Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with ...insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the “culprits” were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount.
Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties.
Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance.
There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear.
Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.
Visceral obesity is intimately associated with metabolic disease and adverse health outcomes. However, a direct association between increasing amounts of visceral fat and end‐organ inflammation and ...scarring has not been demonstrated. We examined the association between visceral fat and liver inflammation in patients with nonalcoholic fatty liver disease (NAFLD) to delineate the importance of visceral fat to progressive steatohepatitis and hence the inflammatory pathogenesis of the metabolic syndrome. We undertook a cross‐sectional, proof of concept study in 38 consecutive adults with NAFLD at a tertiary liver clinic. All subjects had a complete physical examination, anthropometric assessment, and fasting blood tests on the day of liver biopsy. Abdominal fat volumes were assessed by magnetic resonance imaging within 2 weeks of liver biopsy. The extent of hepatic inflammation and fibrosis augmented incrementally with increases in visceral fat (P < 0.01). For each 1% increase in visceral fat, the odds ratio for increasing liver inflammation and fibrosis was 2.4 (confidence interval CI: 1.3‐4.2) and 3.5 (CI: 1.7‐7.1), respectively. Visceral fat remained an independent predictor of advanced steatohepatitis (odds ratio OR 2.1, CI: 1.1‐4.2, P = 0.05) and fibrosis (OR 2.9, CI: 1.4‐6.3, P = 0.006) even when controlled for insulin resistance and hepatic steatosis. Interleukin‐6 (IL‐6) levels, which correlated with visceral fat, also independently predicted increasing liver inflammation. Visceral fat was associated with all components of the metabolic syndrome. Conclusion: Visceral fat is directly associated with liver inflammation and fibrosis independent of insulin resistance and hepatic steatosis. Visceral fat should therefore be a central target for future interventions in nonalcoholic steatohepatitis and indeed all metabolic disease. (HEPATOLOGY 2008.)
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Type 2 diabetes mellitus (T2D) is predicted by central obesity and circulating adipokines regulating inflammation. We hypothesized that visceral adipose tissue (VAT) in T2D expresses greater levels ...of proinflammatory molecules. Paired samples of subcutaneous (SAT) and VAT were excised at elective surgery (n = 16, 6 with T2D, n = 8 age- and gender- matched controls). Metabolic parameters were measured in the fasted state: body composition by dual-energy X-ray absorptiometry and insulin action by hyperinsulinemic–euglycemic clamp. Adipose tissue mRNA gene expression was measured by quantitative reverse transcriptase-PCR. Subjects with T2D had higher VAT expression of molecules regulating inflammation (tumor necrosis factor-α (TNFα), macrophage inflammatory protein (MIP), interleukin-8 (IL-8)). Fasting glucose related to VAT expression of TNFα, MIP, serum amyloid A (SAA), IL-1α, IL-1β, IL-8, and IL-8 receptor. Abdominal fat mass was related to VAT expression of MIP, SAA, cAMP response element–binding protein (CREBP), IL-1β, and IL-8. Insulin action related inversely to VAT complement C3 expression only. There were depot-specific differences in expression of serum T2D predictors: VAT expressed higher levels of complement C3; SAT expressed higher levels of retinol-binding protein-4 (RBP4), adiponectin, and leptin. In summary, VAT in T2D expresses higher levels of adipokines involved in inflammation. VAT expression of these molecules is related to fasting glucose and insulin action. Increased production of these proinflammatory molecules by VAT may explain the links observed between visceral obesity, insulin resistance, and diabetes risk.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Objective
Alterations in lipids in muscle and plasma have been documented in insulin‐resistant people with obesity. Whether these lipid alterations are a reflection of insulin resistance or obesity ...remains unclear.
Methods
Nondiabetic sedentary individuals not treated with lipid‐lowering medications were studied (n = 51). Subjects with body mass index (BMI) > 25 kg/m2 (n = 28) were stratified based on median glucose infusion rate during a hyperinsulinemic‐euglycemic clamp into insulin‐sensitive and insulin‐resistant groups (above and below median, obesity/insulin‐sensitive and obesity/insulin‐resistant, respectively). Lean individuals (n = 23) served as a reference group. Lipidomics was performed in muscle and plasma by liquid chromatography electrospray ionization‐tandem mass spectrometry. Pathway analysis of gene array in muscle was performed in a subset (n = 35).
Results
In muscle, insulin resistance was characterized by higher levels of C18:0 sphingolipids, while in plasma, higher levels of diacylglycerol and cholesterol ester, and lower levels of lysophosphatidylcholine and lysoalkylphosphatidylcholine, indicated insulin resistance, irrespective of overweight/obesity. The sphingolipid metabolism gene pathway was upregulated in muscle in insulin resistance independent of obesity. An overweight/obesity lipidomic signature was only apparent in plasma, predominated by higher triacylglycerol and lower plasmalogen species.
