A family of tissue-specific resistin-like molecules

Alternative splicing generates a novel non-secretable resistin isoform in Wistar rats1

Resistin is a secreted adipose tissue hormone that belongs to the resistin-like molecule family. We report here a new alternatively spliced isoform of the rat resistin gene, named S-resistin (short resistin), detected in adipose tissue by reverse transcription-polymerase chain reaction (RT-PCR). A comparison of this cDNA variant and genomic sequences indicates the lack of the second exon containing the secretory consensus signal. Both cDNAs, resistin and S-resistin, were carboxy-tagged with FLAG epitope and transiently expressed in cultured cell lines. While the resistin-FLAG construct gives the expected pattern for a secretion protein, the S-resistin-FLAG construct yielded a predominant nuclear staining. These results indicate that this splicing event regulates the fate and probably the function of the mature protein.

Bacterial colonization leads to the colonic secretion of RELMbeta/FIZZ2, a novel goblet cell-specific protein

BACKGROUND & AIMS

Goblet cells are highly polarized exocrine cells found throughout the small and large intestine that have a characteristic morphology due to the accumulation of apical secretory granules. These granules contain proteins that play important physiologic roles in cellular protection, barrier function, and proliferation. A limited number of intestinal goblet cell-specific proteins have been identified. In this study, we investigate the expression and regulation of RELMbeta, a novel colon-specific gene.

METHODS

The regulation of RELMbeta messenger RNA expression was determined in LS174T, Caco-2, and HT-29 cell lines in response to stimulation with interleukin 13 and lipopolysaccharide. Quantitative reverse-transcription polymerase chain reaction, immunoblots, and immunohistochemistry were used to examine the expression of RELMbeta in BALB/c and C.B17.SCID mice housed in conventional, germ-free, and gnotobiotic environments.

RESULTS

Messenger RNA for RELMbeta is restricted to the undifferentiated, proliferating colonic epithelium. Immunohistochemistry shows that this protein is expressed in goblet cells located primarily in the distal half of the colon and cecum with lower levels detectable in the proximal colon. High levels of RELMbeta can be detected in the stool of mice and humans, where it exists as a homodimer under nonreducing conditions. Interestingly, the secretion of RELMbeta is dramatically reduced in germ-free mice. Furthermore, introduction of germ-free mice into a conventional environment results in enhanced expression and robust secretion of RELMbeta within 48 hours.

CONCLUSIONS

These studies define a new goblet cell-specific protein and provide the first evidence that colon-specific gene expression can be regulated by colonization with normal enteric bacteria.

The chemical form of selenium affects insulinomimetic properties of the trace element: investigations in type II diabetic dbdb mice

The objective of the present study was to investigate the effects of oral selenate application in comparison to selenium deficiency and selenite treatment on the development of the diabetic status (glucose tolerance, insulin resistance and activities of glycolytic and gluconeogenic marker enzymes) in dbdb mice, representing a type II diabetic animal model. Therefore 21 adult male dbdb mice were assigned to 3 experimental groups of 7 animals each and put on a selenium deficient diet (< 0.03 mg/kg diet) based on torula yeast. Group 0Se was kept on selenium deficiency for 10 weeks while the mice of the groups SeIV and SeVI were supplemented daily with 15% of their individual LD(50) of sodium selenite or sodium selenate in addition to the diet. After 10 weeks a distinct melioration of the diabetic status indicated by a corrected glucose tolerance and a lowered insulin resistance was measured in selenate treated mice (group SeVI) in comparison to their selenium deficient and selenite treated companions and to their initial status. Activities of the glycolytic marker enzymes hexokinase, phosphofructokinase and pyruvate kinase were increased 1.7 to 3-fold in liver and/or adipose tissue by selenate treatment as compared to mice on selenium deficiency and mice with selenite administration. In contrast selenate treatment (SeVI) repressed the activity of liver pyruvate carboxylase the first enzyme in gluconeogenesis by about 33% in comparison to the selenium deficient (0Se) and selenite treated mice (SeIV). However the current study revealed an insulinomimetic role for selenate (selenium VI) also in type II diabetic animals due to a melioration of insulin resistance. In contrast selenium deficiency and especially selenite (selenium IV) impaired the diabetic status of dbdb mice, demonstrating the need for investigations on the insulinomimetic action of selenium due to the metabolism of different selenium compounds.

Serum concentrations of adipocytokines in patients with hyperthyroidism and hypothyroidism before and after control of thyroid function

OBJECTIVE

Adipose tissue is a hormonally active system that produces and releases different bioactive substances. Leptin, adiponectin and resistin are some of the recently discovered adipocytokines that participate in the regulation of intermediate metabolism. The aim of this study was to evaluate the circulating levels of leptin, adiponectin and resistin in patients with thyroid dysfunction before and after normalization of thyroid function with appropriate therapy.

PATIENTS AND MEASUREMENTS

We studied 20 patients with hyperthyroidism (16 women and 4 men; mean age 47.2 +/- 3.9 years) and 20 patients with hypothyroidism (17 women and 3 men; 51.5 +/- 4.1 years). A group of 20 euthyroid subjects served as control group. Patients were evaluated at the time of diagnosis and again after normalization of thyroid function with appropriate therapy. Serum concentrations of free T4 (FT4), total T3, TSH, insulin, leptin, adiponectin and resistin were measured in all subjects.

RESULTS

Hyperthyroid patients showed significantly decreased leptin levels in comparison with controls (11.0 +/- 1.1 vs. 30.4 +/- 5.0 microg/l, P < 0.001). No significant differences in adiponectin levels between hyperthyroid and control groups were found (27.8 +/- 4.0 vs. 46.0 +/- 12.0 mg/l, NS). Patients with hyperthyroidism exhibited reduced resistin levels in comparison with euthyroid subjects (6.4 +/- 0.8 vs. 8.4 +/- 0.7 microg/l, P < 0.05). Normalization of circulating thyroid hormone was accompanied by a nonsignificant increase in leptin levels (12.9 +/- 1.7 microg/l, P < 0.01 vs. control) and no significant modification both in adiponectin (32.0 +/- 7.1 mg/l, NS) and resistin (5.4 +/- 0.7 microg/l, NS) levels. Adjustment of adipocytokine concentrations for body mass index (BMI) showed that treatment of hyperthyroidism induced a significant reduction in adjusted resistin concentrations (0.21 +/- 0.03 vs. 0.28 +/- 0.03 microg/l/BMI units, P < 0.05), with no changes in adjusted leptin and adiponectin. Hypothyroid patients showed significantly lower leptin levels compared with the controls (16.0 +/- 3.5 vs. 30.4 +/- 5.0 microg/l, P < 0.05). Adiponectin levels in patients with hypothyroidism (71.8 +/- 16.0 mg/l) were similar to those in the control group and were not modified with therapy. Resistin levels were significantly reduced among hypothyroid patients (5.8 +/- 1.0 microg/l, P < 0.05), and were not increased after levothyroxine therapy. A significant rise in BMI-corrected leptin levels was observed after replacement therapy, with no changes in adiponectin- and resistin-corrected values.

