Role of hexosamines in insulin resistance and nutrient sensing in human adipose and muscle tissue.

It has been proposed that the hexosamine pathway acts as a nutrient-sensing pathway, protecting the cell against abundant fuel supply, and that accumulation of hexosamines represents a biochemical mechanism by which hyperglycemia and hyperlipidemia induce insulin resistance. We hypothesized that if an increased flux through the hexosamine pathway caused insulin resistance in humans, the hexosamine levels should be increased in adipose and/or muscle tissue in insulin-resistant subjects, such as patients with type 2 diabetes and obese individuals. In addition, we reasoned that if the hexosamine pathway were a nutrient-sensing pathway, hexosamine levels in adipose and skeletal muscle tissue should be correlated with levels of circulating nutrients, such as glucose and free fatty acids (FFAs) and leptin concentrations. In a human cross-sectional study of 55 patients [20 with type 2 diabetes mellitus (DM) and 21 normal-lean (NL) and 14 normal-obese (NO) subjects] who underwent hip replacement surgery, adipose and muscle tissue biopsies were obtained and analyzed for levels of hexosamines [UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylgalactosamine] and hexoses (UDP-glucose and UDP-galactose). Fasting plasma glucose, glycosylated hemoglobin, serum insulin and homeostasis model assessment calculations, serum lipids, and leptin were measured on the same day. Hexosamines were not elevated in adipose and muscle tissue of patients with type 2 DM compared with NL and NO subjects (UDP-GlcNac DM vs. NL vs. NO, 3.3 +/- 2.3 vs. 2.2 +/- 2.1 vs. 3.0 +/- 2.0 nmol/g tissue in adipose tissue and 8.1 +/- 2.9 vs. 7.8 +/- 2.8 vs. 7.6 +/- 2.8 nmol/g tissue in muscle tissue, respectively). Hexosamines in adipose tissue were positively correlated with circulating levels of FFA (UDP-GlcNAc, r = 0.33, P < 0.05; UDP-N-acetylgalactosamine, r = 0.41, P < 0.01). Adipose tissue UDP-GlcNAc was correlated with leptin levels (r = 0.33; P < 0.05). No such relationship was identified in muscle tissue. In conclusion, these findings argue against a pathophysiological role of the hexosamine pathway in insulin resistance in humans but support the hypothesis that the hexosamine pathway in adipose tissue, not in muscle, is a FFA-sensing pathway and could be involved in the regulation of leptin expression.

Effect of 2 weeks of theophylline on glucose counterregulation in patients with type 1 diabetes and unawareness of hypoglycemia.

BACKGROUND AND OBJECTIVE: A single dose of theophylline improves hypoglycemia unawareness in type 1 diabetic patients. Prolonged theophylline use is, however, associated with emergence of tolerance. This study investigated whether prolonged use of theophylline retains efficacy for counterregulatory defects in patients with type 1 diabetes and hypoglycemia unawareness. METHODS: Experiments were performed with 12 subjects with type 1 diabetes and hypoglycemia unawareness. All subjects participated in a crossover study of 2 randomly scheduled 15-day study periods during which 250 mg theophylline twice daily or matching placebo was used. On the final day of each period, hyperinsulinemic (360 pmol x m(-2) x min(-1)) hypoglycemic (5.0, 3.5, 2.5 mmol x L(-1)) glucose clamps were used to assess counterregulatory and cardiovascular responses. RESULTS: Under normoglycemic conditions, there were no differences between theophylline and placebo. Under hypoglycemic conditions, theophylline enhanced responses of growth hormone, symptoms, heart rate, and pulse pressure (all P <.05), induced sweating at higher plasma glucose levels (P =.039), and reduced exogenous glucose requirements (P =.018). Hypoglycemia-induced responses of epinephrine, norepinephrine, and cortisol were not enhanced by theophylline. CONCLUSIONS: Prolonged use of theophylline has a sustained effect on cardiovascular, metabolic, and symptom responses to hypoglycemia in patients with type 1 diabetes and hypoglycemia unawareness. Whether these results translate into clinical benefit remains to be determined.

Muscle uridine diphosphate-hexosamines do not decrease despite correction of hyperglycemia-induced insulin resistance in type 2 diabetes.

