DsbA-L alleviates endoplasmic reticulum stress-induced adiponectin downregulation

New insights of DsbA-L in the pathogenesis of metabolic diseases

Metabolic diseases, such as obesity, diabetes mellitus, and non-alcoholic fatty liver disease (NAFLD), are abnormal conditions that result from disturbances of metabolism. With the improvement of living conditions, the morbidity and mortality rates of metabolic diseases are steadily rising, posing a significant threat to human health worldwide. Therefore, identifying novel effective targets for metabolic diseases is crucial. Accumulating evidence has indicated that disulfide bond A oxidoreductase-like protein (DsbA-L) delays the development of metabolic diseases. However, the underlying mechanisms of DsbA-L in metabolic diseases remain unclear. In this review, we will discuss the roles of DsbA-L in the pathogenesis of metabolic diseases, including obesity, diabetes mellitus, and NAFLD, and highlight the potential mechanisms. These findings suggest that DsbA-L might provide a novel therapeutic strategy for metabolic diseases.

Decreased Proteasomal Function Exacerbates Endoplasmic Reticulum Stress-Induced Chronic Inflammation in Obese Adipose Tissue

Low-grade chronic inflammation contributes to both aging and the pathogenesis of age-related diseases. White adipose tissue (WAT) in obese individuals exhibits chronic inflammation, which is associated with obesity-related disorders. Aging exacerbates obesity-related inflammation in WAT; however, the molecular mechanisms underlying chronic inflammation and its exacerbation by aging remain unclear. Age-related decline in activity of the proteasome, a multisubunit proteolytic complex, has been implicated in age-related diseases. This study employed a mouse model with decreased proteasomal function that exhibits age-related phenotypes to investigate the impact of adipocyte senescence on WAT inflammation. Transgenic mice expressing proteasomal subunit β5t with weak chymotrypsin-like activity experience reduced lifespan and develop age-related phenotypes. Mice fed with a high-fat diet and experiencing proteasomal dysfunction exhibited increased WAT inflammation, increased infiltration of proinflammatory M1-like macrophages, and increased proinflammatory adipocytokine-like monocyte chemoattractant protein-1, plasminogen activator inhibitor-1, and tumor necrosis factor-α, which are all associated with activation of endoplasmic reticulum (ER) stress-related pathways. Impaired proteasomal activity also activated ER stress-related molecules and induced expression of proinflammatory adipocytokines in adipocyte-like cells differentiated from 3T3-L1 cells. Collective evidence suggests that impaired proteasomal activity increases ER stress and that subsequent inflammatory pathways play pivotal roles in WAT inflammation. Because proteasomal function declines with age, age-related proteasome impairment may be involved in obesity-related inflammation among elderly individuals.

The role of endoplasmic reticulum-mitochondria-associated membranes in diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide. The pathomechanisms of DKD are multifactorial, yet haemodynamic and metabolic changes in the early stages of the disease appear to predispose towards irreversible functional loss and histopathological changes. Recent studies highlight the importance of endoplasmic reticulum-mitochondria-associated membranes (ER-MAMs), structures conveying important cellular homeostatic and metabolic effects, in the pathology of DKD. Disruption of ER-MAM integrity in diabetic kidneys is associated with DKD progression, but the regulation of ER-MAMs and their pathogenic contribution remain largely unknown. Exploring the cell-specific components and dynamic changes of ER-MAMs in diabetic kidneys may lead to the identification of new approaches to detect and stratify diabetic patients with DKD. In addition, these insights may lead to novel therapeutic approaches to target and/or reverse disease progression. In this review, we discuss the association of ER-MAMs with key pathomechanisms driving DKD such as insulin resistance, dyslipidaemia, ER stress, and inflammasome activation and the importance of further exploration of ER-MAMs as diagnostic and therapeutic targets in DKD.

The emerging role of bile acids in white adipose tissue

The effects of bile acids (BAs) on liver, enteroendocrine function, small intestine, and brown adipose tissue have been described extensively. Outside the liver, BAs in the peripheral circulation system represent a specific but underappreciated physiological compartment. We discuss how systemic BAs can be regarded as specific steroidal hormones that act on white adipocytes, and suggest the name 'bilokines' ('bile hormones') for the specific FXR/TGR5 receptor interaction in adipocytes. Some BAs and their agonists regulate adipocyte differentiation, lipid accumulation, hypoxia, autophagy, adipokine and cytokine secretion, insulin signaling, and glucose uptake. BA signaling could provide a new therapeutic avenue for adipoflammation and metaflammation in visceral obesity, the causal mechanisms underlying insulin resistance and type 2 diabetes mellitus (T2D).

The Role of Adipocyte Endoplasmic Reticulum Stress in Obese Adipose Tissue Dysfunction: A Review

Adipose tissue dysfunction plays an important role in metabolic diseases associated with chronic inflammation, insulin resistance and lipid ectopic deposition in obese patients. In recent years, it has been found that under the stimulation of adipocyte endoplasmic reticulum stress (ERS), the over-activated ER unfolded protein response (UPR) exacerbates the inflammatory response of adipose tissue by interfering with the normal metabolism of adipose tissue, promotes the secretion of adipokines, and affects the browning and thermogenic pathways of adipose tissue, ultimately leading to the manifestation of metabolic syndrome such as ectopic lipid deposition and disorders of glucolipid metabolism in obese patients. This paper mainly summarizes the relationship between adipocyte ERS and obese adipose tissue dysfunction and provides an overview of the mechanisms by which ERS induces metabolic disorders such as catabolism, thermogenesis and inflammation in obese adipose tissue through the regulation of molecules and pathways such as NF-κB, ADPN, STAMP2, LPIN1, TRIP-Br2, NF-Y and SIRT2 and briefly describes the current mechanisms targeting adipocyte endoplasmic reticulum stress to improve obesity and provide ideas for intervention and treatment of obese adipose tissue dysfunction.

