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W T Garvey

Publications and source records attributed to W T Garvey.

At least 55 records · Page 3Linked to original sources
PubMed1994

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)

Adipocytes↗
PubMed1994

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.

Adipose Tissue↗
PubMed1993

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)

Analysis of Variance↗
PubMed1993

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.

3-O-Methylglucose↗
PubMed1993

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).

Adipocytes↗
PubMed1993

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.

Adipose Tissue↗
PubMed1993

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.

Adipocytes↗
PubMed1992

Glucose transporters of rat proximal tubule: differential expression and subcellular distribution.

In the late proximal tubule, glucose reabsorption progressively lowers the concentration of luminal glucose, and concentrative glucose influx increases to ensure complete glucose reabsorption. The change in glucose influx is effected by luminal Na(+)-dependent glucose transporters (Na(+)-GLUT), which exhibit higher Na(+)-to-glucose stoichiometric ratios in the late proximal tubule. In this work, the corresponding changes in the axial distribution of basolateral glucose efflux transporters (GLUTs) were examined. mRNAs encoding high-affinity facilitative basolateral transporter GLUT1, low-affinity GLUT2, and apical Na(+)-GLUT were identified in mixed populations of proximal convoluted and straight tubules. The organization of the cognate proteins was also appraised on Western blots. GLUT1 was present in glomeruli, proximal convoluted, and straight tubules, GLUT2 was only expressed in the proximal convoluted tubule, and Na(+)-GLUT was present in both proximal convoluted and straight segments. GLUT1 and GLUT2 were confined to the basolateral membrane, whereas Na(+)-GLUT was preferentially localized to the brush-border membrane. These data are consistent with the idea that glucose influx in early and late proximal tubule is achieved through Na(+)-GLUT, that GLUT1 and GLUT2 are responsible for glucose efflux in the early proximal tubule, and that in the late proximal tubule, where transcellular glucose flux is lower, only GLUT1 mediates glucose efflux.

Animals↗
PubMed1992

Glucose and insulin chronically regulate insulin action via different mechanisms in BC3H1 myocytes. Effects on glucose transporter gene expression.

We previously reported that, in primary cultured adipocytes, chronic exposure to glucose plus insulin impairs the insulin-responsive glucose transport system. In this study, we examined regulation of glucose transport in BC3H1 myocytes as a model for muscle and found important differences between BC3H1 cells and adipocytes. In myocytes, chronic glucose exposure per se (25 mM) decreased basal glucose transport activity by 78% and insulin's acute ability to maximally stimulate transport by 68% (ED50 approximately 2.5 mM; T1/2 approximately 4 h). D-Mannose and 3-O-methyl-glucose diminished transport rates with approximately 100 and 50% of the potency of D-glucose, respectively, whereas L-glucose, D-fructose, and D-galactose were inactive. Chronic glucose exposure also reduced cell surface insulin binding by 30% via an apparent decrease in receptor affinity, and this effect was associated with a comparable rightward shift in the insulin-glucose transport dose-response curve. In other studies, persistent stimulation with 15 nM insulin also decreased maximally stimulated glucose transport activity, which was independent and additive to the regulatory effect of glucose. Moreover, glucose and insulin-induced insulin resistance via different mechanisms. Glucose (25 mM) reduced the number of cellular glucose transporter proteins by 84% and levels of GLUT1 transporter mRNA by 50% (whether normalized to total RNA or CHO-B mRNA). In contrast, chronic insulin exposure led to a 2.1-fold increase in GLUT1 mRNA but did not alter cellular levels of transporter protein. Cotreatment with glucose prevented the insulin-induced rise in GLUT1 mRNA. BC3H1 cells did not express GLUT4 mRNA that encodes the major transporter isoform in skeletal muscle. In conclusion, in BC3H1 myocytes 1) glucose diminished insulin sensitivity by decreasing insulin receptor binding affinity and decreased basal and maximally insulin-stimulated glucose transport rates via cellular depletion of glucose transporters and suppression of GLUT1 mRNA; 2) chronic insulin exposure exerted an independent and additive effect to reduce maximal transport activity; however, insulin increased levels of GLUT1 mRNA and did not alter the cellular content of glucose transporters; and 3) although BC3H1 cells are commonly used as a model for skeletal muscle, studies examining glucose transport should be interpreted cautiously due to the absence of GLUT4 expression. Nevertheless, the data generally support the idea that, in non-insulin-dependent diabetes mellitus, hyperglycemia and hyperinsulinemia can induce or exacerbate insulin resistance in target tissues.

