Increased uncoupling protein-2 and -3 mRNA expression during fasting in obese and lean humans.

Uncoupling protein-2 and -3 (UCP2 and UCP3) are mitochondrial proteins that show high sequence homology with the brown adipocyte-specific UCP1. UCP1 induces heat production by uncoupling respiration from ATP synthesis. UCP2 is widely expressed in human tissues, whereas UCP3 expression seems restricted to skeletal muscle, an important site of thermogenesis in humans. We have investigated the regulation of UCP2 and UCP3 gene expression in skeletal muscle and adipose tissue from lean and obese humans. UCP2 and -3 mRNA levels were not correlated with body mass index (BMI) in skeletal muscle, but a positive correlation (r = 0.55, P < 0.01, n = 22) was found between UCP2 mRNA level in adipose tissue and BMI. The effect of fasting was investigated in eight lean and six obese subjects maintained on a hypocaloric diet (1,045 kJ/d) for 5 d. Calorie restriction induced a similar 2-2.5-fold increase in UCP2 and -3 mRNA levels in lean and obese subjects. To study the effect of insulin on UCP gene expression, six lean and five obese subjects underwent a 3-h euglycemic hyperinsulinemic clamp. Insulin infusion did not modify UCP2 and -3 mRNA levels. In conclusion, the similar induction of gene expression observed during fasting in lean and obese subjects shows that there is no major alteration of UCP2 and -3 gene regulation in adipose tissue and skeletal muscle of obese subjects. The increase in UCP2 and -3 mRNA levels suggests a role for these proteins in the metabolic adaptation to fasting.     

Acute insulin administration does not affect plasma leptin levels in lean or obese subjects

Abstract. Whether leptin levels are related to insulin sensitivity or subject to acute regulation by insulin is not known. In 12 obese [body mass index (BMI) = 34.0 ±1.5 kg m^-2] and 12 lean (BMI = 22.2 ±0.6 kg m^-2) non-diabetic subjects, plasma leptin concentrations were measured in the fasting state and during 2 hours of euglycaemic hyperinsulinaemia (˜600 pmol L^-2). Fasting plasma leptin was significantly higher in obese (26.6 ±3.2) than in lean subjects (6.4 ±1.2 ng mL^-1, P = 0.0001), and in women (21.1 ±3.3) than in men (7.3 = 2.3 ng mL^-1, P = 0.01). In univariate analysis, fasting plasma leptin was strongly related to all anthropometric measures (body weight, fat mass, percent fat mass, waist and hip circumferences). In multiple regression, per cent adiposity, hip circumference and duration of obesity explained 90% of the variability in fasting leptin concentrations. Fasting and stimulated (OGTT) insulin levels, insulin sensitivity (22.6 ±1.9 vs 36.7 ±2.0 μmol min^-1 kg^-1 in lean and obese subjects, respectively, P < 0.0001), glucose area, and serum triglycerides were positively related to fasting plasma leptin concentrations; none of these associations, however, was statistically significant after adjusting for BMI. During the clamp, plasma leptin concentrations remained constant in both lean and obese subjects. We conclude that neither insulin levels nor sensitivity relate to leptin levels independently of fat mass, and that leptin is not subject to acute (2 hours) regulation by insulin in lean or obese humans.     

Peroxisome proliferator-activated receptor gene expression in human tissues. Effects of obesity,weight loss,and regulation by insulin and glucocorticoids.

