(-)-Epigallocatechin-3-gallate induces Du145 prostate cancer cell death via downregulation of inhibitor of DNA binding 2, a dominant negative helix-loop-helix protein.

Prostate cancer is one of the leading causes of cancer death for men in developed countries; almost one in every six men develop this disease. It mostly occurs at an advanced age of over 65 years and is more prevalent as men get older. A large number of prostate cancer cases are diagnosed each year through annual serum prostate-specific antigen (PSA) screening. Although the disease shows very slow progression,(1) almost 30 000 men die of the disease, annually. In the early stage, growth of prostate cancer cells depends on androgen. Castration or other androgen-ablation treatment has been effective in controlling the progression of tumors. However, androgen receptor mutation and amplification may cause prostate cancer resistance to hormonal ablation. The high prevalence of prostate cancer in Western countries has been attributed to a variety of factors, including age, race, genetics, and environmental factors or diets. The difference in dietary habits has been suggested to explain the relatively low incidence of prostate cancer in Far East Asia, such as in Japan and China, versus those in America, as tea is the most popular beverage among Chinese and Japanese people. It turns tea into the focus of epidemiological studies for cancer chemoprevention. (−)-Epigallocatechin-3-gallate (EGCG), the main polyphenol component of tea, has been considered a cancer-preventive agent for its apoptotic and growth-inhibitory effects on cancer cells in vitro and in animal models over the past several years.(2) EGCG plays a critical role in cell growth arrest and enhancement of apoptosis in tumors of a variety of organs and tissues, such as nasopharyngeal carcinoma,(3) cervical cancer cell lines,(4) bladder tumor cells,(5) prostate cancer cell lines,(6) liver cancer cell line,(7) gastrointestinal tract,(8) and lung and mammary cancer cell lines.(9) Some of the anticancer effects of EGCG may result from its protection of DNA from methylation and activation of silenced genes.(10) EGCG also increases the expression of p53 (wild type), p21/wild type P53-activated fragment 1 (WAF1) protein, and Bcl2-associated X protein (Bax)/B-cell lymphoma 2 (Bcl-2) protein in LNCaP prostate cancer cells, leading to cell cycle arrest and apoptotic cell death.(11,12) EGCG activates ERK1/ 2 via the phosphoinositide-3-kinase (PI3-K)-dependent signaling pathway to inhibit the cell proliferation of PC3 prostate cancer cells, but not in RWPE-1, an immortalized prostate cell line.(13) Du145 is an androgen-independent prostate cancer cell line that is insensitive to hormone ablation and has distinctive metastatic behavior from that of PC3 and LNCaP cells. EGCG was shown to suppresses Du145 cell growth and induce apoptosis through an increase in reactive oxygen species and induction of mitochondrial depolarization, without changing the expression of BCL-2, BCL-XL, and Bcl 2 antagonist of cell death (BAD).(14) Despite our knowledge about the anticancer effects of EGCG, the molecular mechanism of EGCG on the androgen receptornull prostate cancer cell line Du145 is still fragmented and not well characterized. In the present study, we investigated the EGCG-regulated gene expression effect on Du145 using Affymetrix (Santa Clara, CA, USA) gene chip U133 2.0. By comparing the gene expression levels of Du145 cells treated with and without EGCG, we found that inhibitor of DNA binding 2 (ID2) was dramatically downregulated by EGCG. Restoration of ID2 expression showed protection against cell death induced by EGCG. Downregulation of ID2 without treatment of EGCG mimicked the EGCG effect on Du145 cells.