Neutrophil-derived resistin release induced by Aggregatibacter actinomycetemcomitans

Resistin is an adipokine that induces insulin resistance in mice. In humans, resistin is not produced in adipocytes, but in various leukocytes instead, and it acts as a proinflammatory molecule. The present investigation demonstrated high levels of resistin in culture supernatants of neutrophils that are stimulated by a highly leukotoxic strain of Aggregatibacter actinomycetemcomitans. In contrast, the level of resistin was remarkably low when neutrophils were exposed to two other strains that produce minimal levels of leukotoxin and a further isogenic mutant strain incapable of producing leukotoxin. Pretreatment of neutrophils with a monoclonal antibody to CD18, β chain of lymphocyte function-associated molecule 1 (LFA-1), or an Src family tyrosine kinase inhibitor before incubation with the highly leukotoxic strain inhibited the release of resistin. These results show that A. actinomycetemcomitans-expressed leukotoxin induces extracellular release of human neutrophil-derived resistin by interacting with LFA-1 on the surface of neutrophils and, consequently, activating Src family tyrosine kinases.     

Adipokine expression profile in adipocytes of different mouse models of obesity.

Adipose tissue produces and secretes multiple adipokines. Most studies on adipokine production/expression have been performed on whole adipose tissue. In addition, data concerning an overall of adipokine expression are scarce and can be heterogeneous depending on the obesity model studied. Our first aim was to compare the expression of adipokines involved in the interplay between obesity and insulin resistance in isolated adipocytes from different mouse models of obesity displaying different levels of weight gain and insulin sensitivity. The second aim was to determine perigonadal/subcutaneous ratio of each adipokine. Only resistin expression was decreased in obese mice without modifications in glucose and insulin blood levels. In addition to decreased levels of resistin, obesity models associated with hyperglycemia and hyperinsulinemia presented an increased expression of leptin and tumor necrosis factor-alpha (TNFalpha). Obese and diabetic mice were the only animals to exhibit high expression of plasminogen activator inhibitor type-1 and interleukin-6. All adipokines except TNFalpha were more heavily expressed in perigonadal than in subcutaneous adipocytes. Interestingly, fat-enriched diet and overweight on their own did not modify the distribution of adipokines between the two fat depots. However, severe obesity modified the distribution of proinflammatory adipokines. In conclusion, the level and number of adipokines with altered expression increased with obesity and hyperinsulinemia in mice. The physiopathological impact of depot-specific differences of adipokine expression in adipocytes remains to be clarified.     

Resistin Gene Expression in Human Adipocytes Is Not Related to Insulin Resistance

Objectives: Obesity is an important risk factor for the development of insulin resistance and type 2 diabetes. Recently, a newly described circulating hormone resistin, which is expressed primarily in adipocytes, has been shown to antagonize insulin action in mice. Resistin, therefore, has been suggested to play a role in the pathogenesis of insulin resistance. Research Methods and Procedures: We studied the expression of the resistin gene in primary cultured human adipocytes and preadipocytes. We also examined resistin gene expression in subcutaneous abdominal adipocytes in women (n = 24) over a wide range of body weight and insulin sensitivity. Results: Whereas resistin gene expression was barely detectable in mature adipocytes, it was highly expressed in preadipocytes. Adipogenic differentiation of preadipocytes was associated with a time-dependent down-regulation of resistin gene expression. There was no relationship between body weight, insulin sensitivity, or other metabolic parameters and adipocyte resistin gene expression in the clinical study. Discussion: Together these findings do not support an important role of adipose-tissue resistin gene expression in human insulin resistance.     

Resistin messenger-RNA expression is increased by proinflammatory cytokines in vitro

Resistin is a recently discovered polypeptide that induces insulin resistance in rodents. While in rodents resistin is predominantly expressed in adipocytes, in humans peripheral blood mononuclear cells (PBMC) seem to a be a major source of resistin. In the present study, we show that in human PBMC resistin mRNA expression-determined by fluorescence-based real-time polymerase chain reaction-is strongly increased by the proinflammatory cytokines interleukin (IL)-1, IL-6, tumor necrosis factor alpha (TNF-alpha), and also by lipopolysaccharides (LPS), respectively, while no effect was found by interferon-gamma (IFN-gamma) or leptin. Our results suggest that in humans resistin may be a link in the well-known association between inflammation and insulin resistance.     

