Differential dimerization and association among resistin family proteins with implications for functional specificity

Adipocyte-specific ablation of the Ca2+ pump SERCA2 impairs whole-body metabolic function and reveals the diverse metabolic flexibility of white and brown adipose tissue

OBJECTIVE

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) transports Ca²⁺ from the cytosol into the ER and is essential for appropriate regulation of intracellular Ca²⁺ homeostasis. The objective of this study was to test the hypothesis that SERCA pumps are involved in the regulation of white adipocyte hormone secretion and other aspects of adipose tissue function and that this control is disturbed in obesity-induced type-2 diabetes.

METHODS

SERCA expression was measured in isolated human and mouse adipocytes as well as in whole mouse adipose tissue by Western blot and RT-qPCR. To test the significance of SERCA2 in adipocyte functionality and whole-body metabolism, we generated adipocyte-specific SERCA2 knockout mice. The mice were metabolically phenotyped by glucose tolerance and tracer studies, histological analyses, measurements of glucose-stimulated insulin release in isolated islets, and gene/protein expression analyses. We also tested the effect of pharmacological SERCA inhibition and genetic SERCA2 ablation in cultured adipocytes. Intracellular and mitochondrial Ca²⁺ levels were recorded with dual-wavelength ratio imaging and mitochondrial function was assessed by Seahorse technology.

RESULTS

We demonstrate that SERCA2 is downregulated in white adipocytes from patients with obesity and type-2 diabetes as well as in adipocytes from diet-induced obese mice. SERCA2-ablated adipocytes display disturbed Ca²⁺ homeostasis associated with upregulated ER stress markers and impaired hormone release. These adipocyte alterations are linked to mild lipodystrophy, reduced adiponectin levels, and impaired glucose tolerance. Interestingly, adipocyte-specific SERCA2 ablation leads to increased glucose uptake in white adipose tissue while glucose uptake is reduced in brown adipose tissue. This dichotomous effect on glucose uptake is due to differently regulated mitochondrial function. In white adipocytes, SERCA2 deficiency triggers an adaptive increase in FGF21, increased mitochondrial UCP1 levels, and increased oxygen consumption rate (OCR). In contrast, brown SERCA2 null adipocytes display reduced OCR despite increased mitochondrial content and UCP1 levels compared to wild type controls.

CONCLUSIONS

Our data suggest causal links between reduced white adipocyte SERCA2 levels, deranged adipocyte Ca²⁺ homeostasis, adipose tissue dysfunction and type-2 diabetes.

Type 2 diabetes and metabolic syndrome - adipokine levels and effect of drugs

Type 2 diabetes mellitus (T2DM) is a consequence of complex interactions among multiple genetic variants and environmental risk factors. This complex disorder is also characterized by changes in various adipokines. In this study, our objective was to estimate the levels of adiponectin, leptin, and resistin (ALR) in T2DM patients, besides studying the effect of various drugs on their levels. Study participants included 400 diabetic and 300 normal patients from the Department of Endocrinology and Department of Biochemistry, Govt Medical College Srinagar. Subjects were categorized under various groups, i.e., Group 1 (metformin treated) and Group 2 (glimepiride treated), and cases were also categorized as obese with T2DM (Group A), obese without T2DM (Group B), and T2DM only (Group C). The serum ALR levels were estimated by ELISA (Alere), and biochemical parameters were also evaluated before and after treatment. Adiponectin levels were found to be significantly lower in T2DM cases as compared to controls (12 ± 5.5 versus 22.5 ± 7.9 μg/ml), while leptin and resistin levels were found to be significantly higher than controls (14.3 ± 7.4 versus 7.36 ± 3.73 ng/ml) (13.4 ± 1.56 versus 7.236 ± 2.129 pg/ml). Taking the effect of drugs into consideration, the effect on adiponectin and resistin levels was found to be highly significant in Group 2 before and after treatment (11 ± 5 versus 19.2 ± 4.5 μg/ml) (13.6 ± 2.5 versus 7.3 ± 2.9 pg/ml), while more effect was observed in leptin among Group 1 (metformin)-treated cases (27 ± 15 ng/ml versus 15 ± 15 ng/ml). Further the adiponectin levels were found to be significantly lower in Group B, while leptin and resistin levels were found to be significantly higher among obese cases when compared to T2DM cases only. Glimepiride also shows more effect on FBG, HbA1c% levels, while metformin shows more effect on Lipid profile levels. From the study, it can be concluded that ALR levels are affected by use of antidiabetic drugs among which glimepiride shows more effect on adiponectin and resistin levels, while leptin gets affected more by metformin. It can also be proposed that ALR levels are not affected by diabetes only, suggesting that their alterations in T2DM may be due to obesity as we observed more ALR changes in obese cases when compared to T2DM cases, and so there might be an important link between adiposity and insulin resistance.

Mouse Resistin (mRetn): cloning, expression and purification in Escherichia coli and the potential regulative effects on murine bone marrow hematopoiesis

Background

Resistin (Retn) is a cytokine which has a controversial physiological role regarding its involvement with obesity and type II diabetes mellitus. Recently, murine Retn was found to be a possibly potential regulator of hematopoiesis in mice shown in the screening results of a set of gene chips which mapped the expression level of murine genes during regeneration of impaired bone marrow (BM) by 5-fluorouracil.

