Action of insulin on glucose metabolism in vivo

Surveillance of the liver in type 2 diabetes: important but unfeasible?

Fatty liver plays a pivotal role in the pathogenesis of the metabolic syndrome and type 2 diabetes. According to an updated classification, any individual with liver steatosis and one or more features of the metabolic syndrome, without excess alcohol consumption or other known causes of steatosis, has metabolic dysfunction-associated steatotic liver disease (MASLD). Up to 60-70% of all individuals with type 2 diabetes have MASLD. However, the prevalence of advanced liver fibrosis in type 2 diabetes remains uncertain, with reported estimates of 10-20% relying on imaging tests and likely overestimating the true prevalence. All stages of MASLD impact prognosis but fibrosis is the best predictor of all-cause and liver-related mortality risk. People with type 2 diabetes face a two- to threefold increase in the risk of liver-related death and hepatocellular carcinoma, with 1.3% progressing to severe liver disease over 7.7 years. Because reliable methods for detecting steatosis are lacking, MASLD mostly remains an incidental finding on imaging. Regardless, several medical societies advocate for universal screening of individuals with type 2 diabetes for advanced fibrosis. Proposed screening pathways involve annual calculation of the Fibrosis-4 (FIB-4) index, followed by a secondary test such as transient elastography (TE) for intermediate-to-high-risk individuals. However, owing to unsatisfactory biomarker specificity, these pathways are expected to channel approximately 40% of all individuals with type 2 diabetes to TE and 20% to tertiary care, with a false discovery rate of up to 80%, raising concerns about feasibility. There is thus an urgent need to develop more effective strategies for surveying the liver in type 2 diabetes. Nonetheless, weight loss through lifestyle changes, pharmacotherapy or bariatric surgery remains the cornerstone of management, proving highly effective not only for metabolic comorbidities but also for MASLD. Emerging evidence suggests that fibrosis biomarkers may serve as tools for risk-based targeting of weight-loss interventions and potentially for monitoring response to therapy.

A glucose-responsive insulin therapy protects animals against hypoglycemia

Hypoglycemia is commonly associated with insulin therapy, limiting both its safety and efficacy. The concept of modifying insulin to render its glucose-responsive release from an injection depot (of an insulin complexed exogenously with a recombinant lectin) was proposed approximately 4 decades ago but has been challenging to achieve. Data presented here demonstrate that mannosylated insulin analogs can undergo an additional route of clearance as result of their interaction with endogenous mannose receptor (MR), and this can occur in a glucose-dependent fashion, with increased binding to MR at low glucose. Yet, these analogs retain capacity for binding to the insulin receptor (IR). When the blood glucose level is elevated, as in individuals with diabetes mellitus, MR binding diminishes due to glucose competition, leading to reduced MR-mediated clearance and increased partitioning for IR binding and consequent glucose lowering. These studies demonstrate that a glucose-dependent locus of insulin clearance and, hence, insulin action can be achieved by targeting MR and IR concurrently.

Selective control of up-regulated and down-regulated genes by temporal patterns and doses of insulin

Secretion of insulin transiently increases after eating, resulting in a high circulating concentration. Fasting limits insulin secretion, resulting in a low concentration of insulin in the circulation. We analyzed transcriptional responses to different temporal patterns and doses of insulin in the hepatoma FAO cells and identified 13 up-regulated and 16 down-regulated insulin-responsive genes (IRGs). The up-regulated IRGs responded more rapidly than did the down-regulated IRGs to transient stepwise or pulsatile increases in insulin concentration, whereas the down-regulated IRGs were repressed at lower concentrations of insulin than those required to stimulate the up-regulated IRGs. Mathematical modeling of the insulin response as two stages-(i) insulin signaling to transcription and (ii)transcription and mRNA stability-indicated that the first stage was the more rapid stage for the down-regulated IRGs, whereas the second stage of transcription was the more rapid stage for the up-regulated IRGs. A subset of the IRGs that were up-regulated or down-regulated in the FAO cells was similarly regulated in the livers of rats injected with a single dose of insulin. Thus, not only can cells respond to insulin but they can also interpret the intensity and pattern of signal to produce distinct transcriptional responses. These results provide insight that may be useful in treating obesity and type 2 diabetes associated with aberrant insulin production or tissue responsiveness.

Reduced fat oxidation rates during submaximal exercise in adolescents with Crohn's disease

BACKGROUND

Children with Crohn's disease (CD) suffer from malnutrition. Understanding substrate utilization during exercise may help patients with CD sustain a healthy active lifestyle without compromising nutrition. The aim of this study was to determine whether substrate utilization and bioavailability during exercise are altered in children with CD compared with controls.

METHODS

Seven children with CD (mean age ± SD: 15.2 ± 2.3 yr) and 7 controls (14.4 ± 2.3 yr) were matched by sex and biological age. Participants completed 60 minutes of cycling at an intensity equivalent to 50% of their peak mechanical power. Rates of total fat and carbohydrate (CHO) oxidation, the amount of fat and CHO oxidized, and the contribution of fat and CHO to total energy expenditure were calculated from expired gases collected during exercise. Blood was collected before, during, and at the end of exercise and analyzed for insulin, free fatty acids, and glucose.

