An autosomal genomic scan for loci linked to prediabetic phenotypes in Pima Indians

Type 2 diabetes mellitus is a common chronic disease that is thought to have a substantial genetic basis. Identification of the genes responsible has been hampered by the complex nature of the syndrome. Abnormalities in insulin secretion and insulin action predict the development of type 2 diabetes and are, themselves, highly heritable traits. Since fewer genes may contribute to these precursors of type 2 diabetes than to the overall syndrome, such genes may be easier to identify. We, therefore, undertook an autosomal genomic scan to identify loci linked to prediabetic traits in Pima Indians, a population with a high prevalence of type 2 diabetes. 363 nondiabetic Pima Indians were genotyped at 516 polymorphic microsatellite markers on all 22 autosomes. Linkage analyses were performed using three methods (single-marker, nonparametric multipoint [MAPMAKER/SIBS], and variance components multipoint). These analyses provided evidence for linkage at several chromosomal regions, including 3q21-24 linked to fasting plasma insulin concentration and in vivo insulin action, 4p15-q12 linked to fasting plasma insulin concentration, 9q21 linked to 2-h insulin concentration during oral glucose tolerance testing, and 22q12-13 linked to fasting plasma glucose concentration. These results suggest loci that may harbor genes contributing to type 2 diabetes in Pima Indians. None of the linkages exceeded a LOD score of 3.6 (a 5% probability of occurring in a genome-wide scan). These findings must, therefore, be considered tentative until extended in this population or replicated in others.

Autosomal genomic scan for loci linked to obesity and energy metabolism in Pima Indians

An autosomal genomic scan to search for linkage to obesity and energy metabolism was completed in Pima Indians, a population prone to obesity. Obesity was assessed by percent body fat (by hydrodensitometry) and fat distribution (the ratio of waist circumference to thigh circumference). Energy metabolism was measured in a respiratory chamber as 24-h metabolic rate, sleeping metabolic rate, and 24-h respiratory quotient (24RQ), an indicator of the ratio of carbohydrate oxidation to fat oxidation. Five hundred sixteen microsatellite markers with a median spacing of 6.4 cM were analyzed, in 362 siblings who had measurements of body composition and in 220 siblings who had measurements of energy metabolism. These comprised 451 sib pairs in 127 nuclear families, for linkage analysis to obesity, and 236 sib pairs in 82 nuclear families, for linkage analysis to energy metabolism. Pointwise and multipoint methods for regression of sib-pair differences in identity by descent, as well as a sibling-based variance-components method, were used to detect linkage. LOD scores >=2 were found at 11q21-q22, for percent body fat (LOD=2.1; P=.001), at 11q23-q24, for 24-h energy expenditure (LOD=2.0; P=.001), and at 1p31-p21 (LOD=2.0) and 20q11.2 (LOD=3.0; P=.0001), for 24RQ, by pointwise and multipoint analyses. With the variance-components method, the highest LOD score (LOD=2.3 P=.0006) was found at 18q21, for percent body fat, and at 1p31-p21 (LOD=2.8; P=.0003), for 24RQ. Possible candidate genes include LEPR (leptin receptor), at 1p31, and ASIP (agouti-signaling protein), at 20q11.2.

Exercise in transgenic mice overexpressing GLUT4 glucose transporters: effects on substrate metabolism and glycogen regulation

We assessed the effects of GLUT4 glucose transporter expression on substrate metabolism and glycogen regulation during exercise. Transgenic mice overexpressing human (h)GLUT4 in muscle and fat (TG) and their wild-type littermates (WT) were studied by indirect calorimetry at rest and during acute treadmill exercise (30 minutes) and recovery (30 minutes). The rate of carbon dioxide production (VCO2) increased to a greater degree in TG during exercise, whereas resting VCO2, resting oxygen production (VO2), and exercise-induced increments in VO2 were similar in TG and WT. As a result, the respiratory quotient (RQ) was increased by .03 to .05 in TG during exercise, due to greater consumption of carbohydrate (up to approximately 64% more) and less consumption of lipid (up to approximately 40% less) compared with WT, without differences in overall energy expenditure. These differences in substrate metabolism were observed despite relative hypoglycemia and elevated free fatty acids (FFAs) in TG that persisted throughout resting, exercise, and recovery periods. To further assess substrate availability, glycogen content and glycogen synthase activity were measured in skeletal muscle and liver. At rest, muscle glycogen content was 50% higher and glycogen synthase I was 40% lower in TG compared with WT. During exercise and recovery, muscle glycogen was more profoundly depleted in TG than in WT, and glycogen synthase I increased to levels observed in WT, with no change in total glycogen synthase. In the liver, glycogen content and total glycogen synthase were similar in TG and WT under resting conditions, while glycogen synthase I was reduced by 48%. Exercise and recovery induced a more profound depletion of liver glycogen (76% v 30%) and greater increments in both I-form and total glycogen synthase in TG. In conclusion, (1) TG overexpressing GLUT4 exhibit greater muscle glycogen content at rest than WT; (2) during exercise, TG metabolize more carbohydrate, made possible by increased glycogenolysis in muscle and liver, and this predominates as a fuel source despite hypoglycemia and increased availability of FFA; (3) increased carbohydrate metabolism is linked to a decrease in lipid metabolism such that there is no change in overall energy expenditure; and (4) glycogen synthase I activity is inversely proportional to tissue glycogen content despite differences in circulating glucose, insulin, and FFA concentrations, indicating that glycogen content has an overriding regulatory influence on glycogen synthase.

