Expression of the Aryl Hydrocarbon Receptor Is Regulated by Serum and Mitogenic Growth Factors in Murine 3T3 Fibroblasts

Abstract

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates biological responses to planar aromatic hydrocarbons such as benzo[alpha]pyrene. However, no endogenous physiological ligand for the AhR has been identified. Since the AhR regulates bioactivity to common environmental pollutants, and since it is predicted to play an important physiological function, we have investigated the expression of the AhR during the cell cycle of murine 3T3 fibroblasts. We show here that stimulation of growth-arrested 3T3 cells with serum results in increased expression of AhR protein. Serum-induced expression of AhR in synchronized, serum-stimulated cells occurs at the onset of DNA synthesis (S phase) and is maximal at time points corresponding to late S phase. Transient transfections with an AhR-promoter-luciferase construct demonstrate that reporter gene transcription from the AhR promoter is regulated in a serum-dependent manner. Serum-dependent induction of AhR expression is prevented by an inhibitor of tyrosine kinase activity. Ligand-activated growth factor receptors (platelet-derived growth factor receptor basic fibroblast growth factor receptor) as well as an ectopically expressed tyrosine kinase (the v-Src oncoprotein) induce AhR expression in the absence of serum. Therefore, tyrosine kinase signaling is both necessary and sufficient for induction of AhR expression. Studies with the G1 blocker sodium butyrate show that the signal transduction pathways mediating serum-stimulated progression through the cell cycle are distinct from those that induce AhR expression. These data suggest that transcriptional regulation of the AhR is important in determining cellular sensitivity to the actions of AhR ligand(s) and that the AhR may play a role during the cellular proliferative response.