The peroxisome proliferator activated receptor (PPAR gamma) plays a key role in adipogenesis and adipocyte gene expression and is the receptor for the thiazolidinedione class of insulin-sensitizing drugs. The tissue expression and potential for regulation of human PPAR gamma gene expression in vivo are unknown. We have cloned a partial human PPAR gamma cDNA, and established an RNase protection assay that permits simultaneous measurements of both PPAR gamma1 and PPAR gamma2 splice variants. Both gamma1 and gamma2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma1 was detected at lower levels in liver and heart, whereas both gamma1 and gamma2 mRNAs were expressed at low levels in skeletal muscle. To examine the hypothesis that obesity is associated with abnormal adipose tissue expression of PPAR gamma, we quantitated PPARgamma mRNA splice variants in subcutaneous adipose tissue of 14 lean and 24 obese subjects. Adipose expression of PPARgamma 2 mRNA was increased in human obesity (14.25 attomol PPAR gamma2/18S in obese females vs 9.9 in lean, P = 0.003). This increase was observed in both male and females. In contrast, no differences were observed in PPAR gamma1/18S mRNA expression. There was a strong positive correlation (r = 0.70, P < 0.001) between the ratio of PPAR gamma2/gamma1 and the body mass index of these patients. We also observed sexually dimorphic expression with increased expression of both PPAR gamma1 and PPAR gamma2 mRNAs in the subcutaneous adipose tissue of women compared with men. To determine the effect of weight loss on PPAR gamma mRNA expression, seven additional obese subjects were fed a low calorie diet (800 Kcal) until 10% weight loss was achieved. Mean expression of adipose PPAR gamma2 mRNA fell 25% (P = 0.0250 after a 10% reduction in body weight), but then increased to pretreatment levels after 4 wk of weight maintenance. Nutritional regulation of PPAR gamma1 was not seen. In vitro experiments revealed a synergistic effect of insulin and corticosteroids to induce PPAR gamma expression in isolated human adipocytes in culture. We conclude that: (a) human PPAR gamma mRNA expression is most abundant in adipose tissue, but lower level expression of both splice variants is seen in skeletal muscle; to an extent that is unlikely to be due to adipose contamination. (b) RNA derived from adipose tissue of obese humans has increased expression of PPAR gamma 2 mRNA, as well as an increased ratio of PPAR gamma2/gamma1 splice variants that is proportional to the BMI; (c) a low calorie diet specifically down-regulates the expression of PPAR gamma2 mRNA in adipose tissue of obese humans; (d) insulin and corticosteroids synergistically induce PPAR gamma mRNA after in vitro exposure to isolated human adipocytes; and (e) the in vivo modulation of PPAR gamma2 mRNA levels is an additional level of regulation for the control of adipocyte development and function, and could provide a molecular mechanism for alterations in adipocyte number and function in obesity.     

Marked impairment of protein tyrosine phosphatase 1B activity in adipose tissue of obese subjects with and without type 2 diabetes mellitus.

Protein tyrosine phosphatases (PTPs) are required for the dephosphorylation of the insulin receptor (IR) and its initial cellular substrates, and it has recently been reported that PTP-1B may play a role in the pathogenesis of insulin resistance in obesity and type 2 diabetes mellitus (DM). We therefore determined the amount and activity of PTP-1B in abdominal adipose tissue obtained from lean nondiabetic subjects (lean control (LC)), obese nondiabetic subjects (obese control (OC)), and subjects with both type 2 DM (DM2) and obesity (obese diabetic (OD)). PTP-1B protein levels were 3-fold higher in OC than in LC (1444 +/- 195 U vs 500 +/- 146 U (mean +/- SEM), P < .015), while OD exhibited a 5.5-fold increase (2728 +/- 286 U, P < .01). PTP activity was assayed by measuring the dephosphorylating activity toward a phosphorus 32-labeled synthetic dodecapeptide. In contrast to the increased PTP-1B protein levels, PTP-1B activity per unit of PTP-1B protein was markedly reduced, by 71% and 88% in OC and OD, respectively. Non-PTP-1B tyrosine phosphatase activity was comparable in all three groups. Similar results were obtained when PTP-1B activity was measured against intact human IR. A significant correlation was found between body mass index (BMI) and PTP-1B level (r = 0.672, P < .02), whereas BMI and PTP-1B activity per unit of PTP-1B showed a strong inverse correlation (r = -0.801, P < .002). These data suggest that the insulin resistance of obesity and DM2 is characterized by the increased expression of a catalytically impaired PTP-1B in adipose tissue and that impaired PTP-1B activity may be pathogenic for insulin resistance in these conditions.     

