E Van Obberghen

Insulin-receptor antibodies mimic a late insulin effect.

AUTOANTIBODIES against the insulin receptor are found in patients with a rare form of extreme insulin resistance associated with acanthosis nigricans1. These antibodies are primarily polyclonal IgGs, compete with insulin for binding to its receptor2,3, immunoprecipitate solubilised insulin receptors4, and have been used as probes of insulin action. The acute action of the antibodies is to mimic many of insulins biological effects, particularly those which occur rapidly after hormone binding to its cell-surface receptors5–9. In addition, insulin exerts biological effects which appear more slowly. For example, insulin treatment of 3T3-L1 fatty fibroblasts increases the activity of lipo-protein lipase maximally after 2–4 d but is without effect during the first 4 h (refs 10, 11). Some investigators have suggested that these slower effects may be mediated by different mechanisms from those involved in the production of the acute effects12,13. We now report that autoantibodies against the insulin receptor mimic this long-term insulin effect on lipoprotein lipase.

The Insulin Receptor and Insulin of the Atlantic Hagfish: Extraordinary Conservation of Binding Specificity and Negative Cooperativity in the Most Primitive Vertebrate

The North Atlantic hagfish, a cyclostome that is representative of the most primitive vertebrates still alive, diverged from the other vertebrates about 500 million years ago. Hagfish insulin, which differs from porcine insulin in 18 (38%) of its amino acids, had a potency of 5-10% that of porcine insulin in stimulating glucose oxidation and deoxyglucose transport in rat adipocytes and was 5-10% as potent as porcine insulin in binding to insulin receptors on rat adipocytes and human (IM-9) lymphocytes. Like all other naturally occurring insulins, hagfish insulin accelerated the dissociation of 125I-porcine insulin from insulin receptors and the degree of the acceleration was related to its occupancy of the receptor. The insulin receptor of the hagfish erythrocyte showed the time, temperature, and pH dependence of binding and the negative cooperativity that are characteristic of all other insulin receptors. That the negative cooperativity is fully conserved in such an ancient insulin and receptor suggests that it is an important functional feature of this hormone-receptor system. The hagfish receptor showed the same absolute affinity and rank order of preference for insulins and insulin analogues (chicken > pork > proinsulin > guinea pig > desoctapeptide) found with other receptors of less primitive vertebrates, which supports the conclusion that the receptor for insulin is functionally better conserved evolutionary than the hormone. However, uniquely, hagfish insulin was more potent in binding to hagfish receptors than to mammalianreceptors; with all other species of insulins studied, the affinity of the hormone for homologous receptor was the same as for receptors of heterologous species.

The insulin receptor on the human lymphocyte: Insulin-induced down-regulation of 126,000 and 90,000 glycosylated subunits

SummaryCultured human lymphoblastoid B lymphocytes were surface-labelled with iodine125 and solubilized in 1% Triton X-100 in the presence of protease inhibitors. After purification of labelled glycoproteins by elution from immobilized wheat germ leetin with 0.3 mol/l N-acetyl-D-glucosamine, insulin receptors were quantitatively immunoprecipitated using IgG receptor auto-antibodies. The overall recovery of labelled glycoprotein was 0.02–0.04%; analysis by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography under reducing conditions revealed two major bands with molecular weights of 126,000 and 90,000, and a minor band of 67,000 daltons. The mobilities of both major receptor subunits were increased after treatment with neuraminidase. When lymphocyte receptor binding was ‘down-regulated’ before surface labelling, there was a concomitant decrease in the recovery of both the 126,000 and 90,000 subunits. These data indicate that ‘down-regulation’ of binding probably involves degradation of the receptor molecule.

Autoantibodies to the Insulin Receptor and Insulin-Resistant Diabetes

Autoantibodies to the insulin receptor are a rare cause of insulin-resistant diabetes, but when they occur they produce a profound clinical syndrome. These antibodies block insulin binding, immunoprecipitate solubilized insulin receptors, and their acute effect is to mimic the biological effects of insulin. However, prolonged exposure of cells to these antibodies produces a state of insulin resistance. Since the antigen to which the antibody is directed is relatively well-characterized, many of the observations in this syndrome can serve as a model for elucidating molecular mechanisms in other diseases with antibodies against membrane components. The autoantibodies to the insulin receptor have also provided valuable probes in the study of insulin receptor structure and insulin action.