Adiponectin But Not Resistin Is Associated with Insulin Resistance-Related Phenotypes in Baboons

OBJECTIVE

The hormones adiponectin and resistin have been associated with insulin resistance. This paper analyzed the potential relationship between adiponectin and resistin and insulin resistance-related phenotypes in baboons.

RESEARCH METHODS AND PROCEDURES

One hundred eight adult baboons (84 female and 24 male) were studied. Weight was measured, and a blood sample was collected under fasting conditions for plasma and monocyte isolation. Fasting glucose, insulin, C-peptide, and adiponectin levels in plasma were measured by standard methods. Insulin resistance was calculated by the homeostasis model assessment index. Resistin mRNA abundance in monocytes was determined by real-time quantitative reverse transcription-polymerase chain reaction. Data were clustered by weight tertiles for statistical analysis.

RESULTS

As observed in humans, the insulin resistance-related phenotypes were related to weight, plasma levels of adiponectin, and C-peptide. No significant relationship between resistin circulating levels or expression in monocytes and insulin resistance-related phenotypes was found in baboons.

DISCUSSION

These findings suggest that resistin is not associated with insulin resistance. However, previous observations of relationships among weight, adiponectin, and insulin resistance are confirmed.

Sexual Dimorphism and Regulation of Resistin, Adiponectin, and Leptin Expression in the Mouse

OBJECTIVE

To examine gender differences and hormonal regulation of resistin, adiponectin, and leptin.

RESEARCH METHODS AND PROCEDURES

Plasma levels were measured, and mRNA expression in perigonadal fat was quantified by RNase protection assays.

RESULTS

Plasma resistin declined with age despite an increase in adiposity in both genders. In male mice, plasma leptin increased, whereas adiponectin levels were constant. In females, both adiponectin and leptin levels increased with age. Resistin mRNA levels were significantly higher in female than male mice at all ages, whereas leptin and adiponectin mRNA levels were similar in fat from 6-week-old male and female mice, and sexual dimorphism was apparent only in the older mice, with higher levels apparent in females. Castration did not abolish gender differences in plasma levels or resistin, adiponectin, or leptin mRNAs. Castration of male mice did not significantly change adipokine mRNA levels or plasma levels of resistin or leptin; however, adiponectin was significantly increased. Dihydrotestosterone treatment had no effect on adipokine mRNA expression or resistin and adiponectin levels but increased leptin levels. In contrast, ovariectomy significantly increased resistin mRNA abundance and decreased leptin and adiponectin mRNAs. Plasma leptin levels were also increased by ovariectomy, whereas resistin and adiponectin levels were unchanged. Estrogen replacement significantly reduced resistin mRNA and increased leptin and adiponectin mRNA levels but had no effect on plasma adipokine levels.

DISCUSSION

The gender differences in adipokine mRNA expression and plasma levels were not ablated by castration and seem to be dependent on other factors in addition to gonadal steroids.

Regulation of gut-derived resistin-like molecule beta expression by nutrients

Resistin was initially identified as a protein, secreted by adipocytes, which inhibits insulin action and adipose differentiation. The three proteins homologous to resistin were identified and given the names resistin-like molecules (RELM) alpha, beta and gamma. Resistin and RELMalpha are abundantly expressed in adipose, but RELMbeta and RELMgamma are secreted mainly from the gut. Since nutrient composition greatly affects insulin sensitivity, we investigated the regulatory effects of various nutritional factors in food on the expressions of resistin family proteins. First, mice were given diets with different nutritional compositions (high-carbohydrate, high-protein and high-fat) for 2 weeks. RELMbeta mRNA expression in the intestines was markedly suppressed by the high-protein and high-carbohydrate diets, while slightly but not significantly upregulated by the high-fat diet. In the epididymal fat, resistin expression was unchanged, while RELMalpha expression was markedly decreased by the high-carbohydrate diet. Taking into consideration that humans have neither RELMalpha nor RELMgamma, our subsequent studies focused on RELMbeta expression. We used the human colon cancer cell line LS174T. Treatments with insulin and TNFalpha as well as stearic acid, a saturated free fatty acid, upregulated RELMbeta expression, while d-glucose downregulated RELMbeta. These results suggest RELMbeta expression to be regulated directly by nutrients such as glucose and saturated free fatty acids including stearic acid, as well as by hormones including insulin and TNFalpha. These regulations may play an important role in the nutrient-associated induction of insulin resistance.

Resistin-like molecule-beta is an allergen-induced cytokine with inflammatory and remodeling activity in the murine lung

Resistin-like molecule (RELM)-beta is a cysteine-rich cytokine implicated in insulin resistance and asthmatic responses, but its function remains an enigma. We now report that RELM-beta has a role in promoting airway inflammation and lung remodeling in the mouse lung. RELM-beta is strongly induced by diverse allergens and T helper type 2 (Th2) cytokines by an IL-13- and STAT6-dependent mechanism. To understand the in vivo role of RELM-beta, we delivered recombinant murine RELM-beta intratracheally to naïve mice. RELM-beta induced dose-dependent leukocyte accumulation (most prominently involving macrophages) and goblet cell hyperplasia. The most prominent effect induced by RELM-beta was increased perivascular and peribronchial collagen deposition. Mice genetically deficient in RELM-beta had reduced accumulation of collagen and goblet cell hyperplasia in an experimental model of allergic airway inflammation. In vitro experiments demonstrated that RELM-beta had fibroblast motogenic activity. These results identify RELM-beta as a Th2-associated cytokine with potent inflammatory and remodeling activity.

Absence of bacterially induced RELMbeta reduces injury in the dextran sodium sulfate model of colitis

Although inflammatory bowel disease (IBD) is the result of a dysregulated immune response to commensal gut bacteria in genetically predisposed individuals, the mechanism(s) by which bacteria lead to the development of IBD are unknown. Interestingly, deletion of intestinal goblet cells protects against intestinal injury, suggesting that this epithelial cell lineage may produce molecules that exacerbate IBD. We previously reported that resistin-like molecule beta (RELMbeta; also known as FIZZ2) is an intestinal goblet cell-specific protein that is induced upon bacterial colonization whereupon it is expressed in the ileum and colon, regions of the gut most often involved in IBD. Herein, we show that disruption of this gene reduces the severity of colitis in the dextran sodium sulfate (DSS) model of murine colonic injury. Although RELMbeta does not alter colonic epithelial proliferation or barrier function, we show that recombinant protein activates macrophages to produce TNF-alpha both in vitro and in vivo. RELMbeta expression is also strongly induced in the terminal ileum of the SAMP1/Fc model of IBD. These results suggest a model whereby the loss of epithelial barrier function by DSS results in the activation of the innate mucosal response by RELMbeta located in the lumen, supporting the hypothesis that this protein is a link among goblet cells, commensal bacteria, and the pathogenesis of IBD.

Altered expression of goblet cell- and mucin glycosylation-related genes in the intestinal epithelium during infection with the nematode Nippostrongylus brasiliensis in rat

Intestinal nematode infection induces marked goblet cell hyperplasia and mucus secretion, but the mechanisms of regulation of the changes still remain to be elucidated. In the present study, epithelial cells were isolated from the rat small intestine at various times after Nippostrongylus brasiliensis infection, and the levels of expression of goblet cell- and mucin glycosylation-related genes were estimated by semi-quantitative reverse transcription (RT)-PCR. Among the genes investigated, mucin core peptide (MUC) 2, sialyltransferase (Siat) 4c and trefoil factor family (TFF) 3 were upregulated as early as 2-4 days post-infection, suggesting that they are associated with an early innate protective response. Seven days post-infection and thereafter, when the nematodes reached maturity, significant upregulation of MUC3, MUC4, resistin-like molecule beta (Relmbeta) and 3O-sulfotransferase (3ST)1 was observed, while 3ST2 expression levels increased after the majority of the worms were expelled from the intestine. Similar alterations of glycosylation-related gene expression were also observed in mast-cell-deficient Ws/Ws rats, suggesting that mast cells in the epithelium are not relevant to the upregulation of these genes. The present finding that the expression level of each goblet cell- or glycosylation-related gene was altered differently during the time course of infection indicates the progression of sequential qualitative changes in the mucus layer after infection.