Identification of Rad's effector-binding domain, intracellular localization, and analysis of expression in Pima Indians

In order to characterize the endogenous gene product for rad (ras-related protein associated with diabetes), we prepared antibodies to synthetic peptides that correspond to amino acids (109-121, 178-195, 254-271) within the protein. These antibodies were used to analyze the expression, structure, and function of rad. Western analysis with these antibodies revealed that rad was a 46 kDa protein which was expressed during myotube formation. Further, immunolocalization studies showed that rad localized to thin filamentous regions in skeletal muscle. Interestingly, when muscle biopsies from diabetic and control Pima Indians were compared, no differences in rad protein or mRNA expression were observed. Similarly, no differences were observed in protein expression in diabetic and control Zucker diabetic fatty (ZDF) rats. Functional analysis of muscle rad revealed that its GTP-binding activity was inhibited by the addition of N-ethylmaliemide, GTP, GTP gamma S, and GDP beta S but not ATP or dithiothreitol. Moreover, cytosol-dependent rad-GTPase activity was stimulated by the peptide corresponding to amino acids 109-121. Antibodies corresponding to this epitope inhibited cytosol-dependent rad-GTPase activity. Taken together, the results indicate that 1) rad is a 46 kDa GTP-binding protein localized to thin filaments in muscle and its expression increases during myoblast fusion, 2) expression of rad in Pima Indians and ZDF rats does not correlate with diabetes, and 3) the amino acids (109-121) may be involved in regulating rad-GTPase activity, perhaps by interacting with a cytosolic factor(s) regulating nucleotide exchange and/or hydrolysis.

Gestational diabetes mellitus and gene mutations which affect insulin secretion

We investigated whether genetic mutations known to impair insulin secretion and glucose tolerance are operative in a group of American women with gestational diabetes mellitus. Study groups were comprised of elderly non-diabetic controls (n = 55) with normal glucose tolerance and patients with gestational diabetes (n = 50), together with one family with maturity-onset diabetes of the young (three controls and three affected). No mutations were detected in any exon of the human glucokinase gene or the mitochondrial tRNA[Leu](UUR) gene by single strand conformational analysis and direct exon sequencing. Also, chi2 analysis showed no significant association with gestational diabetes for a polymorphism at position -30 (G --> A) of the beta-cell-specific glucokinase gene promoter. We have determined that glucokinase and mitochondrial tRNA[Leu](UUR) gene mutations, which are known to impair insulin secretion are relatively uncommon and do not constitute a large component of genetic risk for gestational diabetes in the study population.

The metabolic significance of leptin in humans: gender-based differences in relationship to adiposity, insulin sensitivity, and energy expenditure

Leptin is an adipocyte-derived hormone that interacts with a putative receptor(s) in the hypothalamus to regulate body weight. The relationship of leptin to metabolic abnormalities associated with obesity together with hormonal and substrate regulation of leptin have not been extensively studied. Therefore, 116 subjects (62 men and 54 women) with a wide range of body weight [body mass index (BMI), 17-54 kg/m2] were characterized on a metabolic ward with regard to body composition, glucose intolerance, insulin sensitivity, energy expenditure, substrate utilization, and blood pressure. Eighty-five of the subjects had normal glucose tolerance (50 men and 35 women), and 31 had noninsulin-dependent diabetes mellitus (12 men and 19 women). In both men and women, fasting leptin levels were highly correlated with BMI (r = 0.87 and r = 0.88, respectively) and percent body fat (r = 0.82 and r = 0.88, respectively; all P < 0.0001). However, men exhibited lower leptin levels at any given measure of obesity. Compared with those in men, leptin levels rose 3.4-fold more rapidly as a function of BMI in women [leptin = 1.815 (BMI)-31.103 in women; leptin = 0.534 (BMI)-8.437 in men] and 3.2 times more rapidly as a function of body fat [leptin = 1.293 (% body fat)-24.817 in women; leptin = 0.402 (% body fat)-3.087 in men]. Hyperleptinemia was associated with insulin resistance (r = -0.57; P < 0.0001) and high waist to hip ratio (r = 0.75; P < 0.0001) only in men. On the other hand, during the hyperinsulinemic euglycemic clamp studies, hyperinsulinemia acutely increased leptin concentrations (20%) only in women. There was no correlation noted between fasting leptin levels and either resting energy expenditure or insulin-induced thermogenesis in men or women (P = NS). In stepwise and multiple regression models with leptin as the dependent variable, noninsulin-dependent diabetes mellitus did not enter the equations at a statistically significant level. The data indicate that there are important gender-based differences in the regulation and action of leptin in humans. Serum leptin levels increase with progressive obesity in both men and women. However, for any given measure of obesity, leptin levels are higher in women than in men, consistent with a state of relative leptin resistance. These findings have important implications regarding differences in body composition in men and women. The observation that serum leptin is not related to energy expenditure rates suggests that leptin regulates body fat predominantly by altering eating behavior rather than calorigenesis.

