Activation of MAP kinase in Swiss 3T3 fibroblasts by insulin-like growth factor-I

A peak of cytosolic myelin basic protein kinase activity was observed at 2-5 min after addition of 10 nM insulin-like growth factor-I (IGF-I) to Swiss 3T3 fibroblasts. Analysis of the induced kinase activity by chromatography, immunodetection and in situ kinase assay suggests that this represents a 1.8- to 3.5-fold increase in the activity of p42 mitogen-activated protein kinase. Addition of insulin at 10 nM also resulted in an increased kinase activity in these respects, however, only to approximately 60% of that induced by IGF-I. A similar quantitative relation between the effects of insulin and IGF-I was found for induction of lipid and glycogen synthesis from [14C- (U)]-D-glucose. However, incorporation of 3H-L-leucine into protein was stimulated to the same extent by the two agents. The effect of 10 nM insulin on proliferation, measured as tetrazolium dye reduction, was only 18% of that induced by 10 nM IGF-I. The obtained data suggest that cytosolic mitogen-activated protein kinase activation may constitute a signalling pathway for glucose metabolism, and especially glycogen synthesis, induced by peptides of the insulin superfamily.