Mitochondrial Dysfunction Associated with mtDNA in Metabolic Syndrome and Obesity

Metabolic syndrome (MetS) is a precursor to the major health diseases associated with high mortality in industrialized countries: cardiovascular disease and diabetes. An important component of the pathogenesis of the metabolic syndrome is mitochondrial dysfunction, which is associated with tissue hypoxia, disruption of mitochondrial integrity, increased production of reactive oxygen species, and a decrease in ATP, leading to a chronic inflammatory state that affects tissues and organ systems. The mitochondrial AAA + protease Lon (Lonp1) has a broad spectrum of activities. In addition to its classical function (degradation of misfolded or damaged proteins), enzymatic activity (proteolysis, chaperone activity, mitochondrial DNA (mtDNA)binding) has been demonstrated. At the same time, the spectrum of Lonp1 activity extends to the regulation of cellular processes inside mitochondria, as well as outside mitochondria (nuclear localization). This mitochondrial protease with enzymatic activity may be a promising molecular target for the development of targeted therapy for MetS and its components. The aim of this review is to elucidate the role of mtDNA in the pathogenesis of metabolic syndrome and its components as a key component of mitochondrial dysfunction and to describe the promising and little-studied AAA + LonP1 protease as a potential target in metabolic disorders.

Vitamin C protects against hypoxia, inflammation, and ER stress in primary human preadipocytes and adipocytes

Dysregulation of adipose tissue involves increased cellular hypoxia, ER stress, and inflammation and altered adipokine production, contributing to the aetiology of obesity-related diseases including type 2 diabetes and cardiovascular disease. This study aimed to investigate the effects of Vitamin C supplementation on these processes in primary human preadipocytes and adipocytes. Treatment of preadipocytes and adipocytes with the proinflammatory cytokine TNFα and palmitic acid (PA), to mimic the obesogenic milieu, significantly increased markers of hypoxia, ER stress and inflammation and reduced secretion of high molecular weight (HMW) adiponectin. Importantly, Vitamin C abolished TNFα+PA induced hypoxia and significantly reduced the increases in ER stress and inflammation in both cell types. Vitamin C also significantly increased the secretion of HMW adiponectin from adipocytes. These findings indicate that Vitamin C can reduce obesity-associated cellular stress and thus provide a rationale for future investigations.

PDIA4, a novel ER stress chaperone, modulates adiponectin expression and inflammation in adipose tissue

Increasing evidence supporting a causal link between obesity and endoplasmic reticulum (ER) stress in adipose tissue is being reported. Protein disulfide isomerase 4 (PDIA4) is a novel ER chaperone involved in the pancreatic β-cells pathogenesis in diabetes. However, the role of PDIA4 in obesity progression remains poorly understood. To assess the relationship between PDIA4, adiponectin, and metformin, we used the palmitate-induced inflammation in hypertrophic adipocytes and the high-fat diet-induced obesity mouse model. Our results revealed that palmitate-induced hypertrophic adipocytes exhibit obesity-associated conditions such as increased lipid accumulation, inflammation, and reduced glucose uptake. Pharmacological and genetic inhibition of PDIA4 significantly reverses these obesity-associated conditions in adipocytes. PDIA4 mechanistically promotes obesity progression via adiponectin downregulation. Furthermore, metformin modulates PDIA4 and adiponectin expression and improves obesity-associated conditions in both in vitro adipocytes and in vivo mouse models. Serum PDIA4 concentrations are also associated with body mass index, adiponectin, triglycerides, and inflammatory cytokines in humans. This is the first study demonstrating that PDIA4 modulates adipocytes by downregulating adiponectin. Moreover, metformin may serve as a potential therapeutic for preventing obesity via PDIA4-targeting.

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.

Untargeted metabolomics analysis of the anti-diabetic effect of Red ginseng extract in Type 2 diabetes Mellitus rats based on UHPLC-MS/MS

Red ginseng is a traditional Chinese herbal medicine that has long been used to treat diabetes, and its blood sugar-lowering activity has been confirmed. However, the mechanism of action of red ginseng on type 2 diabetes mellitus (T2DM) at the metabolic level is still unclear. The purpose of this study is to investigate the effect of red ginseng extract in the treatment of T2DM rats based on untargeted metabolomics. The rat model of T2DM was induced by a high-fat diet (HFD) combined with streptozotocin (STZ), and serum samples were collected after four weeks of treatment. The ultra-high-performance liquid chromatography coupled with Q Exactive HF-X Mass Spectrometer was used to analyze the level of metabolites in serum to evaluate the differences in metabolic levels between different groups. The results of biochemical analysis showed that red ginseng extract intervention significantly improved the levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), serum glucose (GLU), and fasting insulin (FINS) after four weeks. Orthogonal partial least squares discriminant analysis was used to study the overall changes of rat metabolomics. After the intervention of red ginseng extract, 50 biomarkers showed a callback trend. Metabolic pathway enrichment analysis showed that the regulated pathways were D-arginine and D-ornithine metabolism, D-glutamine and D-glutamate metabolism, taurine and hypotaurine metabolism, arginine biosynthesis, and tryptophan metabolism. Generally, the results demonstrated that red ginseng extract had beneficial effects on T2DM, which could be mediated via ameliorating the metabolic disorders.

The cGAS-STING signaling in cardiovascular and metabolic diseases: Future novel target option for pharmacotherapy

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling exert essential regulatory function in microbial-and onco-immunology through the induction of cytokines, primarily type I interferons. Recently, the aberrant and deranged signaling of the cGAS-STING axis is closely implicated in multiple sterile inflammatory diseases, including heart failure, myocardial infarction, cardiac hypertrophy, nonalcoholic fatty liver diseases, aortic aneurysm and dissection, obesity, etc. This is because of the massive loads of damage-associated molecular patterns (mitochondrial DNA, DNA in extracellular vesicles) liberated from recurrent injury to metabolic cellular organelles and tissues, which are sensed by the pathway. Also, the cGAS-STING pathway crosstalk with essential intracellular homeostasis processes like apoptosis, autophagy, and regulate cellular metabolism. Targeting derailed STING signaling has become necessary for chronic inflammatory diseases. Meanwhile, excessive type I interferons signaling impact on cardiovascular and metabolic health remain entirely elusive. In this review, we summarize the intimate connection between the cGAS-STING pathway and cardiovascular and metabolic disorders. We also discuss some potential small molecule inhibitors for the pathway. This review provides insight to stimulate interest in and support future research into understanding this signaling axis in cardiovascular and metabolic tissues and diseases.

Adiponectin Synthesis, Secretion and Extravasation from Circulation to Interstitial Space

Adiponectin, an adipokine that circulates as multiple multimeric complexes at high levels in serum, has antidiabetic, anti-inflammatory, antiatherogenic, and cardioprotective properties. Understanding the mechanisms regulating adiponectin's physiological effects is likely to provide critical insight into the development of adiponectin-based therapeutics to treat various metabolic-related diseases. In this review, we summarize our current understanding on adiponectin action in its various target tissues and in cellular models. We also focus on recent advances in two particular regulatory aspects; namely, the regulation of adiponectin gene expression, multimerization, and secretion, as well as extravasation of circulating adiponectin to the interstitial space and its degradation. Finally, we discuss some potential therapeutic approaches using adiponectin as a target and the current challenges facing adiponectin-based therapeutic interventions.

