Numerous studies with PC12 cells have suggested that the mitogen-activated protein (MAP) kinase pathway might play a major role in the neuronal differentiation that is induced by nerve growth factor (NGF). Cells of the PC12D subline extend neurites within several hours in response to NGF in the presence of inhibitors of the synthesis of RNA and protein. We examined the effects of a specific inhibitor 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) of the MAP kinase kinase (MEK)/MAP kinase pathway on the NGF-induced outgrowth of neurites in PC12D cells. The increase in MAP kinase activity in response to NGF was reduced by 80% upon treatment of PC12D cells with 50 microM PD98059, whereas the NGF-dependent formation of ruffles and the subsequent outgrowth of neurites were not blocked by PD98059 at this concentration. The outgrowth of neurites from conventional PC12 cells by NGF was suppressed by the addition of 50 microM PD98059 as reported by Pang et al. [L. Pang, T. Sawada, J. Stuart,S.J. Decker, A.R. Saltiel, Inhibition of MAP kinase kinase blocks the differentiation of PC12 cells induced by nerve growth factor, J. Biol. Chem. 270 (1995) 13585-13588]. In contrast, the rapid regeneration of neurites from PC12 cells primed with NGF, was not altered in the presence of the same dose of the inhibitor of MEK. It appeared, therefore, that the activation of the MAP kinase pathway was not necessarily required for the NGF-dependent extension of neurites. When PC12D cells were transfected with the dominant inhibitory Ha-ras Asn-17 gene, the induction of the mutant Ras protein led the suppression of the rapid outgrowth of neurites in response to NGF but not to dibutyryl cyclic AMP (dbcAMP). The result implies a direct involvement of Ras protein in the NGF-induced signal transduction that lead to the formation of neurites in PC12D cells. We can conclude that the activation of MAP kinase and selective gene expression are required for the differentiation of conventional PC12 cells to sympathetic neuron-like cells and that activation of Ras protein and, subsequently, of a MAP kinase-independent pathway might be involved in the extension of neurites from PC12D cells or in the regeneration of neurites from primed PC12 cells in response to NGF.
Copyright 1998 Elsevier Science B.V.