Regulation of aryl hydrocarbon receptor signal transduction by protein tyrosine kinases

Abstract

The involvement of protein tyrosine kinases (PTKs) in aryl hydrocarbon receptor (AhR)-mediated signalling by omeprazole and 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) was investigated in hepatoma cells. Both omeprazole- and TCDD-dependent AhR signalling was attenuated by inhibition of c- src kinase, either by using pyrazolopyrimidine 4-amino-5-(4-methylphenyl)-7-( t-butyl)pyrazolo[3,4 ]pyrimidine (PP1) and 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4- d]pyrimidine (PP2) inhibitors or by expression of dominant-negative c- src. These results indicate that the overall AhR function is modulated by c- src kinase activity. In contrast, a selective inhibition of omeprazole-mediated AhR signalling was revealed by tyrosine kinase inhibitors, tyrphostins AG17 and AG879. Furthermore, omeprazole-dependent AhR activation was abolished by mutation of Tyr 320 to Phe, suggesting that this residue is a putative phosphorylation site. TCDD-dependent AhR signalling was neither affected by tyrphostins nor by this mutation. Our results are consistent with activation of the AhR by omeprazole in a ligand-independent manner, via a signal transduction pathway that involves protein tyrosine kinases, and are different from the mechanism exerted by high-affinity ligands.