Activation of mitogen-activated protein kinase in freshly isolated rat hepatocytes by both a calcium- and a protein kinase C-dependent pathway.
In the present study, we investigated the role of calcium and protein kinase C (PKC) in the activation of mitogen-activated protein kinase (MAPK) in isolated rat hepatocytes. We found that the glycogenolytic hormone norepinephrine (NE), acting through the alpha1-adrenergic receptor and the G protein Gq, was able to induce a dose- and time-dependent activation of MAPK in hepatocytes. Vasopressin, which acts through a different receptor but also through stimulation of the Gq-dependent pathway, also caused a twofold activation of MAPK. Activation of MAPK by both agonists required the presence of free extracellular calcium and was blocked by the specific PKC inhibitor, Ro 31-8220. MAPK activation was also induced by phorbol myristate acetate (PMA), confirming that a PKC-dependent pathway exists for MAPK activation in liver. Furthermore, calcium-mobilizing agents such as thapsigargin and ionomycin were able to induce an activation of MAPK by a PKC-independent pathway that was totally abolished by preincubation of cells with EGTA. A second pathway for MAPK activation that relies solely on calcium may therefore exist. Ro 31-8220 did not affect phosphorylase activation by NE, vasopressin, thapsigargin, and ionomycin, indicating that PKC inhibition did not interfere with the signaling pathway leading to inositol-1,4,5-triphosphate (IP3)-induced calcium mobilization or with changes in calcium fluxes. The role of MAPK activation by NE and vasopressin in the regulation of hepatic carbohydrate metabolism is discussed.
Romanelli
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A
Department of Nutrition, Universite de Montreal, Quebec, Canada.
van de Werve
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G
D000818:Animals; D002118:Calcium / Q000502:physiology*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D002469:Cell Separation; D004789:Enzyme Activation; D008099:Liver / Q000166:cytology / Q000201:enzymology*; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010762:Phosphorylase a / Q000378:metabolism; D011493:Protein Kinase C / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D014667:Vasopressins / Q000494:pharmacology
D014667:Vasopressins; D010762:Phosphorylase a; D011493:Protein Kinase C; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002118:Calcium; D009638:Norepinephrine