Opposing effects of adiponectin receptors 1 and 2 on energy metabolism

Apigenin-6-C-glucoside ameliorates MASLD in rodent models via selective agonism of adiponectin receptor 2

Adiponectin plays key roles in energy metabolism and ameliorates inflammation, oxidative stress, and mitochondrial dysfunction via its primary receptors, adiponectin receptors -1 and 2 (AdipoR1 and AdipoR2). Systemic depletion of adiponectin causes various metabolic disorders, including MASLD; however adiponectin supplementation is not yet achievable owing to its large size and oligomerization-associated complexities. Small-molecule AdipoR agonists, thus, may provide viable therapeutic options against metabolic disorders. Using a novel luciferase reporter-based assay here, we have identified Apigenin-6-C-glucoside (ACG), but not apigenin, as a specific agonist for the liver-rich AdipoR isoform, AdipoR2 (EC50: 384 pM) with >10000X preference over AdipoR1. Immunoblot analysis in HEK-293 overexpressing AdipoR2 or HepG2 and PLC/PRF/5 liver cell lines revealed rapid AMPK, p38 activation and induction of typical AdipoR targets PGC-1α and PPARα by ACG at a pharmacologically relevant concentration of 100 nM (reported cMax in mouse; 297 nM). ACG-mediated AdipoR2 activation culminated in a favorable modulation of key metabolic events, including decreased inflammation, oxidative stress, mitochondrial dysfunction, de novo lipogenesis, and increased fatty acid β-oxidation as determined by immunoblotting, QRT-PCR and extracellular flux analysis. AdipoR2 depletion or AMPK/p38 inhibition dampened these effects. The in vitro results were recapitulated in two different murine models of MASLD, where ACG at 10 mg/kg body weight robustly reduced hepatic steatosis, fibrosis, proinflammatory macrophage numbers, and increased hepatic glycogen content. Together, using in vitro experiments and rodent models, we demonstrate a proof-of-concept for AdipoR2 as a therapeutic target for MASLD and provide novel chemicobiological insights for the generation of translation-worthy pharmacological agents.

Aging AdipoR2-deficient mice are hyperactive with enlarged brains excessively rich in saturated fatty acids

To investigate how the fatty acid composition of brain phospholipids influences brain-specific processes, we leveraged the AdipoR2 (adiponectin receptor 2) knockout mouse model in which the brain is enlarged, and cellular membranes are excessively rich in saturated fatty acids. Lipidomics analysis of brains at 2, 7, and 18 months of age showed that phosphatidylcholines, which make up about two-thirds of all cerebrum membrane lipids, contain a gross excess of saturated fatty acids in AdipoR2 knockout mice, and that this is mostly attributed to an excess palmitic acid (C16:0) at the expense of oleic acid (C18:1), consistent with a defect in fatty acid desaturation and elongation in the mutant. Specifically, there was a ~12% increase in the overall saturated fatty acid content within phosphatidylcholines and a ~30% increase in phosphatidylcholines containing two palmitic acids. Phosphatidylethanolamines, sphingomyelins, ceramides, lactosylceramides, and dihydroceramides also showed an excess of saturated fatty acids in the AdipoR2 knockout mice while nervonic acid (C24:1) was enriched at the expense of shorter saturated fatty acids in glyceroceramides. Similar defects were found in the cerebellum and myelin sheaths. Histology showed that cell density is lower in the cerebrum of AdipoR2 knockout mice, but electron microscopy did not detect reproducible defects in the ultrastructure of cerebrum neurons, though proteomics analysis showed an enrichment of electron transport chain proteins in the cerebellum. Behavioral tests showed that older (33 weeks old) AdipoR2 knockout mice are hyperactive and anxious compared to control mice of a similar age. Also, in contrast to control mice, the AdipoR2 knockout mice do not gain weight in old age but do have normal lifespans. We conclude that an excess fatty acid saturation in brain phospholipids is accompanied by hyperactivity but seems otherwise well tolerated.

Regulation of meiotic telomere dynamics through membrane fluidity promoted by AdipoR2-ELOVL2

The cellular membrane in male meiotic germ cells contains a unique class of phospholipids and sphingolipids that is required for male reproduction. Here, we show that a conserved membrane fluidity sensor, AdipoR2, regulates the meiosis-specific lipidome in mouse testes by promoting the synthesis of sphingolipids containing very-long-chain polyunsaturated fatty acids (VLC-PUFAs). AdipoR2 upregulates the expression of a fatty acid elongase, ELOVL2, both transcriptionally and post-transcriptionally, to synthesize VLC-PUFA. The depletion of VLC-PUFAs and subsequent accumulation of palmitic acid in AdipoR2 knockout testes stiffens the cellular membrane and causes the invagination of the nuclear envelope. This condition impairs the nuclear peripheral distribution of meiotic telomeres, leading to errors in homologous synapsis and recombination. Further, the stiffened membrane impairs the formation of intercellular bridges and the germ cell syncytium, which disrupts the orderly arrangement of cell types within the seminiferous tubules. According to our findings we propose a framework in which the highly-fluid membrane microenvironment shaped by AdipoR2-ELOVL2 underpins meiosis-specific chromosome dynamics in testes.

Adipose tissue as a therapeutic target for vascular damage in Alzheimer's disease

Adipose tissue has recently been recognized as an important endocrine organ that plays a crucial role in energy metabolism and in the immune response in many metabolic tissues. With this regard, emerging evidence indicates that an important crosstalk exists between the adipose tissue and the brain. However, the contribution of adipose tissue to the development of age-related diseases, including Alzheimer's disease, remains poorly defined. New studies suggest that the adipose tissue modulates brain function through a range of endogenous biologically active factors known as adipokines, which can cross the blood-brain barrier to reach the target areas in the brain or to regulate the function of the blood-brain barrier. In this review, we discuss the effects of several adipokines on the physiology of the blood-brain barrier, their contribution to the development of Alzheimer's disease and their therapeutic potential. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

The Effect and Mechanism of Oleanolic Acid in the Treatment of Metabolic Syndrome and Related Cardiovascular Diseases

Metabolic syndromes (MetS) and related cardiovascular diseases (CVDs) pose a serious threat to human health. MetS are metabolic disorders characterized by obesity, dyslipidemia, and hypertension, which increase the risk of CVDs' initiation and development. Although there are many availabile drugs for treating MetS and related CVDs, some side effects also occur. Considering the low-level side effects, many natural products have been tried to treat MetS and CVDs. A five-cyclic triterpenoid natural product, oleanolic acid (OA), has been reported to have many pharmacologic actions such as anti-hypertension, anti-hyperlipidemia, and liver protection. OA has specific advantages in the treatment of MetS and CVDs. OA achieves therapeutic effects through a variety of pathways, attracting great interest and playing a vital role in the treatment of MetS and CVDs. Consequently, in this article, we aim to review the pharmacological actions and potential mechanisms of OA in treating MetS and related CVDs.

