Divergence of signaling pathways for insulin in PC-12 pheochromocytoma cells

A PC-12 pheochromocytoma cell line is described with roughly equivalent levels of functional receptors for nerve growth factor (NGF), epidermal growth factor (EGF), and insulin. Each of these receptors undergoes autophosphorylation upon binding of their respective ligands, and causes the activation of phosphatidylinositol-3 kinase via a mechanism involving tyrosine phosphorylation. In the case of insulin, this activation is due to the tyrosine phosphorylation of its major cellular substrate, IRS-1. Despite the presence of functional receptors in these cells, 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 MAP kinase was not due to the insulin-dependent dephosphorylation of the enzyme, but correlated with the lack of activation of the MAP kinase kinase, although this enzyme was also activated by NGF and EGF. Similarly, the activation of the raf and ras protooncogenes in these cells was not observed with insulin, whereas NGF and EGF produced marked activation. In addition, insulin-dependent induction of the c-fos protein was impaired, in comparison to NGF. In contrast to a lack of effect on the MAP kinase pathway, these PC-12 cells were metabolically responsive to insulin, exhibiting increases in glucose, lipid, and protein synthesis in response to the hormone. The differential responses of phosphorylation events to insulin, NGF, and EGF in these cells indicates that divergence of signaling pathways may occur at or near the insulin receptor.