[show abstract][hide abstract] ABSTRACT: The incretins are gut hormones secreted in response to nutrient/carbohydrate ingestion and act on the pancreatic beta cell to amplify glucose-stimulated insulin secretion. Incretin hormone-based treatments for patients with type 2 diabetes represent a major advance in diabetes therapeutics. The ability of the incretin agents (glucagon-like peptide 1 [GLP-1] agonists and dipeptidyl peptidase IV [DPP-4] inhibitors) to improve glycaemia with a low associated risk of hypoglycaemia, together with beneficial/neutral effects on body weight, offers a significant advantage for both patients and treating clinicians. In this edition of 'Then and Now,' it is useful to look back 25 years and reflect upon the developments in this field since Nauck and colleagues published two seminal papers. In 1986 they first documented a reduced incretin effect in patients with type 2 diabetes (Diabetologia 29:46-52), and then in 1993 they demonstrated that, in patients with poorly controlled type 2 diabetes, a single exogenous infusion of an incretin (GLP-1) increased insulin levels in a glucose-dependent manner and normalised fasting hyperglycaemia (Diabetologia 36:741-744). In the ensuing 26 years, progress in the field of incretin hormones has resulted in a greater understanding of the relative roles of GLP-1 and glucose-dependent insulinotropic polypeptide secretion and activity in the pathogenesis of type 2 diabetes and the important recognition that native GLP-1 is quickly degraded by the ubiquitous protease DPP-4. This has led to the development of GLP-1 agonists that are resistant to degradation by DPP-4 and of selective inhibitors of DPP-4 activity as therapeutic agents. GLP-1 agonists (exenatide and liraglutide) and DPP-4 inhibitors (sitagliptin, vildagliptin, saxagliptin and linagliptin) currently represent effective treatment options for patients with type 2 diabetes. Several additional agents are in the pipeline, including longer acting DPP-4-resistant GLP-1 agonists. More exciting, however, is the increasing recognition that the incretin agents have numerous extra-glycaemic effects that could translate into potential cardiovascular and other benefits.
[show abstract][hide abstract] ABSTRACT: To assess changes in insulin sensitivity in non-diabetic adults with schizophrenia or schizoaffective disorder treated with olanzapine or risperidone.
One hundred and thirty patients were randomly assigned to 12 weeks double-blind treatment with olanzapine or risperidone. Insulin sensitivity was measured using a two-step euglycaemic, hyperinsulinaemic clamp procedure. Whole-body adiposity was measured using dual-energy X-ray absorptiometry. The primary endpoint was the within-group change from baseline in insulin sensitivity normalized to fat-free mass (M(ffm) /I) during the clamp procedure's low-insulin phase, using an analysis of covariance model including the covariate weight change.
Forty-one olanzapine-treated and 33 risperidone-treated patients completed baseline and endpoint clamp measurements. Mean M(ffm) /I during the low-insulin phase declined 9.0% (p = 0.226) in olanzapine-treated patients and 13.2% (p = 0.047) in risperidone-treated patients (between-group difference p = 0.354). During the high-insulin phase, M(ffm) /I declined 10.4% (p = 0.036) in olanzapine-treated patients and 2.1% (p = 0.698) in risperidone-treated patients (between-group difference p = 0.664). Changes in M(ffm) /I correlated inversely with changes in body weight and adiposity, which were generally higher in olanzapine-treated patients. Significant within-group increases in fasting glucose, but not haemoglobin A1c (HbA1c), were observed during olanzapine treatment. The fasting glucose change was not correlated with M(ffm) /I changes.
Small, but statistically significant, decrements in insulin sensitivity were observed in olanzapine- and risperidone-treated patients at 1 of 2 insulin doses tested. Significant increases in fasting glucose and insulin and total fat mass were observed only in olanzapine-treated patients. Changes in insulin sensitivity correlated significantly with changes in weight or adiposity, but not with changes in glucose.
Diabetes Obesity and Metabolism 03/2011; 13(8):726-35. · 5.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: To evaluate the effects of intensive insulin therapy alone and with added pioglitazone on body weight, fat distribution, lean body mass (LBM) and liver fat in type 2 diabetic patients.
