A cysteine-rich adipose tissue-specific secretory factor inhibits adipocyte differentiation

Increased resistin expression in the adipose tissue of male prolactin transgenic mice and in male mice with elevated androgen levels

The aim of this study was to investigate the regulation of resistin, a recently identified adipocyte-secreted peptide, in the adipose tissue of prolactin (PRL)-transgenic (tg) mice using ribonuclease protection assay. The level of resistin mRNA increased 3.5-fold in the adipose tissue of untreated male PRL-tg mice compared to controls. However, there was no difference in resistin expression in the adipose tissue of female PRL-tg mice compared to control mice. PRL-tg male mice have elevated serum testosterone levels and we therefore analyzed the effects of testosterone alone on resistin mRNA expression. Furthermore, the effects of elevated androgen levels on PRL receptor (PRLR) mRNA expression in the adipose tissue were investigated. Resistin mRNA increased 2.6-fold in the adipose tissue of control male mice with elevated serum androgen levels. In addition, PRLR mRNA expression was increased in the adipose tissue of male mice with elevated testosterone. These results suggest testosterone to be a regulator of resistin and PRLR mRNA expression in the adipose tissue of male mice.

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.

Resistin / Fizz3 expression in relation to obesity and peroxisome proliferator-activated receptor-gamma action in humans

Recent studies in murine models suggest that resistin (also called Fizz3 [1]), a novel cysteine-rich protein secreted by adipocytes, may represent the long-sought link between obesity and insulin resistance (2). Furthermore, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists appear to inhibit resistin expression in murine adipocytes, providing a possible explanation for the mode of action of this class of insulin sensitizers (2). Using a fluorescent real-time reverse transcriptase-polymerase chain reaction-based assay, we found that resistin mRNA levels in whole adipose tissue samples were increased in morbidly obese humans compared with lean control subjects. However, in freshly isolated human adipocytes, resistin mRNA levels were very low and showed no correlation with BMI. Resistin mRNA was undetectable in preadipocytes, endothelial cells, and vascular smooth muscle cells, but it was readily detectable in circulating mononuclear cells. Although exposure of human mononuclear cells to PPAR-gamma agonists markedly upregulated fatty acid-binding protein-4 expression, these agents had no effect on mononuclear cell resistin expression. Finally, resistin mRNA was undetectable in adipocytes from a severely insulin-resistant subject with a dominant-negative mutation in PPAR-gamma (3). We conclude that the recently described relationships of murine resistin/Fizz3 expression with obesity, insulin resistance, and PPAR-gamma action may not readily translate to humans. Further studies of this novel class of proteins are needed to clarify their roles in human metabolism.

Expression of p68 RNA helicase is closely related to the early stage of adipocyte differentiation of mouse 3T3-L1 cells

We have previously identified the genes expressed early in the differentiation of mouse 3T3-L1 cells into adipocytes. Since these genes were isolated as small fragments, many were unknown. In this study, we have cloned two full-length cDNAs and identified them as p68 RNA helicase and mc3s5/mtCLIC. The expression of these genes was rapidly induced, and specific to the adipocyte differentiation. When the expression of p68 RNA helicase was inhibited using an inducible antisense system, the differentiation into adipocytes was partially blocked, and the expression levels of some marker genes decreased. These findings strongly indicate that the expression of the above two genes was closely related to the adipocyte differentiation, and p68 RNA helicase in particular is crucial to the differentiation.

Adiponutrin, a transmembrane protein corresponding to a novel dietary- and obesity-linked mRNA specifically expressed in the adipose lineage

We have used a mRNA differential display technique to identify new genes involved in the reprogramming of gene expression during the adipose conversion of mouse 3T3 preadipocyte cell lines. We report here on the identification and cloning of a novel adipose-specific cDNA encoding a predicted membrane protein of 413 amino acids. The level of the corresponding 3.2-kilobase mRNA is tremendously increased during 3T3-L1 and 3T3-F442A differentiation into adipocytes. A single, very abundant 3.2-kilobase transcript is also found in inguinal and epididymal white adipose tissues and in interscapular brown adipose tissue but not in any other tissues examined. Its expression in adipose tissue is under tight nutritional regulation. The level of this novel 3.2-kilobase transcript becomes virtually nondetectable during fasting but is dramatically increased when fasted mice are refed a high carbohydrate diet. Based on its adipose specificity and dietary regulation, the novel gene product has been designated adiponutrin. The expression of adiponutrin mRNA is also 50-fold elevated in genetically obese fa/fa rats, indicating a link between adiponutrin and obesity. Western blot and confocal imagery analyses with epitope-tagged protein transiently expressed in 3T3-L1 adipocytes, and COS cells show that adiponutrin strictly localizes to membranes and is absent from the cytosol. Sequence analysis reveals homologies with several other members of related eukaryotic proteins, including a human paralog, which has been recently described in vesicular transport mechanisms. This leads us to suggest that adiponutrin could be involved in vesicular targeting and protein transport restricted to the adipocyte function.