Conjugated linoleic acid deteriorates insulin resistance in obese/diabetic mice in association with decreased production of adiponectin and leptin

Dietary supplementation of conjugated linoleic acids (CLA) is known to have some beneficial effects such as anti-carcinogenic and anti-obesity effects in several animal species, while it also induces insulin resistance and fatty liver, especially in mice. To explore the possible factors responsible for the CLA-induced insulin resistance, we examined the plasma and mRNA expression levels of several adipocytokines, which are likely involved in the regulation of insulin sensitivity, in normal C5 7BL, mildly obese/diabetic KK and morbidly obese/diabetic KKAy mice. Feeding a diet supplemented with 0.5%, CLA oil consisting of 30.5/% c9, t11-CLA and 28.9% t10, c12-CLA for 4 wk resulted in a decrease in white adipose tissue (WAT), an increase in liver weight with excess accumulation of triglyceride, and insulin resistance associated with hyperglycemia and hyperinsulinemia. The plasma and WAT mRNA levels of leptin were higher in KK and KKAy mice than C57BI. mice, whereas those of adiponectin were higher in C5 7BL mice. CLA-feeding decreased the levels of leptin, adiponectin and resistin, especially in KK and KKAy mice. In contrast, tumor necrosis factor-alpha (TNFalpha) mRNA levels were higher in KK and KKAy mice than C57BL mice, and were increased by CLA feeding. The present results thus indicate that CLA feeding promotes insulin resistance in obese/diabetic mice by at least inverse regulation of leptin and adiponectin, and TNFalpha, adipocytokines known to either ameliorate or deteriorate insulin sensitivity, respectively.

Resistin SNP-420 determines its monocyte mRNA and serum levels inducing type 2 diabetes

Resistin, secreted from adipocytes, causes insulin resistance in rodents. Its roles and main source in humans remain unknown. The G/G genotype of resistin single nucleotide polymorphism, SNP-420, induces type 2 diabetes mellitus (T2DM) by increasing promoter activity. We elucidated factors correlated with serum resistin and effects of SNP-420 on monocyte resistin mRNA. In 198 T2DM and 157 controls, fasting serum resistin was higher in T2DM. Multiple regression analysis revealed that SNP-420 genotype was the strongest determinant of serum resistin. In T2DM, 1-year duration of T2DM and 1% HbA1c was also correlated with 0.19 and 0.54 ng/ml serum resistin, respectively. Logistic regression analysis revealed that serum resistin was an independent factor for T2DM. In 23 healthy volunteers, monocyte resistin mRNA was positively correlated with its simultaneous serum levels and was higher in G/G genotype. Thus, SNP-420 determines monocyte mRNA and serum levels of resistin, which could induce T2DM.

Increased serum resistin in adults with prader-willi syndrome is related to obesity and not to insulin resistance

CONTEXT

Determinants of insulin resistance in Prader-Willi syndrome (PWS) are not completely understood. The discovery of several adipokines with relevant effects on insulin resistance and cardiovascular complications of metabolic syndrome offered new tools of investigation of insulin resistance in PWS.

OBJECTIVE

The purpose of this study was to measure serum resistin and mRNA in adipose tissue of patients with PWS, those with simple obesity, and healthy controls and correlate resistin levels with anthropometric and biochemical features.

DESIGN

Twenty-eight adult PWS patients, 29 obese patients, and 25 healthy controls were studied. Anthropometric variables were measured and fasting serum and plasma were collected for measurement of resistin, adiponectin, leptin, lipid profile, glucose, and insulin.

RESULTS

Serum resistin and resistin mRNA expression in adipose tissue was significantly higher in PWS patients, compared with both healthy lean controls and obese patients. Moreover, on regression analysis resistin was significantly correlated with body mass index, whereas no significant association was found between resistin and homeostasis model assessment index. A weak association between resistin and adiponectin was found in the PWS group only. However, on multivariate analysis only the correlation between resistin and body mass index remained significant.

CONCLUSIONS

These results support a link between circulating resistin and obesity in humans but do not support a role for resistin in human insulin resistance.

Differential effects of dietary saturated and trans-fatty acids on expression of genes associated with insulin sensitivity in rat adipose tissue

OBJECTIVE

Trans-fatty acids (TFAs) are formed during partial hydrogenation of vegetable oils and are shown to be more atherogenic than saturated fatty acids (SFAs). Our previous study showed that dietary TFAs decrease adipose tissue insulin sensitivity to a greater extent than SFAs in rats. We hypothesized that the effects of these fatty acids on insulin sensitivity could be mediated through an alteration in gene expression. In the current study we have investigated the effects of dietary TFAs or SFAs on expression of genes associated with insulin sensitivity in rat adipose tissue.

DESIGN AND METHODS

Male weanling Wistar/NIN rats were divided into four groups and fed one of the following diets containing 10% fat (g/100 g diet) differing only in the fatty acid composition for 3 months: control diet (3.7% linoleic acid (LA)), SFA diet (5% SFA), TFA diet 1 (1.5% TFA + 1% LA) and TFA diet 2 (1.5% TFA + 2% LA). The mRNA expression of peroxisome proliferator-activated receptor gamma (PPARgamma), lipoprotein lipase (LPL), glucose transporter-4 (GLUT4), resistin and adiponectin was analyzed in epididymal fat using RT-PCR. The effects of TFA were studied at two levels of LA to understand the beneficial effects of LA over the effects of TFA.

RESULTS

Both dietary SFA and TFA upregulated the mRNA levels of resistin. Dietary SFA downregulated adiponectin and GLUT4 and upregulated LPL, while TFA downregulated PPARgamma and LPL. The effects of dietary TFA on PPARgamma and resistin were not counteracted by increased LA (TFA diet 2).

CONCLUSION

The effects of SFAs on the aforementioned genes except PPARgamma could be extrapolated towards decreased insulin sensitivity, while only the alteration in the mRNA levels of PPARgamma and resistin could be associated with insulin resistance in TFA-fed rats. These findings suggest that dietary SFAs and TFAs alter the expression of different genes associated with insulin sensitivity in adipose tissue.

Mouse and human resistins impair glucose transport in primary mouse cardiomyocytes, and oligomerization is required for this biological action

The adipocytokine resistin impairs glucose tolerance and insulin sensitivity in rodents. Here, we examined the effect of resistin on glucose uptake in isolated adult mouse cardiomyocytes. Murine resistin reduced insulin-stimulated glucose uptake, establishing the heart as a resistin target tissue. Notably, human resistin also impaired insulin action in mouse cardiomyocytes, providing the first evidence that human and mouse resistin homologs have similar functions. Resistin is a cysteine-rich molecule that circulates as a multimer of a dimeric form dependent upon a single intermolecular disulfide bond, which, in the mouse, involves Cys26; mutation of this residue to alanine (C26A) produces a monomeric molecule that appears to be bioactive in the liver. Remarkably, unlike native resistin, monomeric C26A resistin had no effect on basal or insulin-stimulated glucose uptake in mouse cardiomyocytes. Resistin impairs glucose uptake in cardiomyocytes by mechanisms that involve altered vesicle trafficking. Thus, in cardiomyocytes, both mouse and human resistins directly impair glucose transport; and in contrast to effects on the liver, these actions of resistin require oligomerization.

Genome-wide scan of resistin mRNA expression in omental adipose tissue of baboons

INTRODUCTION

The hormone resistin was recently discovered in adipose tissue of mice. Functional tests suggest a role for resistin in the regulation of insulin sensitivity. However, human studies have reported controversial results on the metabolic function of this hormone.

METHODS

A 1 g omental adipose tissue biopsy was obtained from 404 adult baboons. Resistin mRNA expression was assayed by real-time, quantitative RT-PCR, and univariate and bivariate quantitative genetic analyses were performed, via the variance decomposition approach. A genome scan analysis was conducted using resistin mRNA abundance in omental adipose tissue as a quantitative phenotype.

RESULTS

A significant heritability of h2 = 0.23 (P = 0.003) was found for resistin mRNA abundance in omental adipose tissue. A genome scan detected a quantitative trait locus for resistin expression with an LOD score of 3.8, in the region between markers D19S431 and D19S714, corresponding to human chromosome 19 p13. This chromosomal region contains genes related to insulin resistance phenotypes, such as resistin, insulin receptor, angiopoietin-like 4 protein and LDL receptor.

CONCLUSIONS

Individual variation in resistin mRNA expression has a significant genetic component, and a gene or genes on chromosome 19 p13 may regulate resistin mRNA levels in baboon omental adipose tissue.