3-O-Methylglucose↗
PubMed1992

Gene expression of GLUT4 in skeletal muscle from insulin-resistant patients with obesity, IGT, GDM, and NIDDM.

In obesity, impaired glucose tolerance (IGT), non-insulin-dependent diabetes mellitus (NIDDM), and gestational diabetes mellitus (GDM), defects in glucose transport system activity, contribute to insulin resistance in target tissues. In adipocytes from obese and NIDDM patients, we found that pretranslational suppression of the insulin-responsive GLUT4 glucose transporter isoform is a major cause of cellular insulin resistance; however, whether this process is operative in skeletal muscle is not clear. To address this issue, we performed percutaneous biopsies of the vastus lateralis in lean and obese control subjects and in obese patients with IGT and NIDDM and open biopsies of the rectus abdominis at cesarian section in lean and obese gravidas and gravidas with GDM. GLUT4 was measured in total postnuclear membrane fractions from both muscles by immunoblot analyses. The maximally insulin-stimulated rate of in vivo glucose disposal, assessed with euglycemic glucose clamps, decreased 26% in obesity and 74% in NIDDM, reflecting diminished glucose uptake by muscle. However, in vastus lateralis, relative amounts of GLUT4 per milligram membrane protein were similar (NS) among lean (1.0 +/- 0.2) and obese (1.5 +/- 0.3) subjects and patients with IGT (1.4 +/- 0.2) and NIDDM (1.2 +/- 0.2). GLUT4 content was also unchanged when levels were normalized per wet weight, per total protein, and per DNA as an index of cell number. Levels of GLUT4 mRNA were similarly not affected by obesity, IGT, or NIDDM whether normalized per RNA or for the amount of an unrelated constitutive mRNA species. Because muscle fibers (types I and II) exhibit different capacities for insulin-mediated glucose uptake, we tested whether a change in fiber composition could cause insulin resistance without altering overall levels of GLUT4. However, we found that quantities of fiber-specific isoenzymes (phopholamban and types I and II Ca(2+)-ATPase) were similar in all subject groups. In rectus abdominis, GLUT4 content was similar in the lean, obese, and GDM gravidas whether normalized per milligram membrane protein (relative levels were 1.0 +/- 0.2, 1.3 +/- 0.1, and 1.0 +/- 0.2, respectively) or per wet weight, total protein, and DNA. We conclude that in human disease states characterized by insulin resistance, i.e., obesity, IGT, NIDDM, and GDM, GLUT4 gene expression is normal in vastus lateralis or rectus abdominis. To the extent that these muscles are representative of total muscle mass, insulin resistance in skeletal muscle may involve impaired GLUT4 function or translocation and not transporter depletion as observed in adipose tissue.

Adult↗
PubMed1992

Glucose transport and NIDDM.

Three major metabolic abnormalities contribute to hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM) including defective glucose-induced insulin secretion, elevated rates of hepatic glucose output, and insulin's impaired ability to stimulate glucose uptake in peripheral target tissues (insulin resistance). These functions involve cellular glucose transport in beta-cells, liver, adipose tissue, and skeletal muscle; and, in some instances, abnormalities in glucose transporter isoforms (GLUT) specifically expressed in these tissues may constitute key biochemical lesions underlying defective glucose homeostasis. In animal models of NIDDM, suppression of GLUT2 in beta-cells is correlated with loss of high-Km glucose transport and glucose-sensitive insulin secretion. Although there are no data on humans with NIDDM, GLUT2 loss would constitute an attractive mechanism for defective glucose sensing in beta-cells if it can be shown that transport then becomes rate limiting for glucose metabolism. In the liver, however, hepatocyte glucose transport via GLUT2 probably plays only a permissive role in sustaining increased glucose efflux. Peripheral insulin resistance is associated with decreased glucose transport activity, the likely rate-limiting step for glucose uptake in fat and muscle. Accordingly, the insulin-responsive GLUT4 isoform expressed exclusively in insulin target tissues has been studied intensively in NIDDM. In these studies, pretranslational suppression of GLUT4 appears to be the key mechanism of insulin resistance in adipocytes. However, levels of GLUT4 protein and mRNA are normal in vastus lateralis and rectus abdominis, inferring that defects in GLUT4 functional activity or insulin-mediated translocation cause insulin resistance in muscle. Thus, the intensified study of glucose transport has provided important new insights into NIDDM pathogenesis over the past 5 yr and has presented investigators with additional intriguing hypotheses.

