DIFFERENTIAL RAS-DEPENDENCE OF GENE INDUCTION BY NERVE GROWTH-FACTOR AND 2ND-MESSENGER ANALOGS IN PC12 CELLS

Induction of neurite formation by nerve growth factor (NGF) in PC12 ph eochromocytoma cells can be efficiently inhibited by expressing a domi nant negative mutant form of the small guanine nucleotide binding Ha-R as protein in these cells. The block in NGF-induced neuritogenesis cau sed by inhibition of endogenous Ras proteins was found to be partially relieved by simultaneous stimulation of cAMP- or Ca++-dependent signa ling pathways. Since expression of certain genes is believed to be inv olved in NGF-signaling leading to morphological differentiation, we de cided to study the combined effects of NGF and second messenger analog s on gene expression in PC12 cell lines expressings different levels o f the interfering Ras protein. We found NGF-second messenger combinati ons that induced normal c-fos, zif268 and nur77 early-response gene ex pression without neuritogenesis, and, conversely, cell lines in which certain combination treatments caused partial neuronal differentiation in the absence of substantial activation of these genes. Similarly, n eurite outgrowth induced by combination treatments does not seem to re quire the activation of the late-response transin gene. Our results th us suggest a lack of strong correlation between NGF-stimulated early- and secondary-response gene induction and morphological differentiatio n.