Resistin expression and plasma concentration peak at different times during pregnancy in rats

Resistin has been proposed as both an anti-adipogenic factor and an inducer of insulin resistance. During late pregnancy, white adipose tissue mass increases and insulin sensitivity decreases. To check for the involvement of resistin in these processes, we measured plasma resistin in pregnant and non-pregnant rats and in lactating dams. Plasma resistin increased by day 15 of pregnancy and remained high 5 days post partum. The simultaneous increase in plasma resistin concentration and the decrease in insulin sensitivity is compatible with resistin depressing maternal insulin sensitivity. Resistin expression increased 5-15 times in visceral white adipose tissue depots by day 8 of pregnancy but was similar to pre-pregnancy values by day 19. Resistin expression in the placenta and mammary gland was similar to that in the parametrial adipose depot by day 8 but was almost null by day 19. There was therefore a time-lag between the peaks in expression and in plasma concentration. White adipose tissue mass increased without changes in adipocyte size once peaks in resistin expression had passed, which is compatible with an anti-adipogenic role for enhanced resistin expression. A bolus injection of chorionic gonadotrophin - which peaks in early pregnancy - to non-pregnant rats increased resistin expression in white adipose tissue, indicating that this hormone is involved in controlling resistin expression. Resistin was not detected in cerebrospinal fluid. Our results have suggested a role for resistin in pregnancy.

Regulation of human resistin gene expression in cell systems: an important role of stimulatory protein 1 interaction with a common promoter polymorphic site

AIMS/HYPOTHESIS

Resistin is an adipokine that might link obesity and insulin resistance. A common polymorphism of the human resistin gene, -420C >G, is a major determinant of plasma resistin concentrations as well as resistin mRNA expression in human adipose tissue. In this study, we investigated the regulatory mechanism by which this polymorphism affects resistin expression.

METHODS

Electrophoretic mobility shift assay was performed to identify the transcription factors binding to the -420G region. Transient transfection and reporter assay were used to measure promoter activities of the resistin gene. The binding ability of stimulatory protein 1 (Sp1) in response to adipocyte differentiation or high glucose concentrations was also measured.

RESULTS

Sp1 and stimulatory protein 3 (Sp3) specifically bound to the region around -420G of the human resistin gene. Overexpression of Sp1 increased the promoter activity regardless of -420 genotypes, while the promoter activity of the -420G construct was two-fold higher than that of the -420C construct. In contrast, overexpression of Sp3 scarcely increased the promoter activity. The binding ability of Sp1 to the -420G region was increased in response to adipocyte differentiation. Mithramycin A, an inhibitor of DNA binding of Sp1, reduced the effect of high glucose on transcription induction of the resistin gene in adipocytes.

CONCLUSIONS/INTERPRETATION

These results suggest that Sp1 is an important factor regulating transcription of human resistin gene. A common polymorphism of the human resistin promoter, -420C >G, is critical for the binding of Sp1 and modulates the transcriptional activity of the resistin gene by changing the binding ability of Sp1. In addition, Sp1 may be involved in the increase of resistin expression by hyperglycaemia.

Resistin polymorphisms are associated with cerebrovascular disease in Finnish Type 2 diabetic patients

AIMS

Resistin is a hormone secreted by adipocytes and it is also expressed in monocytes. Resistin has been found to increase insulin resistance, a key feature in Type 2 diabetes. The aim of this study was to investigate whether resistin polymorphisms are associated with Type 2 diabetes and its clinical characteristics.

METHODS

We studied the allele and genotype frequencies of the single-nucleotide polymorphisms (SNPs)-420 (C>G), +157 (T>C) and +299 (G>A) in the resistin gene in 258 Finnish Type 2 diabetics and 494 controls.

RESULTS

These three markers were in significant linkage disequilibrium with each other. No significant (P<0.05) differences in the allele or genotype frequencies were observed between the study groups. Subjects with Type 2 diabetes showed a significant association between cerebrovascular disease and the SNPs-420 (P=0.004) and +299 (P=0.007), the G-G and A-A genotypes, respectively, had the highest frequencies. SNPs-420 (P=0.000) and +299 (P=0.002) in men and SNP+157 in men (P=0.005) and in women (P=0.019) showed significant association with higher mean blood glucose. The rare allele homozygotes also had the highest mean blood glucose values. We also observed associations between at least one of the SNPs and fasting blood glucose, glycosylated haemoglobin A1 (GHbA1), low-density lipoprotein (LDL) cholesterol and systolic and diastolic blood pressure. After correction for multiple comparisons, the association between the promoter variant SNP-420 and cerebrovascular disease in both genders and the associations between mean blood glucose and SNP-420 and SNP+299 in men remained significant.

CONCLUSIONS

The results suggest that resistin may play a role in atherogenesis probably through increasing insulin resistance.

Serum concentrations of resistin-like molecules beta and gamma are elevated in high-fat-fed and obese db/db mice, with increased production in the intestinal tract and bone marrow

AIMS/HYPOTHESIS

Resistin and the resistin-like molecules (RELMs) comprise a novel class of cysteine-rich proteins. Among the RELMs, RELMbeta and RELMgamma are produced in non-adipocyte tissues, but the regulation of their expression and their physiological roles are largely unknown. We investigated in mice the tissue distribution and dimer formation of RELMbeta and RELMgamma and then examined whether their serum concentrations and tissue expression levels are related to insulin resistance.

METHODS

Specific antibodies against RELMbeta and RELMgamma were generated. Dimer formation was examined using COS cells and the colon. RELMbeta and RELMgamma tissue localisation and expression levels were analysed by an RNase protection assay, immunoblotting and immunohistochemical study. Serum concentrations in high-fat-fed and db/db mice were also measured using the specific antibodies.

RESULTS

The intestinal tract produces RELMbeta and RELMgamma, and colonic epithelial cells in particular express both RELMbeta and RELMgamma. In addition, RELMbeta and RELMgamma were shown to form a homodimer and a heterodimer with each other, in an overexpression system using cultured cells, and in mouse colon and serum. Serum RELMbeta and RELMgamma levels in high-fat-fed mice were markedly higher than those in mice fed normal chow. Serum RELMbeta and RELMgamma concentrations were also clearly higher in db/db mice than in lean littermates. Tissue expression levels revealed that elevated serum concentrations of RELMbeta and RELMgamma are attributable to increased production in the colon and bone marrow.

CONCLUSIONS/INTERPRETATION

RELMbeta and RELMgamma form homo/heterodimers, which are secreted into the circulation. Serum concentrations of RELMbeta and RELMgamma may be a novel intestinal-tract-mediating regulator of insulin sensitivity, possibly involved in insulin resistance induced by obesity and a high-fat diet.

Hypothyroidism in rats decreases peripheral glucose utilisation, a defect partially corrected by central leptin infusion

AIMS/HYPOTHESIS

The aims of this work were to determine the effect of hypothyroidism on insulin-stimulated glucose turnover and to unravel the potential mechanisms involved in such an effect.

METHODS

Hypothyroidism was induced by administration of propylthiouracil, with partial T4 substitution. Euglycaemic-hyperinsulinaemic clamps, associated with the labelled 2-deoxy-D-glucose technique for measuring tissue-specific glucose utilisation, were used. To assess a possible involvement of leptin in the modulation of glucose metabolism by hypothyroidism, leptin was infused intracerebroventricularly for 6 days. A group of leptin-infused rats was treated with rT3 to determine a potential role of T3 in mediating the leptin effects.

RESULTS

Compared with euthyroid rats, hypothyroid animals exhibited decreased overall glucose turnover and decreased glucose utilisation indices in skeletal muscle and adipose tissue. Leptinaemia in hypothyroid rats was lower while resistin mRNA expression in adipose tissue was higher than in euthyroid animals. Intracerebroventricular leptin infusion in hypothyroid rats partially restored overall, muscle and adipose tissue insulin-stimulated glucose utilisation and improved the reduced glycaemic response observed during insulin tolerance tests. The leptin effects were due neither to the observed increase in plasma T3 levels nor to changes in the high adipose tissue resistin expression of hypothyroid rats. The administration of leptin to hypothyroid animals was accompanied by increased expression of muscle and adipose tissue carnitine palmitoyl transferases, decreased plasma NEFA levels and reduced muscle triglyceride content.

CONCLUSIONS/INTERPRETATION

Hypothyroidism is characterised by decreased insulin responsiveness, partly mediated by an exaggerated glucose-fatty acid cycle that is partly alleviated by intracerebroventricular leptin administration.

Stimulated resistin expression in white adipose of rats with bile duct ligation-induced liver cirrhosis: relationship to cirrhotic hyperinsulinemia and increased tumor necrosis factor-alpha

Resistin, an adipose-derived polypeptide hormone, is proposed as a candidate of insulin resistance, although its roles in inhibiting adipogenesis and in inflammation have also been suggested. Liver cirrhosis is characterized by elevated circulating proinflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha), hyperinsulinemia and insulin resistance. The study aimed to examine resistin expression and its association with insulin and TNF-alpha in a cirrhotic rat model using bile duct ligation (BDL). The BDL-induced cirrhotic rats showed significantly lower fat mass, insulin sensitivity and elevated plasma insulin and TNF-alpha compared to sham animals. In addition, epididymal TNF-alpha and resistin mRNA and protein levels were higher in cirrhotic rats. In normal control rats, in vivo insulin infusion and ex vivo administration of TNF-alpha to cultured fat pads increased resistin gene expression significantly. These results implied that hyperinsulinemia and increased TNF-alpha levels might upregulate adipose resistin gene in BDL-induced liver cirrhosis. Further study is necessary to document the role of resistin in metabolic abnormalities of liver cirrhosis.

