Discovery of a small molecule insulin mimetic with antidiabetic activity in mice.

Insulin elicits a spectrum of biological responses by binding to its cell surface receptor. In a screen for small molecules that activate the human insulin receptor tyrosine kinase, a nonpeptidyl fungal metabolite (L-783,281) was identified that acted as an insulin mimetic in several biochemical and cellular assays. The compound was selective for insulin receptor versus insulin-like growth factor I (IGFI) receptor and other receptor tyrosine kinases. Oral administration of L-783,281 to two mouse models of diabetes resulted in significant lowering in blood glucose levels. These results demonstrate the feasibility of discovering novel insulin receptor activators that may lead to new therapies for diabetes.

Alternatively spliced variants of the insulin receptor protein. Expression in normal and diabetic human tissues.

Two insulin receptor mRNA transcripts resulting from alternative splicing of exon 11 in the receptor gene are expressed in a highly regulated tissue-specific fashion. To date, there is no information about the relative abundance of the protein isoforms encoded by these mRNAs in tissues of normal or diabetic subjects. We employed an antibody raised against the peptide sequence encoded by exon 11 to develop a specific immunoprecipitation assay that is capable of determining the fraction of receptors that include this amino acid sequence. The assay is based on the relative ability of the exon 11 specific monoclonal antibody (alpha IR alpha) compared to a nonspecific anti-receptor antiserum (B-2) to immunoprecipitate solubilized receptors that are first labeled with 125I-insulin. The assay was validated using standard curves generated with samples composed of known ratios of the two receptor isoforms. Our results in general confirm observations regarding the relative abundance of the two mRNA species in human tissues, with marked predominance of the exon 11+ isoform in liver, and the exon 11- isoform in leukocytes. Similar amounts of both variants are present in placenta, skeletal muscle, and adipose tissue. In studies with this assay using skeletal muscle extracts from control and noninsulin-dependent diabetes mellitus (NIDDM) subjects, as well as in studies of the two mRNAs in control versus NIDDM muscle using a quantitative polymerase chain reaction assay, we could find no significant difference between control and diabetic subjects. This data contradicts a recent report claiming that normal individuals have only the exon 11- mRNA transcript in their skeletal muscle, whereas NIDDM subjects have similar expression of both mRNAs. Given the emerging evidence that functional differences exist between the two receptor isoforms, these studies are relevant to our understanding of insulin receptor function in health and disease.

Regulation of PPAR gamma gene expression by nutrition and obesity in rodents.

The orphan nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma, is implicated in mediating expression of fat-specific genes and in activating the program of adipocyte differentiation. The potential for regulation of PPAR gamma gene expression in vivo is unknown. We cloned a partial mouse PPAR gamma cDNA and developed an RNase protection assay that permits simultaneous quantitation of mRNAs for both gamma l and gamma 2 isoforms encoded by the PPAR gamma gene. Probes for detection of adipocyte P2, the obese gene product, leptin, and 18S mRNAs were also employed. Both gamma l and gamma 2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma 1 expression was also detected at lower levels in liver, spleen, and heart; whereas, gamma l and gamma 2 mRNA were expressed at low levels in skeletal muscle. Adipose tissue levels of gamma l and gamma 2 were not altered in two murine models of obesity (gold thioglucose and ob/ob), but were modestly increased in mice with toxigene-induced brown fat ablation uncoupling protein diphtheria toxin A mice. Fasting (12-48 h) was associated with an 80% fall in PPAR gamma 2 and a 50% fall in PPAR gamma mRNA levels in adipose tissue. Western blot analysis demonstrated a marked effect of fasting to reduce PPAR gamma protein levels in adipose tissue. Similar effects of fasting on PPAR gamma mRNAs were noted in all three models of obesity. Insulin-deficient (streptozotocin) diabetes suppressed adipose tissue gamma l and gamma 2 expression by 75% in normal mice with partial restoration during insulin treatment. Levels of adipose tissue PPAR gamma 2 mRNA were increased by 50% in normal mice exposed to a high fat diet. In obese uncoupling protein diphtheria toxin A mice, high fat feeding resulted in de novo induction of PPAR gamma 2 expression in liver. We conclude (a) PPAR gamma 2 mRNA expression is most abundant in adipocytes in normal mice, but lower level expression is seen in skeletal muscle; (b) expression of adipose tissue gamma1 or gamma2 mRNAs is increased in only one of the three models of obesity; (c) PPAR gamma 1 and gamma 2 expression is downregulated by fasting and insulin-deficient diabetes; and (d) exposure of mice to a high fat diet increases adipose tissue expression of PPAR gamma (in normal mice) and induces PPAR gamma 2 mRNA expression in liver (in obese mice). These findings demonstrate in vivo modulation of PPAR gamma mRNA levels over a fourfold range and provide an additional level of regulation for the control of adipocyte development and function.

