Green Tea Polyphenol Epigallocatechin 3-Gallate , Contributes to the Degradation of DNMT 3 A and HDAC 3 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 methylationsensitive 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. Green tea is the world’s most popular beverage and substantial evidence supports its success in the prevention of carcinogenesis in animal models (11). Green tea has been found to re-activate genes in carcinogen-induced rodent models of colon cancer, which ultimately led to the suppression of intestinal tumorigenesis (25). The most active compound in green tea, epigallocatechin gallate (EGCG) induces cell-cycle arrest and apoptosis of cancer cells (1, 23). In colon cancer models, EGCG has been shown to inhibit epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), and cyclooxygenase-2 (COX2), as well as human epidermal growth factor-3 (HER3) (20, 21). This includes EGCG in a host of compounds that may be useful in preventing tumor metastasis through inhibiting angiogenesis, as well as supports its inclusion as an anti-inflammatory agent. EGCG also inhibits DNA methyltransferases (DNMTs), as well as re-activating key regulatory genes silenced in colon cancer (7). EGCG targets multiple signaling pathways, making it a good subject for inclusion as a chemopreventive or therapeutic agent. The inhibition of DNMTs by EGCG provides a potential mechanism as to how tumorigenesis is halted by green tea but specific details on the role of EGCG as it pertains to DNMTs or other epigenetic players is limited. It is possible that EGCG may work not only through inhibition of enzymatic activity but also through classic pathways involving protein degradation. More recently, with categorization of the different molecular genetic profiles of colon carcinomas, it has become clear that variations in response of human cancer to different therapies may depend on genetic and epigenetic profiles. One subset of genetically distinct colon carcinomas are considered to be microsatellite-instable (MSI) carcinomas in which the mismatch repair gene, human MutL homolog 1 (hMLH1), is silenced due to aberrant methylation of its promoter (17). MSI is associated with colorectal cancer that has aberrant methylation in the CpG islands of genes (24). This is referred to as the CpG island methylator phenotype, or CIMP. CIMPpositive tumors in colorectal cancer exhibit methylation of tumor-suppressor genes and silencing of other regulatory genes (24). The belief is that if these regulatory genes can be de-silenced, cancer progression can be halted or reversed. DNA hypermethylation and histone de-acetylation are key epigenetic mechanisms for the silencing of many genes including tumor-suppressor genes (18), genes responsible for 5325 Correspondence to: Vondina R. Moseley, 86 Jonathan Lucas St. HCC 710, Charleston SC, U.S.A. Tel: +1 8437926027, Fax: +1 8437923200, email: vondinab@gmail.com