Comparative studies of resistin expression and phylogenomics in human and mouse

Resistin is a newly identified adipocytokine that has been proposed to be a link between obesity and type 2 diabetes based on animal studies. However, the role of resistin in the pathogenesis of insulin resistance associated with obesity in humans remains unclear. We comparatively and quantitatively studied the tissue distributions of resistin mRNA between human and mouse. The expression level of resistin mRNA in human adipose tissue is extremely low but detectable by real-time PCR and is about 1/250 of that in the mouse. Remarkably, resistin mRNA is abundant in human primary acute leukemia cells and myeloid cell lines U937 and HL60, but not in the Raw264 mouse myeloid cell line. Resistin expression in U937 cells was not affected by lipopolysaccharide (LPS) or by ciglitazone, a PPARgamma ligand. Phylogenomics revealed that the human resistin gene is the ortholog of its murine counterpart and is located in a region of chromosome 19p13.3, which is syntenic to mouse chromosome 8A1. In addition to the resistin-like molecule (RELM) sequences already reported, bioinformatics analysis disclosed another RELM sequence in the vicinity of RELMbeta on human chromosome 3q13.1, but this sequence is unlikely to encode an expressed gene. Therefore, only two RELMs, resistin and RELMbeta, exist in humans, instead of the three RELMs, resistin, RELMalpha, and RELMbeta, that exist in mice. This finding provides a possible answer to the question of why only two RELMs have been cloned in humans and suggests that the RELM family is not well conserved in evolution and may function differently between species. Therefore, caution should be exercised in interpreting resistin as a link between obesity and insulin resistance in humans. The high expression of resistin in human leukemia cells suggests a hitherto unidentified biological function of resistin in leukocytes.     

The control of insulin secretion by adipokines: current evidence for adipocyte-beta cell endocrine signalling in metabolic homeostasis

Metabolic homeostasis is maintained by the coordinated action of multiple organ systems. Insulin secretion is often enhanced during obesity or insulin resistance to maintain glucose and lipid homeostasis, whereas a loss of insulin secretion is associated with type 2 diabetes. Adipocytes secrete hormones known as adipokines which act on multiple cell types to regulate metabolism. Many adipokines have been shown to influence beta cell function by enhancing or inhibiting insulin release or by influencing beta cell survival. Insulin, in turn, regulates lipolysis and promotes glucose uptake and lipid storage in adipocytes. As adipokine secretion and action is strongly influenced by obesity, this provides a potential route by which beta cell function is coordinated with adiposity, independently of alterations in blood glucose or lipid levels. In this review, I assess the evidence for the direct regulation of beta cell function by the adipokines leptin, adiponectin, extracellular nicotinamide phosphoribosyltransferase, apelin, resistin, retinol binding protein 4, fibroblast growth factor 21, nesfatin-1 and fatty acid binding protein 4. I summarise in vitro and in vivo data and discuss the influence of obesity and diabetes on circulating adipokine concentrations, along with the potential for influencing beta cell function in human physiology. Finally, I highlight future research questions that are likely to yield new insights into the exciting field of insulinotropic adipokines.     

Inhibition by insulin of resistin gene expression in 3T3-L1 adipocytes.

Expression of the gene encoding resistin, a low molecular weight protein secreted from adipose tissue postulated to link obesity and type II diabetes, was examined in 3T3-L1 adipocytes. Resistin mRNA was detected in 3T3-L1 cells by day 3 following induction of differentiation into adipocytes; by day 4 the level of resistin mRNA peaked and remained high. The PPARgamma activators, rosiglitazone or darglitazone, reduced the level of resistin mRNA. Dexamethasone upregulated resistin mRNA level, but no effect was observed with the beta(3)-adrenoceptor agonist, BRL 37344. A substantial reduction in resistin mRNA level was observed with insulin, which induced decreases at physiological concentrations. Insulin may be a major inhibitor of resistin production, and this does not support a role for resistin in insulin resistance.     