Silencing of DsbA-L gene impairs the PPARγ agonist function of improving insulin resistance in a high-glucose cell model

Disulfide-bond A oxidoreductase-like protein (DsbA-L) is a molecular chaperone involved in the multimerization of adiponectin. Recent studies have found that DsbA-L is related to metabolic diseases including gestational diabetes mellitus (GDM), and can be regulated by peroxisome proliferator-activated receptor γ (PPARγ) agonists; the specific mechanism, however, is uncertain. Furthermore, the relationship between DsbA-L and the novel adipokine chemerin is also unclear. This article aims to investigate the role of DsbA-L in the improvement of insulin resistance by PPARγ agonists in trophoblast cells cultured by the high-glucose simulation of GDM placenta. Immunohistochemistry and western blot were used to detect differences between GDM patients and normal pregnant women in DsbA-L expression in the adipose tissue. The western blot technique was performed to verify the relationship between PPARγ agonists and DsbA-L, and to explore changes in key molecules of the insulin signaling pathway, as well as the effect of chemerin on DsbA-L. Results showed that DsbA-L was significantly downregulated in the adipose tissue of GDM patients. Both PPARγ agonists and chemerin could upregulate the level of DsbA-L. Silencing DsbA-L affected the function of rosiglitazone to promote the phosphatidylinositol 3-kinase (PI3K)-protein kinase B (PKB)/AKT pathway. Therefore, it is plausible to speculate that DsbA-L is essential in the environment of PPARγ agonists for raising insulin sensitivity. Overall, we further clarified the mechanism by which PPARγ agonists improve insulin resistance.

Co-activation of Akt, Nrf2, and NF-κB signals under UPRER in torpid Myotis ricketti bats for survival

Bats hibernate to survive stressful conditions. Examination of whole cell and mitochondrial proteomes of the liver of Myotis ricketti revealed that torpid bats had endoplasmic reticulum unfolded protein response (UPRER), global reduction in glycolysis, enhancement of lipolysis, and selective amino acid metabolism. Compared to active bats, torpid bats had higher amounts of phosphorylated serine/threonine kinase (p-Akt) and UPRER markers such as PKR-like endoplasmic reticulum kinase (PERK) and activating transcription factor 4 (ATF4). Torpid bats also had lower amounts of the complex of Kelch-like ECH-associated protein 1 (Keap1), nuclear factor erythroid 2-related factor 2 (Nrf2), and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) (p65)/I-κBα. Cellular redistribution of 78 kDa glucose-regulated protein (GRP78) and reduced binding between PERK and GRP78 were also seen in torpid bats. Evidence of such was not observed in fasted, cold-treated, or normal mice. These data indicated that bats activate Akt, Nrf2, and NF-κB via the PERK-ATF4 regulatory axis against endoplasmic reticulum stresses during hibernation.

Adiponectin: Role in Physiology and Pathophysiology

Adiponectin, an adipokine secreted by adipocytes, is a well-known homeostatic factor for regulating glucose levels, lipid metabolism, and insulin sensitivity through its anti-inflammatory, anti-fibrotic, and antioxidant effects. All these metabolic processes are mediated via two adiponectin receptors, AdipoR1 and AdipoR2. In addition, adiponectin is one of the hormones with the highest plasma concentrations. Weight loss or caloric restriction leads to increasing adiponectin levels, and this increase is associated with increased insulin sensitivity. Therefore, the adiponectin pathway can play a crucial role in the development of drugs to treat type 2 diabetes mellitus and other obesity-related diseases affected by insulin resistance like cancers or cardiovascular diseases. Adiponectin appears to increase insulin sensitivity by improving glucose and lipid metabolisms. The objective of this review is to analyze current knowledge concerning adiponectin and, in particular, its role in physiology and pathophysiology.

PPARγ enhanced Adiponectin polymerization and trafficking by promoting RUVBL2 expression during adipogenic differentiation

Adipocyte differentiation is an essential part of adipose tissue development, and is closely related to obesity and obesity-related diseases. In this study, we found that the expression of PPARγ, RUVBL2 and Adiponectin were concurrently obviously increased in the 5th-7th day of 3T3-L1 cell differentiation. PPARγ overexpression or the PPARγ activator facilitated Adiponectin trafficking and secretion and upregulated RUVBL2 expression as well as AS160 phosphorylation during adipogenic differentiation of 3T3-L1 cells. Consistently RUVBL2 overexpression also enhanced the polymerization and secretion of Adiponectin, in contrast, RUVBL2 knockdown reduced Adiponectin secretion. Further, PPARγ significantly enhanced RUVBL2 promoter activity and transcription. The progressive deletions and mutations of RUVBL2 promoter for PPARγ binding sites suggested that the PPARγ binding motif situated at -804/-781 bp is an essential component required for RUVBL2 promoter activity. Chromatin immunoprecipitation (ChIP) assays determined that PPARγ can directly interact with the RUVBL2 promoter DNA. Taken together, these data suggest that PPARγ promotes the expression, polymerization and secretion of Adiponectin by activating RUVBL2 transcriptionally, which accelerates 3T3-L1 cell differentiation.

DsbA-L deficiency exacerbates mitochondrial dysfunction of tubular cells in diabetic kidney disease

Excessive mitochondrial fission has been identified as the central pathogenesis of diabetic kidney disease (DKD), but the precise mechanisms remain unclear. Disulfide-bond A oxidoreductase-like protein (DsbA-L) is highly expressed in mitochondria in tubular cells of the kidney, but its pathophysiological role in DKD is unknown. Our bioinformatics analysis showed that tubular DsbA-L mRNA levels were positively associated with eGFR but negatively associated with Scr and 24h-proteinuria in CKD patients. Furthermore, the genes that were coexpressed with DsbA-L were mainly enriched in mitochondria and were involved in oxidative phosphorylation. In vivo, knockout of DsbA-L exacerbated diabetic mice tubular cell mitochondrial fragmentation, oxidative stress and renal damage. In vitro, we found that DsbA-L was localized in the mitochondria of HK-2 cells. High glucose (HG, 30 mM) treatment decreased DsbA-L expression followed by increased mitochondrial ROS (mtROS) generation and mitochondrial fragmentation. In addition, DsbA-L knockdown exacerbated these abnormalities, but this effect was reversed by overexpression of DsbA-L. Mechanistically, under HG conditions, knockdown DsbA-L expression accentuated JNK phosphorylation in HK-2 cells. Furthermore, administration of a JNK inhibitor (SP600125) or the mtROS scavenger MitoQ significantly attenuated JNK activation and subsequent mitochondrial fragmentation in DsbA-L-knockdown HK-2 cells. Additionally, the down-regulation of DsbA-L also amplified the gene and protein expression of mitochondrial fission factor (MFF) via the JNK pathway, enhancing its ability to recruit DRP1 to mitochondria. Taken together, these results link DsbA-L to alterations in mitochondrial dynamics during tubular injury in the pathogenesis of DKD and unveil a novel mechanism by which DsbA-L modifies mtROS/JNK/MFF-related mitochondrial fission.

