Standard isolation of primary adipose cells from mouse epididymal fat pads induces inflammatory mediators and down-regulates adipocyte genes

Bradykinin augments insulin-stimulated glucose transport in rat adipocytes via endothelial nitric oxide synthase-mediated inhibition of Jun NH2-terminal kinase

An increase in bradykinin has been suggested to contribute to the enhanced insulin sensitivity observed in the presence of ACE inhibitors. To investigate a potential direct, nonvascular effect on an insulin target tissue, the effect of bradykinin on glucose uptake and insulin signaling was studied in primary rat adipocytes. Whereas basal glucose uptake was not altered, bradykinin augmented insulin-stimulated glucose uptake twofold, which was blocked by HOE-140, a bradykinin B2 receptor antagonist. The bradykinin effect on glucose uptake was nitric oxide (NO) dependent, mimicked by NO donors and absent in adipocytes from endothelial NO synthase-/- mice. Investigation of insulin signaling revealed that bradykinin enhanced insulin receptor substrate-1 (IRS-1) Tyr phosphorylation, Akt/protein kinase B phosphorylation, and GLUT4 translocation. In contrast, insulin-stimulated extracellular signal-regulated kinase1/2 and Jun NH2-terminal kinase (JNK) activation were decreased in the presence of bradykinin, accompanied by decreased IRS-1 Ser307 phosphorylation. Furthermore, bradykinin did not enhance insulin action in the presence of the JNK inhibitor, SP-600125, or in adipocytes from JNK1-/- mice. These data indicate that bradykinin enhances insulin sensitivity in adipocytes via an NO-dependent pathway that acts by modulating the feedback inhibition of insulin signaling at the level of IRS-1.

11beta-hydroxysteroid dehydrogenase type 1 in adipose tissue and prospective changes in body weight and insulin resistance

OBJECTIVE

Increased mRNA and activity levels of 11beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1) in human adipose tissue (AT) are associated with obesity and insulin resistance. The aim of our study was to investigate whether 11betaHSD1 expression or activity in abdominal subcutaneous AT of non-diabetic subjects are associated with subsequent changes in body weight and insulin resistance [homeostasis model assessment of insulin resistance (HOMA-IR)].

RESEARCH METHODS AND PROCEDURES

Prospective analyses were performed in 20 subjects (two whites and 18 Pima Indians) who had baseline measurements of 11betaHSD1 mRNA and activity in whole AT (follow-up, 0.3 to 4.9 years) and in 47 Pima Indians who had baseline assessments of 11betaHSD1 mRNA in isolated adipocytes (follow-up, 0.8 to 5.3 years).

RESULTS

In whole AT, although 11betaHSD1 mRNA levels showed positive associations with changes in weight and HOMA-IR, 11betaHSD1 activity was associated with changes in HOMA-IR but not in body weight. 11betaHSD1 mRNA levels in isolated adipocytes were not associated with follow-up changes in any of the anthropometric or metabolic variables.

DISCUSSION

Our results indicate that increased expression of 11betaHSD1 in subcutaneous abdominal AT may contribute to risk of worsening obesity and insulin resistance. This prospective relationship does not seem to be mediated by increased 11betaHSD1 expression in adipocytes.

Albumin inhibits adipogenesis and stimulates cytokine release from human adipocytes

Bovine serum albumin (BSA) is commonly used in adipocyte experiments as a binding protein for fat-soluble substances. Therefore, it is of interest to investigate whether BSA per se is influencing the functioning of human adipocytes in vitro. In the present study, we investigated the potential of BSA to affect the proliferation and differentiation capacity of human preadipocytes. BSA was found to inhibit adipose differentiation in a dose-dependent manner (being significant at concentrations of 2.5 μM), whereas proliferation was not affected. We further investigated the effect of BSA on the secretory function of adipocytes focusing on the release of selected cytokines. Preadipocytes and freshly isolated adipocytes incubated with BSA secreted significantly higher amounts of IL-6, -8, and -10, and TNF-α compared with cells incubated without BSA. The effects on cytokine secretion seemed to reside at the level of gene expression because BSA increased TNF-α and IL-6 mRNA in a dose-dependent manner. The results of the present study indicate that the presence of BSA in the culture medium has considerable effects on adipocyte function in vitro. These effects should be carefully considered for in vitro studies of adipose tissue.

