Mutant insulin receptors in syndromes of insulin resistance

Mitigating effect of single or combined administration of nanoparticles of zinc oxide, chromium oxide, and selenium on genotoxicity and metabolic insult in fructose/streptozotocin diabetic rat model

This research was intended to evaluate the antidiabetic effect of single or combined administration of nanoparticles of zinc oxide nanoparticles (ZnONPs), chromium oxide nanoparticles (Cr₂O₃NPs), and selenium nanoparticles (SeNPs), on genetic and metabolic insult in fructose/streptozotocin diabetic rat model. Type 2 diabetes mellitus was induced by feeding sixty adult male albino rats with a high fructose diet accompanied by a single i.p. injection of streptozotocin (STZ). The rats were divided into 6 groups (10 rats/each) and the doses of nanoparticles were 10 mg/kg b.wt for ZnONPs, 1 mg/kg b.wt for Cr₂O₃, and 0.4 mg/kg b.wt for SeNPs. The results displayed that diabetes significantly decreased bodyweight, serum insulin, C-peptide, adiponectin levels, erythrocyte glutathione peroxidase, serum superoxide dismutase activities, high-density lipoprotein cholesterol (HDL-C), and total antioxidant capacity while causing a substantial increase in serum glucose, C-reactive protein, atherogenic index, HOMA-IR, malondialdehyde, lipid profile, interleukin-6 levels, and liver function and kidney function parameters. Furthermore, the findings showed a decrease in insulin receptor substrate-1 (IRS-1) hepatic mRNA expression level and peroxisome proliferator-activated receptor (PPAR-γ) adipocyte mRNA expression level in type 2 diabetic rats. DNA damage was confirmed by performing the comet assay. Moreover, histological observation of pancreatic and hepatic tissues was performed, which were consistent with the biochemical results. The present study confirmed that oral administration of ZnONPs, Cr₂O₃NPs, SeNPs, and their mixture improved all the biochemical and genetic parameters toward normal levels and ameliorated the diabetic consequences that were manifested by restricting cellular DNA damage which maintaining pancreatic and hepatic tissues from oxidative damage. The best reported antidiabetic effect was observed in the mixture administered group.

Monogenic diabetes: a gateway to precision medicine in diabetes

Monogenic diabetes refers to diabetes mellitus (DM) caused by a mutation in a single gene and accounts for approximately 1%-5% of diabetes. Correct diagnosis is clinically critical for certain types of monogenic diabetes, since the appropriate treatment is determined by the etiology of the disease (e.g., oral sulfonylurea treatment of HNF1A/HNF4A-diabetes vs. insulin injections in type 1 diabetes). However, achieving a correct diagnosis requires genetic testing, and the overlapping of the clinical features of monogenic diabetes with those of type 1 and type 2 diabetes has frequently led to misdiagnosis. Improvements in sequencing technology are increasing opportunities to diagnose monogenic diabetes, but challenges remain. In this Review, we describe the types of monogenic diabetes, including common and uncommon types of maturity-onset diabetes of the young, multiple causes of neonatal DM, and syndromic diabetes such as Wolfram syndrome and lipodystrophy. We also review methods of prioritizing patients undergoing genetic testing, and highlight existing challenges facing sequence data interpretation that can be addressed by forming collaborations of expertise and by pooling cases.

Structural Basis and Genotype-Phenotype Correlations of INSR Mutations Causing Severe Insulin Resistance

The insulin receptor (INSR) gene was analyzed in four patients with severe insulin resistance, revealing five novel mutations and a deletion that removed exon 2. A patient with Donohue syndrome (DS) had a novel p.V657F mutation in the second fibronectin type III domain (FnIII-2), which contains the α-β cleavage site and part of the insulin-binding site. The mutant INSR was expressed in Chinese hamster ovary cells, revealing that it reduced insulin proreceptor processing and impaired activation of downstream signaling cascades. Using online databases, we analyzed 82 INSR missense mutations and demonstrated that mutations causing DS were more frequently located in the FnIII domains than those causing the milder type A insulin resistance (P = 0.016). In silico structural analysis revealed that missense mutations predicted to severely impair hydrophobic core formation and stability of the FnIII domains all caused DS, whereas those predicted to produce localized destabilization and to not affect folding of the FnIII domains all caused the less severe Rabson-Mendenhall syndrome. These results suggest the importance of the FnIII domains, provide insight into the molecular mechanism of severe insulin resistance, will aid early diagnosis, and will provide potential novel targets for treating extreme insulin resistance.

