A novel index of whole body antilipolytic insulin action

Objective:

Assessment of antilipolytic insulin action is important in obesity research, but extensive isotopic tracer studies are not always feasible. We evaluated whether an index of antilipolytic insulin action could be derived from readily available insulin and glycerol concentrations obtained during clamps or oral glucose tolerance tests (OGTT).

Design and Methods:

We evaluated data collected from 29 subjects who had undergone a 3‐stage hyperinsulinemic‐euglycemic clamp (4, 8, and 40 mU/m²/min) with infusion of [²H₅]glycerol to calculate the glycerol rate of appearance (GLY_RA). Exponential decay curves for GLY_RA across insulin concentrations were generated for each individual and suppression of lipolysis was calculated as the insulin concentration needed to half‐maximally suppress GLY_RA (GLY_RA EC₅₀). Areas under the curve for glycerol (GLY_AUC) and insulin (INS_AUC) were calculated and their products (INS_AUC × GLY_AUC) were calculated as an index of insulin suppression of lipolysis.

Results:

The clamp index was highly correlated with GLY_RA EC₅₀ (r = 0.862, P < 0.001), as was an OGTT‐derived index (r = 0.720, P < 0.01).

Conclusions:

These findings suggest that the products of the insulin and glycerol AUC from either a clamp or an OGTT are good biomarkers of the antilipolytic action of insulin and are comparable with direct measurement by isotopic tracer methods.     

Intravenous estrogens increase insulin clearance and action in postmenopausal women

To test the hypothesis that estrogens alter insulin action, we evaluated the effects of intravenous conjugated estrogens (CE) on insulin-stimulated steady-state glucose infusion rate (SSGIR) and suppression of plasma glycerol in postmenopausal women (mean +/- SD; 56 +/- 4 yr; n = 12) not using hormone replacement. SSGIR and glycerol were measured during a two-stage (8 and 40 mU. m-2. min-1) hyperinsulinemic euglycemic clamp on 2 days, with or without a 2.5-mg intravenous CE bolus. Serum estradiol concentrations were increased approximately 200% on the estrogen (EST) compared with the control (CON) days. Serum insulin was reduced (P < 0.01) during stage 2 of the clamp for EST (63.3 +/- 12.8 micro U/ml) vs. CON (78.2 +/- 15.8 micro U/ml). Mean SSGIR and plasma glycerol did not differ between CON and EST days. With adjustment for differences in insulin concentration between conditions, stage 2 glucose disposals were significantly higher (8.63 vs. 7.20 mg. kg-1. min-1) and plasma glycerol concentrations were significantly lower (29.4 vs. 35.0 micro mol/l) for EST vs. CON. Our findings suggest that acute CE administration increases insulin clearance and action in postmenopausal women.     

Lipolysis in Adipocytes Isolated from Deep and Superficial Subcutaneous Adipose Tissue

Objective: Abdominal subcutaneous adipose tissue (SAT) occurs in two depots separated by a fascial plane: deep SAT and superficial SAT. In a recent study it was demonstrated that the amount of deep SAT has a much stronger relationship to insulin resistance than does superficial SAT. Because insulin resistance may be related to fatty acid release from adipose tissue, we hypothesized that the two SAT depots may have a different lipolytic activity. Research Methods and Procedures: To test this hypothesis, we obtained samples of deep and superficial SAT from patients undergoing elective abdominal surgery. The rate of lipolysis was determined in the collagenase‐digested adipocytes obtained from the two fat depots by measuring glycerol release in the presence and absence of isoproterenol. In addition, the relative concentration of hormone‐sensitive lipase was determined in both SAT depots by Western blot analysis. Results: Our results showed that the rate of isoproterenol‐stimulated lipolysis was ～20% higher in cells from deep SAT compared with those from superficial SAT, indicating that the deep SAT is more lipolytically active. The concentration of hormone‐sensitive lipase did not differ between the two adipose tissue depots. Discussion: These findings suggest that the higher lipolytic activity of deep SAT may account for its stronger association with insulin resistance. The mechanism seems to be independent of differences in hormone‐sensitive lipase concentration.     

