Enhanced ANGPTL2 expression in adipose tissues and its association with insulin resistance in obese women

Sexually dimorphic effects of angiopoietin-like 2 on energy metabolism and hypothalamic neuropeptide regulation

BACKGROUND

Adipokines regulate body weight and metabolism by targeting the hypothalamus, influencing feeding, energy expenditure (EE) and insulin sensitivity. Angiopoietin-like 2 (Angptl2) is a pro-inflammatory adipokine linking obesity to insulin resistance. Both Angptl2 and its receptor are expressed in the central nervous system. Yet, the contribution of Angptl2 to the regulation of energy metabolism and relevant hypothalamic neuropeptides in male and female mice is unknown. We aim at determining the impact of Angptl2 knockdown (KD) on energy balance, nutrient partitioning and hypothalamic responses to a standard (STD) or high-fat diet (HFD) in mice.

METHODS

Three-month-old male and female Angptl2-KD mice and wildtype (WT) littermates were fed 16 weeks either a STD or a HFD. Body weight, food consumption and insulin sensitivity were assessed along with measurements of EE, respiratory exchange ratio (RER) and locomotor activity. We quantified the expression of Angptl2 and its receptors itga5, mag and pirb in the medio-basal hypothalamus (MBH) of WT mice, and MBH neuropeptide Y (NPY), agouti-related neuropeptide (AgRP) and proopiomelanocortin (POMC) gene expression in both KD and control fasting mice.

RESULTS

Lack of Angptl2 reduced food intake in males on both diets, and in females on HFD. In KD males, this anorexigenic effect was associated with lower body weight, increased EE, improved insulin sensitivity and lower hypothalamic orexigenic NPY expression compared to controls. Female Angptl2-KD mice however, exhibited unaltered body weight, EE and insulin sensitivity, and elevated NPY, AgRP and MC4R expression compared to controls. Fasting caused an increase in the MBH of mag expression in males and females but Angptl2 expression only in female mice.

CONCLUSIONS

Angptl2 KD improved diet-induced obesity and associated metabolic dysfunction in male mice. The lack of similar changes in female mice and divergent MBH neuropeptide profile suggest that sex-dependent mechanisms underly the anabolic effects of this proinflammatory adipokine.

Potential Therapeutic Exploitation of G Protein-Coupled Receptor 120 (GPR120/FFAR4) Signaling in Obesity-Related Metabolic Disorders

The increasing prevalence of overweight and obesity not only in adults but also among children and adolescents has become one of the most alarming health problems worldwide. Metabolic disorders accompanying fat accumulation during pathological weight gain induce chronic low-grade inflammation, which, in a vicious cycle, increases the immune response through pro-inflammatory changes in the cytokine (adipokine) profile. Obesity decreases life expectancy, largely because obese individuals are at an increased risk of many medical complications, often referred to as metabolic syndrome, which refers to the co-occurrence of insulin resistance (IR), impaired glucose tolerance, type 2 diabetes (T2D), atherogenic dyslipidemia, hypertension, and premature ischemic heart disease. Metabotropic G protein-coupled receptors (GPCRs) constitute the most numerous and diverse group of cell surface transmembrane receptors in eukaryotes. Among the GPCRs, researchers are focusing on the connection of G protein-coupled receptor 120 (GPR120), also known as free fatty acid receptor 4 (FFAR4), with signaling pathways regulating the inflammatory response and insulin sensitivity. This review presents the current state of knowledge concerning the involvement of GPR120 in anti-inflammatory and metabolic signaling. Since both inflammation in adipose tissue and insulin resistance are key problems in obesity, there is a rationale for the development of novel, GPR120-based therapies for overweight and obese individuals. The main problems associated with introducing this type of treatment into clinical practice are also discussed.

Comprehensive Matrisome Profiling of Human Adipose Tissue for Soft Tissue Reconstruction

For effective translation of research from tissue engineering and regenerative medicine domains, the cell-instructive extracellular matrix (ECM) of specific tissues must be accurately realized. As adipose tissue is gaining traction as a biomaterial for soft tissue reconstruction, with highly variable clinical outcomes obtained, a quantitative investigation of the adipose tissue matrisome is overdue. In this study, the human adipose tissue matrisome is profiled using quantitative sequential windowed acquisition of all theoretical fragment ion spectra - mass spectrometry (SWATH-MS) proteomics across a cohort of 13 fat-grafting patients, to provide characterization of ECM proteins within the tissue, and to understand human population variation. There are considerable differences in the expression of matrisome proteins across the patient cohort, with age and lipoaspirate collection technique contributing to the greatest variation across the core matrisome. A high abundance of basement membrane proteins (collagen IV and heparan sulfate proteoglycan) is detected, as well as fibrillar collagens I and II, reflecting the hierarchical structure of the tissue. This study provides a comprehensive proteomic evaluation of the adipose tissue matrisome and contributes to an enhanced understanding of the influence of the matrisome in adipose-related pathologies by providing a healthy reference cohort and details an experimental pipeline that can be further exploited for future biomaterial development.