Multiple endocrinopathies in an infant with fatal neurodegenerative disease

We report on a male infant with congenital hypoparathyroidism who developed primary hypothyroidism at 3 months and insulin-dependent diabetes mellitus at 25 months. He had evidence of widespread and progressive neurologic dysfunction characterized by severe developmental delay, blindness, deafness, seizures, atrophy of the cerebellar and frontal lobes, and elevated spinal fluid protein. Also noted were renal hypoplasia, hyporeninemic hypoaldosteronism, chronic anemia, persistent elevation of liver transaminase levels, abnormal intraventricular cardiac conduction, reduction in numbers of helper T-cells, and distinctive facial anomalies. The child died of multiorgan failure at 29 months. A mitochondrial basis for the syndrome was considered but a molecular mechanism has, as yet, not been identified.

Muscle Rad expression and human metabolism: potential role of the novel Ras-related GTPase in energy expenditure and body composition

Ras associated with diabetes (Rad), a new ras-related GTPase, was recently identified by subtractive cloning as an mRNA in skeletal muscle that is overexpressed in NIDDM. To better understand its metabolic significance, we measured skeletal muscle Rad expression in well-characterized insulin sensitive (IS) and insulin resistant (IR) subjects with normal glucose tolerance and in untreated NIDDM patients. We found no differences in expression of Rad mRNA levels among IS, IR, and NIDDM groups using a ribonuclease protection assay (0.22 +/- 0.06, 0.13 +/- 0.01, and 0.16 +/- 0.02 relative units, respectively; NS) and no differences in Rad protein expression using a specific anti-peptide Rad antibody (1.05 +/- 0.18, 1.14 +/- 0.08, and 1.08 +/- 0.21 units/mg protein, respectively; NS). However, Rad protein levels were positively correlated with BMI (r = 0.43, P = 0.03) and percentage body fat (r = 0.55, P < 0.005), two independent measures of obesity, and negatively correlated with resting metabolic rate (r = 0.49, P = 0.01). In multiple regression analyses, percentage body fat and resting metabolic rate independently accounted for 30 and 10% of individual variability in muscle Rad protein expression. In conclusion, Rad expression in skeletal muscle is not altered as a function of insulin resistance or NIDDM in humans. However, these data, for the first time, implicate a role for Rad in regulating body composition and energy expenditure and provide a framework for studies designed to elucidate Rad's cellular functions.

Genomewide search for genes influencing percent body fat in Pima Indians: suggestive linkage at chromosome 11q21-q22. Pima Diabetes Gene Group

On the basis of accumulating evidence that obesity has a substantial genetic component, a genomewide search for linkages of DNA markers to percent body fat is ongoing in Pima Indians, a population with a very high prevalence of obesity. An initial screen of the genome (>600 markers in 874 individuals) has been completed using highly polymorphic markers (mean heterozygosity = .67). Reported here are the sib-pair linkage results for percent body fat (277 siblings), the best available indicator of overall obesity. Single-marker linkages to percent body fat were evaluated by sib-pair analysis for quantitative traits. From these analyses, the best evidence of genes influencing body fat came from markers at chromosome 11q21-q22 and 3p24.2-p22 (P = .001; LOD = 2.0). Regions flanking these markers were further investigated by multipoint linkage. The evidence for linkage at 11q21-q22 increased to P = .0002 (LOD = 2.8), peaking between markers D11S2000 and D11S2366. Evidence for linkage at 3p24.2-p22 did not change. No association was detected for any marker in the region. Although several genes are known in the 11q21-q22 region, none have been implicated as candidate genes for obesity.

