Suppression of systemic, intramuscular, and subcutaneous adipose tissue lipolysis by insulin in humans

Association between new markers of cardiovascular risk and hepatic insulin resistance in those at high risk of developing type 2 diabetes

AIM/HYPOTHESIS

Hepatic insulin resistance (HIR) is considered to be an independent predictor of metabolic disorders and plays an important role in systemic inflammation, which contributes to abnormalities in cardiovascular disease (CVD) risk factors. The aim of this study was to investigate the relationship between HIR and new markers of cardiovascular risks, including leptin/adiponectin ratio (L/A), lipoprotein(a) [Lp(a)], and tumor necrosis factor alpha (TNF-α), at comparable whole body insulin sensitivity in non-diabetic individuals with or without CVD and at high risk of developing type 2 diabetes.

METHODS

The HIR index, L/A, Lp(a), and TNF-α were measured in 50 participants with CVD and in 200 without CVD (1:4 ratio). These were also matched for the homeostatic model assessment for insulin resistance (HOMA-IR) and Matsuda-insulin sensitivity index (ISI) in an observational study design.

RESULTS

The HIR index (1.52 ± 0.14 vs. 1.45 ± 0.17, p < 0.02), L/A (3.22 ± 3.10 vs. 2.09 ± 2.27, p < 0.004), and levels of Lp(a) (66.6 ± 49.5 vs. 37.9 ± 3 6.8 mg/dL, p < 0.0001) and TNF-α (18.9 ± 21.8 vs. 5.4 ± 7.1 pg/mL, p < 0.0001) were higher in those with CVD than those without CVD. HOMA-IR and ISI were not significantly different (p = 0.88 and p = 0.35, respectively). The HIR index was directly correlated with L/A (r = 0.41, p < 0.0001), Lp(a) (r = 0.20, p < 0.002), TNF- α (r = 0.14, p < 0.03), and diastolic blood pressure (DBP) (r = 0.13, p < 0.03). The stepwise model analysis showed that L/A, Lp(a), and TNF-α explained about 20% of the variation in the HIR indices of all the participants (p < 0.02).

CONCLUSIONS/INTERPRETATIONS

Our results suggest a positive association between HIR and new markers of cardiovascular risk [L/A, Lp(a), and TNF- α] at comparable whole body insulin sensitivity in those with or without CVD and at high risk of developing type 2 diabetes.

Fetuin-B, a potential link of liver-adipose tissue cross talk during diet-induced weight loss-weight maintenance

Background/objectives

Numerous hepatokines are involved in inter-organ cross talk regulating tissue-specific insulin sensitivity. Adipose tissue lipolysis represents a crucial element of adipose insulin sensitivity and is substantially involved in long-term body weight regulation after dietary weight loss. Thus, we aimed to analyze the impact of the hepatokine Fetuin-B in the context of weight loss induced short- and long-term modulation of adipose insulin sensitivity.

Subjects/methods

143 subjects (age > 18; BMI ≥ 27 kg/m²) were analyzed before (T-3) and after (T0) a standardized 12-week dietary weight reduction program. Afterward, subjects were randomized to a 12-month lifestyle intervention or a control group. After 12 months (T12) no further intervention was performed until 6 months later (T18) (Maintain-Adults trial). Tissue-specific insulin sensitivity was estimated by HOMA-IR (predominantly liver), ISI_Clamp (predominantly skeletal muscle), and free fatty acid suppression during hyperinsulinemic-euglycemic clamp (FFA_Supp) (predominantly adipose tissue). Fetuin-B was measured at all concomitant time points.

Results

Circulating Fetuin-B levels correlated significantly with estimates of obesity, hepatic steatosis as well as HOMA-IR, ISI_Clamp, FFA_Supp at baseline. Fetuin-B decreased during dietary weight loss (4.2 (3.5–4.9) vs. 3.8 (3.2–4.6) µg/ml; p = 2.1 × 10⁻⁵). This change was associated with concomitant improvement of HOMA-IR (r = 0.222; p = 0.008) and FFA_Supp (r = −0.210; p = 0.013), suggesting a particular relationship to hepatic and adipose tissue insulin sensitivity. Weight loss induced improvements of insulin resistance were almost completely preserved until months 12 and 18 and most interestingly, the short and long-term improvement of FFA_Supp was partially predicted by baseline level of Fetuin-B.

Conclusions

Our data suggest that Fetuin-B might be a potential mediator of liver-adipose cross talk involved in short- and long-term regulation of adipose insulin sensitivity, especially in the context of diet-induced weight changes.