Differential regulation of adipocytokine mRNAs by rosiglitazone in db/db mice

The precise mechanism by which PPARgamma activation by thiazolidinediones (TZDs) improves insulin sensitivity is still unclear. Recent studies have focused on the role of adipocytokines in metabolic control and their regulation by TZDs. In this study, we compared the chronic effects of antihyperglycemic doses of the TZD rosiglitazone, the beta3-adrenoceptor agonist BRL-35135, and the PPARalpha agonist Wy-14,643 on the mRNA expression of adipocytokines in WAT of db/db mice. Rosiglitazone treatment decreased adiponectin and resistin mRNA levels by 57 and 72%, respectively (P < 0.001), with no effect on the level of TNFalpha or RELMalpha transcripts. In comparison, Wy-14,643 reduced adiponectin transcript levels by 31% (P = 0.015) while BRL-35135 increased RELMalpha mRNA expression by 245% (P < 0.001) without effect on the other transcripts. Our results indicate that although a reduction in adiponectin and resistin mRNA levels in WAT by rosiglitazone treatment of diabetic mice may contribute to the antidiabetic effects, an alteration in TNFalpha, adiponectin, resistin, or RELMalpha mRNA expression is not absolutely required for the regulation of blood glucose concentration in the db/db mouse.

RGS2 promotes adipocyte differentiation in the presence of ligand for peroxisome proliferator-activated receptor gamma

The events at the earliest stage of adipocyte differentiation are yet to be fully elucidated. Previously, we cloned the genes that are induced at the beginning of the differentiation of mouse 3T3-L1 preadipocyte cells. We found that the gene expression of regulators of G protein signaling-2 (RGS2) rapidly increased after the addition of inducers and decreased at 3-12 h. The expression pattern of RGS2 mRNAs differed among growth-arrested and proliferating 3T3-L1 cells and NIH-3T3 cells, indicating a specificity for adipogenesis. Here we report that the ectopic expression of RGS2 using a retroviral system in mouse NIH-3T3 cells promotes adipogenesis only in the presence of BRL49653, which is a ligand for the peroxisome proliferator-activated receptor gamma (PPARgamma). These results strongly suggest that RGS2 play a crucial role in the program of adipocyte differentiation and may contribute to the function of PPARgamma.

Isoproterenol inhibits resistin gene expression through a G(S)-protein-coupled pathway in 3T3-L1 adipocytes

Resistin was recently identified as a hormone secreted by adipocytes which leads to insulin resistance in vivo and in vitro and might therefore be an important link between obesity and diabetes. To clarify the regulation of resistin gene expression, 3T3-L1 adipocytes were treated with various agents known to modulate insulin sensitivity, and resistin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, isoproterenol treatment reduced the level of resistin mRNA to 20% of non-treated control cells. This effect was dose-dependent with significant inhibition occurring at concentrations as low as 10 nM isoproterenol. Moreover, pretreatment of adipocytes with the beta-adrenergic antagonist propranolol almost completely reversed the inhibitory effect of isoproterenol, whereas addition of the alpha-adrenergic antagonist phentolamine did not have any effect. Furthermore, the effect of isoproterenol could be mimicked by activation of G(S)-proteins and adenylyl cyclase. Thus, both cholera toxin and forskolin decreased resistin mRNA expression in a dose-dependent fashion by up to 90% of control levels. Taken together, these results suggest that resistin gene expression is regulated by a protein kinase A-dependent pathway in 3T3-L1 adipocytes.

Adipose tissue resistin expression is severely suppressed in obesity and stimulated by peroxisome proliferator-activated receptor gamma agonists

Elevated levels of the hormone resistin, which is secreted by fat cells, are proposed to cause insulin resistance and to serve as a link between obesity and type 2 diabetes. In this report we show that resistin expression is significantly decreased in the white adipose tissue of several different models of obesity including the ob/ob, db/db, tub/tub, and KKA(y) mice compared with their lean counterparts. Furthermore, in response to several different classes of antidiabetic peroxisome proliferator-activated receptor gamma agonists, adipose tissue resistin expression is increased in both ob/ob mice and Zucker diabetic fatty rats. These data demonstrate that experimental obesity in rodents is associated with severely defective resistin expression, and decreases in resistin expression are not required for the antidiabetic actions of peroxisome proliferator-activated receptor gamma agonists.

Dimerization of resistin and resistin-like molecules is determined by a single cysteine

Resistin is a peptide hormone secreted by adipocytes. Cysteine residues comprise 11 of 94 (12%) amino acids in resistin. The arrangement of these cysteines is unique to resistin and its recently discovered family of tissue-specific secreted proteins, which have been independently termed resistin-like molecules (RELMs) and the FIZZ (found in inflammatory zone) family. Here we show that resistin is a disulfide-linked homodimer that can be converted to a monomer by reducing conditions. The intestine-specific RELM beta has similar characteristics. Remarkably, however, the adipose-enriched RELM alpha is a monomer under non-reducing conditions. We note that RELM alpha lacks a cysteine residue, closest to the cleaved N terminus, that is present in resistin and RELM beta in multiple species. Conversion of this cysteine to alanine abolishes dimerization of resistin. Thus, a single disulfide bond is necessary to connect two resistin subunits in a homodimer. The additional 10 cysteines most likely participate in intramolecular disulfide bonds that define the conserved structure of the family members. The monomeric nature of RELM alpha suggests structural and potentially functional divergence between resistin and this close family member.