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Four polyclonal antisera directed against the insulin receptor were tested for their capability to activate the tyrosine-specific protein kinase associated with the receptor. All four antisera were shown to inhibit insulin binding to the receptor in cultured human lymphoblastoid cells and to stimulate lipogenesis in isolated rat adipocytes. Although two antisera (B-d, B-8) stimulated the activity of the tyrosine kinase of partially purified receptor preparations from rat liver, two other antisera (B-2 and B-10) failed to do so. This failure could not be explained by lack of antibody binding to receptor, by interference with the receptor as a substrate for the kinase, or by blocking of the enzyme's active site. We conclude that these two antireceptor antibodies bind to the receptor but fail to activate the kinase. The simplest interpretation of these observations is that activation of the tyrosine-specific protein kinase might not be an obligatory step in coupling insulin binding to insulin action. However, it is also possible that the mechanism by which polyclonal antireceptor antisera mimic insulin's bioactivity may differ from the mechanism of action of insulin itself.

We studied a 23-yr-old woman with scleroderma and type B insulin resistance. The association with autoimmune disease suggested that the insulin resistance resulted from autoantibodies to the insulin receptor. However, in preliminary studies, serum antireceptor antibodies were not detected in an assay that measures the ability of the antibodies to inhibit insulin binding to the insulin receptor. Antireceptor antibodies were subsequently detected by their ability to immunoprecipitate affinity-labeled receptors. After the patient had received immunosuppressive therapy with prednisone and cyclophosphamide for 3 months, her insulin resistance remitted, and she developed hypoglycemia. Simultaneously with the remission of insulin resistance, the titer of serum antireceptor antibody (measured by the immunoprecipitation assay) fell to less than 1% of the previous level. In a series of 21 patients, this is the first patient with antireceptor antibodies that bound to the insulin receptor without inhibiting insulin binding.

We examined the structure of the affinity-labeled insulin receptors in rat brain, rat liver, and human IM-9 lymphocytes using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In gels run under reducing conditions, the alpha-subunit of the insulin receptor in brain had an apparent Mr of 127,000 distinctly lower than that seen in both rat liver and human lymphocytes (apparent Mr = 136,000). Exposure to neuraminidase increased the electrophoretic mobility of the liver receptor, but had no effect on the insulin receptor in brain. The carbohydrate moieties of the insulin receptors in rat brain and liver were further examined by chromatography on wheat-germ agglutinin agarose. The receptors in both tissues adsorbed to the wheat-germ agglutinin; elution with 0.3 M N-acetyl glucosamine resulted in slightly better recovery of the brain than of the liver receptor. Exposure to neuraminidase virtually abolished the interaction of the liver receptor with the lectin, whereas adsorption of the brain receptor was unaffected by neuraminidase. These results indicate that the insulin receptor in brain is distinguished from those in peripheral tissues by structural alterations, including changes in the carbohydrate moiety of the receptor. Such alterations contrast sharply with the previously observed similarities in insulin binding properties between insulin receptors in brain and other tissues. The implications of such structural alterations for the program of insulin action expressed by the receptors in brain remain to be explored.

[125I]Insulin binding has been studied in two patients with extreme insulin resistance using cultured B-lymphocytes transformed with Epstein-Barr virus. A cell line from a female infant with leprechaunism had insulin binding which was decreased 90% below the lower limit of normal. Lymphocytes from a young woman with type A extreme insulin resistance (associated with acanthosis nigricans and virilization) had insulin binding which was 80% depressed. In both cases, the defect in binding resulted from a decrease in the number of receptors per cell. The remaining receptors had normal properties, including a normal affinity for insulin and a normal specificity for insulin analogs. Insulin binding in cultured lymphocytes from these two insulin-resistant patients was also inhibited normally by antibodies to the insulin receptor. Immunological assays using anti-receptor antibodies confirmed the conclusion that the number of receptors was decreased. Affinity labeling of the leprechaun insulin receptor with [125I]insulin demonstrated the existence of an alpha-subunit with apparently normal molecular weight (130,000 daltons). However, the number of receptor molecules per cell appeared reduced.

Defects in insulin-receptor structure can impair insulin-receptor function. We have previously identified qualitative abnormalities in insulin binding to insulin receptors from an insulin-resistant patient (Lep/Ark-1). The defects in insulin binding are probably caused by a defect in receptor structure. In this study, we used immunological probes to investigate the structural defect(s) responsible for the abnormal function. Several anti-receptor antibodies were impaired in their abilities to bind to the insulin receptor of Lep/Ark-1. For example, monoclonal antibody MoAb-51 was much less effective in inhibiting binding to insulin receptors from Lep/Ark-1 (ID50 70 nM) than to those of normal subjects (ID50 8 nM). In addition, there was a 10-fold reduction of the avidity with which human polyclonal antibody B-d immunoprecipitated the patient's insulin receptors. The avidity of antibody B-10 was also reduced, although to a lesser extent. In contrast, several site-specific antibodies against epitopes on the β-subunit of the receptor bound to receptors from Lep/Ark-1 with normal avidity. The data with monoclonal and polyclonal antibodies are consistent with the hypothesis that the structural defect resides in the extracellular domain of this patient's insulin receptor. The normal immunoreactivity of two putative phosphorylation sites on the β-subunit with site-specific antibodies gives further support to the conclusion that this patient's receptors have normal tyrosine kinase activity.

The insulin receptor possesses protein kinase activity, which may play a role in mediating insulin action. Recently, we have identified a glycoprotein (pp120) in rat liver plasma membranes that is phosphorylated by the solubilized insulin receptor in a cell-free system. We now report that insulin stimulates phosphorylation of pp120 in intact H-35 cells. H-35 cells were preloaded with [32P]orthophosphate to label the intracellular ATP pool. Insulin caused a 10-fold increase in the phosphorylation of its receptor and a 2-fold increase in phosphorylation of pp120 (P less than 0.001). The time course of insulin's stimulation of pp120 closely paralleled that of insulin receptor phosphorylation over the time period investigated (15-45 min). This effect had the specificity corresponding to the insulin receptor. Epidermal growth factor was inactive, and insulin-like growth factor I had approximately equal to 1% the potency of insulin in this regard. Insulin increased 32P incorporation into pp120 in a linkage that was stable to alkaline hydrolysis, as would be expected for tyrosine-specific phosphorylation. Direct phosphoamino acid analysis confirmed that insulin increased 32P incorporation into phosphotyrosine residues in pp120.