Tumor necrosis factor-α and interleukin-1β increases CTRP1 expression in adipose tissue

CTRP1, a member of the CTRP superfamily, consists of an N-terminal signal peptide sequence followed by a variable region, a collagen repeat domain, and a C-terminal globular domain. CTRP1 is expressed at high levels in adipose tissues of LPS-stimulated Sprague-Dawley rats. The LPS-induced increase in CTRP1 gene expression was found to be mediated by TNF-alpha and IL-1beta. Also, a high level of expression of CTRP1 mRNA was observed in adipose tissues of Zucker diabetic fatty (fa/fa) rats, compared to Sprague-Dawley rats in the absence of LPS stimulation. These findings indicate that CTRP1 expression may be associated with a low-grade chronic inflammation status in adipose tissues.

Macrophages in human visceral adipose tissue: increased accumulation in obesity and a source of resistin and visfatin

AIMS/HYPOTHESIS

Increased visceral white adipose tissue (WAT) is linked to the risk of developing diabetes.

METHODS/RESULTS

We showed by fluorescence activated cell sorting analysis that human visceral WAT contains macrophages, the proportion of which increased with obesity. Selective isolation of mature adipocytes and macrophages from human visceral WAT by CD14 immunoselection revealed that macrophages expressed higher levels of chemokines (monocyte chemotactic protein 1, macrophage inflammatory protein 1alpha, IL-8) and the adipokines resistin and visfatin than did mature adipocytes, as assessed by real-time PCR analysis. Moreover, resistin and visfatin proteins were found to be released predominantly by visceral WAT macrophages. Macrophage-derived secretory products stimulated phosphorylation of protein kinase B in human hepatocytes.

CONCLUSIONS/INTERPRETATION

Resistin and visfatin might be considered to be proinflammatory markers. The increased macrophage population in obese human visceral WAT might be responsible for the enhanced production of chemokines as well as resistin and visfatin.

Inhibition of p38MAPK increases adipogenesis from embryonic to adult stages

Formation of new adipocytes from precursor cells contributes to adipose tissue expansion and obesity. In this study, we asked whether p38 mitogen-activated protein kinase (MAPK) pathway regulates normal and pathological adipogenesis. In both dietary and genetically (ob/ob) obese mice, adipose tissues displayed a marked decrease in p38MAPK activity compared with the same tissues from lean mice. Furthermore, p38MAPK activity was significantly higher in preadipocytes than in adipocytes, suggesting that p38MAPK activity decreases during adipocyte differentiation. In agreement with an inhibitory role of p38MAPK in this process, we found that in vitro inhibition of p38MAPK, with the specific inhibitor PD169316, increased the expression of adipocyte markers in several cellular models, from embryonic to adult stages. Importantly, the expression of adipocyte markers was higher in p38MAPKalpha knockout cells than in their wild-type counterparts. Phosphorylation of C/EBPbeta, which enhances its transcriptional activity, is increased after p38MAPK inhibition. Finally, either inhibition or disruption of p38MAPK increased peroxisome proliferator-activated receptor (PPAR)gamma expression and transactivation. Rescue of p38MAPK in knockout cells reduced PPARgamma activity to the low basal level of wild-type cells. We demonstrate here, by using multipronged approaches involving p38 chemical inhibitor and p38MAPKalpha knockout cells, that p38MAPK plays a negative role in adipogenesis via inhibition of C/EBPbeta and PPARgamma transcriptional activities.

SGK1 kinase upregulates GLUT1 activity and plasma membrane expression

Phosphatidylinositol 3-kinase (PI3 kinase) inhibition disrupts the ability of insulin to stimulate GLUT1 and GLUT4 translocation into the cell membrane and thus glucose transport. The effect on GLUT4 but not on GLUT1 is mediated by activation of protein kinase B (PKB). The serum- and glucocorticoid-inducible kinase SGK1, a further kinase downstream of PI3 kinase, regulates several transporters by enhancing their plasma membrane abundance. GLUT1 contains a consensus site ((95)Ser) for phosphorylation by SGK1. Thus, the present study investigated whether GLUT1 is regulated by the kinase. Tracer-flux studies in Xenopus oocytes and HEK-293 cells demonstrated that GLUT1 transport is enhanced by constitutively active (S422D)SGK1. The effect requires the kinase catalytical activity since the inactive mutant (K127N)SGK1 failed to modulate GLUT1. GLUT1 stimulation by (S422D)SGK1 is not due to de novo protein synthesis but rather to an increase of the transporter's abundance in the plasma membrane. Kinetic analysis revealed that SGK1 enhances maximal transport rate without altering GLUT1 substrate affinity. These observations suggest that SGK1 regulates GLUT1 and may contribute to or account for the PI3 kinase-dependent but PKB-independent stimulation of GLUT1 by insulin.

