Insulin activation of plasma nonesterified fatty acid uptake in metabolic syndrome

High-Fat, High-Calorie Breast Milk in Women with Overweight or Obesity and Its Association with Maternal Serum Insulin Concentration and Triglycerides Levels

The childhood obesity epidemic continues to be a challenge. Maternal obesity and excessive infant weight gain are strong predictors of childhood obesity, which itself is a major risk factor for adult obesity. The primary source of nutrition during early life is breast milk, and its composition is impacted by maternal habitus and diet. We thus studied the relationship between maternal BMI, serum lipids and insulin, and breast milk fat and calorie content from foremilk to hindmilk. Women who were exclusively breastfeeding at 7-8 weeks postpartum were BMI classified as Normal (18.5-24.9, n = 9) and women with Overweight/Obese (OW/OB ≥ 25, n = 13). Maternal blood and continuous breast milk samples obtained from foremilk to hindmilk were analyzed, and infant milk intake was assessed. Women with OW/OB had significantly higher milk fat and calorie content in the first foremilk and last hindmilk sample as compared to Normal BMI women. Amongst all women, maternal serum triglycerides, insulin, and HOMA were significantly correlated with foremilk triglyceride concentration, suggesting that maternal serum triglyceride and insulin action contribute to human milk fat content. As the milk fat content of OW/OB women has caloric implications for infant growth and childhood obesity, these results suggest the potential for modulating milk fat content by a reduction in maternal serum lipids or insulin.

Mathematical modeling reveals differential dynamics of insulin action models on glycerol and glucose in adolescent girls with obesity

Under healthy conditions, the pancreas responds to a glucose challenge by releasing insulin. Insulin suppresses lipolysis in adipose tissue, thereby decreasing plasma glycerol concentration, and it regulates plasma glucose concentration through action in muscle and liver. Insulin resistance (IR) occurs when more insulin is required to achieve the same effects, and IR may be tissue-specific. IR emerges during puberty as a result of high concentrations of growth hormone and is worsened by youth-onset obesity. Adipose, liver, and muscle tissue exhibit distinct dose-dependent responses to insulin in multi-phase hyperinsulinemic-euglycemic (HE) clamps, but the HE clamp protocol does not address potential differences in the dynamics of tissue-specific insulin responses. Changes to the dynamics of insulin responses would alter glycemic control in response to a glucose challenge. To investigate the dynamics of insulin acting on adipose tissue, we developed a novel differential-equations based model that describes the coupled dynamics of glycerol concentrations and insulin action during an oral glucose tolerance test in female adolescents with obesity and IR. We compared these dynamics to the dynamics of insulin acting on muscle and liver as assessed with the oral minimal model applied to glucose and insulin data collected under the same protocol. We found that the action of insulin on glycerol peaks approximately 67 min earlier (p < 0.001) and follows the dynamics of plasma insulin more closely compared to insulin action on glucose as assessed by the parameters representing the time constants for insulin action on glucose and glycerol (p < 0.001). These findings suggest that the dynamics of insulin action show tissue-specific differences in our IR adolescent population, with adipose tissue responding to insulin more quickly compared to muscle and liver. Improved understanding of the tissue-specific dynamics of insulin action may provide novel insights into the progression of metabolic disease in patient populations with diverse metabolic phenotypes.

Improvement in insulin sensitivity and prevention of high fat diet-induced liver pathology using a CXCR2 antagonist

BACKGROUND

Liver pathology (LP) characteristic of non-alcoholic fatty acid disease (NAFLD)/non-alcoholic steatohepatitis (NASH) is a prevalent co-morbidity of type 2 diabetes (T2D). Accumulating evidence indicates that neutrophils driving insulin resistance (IR), including hepatic IR, precipitate T2D-associated NAFLD/NASH. We hypothesized that targeting neutrophil accumulation into insulin-sensitive tissues in mice using a CXCR2 antagonist under T2D-precipitating high fat diet (HFD) could improve insulin sensitivity and prevent the progression towards liver pathology reminiscent of NAFLD/NASH.

METHODS

Mice were age-matched and on standard rodent chow prior to 1:1 randomization into control and HFD formulated with the CXCR2 antagonist AZD5069 or with biologically inactive substitute. They were monitored for metabolic changes including insulin sensitivity using the hyperinsulinemic-euglycemic clamp and hepatic histopathologic evaluation in H&E-stained sections as well as via immunofluorescence microscopy of liver sections for leukocyte markers, collagen 1A1 formation, α-smooth muscle actin (SMA), and galectin-3 expression, for 16 weeks. Statistical tests used to determine significant differences among study groups and outcomes include Student's t-test, one-way ANOVA, repeated measures two-way ANOVA, and Fisher's exact test, depending on the analytical question.