RESULTS

Whole-body fat oxidation rate (expressed in mg · kg(-1) of body weight per min) during exercise was lower in children with CD (5.8 ± 1.0) compared with controls (8.0 ± 2.2, P < 0.05). Children with CD relied significantly more on CHO, with approximately 10% greater contribution toward total energy expenditure (P < 0.05) than controls. There were no differences in plasma insulin, free fatty acids, or glucose between the groups.

CONCLUSIONS

Fat metabolism during exercise seems to be impaired in children with CD. A greater reliance on CHO is required to meet the energy demands of submaximal exercise.

Roles of insulin, age, and asymmetric dimethylarginine on nitric oxide synthesis in vivo

We tested the effects of insulin on production of nitrous oxide (NO)-related substances (nitrites and nitrates [NOx]) after (15)N-arginine intravenous infusion and on asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) concentrations in conditions reportedly associated with altered NO availability, i.e., aging, hypertension, hypercholesterolemia, and type 2 diabetes mellitus (T2DM). A total of 26 male subjects (age 23-71 years, BMI 23-33 kg/m(2)), some of whom were affected by mixed pathologic features, were enrolled. NOx fractional synthesis rate (FSR) was lower in elderly (P < 0.015) and T2DM subjects (P < 0.03) than in matched control subjects. Hyperinsulinemia generally increased both NOx FSR and absolute synthesis rate (ASR) and reduced NOx, ADMA, and SDMA concentrations. Insulin sensitivity was impaired only in T2DM. With use of simple linear regression analysis across all subjects, age was inversely correlated with both NOx FSR (R(2) = 0.23, P < 0.015) and ASR (R(2) = 0.21, P < 0.02). NOx FSR inversely correlated with both ADMA and SDMA. With use of multiple regression analysis and various models, NOx FSR remained inversely associated with age and ADMA, whereas ASR was inversely associated with age and diabetes. No association with insulin sensitivity was found. We conclude that whole-body NOx production is decreased in aging and T2DM. Age, ADMA concentration, and T2DM, but not insulin resistance, appear as negative regulators of whole-body NOx production.

The selective control of glycolysis, gluconeogenesis and glycogenesis by temporal insulin patterns

The regulation of glucose metabolism by pulse stimulations of insulin is compared with the effect of ramp stimulations. Specific network motifs mediate the differential response to these temporal patterns of stimulations that mimic in vivo patterns of insulin secretion.

Temporal patterns and absolute concentration of insulin selectively control glycolysis, gluconeogenesis and glycogenesis through the different network motif in FAO hepatoma cells.

Step stimulation of insulin induces the transient responses and adaptations of glycolysis (via F16P) and glycogenesis through a feedforward with substrate depletion and though an incoherent feedforward loop, respectively, and induces the sustained response of gluconeogenesis (via PEPCK) through a feedforward inhibition.

Pulse stimulation of insulin, like additional secretory pattern in vivo, induces responses of glycolysis (via F16P), gluconeogenesis (via PEPCK) and glycogenesis.

Ramp stimulation of insulin, like basal secretory pattern in vivo, induces only the response of gluconeogenesis (via PEPCK), but not the responses of glycolysis (via F16P) and glycogenesis.

Insulin governs systemic glucose metabolism, including glycolysis, gluconeogenesis and glycogenesis, through temporal change and absolute concentration. However, how insulin-signalling pathway selectively regulates glycolysis, gluconeogenesis and glycogenesis remains to be elucidated. To address this issue, we experimentally measured metabolites in glucose metabolism in response to insulin. Step stimulation of insulin induced transient response of glycolysis and glycogenesis, and sustained response of gluconeogenesis and extracellular glucose concentration (GLC_ex). Based on the experimental results, we constructed a simple computational model that characterises response of insulin-signalling-dependent glucose metabolism. The model revealed that the network motifs of glycolysis and glycogenesis pathways constitute a feedforward (FF) with substrate depletion and incoherent feedforward loop (iFFL), respectively, enabling glycolysis and glycogenesis responsive to temporal changes of insulin rather than its absolute concentration. In contrast, the network motifs of gluconeogenesis pathway constituted a FF inhibition, enabling gluconeogenesis responsive to absolute concentration of insulin regardless of its temporal patterns. GLC_exwas regulated by gluconeogenesis and glycolysis. These results demonstrate the selective control mechanism of glucose metabolism by temporal patterns of insulin.

Increases in muscle blood flow after a mixed meal are impaired at all stages of type 2 diabetes

OBJECTIVE

In type 2 diabetes, although the impairment of postprandial muscle blood flow response is well established, information on the effect of this impairment on glucose uptake and lipid metabolism is controversial.

DESIGN

Postprandial forearm blood flow responses and metabolic parameters were assessed in a cross-sectional study of subjects at various stages of insulin resistance.

