Insulin Resistance Is Mediated by a Proteolytic Fragment of the Insulin Receptor

Abstract

Insulin resistance is a common clinical feature of obesity and non-insulin-dependent diabetes mellitus, and is characterized by elevated serum levels of glucose, insulin, and lipids. The mechanism by which insulin resistance is acquired is unknown. We have previously demonstrated that upon chronic treatment of fibroblasts with insulin, conditions that mimic the hyperinsulinemia associated with insulin resistance, the membrane-associated insulin receptor ␤ subunit is proteolytically cleaved, resulting in the generation of a cytosolic fragment of the ␤ subunit, ␤, and that the generation of ␤ is inhibited by the thiol protease inhibitor E64 (Knutson, V. P. (1991) J. Biol. Chem. 266, 15656 -15662). In this report, we demonstrate that in 3T3-L1 adipocytes: 1) cytosolic ␤ is generated by chronic insulin administration to the cells, and that E64 inhibits the production of ␤; 2) chronic administration of insulin to the adipocytes leads to an insulinresistant state, as measured by lipogenesis and glycogen synthesis, and E64 totally prevents the generation of this insulin-induced cellular insulin resistance; 3) E64 has no effect on the insulin-induced down-regulation of insulin receptor substrate-1, and therefore insulin resistance is not mediated by the down-regulation of insulin receptor substrate-1; 4) under in vitro conditions, partially purified ␤ stoichiometrically inhibits the insulin-induced autophosphorylation of the insulin receptor ␤ subunit; and 5) administration of E64 to obese Zucker fatty rats improves the insulin resistance of the rats compared to saline-treated animals. These data indicate that ␤ is a mediator of insulin resistance, and the mechanism of action of ␤ is the inhibition of the insulin-induced autophosphorylation of the ␤ subunit of the insulin receptor.Insulin resistance is a characteristic clinical feature of a number of disease states, chief among them diabetes mellitus, and is associated with hyperglycemia, hyperinsulinemia, hyperlipidemia, and hypertension (reviewed in Refs. 1 and 2). The potential mechanisms by which cellular insulin resistance is generated are many: a mutation in the gene coding for the insulin receptor protein, resulting in a decreased expression of the protein; a decrease in the binding of insulin; a decrease in the number of insulin receptor molecules expressed on the plasma membrane of the target cell; or a so-called "post-receptor defect" in which there is a decreased interaction between the insulin receptor and downstream effector molecules.Natural mutations in the primary sequence of the insulin receptor have been identified in patients with extreme forms of insulin resistance (3, 4). Mutations in the extracellular ligand binding domain of the receptor have been shown to result in a decreased affinity of insulin for the receptor (5-9). Mutations have also been documented in the intracellular ␤ subunit of the receptor, especially in the ATP binding domain and the autophosphorylation domain of the receptor protein, resulting in a decreased tyrosine kinase activity...

Reduced Expression of Insulin Receptors in the Kidneys of Insulin-Resistant Rats

“…Furthermore, insulin-induced proteolysis selectively of the IR-␤ subunit has been described, which might result in increased turnover of IR-␤ relative to IR-␣ with insulin infusion. 35,36 In our insulin-resistant rat models (the obese Zucker rat and the highfat-fed rat), we found decreased abundance of both subunits, although there was a tendency for the percentage of decrease to be greater for IR-␤, the signaling subunit.…”

Growth Hormone (GH)-induced Dimerization Inhibits Phorbol Ester-stimulated GH Receptor Proteolysis

“…for each is shown for n ϭ 3 determinations. For samples treated with PMA in the presence versus absence of GH: ‫,ء‬ p Ͻ 0.01. cytoplasmic domain-containing remnant protein (13,15); although we have no evidence as yet of a role for it, such a cytoplasmic domain-containing fragment has been shown to affect signaling in other receptor systems (57,58). These observations have suggested to us that, in addition to generating soluble GHBP, inducible GHR proteolysis may act in various ways to affect GHR function and GH signaling.…”

Deficiency and Inhibition of Cathepsin K Reduce Body Weight Gain and Increase Glucose Metabolism in Mice

“…Prior studies suggested a role of thio-proteases in IR catabolism 24. Despite the observation that the rate of insulin-IR internalization and degradation is reduced in cells from obese patients, the pathological implication of altered IR processing remains unknown.…”