CONCLUSIONS

The results suggest that (1) low serum leptin levels are present in both hyperthyroid and hypothyroid patients but are only increased after therapy in the latter; (2) resistin might be implicated in the insulin resistance state that accompanies thyrotoxicosis; and (3) inadequate secretion of adiponectin seems to have no role in metabolic changes associated with thyroid dysfunction.

Rosiglitazone improves glucose metabolism in nondiabetic uremic patients on CAPD

BACKGROUND

Insulin resistance, a strong risk factor for atherosclerotic vascular disease, is present in uremic patients without diabetes on continuous ambulatory peritoneal dialysis (CAPD) therapy. Amelioration of insulin resistance may reduce associated long-term cardiovascular complications. The aim of the study is to investigate the effects of rosiglitazone (ROS), an insulin sensitizer, on glucose metabolism in CAPD patients without diabetes.

METHODS

Fifteen uremic patients without diabetes on CAPD therapy were enrolled. All were administered ROS, 4 mg/d, for 12 weeks. A control group consisted of 15 age- and sex-matched healthy subjects. Oral glucose tolerance test (OGTT) results, fasting glucose and insulin levels, related blood biochemistry results, and C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) levels were determined before initiation and at 4 and 12 weeks of therapy. Insulin resistance was evaluated using the homeostasis model assessment method (HOMA-IR). A whole-body insulin sensitivity index (ISI) and insulinogenic index for insulin production were calculated from OGTT results.

RESULTS

CAPD patients showed significantly greater HOMA-IR and glucose intolerance compared with healthy controls. After 4 and 12 weeks of ROS therapy, there were no significant changes in body weight, blood pressure, dialysis adequacy, hemoglobin level, hemoglobin A(1c) level, liver function, lipid profile, or intact parathyroid hormone, CRP, IL-6, or TNF-alpha levels. There was a significant decrease in HOMA-IR (3.2 +/- 0.6, 2.2 +/- 0.4, and 2.1 +/- 0.4; P < 0.05). During the OGTT, there was a significant decrease in the area under the glucose curve and a significant increase in ISI (3.5 +/- 0.4, 5.0 +/- 0.7, and 5.3 +/- 0.7; P < 0.05), but no significant change in insulinogenic index.

CONCLUSION

ROS improved insulin resistance in CAPD patients without diabetes. Whether long-term use of ROS reduces cardiovascular risk needs further study.

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.

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.

Functional characterization of the human resistin promoter with adipocyte determination- and differentiation-dependent factor 1/sterol regulatory element binding protein 1c and CCAAT enhancer binding protein-alpha

Recent studies with murine models propose that resistin would be a possible mediator to link between obesity and insulin resistance. Although it has been reported that resistin is highly expressed and secreted by adipocytes, transcription factors that are involved in resistin gene expression have not been well characterized. To investigate the molecular mechanisms of resistin gene expression, we cloned and characterized the human resistin promoter. Sequence analysis of the resistin promoter revealed several putative binding sites for adipogenic transcription factors including adipocyte determination- and differentiation-dependent factor 1 (ADD1)/sterol regulatory element binding protein 1c (SREBP1c) and CCAAT enhancer binding protein-alpha (C/EBP alpha). EMSA and chromatin immunoprecipitation assays demonstrated that ADD1/SREBP1c binds to the human resistin promoter in vitro and in vivo. Expression of ADD1/SREBP1c transactivated the luciferase reporter gene activity, the promoter region of which contains a human resistin promoter in a sterol regulatory element (SRE)-dependent manner. Furthermore, ectopic expression of ADD1/SREBP1c by adenovirus significantly increased the expression of resistin mRNA in adipocytes. Human resistin promoter was also activated by C/EBP alpha expression, although ectopic expression of both transcription factors did not show any synergistic effects on the activation of resistin promoter. Together, these data suggest that ADD1/SREBP1c and C/EBP alpha may play discrete roles in the regulation of the resistin gene expression.

Correlation between serum resistin level and adiposity in obese individuals

OBJECTIVE

Resistin is associated with insulin resistance in mice and may play a similar role in humans. The aim of our study was to examine the relationship of serum resistin level to body composition, insulin resistance, and related obesity phenotypes in humans.

RESEARCH METHODS AND PROCEDURES

Sixty-four young (age 32 +/- 10 years), obese (BMI 32.9 +/- 5.6), nondiabetic subjects taking no medication, and 15 lean (BMI 21.1 +/- 1.3) volunteers were studied cross-sectionally. Thirty-five of the subjects were also reevaluated after 1.5 years on a weight reduction program entailing dieting and exercise; changes of serum resistin were compared with changes of BMI, body composition, fat distribution, and several indices of insulin sensitivity derived from plasma glucose and serum insulin levels measured during 75-g oral glucose tolerance test.

RESULTS

In a cross-sectional analysis, serum resistin was significantly higher in obese subjects than in lean volunteers (24.58 +/- 12.93 ng/mL; n = 64 vs. 12.83 +/- 8.30 ng/mL; n = 15; p < 0.01), and there was a correlation between resistin level and BMI, when the two groups were combined (rho = 0.35, p < 0.01). Although cross-sectional analysis in obese subjects revealed no correlation between serum resistin and parameters related to adiposity or insulin resistance, longitudinal analysis revealed change in serum resistin to be positively correlated with changes in BMI, body fat, fat mass, visceral fat area, and mean glucose and insulin (rho = 0.39, 0.40, 0.44, 0.50, 0.40, and 0.50; p = 0.02, 0.03, 0.02, <0.01, 0.02, and <0.01, respectively).

DISCUSSION

Resistin appears to be related to human adiposity and to be a possible candidate factor in human insulin resistance.

Resistin is expressed in different rat tissues and is regulated in a tissue- and gender-specific manner

Resistin is a polypeptide hormone first reported from human and rodent adipocytes. In order to better define the potential biological role of resistin we undertook a detailed analysis of its expression in different rat tissues. We demonstrate by real-time reverse transcription polymerase chain reaction, Southern blotting and immunohistochemistry that resistin is expressed not only in brown and white adipose tissue, but also in the stomach, small and large intestines, adrenal gland, and skeletal muscle. Food deprivation led to a decrease in resistin mRNA expression only in adipose tissue, not in any of the other tissues studied. Furthermore, resistin mRNA expression is higher in males than in females in adipose tissue, not in any of the other tissues. Thus, our data suggest that resistin is not exclusively localized in adipocytes, and indicate that its expression is regulated in a tissue- and sex-specific manner.