Influence of lipolysis and fatty acid availability on fuel selection during exercise

The aim of the present study was to investigate the influence of substrate availability on fuel selection during exercise. Eight endurance-trained male cyclists performed 90-min exercise at 70 % of their maximal oxygen uptake in a cross-over design, either in rested condition (CON) or the day after 2-h exercise practised at 70 % of maximal oxygen uptake (EX). Subjects were given a sucrose load (0.75 g kg^?1 body weight) 45 min after the beginning of the 90-min exercise test. Lipolysis was measured in subcutaneous abdominal adipose tissue (SCAT) by microdialysis and substrate oxidation by indirect calorimetry. Lipid oxidation increased during exercise and tended to decrease during sucrose ingestion in both conditions. Lipid oxidation was higher during the whole experimental period in the EX group (p?=?0.004). Interestingly, fuel selection, assessed by the change in respiratory exchange ratio (RER), was increased in the EX session (p?=?0.002). This was paralleled by a higher rate of SCAT lipolysis reflected by dialysate glycerol, plasma glycerol, and fatty acids (FA) levels (p?<?0.001). Of note, we observed a significant relationship between whole-body fat oxidation and dialysate glycerol in both sessions (r ²?=?0.33, p?=?0.02). In conclusion, this study highlights the limiting role of lipolysis and plasma FA availability to whole-body fat oxidation during exercise in endurance-trained subjects. This study shows that adipose tissue lipolysis is a determinant of fuel selection during exercise in healthy subjects.     

Sex differences in lipolysis-regulating mechanisms in overweight subjects: effect of exercise intensity

Objective: To explore sex differences in the regulation of lipolysis during exercise, the lipid‐mobilizing mechanisms in the subcutaneous adipose tissue (SCAT) of overweight men and women were studied using microdialysis. Research Methods and Procedures: Subjects matched for age, BMI, and physical fitness performed two 30‐minute exercise bouts in a randomized fashion: the first test at 30% and 50% of their individual maximal oxygen uptake (Vo _2max) and the second test at 30% and 70% of their Vo _2max. Results: In both groups, an exercise‐dependent increment in extracellular glycerol concentration (EGC) was observed. Whatever the intensity, phentolamine [α‐adrenergic receptor (AR) antagonist] added to a dialysis probe potentiated exercise‐induced lipolysis only in men. In a probe containing phentolamine plus propranolol (β‐AR antagonist), no changes in EGC occurred when compared with the control probe when exercise was performed at 30% and 50% Vo _2max. A significant reduction of EGC (when compared with the control probe) was observed in women at 70% Vo _2max. At each exercise power, the plasma non‐esterified fatty acid and glycerol concentrations were higher in women. Exercise‐induced increase in plasma catecholamine levels was lower in women compared with men. Plasma insulin decreased and atrial natriuretic peptide increased similarly in both groups. Discussion: Overweight women mobilize more lipids (assessed by glycerol) than men during exercise. α₂‐Anti‐lipolytic effect was functional in SCAT of men only. The major finding is that during low‐to‐moderate exercise periods (30% and 50% Vo _2max), lipid mobilization in SCAT relies less on catecholamine‐dependent stimulation of β‐ARs than on an increase in plasma atrial natriuretic peptide concentrations and the decrease in plasma insulin.     

Effect of endothelin-1 on lipolysis in rat adipocytes

Objective : To explore the role of endothelin‐1 (ET‐1) on lipid metabolism, we examined the effect of ET‐1 on lipolysis in rat adipocytes. Research Methods and Procedure : Adipocytes isolated from male Sprague‐Dawley rats, weighing 400 to 450 grams, were incubated in Krebs‐Ringer buffer with or without 10^?7 M ET‐1 for various times or with various concentrations of ET‐1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET_AR and ET_BR blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET‐1‐stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET‐1 treatment. To further explore the underlying mechanism of ET‐1 action, we examined the involvement of the extracellular signal‐regulated kinase (ERK)‐mediated pathways. Results : Our results showed that ET‐1 caused lipolysis in rat adipocytes in a time‐ and dose‐dependent manner. BQ610, a selective ET_AR blocker, blocked this effect. The adenylyl cyclase inhibitor, 2′, 5′‐dideoxyadenosine, had no effect on ET‐1‐stimulated lipolysis. ET‐1 did not induce an increase in intracellular cAMP levels. In addition, ET‐1‐induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET‐1 and insulin suppressed ET‐1‐stimulated lipolysis. Discussion : These findings show that ET‐1 stimulates lipolysis in rat adipocytes through the ET_AR and activation of the ERK pathway. The underlying mechanism is cAMP‐independent. However, this non‐conventional lipolytic effect of ET‐1 is inhibited by the anti‐lipolytic effect of insulin.     