RESULTS

Compared to mice on HFD, mice in the AZD5069-formulated HFD exhibited improved insulin sensitivity, a modest reduction in weight gain, and a significant improvement in LP and markers related to NAFLD/NASH. Mice in the AZD5069-formulated HFD also exhibited reduced neutrophil accumulation into the liver at the end of the 16 week study period.

CONCLUSIONS

These results show, for the first time, the effectiveness of a selective CXCR2 antagonist to improve insulin sensitivity, concomitantly preventing the progression towards LP characteristic of NAFLD/NASH. This represents a novel approach to target IR and developing LP under T2D-susceptible conditions using a single agent. Furthermore, our data extend the growing evidence in support of neutrophils as a leukocyte population that imprints and maintains a chronic inflammatory state in the progression of dysregulated metabolism in liver-specific co-morbid conditions.

Interactions between insulin and exercise

The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.

Insulin signaling in health and disease

The molecular mechanisms of cellular insulin action have been the focus of much investigation since the discovery of the hormone 100 years ago. Insulin action is impaired in metabolic syndrome, a condition known as insulin resistance. The actions of the hormone are initiated by binding to its receptor on the surface of target cells. The receptor is an α2β2 heterodimer that binds to insulin with high affinity, resulting in the activation of its tyrosine kinase activity. Once activated, the receptor can phosphorylate a number of intracellular substrates that initiate discrete signaling pathways. The tyrosine phosphorylation of some substrates activates phosphatidylinositol-3-kinase (PI3K), which produces polyphosphoinositides that interact with protein kinases, leading to activation of the kinase Akt. Phosphorylation of Shc leads to activation of the Ras/MAP kinase pathway. Phosphorylation of SH2B2 and of Cbl initiates activation of G proteins such as TC10. Activation of Akt and other protein kinases produces phosphorylation of a variety of substrates, including transcription factors, GTPase-activating proteins, and other kinases that control key metabolic events. Among the cellular processes controlled by insulin are vesicle trafficking, activities of metabolic enzymes, transcriptional factors, and degradation of insulin itself. Together these complex processes are coordinated to ensure glucose homeostasis.

Human intestinal lipid storage through sequential meals reveals faster dinner appearance is associated with hyperlipidemia

Background

It is increasingly recognized that intestinal cells can store lipids after a meal, yet the effect of this phenomenon on lipid absorption patterns in insulin resistance remains unknown.

Methods

The kinetics of meal fat appearance were measured in insulin-sensitive (IS, n = 8) and insulin-resistant (IR, n = 8) subjects after sequential, isotopically labeled lunch and dinner meals. Plasma dynamics on triacylglycerol-rich (TAG-rich) lipoproteins and plasma hormones were analyzed using a nonlinear, non–steady state kinetic model.

Results

At the onset of dinner, IS subjects showed an abrupt plasma appearance of lunch lipid consistent with the “second-meal effect,” followed by slower appearance of dinner fat in plasma, resulting in reduced accumulation of dinner TAG of 48% compared with lunch. By contrast, IR subjects exhibited faster meal TAG appearance rates after both lunch and dinner. This effect of lower enterocyte storage between meals was associated with greater nocturnal and next-morning hyperlipidemia. The biochemical data and the kinetic analysis of second-meal effect dynamics are consistent with rapid secretion of stored TAG bypassing lipolysis and resynthesis. In addition, the data are consistent with a role for the diurnal pattern of plasma leptin in regulating the processing of dietary lipid.

Conclusion

These data support the concept that intestinal lipid storage may be physiologically beneficial in IS subjects.

Trial registration

ClinicalTrials.gov NCT02020343.

Funding

This study was supported by a grant from the American Diabetes Association (grant 1-13-TS-12).

Prolonged Glucagon Infusion Does Not Affect Energy Expenditure in Individuals with Overweight/Obesity: A Randomized Trial

OBJECTIVE

The aim of this study was to determine the effects of prolonged (72 hours) glucagon administration at a low dose (LD) (12.5 ng/kg/min) and high dose (HD) (25 ng/kg/min) on energy expenditure (EE) in healthy individuals with overweight or obesity.

METHODS

Thirty-one healthy participants with overweight or obesity (BMI of 27-45 kg/m² , 26-55 years old, 23 females) were randomized into LD, HD, or placebo groups and underwent 72-hour intravenous infusion of glucagon. Whole-room calorimetry was used to assess EE and substrate use during five overnight stays (2 days at baseline, 3 days of infusion) and during two 24-hour stays (baseline vs. day 3). Blood was sampled at regular intervals throughout the inpatient stay and analyzed for glucagon and biomarkers of metabolism.