Increased plasma-soluble tumor necrosis factor-alpha receptor 2 level in lean nondiabetic offspring of type 2 diabetic subjects

OBJECTIVE: Tumor necrosis factor-alpha (TNF-alpha) is one of the proposed mediators of insulin resistance, upregulated in human obesity. Insulin resistance, however, might precede the development of obesity, especially in subjects with a family history of type 2 diabetes. Therefore, the aim of the present study was to assess plasma levels of TNF-alpha and soluble forms of its receptors (soluble TNF-alpha receptors 1 [sTNFR1] and 2 [sTNFR2]) and to evaluate the relationship of the TNF-alpha system with insulin resistance in lean, nondiabetic offspring of type 2 diabetic patients. RESEARCH DESIGN AND METHODS: We compared 20 lean offspring (BMI <25 kg/m(2), 8 men and 12 women) of type 2 diabetic patients with 20 lean subjects with no family history of diabetes, matched for age, sex, and BMI (control group). Anthropometry and blood biochemical parameters were measured, and insulin sensitivity was evaluated with the euglycemic-hyperinsulinemic clamp technique. RESULTS: Both men and women in the offspring group were markedly more insulin-resistant and had higher plasma levels of sTNFR2 (all P < 0.05). TNF-alpha, sTNFR1, and other examined parameters did not differ between the studied groups. Both TNF-alpha receptors were related to waist-to-hip ratio (WHR), fat-free mass (FFM), plasma total cholesterol, HDL cholesterol, LDL cholesterol, and nonesterified fatty acids (NEFAs). sTNFR2, but not sTNFR1, was also associated with insulin sensitivity (r = -0.49, P = 0.001). This relationship remained significant after adjustment for WHR, FFM, plasma insulin, and NEFA. CONCLUSIONS: TNF-alpha system might be involved in modulating insulin action before the onset of obesity in subjects at high risk for type 2 diabetes.     

Alterations in skeletal muscle protein-tyrosine phosphatase activity and expression in insulin-resistant human obesity and diabetes.

Obese human subjects have increased protein-tyrosine phosphatase (PTPase) activity in adipose tissue that can dephosphorylate and inactivate the insulin receptor kinase. To extend these findings to skeletal muscle, we measured PTPase activity in the skeletal muscle particulate fraction and cytosol from a series of lean controls, insulin-resistant obese (body mass index > 30) nondiabetic subjects, and obese individuals with non-insulin-dependent diabetes. PTPase activities in subcellular fractions from the nondiabetic obese subjects were increased to 140-170% of the level in lean controls (P < 0.05). In contrast, PTPase activity in both fractions from the obese subjects with non-insulin-dependent diabetes was significantly decreased to 39% of the level in controls (P < 0.05). By immunoblot analysis, leukocyte antigen related (LAR) and protein-tyrosine phosphatase 1B had the greatest increase (threefold) in the particulate fraction from obese, nondiabetic subjects, and immunodepletion of this fraction using an affinity-purified antibody directed at the cytoplasmic domain of leukocyte antigen related normalized the PTPase activity when compared to the activity from control subjects. These findings provide further support for negative regulation of insulin action by specific PTPases in the pathogenesis of insulin resistance in human obesity, while other regulatory mechanisms may be operative in the diabetic state.     

Obesity-associated improvements in metabolic profile through expansion of adipose tissue

Excess caloric intake can lead to insulin resistance. The underlying reasons are complex but likely related to ectopic lipid deposition in nonadipose tissue. We hypothesized that the inability to appropriately expand subcutaneous adipose tissue may be an underlying reason for insulin resistance and beta cell failure. Mice lacking leptin while overexpressing adiponectin showed normalized glucose and insulin levels and dramatically improved glucose as well as positively affected serum triglyceride levels. Therefore, modestly increasing the levels of circulating full-length adiponectin completely rescued the diabetic phenotype in ob/ob mice. They displayed increased expression of PPARgamma target genes and a reduction in macrophage infiltration in adipose tissue and systemic inflammation. As a result, the transgenic mice were morbidly obese, with significantly higher levels of adipose tissue than their ob/ob littermates, leading to an interesting dichotomy of increased fat mass associated with improvement in insulin sensitivity. Based on these data, we propose that adiponectin acts as a peripheral "starvation" signal promoting the storage of triglycerides preferentially in adipose tissue. As a consequence, reduced triglyceride levels in the liver and muscle convey improved systemic insulin sensitivity. These mice therefore represent what we believe is a novel model of morbid obesity associated with an improved metabolic profile.     

Increased abundance of the receptor-type protein-tyrosine phosphatase LAR accounts for the elevated insulin receptor dephosphorylating activity in adipose tissue of obese human subjects.

