SIGNAL TRANSDUCTION-MEDIATED ACTIVATION OF THE ARYL-HYDROCARBON RECEPTOR IN RAT HEPATOMA H4IIE CELLS

We have investigated mechanisms of omeprazole (OME)-mediated induction of CYP1A1 and CYP3A, using the rat hepatoma H4IIE cell line, in compa rison with mechanisms exerted by traditional aryl hydrocarbon receptor (AhR) ligands such as benso(a)pyrene (B(a)P) and 2,3,7,8-tetrachlorod ibenzo-p-dioxin (TCDD). OME did not bind specifically to AhR, and it c ould not activate the AhR complex in rat cytosol to a xenobiotic-respo nsive element (XRE)-binding form in vitro. Genistein, a tyrosine kinas e inhibitor, and daidzein, an inhibitor of casein kinase II, efficient ly inhibited OME-mediated but not B(a)P-or TCDD-mediated induction of CYP1A1, as monitored at the transcriptional, mRNA, and protein levels as well as by analysis of activation of XRE-luciferase reporter constr ucts transfected into H4IIE cells. The protease inhibitor N-alpha-p-to syl-L-lysine chloromethyl ketone (TLCK) and lavendustin A also had sim ilar OME-specific effects. In addition, insulin pretreatment caused an almost complete inhibition of OME-dependent CYP1A1 induction but only partially affected TCDD and B(a)P-mediated induction of CYPIA1. Staur osporine, an inhibitor of protein kinase C, impaired the induction by both B(a)P and OME. OME caused an approximately 2-fold increase in the level of CYP3A expression, but all inhibitors used were ineffective i n preventing this induction. Gel shift analysis with radiolabeled XRE and specific peptide antibodies toward AhR and aryl hydrocarbon recept or nuclear translocator protein (Arnt) revealed an OME-mediated transl ocation of the AhR.Arnt complex into the nuclei. Genistein inhibited t he specific nuclear XRE binding caused by OME, but it potentiated the formation of the TCDD-induced XRE.AhR complex. Although daidzein was a ble to effectively inhibit the OME-stimulated CYP1A1 gene transcriptio n, it did not influence the OME-dependent AhR.XRE complex formation, T he data are consistent with a mechanism for OME-mediated induction of CYP1A1 that involves activation of the AhR complex via intracellular s ignal transduction systems and that is distinct from induction mediate d by AhR ligands.