Sylvia Kehlenbrink

Activation of KATP channels suppresses glucose production in humans

Increased endogenous glucose production (EGP) is a hallmark of type 2 diabetes mellitus. While there is evidence for central regulation of EGP by activation of hypothalamic ATP-sensitive potassium (K(ATP)) channels in rodents, whether these central pathways contribute to regulation of EGP in humans remains to be determined. Here we present evidence for central nervous system regulation of EGP in humans that is consistent with complementary rodent studies. Oral administration of the K(ATP) channel activator diazoxide under fixed hormonal conditions substantially decreased EGP in nondiabetic humans and Sprague Dawley rats. In rats, comparable doses of oral diazoxide attained appreciable concentrations in the cerebrospinal fluid, and the effects of oral diazoxide were abolished by i.c.v. administration of the K(ATP) channel blocker glibenclamide. These results suggest that activation of hypothalamic K(ATP) channels may be an important regulator of EGP in humans and that this pathway could be a target for treatment of hyperglycemia in type 2 diabetes mellitus.

Reduced Adipose Tissue Macrophage Content Is Associated With Improved Insulin Sensitivity in Thiazolidinedione-Treated Diabetic Humans

Obesity is associated with increased adipose tissue macrophage (ATM) infiltration, and rodent studies suggest that inflammatory factors produced by ATMs contribute to insulin resistance and type 2 diabetes. However, a relationship between ATM content and insulin resistance has not been clearly established in humans. Since thiazolidinediones attenuate adipose tissue inflammation and improve insulin sensitivity, we examined the temporal relationship of the effects of pioglitazone on these two parameters. The effect of 10 and 21 days of pioglitazone treatment on insulin sensitivity in 26 diabetic subjects was assessed by hyperinsulinemic-euglycemic clamp studies. Because chemoattractant factors, cytokines, and immune cells have been implicated in regulating the recruitment of ATMs, we studied their temporal relationship to changes in ATM content. Improved hepatic and peripheral insulin sensitivity was seen after 21 days of pioglitazone. We found early reductions in macrophage chemoattractant factors after only 10 days of pioglitazone, followed by a 69% reduction in ATM content at 21 days and reduced ATM activation at both time points. Although markers for dendritic cells and neutrophils were reduced at both time points, there were no significant changes in regulatory T cells. These results are consistent with an association between adipose macrophage content and systemic insulin resistance in humans.

Adipocyte-Derived Factors Potentiate Nutrient-Induced Production of Plasminogen Activator Inhibitor–1 by Macrophages