A point mutation in transthyretin increases affinity for thyroxine and produces euthyroid hyperthyroxinemia.

In a family expressing euthyroid hyperthyroxinemia, an increased association of plasma thyroxine (T4) with transthyretin (TTR) is transmitted by autosomal dominant inheritance and is secondary to a mutant TTR molecule with increased affinity for T4. Eight individuals spanning three generations exhibited the abnormality. Although five of eight individuals had elevated total T4 concentrations, all affected individuals were clinically euthyroid and all had normal free T4 levels. Purified TTR from the propositus had an affinity for 125I-T4 three times that of control TTR. Exons 2, 3, and 4 (representing greater than 97% of the coding sequence) of the TTR gene of DNA prepared from the propositus' peripheral blood leukocytes were amplified using the polymerase chain reaction (PCR) and were sequenced after subcloning. Exons 2 and 3 were indistinguishable from normal. In 50% of clones amplified from exon 4, a substitution of adenine (ACC) for guanine (GCC) in codon 109 resulted in the replacement of threonine-for-alanine, a mutation confirmed by amino acid sequencing of tryptic peptides derived from purified plasma TTR. The adenine-for-guanine substitution abolishes one of two Fnu 4H I restriction sites in exon 4. PCR amplification of exon 4 of TTR and restriction digestion with Fnu 4H I confirmed that five affected family members with increased binding of 125I-T4 to TTR are heterozygous for the threonine 109 substitution that increases the affinity of this abnormal TTR for T4.

Role of AMP-activated protein kinase in mechanism of metformin action.

Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin's beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin's inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.

Cloning and characterization of p97MAPK, a novel human homolog of rat ERK-3.

Mitogen-activated protein kinases, or extracellular signal-regulated kinases (ERKs), are serine/threonine protein kinases that are activated in response to a wide variety of extracellular stimuli and are encoded by a multigene family. Little is known about the function of the ERK-3 subfamily. To explore the molecular diversity of the ERK-3 subfamily, we isolated a novel human cDNA, designated Hu-ERK-3, from a fetal skeletal muscle library. Analysis of the complete 3,920-bp nucleotide sequence revealed that this clone encodes a predicted protein of 721 amino acids. In vitro transcription-translation generates a 97-kDa protein referred to as p97MAPK. Of all of the sequences compared, p97MAPK is the most homologous to rat ERK-3. Interestingly, although p97MAPK is highly (98%) homologous to ERK-3 at the amino acid level within the N-terminal two-thirds of the coding region, it diverges at the carboxyl terminus as a result of a unique extension of 178 amino acids. Although expression of p97MAPK was detected in all of the tissues tested by Northern (RNA) analysis, the most abundant expression was seen in skeletal muscle. An antibody raised against the unique C terminus recognized a 97-kDa protein in human cells. By using this antibody in an immune complex protein kinase assay, we have shown that treatment of human fibroblasts with serum or phorbol esters activates a myelin basic protein and histone H1 kinase activity in immunoprecipitates. p97MAPK appears to be the human homolog of rat ERK-3, and a member of this family is an active protein kinase.

RSK3 encodes a novel pp90rsk isoform with a unique N-terminal sequence: growth factor-stimulated kinase function and nuclear translocation.

