Separation and characterization of three insulin receptor species that differ in subunit composition

Substrates for protein kinase CK2 in insulin receptor preparations from rat liver membranes: identification of a 210-kDa protein substrate as the dimeric form of endoplasmin

Chromatography of extracts from rat liver membranes on wheat-germ lectin-Sepharose resulted in a partial resolution of the insulin receptor from other phosphorylatable proteins. Among the latter, a protein (p210, with an apparent M(r) of 210 kDa on SDS/PAGE under nonreducing conditions) was found to be phosphorylated by protein kinase CK2 on Thr and Ser residues. Under reducing conditions p210 was resolved into two phosphopolypeptides with apparent M(r) of 95 and 105 kDa. Neither the 95-kDa nor the 105-kDa polypeptides were recognized by antibodies against the beta-subunit of the insulin receptor. Both polypeptides gave identical phosphopeptide maps after protease V8 digestion and contained the same N-terminal amino acid sequence. This sequence coincided with that of endoplasmin, and both polypeptides as well as p210 were recognized by antibodies against this protein. This shows that p210 corresponds to the dimeric form of rat liver endoplasmin. DEAE-Sepharose chromatography of p210 preparations removed most other contaminating proteins and revealed the presence of a protein kinase activity that coeluted with p210. This protein kinase possessed the properties (substrate specificity and inhibition by heparin) that are characteristic of the protein kinase CK2 enzymes. Furthermore, phosphoamino acid analysis and phosphopeptide maps of the 95/105-kDa polypeptides phosphorylated either by the endogenous protein kinase or by exogenous protein kinase CK2 gave similar results. The phosphorylation of p210/endoplasmin by protein kinase CK2 and its coelution gives support to the involvement of this protein kinase in membrane-associated processes.

Impairment of the liver insulin receptor autoactivation cascade at full-term pregnancy in the rat

Partially purified liver insulin receptors from full-term pregnant rats show decreased autophosphorylation rates if compared with receptors from virgins. We studied the molecular mechanism of this phenomenon, looking at possible structural and functional changes of several domains. The ATP-binding domain seems to be unaltered in receptors from pregnant rats since Km for ATP was similar to that observed in virgins. In contrast, the Vmax. is decreased some 45%, suggesting changes in the kinase domain. Truncation of a fragment of 10 kDa from the C-terminal tail does not normalize the kinase activity in receptors from pregnant rats, suggesting that this domain is not involved in the inhibitory regulation. Treatment with alkaline phosphatase increases the [32P]Pi incorporation into receptors from pregnant rats; however, the autophosphorylation remains lower than that observed in virgin rats. Tryptic phosphopeptide maps of phosphorylated receptors show that the same phosphopeptides are present in receptors from virgin and pregnant rats. However, the progression through the autoactivation cascade in the kinase domain is impaired in receptors from pregnant rats. Differences in the cleavage by trypsin at the two alternative sites in the kinase domain were observed, indicating possible structural changes in receptors from pregnant rats that could be related to the impairment of the autoactivation cascade. Integrity of the alpha- and beta-subunits, as well as differential expression of the two receptor isotypes, were shown to be unaltered. We conclude that (1) the decreased autophosphorylation rate of the liver insulin receptor from pregnant rats is associated with the impairment of its autoactivation cascade, probably as a consequence of the basal Ser/Thr phosphorylation; and (2) the inhibition of the autoactivation cascade does not account for the overall inhibition of autophosphorylation observed in receptors from pregnant rats.

