INHIBITORY ACTIONS OF CYCLIC ADENOSINE-MONOPHOSPHATE AND PERTUSSIS TOXIN DEFINE 2 DISTINCT EPIDERMAL GROWTH FACTOR-REGULATED PATHWAYS LEADING TO ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE IN RAT HEPATOCYTES

Increased intracellular cyclic adenosine monophosphate (cAMP) levels h ave been shown in some reports to inhibit and in other studies to stim ulate growth factor-mediated activation of the mitogen-activated prote in kinase (MAP kinase) pathway, depending on the cell type examined, T he relationship between cAMP and MAP kinase in hepatocytes has not bee n examined, In the current study, stimulation of primary cultures of r at hepatocytes with hepatocyte growth factor (EGF) or epidermal growth factor (EGF) increased Ras, Raf, and MAP kinase activity, Incubation of hepatocytes with cAMP-increasing agents blocked activation of Raf b y both HGF and EGF, whereas activation of Pas was unaffected, MAP kina se activation by HGF was completely inhibited, whereas EGF-stimulated MAP kinase activity was only slightly reduced, Incubation of hepatocyt es with pertussis toxin slightly blunted MAP kinase activation by EGF but not HGF. Increasing cAMP in hepatocytes preincubated with pertussi s toxin completely inhibited the activation of MAP kinase by EGF. In c onclusion, HGF activates MAP kinase in hepatocytes exclusively through an Raf-dependent pathway and this activation may be completely blocke d by increasing cAMP, In contrast, EGF activates MAP kinase in hepatoc ytes through both Raf-dependent and Raf-independent pathways: the latt er pathway probably involves a pertussis toxin-sensitive G protein.