Adipocyte hypertrophy associates with in vivo postprandial fatty acid metabolism and adipose single-cell transcriptional dynamics

Adipocyte hypertrophy is associated with metabolic complications independent of obesity. We aimed to determine: 1) the association between adipocyte size and postprandial fatty acid metabolism; 2) the potential mechanisms driving the obesity-independent, hypertrophy-associated dysmetabolism in vivo and at a single-cell resolution. Tracers with positron emission tomography were used to measure fatty acid metabolism in 40 men and women with normal or impaired glucose tolerance (NCT02808182), and single nuclei RNA-sequencing (snRNA-seq) to determine transcriptional dynamics of subcutaneous adipose tissue (AT) between individuals with AT hypertrophy vs. hyperplasia matched for sex, ethnicity, glucose-tolerance status, BMI, total and percent body fat, and waist circumference. Adipocyte size was associated with high postprandial total cardiac fatty acid uptake and higher visceral AT dietary fatty acid uptake, but lower lean tissue dietary fatty acid uptake. We found major shifts in cell transcriptomal dynamics with AT hypertrophy that were consistent with in vivo metabolic changes.

Molecular Mechanisms for the Vicious Cycle between Insulin Resistance and the Inflammatory Response in Obesity

The comprehensive anabolic effects of insulin throughout the body, in addition to the control of glycemia, include ensuring lipid homeostasis and anti-inflammatory modulation, especially in adipose tissue (AT). The prevalence of obesity, defined as a body mass index (BMI) ≥ 30 kg/m², has been increasing worldwide on a pandemic scale with accompanying syndemic health problems, including glucose intolerance, insulin resistance (IR), and diabetes. Impaired tissue sensitivity to insulin or IR paradoxically leads to diseases with an inflammatory component despite hyperinsulinemia. Therefore, an excess of visceral AT in obesity initiates chronic low-grade inflammatory conditions that interfere with insulin signaling via insulin receptors (INSRs). Moreover, in response to IR, hyperglycemia itself stimulates a primarily defensive inflammatory response associated with the subsequent release of numerous inflammatory cytokines and a real threat of organ function deterioration. In this review, all components of this vicious cycle are characterized with particular emphasis on the interplay between insulin signaling and both the innate and adaptive immune responses related to obesity. Increased visceral AT accumulation in obesity should be considered the main environmental factor responsible for the disruption in the epigenetic regulatory mechanisms in the immune system, resulting in autoimmunity and inflammation.

AMP-activated protein kinase is a key regulator of obesity-associated factors

An imbalance between caloric intake and energy expenditure leads to obesity. Obesity is an important risk factor for the development of several metabolic diseases including insulin resistance, metabolic syndrome, type 2 diabetes mellitus, and cardiovascular disease. So, controlling obesity could be effective in the improvement of obesity-related diseases. Various factors are involved in obesity, such as AMP-activated protein kinases (AMPK), silent information regulators, inflammatory mediators, oxidative stress parameters, gastrointestinal hormones, adipokines, angiopoietin-like proteins, and microRNAs. These factors play an important role in obesity by controlling fat metabolism, energy homeostasis, food intake, and insulin sensitivity. AMPK is a heterotrimeric serine/threonine protein kinase known as a fuel-sensing enzyme. The central role of AMPK in obesity makes it an attractive molecule to target obesity and related metabolic diseases. In this review, the critical role of AMPK in obesity and the interplay between AMPK and obesity-associated factors were elaborated.

Adipose tissue LECT2 expression is associated with obesity and insulin resistance in Korean women

OBJECTIVE

Leukocyte cell-derived chemotaxin 2 (LECT2) is an obesity-upregulated hepatokine inducing skeletal muscle insulin resistance. The study's aim was to explore whether LECT2 is expressed in human adipose tissue and whether the expression is dysregulated during obesity and associated with obesity-related metabolic disorders.

