Down-Regulation of p23 in Normal Lung Epithelial Cells Reduces Toxicities From Exposure to Benzo[a]pyrene and Cigarette Smoke Condensate via an Aryl Hydrocarbon Receptor-Dependent Mechanism

The aryl hydrocarbon receptor (AHR) is a ligand-activated signaling molecule which controls tumor growth and metastasis, T cell differentiation, and liver development. Expression levels of this receptor protein is sensitive to the cellular p23 protein levels in immortalized cancer cell lines. As little as 30% reduction of the p23 cellular content can suppress the AHR function. Here we reported that down-regulation of the p23 protein content in normal, untransformed human bronchial/tracheal epithelial cells to 48% of its content also suppresses the AHR protein levels to 54% of its content. This p23-mediated suppression of AHR is responsible for the suppression of (1) the ligand-dependent induction of the cyp1a1 gene transcription; (2) the benzo[a]pyrene- or cigarette smoke condensate-induced CYP1A1 enzyme activity, and (3) the benzo[a]pyrene and cigarette smoke condensate-mediated production of reactive oxygen species. Reduction of the p23 content does not alter expression of oxidative stress genes and production of PGE(2). Down regulation of p23 suppresses the AHR protein levels in two other untransformed cell types, namely human breast MCF-10A and mouse immune regulatory Tr1 cells. Collectively, down-regulation of p23 suppresses the AHR protein levels in normal and untransformed cells and can in principle protect our lung epithelial cells from AHR-dependent oxidative damage caused by exposure to agents from environment and cigarette smoking.