PATIENTS

Eleven healthy subjects (CONTROLS), 11 first-degree relatives of type-2 diabetics (RELATIVES), 10 patients with impaired glucose tolerance (IGT), 10 diabetic patients with postprandial hyperglycaemia (DMA), and 13 diabetic patients with both fasting and postprandial hyperglycaemia (DMB).

MEASUREMENTS

All subjects received a meal. Blood was drawn from a forearm deep vein and the radial artery at specific time-points during a period of 360 min for measurements of glucose, insulin, triglycerides and nonesterified-fatty acids. Forearm muscle blood flow was measured with strain-gauge plethysmography. Glucose uptake and ISI Index were calculated.

RESULTS

Peak-baseline muscle blood flow was higher in CONTROLS (3.32 ± 0.4) than in RELATIVES (0.53 ± 0.29), IGT (0.82 ± 0.2), DMA (1.44 ± 0.34), DMB (1.23 ± 0.35 ml/min/100 ml tissue), P < 0.001. Glucose uptake (AUC(0-360,) μmol/100 ml tissue) was higher in CONTROLS (1023 ± 132) than in RELATIVES (488 ± 42), IGT (458 ± 43), DMA (347 ± 63), DMB (543 ± 53), P < 0.001. ISI index, postprandial triglycerides and nonesterified-fatty acids behaved in a similar way. Peak-baseline muscle blood flow correlated positively with glucose uptake (r = 0.440, P = 0.001) and ISI index (r = 0.397, P = 0.003), and negatively with postprandial triglycerides (r = -0.434, P = 0.001) and nonesterified-fatty acids (r = -0.370, P = 0.005).

CONCLUSIONS

These results suggest that increase in muscle blood flow after a meal is impaired at all stages of type-2 diabetes. This defect influences glucose uptake and is associated with impaired lipid metabolism in the postprandial state.

Antihyperglycemic effect of the alcoholic seed extract of Swietenia macrophylla on streptozotocin-diabetic rats

Background:

Streptozotocin (STZ) selectively destroys the pancreatic insulin secreting cells, leaving less active cells and resulting in a diabetic state. The present study was designed to investigate the antihyperglycemic effect of the ethanolic seed extract of Swietenia macrophylla (SME) in normal and STZ-diabetic rats.

Materials and Methods:

The experimental groups were rendered diabetic by intraperitoneal injection of a single dose of STZ (40 mg/kg body weight [BW]). Rats with glucose levels > 200 mg/dL were considered diabetic and were divided into 5 groups. Three groups of diabetic animals were orally administered, daily with seed extract at a dosage of 50, 100, and 200 mg/kg BW. One group of STZ rats was treated as diabetic control and the other group was orally administered 600 μg/kg BW glibenclamide daily.

Results:

Graded doses of seed extract and glibenclamide showed a significant reduction in blood glucose levels and improvement in serum insulin levels. The extract also improved body weight and promoted liver glycogen content. After treatment, hemoglobin (Hb) level increased and glycosylated Hb level significantly decreased in diabetic rats. The activities of the carbohydrate metabolic enzymes showed significant changes in the rats. Of the 3 doses, 100 mg dose showed maximum activity. Histological investigations of pancreas also supported the biochemical findings.

Conclusions:

Thus, our findings indicate the folklore use of the seed for diabetes and the mechanism seems to be insulin secretion.

Increased liver fat content and unfavorable glucose profiles in subjects without diabetes

BACKGROUND

Liver fat content (LFC) in subjects without diabetes may be correlated with adverse glucose profiles.

METHODS

LFC was measured by (1)H magnetic resonance spectroscopy in 47 healthy subjects, 35 of whom also completed continuous glucose monitoring (CGM) studies. Subjects were divided into two groups according to LFC, and between-group comparisons of glucose profiles were made.

RESULTS

The median LFC value was 3.8% (interquartile range, 2.4-7.6%), and 29.8% of the subjects had LFC values ≥ 5.5%. LFC was positively correlated with triglyceride, aspartate aminotransferase, γ-glutamyltransferase, fasting C-peptide, fasting insulin, insulin concentration at 120 min, area under the time-concentration curve for insulin, and change in insulin from 0 to 30 min, homeostatic model assessment of insulin resistance, fasting plasma glucose, nocturnal mean blood glucose concentration (MBG) (all P < 0.05). The 35 CGM profiles revealed that nocturnal MBG was higher in subjects with LFC ≥ 3.8% (P < 0.05). In subjects with LFC ≥ 5.5%, both diurnal and nocturnal MBG values were elevated compared with subjects with lower LFC (P < 0.05). LFC was the strongest predictive factor of nocturnal MBG.

CONCLUSIONS

Even modestly elevated LFC was associated with unfavorable glucose profiles in subjects without diabetes.

From the metabolic syndrome to NAFLD or vice versa?

