Constitutive overexpression of Nrf2-dependent heme oxygenase-1 in A549 cells contributes to resistance to apoptosis induced by epigallocatechin 3-gallate

Epigallocatechin 3-gallate (EGCG), the major polyphenol found in green tea, exerts antiproliferative and proapoptotic effects in many cancer cells. However, we found that among many cancer cells human lung adenocarcinoma A549 cells are markedly resistant to apoptosis induction by EGCG ( even at 100 mu M for 72 h). Heme oxygenase-1 (HO-1) induced by stress stimuli represents a prime cellular defense mechanism, but it may be associated with enhanced cell proliferation and chemoresistance in some cancer cells. Because we found that A549 cells constitutively overexpress HO-1 and its associated transcription factor Nrf2, we tested an hypothesis that EGCG resistance in these cells may be linked with Nrf2-mediated HO-1 overexpression. HO-1 inhibition with tin-protoporphyrin IX and silencing with RNA interference rendered cells more sensitive to apoptosis induction by EGCG and classical prooxidants. Interestingly, EGCG at high concentration (> 200 mu M) induced apoptosis by suppressing expression of HO-1 protein and mRNA, and this effect correlated with a decrease in both Nrf2-ARE binding and HO-1-ARE- luciferase activity, suggesting Nrf2-driven transcriptional activation of ho-1. Because we observed notably high levels of phosphorylated protein kinase C alpha and its suppression by EGCG and deferoxamine (an iron chelator), a possible mechanism involving phosphorylated protein kinase C alpha and iron in Nrf2-HO-1 activation was further investigated. Collectively, our findings suggest that Nrf2-mediated HO- 1 overexpression confers resistance to apoptosis induction by EGCG; therefore, its inactivation may be a target for overcoming the resistance to chemoprevention and chemotherapy.