Adiponectin-mediated regulation of the adiponectin cascade in cardiovascular disease: Updates

The endocrine function of white adipose tissue is characterized by the synthesis of one its main hormones: adiponectin. Although the biological role of adiponectin has not been fully defined, clinical and experimental observations have shown that low plasma concentrations of adiponectin participate in the prevalence of insulin resistance and cardiovascular diseases, mainly in obese patients. Adiponectin also exerts its effects on the heart and blood vessels, thereby influencing their physiology. Studying the effects of adiponectin presents some complexities, primarily due to potential cross-interactions and interference with other pathways, such as the AdipoR1/R2 pathways. Under optimal conditions, the activation of the adiponectin cascade may involve signals such as AMPK and PPARα. Interestingly, these pathways may trigger similar responses, such as fatty acid oxidation. Understanding the downstream effectors of these pathways is crucial to comprehend the extent to which adiponectin signaling impacts metabolism. In this review, the aim is to explore the current mechanisms that regulate the adiponectin pathways. Additionally, updates on the major downstream factors involved in adiponectin signaling are provided, specifically in relation to metabolic syndrome and atherosclerosis.

The Role of Adipokines in the Control of Pituitary Functions

The pituitary gland is a key endocrine gland in all classes of vertebrates, including mammals. The pituitary gland is an important component of hypothalamus-pituitary-target organ hormonal regulatory axes and forms a functional link between the nervous system and the endocrine system. In response to hypothalamic stimuli, the pituitary gland secretes a number of hormones involved in the regulation of metabolism, stress reactions and environmental adaptation, growth and development, as well as reproductive processes and lactation. In turn, hormones secreted by target organs at the lowest levels of the hormonal regulatory axes regulate the functions of the pituitary gland in the process of hormonal feedback. The pituitary also responds to other peripheral signals, including adipose-tissue-derived factors. These substances are a broad group of peptides known as adipocytokines or adipokines that act as endocrine hormones mainly involved in energy homeostasis. Adipokines, including adiponectin, resistin, apelin, chemerin, visfatin, and irisin, are also expressed in the pituitary gland, and they influence the secretory functions of this gland. This review is an overview of the existing knowledge of the relationship between chosen adipose-derived factors and endocrine functions of the pituitary gland, with an emphasis on the pituitary control of reproductive processes.

The Antiviral Potential of AdipoRon, an Adiponectin Receptor Agonist, Reveals the Ability of Zika Virus to Deregulate Adiponectin Receptor Expression

Zika virus (ZIKV) is a pathogenic member of the flavivirus family, with several unique characteristics. Unlike any other arbovirus, ZIKV can be transmitted sexually and maternally, and thus produce congenital syndromes (CZS) due to its neurotropism. This challenges the search for safe active molecules that can protect pregnant women and their fetuses. In this context, and in the absence of any existing treatment, it seemed worthwhile to test whether the known cytoprotective properties of adiponectin and its pharmacological analog, AdipoRon, could influence the outcome of ZIKV infection. We showed that both AdipoRon and adiponectin could significantly reduce the in vitro infection of A549 epithelial cells, a well-known cell model for flavivirus infection studies. This effect was particularly observed when a pre-treatment was carried out. Conversely, ZIKV revealed an ability to downregulate adiponectin receptor expression and thereby limit adiponectin signaling.

Intermittent fasting favorably modulates adipokines and potentially attenuates atherosclerosis

Adipose tissue is now recognized as an endocrine organ that secretes bioactive molecules called adipokines. These biomolecules regulate key physiological functions, including insulin sensitivity, energy metabolism, appetite regulation, endothelial function and immunity. Dysregulated secretion of adipokines is intimately associated with obesity, and translates into increased risk of obesity-related cardiovasculo-metabolic diseases. In particular, emerging evidence suggests that adipokine imbalance contributes to the pathogenesis of atherosclerosis. One of the promising diet regimens that is beneficial in the fight against obesity and cardiometabolic disorders is intermittent fasting (IF). Indeed, IF robustly suppresses inflammation, meditates weight loss and mitigates many aspects of the cardiometabolic syndrome. In this paper, we review the main adipokines and their role in atherosclerosis, which remains a major contributor to cardiovascular-associated morbidity and mortality. We further discuss how IF can be employed as an effective management modality for obesity-associated atherosclerosis. By exploring a plethora of the beneficial effects of IF, particularly on inflammatory markers, we present IF as a possible intervention to help prevent atherosclerosis.

Elevated Adipocyte Membrane Phospholipid Saturation Does Not Compromise Insulin Signaling

Increased saturated fatty acid (SFA) levels in membrane phospholipids have been implicated in the development of metabolic disease. Here, we tested the hypothesis that increased SFA content in cell membranes negatively impacts adipocyte insulin signaling. Preadipocyte cell models with elevated SFA levels in phospholipids were generated by disrupting the ADIPOR2 locus, which resulted in a striking twofold increase in SFA-containing phosphatidylcholines and phosphatidylethanolamines, which persisted in differentiated adipocytes. Similar changes in phospholipid composition were observed in white adipose tissues isolated from the ADIPOR2-knockout mice. The SFA levels in phospholipids could be further increased by treating ADIPOR2-deficient cells with palmitic acid and resulted in reduced membrane fluidity and endoplasmic reticulum stress in mouse and human preadipocytes. Strikingly, increased SFA levels in differentiated adipocyte phospholipids had no effect on adipocyte gene expression or insulin signaling in vitro. Similarly, increased adipocyte phospholipid saturation did not impair white adipose tissue function in vivo, even in mice fed a high-saturated fat diet at thermoneutrality. We conclude that increasing SFA levels in adipocyte phospholipids is well tolerated and does not affect adipocyte insulin signaling in vitro and in vivo.

A novel small molecule AdipoR2 agonist ameliorates experimental hepatic steatosis in hamsters and mice

Adiponectin receptor 2 (AdipoR2) can be activated by its endogenous ligand adiponectin to reduce hepatic steatosis, and is regarded as a therapeutic target for metabolic associated fatty liver disease (MAFLD). This study proposes a novel anthraquinone compound, emodin succinate monoethyl ester (ESME), which activates AdipoR2, inhibits hepatic lipogenesis, promotes fatty acid oxidation, and alleviates experimental hepatic steatosis in hamsters and mice. Molecular docking shows that ESME has strong binding potential with AdipoR2 by forming a arene-arene interaction. AdipoR2 on the cytomembrane of HepG2 cells can be labeled by fluorescent ESME (Cy5-ESME). ESME activates AdipoR2, AMPK and PPARα, and reduces lipid deposition in palmitic acid or oleic acid-induced HepG2 and L02 cells. Suppression of AdipoR2 expression or AMPK activation completely eliminates the effect of ESME on reducing lipid accumulation in hepatocytes. Oral administration of ESME reduces liver lipid production and accumulation, and alleviates hepatic steatosis in hamsters and Apoe^-/- mice induced by high-fat diet. Compared with statins and emodin, ESME showed prepotent efficacy and safety in reducing hepatic steatosis and protecting hepatocytes. Furthermore, ESME activates CaMKK2 and LKB1 in liver to activate AMPK and reduce lipogenesis through stimulating AdipoR2. Taken together, ESME reduces hepatic lipid accumulation and alleviates hepatic steatosis by agonizing AdipoR2. ESME is a promising new agent for clinical treatment of MAFLD.