Conclusions
Muscle C18:0 sphingolipids may play a role in insulin resistance independent of excess adiposity.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Adipocyte differentiation and its impact on restriction or expansion of particular adipose tissue depots have physiological and pathophysiological significance in view of the different functions of ...these depots. Brown or "beige" fat brown adipose tissue (BAT) expansion can enhance thermogenesis, lipid oxidation, insulin sensitivity, and glucose tolerance; conversely expanded visceral fat visceral white adipose tissue (VAT) is associated with insulin resistance, low grade inflammation, dyslipidemia, and cardiometabolic risk. The largest depot, subcutaneous white fat subcutaneous white adipose tissue (SAT), has important beneficial characteristics including storage of lipid "out of harms way" and secretion of adipokines, especially leptin and adiponectin, with positive metabolic effects including lipid oxidation, energy utilization, enhanced insulin action, and an anti-inflammatory role. The absence of these functions in lipodystrophies leads to major metabolic disturbances. An ability to expand white adipose tissue adipocyte differentiation would seem an important defense mechanism against the detrimental effects of energy excess and limit harmful accumulation of lipid in "ectopic" sites, such as liver and muscle. Adipocyte differentiation involves a transcriptional cascade with PPARγ being most important in SAT but less so in VAT, with increased angiogenesis also critical. The transcription factor, Islet1, is fairly specific to VAT and in vitro inhibits adipocyte differentiation. The physiological importance of Islet1 requires further study. Basic control of differentiation is similar in BAT but important differences include the effect of PGC-1α on mitochondrial biosynthesis and upregulation of UCP1; also PRDM16 plays a pivotal role in expression of the BAT phenotype. Modulation of the capacity or function of these different adipose tissue depots, by altering adipocyte differentiation or other means, holds promise for interventions that can be helpful in human disease, particularly cardiometabolic disorders associated with the world wide explosion of obesity.
HIV-1 protease-inhibitor treatments are associated with a syndrome of peripheral lipodystrophy, central adiposity, breast hypertrophy in women, hyperlipidaemia, and insulin resistance. The catalytic ...region of HIV-1 protease, to which protease inhibitors bind, has approximately 60% homology to regions within two proteins that regulate lipid metabolism: cytoplasmic retinoic-acid binding protein type 1 (CRABP-1) and low density lipoprotein-receptor-related protein (LRP). We hypothesise that protease inhibitors inhibit CRABP-1-modified, and cytochrome P450 3A-mediated synthesis of cis-9-retinoic acid, a key activator of the retinoid X receptor; and peroxisome proliferator activated receptor type gamma (PPAR-γ) heterodimer, an adipocyte receptor that regulates peripheral adipocyte differentiation and apoptosis. Protease-inhibitor binding to LRP would impair hepatic chylomicron uptake and triglyceride clearance by the endothelial LRP-lipoprotein lipase complex. The resulting hyperlipidaemia contributes to central fat deposition (and in the breasts in the presence of oestrogen), insulin resistance, and, in susceptible individuals, type 2 diabetes. Understanding the syndrome's pathogenesis should lead to treatment strategies and to the design of protease inhibitors that do not cause this syndrome.
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DOBA, GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SIK, UILJ, UKNU, UL, UM, UPCLJ, UPUK, VSZLJ
Several circulating cytokines are increased with obesity and may combine with the influence of visceral fat to generate insulin resistance, inflammation, and fibrosis in nonalcoholic fatty liver ...disease (NAFLD). Little information exists in NAFLD about three recently recognized tissue‐derived cytokines that are all lipid‐binding and involved in inflammation, namely adipocyte fatty acid–binding protein (AFABP), lipocalin‐2, and retinol‐binding protein 4 (RBP4). We examined the association of these three peptides with hepatic steatosis, inflammation, and fibrosis plus indices of adiposity, insulin resistance, and dyslipidaemia in 100 subjects with NAFLD and 129 matched controls. Levels of AFABP and lipocalin‐2, but not RBP4, were significantly elevated in NAFLD versus control (AFABP, 33.5 ± 14.4 versus 23.1 ± 12.1 ng/mL P < 0.001; lipocalin‐2, 63.2 ± 26 versus 48.6 ± 20 ng/mL P < 0.001) and correlated with indices of adiposity. AFABP correlated with indices of subcutaneous rather than visceral fat. AFABP alone distinguished steatohepatitis from simple steatosis (P= 0.02). Elevated AFABP independently predicted increasing inflammation and fibrosis, even when insulin resistance and visceral fat were considered; this applied to lobular inflammation and ballooning (odds ratio 1.4, confidence interval 1.0‐1.8) and fibrosis stage (odds ratio 1.3, confidence interval 1.0‐1.7) (P ≤ 0.05 for all). None of the cytokines correlated with steatosis grade. AFABP levels correlated with insulin resistance (homeostasis model assessment of insulin resistance) in controls and NAFLD, whereas lipocalin‐2 and RBP4 only correlated positively with insulin resistance in controls. Conclusion: Circulating AFABP, produced by adipocytes and macrophages, and lipocalin‐2, produced by multiple tissues, are elevated and may contribute to the metabolic syndrome in NAFLD. AFABP levels, which correlate with subcutaneous, but not visceral fat, independently predict inflammation and fibrosis in NAFLD and may have a direct pathogenic link to disease progression. (HEPATOLOGY 2009;49:1926–1934.)