Endothelin‐1 Suppresses Long‐Chain Fatty Acid Uptake and Glucose Uptake Via Distinct Mechanisms in 3T3‐L1 Adipocytes

Endothelin‐1 (ET‐1) has been demonstrated to induce insulin resistance (IR) and lipolysis, raising the possibility that ET‐1 may also contribute to the elevated fatty acid levels in IR‐associated comorbidities. We attempted to evaluate whether ET‐1 also affects the long‐chain fatty acid (LCFA) utilization in 3T3‐L1 adipocytes. The effects of chronic ET‐1 exposure on basal and insulin‐stimulated LCFA uptake, and LCFA uptake kinetics were examined in 3T3‐L1 adipocytes. Chronic exposure to ET‐1 induced IR and suppressed basal and insulin‐stimulated LCFA uptake. Given that insulin acutely stimulates LCFA uptake, there was dramatically similar trend of dose‐response curves for ET‐1‐suppressed LCFA uptake, and also similar corresponding IC₅₀ values, between basal and insulin‐stimulated states, reflecting that ET‐1 predominantly suppresses basal LCFA uptake. Results of LCFA kinetics, western blots, and CD36 inhibition using sulfosuccinimidyl oleate (SSO) revealed that suppression of LCFA uptake by ET‐1 is associated with downregulation of CD36. ET type A receptor (ET_AR) antagonist BQ‐610 reversed the IR induction and the ET‐1‐suppressed LCFA uptake. Exogenous replenishment of phosphatidylinositol (PI) 4, 5‐bisphosphate (PIP₂) prevented IR induction, but not the suppression of LCFA uptake by ET‐1. Pharmacological inhibition of the activation of mitogen‐activated protein kinase (MAPK)/extracellular signal‐regulated kinase (ERK) completely blocked the ET‐1‐suppressed LCFA uptake. Serving as an inducer of IR, ET‐1 also chronically suppresses LCFA uptake via PIP₂‐independent and ERK‐dependent pathway. The interplay between impaired glucose disposal and diminished LCFA utilization, induced by ET‐1, could worsen the dysregulation of adipose metabolism and energy homeostasis in insulin‐resistant states.     

Both Subcutaneous and Visceral Adipose Tissue Correlate Highly with Insulin Resistance in African Americans

Objective: The contribution of visceral adipose tissue (VAT) to insulin resistance is well‐established; however, the role of subcutaneous abdominal adipose tissue (SAT) in insulin resistance remains controversial. Sex may determine which of these two components of abdominal obesity is more strongly related to insulin resistance and its consequences. The aim of this study was to determine whether both VAT and SAT contribute to insulin resistance in African Americans and to examine the effects of sex on this relationship. Research Methods and Procedures: This was a cross‐sectional study of 78 nondiabetic African‐American volunteers (44 men, 35 women; age 33.8 ± 7.3 years; BMI 30.9 ± 7.4 kg/m²). VAT and SAT volumes were measured using serial computerized tomography slices from the dome of the diaphragm to the iliac crest. The insulin sensitivity index (S_I) was determined from the minimal model using data obtained from the frequently sampled intravenous glucose tolerance test. Results: In men, both VAT and SAT were negatively correlated with S_I (r for both correlations = ?0.57; p < 0.01). In women, the correlation coefficient between VAT and S_I was ?0.50 (p < 0.01) and between SAT and S_I was ?0.67 (p < 0.01). In women, the correlation coefficient for S_I with SAT was significantly greater than the correlation coefficient with VAT (p = 0.02). Discussion: Both SAT and VAT are strongly correlated with insulin resistance in African Americans. For African‐American women, SAT may have a greater effect than VAT on insulin resistance.     