The metabolic syndrome encompasses metabolic and cardiovascular risk factors which predict diabetes and cardiovascular disease (CVD) better than any of its individual components. Nonalcoholic fatty liver disease (NAFLD) comprises a disease spectrum which includes variable degrees of simple steatosis (nonalcoholic fatty liver, NAFL), nonalcoholic steatohepatitis (NASH) and cirrhosis. NAFLD is the hepatic manifestation of the metabolic syndrome, with insulin resistance as the main pathogenetic mechanism. Recent data indicate that hyperinsulinemia is probably the consequence rather than cause of NAFLD and NAFLD can be considered an independent predictor of cardiovascular disease. Serum free fatty acids derived from lipolysis of visceral adipose tissue are the main source of hepatic triglycerides in NAFLD, although hepatic de novo lipogenesis and dietary fat supply contribute to the pathogenesis of NAFLD. Approximately 10-25% NAFLD patients develop NASH, the evolutive form of hepatic steatosis. Presumably in a genetically predisposed environment, this increased lipid overload overwhelms the oxidative capacity and reactive oxygen species are generated, leading to lipid peroxidation, cytokine induction, chemoattraction of inflammatory cells, hepatic stellate cell activation and finally fibrogenesis with extracellular matrix deposition. No currently available therapies for NAFLD and NASH exist. Recently nuclear receptors have emerged as key regulators of lipid and carbohydrate metabolism for which specific pharmacological ligands are available, making them attractive therapeutic targets for NAFLD and NASH.

Insulin-stimulated rates of glucose uptake in muscle in hyperthyroidism: the importance of blood flow

BACKGROUND

In hyperthyroidism, although hepatic insulin resistance is well established, information on the effects of insulin on glucose uptake in skeletal muscle is variable.

METHODS

To investigate this, a meal was given to nine hyperthyroid (HR) and seven euthyroid (EU) subjects. Blood was withdrawn for 360 min from a forearm deep vein and the radial artery for measurements of insulin and glucose. Forearm blood flow (BF) was measured with strain-gauge plethysmography. Glucose flux was calculated as arteriovenous difference multiplied by BF and fractional glucose extraction as arteriovenous difference divided by arterial glucose concentrations.

RESULTS

Both groups displayed comparable postprandial glucose levels, with the HR having higher insulin levels than the EU. In the forearm of HR vs. EU: 1) glucose flux was similar [area under the curve (AUC)(0-360) 673 +/- 143 vs. 826 +/- 157 micromol per 100 ml tissue]; 2) BF was increased (AUC(0-360) 3076 +/- 338 vs. 1745 +/- 145 ml per 100 ml tissue, P = 0.005); and 3) fractional glucose extraction was decreased (AUC(0-360) 14.5 +/- 3 vs. 32 +/- 5%min, P = 0.03).

CONCLUSIONS

These results suggest that, in hyperthyroidism, insulin-stimulated glucose uptake in muscle is impaired; this defect is corrected, at least in part, by the increases in BF.

Rosiglitazone reduces liver fat and insulin requirements and improves hepatic insulin sensitivity and glycemic control in patients with type 2 diabetes requiring high insulin doses

BACKGROUND

Liver fat is an important determinant of insulin requirements during insulin therapy. Peroxisome proliferator-activated receptor (PPAR)-gamma agonists reduce liver fat. We therefore hypothesized that type 2 diabetic patients using exceptionally high doses of insulin might respond well to addition of a PPARgamma agonist.

METHODS

We determined the effect of the PPARgamma agonist rosiglitazone on liver fat and directly measured hepatic insulin sensitivity in 14 patients with type 2 diabetes (aged 51 +/- 3 yr, body mass index 36.7 +/- 1.1 kg/m2), who were poorly controlled (glycosylated hemoglobin A 1c (HbA 1c) 8.9 +/- 0.4%) despite using high doses of insulin (218 +/- 22 IU/d) in combination with metformin. Liver fat content (1H-magnetic resonance spectroscopy), hepatic insulin sensitivity [6 h hyperinsulinemic euglycemic clamp (insulin 0.3 mU/kg.min) combined with [3-3H]glucose], body composition (magnetic resonance imaging), substrate oxidation rates (indirect calorimetry), clinical parameters, and liver enzymes were measured before and after rosiglitazone treatment (8 mg/d) for 8 months.

RESULTS

During rosiglitazone, HbA(1c) decreased from 8.9 +/- 0.4% to 7.8 +/- 0.3% (P = 0.007) and insulin requirements from 218 +/- 22 to 129 +/- 20 IU/d (P = 0.002). Liver fat content decreased by 46 +/- 9% from 20 +/- 3% to 11 +/- 3% (P = 0.0002). Hepatic insulin sensitivity, measured from the percent suppression of endogenous glucose production by insulin, increased from -40 +/- 7% to -89 +/- 12% (P = 0.001). The percent change in liver fat correlated with the percent decrease in HbA 1c (r = 0.53, P = 0.06), insulin dose (r = 0.66, P = 0.014), and suppression of endogenous glucose production (r = 0.76, P = 0.003).

CONCLUSIONS

Our results suggest that rosiglitazone may be particularly effective in type 2 diabetic patients who are poorly controlled despite using high insulin doses. The mechanism is likely to involve reduced liver fat and enhanced hepatic insulin sensitivity.

