| Tea, one of the most widely consumed beverages worldwide, has been shown to have anti-cancer activity in various cancers including colon cancer. It has been demonstrated that overexpression of cyclooxygenase (COX-2) occurs during colon tumorigenesis and inhibition of COX-2 by non-steroidal anti-inflammatory drugs (NSAIDs) is chemopreventive. To determine whether the anti-cancer effect of green tea is associated with the alteration of COX-2 expression, human colon cancer cell lines HT-29 and HCA-7, which constitutively express COX-2, were treated with (-)-epigallocatechin-3-gallate (EGCG), the most abundant and effective polyphenol of green tea. We found that EGCG treatment significantly inhibited COX-2 mRNA and protein expression and decreased prostaglandin levels. Furthermore, EGCG inhibited cell growth in both cell lines. The signaling pathways related to the inhibitory effect of EGCG on COX-2 expression were determined. We observed that EGCG inhibited the constitutive activation of ERK1/2 and Akt in colon cancer cells. In contrast, EGCG slightly increased the activation of p38. Decreased ERK1/2 activation was associated with block of translocation of c-Raf, an upstream kinase of ERK, from cytosol to the membrane. Modulation of multiple signaling pathways by EGCG led us to investigate the mechanism for inhibition of COX-2 by EGCG. Luciferase reporter assay indicated that EGCG inhibited COX-2 promoter activity and promoted rapid mRNA decay through the COX-2 3' untranslated region (3' UTR). Electrophoretic mobility shift assay (EMSA) and luciferase reporter constructs containing a series of deletion in COX-2 promoter demonstrated that inhibition of COX-2 was mediated via inhibition of nuclear factor kappaB (NF-kappaB) activation. We further demonstrated that EGCG inhibited COX-2 expression induced by interleukin-1beta (IL-1beta) and mutated Ras oncogene in rat intestinal epithelial cells (RIE-1). Moreover, EGCG inhibited cell growth and arrested cell cycle transition at G1 phase via inhibition of cyclin D1 expression in RIE-1 cells induced by Ras oncogene. Anti-proliferative activity of EGCG on Ras induced transformation of RIE-1 cells might be mediated by activation of p38. This study indicates that inhibition of COX-2 is a mechanism for the anti-proliferative effect of green tea and implies that dietary factors have the potential to be anti-cancer agents. |