RESULTS

HD infusion elevated plasma glucagon levels compared with the placebo and LD infusion (P < 0.001). Sleeping, basal, and 24-hour EE was not significantly different among groups at any time point. Those receiving HD had significantly higher basal fat oxidation (Fat Ox) at days 2 and 3 than those receiving the placebo (P < 0.05); however, no differences in 24-hour Fat Ox were observed among groups (baseline vs. day 3).

CONCLUSIONS

An HD plasma glucagon infusion over 72 hours does not increase any aspects of EE in healthy individuals with overweight or obesity.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Insulin Action, Glucose Homeostasis and Free Fatty Acid Metabolism: Insights From a Novel Model

Glucose and free fatty acids (FFA) are essential nutrients that are both partly regulated by insulin. Impaired insulin secretion and insulin resistance are hallmarks of aberrant glucose disposal, and type 2 diabetes (T2DM). In the current study, a novel model of FFA kinetics is proposed to estimate the role insulin action on FFA lipolysis and oxidation allowing estimation of adipose tissue insulin sensitivity (S_IFFA ). Twenty-five normal volunteers were recruited for the current study. To participate, volunteers had to be less than 40 years of age and have a body mass index (BMI) < 30 kg/m², and be free of medical comorbidity. The proposed model of FFA kinetics was used to analyze the data derived from the insulin-modified FSIGT. Mean fractional standard deviations of the parameter estimates were all less than 20%. Standardized residuals of the fit of the model to the FFA temporal data were randomly distributed, with only one estimated point lying outside the 2-standard deviation range, suggesting an acceptable fit of the model to the FFA data. The current study describes a novel one-compartment non-linear model of FFA kinetics during an FSIGT that provides an FFA metabolism insulin sensitivity parameter (S_IFFA ). Furthermore, the models suggest a new role of glucose as the modulator of FFA disposal. Estimates of S_IFFA confirmed previous findings that FFA metabolism is more sensitive to changes in insulin than glucose metabolism. Novel derived indices of insulin sensitivity of FFA (S_IFFA ) were correlated with minimal model indices. These associations suggest a cooperative rather than competitive interplay between the two primary nutrients (glucose and FFA) and allude to the FFA acting as the buffer, such that glucose homeostasis is maintained.

Advances in stable isotope tracer methodology part 1: hepatic metabolism via isotopomer analysis and postprandial lipolysis modeling

Stable isotope tracers have been used to gain an understanding of integrative animal and human physiology. More commonly studied organ systems include hepatic glucose metabolism, lipolysis from adipose tissue, and whole body protein metabolism. Recent improvements in isotope methodology have included the use of novel physiologic methods/models and mathematical modeling of data during different physiologic states. Here we review some of the latest advancements in this field and highlight future research needs. First we discuss the use of an oral [U-13C3]-glycerol tracer to determine the relative contribution of glycerol carbons to hepatic glucose production after first cycling through the tricarboxylic acid cycle, entry of glycerol into the pentose phosphate pathway or direct conversion of glycerol into the glucose. Second, we describe an adaptation of the established oral minimal model used to define postprandial glucose dynamics to include glycerol dynamics in an oral glucose tolerance test with a [2H5]-glycerol tracer to determine dynamic changes in lipolysis. Simulation results were optimized when parameters describing glycerol flux were determined with a hybrid approach using both tracer-based calculations and constrained parameter optimization. Both of these methodologies can be used to expand our knowledge of not only human physiology, but also the effects of various nutritional strategies and medications on metabolism.

Effects of body weight gain on insulin and lipid metabolism in equines

The objective of this study was to illuminate the changes in serum NEFA concentrations during a combined glucose-insulin test (CGIT) and basal serum triacylglycerides (TGs) with increasing BW in Shetland ponies and warmblood horses. Therefore, basal blood samples were taken during fasting and a CGIT was performed in 19 healthy equines (10 Shetland ponies, 9 warmblood horses) (t0). After one (t1) and two (t2) year(s) of receiving 200% of their maintenance metabolizable energy requirement, procedures were repeated in the same equines. Sixteen of 19 equines had no signs of insulin dysregulation confirmed by CGIT. Basal plasma glucose concentrations increased in ponies (P = 0.001) when comparing t0 and t2, and basal serum insulin concentrations increased in ponies (P = 0.009) and horses (P = 0.024) from t0 to t2. Basal serum NEFA concentrations increased in ponies comparing t0 and t2 (P = 0.01). During CGIT, NEFA levels dropped until reaching a nadir and subsequently recovered until reaching basal concentrations. The minimum serum NEFA value did not change over time in ponies or horses. However, a strong correlation between basal serum NEFA concentrations and the percentage drop to minimum NEFA levels was found in ponies. Two of three equines (one horse and one pony) graded as insulin-dysregulated suffered from laminitis at t2. The serum NEFA concentrations of these animals had a delay in recovery of the minimum NEFA levels. Basal serum TG levels did not change with BW gain, and no breed differences were observed. In conclusion, serum NEFA concentrations are useful parameters during CGIT to detect insulin dysregulation in equines. Thus, further investigation should be focused on lipid metabolism during insulin dysregulation.

