EPIDERMAL GROWTH-FACTOR INDUCES PC12 CELL-DIFFERENTIATION IN THE PRESENCE OF THE PROTEIN-KINASE INHIBITOR K-252A

The protein kinase inhibitors K-252a and K-252b have been shown earlie r to block the actions of nerve growth factor and other neurotrophins and, at lower concentrations, to selectively potentiate neurotrophin-3 actions. In the present study we show that K-252a, but not K-252b, en hances epidermal growth factor (EGF)- and basic fibroblast growth fact or (bFGF)-induced neurite outgrowth of PC12 cells at higher concentrat ions than required for neurotrophin inhibition. In parallel, tyrosine phosphorylation of extracellular signal-regulated kinases (Erks) elici ted by EGF or bFGF was also increased in the presence of K-252a, and t his signal was prolonged for 6 h. EGF- and bFGF-induced phosphorylatio n of phospholipase C-gamma 1 were not changed. The effect of K-252a on Erks was resistant to chronic treatment with phorbol ester, indicatin g that protein kinase C is not involved in this potentiation. In parti al contrast to the actions of K-252a, the neurotrophin-3- potentiating effect of K-252b was accompanied by an increase in tyrosine phosphory lation of the Erks and of phospholipase C-gamma 1. Finally, although K 252a alone did not induce neurite outgrowth or tyrosine phosphorylatio n of Erks or phospholipase C-gamma 1, this compound alone stimulated p hosphatidylinositol hydrolysis. Our findings identify activities of K- 252a besides the direct interaction with neurotrophin receptors and su ggest that a K-252a-sensitive protein kinase or phosphatase might be i nvolved in signal transduction for EGF and bFGF. Our results are furth er compatible with the hypothesis that sustained activation of Erks ma y be important in PC12 differentiation.