Imbalanced Insulin Actions in Obesity and Type 2 Diabetes: Key Mouse Models of Insulin Signaling Pathway

Since the discovery of the tyrosine kinase activity of the insulin receptor (IR), researchers have been engaged in intensive efforts to resolve physiological functions of IR and its major downstream targets, insulin receptor substrate 1 (Irs1) and Irs2. Studies conducted using systemic and tissue-specific gene-knockout mice of IR, Irs1, and Irs2 have revealed the physiological roles of these molecules in each tissue and interactions among multiple tissues. In obesity and type 2 diabetes, selective downregulation of Irs2 and its downstream actions to cause reduced insulin actions was associated with increased insulin actions through Irs1 in variety tissues. Thus, we propose the novel concept of "organ- and pathway-specific imbalanced insulin action" in obesity and type 2 diabetes, which includes and extends "selective insulin resistance." This Review focuses on recent progress in understanding insulin signaling and insulin resistance using key mouse models for elucidating pathophysiology of human obesity and type 2 diabetes.

Identification and Functional Characterization of Two Novel Nonsense Mutations in the β-Subunit of INSR That Cause Severe Insulin Resistance Syndrome

BACKGROUND/AIMS

Donohue syndrome is an extremely rare autosomal recessive disorder caused by mutations in INSR. This study describes the clinical course of a patient with Donohue syndrome, and we also evaluated the molecular and functional characteristics of 2 novel INSR mutations.

METHODS

Our patient was a male newborn with acanthosis nigricans, lack of subcutaneous fat, hirsutism, thick lips, and high serum insulin levels, all of which are characteristic of Donohue syndrome. INSR mutation analysis was performed, and Western blot analysis was used to verify the effects of the novel mutations on INSR protein expression.

RESULTS

Direct INSR sequencing identified the following 2 novel compound heterozygous mutations in the β-subunit of INSR: p.Arg1066* and p.Gln1232*. Western blot analysis of skin fibroblasts revealed a comparable expression of the α-subunit of INSR in mutant and control samples, but reduced levels of mature INSR β-subunit protein were found in mutant INSR-expressing cells in comparison to the controls.

CONCLUSIONS

This study describes the clinical course of a male patient with Donohue syndrome and the molecular characteristics of 2 novel compound heterozygous mutations in INSR. These novel nonsense mutations are associated with reduced expression of the mature INSR β-subunit, which was most likely due to impaired proreceptor processing.

A protective role of arecoline hydrobromide in experimentally induced male diabetic rats

Objectives. Arecoline, the most potent and abundant alkaloid of betel nut, causes elevation of serum testosterone and androgen receptor expression in rat prostate, in addition to increase in serum insulin levels in rats, leading to insulin resistance and type 2 diabetes-like conditions. This study investigated the role of arecoline on the reproductive status of experimentally induced type 1 diabetic rats. Methods. Changes in the cellular architecture were analyzed by transmission electron microscopy. Blood glucose, serum insulin, testosterone, FSH, and LH were assayed. Fructose content of the coagulating gland and sialic acid content of the seminal vesicles were also analyzed. Results. Arecoline treatment for 10 days at a dose of 10 mg/kg of body weight markedly facilitated β-cell regeneration and reversed testicular and sex accessory dysfunctions by increasing the levels of serum insulin and gonadotropins in type 1 diabetic rats. Critical genes related to β-cell regeneration, such as pancreatic and duodenal homeobox 1 (pdx-1) and glucose transporter 2 (GLUT-2), were found to be activated by arecoline at the protein level. Conclusion. It can thus be suggested that arecoline is effective in ameliorating the detrimental effects caused by insulin deficiency on gonadal and male sex accessories in rats with type 1 diabetes.

Severe insulin resistance and intrauterine growth deficiency associated with haploinsufficiency for INSR and CHN2: new insights into synergistic pathways involved in growth and metabolism

OBJECTIVE

Digenic causes of human disease are rarely reported. Insulin via its receptor, which is encoded by INSR, plays a key role in both metabolic and growth signaling pathways. Heterozygous INSR mutations are the most common cause of monogenic insulin resistance. However, growth retardation is only reported with homozygous or compound heterozygous mutations. We describe a novel translocation [t(7,19)(p15.2;p13.2)] cosegregating with insulin resistance and pre- and postnatal growth deficiency. Chromosome translocations present a unique opportunity to identify modifying loci; therefore, our objective was to determine the mutational mechanism resulting in this complex phenotype.

