An Extracellular Domain of the Insulin Receptor β-Subunit with Regulatory Function on Protein-Tyrosine Kinase

Anti-insulin receptor monoclonal antibody MA-10 inhibits insulin receptor autophosphorylation of purified rat liver insulin receptors without affecting insulin binding (Cordera, R., Andraghetti, G., Gherzi, R., Adezati, L., Montemurro, A., Lauro, R., Goldfine, I. D., and De Pirro, R. (1987) Endocrinology 121, 2007–2010). The effect of MA-10 on insulin receptor autophosphorylation and on two insulin actions (thymidine incorporation into DNA and receptor down-regulation) was investigated in rat hepatoma Fao cells. MA-10 inhibits insulin-stimulated receptor autophosphorylation, thymidine incorporation into DNA, and insulin-induced receptor down-regulation without affecting insulin receptor binding. We show that MA-10 binds to a site of rat insulin receptors different from the insulin binding site in intact Fao cells. Insulin does not inhibit MA-10 binding, and MA-10 does not inhibit insulin binding to rat Fao cells. Moreover, MA-10 binding to down-regulated cells is reduced to the same extent as insulin binding. In rat insulin receptors the MA-10 binding site has been tentatively localized in the extracellular part of the insulin receptor β-subunit based on the following evidence: (i) MA-10 binds to insulin receptor in intact rat cells; (ii) MA-10 immunoprecipitates isolated insulin receptor β-subunits labeled with both [35S]methionine and 32P; (iii) MA-10 reacts with rat insulin receptor β-subunits by the method of immunoblotting, similar to an antipeptide antibody directed against the carboxyl terminus of the insulin receptor β-subunit. Moreover, MA-10 inhibits autophosphorylation and protein-tyrosine kinase activity of reduced and purified insulin receptor β-subunits. The finding that MA-10 inhibits insulin-stimulated receptor autophosphorylation and reduces insulin-stimulated thymidine incorporation into DNA and receptor down-regulation suggests that the extracellular part of the insulin receptor β-subunit plays a role in the regulation of insulin receptor protein-tyrosine kinase activity.