Metabolic dysregulation and decreased capillarization in skeletal muscles of male adolescent offspring rats exposed to gestational intermittent hypoxia

Gestational intermittent hypoxia (IH) is a hallmark of obstructive sleep apnea that occurs frequently during pregnancy, and effects caused by this environmental change during pregnancy may be transmitted to the offspring. In this study, we aimed to clarify the effects of IH in pregnant rats on the skeletal muscle of adolescent offspring rats. Mother rats underwent IH from gestation day 7-21, and their 5-weeks-old male offspring were analyzed. All male offspring rats were born and raised under normoxia conditions. Although no general growth retardation was observed, we found that exposure to gestational IH reduces endurance running capacity of adolescent offspring rats. Both a respiratory muscle (diaphragm; DIA) and a limb muscle (tibialis anterior; TA) showed no histological abnormalities, including fiber size and fiber type distribution. To identify the possible mechanism underlying the reduced running capacity, regulatory factors associated with energy metabolism were analyzed in different parts of skeletal muscles. Compared with rats born under conditions of gestational normoxia, gestational IH offspring rats showed significantly lower expression of genes associated with glucose and lipid metabolism, and lower protein levels of phosphorylated AMPK and AKT. Furthermore, gene expression of adiponectin receptors one and two was significantly decreased in the DIA and TA muscles. In addition, the DIA muscle from adolescent rats had significantly decreased capillary density as a result of gestational IH. However, these changes were not observed in a sucking muscle (geniohyoid) and a masticating muscle (masseter) of these rats. These results suggest that respiratory and limb muscles are vulnerable to gestational IH, which induces altered energy metabolism with decreased aerobic motor function. These changes were partially owing to the decreased expression of adiponectin receptors and decreased capillary density in adolescent offspring rats.

Genetics of Cholesterol-Related Genes in Metabolic Syndrome: A Review of Current Evidence

Metabolic syndrome (MetS) refers to a cluster of metabolic dysregulations, which include insulin resistance, obesity, atherogenic dyslipidemia and hypertension. The complex pathogenesis of MetS encompasses the interplay between environmental and genetic factors. Environmental factors such as excessive nutrients and sedentary lifestyle are modifiable and could be improved by lifestyle modification. However, genetic susceptibility to MetS, a non-modifiable factor, has attracted the attention of researchers, which could act as the basis for future diagnosis, prognosis, and therapy for MetS. Several cholesterol-related genes associated with each characteristic of MetS have been identified, such as apolipoprotein, lipoprotein lipase (LPL), cholesteryl ester transfer protein (CETP) and adiponectin. This review aims to summarize the genetic information of cholesterol-related genes in MetS, which may potentially serve as biomarkers for early prevention and management of MetS.

Adipokines: Deciphering the cardiovascular signature of adipose tissue

Obesity and hypertension are intimately linked due to the various ways that the important cell types such as vascular smooth muscle cells (VSMC), endothelial cells (EC), immune cells, and adipocytes, communicate with one another to contribute to these two pathologies. Adipose tissue is a very dynamic organ comprised primarily of adipocytes, which are well known for their role in energy storage. More recently adipose tissue has been recognized as the largest endocrine organ because of its ability to produce a vast number of signaling molecules called adipokines. These signaling molecules stimulate specific types of cells or tissues with many adipokines acting as indicators of adipocyte healthy function, such as adiponectin, omentin, and FGF21, which show anti-inflammatory or cardioprotective effects, acting as regulators of healthy physiological function. Others, like visfatin, chemerin, resistin, and leptin are often altered during pathophysiological circumstances like obesity and lipodystrophy, demonstrating negative cardiovascular outcomes when produced in excess. This review aims to explore the role of adipocytes and their derived products as well as the impacts of these adipokines on blood pressure regulation and cardiovascular homeostasis.

Sphingosine 1-phosphate mediates adiponectin receptor signaling essential for lipid homeostasis and embryogenesis

Cells and organisms require proper membrane composition to function and develop. Phospholipids are the major component of membranes and are primarily acquired through the diet. Given great variability in diet composition, cells must be able to deploy mechanisms that correct deviations from optimal membrane composition and properties. Here, using lipidomics and unbiased proteomics, we found that the embryonic lethality in mice lacking the fluidity regulators Adiponectin Receptors 1 and 2 (AdipoR1/2) is associated with aberrant high saturation of the membrane phospholipids. Using mouse embryonic fibroblasts (MEFs) derived from AdipoR1/2-KO embryos, human cell lines and the model organism C. elegans we found that, mechanistically, AdipoR1/2-derived sphingosine 1-phosphate (S1P) signals in parallel through S1PR3-SREBP1 and PPARγ to sustain the expression of the fatty acid desaturase SCD and maintain membrane properties. Thus, our work identifies an evolutionary conserved pathway by which cells and organisms achieve membrane homeostasis and adapt to a variable environment.

Regulation of membrane fluidity by RNF145-triggered degradation of the lipid hydrolase ADIPOR2

The regulation of membrane lipid composition is critical for cellular homeostasis. Cells are particularly sensitive to phospholipid saturation, with increased saturation causing membrane rigidification and lipotoxicity. How mammalian cells sense membrane lipid composition and reverse fatty acid (FA)-induced membrane rigidification is poorly understood. Here we systematically identify proteins that differ between mammalian cells fed saturated versus unsaturated FAs. The most differentially expressed proteins were two ER-resident polytopic membrane proteins: the E3 ubiquitin ligase RNF145 and the lipid hydrolase ADIPOR2. In unsaturated lipid membranes, RNF145 is stable, promoting its lipid-sensitive interaction, ubiquitination and degradation of ADIPOR2. When membranes become enriched in saturated FAs, RNF145 is rapidly auto-ubiquitinated and degraded, stabilising ADIPOR2, whose hydrolase activity restores lipid homeostasis and prevents lipotoxicity. We therefore identify RNF145 as a FA-responsive ubiquitin ligase which, together with ADIPOR2, defines an autoregulatory pathway that controls cellular membrane lipid homeostasis and prevents acute lipotoxic stress.

Adiponectin: friend or foe in obesity and inflammation

Adiponectin is an adipokine predominantly produced by fat cells, circulates and exerts insulin-sensitizing, cardioprotective and anti-inflammatory effects. Dysregulation of adiponectin and/or adiponectin signaling is implicated in a number of metabolic diseases such as obesity, insulin resistance, diabetes, and cardiovascular diseases. However, while the insulin-sensitizing and cardioprotective effects of adiponectin have been widely appreciated in the field, the obesogenic and anti-inflammatory effects of adiponectin are still of much debate. Understanding the physiological function of adiponectin is critical for adiponectin-based therapeutics for the treatment of metabolic diseases.