RESEARCH DESIGN AND METHODS

Breakpoint mapping was performed by fluorescence in situ hybridization (FISH) on patient chromosomes. Sequencing and gene expression studies of disrupted and adjacent genes were performed on patient-derived tissues. RESULTS Affected individuals had increased insulin, C-peptide, insulin-to-C-peptide ratio, and adiponectin levels consistent with an insulin receptoropathy. FISH mapping established that the translocation breakpoints disrupt INSR on chromosome 19p15.2 and CHN2 on chromosome 7p13.2. Sequencing demonstrated INSR haploinsufficiency accounting for elevated insulin levels and dysglycemia. CHN2 encoding beta-2 chimerin was shown to be expressed in insulin-sensitive tissues, and its disruption was shown to result in decreased gene expression in patient-derived adipose tissue.

CONCLUSIONS

We present a likely digenic cause of insulin resistance and growth deficiency resulting from the combined heterozygous disruption of INSR and CHN2, implicating CHN2 for the first time as a key element of proximal insulin signaling in vivo.

IGF-I treatment of insulin resistance

Severe insulin resistance resulting from known or putative genetic defects affecting the insulin receptor or post-insulin receptor signalling represents a clinical spectrum ranging from Donohue's and Rabson-Mendenhall syndrome, where the genetic defect is identified, through to the milder phenotype of type A insulin resistance, where a genetic defect can only be detected in around 10% of cases. Paradoxically, subjects with these conditions may present with hypoglycaemia due to mismatch of post-prandial glucose excursion and compensatory hyperinsulinaemia. Ultimately, treatment with insulin and insulin sensitisers will be unsuccessful and subjects may succumb to diabetes or its complications. Recombinant human IGF-I alone or combined with its binding protein (IGFBP-3) provides an alternative therapy as IGF-I receptor shares structural and functional homology with the insulin receptor and recombinant human insulin-like growth factor I (rhIGF-I) therapy could improve glucose disposal by signalling through the IGF-I receptor, whilst reducing the adverse effects of high insulin concentrations. There are also data which indicate that IGF-I signalling through the IGF-I receptor on the pancreatic beta-cell may be important in maintaining insulin secretion. Pilot studies confirmed that rhIGF-I could reduce glucose and insulin levels in subjects with type A insulin resistance and those with Rabson-Mendenhall syndrome with sustained beneficial effects on HbA1c. Continued study has confirmed efficacy of rhIGF-I when combined with IGFBP-3 in the treatment of Donohue's and type A insulin resistance subjects. Observations that IGF-I treatment can improve C-peptide levels in these subjects may indicate that it might be more valuable as a first line intervention to preserve beta-cell function, rather than its current use as a medication of last resort in subjects where all other therapies have failed.

Functional characterization of a novel insulin receptor mutation contributing to Rabson-Mendenhall syndrome

OBJECTIVE/PATIENTS

Rabson-Mendenhall syndrome (RMS) is a rare, recessively inherited disorder of extreme insulin resistance due to mutations in the insulin receptor gene. We have identified a pair of siblings with RMS attributable to compound heterozygosity for two insulin receptor mutations, one previously unreported, and have characterized the novel receptor mutation functionally.

MEASUREMENTS

Insulin receptor sequencing was performed to identify the mutations. Expression levels of the mature receptor were determined in lymphoblastoid cells from the affected subjects. Further studies of immortalized cell lines transfected with mutant and wild type (WT) receptors were undertaken to characterize the effects of the novel mutation on [(125)I]-labelled insulin binding, proreceptor processing and insulin-stimulated receptor autophosphorylation.

RESULTS

Sequencing of the insulin proreceptor coding sequence revealed both siblings to be compound heterozygotes for the missense mutations Arg209His and Gly359Ser in the mature insulin receptor. The former mutation has been described in homozygous form in Donohue syndrome, while the latter is novel. Insulin receptor expression in lymphoblastoid cell lines was present at only 10-30% of that in control cells; studies of immortalized cells transfected with mutant and WT receptors confirmed the reduced expression of the mutant. The degree of impairment of insulin binding and insulin-stimulated receptor autophosphorylation were commensurate with the decrease in expression of the mature receptor.

