title: Insulin inhibits pertussis toxin-catalyzed ADP-ribosylation of G-proteins. Evidence for a novel interaction between insulin receptors and G-proteins.
abstract: Insulin modifies cellular responsiveness to some hormones which operate via guanine nucleotide binding proteins (G-proteins); also, G-proteins have been implicated in some actions of insulin. Using pertussis toxin-catalyzed [32P]ADP-ribosylation of Gi as an index of G-protein conformation, we evaluated interaction of insulin receptors with G-proteins. In isolated rat liver plasma membranes, insulin treatment for 10 min inhibited [32P]ADP-ribosylation of Gi by 50%. This effect was half-maximal at 2 x 10(-8) M. A similar effect was observed with rat adipocyte plasma membranes with half-maximal effect at 1 x 10(-8) M. Pertussis toxin activity itself was uninfluenced by insulin, as ribosylation of tubulin or heat-treated bovine serum albumin was unaltered. Elevated Mg2+ diminished basal ADP-ribosylation, but insulin inhibition occurred at all Mg2+ levels between 0 and 1 mM. Insulin inhibition was independent of ATP (20 microM to 10 mM), and GTP (0-100 microM) concentrations. Because both protein kinase C and purified insulin receptor phosphorylate purified Gi in vitro, we examined Gi as a substrate for the insulin receptor tyrosine kinase in vivo. Triton-extracts of isolated rat hepatocytes which had been 32Pi labeled and treated with insulin were immunoprecipitated with a polyclonal anti-Gi antiserum. The dominant labeled phosphoprotein had a molecular weight of 42 kDa, consistent with the alpha-subunit of Gi, contained only phosphoserine, and was unaffected in its phosphorylation by insulin. These results indicate the existence of a novel pathway for physiological "cross-talk" between insulin and other hormones and further suggests that the insulin receptor may interact with regulatory G-proteins via biochemical mechanisms not directly involving the tyrosine kinase activity of the insulin receptor.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000273:Adipose Tissue / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008297:Male; D008970:Molecular Weight; D037342:Pertussis Toxin; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011972:Receptor, Insulin / Q000378:metabolism*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
chemical_list: D007328:Insulin; D010566:Virulence Factors, Bordetella; D015636:Magnesium Chloride; D000255:Adenosine Triphosphate; D011065:Poly(ADP-ribose) Polymerases; D037342:Pertussis Toxin; D011972:Receptor, Insulin; D019204:GTP-Binding Proteins; D008274:Magnesium