Regulation of Adipsin Expression by Endoplasmic Reticulum Stress in Adipocytes

Adpsin is an adipokine that stimulates insulin secretion from β-cells and improves glucose tolerance. Its expression has been found to be markedly reduced in obese animals. However, it remains unclear what factors lead to downregulation of adipsin in the context of obesity. Endoplasmic reticulum (ER) stress response is activated in various tissues under obesity-related conditions and can induce transcriptional reprogramming. Therefore, we aimed to investigate the relationship between adipsin expression and ER stress in adipose tissues during obesity. We observed that obese mice exhibited decreased levels of adipsin in adipose tissues and serum and increased ER stress markers in adipose tissues compared to lean mice. We also found that ER stress suppressed adipsin expression via adipocytes-intrinsic mechanisms. Moreover, the ER stress-mediated downregulation of adipsin was at least partially attributed to decreased expression of peroxisome proliferator-activated receptor γ (PPARγ), a key transcription factor in the regulation of adipocyte function. Finally, treatment with chemical chaperones recovered the ER stress-mediated downregulation of adipsin and PPARγ in vivo and in vitro. Our findings suggest that activated ER stress in adipose tissues is an important cause of the suppression of adipsin expression in the context of obesity.

Adrenergic stimulation of adiponectin secretion in visceral mouse adipocytes is blunted in high-fat diet induced obesity

The hormone adiponectin is secreted by white adipocytes and has been put forward as a key mediator of obesity-linked insulin resistance and the metabolic syndrome. Although adiponectin was discovered two decades ago, the knowledge about the molecular and cellular regulation of its secretion is incomplete. Here we have investigated the adrenergic regulation of adiponectin secretion in primary visceral (gonadal) adipocytes isolated from lean or obese/diabetic mice. We show that visceral adipocyte adiponectin release is triggered by cAMP/catecholamines via signalling pathways involving adrenergic beta-3-receptors (β₃ARs) and Exchange Protein directly Activated by cAMP, isoform 1 (Epac1). The adrenergically stimulated adiponectin secretion is blunted in visceral adipocytes isolated from obese and diabetic mice and our results suggest the existence of a secretory defect. We have previously shown that adiponectin secretion in subcutaneous adipocytes is abolished in the obese/diabetic state due to reduced abundance of β₃ARs and Epac1. However, here we show that protein levels of β₃ARs and Epac1 are maintained in visceral adipocytes from obese/diabetic mice proposing that other molecular defects underlie the blunted adiponectin release. Gene expression analysis indicate diabesity-associated disturbances of the signalling downstream of Epac1 and/or the exocytotic process itself. Our study proposes that visceral adipocytes partake in the regulated secretion of adiponectin and may thus influence circulating levels of the hormone, in health and in metabolic disease.

DsbA-L ameliorates high glucose induced tubular damage through maintaining MAM integrity

Background

The mitochondrial associated endoplasmic reticulum (ER) membrane (MAM) provides a platform for communication between the mitochondria and ER, and it plays a vital role in many biological functions. Disulphide-bond A oxidoreductase-like protein (DsbA-L), expressed in the MAM, serves as an antioxidant and reduces ER stress. However, the role of DsbA-L and MAM in kidney pathobiology remains unclear.

Methods

Molecular biology techniques, transmission electron microscopy (TEM), in situ proximity ligation assays (PLAs), confocal microscopy, TUNEL staining and flow cytometry were utilized to analyse apoptosis and status of MAM in DsbA-L mutant mice.

Findings

We showed that MAM was significantly reduced in the kidneys of streptozotocin-induced diabetic mice, which correlated with the extent of renal injury. We also observed a correlation between the loss of MAM integrity and increased apoptosis and renal injury in diabetic nephropathy (DN). These alterations were further exacerbated in diabetic DsbA-L gene-deficient mice (DsbA-L^−/−). In vitro, overexpression of DsbA-L in HK-2 cells restored MAM integrity and reduced apoptosis induced by high-glucose ambience. These beneficial effects were partially blocked by overexpression of FATE-1, a MAM uncoupling protein. Finally, the expression of DsbA-L was positively correlated with MAM integrity in the kidneys of DN patients but negatively correlated with apoptosis and renal injury.

Interpretation

Our results indicate that DsbA-L exerts an antiapoptotic effect by maintaining MAM integrity, which is apparently disrupted in DN.

Fund

This work was supported by the National Natural Science Foundation of China (81730018), the National Key R&D Program of China (2016YFC1305501) and NIH (DK60635).

Adiponectin for the treatment of diabetic nephropathy

The metabolic burden caused by hyperglycemia can result in direct and immediate metabolic injuries, such as oxidative stress and tissue inflammation, in the kidney. Furthermore, chronic hyperglycemia can lead to substantial structural changes such as formation of advanced glycation end-products, glomerular and tubular hypertrophy, and tissue fibrosis. Glomerular hypertrophy renders podocytes vulnerable to increased glomerular filtration, leading to podocyte instability and loss. Thus, prevention of glomerular hypertrophy and attenuation of glomerular hyperfiltration may have therapeutic potential for diabetic nephropathy (DN). Adiponectin is an adipokine that improves insulin sensitivity in obesity-related metabolic disorders, including diabetes, but its efficacy is unknown. Moreover, the recently developed adiponectin receptor agonist, AdipoRon, shows therapeutic potential for DN. In this review, we focus on the role of glomerular hypertrophy in the pathogenesis of DN and discuss the role of adiponectin in its prevention.