Nonalcoholic fatty liver disease in obese adolescent females is associated with multi-tissue insulin resistance and visceral adiposity markers

Objective

Nonalcoholic fatty liver disease (NAFLD) is associated with insulin resistance (IR) and visceral adiposity in adults and boys, but girls with NAFLD are understudied. We sought to evaluate adipose, liver, and skeletal muscle insulin sensitivity in obese adolescent females with or without hepatic steatosis (HS) (intrahepatic triglyceride (IHTG) content >5.5%) along with cardiometabolic components typically associated with IR.

Study design

73 obese adolescent girls at high risk for NAFLD were enrolled. Participants underwent fasting labs, an MRI to measure IHTG and visceral fat, ³¹phosphorous MR spectroscopy for muscle mitochondrial function, ¹H MR spectroscopy for intramyocellular lipid (IMCL), bicycle ergometry to assess VO₂peak and a 4-phase hyperinsulinemic euglycemic clamp with isotope tracers to measure hepatic and peripheral IR. 29 participants had HS [age 15 yrs(13,16), BMI%ile 98.7(97.4,99.0), IHTG 10.4%(8.0,13.5)] and 44 did not [age 15 yrs(13,17), BMI%ile 98.5(96.2,99.0), IHTG 2.0%(1.1,3.0)].

Results

During hyperinsulinemia, participants with HS vs. non-HS had failure to suppress free fatty acids (p = 0.008), endogenous glucose release (p = 0.002), and a lower glucose metabolic rate of disappearance (Rd) (p = 0.012). Girls with NALFD also had higher visceral fat (p < 0.001), systolic blood pressure (p = 0.026), triglycerides (p = 0.02), ALT (p < 0.01) and white blood cell count (p < 0.01), and lower adiponectin (p = 0.02). There was no difference between girls with and without HS in systemic glycerol turnover measured with glycerol release, or in IMCL, mitochondrial function or VO₂peak.

Conclusions

Obese adolescent girls with HS have evidence of multi-tissue IR, visceral adiposity, inflammation and multiple components of the metabolic syndrome, arguing for close cardiometabolic surveillance over time of girls with HS.

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We described tissue specific insulin sensitivity in adolescent girls ± NAFLD.

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Girls with NAFLD have higher hepatic and muscular insulin resistance.

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Intramyocellular lipids and muscle mitochondrial function were not different between groups.

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Adipose tissue insulin resistance was not different between groups.

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Girls with NAFLD have worst metabolic profile than those without.

The Development and Enhancement of FRAP as a Key Tool for Investigating Protein Dynamics

The saga of fluorescence recovery after photobleaching (FRAP) illustrates how disparate technical developments impact science. Starting with the classic 1976 Axelrod et al. work in Biophysical Journal, FRAP (originally fluorescence photobleaching recovery) opened the door to extraction of quantitative information from photobleaching experiments, laying the experimental and theoretical groundwork for quantifying both the mobility and the mobile fraction of a labeled population of proteins. Over the ensuing years, FRAP's reach dramatically expanded, with new developments in GFP technology and turn-key confocal microscopy, which enabled measurement of protein diffusion and binding/dissociation rates in virtually every compartment within the cell. The FRAP technique and data catalyzed an exchange of ideas between biophysicists studying membrane dynamics, cell biologists focused on intracellular dynamics, and systems biologists modeling the dynamics of cell activity. The outcome transformed the field of cellular biology, leading to a fundamental rethinking of long-held theories of cellular dynamism. Here, we review the pivotal FRAP studies that made these developments and conceptual changes possible, which gave rise to current models of complex cell dynamics.

