Phosphorylation of receptors for insulin and insulin-like growth factor I. Effects of hormones and phorbol esters.

The phosphorylation of receptors for insulin and insulin-like growth factor I was studied by phosphoamino acid analysis and tryptic phosphopeptide maps in an attempt to determine if protein kinase C is involved in their phosphorylation in response to insulin and insulin-like growth factor I, respectively. Two cell lines were utilized, Hep G2 and IM-9 cells. sn-1,2-Dioctanoylglycerol and 12-O-tetradecanoylphorbol 13-acetate (TPA), agents known to activate protein kinase C, stimulated the phosphorylation of the beta subunits of both receptors, as did their hormones. In unstimulated cells, phosphorylation of the insulin receptor occurred on seryl and to a lesser extent on threonyl residues. TPA stimulated seryl and threonyl phosphorylation that resulted in the appearance of four major phosphoserine-containing phosphopeptides which were not detected in the basal state and an increase in phosphorylation of a phosphothreonine-containing peptide which was present in the basal state. Insulin treatment resulted in the appearance of three major phosphotyrosine-containing tryptic peptides. In IM-9 cells, insulin also increased the phosphoserine and possibly the phosphothreonine content of the beta subunit. In both cells, the major phosphoserine-containing peptides that were stimulated by TPA were not detected following treatment with insulin. Very similar results, including similar peptide maps, were obtained for the insulin-like growth factor I receptor from cells treated with TPA and insulin-like growth factor I. Although not entirely conclusive, these results suggest that the insulin- and insulin-like growth factor I-stimulated phosphorylation of their receptors does not result from activation of protein kinase C.