A novel pp90rsk Ser/Thr kinase (referred to as RSK3) was cloned from a human cDNA library. The RSK3 cDNA encodes a predicted 733-amino-acid protein with a unique N-terminal region containing a putative nuclear localization signal. RSK3 mRNA was widely expressed (but was predominant in lung and skeletal muscle). By using fluorescence in situ hybridization, the human RSK3 gene was localized to band q27 of chromosome 6. Hemagglutinin epitope-tagged RSK3 was expressed in transiently transfected COS cells. Growth factors, serum, and phorbol ester stimulated autophosphorylation of recombinant RSK3 and its kinase activity toward several protein substrates known to be phosphorylated by RSKs. However, the relative substrate specificity of RSK3 differed from that reported for other isoforms. RSK3 also phosphorylated potential nuclear target proteins including c-Fos and histones. Furthermore, although RSK3 was inactivated by protein phosphatase 2A in vitro, the enzyme was not activated by ERK2/mitogen-activated protein (MAP) kinase. In contrast, the kinase activity of another epitope-tagged RSK isoform (RSK-1) was significantly increased by in vitro incubation with ERK2/MAP kinase. Finally, we used affinity-purified RSK3 antibodies to demonstrate by immunofluorescence that endogenous RSK3 undergoes serum-stimulated nuclear translocation in cultured HeLa cells. These results provide evidence that RSK3 is a third distinct isoform of pp90rsk which translocates to the cell nucleus, phosphorylates potential nuclear targets, and may have a unique upstream activator. RSK3 may therefore subserve a discrete physiologic role(s) that differs from those of the other two known mammalian RSK isoforms.

Altered extracellular signal-regulated kinase signaling and glycogen metabolism in skeletal muscle from p90 ribosomal S6 kinase 2 knockout mice.

The p90 ribosomal S6 kinase (RSK), a cytosolic substrate for the extracellular signal-regulated kinase (ERK), is involved in transcriptional regulation, and one isoform (RSK2) has been implicated in the activation of glycogen synthase by insulin. To determine RSK2 function in vivo, mice lacking a functional rsk2 gene were generated and studied in response to insulin and exercise, two potent stimulators of the ERK cascade in skeletal muscle. RSK2 knockout (KO) mice weigh 10% less and are 14% shorter than wild-type (WT) mice. They also have impaired learning and coordination. Hindlimb skeletal muscles were obtained from mice 10, 15, or 30 min after insulin injection or immediately after strenuous treadmill exercise for 60 min. While insulin and exercise significantly increased ERK phosphorylation in skeletal muscle from both WT and KO mice, the increases were twofold greater in the KO animals. This occurred despite 27% lower ERK2 protein expression in skeletal muscle of KO mice. KO mice had 18% less muscle glycogen in the fasted basal state, and insulin increased glycogen synthase activity more in KO than WT mice. The enhanced insulin-stimulated increases in ERK and glycogen synthase activities in KO mice were not associated with higher insulin receptor or with IRS1 tyrosine phosphorylation or with IRS1 binding to phosphatidylinositol 3-kinase. However, insulin-stimulated serine phosphorylation of Akt was significantly higher in the KO animals. c-fos mRNA was increased similarly in muscle from WT and KO mice in response to insulin (2. 5-fold) and exercise (15-fold). In conclusion, RSK2 likely plays a major role in feedback inhibition of the ERK pathway in skeletal muscle. Furthermore, RSK2 is not required for activation of muscle glycogen synthase by insulin but may indirectly modulate muscle glycogen synthase activity and/or glycogen content by other mechanisms, possibly through regulation of Akt. RSK2 knockout mice may be a good animal model for the study of Coffin-Lowry syndrome.

Transgenic approaches to the pathogenesis of NIDDM.

The pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM) involves complex interactions between multiple physiological defects, both genetic and acquired. The application of transgenic technology to create animal models that address questions concerning NIDDM (and obesity) is a very recent development that is now gaining rapid momentum and receiving deserved attention. In general, transgenic methods afford new opportunities to alter the site or level of expression of functional genes in vivo, to transfer novel foreign genes into animals, to prevent the expression of specific genes, or to replace genes with specific genetic variants. Two general approaches can be applied: 1) conventional transgenics, the transfer to and expression of new genetic information in animals; and 2) gene targeting, the disruption or replacement of specific endogenous genes. Recent transgenic initiatives have provided important insights into 1) the mechanism of glucose-stimulated insulin secretion and the role of potential defects in this system, 2) the regulated expression of genes that control hepatic glucose production, 3) the role of specific molecules that mediate the actions of insulin, and 4) the elucidation of factors that contribute to in vivo regulation of energy balance and body composition. Emerging transgenic strategies should have a dramatic impact on future efforts to assess the function of newly identified molecules implicated in the regulation of in vivo glucose homeostasis and to determine the roles of candidate loci or specific mutations uncovered during the search for new NIDDM susceptibility genes.

