Background: Stimulation of phaeochromocytoma PC12 cells by nerve growth factor leads to growth arrest and neuronal differentiation, whereas insulin induces various metabolic responses such as metabolism of glucose and lipids. Moreover, both insulin and epidermal growth factor stimulate the proliferation of PC12 cells. In spite of their different biological effects, nerve growth factor, insulin and epidermal growth factor induce very similar early responses in PC12 cells. Stimulation with nerve growth factor leads to the sustained activation and nuclear translocation of mitogen-activated protein (MAP) kinase. By contrast, both insulin and epidermal growth factor induce the transient activation of MAP kinase, without pronounced nuclear translocation of the enzyme. We have investigated whether the differential activation of signaling pathway components can account for the distinct cellular responses to these different growth factors.
Results: By overexpressing insulin receptors in PC12 cells, we observed insulin-dependent neurite outgrowth, similar to that induced by nerve growth factor in both non-transfected and overexpressing cells. Overexpression of insulin receptors in PC12 cells leads to a more pronounced, but similar pattern of insulin-induced tyrosine-phosphorylated proteins in PC12 cells, including enhanced recruitment of Grb2/Sos into a complex with either Shc or IRS1. MAP kinase activation in response to insulin stimulation of cells overexpressing the insulin receptor is similar to MAP kinase activation in response to NGF stimulation of parental or overexpressing PC12 cells: the activation is prolonged and nuclear translocation of the enzyme occurs.
Conclusion: The differential subcellular localization and duration of MAP kinase activation induced by insulin and NGF may explain the difference in the biological actions of these two factors on PC12 cells. Our results show that the strength of the signal generated by a receptor with tyrosine kinase activity can influence the downstream signaling pathway, leading to cell differentiation instead of cell proliferation.