Circulating resistin in lean, obese, and insulin-resistant mouse models: lack of association with insulinemia and glycemia

Resistin is an adipocyte-secreted hormone proposed to link obesity with insulin resistance and diabetes, but no previous study has performed a joint quantitative evaluation of white adipose tissue (WAT) resistin mRNA expression and serum levels in relation to insulinemia and glycemia in mice. We have thus comparatively assessed WAT resistin mRNA expression and serum resistin levels in lean C57BL/6J mice and various mouse models of obesity, including diet-induced obese (DIO) C57BL/6J mice, high fat-fed TNF-alpha-/- mice, and brown adipose tissue (BAT)-deficient uncoupling protein-diphtheria toxin A chain (UCP1-DTA) mice. We also studied whether treatment with the weight-reducing and insulin-sensitizing compounds, MTII, an alpha-melanocyte-stimulating hormone analog, or CNTF(Ax15), a ciliary neurotrophic factor analog, alters resistin mRNA expression and/or circulating levels in lean and DIO C57BL/6J mice. We find that resistin mRNA expression is similar in DIO and lean C57BL/6J mice, as well as in TNF-alpha-/- and wild-type (WT) mice. Circulating resistin levels, however, are higher in DIO C57BL/6J, high fat-fed TNF-alpha-/-, and UCP1-DTA mice compared with lean controls. Moreover, although resistin mRNA expression is upregulated by MTII treatment for 24 h and downregulated by CNTF(Ax15) treatment for 3 or 7 days, circulating resistin levels are not altered by MTII or CNTF(Ax15) treatment. In addition, serum resistin levels, but not resistin mRNA expression levels, are correlated with body weight, and neither resistin mRNA expression nor serum resistin levels are correlated with serum insulin or glucose levels. We conclude that transcriptional regulation of resistin in WAT does not correlate with circulating resistin levels and that circulating resistin is unlikely to play a major endocrine role in insulin resistance or glycemia in mice.

Mapping and association studies of diabetes related genes in the pig

The mitogen-activated protein kinase 8 (MAPK8), resistin (RETN), 11 beta hydroxysteroid dehydrogenase isoform 1 (HSD11B1) and protein kinase B Akt2 (AKT2) genes are all genes known to affect insulin signalling and have been implicated in the progression of obesity and type 2 diabetes in humans. In this study, polymorphisms in the porcine diabetes related MAPK8, RETN, HSD11B1 and AKT2 genes were identified, mapped and their associations with phenotypic measurements in swine were analysed. Polymorphisms detected in the MAPK8, RETN and HSD11B1 loci were used to genotype a Berkshire-Yorkshire pig breed reference family. Using linkage analysis, RETN, HSD11B1 and MAPK8 genes were mapped to pig chromosomes 2, 9 and 14, respectively, while the AKT2 gene was physically mapped to pig chromosome 6q21. Results presented here suggest associations between the polymorphisms in the MAPK8, RETN and HSD11B1 genes with several phenotypic measurements, including fat deposition traits in the pig. Because these genes have been implicated in obesity and diabetes in humans, and this study suggests associations with fat related traits, further research on these genes in swine may provide useful information on genetic factors underlying lean pork production.

Pituitary resistin gene expression is upregulated in vitro and in vivo by dexamethasone but is unaffected by rosiglitazone

A physiological role for the adipose-derived hormone, resistin, remains unsolved and its putative involvement in insulin resistance is largely controversial. Like leptin and other adipokines, we detected resistin in the rodent hypothalamic-pituitary system. In addition, the pituitary corticotrope cell line, AtT20, is also a source of resistin. These data suggested that resistin could be involved in diverse physiological processes in non-adipose tissue. Initial studies indicated that pituitary resistin gene expression was regulated in a nutritional (fed>fasted), age- (young>old) and gender-specific manner (male>female). In the present work we hypothesized that pituitary resistin expression should be regulated through signalling pathways similar to those reported for adipose tissue. For example, dexamethasone (DEX) potently stimulates production of resistin in murine adipose tissue, whereas thiazolidinediones such as rosiglitazone (ROSI), acting via peroxisome proliferator-activated receptor (PPAR)gamma, reportedly inhibit or stimulate resistin expression. Using quantitative real-time RT-PCR we determined that injection of DEX (10 and 50 microg) yielded 7- and 9-fold increases in pituitary resistin gene expression in prepubertal, but not adult mice. In addition, adrenalectomy attenuated pituitary resistin gene expression, which was restored by DEX (10 microg). In AtT20 cells, exposure to corticosterone (10(-7) and 10(-6)M; 24 h) and DEX (10(-9)-10(-6)M) stimulated resistin mRNA more than 65% (p<0.05) and 115% (p<0.0001), respectively. In contrast, ROSI, injected (5 and 20 mg/kg s.c. for 14 days) or given orally (3 mg/kg/day to 10 mg/kg/day for up to 7 weeks), failed to alter pituitary resistin gene expression in healthy male CD1 mice. Treatment of AtT20 cells with ROSI (10(-5)-10(-10)M; 24 h or 96 h) or the PPARgamma agonist GW 1929 (10(-9) and 10(-5)M; 24 h) had no effect on resistin mRNA. The PPARgamma antagonist GW 9662 (10(-6) and 10(-5)M) was also ineffective. In conclusion, pituitary resistin mRNA levels are robustly stimulated by corticosteroids, particularly at the time of puberty. This is consistent with our previous suggestion that resistin may be involved in maturation of the hypothalamic-pituitary axis. In contrast, pituitary resistin gene expression appears to be PPARgamma-independent and therefore different from the situation in adipose tissue.

Adenovirus-mediated high expression of resistin causes dyslipidemia in mice

The adipocyte-derived hormone resistin has been proposed as a possible link between obesity and insulin resistance in murine models. Many recent studies have reported physiological roles for resistin in glucose homeostasis, one of which is enhancement of glucose production from the liver by up-regulating gluconeogenic enzymes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. However, its in vivo roles in lipid metabolism still remain to be clarified. In this study, we investigated the effects of resistin overexpression on insulin action and lipid metabolism in C57BL/6 mice using an adenoviral gene transfer technique. Elevated plasma resistin levels in mice treated with the resistin adenovirus (AdmRes) were confirmed by Western blotting analysis and RIAs. Fasting plasma glucose levels did not differ between AdmRes-treated mice and controls, but the basal insulin concentration was significantly elevated in AdmRes-treated mice. In AdmRes-treated mice, the glucose-lowering effect of insulin was impaired, as evaluated by insulin tolerance tests. Furthermore, total cholesterol and triglyceride concentrations were significantly higher, whereas the high-density lipoprotein cholesterol level was significantly lower. Lipoprotein analysis revealed that low-density lipoprotein was markedly increased in AdmRes-treated mice, compared with controls. In addition, in vivo Triton WR-1339 studies showed evidence of enhanced very low-density lipoprotein production in AdmRes-treated mice. The expressions of genes involved in lipoprotein metabolism, such as low-density lipoprotein receptor and apolipoprotein AI in the liver, were decreased. These results suggest that resistin overexpression induces dyslipidemia in mice, which is commonly seen in the insulin-resistant state, partially through enhanced secretion of lipoproteins.

A polymorphism in the resistin gene promoter is associated with body mass index in women with polycystic ovary syndrome

Resistin does not appear to be a major gene predisposing to polycystic ovary syndrome (PCOS). However, an association of a resistin variant with body mass index was found in women with PCOS, suggesting that resistin may be related to adiposity in PCOS.

Resistin mRNA levels are downregulated by estrogen in vivo and in vitro

Resistin, a hormone secreted by adipocytes, is suggested to be an important link between obesity and diabetes. The aim of this study was to evaluate the regulatory effect of estrogen on adipocyte resistin gene expression in ovariectomized (OVX) rats and in isolated rat adipocytes in vitro. Subcutaneous injection of estradiol benzoate reduced resistin mRNA levels in adipocytes isolated from the inguinal, parametrial, perirenal, retroperitoneal, or periovarian fat deposits of OVX rats, while an in vitro study showed that estradiol treatment decreased resistin mRNA levels in cultured rat periovarian fat adipocytes. Results of Western blotting analysis also showed that estrogen decreased adipose resistin contents in vivo and in vitro. These data suggest that estrogen is a pivotal negative regulator of resistin gene expression.