Adiponectin and Its Isoforms in Pathophysiology

Adiponectin circulates in blood in multiple isoforms. High molecular weight (HMW) adiponectin is thought to be most biologically active and promotes glucose uptake, insulin sensitivity, and fatty acid oxidation. In obesity, adiponectin isoform formation is disrupted, leading to an inverse association between metabolic disease and HMW and total adiponectin. Adiponectin isoforms also function as acute-phase reactants influencing inflammation in acute and chronic disease. Interestingly, adiponectin and mortality have a U-shaped association. Unfortunately, data concerning adiponectin and its pathophysiologic function conflict. This is predominantly due to difficulties in adequate measurement of adiponectin isoforms and lack of a gold standard. In this review we provide a general overview of the formation and function of adiponectin and its isoforms under physiologic conditions. We highlight the ways adiponectin isoform formation is disrupted in obesity and its ensuing pathologic conditions. Furthermore, we will elaborate on the role of adiponectin isoforms as inflammatory proteins with respect to cardiac and kidney disease and discuss the association of adiponectin with mortality. Finally, we will provide a historical perspective on the measurement of adiponectin isoforms, current limitations, and future challenges.

Leucine reduces the proliferation of MC3T3-E1 cells through DNA damage and cell senescence

Leucine (Leu) is an essential branched-chain amino acid, present in dairy products, which has been investigated for its important role in cell signaling. The effects of Leu on several kinds of cells have been studied, altough little is known on its action upon bone cells and cell proliferation. Thus, the aim of this study is to investigate the effects of Leu supplementation on the proliferation of pre-osteoblasts from MC3T3-E1 lineage. MC3T3-E1 cells were kept in Alpha medium supplemented with 10% fetal bovine serum and 1% antibiotic-antimitotic. Cells were treated during 48h by adding 50μM of Leu, which corresponds to a 12.5% increase of the amino acid in the culture medium. The evaluation of viability and proliferation of cultured cells was performed using Trypan Blue dye. In order to identify the mechanisms related to the decreased cellular proliferation, assays were performed to assess cytotoxicity, apotosis, oxidative stress, inflammation, autophagy, senescence and DNA damage. Results showed that Leu supplementation decreased cell proliferation by 40% through mechanisms not related to cell necrosis, apoptosis, oxidative stress, autophagy or inhibition of the mTORC1 pathway. On the other hand, Leu supplementation caused DNA damage. In conclusion, Leu caused a negative impact on bone cell proliferation by inducing cell senescence through DNA damage.

Succination of Protein Disulfide Isomerase Links Mitochondrial Stress and Endoplasmic Reticulum Stress in the Adipocyte During Diabetes

AIMS

Protein succination by fumarate increases in the adipose tissue of diabetic mice and in adipocytes matured in high glucose as a result of glucotoxicity-driven mitochondrial stress. The endoplasmic reticulum (ER) oxidoreductase protein disulfide isomerase (PDI) is succinated in adipocytes that are matured in high glucose, and in this study we investigated whether succination would alter PDI oxidoreductase activity, directly linking mitochondrial stress and ER stress.

RESULTS

Protein succination and the ER stress marker C/EBP homologous protein (CHOP) were diminished after pharmaceutical targeting of mitochondrial stress with the chemical uncoupler niclosamide in adipocytes matured in high-glucose concentrations. PDI was succinated by fumarate on both CXXC-containing active sites, contributing to reduced enzymatic activity. Succinated PDI decreased reductase activity in adipocytes matured in high glucose, and in db/db epididymal adipose tissue, in association with increased levels of CHOP. PDI succination was increased in fumarase knockdown adipocytes, leading to reduced PDI oxidoreductase activity, increased CHOP levels, and pro-inflammatory cytokine secretion, confirming the specific role of elevated fumarate levels in contributing to ER stress. In addition, PDI succination and ER stress were decreased, and PDI reductase activity was restored when exposure to chronic high glucose was limited, highlighting the importance of calorie restriction in the improvement of adipocyte metabolic function.

INNOVATION

These experiments identify PDI succination as a novel biochemical mechanism linking altered mitochondrial metabolism to ER stress in the adipocyte during diabetes.

CONCLUSION

The current study demonstrates that early biochemical changes in mitochondrial metabolism have important implications for the development of adipocyte stress. Antioxid. Redox Signal. 27, 1281-1296.

Resveratrol Cardioprotection Against Myocardial Ischemia/Reperfusion Injury Involves Upregulation of Adiponectin Levels and Multimerization in Type 2 Diabetic Mice

Downregulation of adiponectin (APN) multimerization is significantly correlated with the aggravation of myocardial ischemia/reperfusion (MI/R) injury in type 2 diabetes mellitus (T2DM). Resveratrol (RSV) upregulates APN multimerization in adipocytes, but whether RSV improves endogenous APN multimerization and thus attenuates MI/R injury in T2DM mice has never been investigated. T2DM mice were treated with 10 mg/kg RSV daily for 3 weeks, followed by 30 minutes of myocardial ischemia and 3 hours or 24 hours of reperfusion. RSV administration alleviated MI/R injury in diabetic mice, as evidenced by reduced infarct size, cardiomyocyte apoptosis, and caspase-3 activity, and improved cardiac function. Moreover, RSV reversed the downregulated APN levels and multimerization both in plasma and adipose tissue, accompanied by increased disulfide bond A oxidoreductase-like protein (DsbA-L) expression in T2DM mice. Conversely, serving as a key downstream molecule of APN in ameliorating MI/R injury, inhibition of AMP-activated protein kinase (AMPK) significantly attenuated the cardioprotective effects of RSV. In conclusion, long-term administration of RSV upregulates adiponectin levels and multimerization in T2DM mice, consequently attenuating MI/R injury partially through APN-AMPK signaling.

Synthetic peptides designed to modulate adiponectin assembly improve obesity-related metabolic disorders

BACKGROUND AND PURPOSE

Adiponectin, an adipokine possessing profound insulin-sensitizing and anti-inflammatory properties, is a potent biotherapeutic agent . The trimeric adiponectin subunit assembles into hexameric and functionally important higher molecular weight (HMW) forms, controlled by the endoplasmic reticulum protein 44 (ERp44). Obesity-induced ER stress decreases the HMW form in serum, contributing to the development of insulin resistance and Type 2 diabetes. In this study, a panel of synthetic peptides, designed to target ERp44-adiponectin interactions, were tested for their effects on circulating levels of HMW adiponectin.