Trial registration

ClinicalTrials.gov number: NCT00850629, https://clinicaltrials.gov/ct2/show/NCT00850629, date of registration: February 25, 2009.

Hyperinsulinemia in Obesity, Inflammation, and Cancer

The relative insufficiency of insulin secretion and/or insulin action causes diabetes. However, obesity and type 2 diabetes mellitus can be associated with an absolute increase in circulating insulin, a state known as hyperinsulinemia. Studies are beginning to elucidate the cause-effect relationships between hyperinsulinemia and numerous consequences of metabolic dysfunctions. Here, we review recent evidence demonstrating that hyperinsulinemia may play a role in inflammation, aging and development of cancers. In this review, we will focus on the consequences and mechanisms of excess insulin production and action, placing recent findings that have challenged dogma in the context of the existing body of literature. Where relevant, we elaborate on the role of specific signal transduction components in the actions of insulin and consequences of chronic hyperinsulinemia. By discussing the involvement of hyperinsulinemia in various metabolic and other chronic diseases, we may identify more effective therapeutics or lifestyle interventions for preventing or treating obesity, diabetes and cancer. We also seek to identify pertinent questions that are ripe for future investigation.

Body Mass Dynamics Is Determined by the Metabolic Ohm's Law and Adipocyte-Autonomous Fat Mass Homeostasis

An ODE model integrating metabolic mechanisms with clinical data reveals an Ohm's law governing lifetime body mass dynamics, where fat and lean tissues are analogous to a parallel nonlinear capacitor and resistor, respectively. The law unexpectedly decouples weight stability (a cell-autonomous property of adipocytes) and weight change (a parabolic trajectory governed by Ohm's law). In middle age, insulin resistance causes fat accumulation to avoid excessive body shrinkage in old age. Moderate middle-age spread is thus natural, not an anomaly caused by hypothalamic defects, as proposed by lipostatic theory. These discoveries provide valuable insights into health care practices such as weight control and health assessment, explain certain observed phenomena, make testable predictions, and may help to resolve major conundrums in the field. The ODE model, which is more comprehensive than Ohm's law, is useful to study metabolism at the detailed microscopic levels.

Skeletal muscle ceramides do not contribute to physical-inactivity-induced insulin resistance

Physical inactivity increases the risk to develop type 2 diabetes, a disease characterized by a state of insulin resistance. By promoting inflammatory state, ceramides are especially recognized to alter insulin sensitivity in skeletal muscle. The present study was designed to analyze, in mice, whether muscle ceramides contribute to physical-inactivity-induced insulin resistance. For this purpose, we used the wheel lock model to induce a sudden reduction of physical activity, in combination with myriocin treatment, an inhibitor of de novo ceramide synthesis. Mice were assigned to 3 experimental groups: voluntary wheel access group (Active), a wheel lock group (Inactive), and wheel lock group treated with myriocin (Inactive-Myr). We observed that 10 days of physical inactivity induces hyperinsulinemia and increases basal insulin resistance (HOMA-IR). The muscle ceramide content was not modified by physical inactivity and myriocin. Thus, muscle ceramides do not play a role in physical-inactivity-induced insulin resistance. In skeletal muscle, insulin-stimulated protein kinase B phosphorylation and inflammatory pathway were not affected by physical inactivity, whereas a reduction of glucose transporter type 4 content was observed. Based on these results, physical-inactivity-induced insulin resistance seems related to a reduction in glucose transporter type 4 content rather than defects in insulin signaling. We observed in inactive mice that myriocin treatment improves glucose tolerance, insulin-stimulated protein kinase B, adenosine-monophosphate-activated protein kinase activation, and glucose transporter type 4 content in skeletal muscle. Such effects occur regardless of changes in muscle ceramide content. These findings open promising research perspectives to identify new mechanisms of action for myriocin on insulin sensitivity and glucose metabolism.

Acute physiological effects of glucocorticoids on fuel metabolism in humans are permissive but not direct

BACKGROUND AND AIMS

The effects of glucocorticoids on fuel metabolism are complex. Acute glucocorticoid excess promotes lipolysis but chronic glucocorticoid excess causes visceral fat accumulation. We hypothesized that interactions between cortisol and insulin and adrenaline account for these conflicting results. We tested the effect of cortisol on lipolysis and glucose production with and without insulin and adrenaline in humans both in vivo and in vitro.