Microarray technology in the study of obesity and non-alcoholic fatty liver disease

The recent development of high-throughput gene expression technology permits simultaneous investigation of thousands of genes, providing a snapshot of the transcription state of diseased tissue. Microarray-based expression profiling is well suited to investigate the molecular basis of complex diseases such as obesity and chronic liver disease. With the help of microarray technology, functional genomics will surely advance our understanding of these diseases, and lead to more effective, targeted interventions that lack the toxicity of many conventional treatments. Despite their tremendous potential, microarray studies are subject to potential flaws in experimental design, experimental techniques, data analysis, and data interpretation. Besides the technical issues, the most important challenge is to develop integrative databases that combine gene expression data with the clinical data. Over the next few years, advances in technology and refinements in study design and data analysis will make clinically relevant translational research even more engaging and productive.

Metabolic and cellular plasticity in white adipose tissue I: effects of beta3-adrenergic receptor activation

Selective agonists of beta(3)-adrenergic receptors (Adrb3) exhibit potent anti-diabetes properties in rodent models when given chronically, yet the mechanisms involved are poorly understood. A salient feature of chronic Adrb3 activation is pronounced remodeling of white adipose tissue (WAT), which includes mitochondrial biogenesis and elevation of metabolic rate. To gain insights into potential mechanisms underlying WAT remodeling, the time course of remodeling induced by the Adrb3 agonist CL-316,243 (CL) was analyzed using histological, physiological, and global gene profiling approaches. The results indicate that continuous CL treatment induced a transient proinflammatory response that was followed by cellular proliferation among stromal cells and multilocular adipocytes. CL treatment strongly fragmented the central lipid storage droplet of mature adipocytes and induced mitochondrial biogenesis within these cells. Mitochondrial biogenesis was correlated with the upregulation of genes involved in fatty acid oxidation and mitochondrial electron transport activity. The elevated catabolic activity of WAT was temporally correlated with upregulation of peroxisome proliferator-activated receptor-alpha and its target genes, suggesting involvement of this transcription factor in coordinating the gene program that elevates WAT catabolic activity.

Microarray profiling of isolated abdominal subcutaneous adipocytes from obese vs non-obese Pima Indians: increased expression of inflammation-related genes

AIMS/HYPOTHESIS

Obesity increases the risk of developing major diseases such as diabetes and cardiovascular disease. Adipose tissue, particularly adipocytes, may play a major role in the development of obesity and its comorbidities. The aim of this study was to characterise, in adipocytes from obese people, the most differentially expressed genes that might be relevant to the development of obesity.

METHODS

We carried out microarray gene profiling of isolated abdominal subcutaneous adipocytes from 20 non-obese (BMI 25+/-3 kg/m2) and 19 obese (BMI 55+/-8 kg/m2) non-diabetic Pima Indians using Affymetrix HG-U95 GeneChip arrays. After data analyses, we measured the transcript levels of selected genes based on their biological functions and chromosomal positions using quantitative real-time PCR.

RESULTS

The most differentially expressed genes in adipocytes of obese individuals consisted of 433 upregulated and 244 downregulated genes. Of these, 410 genes could be classified into 20 functional Gene Ontology categories. The analyses indicated that the inflammation/immune response category was over-represented, and that most inflammation-related genes were upregulated in adipocytes of obese subjects. Quantitative real-time PCR confirmed the transcriptional upregulation of representative inflammation-related genes (CCL2 and CCL3) encoding the chemokines monocyte chemoattractant protein-1 and macrophage inflammatory protein 1alpha. The differential expression levels of eight positional candidate genes, including inflammation-related THY1 and C1QTNF5, were also confirmed. These genes are located on chromosome 11q22-q24, a region with linkage to obesity in the Pima Indians.

CONCLUSIONS/INTERPRETATION

This study provides evidence supporting the active role of mature adipocytes in obesity-related inflammation. It also provides potential candidate genes for susceptibility to obesity.