Blackberry Feeding Increases Fat Oxidation and Improves Insulin Sensitivity in Overweight and Obese Males

Berries and other anthocyanin-rich treatments have prevented weight gain and adiposity in rodent models of diet-induced obesity. Their efficacy may be explained by modulation of energy substrate utilization. However, this effect has never been translated to humans. The objective of this study was to evaluate the effects of berry intake on energy substrate use and glucoregulation in volunteers consuming a high-fat diet. Twenty-seven overweight or obese men were enrolled in a randomized, placebo-controlled crossover study with two treatment periods. Subjects were fed an investigator controlled, high-fat (40% of energy from fat) diet which contained either 600 g/day blackberries (BB, 1500 mg/day flavonoids) or a calorie and carbohydrate matched amount of gelatin (GEL, flavonoid-free control) for seven days prior to a meal-based glucose tolerance test (MTT) in combination with a 24 h stay in a room-sized indirect calorimeter. The washout period that separated the treatment periods was also seven days. The BB treatment resulted in a significant reduction in average 24 h respiratory quotient (RQ) (0.810 vs. 0.817, BB vs. GEL, p = 0.040), indicating increased fat oxidation. RQ during the MTT was significantly lower with the BB treatment (0.84) compared to GEL control (0.85), p = 0.004. A 4 h time isolation during dinner showed similar treatment effects, where RQ was reduced and fat oxidation increased with BB (0.818 vs. 0.836, 28 vs. 25 g, respectively; BB vs. GEL treatments). The glucose AUC was not different between the BB and GEL treatments during the MTT (3488 vs. 4070 mg·min/dL, respectively, p = 0.12). However, the insulin AUC was significantly lower with the BB compared to the GEL control (6485 vs. 8245 µU·min/mL, p = 0.0002), and HOMA-IR improved with BB (p = 0.0318). Blackberry consumption may promote increased fat oxidation and improved insulin sensitivity in overweight or obese males fed a high fat diet.

Basal insulin peglispro increases lipid oxidation, metabolic flexibility, thermogenesis and ketone bodies compared to insulin glargine in subjects with type 1 diabetes mellitus

AIMS

When treated with basal insulin peglispro (BIL), patients with type 1 diabetes mellitus (T1DM) exhibit weight loss and lower prandial insulin requirements versus insulin glargine (GL), while total insulin requirements remain similar. One possible explanation is enhanced lipid oxidation and improved ability to switch between glucose and lipid metabolism with BIL. This study compared the effects of BIL and GL on glucose and lipid metabolism in subjects with T1DM.

MATERIALS AND METHODS

Fifteen subjects with T1DM were enrolled into this open-label, randomised, crossover study, and received once-daily stable, individualised, subcutaneous doses of BIL and GL for 4 weeks each. Respiratory quotient (RQ) was measured using whole-room calorimetry, and energy expenditure (EE) and concentrations of ketone bodies (3-hydroxybutyrate) and acylcarnitines were assessed.

RESULTS

Mean sleep RQ was lower during the BIL (0.822) than the GL (0.846) treatment period, indicating greater lipid metabolism during the post-absorptive period with BIL. Increases in carbohydrate oxidation following breakfast were greater during BIL than GL treatment (mean change in RQ following breakfast 0.111 for BIL, 0.063 for GL). Furthermore, BIL treatment increased total daily EE versus GL (2215.9 kcal/d for BIL, 2135.5 kcal/d for GL). Concentrations of ketone bodies and acylcarnitines appeared to be higher following BIL than GL treatment.

CONCLUSIONS

BIL increased sleeping fat oxidation, EE, ketone bodies, acylcarnitines and post-prandial glucose metabolism when switching from conventional insulin, thus, restoring metabolic flexibility and increasing thermogenesis. These changes may explain the previously observed weight loss with BIL versus GL.

Aromatase Inhibition Reduces Insulin Sensitivity in Healthy Men

CONTEXT

Deficiency of aromatase, the enzyme that catalyzes the conversion of androgens to estrogens, is associated with insulin resistance in humans and mice.

OBJECTIVE

We hypothesized that pharmacological aromatase inhibition results in peripheral insulin resistance in humans.

DESIGN

This was a double-blind, randomized, controlled, crossover study.

SETTING

The study was conducted at a clinical research facility.

PARTICIPANTS

Seventeen healthy male volunteers (18-50 y) participated in the study.

INTERVENTION

The intervention included oral anastrozole (1 mg daily) and placebo, each for 6 weeks with a 2-week washout period.

MAIN OUTCOME MEASURE

Glucose disposal and rates of lipolysis were measured during a stepwise hyperinsulinemic euglycemic clamp. Data are mean (SEM).