Animals↗
PubMed1991

Demonstration of a 1,25-dihydroxyvitamin D3-responsive protein in human lymphocytes: immunologic crossreactivity and inverse regulation with the vitamin D receptor.

Using Western blot analysis with a monoclonal antibody recognizing a 17-amino acid epitope of the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]receptor, we have detected two crossreacting proteins in activated normal human lymphocytes. The smaller of the two proteins (50 kDa) was indistinguishable from the classical 1,25(OH)2D3 receptor and, similar to the classical 1,25(OH)2D3 receptor, was upregulated in a dose-dependent fashion by 1,25(OH)2D3. The larger crossreacting protein exhibited an electrophoretic mobility of 80 kDa, was localized in the cell cytosol, and appeared to be specific for activated lymphocytes since it was not detected in several other human cells including monocytes. More strikingly, the 80-kDa protein was downregulated in a dose-dependent fashion by 1,25(OH)2D3; this effect was independent of the mode of lymphocyte activation and specific for the 1,25(OH)2D3 metabolite of vitamin D3. However, two potent immunosuppressive agents, glucocorticoids and cyclosporin A, also inhibited the 80-kDa protein.

Blotting, Western↗
PubMed1991

New insights into the metabolic regulation of insulin action and insulin resistance: role of glucose and amino acids.

In primary cultured adipocytes, metabolic substrates such as glucose and amino acids have profound effects on modulating insulin's stimulatory actions on glucose uptake and protein synthesis. Insights into how substrates modulate insulin action were recently obtained when we discovered that the routing of incoming glucose through the hexosamine biosynthesis pathway leads to a refractory state over a period of several hours in which the ability of insulin to stimulate glucose uptake is severely impaired--a state known as insulin resistance. Glutamine:fructose-6-phosphate amidotransferase was found to play a central role in the development of insulin resistance as this enzyme catalyzes the first and rate-limiting step in the formation of hexosamine products. Collectively, these results are consistent with the idea that the hexosamine biosynthesis pathway serves as a glucose sensor coupled to a negative feedback system that can limit the extent of glucose uptake in response to hyperglycemic and hyperinsulinemic conditions.

Adipose Tissue↗
PubMed1991

Pretranslational suppression of a glucose transporter protein causes insulin resistance in adipocytes from patients with non-insulin-dependent diabetes mellitus and obesity.

A major portion of insulin-mediated glucose uptake occurs via the translocation of GLUT 4 glucose transporter proteins from an intracellular depot to the plasma membrane. We have examined gene expression for the GLUT 4 transporter isoform in subcutaneous adipocytes, a classic insulin target cell, to better understand molecular mechanisms causing insulin resistance in non-insulin-dependent diabetes mellitus (NIDDM) and obesity. In subgroups of lean (body mass index [BMI] = 24 +/- 1) and obese (BMI = 32 +/- 2) controls and in obese NIDDM (BMI = 35 +/- 2) patients, the number of GLUT 4 glucose transporters was measured in total postnuclear and subcellular membrane fractions using specific antibodies on Western blots. Relative to lean controls, the cellular content of GLUT 4 was decreased 40% in obesity and 85% in NIDDM in total cellular membranes. In obesity, cellular depletion of GLUT 4 primarily involved low density microsomes (LDM), leaving fewer transporters available for insulin-mediated recruitment to the plasma membrane (PM). In NIDDM, loss of GLUT 4 was profound in all membrane subfractions, PM, LDM, as well as high density microsomes. These observations corresponded with decrements in maximally stimulated glucose transport rates in intact cells. To assess mechanisms responsible for depletion of GLUT 4, we quantitated levels of mRNA specifically hybridizing with human GLUT 4 cDNA on Northern blots. In obesity, GLUT 4 mRNA was decreased 36% compared with lean controls, and the level was well correlated (r = + 0.77) with the cellular content of GLUT 4 protein over a wide spectrum of body weight. GLUT 4 mRNA in adipocytes from NIDDM patients was profoundly reduced by 86% compared with lean controls and by 78% relative to their weight-matched nondiabetic counterparts (whether expressed per RNA, per cell, or for the amount of CHO-B mRNA). Interestingly, GLUT 4 mRNA levels in patients with impaired glucose tolerance (BMI = 34 +/- 4) were decreased to the same level as in overt NIDDM. We conclude that, in obesity, insulin resistance in adipocytes is due to depletion of GLUT 4 glucose transporters, and that the cellular content of GLUT 4 is determined by the level of encoding mRNA over a wide range of body weight. In NIDDM, more profound insulin resistance is caused by a further reduction in GLUT 4 mRNA and protein than is attributable to obesity per se. Suppression of GLUT 4 mRNA is observed in patients with impaired glucose tolerance, and therefore, may occur early in the evolution of diabetes. Thus, pretranslational suppression of GLUT 4 transporter gene expression may be an important mechanism that produces and maintains cellular insulin resistance in NIDDM.