Normal insulin-receptor cDNA sequence in Pima Indians with NIDDM.

Pima Indians have served as a model of non-insulin-dependent diabetes mellitus (NIDDM). Within this population, inherited insulin resistance is a primary determinant of abnormal glucose metabolism. The insulin receptor is regarded as a "candidate gene" that could potentially be defective in Pima Indians or other populations with NIDDM. To directly address the question of potential insulin-receptor genetic defects in Pima Indians, we isolated and sequenced insulin-receptor cDNA from two Pima Indians with NIDDM. Small amounts of lymphoblast RNA were used to generate first-strand cDNA, which was then amplified via the polymerase chain reaction (PCR). In this way, seven overlapping segments of insulin-receptor cDNA were obtained. With the exception of the alternatively spliced 36-base pair exon 11, which is not expressed in lymphoblasts, the complete coding region of the mature proreceptor was examined with a combination of direct sequencing and sequencing of subcloned PCR segments. The nucleotide sequence in both subjects was identical to previously published insulin-receptor cDNA sequences obtained from healthy subjects. These data indicate that abnormalities of insulin binding and receptor function that have been previously observed in vitro with fresh and cultured cells from Pima Indians may be consequences of the diabetic milieu and/or genetic abnormalities in molecules that interact with the insulin receptor.

Analysis of the hexokinase II gene in subjects with insulin resistance and NIDDM and detection of a Gln142-->His substitution.

Hexokinase II (HKII) is the predominant hexokinase isozyme expressed in insulin-responsive tissues. Since defects involving glucose transport and/or its phosphorylation to glucose-6-phosphate are present in muscle of insulin-resistant humans, HKII should be viewed as a candidate gene for inherited insulin resistance and susceptibility to non-insulin-dependent diabetes mellitus (NIDDM). To investigate the prevalence of potential mutations in the gene encoding HKII, we used the polymerase chain reaction (PCR) to amplify each of the 18 exons of the HKII gene from genomic DNA derived from 59 subjects: 25 insulin-resistant probands with clinical features of the type A syndrome and 34 NIDDM subjects enrolled in the United Kingdom Prospective Study of Therapies of NIDDM (UKPDS) who represented the highest percentile of fasting hyperinsulinemia in the UKPDS population of 5,098 subjects. PCR products corresponding to individual HKII exons derived from each subject were screened for the presence of nucleotide variation using a sensitive nonradioactive single-strand conformation polymorphism (SSCP) protocol. Variant SSCP patterns indicative of genetic variation were detected only in PCR amplimers containing exons 4-7, 10, 15, and 17. Direct sequencing of amplified DNA from individuals affected with variant SSCP patterns revealed the presence of the following silent polymorphisms: Asp251 (GAT/C) in exon 7 and Asn692 (AAT/C) in exon 15. SSCP variants detected in PCR products containing exons 5, 10, and 17 were due to single base substitutions in flanking intronic sequences. A polymorphic GGA repeat was identified within intron 5.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of mutations in insulin-receptor gene in NIDDM patients by analysis of single-stranded conformation polymorphisms.

We used the recently described technique of single-stranded conformation-polymorphism (SSCP) analysis to examine the insulin-receptor locus. First, the ability of the method to detect known mutations and polymorphisms in the insulin-receptor coding sequence was assessed. Regions of the insulin-receptor sequence containing 16 different nucleotide changes, 9 in patient genomic DNA and 7 as cloned cDNA in plasmids, were analyzed. All 9 patient genomic DNA mutants and 5 of 7 plasmid mutants exhibited variant SSCP patterns. To investigate the potential of the technique for screening many patients, the 5 exons that encode the tyrosine kinase domain of the insulin receptor were examined in 30 unrelated white subjects with non-insulin-dependent diabetes mellitus (NIDDM). Exons 17-21 were amplified from genomic DNA with polymerase chain reaction and subjected to SSCP analysis. Exons 19, 20, and 21 revealed no bands of aberrant migration, suggesting a high degree of conservation of these sequences. One diabetic subject had an SSCP variant in exon 18. Direct sequencing of this subject's genomic DNA revealed a heterozygous missense mutation (Lys1068----Glu1068). Five different SSCP patterns were detected in exon 17. Based on direct sequencing, these patterns were explained by combinations of three different nucleotide substitutions, two of which were common silent polymorphisms. One subject had a heterozygous missense mutation Val985---- Met985. Allele-specific oligonucleotide hybridization confirmed the presence of these mutations in the appropriate diabetic subjects and also detected the Val985 mutation in heterozygous form in 1 of 13 nondiabetic white subjects. SSCP analysis is a sensitive rapid method for screening for mutations in the insulin-receptor gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle-specific expression of human insulin receptor in transgenic mice.