Genomic organization, chromosomal localization and adipocytic expression of the murine gene for CORS-26 (collagenous repeat-containing sequence of 26 kDa protein)

The murine gene for CORS-26 shows striking homologies to the adipocyte-specific secretory protein adiponectin (belonging to the newly discovered C1q/TNF molecular superfamily) and its expression has been reported to be restricted to fibroblasts, cartilage and kidney. However, the present data demonstrate specific induction of CORS-26 mRNA expression in hormonally differentiated 3T3-L1 adipocytes, but not in preadipocytes. Furthermore, CORS-26 mRNA expression could be demonstrated in human synovial adipocytes of the knee by in situ hybridization. Since the genes for CORS-26 and adiponectin are homologous for their COOH-terminal globular domain and of their N-terminal collagenous domain, they might have originated by divergence from an innate mesenchymal precursor molecule directing the development of myocytes, adipocytes and chondrocytes from a mesenchymal stem cell. Here, the complete genomic organization with exon/intron boundaries together with exon-specific primer combinations are presented. Additionally, approximately 1 kb of the TATA-box-containing promoter region was cloned and analyzed for putative transcription factor binding sites. The chromosomal localization of the murine CORS-26 gene was mapped to mouse chromosome 15 A2 by fluorescence in situ hybridization (FISH). Since the linkage loci for proteoglycan-induced arthritis and MRL/lpr arthritis in mice have been mapped to that chromosomal region, CORS-26 might represent the underlying mechanism of disease. The present data provide the basis for further investigation of the CORS-26 gene regulation in the context of mesenchymal tissue development, chondrocyte/adipocyte function and bone or skeletal disease.

Identification of RELMgamma, a novel resistin-like molecule with a distinct expression pattern

We have identified RELMgamma, a novel member of the resistin-like molecule/found in inflammatory zone (RELM/FIZZ) family in mice and rats. Microarray and real-time RT-PCR experiments revealed a repression of RELMgamma mRNA in nasal respiratory epithelium of cigarette smoke-exposed versus untreated rats. The analysis of the physiological tissue-specific expression revealed highest expression in hematopoietic tissues, suggesting a cytokine-like role for RELMgamma. RELMgamma is most closely related to RELMalpha/FIZZ1. Despite the high similarity, the expression properties of the two genes are clearly distinct. While RELMgamma (approved symbol retnlg) is expressed in rat white adipose tissue, minute to no expression of RELMalpha was detected in that system. Thus, previous reports analyzing RELMalpha expression in rat adipose tissue might have been influenced by cross-hybridization with RELMgamma. Finally we could demonstrate that white adipose tissue of mice shows strong RELMalpha expression but only low levels of RELMgamma, indicating a species-specific gene regulation.

FIZZ1/RELMalpha, a novel hypoxia-induced mitogenic factor in lung with vasoconstrictive and angiogenic properties

In a mouse chronic hypoxia model of pulmonary hypertension, we discovered a novel hypoxia-inducible gene in lung, FIZZ1/RELMalpha, first through a cDNA array analysis and then confirmed by RT-PCR. Western blot and immunohistochemistry revealed that its expression was induced by hypoxia only in lung. The hypoxia-upregulated gene expression was located in the pulmonary vasculature, bronchial epithelial cells, and type II pneumocytes. 3H-thymidine incorporation demonstrated that the recombinant protein stimulated rat pulmonary microvascular smooth muscle cell (RPSM) proliferation dose-dependently ranging from 3.3x10(-9) to 3.3x10(-8) mol/L. Therefore, we renamed this gene as hypoxia-induced mitogenic factor (HIMF). HIMF strongly activated Akt phosphorylation. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (10 micromol/L) inhibited HIMF-activated Akt phosphorylation. It also inhibited HIMF-stimulated RPSM proliferation. Thus, the PI3K/Akt pathway, at least in part, mediates the proliferative effect of HIMF. Further studies showed that HIMF had angiogenic and vasoconstrictive properties. HIMF increased pulmonary arterial pressure and vascular resistance more potently than either endothelin-1 or angiotensin II.

Resistin: molecular history and prognosis

Obesity and diabetes have reached epidemic proportions worldwide. The antidiabetic thiazolidinedione (TZD) drugs are insulin-sensitizing agents now widely used in the treatment of type 2 diabetes. TZDs are ligands for the nuclear hormone receptor peroxisome proliferator activated receptor gamma, which is a master regulator of adipogenesis and adipocyte metabolism. The molecular mechanisms by which TZDs improve insulin sensitivity have not been fully identified. Here we consider a novel secreted factor first identified as a TZD-suppressible gene in mouse adipocytes, called resistin, and discuss what is currently known about resistin regulation and function in mouse and human.

Reductions in plasma cholesterol levels after fenofibrate treatment are negatively correlated with resistin expression in human adipose tissue

The adipocyte-derived cytokine, resistin, has been proposed as the link between obesity and type 2 diabetes mellitus in murine models. In humans, resistin is identical to FIZZ3 (found in inflammatory zone 3), which belongs to a family of proteins that appears to be involved in inflammatory processes. To study the mechanisms by which fibrates improve glucose homeostasis, we determined resistin mRNA levels by using relative quantitative reverse-transcriptase-polymerase chain reaction (RT-PCR) in omental white adipose tissue samples obtained from patients treated with placebo or fenofibrate (200 mg/d) for 8 weeks before elective cholecystectomy. Fenofibrate treatment reduced total plasma cholesterol and low-density lipoprotein (LDL)-cholesterol levels by 24% and 35%, respectively. Compared with placebo values, a 2.4-fold induction in resistin mRNA levels was observed in white adipose tissue of fenofibrate-treated patients, whereas no changes were observed in the mRNA levels of the well-known perosixome proliferator-activated receptor (PPAR) target genes CD36, acyl-CoA oxidase, and carnitine palmitoyltransferase. These findings indicate that resistin changes were not related to PPAR activation by fenofibrate. Interestingly, resistin mRNA levels showed a negative correlation with plasma cholesterol levels (r2 =.53, P =.039, n = 8), but not with triglyceride levels (r2 =.02, P =.73, n = 8). These results suggest that cholesterol regulates resistin expression in human white adipose tissue.