Adipose Tissue Dysfunction and Obesity-Related Male Hypogonadism

Obesity is a chronic illness associated with several metabolic derangements and comorbidities (i.e., insulin resistance, leptin resistance, diabetes, etc.) and often leads to impaired testicular function and male subfertility. Several mechanisms may indeed negatively affect the hypothalamic–pituitary–gonadal health, such as higher testosterone conversion to estradiol by aromatase activity in the adipose tissue, increased ROS production, and the release of several endocrine molecules affecting the hypothalamus–pituitary–testis axis by both direct and indirect mechanisms. In addition, androgen deficiency could further accelerate adipose tissue expansion and therefore exacerbate obesity, which in turn enhances hypogonadism, thus inducing a vicious cycle. Based on these considerations, we propose an overview on the relationship of adipose tissue dysfunction and male hypogonadism, highlighting the main biological pathways involved and the current therapeutic options to counteract this condition.

Leptin and adiponectin regulate the activity of nuclei involved in sleep-wake cycle in male rats

Epidemiological and experimental evidence recognize a relationship between sleep-wake cycles and adiposity levels, but the mechanisms that link both are not entirely understood. Adipose tissue secretes adiponectin and leptin hormones, mainly involved as indicators of adiposity levels and recently associated to sleep. To understand how two of the main adipose tissue hormones could influence sleep-wake regulation, we evaluated in male rats, the effect of direct administration of adiponectin or leptin in the ventrolateral preoptic nuclei (VLPO), a major area for sleep promotion. The presence of adiponectin (AdipoR1 and AdipoR2) and leptin receptors in VLPO were confirmed by immunohistochemistry. Adiponectin administration increased wakefulness during the rest phase, reduced delta power, and activated wake-promoting neurons, such as the locus coeruleus (LC), tuberomammillary nucleus (TMN) and hypocretin/orexin neurons (OX) within the lateral hypothalamus (LH) and perifornical area (PeF). Conversely, leptin promoted REM and NREM sleep, including increase of delta power during NREM sleep, and induced c-Fos expression in VLPO and melanin concentrating hormone expressing neurons (MCH). In addition, a reduction in wake-promoting neurons activity was found in the TMN, lateral hypothalamus (LH) and perifornical area (PeF), including in the OX neurons. Moreover, leptin administration reduced tyrosine hydroxylase (TH) immunoreactivity in the LC. Our data suggest that adiponectin and leptin act as hormonal mediators between the status of body energy and the regulation of the sleep-wake cycle.

Role of Leptin and Adiponectin in Endometrial Cancer

Endometrial cancer is the most common malignancy of the female genital tract. Obesity is a strong risk factor for endometrial cancer. Adipose tissue is an active endocrine organ that synthesizes biologically active cytokine peptides, called adipokines. Adiponectin and leptin are the main cytokines of adipose tissue, which may influence the development of metabolic diseases and carcinogenesis. In this scenario, we describe the role of leptin and adiponectin in the development of endometrial cancer. A better understanding of the signalling pathway of these cytokines in endometrial cancerogenesis will provide an opportunity for effective target therapy and may be usable in fertility-sparing treatment. In the future, clinical trials focusing on adipokines, molecular biology, and genetics of the tumour will be needed.

An Overview of the TRP-Oxidative Stress Axis in Metabolic Syndrome: Insights for Novel Therapeutic Approaches

Metabolic syndrome (MS) is a complex pathology characterized by visceral adiposity, insulin resistance, arterial hypertension, and dyslipidaemia. It has become a global epidemic associated with increased consumption of high-calorie, low-fibre food and sedentary habits. Some of its underlying mechanisms have been identified, with hypoadiponectinemia, inflammation and oxidative stress as important factors for MS establishment and progression. Alterations in adipokine levels may favour glucotoxicity and lipotoxicity which, in turn, contribute to inflammation and cellular stress responses within the adipose, pancreatic and liver tissues, in addition to hepatic steatosis. The multiple mechanisms of MS make its clinical management difficult, involving both non-pharmacological and pharmacological interventions. Transient receptor potential (TRP) channels are non-selective calcium channels involved in a plethora of physiological events, including energy balance, inflammation and oxidative stress. Evidence from animal models of disease has contributed to identify their specific contributions to MS and may help to tailor clinical trials for the disease. In this context, the oxidative stress sensors TRPV1, TRPA1 and TRPC5, play major roles in regulating inflammatory responses, thermogenesis and energy expenditure. Here, the interplay between these TRP channels and oxidative stress in MS is discussed in the light of novel therapies to treat this syndrome.

Palmitic acid causes increased dihydroceramide levels when desaturase expression is directly silenced or indirectly lowered by silencing AdipoR2

Background

AdipoR1 and AdipoR2 (AdipoRs) are plasma membrane proteins often considered to act as adiponectin receptors with a ceramidase activity. Additionally, the AdipoRs and their yeast and C. elegans orthologs are emerging as membrane homeostasis regulators that counter membrane rigidification by promoting fatty acid desaturation and incorporation of unsaturated fatty acids into phospholipids, thus restoring fluidity.

Methods

Using cultured cells, the effects of AdipoR silencing or over-expression on the levels and composition of several sphingolipid classes were examined.

Results

AdipoR2 silencing in the presence of exogenous palmitic acid potently causes increased levels of dihydroceramides, a ceramide precursor in the de novo ceramide synthesis pathway. Conversely, AdipoR2 over-expression caused a depletion of dihydroceramides.

Conclusions

The results are consistent with AdipoR2 silencing leading to increased intracellular supply of palmitic acid that in turn leads to increased dihydroceramide synthesis via the rate-limiting serine palmitoyl transferase step. In agreement with this model, inhibiting the desaturase SCD or SREBF1/2 (positive regulators of SCD) also causes a strong increase in dihydroceramide levels.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01600-y.

Effects of Obesity on Adiponectin System Skin Expression in Dogs: A Comparative Study

Simple Summary

Adipokines are biologically active molecules with hormonal action, produced mainly by white adipose tissue and related to the individual’s nutritional status. Adiponectin with its receptors (ADIPOR1, ADIPOR2) intervenes in the control of energy metabolism, as well as in the regulation of peripheral tissue functions. Adiponectin has a primary role in the skin in both physiological and pathological conditions, in addition, this molecule is greatly affected by nutritional status, and its serum level is lowered in the obese. In this work, the adiponectin system was evaluated in the skin of obese dogs along with adiponectin serum levels. Results were compared to normal weight dogs to evidence modifications in the obesity condition. Obesity is a widespread phenomenon in dogs, with a growing trend, as well, in humans; this condition may interfere with the local functionality of tissues, including the skin. The evaluation performed evidenced that adiponectin and ADIPOR2 skin expression is negatively correlated with the serum adiponectin level and accordingly with obesity. These findings evidence that the adiponectin system changes in the skin of obese dogs; this study also explores the role of adipokines in skin biology.

Abstract

Obesity is an important health issue in dogs since it influences a plethora of associated pathologies, including dermatological disorders. Considering the scarcity of information in pets, this work aimed to evaluate the localization and expression of adiponectin (ADIPOQ) and its two receptors (ADIPOR1 and ADIPOR2) in the skin of 10 obese dogs, compared with serum ADIPOQ level. Through immunohistochemistry, ADIPOQ and ADIPOR2 were observed in the adipose tissue, sweat and sebaceous glands, endothelium, and some connective cells. Both receptors were observed in the epidermis and the hair follicles, other than in the sweat and sebaceous glands. Real-time PCR evidenced that the ADIPOQ and ADIPOR2 transcripts were expressed 5.4-fold (p < 0.01) and 2.3-fold less (p < 0.01), respectively, in obese than in normal weight dogs, while ADIPOR1 expression did not change. Obese dogs showed lower serum ADIPOQ levels than the normal weight group. Accordingly, ADIPOQ and ADIPOR2 expression in the skin appear negatively correlated with obesity in the same way as the serum ADIPOQ level. These findings evidence that ADIPOQ system changes in the skin of obese dogs and suggest that the ADIPOQ effect on the skin is at least in part regulated by the reduced expression of ADIPOR2.