Insulin binding to its receptor stimulates a tyrosine-specific protein kinase. This enzyme phosphorylates the insulin receptor, as well as a variety of exogenous substrates in vitro. In the present studies, we have identified an endogenous substrate for the insulin receptor-associated kinase. We studied insulin-stimulated protein phosphorylation in partially purified insulin receptor preparations from the livers of dexamethasone-treated rats. In this cell-free system, insulin stimulated the phosphorylation of its own receptor as well as of a phosphoprotein of apparent Mr = 120,000 (termed pp120). pp120 was not immunoprecipitated by three anti-receptor antisera, nor was the receptor immunoprecipitated by antisera raised against pp120, suggesting that pp120 is not antigenically related or tightly bound to the insulin receptor. Dose-response curves for receptor and pp120 phosphorylation stimulated by pork insulin were essentially identical, and showed the appropriate specificity (insulin much greater than proinsulin) for a receptor-mediated event. Phosphoamino acid analysis revealed that insulin stimulated the incorporation of 32P predominantly into tyrosine residues of pp120. Casein, an artificial substrate for the insulin receptor kinase, competed with pp120 for insulin-stimulated phosphorylation. Phosphorylation of pp120 was rapid (half-maximal effect within 2 min at 24 degrees C) and, like receptor phosphorylation, was supported with Mn2+ or Mg2+ as divalent cation and ATP as the phosphate donor. While receptor autophosphorylation and artificial substrate phosphorylation were not altered by prior treatment of the rats with dexamethasone, insulin-stimulated pp120 phosphorylation was enhanced in preparations derived from dexamethasone-treated rats, suggesting an alteration of pp120, not the receptor, as a result of dexamethasone-treatment. Further studies of this newly identified endogenous substrate may help clarify the physiologic role of the insulin receptor-associated kinase.

Incubation of cultured human (IM-9) lymphocytes with glucocorticoids increases the number of insulin receptors on the cell surface. Recently it has been shown that this results from a 3-fold increase in the rate of proreceptor synthesis. In the case of homologous insulin receptor down-regulation, a moderate rise in proreceptor biosynthesis has also been demonstrated. To further delineate the mechanisms of insulin receptor regulation, we have measured insulin receptor mRNA levels in hydrocortisone-treated and insulin-treated IM-9 lymphocytes. An increase in insulin receptor mRNA could be detected after 2 h of incubation with hydrocortisone and a plateau was reached by 4-6 h. The response was dose-dependent, being detectable with 50 nM hydrocortisone and reaching a maximal 3.7-fold increase at 200 nM. Actinomycin D completely suppressed the effect, whereas cycloheximide inhibited the effect by no more than 50%. These findings were extended by performing in vitro nuclear transcription assays which revealed that the 3-4-fold increase in insulin receptor mRNA could be attributed to increases in transcription. In contrast, homologous down-regulation was not associated with any change in total insulin receptor mRNA levels. The present study demonstrates that hydrocortisone, but not insulin, stimulates insulin receptor biosynthesis by increasing the rate of transcription and that de novo protein synthesis is probably required for a maximal effect.

The interaction between insulin and its receptor was investigated using both monoclonal and polyclonal anti-insulin antibodies. After covalent cross-linking of 125I-insulin to the insulin receptor on cultured human lymphocytes (IM-9 cells) using disuccinimidyl su berate, we inquired whether the insulin-receptor complex could be immunoprecipitated with anti-insulin antibodies. While a polyclonal guinea pig anti-insulin anti-serum succeeded in immunoprecipitating receptor-bound 125I-insulin, binding to the receptor decreased the avidity of the antiserum for the insulin moiety by a factor of approximately 1000-fold. Sixteen distinct monoclonal murine anti-insulin antibodies were employed to immunoprecipitate receptor-bound 125I-insulin. Of these 16 monoclonal antibodies, only one (antibody 5.9F4) could be shown to recognize receptor-bound 125I-insulin. Moreover, even with antibody 5.9F4, binding of 125I-insulin to its receptor reduced the affinity of the antibody by a factor of 10- to 100-fold. These data strongly suggest that, when insulin binds to its receptor, the majority of the insulin molecule is unavailable for binding by anti-insulin antibodies. It seems likely that the hormone binding site on the receptor may be very large, thereby allowing for sequestration of the majority of the insulin molecule with relatively little of the hormone remaining exposed.

A patient with extreme insulin resistance (leprechaun/Ark-1) had an 80-90% decrease in the number of insulin receptors on her circulating monocytes. In contrast, while a normal number of insulin receptors was expressed on the surface of Epstein-Barr (EB) virus-transformed lymphocytes from the patient, the receptors had decreased sensitivity to changes in temperature and pH. The father, who had a moderate degree of insulin resistance, resembled the patient in that his monocytes had a 60-80% decrease in the number of insulin receptors. Binding to the father's EB virus-transformed lymphocytes was normal. The mother was normally sensitive to insulin and had a normal number of insulin receptors on her circulating monocytes. In contrast, insulin receptors on the mother's EB virus-transformed lymphocytes were qualitatively abnormal, closely resembling the daughter's cultured cells. These observations suggest that each parent has transmitted a different genetic defect to the patient. When both mutations coexist in the same individual, they fail to complement, but, rather, result in extreme insulin resistance.

The immunoglobulin fraction of a polyclonal anti-insulin receptor antibody (B-10) derived from a patient with severe insulin resistance and acanthosis nigricans was tested for its ability to activate the protein kinase activity of the insulin receptor and to mimic insulin action in Chinese hamster ovary cells expressing either wild type or kinase-deficient human insulin receptors. This antiserum had previously been reported to be insulinmimetic without activating the insulin receptor protein tyrosine kinase. Antibody B-10 bound to both wild type and mutant human insulin receptors, but it induced receptor down-regulation and stimulated hexose transport and thymidine incorporation into DNA only in cells expressing the wild type receptor. Furthermore, this antibody activated the kinase activity of the wild type insulin receptor in intact cells and in vitro. It is likely, therefore, that the biological activities of antibody B-10, like those of insulin, depend upon the protein tyrosine kinase activity of the insulin receptor.

Insulin binds to a receptor on the cell surface, thereby triggering a biological response within the target cell. Mutations in the insulin receptor gene can render the cell resistant to the biological action of insulin. We have studied a family in which two sisters have a genetic form of insulin-resistant diabetes mellitus. The technique of homozygosity mapping has been used to demonstrate that the mutation causing diabetes in this consanguineous family is genetically linked to the insulin receptor gene. The two insulin-resistant sisters are homozygous for a mutation encoding substitution of valine for phenylalanine at position 382 in the alpha-subunit of the insulin receptor. Transfection of mutant insulin receptor cDNA into NIH3T3 cells demonstrated that the Val382 mutation impaired post-translational processing and retarded transport of the insulin receptor to the plasma membrane. Thus, the mutation causes insulin resistance by decreasing the number of insulin receptors on the surface of the patients' cells.