The genomic organization of mouse resistin reveals major differences from the human resistin: functional implications

The resistin gene is a potential candidate for the etiology of insulin resistance and type 2 diabetes and has been implicated as the molecular link between type 2 diabetes and obesity. Unlike the mouse resistin, expression of the human resistin appears to be regulated differently. We report comparative analyses of the mouse and human genomic fragments encoding the resistin gene. At the amino acid level the two proteins exhibit 59% identity. While at the mRNA level the human resistin shows 64.4% sequence identity with its mouse counterpart, the mouse resistin genomic sequence displays only 46.7% sequence identity with the human resistin and is almost three times bigger than the human resistin. The intronic sequences per se displayed the least identities (28.7%), however the intron boundaries were highly conserved between human and mouse. The mouse resistin carries a very large intron in the 3' UTR, which has a number of regulatory sequences possibly involved in differential gene expression. Of particular significance is the presence of a PPAR/RXR heterodimer binding site within intron X (IntX-PPRE) which may possibly confer TZD responsiveness. Oligonucleotides carrying the authentic PPAR/RXR binding element (Aco-PPRE) as well as IntX-PPRE specifically bound factors (PPAR/RXR heterodimers) present in differentiated 3T3-L1 adipocyte cells in an electrophoretic mobility shift assay. IntX-PPRE oligonucleotide modulated the expression of the luciferase reporter gene in transient transfection assays using 3T3-L1 cells.

Adipose-derived resistin and gut-derived resistin-like molecule-beta selectively impair insulin action on glucose production

The adipose-derived hormone resistin is postulated to link obesity to insulin resistance and diabetes. Here, the infusion of either resistin or the resistin-like molecule-beta (RELMbeta) rapidly induced severe hepatic but not peripheral insulin resistance. In the presence of physiologic hyperinsulinemia, the infusion of purified recombinant resistin, increasing circulating resistin levels by approximately twofold to 15-fold, inhibited glucose metabolism such that lower rates of glucose infusion were required to maintain the plasma glucose concentration at basal levels. The effects of resistin and RELMbeta on in vivo insulin action were completely accounted for by a marked increase in the rate of glucose production. These results support the notion that a novel family of fat- and gut-derived circulating proteins modulates hepatic insulin action.

Insulin down-regulates resistin mRNA through the synthesis of protein(s) that could accelerate the degradation of resistin mRNA in 3T3-L1 adipocytes

AIMS/HYPOTHESIS

Resistin is a peptide secreted by adipocytes and recognized as a hormone that could link obesity to insulin resistance. This study was designed to examine the effect and mechanism(s) of insulin on resistin expression in 3T3-L1 adipocytes.

METHODS

Differentiated 3T3-L1 adipocytes were stimulated with insulin and resistin mRNA expression was examined by Northern blot analysis. In some experiments, the insulin signal was blocked by several chemical inhibitors or overexpression of a dominant negative form (Deltap85) of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase).

RESULTS

Insulin treatment caused a reduction of resistin mRNA in time-dependent and dose-dependent manners in 3T3-L1 adipocytes. Pre-treatment with PD98059, an inhibitor of extracellular signal-regulated kinase 1/2 (ERK1/2) pathway, or SB203580, an inhibitor of p38 mitogen-activated protein-kinase (p38 MAP-kinase) pathway, did not influence insulin-induced reduction of resistin mRNA. Inhibition of PI 3-kinase by LY294002 or Deltap85 also failed to block insulin-induced reduction of resistin mRNA. Cycloheximide, a protein synthesis inhibitor, completely blocked insulin-induced reduction of resistin mRNA. Actinomycin D, a RNA synthesis inhibitor, also blocked insulin-induced reduction of resistin mRNA, and the decreasing rate of resistin mRNA in cells treated with insulin alone was faster than that with actinomycin D.

CONCLUSION/INTERPRETATION

Insulin downregulates resistin mRNA via PI 3-kinase, ERK or p38 MAP-kinase independent pathways in 3T3-L1 adipocytes. The downregulation mechanism of resistin mRNA by insulin would be an indirect event through the synthesis of novel protein(s) that could accelerate the degradation of resistin mRNA.

Macrophages in chronic type 2 inflammation have a novel phenotype characterized by the abundant expression of Ym1 and Fizz1 that can be partly replicated in vitro

Using a murine model of nematode infection, we have discovered macrophages that display a novel phenotype that may be characteristic of macrophages in chronic type 2 inflammation. These nematode-elicited macrophages (NeMphi) are characterized by two unique features: the ability to actively suppress proliferation of a broad range of cell types and the high level expression of two novel macrophage genes, Ym1 and Fizz1. NeMphi also show some similarities with in vitro-derived 'alternatively activated macrophages' such as the downregulation of inflammatory cytokines. We therefore investigated how much of the phenotype discovered in vivo could be replicated by activation with Th2 cytokines in vitro. Fizz1 and Ym1 were upregulated by IL-4 and IL-13 in vitro but at a considerably lower level than in NeMphi. In vitro treatment with IL-4 could also partly replicate the ability of NeMphi to block cellular proliferation. As well as the quantitative differences in gene expression and suppressive phenotype, we also observed phenotypic differences in the cell morphology between macrophages activated in vivo and in vitro. Although this study illustrated that macrophages activated in chronic inflammation have distinct features that cannot be readily reproduced in vitro it also demonstrated that some features of the complex NeMphi phenotype can be replicated by treatment of cultured macrophages with Th2 cytokines. In future, we hope to use in vitro analysis to help define the pathways that lead to this distinctive in vivo macrophage phenotype.

Adipose-derived resistin and gut-derived resistin-like molecule-beta selectively impair insulin action on glucose production

The adipose-derived hormone resistin is postulated to link obesity to insulin resistance and diabetes. Here, the infusion of either resistin or the resistin-like molecule-beta (RELMbeta) rapidly induced severe hepatic but not peripheral insulin resistance. In the presence of physiologic hyperinsulinemia, the infusion of purified recombinant resistin, increasing circulating resistin levels by approximately twofold to 15-fold, inhibited glucose metabolism such that lower rates of glucose infusion were required to maintain the plasma glucose concentration at basal levels. The effects of resistin and RELMbeta on in vivo insulin action were completely accounted for by a marked increase in the rate of glucose production. These results support the notion that a novel family of fat- and gut-derived circulating proteins modulates hepatic insulin action.