The insulin receptor: structure, function, and signaling

The insulin receptor is a member of the ligand-activated receptor and tyrosine kinase family of transmembrane signaling proteins that collectively are fundamentally important regulators of cell differentiation, growth, and metabolism. The insulin receptor has a number of unique physiological and biochemical properties that distinguish it from other members of this large well-studied receptor family. The main physiological role of the insulin receptor appears to be metabolic regulation, whereas all other receptor tyrosine kinases are engaged in regulating cell growth and/or differentiation. Receptor tyrosine kinases are allosterically regulated by their cognate ligands and function as dimers. In all cases but the insulin receptor (and 2 closely related receptors), these dimers are noncovalent, but insulin receptors are covalently maintained as functional dimers by disulfide bonds. The initial response to the ligand is receptor autophosphorylation for all receptor tyrosine kinases. In most cases, this results in receptor association of effector molecules that have unique recognition domains for phosphotyrosine residues and whose binding to these results in a biological response. For the insulin receptor, this does not occur; rather, it phosphorylates a large substrate protein that, in turn, engages effector molecules. Possible reasons for these differences are discussed in this review. The chemistry of insulin is very well characterized because of possible therapeutic interventions in diabetes using insulin derivatives. This has allowed the synthesis of many insulin derivatives, and we review our recent exploitation of one such derivative to understand the biochemistry of the interaction of this ligand with the receptor and to dissect the complicated steps of ligand-induced insulin receptor autophosphorylation. We note possible future directions in the study of the insulin receptor and its intracellular signaling pathway(s).

A mutation (Trp1193-->Leu1193) in the tyrosine kinase domain of the insulin receptor associated with type A syndrome of insulin resistance

We evaluated a 35-year-old diabetic male patient with type A insulin resistance, showing acanthosis nigricans. Insulin binding to the patient's Epstein-Barr-virus transformed lymphocytes was mildly reduced. The maximal insulin-stimulated autophosphorylation of the insulin receptor from the patient's transformed lymphocytes was decreased to 45% of that from the control subjects. On examination, the biological activities of insulin and insulin-like growth factor I in the patient's cultured fibroblasts, insulin sensitivity of amino isobutyric acid uptake and thymidine incorporation was decreased, but insulin-like growth factor I action was normal. The sequence analysis of amplified genomic DNA revealed that the patient was heterozygous for a mutation substituting Leu for Trp at codon 1193 in exon 20 of the insulin receptor gene. The patient's mother and sister were also heterozygous for a mutation in the insulin receptor gene that substituted Leu for Trp1193 in the beta subunit of the receptor. Therefore, the mutation causes insulin resistance in a dominant fashion. They were less hyperglycaemic and more hyperinsulinaemic than the proband after glucose loading. The mother had diabetes mellitus but did not show acanthosis nigricans, while the sister did not have diabetes and showed acanthosis nigricans. These results suggest that this mutation causes defective tyrosine kinase activity of the insulin receptor, which results in insulin resistance. Insulin action and phenotypic appearance may be mediated by different factors.

Insulin/IGF-1 hybrid receptors: implications for the dominant-negative phenotype in syndromes of insulin resistance

Classical insulin and IGF-1 receptors are alpha 2 beta 2 heterotetrameric complexes synthesized from two identical alpha beta half-receptor precursors. Recent data strongly suggests, however, that nonidentical alpha beta half-receptor precursors can assemble to generate hybrid holoreceptor species both in vivo and in vitro. This review focuses primarily on two types of hybrid receptors. The first type is an insulin/IGF-1 hybrid receptor generated by the association of an alpha beta insulin half-receptor with an alpha beta IGF-1 half-receptor. The second type is one formed from a wildtype (kinase-active) insulin or IGF-1 alpha beta half-receptor and a mutant (kinase-inactive) insulin alpha beta half-receptor. Although the functional properties of insulin/IGF-1 hybrid receptors have not yet been completely defined, wildtype/mutant hybrid receptors are essentially substrate kinase inactive. These data indicate that the mutant alpha beta half-receptor exerts a transdominant inhibition upon the wildtype alpha beta half-receptor within the alpha 2 beta 2 holoreceptor complex. This defect in substrate kinase activity may contribute to the molecular defect underlying some syndromes of severe insulin resistance and diabetes. Heterozygous individuals expressing both wildtype and mutant tyrosine kinase-defective insulin receptor precursors demonstrate varying degrees of insulin resistance and diabetes. In addition, cell lines which express both endogenous wildtype and transfected kinase-defective insulin receptors display markedly decreased insulin and IGF-1 sensitivity and responsiveness. Formation of hybrid receptors which results in premature termination of insulin signal transduction may be one mechanism underlying the observation that kinase-inactive receptors inhibit the function of native receptors.

