Differential phosphorylation of IRS-1 and IRS-2 by insulin and IGF-I receptors.

The specific contribution of insulin and IGF-I receptors to IRS-protein activation remains elusive. We studied the signalling properties of AspB10-insulin, an analog with enhanced affinity for the IGF-I receptor, in comparison to native insulin using primary human skeletal muscle cells. In myoblasts regular insulin and AspB10-insulin were equipotent in stimulating the IRS cascade, whereas this analog induced a significantly higher Shc phosphorylation. Phosphorylation of IRS-1 in response to insulin was inhibited equally by blocking either the insulin or the IGF-I receptor. IRS-1 activation by AspB10-insulin was only inhibited by blocking the IGF-I receptor. IRS-2 phosphorylation induced by both insulin and AspB10-insulin was nearly insensitive to blocking the insulin receptor, being predominantly mediated by the IGF-I receptor. We conclude that in myoblasts IRS-2, but not IRS-1, functions as preferred substrate for the IGF-I receptor. These data suggest a specific role for IRS-2 in growth and differentiation of human skeletal muscle.