In response to free fatty acids and factors from fat cells, macrophages within the fat tissue of obese individuals release a hormone that contributes to atherosclerosis and insulin resistance. Deleterious Cytokines from Adipose Tissue Obesity is bad for your metabolism. One possible contributor to obesity-induced diabetes and atherosclerosis is plasminogen activator inhibitor-1 or PAI-1, a hormone secreted from fat cells. PAI-1, a protease inhibitor and hemostatic agent, is correlated with cardiovascular events and atherosclerosis, and mice engineered without this hormone are protected from these diseases. Free fatty acids in blood, which are derived from the diet, are also correlated with PAI-1, with higher fatty acid concentrations resulting in more blood PAI-1. Kishore et al. tested whether diet-derived free fatty acids cause increased PAI-1 production in human adipose tissue, focusing on macrophages, immune cells resident in fat that secrete PAI-1. By delivering free fatty acids directly to the bloodstream of their subjects, the authors elicited increased serum PAI-1 concentrations, as well as decreased glucose uptake into tissues. This increased PAI-1 in blood was accompanied by PAI-1 gene expression in macrophages from adipose tissue, although similar cells in the bloodstream did not respond in this way to the fatty acids. Suspecting that the fat cells surrounding the adipose tissue macrophages were somehow sensitizing the macrophages, the authors added adipocyte-conditioned medium to macrophages. This treatment augmented the response of the macrophages to fatty acids, boosting their production of PAI-1. Thus the adipose-tissue macrophages seemed to be uniquely stimulated by their surrounding environment to respond to free fatty acids by secreting the undesirable hormone PAI-1, which can increase propensity to cardiovascular disease. Adipose tissue is gaining a reputation for being more than an inert fat depot. The results from this study by Kishore et al. reinforce this idea, suggesting that in humans adipose tissue delivers elevated concentrations of PAI-1 by creating a unique, local paracrine environment to promote its synthesis. Macrophages are more abundant in adipose tissue from obese individuals than from those of normal weight and may contribute to the metabolic consequences of obesity by producing various circulating factors. One of these factors is plasminogen activator inhibitor–1 (PAI-1), which contributes to both atherosclerosis and insulin resistance. Because nutritional factors appear to regulate PAI-1 expression, we hypothesized that exposure to fatty acids and adipocyte secretory products could stimulate production of PAI-1 by adipose macrophages. Increased free fatty acid (FFA) concentrations in blood for 5 hours in nondiabetic, overweight subjects markedly suppressed insulin-stimulated glucose uptake and raised circulating PAI-1 concentrations, with a concomitant increase in the expression of the PAI-1 gene in adipose tissue. FFAs also rapidly increased PAI-1 gene expression in adipose macrophages and PAI-1 protein immunofluorescence surrounding these cells. By contrast, PAI-1 expression in circulating monocytes was very low and was not affected by raising the concentration of FFAs. Medium from cultured adipocytes stimulated PAI-1 expression in cultured macrophages and potentiated the increase in PAI-1 messenger RNA expression in response to FFAs. Together, our data suggest that adipocyte-derived factors prime adipose macrophages so that they respond to nutritional signals (FFAs) by releasing a key inflammatory adipokine, PAI-1.

Evidence for Central Regulation of Glucose Metabolism

Evidence for central regulation of glucose homeostasis is accumulating from both animal and human studies. Central nutrient and hormone sensing in the hypothalamus appears to coordinate regulation of whole body metabolism. Central signals activate ATP-sensitive potassium (KATP) channels, thereby down-regulating glucose production, likely through vagal efferent signals. Recent human studies are consistent with this hypothesis. The contributions of direct and central inputs to metabolic regulation are likely of comparable magnitude, with somewhat delayed central effects and more rapid peripheral effects. Understanding central regulation of glucose metabolism could promote the development of novel therapeutic approaches for such metabolic conditions as diabetes mellitus.

Insulin sensitizing and anti-inflammatory effects of thiazolidinediones are heightened in obese patients.

Objective The American Diabetes Association has called for further research on how patients’ demographics should determine drug choices for individuals with type 2 diabetes mellitus (T2DM). Here, using in-depth physiology studies, we investigate whether obese patients with T2DM are likely to benefit from thiazolidinediones, medications with a known adverse effect of weight gain. Materials and Methods Eleven obese and 7 nonobese individuals with T2DM participated in this randomized, placebo-controlled, double-blind, crossover study. Each subject underwent a pair of “stepped” pancreatic clamp studies with subcutaneous adipose tissue biopsies after 21 days of pioglitazone (45 mg) or placebo. Results Obese subjects demonstrated significant decreases in insulin resistance and many adipose inflammatory parameters with pioglitazone relative to placebo. Specifically, significant improvements in glucose infusion rates, suppression of hepatic glucose production, and whole fat expression of certain inflammatory markers (IL-6, IL-1B, and inducible nitric oxide synthase) were observed in the obese subjects but not in the nonobese subjects. Additionally, adipose tissue from the obese subjects demonstrated reduced infiltration of macrophages, dendritic cells, and neutrophils as well as increased expression of factors associated with fat “browning” (peroxisome proliferator–activated receptor gamma coactivator-1α and uncoupling protein-1). Conclusions These findings support the efficacy of pioglitazone to improve insulin resistance and reduce adipose tissue inflammation in obese patients with T2DM.