Results

Recombinant mice Retn was expressed in Escherichia coli and purified using ion exchange chromatography. Totally 11.4 mg rmRetn was obtained from 500 ml culture with endotoxin level less than 1.0 EU/ug. The purity of recombinant murine Resistin reached to at least 97.6 % via SDS-PAGE analysis and HPLC. The protein possessed chemotaxis effects in the mouse aortic endothelial cells in vitro in transwell analysis. In vitro, rmRetn could up regulate the CFU number of mice BM and after rmRetn was administered, the cell number of murine bone marrow was significantly increased in vivo after chemotherapy. Finally, rmRetn was found able to protect mice from the chemotoxicity of 5-fluorouracil.

Conclusions

The discovery demonstrated a new function of murine Retn and suggested that it could potentially accelerate bone marrow regeneration post chemotherapy.

Resistin: functional roles and therapeutic considerations for cardiovascular disease

Resistin, originally described as an adipocyte-specific hormone, has been suggested to be an important link between obesity, insulin resistance and diabetes. Although its expression was initially defined in adipocytes, significant levels of resistin expression in humans are mainly found in mononuclear leukocytes, macrophages, spleen and bone marrow cells. Increasing evidence indicates that resistin plays important regulatory roles apart from its role in insulin resistance and diabetes in a variety of biological processes: atherosclerosis and cardiovascular disease (CVD), non-alcoholic fatty liver disease, autoimmune disease, malignancy, asthma, inflammatory bowel disease and chronic kidney disease. As CVD accounts for a significant amount of morbidity and mortality in patients with diabetes and without diabetes, it is important to understand the role that adipokines such as resistin play in the cardiovascular system. Evidence suggests that resistin is involved in pathological processes leading to CVD including inflammation, endothelial dysfunction, thrombosis, angiogenesis and smooth muscle cell dysfunction. The modes of action and signalling pathways whereby resistin interacts with its target cells are beginning to be understood. In this review, the current knowledge about the functions and pathophysiological implications of resistin in CVD development is summarized; clinical translations, therapeutic considerations and future directions in the field of resistin research are discussed.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Resistin: functional roles and therapeutic considerations for cardiovascular disease

Resistin, originally described as an adipocyte-specific hormone, has been suggested to be an important link between obesity, insulin resistance and diabetes. Although its expression was initially defined in adipocytes, significant levels of resistin expression in humans are mainly found in mononuclear leukocytes, macrophages, spleen and bone marrow cells. Increasing evidence indicates that resistin plays important regulatory roles apart from its role in insulin resistance and diabetes in a variety of biological processes: atherosclerosis and cardiovascular disease (CVD), non-alcoholic fatty liver disease, autoimmune disease, malignancy, asthma, inflammatory bowel disease and chronic kidney disease. As CVD accounts for a significant amount of morbidity and mortality in patients with diabetes and without diabetes, it is important to understand the role that adipokines such as resistin play in the cardiovascular system. Evidence suggests that resistin is involved in pathological processes leading to CVD including inflammation, endothelial dysfunction, thrombosis, angiogenesis and smooth muscle cell dysfunction. The modes of action and signalling pathways whereby resistin interacts with its target cells are beginning to be understood. In this review, the current knowledge about the functions and pathophysiological implications of resistin in CVD development is summarized; clinical translations, therapeutic considerations and future directions in the field of resistin research are discussed.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Adipokines: new emerging roles in fertility and reproduction

Adipose tissue is a specialized endocrine and paracrine organ producing specific factors called adipokines. It is well known that adipokines balance is fundamental to prevent obesity, metabolic syndrome, and cardiovascular diseases. During the last years, new roles of adipokines have been emerging in the field of fertility and reproduction. Although the literature is still quite controversial, this review serves to resume current knowledge on this topic. Alterations in adipokine levels or in their mechanism of action are associated with fertility impairment and pregnancy diseases, as well as with obesity, metabolic syndrome, and cardiovascular diseases. Normal levels of adipokines are fundamental to maintain integrity of hypothalamus-pituitary-gonadal axis, regular ovulatory processes, successful embryo implantation, and physiologic pregnancy. More efforts are needed to understand the mechanisms and to the extent to which adipokine changes are involved in the impairment of fertility and pregnancy outcome, to find possible medical treatments.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians Learning Objectives: After completion of this educational activity, the obstetrician/gynecologist should be better able to demonstrate current knowledge in the research field of adipokines in fertility and reproduction; evaluate the central role of metabolism balance in good pregnancy outcome; and apply new perspectives of studies.

Resistin

PURPOSE OF REVIEW

Resistin, a cysteine-rich 12.5 kDa polypeptide, is a recently discovered adipocytokine with a controversial history regarding its role in the pathogenesis of obesity-mediated insulin resistance and type 2 diabetes mellitus. Whilst current studies appear to re-affirm the role of resistin on glucose homeostasis in rodent systems, we are still unravelling the functionality of resistin in human biology in respect to glucose metabolism and insulin signalling. This review will summarize the current knowledge, put into context the developments to date and discuss the controversial points.