Variation in resistin gene promoter not associated with polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a leading cause of anovulatory infertility and affects approximately 4-7% of reproductive age women in the U.S. It is characterized by hyperandrogenemia and chronic anovulation and is associated with insulin resistance, obesity, and increased risk for type 2 diabetes. In a screen of candidate genes, a region on chromosome 19p13.3 was identified that shows significant evidence for both linkage and association with PCOS. A promising candidate gene for PCOS, resistin, maps to exactly this region. Resistin is a protein hormone thought to modulate glucose tolerance and insulin action. We tested for association between a single nucleotide polymorphism in the promoter region of the resistin gene and three phenotypes: PCOS, obesity, and insulin resistance. We did not find evidence for association with any of the phenotypes. It is therefore unlikely that variation in the resistin gene accounts for the strong association that we observe between chromosome 19p13.3 and PCOS. Instead, this association is most likely due to a gene or genetic element in this region that has not been identified.

Inhibition of adipocyte differentiation by resistin-like molecule alpha. Biochemical characterization of its oligomeric nature

A novel family of cysteine-rich secreted proteins with unique tissue distribution has recently been identified. One of the members, resistin (for "resistance to insulin"), also called FIZZ3, was identified in a screen for molecules that are down-regulated in mature adipocytes upon administration of thiazolidinediones. The prototypical member of this family was originally identified from bronchoalveolar lavage fluid of inflamed lungs and designated FIZZ1 ("found in inflammatory zone"). This molecule was also found to be highly expressed in adipose tissue and was named resistin-like molecule alpha (RELMalpha). Here we demonstrate that RELMalpha inhibits the differentiation of 3T3-L1 preadipocytes into adipocytes. RELMalpha has no effect on proliferation of 3T3-L1 preadipocytes. Pretreatment of 3T3-L1 preadipocytes with RELMalpha does not affect insulin- or platelet-derived growth factor-induced mitogenesis. IRS-1 phosphorylation and glucose transport stimulated by insulin in mature adipocytes were also unaffected by RELMalpha. We show that RELMalpha forms disulfide-linked homooligomers based on results from electrophoresis under reducing and nonreducing conditions, coimmunoprecipitation experiments as well as by mass spectrometry. In addition, RELMalpha is able to form heterooligomers with resistin but not RELMbeta. Since RELMalpha is expressed by adipose tissue and it is a secreted factor, our findings suggest that RELMalpha may be involved in the control of the adipogenesis as well as in the process of muscle differentiation.

Resistin and adiponectin expression in visceral fat of obese rats: effect of weight loss

OBJECTIVE

Obesity-related insulin resistance is closely associated with visceral fat accumulation. Several adipocyte-secreted molecules have been implicated in the development of type 2 diabetes, among them, the recently discovered adiponectin and resistin proteins. Some of these adipocytokines are also present in the immune system, thus suggesting an intriguing functional connection.

RESEARCH METHODS AND PROCEDURES

We determined adiponectin and resistin expressions in visceral (VAT) and subcutaneous adipose tissue of lean and obese Zucker (fa/fa) rats using reverse-transcription polymerase chain reaction. Moreover, we analyzed the variations after body-weight reduction in food-restricted obese rats.

RESULTS

Resistin and adiponectin expression was significantly lower in VAT of genetically obese in comparison with lean rats; no differences were observed when subcutaneous adipose tissues of the same animals were compared. Weight loss resulted in an increase of adiponectin expression in VAT, whereas a further significant decrease in resistin mRNA level was observed. Resistin is also present and equally expressed in splenocytes of lean and obese rats.

DISCUSSION

Adiponectin and resistin are down-regulated in VAT of obese rats. Adiponectin expression is restored to normal levels after body-weight reduction, supporting its link with obesity-related insulin resistance. On the contrary, the further decrease of resistin mRNA after weight loss does not support the hypothesis that resistin may play a causative role in insulin resistance in obese rats. Moreover, we demonstrated the presence of resistin in immunocompetent cells in both humans and rats, thus adding another factor to the list of molecules that adipose tissue shares with the immune system.

The metabolic abnormalities associated with non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a common disorder in the Western hemisphere. It encompasses two histological lesions: fatty liver and steatohepatitis. A large body of literature indicates that insulin resistance is a key pathophysiological abnormality in patients with NAFLD. Insulin resistance results from a complex interplay between the major targets of insulin action, i.e. muscle, adipose tissue and liver, versus the ability of the pancreatic islet beta cells to compensate for insulin resistance by increasing insulin production. The metabolic and clinical profile associated with insulin resistance is thus defined by the factors that produce and maintain insulin resistance and the effects of decreased insulin sensitivity on various insulin-dependent pathways. The major metabolic defects associated with insulin resistance are increased peripheral lipolysis, increased hepatic glucose output due to increased gluconeogenesis and increased lipid oxidation. This is associated with an oxidative stress in the liver that may be compounded by additional pathophysiological abnormalities. While much work remains to be done, the current understanding of the pathogenesis of NAFLD provides direction for both future investigation and development of therapeutic trials.

Insulin resistance as the core defect in type 2 diabetes mellitus

Insulin resistance is a major contributor to the pathogenesis of type 2 diabetes and plays a key role in associated metabolic abnormalities, such as dyslipidemia and hypertension. Obesity, especially visceral adiposity, is negatively correlated with insulin sensitivity. The release of free fatty acids from adipocytes can block insulin-signaling pathways and lead to insulin resistance. In addition, recently identified adipocyte-specific chemical messengers, the adipocytokines, such as tumor necrosis factor-alpha, adiponectin, and resistin, appear to modulate the underlying insulin resistance. When insulin resistance is combined with beta-cell defects in glucose-stimulated insulin secretion, impaired glucose tolerance, hyperglycemia, or type 2 diabetes can result. The thiazolidinediones are potent peroxisome proliferator-activated receptor-gamma agonists and directly improve insulin resistance and glycemic control in patients with type 2 diabetes. Increasing evidence supports the early use of thiazolidinediones for preventing, delaying, or treating diabetes by improving insulin sensitivity and beta-cell insulin secretion.

Genetic variants at the resistin locus and risk of type 2 diabetes in Caucasians

Resistin is a newly identified hormone secreted by adipocytes that inhibits insulin action on peripheral tissues. The aim of our study was to investigate whether genetic variability at this locus is associated with the risk of type 2 diabetes. By sequencing 32 subjects with type 2 diabetes, we identified 8 single nucleotide polymorphisms (SNPs) in the 5'-flanking region and introns of the resistin gene. Allele and genotype distributions were determined for all 8 SNPs in 312 cases with type 2 diabetes and 303 nondiabetic controls, all of Caucasian origin. No significant association with type 2 diabetes was found at any of the polymorphic loci. However, an interactive effect of genotype at SNP 6 (IVS2 + 181G-->A) and obesity was a significant determinant of type 2 diabetes risk in this population. The relative risk of diabetes for the A/A genotype was 4.8 (95% confidence interval, 1.1-21.0) in individuals above the median for body weight, but only 0.7 (95% confidence interval, 0.2-2.1) in those below the median. This difference between relative risks was significant (chi(2) = 4.5; P = 0.03). A similar, but much weaker, interaction with obesity was observed for SNPs in linkage disequilibrium with SNP6. In conclusion, resistin does not appear to be a major gene for type 2 diabetes. However, our data suggest a synergistic effect of sequence differences at the resistin locus and obesity on risk of type 2 diabetes. Further studies are needed to confirm this finding in other populations.