MATERIALS AND METHODS

A total of 20 healthy men were randomized to low and high insulin groups (both n = 10). Subjects attended on 3 occasions and received low (c. 150 nM), medium (c. 400 nM) or high (c. 1400 nM) cortisol infusion in a randomized crossover design. Deuterated glucose and glycerol were infused intravenously along with a pancreatic clamp (somatostatin with replacement of glucagon, insulin and growth hormone) and adrenaline. Subcutaneous adipose tissue was obtained for analysis. In parallel, the effect of cortisol on lipolysis was tested in paired primary cultures of human subcutaneous and visceral adipocytes.

RESULTS

In vivo, high cortisol increased lipolysis only in the presence of high insulin and/or adrenaline but did not alter glucose kinetics. High cortisol increased adipose mRNA levels of ATGL, HSL and CGI-58 and suppressed G0S2. In vitro, high cortisol increased lipolysis in the presence of insulin in subcutaneous, but not visceral, adipocytes.

CONCLUSIONS

The acute lipolytic effects of cortisol require supraphysiological concentrations, are dependent on insulin and adrenaline and are observed only in subcutaneous adipose tissue. The resistance of visceral adipose tissue to cortisol's lipolytic effects may contribute to the central fat accumulation observed with chronic glucocorticoid excess.

Estradiol-mediated improvements in adipose tissue insulin sensitivity are related to the balance of adipose tissue estrogen receptor α and β in postmenopausal women

We recently demonstrated that short-term estradiol (E₂) treatment improved insulin-mediated suppression of lipolysis in postmenopausal women, but to a greater extent in those who were late compared to early postmenopausal. In this follow-up study we tested whether subcutaneous adipose tissue (SAT) expression of estrogen receptors (ER) α and β differs between early and late postmenopausal women. We further tested whether the balance of ERα to ERβ in SAT determined the effect of E₂ on SAT insulin sensitivity. The present study included 35 women who were ≤6 years past menopause (EPM; n = 16) or ≥10 years past menopause (LPM; n = 19). Fasted SAT samples were taken following 1-week transdermal E₂ treatment or placebo (PL) in a random cross-over design. Samples were analyzed for nuclear/cytosolic protein content and mRNA expression using Western blot and qPCR, respectively. While ESR1 increased slightly (~1.4-fold) following E₂ treatment in both groups, ERα and ERβ protein expression did not differ between groups at baseline or in response to E₂. However, the balance of ERα/ERβ protein in the SAT nuclear fraction increased 10% in EPM compared to a 25% decrease in LPM women (group x treatment interaction, p<0.05). A greater proportion of ERα/ERβ protein in the nuclear fraction of SAT at baseline (placebo day) was associated with greater reduction in SAT insulin resistance (i.e., better suppression of lipolysis, EC₅₀) in response to E₂ (r = -0.431, p<0.05). In conclusion, there do not appear to be differences in the proportion of adipose tissue ERα/ERβ protein in late, compared to early, postmenopausal women. However, the balance of ERα/ERβ may be important for E₂-mediated improvement in adipose tissue insulin sensitivity.

Trial Registration: Clinical Trials#:NCT01605071

Use of the hyperinsulinemic euglycemic clamp to assess insulin sensitivity in guinea pigs: dose response, partitioned glucose metabolism, and species comparisons

The guinea pig is an alternate small animal model for the study of metabolism, including insulin sensitivity. However, only one study to date has reported the use of the hyperinsulinemic euglycemic clamp in anesthetized animals in this species, and the dose response has not been reported. We therefore characterized the dose-response curve for whole body glucose uptake using recombinant human insulin in the adult guinea pig. Interspecies comparisons with published data showed species differences in maximal whole body responses (guinea pig ≈ human < rat < mouse) and the insulin concentrations at which half-maximal insulin responses occurred (guinea pig > human ≈ rat > mouse). In subsequent studies, we used concomitant d-[3-³H]glucose infusion to characterize insulin sensitivities of whole body glucose uptake, utilization, production, storage, and glycolysis in young adult guinea pigs at human insulin doses that produced approximately half-maximal (7.5 mU·min^-1·kg^-1) and near-maximal whole body responses (30 mU·min^-1·kg^-1). Although human insulin infusion increased rates of glucose utilization (up to 68%) and storage and, at high concentrations, increased rates of glycolysis in females, glucose production was only partially suppressed (~23%), even at high insulin doses. Fasting glucose, metabolic clearance of insulin, and rates of glucose utilization, storage, and production during insulin stimulation were higher in female than in male guinea pigs (P < 0.05), but insulin sensitivity of these and whole body glucose uptake did not differ between sexes. This study establishes a method for measuring partitioned glucose metabolism in chronically catheterized conscious guinea pigs, allowing studies of regulation of insulin sensitivity in this species.