METHODS

This study measured metabolic parameters, serum LECT2, and expression of LECT2 and CD209, a gene encoding a putative receptor for LECT2, in abdominal subcutaneous and visceral adipose tissues in women with obesity (with or without type 2 diabetes) and women with normal weight. The expression/secretion of LECT2 and its putative effects were assessed in human adipocytes.

RESULTS

Adipose tissue LECT2 mRNA and serum LECT2 were higher in women with obesity and were significantly correlated with parameters related to insulin resistance. LECT2 was mainly expressed by adipocytes. Both LECT2 and CD209 expression was higher in adipocytes from women with obesity. Incubating adipocytes with substances mimicking the microenvironment of obesity adipose tissue increased LECT2 expression/secretion. LECT2 treatment of adipocytes suppressed insulin-stimulated Akt phosphorylation; it reduced adiponectin (ADIPOQ) and increased leptin (LEP) expression in a CD209-dependent manner.

CONCLUSIONS

This study demonstrates that LECT2 expression in adipose tissue is high in patients with obesity and associated with insulin resistance and suggests that adipocyte-derived LECT2 may contribute to adipose tissue dysfunction.

TRIB3 Is Highly Expressed in the Adipose Tissue of Obese Patients and Is Associated With Insulin Resistance

CONTEXT

The upregulation of TRIB3 (Tribbles homolog 3), a stress-inducible gene encoding a pseudokinase, has been implicated in the development of insulin resistance in the skeletal muscle and liver of patients with obesity and type 2 diabetes. However, there is little information regarding TRIB3 expression in human adipose tissue.

OBJECTIVE

To investigate whether TRIB3 expression is dysregulated in human adipose tissue in the context of obesity and type 2 diabetes and whether TRIB3 expression in adipose tissues is associated with insulin resistance.

METHODS

We measured metabolic parameters and TRIB3 expression in abdominal subcutaneous and visceral adipose tissue in obese (with or without type 2 diabetes) and normal-weight women. Regulation of TRIB3 expression was studied in human adipocytes.

RESULTS

TRIB3 expression in both fat depots was higher in patients with obesity and/or type 2 diabetes; in addition, the expression level was significantly associated with insulin resistance. Incubating adipocytes under conditions mimicking the microenvironment of obese adipose tissue, including increased endoplasmic reticulum (ER) stress, induced TRIB3 expression. In human adipocytes, the overexpression of TRIB3 impaired insulin-stimulated protein kinase B (AKT) phosphorylation and caused dysregulation of the transcription of genes encoding bioactive molecules released from adipocytes, such as proinflammatory cytokines, adiponectin, and leptin. Pioglitazone, an insulin-sensitizing agent, reduced both these effects of TRIB3 and the ER stressor-induced expression of TRB3.

CONCLUSION

Our data indicate that TRIB3 expression in adipose tissue is enhanced in patients with obesity and suggest that increased TRIB3 dysregulates adipocyte function, which may contribute to the development of insulin resistance.

miR-124-3p Combined with ANGPTL2 Has High Diagnostic Values for Obese and Nonobese Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a hormonal disorder that affects 5-20% of women of reproductive age. Interestingly, serum miR-124-3p and ANGPTL2 are differentially expressed in PCOS patients. Accordingly, this study set out to explore the clinical roles of serum miR-124-3p/ANGPTL2 in PCOS. Firstly, miR-124-3p/ANGPTL2 expression patterns were detected in the serum of 102 PCOS patients and 100 healthy subjects. miR-124-3p or/and ANGPTL2 diagnostic efficacy on PCOS was further analyzed, in addition to the measurement of lipid metabolism, glucose metabolism, sex hormone indexes, and inflammation levels. Correlations between serum miR-124-3p/ANGPTL2 expressions and age, BMI, Ferriman-Gallwey score, lipid metabolism, glucose metabolism, sex hormone indexes, TNF-α, and IL-6 in PCOS patients were determined. The expression correlation and binding relationship of ANGPTL2 and miR-124-3p were identified. In addition, miR-124-3p was downregulated and ANGPTL2 was upregulated in the serum of obese and nonobese PCOS patients. miR-124-3p expression was found to be negatively correlated with Ferriman-Gallwey score and serum total testosterone (T), and negatively related to prolactin (PRL). ANGPTL2 expression was positively correlated with FNS and inversely linked with PRL. TNF-α and IL-6 were negatively correlated with miR-124-3p, but positively correlated with ANGPTL2. Furthermore, there was a negative correlation and a targeting relationship between ANGPTL2 and miR-124-3p expression in the serum of obese and nonobese PCOS patients. Collectively, our findings indicated that miR-124-3p might target ANGPTL2 expression in obese and nonobese PCOS patients, and further underscored the diagnostic value of their combination.