Elevated resistin levels in cirrhosis are associated with the proinflammatory state and altered hepatic glucose metabolism but not with insulin resistance

The adipokine resistin has been implicated in obesity and insulin resistance. Liver cirrhosis is associated with decreased body fat mass and insulin resistance. We determined plasma resistin levels in 57 patients with cirrhosis, 13 after liver transplantation, and 30 controls and correlated these with hemodynamic as well as hepatic and systemic metabolic parameters. Patients with cirrhosis had, dependent on the clinical stage, an overall 86% increase in resistin levels (P < 0.001) with hepatic venous resistin being higher than arterial levels (P < 0.001). Circulating resistin was significantly correlated with plasma TNF-alpha levels (r = 0.62, P < 0.001). No correlation was observed between resistin and hepatic hemodynamics, body fat mass, systemic energy metabolism, and the degree of insulin resistance. However, plasma resistin in cirrhosis was negatively associated with hepatic glucose production (r = -0.47, P < 0.01) and positively with circulating free fatty acids (FFA; r = 0.40, P < 0.01) and ketone bodies (r = 0.48, P < 0.001) as well as hepatic ketone body production (r = 0.40, P < 0.01). After liver transplantation, plasma resistin levels remained unchanged, whereas insulin resistance was significantly improved (P < 0.01). These data provide novel insights into the role of resistin in the pathophysiological background of a catabolic disease in humans and also indicate that resistin inhibition may not represent a suitable therapeutic strategy for the treatment of insulin resistance and diabetes in patients with liver cirrhosis.     

Effects of the new C1q/TNF-related protein (CTRP-3) "cartonectin" on the adipocytic secretion of adipokines

Background: Cartonectin (collagenous repeat‐containing sequence of 26‐kDa protein; CORS‐26) was described as a new adipokine of the C1q/TNF molecular superfamily C1q/TNF‐related protein‐3 (CTRP‐3), secreted by the adipocytes of mice and humans. The receptor and function of cartonectin are unknown and the recombinant protein is not commercially available. Objective: To investigate the effects of recombinant cartonectin on the secretion of adipokines such as adiponectin, leptin, and resistin from adipocytes of human and murine origin. The effect of the BMI of the adipocyte donor was also investigated. Methods and Procedures: Human adipocytes from pooled lean and preobese healthy individuals and murine 3T3‐L1 adipocytes were used for stimulation experiments. Recombinant cartonectin was expressed in insect H5 cells. Adipokine secretion was measured using enzyme‐linked immunosorbent assay. In addition, western blot analysis and luciferase reporter gene assays were employed. Results: Cartonectin (1, 10, 50, and 250 ng/ml) in higher doses stimulates the secretion of adiponectin and resistin from murine adipocytes. This effect is not caused by an induction of peroxisome proliferator‐activated receptor‐γ (PPAR‐γ) protein expression, as confirmed by western blot analysis. Also, luciferase reporter gene assay revealed that cartonectin failed to induce luciferase activity at the peroxisome proliferator‐activated receptor responsive element site containing the adiponectin/luciferase promoter fragment. Human adipocytes from lean individuals secrete higher amounts of adiponectin and leptin when compared with adipocytes of individuals with a preobesity BMI (25–30 kg/m²). Cartonectin failed to stimulate adiponectin or leptin secretion from human adipocytes, irrespective of the BMI value. Discussion: Cartonectin is a new adipokine that differentially regulates the secretion of classical adipokines, with marked differences between the human and the murine systems. These effects are species‐dependent, while basal adipokine secretion is influenced by the BMI.     

Plasma resistin levels in patients with type 1 and type 2 diabetes mellitus and in healthy controls.

Resistin is a recently discovered hormone that is exclusively expressed in adipose tissue. Its expression in rodents was reported to be elevated or suppressed in genetic and diet-induced obesity, respectively. Resistin treatment impaired glucose tolerance and insulin action. Immunoneutralization of resistin improved insulin sensitivity, while thiazolidinedione treatment reduced resistin expression. Therefore, resistin could play a critical role in the development of obesity and type 2 diabetes. In this study were determined resistin plasma levels in humans suffering from type 1 and type 2 diabetes and in healthy controls. Plasma levels of resistin in healthy controls were 38.78 ng/ml. They were not statistically different in individuals with a broad BMI range. Resistin plasma levels in type 2 diabetes were 38.7 ng/ml, and 39.4 ng/ml in type 1 diabetes. Thiazolidinedione treatment did not influence resistin plasma levels. We conclude from our data: 1. resistin can be detected in human plasma, 2. plasma resistin levels are not different in type 1 and type 2 diabetes.     

