Molecular Targets of Green Tea Catechin, EGCG, on Human Colorectal Carcinogenesis

This thesis explores the effect of major green tea catechin, EGCG, on colorectal carcinogenesis particularly on target genes, bFGF and NUDT6. Cancer is a second leading cause and colorectal cancer is a third leading cause of deaths in the Western World. Recently there is a lot of persuasive epidemiological and experimental evidence that phytochemical-enriched diet may be involved in the prevention of colorectal cancer. EGCG exhibits beneficial effects like anti-tumorigenic, anti-oxidant, anti-angiogenic, and pro-apoptotic activity.

In studies presented here, we investigated the molecular mechanisms by which EGCG affects bFGF degradation (Chapter 3): EGCG increased the ubiquitination as well as trypsin like activity of 26S proteasome causing faster bFGF degradation. We also demonstrated that EGCG could decrease the tumor number and volume in mouse models of colorectal cancer.

Our microarray data from catechin-treated colorectal cancer cells revealed down regulation of a gene called NUDT6, an anti-sense of bFGF, which we have shown to regulate the bFGF function. NUDT6 is a mitochondrial protein whose biological function is unknown. EGCG affected the mRNA stability of NUDT6 by decreasing the luciferase activity of its 3‘UTR (Chapter 4). NUDT6 stable cells showed increase in cell proliferation and soft agar colony formation. The electrical impedance bio-analytical assay showed increased attachment of NUDT6 cells to the surface media and to each other. NUDT6 cells showed increased invasion rate and decreased Caspase 3/7 activity.

Injection of CONTROL and NUDT6 cells into nude mice complemented the in vitro data showing increase in tumor volume, mitotic index and decrease in caspase activity and apoptotic index. This was possibly due to binding of p53 to NUDT6 which led to degradation of p53. NUDT6 was highly expressed in human colon tumor tissue sample compared to normal and NUDT6 could be used as a marker in colon tumorigenesis.

The findings presented in this thesis clearly show that EGCG is an effective chemopreventive dietary agent, decreasing the expression of pro-angiogenic and cell proliferation genes, bFGF and NUDT6 in colorectal cancer cells. Further characterization of NUDT6 as a potential biomarker in colorectal carcinogenesis may provide a better prediction of disease progression in the future.