CONCLUSIONS

Loss of function of the novel insulin receptor (INSR) G359S variant is largely accounted for by aberrant proreceptor processing rather than intrinsically impaired signal transduction by the mutant receptor.

One gene, two phenotypes: ROR2 mutations in autosomal recessive Robinow syndrome and autosomal dominant brachydactyly type B

Autosomal recessive Robinow syndrome (RRS) is a severe skeletal dysplasia with short stature, generalized limb shortening, segmental defects of the spine, brachydactyly, and a dysmorphic facial appearance. The gene encoding receptor orphan receptor tyrosine kinase 2 (ROR2) is located on chromosome 9q22 and homozygous loss-of-function mutations in this gene are responsible for RRS. Moreover, knocking out the mouse Ror2 gene causes mesomelic dwarfism in the homozygous state, with almost identical features to recessive Robinow syndrome. The protein product of this gene is a cell membrane receptor, containing distinct motifs including an immunoglobulin-like (Ig) domain, a Frizzled-like cysteine-rich domain (FRZ or CRD), and a kringle domain (KD) in the extracellular region; and an intracellular region with tyrosine kinase (TK), serine/threonine-rich, and proline-rich structures. The extracellular motifs of the ROR2 protein are known to be involved in protein-protein interactions. The tyrosine kinase domain is involved in an as yet uncharacterized signaling pathway. Interestingly, heterozygous mutations in ROR2 have recently been shown to give rise to autosomal dominant brachydactyly type B1 (BDB1). This condition is characterized by terminal deficiency of fingers and toes. A variety of mutations have been reported in ROR2. Here, these genetic defects are compiled and possible genotype-phenotype correlations are discussed.

Mitochondrial diabetes, DIDMOAD and other inherited diabetes syndromes

Inherited diabetes syndromes are individually rare but collectively make up a significant proportion of patients attending diabetes clinics, some of whom have multiple handicaps. This chapter focuses on syndromes in which major advances have been made in our understanding of the underlying molecular genetics. These conditions demonstrate novel genetic mechanisms such as maternal inheritance and genetic imprinting. They are also fascinating as they aid our understanding of insulin metabolism, both normal and abnormal. As the causative genes are identified, future issues will be the availability of genetic testing, their contribution to the genetic heterogeneity of the more common types of diabetes, and functional studies of the relevant proteins. It is probable that other subtypes of diabetes will be identified as the relevant metabolic pathways are characterized. This is an exciting time to be a diabetes physician as diabetology returns to being a diagnostic rather than a mainly management-based speciality.

Two aberrant splicings caused by mutations in the insulin receptor gene in cultured lymphocytes from a patient with Rabson-Mendenhall's syndrome

Rabson-Mendenhall's syndrome is one of the most severe forms of insulin resistance syndrome. We analyzed an English patient described elsewhere and found novel mutations in both alleles of the insulin receptor gene. One is a substitution of G for A at the 3' splice acceptor site of intron 4, and the other is an eight-base pair deletion in exon 12. Both decrease mRNA expression in a cis-dominant manner, and are predicted to produce severely truncated proteins. Surprisingly, nearly normal insulin receptor levels were expressed in the patient's lymphocytes, although the level of expression assessed by immunoblot was approximately 10% of the control cells. Insulin binding affinity was markedly reduced, but insulin-dependent tyrosine kinase activity was present. Analyzing the insulin receptor mRNA of the patient's lymphocytes by reverse transcription PCR, we discovered aberrant splicing caused by activation of a cryptic splice site in exon 5, resulting in a four-amino acid deletion and one amino acid substitution, but restoring an open reading frame. Skipped exon 5, another aberrant splicing, was found in both the patient and the mother who had the heterozygotic mutation, whereas activation of the cryptic splice site occurred almost exclusively in the patient. Transfectional analysis in COS cells revealed that the mutant receptor produced by cryptic site activation has the same characteristics as those expressed in patient's lymphocytes. We speculate that this mutant receptor may be involved in the relatively long survival of the patient by rescuing otherwise more severe phenotypes resulting from the complete lack of functional insulin receptors.