Metabolic Effects of Mental Stress during Over- and Underfeeding in Healthy Women

Objective: To assess the short-term consequences of carbohydrate or fat overfeeding or of food restriction on the metabolic effects of mental stress in healthy lean women. Research Methods and Procedures: The effects of a sympathetic activation elicited by mental stress were evaluated in a group of healthy women after standardized isocaloric feeding (ISO) or after a 3-day overfeeding with 40% excess calories as either carbohydrate overfeeding (CHO OF) or fat overfeeding (FAT OF). Oxygen consumption rate (VO ₂) was measured as an index of energy expenditure, and subcutaneous glycerol concentrations were monitored with microdialysis. The same measurements were performed in another group of healthy women after ISO and after a 3-day period of underfeeding with a protein sparing modified fast (UF). Results: In all conditions, mental stress significantly increased heart rate, blood pressure, plasma norepinephrine and epinephrine concentrations, and VO ₂, and produced a nonsignificant increase in subcutaneous glycerol concentrations. CHO OF and FAT OF did not alter the effects of mental stress on VO ₂ and subcutaneous glycerol concentrations. In contrast, UF increased basal VO ₂ but significantly reduced its stimulation by mental stress. UF also enhanced the increase in subcutaneous glycerol concentrations during mental stress. Discussion: UF reduces the stimulation of energy expenditure and enhances lipolysis during sympathetic activation. These adaptations may be involved in mobilization of endogenous fat while limiting weight loss. In contrast, short-term overfeeding fails to alter the sympathetic control of energy expenditure and lipolysis.     

Metabolic Adaptations to Dexamethasone‐Induced Insulin Resistance in Healthy Volunteers

Objective : Insulin resistance is observed in individuals with normal glucose tolerance. This indicates that increased insulin secretion can compensate for insulin resistance and that additional defects are involved in impaired glucose tolerance or type 2 diabetes. The objective of this study was to evaluate a procedure aimed at assessing the compensatory mechanisms to insulin resistance. Research Methods and Procedures : Eight healthy nonobese female patients were studied on two occasions, before and after administration of 2 mg/d dexamethasone for 2 days during a two‐step hyperglycemic clamp. Insulin secretion was assessed from plasma insulin concentrations. Insulin sensitivity was assessed from the ratio of whole‐body glucose use (6, 6 ²H₂ glucose) to plasma insulin concentrations. This procedure is known to induce a reversible impairment of glucose tolerance and insulin resistance. Results : In all subjects, dexamethasone induced a decrease in insulin sensitivity and a proportionate increase in first‐phase insulin secretion and in insulin concentrations at both steps of glycemia. The resulting hyperinsulinemia allowed the restoration of normal whole‐body glucose uptake and the suppression of plasma free fatty acids and triglycerides. In contrast, the suppression of endogenous glucose production was impaired after dexamethasone (p < 0.01). Discussion : Increased insulin secretion fully compensates dexamethasone‐induced insulin resistance in skeletal muscle and adipose tissue but not in the liver. This suggests that failure to overcome hepatic insulin resistance can impair glucose tolerance. The compensatory insulin secretion in response to insulin resistance can be assessed by means of a hyperglycemic clamp after a dexamethasone challenge.     

Preferential Stimulation of Abdominal Subcutaneous Lipolysis after Prednisolone Exposure in Humans

Objective: The role of cortisol in the regulation of lipolysis is not clear. This study was undertaken to explore whether a standard dose of prednisolone for 1 week would influence lipolysis in abdominal and femoral tissue. Research Methods and Procedures: We used the microdialysis technique, the forearm technique, and indirect calorimetry, in the fasting state, after 1 week of treatment with prednisolone (30 mg daily) or placebo. Eight healthy young men (age: 25 ± 3 years; height: 181 ± 1 cm; body mass index [BMI]: 23.3 ± 0.7 kg/m²) were studied. Results: Treatment with prednisolone induced insulin resistance (Homeostasis Model Assessment index: placebo vs. prednisolone: 7.15 ± 1.63 vs. 17.00 ± 14.26, p = 0.03), hyperinsulinemia (p = 0.01), and hyperglucagonemia (p = 0.001), whereas growth hormone concentrations were unaffected. Abdominal adipose tissue interstitial glycerol was increased during treatment with prednisolone in the face of significant hyperinsulinemia, although it barely reached statistical significance (p = 0.06). At the femoral adipose tissue depot, no difference in lipolysis was found. Arterial and venous free fatty acids (FFA) were comparable in the two situations, whereas the arteriovenous difference across the forearm was significantly decreased during treatment with prednisolone, indicating increased uptake, or decreased release of FFA. Energy expenditure (p = 0.3), repiratory quotient (p = 0.9), glucose oxidation (p = 0.9), lipid oxidation (p = 1.0), and protein oxidation (p = 0.1) were unaltered on the 2 study days. Discussion: Short‐term treatment with a standard dose of corticosteroids induces increased abdominal adipose tissue lipolysis, as well as hyperinsulinemia, hyperglucagonemia, and insulin resistance.     