Isolation by preparative free-flow electrophoresis and aqueous two-phase partition from rat adipocytes of an insulin-responsive small vesicle fraction with glucose transport activity

Preparative free-flow electrophoresis and aqueous two-phase polymer partition were used to obtain a plasma membrane-enriched fraction of adipocytes isolated from epididymal fat pads of the rat together with a fraction enriched in small vesicles with plasma membrane characteristics (thick membranes, clear dark-light-dark pattern). The electrophoretic mobility of the small vesicles was much less than that of the plasma membrane consistent with an inside-out orientation whereby charged molecules normally directed to the cell surface were on the inside. When plasma membranes and the small vesicle fraction were isolated from fat cells treated or not treated with 100 microU/ml insulin and the resident proteins of the two fractions analyzed by SDS-PAGE, the two fractions exhibited characteristic responses involving specific protein bands. Insulin treatment for 2 min resulted in the loss of a 90 kDa band from the plasma membrane. At the same time, a ca. 55-kDa peptide band that was enhanced in the plasma membrane was lost from the small vesicle fraction. The latter corresponded on Western blots to the GLUT-4 glucose transporter. Thus, we suggest that the small vesicle fraction with characteristics of inside-out plasma membrane vesicles may represent the internal vesicular pool of plasma membrane subject to modulation by treatment of adipocytes with insulin.

The expression of TNF alpha by human muscle. Relationship to insulin resistance

TNFalpha is orverexpressed in the adipose tissue of obese rodents and humans, and is associated with insulin resistance. To more closely link TNF expression with whole body insulin action, we examined the expression of TNF by muscle, which is responsible for the majority of glucose uptake in vivo. Using RT-PCR, TNF was detected in human heart, in skeletal muscle from humans and rats, and in cultured human myocytes. Using competitive RT-PCR, TNF was quantitated in the muscle biopsy specimens from 15 subjects whose insulin sensitivity had been characterized using the glucose clamp. technique. TNF expression in the insulin resistant subjects and the diabetic patients was fourfold higher than in the insulin sensitive subjects, and there was a significant inverse linear relationship between maximal glucose disposal rate and muscle TNF (r = -0.60, P < 0.02). In nine subjects, muscle cells from vastus lateralis muscle biopsies were placed into tissue culture for 4 wk, and induced to differentiate into myotubes. TNF was secreted into the medium from these cells, and cells from diabetic patients expressed threefold more TNF than cells from nondiabetic subjects. Thus, TNF is expressed in human muscle, and is expressed at a higher level in the muscle tissue and in the cultured muscle cells from insulin resistant and diabetic subjects. These data suggest another mechanism by which TNF may play an important role in human insulin resistance.

The effects of brefeldin A on the glucose transport system in rat adipocytes. Implications regarding the intracellular locus of insulin-sensitive Glut4

Insulin activates glucose transport by recruiting Glut4 glucose transporters from an intracellular pool to plasma membrane (PM). To localize intracellular translocating Glut4, we studied the effects of brefeldin A (BFA), which disassembles Golgi and prevents trans-Golgi vesicular budding, on the glucose transport system. Isolated rat adipocytes were treated with and without both BFA (10 micrograms/ml) and insulin. BFA did not affect maximal rates of either 2-deoxyglucose or 3-O-methyl-glucose transport or the insulin:glucose transport dose-response curve but did increase basal transport by approximately 2-fold (p < 0.05). We also measured Glut4 in PM, low (LDM) and high density microsome subfractions. In basal cells, BFA increased PM Glut4 by 58% concomitant with a 18% decrease in LDM (p < 0.05). Insulin alone increased PM Glut4 by 3-fold concomitant with a 56% decrease in LDM. BFA did not affect insulin-induced changes in Glut4 levels in PM or LDM. Most intracellular Glut4 was localized to sub-PM vesicles by immunoelectron microscopy in basal cells, and BFA did not affect insulin-mediated recruitment of immunogold-labeled Glut4 to PM. In summary, 1) in basal cells, BFA led to a small increase in glucose transport activity and redistribution of a limited number of transporters from LDM to PM; 2) BFA did not affect insulin's ability to stimulate glucose transport or recruit normal numbers of LDM Glut4 to PM; and 3) insulin action is predominantly mediated by a BFA-insensitive pool of intracellular Glut4, which localizes to sub-PM vesicles. Thus, the major translocating pool of Glut4 in rat adipocytes does not involve trans-Golgi.