Insulin stimulates a kinase that phosphorylates tyrosines in the insulin receptor; this kinase is tightly associated with the insulin receptor itself. We now show that the insulin-stimulated casein kinase, present in solubilized, lectin-purified receptor preparations from rat liver, is indistinguishable from the insulin receptor kinase. As with phosphorylation of the insulin receptor, insulin selectively enhanced by 2-3-fold the phosphorylation of tyrosines in casein. The insulin-stimulated activities of both kinases were inactivated at 37 degrees C with the same t0.5 of 5 min and were identically affected by alkylating agents. Both receptor and casein kinase activities were specifically coprecipitated by anti-receptor antibodies or by insulin and anti-insulin antibodies. When the latter type of immune complexes were incubated with an excess of insulin, both kinase activities were quantitatively recovered. We therefore conclude that insulin-stimulated receptor and casein phosphorylations are probably catalyzed by a single enzyme which is tightly associated with the receptor itself. Now, by replacing casein for receptor as substrate, it is possible to measure the enzymatic activity of this receptor-related kinase itself, i.e. independent of the receptor as substrate. Detection of this activity is improved in the presence of certain alkylating agents. Use of artificial substrates (in combination with alkylating agents) is particularly important to dissect the functional components of the receptor complex, to study mechanisms of enzyme regulation and especially in situations where the available receptor for study is limited, e.g. fresh or cultured cells from patients.

Cultured lymphocytes transformed by Epstein--Barr virus were employed to study insulin receptors from a patient with extreme insulin resistance associated with the syndrome of leprechaunism. With cultured lymphocytes from normal subjects, insulin binding to its receptor is exquisitely sensitive to changes in temperature and pH. In cells from normal subjects, insulin binding was increased by approximately 250% as the temperature was decreased from 37 degrees C to 12 degrees C. In contrast, with cells from the leprechaun, insulin binding was only approximately 30% higher at 12 degrees C than at 37 degrees C. Similarly, insulin binding to cells from the leprechaun was markedly less sensitive to changes in pH, as compared to cells from normal subjects. Binding studies suggested that the number of insulin receptors per cell was within the normal range in this patient. Despite the unusual characteristics of insulin binding in cells from this insulin-resistant patient, the receptors were typical in at least two respects: (i) binding was inhibited normally by antibodies to the receptor; and (ii) the specificity for insulin analogs was normal (chicken insulin greater than porcine insulin much greater than guinea pig insulin greater than porcine proinsulin). This patient has an inborn error affecting insulin receptor function. The receptor's binding function was abnormal in having decreased sensitivity to alterations in temperature and pH. However, the level of insulin binding to cells from the leprechaun was within normal limits. Consequently, the hormonal resistance probably results from a decreased ability of the receptor to couple insulin binding to insulin action.

Summary Using lectin affinity-purified receptor preparations from human hepatoma cells, insulin (10 −7 M) specifically stimulated phosphorylation of the 95,000 dalton (beta) subunit of its own receptor. Phospho-amino acid analysis of the receptor subunit revealed that insulin increased at least 2.5-fold the content of phosphoserine and of phosphotyrosine. In intact cells, the major effect of insulin is to increase the phosphoserine content of its receptor. These findings are the first demonstration of an insulin-stimulated serine kinase in a cell-free system.

[125I]Insulin binding has been studied with cultured cells from an insulin-resistant patient with the Rabson-Mendenhall syndrome. Previously, this patient has been demonstrated to have decreased insulin binding to circulating blood cells. Similarly, [125I]insulin binding to cultured lymphocytes and cultured fibroblasts was markedly decreased. Whereas the decrease in [125I]insulin binding to cultured lymphocytes appeared to result from a decrease in receptor number in cultured lymphocytes, there appeared to be a reduction in the affinity of [125I]insulin binding to cultured fibroblasts. This discrepancy between the observations with the two cell types is a general phenomenon which has been described in patients with a variety of syndromes of extreme insulin resistance. It is possible that the interpretation of the studies with the fibroblasts is complicated by the confounding influence of receptors for insulin-like growth factors on the surface of those cells. If the number of insulin receptors is sufficiently reduced on fibroblasts from the patient with extreme insulin resistance, then the low level of residual [125I]insulin binding may involve primarily receptors for insulin-like growth factors, which bind insulin with relatively low affinity.

Summary The binding of insulin to its receptor has been studied under various physiological and pathological conditions. Quantitative studies have involved human circulating cells such as monocytes and erythrocytes, adipocytes, placental cells, and cultured cells such as fibroblasts and transformed lymphocytes. In animals, other target tissues such as liver and muscle have been studied and correlated with the human studies. Various physiological conditions such as diurnal rhythm, diet, age, exercise and the menstrual cycle affect insulin binding; in addition, many drugs perturb the receptor interaction. Disease affecting the insulin receptor can be divided into five general categories: (1) Receptor regulation — this involves diseases characterized by hyper- or hypoinsulinaemia. Hyperinsulinaemia in the basal state usually leads to receptor ‘down’ regulation as seen in obesity, type II diabetes, acromegaly and islet cell tumours. Hypoinsulinaemia such as seen in anorexia nervosa or type I diabetes may lead to elevated binding. (2) Anti-receptor antibodies — these immunoglobulins bind to the receptor and competitively inhibit insulin binding. They may act as agonists, antagonists or partial agonists. (3) Genetic diseases which produce fixed alterations in both freshly isolated and cultured cells. (4) Diseases of receptor specificity where insulin may bind with different affinity to its own receptor or related receptors such as receptors for insulin-like growth factors. (5) Disease of affinity modulation where physical factors such as pH, temperature, ions, etc. may modify binding. In this review, we have considered primarily abnormality in insulin receptor binding. There are numerous other functions of the receptor such as coupling and transmission of the biological signal. These mechanisms are frequently referred to as postreceptor events, but more properly should be referred to as postbinding events since the receptor subserves other functions in addition to recognition and binding of insulin.

The receptors for insulin and epidermal growth factor possess tyrosine-specific protein kinase activity which may play a role in mediating the biological actions of these two peptides. We have identified a 120 kDa glycoprotein (pp120) in rat liver plasma membranes which can be phosphorylated by the insulin receptor in a cell-free system and in intact cultured hepatoma cells. In the present report, we have demonstrated in a cell-free system that solubilized epidermal growth factor receptors can phosphorylate tyrosine residues inpp120.

Previously, we have identified novel abnormalities in insulin binding to cultured lymphocytes from a patient (leprechaun/Ark-1) with a genetic form of extreme insulin resistance. Insulin receptors from this patient are characterized by abnormally high affinity for insulin as well as markedly decreased sensitivity to alterations in pH and temperature. In the present report, we describe further studies investigating the mechanism of the binding abnormalities in this patient. The binding abnormalities persist after solubilization of insulin receptors from lymphocyte plasma membranes. The apparent binding affinity of the insulin receptor from leprechaun/ Ark-1 is 2- to 3-fold higher than observed with receptors from normal subjects. In addition, the solubilized insulin receptor from leprechaun/Ark-1 is markedly less sensitive to alterations in the pH. In further studies, insulin receptors were partially purified by affinity chromatography over wheat germ agglutinin-agarose. With normal insulin receptors, lect...

