Reduction in promotor methylation utilizing EGCG (epigallocatechin-3-gallate) restores RXRα expression in human colon cancer cells.

Silencing of regulatory genes through hypermethylation of CpG islands is an important mechanism in tumorigenesis. In colon cancer, RXRα, an important dimerization partner with other nuclear transcription factors, is silenced through this mechanism. We previously found that colon tumors in ApcMin/+ mice had diminished levels of RXRα protein and expression levels of this gene were restored by treatment with a green tea intervention, due to reduced promoter methylation of RXRα. We hypothesized that CIMP+ cell lines, which epigenetically silence key regulatory genes would also evidence silencing of RXRα and EGCG treatment would restore its expression. We indeed found EGCG to restore RXRα activity levels in the human cell lines, in a dose dependent manner and reduced RXRα promoter methylation. EGCG induced methylation changes in several other colon cancer related genes but did not cause a decrease in global methylation. Numerous epidemiological reports have shown the benefits of green tea consumption in reducing colon cancer risk but to date no studies have shown that the risk reduction may be related to the epigenetic restoration by tea polyphenols. Our results show that EGCG modulates the reversal of gene silencing involved in colon carcinogenesis providing a possible avenue for colon cancer prevention and treatment.

The importance of the retinoid X receptor alpha in modulating inflammatory signaling in acute murine colitis.

BACKGROUND: In order for vitamin D to signal and regulate inflammatory pathways, it must bind to its receptor (VDR) which must heterodimerize with the retinoid X receptor alpha (RXRα). Although the role that vitamin D signaling plays in the development and progression of colitis, a disease characterized by excessive inflammation, has been suggested, little research has been done on determining the role that RXRα plays in acute colitis development. AIMS: This study sought to determine the effects that reduced availability of RXRα would have on the development of acute murine colitis. Expression of inflammatory markers, VDR and RXRα were investigated to determine if the reduction in expression of RXRα in RXRα(+/-) mice would result in increased inflammatory signaling and receptor downregulation as compared to their wild-type littermates. METHODS: An acute murine model of colitis, the axozymethane (AOM) and dextran sulfate sodium (DSS) model was utilized in wild-type and RXRα(+/-) mice. Gross manifestations of colitis measured included weight loss and colitis score. Immunblots and real-time PCR were performed for inflammatory markers and receptor expression. RESULTS: RXRα(+/-) mice induced with AOM/DSS colitis demonstrated increased gene expression of Snail and Snail2, transcription factors downstream of inflammatory mediators, as compared to their wild-type littermates. CONCLUSIONS: This demonstrates the importance of RXRα in regulating inflammation in acute colitis and also identifies RXRα expression as a new consideration when developing successful interventions for acute colitis due to the requirement of numerous receptors for RXRα.

Green tea polyphenol epigallocatechin 3-gallate, contributes to the degradation of DNMT3A and HDAC3 in HCT 116 human colon cancer cells.

BACKGROUND: Colon cancer is still the second leading cause of cancer deaths in the United States. Epigenetic gene silencing involving DNA methyltransferases (DNMTs) and histone deacetylases (HDACs) plays an important role in the progression of colon cancer. MATERIALS AND METHODS: In the present study we found that the sensitivity of colon cancer cells to methylation plays a role in its response to alternative therapy involving the green tea polyphenol, epigallocatechin 3-gallate. HDAC and DNMT protein expression were reduced when methylation-sensitive HCT 116 human colon cancer cells was treated with EGCG, but was relatively stable in the HT-29 cell line. This decrease in expression may be partially explained by our finding that DNMT3A and HDAC3 are degraded in the methylation-sensitive colon cancer cells in part by inhibiting their association with the E3 ubiquitin ligase, UHRF1. CONCLUSION: These findings provide a rationale for the development of a targeted therapy for methylation-sensitive colon cancer that can include EGCG in combination with other DNMT and HDAC inhibitors.

Vitamin D receptor and retinoid X receptor α status and vitamin D insufficiency in models of murine colitis.

The anti-inflammatory actions of vitamin D have long been recognized and its importance in modulating colon cancer and colitis development is becoming apparent. The vitamin D receptor (VDR) is downregulated in human ulcerative colitis and colitis-associated cancer (CAC); however, its status in murine models of colitis has yet to be explored. Snail and Snail2, zinc-finger transcription factors regulated by inflammatory pathways and able to transcriptionally silence VDR, are upregulated in human Ulcerative Colitis and are associated with localized VDR silencing. To signal, VDR must heterodimerize with retinoid X receptor α (RXRα). If either VDR or RXRα are compromised, vitamin D cannot regulate inflammatory pathways. RXRα is downregulated in human colorectal cancer, yet its expression in human and murine colitis has yet to be investigated. To explore the importance of vitamin D and VDR in murine colitis, we used acute and chronic azoxymethane/dextran sulfate sodium models of murine colitis. VDR was downregulated early in the onset of colitis, whereas RXRα downregulation only occurred as colitis became chronic and developed into CAC. Receptor downregulation was associated with an early increase in the expression of the inflammatory markers, Snail and Snail2. The acute colitis model induced in combination with a vitamin D-deficient diet resulted in increased morbidity, receptor downregulation, inflammatory marker expression, and Snail and Snail2 upregulation. These experiments show the importance of vitamin D and VDR in modulating murine colitis development.

Epigenetic mechanisms underlying diet-sourced compounds in the prevention and treatment of gastrointestinal cancer.

The development of colon cancer, the third most diagnosed cancer and third leading cause of cancer deaths in the United States, can be influenced by genetic predispositions and environmental exposures. As 80% of colon cancer cases are sporadic in nature, much interest lies in determining risk factors that may foster its development, as well as identifying compounds that could inhibit colon cancer development or halt progression. A major risk factor for sporadic colon cancer is a high fat, Western diet which has been linked to a cancer-prone, pro-inflammatory state. Cultures which place an emphasis on fresh fruits and vegetables demonstrate lower colon cancer incidences. Diet not only has the potential to encourage colon cancer development, but recent evidence demonstrates that certain dietary natural products can halt colon cancer development and progression via epigenetic regulation. Epigenetic dysregulation may contribute to inflammation-driven diseases, such as cancer, and can lead to the inappropriate silencing of genes necessary to inhibit cancer development. Natural compounds have shown the ability to reverse epigenetic dysregulation in in vitro and in vivo models. As current allopathic medicines aimed at reversing epigenetic silencing are accompanied with the risk of toxicity and side effects, much interest lies in being able to harness the disease preventing properties in natural products. Here, we discuss the epidemiology of colon cancer, describe the need for natural approaches to inhibit disease development and highlight natural products which have been shown to inhibit gastrointestinal cancer initiation and progression in vitro or in vivo through epigenetic modulation.