Positive and negative regulation of JNK1 by protein kinaseC and p42MAP kinase in adult rat hepatocytes

The role of protein kinaseC (PKC) and p42MAP kinase signaling in the regulation of proliferation and apoptosis was investigated in freshly isolated and primary cultured rat hepatocytes. Acute treatment of freshly isolated hepatocytes with phenylephrine and EGF caused rapid phasic activations of p42MAP kinase and JNK1. Acute pre-treatment of hepatocytes with the PKC inhibitors sphingosine, chelerythrine and bis-indolylmaleimide abolished the ability of phenylephrine, but not EGF, to activate p42MAP kinase and JNK1. Acute pre-treatments with all of the PKC inhibitors alone increased JNK1 basal activity ∼2-fold. Acute treatments of primary cultures of hepatocytes with an inhibitor of MEK1 activation (PD98059) also caused inhibition of p42MAP kinase and a ∼2-fold activation of JNK1. These data demonstrate that PKC can function as both a proximal activator and a distal inhibitor of signaling through the JNK1/SAP kinase pathway. Treatments (4 h) of primary cultured hepatocytes with sphingosine, chelerythrine, bis-indolylmaleimide and PD98059 did not induce apoptosis as judged by propidium iodide staining. Similar acute treatments of HepG2 cells rapidly induced cell death. These data demonstrate that acute inhibition of either PKC or p42MAP kinase function is sufficient to rapidly induce apoptosis in transformed, but not in non-transformed hepatocytes.