Humoral autoimmunity plays an important role in the pathogenesis of two forms of hypoglycemia. In one syndrome, antireceptor autoantibodies bind to the insulin receptor, mimic insulin action, and cause fasting hypoglycemia. In most patients with autoantibodies to the insulin receptor, there is other evidence of autoimmune disease as well. Interpretation of the standard tests used in evaluation of hypoglycemia may be confusing in these patients. For example, antireceptor antibodies may inhibit insulin binding, thereby inhibiting insulin clearance and elevating levels of plasma insulin. Nevertheless, because hypoglycemia suppresses beta-cell secretion, C-peptide levels are usually low. This constellation of data is consistent with surreptitious insulin injection. The most important laboratory test in the differential diagnosis is a direct assay for the presence of antibodies directed against the insulin receptor. Therapy with prednisone appears to alleviate the hypoglycemia rapidly, usually within 24 hours. This effect of prednisone appears to result from antagonism of the effects of antireceptor antibodies without actually lowering their titer. The natural history of this syndrome is that the antireceptor antibodies disappear and the syndrome resolves over a time course of several months to several years. In North America, the presence of anti-insulin antibodies in a hypoglycemic patient most commonly suggests that the patient has been immunized with exogenous insulin. However, some patients--especially in Japan--develop spontaneous autoantibodies directed against insulin. These antibodies can cause hypoglycemia, which is generally reactive in that it occurs several hours after a meal or a glucose challenge rather than in a fasting state. The most effective therapy is frequent small feedings and avoidance of large meals.

Simultaneous measurements were made of lipogenesis and pyruvate dehydrogenase activity in segments of rat epididymal adipose tissue incubated with saturating amounts of [U- 14 C]glucose and insulin. Glucose was converted to fatty acids at a rate only 64–79% of that permitted by the tissue's content of the active form of pyruvate dehydrogenase (PDH a ). Addition of either of the electron acceptors, phenazine methosulfate (10 μ m ) or N,N,N′,N′ -tetramethyl- p -phenylenediamine (50 μ m ), increased lipogenesis until it equalled the PDH a activity of the tissue. Pyruvate release was increased 2-fold or more by the electron acceptors, suggesting that the increase in lipogenesis might have resulted from an increase in the intracellular pyruvate levels such that PDH a became saturated with substrate. Higher levels of the electron acceptors decreased PDH a activity, and reduced lipogenesis correspondingly. The data suggest that the maximal rate of lipogenesis in the presence of glucose and insulin is limited by the inability of the tissue to elevate pyruvate levels sufficiently to saturate PDH a . Although glycerol release was increased by either electron acceptor and insulin partially overcame this effect, the effects of the electron acceptors on PDH a activity could not be attributed to an increase in lipolysis.

The β-subunit of the insulin receptor possesses a tyrosine-specific protein kinase activity which may play a role in coupling insulin binding to insulin action. Previously, we have identified a substrate for the receptor-associated protein kinase in a cell-free system. This endogenous substrate (ppl20), which appeared to be a glycoprotein with an apparent mol wt of 120,000, was detected in rat liver microsomes. In the present work, we have demonstrated that ppl20 is localized to a highly purified preparation of rat liver plasma membranes (Neville preparation). Moreover, ppl20 appears to be specific to liver, having been detected in liver from rat, monkey, and rabbit, but not in rat brain, skeletal muscle, heart, kidney, or adipocytes. As a preliminary to addressing the question of whether insulin stimulates phosphorylation of ppl20 in intact cells, we have sought to identify tissue culture cell lines that contain both insulin receptors and ppl20. We have succeeded in identifying ppl20 in two cell lines de...

Although many studies have investigated the complex interrelationships between thyroid hormone levels and insulin-glucose secretion and action, there is at present a lack of information concerning the effects of various thyroid hormone levels on insulin receptors. In the present study, rat liver membranes were prepared from control, hyperthyroid [10 micrograms L-T4 (T4)/100 g BW for 14 days], and hypothyroid rats and the binding characteristics of [125I]iodoinsulin to these membranes were studied. In addition, serum T3, T4, glucose, and insulin levels were measured. The mean (+/- SD) serum T4 and T3 levels were higher (P less than 0.05) in the T4-injected rats than in the hypothyroid rats (T4, 9.1 +/- 0.6 vs. 1.7 +/- 0.4 microgram/dl; T3, 144 +/- 18 vs. 30 +/- 16 ng/dl). When compared to controls, glucose levels were higher in the hypothyroid rats (116 +/- 11 vs. 141 +/- 14 mg/dl; P less than 0.05) and were statistically unaltered in thyrotoxicosis. Plasma insulin levels were increased in hypothyroidism (6.5 +/- 0.8 vs. 10.8 +/- 2.9 microU/ml; P less than 0.05) and decreased in thyrotoxicosis (7.5 +/- 0.5 vs. 28.3 +/- 15 microU/ml; P less than 0.05). Despite these alterations, membrane binding and the derived Scatchard plots were not significantly different in the hyperthyroid or hypothyroid groups. In summary, the present studies indicate that plasma membrane insulin receptor number and affinity are unaltered regardless of the thyroid state of the rat.

Incubation of rat epididymal adipose tissue fragments with insulin led to increases of up to 2.4-fold in the activity of pyruvate dehydrogenase subsequently assayed in tissue homogenates. The activation of pyruvate dehydrogenase by insulin was greatest when tissue was incubated in the presence of bicarbonate ions and when 2 mg of glucose or fructose per ml was added to the incubation medium. Several other agents known to inhibit lipolysis and to decrease cyclic AMP levels in fat cells, including niacin, 5-methylpyrazole-3-carboxylic acid, and prostaglandin E1 were also effective in activating pyruvate dehydrogenase. Severe depletion of tissue ATP levels caused by the addition of oligomycin or dinitrophenol, by anaerobic incubation, or by prolonged incubation with epinephrine in the absence of albumin also activated pyruvate dehydrogenase. Incubation of adipose tissue with 1 mm oleate, 1.5 mm octanoate, 1.5 mm heptanoate, 1 mm butyrate, or 3 mm dl-β-hydroxybutyrate decreased pyruvate dehydrogenase activity 20 to 70%. The addition of 5 mm acetate, 5 mm propionate, or 3 mm 4-pentenoate led to 70 to 160% activation of pyruvate dehydrogenase, whereas 1.5 mm pentanoate had no effect. High concentrations of glucose (20 mg per ml) or of pyruvate-lactate (5 and 30 mm, respectively) also increased tissue pyruvate dehydrogenase activity.

Specifically (tritium) labeled precursors (VIII, X, XIV, XV, and XVI), upon feeding to Penicillium rubrum , are incorporated into ergosta-4,6,8(14),22-tetraen-3-one (IV) to the extent of 14.2, 4.5, 11.4, 16.3, and 5.5% respectively. Proof that the ergostane skeleton was incorporated intact was afforded by a chemical-biosynthetic cycle, the latter stages of which entailed reduction of isolated (IV) to ergosterone (VIII), followed by removal of the label through base-catalyzed exchange. A search of the growth medium of P. rubrum revealed the presence of nonartefactual ergosterol epidioxide (XIII) and ergosta-6,22-dien-3β,5α,8α-triol (XVIII). The incorporation data are consistent with a set of multiple pathways with no unique biosynthetic sequence apparent.