Timing of Estradiol Treatment After Menopause May Determine Benefit or Harm to Insulin Action

CONTEXT

Type 2 diabetes (T2D) is reduced in postmenopausal women randomized to estrogen-based hormone therapy (HT) compared with placebo. Insulin sensitivity is a key determinant of T2D risk and overall cardiometabolic health, and studies indicate that estradiol (E2) directly impacts insulin action.

OBJECTIVE

We hypothesized that the timing of E2 administration after menopause is an important determinant of its effect on insulin action.

DESIGN

We performed a randomized, crossover, placebo-controlled study.

PARTICIPANTS

Study participants were early postmenopausal (EPM; ≤ 6 years of final menses; n = 22) and late postmenopausal (LPM; ≥ 10 years since last menses; n = 24) women naive to HT.

INTERVENTION

Study interventions included short-term (1 week) transdermal E2 and placebo.

MAIN OUTCOMES AND MEASURES

The study's main outcome was insulin-mediated glucose disposal (glucose disposal rate [GDR]) via hyperinsulinemic-euglycemic clamp.

RESULTS

Compared to EPM women, LPM women were older (mean ± SD; 63 ± 3 vs 56 ± 4 years, P < .05) and more years past menopause (12 ± 2 vs 3 ± 2 years, P < .05). Body mass index (24 ± 3 vs 25 ± 7 kg/m(2)) and fat mass (25 ± 7 vs 23 ± 6 kg) did not differ between groups, but fat-free mass (FFM) was lower in LPM women compared to EPM women (40 ± 4 vs 43 ± 5 kg, P < .05). Baseline GDR did not differ between groups (11.7 ± 2.8 vs 11.5 ± 2.9 mg/kg FFM/min). In support of our hypothesis, 1 week of E2 decreased GDR in LPM women compared to an increase in EPM women (+0.44 ± 1.7 vs - 0.76 ± 2.1 mg/kg FFM/min, P < .05).

CONCLUSIONS

There was not an apparent decline in GDR with age or time since menopause per se. However, E2 action on GDR was dependent on time since menopause, such that there was an apparent benefit early (≤ 6 years) compared to harm later (≥ 10 years) in menopause. E2-mediated effects on insulin action may be one mechanism by which HT reduces the incidence of T2D in early postmenopausal women.

Insulin resistance in the liver: deficiency or excess of insulin?

In insulin-resistant states (obesity, pre-diabetes, and type 2 diabetes), hepatic production of glucose and lipid synthesis are heightened in concert, implying that insulin deficiency and insulin excess coexists in this setting. The fact that insulin may be inadequate or excessive at any one point in differing organs and tissues has many biologic ramifications. In this context the concept of metabolic compartmentalization in the liver is offered herein as one perspective of this paradox. In particular, we focus on the hypothesis that insulin resistance accentuates differences in periportal and perivenous hepatocytes, namely periportal glucose production and perivenous lipid synthesis. Subsequently, excessive production of glucose and accumulation of lipids could be expected in the livers of patients with obesity and insulin resistance. Overall, in this review, we provide our integrative perspective regarding how excessive production of glucose in periportal hepatocytes and accumulation of lipids in perivenous hepatocytes interact in insulin resistant states.

Insulin-mediated suppression of lipolysis in adipose tissue and skeletal muscle of obese type 2 diabetic men and men with normal glucose tolerance

AIMS/HYPOTHESIS

Impaired regulation of lipolysis and accumulation of lipid intermediates may contribute to obesity-related insulin resistance and type 2 diabetes mellitus. We investigated insulin-mediated suppression of lipolysis in abdominal subcutaneous adipose tissue (AT) and skeletal muscle (SM) of obese men with normal glucose tolerance (NGT) and obese type 2 diabetic men.

METHODS

Eleven NGT men and nine long-term diagnosed type 2 diabetic men (7 ± 1 years), matched for age (58 ± 2 vs 62 ± 2 years), BMI (31.4 ± 0.6 vs 30.5 ± 0.6 kg/m(2)) and [Formula: see text] (28.9 ± 1.5 vs 29.5 ± 2.4 ml kg(-1) min(-1)) participated in this study. Interstitial glycerol concentrations in AT and SM were assessed using microdialysis during a 1 h basal period and a 6 h stepwise hyperinsulinaemic-euglycaemic clamp (8, 20 and 40 mU m(-2) min(-1)). AT and SM biopsies were collected to investigate underlying mechanisms.