Protein-tyrosine phosphatases (PTPases) have an essential role in the regulation of the steady-state phosphorylation of the insulin receptor and other proteins in the insulin signalling pathway. To examine whether increased PTPase activity is associated with adipose tissue insulin resistance in human obesity we measured PTPase enzyme activity towards the insulin receptor in homogenates of subcutaneous adipose tissue from a series of six lean and six nondiabetic, obese (body mass index > 30) subjects. The obese subjects had a mean 1.74-fold increase in PTPase activity (P < 0.0001) with a striking positive correlation by linear regression analysis between PTPase activity and body mass index among all of the samples (R = 0.918; P < 0.0001). The abundance of three candidate insulin receptor PTPases in adipose tissue was also estimated by immunoblot analysis. The most prominent increase was a 2.03-fold rise in the transmembrane PTPase LAR (P < 0.001). Of the three PTPase examined, only immunodepletion of LAR protein from the homogenates with neutralizing antibodies resulted in normalization of the PTPase activity towards the insulin receptor, demonstrating that the increase in LAR was responsible for the enhanced PTPase activity in the adipose tissue from obese subjects. These studies suggest that increased PTPase activity towards the insulin receptor is a pathogenetic factor in the insulin resistance of adipose tissue in human obesity and provide evidence for a potential role of the LAR PTPase in the regulation of insulin signalling in disease states.     

The expression of TNF alpha by human muscle. Relationship to insulin resistance.

TNFalpha is orverexpressed in the adipose tissue of obese rodents and humans, and is associated with insulin resistance. To more closely link TNF expression with whole body insulin action, we examined the expression of TNF by muscle, which is responsible for the majority of glucose uptake in vivo. Using RT-PCR, TNF was detected in human heart, in skeletal muscle from humans and rats, and in cultured human myocytes. Using competitive RT-PCR, TNF was quantitated in the muscle biopsy specimens from 15 subjects whose insulin sensitivity had been characterized using the glucose clamp. technique. TNF expression in the insulin resistant subjects and the diabetic patients was fourfold higher than in the insulin sensitive subjects, and there was a significant inverse linear relationship between maximal glucose disposal rate and muscle TNF (r = -0.60, P < 0.02). In nine subjects, muscle cells from vastus lateralis muscle biopsies were placed into tissue culture for 4 wk, and induced to differentiate into myotubes. TNF was secreted into the medium from these cells, and cells from diabetic patients expressed threefold more TNF than cells from nondiabetic subjects. Thus, TNF is expressed in human muscle, and is expressed at a higher level in the muscle tissue and in the cultured muscle cells from insulin resistant and diabetic subjects. These data suggest another mechanism by which TNF may play an important role in human insulin resistance.     

Insulin resistance and hypersecretion in obesity. European Group for the Study of Insulin Resistance (EGIR).

Insulin resistance and insulin hypersecretion are established features of obesity. Their prevalence, however, has only been inferred from plasma insulin concentrations. We measured insulin sensitivity (as the whole-body insulin-mediated glucose uptake) and fasting posthepatic insulin delivery rate (IDR) with the use of the euglycemic insulin clamp technique in a large group of obese subjects in the database of the European Group for the Study of Insulin Resistance (1,146 nondiabetic, normotensive Caucasian men and women aged 18-85 yr, with a body mass index (BMI) ranging from 15 to 55 kg.m-2). Insulin resistance, defined as the lowest decile of insulin sensitivity in the lean subgroup (608 subjects with a mean BMI of 29 kg.m-2). Insulin sensitivity declined linearly with BMI at an age- and sex-adjusted rate of 1.2 micromol.min-1.kg FFM-1 per BMI unit (95% confidence intervals = 1.0-1.4). Insulin hypersecretion, defined as the upper decile of IDR, was significantly (P<0.0001) more prevalent (38%) than insulin resistance in the obese group. In the whole dataset, IDR rose as a function of both BMI and insulin resistance in a nonlinear fashion. Neither the waist circumference nor the waist-to-hip ratio, indices of body fat distribution, was related to insulin sensitivity after adjustment for age, gender, and BMI; both, however, were positively associated (P<0.001) with insulin hypersecretion, particularly in women. In nondiabetic, normotensive obese subjects, the prevalence of insulin resistance is relatively low, and is exceeded by the prevalence of insulin hypersecretion, particularly in women with central obesity. In the obese with preserved insulin sensitivity, risk for diabetes, cardiovascular risk, and response to treatment may be different than in insulin resistant obesity.     