Twenty-five insulin-treated, obese patients with type 2 diabetes were randomized to addition of pioglitazone 45 mg (n = 12) or placebo (n = 13) and treated intensively for 12-16 weeks. Dual-energy X-ray absorptiometry/abdominal computed tomography scans were performed before/after treatment. LBM, visceral/subcutaneous adipose tissue (VAT/SAT) and liver/spleen (L/S) attenuation ratios were measured pre-/posttreatment (a ratio <1 represents fatty liver).
Intensive insulin alone and insulin + pioglitazone significantly improved glycaemic control (7.8 ± 0.3 to 7.2 ± 0.3% and 7.6 ± 0.3 to 7.1 ± 0.4%, respectively). Body weight gain was greater with insulin + pioglitazone (4.9 ± 4.5 kg) versus insulin therapy alone (1.7 ± 0.7 kg). SAT increased significantly with pioglitazone + insulin therapy (393.9 ± 48.5 to 443.2 ± 56.7 cm(2) , p < 0.01) compared to a non-significant increase with insulin therapy alone (412.9 ± 42.5 to 420.8 ± 43.8 cm(2) ). VAT decreased non-significantly in both groups (240.3 ± 41.7 to 223.8 ± 38.1 cm(2) with insulin + pioglitazone and 266.6 ± 27.4 to 250.5 ± 22.2 cm(2) with insulin therapy). LBM increased significantly by 1.92 ± 0.74 kg with insulin + pioglitazone treatment. The L/S attenuation ratio in the placebo + insulin group decreased from 1.08 ± 0.1 to 1.04 ± 0.1 (p = ns) and increased from 1.00 ± 0.1 to 1.08 ± 0.05 (p = 0.06) in the pioglitazone + insulin group.
Intensification of insulin therapy in type 2 diabetic patients causes modest weight gain and no change in body fat distribution, LBM or liver fat. In contrast, the addition of pioglitazone, at equivalent glycaemia, increases weight gain, fat mass and SAT; increases LBM and tends to decrease liver fat. These changes in fat distribution may contribute to the beneficial effects of pioglitazone, despite greater weight gain.
Diabetes Obesity and Metabolism 01/2011; 13(6):505-10. · 5.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: The aim of the study was to examine the determinants of oral glucose tolerance in 602 persons with impaired glucose tolerance (IGT) who participated in the Actos Now for Prevention of Diabetes (ACT NOW) study.
In addition to the 602 IGT participants, 115 persons with normal glucose tolerance (NGT) and 50 with impaired fasting glucose (IFG) were identified during screening and included in this analysis. Insulin secretion and insulin sensitivity indices were derived from plasma glucose and insulin during an OGTT. The acute insulin response (AIR) (0-10 min) and insulin sensitivity (S(I)) were measured with the frequently sampled intravenous glucose tolerance test (FSIVGTT) in a subset of participants.
At baseline, fasting plasma glucose, 2 h postprandial glucose (OGTT) and HbA(1c) were 5.8 +/- 0.02 mmol/l, 10.5 +/- 0.05 mmol/l and 5.5 +/- 0.04%, respectively, in participants with IGT. Participants with IGT were characterised by defects in early (DeltaI (0-30)/DeltaG (0-30) x Matsuda index, where DeltaI is change in insulin in the first 30 min and DeltaG is change in glucose in the first 30 min) and total (DeltaI(0-120)/DeltaG(0-120) x Matsuda index) insulin secretion and in insulin sensitivity (Matsuda index and S(I)). Participants with IGT in whom 2 h plasma glucose was 7.8-8.3 mmol/l had a 63% decrease in the insulin secretion/insulin resistance (disposition) index vs participants with NGT and this defect worsened progressively as 2 h plasma glucose rose to 8.9-9.94 mmol/l (by 73%) and 10.0-11.05 mmol/l (by 80%). The Matsuda insulin sensitivity index was reduced by 40% in IGT compared with NGT (p < 0.005). In multivariate analysis, beta cell function was the primary determinant of glucose AUC during OGTT, explaining 62% of the variance.
Our results strongly suggest that progressive beta cell failure is the main determinant of progression of NGT to IGT.
[show abstract][hide abstract] ABSTRACT: To evaluate the effects of intensive insulin therapy alone or with added pioglitazone on renal salt/water balance and body fluid compartment shifts in type 2 diabetes.