A mutation in signal peptide of rat resistin gene inhibits differentiation of 3T3-L1 preadipocytes

AIM

To detect the resistin expression of white adipose tissue in diet-induced obese (DIO) versus diet-resistant (DR) rats, and to investigate the relationship of mutated resistin and 3T3-L1 preadipocytes differentiation.

METHODS

RT-PCR and Western Blot were used to detect gene /protein expression. 3T3-L1 cells were cultured, transfected, and induced to differentiation using 0.5 mmol/L 3-isobutyl-1-methylxanthine (MIX), 1 mg/L insulin, and 1 micromol/L dexamethasone. Oil red O staining was applied to detect the degree of preadipocytes differentiation.

RESULTS

Expression of resistin mRNA was upregulated in DIO rats and downregulated in DR rats. However, the expression levels varied greatly within the groups. Sequencing of the resistin genes from DIO and DR rats revealed a Leu9Val (C25G) missense mutation within the signal peptide in one DR rat. The mutant resistin inhibited preadipocyte differentiation. Local experiments and Western blotting with tagged resistin fusion proteins identified both mutant and wild type proteins in the cytoplasm and secreted into the culture medium. Computer predictions using the Proscan and Subloc programs revealed four putative phosphorylation sites and a possible leucine zipper motif within the rat resistin protein.

CONCLUSION

Resistin-increased differentiation may be inhibited by the mutation-containing precursor protein, or by the mutant non-secretory resistin isoform.

Human resistin gene polymorphism is associated with visceral obesity and fasting and oral glucose stimulated C-peptide in the Québec Family Study

Obesity and insulin resistance are common features of Type 2 Diabetes. A new protein called resistin has been shown to be secreted by adipocytes in mice and to influence insulin sensitivity. The goal of the present study was to investigate the associations between one polymorphism (g-420C>G) of the human resistin gene and phenotypes related to adiposity and glucose metabolism. We genotyped 725 (including 42 diabetics) adult subjects participating in the Quebec Family Study (QFS) by a minisequencing method. Forty-two were diabetic subjects. Phenotypes measured were: body mass index (BMI) and waist circumference (WC), % body fat (PFAT) and fat mass (FM) assessed by under water weighing, abdominal total, subcutaneous and visceral fat assessed by computed tomography and fasting plasma glucose, insulin and C-peptide and their responses to an oral glucose tolerance test (OGTT). Comparisons between genotypes were performed in non-diabetic men (no.=280) and women (no.=403) separately by analyses of covariance (ANCOVA). Among men, g-420 G homozygotes had less visceral fat (p < 0.05), lower levels of acute insulin responses to an OGTT and lower levels of C-peptide in a fasting state and in responses to an OGTT than carriers of the C allele (p < 0.01). These associations were independent of age and adiposity but were not observed in women. These results suggest that in men, the human resistin gene is associated with reduced amount of visceral obesity and lower insulin secretory responses to a glucose load.

Resistin expression in 3T3-L1 adipocytes is reduced by arachidonic acid

The resistin gene is expressed in adipocytes and encodes a protein proposed to link obesity and type 2 diabetes. Increased plasma FFA is associated with insulin resistance. We examined the effect of separate FFAs on the expression of resistin mRNA in cultured murine 3T3-L1 adipocytes. The FFAs tested did not increase resistin expression, whereas both arachidonic acid (AA) and eicosapentaenoic acid (EPA) reduced resistin mRNA levels. AA was by far the most potent FFA, reducing resistin mRNA levels to approximately 20% of control at 60-250 muM concentration. Selective inhibitors of cyclooxygenase-1 and of mitogen-activated protein kinase kinase counteracted AA-induced reduction in resistin mRNA levels. Transient overexpression of sterol-regulatory element binding protein-1a (SREBP-1a) activated the resistin promoter, but there was no reduction in the abundance of approximately 65 kDa mature SREBP-1 after AA exposure. Actinomycin D as well as cycloheximide abolished the AA-induced reduction of resistin mRNA levels, indicating dependence on de novo transcription and translation. Our data suggest that reductions in resistin mRNA levels involve a destabilization of the resistin mRNA molecule. An inhibitory effect of AA and EPA on resistin expression may explain the beneficial effect of ingesting PUFAs on insulin sensitivity.

Impaired glucose tolerance is accompanied by decreased insulin sensitivity in tissues of mice implanted with cells that overexpress resistin

AIM/HYPOTHESIS

Resistin, the expression of which is suppressed by thiazolidinedione treatment in adipocytes, is one of the key molecules for the tight link between adiposity and insulin resistance. Here, we show the in vivo effects of resistin on insulin sensitivity in mature mice using a cell implantation method.

METHODS

Resistin cDNA was transfected into 3T3-L1 pre-adipocytes, which were then implanted into subcutaneous areas of nude mice. Metabolic analyses were performed 4 or 6 weeks after implantation.

RESULTS

The mice implanted with 3T3-L1 cells overexpressing resistin (R-mice) showed significantly (p<0.05) increased plasma resistin levels. After a glucose load plasma insulin levels were significantly greater in R-mice than in mice implanted with mock-transfected cells (M-mice). The AUC of insulin after glucose loading was positively correlated with circulating resistin levels. Significantly decreased glucose responses after insulin injection were observed in R-mice, compared to M-mice. The insulin-induced phosphorylation level of IRS-1 was significantly lower in muscles of R-mice than M-mice. The expression of TNF-alpha mRNA in intra-peritoneal fat tissues was significantly greater in R-mice than in M-mice, but there was no difference between the two groups with regard to subcutaneous fat tissues. The concentration of TNF-alpha in plasma was positively correlated with resistin levels in R-mice.

CONCLUSIONS/INTERPRETATION

Resistin, when actually secreted from cells in mature mice, causes disturbed glucose metabolism, possibly based on decreased insulin sensitivity in muscle. The in vivo effects of resistin on insulin sensitivity might be in part mediated by increased TNF-alpha expression in visceral fat tissues.

The G/G genotype of a resistin single-nucleotide polymorphism at -420 increases type 2 diabetes mellitus susceptibility by inducing promoter activity through specific binding of Sp1/3

Insulin resistance is a major cause of type 2 diabetes mellitus (T2DM). Resistin, an adipocyte-secreted hormone, antagonizes insulin. Transgenic mice that overexpress the resistin gene (Retn) in adipose tissue are insulin-resistant, whereas Retn (-/-) mice show lower fasting blood glucose, suggesting that the altered Retn promoter function could cause diabetes. To determine the role of RETN in human T2DM, we analyzed polymorphisms in its 5' flanking region. We found that the -420G/G genotype was associated with T2DM (397 cases and 406 controls) (P=.008; adjusted odds ratio = 1.97 [by logistic regression analysis]) and could accelerate the onset of disease by 4.9 years (P=.006 [by multiple regression analysis]). Meta-analysis of 1,888 cases and 1,648 controls confirmed this association (P=.013). Linkage disequilibrium analysis revealed that the -420G/G genotype itself was a primary variant determining T2DM susceptibility. Functionally, Sp1 and Sp3 transcription factors bound specifically to the susceptible DNA element that included -420G. Overexpression of Sp1 or Sp3 enhanced RETN promoter activity with -420G in Drosophila Schneider line 2 cells that lacked endogenous Sp family members. Consistent with these findings, fasting serum resistin levels were higher in subjects with T2DM who carried the -420G/G genotype. Therefore, the specific recognition of -420G by Sp1/3 increases RETN promoter activity, leading to enhanced serum resistin levels, thereby inducing human T2DM.

Variation in the resistin gene is associated with obesity and insulin-related phenotypes in Finnish subjects

AIMS/HYPOTHESIS

Resistin is a peptide hormone produced by adipocytes that is present at high levels in sera of obese mice and may be involved in glucose homeostasis through regulation of insulin sensitivity. Several studies in humans have found associations between polymorphisms in the resistin gene and obesity, insulin sensitivity and blood pressure. An association between variation in the resistin gene and type 2 diabetes has been reported in some, but not all studies. The aim of this study was to analyse variants of the resistin gene for association with type 2 diabetes and related traits in a Finnish sample.

METHODS

In 781 cases with type 2 diabetes, 187 spouse controls and 222 elderly controls of Finnish origin, we genotyped four previously identified non-coding single-nucleotide polymorphisms (SNPs): -420C>G from the promoter region, +156C>T and +298G>A from intron 2, and +1084G>A from the 3' untranslated region. We then tested whether these SNPs were associated with type 2 diabetes and related traits.