Lower Total Adipocyte Number but No Evidence for Small Adipocyte Depletion in Patients With Type 2 Diabetes

OBJECTIVE

We hypothesized that, compared with obese subjects, patients with type 2 diabetes have a lower total adipocyte number with fewer small adipocytes.

RESEARCH DESIGN AND METHODS

Abdominal subcutaneous adipose tissue was obtained from lean and obese subjects with or without type 2 diabetes matched for BMI. Adipocyte size was measured by osmium fixation and sizing/counting in a Coulter counter. Adipocyte size and number subdistributions (small, medium, large, and very large) were determined.

RESULTS

Compared with obese subjects, type 2 diabetic patients had larger mean adipocyte size and 67% bigger very large adipocytes; the total adipocyte number was lower, but the fraction of small adipocytes was increased by 27%.

CONCLUSIONS

Total adipocyte cellularity is lower in type 2 diabetic subjects than in obese subjects. We found no evidence for depletion of small adipocytes in patients with type 2 diabetes. This suggests the presence of a defect in early maturation of adipocytes in patients with type 2 diabetes.There is a growing appreciation that adipose tissue dysfunction in type 2 diabetes is due in part to adipocyte hypertrophia. Mean adipocyte size is inversely correlated with insulin sensitivity (1). A greater number of small adipocytes is associated with insulin sensitivity (2,3). Danforth (4) hypothesized that adipocytes are hypertrophic due to defects in preadipocyte differentiation to adipocytes, leading to an inability to buffer excess dietary fat.In this study, we aimed to identify the differences in adipocyte size and number subdistributions in abdominal subcutaneous adipose tissue (SAT) of lean and obese subjects with or without type 2 diabetes. We hypothesized that, compared with obese subjects, type 2 diabetic patients have a lower total adipocyte number with fewer small adipocytes.     

Lipocalin-2 is an inflammatory marker closely associated with obesity, insulin resistance, and hyperglycemia in humans

BACKGROUND: Lipocalin-2, a 25-kDa secreted glycoprotein, is a useful biomarker for early detection of various renal injuries. Because lipocalin-2 is abundantly expressed in adipose tissue and liver, we investigated its relevance to obesity-related pathologies. METHODS: We used real-time PCR and in-house immunoassays to quantify the mRNA and serum concentrations of lipocalin-2 in C57BL/KsJ db/db obese mice and their age- and sex-matched lean littermates. We analyzed the association between serum lipocalin-2 concentrations and various metabolic and inflammatory variables in 229 persons (121 men and 108 women) recruited from a previous cross-sectional study, and we evaluated the effect of the insulin-sensitizing drug rosiglitazone on serum lipocalin-2 concentrations in 32 diabetic patients (21 men and 11 women). RESULTS: Compared with the lean littermates, lipocalin-2 mRNA expression in adipose tissue and liver and its circulating concentrations were significantly increased in db/db diabetic/obese mice (P <0.001). These changes were normalized after rosiglitazone treatment. In humans, circulating lipocalin-2 concentrations were positively correlated (P <0.005) with adiposity, hypertriglyceridemia, hyperglycemia, and the insulin resistance index, but negatively correlated (P = 0.002) with HDL cholesterol. There was also a strong positive association between lipocalin-2 concentrations and high sensitivity C-reactive protein (hs-CRP), independent of age, sex, and adiposity (P = 0.007). Furthermore, rosiglitazone-mediated decreases in lipocalin-2 concentrations correlated significantly with increases in insulin sensitivity (r = 0.527; P = 0.002) and decreases in hs-CRP concentrations (r = 0.509; P = 0.003). CONCLUSIONS: Lipocalin-2 is an inflammatory marker closely related to obesity and its metabolic complications. Measurement of serum lipocalin-2 might be useful for evaluating the outcomes of various clinical interventions for obesity-related metabolic and cardiovascular diseases.     