A total of 25 insulin-treated, obese patients with type 2 diabetes were randomized to pioglitazone 45 mg (n = 12) or placebo (n = 13) and treated intensively for 12-16 weeks to achieve equivalent glycaemic control. We measured total body water (TBW) and extracellular/intracellular fluid by bioimpedance analysis; plasma/RBC volume with I(131)albumin; sodium handling by fractional excretion of sodium/lithium (FeNa/FeLi) and other renal/hormonal parameters.
Intensification of insulin therapy and the addition of pioglitazone significantly improved glycaemia (HbA1C 7.8-7.2% and 7.6-7.1%) and increased body weight (1.7 and 4.9 kg) respectively. TBW increased 1.7 l with insulin alone (65% intracellular) and 1.6 l with added pioglitazone (75% extracellular) (p = 0.06 and 0.09 respectively). Plasma volume increased 0.2 +/- 0.1 l with insulin alone (p = 0.05) and 0.4 +/- 0.1 l with added pioglitazone (p < 0.05). Extravascular, extracellular (interstitial) fluid increased significantly and more with added pioglitazone (0.8 +/- 0.2 l, p < 0.01) than with insulin alone (0.4 +/- 0.2 l, p = ns). At steady-state, FeLi (marker of proximal-tubular sodium delivery to the distal nephron) increased significantly with added pioglitazone (12.4 +/- 1.3 to 18.0 +/- 3.2%) vs. no significant change with insulin alone (15.4 +/- 1.2 to 14.5 +/- 2.3%). There were no significant changes in the other parameters.
In intensively insulin-treated obese type 2 diabetic patients, at equivalent glycaemic control, the addition of pioglitazone causes greater weight gain, but a similar increase in body water that is mainly extracellular and interstitial compared with intracellular increase with insulin therapy alone. Pioglitazone also increases the filtered load of sodium reabsorbed at the distal nephron with no net change in FeNa.
Diabetes Obesity and Metabolism 11/2009; 12(2):133-8. · 5.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: The involvement of the beta-isoform of glycogen synthase kinase (GSK-3) in glucose metabolism and insulin action was investigated in cultured human skeletal muscle cells. A 60% reduction in GSK-3beta protein expression was attained by treatment with siRNA; GSK-3alpha expression was unaltered. GSK-3beta knockdown did not influence total glycogen synthase (GS) activity, but increased the phosphorylation-dependent activity (fractional velocity-FV) in the basal state. Insulin responsiveness of GSFV was doubled by GSK-3beta knockdown (p<0.05). Basal rates of glucose uptake (GU) were not significantly influenced by GSK-3beta knockdown, while insulin stimulation of GU was increased. Improvements in insulin action on GS and GU did not involve changes in protein expression of either IRS-1 or Akt 1/2. Maximal insulin stimulation of phosphorylation of Akt was unaltered by GSK-3beta knockdown. Unlike GSK-3alpha, GSK-3beta directly regulates both GS activity in the absence of added insulin and through control of insulin action.
Molecular and Cellular Endocrinology 06/2009; 315(1-2):153-8. · 4.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: Type 2 diabetes is a common disorder often accompanied by numerous metabolic abnormalities leading to a high risk of cardiovascular morbidity and mortality. Results from the UKPDS have confirmed that intensive glucose control delays the onset and retards the progression of microvascular disease and possibly of macrovascular disease in patients with type 2 diabetes. In the early stages of the disease, insulin resistance plays a major role in the development of hyperglycemia and other metabolic abnormalities, and patients with type 2 diabetes often benefit from measures to improve insulin sensitivity such as weight loss, dietary changes, and exercise. Later, the use of oral insulin secretagogues and insulin sensitizers as monotherapy and in combination helps maintain glycemia for varying periods of time. Ultimately, because of the progressive nature of the disease and the progressive decline in pancreatic beta-cell function, insulin therapy is almost always obligatory to achieve optimal glycemic goals. Not all patients are candidates for aggressive insulin management; therefore, the goals of therapy should be modified, especially in elderly individuals and those with co-morbid conditions. Candidates for intensive management should be motivated, compliant, and educable, without other major medical conditions and physical limitations that would preclude accurate and reliable HGM and insulin administration. In selected patients, combination therapy with insulin and oral antidiabetic medications can be an effective method for normalizing glycemia without the need for rigorous multiple-injection regimens. The patients for whom combination therapy is most commonly successful are those who do not achieve adequate glycemic control using daytime oral agents but who still show some evidence of responsiveness to the medications. Bedtime intermediate-acting or predinner premixed intermediate- and rapid-acting insulin is administered and progressively increased until the FPG concentration is normalized. If combination therapy