Mutations of the extracellular domain of the insulin receptor impair processing and transport of receptors to the plasma membrane. We have previously reported that a mutation substituting Val for Phe382 in the α-subunit of the insulin receptor impairs intracellular processing and insulin-induced autophosphorylation of the mutant receptor. In this investigation, we have generated two independent mutations of amino acids Phe381 and Phe382 of the insulin receptor: Val for Phe381 and Leu for Phe382. These substitutions cause a slight impairment of intracellular processing and transport of the mutant receptors. Furthermore, insulin-dependent internalization of the mutant receptors is unaffected by these mutations. Thus, of the three substitutions studied to date, Val for Phe382 is the only mutation of the Phe381-Phe382 sequence that causes a major defect in posttranslational processing of the receptor.

There is a natural temptation to extrapolate insights from these animal models to explain the pathophysiology of human diabetes. In some cases, the animal model closely resembles the human disease, as is the case with glucokinase knockout mice and MODY2. In contrast, insulin receptor knockout mice differ dramatically from patients with leprechaunism, the human disease resulting from absence of insulin receptors (9xElders, M., Schedewie, H., Olefsky, J., Givens, B., Char, F., Bier, D., Baldwin, D., Fiser, R., Seyedabadi, S., and Rubenstein, A. J. Natl. Med. Assoc. 1982; 74: 1195–1210PubMedSee all References, 20xTaylor, S.I. Diabetes. 1992; 41: 1473–1490Crossref | PubMedSee all References). While insulin receptor knockout mice develop a severe form of diabetes associated with marked hyperglycemia and ketoacidosis, patients with leprechaunism exhibit relatively mild hyperglycemia. Furthermore, intrauterine growth retardation, a key feature of the human disease, was not found in the knockout mouse. Why might the same mutation cause different phenotypes in different species? There may be differences in the genetic background; for example, parallel or redundant pathways may vary in importance in different species. Indeed, there can be striking differences in phenotype even when a specific mutation is introduced into different strains of mice. By mapping and ultimately cloning modifier genes that account for variations in phenotype, we will advance our understanding of polygenic diseases such as diabetes.The differences between insulin receptor knockout mice and the corresponding human disease invite the question: Are mice with tissue-specific insulin resistance good models for human disease?Insulin resistance in skeletal muscle. The phenotype of mice with insulin resistance at the level of muscle (16xMoller, D., Chang, P., Yaspelkis, B., Flier, J., Wallberg-Henriksson, H., and Ivy, J. Endocrinology. 1996; 137: 2397–2405PubMedSee all References, 6xBruning, J., Michael, M., Winnay, J., Hayashi, T., Horsch, D., Accili, D., Goodyear, L., and Kahn, C. Mol. Cell. 1998; 2: 559–569Abstract | Full Text | Full Text PDF | PubMedSee all References) closely resembles an early stage of human type 2 diabetes (DeFronzo 1997xDeFronzo, R. Diabetes Rev. 1997; 5: 177–269See all ReferencesDeFronzo 1997). Like “prediabetic” humans, these mice exhibit insulin resistance at the level of muscle, but maintain normal (or only minimally elevated) fasting plasma glucose levels. Unlike the animal models, the precise molecular cause of insulin resistance has not yet been elucidated in humans although it is known that there is a block at the level of glucose uptake/phosphorylation in muscle of patients with type 2 diabetes.Insulin resistance in pancreatic β cells. Because insulin resistance develops early in the natural history of type 2 diabetes, it has long been suspected that insulin resistance may play a causal role in the development of the defect in insulin secretion. Novel insights provided by βIRKO mice suggest a unified theory to explain the pathogenesis of type 2 diabetes. A genetic defect in the insulin action pathway may not only cause target cells to become insulin resistant, but also might directly cause β cell failure and insulin deficiency.However, Kulkarni et al. 1999xKulkarni, R., Bruning, J.C., Winnay, J., Postic, C., Magnuson, M., and Kahn, C. Cell. 1999; 96: 329–339Abstract | Full Text | Full Text PDF | PubMed | Scopus (770)See all ReferencesKulkarni et al. 1999 identified an important difference in phenotypes between βIRKO mice and human patients. Whereas insulin secretion is impaired in βIRKO mice, humans with mutations in the insulin receptor gene (e.g., patients with leprechaunism) exhibit striking hyperplasia of pancreatic islets and hypersecretion of insulin (Elders et al. 1982xElders, M., Schedewie, H., Olefsky, J., Givens, B., Char, F., Bier, D., Baldwin, D., Fiser, R., Seyedabadi, S., and Rubenstein, A. J. Natl. Med. Assoc. 1982; 74: 1195–1210PubMedSee all ReferencesElders et al. 1982). Furthermore, even in mice, a defect in insulin signaling does not necessarily cause deficiency in insulin secretion. Bruning et al. 1997xBruning, J.C., Winnay, J., Bonner-Weir, S., Taylor, S.I., Accili, D., and Kahn, C.R. Cell. 1997; 88: 561–572Abstract | Full Text | Full Text PDF | PubMed | Scopus (416)See all ReferencesBruning et al. 1997 engineered mice to become insulin resistant by introducing heterozygous null mutations into both the Insr gene and the Irs1 gene; these Insr+/−Irs1+/− mice developed striking hyperplasia of β cells. Interestingly, the phenotype of β cell hyperplasia observed in the Insr+/−Irs1+/− mice closely resembles the phenotype of patients with leprechaunism. Furthermore, Insr+/−Irs1+/− mice, like most patients with type 2 diabetes, exhibit generalized, partial insulin resistance rather than the total, tissue-specific insulin resistance of βIRKO mice. Thus, there appear to be complex interactions between insulin signaling and β cell function; depending upon the circumstances, insulin resistance may lead either to insulin deficiency (as in βIRKO mice) or to β cell hyperplasia plus hyperinsulinemia (as in humans with leprechaunism and Insr+/−Irs1+/− mice).Because type 2 diabetes is a complex heterogeneous disease with multiple genes contributing to the cause of this polygenic disorder, it is likely that these murine animal models will shed light on disease mechanisms that contribute to the pathophysiology of human diabetes. Defects in the insulin action pathway may be the cause of insulin deficiency in some patients, but there may also be patients in whom there are primary genetic defects that directly impair the machinery for insulin secretion. The approach of positional cloning, currently underway in several laboratories, holds great promise to resolve these controversies by directly identifying the genes that cause both insulin deficiency and insulin resistance.