RESULTS

The SNPs were not significantly associated with type 2 diabetes. However, SNPs -420C>G, +156C>T and +298G>A and the common haplotype for these three markers were associated with increased values of weight-related traits and diastolic blood pressure in cases, lower weight in elderly control subjects, and lower insulin sensitivity and greater acute insulin response in spouses. Furthermore, the +1084G allele was associated with lower HDL cholesterol in both cases and controls, higher systolic blood pressure and waist circumference in cases, and greater acute insulin response in spouse controls.

CONCLUSIONS/INTERPRETATION

Our results add to growing evidence that resistin is associated with variation in weight, fat distribution and insulin resistance.

RELMbeta/FIZZ2 is a goblet cell-specific immune-effector molecule in the gastrointestinal tract

Gastrointestinal (GI) nematode infections are an important public health and economic concern. Experimental studies have shown that resistance to infection requires CD4(+) T helper type 2 (Th2) cytokine responses characterized by the production of IL-4 and IL-13. However, despite >30 years of research, it is unclear how the immune system mediates the expulsion of worms from the GI tract. Here, we demonstrate that a recently described intestinal goblet cell-specific protein, RELMbeta/FIZZ2, is induced after exposure to three phylogenetically distinct GI nematode pathogens. Maximal expression of RELMbeta was coincident with the production of Th2 cytokines and host protective immunity, whereas production of the Th1 cytokine, IFN-gamma, inhibited RELMbeta expression and led to chronic infection. Furthermore, whereas induction of RELMbeta was equivalent in nematode-infected wild-type and IL-4-deficient mice, IL-4 receptor-deficient mice showed minimal RELMbeta induction and developed persistent infections, demonstrating a direct role for IL-13 in optimal expression of RELMbeta. Finally, we show that RELMbeta binds to components of the nematode chemosensory apparatus and inhibits chemotaxic function of a parasitic nematode in vitro. Together, these results suggest that intestinal goblet cell-derived RELMbeta may be a novel Th2 cytokine-induced immune-effector molecule in resistance to GI nematode infection.

Increased insulin sensitivity in paternal Gnas knockout mice is associated with increased lipid clearance

The G protein alpha-subunit Gsalpha is required for hormone-stimulated cAMP generation. The Gsalpha gene Gnas is a complex gene with multiple imprinted gene products. Mice with heterozygous disruption of the Gnas paternal allele (+/p-) are partially Gsalpha deficient and totally deficient in XLalphas, a neuroendocrine-specific Gsalpha isoform that is expressed only from the paternal Gnas allele. We previously showed that these mice are hypermetabolic and lean and have increased insulin sensitivity. We now performed hyperinsulinemic-euglycemic clamp studies, which confirmed the markedly increased whole body insulin sensitivity in +/p- mice. +/p- mice had 1.4-, 7- and 3.8-fold increases in insulin-stimulated glucose uptake in muscle and white and brown adipose tissue, respectively, and markedly suppressed endogenous glucose production from the liver. This was associated with increased phosphorylation of insulin receptor and a downstream effector (Akt kinase) in both liver and muscle in response to insulin. Triglycerides cleared more rapidly in +/p- mice after a bolus administered by gavage. This was associated with decreased liver and muscle triglyceride content and increased muscle acyl-CoA oxidase mRNA expression. Resistin and adiponectin were overexpressed in white adipose tissue of +/p- mice, although there was no difference in serum adiponectin levels. The lean phenotype and increased insulin sensitivity observed in +/p- mice is likely a consequence of increased lipid oxidation in muscle and possibly other tissues. Further studies will clarify whether XLalphas deficiency is responsible for these effects and if so, the mechanism by which XLalphas deficiency leads to this metabolic phenotype.

Adenovirus-mediated chronic "hyper-resistinemia" leads to in vivo insulin resistance in normal rats

We investigated the chronic in vivo effect of resistin on insulin sensitivity and glucose metabolism by overexpressing resistin protein in male Wistar rats using intravenous administration of an adenovirus encoding mouse resistin. After 7 days of elevated resistin levels at a supraphysiological concentration, the animals displayed glucose intolerance and hyperinsulinemia during glucose tolerance tests, and insulin tolerance tests demonstrated an impaired glucose-lowering effect of insulin. The glucose clamp studies were performed at submaximal (4 mU/kg/min) and maximal (25 mU/kg/min) insulin infusion rates and demonstrated the presence of insulin resistance induced by elevated resistin levels. Indeed, the insulin-stimulated glucose infusion rate was decreased by 12-31%; suppression of hepatic glucose output was attenuated by 28-55%; and insulin suppression of circulating FFA levels was inhibited by 7%. Insulin receptor substrate-1 and -2 phosphorylation and Akt activation were impaired in muscle and adipose tissue. Interestingly, activation of AMP-activated protein kinase in skeletal muscle, liver, and adipose tissue was also significantly downregulated. Together, these results indicate that chronic "hyper-resistinemia" leads to whole-body insulin resistance involving impaired insulin signaling in skeletal muscle, liver, and adipose tissue, resulting in glucose intolerance, hyperinsulinemia, and hypertriglyceridemia. Thus elevated resistin levels in normal rats fed a regular chow diet produce many of the features of human syndrome X.

Differences in mRNA expression of adipocyte-derived factors in response to fasting, refeeding and leptin

The present study examines whether and to what extent the profiles of adipose-derived factors are altered in epididymal and subcutaneous adipose tissues of long-term fasted/refed and of fasted rats treated by recombinant leptin. Fasting was characterized by three successive metabolic phases. Minor differences in the time-course and magnitude of response were detected between the two adipose sites. Leptin, adiponectin, resistin, adiponutrin, and insulin-like growth factor-1 (IGF-1) gene expressions differentially decreased according to the fasting duration. mRNA levels reached a minimum in late fasting for these secreted factors, being decreased by 60-90% for adiponectin, resistin, and IGF-1, 95-98% for leptin and by 100% for adiponutrin. Refeeding partially or totally restored their mRNA expression in epididymal adipose. Expression levels of apolipoprotein E (ApoE), angiotensinogen (AGT), adipsin and macrophage migration inhibitory factor (MIF) were either unchanged or slightly affected. In leptin-treated rats, leptin mRNA concentrations were significantly decreased in phase 2 of fasting (by 85%) from levels in control phosphate-buffered saline (PBS)-treated rats in both tissues. Leptin treatment also decreased resistin mRNA levels (by 78% in P2L and 63% in P3L relative to control groups) in subcutaneous adipose. These data suggest that adiponectin, resistin, adiponutrin, and IGF-1 could be involved in overall energy homeostasis during prolonged fasting, as leptin is. The mechanisms that underlie the expressions of these adipose-secreted factors remain to be determined.

Identification of murine and human XCP1 genes as C/EBP-epsilon-dependent members of FIZZ/Resistin gene family

The CCAAT enhancer binding protein epsilon (C/EBP-epsilon) transcription factor is expressed predominantly in granulocytes. Mice with a disruption of the C/EBP-epsilon gene fail to produce mature granulocytes and eosinophils. Cells derived from the peritoneal exudates of C/EBP-epsilon -/- mice lack the expression of a number of chemokines and chemokine receptor genes. We have found a novel C/EBP-epsilon-dependent promyelocyte-specific gene, mXCP1. mXCP1 belongs to a family of XCP/FIZZ/Resistin genes, which includes four murine genes and two human genes, hXCP1 and hXCP2. These genes have four exons and encode short secreted proteins sharing a ten-cysteine motif. Murine mXCP1, mXCP2 and mXCP3 genes map to murine chromosome 16 and mXCP4 is positioned on chromosome 8; the hXCP1 and hXCP2 genes are located at homologous regions of chromosomes 3 and 19. Introduction of an inducible C/EBP-epsilon gene into the NIH3T3 and myeloid cells from C/EBP-epsilon-null mice line revealed that the conditional expression of C/EBP-epsilon induced mXCP1. The HXCP1 gene was identified as a C/EBP-epsilon-dependent regulatory homologue of mXCP1. The expression data for other members of XCP/FIZZ gene family are presented. Further studies indicate that XCP1 is a secreted protein that is chemotactic to myeloid cells from C/EBP-epsilon-null mice and is able to interact directly with alpha-defensin.