Loss of resistin ameliorates hyperlipidemia and hepatic steatosis in leptin-deficient mice

Resistin has been linked to components of the metabolic syndrome, including obesity, insulin resistance, and hyperlipidemia. We hypothesized that resistin deficiency would reverse hyperlipidemia in genetic obesity. C57Bl/6J mice lacking resistin [resistin knockout (RKO)] had similar body weight and fat as wild-type mice when fed standard rodent chow or a high-fat diet. Nonetheless, hepatic steatosis, serum cholesterol, and very low-density lipoprotein (VLDL) secretion were decreased in diet-induced obese RKO mice. Resistin deficiency exacerbated obesity in ob/ob mice, but hepatic steatosis was drastically attenuated. Moreover, the levels of triglycerides, cholesterol, insulin, and glucose were reduced in ob/ob-RKO mice. The antisteatotic effect of resistin deficiency was related to reductions in the expression of genes involved in hepatic lipogenesis and VLDL export. Together, these results demonstrate a crucial role of resistin in promoting hepatic steatosis and hyperlipidemia in obese mice.     

Resistin is expressed in pancreatic islets

Resistin, a recently described adipocyte factor, is regulated by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists. While resistin has been proposed to mediate insulin resistance in rodents, little is known about human resistin and its expression in pancreatic islets has not been tested. The goal of the present study was therefore to analyze whether resistin, like PPARgamma, is expressed in islets. Human islets from seven donors were analyzed by quantitative RT-PCR revealing resistin expression in all samples. Immunohistochemistry using a resistin-specific antibody on human pancreatic sections localized resistin protein to the islets. Mouse resistin was also detected in the Min6 beta cell line. Interestingly, we found a 4-fold increase in islet resistin expression in insulin resistant A-ZIP transgenic compared to wild-type mice. Our results demonstrate that resistin is expressed in islets and up-regulated in insulin resistance and thereby shed new light on the role of resistin in mice and humans.     

抵抗素的研究进展

抵抗素（resistin）是近年来发现的一个新脂肪细胞因子,最初在动物实验被用于联系肥胖、胰岛素抵抗和2型糖尿病。但随着进一步在人类实验的研究中发现,抵抗素能调节炎症反应而非胰岛素敏感性,研究发现抵抗素能促进炎症发生,炎症因子也能调节抵抗素的表达。动脉硬化是一种慢性亚临床性炎症,大量的体外研究提示抵抗素有可能作为一种促炎因子,通过激活内皮功能,诱导平滑肌增殖迁移,促巨噬细胞的脂质沉积,导致脂质代谢紊乱等途径,促进动脉粥样硬化的发生与发展。     

Human recombinant resistin protein displays a tendency to aggregate by forming intermolecular disulfide linkages

Resistin, a small cysteine rich protein secreted by adipocytes, has been proposed to be a link between obesity and type II diabetes by modulating the insulin signaling pathway and thus inducing insulin resistance. Resistin protein, with 11 cysteine residues, was not significantly homologous at the amino acid level to any other known cysteine rich proteins. Resistin cDNA derived from human subcutaneous adipose tissue was expressed in Escherichia coli as an N-terminal six-His-tag fusion protein. The overexpressed recombinant resistin was purified to homogeneity from inclusion bodies, after solubilization in 8 M urea, using a metal affinity column. While MALDI-TOF mass spectrometric analysis of the purified protein generated a single peak corresponding to the estimated size of 11.3 kDa, the protein exhibited a concentration-dependent oligomerization which is evident from size exclusion chromatography. The oligomeric structure was SDS-insensitive but beta-mercaptoethanol-sensitive, pointing to the importance of disulfide linkages in resistin oligomerization. Estimation of free cysteine residues using the NBD-Cl assay revealed a concentration- and time-dependent increase in the extent of formation of disulfide linkages. The presence of intermolecular disulfide bond(s), crucial in maintaining the global conformation of resistin, was further evident from fluorescence emission spectra. Circular dichroism spectra revealed that recombinant resistin has a tendency to reversibly convert from alpha-helical to beta-sheet structure as a direct function of protein concentration. Our novel observations on the biophysical and biochemical features of human resistin, particularly those shared with prion proteins, may have a bearing on its likely physiological function.     