Glucose transport is equally sensitive to insulin stimulation, but basal and insulin-stimulated transport is higher, in human omental compared with subcutaneous adipocytes

Excess visceral fat has been found to correlate more closely with morbidity than subcutaneous fat. We found that isolated adipocytes from omental fat of nondiabetic women expressed significantly more of the insulin-regulated glucose transporter glucose transporter 4 protein and exhibited a higher basal and insulin-stimulated rate of glucose transport, at all concentrations of insulin, than subcutaneous adipocytes from the same individuals. In contrast, dose-response relationships for insulin stimulation of glucose transport demonstrated identical sensitivity to insulin in adipocytes from the 2 locations. The results demonstrate that there is no relative insulin resistance to stimulate glucose uptake in visceral compared with subcutaneous fat cells.

Autocrine/paracrine role of inflammation-mediated calcitonin gene-related peptide and adrenomedullin expression in human adipose tissue

Human adipose tissue is a contributor to inflammation- and sepsis-induced elevation of serum procalcitonin (ProCT). Several calcitonin (CT) peptides, including ProCT, CT gene-related peptide (CGRP), and adrenomedullin (ADM) are suspected mediators in human inflammatory diseases. Therefore, we aimed to explore the expression, interactions, and potential roles of adipocyte-derived CT peptide production. Expression of CT peptide-specific transcripts was analyzed by RT-PCR and quantitative real-time PCR in human adipose tissue biopsies and three different inflammation-challenged human adipocyte models. ProCT, CGRP, and ADM secretions were assessed by immunological methods. Adipocyte transcriptional activity, glycerol release, and insulin-mediated glucose transport were studied after exogenous CGRP and ADM exposure. With the exception of amylin, CT peptides were expressed in adipose tissue biopsies from septic patients, inflammation-activated mature explanted adipocytes, and macrophage-activated preadipocyte-derived adipocytes. ProCT and CGRP productions were significantly augmented in IL-1beta and lipopolysaccharide-challenged mesenchymal stem cell-derived adipocytes but not in undifferentiated mesenchymal stem cells. In contrast, ADM expression occurred before and after adipogenic differentiation. Interferon-gamma coadministration inhibited IL-1beta-mediated ProCT and CGRP secretion by 78 and 34%, respectively but augmented IL-1beta-mediated ADM secretion by 50%. Exogenous CGRP and ADM administration induced CT, CGRP I, and CGRP II mRNAs and dose-dependently (10(-10) and 10(-6) m) enhanced glycerol release. In contrast, no CGRP- and ADM-mediated effects were noted on ADM, TNFalpha, and IL-1beta mRNA abundances. In summary, CGRP and ADM are two differentially regulated novel adipose tissue secretion factors exerting autocrine/paracrine roles. Their lipolytic effect (glycerol release) suggests a metabolic role in adipocytes during inflammation.

Fat cells: afferent and efferent messages define new approaches to treat obesity

For a long time neural and endocrine messages were studied for their impact on adipocyte metabolism and control of storage/release of fatty acids. In fact, bidirectional communication exists between adipocytes and other tissues. Several molecules secreted from adipocytes are involved in fat cell signaling to other tissues. Adipocyte products could initiate antagonistic effects on target tissues. Fat cells produce peptides that can elicit insulin resistance, such as tumor necrosis factor-alpha and resistin, as well as hormones that can improve insulin resistance, such as leptin and adiponectin. Secretion of complement proteins, proinflammatory cytokines, procoagulant, and acute phase reactant proteins have also been observed in adipocytes. There is much to learn about how these signals function. It is unlikely that all the adipocyte's endocrine and paracrine signals have been identified. Putative pharmacological strategies aiming at modulation of afferent and efferent fat cell messages are reviewed and discussed.

Involvement of multiple signaling pathways in the post-bariatric induction of IL-6 and IL-8 mRNA and release in human visceral adipose tissue

The present studies were designed to determine the site of and the mechanism for the rapid increase in IL-6 and IL-8 mRNA observed in human visceral adipose tissue after removal during laparoscopic bariatric surgery. Upregulation of IL-6 and IL-8 mRNA as well as their release were seen within 3h whether one intact piece of tissue or minced pieces of adipose tissue were incubated in vitro. Most of the IL-6 and IL-8 mRNA content of visceral adipose tissue after 3h of incubation was in the non-fat cells. Actinomcyin D markedly reduced the upregulation of IL-6 and IL-8 mRNA. Incubation of adipose tissue explants with a soluble TNFalpha receptor (etanercept) plus a blocking antibody against IL-lbeta reduced by 55% the increase in IL-6 mRNA and by 42% that of IL-8 mRNA seen between 1 and 5h of incubation. The upregulation of IL-8 and IL-6 mRNA accumulation as well as their release over a 2 or 4h incubation was reduced by around 50% in the presence of an inhibitor of the p38 MAPK or an inhibitor of the NFkappaB pathway and by 85% in the presence of both inhibitors. The data suggest that the relative trauma and/or hypoxia that occurs when adipose tissue is removed results in the release of TNFalpha and IL-1beta. These cytokines, and probably other factors as well, enhance IL-6 and IL-8 mRNA accumulation in human adipose tissue explants through mechanisms involving the p38 MAPK and NFkappaB pathways.