Angptl2 gene knockdown is critical for abolishing angiotensin II-induced vascular smooth muscle cell proliferation and migration

Angiopoietin-like 2 (Angptl2) is reported to be correlated with cardiovascular diseases, but its role in hypertension remains unclear. This study aimed to investigate the role and potential mechanism of Angptl2 in hypertension. Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) were used to detect the expression of Angptl2. Angiotensin II (Ang II) stimulates vascular smooth muscle cells (VSMCs) to mimic hypertension in vitro. Cell proliferation, migration, and invasion abilities were determined using CCK-8, cell colony formation, wound healing, and transwell assays, respectively. The cell cycle distribution was detected by flow cytometry. The expression of Ki67 was determined by immunofluorescence, and protein expression was measured using western blotting. Angptl2 was found to be elevated in hypertensive rats in vivo and in VSMCs upon Ang II stimulation in vitro. Angptl2 knockdown suppressed cell proliferation, colony formation, cell migration, and invasion as well as the downregulation of Ki67. Additionally, Angptl2 knockdown hindered cell cycle progression and downregulated protein expression of CDK2/4 and cyclin D1, but upregulated p21 expression. Furthermore, Angptl2 knockdown inhibited activation of the NLRP3 inflammasome. Our findings suggest that Angptl2 knockdown suppresses VSMC proliferation, migration, and invasion induced by Ang II. Angptl2 may be a new target for vascular remodeling in hypertension.

Angiopoietin-like protein 2 deficiency promotes periodontal inflammation and alveolar bone loss

BACKGROUND

Human periodontitis is a highly prevalent inflammatory disease that leads to connective tissue degradation, alveolar bone resorption, and tooth loss. Angiopoietin-like 2 (ANGPTL2) regulates chronic inflammation in various diseases and is functionally involved in maintaining tissue homeostasis and promoting tissue regeneration, but there is limited information about its function in periodontitis. Here we investigated the expression and explicit role of ANGPTL2 in periodontitis.

METHODS

Immunohistochemistry and quantitative real-time PCR (qRT-PCR) were used to detect the ANGPTL2 expression in periodontal tissues and periodontal ligament cells (PDLCs). A ligature-induced periodontitis model was generated in wild-type and ANGPTL2 knockout mice. qRT-PCR and enzyme-linked immunosorbent assay were used to assess the production of inflammatory cytokines and matrix metalloproteinases (MMPs) in cultured PDLCs. Western blot was performed to detect proteins in relevant signaling pathways.

RESULTS

Increased ANGPTL2 expression was observed in inflamed periodontal tissues and PDLCs. ANGPTL2 deficiency promoted alveolar bone loss with enhanced osteoclastogenesis and inflammatory reactions in ligature-induced periodontitis. Downregulation of ANGPTL2 remarkably enhanced expression levels of interleukin (IL)-6, IL-8, MMP1, and MMP13 in Porphyromonas gingivalis lipopolysaccharide-induced PDLCs, whereas ANGPTL2-overexpressing PDLCs showed opposite trends. ANGPTL2 downregulation activated STAT3 and nuclear factor-κB pathways and blocked Akt signaling under inflammatory environment. Treatment with a STAT3 inhibitor partially suppressed the inflammatory reaction of PDLCs mediated by ANGPTL2 knockdown.

CONCLUSIONS

Our study provides the first evidence of an anti-inflammatory effect of ANGPTL2 in murine periodontitis. The findings demonstrate the critical and protective role of ANGPTL2 in alveolar bone loss and periodontal inflammation.

Angiopoietin-Like Proteins 2 and 3 in Children and Adolescents with Obesity and Their Relationship with Hypertension and Metabolic Syndrome

Background

Angiopoietin-like protein 2 (ANGPTL2) is one of the adipocyte-derived inflammatory factors which connects obesity to insulin resistance. ANGPTL3 has a direct role in regulation of lipid metabolism. The objective of this study was to evaluate ANGPTL2 and ANGPTL3 in childhood obesity and their relationship with metabolic syndrome.