EXPERIMENTAL APPROACH

Peptides derived from the ERp44 binding region of adiponectin and immunoglobulin IgM were synthesized with or without a cell-penetrating sequence. Cultures of 3T3-L1 adipocytes were incubated with the peptides for assessing the assembly and secretion of HMW adiponectin. Mice given standard chow or a high-fat diet were treated acutely or chronically, with the peptides to investigate the therapeutic effects on insulin sensitivity and energy metabolism.

RESULTS

The designed peptides interfered with ERp44-adiponectin interactions and modulated adiponectin assembly and release from adipocytes. In particular, IgM-derived peptides facilitated the release of endogenous adiponectin (especially the HMW form) from adipose tissue, enhanced its circulating level and the ratio of HMW-to-total-adiponectin in obese mice. Long-term treatment of mice fed with high-fat diet by IgM-derived peptides reduced the circulating lipid levels and improved insulin sensitivity.

CONCLUSIONS AND IMPLICATIONS

Targeting ERp44-adiponectin interactions with short peptides represents an effective strategy to treat of obesity-related metabolic disorders, such as insulin resistance and Type 2 diabetes.

DsbA-L prevents obesity-induced inflammation and insulin resistance by suppressing the mtDNA release-activated cGAS-cGAMP-STING pathway

Chronic inflammation in adipose tissue plays a key role in obesity-induced insulin resistance. However, the mechanisms underlying obesity-induced inflammation remain elusive. Here we show that obesity promotes mtDNA release into the cytosol, where it triggers inflammatory responses by activating the DNA-sensing cGAS-cGAMP-STING pathway. Fat-specific knockout of disulfide-bond A oxidoreductase-like protein (DsbA-L), a chaperone-like protein originally identified in the mitochondrial matrix, impaired mitochondrial function and promoted mtDNA release, leading to activation of the cGAS-cGAMP-STING pathway and inflammatory responses. Conversely, fat-specific overexpression of DsbA-L protected mice against high-fat diet-induced activation of the cGAS-cGAMP-STING pathway and inflammation. Taken together, we identify DsbA-L as a key molecule that maintains mitochondrial integrity. DsbA-L deficiency promotes inflammation and insulin resistance by activating the cGAS-cGAMP-STING pathway. Our study also reveals that, in addition to its well-characterized roles in innate immune surveillance, the cGAS-cGAMP-STING pathway plays an important role in mediating obesity-induced metabolic dysfunction.

Maternal intake of grape seed procyanidins during lactation induces insulin resistance and an adiponectin resistance-like phenotype in rat offspring

Previously, we demonstrated that a grape seed procyanidin extract (GSPE) supplementation in pregnant and lactating rats exerted both healthy and deleterious programming effects on their offspring. Here, we evaluated whether the administration of GSPE during lactation (100 mg.kg⁻¹.day⁻¹) in rats elicited beneficial effects in their normoweight (STD-GSPE group) and cafeteria-fed obese (CAF-GSPE group) adult male offspring. STD-GSPE and CAF-GSPE offspring showed increased energy expenditure and circulating total and high-molecular-weight adiponectin. However, these rats showed hyperinsulinemia, decreased insulin sensitivity, increased insulin resistance, down-regulated mRNA levels of adiponectin receptors in inguinal white adipose tissue (Adipor1 and Adipor2) and soleus muscle (Adipor2), and decreased levels of phosphorylated AMPK, the downstream post-receptor target of adiponectin, in the soleus muscle. These deleterious effects could be related to an increased lipid transfer to the pups through the milk, since GSPE-supplemented dams displayed decreased fat content and increased expression of lipogenic genes in their mammary glands, in addition to increased circulating total adiponectin and non-esterified free fatty acids. In conclusion, maternal intake of GSPE during lactation induced insulin resistance and an adiponectin resistance-like phenotype in their normoweight and obese offspring. These findings raise concerns about the possibility of using GSPE as a nutraceutical supplement during this period.

Hepatic DsbA-L protects mice from diet-induced hepatosteatosis and insulin resistance

Hepatic insulin resistance and hepatosteatosis in diet-induced obesity are associated with various metabolic diseases, yet the underlying mechanisms remain to be fully elucidated. Here we show that the expression levels of the disulfide-bond A oxidoreductase-like protein (DsbA-L) are significantly reduced in the liver of obese mice and humans. Liver-specific knockout or adenovirus-mediated overexpression of DsbA-L exacerbates or alleviates, respectively, high-fat diet-induced mitochondrial dysfunction, hepatosteatosis, and insulin resistance in mice. Mechanistically, we found that DsbA-L is localized in mitochondria and that its deficiency is associated with impairment of maximum respiratory capacity, elevated cellular oxidative stress, and increased JNK activity. Our results identify DsbA-L as a critical regulator of mitochondrial function, and its down-regulation in the liver may contribute to obesity-induced hepatosteatosis and whole body insulin resistance.-Chen, H., Bai, J., Dong, F., Fang, H., Zhang, Y., Meng, W., Liu, B., Luo, Y., Liu, M., Bai, Y., Abdul-Ghani, M. A., Li, R., Wu, J., Zeng, R., Zhou, Z., Dong, L. Q., Liu, F. Hepatic DsbA-L protects mice from diet-induced hepatosteatosis and insulin resistance.

Endoplasmic Reticulum (ER) Stress and Endocrine Disorders

The endoplasmic reticulum (ER) is the organelle where secretory and membrane proteins are synthesized and folded. Unfolded proteins that are retained within the ER can cause ER stress. Eukaryotic cells have a defense system called the “unfolded protein response” (UPR), which protects cells from ER stress. Cells undergo apoptosis when ER stress exceeds the capacity of the UPR, which has been revealed to cause human diseases. Although neurodegenerative diseases are well-known ER stress-related diseases, it has been discovered that endocrine diseases are also related to ER stress. In this review, we focus on ER stress-related human endocrine disorders. In addition to diabetes mellitus, which is well characterized, several relatively rare genetic disorders such as familial neurohypophyseal diabetes insipidus (FNDI), Wolfram syndrome, and isolated growth hormone deficiency type II (IGHD2) are discussed in this article.

Reciprocal Crosstalk Between Autophagic and Endocrine Signaling in Metabolic Homeostasis

Autophagy is a cellular quality control and energy-providing process that is under strict control by intra- and extracellular stimuli. Recently, there has been an exponential increase in autophagy research and its implications for mammalian physiology. Autophagy deregulation is now being implicated in many human diseases, and its modulation has shown promising results in several preclinical studies. However, despite the initial discovery of autophagy as a hormone-regulated process by De Duve in the early 1960s, endocrine regulation of autophagy still remains poorly understood. In this review, we provide a critical summary of our present understanding of the basic mechanism of autophagy, its regulation by endocrine hormones, and its contribution to endocrine and metabolic homeostasis under physiological and pathological settings. Understanding the cross-regulation of hormones and autophagy on endocrine cell signaling and function will provide new insight into mammalian physiology as well as promote the development of new therapeutic strategies involving modulation of autophagy in endocrine and metabolic disorders.