RESULTS

Hyperinsulinaemia suppressed interstitial SM glycerol concentrations less in men with type 2 diabetes (-7 ± 6%, -13 ± 9% and -27 ± 9%) compared with men with NGT (-21 ± 7%, -38 ± 8% and -53 ± 8%) (p = 0.014). This was accompanied by increased circulating fatty acid and glycerol concentrations, a lower glucose infusion rate (21.8 ± 3.1 vs 30.5 ± 2.0 μmol kg body weight(-1) min(-1); p < 0.05), higher hormone-sensitive lipase (HSL) serine 660 phosphorylation, increased saturated diacylglycerol (DAG) lipid species in the muscle membrane and increased protein kinase C (PKC) activation in type 2 diabetic men vs men with NGT. No significant differences in insulin-mediated reduction in AT interstitial glycerol were observed between groups.

CONCLUSIONS/INTERPRETATION

Our results suggest that a blunted insulin-mediated suppression of SM lipolysis may promote the accumulation of membrane saturated DAG, aggravating insulin resistance, at least partly mediated by PKC. This may represent an important mechanism involved in the progression of insulin resistance towards type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01680133.

PET imaging reveals distinctive roles for different regional adipose tissue depots in systemic glucose metabolism in nonobese humans

Excess amounts of abdominal subcutaneous (SAT) and visceral (VAT) adipose tissue (AT) are associated with insulin resistance, even in normal-weight subjects. In contrast, gluteal-femoral AT (GFAT) is hypothesized to offer protection against insulin resistance. Dynamic PET imaging studies were undertaken to examine the contributions of both metabolic activity and size (volume) of these depots in systemic glucose metabolism. Nonobese, healthy volunteers (n = 15) underwent dynamic PET imaging uptake of [¹⁸F]FDG at a steady-state (20 mU·m⁻²·min⁻¹) insulin infusion. PET images of tissue [¹⁸F]FDG activity were coregistered with MRI to derive K values for insulin-stimulated rates of fractional glucose uptake within tissue. Adipose tissue volume was calculated from DEXA and MRI. VAT had significantly higher rates of fractional glucose uptake per volume than SAT (P < 0.05) or GFAT (P < 0.01). K(GFAT) correlated positively (r = 0.67, P < 0.01) with systemic insulin sensitivity [glucose disappearance rate (R(d))] and negatively with insulin-suppressed FFA (r = -0.71, P < 0.01). SAT (r = -0.70, P < 0.01) and VAT mass (r = -0.55, P < 0.05) correlated negatively with R(d), but GFAT mass did not. We conclude that rates of fractional glucose uptake within GFAT and VAT are significantly and positively associated with systemic insulin sensitivity in nonobese subjects. Furthermore, whereas SAT and VAT amounts are confirmed to relate to systemic insulin resistance, GFAT amount is not associated with insulin resistance. These dynamic PET imaging studies indicate that both quantity and quality of specific AT depots have distinct roles in systemic insulin resistance and may help explain the metabolically obese but normal-weight phenotype.

Metabolomics approach for analyzing the effects of exercise in subjects with type 1 diabetes mellitus

The beneficial effects of exercise in patients with type 1 diabetes (T1D) are not fully proven, given that it may occasionally induce acute metabolic disturbances. Indeed, the metabolic disturbances associated with sustained exercise may lead to worsening control unless great care is taken to adjust carbohydrate intake and insulin dosage. In this work, pre- and post-exercise metabolites were analyzed using a ¹H-NMR and GC-MS untargeted metabolomics approach assayed in serum. We studied ten men with T1D and eleven controls matched for age, body mass index, body fat composition, and cardiorespiratory capacity, participated in the study. The participants performed 30 minutes of exercise on a cycle-ergometer at 80% VO₂max. In response to exercise, both groups had increased concentrations of gluconeogenic precursors (alanine and lactate) and tricarboxylic acid cycle intermediates (citrate, malate, fumarate and succinate). The T1D group, however, showed attenuation in the response of these metabolites to exercise. Conversely to T1D, the control group also presented increases in α-ketoglutarate, alpha-ketoisocaproic acid, and lipolysis products (glycerol and oleic and linoleic acids), as well as a reduction in branched chain amino acids (valine and leucine) determinations. The T1D patients presented a blunted metabolic response to acute exercise as compared to controls. This attenuated response may interfere in the healthy performance or fitness of T1D patients, something that further studies should elucidate.

Comparison of insulins detemir and glargine: effects on glucose disposal, hepatic glucose release and the central nervous system

AIMS

The effects of insulins detemir (Det) and glargine (Glar) on endogenous glucose production (EGP) and net hepatic glucose output (NHGO) were compared.