Variations in skeletal muscle insulin signaling are thought to have important effects on in vivo glucose homeostasis. To address the role of the insulin receptor in insulin action in muscle, we overexpressed human insulin receptors in the skeletal muscle of transgenic mice. A muscle-specific transgene (TMPE/HIR) was constructed by using promotor and enhancer elements derived from the rat MLC1/3 locus coupled to the intact protein-coding region of the human insulin-receptor cDNA. After testing the transgene for expression in cultured C2C12 myotubes, six founder mice transgenic for TMPE/HIR were generated. We determined that one line of mice had significant expression of human insulin-receptor mRNA in skeletal muscle. The analysis of several tissues from these mice by immunoprecipitation of labeled insulin receptors with a human-specific antireceptor antibody, revealed exclusive expression of human insulin receptors in skeletal muscle. Using both human-specific and non--species-specific anti-insulin receptor antibodies, we developed two immunoassays capable of quantitating the relative amounts of human and total insulin receptors in muscle. Compared with nontransgenic littermate controls, the total number of insulin receptors was increased by 30% in heterozygous transgenics and 68% in homozygotes. Human insulin-receptor protein contributed substantially to the total insulin-receptor pool present in transgenic muscle (42% for heterozygotes, 61% for homozygotes). Intraperitoneal glucose and insulin tolerance tests were performed with homozygous transgenic and nontransgenic littermate mice. Results with both approaches were significantly different for the two groups of mice, suggesting that the modest increase in insulin receptors in the muscle of transgenic mice causes a direct increase in insulin responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)

Targeted disruption of the tumor necrosis factor-alpha gene: metabolic consequences in obese and nonobese mice.

To address the hypothesis that tumor necrosis factor (TNF)-alpha has a role in obesity-associated insulin resistance or the regulation of in vivo lipid metabolism, mice with targeted disruption of the TNF-alpha gene were generated and studied. The absence of TNF-alpha protein in TNF-null (-/-) mice was confirmed. Lean or obese (gold-thioglucose [GTG]-injected) homozygous (-/-) mice were compared with lean or obese age- and sex-matched wild-type (+/+) mice derived from the same line at 13, 19, and 28 weeks of age. The following parameters were significantly affected in lean -/- versus +/+ mice: Body weight was not affected until week 28 (decreased by 14%); epididymal fat pad weight also decreased (25%) at this time, as did percentage body fat (16%), while percentage body protein was increased 13%. Fed plasma insulin levels decreased 47% (28 weeks), triglyceride levels decreased (all three ages; maximum 35% at 19 weeks), and fed plasma leptin decreased 33% (28 weeks). Fasting glucose was slightly (10%) reduced, but the glucose response to an oral glucose tolerance test (OGTT) was not affected. There was a trend (NS) toward increased total adipose tissue lipoprotein lipase in -/- versus +/+ mice. GTG-treatment resulted in obese -/- and +/+ mice with equal mean body weights (42 and 58% increased weight versus lean mice). The following parameters were significantly different in obese -/- mice: fasting plasma glucose decreased 13% (28 weeks), fed plasma insulin decreased 67% (28 weeks), and insulin response to OGTT was decreased by 50%. For both groups of obese mice, glucose levels during the OGTT were substantially increased compared with those in lean mice; however, mean stimulated glucose levels were 20% lower in obese -/- versus +/+ mice. We conclude 1) that TNF-alpha functions to regulate plasma triglycerides and body adiposity and 2) that although TNF-alpha contributes to reduced insulin sensitivity in older or obese mice, the absence of TNF-alpha is not sufficient to substantially protect against insulin resistance in the GTG hyperphagic model of rodent obesity.