Wogonin enhances intracellular adiponectin levels and suppresses adiponectin secretion in 3T3-L1 adipocytes

As an insulin sensitizer and modulator of inflammatory responses, adiponectin has become a therapeutic target for insulin resistance, diabetes, and diabetes-related complications. Wogonin possesses anti-oxidative, anti-inflammatory, and anti-diabetic abilities. However, its effect on generation and secretion of adiponectin is ill-defined in adipocytes. Here, we demonstrated that wogonin administration augmented intracellular adiponectin levels and attenuated adiponectin release in a dose- and time-dependent manner in mature 3T3-L1 adipocytes, along with a suppression of PKCδ phosphorylation. Wogonin treatment also prevented PKCδ overexpression-induced reduction of intracellular adiponectin levels and enhancement of adiponectin release. In addition, wogonin supplementation dramatically increased AMPK phosphorylation and SirT1 expression. Inhibition of either AMPK or SirT1 mitigated wogonin action on adiponectin production and release. Furthermore, inhibition of AMPK by its specific inhibitor markedly reduced wogonin-enhanced mRNA and protein expressions of SirT1. These results suggested that wogonin regulated expression and secretion of adiponectin via PKCδ/AMPK/SirT1 signaling pathway in mature 3T3-L1 adipocytes.

The Influence of a KDT501, a Novel Isohumulone, on Adipocyte Function in Humans

OBJECTIVE

In a phase II clinical trial in nine obese, insulin-resistant humans, we observed that treatment with KDT501, a novel isohumulone drug, increased total and high-molecular weight (HMW) adiponectin in plasma. The objective was to determine whether KDT501 increased adiponectin secretion from subcutaneous white adipose tissue (SC WAT) and the underlying mechanism(s).

METHODS

Nine obese participants with either prediabetes or with normal glucose tolerance plus three features of metabolic syndrome were part of the study. SC WAT biopsies were performed before and after 28 days of KDT501 treatment in a clinical research setting. In addition, a cold stimulus was used to induce thermogenic gene expression. Adiponectin secretion was measured, and gene expression of 130 genes involved in adipose tissue function was determined. The effect of KDT501 on adipocyte mitochondrial function was analyzed in vitro.

RESULTS

SC WAT explants secreted more total and HMW adiponectin after KDT501 treatment (P < 0.05). After KDT501 treatment, a number of genes involved in thermogenesis and lipolysis were induced by cold (P < 0.05). KDT501 also potentiated β-adrenergic signaling (P < 0.001) and enhanced mitochondrial function in adipocytes (P < 0.001).

CONCLUSION

KDT501 induced adiponectin secretion posttranscriptionally and increased gene expression of thermogenic and lipolytic genes in response to cold stimulation. These beneficial effects on SC WAT may be explained by the ability of KDT501 to potentiate β-adrenergic signaling and enhance mitochondrial function in adipocytes.

CLINICAL TRIAL REGISTRATION

https://www.ClinicalTrials.gov, ID number: NCT02444910.

Fibroblast growth factor 21 reverses suppression of adiponectin expression via inhibiting endoplasmic reticulum stress in adipose tissue of obese mice

Fibroblast growth factor 21 (FGF21) has recently emerged as a novel endocrine hormone involved in the regulation of glucose and lipid metabolism. However, the exact mechanisms whereby FGF21 mediates insulin sensitivity remain not fully understood. In the present study, FGF21was administrated in high-fat diet-induced obese mice and tunicamycin-induced 3T3-L1 adipocytes, and metabolic parameters, endoplasmic reticulum (ER) stress indicators, and insulin signaling molecular were assessed by Western blotting. The administration of FGF21 in obese mice reduced body weight, blood glucose and serum insulin, and increased insulin sensitivity, resulting in alleviation of insulin resistance. Meanwhile, FGF21 treatment reversed suppression of adiponectin expression and restored insulin signaling via inhibiting ER stress in adipose tissue of obese mice. Additionally, suppression of ER stress via the ER stress inhibitor tauroursodeoxycholic acid increased adiponectin expression and improved insulin resistance in obese mice and in tunicamycin-induced adipocytes. In conclusion, our results showed that the administration of FGF21 reversed suppression of adiponectin expression and restored insulin signaling via inhibiting ER stress under the condition of insulin resistance, demonstrating the causative role of ER stress in downregulating adiponectin levels.

C1q/TNF-Related Protein 9 Protects Diabetic Rat Heart against Ischemia Reperfusion Injury: Role of Endoplasmic Reticulum Stress

As a newly identified adiponectin paralog, C1q/TNF-related protein 9 (CTRP9) reduces myocardial ischemia reperfusion (IR) injury through partially understood mechanisms. In the present study, we sought to identify the role of endoplasmic reticulum stress (ERS) in CTRP9 induced cardioprotection in diabetic heart. Isolated hearts from high-fat-diet (HFD) induced type 2 diabetic Sprague-Dawley rats were subjected to ex vivo IR protocol via a Langendorff apparatus at the presence of globular CTRP9. CTRP9 significantly improved post-IR heart function and reduced cardiac infarction, cardiomyocytes apoptosis, Caspase-3, Caspase-9, Caspase-12, TNF-α expression, and lactate dehydrogenase activity. The cardioprotective effect of CTRP9 was associated with reduced ERS and increased expression of disulfide-bond A oxidoreductase-like protein (DsbA-L) in diabetic heart. CTRP9 reduced ERS in thapsigargin (TG) treated cardiomyocytes and protected endoplasmic reticulum (ER) stressed H9c2 cells against simulated ischemia reperfusion (SIR) injury, concurrent with increased expression of DsbA-L. Knockdown of DsbA-L increased ERS and attenuated CTRP9 induced protection against SIR injury in H9c2 cells. Our findings demonstrated for the first time that CTRP9 exerts cardioprotection by reducing ERS in diabetic heart through increasing DsbA-L.