is not successful, a split-mixed regimen of intermediate- and rapid-acting insulin equally divided between the prebreakfast and pre-dinner periods is advised for oese patients, and more intensive regimens are advised for thin patients. Insulin therapy is invariably associated with weight gain and hypoglycemia. The use of metformin or glitazones in combination with insulin has been demonstrated to have insulin-sparing properties. Also, metformin use may ameliorate weight gain. The use of continuous subcutaneous insulin infusion pumps can be particularly beneficial in treating patients with type 2 diabetes mellitus who do not respond satisfactorily to more conventional treatment strategies. Intraperitoneal insulin delivery systems hold considerable promise in type 2 diabetes because of their more physiologic delivery of insulin and their ability to inhibit hepatic glucose production selectively, with less peripheral insulinemia than with subcutaneous insulin injections. Newer insulin analogues such as the rapidly acting Lispro insulin and the peakless, long-acting glargine insulin are increasingly being used because of their unique physiologic pharmacokinetics. New developments such as inhaled and buccal insulin preparations will also make it easier for many patients to initiate and maintain a proper insulin regimen. Finally, a new generation of gut peptides such as amylin and GLP-1 will add a new dimension to glycemic control through modification of nutrient delivery and other mechanisms; however, the ultimate goal in the management of type 2 diabetes is the primary prevention of the disease. The Diabetes Prevention Program (DPP) sponsored by the National Institutes of Health has currently randomly assigned more than 3000 persons with impaired glucose tolerance and at high risk of developing diabetes into three treatment arms: metformin arm, an intensive lifestyle-modification arm, and a placebo arm. The study will conclude in 2002 after all participants have been followed for 3 to 6 years.
Endocrinology & Metabolism Clinics of North America 01/2002; 30(4):935-82. · 3.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: The aim of this study was to determine whether the long-acting insulin analog, insulin glargine, behaves like human insulin for metabolic and mitogenic responses in differentiated cultured human skeletal muscle cells from nondiabetic and diabetic subjects. Human insulin and insulin glargine were equipotent in their ability to compete for [(125)I]insulin binding. Insulin glargine displaced [(125)I]IGF-I from the IGF-I-binding site with approximately 0.5% the potency of IGF-I. In nondiabetic muscle cells, all three ligands stimulated glucose uptake similarly, whereas the sensitivity of glucose uptake was greatest in response to IGF-I and lower and equal for human insulin and insulin glargine. In diabetic muscle cells, the final responsiveness of glucose uptake was greatest for IGF-I and equivalent for human insulin and insulin glargine; sensitivities were the same as those for nondiabetic cells. Thymidine uptake into DNA was stimulated foremost by IGF-I, whereas human insulin and insulin glargine showed equivalent, but greatly reduced, sensitivities and potencies (<1% IGF-I). Stimulation of Akt phosphorylation was slightly more responsive to IGF-I compared with human insulin and insulin glargine, with sensitivities similar to glucose uptake stimulation. We conclude that in human skeletal muscle cells, insulin glargine is equivalent to human insulin for metabolic responses and does not display augmented mitogenic effects.
[show abstract][hide abstract] ABSTRACT: Insulin signaling pathways potentially involved in regulation of skeletal muscle glycogen synthase were compared in differentiated human muscle cell cultures from nondiabetic and type 2 diabetic patients. Insulin stimulation of glycogen synthase activity as well as phosphorylation of MAPK, p70 S6 kinase, and protein kinase B (Akt) were blocked by the phosphatidylinositol 3-kinase inhibitors wortmannin (50 nM) and LY294002 (10 microM). In contrast to lean and obese nondiabetic subjects, where there were minimal effects (15-20% inhibition), insulin stimulation of glycogen synthase in muscle cultures from diabetic subjects was greatly diminished ( approximately 75%) by low concentrations of wortmannin (25 nM) or LY294002 (2 microM). This increased sensitivity of diabetic muscle to impairment of insulin-stimulated glycogen synthase activity occurs together with diminished insulin-stimulation (by 40%) of IRS-1-associated phosphatidylinositol 3-kinase activity in the same cells. Protein expression of IRS-1, p85, p110, Akt, p70 S6 kinase, and MAPK were normal in diabetic cells, as was insulin-stimulated phosphorylation of Akt, p70 S6 kinase, and MAPK. These studies indicate that, despite prolonged growth and differentiation of diabetic muscle under normal metabolic culture conditions, defects of insulin-stimulated phosphatidylinositol 3-kinase and glycogen synthase activity in diabetic muscle persist, consistent with intrinsic (rather than acquired) defects of insulin action.