Resistin/ADSF/FIZZ3 in obesity and diabetes

The role of adipocyte-secreted resistin/adipocyte-specific secretory factor (ADSF)/FIZZ3 in obesity and diabetes has been controversial at best. Recently generated resn knockout mice showed normal glucose and insulin sensitivity with lower fasting glucose levels. Upon feeding with a high-fat diet, the knockout mice exhibited increased glucose tolerance with decreased hepatic glucose output, possibly due to phosphorylation and activation of AMP-activated protein kinase and suppression of gluconeogenic genes. In comparison, transgenic mice overexpressing a dominant negative form of resistin/ADSF/FIZZ3 showed increased adiposity with elevated leptin and adiponectin levels, accompanying enhanced glucose tolerance and insulin sensitivity both on chow and high-fat diets. Although its underlying mechanisms need further elucidation, the in vivo studies demonstrate a role of resistin/ADSF/FIZZ3 in obesity and insulin resistance.

Novel resistin promoter polymorphisms: association with serum resistin level in Japanese obese individuals

OBJECTIVES

Resistin, an adipocyte-secreted cytokine recently discovered in mice, has been proposed as a link between obesity and diabetes. We analyzed resistin gene polymorphisms and examined their association with serum resistin level and obesity phenotypes in humans.

SUBJECTS AND METHODS

Sixty young, obese, non-diabetic subjects taking no medication were studied. DNA sequencing and genotyping of identified single nucleotide polymorphisms were performed. Associations between polymorphisms and serum resistin level, BMI, body composition, fat distribution, and several indices of insulin sensitivity were examined. Moreover, single nucleotide polymorphisms in the promoter region were examined for their influence on resistin gene transcriptional activity using luciferase reporter vectors.

RESULTS

Ten non-coding single nucleotide polymorphisms were found. The -638G>A, -420C>G, and -358G>A polymorphisms in the promoter region showed marked linkage disequilibrium with each other, and were associated with serum resistin level; however, there was no association between these polymorphisms and parameters related to adiposity or insulin resistance. The results of luciferase assay revealed that -638G>A together with the -420C>G polymorphism influenced resistin gene transcriptional activity.

CONCLUSION

We found that variability in the serum resistin level might be related to polymorphic variants of the promoter region of the gene.

Two variants in the resistin gene and the response to long-term overfeeding

OBJECTIVE AND SUBJECTS

To investigate the role of resistin gene variants on the adiposity and metabolic changes observed in response to a 100-day overfeeding protocol conducted with 12 pairs of monozygotic twins.

MEASUREMENTS

Body-fat measurements included hydrodensitometry and abdominal fat from computed tomography. Plasma glucose and insulin during fasting and in response to an oral glucose tolerance test (OGTT) were assayed. A 4.2 MJ test meal was consumed, after which calorimetric measurements were performed for 240 min.

RESULTS

Respiratory quotient (RQ) decreased (P=0.001) more in AA/AG than in GG subjects of the IVS2+181G>A polymorphism after the caloric surplus and the significance persisted when correction for multiple testing was performed. Total abdominal (P=0.027) and visceral (P=0.004) fat increased more in TC than in TT subjects of the IVS2+39C>T polymorphism. In response to overfeeding, glucose area under the curve during the OGTT showed a slight decrease (P=0.031) in the TC while it increased in TT subjects. OGTT insulin area tended to increase less (P=0.055) in TC than in TT subjects. After overfeeding, fasting insulin was lower in TC than in TT subjects (P=0.010). In addition, TC subjects experienced more decrease in RQ than TT subjects (P=0.034).

CONCLUSION

The IVS2+181G>A variant was associated with the changes in RQ in response to overfeeding. The IVS2+39C>T polymorphism was associated with overfeeding-induced changes in abdominal visceral fat, OGTT glucose area and RQ. The results suggest that sequence variation in the resistin gene is involved in the adaptation to chronic positive energy balance.

Identification of an alternative splicing transcript for the resistin gene and distribution of its mRNA in human tissue

OBJECTIVE

Adipocytes secrete a number of molecules such as tumor necrosis factor-alpha, leptin and free fatty acids that can influence the ability of the body to metabolize glucose. Recently, a novel 12.5 kDa cysteine-rich protein, termed resistin, was shown to be secreted by adipocytes. Resistin expression was markedly induced during the conversion of 3T3-L1 cells to mature adipocytes. Expression of resistin has been studied in human, mouse and rat; however, sequence information about an alternative splicing variant (ASV) of resistin mRNA has not been reported. In the present study, we investigated the occurrence of a novel ASV of the resistin gene in human normal tissues.

DESIGN AND METHODS

We identified a novel ASV of resistin mRNA in human lung tissue by RT-PCR analysis in human lung tissue. We then investigated a novel ASV of resistin mRNA by real-time PCR analysis in 26 different types of normal human tissues.

RESULTS AND CONCLUSIONS

We identified a novel deletion variant of the resistin transcript in the normal human tissues. The deleted transcript of resistin was characterized by an in-frame deletion of 78 bp, corresponding to the complete loss of exon 2 (resistin delta2 ASV). Thus, resistin delta2 ASV causes protein truncation. Our results provide the basis for more detailed studies on the regulation of resistin activity, and should assist in the development of clinical trials with resistin for the central regulation of adipogenesis and adipocyte metabolism.

Regulation of resistin expression and circulating levels in obesity, diabetes, and fasting

Resistin was originally reported as an adipose tissue-specific hormone that provided a link between obesity and diabetes. Resistin protein level was elevated in obese mice and decreased by insulin-sensitizing thiazolidinediones. Immunoneutralization of resistin improved insulin sensitivity in diet-induced obese mice, while the administration of exogenous resistin induced insulin resistance. More recently, we have shown that ablation of the resistin gene in mice decreased fasting glucose through impairment of gluconeogenesis, while resistin treatment in these knockout mice increased hepatic glucose production. However, the link between resistin and glucose homeostasis has been questioned by studies demonstrating reduced, rather than increased, resistin mRNA expression in obese and diabetic mice. To better understand the regulation of resistin, we developed a sensitive and specific RIA resistin that could accurately measure serum resistin levels in several mouse models. We show that while resistin mRNA is indeed suppressed in obese mice, the circulating resistin level is significantly elevated and positively correlated with insulin, glucose, and lipids. Both resistin mRNA expression and protein levels in Lep(ob/ob) mice are suppressed by leptin treatment in parallel with reductions in glucose and insulin. In wild-type mice, serum resistin increases after nocturnal feeding, concordant with rising levels of insulin. Resistin mRNA and protein levels decline in parallel with glucose and insulin during fasting and are restored after refeeding. We performed clamp studies to determine whether resistin is causally related to insulin and glucose. Adipose resistin expression and serum resistin increased in response to hyperinsulinemia and further in response to hyperglycemia. Taken together, these findings suggest that the nutritional regulation of resistin and changes in resistin gene expression and circulating levels in obesity are mediated, at least in part, through insulin and glucose.

Role of resistin in diet-induced hepatic insulin resistance

Resistin is an adipose-derived hormone postulated to link adiposity to insulin resistance. To determine whether resistin plays a causative role in the development of diet-induced insulin resistance, we lowered circulating resistin levels in mice by use of a specific antisense oligodeoxynucleotide (ASO) directed against resistin mRNA and assessed in vivo insulin action by the insulin-clamp technique. After 3 weeks on a high-fat (HF) diet, mice displayed severe insulin resistance associated with an approximately 80% increase in plasma resistin levels. In particular, the rate of endogenous glucose production (GP) increased more than twofold compared with that in mice fed a standard chow. Treatment with the resistin ASO for 1 week normalized the plasma resistin levels and completely reversed the hepatic insulin resistance. Importantly, in this group of mice, the acute infusion of purified recombinant mouse resistin, designed to acutely elevate the levels of circulating resistin up to those observed in the HF-fed mice, was sufficient to reconstitute hepatic insulin resistance. These results provide strong support for a physiological role of resistin in the development of hepatic insulin resistance in this model.

A resistin gene polymorphism is associated with body mass index in women

The potential association of resistin (RETN) gene variability with obesity-related phenotypes was investigated in 585 non-diabetic individuals of European descent. The polymorphism studied (-420 C>G) is located in the RETN gene 5'-flanking region. A significant association between the polymorphism and body mass index and waist circumference was observed in the women subsample (n = 356), where the G allele was somewhat less frequent in the overweight/obese group than in normal-weight individuals (0.25 vs. 0.32; p = 0.040; OR=0.70 [0.50-0.98]). Female carriers of the G-allele presented a lower mean BMI than C/C homozygotes (25.5 vs. 26.8 kg/m(2); p = 0.010). Furthermore, when women were stratified by menopausal status, the association was restricted to premenopausal women (C/C homozygotes, mean BMI = 26.3 kg/m2; G-carriers, 24.4 kg/m2; p = 0.014). Our findings suggest that RETN gene variation has gender-specific effects on BMI and warrants further investigation of its implications for the development of obesity.