RECENT FINDINGS

Current evidence appears to suggest that resistin is a pro-inflammatory cytokine. Thus, like many other adipocytokines, resistin may possess a dual role in contributing to metabolic disease: first through its direct effects on substrate metabolism and second, through regulating inflammation within its target tissues. The chemistry of resistin has also been the subject of investigation and like adiponectin, the homo-oligomerization of this protein has a bearing on its function.

SUMMARY

The most recent advances include the identification of circulating higher molecular weight structures of resistin in both rodent and human serum. This has been complemented by work casting light on the function and purpose of multimeric resistin in mice. Resistin appears to have effects on substrate metabolism through impairment of insulin action, particularly in the liver, but in addition, also has effects on insulin independent pathways.

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.

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.

The direct peroxisome proliferator-activated receptor target fasting-induced adipose factor (FIAF/PGAR/ANGPTL4) is present in blood plasma as a truncated protein that is increased by fenofibrate treatment

The fasting-induced adipose factor (FIAF, ANGPTL4, PGAR, HFARP) was previously identified as a novel adipocytokine that was up-regulated by fasting, by peroxisome proliferator-activated receptor agonists, and by hypoxia. To further characterize FIAF, we studied regulation of FIAF mRNA and protein in liver and adipose cell lines as well as in human and mouse plasma. Expression of FIAF mRNA was up-regulated by peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARbeta/delta agonists in rat and human hepatoma cell lines and by PPARgamma and PPARbeta/delta agonists in mouse and human adipocytes. Transactivation, chromatin immunoprecipitation, and gel shift experiments identified a functional PPAR response element within intron 3 of the FIAF gene. At the protein level, in human and mouse blood plasma, FIAF was found to be present both as the native protein and in a truncated form. Differentiation of mouse 3T3-L1 adipocytes was associated with the production of truncated FIAF, whereas in human white adipose tissue and SGBS adipocytes, only native FIAF could be detected. Interestingly, truncated FIAF was produced by human liver. Treatment with fenofibrate, a potent PPARalpha agonist, markedly increased plasma levels of truncated FIAF, but not native FIAF, in humans. Levels of both truncated and native FIAF showed marked interindividual variation but were not associated with body mass index and were not influenced by prolonged semistarvation. Together, these data suggest that FIAF, similar to other adipocytokines such as adiponectin, may partially exert its function via a truncated form.

Dominant inhibitory adipocyte-specific secretory factor (ADSF)/resistin enhances adipogenesis and improves insulin sensitivity

Adipocyte-specific secretory factor (ADSF)/resistin is a small cysteine-rich protein secreted from adipose tissue that belongs to a gene family found in inflammatory zone (FIZZ) or found in resistin-like molecule (RELM). ADSF has been implicated in modulating adipogenesis and insulin resistance. To examine the long-term function of ADSF in adipogenesis and glucose homeostasis, we constructed an expression vector for a dominant inhibitory form of ADSF by fusing it to the human IgGgamma constant region (hFc). ADSF-hFc not only homodimerizes but heterooligomerizes with ADSF/resistin and prevents ADSF/resistin inhibition of adipocyte differentiation of 3T3-L1 cells in a dominant negative manner. Transgenic mice overexpressing ADSF-hFc in adipose tissue show increased adiposity with elevated expression of adipocyte markers as well as enlarged adipocyte size. This finding clearly demonstrates in vivo the inhibitory role of ADSF in adipogenesis. ADSF-hFc transgenic mice with impaired ADSF function exhibit improved glucose tolerance and insulin sensitivity either on chow or high-fat diets. Because of the enhanced adipocyte differentiation, the ADSF-hFc transgenic mice show increased expression of leptin and adiponectin in adipose tissue. The elevated circulating levels for these adipocyte-derived hormones with decreased plasma triglyceride and free fatty acid levels may account for the improved glucose and insulin tolerance in these transgenic mice.

Dimerization of human recombinant resistin involves covalent and noncovalent interactions

Resistin, an adipocyte secreted cysteine rich hormone has been implicated as molecular link between obesity and type 2 diabetes in a murine model. Although, at the protein level mouse and human resistin show remarkable similarities with respect to conserved cysteine residues, the physiological role of human resistin is not yet clear. In the present study we describe the purification and refolding of human recombinant resistin using two different refolding processes. Gel filtration analysis of protein refolded by both the methods revealed that human recombinant resistin, like mouse resistin, has a tendency to form dimers. Interestingly, dimerization of resistin appears to be mediated by both covalent (disulfide bond mediated) and non-covalent interactions as seen on reducing and non-reducing SDS-PAGE. Circular dichroism spectral analysis revealed that human resistin peptide backbone is a mixture of alpha-helical and beta-sheet conformation with significant amounts of unordered structure, similar to the mouse resistin. It is likely that the first cysteine (Cyst22) of human resistin, which is equivalent to mouse Cyst26, may be involved in stabilizing the dimers through covalent interaction.

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.

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.

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.