[show abstract][hide abstract] ABSTRACT: Retinoid X receptor (RXR) is a nuclear receptor that functions as an obligate heterodimeric partner of peroxisome proliferator-activator receptor (PPAR). Studies have shown that the alpha isoform of RXR and PPARgamma act synergistically to regulate gene expression and insulin action. The aim of the current study was to compare the expression and regulation of RXR in the primary insulin-sensitive tissue, skeletal muscle, of various degrees of insulin-resistant states including obese type 2 diabetic (T2D), obese nondiabetic (OND), and lean nondiabetic (LND) subjects. Insulin action/resistance was determined by a 3-hour hyperinsulinemic, euglycemic (5.0 to 5.5 mmol/L) clamp. Percutaneous biopsy of the vastus lateralis muscle was performed before and after the clamp. RXRalpha mRNA was measured using a quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay, while protein was determined by Western blotting. All 3 isoforms of RXR, alpha, beta, and gamma, were present in skeletal muscle. Protein expression of RXR isoforms did not differ between groups; RXR alpha mRNA was also similar between groups. Neither RXR alpha mRNA, RXR -beta nor -gamma protein displayed significant relationships with any of the clinical or laboratory parameters measured, including insulin sensitivity. RXR alpha exhibited a negative correlation with free fatty acids levels (r, -.42, P <.05). There was also no relationship between RXR alpha and PPARgamma protein levels. RXR alpha mRNA was unaltered following insulin infusion. We conclude that RXR isoform (alpha, beta, gamma) expression is not tightly controlled by insulin, insulin resistance or type 2 diabetes. Instead, RXR isoforms are likely constitutive proteins or controlled by other factors.
[show abstract][hide abstract] ABSTRACT: To determine the independent and potentially synergistic effects of agonists for PPAR gamma and RXR on glucose and lipid metabolism, as well as gene expression, in human skeletal muscle cell cultures.
Fully differentiated myotubes from non-diabetic subjects and subjects with Type II (non-insulin-dependent) diabetes mellitus were chronically (2 days) treated with LG100268 (4 mumol/l), an RXR agonist, or troglitazone (4.6 mumol/l), a PPAR gamma agonist or both, to determine the effects on glucose uptake, activity of glycogen synthase and palmitate oxidation.
The combination of both agents increased glucose uptake (60 +/- 9% compared to control subjects) but not either agent alone (16 +/- 9 and 26 +/- 6% for LG100268 and troglitazone, p < 0.01, respectively). The agent LG100268 alone had little effect on the activity of glycogen synthase but the effect of troglitazone increased with LG100268 (p < 0.05). With chronic exposure, LG100268 upregulated palmitate oxidation (53 +/- 12% increase, p < 0.005), in a way similar to troglitazone (68 +/- 23%, p < 0.005). Synergism was observed when both agonists were combined (146 +/- 38%, p < 0.005 vs either agent alone). Treatment with either agent led to about a twofold increase in the expression of fatty acid transporter (FAT/CD36). Troglitazone upregulated PPAR gamma protein expression, whereas LG100268 had no effect. Furthermore, neither LG100268 nor troglitazone had any effect on the protein expression of RXR isoforms or PPAR alpha.
Co-activation of PPAR gamma and RXR results in additive or synergistic effects on glucose and lipid metabolism in skeletal muscle, but unlike troglitazone, LG100268 does not alter expression of its own receptor.
