EVIDENCE THAT HUMAN AND PORCINE INSULIN DIFFERENTLY AFFECT THE HUMAN INSULIN-RECEPTOR - STUDIES WITH MONOCLONAL ANTIINSULIN RECEPTOR ANTIBODIES

Binding studies have been carried out with radioiodinated monoclonal antibodies directed to various epitopes of the insulin receptor in order to detect differences between human and porcine insulin in the interaction with the human insulin receptor. Human insulin was more effective that porcine insulin at inhibiting the binding of I-125-MA-5 to IM-9 cells, Hep-2 human larynx cells and human placenta membranes. On the contrary, human and porcine insulin showed similar inhibitory effect on the binding of two other labeled anti-insulin receptor monoclonal antibodies, thus ruling out the possibility that results were due to experimental artifacts. Although several interpretations are possible, data reported suggest that human insulin and porcine insulin might differently affect the insulin receptor, even if, the biological significance of these findings remains unknown

EVIDENCE THAT 2 NATURALLY-OCCURRING HUMAN INSULIN-RECEPTOR ALPHA-SUBUNIT VARIANTS ARE IMMUNOLOGICALLY DISTINCT

The IgG from a patient (Italy 2 [I-2]) with hypoglycemia, due to autoantibodies to the insulin receptor, was purified on protein A Sepharose into two fractions that were tested in various human tissues and cells. The IgG fraction that bound protein A (absorbed IgG [IgGa]) nearly completely inhibited the binding of I-125-labeled insulin to various cells or tissues (placenta, IM-9, adipocytes, HEp-2-larynx cells, Epstein-Barr virus lymphocytes) but not > 50% of I-125-labeled insulin binding to human liver membranes. Conversely, both the IgG fraction from this patient, which did not bind protein A (flow-through IgG [IgGb]), and the IgGa fraction from a second similar patient (Italy 1 [I-1]) almost completely inhibited the binding of I-125-labeled insulin to liver membranes. The IgGa fraction from patient 1-2 did not change receptor affinity because 50% inhibition of I-125-labeled insulin binding was not affected by either the presence or absence of these IgG fractions. Furthermore, liver binding data were not due to cross-reaction of I-125-labeled insulin to the insulinlike growth factor I receptor, and treatment of liver membranes with neuraminidase did not alter the inhibitory effect of the IgGa fraction from patient I-2 on I-125-labeled insulin binding to liver. Binding inhibition experiments performed with cells transfected with and overexpressing the -12 (human insulin receptor [HIR]-A) or the +12 (HIR-B) variant of HIR revealed that the IgGa fraction from patient I-2 inhibited I-125-labeled insulin binding to the HIR-A receptor but not to the HIR-B receptor. The data reported herein demonstrate that the two naturally occurring insulin-receptor variants are immunologically distinct and indicate that the HIR-B variant is predominantly expressed in liver compared with other human tissues