RESULTS

Anastrozole therapy resulted in significant estradiol suppression (59.9 ± 3.6 vs 102.0 ± 5.7 pmol/L, P = < .001) and a more modest elevation of total T (25.8 ± 1.2 vs 21.4 ± 0.7 nmol/L, P = .003). Glucose infusion rate, during the low-dose insulin infusion, was lower after anastrozole administration (12.16 ± 1.33 vs 14.15 ± 1.55 μmol/kg·min, P = .024). No differences in hepatic glucose production or rate of lipolysis were observed.

CONCLUSION

Aromatase inhibition reduces insulin sensitivity, with respect to peripheral glucose disposal, in healthy men. Local generation and action of estradiol, at the level of skeletal muscle, is likely to be an important determinant of insulin sensitivity.

Modeling glucose and free fatty acid kinetics in glucose and meal tolerance test

BACKGROUND

Quantitative evaluation of insulin regulation on plasma glucose and free fatty acid (FFA) in response to external glucose challenge is clinically important to assess the development of insulin resistance (World J Diabetes 1:36-47, 2010). Mathematical minimal models (MMs) based on insulin modified frequently-sampled intravenous glucose tolerance tests (IM-FSIGT) are widely applied to ascertain an insulin sensitivity index (IEEE Rev Biomed Eng 2:54-96, 2009). Furthermore, it is important to investigate insulin regulation on glucose and FFA in postprandial state as a normal physiological condition. A simple way to calculate the appearance rate (Ra) of glucose and FFA would be especially helpful to evaluate glucose and FFA kinetics for clinical applications.

METHODS

A new MM is developed to simulate the insulin modulation of plasma glucose and FFA, combining IM-FSIGT with a mixed meal tolerance test (MT). A novel simple functional form for the appearance rate (Ra) of glucose or FFA in the MT is developed. Model results are compared with two other models for data obtained from 28 non-diabetic women (13 African American, 15 white).

RESULTS

The new functional form for Ra of glucose is an acceptable empirical approximation to the experimental Ra for a subset of individuals. When both glucose and FFA are included in FSIGT and MT, the new model is preferred using the Bayes Information Criterion (BIC).

CONCLUSIONS

Model simulations show that the new MM allows consistent application to both IM-FSIGT and MT data, balancing model complexity and data fitting. While the appearance of glucose in the circulation has an important effect on FFA kinetics in MT, the rate of appearance of FFA can be neglected for the time-period modeled.

Fatty Acid Transport Proteins: Targeting FATP2 as a Gatekeeper Involved in the Transport of Exogenous Fatty Acids

The fatty acid transport proteins (FATP) are classified as members of the Solute Carrier 27 (Slc27) family of proteins based on their ability to function in the transport of exogenous fatty acids. These proteins, when localized to the plasma membrane or at intracellular membrane junctions with the endoplasmic reticulum, function as a gate in the regulated transport of fatty acids and thus represent a therapeutic target to delimit the acquisition of fatty acids that contribute to disease as in the case of fatty acid overload. To date, FATP1, FATP2, and FATP4 have been used as targets in the selection of small molecule inhibitors with the goal of treating insulin resistance and attenuating dietary absorption of fatty acids. Several studies targeting FATP1 and FATP4 were based on the intrinsic acyl CoA synthetase activity of these proteins and not on transport directly. While several classes of compounds were identified as potential inhibitors of fatty acid transport, in vivo studies using a mouse model failed to provide evidence these compounds were effective in blocking or attenuating fatty acid transport. Studies targeting FATP2 employed a naturally occurring splice variant, FATP2b, which lacks intrinsic acyl CoA synthetase due to the deletion of exon 3, yet is fully functional in fatty acid transport. These studies identified two compounds, 5'-bromo-5-phenyl-spiro[3H-1,3,4-thiadiazole-2,3'-indoline]-2'-one), now referred to as Lipofermata, and 2-benzyl-3-(4-chlorophenyl)-5-(4-nitrophenyl)pyrazolo[1,5-a]pyrimidin-7(4H)-one, now called Grassofermata, that are effective fatty acid transport inhibitors both in vitro using a series of model cell lines and in vivo using a mouse model.