The insulin receptor: signalling mechanism and contribution to the pathogenesis of insulin resistance

The insulin receptor is a heterotetrameric structure consisting of two alpha-subunits of Mr 135 kilodalton on the outside of the plasma membrane connected by disulphide bonds to beta-subunits of Mr 95 kilodalton which are transmembrane proteins. Insulin binding to the alpha-subunit induces conformational changes which are transduced to the beta-subunit. This leads to the activation of a tyrosine kinase activity which is intrinsic to the cytoplasmatic domains of the beta-subunit. Activation of the tyrosine kinase activity of the insulin receptor represents an essential step in the transduction of an insulin signal across the plasma membrane of target cells. Signal transduction on the post-kinase level is not yet understood in detail, possible mechanisms involve phosphorylation of substrate proteins at tyrosine residues, activation of serine kinases, the interaction with G-proteins, phospholipases and phosphatidylinositol kinases. Studies in multiple insulin-resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistance. An impairment of the insulin effect on tyrosine kinase activation in all major target tissues of insulin, in particular the skeletal muscle was demonstrated in Type 2 (non-insulin-dependent) diabetic patients. There is no evidence that the impaired tyrosine kinase response in the skeletal muscle is a primary defect, however, it is likely that this abnormality of insulin signal transduction contributes significantly to the pathogenesis of the insulin-resistant state in Type 2 diabetes.

Transdominant inhibition of tyrosine kinase activity in mutant insulin/insulin-like growth factor I hybrid receptors

Classical insulin and insulin-like growth factor I (IGF-I) receptors exist as well defined alpha 2 beta 2 heterotetrameric complexes that are assembled from two identical alpha beta heterodimeric half-receptor precursors. Recent evidence suggests that insulin and IGF-I half-receptors can heterologously assemble to form alpha 2 beta 2 insulin/IGF-I hybrid receptor complexes in vivo and in vitro. We have utilized hybrid receptor complexes to examine ligand-stimulated transmembrane signaling of wild-type insulin (alpha beta INS.WT) or IGF-I (alpha beta IGF.WT) half-receptors assembled with a kinase-defective insulin half-receptor mutant (alpha beta INS.A/K). In vitro assembly of either (alpha beta)IGF.WT/(alpha beta)INS.A/K or (alpha beta)INS.WT/(alpha beta)INS.A/K hybrid receptors resulted in decreased substrate protein kinase activity. The degree of protein kinase inactivation directly correlated with the amount of immunologically cross-reactive hybrid receptors formed. In contrast to substrate kinase activity, insulin-stimulated autophosphorylation of the (alpha beta)INS.WT/(alpha beta)INS.A/K hybrid receptor complex was completely unaffected in comparison to the wild-type (alpha beta)INS.WT/(alpha beta)INS.WT receptor. To assess a molecular basis for this difference, autophosphorylation of a hybrid receptor composed of a truncated beta-subunit insulin half-receptor with the kinase-defective half-receptor, (alpha beta)INS. delta CT/(alpha beta)INS.A/K, demonstrated the exclusive autophosphorylation of the (alpha beta)INS.A/K half-receptor beta subunit. These results demonstrate that ligand-dependent substrate phosphorylation by insulin and IGF-I holoreceptors requires interactions between two functional beta subunits within the alpha 2 beta 2 heterotetrameric complex and occurs through an intramolecular trans-phosphorylation reaction.