Glucosamine induces insulin resistance in vivo by affecting GLUT 4 translocation in skeletal muscle. Implications for glucose toxicity

Glucosamine (Glmn), a product of glucose metabolism via the hexosamine pathway, causes insulin resistance in isolated adipocytes by impairing insulin-induced GLUT 4 glucose transporter translocation to the plasma membrane. We hypothesized that Glmn causes insulin resistance in vivo by a similar mechanism in skeletal muscle. We performed euglycemic hyperinsulinemic clamps (12 mU/kg/min + 3H-3-glucose) in awake male Sprague-Dawley rats with and without Glmn infusion at rates ranging from 0.1 to 6.5 mg/kg/min. After 4h of euglycemic clamping, hindquarter muscles were quick-frozen and homogenized, and membranes were subfractionated by differential centrifugation and separated on a discontinuous sucrose gradient (25, 30, and 35% sucrose). Membrane proteins were solubilized and immunoblotted for GLUT 4. With Glmn, glucose uptake (GU) was maximally reduced by 33 +/- 1%, P < 0.001. The apparent Glmn dose to reduce maximal GU by 50% was 0.1 mg/kg/min or 1/70th the rate of GU on a molar basis. Control galactosamine and mannosamine infusions had no effect on GU. Relative to baseline, insulin caused a 2.6-fold increase in GLUT 4 in the 25% membrane fraction (f), P < 0.01, and a 40% reduction in the 35%f, P < 0.05, but had no effect on GLUT 4 in the 30% f, P= NS. Addition of Glmn to insulin caused a 41% reduction of GLUT 4 in the 25%f, P < 0.05, a 29% fall in the 30%f, and prevented the reduction of GLUT 4 in the 35% f. The 30%f membranes were subjected to a second separation with a 27 and 30% sucrose gradient. Insulin mobilized GLUT 4 away from the 30%f, P < 0.05, but not the 27% f. In contrast, Glmn reduced GLUT 4 in the 27%f, P < 0.05, but not the 30%f. Thus Glmn appears to alter translocation of an insulin-insensitive GLUT 4 pool. Coinfusion of Glmn did not alter enrichment of the sarcolemmal markers 5'-nucleotidase, Na+/K+ATPase, and phospholemman in either 25, 30, or 35% f. Thus Glmn completely blocked movement of Glut 4 induced by insulin. Glmn is a potent inducer of insulin resistance in vivo by causing (at least in part) a defect intrinsic to GLUT 4 translocation and/or trafficking. These data support a potential role for Glmn to cause glucose-induced insulin resistance (glucose toxicity).

Mechanisms of enhanced insulin sensitivity in endurance-trained athletes: effects on blood flow and differential expression of GLUT 4 in skeletal muscles

Exercise is associated with increased insulin sensitivity. To better understand mechanisms that could be responsible for this association, we studied seven controls and seven endurance-trained athletes. A 600 mU/m2.min hyperinsulinemic euglycemic glucose clamp with the limb balance technique assessed insulin sensitivity as whole body glucose uptake (WBGU) and leg glucose uptake (LGU). Indirect calorimetry and hemodynamic measurements, such as leg blood flow (LBF) and cardiac output, were performed at baseline and maximal insulin stimulation. The content of the glucose transporter GLUT 4 and muscle fiber type were evaluated in three muscle groups: vastus lateralis, gastrocnemius, and biceps. Athletes exhibited 35% higher WBGU and 30% higher LGU than controls. Basal LBF (liters per min) was higher in athletes, but the difference was not statistically significant. After insulin stimulation, LBF was 31% higher in athletes than controls (P = 0.05). Indirect calorimetry revealed that athletes had a 44% higher rate of nonoxidative glucose metabolism than controls (P = 0.01). GLUT 4 levels in vastus were 90% (P < 0.05) greater in athletes, whereas smaller differences were noted between athletes and controls in biceps and gastrocnemius. Importantly, the vastus lateralis GLUT 4 content was correlated with WBGU (r = 0.60; P < 0.05) and LGU (r = 0.62; P < 0.05). Relative numbers of oxidative fibers were increased in vastus from athletes and were positively correlated with maximal oxygen consumption (VO2 max), but GLUT 4 content could not be correlated with oxidative fiber content in individual controls or athletes. We conclude that in humans 1) endurance training enhances insulin's ability to increase LBF; 2) GLUT 4 is differentially expressed as a function of muscle group and is up-regulated by exercise in a muscle-specific manner; 3) in vastus lateralis, GLUT 4 levels are well correlated with insulin-stimulated rates of both WBGU and LGU; and 4) GLUT 4 content and in vivo insulin sensitivity do not vary as a function of fiber type composition. Thus, blood flow and GLUT 4 expression in muscle are important mechanisms that mediate greater insulin sensitivity in athletes.