Endofacial competitive inhibition of glucose transporter-4 intrinsic activity by the mitogen-activated protein kinase inhibitor SB203580

The translocation of glucose transporter-4 (GLUT4) to the cell surface is a complex multistep process that involves movement of GLUT4 vesicles from a reservoir compartment, and docking and fusion of the vesicles with the plasma membrane. It has recently been proposed that a p38 mitogen-activated protein kinase (MAPK)-dependent step may lead to intrinsic activation of the transporters exposed at the cell surface. In contrast to data obtained in muscle and adipocyte cell lines, we found that no insulin activation of p38 MAPK occurred in rat adipose cells. However, the p38 MAPK inhibitor SB203580 consistently inhibited transport activity after preincubation with the adipose cells. These apparently contradictory findings led us to hypothesize that the inhibitor may have a direct effect on the transport catalytic activity of GLUT4 that was independent of inhibition of the kinase. Kinetic analysis of 3-O-methyl-d-glucose transport activity revealed that SB203580 was a noncompetitive inhibitor of zero-trans (substrate outside but not inside) transport, but was a competitive inhibitor of equilibrium-exchange (substrate inside and outside) transport. This pattern of inhibition of GLUT4 was also observed with cytochalasin B. The pattern of inhibition is consistent with interaction at the endofacial surface, but not the exofacial surface of the transporter. Occupation of the endofacial substrate site reduces maximum velocity under zero-trans conditions, because return of the substrate site to the outside is blocked, and no substrate is present inside to displace the inhibitor. Under equilibrium-exchange conditions, internal substrate competitively displaces the inhibitor, and the transport K(m) is increased.

Change in β1-adrenergic receptor protein concentration in adipose tissue correlates with diet-induced weight loss

The aim of the present study was to examine gene expression and protein concentrations of β1- and β2-adrenergic receptors in subcutaneous adipose tissue in obese subjects in response to weight loss. Eighteen obese subjects were studied during diet-induced weight loss. β-Adrenergic receptor mRNA levels were quantified by reverse transcription-PCR–HPLC. β-Adrenergic receptor protein concentrations were measured by Western blotting using fluorescence laser scanning for detection. Subjects lost 12.8±0.8 kg (mean±S.E.M.) during diet treatment. There was a 34% decrease in the β1-adrenergic receptor mRNA level (0.92±0.09 compared with 0.61±0.06 amol/μg of DNA; P<0.002). β2-Adrenergic receptor mRNA did not decrease significantly. β2-Adrenergic receptor protein concentration decreased 37% (25.5±7.1 compared with 16.0±5.6 arbitrary units/ng of DNA; P=0.008), whereas β1-adrenergic receptor protein concentration did not decrease significantly. The degree of weight loss was correlated with the concentration of β1-adrenergic receptor protein (r=0.65, P<0.003) and changes in receptor protein concentration (r=0.50, P=0.035) during the very-low-calorie diet. In conclusion, the present study demonstrates a relationship between β1-adrenergic receptor protein concentration in adipose tissue and the degree of weight loss. This relationship is not directly related to energy expenditure and deserves further investigation.