METHODS

Arteriovenous difference and tracer ([3-(3) H]glucose) techniques were employed during a two-step hyperinsulinemic euglycaemic clamp in conscious dogs (6 groups, n = 5-6/group). After equilibration and basal sampling (0-120 min), somatostatin was infused and basal glucagon was replaced intraportally. Det or Glar was infused via portal vein (Po), peripheral vein (IV), or bilateral carotid and vertebral arteries (H) at 0.1 and 0.3 mU/kg/min (low Insulin; Glar vs. Det, respectively, 120-420 min) and 4× the low insulin rate (high insulin; 420-540 min).

RESULTS

NHGO and EGP were suppressed and glucose R(d) and infusion rate were stimulated similarly by Det and Glar at both Low and high insulin with each infusion route. Non-esterified fatty acid (NEFA) concentrations during low insulin were 202 ± 37 versus 323 ± 75 µM in DetPo and GlarPo (p < 0.05) and 125 ± 39 versus 263 ± 48 µM in DetIV and GlarIV, respectively (p < 0.05). In DetH versus GlarH, pAkt/Akt (1.7 ± 0.2 vs. 1.0 ± 0.2) and pSTAT3/STAT3 (1.4 ± 0.2 vs. 1.0 ± 0.1) were significantly increased in the liver but not in the hypothalamus.

CONCLUSIONS

Det and Glar have similar net effects on acute regulation of hepatic glucose metabolism in vivo regardless of delivery route. Portal and IV detemir delivery reduces circulating NEFA to a greater extent than glargine, and head detemir infusion enhances molecular signalling in the liver. These findings indicate a need for further examination of Det's central and hepatic effects.

Serum Concentrations of Fibroblast Growth Factor 19 in Patients With Obesity and Type 2 Diabetes Mellitus: the Influence of Acute Hyperinsulinemia, Very-Low Calorie Diet and PPAR-α Agonist Treatment

The aim of our study was to measure serum concentrations of fibroblast growth factor 19 (FGF-19) in patients with obesity (OB), obesity and type 2 diabetes mellitus (T2DM) and healthy subjects (C) at baseline and after selected interventions. We measured serum FGF-19 levels and other biochemical and hormonal parameters in 29 OB and 19 T2DM females and 30 sex- and age-matched control subjects. The interventions were acute hyperinsulinemia during isoglycemic-hyperinsulinemic clamp (n=11 for T2DM and 10 for C), very-low calorie diet (VLCD, n=12 for OB) and 3 months treatment with PPAR-α agonist fenofibrate (n=11 for T2DM). Baseline serum FGF-19 levels were significantly lower in OB relative to C group (132.1±12.7 vs. 202.2±16.7 pg/ml, p<0.05), while no significant difference was observed between T2DM and OB or control group. Acute hyperinsulinemia tended to decrease FGF-19 levels in both healthy and T2DM subjects. Three weeks of VLCD in OB group had no significant effect on FGF-19, whereas three months of fenofibrate treatment markedly reduced FGF-19 levels in T2DM patients (194.58±26.2 vs. 107.47±25.0 pg/ml, p<0.05). We conclude that FGF-19 levels in our study were at least partially dependent upon nutritional status, but were not related to parameters of glucose metabolism or insulin sensitivity.

Regulation of Adipose Tissue Metabolism in Humans: Analysis of Responses to the Hyperinsulinemic-Euglycemic Clamp Experiment

The suppression of lipolysis is one of the key metabolic responses of the adipose tissue during hyperinsulinemia. The failure to respond and resulting increase in plasma fatty acids could contribute to the development of insulin resistance and perturbations in the fuel homeostasis in the whole body. In this study, a mechanistic, computational model of adipose tissue metabolism in vivo has been enhanced to simulate the physiological responses during hyperinsulinemic-euglycemic clamp experiment in humans. The model incorporates metabolic intermediates and pathways that are important in the fed state. In addition, it takes into account the heterogeneity of triose phosphate pools (glycolytic vs. glyceroneogenic), within the adipose tissue. The model can simulate not only steady-state responses at different insulin levels, but also concentration dynamics of major metabolites in the adipose tissue venous blood in accord with the in vivo data. Simulations indicate that (1) regulation of lipoprotein lipase (LPL) reaction is important when the intracellular lipolysis is suppressed by insulin; (2) intracellular diglyceride levels can affect the regulatory mechanisms; and (3) glyceroneogenesis is the dominant pathway for glycerol-3-phosphate synthesis even in the presence of increased glucose uptake by the adipose tissue. Reduced redox and increased phosphorylation states provide a favorable milieu for glyceroneogenesis in response to insulin. A parameter sensitivity analysis predicts that insulin-stimulated glucose uptake would be more severely affected by impairment of GLUT4 translocation and glycolysis than by impairment of glycogen synthesis and pyruvate oxidation. Finally, simulations predict metabolic responses to altered expression of phosphoenolpyruvate carboxykinase (PEP-CK). Specifically, the increase in the rate of re-esterification of fatty acids observed experimentally with the overexpression of PEPCK in the adipose tissue would be accompanied by the up-regulation of acyl Co-A synthase.