Role of Perivascular Adipose Tissue-Derived Adiponectin in Vascular Homeostasis

Studies of adipose tissue biology have demonstrated that adipose tissue should be considered as both passive, energy-storing tissue and an endocrine organ because of the secretion of adipose-specific factors, called adipokines. Adiponectin is a well-described homeostatic adipokine with metabolic properties. It regulates whole-body energy status through the induction of fatty acid oxidation and glucose uptake. Adiponectin also has anti-inflammatory and antidiabetic properties, making it an interesting subject of biomedical studies. Perivascular adipose tissue (PVAT) is a fat depot that is conterminous to the vascular wall and acts on it in a paracrine manner through adipokine secretion. PVAT-derived adiponectin can act on the vascular wall through endothelial cells and vascular smooth muscle cells. The present review describes adiponectin's structure, receptors, and main signaling pathways. We further discuss recent studies of the extent and nature of crosstalk between PVAT-derived adiponectin and endothelial cells, vascular smooth muscle cells, and atherosclerotic plaques. Furthermore, we argue whether adiponectin and its receptors may be considered putative therapeutic targets.

AdipoRon: A Novel Insulin Sensitizer in Various Complications and the Underlying Mechanisms: A Review

BACKGROUND

AdipoRon is the first synthetic analog of endogenous adiponectin, an adipose tissue-derived hormone. AdipoRon possesses pharmacological properties similar to adiponectin and its ability to bind and activate the adipoR1 and adipoR2 receptors makes it a suitable candidate for the treatment of a multitude of disorders.

OBJECTIVE

In the present review, an attempt was made to compile and discuss the efficacy of adipoRon against various disorders.

RESULTS

AdipoRon is a drug that acts not only in metabolic diseases but in other conditions unrelated to energy metabolism. It is well- reported that adipoRon exhibits strong anti-obesity, anti-diabetic, anticancer, anti-depressant, anti-ischemic, anti-hypertrophic properties and also improves conditions like post-traumatic stress disorder, anxiety, and systemic sclerosis.

CONCLUSION

A lot is known about its effects in experimental systems, but the translation of this knowledge to the clinic requires studies which, for many of the potential target conditions, have yet to be carried out. The beneficial effects of AdipoRon in novel clinical conditions will suggest an underlying pathophysiological role of adiponectin and its receptors in previously unsuspected settings.

Adiponectin/AdipoR1 Axis Promotes IL-10 Release by Human Regulatory T Cells

Background

Adiponectin is an important immunomodulatory mediator in inflammatory conditions. While we previously showed that adiponectin receptor 1 (AdipoR1) is expressed in murine regulatory T cells (Tregs), its expression in human Tregs remain unknown. Here, we examined the expression of AdipoR1 in human Tregs and whether its ligand, globular adiponectin (gAd) affects the Treg ability to secrete IL-10 and the role of Type 2 (T2) inflammation in such process.

Methods

Human Tregs from peripheral blood were analyzed by flow cytometry for AdipoR1, Helios and IL-10 expression. CD4⁺ T cells enriched from peripheral blood mononuclear cells (PBMCs) were cultured in the presence or the absence of gAd or the chemical adiponectin receptor agonist, AdipoRon, or in a T2 cytokine milieu. Flow cytometry was then used to assess intracellular IL-10, IL-10 secreting cells, FOXP3 and Helios expression, and phosphorylated p38 MAP kinase (MAPK). IL-10 levels in CD4⁺ T cell supernatants were quantified by ELISA.

Results

We found that a subset of human Tregs expressed AdipoR1. Importantly, more Helios^- cells expressed AdipoR1 than Helios⁺ cells. Likewise, there was a higher frequency of IL-10⁺ cells within Helios^- AdipoR1⁺ Tregs compared to Helios⁺ AdipoR1⁺ Tregs. In contrast, the IL-10 mean fluorescence intensity (MFI) was higher in Helios⁺ AdipoR1⁺ Tregs compared to Helios^-AdipoR1⁺ Tregs. When human CD4⁺ T cells were treated with gAd or AdipoRon, a significant increase in IL-10 secretion, FOXP3 expression, and p38 MAPK phosphorylation was observed in Helios^- AdipoR1⁺ Tregs. Interestingly, gAd under T2 cytokine milieu significantly increased the intracellular levels of IL-10, mainly in Helios⁺ AdipoR1⁺ Tregs, and IL-10 levels in supernatants of CD4⁺ T cells.

Conclusions

Collectively, our findings suggest that adiponectin/AdipoR1 axis promotes IL-10 release by Tregs, mainly in Helios^- Tregs, and the effect was amplified by T2 inflammation in Helios⁺ Tregs.

Systems genetic analysis of binge-like eating in a C57BL/6J x DBA/2J-F2 cross

Binge eating is a heritable trait associated with eating disorders and refers to the rapid consumption of a large quantity of energy-dense food that is, associated with loss of control and negative affect. Binge eating disorder is the most common eating disorder in the United States; however, the genetic basis is unknown. We previously identified robust mouse inbred strain differences between C57BL/6J and DBA/2J in binge-like eating of sweetened palatable food in an intermittent access, conditioned place preference paradigm. To map the genetic basis of changes in body weight and binge-like eating (BLE) and to identify candidate genes, we conducted quantitative trait locus (QTL) analysis in 128 C57BL/6J x DBA/2J-F2 mice combined with PheQTL and trait covariance analysis in GeneNetwork2 using legacy BXD-RI trait datasets. We identified a QTL on Chromosome 18 influencing changes in body weight across days in females (log of the odds [LOD] = 6.3; 1.5-LOD: 3-12 cM) that contains the candidate gene Zeb1. We also identified a sex-combined QTL influencing initial palatable food intake on Chromosome 5 (LOD = 5.8; 1.5-LOD: 21-28 cM) that contains the candidate gene Lcorl and a second QTL influencing escalated palatable food intake on Chromosome 6 in males (LOD = 5.4; 1.5-LOD: 50-59 cM) that contains the candidate genes Adipor2 and Plxnd1. Finally, we identified a suggestive QTL in females for slope of BLE on distal Chromosome 18 (LOD = 4.1; p = 0.055; 1.5-LOD: 23-35 cM). Future studies will use BXD-RI strains to fine map loci and support candidate gene nomination for gene editing.

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.