Autophagy and Obesity-Related Lung Disease

Obesity-related disease is a significant source of premature death and economic burden globally. It is also a common comorbidity in patients suffering from lung disease, affecting both severity and treatment success. However, this complex association between obesity and the lung is poorly understood. Autophagy is a self-recycling homeostatic process that has been linked to beneficial or deleterious effects, depending on the specific lung disease. Obesity affects autophagy in a tissue-specific manner, activating autophagy in adipocytes and impairing autophagy in hepatocytes, immune cells, and pancreatic β-cells, among others. Obesity is also characterized by chronic low-grade inflammation that can be modulated by the pro- and antiinflammatory effects of the autophagic machinery. Scant evidence exists regarding the impact of autophagy in obesity-related lung diseases, but there are communal pathways that could be related to disease pathogenesis. Important signaling molecules in obesity, including IL-17, leptin, adiponectin, NLRP3 inflammasome, and TLR-4, have been implicated in the pathogenesis of lung disease. These mediators are known to be modulated by autophagy activity. In this perspective, we highlight the recent advances in the understanding of autophagy in obesity-related conditions, as well as the potential mechanisms that can link autophagy and obesity in the pathogenesis of lung disease.

Stress signaling between organs in metazoa

Many organisms have developed a robust ability to adapt and survive in the face of environmental perturbations that threaten the integrity of their genome, proteome, or metabolome. Studies in multiple model organisms have shown that, in general, when exposed to stress, cells activate a complex prosurvival signaling network that includes immune and DNA damage response genes, chaperones, antioxidant enzymes, structural proteins, metabolic enzymes, and noncoding RNAs. The manner of activation runs the gamut from transcriptional induction of genes to increased stability of transcripts to posttranslational modification of important biosynthetic proteins within the stressed tissue. Superimposed on these largely autonomous effects are nonautonomous responses in which the stressed tissue secretes peptides and other factors that stimulate tissues in different organs to embark on processes that ultimately help the organism as a whole cope with stress. This review focuses on the mechanisms by which tissues in one organ adapt to environmental challenges by regulating stress responses in tissues of different organs.

Adiporedoxin, an upstream regulator of ER oxidative folding and protein secretion in adipocytes

Objective

Adipocytes are robust protein secretors, most notably of adipokines, hormone-like polypeptides, which act in an endocrine and paracrine fashion to affect numerous physiological processes such as energy balance and insulin sensitivity. To understand how such proteins are assembled for secretion we describe the function of a novel endoplasmic reticulum oxidoreductase, adiporedoxin (Adrx).

Methods

Adrx knockdown and overexpressing 3T3-L1 murine adipocyte cell lines and a knockout mouse model were used to assess the influence of Adrx on secreted proteins as well as the redox state of ER resident chaperones. The metabolic phenotypes of Adrx null mice were characterized and compared to WT mice. The correlation of Adrx levels BMI, adiponectin levels, and other inflammatory markers from adipose tissue of human subjects was also studied.

Results

Adiporedoxin functions via a CXXC active site, and is upstream of protein disulfide isomerase whose direct function is disulfide bond formation, and ultimately protein secretion. Over and under expression of Adrx in vitro enhances and reduces, respectively, the secretion of the disulfide-bonded proteins including adiponectin and collagen isoforms. On a chow diet, Adrx null mice have normal body weights, and glucose tolerance, are moderately hyperinsulinemic, have reduced levels of circulating adiponectin and are virtually free of adipocyte fibrosis resulting in a complex phenotype tending towards insulin resistance. Adrx protein levels in human adipose tissue correlate positively with adiponectin levels and negatively with the inflammatory marker phospho-Jun kinase.

Conclusion

These data support the notion that Adrx plays a critical role in adipocyte biology and in the regulation of mouse and human metabolism via its modulation of adipocyte protein secretion.

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Adrx is an adipocyte specific, endoplasmic reticulum oxidoreductase upstream of disulfide bond formation.

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Adrx over and under expression in vitro results enhanced and decreased protein secretion, respectively.

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Mice lacking Adrx have lower levels of circulating adiponectin and decreased fibrosis.

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Adrx is expressed in human adipocytes and down regulated in proportion to the level of inflammation.

Peroxisome proliferator-activated receptor γ enhances adiponectin secretion via up-regulating DsbA-L expression

Disulfide-bond A oxidoreductase like-protein (DsbA-L) was identified as a molecular chaperone facilitating the assembly and secretion of adiponectin, an adipokine with multiple beneficial effects. In obesity the level of DsbA-L is reduced with a concomitant decrease of the circulating adiponectin level, especially of the high molecular weight form (HMW). Both rodent and human studies have shown that the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-γ agonists increase adiponectin levels in serum by activating PPARγ, which up-regulates critical endoplasmic reticulum (ER) chaperones thus facilitating protein folding. As shown in the present study, overexpression of PPARγ in human embryonic kidney (HEK) 293 cells elicited the cellular release of HMW adiponectin. PPARγ enhanced expression of DsbA-L by binding directly to peroxisome proliferator response element (PPRE) site within the DsbA-L promoter. Conversely, in differentiated 3T3-L1 cells, PPARγ knockdown resulted in decreased expression of Adiponectin, DsbA-L and ERp44. DsbA-L expression increased after PPARγ agonist treatment and decreased upon treatment with PPARγ antagonist in 3T3-L1 adipocytes. DsbA-L deficiency in differentiated 3T3-L1 cells impaired the secretion of adiponectin. We therefore propose that DsbA-L plays an important role in facilitating HMW adiponectin formation and release from cells under the regulation of PPARγ.

Obesity-induced DNA hypermethylation of the adiponectin gene mediates insulin resistance

Adiponectin plays a key role in the regulation of the whole-body energy homeostasis by modulating glucose and lipid metabolism. Although obesity-induced reduction of adiponectin expression is primarily ascribed to a transcriptional regulation failure, the underlying mechanisms are largely undefined. Here we show that DNA hypermethylation of a particular region of the adiponectin promoter suppresses adiponectin expression through epigenetic control and, in turn, exacerbates metabolic diseases in obesity. Obesity-induced, pro-inflammatory cytokines promote DNMT1 expression and its enzymatic activity. Activated DNMT1 selectively methylates and stimulates compact chromatin structure in the adiponectin promoter, impeding adiponectin expression. Suppressing DNMT1 activity with a DNMT inhibitor resulted in the amelioration of obesity-induced glucose intolerance and insulin resistance in an adiponectin-dependent manner. These findings suggest a critical role of adiponectin gene epigenetic control by DNMT1 in governing energy homeostasis, implying that modulating DNMT1 activity represents a new strategy for the treatment of obesity-related diseases.