Gene expression of epithelial glucose transporters: the role of diabetes mellitus

The functions of absorption of dietary glucose by the small intestine and reabsorption of filtered glucose by the renal proximal tubule are strikingly similar in their organization and in the way they adapt to uncontrolled diabetes mellitus. In both cases, transepithelial glucose and Na+ fluxes are augmented. The epithelial adaptations to hyperglycemia of uncontrolled diabetes are accomplished by increasing the glucose transport surface area and the number of the efflux glucose transporter GLUT2 located in the basolateral membrane. The signals that modify the size of the epithelium and the overexpression of basolateral GLUT2 are not known. It was speculated that high glucose levels and enhanced Na+ flux may be important factors in the signaling event that culminates in a renal and intestinal epithelium that is modified to transport higher rates of glucose against a higher extracellular level of glucose.

Glucose transporter proteins and insulin sensitivity in humans

1. Pretranslational suppression of glucose transport protein, isozyme 4 (GLUT 4), is a major mechanism of insulin resistance in adipocytes in obesity and non-insulin-dependent diabetes mellitus (NIDDM). 2. Patients with gestational diabetes mellitus (GDM) are heterogeneous; adipocyte GLUT 4 levels are either normal or markedly reduced but all patients exhibit abnormalities in GLUT 4 subcellular distribution and insulin-mediated translocation. 3. Skeletal muscle GLUT 4 expression is normal in obesity, impaired glucose tolerance (IGT), GDM, and NIDDM, indicating that functional activity or translocation of GLUT 4 may be impaired. 4. Adipocyte defects in GDM consistent with abnormalities in GLUT 4-vesicle traffic have implications with respect to potential mechanisms of insulin resistance in human muscle. Given the central role of insulin resistance in NIDDM and Syndrome 'X', elucidating the underlying mechanism in muscle is critical for developing more effective treatment and disease prevention.

Molecular adaptations of GLUT1 and GLUT2 in renal proximal tubules of diabetic rats

The renal reabsorption of glucose is mediated by two major classes of transporters. Initially, luminal glucose is concentrated in tubules by Na(+)-glucose cotransporters (Na(+)-GLUT). Afterwards, glucose reaches the blood space through facilitative glucose transporters, low-Michaelis constant (Km) GLUT1 and high-Km GLUT2. Hence, the transtubular flux of glucose could be impaired in hyperglycemia because the outwardly directed glucose gradient, from tubule to blood, is potentially lowered. However, in diabetic rats, transtubular glucose flux is not reduced but increased. In this work the molecular mechanism underlying this adaptation was examined. We tested the hypothesis that upregulation of renal tubular high-Km GLUT2 gene may compensate for the decrease in the tubule to blood glucose gradient. In rat tubules, GLUT1 protein and mRNA steady-state levels were reduced, and GLUT2 protein and mRNA levels were increased in rats after 2, 3, and 4 wk of uncontrolled streptozotocin-induced diabetes. These molecular adaptations were associated with augmented facilitative glucose flux. In summary, changes in GLUT1 and GLUT2 gene expression are important to the preservation of renal glucose reabsorption in hyperglycemia.

Biological actions of insulin are differentially regulated by glucose and insulin in primary cultured adipocytes. Chronic ability to increase glycogen synthase activity

We have shown previously that prolonged exposure to insulin and glucose impairs the insulin-responsive glucose transport system in primary cultured adipocytes. To assess the ability of insulin and glucose to regulate other cellular insulin actions, epididymal rat adipocytes were cultured in media containing 0-15 mM D-glucose and with or without insulin (50 ng/ml). After 24 h, cells were washed and basal and maximally insulin-stimulated rates of 2-deoxy-D-glucose uptake, L-leucine incorporation into protein, glucose oxidation to CO2, glucose incorporation into lipids, and glycogen synthase activity were measured. The results confirmed that glucose potentiates insulin's chronic ability to decrease basal and maximal glucose transport rates by approximately 50% at 5 mM glucose and by approximately 70% at 15 mM glucose compared with control cells. However, neither glucose nor insulin, alone or in combination, affected rates of leucine incorporation into protein. In addition, basal and maximal rates of glucose oxidation and of glucose incorporation into lipids were not regulated by glucose, and maximal responses declined approximately 50% over 24 h only when insulin was not present during preincubation (i.e., chronic insulin exposure was necessary to maintain full maximal responses). Glycogen synthase activity was measured in a cell-free system (0.5 mM UDP-glucose, with 10 or 0.01 mM glucose-6-phosphate) after exposing intact cells to glucose and insulin. Both short-term (1 h) and long-term (24 h) exposure to glucose alone led a dose-dependent increase in I-form and D-form glycogen synthase activity. Chronic exposure to insulin also increased total glycogen synthase activity (I- plus D-form) but did not affect absolute rates of maximally stimulated I-form activity. Glucose (but not insulin) increased the cellular content of immunoreactive glycogen synthase by 70% after 1 h. These results show that 1) chronic exposure to glucose and insulin impairs insulin responsiveness of the glucose transport system but does not affect rates of amino acid incorporation into protein; 2) the chronic presence of insulin is necessary for the maintenance of normal maximally stimulated rates of glucose oxidation and of glucose incorporation into lipids in cultured cells; and 3) glucose increases both D-form and I-form glycogen synthase activity, in part by increasing the amount of synthase protein, whereas chronic insulin exposure increases total glycogen synthase activity without altering maximal absolute rates of I-form activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Potential role for insulin and cycloheximide in regulating the intrinsic activity of glucose transporters in isolated rat adipocytes

