EFFECT OF NERVE GROWTH-FACTOR AND FIBROBLAST GROWTH-FACTOR ON PC12 CELLS - INHIBITION BY ORTHOVANADATE

Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, c auses increased levels of tyrosine phosphorylation and blocks, at nonc ytotoxic concentrations, the differentiative response of rat pheochrom ocytoma (PC12) cells to beta-nerve growth factor (betaNGF) and basic f ibroblast growth factor (bFGF) in a reversible manner. It also prevent s growth factor-induced neurite proliferation in primed cells and caus es the retraction of previously formed neurites, even in the presence of betaNGF or bFGF. It is equally effective in blocking neurite prolif eration by 8-Br-cAMP. Zinc chloride and ammonium molybdate, two other inhibitors of tyrosine phosphatases, also cause parallel decreases in neurite proliferation. Orthovanadate generally reduces the transcripti on of immediate early response genes (TIS 8 and c-fos) and secondary r esponse genes (ornithine decarboxylase (ODC), acetylcholinesterase (AC hE) and SCG 10) induced by betaNGF, bFGF, EGF, and PMA, albeit in a va riable fashion. There was no observed effect on the kinetics of expres sion as judged by TIS 8 induction by betaNGF and protein kinase C (PKC ) downregulation did not change the levels of inhibition by orthovanad ate seen in control cells. Orthovanadate does not affect the productio n of diacylglycerol induced by betaNGF or bFGF. These observations are consistent with the view that growth factor stimulation of differenti ation in PC12 cells involves at least one other PKC independent pathwa y, and that cAMP and PMA (and their active analogs) activate tyrosine kinases (albeit probably secondarily), which are at least partially re sponsible for their actions. Although the exact site(s) of action of o rthovanadate that lead to the inhibition of growth factor-induced neur ite proliferation are unknown, the results presented suggest that it p rolongs tyrosine phosphorylations by nonreceptor tyrosine kinases that act downstream from the receptor kinases.