GH is a major determinant of cytochrome P4502C12 and insulin-like growth factor-I (IGF-I) mRNA expression in rat liver. In the present study, a possible role for protein kinase C (PKC) in the GH-mediated regulation of these two genes was investigated. Addition of bovine GH (bGH) to cultured primary adult rat hepatocytes lead to the formation of diacylglycerol and subsequent induction of P4502C12 and IGF-I mRNA, indicating a PKC-dependent signal transduction. However, stimulation of PKC by phorbol 12-myristate 13-acetate (PMA) or sn-1,2-dioctanoylglycerol treatment, in dose and time-course experiments in the presence or absence of ionomycin, failed to induce either P4502C12 or IGF-I mRNA. On the other hand, down-regulation of PKC by PMA treatment, i.e. 24 h pretreatment, attenuated the bGH induction of both P4502C12 and IGF-I mRNA. One hundred nanomolar PMA reduced the bGH-stimulated expression of both IGF-I mRNA and P4502C12 mRNA (∼50%). Treatment with the potent kinase inhibitor staurosporine in combination with bGH caused a dose-dependent decrease of the bGH response with different sensitivities toward the inhibitor for the different mRNA species, IGF-I being less sensitive. These data indicate a permissive role for PKC in the GH-mediated induction of P4502C12 and IGF-I mRNA. When activators of protein kinase A, such as forskolin and 8-Br-cAMP were added to the culture medium opposite effects were observed on the mRNA levels of P4502C12 and IGF-I. The basal expression of P4502C12 mRNA decreased, whereas that of IGF-I increased, effects which were observed independent of the presence or absence of bGH. Glucagon, a ligand of an adenylate cyclase coupled receptor was found to exert the same effects, indicating a role for a cAMP-dependent signaling pathway in the overall regulation of P4502C12 and IGF-I mRNA expression in hepatocytes.
ASJC Scopus subject areas
- Molecular Biology