Antibodies to the insulin receptor have provided important experimental probes of receptor structure and function. In the present study, we have characterized the insulin receptor on human lymphoblastoid cell lines using polyclonal and monoclonal anti-receptor antibodies and fluorescence flow cytometry. The cell lines were derived by Epstein-Barr virus transformation of peripheral mononuclear leucocytes from normal subjects or patients with disorders that affect the insulin receptor. Fluorescence analysis revealed a high level of specific fluorescence on lymphoid cell lines from normal individuals (mean peak fluorescence 30-50 units above the control) and was similar to the labelling of the spontaneously transformed lymphoblastoid cell line IM-9. Transformed cells from patients with syndromes of insulin resistance, such as the Rabson Mendenhall syndrome, leprechaunism and the type A syndrome of insulin resistance and acanthosis nigricans, exhibited little or no specific fluorescence. In all cases, there was an unimodal distribution of receptors on cells. In addition, there was a good correlation between specific binding of 125I-insulin and percentage peak fluorescence. The data indicate that fluorescence flow cytometry can be used to study the distribution of insulin receptor on different cell lines and to study cells derived from patients with disease states.

The family of tyrosine-specific protein kinases includes proteins encoded by retroviral oncogenes as well as receptors for insulin and several growth factors. Antibodies to pp60src, the protein encoded by the src oncogene of Rous sarcoma virus (RSV), can specifically immunoprecipitate affinity-labeled insulin receptors from cultured human lymphocytes (IM-9 cells). This precipitation is specifically inhibited by the src gene product purified from RSV-transformed rat cells. These observations provide evidence that there is structural homology between the insulin receptors and pp60src.

A 43-year-old woman with spontaneous episodes of neuroglycopenic hypoglycemia was found to have immune-mediated thrombocytopenic purpura and primary biliary cirrhosis. Hypoglycemia along with hyperinsulinemia suggested insulinoma. Serum c-peptide levels were disproportionately low, raising the possibility of factitious hypoglycemia. The patient's plasma contained circulating insulin receptor autoantibodies, thought to cause hypoglycemia by their insulin-like actions. With prednisone therapy, her other autoimmune features improved, and the hypoglycemia eventually resolved. Hypoglycemia mediated by insulin receptor autoantibodies should be considered in patients with fasting hypoglycemia and features suggesting an underlying autoimmune disorder before pursuing more invasive procedures. High-dose steroids may be life-saving in this disorder.

Solubilized lectin-purified extracts from human monocyte-like cells (U-937) and freshly isolated human mononuclear cells preincubated in the presence of phorbol 12-myristate 13-acetate (PMA) stimulated phosphorylation of synthetic tyrosine-containing polymers and of casein. Tyrosine phosphorylation was confirmed by phospho amino acid analysis. PMA stimulated phosphorylation of exogenous substrates in a time- and concentration-dependent manner. This phosphorylation reaction did not require addition of phospholipid, diolein, or calcium. Biologically inactive phorbol compounds did not stimulate phosphorylation in this system. In addition, PMA enhanced phosphorylation of a Mr approximately equal to 140,000 protein as well as several other endogenous proteins in the U-937 extracts. PMA treatment stimulated predominantly phosphorylation on tyrosine residues of the Mr 140,000 protein. Tyrosine phosphorylation, typical of growth-promoting peptides such as insulin or epidermal growth factor, is believed to play a role in regulating normal and disordered cellular growth and proliferation. The demonstration of PMA-stimulated tyrosine phosphorylation might provide a clue to the mechanism of cellular differentiation and proliferation induced by the tumor promoter.

Insulin-stimulated phosphorylation of the insulin receptor was studied in cultured B-lymphocytes transformed by Epstein-Barr virus. In studies with cell lines derived from six normal subjects, insulin (10(-7) M) caused an average increase of approximately 200% in 32P incorporation into the 95K subunit of the insulin receptor. Phosphorylation was rapid (detectable within 1-2 min) and reached a maximum level by 15 min. Dose-response curves for receptor occupancy and phosphorylation were nearly superimposable, indicating few or absent spare receptors for this response to insulin. These data suggest that insulin receptor phosphorylation is an early response to insulin in cultured lymphocytes transformed with Epstein-Barr virus. We studied insulin receptor phosphorylation in cell lines derived from nine patients with clinical syndromes associated with extreme insulin resistance, all of whom had normal [125I] insulin binding. While the magnitude of insulin's stimulation varied widely among the individual cell lines, no significant differences were found between cell lines from normal subjects and those from patients with extreme insulin resistance.

In some patients with genetic forms of extreme insulin resistance, the cause of insulin resistance is a marked (greater than or equal to 90%) reduction in the number of insulin receptors on the cell surface. In the present work, we describe studies of insulin receptor biosynthesis in Epstein-Barr virus (EBV)-transformed lymphocytes from three patients (A-1, A-5, and A-8) with type A extreme insulin resistance. Insulin receptors are composed of two major glycoprotein subunits (apparent molecular weight [Mr] of 135 and 95 kD), which are both derived from a common precursor molecule with Mr of 190 kD. In one patient (A-1), there was a marked reduction in the biosynthesis of both the 190-kD precursor and the mature receptor. Thus, in this patient, the defect appears to occur early in the biosynthetic pathway (i.e., before the synthesis of the 190-kD precursor). In contrast, in two sisters (A-5 and A-8) with type A extreme insulin resistance, biosynthesis of the 190-kD precursor proceeds at a normal rate. However, there appears to be a defect subsequent to the biosynthesis of the 190-kD precursor, but before the insertion of the mature receptor in the plasma membrane. Our observations suggest the existence of at least two distinct types of biosynthetic defects which may give rise to a marked reduction in the number of insulin receptors on the cell surface. In addition, for comparison, we have studied receptor biosynthesis in cultured EBV lymphocytes from a fourth patient (A-7) with type A extreme insulin resistance. Although the cells of patient A-7 have a normal number of insulin receptors, we have detected subtle abnormalities in the posttranslational processing of the insulin receptor precursor, which may be a biochemical marker for a postbinding defect that causes insulin resistance in this patient.

Defects in insulin receptor function can impair the response of target cells to insulin. Previously we have described an insulin resistant patient (leprechaun/Ark-1) with qualitative abnormalities in insulin binding suggestive of a structural defect in her insulin receptors. In the present work, we have studied the tyrosine kinase activity associated with insulin receptors from cultured Epstein-Barr virus-transformed lymphocytes. In studies of insulin receptors from leprechaun/Ark-1, we observed that both the magnitude and the dose-dependency of insulin's effect to stimulate the tyrosine kinase activity were normal. This suggests that the defect causing this patient's insulin resistance is independent of the receptor-associated tyrosine kinase. In the course of these studies, we noted that an anti-receptor antiserum (B-d) had a markedly decreased ability to immunoprecipitate insulin receptors from leprechaun/Ark-1. This observation further supports our previous conclusion that the insulin receptor from leprechaun/Ark-1 is abnormal in structure. Moreover, it emphasizes the importance of choosing anti-receptor antisera which are equally effective at immunoprecipitating receptors from both patients and normal subjects when the anti-receptor antisera are employed as reagents in investigations of receptors from insulin-resistant patients.