[show abstract][hide abstract] ABSTRACT: Type 2 diabetes mellitus is a growing problem not only in the United States but also across the world. There is now strong evidence that intensive control of blood glucose can significantly reduce and retard the microvascular complications of retinopathy, nephropathy, and neuropathy. Ultimately however, up to 80% of type 2 diabetics die from macrovascular cardiovascular disease. This increased incidence of atherosclerotic disease is intricately associated with insulin resistance, which is a major pathophysiologic abnormality in type 2 diabetes. There is strong evidence that insulin resistance is involved in the development of not only hyperglycemia, but also dyslipidemia, hypertension, hypercoagulation, vasculopathy, and ultimately atherosclerotic cardiovascular disease. This cluster of metabolic abnormalities has been termed the insulin resistance or cardiovascular dysmetabolic syndrome. The thiazolidinediones (rosiglitazone and pioglitazone), a new class of oral antidiabetic agents, are "insulin sensitizers" and exert direct effects on the mechanisms of insulin resistance. These effects not only improve insulin sensitivity and glycemic control with reduced insulin requirements, but also have potentially favorable effects on other components of the cardiovascular dysmetabolic syndrome. Long-term studies are needed to determine whether the insulin-sensitizing effects of the glitazones can prevent or delay premature atherosclerotic cardiovascular disease, morbidity, and death.
Annual Review of Medicine 02/2001; 52:239-57. · 14.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Aims/hypothesis. To determine the independent and potentially synergistic effects of agonists for PPARγ and RXR on glucose and lipid metabolism, as well as gene expression, in human skeletal muscle cell cultures. Methods. Fully differentiated myotubes from non-diabetic subjects and subjects with Type II (non-insulin-dependent) diabetes mellitus
were chronically (2 days) treated with LG100 268 (4 μmol/l), an RXR agonist, or troglitazone (4.6 μmol/l), a PPARγ agonist or both, to determine the effects on glucose uptake, activity of glycogen synthase and palmitate oxidation. Results. The combination of both agents increased glucose uptake (60 ± 9 % compared to control subjects) but not either agent alone
(16 ± 9 and 26 ± 6 % for LG100 268 and troglitazone, p < 0.01, respectively). The agent LG100 268 alone had little effect on the activity of glycogen synthase but the effect of
troglitazone increased with LG100 268 (p p p p < 0.005 vs either agent alone). Treatment with either agent led to about a twofold increase in the expression of fatty acid
transporter (FAT/CD36). Troglitazone upregulated PPARγ protein expression, whereas LG100 268 had no effect. Furthermore, neither LG100 268 nor troglitazone had any effect on the
protein expression of RXR isoforms or PPARα. Conclusion/interpretation. Co-activation of PPARγ and RXR results in additive or synergistic effects on glucose and lipid metabolism in skeletal muscle, but unlike troglitazone,
LG100 268 does not alter expression of its own receptor. [Diabetologia (2001) 44: 444–452]
[show abstract][hide abstract] ABSTRACT: Glycogen synthase (GS) is the rate-limiting enzyme controlling nonoxidative glucose disposal in skeletal muscle. A reduction in GS activity and an impaired insulin responsiveness are characteristic features of skeletal muscle in type 2 diabetes. These properties also exist in human skeletal muscle cell cultures from type 2 diabetic subjects. To determine the effect of an isolated reduction in GS on skeletal muscle insulin action, cultures from nondiabetic subjects were treated with antisense oligonucleotides (ODNs) to GS to interfere with expression of the gene. Treatment with antisense ODNs reduced GS protein expression by 70% compared with control (scrambled) ODNs (P < .01). GS activity measured at 0.01 mmol/L glucose-6-phosphate (G-6-P) was reduced by antisense ODN treatment. The insulin responsiveness of GS was impaired. Insulin also failed to stimulate glucose incorporation into glycogen after antisense ODN treatment. The cellular glycogen content was lower in antisense ODN-treated cells compared with control ODN. The insulin responsiveness of glucose uptake was abolished by antisense ODN treatment. Thus, reductions in GS expression in human skeletal muscle cells lead to impairments in insulin responsiveness and may play an important role in insulin-resistant states.
[show abstract][hide abstract] ABSTRACT: To evaluate the tissue distribution and possible role of the peroxisome proliferator-activated receptors (PPARs) in insulin action in fat and muscle biopsy specimens from lean, obese and subjects with Type II (non-insulin-dependent) diabetes mellitus.
We measured PPAR alpha, PPAR beta (delta) and PPAR gamma protein expression by western blot analysis. The PPAR gamma protein was also measured in muscle before and after 3-h hyperinsulinaemic (300 mU.m-2.min-1) euglycaemic clamps.