Fatty Liver

Although the epidemic of obesity has been accompanied by an increase in the prevalence of the metabolic syndrome, not all obese develop the syndrome and even lean individuals can be insulin resistant. Both lean and obese insulin resistant individuals have an excess of fat in the liver which is not attributable to alcohol or other known causes of liver disease, a condition defined as nonalcoholic fatty liver disease (NAFLD) by gastroenterologists. The fatty liver is insulin resistant. Liver fat is highly significantly and linearly correlated with all components of the metabolic syndrome independent of obesity. Overproduction of glucose, VLDL, CRP, and coagulation factors by the fatty liver could contribute to the excess risk of cardiovascular disease associated with the metabolic syndrome and NAFLD. Both of the latter conditions also increase the risk of type 2 diabetes and advanced liver disease. The reason why some deposit fat in the liver whereas others do not is poorly understood. Individuals with a fatty liver are more likely to have excess intraabdominal fat and inflammatory changes in adipose tissue. Intervention studies have shown that liver fat can be decreased by weight loss, PPARγ agonists, and insulin therapy.

Tissue specificity of insulin resistance in humans: fat in the liver rather than muscle is associated with features of the metabolic syndrome

AIMS/HYPOTHESIS

The aim of this study was to investigate whether intrahepatic and intramyocellular fat are related to insulin resistance in these respective tissues or to the metabolic syndrome.

METHODS

Hepatic (insulin 1.8 pmol kg(-1) min(-1) combined with [3-3H]glucose) and muscle (insulin 6.0 pmol kg(-1) min(-1)) insulin sensitivity were measured on separate occasions in 45 non-diabetic men (age 42 +/- 1 years, BMI 26.2 +/- 0.6 kg/m2) using the euglycaemic-hyperinsulinaemic clamp. Liver fat and intramyocellular lipid (IMCL) were measured by proton magnetic resonance spectroscopy and body composition by magnetic resonance imaging. We also determined fasting serum insulin and adiponectin concentrations, components of the metabolic syndrome and maximal oxygen consumption.

RESULTS

In participants with high [median 12.0% (interquartile range 5.7-18.5%)] vs low [2.0% (1.0-2.0%)] liver fat, fasting serum triacylglycerols (1.6 +/- 0.2 vs 1.0 +/- 0.1 mmol/l, p = 0.002) and fasting serum insulin (55 +/- 4 vs 32 +/- 2 pmol/l, p < 0.0001) were increased and serum HDL-cholesterol (1.26 +/- 0.1 vs 1.48 +/- 0.1 mmol/l, p = 0.02) and fasting serum adiponectin (9.5 +/- 1.2 vs 12.2 +/- 1.2 microg/ml, p = 0.05) decreased. In participants with high [19.5% (16.0-26.0%)] vs low [5.0% (2.3-7.5%)] IMCL, these parameters were comparable. Liver fat was higher in participants with [10.5% (3.0-18.0%)] than in those without [2.0% (1.5-6.0%), p = 0.010] the metabolic syndrome, even independently of obesity, while IMCL was comparable. Insulin suppression of glucose rate of appearance and serum NEFA was significantly impaired in the high liver fat group.

CONCLUSIONS/INTERPRETATION

Fat accumulation in the liver rather than in skeletal muscle is associated with features of the metabolic syndrome, i.e. increased fasting serum triacylglycerols and decreased fasting serum HDL-cholesterol, as well as with hyperinsulinaemia and low adiponectin.

Severity of insulin resistance in critically ill medical patients

Critical illness is characterized by a hypermetabolic state associated with increased mortality, which is partly ascribed to the occurrence of hyperglycemia caused by enhanced endogenous glucose production and insulin resistance (IR). Insulin resistance is well described in patients after surgery and trauma. However, it is less clearly quantified in critically ill medical patients. In this clinical cohort study, IR (M value) was quantified in 40 critically ill medical patients and 25 matched, healthy controls by isoglycemic hyperinsulinemic clamps after an overnight fast on the day after admission to a medical intensive care unit. Energy and substrate metabolism were measured by using indirect calorimetry in the patients before and during the clamp. The severity of illness was assessed by the acute physiology and chronic health evaluation (APACHE) III score. M values of critically ill medical patients were significantly lower compared with healthy controls (2.29 +/- 1.0 and 7.6 +/- 2.9 mg/kg per minute, respectively; P < .001) and were closely related to APACHE III scores (r = -0.43, P < .01), body mass index (r = -0.41, P < .01), and resting energy expenditure (r = 0.40, P < .05). The M value was not associated with age, basal glucose concentrations, and respiratory quotient, and it did not differ among patients with various admission diagnoses. In conclusion, insulin sensitivity was found to be reduced by 70% in critically ill medical patients. The severity of IR was associated with the severity of illness, body mass index, and resting energy expenditure, but not with substrate oxidation rates. In addition, the severity of IR did not vary among patients with different admission diagnoses.