Evaluation of CETP activity in vivo under non-steady-state conditions: influence of anacetrapib on HDL-TG flux

Studies in lipoprotein kinetics almost exclusively rely on steady-state approaches to modeling. Herein, we have used a non-steady-state experimental design to examine the role of cholesteryl ester transfer protein (CETP) in mediating HDL-TG flux in vivo in rhesus macaques, and therefore, we developed an alternative strategy to model the data. Two isotopomers ([(2)H11] and [(13)C18]) of oleic acid were administered (orally and intravenously, respectively) to serve as precursors for labeling TGs in apoB-containing lipoproteins. The flux of a specific TG (52:2) from these donor lipoproteins to HDL was used as the measure of CETP activity; calculations are also presented to estimate total HDL-TG flux. Based on our data, we estimate that the peak total postprandial TG flux to HDL via CETP is ∼ 13 mg · h(-1) · kg(-1) and show that this transfer was inhibited by 97% following anacetrapib treatment. Collectively, these data demonstrate that HDL TG flux can be used as a measure of CETP activity in vivo. The fact that the donor lipoproteins can be labeled in situ using well-established stable isotope tracer techniques suggests ways to measure this activity for native lipoproteins in free-living subjects under any physiological conditions.

Effects of the rate of insulin infusion during isoglycemic, hyperinsulinemic clamp procedures on measures of insulin action in healthy, mature thoroughbred mares

The objective of this study was to determine whether the rate of insulin infusion during isoglycemic hyperinsulinemic clamp procedures affected measures of insulin action, including glucose disposal and plasma non-esterified fatty acid, endothelin-1, and nitric oxide concentrations, in mature, healthy horses. Eight thoroughbred mares were studied during a 2-h hyperinsulinemic clamp procedure, conducted at each of 4 rates of insulin infusion: 0 (CON), 1.2 (LOWINS), 3 (MEDINS), and 6 (HIGHINS) mU · kg(-1) · min(-1). The infusion rate of a dextrose solution was adjusted throughout the clamp procedures to maintain blood glucose levels within 10% of baseline glucose concentrations. Plasma insulin concentrations were measured throughout the clamp procedures, and used with the rate of glucose infusion to calculate the plasma insulin concentration-to-rate of glucose infusion ratio, a measure of insulin action on glucose disposal. The rate of glucose infusion increased with rate of insulin infusion (P < 0.05). The plasma insulin concentration-to-rate of glucose infusion ratio was highest for the LOWINS treatment (P < 0.05) and decreased by 62% (P < 0.05) and 84% (P < 0.05) for the MEDINS and HIGHINS treatments, respectively. Although plasma non-esterified fatty acid concentrations were lower than baseline by t = 30 min of the clamp procedures in the LOWINS, MEDINS, and HIGHINS treatments (P < 0.05), the decline was similar for all 3 rates of insulin infusion. Jugular vein plasma nitric oxide and endothelin-1 concentrations were not affected by insulin infusion rate (P > 0.05). The data indicate that it is important to standardize insulin infusion rate if data are to be compared between hyperinsulinemic clamp studies.

Increased de novo lipogenesis is a distinct characteristic of individuals with nonalcoholic fatty liver disease

BACKGROUND & AIMS

There have been few studies of the role of de novo lipogenesis in the development of nonalcoholic fatty liver disease (NAFLD). We used isotope analyses to compare de novo lipogenesis and fatty acid flux between subjects with NAFLD and those without, matched for metabolic factors (controls).

METHODS

We studied subjects with metabolic syndrome and/or levels of alanine aminotransferase and aspartate aminotransferase >30 mU/L, using magnetic resonance spectroscopy to identify those with high levels (HighLF, n = 13) or low levels (LowLF, n = 11) of liver fat. Clinical and demographic information was collected from all participants, and insulin sensitivity was measured using the insulin-modified intravenous glucose tolerance test. Stable isotopes were administered and gas chromatography with mass spectrometry was used to analyze free (nonesterified) fatty acid (FFA) and triacylglycerol flux and lipogenesis.

RESULTS

Subjects with HighLF (18.4% ± 3.6%) had higher plasma levels of FFAs during the nighttime and higher concentrations of insulin than subjects with LowLF (3.1% ± 2.7%; P = .04 and P < .001, respectively). No differences were observed between groups in adipose flux of FFAs (414 ± 195 μmol/min for HighLF vs 358 ± 105 μmol/min for LowLF; P = .41) or production of very-low-density lipoprotein triacylglycerol from FFAs (4.06 ± 2.57 μmol/min vs 4.34 ± 1.82 μmol/min; P = .77). In contrast, subjects with HighLF had more than 3-fold higher rates of de novo fatty acid synthesis than subjects with LowLF (2.57 ± 1.53 μmol/min vs 0.78 ± 0.42 μmol/min; P = .001). As a percentage of triacylglycerol palmitate, de novo lipogenesis was 2-fold higher in subjects with HighLF (23.2% ± 7.9% vs 10.1% ± 6.7%; P < .001); this level was independently associated with the level of intrahepatic triacylglycerol (r = 0.53; P = .007).