Metabonomic fingerprints of fasting plasma and spot urine reveal human pre-diabetic metabolic traits

Impaired glucose tolerance (IGT) which precedes overt type 2 diabetes (T2DM) for decades is associated with multiple metabolic alterations in insulin sensitive tissues. In an UPLC-qTOF-mass spectrometry-driven non-targeted metabonomics approach we investigated plasma as well as spot urine of 51 non-diabetic, overnight fasted individuals aiming to separate subjects with IGT from controls thereby identify pathways affected by the pre-diabetic metabolic state. We could clearly demonstrate that normal glucose tolerant (NGT) and IGT subjects clustered in two distinct groups independent of the investigated metabonome. These findings reflect considerable differences in individual metabolite fingerprints, both in plasma and urine. Pre-diabetes associated alterations in fatty acid-, tryptophan-, uric acid-, bile acid-, and lysophosphatidylcholine-metabolism, as well as the TCA cycle were identified. Of note, individuals with IGT also showed decreased levels of gut flora-associated metabolites namely hippuric acid, methylxanthine, methyluric acid, and 3-hydroxyhippuric acid. The findings of our non-targeted UPLC-qTOF-MS metabonomics analysis in plasma and spot urine of individuals with IGT vs NGT offers novel insights into the metabolic alterations occurring in the long, asymptomatic period preceding the manifestation of T2DM thereby giving prospects for new intervention targets. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-010-0203-1) contains supplementary material, which is available to authorized users.

Alterations in fatty acid kinetics in obese adolescents with increased intrahepatic triglyceride content

OBJECTIVE

It has been hypothesized that excessive fatty acid availability contributes to steatosis and the metabolic abnormalities associated with nonalcoholic fatty liver disease (NAFLD). The purpose of this study was to evaluate whether adipose tissue lipolytic activity and the rate of fatty acid release into plasma are increased in obese adolescents with NAFLD.

METHODS

Palmitate kinetics were determined in obese adolescents with normal (n = 9; BMI = 37 +/- 2 kg/m(2); intrahepatic triglyceride (IHTG) <or=5.5% of liver volume) and increased (n = 9; BMI = 36 +/- 2 kg/m(2); IHTG >or= 10% of liver volume) IHTG content during the basal state (postabsorptive condition) and during physiological hyperinsulinemia (postprandial condition). Both groups were matched on body weight, BMI, percent body fat, age, sex, and Tanner stage. The hyperinsulinemic-euglycemic clamp procedure, in conjunction with a deuterated palmitate tracer infusion, was used to determine free-fatty acid (FFA) kinetics, and magnetic resonance spectroscopy was used to determine IHTG content.

RESULTS

The rate of whole-body palmitate release into plasma was greater in subjects with NAFLD than those with normal IHTG content during basal conditions, (87 +/- 7 vs. 127 +/- 13 micromol/min; P < 0.01) and during physiological hyperinsulinemia, (24 +/- 2 vs. 44 +/- 8 micromol/min; P < 0.01).

DISCUSSION

These results demonstrate that adipose tissue lipolytic activity is increased in obese adolescents with NAFLD and results in an increase in the rate of fatty acid release into plasma throughout the day. This continual excess in fatty acid flux supports the hypothesis that adipose insulin resistance is involved in the pathogenesis of steatosis and contributes to the metabolic complications associated with NAFLD.