Resistin expression and regulation in mouse pituitary

Resistin, a new adipocytokine, is expressed in human, rat and mouse adipose tissue. Its putative role as a mediator of insulin resistance is controversial. We hypothesized that resistin, in common with leptin, has multiple roles in non-adipose tissues. Using reverse transcription polymerase chain reaction (RT-PCR) we show that the resistin gene (Retn) is expressed in mouse brain (hypothalamus and cortex) and pituitary gland. Immunohistochemistry revealed resistin protein in the arcuate nucleus and pituitary gland. Semi-quantitative RT-PCR analysis indicated that Retn mRNA is developmentally regulated in the pituitary. Expression was lowest at birth, increased abruptly between postnatal days 14 and 25 (four-fold; P<0.001), and declined thereafter. This peak in pituitary Retn mRNA was unaffected by early weaning but was abolished by neonatal treatment with monosodium glutamate, suggesting that the basal hypothalamus regulates pituitary Retn. Although the role(s) of endogenous resistin in mouse brain and pituitary remains to be determined, it may be distinct from its controversial involvement in insulin resistance. Our data suggest that local resistin expression could have functional implications during prepubertal maturation of the hypothalamic-pituitary system.

Resistin and RELM-alpha gene expression in white adipose tissue of lactating mice

The adipose tissue-derived factor resistin has been suggested to induce insulin resistance in obesity and inhibit adipocyte differentiation. Lactation is associated with major metabolic adaptations, which cause a profound loss of adipose tissue to support milk production by the mammary gland. This study has examined the expression of the resistin and resistin-like molecule-alpha (RELM-alpha) genes in white adipose tissue of lactating mice. Lactation induced significant increases in food intake and body weight, whereas body fat was substantially decreased at peak lactation. Resistin and RELM-alpha mRNAs were both detectable in gonadal, subcutaneous, and mammary gland fat; mRNA level was highest in gonadal fat and lowest in mammary tissue. There was no difference in resistin mRNA level in gonadal fat of lactating mice compared with controls. However, RELM-alpha mRNA fell by approximately 40% in early lactation and there was a parallel fall in the leptin mRNA level proportional to the loss of fat mass. These results indicate that the substantial reduction of adiposity in lactation does not lead to any changes in resistin gene expression; however, the fall in RELM-alpha mRNA might indicate a role for RELM-alpha in the metabolic adaptations of lactation.

Cell type-specific expression and coregulation of murine resistin and resistin-like molecule-alpha in adipose tissue

Adipocytes are the exclusive or predominant source of several secreted proteins that exert profound effects on systemic carbohydrate and lipid metabolism. Resistin, a 10-kDa adipose tissue specific secretory protein, has recently been implicated in exerting a negative effect on systemic insulin sensitivity. It is, however, not known how resistin mediates this insulin-desensitizing effect or what regulatory mechanisms control resistin expression. Resistin-like molecule-alpha (RELMalpha), a homolog of resistin originally identified by its upregulation in asthmatic lung, is another secreted protein expressed in adipose tissue. The regulation of RELMalpha in adipose tissue and its relationship to resistin expression has not been addressed so far. Here, we demonstrate that the expression of resistin and RELMalpha are similarly regulated in adipose tissue despite the fact that RELMalpha is exclusively expressed in the stromal vascular fraction of adipose tissue and not in adipocytes. Interestingly, this coregulation is limited to adipose tissue as the expression of RELMalpha in lung is independent of metabolic regulation. Additionally, we show that resistin and RELMalpha levels are not subject to regulation by proinflammatory stimuli. Finally, acute hyperglycemia leads to up-regulation of resistin and RELMalpha transcription in various adipose depots.

IL-4 dependent alternatively-activated macrophages have a distinctive in vivo gene expression phenotype

BACKGROUND

"Alternatively-activated" macrophages are found in Th2-mediated inflammatory settings such as nematode infection and allergic pulmonary inflammation. Due in part to a lack of markers, these cells have not been well characterized in vivo and their function remains unknown.

RESULTS

We have used murine macrophages elicited by nematode infection (NeM(phi)) as a source of in vivo derived alternatively activated macrophages. Using three distinct yet complementary molecular approaches we have established a gene expression profile of alternatively activated macrophages and identified macrophage genes that are regulated in vivo by IL-4. First, genes abundantly expressed were identified by an expressed sequence tag strategy. Second, an array of 1176 known mouse genes was screened for differential expression between NeM(phi) from wild type or IL-4 deficient mice. Third, a subtractive library was screened to identify novel IL-4 dependent macrophage genes. Differential expression was confirmed by real time RT-PCR analysis.

CONCLUSIONS

Our data demonstrate that alternatively activated macrophages generated in vivo have a gene expression profile distinct from any macrophage population described to date. Several of the genes we identified, including those most abundantly expressed, have not previously been associated with macrophages and thus this study provides unique new information regarding the phenotype of macrophages found in Th2-mediated, chronic inflammatory settings. Our data also provide additional in vivo evidence for parallels between the inflammatory processes involved in nematode infection and allergy.

Human resistin gene: molecular scanning and evaluation of association with insulin sensitivity and type 2 diabetes in Caucasians

Insulin resistance is strongly associated with obesity, but even among obese subjects insulin sensitivity varies widely. Recently, a new adipocyte hormone, resistin, was identified, shown to reduce insulin-mediated glucose uptake, and shown to be increased in obese mice. We used the chromosome 19 draft sequence to determine the genomic structure of human resistin and to screen the exons, introns, and flanking sequences for variation. We screened 44 subjects with type 2 diabetes and 20 nondiabetic family members who were at the extremes of insulin sensitivity. We identified eight noncoding single nucleotide polymorphisms (SNPs) and one GAT microsatellite repeat. Three SNPs, which were in incomplete linkage disequilibrium with each other and had allelic frequencies exceeding 5%, were selected for further study. No SNP was associated with type 2 diabetes, but the SNP in the promoter region was a significant determinant of insulin sensitivity index (P = 0.04) among nondiabetic family members who had undergone iv glucose tolerance tests. The three common SNPs showed statistical significance as determinants of insulin sensitivity index (P < 0.01) in interaction with body mass index. Noncoding SNPs in the resistin gene may influence insulin sensitivity in interaction with obesity, but this finding will need to be confirmed in other populations.