Defects of insulin and IGF-1 action at receptor and postreceptor level in a patient with type A syndrome of insulin resistance

The action of insulin and IGF-1 in comparison to non-diabetic controls was studied in cultured fibroblasts of a patient with an inherited syndrome of insulin resistance (Type A syndrome). Insulin binding was reduced due to decreased receptor affinity, but sequence analyses revealed no alterations of splicing or primary insulin receptor (IR) structure. Most likely due to the IR affinity defect analyses of signal transduction pathways showed an impairment of insulin action on glucose uptake, total RNA synthesis and phosphorylation as well as activity of MAP-kinase. In addition inducibility of c-fos mRNA level was strongly impaired by insulin and IGF-1, but comparable to controls by PDGF indicating a postreceptor defect. In conclusion, we provide evidence that genetic syndromes of insulin resistance can be associated with both, receptor and postreceptor defects.

Non-insulin-dependent diabetes mellitus in childhood and adolescence

NIDDM in children and adolescents represents a heterogeneous group of disorders with different underlying pathophysiologic mechanisms. Most subtypes of NIDDM that occur in childhood are uncommon, but some, such as early onset of "classic" NIDDM, seem to be increasing in prevalence. This observed increase is thought to be caused by societal factors that lead to sedentary lifestyles and an increased prevalence of obesity. In adults, hyperglycemia frequently exists for years before a diagnosis of NIDDM is made and treatment is begun. Microvascular complications, such as retinopathy, are often already present at the time of diagnosis. Children are frequently asymptomatic at the time of diagnosis, so screening for this disorder in high-risk populations is important. Screening should be considered for children of high-risk ethnic populations with a strong family history of NIDDM with obesity or signs of hyperinsulinism, such as acanthosis nigricans. Even for children in these high-risk groups who do not yet manifest hyperglycemia, primary care providers can have an important role in encouraging lifestyle modifications that might delay or prevent onset of NIDDM.

Functional activation of mutant human insulin receptor by monoclonal antibody

BACKGROUND

A mutant insulin receptor, Ser323Leu, has been reported in two severely insulin-resistant patients with Rabson-Mendenhall syndrome. In both cases, extreme hyperglycaemia could not be controlled by conventional antidiabetic therapy. The SER323Leu mutant insulin receptor is inserted normally in the plasma membrane but has very low binding affinity for insulin. A monoclonal antibody directed against the extracellular domain of the insulin receptor (83.14) can mimic the natural ligand as far as the first step after ligand binding--autophosphorylation of the intracellular domain of the receptor. We have investigated whether antibody binding can imitate autophosphorylation of the Ser323Leu mutant receptor and lead to metabolic events within the cell.

METHODS

The effects of insulin and the insulin-receptor monoclonal antibody on receptor autophosphorylation and glycogen synthesis were compared in Chinese hamster ovary cells expressing the wild-type human insulin receptor, mock-transfected cells, cells expressing an insulin-receptor mutant without autophosphorylation capacity, and cells expressing the Ser323Leu mutant receptor.

FINDINGS

Cells expressing the SER323Leu mutant receptor had very low specific insulin binding and, unlike cells expressing wild-type insulin receptors, did not show autophosphorylation or stimulation of glycogen synthesis in response to insulin. However, exposure of cells expressing the Ser323Leu mutant receptor to monoclonal antibody 83.14 resulted in autophosphorylation and stimulation of glycogen synthesis similar to that seen in cells expressing wild-type insulin receptors.

INTERPRETATION

Although insulin does not bind to cells expressing the Ser323Leu mutation, insulin signalling can be mimicked by exposure of the cells to an antibody to the extracellular domain of the insulin receptor. Activation by monoclonal antibodies of mutant transmembrane receptors that show normal cell-surface expression but defective ligand binding may provide an approach to the therapy of some subtypes of inherited hormone resistance for which little effective treatment is available.

Insulin resistance is mediated by a proteolytic fragment of the insulin receptor

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 beta subunit is proteolytically cleaved, resulting in the generation of a cytosolic fragment of the beta subunit, beta', and that the generation of beta' 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 beta' is generated by chronic insulin administration to the cells, and that E64 inhibits the production of beta'; 2) chronic administration of insulin to the adipocytes leads to an insulin-resistant 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 beta' stoichiometrically inhibits the insulin-induced autophosphorylation of the insulin receptor beta 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 beta' is a mediator of insulin resistance, and the mechanism of action of beta' is the inhibition of the insulin-induced autophosphorylation of the beta subunit of the insulin receptor.