Paradigm shift: the primary function of the "Adiponectin Receptors" is to regulate cell membrane composition

The ADIPOR1 and ADIPOR2 proteins (ADIPORs) are generally considered as adiponectin receptors with anti-diabetic properties. However, studies on the yeast and C. elegans homologs of the mammalian ADIPORs, and of the ADIPORs themselves in various mammalian cell models, support an updated/different view. Based on findings in these experimental models, the ADIPORs are now emerging as evolutionarily conserved regulators of membrane homeostasis that do not require adiponectin to act as membrane fluidity sensors and regulate phospholipid composition. More specifically, membrane rigidification activates ADIPOR signaling to promote fatty acid desaturation and incorporation of polyunsaturated fatty acids into membrane phospholipids until fluidity is restored. The present review summarizes the evidence supporting this new view of the ADIPORs, and briefly examines physiological consequences.

Effects of Obesity and Diabetes on Sperm Cell Proteomics in Rats

Infertility caused by male factors is potentially associated with metabolic disorders such as obesity and/or diabetes. This experimental study was conducted in a male rodent model to assess the effects of different diseases on semen quality and sperm proteomics. Ten Wistar rats were used for each treatment. Rats were fed commercial food provided controllably to the control group and the diabetic group, and a hypercaloric diet supplemented with 5% sucrose in water was provided ad libitum to the obese group for 38 weeks. Diabetes was induced with 35 mg/kg streptozotocin. After euthanasia, testicles, spermatozoa, fat, and blood (serum) samples were collected. Spermatozoa were evaluated for quality and subjected to proteomics analyses. Histology and cytology of the testis, and serum leptin, adiponectin, interleukin 8 (IL-8), blood glucose, and testosterone levels, were also assessed. Body weight, retroperitoneal and testicular fat, and the Lee index were also measured. Obesity and diabetes were induced. The diabetic group showed noticeable changes in spermatogenesis and sperm quality. The mass spectrometry proteomics data have been deposited in Mendeley Data (doi: 10.17632/rfp7kfjcsd.5). Fifteen proteins varied in abundance between groups, especially proteins related to energy production and structural function of the spermatozoa, suggesting disturbances in energy production with a subsequent alteration in sperm motility in both groups, but with a compensatory response in the obese group.

Adiponectin/AdipoRs signaling as a key player in testicular aging and associated metabolic disorders

Aging undergoes serious worsening of peripheral organs and vital physiological processes including reproductive performances. Altered white adipose tissue and adipocyte functioning during aging results in ectopic lipid storage/obesity or metabolic derangements, leading to insulin resistance state. Eventually, accelerating cellular senescence thereby enhancing the high risk of age-associated metabolic alterations. Such alterations may cause derangement of numerous physiologically active obesity hormones, known as "adipokines." Specifically, adiponectin exhibits insulin sensitizing action causing anti-aging and anti-obesity effects via activation of adiponectin receptors (AdipoRs). The male reproductive physiology from reproductive mature stage to advanced senescent stage undergoes insidious detrimental changes. The mechanisms by which testicular functions decline with aging remain largely speculative. Adiponectin has also recently been shown to regulate metabolism and longevity signaling thus prolonging lifespan. Therefore, the strategy for activating adiponectin/AdipoRs signaling pathways are expected to provide a solid basis for the prevention and treatment of aging and obesity-associated reproductive dysfunctions, as well as for ensuring healthy reproductive longevity in humans.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

Comparative influence of differentiation and proliferation on gene expression in human meibomian gland epithelial cells

We recently discovered that by changing environmental signals, differentiated immortalized human meibomian gland epithelial cells (IHMGECs) de-differentiate into proliferating cells. We also discovered that following exposure to appropriate stimuli, these proliferative cells re-differentiate into differentiated IHMGECs. We hypothesize that this plasticity of differentiated and proliferative IHMGECs is paralleled by very significant alterations in cellular gene expression. To begin to test this hypothesis, we compared the gene expression patterns of IHMGECs during differentiation and proliferation. IHMGECs were cultured for four days in either differentiating or proliferating media. After four days of culture, cells were processed for the analysis of gene expression by using Illumina BeadChips and bioinformatic software. Our study identified significant differences in the expression of more than 9200 genes in differentiated and proliferative IHMGECs. Differentiation was associated with significant increases in the expression of specific genes (e.g. S100 calcium binding protein P; 7,194,386-fold upregulation) and numerous ontologies (e.g. 83 biological process [bp] ontologies with ≥100 genes were upregulated), such as those related to development, transport and lysosomes. Proliferation also led to a significant rise in specific gene expressions (e.g. cathelicidin antimicrobial peptide; 859,100-fold upregulation) and many ontologies (115 biological process [bp] ontologies with ≥100 genes were upregulated), with most of the highly significant ontologies related to cell cycle (z scores > 13.9). Our findings demonstrate that gene expression in differentiated and proliferative IHMGECs is extremely different. These results may have significant implications for the regeneration of HMGECs and the reversal of MG dropout in MG dysfunction.

Adiponectin receptor 1 variants contribute to hypertrophic cardiomyopathy that can be reversed by rapamycin

Hypertrophic cardiomyopathy (HCM) is a heterogeneous genetic heart muscle disease characterized by hypertrophy with preserved or increased ejection fraction in the absence of secondary causes. However, recent studies have demonstrated that a substantial proportion of individuals with HCM also have comorbid diabetes mellitus (~10%). Whether genetic variants may contribute a combined phenotype of HCM and diabetes mellitus is not known. Here, using next-generation sequencing methods, we identified novel and ultrarare variants in adiponectin receptor 1 (ADIPOR1) as risk factors for HCM. Biochemical studies showed that ADIPOR1 variants dysregulate glucose and lipid metabolism and cause cardiac hypertrophy through the p38/mammalian target of rapamycin and/or extracellular signal-regulated kinase pathways. A transgenic mouse model expressing an ADIPOR1 variant displayed cardiomyopathy that recapitulated the cellular findings, and these features were rescued by rapamycin. Our results provide the first evidence that ADIPOR1 variants can cause HCM and provide new insights into ADIPOR1 regulation.

Nobiletin alleviates high-fat diet-induced nonalcoholic fatty liver disease by modulating AdipoR1 and gp91phox expression in rats

Nobiletin, one of the polymethoxylated flavonoids isolated from citrus peels, is reported to possess various biological activities. The current study investigates the effect and possible mechanisms of nobiletin on nonalcoholic fatty liver disease (NAFLD) in high-fat diet (HFD)-fed rats. Male Sprague-Dawley rats were administrated with HFD and fructose (15%) in drinking water for 16 weeks to induce NAFLD. HFD-fed rats were treated with nobiletin (20 or 40 mg/kg/day) or vehicle for the last 4 weeks. Treatment of HFD-fed rats with nobiletin significantly reduced systolic blood pressure, adiposity, hyperlipidemia, insulin resistance, hepatic lipids content, NAFLD activity score and liver fibrosis. Nobiletin significantly increased plasma adiponectin levels, together with up-regulation of liver adiponectin receptor 1 (AdipoR1) expression. Additionally, decreased malondialdehyde levels and increased superoxide dismutase activity in plasma and hepatic tissue, consistent with down-regulation of liver NADPH oxidase subunit gp91phox expression, were also observed after nobiletin treatment. Furthermore, high dose of nobiletin exhibited higher therapeutic effect as a compared to low dose. These findings suggest that nobiletin alleviates HFD-induced NAFLD and metabolic dysfunction in rats. There might be an association between the observed inhibitory effect of nobiletin on NAFLD and modulation of AdipoR1 and gp91phox.