Glucose-dependent insulinotropic polypeptide modulates adipocyte lipolysis and reesterification

Objective: Glucose‐dependent insulinotropic polypeptide (GIP) is an incretin released from intestinal K‐cells during the postprandial period. Previous studies have suggested that GIP may play an etiologic role in obesity; thus, the GIP receptor may represent a target for anti‐obesity drugs. The present studies were conducted to elucidate mechanisms by which GIP might promote obesity by examining the effect of GIP on both glycerol release (indicative of lipolysis) and free fatty acid (FFA) release (indicative of both lipolysis and reesterification), as well as the ability of a GIP‐specific receptor antagonist (ANTGIP) to attenuate these effects. Research Methods and Procedures: Isolated rat adipocytes were perifused on a column with 10 nM GIP alone or in combination with 10 μU/mL insulin, 1 μM isoproterenol, or 1 μM ANTGIP. Samples were collected every minute and assayed for FFA, glycerol, and lactate. Results: GIP significantly increased FFA reesterification (decreased FFA release by 25%), stimulated lipolysis (increased glycerol release by 22%), and attenuated the lipolytic response to isoproterenol by 43%. These properties were similar to those of insulin in vitro, suggesting that GIP possesses insulin‐like lipogenic effects on adipocytes. Finally, ANTGIP reversed the effects of GIP on both basal and stimulated adipocyte metabolism. Discussion: These studies provide further evidence for an important physiological role for GIP in lipid homeostasis and possibly in the pathogenesis of obesity. They also suggest that the GIP receptor may represent an excellent target for the prevention and treatment of obesity and obesity‐related type 2 diabetes.     

In vivo 2H2O administration reveals impaired triglyceride storage in adipose tissue of insulin-resistant humans

Indirect evidence suggests that impaired triglyceride storage in the subcutaneous fat depot contributes to the development of insulin resistance via lipotoxicity. We directly tested this hypothesis by measuring, in vivo, TG synthesis, de novo lipogenesis (DNL), adipocyte proliferation, and insulin suppression of lipolysis in subcutaneous adipose tissue of BMI-matched individuals classified as insulin resistant (IR) or insulin sensitive (IS). Nondiabetic, moderately obese subjects with BMI 25–35 kg/m², classified as IR or IS by the modified insulin suppression test, consumed deuterated water (²H₂O) for 4 weeks. Deuterium incorporation into glycerol, palmitate, and DNA indicated TG synthesis, DNL, and adipocyte proliferation, respectively. Net TG synthesis and DNL in adipose cells were significantly lower in IR as compared with IS subjects, whereas adipocyte proliferation did not differ significantly. Plasma FFAs measured during an insulin suppression test were 2.5-fold higher in IR subjects, indicating resistance to insulin suppression of lipolysis. Adipose TG synthesis correlated directly with DNL but not with proliferation. These results provide direct in vivo evidence for impaired TG storage in subcutaneous adipose tissue of IR as compared with IS. Relative inability to store TG in the subcutaneous depot may represent a mechanism contributing to the development of insulin resistance in the setting of obesity.     

Medium‐Chain Fatty Acids Attenuate Agonist‐Stimulated Lipolysis,Mimicking the Effects of Starvation