Adipose Tissue↗
PubMed1991

Glucose and insulin regulate insulin sensitivity in primary cultured adipocytes without affecting insulin receptor kinase activity.

We have previously shown in primary cultured adipocytes that chronic insulin exposure decreases insulin's subsequent ability to maximally restimulate the glucose transport system, and that extracellular glucose potentiates this ligand-induced defect in maximal insulin responsiveness. To examine whether glucose could also modulate insulin sensitivity (i.e. acute insulin effects at submaximal concentrations), adipocytes were cultured for 5 and 24 h in the absence and presence of various glucose and insulin concentrations. Then, after washing cells to remove any insulin and allow for full deactivation of transport, we assessed the dose response of insulin's acute ability to stimulate 2-deoxyglucose transport, bind to cell surface receptors, and activate insulin receptor tyrosine kinase activity. After 5 h, glucose and insulin alone had no chronic regulatory effects; however, in combination, these agents were able to decrease insulin sensitivity. In cells preincubated with 50 ng/ml insulin, the insulin ED50 for acute stimulation of glucose transport was increased by 65% and 116% as medium glucose was raised to 5 and 20 mM, respectively, relative to that at 0 mM glucose. After 24 h, chronic exposure to either glucose (20 mM) or insulin (50 ng/ml) alone increased the ED50 value by 52%, and, together they acted synergistically to increase the ED50 by 183%. While glucose and insulin independently and synergistically impaired insulin sensitivity, both agents were necessary for coregulation of maximal insulin responsiveness (confirming our previous observation). Insulin receptor down-regulation (18%) was observed after 24 h (but not 5 h) in insulin-treated cells; however, the major portion of the decrease in insulin sensitivity was due to uncoupling of occupied insulin receptors from stimulation of the glucose transport system. To further determine the mechanism for postbinding desensitization, we tested for concordant regulation of insulin receptor kinase activity. Insulin's ability to stimulate the receptor tyrosine kinase was assessed by multiple methods, including 1) receptor autophosphorylation and phosphorylation of Glu4-Tyr1 by solubilized insulin receptors activated in vitro, 2) histone-2B phosphorylation by receptors that were stimulated in intact cells and then solubilized under conditions that preserve the in cellulo phosphorylation state, and 3) receptor autophosphorylation and phosphorylation of pp180 in intact cells. Long term treatment (24 h) with glucose (10 mM) and insulin (50 ng/ml) markedly decreased insulin sensitivity (and receptor coupling), but did not affect insulin receptor kinase activity in any of these studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Adipose Tissue↗
PubMed1989

Pretranslational suppression of an insulin-responsive glucose transporter in rats with diabetes mellitus.

A prominent feature of diabetes mellitus is the inability of insulin to appropriately increase the transport of glucose into target tissues. The contributions of different glucose transport proteins to insulin resistance in rats with streptozotocin-induced diabetes was evaluated. A glucose transporter messenger RNA and its cognate protein that are exclusively expressed in muscle and adipose tissue were specifically depleted in diabetic animals, and these effects were reversed after insulin therapy; a different glucose transporter and its messenger RNA that exhibit a less restricted tissue distribution were not specifically modulated in this way. Depletion of the muscle- and adipose-specific glucose transporter species correlates with and may account for the major portion of cellular insulin resistance in diabetes in these animals.

3-O-Methylglucose↗