Adipokines in the Skin and in Dermatological Diseases

Adipokines are the primary mediators of adipose tissue-induced and regulated systemic inflammatory diseases; however, recent findings revealed that serum levels of various adipokines correlate also with the onset and the severity of dermatological diseases. Importantly, further data confirmed that the skin serves not only as a target for adipokine signaling, but may serve as a source too. In this review, we aim to provide a complex overview on how adipokines may integrate into the (patho) physiological conditions of the skin by introducing the cell types, such as keratinocytes, fibroblasts, and sebocytes, which are known to produce adipokines as well as the signals that target them. Moreover, we discuss data from in vivo and in vitro murine and human studies as well as genetic data on how adipokines may contribute to various aspects of the homeostasis of the skin, e.g., melanogenesis, hair growth, or wound healing, just as to the pathogenesis of dermatological diseases such as psoriasis, atopic dermatitis, acne, rosacea, and melanoma.

Sex- and season-dependent differences in the expression of adiponectin and adiponectin receptors (AdipoR1 and AdipoR2) in the hypothalamic-pituitary-adrenal axis of the Eurasian beaver (Castor fiber L.)

Adiponectin, a product of the Adipoq gene, is an adipocyte-derived protein hormone of the cytokine family and the most abundantly expressed adipokine. Adiponectin and its receptors AdipoR1 and AdipoR2 (collectively referred to as the adiponectin system) are widely expressed in the central nervous system and other tissues, which suggests that this hormone has pleiotropic effects. Adiponectin could also play a role in the modulation of the hypothalamic-pituitaryadrenal (HPA) hormonal regulatory axis. There is a general scarcity of data on the adiponectin system in wild animals where annual changes in reproductive activity are linked with fluctuations in the activity of the HPA axis. The Eurasian beaver (Castor fiber L.) could be an interesting and suitable model for investigating the above processes. We hypothesized that the expression of the adiponectin system in the tissues of the beaver HPA axis is sex- and season-dependent. The study was performed on adult animals harvested during three different stages of reproductive activity: April ('breeding'), July ('post-breeding') and November ('pre-breeding'). The expression of the adiponectin system was confirmed in all branches (mediobasal hypothalamus, pituitary, adrenal cortex) of the HPA axis in both sexes and during all periods of reproductive activity. The expression of Adipoq, AdipoR1 and AdipoR2 was generally dependent on sex and the period of the reproductive season. The expression of adiponectin system genes was particularly pronounced in the adrenal cortex. These findings suggest that the adiponectin system in the Eurasian beaver could link reproductive processes with stress responses and energy metabolism.

Taste Receptor Cells in Mice Express Receptors for the Hormone Adiponectin

The metabolic hormone adiponectin is secreted into the circulation by adipocytes and mediates key biological functions, including insulin sensitivity, adipocyte development, and fatty acid oxidation. Adiponectin is also abundant in saliva, where its functions are poorly understood. Here we report that murine taste receptor cells (TRCs) express specific adiponectin receptors and may be a target for salivary adiponectin. This is supported by the presence of all three known adiponectin receptors in transcriptomic data obtained by RNA-seq analysis of purified circumvallate (CV) taste buds. As well, immunohistochemical analysis of murine CV papillae showed that two adiponectin receptors, ADIPOR1 and T-cadherin, are localized to subsets of TRCs. Immunofluorescence for T-cadherin was primarily co-localized with the Type 2 TRC marker phospholipase C β2, suggesting that adiponectin signaling could impact sweet, bitter, or umami taste signaling. However, adiponectin null mice showed no differences in behavioral lick responsiveness compared with wild-type controls in brief-access lick testing. AAV-mediated overexpression of adiponectin in the salivary glands of adiponectin null mice did result in a small but significant increase in behavioral lick responsiveness to the fat emulsion Intralipid. Together, these results suggest that salivary adiponectin can affect TRC function, although its impact on taste responsiveness and peripheral taste coding remains unclear.

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.

Altered Brain Adiponectin Receptor Expression in the 5XFAD Mouse Model of Alzheimer's Disease

Metabolic syndromes share common pathologies with Alzheimer's disease (AD). Adiponectin, an adipocyte-derived protein, regulates energy metabolism via its receptors, AdipoR1 and AdipoR2. To investigate the distribution of adiponectin receptors (AdipoRs) in Alzheimer's, we examined their expression in the aged 5XFAD mouse model of AD. In age-matched wild-type mice, we observed neuronal expression of both ARs throughout the brain as well as endothelial expression of AdipoR1. The pattern of receptor expression in the aged 5XFAD brain was significantly perturbed. Here, we observed decreased neuronal expression of both ARs and decreased endothelial expression of AdipoR1, but robust expression of AdipoR2 in activated astrocytes. We also observed AdipoR2-expressing astrocytes in the dorsomedial hypothalamic and thalamic mediodorsal nuclei, suggesting the possibility that astrocytes utilise AdipoR2 signalling to fuel their activated state in the AD brain. These findings provide further evidence of a metabolic disturbance and demonstrate a potential shift in energy utilisation in the AD brain, supporting imaging studies performed in AD patients.

Mechanistic Links Between Obesity, Diabetes, and Blood Pressure: Role of Perivascular Adipose Tissue

Obesity is increasingly prevalent and is associated with substantial cardiovascular risk. Adipose tissue distribution and morphology play a key role in determining the degree of adverse effects, and a key factor in the disease process appears to be the inflammatory cell population in adipose tissue. Healthy adipose tissue secretes a number of vasoactive adipokines and anti-inflammatory cytokines, and changes to this secretory profile will contribute to pathogenesis in obesity. In this review, we discuss the links between adipokine dysregulation and the development of hypertension and diabetes and explore the potential for manipulating adipose tissue morphology and its immune cell population to improve cardiovascular health in obesity.

Direct actions of adiponectin on changes in reproductive, metabolic, and anti-oxidative enzymes status in the testis of adult mice

Obesity is a major health problem that is linked to decreased sperm count. It is hypothesized that an obesity-associated reduction in adiponectin secretion may be responsible for impairment of spermatogenesis. Therefore, the aim of the study was to evaluate the direct role of adiponectin in spermatogenesis and steroid synthesis in adult mice. This study showed that adiponectin receptors (AdipoR1 and AdipoR2) were localized in Leydig cells and seminiferous tubules in the testis of adult mice. The result of the in vitro study showed the direct action of adiponectin on spermatogenesis by stimulating cell proliferation (PCNA) and survival (Bcl2) and by suppressing cell apoptosis. Treatment of testis with adiponectin also enhanced transport of the energetic substrates glucose and lactate to protect cells from undergoing apoptosis. Adiponectin treatment further showed a significant reduction in oxidative stress and nitric oxide. Our findings suggest that adiponectin effectively facilitates cell survival and proliferation, as well as protects from apoptosis. Thus, adiponectin treatment may be responsible for enhancing sperm counts. Interestingly, this study showed the stimulatory effect of adiponectin in spermatogenesis but showed an inhibitory effect on testosterone and estradiol synthesis in the testes. Based on the present study, it is hypothesized that systemic adiponectin treatment may be a promising therapeutic strategy for the improvement of spermatogenesis and sperm count.