Differential regulation of glucose transport and transporters by glucose in vascular endothelial and smooth muscle cells.

Hyperglycemia has been implicated in the pathogenesis of both micro- and macrovascular complications in diabetes. Little is known, however, about glucose transporters and their regulation in the vascular system. In this study, the regulation of glucose transporters by glucose was examined in cultured BAECs and BSMCs, and in human arterial smooth muscle cells. Both BAECs and BSMCs transported glucose via the facilitated diffusion transport system. Glucose-transport activity in vascular smooth muscle cells was inversely and reversibly regulated by glucose. Exposure of BSMCs and HSMCs to high glucose decreased Vmax for 2DG and 3-O-MG uptake, whereas Km remained unchanged. The hexose-transport system of BAECs exhibited lower 2DG and 3-O-MG uptake compared with BSMCs and showed little or no adaptation to changes in ambient glucose. Northern blot analysis demonstrated that GLUT1 mRNA levels in BAECs and BSMCs were unaffected by the concentration of glucose in the medium. GLUT2-5 mRNA could not be detected by Northern blot analysis. GLUT1 protein, quantified by Western blot analysis, was more abundant in BSMCs than in BAECs and was decreased by approximately 50% when medium glucose was elevated from 1.2 to 22 mM for 24 h. The alterations in the level of GLUT1 protein correlated with the changes observed in transport activity. These observations suggest differential regulation of glucose transporter in response to glucose between smooth muscle and endothelial cells. The sites of autoregulation may involve translational control and/or the stability of the protein in the smooth muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against altered expression of GLUT1 or GLUT4 in skeletal muscle of patients with obesity or NIDDM.

Studies of experimental diabetes in rodents induced by the beta-cell toxin streptozocin have shown that the insulin-resistant glucose transport of peripheral tissues (muscle and adipose) in these animals can be ascribed in part to a pretranslational reduction of the major insulin-sensitive glucose transporter (GLUT4) in these tissues. Because a central feature of non-insulin-dependent diabetes mellitus (NIDDM) is an imparied ability of insulin to enhance glucose disposal in skeletal muscle, we examined the hypothesis that reduced expression of GLUT4 is a characteristic finding in the skeletal muscle of subjects with NIDDM. Biopsies of skeletal muscles were obtained from 17 patients with NIDDM and 10 lean and 9 obese nondiabetic subjects. Among the diabetic subjects, 7 were newly diagnosed and untreated. Compared with age-matched and body-weight-matched healthy control subjects, there was no significant alteration in the level of GLUT4 mRNA demonstrated by Northern blot and slot blot or GLUT4 protein determined by immunoblotting muscle membranes. Neither GLUT4 mRNA nor protein concentration correlated with the degree of glycemic control, fasting plasma insulin or glucose, diabetes duration, body mass index, sex, or age. GLUT1 mRNA and protein levels were also not significantly different between diabetic and matched control subjects. Thus, unlike streptozocin-induced diabetes in rodents, there is no evidence that impaired expression of the major insulin-responsive glucose transporter is responsible for insulin-resistant glucose transport in the skeletal muscle of these lean and moderately obese NIDDM patients.

Molecular scanning of insulin-responsive glucose transporter (GLUT4) gene in NIDDM subjects.

We investigated the prevalence of mutations in the gene encoding the major insulin-responsive facilitative glucose transporter (GLUT4) in patients with non-insulin-dependent diabetes mellitus (NIDDM). All 11 exons of the GLUT4 gene from 30 British white subjects with NIDDM were amplified using the polymerase chain reaction and screened for nucleotide sequence variation using the single-stranded conformation polymorphism (SSCP) method. No variation between the study subjects was detected in exons 1-3, 4b-8, and 10. Variant SSCP patterns were detected in exons 4a and 9. SSCP variation in exon 4a was revealed by direct nucleotide sequencing to be due to a common silent polymorphism (AAC----AAT at Asn130). One NIDDM patient demonstrated a variant SSCP pattern in exon 9. This was caused by a point mutation (GTC----ATC) at codon 383, which leads to the conservative substitution of isoleucine for valine in the putative fifth extracellular loop of the transporter. Allele-specific oligonucleotide hybridization was used to examine the frequency of this mutation in 240 Welsh white subjects (160 with NIDDM and 80 controls). The Val----Ile383 mutation was found in the heterozygous state in two diabetic subjects and no control subjects. We conclude that mutations of the GLUT4 coding sequence are very uncommon in this population of subjects with typical NIDDM. Determining whether the Ile383 GLUT4 variant present in 3 diabetic subjects contributes in any way to their disease will require further study.