Animal studies suggest that overactivity of the hexosamine pathway, resulting in increased UDP-hexosamines [UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylgalactosamine (UDP-GalNAc)] is an important mechanism by which hyperglycemia causes insulin resistance. This study was performed to test this hypothesis in patients with type 2 diabetes mellitus (DM). Eight obese patients with uncontrolled DM type 2 and severe insulin resistance were treated with iv insulin for 28 +/- 6 d aimed at euglycemia. Before and after iv insulin treatment, insulin sensitivity was measured using a hyperinsulinemic euglycemic clamp, and a muscle biopsy was taken for measurement of UDP-GlcNAc, UDP-GalNAc, UDP-glucose, and UDP-galactose levels. Also, isoelectric focusing patterns of serum transferrin and the urinary excretion of glycosaminoglycans as measures of final products of the hexosamine pathway were examined. After euglycemia, insulin resistance improved, as demonstrated by an increase in the glucose infusion rate during the clamp from 12.7 +/- 5.6 to 22.4 +/- 8.8 micro mol/kg.min (P < 0.0005) and a decrease in insulin requirement from 1.7 +/- 0.9 to 1.1 +/- 0.6 U/kg.d (P < 0.005), whereas metabolic control improved. Surprisingly, both UDP-GlcNAc, from 8.81 +/- 1.21 to 12.31 +/- 2.52 nmol/g tissue (P < 0.005), and UDP-GalNAc concentrations, from 4.49 +/- 0.85 to 5.89 +/- 1.55 nmol/g tissue (P < 0.05) increased. Isoelectric focusing patterns of serum transferrin and excretion of glycosaminoglycans were similar before and after euglycemia. In conclusion, after amelioration of hyperglycemia- induced insulin resistance, UDP-hexosamines increased in skeletal muscle of patients with type 2 DM. These results do not support the hypothesis that accumulation of products of the hexosamine pathway plays a major role in hyperglycemia-induced insulin resistance.

Atrial natriuretic peptide-induced microalbuminuria is associated with endothelial dysfunction in noncomplicated type 1 diabetes patients.

BACKGROUND: Microalbuminuria reflects widespread vascular dysfunction in type 1 diabetes mellitus and results from increased glomerular sieving caused by changes in transglomerular pressure and/or permselectivity characteristics of the glomerular basement membrane. Increased tubular reabsorption or degradation of albumin will offset an early increase in albuminuria. We hypothesized that the infusion of atrial natriuretic peptide (ANP) as a tool to increase glomerular permeability might uncover changes in permselectivity in patients with uncomplicated type 1 diabetes. METHODS: We investigated whether these patients were characterized by endothelial and/or vascular dysfunction. We therefore studied 46 normoalbuminuric patients (urinary albumin excretion [UAE] < 10 microg/min) with type 1 diabetes and 44 healthy controls. Measurements of renal hemodynamics and albuminuria were performed before (baseline) and during the infusion of ANP (0.01 microg/kg/min). On a separate occasion, endothelial function was assessed by the intra-arterial infusion of acetylcholine (ACh), an endothelial-dependent vasodilator. RESULTS: At baseline, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were greater in patients with diabetes (GFR, 121 +/- 3 versus 106 +/- 2 mL/min/1.73 m(2); ERPF, 558 +/- 16 versus 527 +/- 13 mL/min/1.73 m(2); P < 0.001). The infusion of ANP increased filtration fraction. There were no differences in these responses between groups. UAE was significantly greater in patients with diabetes after the ANP infusion (15.8 +/- 1.4 [+183%] versus 9.5 +/- 1.3 microg/min [+96%]; P < 0.01). A subgroup of patients with diabetes with an enhanced albuminuric response (change in UAE > 2 SD of controls) to ANP infusion (mean UAE, 30.3 +/- 1.0 microg/min; 425% +/- 61%) was characterized by a diminished vasodilatory response to ACh (maximal forearm blood flow, 17.2 +/- 2.9 [+563%] versus 26.3 +/- 2.3 mL/min/dL [+800%] in patients with diabetes with a normal albuminuric response; P < 0.05). CONCLUSIONS: In a subgroup of patients with uncomplicated type 1 diabetes, an increase in glomerular permselectivity can be unmasked by the infusion of ANP. These patients are characterized by a diminished vascular response to ACh.

Theophylline improves hypoglycemia unawareness in type 1 diabetes.

Iatrogenic hypoglycemias and the subsequent occurrence of hypoglycemia unawareness are well-known complications of intensive insulin therapy in type 1 diabetic patients that limit glycemic management. From a pharmacological point of view, the adenosine-receptor antagonist theophylline might be beneficial in the management of hypoglycemia unawareness. Theophylline stimulates the release of catecholamines and reduces cerebral blood flow, thereby facilitating stronger metabolic responses to and a prompter perception of decreasing glucose levels. To test the effect of theophylline on responses to hypoglycemia, we performed paired hyperinsulinemic-hypoglycemic clamp studies in 15 diabetic patients with hypoglycemia unawareness and 15 matched healthy control subjects. In random order, we concurrently infused either theophylline or placebo. Measurements included counterregulatory hormones, symptoms, hemodynamic parameters, and sweat detection using a dew-point electrode. Additionally, middle cerebral artery velocities (V(MCA)) using transcranial Doppler were monitored as an estimate of cerebral blood flow. When compared with placebo, theophylline significantly enhanced responses of plasma epinephrine, norepinephrine, and cortisol levels in both diabetic patients and control subjects. Because of the theophylline, sweat production started at approximately 0.3 mmol/l higher glucose levels in both groups (P < 0.01), and symptom scores in diabetic patients approached those in control subjects. Theophylline decreased V(MCA) in both groups (P < 0.001), but significantly greater in diabetic patients (P < 0.01), and prevented the hypoglycemia-induced increase of V(MCA) that occurred during the placebo studies. We conclude that theophylline improves counterregulatory responses to and perception of hypoglycemia in diabetic patients with impaired awareness of hypoglycemia.