Methods

70 children and adolescents, 35 obese and 35 normal-weight subjects, were enrolled in this research after complete clinical examination and anthropometric evaluations. Serum ANGPTL2 and ANGPTL3 and insulin were measured by enzyme-linked immunosorbent assay (ELISA). Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated and used to estimate insulin resistance (IR). Colorimetric methods were used for the assessment of fasting plasma glucose (FPG), LDL-C, HDL-C, total cholesterol (TC), and triglyceride (TG).

Results

The levels of ANGPTL2 and ANGPTL3 were significantly higher in obese subjects than those in controls, but they did not differ significantly in subjects with or without IR. ANGPTL3 was found to be significantly elevated in obese children with metabolic syndrome (MetS) in comparison with those without MetS. Both of the studied ANGPTLs were positively correlated with BMI, systolic blood pressure (SBP), diastolic blood pressure (DBP), TC, and LDL-C. The correlation between ANGPTL3 and either TC or LDL-C remained significant after adjusting for BMI.

Conclusion

Serum ANGPTL2 and ANGPTL3 were elevated in obesity and associated with blood pressure and indices of metabolic syndrome, suggesting that they might be involved in the advancement of obesity-related hypertension and metabolic syndrome.

CD146 is a Novel ANGPTL2 Receptor that Promotes Obesity by Manipulating Lipid Metabolism and Energy Expenditure

Obesity and its related complications pose an increasing threat to human health; however, targetable obesity-related membrane receptors are not yet elucidated. Here, the membrane receptor CD146 is demonstrated to play an essential role in obesity. In particular, CD146 acts as a new adipose receptor for angiopoietin-like protein 2 (ANGPTL2), which is thought to act on endothelial cells to activate adipose inflammation. ANGPTL2 binds to CD146 to activate cAMP response element-binding protein (CREB), which then upregulates CD146 during adipogenesis and adipose inflammation. CD146 is present in preadipocytes and mature adipocytes, where it is mediated by its ligands ANGPTL2 and galectin-1. In preadipocytes, CD146 ablation suppresses adipogenesis, whereas the loss of CD146 in mature adipocytes suppresses lipid accumulation and enhances energy expenditure. Moreover, anti-CD146 antibodies inhibit obesity by disrupting the interactions between CD146 and its ligands. Together, these findings demonstrate that ANGPTL2 directly affects adipocytes via CD146 to promote obesity, suggesting that CD146 can be a potential target for treating obesity.

Selective Peroxisome Proliferator-Activated Receptor Alpha Modulators (SPPARMα) in the Metabolic Syndrome: Is Pemafibrate Light at the End of the Tunnel?

Purpose of Review

Adoption of poor lifestyles (inactivity and energy-dense diets) has driven the worldwide increase in the metabolic syndrome, type 2 diabetes mellitus and non-alcoholic steatohepatitis (NASH). Of the defining features of the metabolic syndrome, an atherogenic dyslipidaemia characterised by elevated triglycerides (TG) and low plasma concentration of high-density lipoprotein cholesterol is a major driver of risk for atherosclerotic cardiovascular disease. Beyond lifestyle intervention and statins, targeting the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα) is a therapeutic option. However, current PPARα agonists (fibrates) have limitations, including safety issues and the lack of definitive evidence for cardiovascular benefit. Modulating the ligand structure to enhance binding at the PPARα receptor, with the aim of maximising beneficial effects and minimising adverse effects, underlies the SPPARMα concept.

Recent Findings

This review discusses the history of SPPARM development, latterly focusing on evidence for the first licensed SPPARMα, pemafibrate. Evidence from animal models of hypertriglyceridaemia or NASH, as well as clinical trials in patients with atherogenic dyslipidaemia, are overviewed.

Summary

The available data set the scene for therapeutic application of SPPARMα in the metabolic syndrome, and possibly, NASH. The outstanding question, which has so far eluded fibrates in the setting of current evidence-based therapy including statins, is whether treatment with pemafibrate significantly reduces cardiovascular events in patients with atherogenic dyslipidaemia. The PROMINENT study in patients with type 2 diabetes mellitus and this dyslipidaemia is critical to evaluating this.