Mechanisms regulating adipocyte expression of resistin

Resistin, also known as Adipocyte Secreted Factor (ADSF) and Found in Inflammatory Zone 3 (FIZZ3), is a mouse protein with potential roles in insulin resistance and adipocyte differentiation. The resistin gene is expressed almost exclusively in adipocytes. Here we show that a proximal 264-base pair fragment of the mouse resistin promoter is sufficient for expression in adipocytes. Ectopic expression of the adipogenic transcription factor CCAAT/enhancer-binding protein (C/EBPalpha) was sufficient for expression in non-adipogenic cells. C/EBPalpha binds specifically to a site that is essential for expression of the resistin promoter. Chromatin immunoprecipitation studies of the endogenous gene demonstrated adipocyte-specific association of C/EBPalpha with the proximal resistin promoter in adipocytes but not preadipocytes. C/EBPalpha binding was associated with the recruitment of coactivators p300 and CREB-binding protein and a dramatic increase in histone acetylation in the vicinity of the resistin promoter. The antidiabetic thiazolidinedione (TZD) drug rosiglitazone reduced resistin expression with an ED(50) similar to its K(d) for binding to peroxisome proliferator activated receptor gamma (PPARgamma). Other TZD- and non-TZD PPARgamma ligands also down-regulated resistin expression. However, no functional PPARgamma binding site was found within 6.2 kb of the transcriptional start site, suggesting that if PPARgamma is involved, it is either acting at a long distance from the start site, in an intron, or indirectly. Nevertheless, rosiglitazone treatment selectively decreased histone acetylation at the resistin promoter without a change in occupation by C/EBPalpha, CREB-binding protein, or p300. Thus, adipocyte specificity of resistin gene expression is because of C/EBPalpha binding, leading to the recruitment of transcriptional coactivators and histone acetylation that is characteristic of an active chromatin environment. TZD reduces resistin gene expression at least in part by reducing histone acetylation associated with the binding of C/EBPalpha in mature adipocytes.

5' flanking variants of resistin are associated with obesity

Diabetes and obesity have long been known to be related. The recently characterized adipocyte hormone resistin (also called FIZZ3/ADSF) has been implicated as a molecular link between impaired glucose tolerance (IGT) and obesity in mice. A search for sequence variants at the human resistin locus identified nine single-nucleotide polymorphisms (SNPs) but no coding variants. An investigation into the association of these SNPs with diabetes and obesity revealed two 5' flanking variants (g.-537 and g.-420), in strong linkage disequilibrium, that are associated with BMI. In nondiabetic individuals from the Quebec City area and the Saguenay-Lac-St-Jean region of Quebec, the g.-537 mutation (allelic frequency = 0.04) was significantly associated with an increase in BMI (P = 0.03 and P = 0.01, respectively). When the data from these two populations were combined and adjusted for age and sex, both the g.-537 (odds ratio [OR] 2.72, 95% CI 1.28-5.81) and the g.-420 variants (1.58, 1.06-2.35) were associated with an increased risk for a BMI > or =30 kg/m(2). In contrast, in case/control and family-based study populations from Scandinavia, we saw no effect on BMI with either of these promoter variants. No association was seen with diabetes in any of the population samples.

Peroxisome proliferator-activated receptor gamma (PPARgamma) and its ligands: a review

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of a class of nuclear hormone receptors intimately involved in the regulation of expression of myriad genes that regulate energy metabolism, cell differentiation, apoptosis and inflammation. Although originally discovered as a pivotal regulator of adipocyte differentiation, the roles that this transcription factor play in physiology and pathophysiology continue to grow as researchers discover its influence in the function of many cell types. This review highlights the roles that PPARgamma play in the regulation of gene expression associated with normal cell physiology as well as the pathophysiology of multiple diseases including obesity, diabetes and cancer. Additionally, naturally occurring and pharmaceutical ligands for the receptor as well as the potential role of PPARgamma as the receptor responsible for fatty acid-induced effects on gene expression will be described.

Systematic search for single nucleotide polymorphisms in the resistin gene: the absence of evidence for the association of three identified single nucleotide polymorphisms with Japanese type 2 diabetes

Resistin is a novel polypeptide specifically secreted from adipocytes, and its serum levels are increased in obese diabetic mice. Resistin antagonizes insulin and could account for insulin resistance. To determine whether there are single nucleotide polymorphisms (SNPs) in the resistin gene associated with type 2 diabetes, sequences for 24 Japanese type 2 diabetic patients were initially analyzed using PCR direct sequencing. Three SNPs were found in the introns, but none were present in the coding regions. The allele frequencies of genomic -167C>T, +157C>T, and +299G>A in 99 Japanese control subjects were determined to be 3.5, 6.6, and 39.4%, respectively. In each pair of these SNPs, linkage disequilibria were found between either -167C>T and +299G>A or +157C>T and +299G>A. A linkage disequilibrium was also detected among -167C>T, +157C>T, and +299G>A, and only four of the eight possible haplotypes defined by these SNPs were found. A comparison of the frequencies of these SNPs and haplotypes between 99 type 2 diabetes and 99 control subjects revealed no evidence for any association. These identified SNPs, which were in linkage disequilibrium, represent potentially useful tools for searching for their association with specific phenotypes of diabetes.

Peroxisome proliferator-activated receptors

An exciting and rapidly evolving area in vascular biology and atherosclerosis research over the past 3 years has been the establishment of peroxisome proliferator-activated receptor (PPAR) expression in the vascular and inflammatory cells, and the emerging picture of the roles these ligand-activated nuclear receptor/transcription factors might play in vascular biology and atherosclerosis. Such work is all the more compelling given the ongoing clinical use of PPAR activators in patients. Thiazolidinediones (PPAR-g agonists) are used as insulin sensitizers in diabetic patients known to be at extraordinarily high risk for cardiovascular disease, whereas fibrates (PPAR-a agonists) are used to treat dyslipidemia, particularly in the case of high triglycerides and low high-density lipoprotein cholesterol.