The PPAR alpha protein was expressed preferentially in muscle relative to fat (more than sevenfold). The PPAR beta protein was similar in fat and muscle. The amount of PPAR gamma protein found in muscle was, on average, two-thirds of that present in fat. There was no statistically significant difference between non-diabetic and diabetic subjects in baseline (preclamp) muscle PPAR (alpha, beta or gamma) protein expression. Subgroup analysis showed, however, significantly higher PPAR gamma protein in the most insulin resistant diabetic subjects with glucose disposal rates of 3-6 mg.kg-1.min-1 compared with their age and weight matched counterparts with glucose disposal rates of 6-9 (147 +/- 23 vs 88 +/- 10 AU/microgram protein, p < or = 0.01 in diabetic and vs 94 +/- 15, p < or = 0.04 in non-diabetic subjects). Muscle PPAR gamma protein and glucose disposal rates were inversely correlated in diabetic subjects (r = -0.47, p < or = 0.05).
All PPARs (alpha, beta or gamma) are present in skeletal muscle and adipose tissue with different relative distributions. The PPAR gamma protein is abundant in skeletal muscle as well as adipose tissue. The altered expression of skeletal muscle PPAR gamma is consistent with a role for this nuclear protein in the impaired insulin action of Type II diabetes.
[show abstract][hide abstract] ABSTRACT: Glycogen synthase (GS) activity is reduced in skeletal muscle of type 2 diabetes, despite normal protein expression, consistent with altered GS regulation. Glycogen synthase kinase-3 (GSK-3) is involved in regulation (phosphorylation and deactivation) of GS. To access the potential role of GSK-3 in insulin resistance and reduced GS activity in type 2 diabetes, the expression and activity of GSK-3 were studied in biopsies of vastus lateralis from type 2 and nondiabetic subjects before and after 3-h hyperinsulinemic (300 mU x m(-2) x min(-1))-euglycemic clamps. The specific activity of GSK-3alpha did not differ between nondiabetic and diabetic muscle and was decreased similarly after 3-h insulin infusion. However, protein levels of both alpha and beta isoforms of GSK-3 were elevated (approximately 30%) in diabetic muscle compared with lean (P < 0.01) and weight-matched obese nondiabetic subjects (P < 0.05) and were unchanged by insulin infusion. Thus, both basal and insulin-stimulated total GSK-3 activities were elevated by approximately twofold in diabetic muscle. GSK-3 expression was related to in vivo insulin action, as GSK-3 protein was negatively correlated with maximal insulin-stimulated glucose disposal rates. In summary, GSK-3 protein levels and total activities are 1) elevated in type 2 diabetic muscle independent of obesity and 2) inversely correlated with both GS activity and maximally insulin-stimulated glucose disposal. We conclude that increased GSK-3 expression in diabetic muscle may contribute to the impaired GS activity and skeletal muscle insulin resistance present in type 2 diabetes.
[show abstract][hide abstract] ABSTRACT: Chronic exposure (48 h) to glucosamine resulted in a dose-dependent reduction of basal and insulin-stimulated glucose uptake activities in human skeletal muscle cell cultures from nondiabetic and type 2 diabetic subjects. Insulin responsiveness of uptake was also reduced. There was no change in total membrane expression of either GLUT1, GLUT3, or GLUT4 proteins. While glucosamine treatment had no significant effects on hexokinase activity measured in cell extracts, glucose phosphorylation in intact cells was impaired after treatment. Under conditions where glucose transport and phosphorylation were down regulated, the fractional velocity (FV) of glycogen synthase was increased by glucosamine treatment. Neither the total activity nor protein expression of glycogen synthase were influenced by glucosamine treatment. The stimulation of glycogen synthase by glucosamine was not due totally to soluble mediators. Reflective of the effects on transport/phosphorylation, total glycogen content and net glycogen synthesis were reduced after glucosamine treatment. These effects were similar in nondiabetic and type 2 cells. In summary: 1) Chronic treatment with glucosamine reduces glucose transport/phosphorylation and storage into glycogen in skeletal muscle cells in culture and impairs insulin responsiveness as well. 2) Down-regulation of glucose transport/phosphorylation occurs at a posttranslational level of GLUTs. 3) Glycogen synthase activity increases with glucosamine treatment. 4) Nondiabetic and type 2 muscle cells display equal sensitivity and responsiveness to glucosamine. Increased exposure of skeletal muscle to glucosamine, a substrate/precursor of the hexosamine pathway, alters intracellular glucose metabolism at multiple sites and can contribute to insulin resistance in this tissue.
