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 comparison with mechanisms exerted by traditional aryl hydrocarbon receptor (AhR) ligands such as benso(a)pyrene (B(a)P) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). OME did not bind specifically to AhR, and it could not activate the AhR complex in rat cytosol to a xenobiotic-responsive element (XRE)-binding form in vitro. Genistein, a tyrosine kinase inhibitor, and daidzein, an inhibitor of casein kinase II, efficiently 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 constructs transfected into H4IIE cells. The protease inhibitor Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) and lavendustin A also had similar 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 CYP1A1. Staurosporine, 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 in preventing this induction. Gel shift analysis with radiolabeled XRE and specific peptide antibodies toward AhR and aryl hydrocarbon receptor nuclear translocator protein (Arnt) revealed an OME-mediated translocation of the AhR.Arnt complex into the nuclei. Genistein inhibited the specific nuclear XRE binding caused by OME, but it potentiated the formation of the TCDD-induced XRE.AhR complex. Although daidzein was able to effectively inhibit the OME-stimulated CYP1A1 gene transcription, it did not influence the OME-dependent AhR.XRE complex formation. The data are consistent with a mechanism for OME-mediated induction of CYP1A1 that involves activation of the AhR complex via intracellular signal transduction systems and that is distinct from induction mediated by AhR ligands.
Backlund
M
M
Division of Molecular Toxicology, Institute of Environmental Medicine and Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden.
Johansson
I
I
Mkrtchian
S
S
Ingelman-Sundberg
M
M
D000818:Animals; D001564:Benzo(a)pyrene / Q000494:pharmacology; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis / Q000235:genetics*; D004790:Enzyme Induction; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic; D008114:Liver Neoplasms, Experimental / Q000378:metabolism*; D009853:Omeprazole / Q000494:pharmacology; D000072317:Polychlorinated Dibenzodioxins / Q000378:metabolism; D047428:Protein Kinase Inhibitors; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism*; D015398:Signal Transduction; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured
D004791:Enzyme Inhibitors; D000072317:Polychlorinated Dibenzodioxins; D047428:Protein Kinase Inhibitors; D018336:Receptors, Aryl Hydrocarbon; D001564:Benzo(a)pyrene; D019363:Cytochrome P-450 CYP1A1; D009853:Omeprazole