Induction of cyclooxygenase-2 by benzo[a]pyrene diol epoxide through inhibition of retinoic acid receptor-β₂ expression

Benzo[a]pyrene diol epoxide (BPDE, a carcinogen present in tobacco smoke and environmental pollution) has been shown to suppress retinoic acid receptor-beta2 (RAR-β₂) and induce cyclooxygenase-2 (COX-2) expression. Restoration of RAR-β₂ inhibited growth and colony formation of esophageal cancer cells, which was correlated with COX-2 suppression. In this study, we investigated the molecular mechanisms for RAR-β₂-mediated suppression of COX-2 expression using BPDE as a tool. We found that BPDE-induced COX-2 expression was through inhibition of RAR-β₂ and consequently, induction of epidermal growth factor receptor (EGFR), extracellular signal-regulated protein kinases 1/2 (Erk1/2) phosphorylation, and c-Jun expression. Esophageal cancer cells that do not express RAR-β₂ did not respond to BPDE for induction of COX-2. BPDE was also unable to induce COX-2 expression after RAR-β₂ expression was manipulated in these esophageal cancer cells. Furthermore, BPDE induced time-dependent methylation of RAR-β₂ gene promoter in esophageal cancer cells. Transfection of RAR-β₂ expression vector into esophageal cancer cells suppressed expression of EGFR, Erk1/2 phosphorylation, c-Jun, and COX-2. In addition, co-treatment of RAR-β₂-positive cells with BPDE and the MEK1/2 inhibitor U0126 caused little change in c-Jun and COX-2 expression. This study demonstrated that BPDE-suppressed expression of RAR-β₂ results in COX-2 induction and restoration of RAR-β₂ expression reduces COX-2 protein in esophageal cancer cells, thereby further supporting our previous finding that RAR-β₂ plays an important role in suppressing esophageal carcinogenesis.