MODULATION OF KIT STEM-CELL FACTOR RECEPTOR-INDUCED SIGNALING BY PROTEIN-KINASE-C

The Kit/stem cell factor receptor (Kit/SCF-R) is a transmembrane tyros ine kinase receptor of importance for the normal development of hemopo ietic cells, melanoblasts, and germ cells. We recently reported that p rotein kinase C (PRC) is involved in a negative feedback loop regulati ng the Kit/SCF-R by direct phosphorylation on serine residues in the r eceptor. Inhibition of Pf(C led to increased SCF-induced tyrosine kina se activity and mitogenicity, but PKC was necessary for SCF-induced mo tility. In this report we have further examined the modulatory role of PKC on SCF-induced signaling. The ligand-activated Kit/SCF-R associat ed weakly with GRB2 and induced only little tyrosine phosphorylation o f phospholipase C-gamma in porcine aortic endothelial cells transfecte d with Kit/SCF-R. In contrast, the SCF-stimulated Kit/SCF-R associated efficiently with, and induced tyrosine phosphorylation of, the p85 al pha regulatory subunit of phosphatidyl inositide-3'-kinase (PI-3'-kina se). Both receptor association and tyrosine phosphorylation of p85 alp ha were increased after inhibition of PKC, while its serine phosphoryl ation was decreased. Concomitantly, the specific activity of receptor- associated PI-3'-kinase activity was increased, Inhibition of PI-3'-ki nase with wortmannin inhibited SCF-induced mitogenicity. SCF-induced p hosphorylation of Raf-1 and activation of ERK2 still occurred after PK C inhibition but was not increased. In conclusion, SCF-induced PI-3'-k inase activation paralleled the increased SCF-induced mitogenicity aft er inhibition of PKC.