OKADAIC ACID POTENTIATES 3-METHYLCHOLANTHRENE-INDUCED CYP2A8 GENE-EXPRESSION IN PRIMARY CULTURES OF SYRIAN-HAMSTER HEPATOCYTES - POSSIBLE INVOLVEMENT OF ACTIVATOR PROTEIN-1

In Syrian hamster liver, treatment with 3-methylcholanthrene (3-MC) ma rkedly induces an isozyme of cytochrome P450 (CYP), CYP2A8. To elucida te the mechanism of this induction, we studied the effect of okadaic a cid (OA), an inhibitor of serine threonine protein phosphatases ? and 2A, on 3-MC-induced CYP2A8 expression in primary cultures of Syrian ha mster hepatocytes, The addition of OA to the cultured hepatocytes at a concentration of 1 nM potentiated 3-MC- (0.1 and I mu M) induced expr ession of mRNA and protein of CYP2A8 and its associated coumarin 7-hyd roxylase activity. In addition, OA not only induced c-fos and jun-D mR NA, components of transcription factor activator protein-1 (AP-I), wit h an increase in AP-1 binding activity in the nucleus, but also activa ted AP-I-dependent gene transcription in the hepatocytes. The dose-dep endent effect of OA on 3-MC-induced CYP2A8 expression corresponded to that of OA on c-fos and jun-D mRNA induction and on the activation of AP-1-dependent gene transcription. The expression oi c-fos and jun-D m RNA induced by OA preceded the expression of CYP2A8 mRNA potentiated b y cotreatment with 3-MC and OA. Treatment with anisomycin and cyclohex imide also potentiated 0.1 mu M 3-MC-induced coumarin 7-hydroxylase ac tivity, induced c-fos and jun-D mRNA expression, and activated AP I-de pendent gene transcription in the hepatocytes. Furthermore, 3-MC-induc ed CYP2A8 expression was potentiated in the hepatocytes transfected wi th c-Jun expression plasmid. These results suggest that AP-I, inducibl e by serine threonine protein kinase, may be one of the components of the signal transduction system from 3-MC to CYP2A8 gene expression.