Tyrosine kinase growth factor receptors but not seven-membrane-spanning receptors or phorbol esters activate mitogen-activated protein kinase in rat hepatocytes.

The response of rat hepatocytes to hormones and growth factors has been extensively studied with respect to phospholipase regulation and calcium mobilization. However, the mitogen-activated protein (MAP) kinase cascade which integrates signals from a wide variety of extracellular stimuli has not been examined in these cells. Thus, in the present study the pathways leading to activation of MAP kinase in primary cultures of adult rat hepatocytes were investigated. Growth factors acting through tyrosine kinase receptors (epidermal growth factor and hepatocyte growth factor) increased Raf and MAP kinase activity through a protein kinase C and calcium-independent pathway. Agonists acting through seven-membrane-spanning receptors (arginine vasopressin and angiotensin II) increased intracellular calcium concentration but did not stimulate Raf or MAP kinase activity. Arginine vasopressin, however, stimulated MAP kinase activity in rat 1a fibroblasts transfected with the hepatic V1a receptor and in rat aortic vascular smooth muscle cells. Phorbol 12-myristate 13-acetate (PMA) was also unable to stimulate Raf and MAP kinase in hepatocytes in spite of a marked activation of protein kinase C. We conclude that only signals arising from tyrosine kinase receptors are able to activate MAP kinase in hepatocytes. Neither agonists acting through seven-membrane-spanning receptors nor phorbol esters stimulate MAP kinase in hepatocytes. The results suggest that specific cellular components that link seven-membrane-spanning receptors with MAP kinase activation in tissues such as vascular smooth muscle are absent in rat hepatocytes.