Phorbol esters stimulate the phosphorylation of receptors for insulin and somatomedin C.

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Abstract
The effect of phorbol esters on the extent of phosphorylation of receptors for insulin and somatomedin C (insulin-like growth factor I) was studied in intact human B-lymphocyte IM-9 cells that were labeled by incubatin with H332PO4. The tumor-promoting phorbol esters phorbol tetradecanoate acetate (TPA) and phorbol dibutyrate, but not the inactive 4.alpha.-phorbol, enhanced phosphorylation of the .beta. subunit of both receptors approximately 4-fold; 70 nM TPA maximally stimulated phosphorylation of both receptors, whereas concentrations less than or equal to 0.7 nM had no observable efefct. Insulin also enhanced the phosphorylation of the .beta. subunit of the insulin receptor, and its effects appeared to be additive to those of TPA. Peptide maps indicated that at least some of the residues phosphorylated by these 2 agents are distinct. A possible role of protein kinase C in regulating insulin and somatomedin C receptors is suggested.