C-reactive protein in type 1 diabetes and its relationship to coronary artery calcification

OBJECTIVE: In 196 type 1 diabetic subjects and 195 nondiabetic subjects aged 30-55 years, we examined whether C-reactive protein (CRP) is elevated in diabetes and whether CRP is associated with coronary artery calcification (CAC). RESEARCH DESIGN AND METHODS: CRP was measured with a highly sensitive immunoassay. CAC was measured using electron beam computed tomography. RESULTS: CRP was elevated in diabetic women compared with nondiabetic women (median 1.62 vs. 0.85 mg/l, P < 0.001) independently of other factors, but was similar in diabetic and nondiabetic men (median 0.82 vs. 0.81 mg/l). Insulin dose per day was positively correlated with CRP in diabetic women (Spearman's rho = 0.36, P = 0.0003) but much less so in men (rho = 0.16, P = 0.09). Being in the top tertile for CRP was associated with CAC in diabetic and nondiabetic men even after adjustment for other risk factors (adjusted odds ratio [OR] = 4.6 and 4.3, respectively, P = 0.02 for both). In nondiabetic women, being in the top tertile for CRP was associated with CAC (OR 3.1, P = 0.04), but not independently of BMI (OR = 1 after adjustment). Among diabetic women the association was not significant even before adjustment for BMI (OR = 2.6, P = 0.07). CONCLUSIONS: Elevated CRP in diabetic women might reflect a particular sensitivity to insulin levels or might reflect insulin resistance. In general, CRP is an important marker of subclinical atherosclerosis, but the clinical significance of elevated CRP in diabetic women needs to be addressed in prospective studies, since CRP was not clearly associated with CAC in this group.     

Relationship between plasma insulin and blood pressure in South African black women in Johannesburg.

OBJECTIVE--To examine the relationship between fasting plasma insulin and blood pressure (BP) in 40 urbanized normotensive South African black women aged 24-60 yr, and to assess the effects of body mass index (BMI) and fasting plasma glucose on BP. RESEARCH DESIGN AND METHODS--The women comprised equal numbers of young nonobese nondiabetic subjects, middle-aged nonobese nondiabetic subjects, middle-aged obese nondiabetic subjects, and middle-aged obese newly diagnosed non-insulin-dependent diabetic subjects. Systolic and diastolic BPs were recorded (in duplicate) after 15 min of recumbency, and fasting plasma glucose and insulin levels were determined thereafter. The data were analyzed by simple and multivariate regression. RESULTS--There was a wide distribution of individual physical and biochemical features. With simple correlations, systolic BP correlated significantly with age, BMI, and fasting glucose but not with insulin. Diastolic BP correlated significantly with all four variables (r = 0.37, P less than 0.05). When adjusted for age, BMI, and glucose, however, the significant correlation between diastolic BP and insulin diminished (r = -0.04). CONCLUSIONS--As in other nonwhite communities, plasma insulin does not appear to play a major role in regulating the BP of South African black women.     

Glycerol production in subcutaneous adipose tissue in lean and obese humans.

To estimate the regional subcutaneous glycerol production rate in normal and obese humans, the venous arterialized plasma glycerol, interstitial glycerol in the subcutaneous adipose tissue together with adipose tissue blood flow (ATBF, ml/100 g.min) were measured in the postabsorptive state and for 2 h after ingestion of 100 g of oral glucose. Eight lean and eight obese men with normal oral glucose tolerance tests were investigated with the subcutaneous microdialysis technique and 133Xe clearance. In the postabsorptive state, the interstitial glycerol concentrations in lean and obese subjects were 170 +/- 21 vs. 282 +/- 28 microM (P less than 0.01) and 156 +/- 23 vs. 225 +/- 12 microM (P less than 0.05) in the abdominal and femoral subcutaneous adipose tissue, respectively. The corresponding arterial glycerol levels were 54 +/- 4 vs. 75 +/- 14 microM (NS). Abdominal ATBF was greater in lean subjects (3.2 +/- 0.6 vs. 1.6 +/- 0.3; P less than 0.05), whereas femoral ATBF was similar in both groups (2.7 +/- 0.4 vs. 2.4 +/- 0.7). Estimated mean local glycerol release (mumol/100 g.min) was similar in the lean and obese group (0.16 +/- 0.03 vs. 0.20 +/- 0.05 and 0.18 +/- 0.02 vs. 0.17 +/- 0.04) in the abdominal and femoral site, respectively. We conclude that glycerol production from the subcutaneous tissue is increased in obesity, irrespective of adipose tissue distribution. This enhancement is due to the increased adipose tissue mass.