Insulin resistance in human adipocytes occurs downstream of IRS1 after surgical cell isolation but at the level of phosphorylation of IRS1 in type 2 diabetes

Insulin resistance is a cardinal feature of type 2 diabetes and also a consequence of trauma such as surgery. Directly after surgery and cell isolation, adipocytes were insulin resistant, but this was reversed after overnight incubation in 10% CO(2) at 37 degrees C. Tyrosine phosphorylation of the insulin receptor and insulin receptor substrate (IRS)1 was insulin sensitive, but protein kinase B (PKB) and downstream metabolic effects exhibited insulin resistance that was reversed by overnight incubation. MAP-kinases ERK1/2 and p38 were strongly phosphorylated after surgery, but was dephosphorylated during reversal of insulin resistance. Phosphorylation of MAP-kinase was not caused by collagenase treatment during cell isolation and was present also in tissue pieces that were not subjected to cell isolation procedures. The insulin resistance directly after surgery and cell isolation was different from insulin resistance of type 2 diabetes; adipocytes from patients with type 2 diabetes remained insulin resistant after overnight incubation. IRS1, PKB, and downstream metabolic effects, but not insulin-stimulated tyrosine phosphorylation of insulin receptor, exhibited insulin resistance. These findings suggest a new approach in the study of surgery-induced insulin resistance and indicate that human adipocytes should recover after surgical procedures for analysis of insulin signalling. Moreover, we pinpoint the signalling dysregulation in type 2 diabetes to be the insulin-stimulated phosphorylation of IRS1 in human adipocytes.

Metabolic Adaptations in the Absence of Perilipin

Targeted disruption of the lipid droplet protein, perilipin, in mice leads to constitutional lipolysis associated with marked reduction in white adipose tissue as a result of unbridled lipolysis. To investigate the metabolic adaptations in response to the constitutive lipolysis, we studied perilipin-null (plin(-/-)) mice in terms of their fatty acid oxidation and glycerol and glucose metabolism homeostasis by using dynamic biochemical testing and clamp and tracer infusion methods. plin(-/-) mice showed increased beta-oxidation in muscle, liver, and adipose tissue resulting from a coordinated regulation of the enzymes and proteins involved in beta-oxidation. The increased beta-oxidation helped remove the extra free fatty acids created by the constitutive lipolysis. An increase in the expression of the transcripts for uncoupling proteins-2 and -3 also accompanied this increase in fatty acid oxidation. Adult plin(-/-) mice had normal plasma glucose but a reduced basal hepatic glucose production (46% that of plin(+/+)). Insulin infusion during low dose hyperinsulinemic-euglycemic clamp further lowered the glucose production in plin(-/-) mice, but plin(-/-) mice also showed a 36% decrease (p < 0.007) in glucose disposal rate during the low dose insulin clamp, indicating peripheral insulin resistance. However, compared with plin(+/+) mice, 14-week-old plin(-/-) mice showed no significant difference in glucose disposal rate during the high dose hyperinsulinemic clamp, whereas 42-week-old plin(-/-) mice displayed significant insulin resistance on high dose hyperinsulinemic clamp. Despite increasing insulin resistance with age, plin(-/-) mice at different ages maintained a normal glucose response during an intraperitoneal glucose tolerance curve, being compensated by the increased beta-oxidation and reduced hepatic glucose production. These experiments uncover the metabolic adaptations associated with the constitutional lipolysis in plin(-/-) mice that allowed the mice to continue to exhibit normal glucose tolerance in the presence of peripheral insulin resistance.

Effects of different hypocaloric diets on protein secretion from adipose tissue of obese women

Little is known about common factors (e.g., macronutrients and energy supply) regulating the protein secretory function of adipose tissue. We therefore compared the effects of randomly assigned 10-week hypoenergetic (-600 kcal/day) diets with moderate-fat/moderate-carbohydrate or low-fat/high-carbohydrate content on circulating levels and production of proteins (using radioimmunoassays and enzyme-linked immunosorbent assays) from subcutaneous adipose tissue in 40 obese but otherwise healthy women. Similar results were obtained by the two diets. Body weight decreased by approximately 7.5%. The secretion rate of leptin decreased by approximately 40%, as did that of tumor necrosis factor-alpha (TNF-alpha), and interleukin (IL)-6 and -8 decreased by 25-30%, whereas the secretion of plasminogen activator inhibitor 1 (PAI-1) and adiponectin did not show any changes. Regarding mRNA expression (by real-time PCR), only that of leptin and IL-6 decreased significantly. Circulating levels of leptin and PAI-1 decreased by 30 and 40%, respectively, but there were only minor changes in circulating TNF-alpha, IL-6, or adiponectin. In conclusion, moderate caloric restriction but not macronutrient composition influences the production and secretion of adipose tissue-derived proteins during weight reduction, leptin being the most sensitive and adiponectin and PAI-1 the least sensitive.