CONCLUSIONS

By administering isotopes to subjects with NAFLD and control subjects, we confirmed that those with NAFLD have increased synthesis of fatty acids. Subjects with NAFLD also had higher nocturnal plasma levels of FFAs and did not suppress the contribution from de novo lipogenesis on fasting. These findings indicate that lipogenesis might be a therapeutic target for NAFLD.

Increased de novo lipogenesis is a distinct characteristic of individuals with nonalcoholic fatty liver disease

BACKGROUND & AIMS

There have been few studies of the role of de novo lipogenesis in the development of nonalcoholic fatty liver disease (NAFLD). We used isotope analyses to compare de novo lipogenesis and fatty acid flux between subjects with NAFLD and those without, matched for metabolic factors (controls).

METHODS

We studied subjects with metabolic syndrome and/or levels of alanine aminotransferase and aspartate aminotransferase >30 mU/L, using magnetic resonance spectroscopy to identify those with high levels (HighLF, n = 13) or low levels (LowLF, n = 11) of liver fat. Clinical and demographic information was collected from all participants, and insulin sensitivity was measured using the insulin-modified intravenous glucose tolerance test. Stable isotopes were administered and gas chromatography with mass spectrometry was used to analyze free (nonesterified) fatty acid (FFA) and triacylglycerol flux and lipogenesis.

RESULTS

Subjects with HighLF (18.4% ± 3.6%) had higher plasma levels of FFAs during the nighttime and higher concentrations of insulin than subjects with LowLF (3.1% ± 2.7%; P = .04 and P < .001, respectively). No differences were observed between groups in adipose flux of FFAs (414 ± 195 μmol/min for HighLF vs 358 ± 105 μmol/min for LowLF; P = .41) or production of very-low-density lipoprotein triacylglycerol from FFAs (4.06 ± 2.57 μmol/min vs 4.34 ± 1.82 μmol/min; P = .77). In contrast, subjects with HighLF had more than 3-fold higher rates of de novo fatty acid synthesis than subjects with LowLF (2.57 ± 1.53 μmol/min vs 0.78 ± 0.42 μmol/min; P = .001). As a percentage of triacylglycerol palmitate, de novo lipogenesis was 2-fold higher in subjects with HighLF (23.2% ± 7.9% vs 10.1% ± 6.7%; P < .001); this level was independently associated with the level of intrahepatic triacylglycerol (r = 0.53; P = .007).

CONCLUSIONS

By administering isotopes to subjects with NAFLD and control subjects, we confirmed that those with NAFLD have increased synthesis of fatty acids. Subjects with NAFLD also had higher nocturnal plasma levels of FFAs and did not suppress the contribution from de novo lipogenesis on fasting. These findings indicate that lipogenesis might be a therapeutic target for NAFLD.

Metabolic and hormonal consequences of two different meals after a moderate intensity exercise bout in obese prepubertal children

BACKGROUND/OBJECTIVES

To investigate the relationship between postprandial nutrient balance, satiety and hormone changes induced by two different meals taken after a moderate intensity exercise bout.

SUBJECTS/METHODS

Ten prepubertal obese children participated in the study. The experiment was designed as a cross-over study for repeated measures. Each test period lasted five consecutive hours during which the children were under medical supervision. The effects of two isocaloric meals were compared after a moderate intensity exercise (4 multiples of resting metabolic rate, 30 min, cycling): a low-fat/high-carbohydrate meal (meal A) and a high-fat/low-carbohydrate meal (meal B). Pre and postprandial (3 h) substrate oxidation, biochemical parameters, gastrointestinal hormone concentrations and appetite were measured.

RESULTS

The main results were: (i) higher fat balance (5.1±5.0 vs -5.0±6.6 g, P=0.001) and lower carbohydrate balance after meal B than A (-9.7±13.3 vs 11.3±18.3 g, P<0.01); (ii) higher energy balance after meal B than after meal A (5.9±21.5 vs -13.9±20.2 kcal, P<0.05); (iii) higher plasma triglyceride concentrations (area under the curve) after meal B than after meal A (2962.5±2095.8 mg*180 min/dl vs -169.5±1633.7 mg*180 min/dl, P<0.01); (iv) higher serum glucagon-like peptide-1 concentrations after meal B than after meal A (1101.5±873.0 pmol*180 min/l vs 478.8±638.3 pmol*180 min/l, P<0.05).

CONCLUSIONS

After a bout of moderate intensity exercise, a meal with a high-fat/low-carbohydrate ratio had a less favorable metabolic impact than an isoenergetic, isoproteic low-fat/high-carbohydrate meal.