Objective: To test the hypothesis that incorporation of medium‐chain fatty acids (FAs) into adipocyte triglycerides alters intracellular lipolysis. Research Methods and Procedures: 3T3‐L1 adipocytes were pretreated with octanoate for various incubation periods. After the removal of exogenous FAs, cells were incubated with different lipolytic agonists. To determine the effects on lipolysis, we measured the following: the release of glycerol and FAs, lipase activity, protein levels of hormone‐sensitive lipase (HSL), and perilipin A; translocation of HSL; phosphorylation of perilipin A; and levels of cellular adenosine triphosphate, cyclic adenosine monophosphate, and H₂O₂. To compare the effects of starvation with those caused by octanoate pretreatment, we measured glycerol release and H₂O₂ generation in rat adipocytes of starved donors. Results: Pretreatment of adipocytes with octanoate in vitro increased basal lipolysis but decreased the cellular response for agonists. The same effects were seen in starvation in vivo. Preincubation with octanoate for 48 hours did not affect basal lipase activity, HSL, and perilipin protein levels, but it reduced agonist‐stimulated perilipin phosphorylation and HSL translocation toward fat droplets. This was associated with a reduction in basal cellular adenosine triphosphate levels and agonist‐stimulated cyclic adenosine monophosphate generation. Starvation and octanoate pretreatment both increased intracellular H₂O₂ concentrations, which might also contribute to the inhibition on agonist‐stimulated lipolysis. Discussion: Pretreatment with octanoate seems to induce changes in adipocyte lipolysis in a pattern mimicking the effects of starvation. Such changes could contribute, in part, to weight loss in animals and humans associated with dietary medium‐chain FAs.     

Conjugated linoleic acid and betaine affect lipolysis in pig adipose tissue explants

Consumers’ demand of leaner meat products is a challenge. Although betaine and conjugated linoleic acid (CLA) have the potential to decrease porcine adipose tissue, their mode of action is poorly understood. The aim of the study was to determine the lipolytic effect of betaine and CLA in the adipose tissue of Iberian pigs. Adipose tissue explants from five pigs (38 kg BW) were prepared from dorsal subcutaneous adipose tissue samples and cultivated for 2 h (acute experiments) or 72 h (chronic experiments). Treatments included 100 µM linoleic acid (control), 100 µM trans-10, cis-12 CLA, 100 µM linoleic acid + 1 mM betaine and 100 µM trans-10, cis-12 CLA + 1 mM betaine (CLABET). To examine the ability of betaine or CLA to inhibit insulin’s suppression of isoproterenol-stimulated lipolysis, test medium was amended with 1 µM isoproterenol ±10 nM insulin. Media glycerol was measured at the end of the incubations. Acute lipolysis (2 h) was increased by CLA and CLABET (85% to 121%; P < 0.05) under basal conditions. When lipolysis was stimulated with isoproterenol (1090%), acute exposure to betaine tended to increase (13%; P = 0.071), while CLA and CLABET increased (14% to 18%; P < 0.05) isoproterenol-stimulated lipolysis compared with control. When insulin was added to isoproterenol-stimulated explants, lipolytic rate was decreased by 50% (P < 0.001). However, supplementation of betaine to the insulin + isoproterenol-containing medium tended to increase (P = 0.07), while CLABET increased (45%; P < 0.05) lipolysis, partly counteracting insulin inhibition. When culture was extended for 72 h, CLA decreased lipolysis under basal conditions (18%; P < 0.05) with no effect of betaine and CLABET (P > 0.10). When lipolysis was stimulated by isoproterenol (125% increase in rate compared with basal), CLA and CLABET decreased glycerol release (27%; P < 0.001) compared with control (isoproterenol alone). When insulin was added to isoproterenol-stimulated explants, isoproterenol stimulation of lipolysis was completely blunted and neither betaine nor CLA altered the inhibitory effect of insulin on lipolysis. Isoproterenol, and especially isoproterenol + insulin, stimulated leptin secretion compared with basal conditions (68% and 464%, respectively; P < 0.001), with no effect of CLA or betaine (P > 0.10). CLA decreased leptin release (25%; P < 0.001) when insulin was present in the media, partially inhibiting insulin stimulation of leptin release. In conclusion, betaine and CLA produced a biphasic response regarding lipolysis so that glycerol release was increased in acute conditions, while CLA decreased glycerol release and betaine had no effect in chronic conditions. Furthermore, CLA and CLABET indirectly increased lipolysis by reducing insulin-mediated inhibition of lipolysis during acute conditions.     