Disruption of cortical actin in skeletal muscle demonstrates an essential role of the cytoskeleton in glucose transporter 4 translocation in insulin-sensitive tissues

Cell culture work suggests that signaling to polymerize cortical filamentous actin (F-actin) represents a required pathway for the optimal redistribution of the insulin-responsive glucose transporter, GLUT4, to the plasma membrane. Recent in vitro study further suggests that the actin-regulatory neural Wiskott-Aldrich syndrome protein (N-WASP) mediates the effect of insulin on the actin filament network. Here we tested whether similar cytoskeletal mechanics are essential for insulin-regulated glucose transport in isolated rat epitrochlearis skeletal muscle. Microscopic analysis revealed that cortical F-actin is markedly diminished in muscle exposed to latrunculin B. Depolymerization of cortical F-actin with latrunculin B caused a time- and concentration-dependent decline in 2-deoxyglucose transport. The loss of cortical F-actin and glucose transport was paralleled by a decline in insulin-stimulated GLUT4 translocation, as assessed by photolabeling of cell surface GLUT4 with Bio-LC-ATB-BMPA. Although latrunculin B impaired insulin-stimulated GLUT4 translocation and glucose transport, activation of phosphatidylinositol 3-kinase and Akt by insulin was not rendered ineffective. In contrast, the ability of insulin to elicit the cortical F-actin localization of N-WASP was abrogated. These data provide the first evidence that actin cytoskeletal mechanics are an essential feature of the glucose transport process in intact skeletal muscle. Furthermore, these findings support a distal actin-based role for N-WASP in insulin action in vivo.

Regulation of insulin sensitivity by adipose tissue-derived hormones and inflammatory cytokines

PURPOSE OF REVIEW

The aim of this review is to assess the role of adipose tissue-derived hormones and inflammatory cytokines in the pathogenesis of obesity-linked type II diabetes, with a special focus on articles published between December 2002 and December 2003.

RECENT FINDINGS

Insulin resistance is widely recognized as a fundamental defect seen in obesity and type II diabetes. Although the molecular mechanisms triggering the development of insulin resistance remain elusive, recent studies have suggested that adipose tissue and adipose tissue-derived hormones and inflammatory cytokines play essential roles in the overall insulin sensitivity in vivo. Dysfunctions of adipose tissue can lead to systemic insulin resistance.

SUMMARY

Understanding the regulation of the metabolic and secretory functions of adipose tissue, as well as its subsequent impact on overall insulin sensitivity, is becoming increasingly important given the therapeutic potential of targeting the root causes of insulin resistance in the treatment of type 2 diabetes and its associated complications, such as cardiovascular and cerebrovascular diseases.

Multi-tissue gene-expression analysis in a mouse model of thyroid hormone resistance

BACKGROUND

Resistance to thyroid hormone (RTH) is caused by mutations of the thyroid hormone receptor beta (TRbeta) gene. To understand the transcriptional program underlying TRbeta mutant-induced phenotypic expression of RTH, cDNA microarrays were used to profile the expression of 11,500 genes in a mouse model of human RTH.

RESULTS

We analyzed transcript levels in cerebellum, heart and white adipose tissue from a knock-in mouse (TRbetaPV/PV mouse) that harbors a human mutation (referred to as PV) and faithfully reproduces human RTH. Because TRbetaPV/PV mice have elevated thyroid hormone (T3), to define T3-responsive genes in the context of normal TRbeta, we also analyzed T3 effects in hyperthyroid wild-type gender-matched littermates. Microarray analysis revealed 163 genes responsive to T3 treatment and 187 genes differentially expressed between TRbetaPV/PV mice and wild-type littermates. Both the magnitude and gene make-up of the transcriptional response varied widely across tissues and conditions. We identified genes modulated in T3-dependent PV-independent, T3- and PV-dependent, and T3-independent PV-dependent pathways that illuminated the biological consequences of PV action in vivo. Most T3-responsive genes that were dysregulated in the heart and white adipose tissue of TRbetaPV/PV mice were repressed in T3-treated wild-type mice and upregulated in TRbetaPV/PV mice, suggesting the inappropriate activation of T3-suppressed genes in RTH.

CONCLUSIONS

Comprehensive multi-tissue gene-expression analysis uncovered complex multiple signaling pathways that mediate the molecular actions of TRbeta mutants in vivo. In particular, the T3-independent mutant-dependent genomic response unveiled the contribution of a novel 'change-of-function' of TRbeta mutants to the pathogenesis of RTH. Thus, the molecular actions of TRbeta mutants are more complex than previously envisioned.