초록
Regulation of nerve growth factor (NGF)-induced neuronal differentiation by GTPase activating protein(GAP) and its mechanism were investigated in rat pheochromocytoma cell line, PCl2. Overexpression of GAP caused the delay in the onset of neurite outgrowth of PCl2 eel Is in response to NGF. GAP has been known to inhibit p21$\^$ras/, the activated form of which induces neuronal differentiation. Therefore, the activity of p21$\^$ras/ was compared in control cells and cells overexpressing GAP indirectly by measuring the activities of B-Raf and MAP kinase that are known to be positively regulated by p21$\^$ras/. Surprisingly, NGF-induced activities of these two proteins were the same in control eells and GAP-overexpressing cells. Activities of Trk, PLC-r and SMC that act at a site upstream to p21$\^$ras/ in NGF signal transduction pathway were not also affected by GAP overexpression. Interestingly, however, the extent of tyrosine phosphorylation of SNT was found to be remarkably low in cells overexpressing GAP. It has been shown previously that neurotrophins and not mitogens induce SNT tyrosine phosphorylation in PCl2 cells. Thus it is possible that the timing of NGF-induced neuronal differntiation may be in part regulated by SNT and the slower onset of neurite outgrowth in cells overexpressing GAP may be through the inhibition of SNT by GAP.