[show abstract][hide abstract] ABSTRACT: The effects of tumor necrosis factor-alpha (TNF alpha) on glucose uptake and glycogen synthase (GS) activity were studied in human skeletal muscle cell cultures from nondiabetic and type 2 diabetic subjects. In nondiabetic muscle cells, acute (90-min) exposure to TNF alpha (5 ng/ml) stimulated glucose uptake (73 +/- 14% increase) to a greater extent than insulin (37 +/- 4%; P < 0.02). The acute uptake response to TNF alpha in diabetic cells (51 +/- 6% increase) was also greater than that to insulin (31 +/- 3%; P < 0.05). Prolonged (24-h) exposure of nondiabetic muscle cells to TNF alpha resulted in a further stimulation of uptake (152 +/- 31%; P < 0.05), whereas the increase in cells from type 2 diabetics was not significant compared with that in cells receiving acute treatment. After TNF alpha treatment, the level of glucose transporter-1 protein was elevated in nondiabetic (4.6-fold increase) and type 2 (1.7-fold) cells. Acute TNF alpha treatment had no effect on the fractional velocity of GS in either nondiabetic or type 2 cells. Prolonged exposure reduced the GS fractional velocity in both nondiabetic and diabetic cells. In summary, both acute and prolonged treatment with TNF alpha up-regulate glucose uptake activity in cultured human muscle cells, but reduce GS activity. Increased skeletal muscle glucose uptake in conditions of TNF alpha excess may serve as a compensatory mechanism in the insulin resistance of type 2 diabetes.
[show abstract][hide abstract] ABSTRACT: Troglitazone, besides improving insulin action in insulin-resistant subjects, is also a specific ligand for the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma). To determine whether troglitazone might enhance insulin action by stimulation of PPARgamma gene expression in muscle, total PPARgamma messenger RNA (mRNA), and protein were determined in skeletal muscle cultures from nondiabetic control and type II diabetic subjects before and after treatment of cultures with troglitazone (4 days +/- troglitazone, 11.5 microM). Troglitazone treatment increased PPARgamma mRNA levels up to 3-fold in muscle cultures from type II diabetics (277 +/- 63 to 630 +/- 100 x 10(3) copies/microg total RNA, P = 0.003) and in nondiabetic control subjects (200 +/- 42 to 490 +/- 81, P = 0.003). PPARgamma protein levels in both diabetic (4.7 +/- 1.6 to 13.6 +/- 3.0 AU/10 microg protein, P < 0.02) and nondiabetic cells (7.4 +/- 1.0 to 12.7 +/- 1.8, P < 0.05) were also upregulated by troglitazone treatment. Increased PPARgamma was associated with stimulation of human adipocyte lipid binding protein (ALBP) and muscle fatty acid binding protein (mFABP) mRNA, without change in the mRNA for glycerol-3-phosphate dehydrogenase, PPARdelta, myogenin, uncoupling protein-2, or sarcomeric alpha-actin protein. In summary, we showed that troglitazone markedly induces PPARgamma, ALBP, and mFABP mRNA abundance in muscle cultures from both nondiabetic and type II diabetic subjects. Increased expression of PPARgamma protein and other genes involved in glucose and lipid metabolism in skeletal muscle may account, in part, for the insulin sensitizing effects of troglitazone in type II diabetes.
[show abstract][hide abstract] ABSTRACT: An association between hyperhomocysteinemia and premature atherosclerosis in patients with non-insulin-dependent diabetes mellitus (NIDDM) has recently been described. Little is known about the role of insulin in homocysteine [H(e)] metabolism. We measured plasma H(e) concentrations in the fasting state and during a hyperinsulinemic-euglycemic clamp in normal subjects and patients with NIDDM. Plasma H(e) decreased significantly from 7.2 +/- 2.6 to 6.0 +/- 2.7 mmol/L (P < .01) in normal subjects, but did not change in patients with NIDDM (6.0 +/- 2.7 to 5.9 +/- 2.5 mmol/L, respectively). These data suggest that plasma H(e) concentrations are regulated by acute hyperinsulinemia in normal subjects, but not in insulin-resistant NIDDM subjects. These abnormalities may have implications for the pathogenesis of premature vascular disease associated with NIDDM.