Defective glucose homeostasis during infection

Infection leads to profound alterations in whole-body metabolism, which is characterized by marked acceleration of glucose, fat and protein, and amino acid flux. One of the complications of infection, especially in the nutritionally supported setting, is hyperglycemia. The hyperglycemia is caused by peripheral insulin resistance and alterations in hepatic glucose metabolism. The defects in hepatic glucose metabolism include overproduction of glucose and a failure of the liver to appropriately adapt when nutritional support is administered. Investigators have suggested that multiple factors contribute to the observed defects. In this review, I focus primarily on alterations in carbohydrate metabolism, examining both the metabolic response to infection and inflammatory stress, the role of the accompanying neuroendocrine and inflammatory responses in the metabolic response, and the interaction between the endocrine response to infection and nutritional support.

High-fat diet induces increased tissue expression of TNF-alpha

In several strains of genetically obese and insulin resistant rodents, adipose tissue over expresses mRNA for tumor necrosis factor alpha (TNF-alpha). Our purpose was to determine whether tissue expression of TNF-alpha protein is elevated in rats that are made obese and insulin resistant by administration of a high-fat diet. Young Wistar rats weighing approximately 50 g were fed for 39 days with either normal rat chow (12.4% fat) or a high-fat diet (50% fat). After 33 days, glucose tolerance was assessed and after 39 days, insulin-stimulated transport of [3H]-2-deoxyglucose was assessed in isolated strips of soleus muscle. Rats on the high-fat diet consumed slightly fewer calories but became obese, displaying significant approximately 2-fold increases in the mass of both visceral and subcutaneous fat depots. High-fat feeding also caused a moderate degree of insulin resistance. Fasting serum insulin was significantly increased, as were insulin and glucose concentrations following glucose loading. In isolated strips of soleus muscle, the high-fat diet produced a trend toward a 33% decrease in the insulin-stimulated component of glucose transport (p=0.064). Western analysis of muscle, liver and fat revealed two forms of TNF-alpha, a soluble 17 Kd form (sTNF-alpha) and a 26 Kd membrane form (mTNF-alpha). Both sTNF-alpha and mTNF-alpha were relatively abundant in fat; whereas sTNF-alpha was the predominant form present in muscle and liver. High-fat feeding caused a significant 2-fold increase in muscle sTNF-alpha, along with a trend toward a 54% increase in visceral fat sTNF-alpha (p=0.055). TNF-alpha was undetectable in serum. We conclude that muscle over expression of TNF-alpha occurs during the development of diet-induced obesity and may, in part cause insulin resistance by an autocrine mechanism.

Effects of rosiglitazone on gene expression in subcutaneous adipose tissue in highly active antiretroviral therapy-associated lipodystrophy

Highly active antiretroviral therapy (HAART) has improved the prognosis of human immunodeficiency virus (HIV)-infected patients but is associated with severe adverse events, such as lipodystrophy and insulin resistance. Rosiglitazone did not increase subcutaneous fat in patients with HAART-associated lipodystrophy (HAL) in a randomized, double-blind, placebo-controlled trial, although it attenuated insulin resistance and decreased liver fat content. The aim of this study was to examine effects of rosiglitazone on gene expression in subcutaneous adipose tissue in 30 patients with HAL. The mRNA concentrations in subcutaneous adipose tissue were measured using real-time PCR. Twenty-four-week treatment with rosiglitazone (8 mg/day) compared with placebo significantly increased the expression of adiponectin, peroxisome proliferator-activated receptor-gamma (PPARgamma), and PPARgamma coactivator 1 and decreased IL-6 expression. Expression of other genes involved in lipogenesis, fatty acid metabolism, or glucose transport, such as acyl-CoA synthase, adipocyte lipid-binding protein, CD45, fatty acid transport protein-1 and -4, GLUT1, GLUT4, keratinocyte lipid-binding protein, lipoprotein lipase, PPARdelta, and sterol regulatory element-binding protein-1c, remained unchanged. Rosiglitazone also significantly increased serum adiponectin concentration. The change in serum adiponectin concentration was inversely correlated with the change in fasting serum insulin concentration and liver fat content. In conclusion, rosiglitazone induced significant changes in gene expression in subcutaneous adipose tissue and ameliorated insulin resistance in patients with HAL. Increased expression of adiponectin might have mediated most of the favorable insulin-sensitizing effects of rosiglitazone in these patients.

Increased fat accumulation in the liver in HIV-infected patients with antiretroviral therapy-associated lipodystrophy

OBJECTIVE

To determine liver fat content in patients with highly active antiretroviral therapy (HAART)-associated lipodystrophy.

BACKGROUND

Lipodystrophy in several animal models is associated with fat accumulation in insulin-sensitive tissues, such as the liver. This causes hyperinsulinaemia, dyslipidaemia and other features of insulin resistance.

DESIGN

A cross-sectional study.

SUBJECTS AND METHODS

Three age- and weight-matched groups were compared: 25 HIV-positive men with HAART-associated lipodystrophy (HAART+LD+), nine HIV-positive men receiving HAART, but without lipodystrophy (HAART+LD-), and 35 HIV-negative healthy men (HIV-). Liver fat content was measured using proton spectroscopy. Intra-abdominal and subcutaneous fat were determined using magnetic resonance imaging.