Protein phosphorylation is an important mechanism of regulation in many metabolic pathways.In muscle glycogen metabolism,the enzymes involved in anabolic steps are activated by a dephosphorylation,the enzymes involved in catabolic steps are activated by a phosphorylation.

Autoantibodies directed against the insulin receptor mimicked the effect of insulin to down-regulate insulin receptors in IM-9 cells. This down-regulation occurred at 37 C, but not at 15 or 22 C. As with insulin itself, antireceptor antibodies caused down-regulation by accelerating the rate of receptor degradation. Down-regulation induced by antireceptor antibody may play a role in desensitizing target cells to the effects of insulin.

Hirsutism, polycystic ovaries, and elevated levels of plasma testosterone are characteristic clinical features in women with extreme insulin resistance and acanthosis nigricans. Extreme insulin resistance resulting from autoantibodies to the insulin receptor (type B extreme insulin resistance) had been considered an exception to this generalization. A woman with type B extreme insulin resistance developed clinical evidence of masculinization in association with a markedly elevated level of plasma testosterone (1000 ng/dL). In nine women with autoantibodies to the insulin receptor, excessive ovarian production of testosterone was a common feature among the premenopausal patients. Postmenopausal patients rarely developed elevated levels of plasma testosterone, presumably as a result of ovarian failure. Overproduction of testosterone may result from a direct effect of hyperinsulinemia on the ovary.

Previous studies of the insulin receptor in disease states have utilized primarily techniques of equilibrium binding and, to a limited extent structural, analysis. Though techniques have been developed to study receptor degradation in normal cells, they have not been applied to disease states. In the present study we have examined insulin receptor degradation rate in B lymphocytes that were obtained from peripheral blood of normal subjects and patients with several syndromes of extreme insulin resistance. B lymphocytes were established in culture from each patient's peripheral cells by transformation with Epstein-Barr virus. The insulin receptors were surface labeled using Na125I/lactoperoxidase and the cells were returned to incubate in growth media. After varying periods of incubation, aliquots of cells were solubilized and the cell content of labeled receptor subunits were measured by immunoprecipitation with anti-receptor antibodies and NaDodSO4/polyacrylamide gel electrophoresis. The fall in 125I-insulin receptor content approximated a single exponential and was quantitated as receptor subunit half-life (t1/2). In cell lines from four patients in whom the number of insulin receptors was reduced by greater than 90%, the rate of receptor loss was greater than normal (t1/2 equals 3.8 +/- 0.9 h vs. 6.5 +/- 1.2 h; mean +/- SD, P less than 0.01). However, a similar acceleration in receptor degradation was seen in cells from five patients with extreme insulin resistance but low-normal insulin receptor concentration (t1/2 equals 4.4 +/- 0.9 h). This group included cells from one patient with a qualitatively abnormal receptor. Thus, all the patients with genetic syndromes of insulin resistance had accelerated receptor degradation, regardless of their receptor concentration. By contrast, insulin receptors on cultured lymphocytes that were obtained from patients with extreme insulin resistance secondary to autoantibodies to the insulin receptor had normal receptor degradation (t1/2 equals 6.1 +/- 1.9 h). We conclude that (a) accelerated insulin receptor degradation is an additional feature of cells from patients with genetic forms of insulin resistance; (b) that accelerated insulin receptor degradation may explain the low-normal receptor concentrations that were seen in some patients with extreme insulin resistance; and (c) that accelerated degradation does not explain the decreased receptor concentration in patients with very low insulin receptor binding and, therefore, by inference, a defect in receptor synthesis must be present in this subgroup.

Antibodies to the insulin receptor are insulinomimetic in vitro, although they generally induce insulin resistance in vivo. We report the novel case of a patient who presented with fasting hypoglycemia as the sole manifestation of autoantibodies to the insulin receptor. Prednisone therapy (120 mg per day) produced a rise in fasting glucose to more than 100 mg per deciliter (6 mmol per liter) within 48 hours, although there was no detectable change in the titer of antireceptor antibodies. After 10 weeks of therapy, the titer of antireceptor antibodies had fallen approximately 100-fold, and prednisone could be discontinued without recurrence of hypoglycemia. This case demonstrates that antireceptor antibodies must be considered in the differential diagnosis of hypoglycemia, especially in patients with other manifestations of autoimmunity.

Levels of fasting plasma insulin are generally inversely correlated with125I-insulin binding to circulating blood cells. In disease states associated with hyperinsulinemia (e.g., obesity and non-insulin-dependent diabetes mellitus), 125I-insulin binding is usually low. In contrast, 125I-insulin binding to circulating cells may be normal in patients with certain forms of extreme insulin resistance despite marked hyperinsulinemia. To explain this paradox, it has been proposed that postbinding defects in insulin action may give rise to defects in downregulation. We have employed cultured Epstein-Barr virus (EBV)-transformed lymphocytes from eight patients with extreme insulin resistance to address the question of whether there is a defect in the downregulation process in vitro. In this cell type, insulin leads to a decrease in the number of insulin receptors on the cell surface by accelerating the rate of degradation of insulin receptors. We could not detect any abnormality in in vitro downregulation with cultured EBV-transformed lymphocytes from insulin-resistant patients. The apparent discrepancy between the in vivo and in vitro studies raises the possibility that some factor in the patient's internal milieu may prevent insulin-induced downregulation. An alternative possible explanation might be that the mechanism of downregulation in vitro differs from the mechanism whereby receptor number is regulated in vivo in insulin's target cells.

The responsiveness of target cells to insulin is carefully regulated under many physiological conditions (Grunberger et al., 1983; Olefsky, 1981). Thus, two mechanisms work in concert to regulate the biological effect of insulin upon target cells: (1) changes in circulating levels of insulin in plasma; and (2) alterations in the sensitivity of the target cell to insulin. Abnormalities in the regulation of insulin sensitivity contribute importantly to the pathophysiology of many disease states: e.g., obesity, type II diabetes (non-insulin-dependent diabetes mellitus), acromegaly, and Cushing’s syndrome.

Binding of insulin to its receptor followed by covalent cross-linking with disuccinimidyl suberate (DSS) dramatically impairs the ability of antiinsulin antibodies (both polyclonal and monoclonal) to bind to the insulin moiety. We have used dithiotheitol, which has major effects on the oligomeric structure of the insulin receptor, to determine if this decreased antibody recognition is due to alteration in the conformation of insulin itself or to steric factors. Treatment of the covalently cross-linked insulin-receptor complex with dithiothreitol (DTT) increased the ability of the polyclonal and two monoclonal antiinsulin antibodies to immunoprecipitate the insulin-receptor complex. This treatment decreased immunoprecipitation by antireceptor antibodies. The effect of DTT may have been due to a reversal of either a binding-induced conformational change in the insulin moiety or an alteration in the conformation of the insulin-receptor complex, thereby decreasing steric hindrance. In an effort to choose between these two possible explanations, we prepared a biotinylated derivative of insulin which was cross-linked to the receptor. Since the biotin moiety is relatively rigid, it seemed improbable that binding to the receptor would alter the conformation of the epitope recognized by antibiotin antibodies and that the change would be reversed by DTT. Treatment of the cross-linked biotin-insulin-receptor complex with DTT did increase the ability of both antiinsulin and antibiotin antibodies to immunoprecipitate the cross-linked receptor complex. Identification of the cross-linked receptor on reduced sodium dodecyl sulfate-polyacrylamide gels confirms that the DTT treatment alters the distribution of the oligomeric forms of the receptor. These studies favor the hypothesis that when bound to its receptor, most of the insulin molecule is sequestered within the receptor-binding site such that there is steric hindrance to the approach of antiinsulin antibodies. Moreover, DTT alters the conformation of the cross-linked insulin-receptor complex so as to decrease this steric hindrance.