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Background: Impaired mitochondrial function in skeletal muscle is implicated in the development of insulin resistance. However, potential differences in fatness and fitness may influence previous ...results.
Methods: Subjects (n =18) were divided into insulin-sensitive (IS) and insulin-resistant (IR) groups by median glucose infusion rate during a hyperinsulinemic euglycemic clamp. Weight, VO2max (maximal aerobic capacity), and percentage body fat were measured before and after 6 continuous weeks of aerobic exercise training at 55–70% VO2max (40 min/session, 4 d/wk).
Results: Age, percentage fat, and VO2max were not different between IS and IR groups at baseline. Expression of the nuclear encoded PGC1α and mitochondrial encoded gene COX1 were significantly lower in the IR group (P < 0.05). Citrate synthase activity and protein levels of subunits from complexes I and III of the respiratory chain were also lower in the IR group (P < 0.05). Insulin sensitivity and aerobic fitness were increased after exercise training in both groups (P < 0.001), and the expression of mitochondrial encoded genes CYTB and COX1 was also increased (P < 0.01). However, there was no change in PGC1α expression, mitochondrial enzyme activity, or protein levels of complexes of the respiratory chain in response to exercise in either group.
Conclusion: This study confirms that IR men have reduced markers of mitochondrial metabolism, independent of fatness and fitness. Moderate exercise training did not alter these markers despite improving fitness and whole body insulin sensitivity. This study suggests that additional mechanisms may be involved in improving insulin resistance after exercise training in obese men.
Abstract
Context
The etiological mechanism of bile acid (BA) effects on insulin resistance and obesity is unknown.
Objective
This work aimed to determine whether plasma BAs are elevated in human ...obesity and/or insulin resistance.
Methods
This observational study was conducted at an academic research center. Seventy-one adult volunteers formed 4 groups: lean insulin-sensitive (body mass index BMI ≤ 25 kg/m2, Homeostatic Model Assessment of Insulin Resistance HOMA-IR < 2.0, n = 19), overweight/obese nondiabetic who were either insulin sensitive (Obsensitive, BMI > 25 kg/m2, HOMA-IR < 1.5, n = 11) or insulin resistant (Obresistant, BMI > 25 kg/m2, HOMA-IR > 3.0, n = 20), and type 2 diabetes (T2D, n = 21). Main outcome measures included insulin sensitivity by hyperinsulinemic-euglycemic clamp, body composition by dual energy x-ray absorptiometry, abdominal fat distribution, and liver density by computed tomography and plasma BA.
Results
In the Obresistant group, glucose infusion rate/fat-free mass (GIR/FFM, an inverse measure of insulin resistance) was significantly lower, and visceral and liver fat higher, compared to lean and Obsensitive individuals, despite similar total adiposity in Obresistant and Obsensitive. Total BA concentrations were higher in Obresistant (2.62 ± 0.333 mmol/L, P = .03) and T2D (3.36 ± 0.582 mmol/L, P < .001) vs Obsensitive (1.16 ± 0.143 mmol/L), but were similar between Obsensitive and lean (2.31 ± 0.329 mmol/L) individuals. Total BAs were positively associated with waist circumference (R = 0.245, P = .041), visceral fat (R = 0.360, P = .002), and fibroblast growth factor 21 (R = 0.341, P = .004) and negatively associated with insulin sensitivity (R = –0.395, P = .001), abdominal subcutaneous fat (R = –0.352, P = .003), adiponectin (R = –0.375, P = .001), and liver fat (Hounsfield units, an inverse marker of liver fat, R = –0.245, P = .04). Conjugated BAs were additionally elevated in T2D individuals (P < .001).
Conclusions
BA concentrations correlated with abdominal, visceral, and liver fat in humans, though an etiological role in insulin resistance remains to be verified.
Visceral adipose tissue (VAT) is more closely linked to insulin resistance than subcutaneous adipose tissue (SAT). We conducted a quantitative analysis of the secretomes of VAT and SAT to identify ...differences in adipokine secretion that account for the adverse metabolic consequences of VAT.
We used lectin affinity chromatography followed by comparison of isotope-labeled amino acid incorporation rates to quantitate relative differences in the secretomes of VAT and SAT explants. Because adipose tissue is composed of multiple cell types, which may contribute to depot-specific differences in secretion, we isolated preadipocytes and microvascular endothelial cells (MVECs) and compared their secretomes to those from whole adipose tissue.
Although there were no discrete depot-specific differences in the secretomes from whole adipose tissue, preadipocytes, or MVECS, VAT exhibited an overall higher level of protein secretion than SAT. More proteins were secreted in twofold greater abundance from VAT explants compared with SAT explants (59% versus 21%), preadipocytes (68% versus 0%), and MVECs (62% versus 15%). The number of proteins in the whole adipose tissue secretome was greater than the sum of its cellular constituents. Finally, almost 50% of the adipose tissue secretome was composed of factors with a role in angiogenesis.
VAT has a higher secretory capacity than SAT, and this difference is an intrinsic feature of its cellular components. In view of the number of angiogenic factors in the adipose tissue secretome, we propose that VAT represents a more readily expandable tissue depot.
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