Prevalence of mutations in the insulin receptor gene in subjects with features of the type A syndrome of insulin resistance.

Mutations of the insulin receptor gene are a cause of the type A syndrome of extreme insulin resistance. This study assessed the prevalence of such mutations in women with clinical features of the type A syndrome including ovarian hyperandrogenism, moderate-to-severe degrees of insulin resistance, and acanthosis nigricans. We studied 22 unrelated women with insulin resistance (fasting insulin > 300 pM [50 microU/ml] and/or peak during an oral glucose tolerance test (OGTT) > 1,800 pM [300 microU/ml]), acanthosis nigricans, and the polycystic ovary syndrome (hyperandrogenemia, oligoamenorrhea, and hirsutism). Two insulin-resistant probands with congenital generalized lipodystrophy and one male proband with severe insulin resistance also were included in the study. Southern blotting experiments were performed to exclude gross gene deletions, insertions, or rearrangements. Exons 2-22 of the insulin receptor gene were polymerase chain reaction (PCR) amplified from genomic DNA and screened for nucleotide variation using single-strand conformation polymorphism (SSCP). No nucleotide variation between study subjects was detected in exons 4-6, 10-12, 15, 16, 18, 19, or 21. Sequencing of amplified DNA revealed that SSCP variants in exons 2, 3, 8, 9, and 17 corresponded to known silent polymorphisms within the coding region. Variants in exons 2, 9, 13, and 14 were caused by novel silent polymorphisms; variants in exons 7 and 22 were caused by nucleotide substitutions in flanking introns. One proband was found to have a heterozygous point mutation in exon 20 (CGG-->CAG, Arg1174-->Gln) that involves the intracellular receptor beta-subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

New approaches in the treatment of type 2 diabetes.

Type 2 diabetes is a chronic metabolic derangement that results from defects in both insulin action and secretion. New thiazolidinedione insulin sensitizers have been recently launched. New approaches with mechanisms different from current therapies are being explored, including novel ligands of peroxisome proliferator-activated receptor, glucagon receptor antagonists, dipeptidyl peptidase IV inhibitors, and insulin receptor activators.

Ligand-induced stabilization of PPARgamma monitored by NMR spectroscopy: implications for nuclear receptor activation.

Nuclear receptors are ligand-dependent transcription factors that are mediators of the action of lipophilic hormones and other endogenous ligands and are the targets of drugs useful in a variety of therapeutic areas. Peroxisome proliferator-activated receptor (PPAR)gamma is a nuclear receptor that, acting as a heterodimer with RXR, mediates a variety of cellular effects including adipocyte-differentiation. Due to its role in modulating insulin sensitivity, it is the target of therapeutically active anti-diabetic agents such as rosiglitazone. We have assigned the chemical shifts of the backbone atoms of the 32 kDa ligand-binding domain of PPARgamma in the presence of bound rosiglitazone. Three-dimensional HNCO spectra of the apo ligand-binding domain (LBD) have less than half the expected number of cross-peaks. The missing cross-peaks are restored upon binding strong agonists such as rosiglitazone. The NMR results indicate that the apo-LBD of PPARgamma is in a conformationally mobile state, and that agonist binding is associated with a marked stabilization of the conformation. Mapping the missing peaks to the 3D X-ray crystallographic structure indicates the region of mobility is extensive and includes the ligand-binding region and the cofactor-binding site. This leads to the conclusion that activation of this nuclear receptor is a result of a population shift of a dynamic ensemble of conformations, rather than a two-state switch from an inactive to an active conformation. Our results have important implications for the mechanisms by which antagonists, partial agonists, and agonists of nuclear receptor function operate.