We examined the hypothesis that insulin stimulation of cellular glucose transport may involve a protein synthesis-dependent regulation of glucose transporter (GTer) activity independent of GTer translocation to the cell surface. Rat adipocytes were isolated, incubated with or without 10 micrograms/ml (36 microM) cycloheximide (CHX) for 60 min, and then with or without 7 nM insulin for 30 min. Glucose transport rates were assessed in intact cells, and both glucose transport rates and GTer levels were assessed in subcellular fractions of membrane vesicles before and after reconstitution into artificial liposomes. GTer functional and intrinsic activities were calculated as the ratio between these transport rates and GTer levels in native and reconstituted membranes, respectively. Insulin increased functional activity by 340% in native plasma membrane (PM) vesicles and intrinsic activity by 60% in reconstituted membranes (from 54 +/- 4 to 86 +/- 4 molecules transported per GTer/sec, P < 0.02). CHX preincubation of cells did not interfere with the insulin effect to stimulate glucose transport rate in either intact cells or in native PMs; it did, however, reduce PM GTer levels by 27-30%, but not affecting those in the intracellular pool. However, CHX additively increased the insulin-stimulated intrinsic activity of PM GTers by 67%. Relative reconstitution efficiencies, assessed by immunoblotting both native and reconstituted membranes against specific antibodies, were similar for GLUT 1 and GLUT 4. Although insulin did not alter this efficiency, CHX slightly decreased it for GLUT 4. Our data suggest that insulin stimulation of glucose transport may involve, as part of its mechanism, modulation of the GTer intrinsic activity. We further hypothesize that CHX effects on increasing this activity state of GTer may involve as yet unknown protein synthesis-dependent regulator(s).

Multiple defects in the adipocyte glucose transport system cause cellular insulin resistance in gestational diabetes. Heterogeneity in the number and a novel abnormality in subcellular localization of GLUT4 glucose transporters

Mechanisms causing cellular insulin resistance in gestational diabetes mellitus are not known. We, therefore, studied isolated omental adipocytes obtained during elective cesarean sections in nondiabetic (control) and GDM gravidas. Cellular insulin resistance was attributed to impaired stimulation of glucose transport; compared with control subjects, basal and maximally insulin-stimulated transport rates (per surface area) were reduced 38 and 60% in GDM patients, respectively. To determine underlying mechanisms, we assessed the number, subcellular distribution, and translocation of GLUT4, the predominant insulin-responsive glucose transporter isoform. The cellular content of GLUT4 was decreased by 44% in GDM patients as assessed by immunoblot analysis of total postnuclear membranes. However, GDM patients segregated into two subgroups; half expected profound (76%) cellular depletion of GLUT4 and half had GLUT4 levels in the normal range. Cellular GLUT4 was negatively correlated with adipocyte size in the control subjects and GDM patients with normal GLUT4 (r = 0.60), but fell way below this continuum in GDM patients with low GLUT4, indicating that heterogeneity was not caused by differences in obesity. All GDM. distribution. In basal cells, increased amounts of GLUT4 were detected in membranes fractionating with (such that the plasma membrane GLUT4 level in GDM (such that the plasma membrane GLUT4 level in GDM patients was equal to that observed in insulin-stimulated cells from control subjects). Furthermore, insulin stimulation induced translocation of GLUT4 from low-density microsomes to plasma membranes in control subjects but did not alter subcellular distribution in GDM patients. In other experiments, cellular content of GLUT1 was normal in GDM patients, and GLUT1 did not undergo insulin-mediated recruitment to plasma membranes in either control subjects or GDM patients. A faint signal was detected for GLUT3 only in low-density microsomes and only with one of two different antibodies. In GDM, we conclude that insulin resistance in adipocytes involves impaired stimulation of glucose transport and arises from a heterogeneity of defects intrinsic to the glucose transport effector system. GLUT4 content in adipocytes is profoundly depleted in approximately 50% of GDM patients, whereas all patients are found to exhibit a novel abnormality in GLUT4 subcellular distribution. This latter defect is characterized by accumulation of GLUT4 in membranes cofractionating with plasma membranes and high-density microsomes in basal cells and absence of translocation in response to insulin. The data suggest that abnormalities in cellular traffic or targeting relegate GLUT4 to a membrane compartment from which insulin cannot recruit transporters to the cell surface and have important implications regarding skeletal muscle insulin resistance in GDM and NIDDM.