Mendenhall's syndrome: clues to the aetiology of human diabetic neuropathy

The pathogenesis of human diabetic neuropathy remains unclear. Mendenhall's syndrome is characterised by a mutation in the insulin receptor gene with consequent lifelong uncontrolled hyperglycaemia. The sural nerve biopsy from a patient with Mendenhall's syndrome showed a gross loss of myelinated fibres that was comparable with the degree of fibre loss in a case matched diabetic patient with established neuropathy. The biopsy from the patient with Mendenhall's syndrome showed only a very mild degree of microangiopathy, however, which failed to relate to the degree of nerve fibre loss and also did not compare with the advanced degree of microangiopathy seen in the diabetic patient. Thus hyperglycaemia itself did not result in appreciable microangiopathy. Furthermore the presence of severe neuropathy without advanced microangiopathy suggests an important and independent role for metabolic factors in the pathogenesis of neuropathy.

Protein-tyrosine-phosphatase SHPTP2 is a required positive effector for insulin downstream signaling

SHPTP2 is a ubiquitously expressed tyrosine-specific protein phosphatase that contains two amino-terminal Src homology 2 (SH2) domains responsible for its association with tyrosine-phosphorylated proteins. In this study, expression of dominant interfering mutants of SHPTP2 was found to inhibit insulin stimulation of c-fos reporter gene expression and activation of the 42-kDa (Erk2) and 44-kDa (Erk1) mitogen-activated protein kinases. Cotransfection of dominant interfering SHPTP2 mutants with v-Ras or Grb2 indicated that SHPTP2 regulated insulin signaling either upstream of or in parallel to Ras function. Furthermore, phosphotyrosine blotting and immunoprecipitation identified the 125-kDa focal adhesion kinase (pp125FAK) as a substrate for insulin-dependent tyrosine dephosphorylation. These data demonstrate that SHPTP2 functions as a positive regulator of insulin action and that insulin signaling results in the dephosphorylation of tyrosine-phosphorylated pp125FAK.

Molecular analysis of insulin receptor gene in Werner's syndrome

Werner's syndrome is characterized by premature aging and frequent impaired glucose tolerance or overt diabetes. Insulin resistance may play an important role and may be caused by a post-receptor defect or dysfunctional insulin receptor. The present study was undertaken to investigate the insulin receptor gene mutation in Werner's syndrome. The genomic DNAs were obtained from four patients with Werner's syndrome. Exons 2-22 of the insulin receptor gene except exon 1 were amplified from genomic DNA by the polymerase chain reaction and screened for nucleotide variation by examining for single-stranded conformational polymorphisms. There were no nucleotide variations in exons 2, 4-->7, 9 and 12-->22. Variants were thus found in exons 3, 8, 10 and 11 and each were sequenced. The variant in exon 8 was due to a silent polymorphism (GAT-->GAC/T, Asp519) and other variants in exons 3, 10 and 11 were caused by nucleotide substitutions in introns. These results suggest that the patients with Werner's syndrome express normal insulin receptors and that the primary genetic lesion for insulin resistance is not in the insulin receptor gene. Insulin resistance in Werner's syndrome is thus likely by a post-receptor defect.

Elevated protein tyrosine phosphatase activity and increased membrane viscosity are associated with impaired activation of the insulin receptor kinase in old rats

Insulin resistance is very common in the elderly, and may be associated with glucose intolerance or frank diabetes. In previous studies we demonstrated that insulin resistance in old Wistar rats is associated with decreased autophosphorylation and activation of the hepatic insulin receptor kinase (IRK) in vivo. We now show that this defect can be reproduced in vitro, where the extent of insulin-induced activation of IRK in liver membranes of old rats was decreased by approximately 50% compared with young controls. The defect could be largely abolished after solubilization of the membranes with Triton X-100. We also show that: (a) the viscosity of membranes from the old rats was significantly (P < 0.001, n = 4) higher (by 15%) compared with young controls; (b) incubation of plasma membranes from old animals with lecithin liposomes, which lowered their cholesterol levels, partially abolished the defect in IRK activation; and (c) Triton extracts of liver membranes prepared from old rats did not interfere with the activation of IRK derived from young controls. Additionally, non-membrane components did contribute to the development of this defect. We observed a significant (approximately 30%) (P < 0.001, n = 18) elevation of cytosolic protein tyrosine phosphatase (PTP) activity directed against the beta subunit of the insulin receptor in livers of old rats. No such elevation of PTP activity could be demonstrated with synthetic substrates. Our findings are consistent with a model in which increased membrane viscosity as well as enhancement of a cytosolic PTP activity both markedly inhibit the activation in vivo of the hepatic IRK in old animals.