RESULTS

Liver fat content was significantly higher in the HAART+LD+ (8 +/- 10%) than the HIV- (5 +/- 7%; P < 0.05) or the HAART+LD- (3 +/- 5%; P < 0.01) group. Liver fat content correlated with serum fasting insulin in the HAART+LD+ (r = 0.47; P < 0.05) and HIV- groups (r = 0.65; < 0.001), but not with the amount of intra-abdominal fat. Within the HAART+LD+ group, serum insulin did not correlate with the amount of intra-abdominal fat. The HAART+LD+ group had a lower serum leptin concentration when compared to the two other groups. Features of insulin resistance, including hepatic fat accumulation, were not found in HAART+LD-group.

CONCLUSIONS

The severity of the insulin resistance syndrome in patients with HAART-associated lipodystrophy is related to the extent of fat accumulation in the liver rather than in the intra-abdominal region. Fat accumulation in the liver may therefore play a causative role in the development of insulin resistance in these patients.

Alterations in fuel metabolism in critical illness: hyperglycaemia

Hyperglycaemia is common during critical illness and may be viewed teleologically as a means of ensuring an adequate supply of glucose for the brain and phagocytic cells. Under normal conditions, euglycaemia is maintained by neural, hormonal and hepatic autoregulatory mechanisms. Critical illness promotes hyperglycaemia through an activation of the hypothalamic-pituitary-adrenal axis, which in turn increases hepatic glucose production and inhibits insulin-mediated glucose uptake to skeletal muscle. Sustained hyperglycaemia is associated with adverse consequences that demand its control. Appropriate management includes discontinuing causative drugs, correcting hypokalaemia, treating infection and administering insulin. Insulin therapy also appears to be useful for promoting an anabolic response in skeletal muscle.

Sustained endotoxemia leads to marked down-regulation of early steps in the insulin-signaling cascade

OBJECTIVES

To determine the effects of sustained, 3-day endotoxin infusion on early steps of the insulin-signaling pathway in rat liver and skeletal muscle in vivo; to examine insulin signaling in well-established acute endotoxin models of insulin resistance.

DESIGN

Prospective, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Male Sprague-Dawley rats: 24 in the 3-day endotoxin study, 22 in each acute endotoxin study.

INTERVENTIONS

In prolonged endotoxemia studies, endotoxin (1 mg.kg-1.24 hrs-1) was administered via jugular venous catheter for 74 hrs. Insulin was then injected, and liver and skeletal muscle were removed after 5 mins. In acute endotoxemia studies, an endotoxin bolus (1 mg/kg) was administered, and insulin-signaling responses were studied after 4 hrs.

MEASUREMENTS AND MAIN RESULTS

In liver of rats with sustained endotoxemia, there were significant decreases in insulin-stimulated tyrosine phosphorylation of insulin receptors (74%), insulin receptor substrate (IRS)-1 (74%), and IRS2 (53%); binding of the p85 subunit of phosphatidylinositide 3-kinase to IRS1 (80%); and IRS1-precipitable phosphatidylinositide 3-kinase activity (>90%). These findings were associated with significant reductions in abundance of insulin receptors (37%), IRS1 (60%), and IRS2 (23%). Signaling in skeletal muscle was similarly affected, with reduced IRS1 phosphorylation (49%), IRS1 abundance (50%), and binding of p85 to IRS1 (57%). Insulin signaling 4 hrs after endotoxin administration was not different from controls.

CONCLUSIONS

Prolonged endotoxemia is associated with marked deficits in early steps of the insulin-signaling pathway, which are at least partly explained by reduced abundance of the insulin receptor and IRS proteins. Signaling defects were not evident 4 hrs after endotoxin administration under conditions of adequate nutrition, indicating that insulin resistance develops gradually, may require concomitant malnutrition, and is not reversed by the development of endotoxin tolerance.

The clinical importance of postprandial glucose

Atherosclerotic lesions develop over a long period of time and result from complex changes in the arterial wall. Although these changes are not fully understood, there is much evidence to suggest that elevated plasma glucose levels contribute to the development of atherosclerotic lesions. Many studies have shown that there is a strong correlation between elevated plasma glucose levels and the risk of developing cardiovascular disease. Effects of glucose on the arterial wall include immediate effects, which occur rapidly in response to elevated plasma glucose levels, and long-term effects, which result from non-enzymatic glycosylation of various proteins. These adverse effects of elevated plasma glucose levels suggest that tight control of blood glucose levels in patients with diabetes could possibly reduce the risk of cardiovascular complications. This is borne out by the results of clinical studies in patients with type 1 diabetes. Therapy to reduce blood glucose levels may also be appropriate in individuals with impaired glucose tolerance, as this condition is associated with postprandial hyperglycaemia and a significant risk of developing cardiovascular disease.

