Stimulation of the activity of protein kinase C by pretreatment of cells with phorbol esters was tested for its ability to inhibit signaling by four members of the insulin receptor family, including the human insulin and insulin-like growth factor-I receptors, the human insulin receptor-related receptor, and the Drosophila insulin receptor. Activation of overexpressed protein kinase C␣ resulted in a subsequent inhibition of the ligand-stimulated increase in antiphosphotyrosine-precipitable phosphatidylinositol 3-kinase mediated by the kinase domains of all four receptors. This inhibition varied from 97% for the insulin receptorrelated receptor to 65% for the Drosophila insulin receptor. In addition, the activation of protein kinase C␣ inhibited the in situ ligand-stimulated increase in tyrosine phosphorylation of the GTPase-activating protein-associated p60 protein as well as Shc mediated by these receptors. The mechanism for this inhibition was further studied in the case of the insulin-like growth factor-I receptor. Although the in situ phosphorylation of insulin-receptor substrate-1 and p60 by this receptor was inhibited by prior stimulation of protein kinase C␣, the in vitro tyrosine phosphorylation of these two substrates by this receptor was not decreased by prior stimulation of the protein kinase C␣ in the cells that served as a source of the substrates. Finally, the insulin-like growth factor-I-stimulated increase in cell proliferation was found to be inhibited by prior activation of protein kinase C␣. These results indicate that the ability of activated protein kinase C␣ to antagonize signaling by the human insulin receptor is shared by the other members of the insulin receptor family despite their considerable differences in amino acid sequence. Moreover, the present study shows that this antagonism is exerted at a very early step, the initial tyrosine phosphorylation of three distinct endogenous substrates. Finally, the present study indicates that this inhibition is not caused by an increased Ser/Thr phosphorylation of these two substrates.Most studies have documented a critical role for the intrinsic tyrosine kinase activity of the insulin receptor (IR) 1 in mediating subsequent biological responses (1, 2). After binding insulin, the receptor autophosphorylates on several specific tyrosines and then tyrosine-phosphorylates several endogenous substrates including insulin-receptor substrate (IRS)-1, an SH2-containing protein called Shc, and various 60-kDa proteins including one that is tightly bound by the GTPase-activating protein of Ras (3-10). The tyrosine phosphorylation of IRS-1 results in its being bound by several SH2-containing proteins including the phosphatidylinositol (PI) 3-kinase, a tyrosine phosphatase, and two SH2 linker proteins called Grb-2 and Nck (3, 11). A number of studies utilizing a variety of approaches have implicated the tyrosine phosphorylation of IRS-1 as being important in initiating several biological responses including stimulation of growth responses as well as stimulat...

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In the present studies, insulin was found to stimulate in a rat hepatoma cell line (called FAO cells) the tyrosine phosphorylation of the 60-kilodalton p21ras GTPase-activating protein (GAP)-associated protein called p60. Surprisingly, the tyrosine phosphorylation of this protein was also almost equally stimulated by an activator of protein kinase C (PKC), the phorbol ester phorbol 12-myristate 13-acetate (PMA). The tyrosine phosphorylation of p60 induced by either agent correlated with the formation of the GAP-p60 complex in situ and an increase in the ability of p60 to directly bind to the SH2 domain of GAP in vitro. Several lines of evidence indicated that the PMA-induced tyrosine phosphorylation of p60 occurred through a different mechanism than that induced by insulin. First, the stimulation of tyrosine phosphorylation of p60 by maximal concentrations of the two agents was almost additive. Second, down-regulation of PKC or pretreatment with a specific inhibitor of PKC abolished the ability of PMA to stimulate tyrosine phosphorylation of p60 but had no effect on the insulin stimulation. And third, long-term pretreatment with insulin abolished the insulin response but did not affect the response to PMA. The PMA effect did seem to be mediated via a tyrosine kinase, since it was blocked by quercetin, an inhibitor of tyrosine kinases. These results indicate that both PMA and insulin can equally stimulate in FAO cells the tyrosine phosphorylation of p60 and its association with GAP, although these two agents seem to act via different signaling systems.

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The insulin receptors from erythrocytes of 50 patients with non-insulin-dependent diabetes mellitus were tested for their ability to autophosphorylate. The assay was performed by a new enzyme-linked immunosorbent assay system that used monoclonal anti-insulin receptor antibodies absorbed to microtiter plates as a first antibody and polyclonal antiphosphotyrosine antibody as a labeled second antibody. By this assay, 3 patients were identified with defects in their insulin receptor kinase, although their defects appeared heterogeneous. Patient 1 had 85% less maximal autophosphorylation with a normal ED50 (1.6 x 10(-9) M insulin). Patient 2, who had polycystic ovary disease, had a 49.2% decrease in maximal autophosphorylation of insulin receptors, and the ED50 was shifted to the right (5.6 x 10(-8) M). Patient 3 with acanthosis nigricans had a normal maximal autophosphorylation, but the ED50 shifted to the right (2.9 x 10(-8) M). The mechanisms for the diversity detected in this assay is not known, but this technique has sufficient specificity and sensitivity to be used to screen for insulin-resistant patients who have a lack of kinase activity.