Insulin and Insulin-Like Growth Factor I Binding to Cultured Rat Glomerular Mesangial Cells*

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Abstract
The potential effects of insulin and insulin-like growth factor I (IGF-I) on mesangial cell (MC) metabolism and growth were examined. Radiolabeled insulin or IGF-I were incubated with cell membranes from rapidly proliferating (subconfluent) or nonproliferating (confluent) MC in the presence of increasing concentrations of unlabeled heterologous and homologous ligands (0-10-6 M). Insulin binding to MC was specific and saturable, with Scatchard analysis of binding data showing the characteristic curvilinear plot. The predicted insulin binding maximum of 4.2 .times. 10-12 M/100 .mu.g protein for a theoretica high affinity site was consistent with a relatively low density of receptors, which were the same in proliferating and nonproliferating cell preparations. Specific binding of IGF-I to MC was also demonstrated. Binding data for membranes from proliferating cultures generated a linear Scatchard plot, which predicted a binding maximum of 3.5-9.7 .times. 10-11 M/100 .mu.g protein and a Kd of 2.0-3.2 .times. 10-9 M. In contrast, membranes from nonproliferating cultures had no demonstrable specific binding of IGF-I. Covalent cross-linking of radiolabeled IGF-I to membranes from subconfluent cells demonstrated specific binding to a 145K membrane protein. A 95K membrane protein from a partially purified receptor preparation demonstrated autophosphorylation when incubated with 5 .times. 10-9 M IGF-I. Incubation of MC with 10-9 M IGF-I doubled cellular growth rates, an effect that could be duplicated only with high concentrations (10-6 M) of insulin. These observations indicate that MC express predominantly receptors for IGF-I, and that growth stimulatory effects of physiological concentrations of IGF-I and pharmacological concentrations of insulin are probably mediated through the IGF-I receptor.