Inhibiting gluconeogenesis prevents fatty acid-induced increases in endogenous glucose production.

Glucose effectiveness, the ability of glucose per se to suppress endogenous glucose production (EGP), is lost in type 2 diabetes mellitus (T2DM). Free fatty acids (FFA) may contribute to this loss of glucose effectiveness in T2DM by increasing gluconeogenesis (GNG) and impairing the response to hyperglycemia. Thus, we first examined the effects of increasing plasma FFA levels for 3, 6, or 16 h on glucose effectiveness in nondiabetic subjects. Under fixed hormonal conditions, hyperglycemia suppressed EGP by 61% in nondiabetic subjects. Raising FFA levels with Liposyn infusion for > or =3 h reduced the normal suppressive effect of glucose by one-half. Second, we hypothesized that inhibiting GNG would prevent the negative impact of FFA on glucose effectiveness. Raising plasma FFA levels increased gluconeogenesis by approximately 52% during euglycemia and blunted the suppression of EGP by hyperglycemia. Infusion of ethanol rapidly inhibited GNG and doubled the suppression of EGP by hyperglycemia, thereby restoring glucose effectiveness. In conclusion, elevated FFA levels rapidly increased GNG and impaired hepatic glucose effectiveness in nondiabetic subjects. Inhibiting GNG could have therapeutic potential in restoring the regulation of glucose production in type 2 diabetes mellitus.

Fatty Acid-Induced Production of Plasminogen Activator Inhibitor-1 by Adipose Macrophages Is Greater in Middle-Aged Versus Younger Adult Participants

BACKGROUND Human aging is associated with heightened risk of diabetes and cardiovascular disease. Increased fat mass may contribute to age-related diseases by harboring inflammatory macrophages that produce metabolically important proteins such as plasminogen activator inhibitor-1 (PAI-1). Elevated PAI-1 concentrations have been implicated in the pathogenesis of such aging-related conditions as insulin resistance, obesity, and atherosclerosis. We have previously reported that increased plasma free fatty acid (FFA) concentrations augment both circulating PAI-1 concentrations and PAI-1 production by adipose tissue macrophages (ATMs). METHODS Because increasing age is associated with increased infiltration and reactivity of adipose macrophages, we performed euglycemic-hyperinsulinemic clamp studies and adipose tissue biopsies with and without elevated FFA concentrations in 31 nondiabetic participants stratified by age, to determine whether middle-aged individuals manifest heightened insulin resistance and PAI-1 production by ATMs in response to elevated nutrient signals relative to their young adult peers. RESULTS We observed that elevating FFA concentrations under euglycemic-hyperinsulinemic clamp conditions induced the same degree of insulin resistance in both middle-aged and younger body mass index-matched adults, whereas systemic PAI-1 concentrations were significantly increased in the middle-aged group. Likewise, elevated FFA and insulin concentrations induced larger increases in PAI-1 gene expression in the whole fat and ATMs of middle-aged compared with younger adult participants. CONCLUSIONS These studies reveal a heightened adipose inflammatory response to increased FFA and insulin availability in middle-aged individuals relative to younger adults, suggesting that increased susceptibility to the effects of fatty acid excess may contribute to the pathogenesis of age-related diseases.

Surrogate measures of insulin sensitivity when compared to euglycemic hyperinsulinemic clamp studies in Asian Indian men without diabetes

AIM Fasting surrogate measures of insulin sensitivity are increasingly used in research and clinical practice. To assess the reliability of these measures, we aimed to evaluate multiple fasting surrogate measures simultaneously in non-diabetic subjects in comparison with the euglycemic hyperinsulinemic clamp study. METHODS Sixteen normoglycemic male South Indian subjects were studied. After an overnight fast, blood samples were collected for glucose, insulin and lipid profile measurements, and stepped euglycemic hyperinsulinemic clamp studies were performed on all subjects. Steady state glucose infusion rates (M value) during low and high insulin phases of the clamp were calculated. Correlation of M value with surrogate markers of insulin sensitivity was performed. Predictive accuracy of surrogate indices was measured in terms of Root Mean Squared Error (RMSE) and leave-one-out cross-validation-type RMSE of prediction using a calibration model. RESULTS M values showed a strong and significant correlation (p<0.01) with the following surrogate markers: Fasting insulin (r=-0.714), Fasting glucose to insulin ratio (FGIR, r=0.747) and Raynaud index (r=0.714). FGIR had a significantly lower RMSE when compared with HOMA-IR and QUICKI. CONCLUSIONS Among the surrogate measures, FGIR had the strongest correlation with M values. FGIR was also the most accurate surrogate measure, as assessed by the calibration model.

