p21-mediated cell cycle arrest predominates and protects colon cancer cells from p53-dependent apoptosis by EGCG

LB-145

Green tea and its polyphenolic constituent, EGCG are well known for their chemopreventive properties against a wide variety of cancers. The anti-tumorigenic efficacy of green tea is mainly due to its ability to induce selective apoptosis of cancer cells without significant effect on normal cells. We have previously reported a critical role of p53 and its two downstream targets, p21 and Bax in EGCG-induced cell cycle arrest and apoptosis in prostate cancer cells. To further explore the role of p53 signaling in colon cancer cells, we employed a pair of isogenic cell lines, generated by homologous recombination, which differ only in their p21 status and both retain wild-type p53. Treatment of HCT116 p21^+/+ cells with EGCG leads to protective arrest in G1. In contrast, HCT116 p21^-/- cells traverse through the cell cycle and eventually undergo apoptosis as revealed by TUNEL staining. Treatment with EGCG leads to induction of p53, Bax and PUMA in both p21^+/+ and p21^-/- cells. Ablation of p53 by RNAi in HCT116 p21^-/- cells protects these cells from EGCG-induced apoptosis. In addition, ablation of p53 in p21^+/+ cells substantially inhibits p21 expression thus escaping p21-dependent G1 arrest. Furthermore, analysis of cells lacking PUMA or Bax with or without p21 but with p53 reveals that all the cells expressing p53 and p21 survived after EGCG treatment. More interestingly, cells lacking both PUMA and p21 survived ECGC treatment whereas those lacking p21 and Bax did not. Taken together, our results suggest that EGCG exerts a p53-dependent protective arrest via p21 or apoptosis involving PUMA and not Bax. The p21-dependent growth arrest pre-empts the PUMA-dependent apoptosis. p21, thus, acts as a molecular switch and may be used as an important therapeutic target for colon cancer treatment.