An in-frame insertion in exon 3 and a nonsense mutation in exon 2 of the insulin receptor gene associated with severe insulin resistance in a patient with Rabson-Mendenhall syndrome

We have studied the structure and function of the insulin receptor in a patient (PK) with severe insulin resistance and Rabson-Mendenhall syndrome. Insulin binding to cultured fibroblasts from PK was almost not detectable and insulin-induced insulin receptor autophosphorylation and glucose uptake was abolished. The structure of the receptor gene was analysed by sequencing amplified products of the 22 exons with the flanking intron regions directly as well as after subcloning in pUCBM20 plasmids. Two mutant alleles of the insulin receptor gene were detected. One allele contains in-frame 12 additional base pairs in exon 3 coding for the amino acids Leu-His-Leu-Val located between Asp-261 and Leu-262 in the receptor's extracellular domain, being the first report of an insertion mutation of the insulin receptor gene. In the other allele Arg-86 in exon 2 is changed into a stop codon. Therefore, PK is compound heterozygous at the insulin receptor locus. Direct cDNA sequencing indicates that both mutant alleles are expressed in the patient's fibroblasts. Studies of the parents' fibroblasts revealed that PK inherited the insertion mutation from the father and the nonsense mutation from the mother. Insulin binding to fibroblasts of the mother was reduced (63% of control cells) and hormone binding to the father's cells shows a larger reduction (37% of control cells), but less severe than the patient's cells (11% of control). This investigation provides further evidence that the Rabson-Mendenhall syndrome is causally related to mutations in the insulin receptor gene.

Molecular dynamics of insulin/IGF-I receptor transmembrane signaling

To examine the molecular basis of ligand-stimulated intramolecular beta-subunit autophosphorylation, hybrid receptors composed of wild-type and mutant insulin and insulin-like growth factor-1 (IGF-I) half-receptor precursors were characterized. Previous studies have demonstrated that assembly of the IGF-I wild-type half-receptor (alpha beta WT) with a kinase-defective half-receptor (alpha beta A/K) produced a substrate kinase-inactive holoreceptor in vitro [Treadway et al. (1991): Proc Natl Acad Sci USA 88:214-218]. To extend these studies, the vaccinia virus/bacteriophage T7 expression system was used to generate various hybrid receptor complexes in cultured cells. As was observed for hybrid receptors assembled in vitro, the wild-type/mutant hybrid receptors formed in situ were also incapable of phosphorylating several peptide substrates. However, ligand-stimulated beta-subunit autophosphorylation was still observed. To determine the molecular basis for this discrepancy, hybrid receptors were assembled from a truncated beta-subunit insulin half-receptor (alpha beta delta 43) and a kinase-defective half-receptor (alpha beta A/K). Under these conditions, insulin-stimulated autophosphorylation primarily occurred on the full-length kinase-inactive beta-subunit (alpha beta A/K) without significant labeling of the kinase-active truncated beta-subunit (alpha beta delta 43). A similar IGF-I hybrid receptor species was characterized, and the same pattern of autophosphorylation was observed in response to IGF-I. These data demonstrate that both insulin and IGF-I stimulate an intramolecular trans-autophosphorylation reaction between two adjacent beta-subunits within the holoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Serious, prolonged hypoglycaemia with glibenclamide in a patient with Mendenhall's syndrome

Mendenhall's syndrome, characterized by familial insulin resistant diabetes, pineal hyperplasia and multiple somatic abnormalities, is associated with defects involving the alpha-subunit of the insulin receptor. The associated insulin-resistant diabetes is extremely difficult to treat; insulin is required in very large doses to control hyperglycaemia and oral hypoglycaemic agents are ineffective. We report a case of severe, prolonged hypoglycaemia that occurred in a 24-year-old patient with Mendenhall's syndrome following therapy with glibenclamide. He had glibenclamide 10 mg daily for 1 week following which he was admitted to hospital in hypoglycaemic coma with blood glucose levels < 1.0 mmol/l. This subject had undergone hypophysectomy at the age of 11 years. Prior to pituitary ablation, oral hypoglycaemic agents did not improve glycaemic control. Thus, previous hypophysectomy in this patient appears to have made it possible for glibenclamide to exert its hypoglycaemic effect. The occurrence of hypoglycaemia in this patient suggests alternative mechanisms for insulin action in conditions characterized by severe insulin resistance due to insulin receptor defects.