Resistin and obesity-associated insulin resistance

Obesity is a major risk factor for insulin resistance and type 2 diabetes mellitus. Adipocytes secrete numerous substances that might contribute to peripheral insulin sensitivity. These include leptin, tumor necrosis factor alpha, Acrp30/adiponectin/adipoQ and interleukin 6, the potential roles of which are briefly reviewed here. Thiazolidinedione (TZD) antidiabetic drugs regulate gene transcription by binding to peroxisome proliferator activated receptor gamma, a nuclear hormone receptor found at its highest levels in adipocytes. A search for genes that are downregulated by TZDs in mouse adipocytes led to the discovery of an adipose-specific secreted protein called resistin. Resistin circulates in the mouse, with increased levels in obesity, and has effects on glucose homeostasis that oppose those of insulin. Thus, resistin is a potential link between TZDs, obesity and insulin resistance in the mouse. Future studies must address the mechanism of action and biological role of resistin and related family members in mice and humans.

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.

Insulin signalling and the regulation of glucose and lipid metabolism

The epidemic of type 2 diabetes and impaired glucose tolerance is one of the main causes of morbidity and mortality worldwide. In both disorders, tissues such as muscle, fat and liver become less responsive or resistant to insulin. This state is also linked to other common health problems, such as obesity, polycystic ovarian disease, hyperlipidaemia, hypertension and atherosclerosis. The pathophysiology of insulin resistance involves a complex network of signalling pathways, activated by the insulin receptor, which regulates intermediary metabolism and its organization in cells. But recent studies have shown that numerous other hormones and signalling events attenuate insulin action, and are important in type 2 diabetes.

Adipocyte differentiation: from fibroblast to endocrine cell

Recent advances regarding the biology of adipose tissue have demonstrated that white adipose tissue (WAT) plays a central role in the regulation of energy balance and acts as a secretory/endocrine organ that mediates numerous physiological and pathological processes. Dysregulation of WAT mass causes obesity or lipoatrophy, two disorders associated with life-threatening pathologies, including cardiovascular diseases and diabetes. Alterations in WAT mass result from changes in adipocyte size and/or number. Change in adipocyte number is achieved through a complex interplay between proliferation and differentiation of preadipocytes. Adipocyte differentiation or adipogenesis is a highly controlled process that has been extensively studied for the last 25 years. In vitro preadipocyte culture systems that recapitulate most of the critical aspects of fat cell formation in vivo have allowed a meticulous dissection of the cellular and molecular events involved in the adipogenesis process. The adipogenic transcription factors peroxisome proliferator-activated receptor-gamma and CCAAT/enhancer binding protein-alpha play a key role in the complex transcriptional cascade that occurs during adipogenesis. Hormonal and nutritional signaling affects adipocyte differentiation in a positive or negative manner, and components involved in cell-cell or cell-matrix interactions are also pivotal in regulating the differentiation process. This knowledge provides a basis for understanding the physiological and pathophysiological mechanisms that underlie adipose tissue formation and for the development of novel and sound therapeutic approaches to treat obesity and its related diseases.

Signals of adiposity

Adipose tissue, a reserve of energy, has played an essential role in mammalian evolution. Adipose tissue differs from other tissues in that its mass has considerable capacity to expand, which while beneficial in decreasing the risk of starvation, increases the risk of predation. Adipose tissue mass is thus under tight control in nondomestic species. Adipose tissue secretes a variety of factors, some of which (leptin, tumor necrosis factor (TNF) alpha, resistin) are thought to be involved in modulation of adipose mass. Leptin has a variety of functions, primarily targetting the hypothalamus where it acts to decrease appetite and increase energy expenditure. Leptin is also involved in the adaptations to fasting, and leptin is also required for normal reproductive and immune function. TNF alpha and resistin appear to have key paracrine roles, attenuating the anabolic effects of insulin on adipose tissue metabolism.

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.

Independent influences of central fat and skeletal muscle lipids on insulin sensitivity

OBJECTIVE

Insulin resistance is closely associated with two disparate aspects of lipid storage: the intracellular lipid content of skeletal muscle and the magnitude of central adipose beds. Our aim was to determine their relative contribution to impaired insulin action.

RESEARCH METHODS AND PROCEDURES

Eighteen older (56 to 75 years of age) men were studied before elective knee surgery. Insulin sensitivity (M/Delta I) was determined by hyperinsulinemic-euglycemic clamp. Central abdominal fat (CF) was assessed by DXA. Skeletal muscle was excised at surgery and assayed for content of metabolically active long-chain acyl-CoA esters (LCAC).

RESULTS

Significant inverse relationships were observed between LCAC and M/Delta I (R(2) = 0.34, p = 0.01) and between CF and M/Delta I (R(2) = 0.38, p = 0.006), but not between CF and LCAC (R(2) = 0.0005, p = 0.93). In a multiple regression model (R(2) = 0.71, p < 0.0001), both CF (p = 0.0006) and LCAC (p = 0.0009) were independent statistical predictors of M/Delta I. Leptin levels correlated inversely with M/Delta I (R(2) = 0.60, p = 0.0002) and positively with central (R(2) = 0.41, p = 0.006) and total body fat (R(2) = 0.63, p = 0.0001).

DISCUSSION

The mechanisms by which altered lipid metabolism in skeletal muscle influences insulin action may not be related directly to those linking central fat and insulin sensitivity. In particular, it is unlikely that muscle accumulation of lipids directly derived from labile central fat depots is a principal contributor to peripheral insulin resistance. Instead, our results imply that circulating factors, other than nonesterified fatty acids or triglyceride, mediate between central fat depots and skeletal muscle tissue. Leptin was not exclusively associated with central fat, but other factors, secreted specifically from central fat cells, could modulate muscle insulin sensitivity.

Rationale for the existence of additional adipostatic hormones

Parabiosis studies with obese rodents demonstrated that circulating factors are involved in the long-term control of food intake and energy balance. More than 40 years ago it was hypothesized that rats made obese by hypothalamic or dietary means, as well as genetically obese fa/fa rats and db/db mice, produce a circulating factor that either inhibits food intake or acts metabolically to reduce the fat content of non-obese ad libitum-fed partners. However, none of these obese rodents showed a significant change in weight when parabiosed to a normal animal. It was therefore postulated that these obese rodents produced a circulating lipostatic factor but were unable to respond to it. In contrast, genetically obese ob/ob mice were thought to be deficient in the circulating signal, as they lost weight when parabiosed to lean or obese db/db mice. The discovery of leptin suggested that the circulating lipostatic signal had been identified. However, a closer look at the outcome of the parabiotic studies reveals that leptin alone does not explain all of the findings of the parabiotic experiments. Another (or more than one) as yet unidentified factor(s) may be involved in energy balance regulation. The evidence for the existence of further leptin-like hormones comes from observations in which the direct effect of leptin has been eliminated or can be excluded.