The present work discloses a procedure for preparation of human liver plasma membranes containing catalytically competent insulin receptor kinase. In addition to insulin promoted phosphorylation of the β-subunit of insulin receptor kinase, insulin promoted phosphorylation of pp120 and two other new proteins was demonstrated. The new proteins with molecular weights of 50,000 and 120,000 do not bind to WGA, pp120 antibody or insulin receptor antibody, but bind to the antiphosphotyrosyl antibody. The identity and physiological significance of these putative substrates for insulin receptor kinase remains to be established.

Binding of 1251-labeled hormones to receptors can usually be inhibited by addition of unlabeled hormone. In analyzing such binding-inhibition data, it is commonly assumed that both labeled and unlabeled hormones are bound with equal affinity. When this assumption is made incorrectly, an artifactually nonlinear Scatchard plot results. Equations to describe these nonlinear Scatchard plots are derived. These results are discussed with regard to previously published observations of nonlinear Scatchard plots for binding of insulin to its receptor.

Publisher Summary This chapter describes the three principal assay techniques that are employed to detect antibodies to the insulin receptor. Similar approaches may be applied to the detection of antibodies directed against other cell surface receptors. Antibodies directed against cell surface receptors for hormones and neurotransmitters play an important role in the pathogenesis of human disease. In some disease states, the interaction between the antibody and receptor may lead to biological effects identical to those of the natural ligand. In other conditions, anti-receptor antibodies may antagonize the bioactivity of the natural ligand. In addition to their role in pathogenesis of human disease, anti-receptor antibodies play a central role as reagents in studies of receptor structure and function. Identification of anti-receptor antibody requires demonstration that (1) the antibody binds to a receptor and (2) the same receptor binds to the ligand. These two properties can be demonstrated using several different assay techniques. No single assay method is perfect. Rather the different approaches complement one another in definitive identification of antireceptor antibodies.

The syndrome of type A insulin resistance in nonobese women is characterized by hyperinsulinemia, resistance to exogenous insulin, acanthosis nigricans, polycystic ovaries, and masculinization. Insulin binding to intact circulating monocytes and cultured Epstein-Barr virus-transformed B-lymphocytes derived from these patients is decreased in some patients but normal in others. Insulin receptors consist of two subunits; the alpha-subunit contains the insulin-binding site, and the beta-subunit possesses an insulin-sensitive tyrosine-specific protein kinase activity. Insulin binding to circulating monocytes was decreased in five patients, suggesting a decreased number of alpha-subunits on the surface of cells from the patients with type A insulin resistance. In the present work, we demonstrated that there is a proportional decrease in the function of the beta-subunit (i.e. tyrosine kinase activity) in cells from these subjects. In one patient, insulin binding to circulating monocytes was normal, and the insulin-stimulated tyrosine kinase activity of the receptors was normal as well. In separate studies, using cultured Epstein-Barr virus-transformed lymphocytes from the same six patients with type A extreme insulin resistance, the results were similar, in that the functions of the alpha- and beta-subunits of the receptor from these cells correlated. Though heterogeneity among the six patients with type A extreme insulin resistance at the level of the kinase activity of their insulin receptors was demonstrated, it does not appear that a selective defect in beta-subunit phosphorylation per se can be implicated in the mechanisms of insulin resistance of these patients. These findings are distinct from our previously reported patient with normal binding and very low insulin-stimulated phosphorylation of the beta-subunit of the receptor of circulating monocytes, in whom it was speculated that selective reduction in beta-subunit phosphorylation was responsible for insulin resistance.

The development of the FRTL and FRTL-5 thyroid cells (Fig. 1A) by F.S. Ambesi-Impiombato and his colleagues (1,2) moved research in the thyroid area from a concern with the thyrotropin receptor to an examination of receptor interactions or cross-talk. These cells require TSH for their growth (Fig. 1B). However, the TSH action requires the presence of insulin and serum factors, one of which we now know to be IGF-I (Fig. 1B); no single factor, TSH, insulin, or IGF-I, can reproduce the mixture of all. Further it is evident that the insulin and IGF-I actions are not identical; thus when concentrations of insulin as high as 10 micrograms per ml are present, the addition of only 50 or 100 ng/ml of IGF-I is still significantly additive to TSH and insulin in stimulating an increase in cell number.

The polymerase chain reaction catalyzed by Taq DNA polymerase has been used for site-specific mutagenesis. The amplification was primed by two oligodeoxyribonucleotides complementary to insulin receptor cDNA. To direct the synthesis of mutant DNA, mismatches were introduced into one of the primers. Six different mutations were constructed by this technique. Of twelve clones whose sequences were determined, ten (83%) had the correct sequence. This technique, which does not require the use of single-stranded DNA templates, provides a simple and efficient approach to site-specific mutagenesis.

Patients with autoantibodies to the insulin receptor (Anti-R) may exhibit either fasting hypoglycemia or hyperglycemia and extreme insulin resistance. Occasionally, both these phenomena are observed in the same patient at different times in the clinical course. In an effort to understand what determines the patient's response to Anti-R, we developed an animal model of these clinical disorders by passive transfer of Anti-R IgG to rats. IgG fractions from the plasma of Anti-R patients and control subjects were prepared by affinity chromatography with staphylococcal protein A-Sepharose. Anti-R IgG, injected into fasting rats, induced severe and persistent hypoglycemia (plasma glucose 30-60 mg/dl). Rats injected with control IgG maintained a plasma glucose within the range of 75 (fasting) to 165 mg/dl (feeding). In comparison with the effects of insulin, the hypoglycemic response to Anti-R IgG had a slower onset (2-4 h) and lasted longer (8-24 h). Similar, dose-dependent hypoglycemic responses were observed in rats whether the Anti-R IgG was derived from an insulin-resistant or hypoglycemic patient. When Anti-R IgG was administered in sufficiently high doses for several days to fed rats, persistent hyperglycemia (plasma glucose 200-400 mg/dl) developed. Based on these in vivo and previous in vitro studies, we attribute the hypoglycemic response to an insulin-like effect of Anti-R, and the hyperglycemic response to a desensitization of host tissues to the effects of insulin, with more prolonged exposure to higher levels of Anti-R.