GROWTH-HORMONE SIGNALING LEADING TO CYP2C12 GENE-EXPRESSION IN RAT HEPATOCYTES INVOLVES PHOSPHOLIPASE A(2)

The expression of CYP2C12 is liver-specific and regulated at the trans criptional level by growth hormone (GH). In attempts to elucidate the nature of signaling molecules mediating the GH regulation of this gene in rat hepatocytes, a role for phospholipase A(2) (PLA(2)) as a trans ducer of GH-induced levels of P4502C12 mRNA was investigated. GH was s hown to induce tyrosyl-phosphorylation of p42 and p44 microtubule-asso ciated protein (MAP) kinases and to reduce the electrophoretic mobilit y of a 100-kDa protein, immunologically related to cPLA(2). These even ts were observed in parallel with GH-stimulated release of [H-3]arachi donic acid ([H-3]AA) from cellular phospholipids of rat hepatocytes la beled with [H-3]AA. These rapid effects of GH action, as well as the G H-induced expression of CYP2C12, were inhibited in cells treated with the tyrosine kinase inhibitor herbimycin A. Similarly, when the GH-ind uced liberation of [H-3]AA was blocked by the PLA(2) inhibitor mepacri ne or the Ca2+ channel blocker verapamil, GH-induced accumulation of P 4502C12 mRNA was absent. These results suggest a correlation between P LA(2) activity and GH regulation of the CYP2C12 gene. The inhibitory e ffect of mepacrine on GH induction of P4502C12 mRNA was reversed by AA addition, further supporting a role for eicosanoids in the regulation of CYP2C12. Finally, inhibitors of P45O-mediated AA metabolism, SKF-5 25A and ketoconazole as well as eicosatetraynoic acid, blocked the GH- mediated induction of P4502C12 mRNA, whereas more specific inhibitors of cyclooxygenase or lipoxygenase metabolism did not. Based on these r esults, we suggest that GH signaling in rat hepatocytes, leading to in creased expression of CYP2C12, involves PLA(2) activation and subseque nt P450-catalyzed formation of an active AA metabolite.