Remodeling adipose tissue inflammasome for type 2 diabetes mellitus treatment: Current perspective and translational strategies

Obesity-associated type 2 diabetes mellitus (T2DM) is characterized by low-grade chronic systemic inflammation that arises primarily from the white adipose tissue. The interplay between various adipose tissue-derived chemokines drives insulin resistance in T2DM and has therefore become a subject of rigorous investigation. The adipocytokines strongly associated with glucose homeostasis include tumor necrosis factor-α, various interleukins, monocyte chemoattractant protein-1, adiponectin, and leptin, among others. Remodeling the adipose tissue inflammasome in obesity-associated T2DM is likely to treat the underlying cause of the disease and bring significant therapeutic benefit. Various strategies have been adopted or are being investigated to modulate the serum/tissue levels of pro- and anti-inflammatory adipocytokines to improve glucose homeostasis in T2DM. These include use of small molecule agonists/inhibitors, mimetics, antibodies, gene therapy, and other novel formulations. Here, we discuss adipocytokines that are strongly associated with insulin activity and therapies that are under investigation for modulation of their levels in the treatment of T2DM.

Depot-specific UCP1 expression in human white adipose tissue and its association with obesity-related markers

BACKGROUND

This study investigated depot-specific mRNA expression of uncoupling protein 1 (UCP1) in human white adipose tissue (WAT) and its association with obesity-related markers.

METHODS

We recruited 39 normal-weight, 41 nondiabetic obese, and 22 diabetic obese women. We measured UCP1 mRNA expression in abdominal visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT), and investigated the associations between UCP1 mRNA expression in VAT and SAT, and obesity-related markers including mRNA expression of leptin, adiponectin, CCAAT-enhancer-binding protein homologous protein (CHOP), and positive regulatory domain-containing protein 16 (PRDM16). We also evaluated UCP1 mRNA expression in differentiated human white adipocytes after treatment with various stressors and metabolic improvement agents in vitro.

RESULTS

UCP1 mRNA in VAT was significantly higher than in SAT in all groups. UCP1 mRNA in SAT was negatively correlated with BMI, total abdominal fat area, visceral fat area, blood pressure, fasting glucose, insulin, HOMA-IR score, triglyceride, hsCRP, fasting leptin levels, and adipocyte size. UCP1 mRNA in SAT was positively correlated with fasting adiponectin levels. UCP1 mRNA in VAT was negatively correlated with visceral-to-subcutaneous fat ratio (VSR), fasting glucose, and triglyceride levels. In SAT, UCP1 mRNA was negatively correlated with mRNA expression of leptin and CHOP, and positively correlated with mRNA expression of adiponectin. The expression of PRDM16 was positively correlated with UCP1 mRNA in both VAT and SAT. UCP1 mRNA expression in differentiated human white adipocytes was significantly reduced after incubation with thapsigargin, tunicamycin, homocysteine, TNF-α, or IL-β, and significantly increased after incubation with exendin 4, dapagliflozin, and telmisartan.

CONCLUSIONS

This study demonstrated depot-specific mRNA expression of UCP1 and its association with obesity-related markers in human WAT. UCP1 mRNA in human white adipocytes was suppressed by inflammatory agents and enhanced by metabolic improvement agents. UCP1 in human WAT might participate in the pathogenesis of obesity-related metabolic diseases.

Altered Expression of Adrenomedullin 2 and its Receptor in the Adipose Tissue of Obese Patients

CONTEXT

Adrenomedullin 2 (AM2) plays protective roles in the renal and cardiovascular systems. Recent studies in experimental animals demonstrated that AM2 is an adipokine with beneficial effects on energy metabolism. However, there is little information regarding AM2 expression in human adipose tissue.

OBJECTIVE

To investigate the pattern and regulation of the expression of AM2 and its receptor component in human adipose tissue, in the context of obesity and type 2 diabetes.

METHODS

We measured metabolic parameters, serum AM2, and expression of ADM2 and its receptor component genes in abdominal subcutaneous and visceral adipose tissue in obese (with or without type 2 diabetes) and normal-weight women. Serum AM2 was assessed before and 6 to 9 months after bariatric surgery. Expression/secretion of AM2 and its receptor were assessed in human adipocytes.

RESULTS

ADM2 mRNA in both fat depots was higher in obese patients, whether diabetic or not. Although serum AM2 was significantly lower in obese patients, it was not changed after bariatric surgery. AM2 and its receptor complex were predominantly expressed by adipocytes, and the expression of CALCRL, encoding a component of the AM2 receptor complex, was lower in both fat depots of obese patients. Incubating adipocytes with substances mimicking the microenvironment of obese adipose tissue increased ADM2 mRNA but reduced both AM2 secretion into culture media and CALCRL mRNA expression.

CONCLUSIONS

Our data indicate that AM2 signaling is suppressed in adipose tissue in obesity, involving lower receptor expression and ligand availability, likely contributing to insulin resistance and other aspects of the pathophysiology associated with obesity.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.