Central Regulation of Glucose Production May Be Impaired in Type 2 Diabetes.

The challenges of achieving optimal glycemic control in type 2 diabetes highlight the need for new therapies. Inappropriately elevated endogenous glucose production (EGP) is the main source of hyperglycemia in type 2 diabetes. Because activation of central ATP-sensitive potassium (KATP) channels suppresses EGP in nondiabetic rodents and humans, this study examined whether type 2 diabetic humans and rodents retain central regulation of EGP. The KATP channel activator diazoxide was administered in a randomized, placebo-controlled crossover design to eight type 2 diabetic subjects and seven age- and BMI-matched healthy control subjects. Comprehensive measures of glucose turnover and insulin sensitivity were performed during euglycemic pancreatic clamp studies following diazoxide and placebo administration. Complementary rodent clamp studies were performed in Zucker Diabetic Fatty rats. In type 2 diabetic subjects, extrapancreatic KATP channel activation with diazoxide under fixed hormonal conditions failed to suppress EGP, whereas matched control subjects demonstrated a 27% reduction in EGP (P = 0.002) with diazoxide. Diazoxide also failed to suppress EGP in diabetic rats. These results suggest that suppression of EGP by central KATP channel activation may be lost in type 2 diabetes. Restoration of central regulation of glucose metabolism could be a promising therapeutic target to reduce hyperglycemia in type 2 diabetes.

MP2: PATHOPHYSIOLOGY AND METABOLIC PHENOTYPE OF LOW BODY MASS INDEX DIABETES

Purpose of Study Millions of individuals with low body mass index (BMI) globally have diabetes of unclear etiology. These include patients with Fibrocalculous Pancreatic Diabetes (FCPD) and Lean Diabetes (LD), defined by the presence or absence of pancreatic calcifications on ultrasound. We present the first studies using gold-standard methodologies to assess their metabolic phenotype. Methods Used Stepped euglycemic-hyperinsulinemic (∼30 and 80 mU/m2/min) clamp studies were performed in n=8 Indian males with LD (age 38±3 y, BMI 18.4±0.1 kg/m2, HbA1c 11.0±0.8%) and n=22 with FCPD (age 30±1 y, BMI 19.7±0.6 kg/m2, HbA1c 10.2±0.6%), compared with n=12 type 2 diabetes subjects (T2DM, BMI 25.7±0.3 kg/m2, HbA1c 9.7±0.6%) and n=12 age and BMI matched non-diabetic (ND) subjects and n=16 with type 1 diabetes (T1DM, HbA1c 9.1±0.3%). Therapeutic regimens were intensified for two weeks to correct glucose toxicity in all groups. Lean body mass was determined for all subjects from percentage of total body fat as assessed by DXA. Summary of Results Peripheral insulin sensitivity (Rd, mg/kg lean body weight/min), was markedly impaired in T2DM (2.3±0.6; p<0.01) compared to LD (9.2±1.6) and FCPD (5.8±0.7). Rd did not differ between T1DM (5.8±0.7), LD and FCPD groups (figure 1). Conclusions Thus, these comprehensive studies suggest patients with LD and FCPD are only mildly insulin resistant once hyperglycemia is corrected. This promotes a paradigm shift in our understanding of low body mass index diabetes and could have profound therapeutic implications for millions of people. Abstract MP2 Figure 1