive Compounds—the Case of miRNA

Abstract

In cancer, genetic mutations have long been considered to be the only driver of neoplasia. However, there is increasing evidence that epigenetic alterations could also play a major role in carcinogenesis and cancer. A number of experimental and epidemiologic studies have shown that many classes of dietary phytochemicals possess cancer-preventive and epigenetic-modifying properties. The report by Derry and colleagues in this issue of the journal shows that grape seed extract (GSE) prevents azoxymethane (AOM)-induced colon colitis via epigenetic miRNA regulation. Although the precise mechanism underlying the control of miRNA expression is not well understood currently, epigenetic changes could play a major role. This report, along with increasing evidence demonstrating the impact of dietary phytochemicals on epigenetic activities, offers new perspectives on miRNA and epigenetic regulation in cancer prevention.

In cancer, genetic mutations have long been considered to be the only driver of neoplasia (1). However, there is increasing evidence that epigenetic alterations, defined as changes in the regulation of the expression of gene activity without alteration of genetic structure, could also play a major role in carcinogenesis and cancer (2). Unlike genetic alterations, for instance, chemicals or drugs could reverse a decrease in gene expression due to epigenetic silencing via aberrant promoter hypermethylation. Therefore, small molecules targeting enzymes responsible for DNA methylation and histone modifications could potentially be important for cancer prevention and therapy. In this context, inhibitors of DNA methyltransferases (DNMTs) and histone deacetylases (HDACs) have been approved for cancer treatment by the FDA and proven to have therapeutic potency against a number of malignancies (3). Many classes of dietary phytochemicals have been shown to possess cancer-preventive properties through studies in in vitro and, in vivo animal models and findings from numerous epidemiological studies (4, 5). Surprisingly, these dietary cancer-preventive agents have also been found to regulate epigenetic activities (6-8). For instance, green tea polyphenol (–)-epigallocatechin-3-gallate inhibits DNA methyltransferase and reactivates methylation-silenced genes in cancer cell lines (9), while sulforaphane and 3, 3′-diindolylmethane from cruciferous vegetables have been reported to inhibit histone deacetylases activities (10). Curcumin from turmeric and resveratrol from red grapes have also been reported to elicit epigenetic-modifying activities, among others (11, 12). Hence, epigenetic modifications appear to play an important role in cancer prevention in that they can be inhibited by dietary cancer-preventive phytochemicals.

Grape seed extract (GSE) has been extensively investigated for the prevention and treatment of cancers in in vitro and in vivo preclinical models (13-15). In this issue of the journal, Derry et al. reports the efficacy of GSE against azoxymethane (AOM)-induced colon tumorigenesis in A/J mice (16). Results from their study show that dietary supplement of GSE dramatically decreases colon tumor incidence and overall tumor size. Biomarker examination shows anti-proliferation and pro-apoptosis effects of GSE on colon tumor tissues. Interestingly, GSE strongly modulates miRNA expression profiles and miRNA processing machinery associated with alterations in NF-κB, β-catenin and MAPK signaling. Immunohistochemistry analyses shows down-regulation of inflammatory signaling pathways including NF-κB activation and its downstream targets COX-2, iNOS, and VEGF, decreased β-catenin signaling and its target gene C-myc, and a reduction in phosphorylated ERK1/2 levels corroborating miRNA expression profiles. Derry et al. suggests that GSE exerts its efficacy against colon tumorigenesis by targeting inflammation, proliferation and apoptosis possibly via modulation of miRNA expression.

miRNAs are endogenous small non-coding RNA molecules that can control gene expression at both transcriptional and post-transcriptional level. miRNAs bind to their target mRNA in either complete or incomplete complementary fashion, thus they down-regulate the stability and/or translation of their target mRNAs (17). miRNAs have been shown to be involved not only in normal tissue differentiation and development (18) but also in carcinogenesis. For instance, Iorio et al. reported that miR-21 is aberrantly up-regulated in human breast cancers, serving as an anti-apoptosis factor (19). Attenuated expression of miR-let7 was reported in human lung cancers, and forced expression of let-7 in A549 lung adenocarcinoma cell line inhibited cell growth (20). More recently, cellular and molecular studies show that miRNAs affect inflammation and cytokine signaling (21, 22), and relatedly, alteration of miRNAs expression by dietary compounds has been shown to be associated with their cancer preventive and therapeutic benefits (23, 24). Prior to the report by Derry et al. in this issue, regulation of miRNA expression by GSE has been reported in human hepatocellular carcinoma HepG2 cell line (25). Meanwhile, Gao et al. reported dysregulation of immune/inflammation-related miRNAs in AOM-DSS induced colitis-associated colorectal cancer model (26). The current work of Derry et al. links the alteration of miRNA expression and those previously identified molecular targets, indicating a potential epigenetic/miRNA mechanism could exist for the cancer chemopreventive efficacy of GSE.

Although increasing evidence showing the importance of miRNA in cancer initiation and development, the mechanism of miRNA regulation and targeting is still not well understood. Many miRNAs are located in the introns of protein-coding genes (27). As reviewed by You and Jones (2), epigenetic mechanisms including DNA methylation and histone modifications can control the expression of many protein-coding genes, and it is possible that epigenetics could play a critical role in miRNA expression control. A recent study by Wilting et al. (28) shows an increase in CpG methylation of miR-149, -203 and -375 during cervical carcinogenesis, whereas expression of these epigenetically-silenced miRNAs was restored upon treatment with a potent DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine. Similarly, inhibition of histone deacetylases (HDACs) leads to a rapid change in miRNA expression profile in human breast cancer cell line SKBr3 (29), while recent studies demonstrate the importance of histone methyltransferases and acetyl-histone recognition proteins in miRNA regulation in aggressive B-cell lymphomas (30, 31). As discussed above, many dietary cancer chemopreventive phytochemicals from our daily-consumed fruits and vegetables have been reported to exhibit epigenetic modification activities. With respect to GSE, Vaid et al. reported GSE treatment decreased the expression and activity of DNA methyltransferase, caused down-regulation of global DNA methylation level, and resulted in reactivation of silenced tumor suppressor genes, such as RASSF1A, p16^INK4a(CDKN2A) and Cip1/p21(CDKN1A) in human squamous cell carcinoma A431 and SCC13 cell lines (32). Together, these findings may provide some insights into the epigenetic regulatory mechanism by GSE from two possible scenarios: GSE may turn on the tumor suppressor genes that are aberrantly silenced by epigenetic mechanism via CpG demethylation and histone modifications, and on the other side of the coin, GSE may reactivate miRNAs controlling tumor-suppressor genes resulting in down-regulation of target oncogenic mRNAs. Further studies would be needed to interrogate the connectivity between epigenetic changes and alteration of miRNA expression profiles by GSE and other cancer-preventive dietary phytochemicals in in vitro and in vivo cancer models.

In conclusion, the findings of Derry et al. and others – epigenetic modifications by dietary phytochemicals in cancer prevention – provide new potential insights into cancer-preventive mechanisms of GSE and other dietary phytochemicals. It is highly conceivable that epigenetically-controlled chromatin remodeling such as CpG methylation, histone modifications as well as miRNA that drive various stages of carcinogenesis could be intervened or interrupted by dietary cancer-preventive phytochemicals (33). Uncovering these potential epigenetic molecular targets by phytochemicals will certainly advance our basic understanding of carcinogenesis and phytochemical-cancer relationships that can ultimately translate to chemopreventive or therapeutic strategies.