Alpha2-HSG, a specific inhibitor of insulin receptor autophosphorylation, interacts with the insulin receptor

Mol Cell Endocrinol. 2000 Jun;164(1-2):87-98. doi: 10.1016/s0303-7207(00)00237-9.


Human fetuin, [alpha2-Heremans Schmid Glycoprotein (alpha2-HSG)], is a natural inhibitor of insulin receptor tyrosine kinase activity (IR-TKA). Previously, we have demonstrated that alpha2-HSG inhibits the mitogenic pathway without affecting the metabolic arm of insulin signal transduction. In this study, we demonstrate the time-course and specificity of inhibition, its interaction with IR and probable physiological role. In intact rat1 fibroblasts overexpressing the human insulin receptor (HIRc B), incubation of recombinant human alpha2-HSGbac (1.8 microM) inhibited insulin-induced IR autophosphorylation by over 80%. This inhibitory effect of alpha2-HSGbac on insulin-induced IR autophosphorylation was blunted by half in 60 min. Interestingly, alpha2-HSGbac at similar concentrations (0.9 or 1.8 microM), had no effect on EGF- or IGF-I-induced cognate receptor autophosphorylation. Anti-alpha2-HSG immunoprecipitates of alpha2-HSGbac-treated HIRc B cell lysates demonstrated the presence of IR. Our data suggest that alpha2-HSGbac preferentially interacts with the activated IR. To further characterize the site(s) of interaction, the effect of alpha2-HSGbac on trypsin-treated IR autophosphorylation was studied. Trypsin-treatment of intact HIRc B cells results in proteolysis of the IR alpha-chain and constitutive activation of IR-TKA. We demonstrate that alpha2-HSGbac (0.1 microM) completely inhibited trypsin-activated IR autophosphorylation and TKA in vitro indicating that this effect was not mediated by its interaction with the proximal 576 amino acid residues of the IR alpha-subunit. The physiological relevance of these observations was explored by characterizing the effects of alpha2-HSG injection in rats. Alpha2-HSGbac (2 microM), acutely injected through the portal vein of normal rats, inhibited insulin-stimulated IR autophosphorylation and IRS-1 phosphorylation in liver and hindlimb muscle. Taken together our results suggest that alpha2-HSG, by interacting with IR, specifically inhibits insulin-stimulated IR autophosphorylation and may play a physiological role in the regulation of insulin signaling.

MeSH terms

  • Animals
  • Blood Proteins*
  • Cystatins / chemistry
  • Cystatins / pharmacology
  • Cystatins / physiology*
  • Humans
  • Male
  • Phosphorylation / drug effects
  • Protein Binding
  • Protein-Tyrosine Kinases / antagonists & inhibitors
  • Rats
  • Rats, Wistar
  • Receptor, Insulin / chemistry
  • Receptor, Insulin / physiology*
  • Signal Transduction / drug effects*
  • Signal Transduction / physiology*
  • Time Factors
  • alpha-2-HS-Glycoprotein
  • alpha-Fetoproteins / chemistry
  • alpha-Fetoproteins / pharmacology
  • alpha-Fetoproteins / physiology*


  • AHSG protein, human
  • Blood Proteins
  • Cystatins
  • alpha-2-HS-Glycoprotein
  • alpha-Fetoproteins
  • Protein-Tyrosine Kinases
  • Receptor, Insulin