Molecular pathways of apoptosis induced by green tea polyphenol epigallocatechin-3-gallate

The chemopreventive effects of green tea are largely attributed to its major polyphenolic constituent (-) epigallocatechin-3-gallate (EGCG). The studies described in this thesis investigate the molecular pathways of apoptosis induced by EGCG. In the first study, we demonstrate that EGCG mediates apoptosis in LNCaP cells (p53+/+) by modulating opposing signals mediated by the transcription factors p53 and NF-kappaB. EGCG upregulated the protein expression and transcriptional activity of p53 via post-translational modification and downregulation of MDM2. The augmented transcriptional activity of p53 enhances the expression of its downstream targets p21WAF1 and Bax. Simultaneously, EGCG inhibits the transcriptional activity of NF-kappaB and the expression of pro-apoptosic protein Bcl-2. We propose that the upregulation of p21WAF1 mediates G1 growth arrest, while the increase in the ratio of Bax: Bcl-2 activates the caspase cascade and triggers apoptosis.; In the second study, we investigated the role of the p53 targets---p21 WAF1 and Bax in EGCG mediated apoptosis using isogenic prostate carcinoma cell lines---PO (p53-/-) and PC3-p53 (p53 +/+) cells. We demonstrate that EGCG upregulates p21WAF1 independent of p53 expression while Bax is upregulated only in PC3-p53 cells. Inhibition of p21WAF1 or Bax expression by RNA interference prevents EGCG mediated apoptosis of both PC3-p53 and PC3 cells, while G1 arrest is abrogated only by inhibiting expression of p21WAF1. Thus, the data indicate that EGCG inhibits cancer cell proliferation regardless of their p53 status, albeit the antiproliferative effects are pronounced in cells with functional p53. Secondly, the apoptotic response induced by EGCG is dependent on the upregulation of p21WAF1 and Bax suggesting that these proteins may act sequentially or in overlapping pathways in a p53-dependent apoptotic response.; In the final study we describe experiments which demonstrate that the inhibition of NF-kappaB during EGCG mediated apoptosis involves the function of caspases. We show that caspase inhibitor Z-VAD-FMK prevents EGCG induced degradation of NF-kappaB/p65 subunit and the subsequent loss of its transcriptional activity indicating a role for caspases in the regulation of NF-kappaB.; In summary, we have demonstrated that EGCG can modulate the pro-apoptotic and pro-survival pathways in favor of apoptosis making it a good candidate for mechanism based studies on cancer chemoprevention. Future studies will help identity additional targets of EGCG.