Low resistin levels in adipose tissues and serum in high-fat fed mice and genetically obese mice: development of an ELISA system for quantification of resistin

Obesity is a major risk factor for insulin resistance. Resistin, an adipocyte-derived hormone-like molecule, is considered to serve as an important link between obesity and insulin resistance. However, the physiological role of resistin and the mechanism by which it neutralizes insulin action are still unclear. There are also conflicting reports that cast doubt on the cause of insulin resistance. In this study, we developed an enzyme-linked immunosorbent assay (ELISA) system for quantification of mouse resistin levels, analyzed in relation to insulin resistance. C57BL/6J mice fed high-fat diet compared with normal diet had low resistin levels (by 70%, P<0.01) in epididymal adipose tissues. Genetically obese mice, db/db and KK-A(y), had hyperinsulinemia and hyperglycemia but low resistin levels (decreases by 83 and 90%, both P<0.01) compared with C57/BL6J mice in epididymal adipose tissues. Serum resistin levels determined by Western blotting showed a similar pattern to those in adipose tissues. Resistin levels in adipose tissues correlated with serum adiponectin concentrations positively (r=0.49). Our results indicate that the novel ELISA system is suitable for measurement of resistin levels in adipose tissues. The results do not support a role for resistin in insulin resistance.     

Immunomodulatory effect of resistin in human dendritic cells stimulated with lipoteichoic acid from Staphylococcus aureus

Resistin is an adipokine whose physiologic role in obesity, type II diabetes mellitus, and inflammatory diseases has been a subject of debate because while it is expressed in adipocytes and adipose tissue in mouse, it is expressed in leukocytes, such as macrophages, in human. In the present study, we attempt to define the effect of resistin on human dendritic cells (DCs) derived from CD14⁺ monocytes. When DCs were stimulated with lipoteichoic acid (LTA) and treated with various concentrations of resistin, antigen-uptake process and the endocytic capacity of DCs were decreased. It is intriguing that resistin attenuated cytokine production in LTA-primed DCs. Consequently, T cell activity was reduced when lymphocytes were mixed with Staphylococcus aureus-primed autologous DCs treated with resistin compared to S. aureus-primed DCs without resistin. Our results suggest that resistin interferes with the efficacy of immune responses activated by Gram-positive bacterial infection in human DCs.     

Suppressed gene expression of adipocyte resistin in an insulin-resistant rat model probably by elevated free fatty acids.

Resistin, the peptide specifically secreted from adipocytes, is a hormone antagonistic to insulin action and, thus, may serve as a link between human obesity due to adiposity and insulin resistance associated with type 2 diabetes. To test this hypothesis, we studied the gene expression of resistin in adipocytes isolated from rats fed with a fructose diet which induced insulin resistance. Compared to the control rats (C) on a normal chow diet, the fructose-fed rats (F) developed hyperinsulinemia, glucose intolerance, hypertriglyceridemia and hypertension, a profile reminiscent of the syndrome X of patients with non-insulin-dependent diabetes mellitus (NIDDM). The F rats had significantly elevated plasma free fatty acids (FFA), enlarged epididymal fat pads, and increased adipocyte size compared with the C rats. We examined the glucose transport and the relative quantity of resistin mRNA produced in the adipocytes of these two groups of rats. Compared to the C rats, the F rats had a clearly reduced insulin-stimulated glucose transport. The gene expression of resistin and other adipocyte peptides was measured on the mRNA by semiquantitative RT-PCR; the validity of this technique was established in advance with a rat-fasting and then refeeding experiment. The F rats showed a decreased expression of the resistin gene, whereas gene expression of leptin and angiotensinogen in contrast increased. Free fatty acids were found to suppress the expression of resistin gene in normal rat adipocytes. These results demonstrate that an insulin-resistant instance in the fructose diet rat model exists with the decreased gene expression of resistin.     

Preliminary experience with resistin assessment in common population

Resistin is a signal peptide produced by adipose tissue. Mice models have confirmed that resistin may play an important role in insulin resistance. Its function in the human organism has not been elucidated yet. Since in common population the resistin concentrations are not known (no validated commercial set is available), we performed resistin assessment using the ELISA method (with satisfying analytical characteristics) in a population of 123 non-obese probands without signs of insulin resistance and/or inflammation. Mean resistin values amounted to 14.3 ng/ml (reference limit of 7.3-21.3 ng/ml).