Green Tea Polyphenol Epigallocatechin-3-Gallate Differentially Modulates Nuclear Factor κB in Cancer Cells versus Normal Cells

Abstract

Green tea has shown remarkable anti-inflammatory and cancer chemopreventive effects in many animal tumor bioassays, cell culture systems, and epidemiological studies. Many of these biological effects of green tea are mediated by epigallocatechin 3-gallate (EGCG), the major polyphenol present therein. We have earlier shown that EGCG treatment results in apoptosis of several cancer cells, but not of normal cells (J. Natl. Cancer Inst. 89, 1881–1886 (1997)). The mechanism of this differential response of EGCG is not known. In this study, we investigated the involvement of NF-κB during these differential responses of EGCG. EGCG treatment resulted in a dose-dependent (i) inhibition of cell growth, (ii) G0/G1-phase arrest of the cell cycle, and (iii) induction of apoptosis in human epidermoid carcinoma (A431) cells, but not in normal human epidermal keratinocytes (NHEK). Electromobility shift assay revealed that EGCG (10–80 μM) treatment results in lowering of NF-κB levels in both the cytoplasm and nucleus in a dose-dependent manner in both A431 cells and NHEK, albeit at different concentrations. EGCG treatment was found to result in a dose-based differential inhibition of TNF-α- and LPS-mediated activation of NF-κB in these cells. The inhibition of NF-κB constitutive expression and activation in NHEK was observed only at high concentrations. The immunoblot analysis also demonstrated a similar pattern of inhibition of the constitutive expression as well as activation of NF-κB/p65 nuclear protein. This inhibition of TNF-α-caused NF-κB activation was mediated via the phosphorylative degradation of its inhibitory protein IκBα. Taken together, EGCG was found to impart differential dose-based NF-κB inhibitory response in cancer cells vs normal cells; i.e., EGCG-mediated inhibition of NF-κB constitutive expression and activation was found to occur at much higher dose of EGCG in NHEK as compared to A431 cells. This study suggests that EGCG-caused cell cycle deregulation and apoptosis of cancer cells may be mediated through NF-κB inhibition.