Disulfide-dependent multimeric assembly of resistin family hormones

Resistin, founding member of the resistin-like molecule (RELM) hormone family, is secreted selectively from adipocytes and induces liver-specific antagonism of insulin action, thus providing a potential molecular link between obesity and diabetes. Crystal structures of resistin and RELMbeta reveal an unusual multimeric structure. Each protomer comprises a carboxy-terminal disulfide-rich beta-sandwich "head" domain and an amino-terminal alpha-helical "tail" segment. The alpha-helical segments associate to form three-stranded coiled coils, and surface-exposed interchain disulfide linkages mediate the formation of tail-to-tail hexamers. Analysis of serum samples shows that resistin circulates in two distinct assembly states, likely corresponding to hexamers and trimers. Infusion of a resistin mutant, lacking the intertrimer disulfide bonds, in pancreatic-insulin clamp studies reveals substantially more potent effects on hepatic insulin sensitivity than those observed with wild-type resistin. This result suggests that processing of the intertrimer disulfide bonds may reflect an obligatory step toward activation.

Resistin expression in different adipose tissue depots during rat development

Resistin is a hormonal factor synthesised by adipocytes that was first thought to be related with the resistance to insulin in obesity, but whose function is not yet completely established. Here we have studied the ontogenic pattern of resistin mRNA expression in different white adipose tissue depots (WAT)--epididymal, inguinal, mesenteric and retroperitoneal--and in brown adipose tissue (BAT), as well as the circulating resistin levels, in rats of different ages (from the suckling period to one year of age). Resistin mRNA was determined by Northern blotting, and serum levels by enzyme immunoassay. In WAT, resistin expression remains almost constant with age, except in early development, where there is a peak of expression in the epididymal and retroperitoneal depots, and a decrease in the inguinal one, while the expression remains constant for the mesenteric depot. Moreover, there is a site-specific difference regarding resistin expression: all the depots express characteristic levels of mRNA, especially at the age of 2 months, the moment when resistin mRNA levels are significantly higher in the epididymal and the retroperitoneal than in the inguinal and mesenteric WAT and than in the BAT. The transient increased resistin expression in the epididymal and the retroperitoneal WAT at a period of time in which there is a change in diet (from milk to chow) suggests a common nutritional regulation of the resistin gene. Circulating resistin levels increase with age probably reflecting the increase in the body fat content.

Resistin promotes 3T3-L1 preadipocyte differentiation

OBJECTIVE

To investigate the relationship between resistin (a potential link between obesity and type 2 diabetes) and preadipocyte differentiation.

DESIGN

A rat resistin expression vector was transfected into 3T3-L1 preadipocytes and differentiation was compared between normal 3T3-L1 cells, rat resistin-transfected cells and non-transfected cells grown in conditioned medium taken from resistin-expressing cultures.

METHODS

The rat resistin gene was inserted into the pDual GC and pEFGP-N2 expression vectors for examination of the effects of resistin overexpression in 3T3-L1 cells before and after differentiation was stimulated with 3-isobutyl-1-methyxanthine (MIX), insulin and dexamethasone (DEX). Smaller conserved fragments were inserted into short interference RNA (siRNA) expression vectors, for examination of the effect of targeted resistin inhibition on differentiation of resistin-overexpressing 3T3-L1 cells.

RESULTS

Prior to stimulation, the resistin-transfected 3T3-L1 cells contained many more small lipid droplets than did non-transfected 3T3-L1 cells. Following stimulation, differentiation in the resistin-transfected 3T3-L1 cells was dramatically promoted, especially in the early stages. Stimulation of differentiation was also observed in non-transfected 3T3-L1 cells grown in resistin protein-containing conditioned medium. The expression of adipocyte differentiation-associated markers such as CCAAT enhancer binding protein (C/EBPalpha), retinoid X receptor (RXRalpha) and lipoprotein lipase (LPL) was upregulated in resistin-overexpressing cells, whereas expression of preadipocyte factor-1 (Pref-1), an inhibitor of preadipocyte differentiation, was downregulated. In addition, expression of two of the three tested siRNAs inhibited the adipoconversion process, providing further evidence that resistin promotes the differentiation of preadipocytes to adipocytes.

CONCLUSION

Resistin can promote preadipocyte differentiation. Based on this, we propose that resistin may be an important candidate mediator of obesity-induced insulin resistance.

Changes in glycemia by leptin administration or high- fat feeding in rodent models of obesity/type 2 diabetes suggest a link between resistin expression and control of glucose homeostasis

Resistin is an adipose-derived hormone that has been proposed as a link among obesity, insulin resistance, and diabetes. In agreement with a role of resistin in insulin resistance, the administration of recombinant resistin led to glucose intolerance in mice and impaired insulin action in rat liver. However, the regulation of resistin expression by physiological conditions, hormones, or agents known to modulate insulin sensitivity does not always support the association between resistin and obesity-induced insulin resistance. In the present study we investigated the effects of leptin administration on adipose resistin expression in insulin-resistant and obese ob/ob mice. We show that the expression of resistin mRNA and protein in adipose tissue is lower in ob/ob than in wild-type control mice, in agreement with the reduced adipocyte resistin mRNA level reported in several models of obesity. Leptin administration in ob/ob mice resulted in improvement of insulin sensitivity concomitant with a decrease in resistin gene expression. The lack of effect of leptin on resistin in db/db mice indicated that the leptin inhibitory action on resistin expression requires the long leptin receptor isoform. In addition, we demonstrated that the effect of leptin on resistin expression was centrally mediated. High-fat feeding in C57BL/6J wild-type mice, which is known to induce the development of obesity and insulin resistance, produced an increase in resistin expression. Interestingly, in both ob/ob and high fat-fed mice we obtained a striking positive correlation between glycemia and resistin gene expression. In conclusion, our results demonstrate that leptin decreases resistin expression and suggest that resistin may influence glucose homeostasis.

Dehydroepiandrosterone up-regulates resistin gene expression in white adipose tissue

Dehydroepiandrosterone (DHEA), the most abundant steroid hormone in human blood, is considered to be one of fat-reducing hormones. However, the molecular mechanisms underlying DHEA mode of action in obesity has not been fully clarified. The pivotal role in the maintenance of cellular lipid and energy balance is played by peroxisome proliferator-activated receptor alpha (PPARalpha) which acts as transcriptional activator of numerous genes encoding enzymes involved in fatty acid catabolism. Lately published papers suggest that resistin, a low molecular-weight protein produced by adipose tissue, may act as an inhibitor of adipocyte differentiation and could regulate adipose tissue mass. Recent studies have established that the promoter region of the resistin gene contains several putative PPAR response elements. Since DHEA has been characterized as a peroxisome proliferator able to induce hepatic genes through PPARalpha, we hypothesised that DHEA might affect PPARalpha and, subsequently, resistin gene expression in adipose tissue. In order to test this hypothesis, an experiment was performed comparing PPARalpha and resistin gene expression in white adipose tissue (WAT) of male Wistar rats fed standard or DHEA-supplemented (0.6% (w/w)) diet for 2 weeks. DHEA administration to the rats induced PPARalpha and resistin gene expression in WAT (3- and 2.25-fold, respectively; as determined by real-time reverse transcription-polymerase chain reaction (RT-PCR)); reduced body weight, epididymal adipose tissue mass and decreased serum leptin levels. We propose that DHEA may impact on the transcription of resistin gene through a mechanism involving PPARalpha and that an elevated resistin level may lead to an inhibition of adipogenesis and a decrease in adipose tissue mass.

Modulation of resistin expression by retinoic acid and vitamin A status

This work identifies retinoic acid (RA), the acid form of vitamin A, as a signal that inhibits the expression of resistin, an adipocyte-secreted protein previously proposed to act as an inhibitor of adipocyte differentiation and as a systemic insulin resistance factor. Both 9-cis and all-trans RA reduced resistin mRNA levels in white and brown adipocyte cell model systems; the effect was time- and dose-dependent, was followed by a reduced secretion of resistin, and was reproduced by selective agonists of both RA receptors and rexinoid receptors. Association of CCAAT/enhancer-binding protein alpha (a positive regulator of the resistin gene) and its coactivators p300, cAMP response element-binding protein binding protein, and retinoblastoma protein with the resistin gene promoter was reduced in RA-treated adipocytes. RA administration to normal mice resulted in reduced resistin mRNA levels in brown and white adipose tissues, reduced circulating resistin levels, reduced body weight, and improved glucose tolerance. Resistin expression was also downregulated after dietary vitamin A supplementation in mice. The results raise the possibility that vitamin A status may contribute to modulate systemic functions through effects on the production of adipocyte-derived protein signals.