Insulin receptor activity and insulin sensitivity in mammary gland of lactating rats

The mammary gland is a tissue that is extremely sensitive to insulin during lactation; during weaning, the effect of insulin is rapidly abolished. The purpose of this study was to characterize the mammary gland insulin receptors and their kinase activity in lactating and weaned mammary gland. The apparent molecular weight of the alpha-subunit was slightly lower in the mammary gland than in liver and white adipose tissue (127,000 vs. 134,000), but the apparent molecular weight of the beta-subunit was similar in the three tissues (95,000). Insulin induced a 10-fold increase in beta-subunit autophosphorylation, and the half-maximal effect was achieved at 2 nM insulin. After 24 h of weaning, the number of insulin receptors was decreased by 30%, but the kinase activity of the beta-subunit was unchanged. During the euglycemic hyperinsulinemic clamp, insulin entirely activated pyruvate dehydrogenase in lactating rat mammary gland, whereas after 24 h of weaning it was unable to increase the proportion of the enzyme in the active form. These results suggest that the site of alteration in the action of insulin on the mammary gland during weaning is distal to the receptor.

Receptors for insulin and insulin-like growth factor-I can form hybrid dimers. Characterisation of hybrid receptors in transfected cells

We have demonstrated the formation of hybrid insulin/insulin-like growth factor-I(IGF-I) receptors in transfected rodent fibroblasts, which overexpress human receptors, by examining reactivity with species- and receptor-specific monoclonal antibodies. In NIH 3T3 and Rat 1 fibroblasts, endogenous IGF-I receptors were unreactive with anti-(human insulin receptor)monoclonal antibodies (47-9, 25-49, 83-14, 83-7, 18-44). However, in transfected cells expressing high levels of insulin receptors, 60-80% of high-affinity IGF-I receptors reacted with these antibodies, as assessed either by inhibition of ligand binding in intact cells or by precipitation of solubilized receptors. Conversely, endogenous insulin receptors in NIH 3T3 cells were unreactive with anti-(IGF-I receptor) antibodies alpha IR-3 and 16-13. However, approx. 50% of high-affinity insulin receptors reacted with these antibodies in cells expressing high levels of human IGF-I receptors. The hybrid receptors in transfected cells bound insulin or IGF-I with high affinity. However, responses to these ligands were asymmetrical, in that binding of IGF-I inhibited subsequent binding of insulin, but prior binding of insulin did not affect the affinity for IGF-I. The existence of hybrid receptors in normal tissues could have important implications for metabolic regulation by insulin and IGF-I.

Intrinsic kinase activity of the insulin receptor

Since the identification of the insulin receptor by insulin-binding activity almost two decades ago, our understanding of the structure and function of the insulin receptor has progressed tremendously. The importance of the intrinsic tyrosine protein kinase activity of the insulin receptor is implied by the fact that the insulin receptor belongs to a family of receptor tyrosine kinases which play a role in growth control, by experiments demonstrating the intimate association of normal kinase activity and insulin action, and by evidence that the intrinsic kinase activity can be regulated under certain conditions. There are still some major gaps in our knowledge concerning the structure/function of the insulin receptor such as how activation of the intrinsic kinase activity of the receptor leads to altered cellular physiology. The kinase may phosphorylate endogenous substrates or autophosphorylation may simply alter beta subunit conformation so it can then interact with an effector system (i.e. a serine kinase) directly, or indirectly through a G-protein. The truth may lie somewhere between these two pathways.

Insulin binding changes the interface region between alpha subunits of the insulin receptor

The homobifunctional cross-linking reagent disuccinimidyl suberate (DSS) was used to probe the interface region between the two alpha subunits of the alpha 2 beta 2 human insulin receptor. The two alpha subunits formed a covalent dimer when affinity-purified receptor or membrane-bound receptor was reacted with DSS. The alpha 2 species was detected on protein blots from SDS gels using an anti-alpha-subunit antibody or 125I-concanavalin A. Alternatively, iodinated receptor was reacted with DSS and the alpha 2 species measured directly in an SDS gel. As shown by all three assay systems, more alpha 2 was formed when insulin was bound to receptor than when insulin was absent. These data indicate that the conformational change which occurs in the alpha subunit in response to insulin binding results in a change in the alpha-alpha interaction within the receptor complex. The results are consistent with a kinase activation mechanism involving communication between the two alpha beta receptor halves.