Selective activation of phospholipase C gamma1 and distinct protein kinase C subspecies in intracellular signaling by hepatocyte growth factor/scatter factor in primary cultured rat neocortical cells

Abstract

Hepatocyte growth factor/scatter factor (HGF) was recently reported to function as a neurotrophic factor in the CNS. To investigate the intracellular signal pathways after activation of the HGF receptor c-Met in primary cultured rat neocortical cells, in vitro kinase assays were performed. HGF stimulation enhances the phosphorylation of endogenous 80- and 45-kDa substrates. Studies with protein kinase inhibitors and phorbol 12-myristate 13-acetate showed that protein kinase C (PKC) is activated intracellularly. The 80-kDa protein was identified to be the major PKC substrate MARCKS. Although four PKC subspecies, PKC alpha, PKC epsilon, PKC gamma, and PKC lambda, were expressed in the cells, only PKC alpha, PKC epsilon, and PKC gamma were selectively translocated in the plasma membrane after HGF stimulation. As expected from these three PKC subspecies, phosphorylation of phospholipase C gamma1 (PLC gamma1) but not phosphatidylinositol 3-kinase was enhanced, although the stimulation of brain-derived neurotrophic factor induced phosphorylation of phosphatidylinositol 3-kinase. In contrast to the neocortical cells, HGF did not enhance phosphorylation of PLC gamma1 in primary astrocytes. We also found that activated PKC(s) served as a major mitogen-activated protein kinase activator in this pathway. These findings suggest that HGF exerts neurotrophic effects through selective phosphorylation of PLC gamma1 and activation of distinct PKC subspecies in neocortical cells, most likely neurons.