DIVERGENCE OF SIGNALING PATHWAYS FOR INSULIN IN PC-12 PHEOCHROMOCYTOMA CELLS

A PC-12 pheochromocytoma cell line is described with roughly equivalen t levels of functional receptors for nerve growth factor (NGF), epider mal growth factor (EGF), and insulin. Each of these receptors undergoe s autophosphorylation upon binding of their respective ligands, and ca uses the activation of phosphatidylinositol-3 kinase via a mechanism i nvolving tyrosine phosphorylation. In the case of insulin, this activa tion is due to the tyrosine phosphorylation of its major cellular subs trate, IRS-1. Despite the presence of functional receptors in these ce lls, insulin does not stimulate the activity of the mitogen-activated protein (MAP) kinase, despite a 5- to 8-fold activation observed with both NGF and EGF under the same conditions. This failure to activate M AP kinase was not due to the insulin-dependent dephosphorylation of th e enzyme, but correlated with the lack of activation of the MAP kinase kinase, although this enzyme was also activated by NGF and EGF. Simil arly, the activation of the raf and ras protooncogenes in these cells was not observed with insulin, whereas NGF and EGF produced marked act ivation. In addition, insulin-dependent induction of the c-fos protein was impaired, in comparison to NGF. In contrast to a lack of effect o n the MAP kinase pathway, these PC-12 cells were metabolically respons ive to insulin, exhibiting increases in glucose, lipid, and protein sy nthesis in response to the hormone. The differential responses of phos phorylation events to insulin, NGF, and EGF in these cells indicates t hat divergence of signaling pathways may occur at or near the insulin receptor.