ACTIVATION OF P42 P44 MITOGEN-ACTIVATED PROTEIN-KINASE BY ANGIOTENSIN-II, VASOPRESSIN, NOREPINEPHRINE, AND PROSTAGLANDIN-F2-ALPHA IN HEPATOCYTES IS SUSTAINED, AND LIKE THE EFFECT OF EPIDERMAL GROWTH-FACTOR, MEDIATED THROUGH PERTUSSIS-TOXIN-SENSITIVE MECHANISMS/

Several agents that act through G-protein-coupled receptors and also s timulate phosphoinositide-specific phospholipase C (PI-PLC), including angiotensin II, vasopressin, norepinephrine, and prostaglandin (PG) F -2 alpha, activated the ERK1 (p44(mapk)) and ERK2 (p42(mapk)) members of the mitogen-activated protein (MAP) kinase family in primary cultur es of rat hepatocytes, measured as phosphorylation of myelin basic pro tein (MBP) by a partially purified enzyme, immunoblotting, and in-gel assays. All these agonists induced a peak activation (two to threefold increase in MBP-phosphorylation) at 3-5 min, followed by a brief decr ease, and then a sustained elevation or a second increase of the MAP k inase activity that lasted for several hours. Although all the above a gents also stimulated PI-PLC, implicating a G(q)-dependent pathway, th e elevations of the concentration of inositol (1,4,5)-trisphosphate di d not correlate well with the MAP kinase activity. Furthermore, pretre atment of the cells with pertussis toxin markedly reduced the MAP kina se activation by angiotensin II, vasopressin, norepinephrine, or PCF2 alpha. In addition, hepatocytes pretreated with pertussis toxin showed a diminished MAP kinase response to epidermal growth factor (EGF). Th e results indicate that agonists acting via G-protein-coupled receptor s have the ability to induce sustained activation of MAP kinase in hep atocytes, and suggest that G(i)-dependent mechanisms are required for full activation of the MAP kinase signal transduction pathway by G-pro tein-coupled receptors as well as the EGF receptor. (C) 1998 Wiley-Lis s, Inc.