A Single mri Slice Does Not Accurately Predict Visceral and Subcutaneous Adipose Tissue Changes During Weight Loss

Earlier cross‐sectional studies found that a single magnetic resonance imaging (MRI) slice predicts total visceral and subcutaneous adipose tissue (VAT and SAT) volumes well. We sought to investigate the accuracy of trunk single slice imaging in estimating changes of total VAT and SAT volume in 123 overweight and obese subjects who were enrolled in a 24‐week CB‐1R inverse agonist clinical trial (weight change, ?7.7 ± 5.3 kg; SAT change, ?5.4 ± 4.9 l, VAT change, ?0.8 ± 1.0 l). VAT and SAT volumes at baseline and 24 weeks were derived from whole‐body MRI images. The VAT area 5–10 cm above L₄—L₅ (A_+5–10) (R² = 0.59–0.70, P < 0.001) best predicted changes in VAT volume but the strength of these correlations was significantly lower than those at baseline (R² = 0.85–0.90, P < 0.001). Furthermore, the L₄—L₅ slice poorly predicted VAT volume changes (R² = 0.24–0.29, P < 0.001). Studies will require 44–69% more subjects if (A_+5–10) is used and 243–320% more subjects if the L₄—L₅ slice is used for equivalent power of multislice total volume measurements of VAT changes. Similarly, single slice imaging predicts SAT loss less well than cross‐sectional SAT (R² = 0.31–0.49 vs. R² = 0.52–0.68, P < 0.05). Results were the same when examined in men and women separately. A single MRI slice 5–10 cm above L₄—L₅ is more powerful than the traditionally used L₄—L₅ slice in detecting VAT changes, but in general single slice imaging poorly predicts VAT and SAT changes during weight loss. For certain study designs, multislice imaging may be more cost‐effective than single slice imaging in detecting changes for VAT and SAT.     

Association Between the 4 bp Proinsulin Gene Insertion Polymorphism (IVS‐69) and Body Composition in Black South African Women

The objective of the study was to examine the association between a functional 4 bp proinsulin gene insertion polymorphism (IVS‐69), fasting insulin concentrations, and body composition in black South African women. Body composition, body fat distribution, fasting glucose and insulin concentrations, and IVS‐69 genotype were measured in 115 normal‐weight (BMI <25 kg/m²) and 138 obese (BMI ≥30 kg/m²) premenopausal women. The frequency of the insertion allele was significantly higher in the class 2 obese (BMI ≥35kg/m²) compared with the normal‐weight group (P = 0.029). Obese subjects with the insertion allele had greater fat mass (42.3 ± 0.9 vs. 38.9 ± 0.9 kg, P = 0.034) and fat‐free soft tissue mass (47.4 ± 0.6 vs. 45.1 ± 0.6 kg, P = 0.014), and more abdominal subcutaneous adipose tissue (SAT, 595 ± 17 vs. 531 ± 17 cm², P = 0.025) but not visceral fat (P = 0.739), than obese homozygotes for the wild‐type allele. Only SAT was greater in normal‐weight subjects with the insertion allele (P = 0.048). There were no differences in fasting insulin or glucose levels between subjects with the insertion allele or homozygotes for the wild‐type allele in the normal‐weight or obese groups. In conclusion, the 4 bp proinsulin gene insertion allele is associated with extreme obesity, reflected by greater fat‐free soft tissue mass and fat mass, particularly SAT, in obese black South African women.     

Metabolic profiling of the human response to a glucose challenge reveals distinct axes of insulin sensitivity

Glucose ingestion after an overnight fast triggers an insulin‐dependent, homeostatic program that is altered in diabetes. The full spectrum of biochemical changes associated with this transition is currently unknown. We have developed a mass spectrometry‐based strategy to simultaneously measure 191 metabolites following glucose ingestion. In two groups of healthy individuals (n=22 and 25), 18 plasma metabolites changed reproducibly, including bile acids, urea cycle intermediates, and purine degradation products, none of which were previously linked to glucose homeostasis. The metabolite dynamics also revealed insulin's known actions along four key axes—proteolysis, lipolysis, ketogenesis, and glycolysis—reflecting a switch from catabolism to anabolism. In pre‐diabetics (n=25), we observed a blunted response in all four axes that correlated with insulin resistance. Multivariate analysis revealed that declines in glycerol and leucine/isoleucine (markers of lipolysis and proteolysis, respectively) jointly provide the strongest predictor of insulin sensitivity. This observation indicates that some humans are selectively resistant to insulin's suppression of proteolysis, whereas others, to insulin's suppression of lipolysis. Our findings lay the groundwork for using metabolic profiling to define an individual's ‘insulin response profile’, which could have value in predicting diabetes, its complications, and in guiding therapy.     