Insulin sensitivity in a long-term crossover trial with celiprolol and other antihypertensive agents

The effects of a vasodilating beta-blocker, celiprolol, on insulin sensitivity and cardiovascular risk factors were compared with those of another beta1-selective adrenoceptor blocker, calcium channel blockers, and angiotensin-converting enzyme (ACE) inhibitors. A randomized 21-month crossover trial was carried out with 25 patients with dyslipidemia receiving antihypertensive monotherapy. The study consisted of a 3-month active run-in period and two treatment periods, during which the patients received celiprolol (200-400 mg daily) or the control drug for 12 and 6 months in a crossover manner. A hyperinsulinemic euglycemic clamp and an oral glucose tolerance test (OGTT) were performed every 6 months. According to the clamp tests, the insulin-sensitivity index increased on average by 32% (p < 0.0001) during celiprolol treatment compared with that with the other antihypertensive agents, including ACE inhibitors. In OGTT, area under the incremental glucose curve decreased by 36% (p = 0.002) during celiprolol treatment, whereas insulin secretion diminished on average by 26% (p = 0.006). The mean decrease in fasting serum triglycerides was 11% (NS), whereas the high-density lipoprotein to low-density lipoprotein (HDL/LDL) ratio increased by 15% (p = 0.012). The results suggest that celiprolol improves insulin sensitivity of hypertensive patients with dyslipidemia in long-term therapy.

Use of positron emission tomography (PET) in the assessment of skeletal muscle glucose metabolism

Non invasive imaging techniques, such as, positronemission tomography (PET), contribute to our present knowledge of glucose metabolism. Besides measurements of net glucose metabolism, PET provides insights into complex processes of intracellular glucose metabolism (i.e., glucose transport and phosphorylation) and is also capable to measure muscular blood flow as a possible determinant of glucose metabolism.

Effects of metformin treatment on glucose transporter proteins in subcellular fractions of skeletal muscle in (fa/fa) Zucker rats

1. The present study was designed to clarify the cellular mechanism through which the antihyperglycaemic drug, metformin, exerts its effects. For this purpose the contents of glucose transporter protein isoforms GLUT1 and GLUT4 were measured in plasma membrane and intracellular membrane fractions of skeletal muscle obtained from genetically obese, insulin-resistant Zucker rats. 2. Hindlimb muscles were dissected from metformin-treated (300 mg kg-1 day-1, p.o., for 12 days) and control rats in basal treatment state, and after acute stimulation with insulin (22 u kg-1, i.p.). Since metformin treatment reduces food intake, we also used a pair-fed control group to investigate the effects of altered insulinaemia per se. Glucose transporter levels were analysed by Western blot and slot blot-techniques. In addition, 2-deoxy-[14C]-glucose uptake in isolated muscle strips was evaluated. 3. No changes were noted in the contents of GLUT1 proteins in any of the subcellular fractions after metformin treatment. The contents of GLUT4 in subcellular fractions were not altered in the basal treatment state. After acute insulin exposure the content of GLUT4 in the intracellular membrane fraction declined significantly in the metformin-treated group, while no significant effect was seen in the plasma membrane fraction. In agreement with these results, metformin treatment did not alter 2-deoxyglucose uptake into isolated muscle strips. 4. In conclusion, the present study does not support the concept that metformin would enhance translocation of glucose transporter proteins from the intracellular compartment to the plasma membrane in skeletal muscle in vivo.

Association between serum lipids, glucose tolerance, and insulin sensitivity during 12 months of celiprolol treatment

The study was undertaken to evaluate the development and association of parameters related to the metabolic syndrome during celiprolol treatment. Hyperinsulinemic euglycemic clamp and independent oral glucose tolerance tests (OGTT) were performed on 25 nondiabetic patients with controlled hypertension and dyslipidemia. The tests were carried out during the patients' previous antihypertensive monotherapy (beta- or Ca-blocker, or an ACE inhibitor), and after 6 and 12 months of celiprolol treatment. About one third of patients were randomized to a control group in which treatment was kept unchanged. Insulin sensitivity index (ISI), measured by the euglycemic clamp test, increased 35% in the celiprolol group at 6 months and remained at that level at 12 months, independent of the previous treatment (p = 0.03, compared to the change in the control group). During a 2 hour OGTT, incremental glucose area under the curve (AUC) decreased from 4.5 to 1.9 hr x mmol/l during 6 months of celiprolol treatment, and decreased further to 1.5 hr x mmol/l at 12 months (p < 0.001). Insulin AUC decreased from 113 to 72 hr x mU/l, and decreased further to 68 hr x mU/l (p < 0.01). All insulin parameters in OGTT were highly significant (p < 0.0001) and inversely associated with ISI. Insulin AUC had the best linear correlation with ISI (r = -0.682, p < 0.0001). Glucose parameters in OGTT correlated only weakly and inversely with insulin sensitivity. From the fasting serum lipids, triglycerides showed an inverse (p < 0.001) and HDL a weak (p < 0.05) positive association with ISI. Four out of 20 metabolic, clinical, and demographic parameters proved to be independently significant predictors for ISI in multiple regression analysis. These were insulin AUC, fasting insulin levels, triglyceride values, and age. The coefficient of determination in this four-parameter linear model was 69%. In this preliminary, observer-masked trial with a limited control group, celiprolol improved the impaired insulin sensitivity and glucose tolerance of dyslipidemic hypertensive patients. A fairly predictive model can be formulated to evaluate the peripheral insulin sensitivity of hypertensive patients with suspected metabolic syndrome using OGTT with serum insulin determinations.