A computational model of adipose tissue metabolism: evidence for intracellular compartmentation and differential activation of lipases

Regulation of lipolysis in adipose tissue is critical to whole body fuel homeostasis and to the development of insulin resistance. Due to the challenging nature of laboratory investigations of regulatory mechanisms in adipose tissue, mathematical models could provide a valuable adjunct to such experimental work. We have developed a computational model to analyze key components of adipose tissue metabolism in vivo in human in the fasting state. The various key components included triglyceride-fatty acid cycling, regulation of lipolytic reactions, and glyceroneogenesis. The model, consisting of spatially lumped blood and cellular compartments, included essential transport processes and biochemical reactions. Concentration dynamics for major substrates were described by mass balance equations. Model equations were solved numerically to simulate dynamic responses to intravenous epinephrine infusion. Model simulations were compared with the corresponding experimental measurements of the arteriovenous difference across the abdominal subcutaneous fat bed in humans. The model can simulate physiological responses arising from the different expression levels of lipases. Key findings of this study are as follows: (1) Distinguishing the active metabolic subdomain ( approximately 3% of total tissue volume) is critical for simulating data. (2) During epinephrine infusion, lipases are differentially activated such that diglyceride breakdown is approximately four times faster than triglyceride breakdown. (3) Glyceroneogenesis contributes more to glycerol-3-phosphate synthesis during epinephrine infusion when pyruvate oxidation is inhibited by a high acetyl-CoA/free-CoA ratio.

Heterogeneity in limb fatty acid kinetics in type 2 diabetes

AIMS/HYPOTHESIS

In order to test the hypothesis that disturbances in skeletal muscle fatty acid metabolism with type 2 diabetes are not equally present in the upper and lower limbs, we studied fatty acid kinetics simultaneously across the arm and leg of type 2 diabetic patients (n=6) and matched control subjects (n=7) for 5 h under baseline conditions and during a 4-h hyperinsulinaemic-euglycaemic clamp.

METHODS

Limb fatty acid kinetics was determined by means of continuous [U-(13)C]palmitate infusion and measurement of arteriovenous differences.

RESULTS

The systemic palmitate rate of appearance was 3.6+/-0.4 and 2.7+/-0.3 micromol.kg lean body mass(-1).min(-1) and decreased during the clamp by 26% (p=0.04) and 43% (p<0.01) in the diabetic patients and in the control subjects respectively. At baseline, palmitate uptake across the arm was similar in the two groups, whereas leg palmitate uptake was lower than in the arm in the diabetic patients. During the clamp, palmitate uptake decreased in the arm (-48%, p=0.02) and the leg (-38%, p=0.04) of the control subjects, whereas it decreased in the arm (-30%, p=0.04) but not in the leg of the diabetic patients. Similarly, during the clamp palmitate release was substantially suppressed in the arm (-47%, p<0.01) and the leg of the control subjects (-45%, p<0.01), but only in the arm of the diabetic patients (-45%, p<0.01).

CONCLUSIONS/INTERPRETATION

The present data indicate that type 2 diabetes is characterised by heterogeneity in the dysregulation of skeletal muscle fatty acid metabolism, with only the leg, but not the arm, showing an impairment of fatty acid kinetics at baseline and during a hyperinsulinaemic-euglycaemic clamp causing a physiological increase in insulin concentration.

Impaired beta-adrenergically mediated lipolysis in skeletal muscle of obese subjects

AIMS/HYPOTHESIS

The aim of this study was to investigate whether the beta2-adrenergically mediated increase in interstitial glycerol concentrations (used as an indicator of local lipolysis) was impaired in the skeletal muscle (the gastrocnemius muscle) of obese subjects compared with in that of lean subjects, and whether any differences in muscle lipolysis were related to differences in intramyocellular muscle triglyceride content.

METHODS

A microdialysis experiment was performed in the gastrocnemius muscle of eight lean and eight obese men (body fat 22.1+/-1.6% vs 32.7+/-1.6% respectively). After determining baseline extracellular glycerol concentrations, the probe was perfused with increasing concentrations of the beta2-agonist, salbutamol (doses of 1, 10 and 100 micromol/l for 45-min periods). Local blood flow was determined using the ethanol dilution technique. Intramyocellular lipid content was determined using 1H-magnetic resonance spectroscopy.

RESULTS

Compared with that in lean subjects, the beta2-adrenergically mediated increase in glycerol concentrations (absolute and percentage change) was blunted in obese subjects (at 100 micromol/l of salbutamol, percentage change 12.0+/-12% vs 48+/-12%, p<0.05). The decrease in ethanol out:in ratio was less pronounced in the obese individuals ( p<0.05), indicating a diminished increase in local muscle blood flow. Intramyocellular lipid content was comparable in both groups.

CONCLUSIONS/INTERPRETATION

The capacity to increase skeletal interstitial glycerol concentrations during direct beta2-adrenergic stimulation is impaired in obese subjects with normal intramyocellular concentrations, suggesting that this may be an early event in the process of triglyceride accumulation.