Tea polyphenol (-)-epigallocatechin-3-gallate inhibits nicotine- and estrogen-induced α9-nicotinic acetylcholine receptor upregulation in human breast cancer cells

Scope: The aim of this research was to explore whether the tea-polyphenol (-)-epigallocatechin-3-gallate (EGCG) could be used as a potential agent for blocking smoking (nicotine, Nic)- or hormone (estradiol, E2)-induced breast cancer cell proliferation through inhibition of a common signaling pathway. Methods and results: To explore whether Nic (> 0.1 mu M, 24 h) and E2 (> 1 nM, 24 h) significantly increased alpha 9-nicotinic acetylcholine (alpha 9-nicotinic acetylcholine receptor (nAChR)) mRNA and protein expression levels, real-time PCR and immunoblotting analysis experiments were performed in human breast cancer (MCF-7) cells. Luciferase promoter activity experiment was performed to test the alpha 9-nAChR promoter activity affected by Nic, E2 or EGCG. The results indicate that treatment with EGCG (1 mM) profoundly decreases Nic- and E2-induced MCF-7 proliferation by down regulating alpha 9-nAChR expression. The alpha 9-nAChR promoter activity is significantly induced by 24-h treatment with Nic (10 mu M) or E2 (10 nM) (> 1.8 and similar to 2.3-fold, respectively) in MCF-7 cells. Pretreatment with EGCG eliminated the Nic- and E2-induced alpha 9-nAChR promoter-dependent luciferase activity. We further demonstrate that combined treatment with EGCG profoundly inhibits [3H]-Nic/alpha 9-nAChR binding activity in breast cancer cells. Conclusions: We found that the EGCG could be used as an agent for blocking smoking (Nic)-or hormone (E2)-induced breast cancer cell proliferation by inhibiting of alpha 9-nAChR signaling pathway. This study reveals the novel antitumor mechanisms of EGCG, and these results may have significant applications for chemopreventive purposes in human breast cancer.