Muscle group-specific regulation of GLUT 4 glucose transporters in control, diabetic, and insulin-treated diabetic rats

Insulin resistance in diabetic rats involves pretranslational suppression of the GLUT 4 glucose transporter in muscle. Because the capacity for insulin-mediated glucose transport varies as a function of muscle group, we hypothesized that GLUT 4 was differentially expressed and regulated by diabetes in a muscle-specific manner. We studied control (C), streptozocin (STZ)-induced diabetic (D), and insulin-treated diabetic (Tx) rats and examined the following muscles that vary in fiber composition: soleus (type I fibers), gastrocnemius (mixed type IIa > IIb), vastus lateralis and rectus abdominis (type IIb > IIa), and cardiac muscle. In C animals, these muscles exhibited significant differences in the baseline expression of GLUT 4. Relative GLUT 4 content was highest in cardiac muscle, intermediate in soleus, and significantly lower in gastrocnemius, rectus abdominis, and vastus lateralis (1.8:1.0:0.6). The impact of diabetes and insulin therapy on GLUT 4 expression also varied as a function of muscle group. After four weeks of diabetes, GLUT 4 levels were reduced by approximately 50% in cardiac muscle, soleus, and gastrocnemius. In contrast, GLUT 4 content in rectus abdominis and vastus lateralis was similar to that in control rats. Exogenous insulin treatment of diabetic rats increased GLUT 4 content in soleus, cardiac muscle, and gastrocnemius, but had no effect in either vastus lateralis or rectus abdominis. Temporal effects of diabetes and insulin treatment were also examined in different skeletal muscle. Soleus showed a significant decrease in GLUT 4 content as early as 2 days with a further decrease at 4 weeks; rectus abdominis showed little change at either time point.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-responsive human adipocytes express two glucose transporter isoforms and target them to different vesicles

We have characterized the insulin-dependent increase in glucose transport in human adipocytes using subcellular fractionation and antibodies specific for the two isoforms of the glucose transporter that are expressed in these cells. Plasma membranes isolated from untreated human fat cells contain the erythroid/GLUT1 isoform of the glucose transporter almost exclusively whereas the muscle-fat/GLUT4 transporter isoform is most abundant in intracellular microsomal membranes in resting cells. After exposure of adipocytes to insulin, the muscle-fat isoform is dramatically increased in the plasma membrane whereas the erythroid isoform barely changes in response to insulin. Thus, the total insulin-mediated increase in plasma membrane glucose transporters, confirmed by affinity labeling of both transporter isoforms, must be due to the increase in the muscle-fat/GLUT4 transporter. The two isoforms exist in different vesicle populations as shown by immunoadsorption of the muscle fat isoform-containing vesicles which are essentially devoid of the erythroid transporter. These data indicate that the insulin-mediated increases in glucose transport in human fat cells is a result of the translocation of vesicles uniquely containing the muscle-fat glucose transporter isoform.

Effects of diabetes on myocardial glucose transport system in rats: implications for diabetic cardiomyopathy

Biochemical mechanisms underlying impaired myocardial glucose utilization in diabetes mellitus have not been elucidated. We studied sarcolemmal vesicles (SL) in control, streptozotocin-induced diabetic (D), and insulin-treated diabetic (Tx) rats and found that 3-O-methylglucose transport rates were decreased 53% in D rats and were normalized by insulin therapy. Immunoblot analyses of SL revealed that GLUT4 glucose transporters were decreased 56% in D and were normal in Tx rats. Thus diminished transport rates could be fully explained by reduced numbers of SL GLUT4 with normal functional activity. To determine whether SL GLUT4 were decreased due to tissue depletion or abnormal subcellular distribution, we measured GLUT4 in total membranes (SL plus intracellular fractions). Total GLUT4 (per mg membrane protein or per DNA) was decreased 45-51% in D [half time = 3.5 days after streptozotocin], and these values were restored to normal in Tx rats. Also, diabetes decreased GLUT4 mRNA levels by 43%, and this effect was reversed by insulin therapy. We conclude that, in diabetes, 1) impaired myocardial glucose utilization is the result of a decrease in glucose transport activity, and 2) transport rates are reduced due to pretranslational suppression of GLUT4 gene expression and can be corrected by insulin therapy. GLUT4 depletion could limit glucose availability under conditions of increased workload and anoxia and could cause myocardial dysfunction.