Circulating IL-6 concentrations and abdominal adipocyte isoproterenol-stimulated lipolysis in women

Objective: To examine the association of plasma interleukin‐6 (IL‐6) concentrations with adiposity and fat cell metabolism in women. Methods and Procedures: Omental (OM) and subcutaneous (SC) adipose tissue samples were obtained from 48 healthy women (age: 47 ± 5 years, BMI: 26.9 ± 5.3 kg/m²) undergoing gynecological surgeries. Total and visceral adiposity were assessed by dual‐energy X‐ray absorptiometry and computed tomography, respectively. Measures of adipocyte lipolysis (basal, isoproterenol‐, forskolin‐, and cyclic dibutyryl‐adenosine monophosphate (AMP)‐stimulated) and adipose tissue lipoprotein lipase (LPL) activity were obtained. Plasma IL‐6 was measured by radioimmunoassay. Results: Plasma IL‐6 was positively correlated with total body fat mass (r = 0.32, P < 0.05), SC adipose tissue area (r = 0.35, P < 0.05), SC adipocyte diameter (r = 0.30, P < 0.05), and a trend was observed with visceral adipose tissue area (r = 0.20, P < 0.07). Plasma IL‐6 was positively correlated with glycerol released in response to isoproterenol (10^?5 to 10^?8 mol/l) by isolated SC (0.31 ≤ r ≤ 0.65, P < 0.05) and OM (0.36 ≤ r ≤ 0.40, P < 0.02) adipocytes, independent of menopausal status. No correlation was found with LPL activity. A subsample of women with high plasma IL‐6 (n = 10) was matched with women with low plasma IL‐6 (n = 10) for total body fat mass. OM adipocyte glycerol release in response to isoproterenol (10^?5 to 10^?8 mol/l) was higher in the subsample of women with elevated plasma IL‐6 (P ≤ 0.07). Discussion: We observed that OM lipolysis was significantly higher in women with elevated plasma IL‐6 for a similar body fat mass and menopausal status. These results suggest that higher circulating IL‐6 concentrations are associated with increased isoproterenol‐stimulated lipolysis especially in OM abdominal adipocytes in women.     

Regulation of Visceral and Subcutaneous Adipocyte Lipolysis by Acute AICAR‐induced AMPK Activation

This study investigated the role of adenosine monophosphate–activated protein kinase (AMPK) in the regulation of lipolysis in visceral (VC) and subcutaneous (SC) rat adipocytes and the molecular mechanisms involved in this process. VC (epididymal and retroperitoneal) and SC (inguinal) adipocytes were isolated from male Wistar rats (160–180 g). Adipocytes were incubated either in the absence or in the presence of the AMPK agonist 5‐aminoimidazole‐4‐carboxamide‐1‐β‐d‐ribofuranoside (AICAR, 0–500 µmol/l). AMPK and acetyl‐CoA carboxylase (ACC) phosphorylation, basal and epinephrine‐stimulated (100 nmol/l) glycerol release, and hormone‐sensitive lipase (HSL) phosphorylation and activity were determined. AICAR‐induced (500 µmol/l) AMPK activation inhibited basal glycerol release by ～42, 41, and 44% in epididymal, retroperitoneal, and inguinal adipocytes, respectively. Epinephrine‐stimulated glycerol release was almost completely prevented by AICAR treatment in adipocytes from all fat depots. The AMPK inhibitor compound C (20 µmol/l) prevented AICAR‐induced phosphorylation of AMPK and significantly increased basal (～1.3‐, 1.4‐, and 1.7‐fold) and epinephrine‐stimulated (～1.3‐, 1.2‐, 1.4‐fold) glycerol release in epididymal, retroperitoneal, and inguinal adipocytes, respectively. AICAR increased phosphorylation of HSL_Ser565 and inhibited epinephrine‐induced phosphorylation of HSL_Ser563 and HSL_Ser660. This was also accompanied by a 73% reduction in epinephrine‐stimulated HSL activity. Compound C prevented the phosphorylation of HSL_Ser565 induced by AICAR and partially prevented the inhibitory effect of this drug on basal and epinephrine‐stimulated lipolysis in adipocytes in VC and SC fat depots. In summary, despite different fat depots eliciting distinct rates of lipolysis, acute AICAR‐induced AMPK activation suppressed HSL phosphorylation/activation and exerted similar antilipolytic effects on both VC and SC adipocytes.