Adiponectin suppresses amyloid-β oligomer (AβO)-induced inflammatory response of microglia via AdipoR1-AMPK-NF-κB signaling pathway

Background

Microglia-mediated neuroinflammation is important in Alzheimer’s disease (AD) pathogenesis. Extracellular deposition of β-amyloid (Aβ), a major pathological hallmark of AD, can induce microglia activation. Adiponectin (APN), an adipocyte-derived adipokine, exerts anti-inflammatory effects in the periphery and brain. Chronic APN deficiency leads to cognitive impairment and AD-like pathologies in aged mice. Here, we aim to study the role of APN in regulating microglia-mediated neuroinflammation in AD.

Methods

Inflammatory response of cultured microglia (BV2 cells) to AβO and effects of APN were studied by measuring levels of proinflammatory cytokines (tumor necrosis factor α [TNFα] and interleukin-1β [IL-1β]) in cultured medium before and after exposure to AβO, with and without APN pretreatment. Adiponectin receptor 1 (AdipoR1) and receptor 2 (AdipoR2) were targeted by small interference RNA. To study the neuroprotective effect of APN, cultured HT-22 hippocampal cells were treated with conditioned medium of AβO-exposed BV2 cells or were co-cultured with BV2 cells in transwells. The cytotoxicity of HT-22 hippocampal cells was assessed by MTT reduction. We generated APN-deficient AD mice (APN^−/−5xFAD) by crossing APN-knockout mice with 5xFAD mice to determine the effects of APN deficiency on microglia-mediated neuroinflammation in AD.

Results

AdipoR1 and AdipoR2 were expressed in BV2 cells and microglia of mice. Pretreatment with APN for 2 h suppressed TNFα and IL-1β release induced by AβO in BV2 cells. Additionally, APN rescued the decrease of AMPK phosphorylation and suppressed nuclear translocation of nuclear factor kappa B (NF-κB) induced by AβO. Compound C, an inhibitor of AMPK, abolished these effects of APN. Knockdown of AdipoR1, but not AdipoR2 in BV2 cells, inhibited the ability of APN to suppress proinflammatory cytokine release induced by AβO. Moreover, pretreatment with APN inhibited the cytotoxicity of HT-22 cells co-cultured with AβO-exposed BV2 cells. Lastly, APN deficiency exacerbated microglia activation in 9-month-old APN^−/−5xFAD mice associated with upregulation of TNFα and IL-1β in the cortex and hippocampus.

Conclusions

Our findings demonstrate that APN inhibits inflammatory response of microglia to AβO via AdipoR1-AMPK-NF-κB signaling, and APN deficiency aggravates microglia activation and neuroinflammation in AD mice. APN may be a novel therapeutic agent for inhibiting neuroinflammation in AD.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1492-6) contains supplementary material, which is available to authorized users.

AdipoR1 and AdipoR2 maintain membrane fluidity in most human cell types and independently of adiponectin

The FA composition of phospholipids must be tightly regulated to maintain optimal cell membrane properties and compensate for a highly variable supply of dietary FAs. Previous studies have shown that AdipoR2 and its homologue PAQR-2 are important regulators of phospholipid FA composition in HEK293 cells and Caenorhabditiselegans, respectively. Here we show that both AdipoR1 and AdipoR2 are essential for sustaining desaturase expression and high levels of unsaturated FAs in membrane phospholipids of many human cell types, including primary human umbilical vein endothelial cells, and for preventing membrane rigidification in cells challenged with exogenous palmitate, a saturated FA. Three independent methods confirm the role of the AdipoRs as regulators of membrane composition and fluidity: fluorescence recovery after photobleaching, measurements of Laurdan dye generalized polarization, and mass spectrometry to determine the FA composition of phospholipids. Furthermore, we show that the AdipoRs can prevent lipotoxicity in the complete absence of adiponectin, their putative ligand. We propose that the primary cellular function of AdipoR1 and AdipoR2 is to maintain membrane fluidity in most human cell types and that adiponectin is not required for this function.

Effect of fasting and subsequent refeeding on the transcriptional profiles of brain in juvenile Spinibarbus hollandi

Starvation is a common stress in fish. The underlying molecular mechanisms associated with growth depression caused by feeding restriction and compensatory growth are not well understood. We investigated the effect of fasting and refeeding on the transcriptome profiles of brain in juvenile S. hollandi using RNA-seq. A total of 4.73 × 10⁸ raw reads were obtained from nine brain samples. De novo transcriptome assembly identified 387,085 unigenes with 2.1×10⁹ nucleotides. A total of 936 annotated unigenes showed significantly differential expression among the control, fasting, and fasting-refeeding groups. The down-regulated differentially expressed genes (DEGs) during fasting were mainly associated with cell cycle, DNA replication, and mitosis. The up-regulated DEGs were mainly related to glucose and lipid metabolism, material transportation, and transcription factors. Most decreased DEGs during fasting were restored to normal levels after refeeding. Comparing with the control group, genes associated with protein synthesis, stimulus response, and carbohydrate metabolism were significantly over-expressed and pro-opio melanocortin (POMC) was down-regulated during the refeeding period. In conclusion, fish mobilized stored energetic materials and reduced energy consumption to prolong survival during fasting. After refeeding, the down-regulation of DEGs, e.g., POMC may be associated with compensatory growth. Up-regulation of DEGs related to ribosomal protein, stimulus response, and carbohydrate metabolism may contribute to eliminate negative effect of starvation on brain. This study provided the first transcriptome data related with impact of short-time starvation and refeeding in S. hollandi brains.

Mechanisms of Adiponectin Action in Fertility: An Overview from Gametogenesis to Gestation in Humans and Animal Models in Normal and Pathological Conditions

Adiponectin is the most abundant plasma adipokine. It mainly derives from white adipose tissue and plays a key role in the control of energy metabolism thanks to its insulin-sensitising, anti-inflammatory, and antiatherogenic properties. In vitro and in vivo evidence shows that adiponectin could also be one of the hormones controlling the interaction between energy balance and fertility in several species, including humans. Indeed, its two receptors—AdipoR1 and AdipoR2—are expressed in hypothalamic–pituitary–gonadal axis and their activation regulates Kiss, GnRH and gonadotropin expression and/or secretion. In male gonads, adiponectin modulates several functions of both somatic and germ cells, such as steroidogenesis, proliferation, apoptosis, and oxidative stress. In females, it controls steroidogenesis of ovarian granulosa and theca cells, oocyte maturation, and embryo development. Adiponectin receptors were also found in placental and endometrial cells, suggesting that this adipokine might play a crucial role in embryo implantation, trophoblast invasion and foetal growth. The aim of this review is to characterise adiponectin expression and its mechanism of action in male and female reproductive tract. Further, since features of metabolic syndrome are associated with some reproductive diseases, such as polycystic ovary syndrome, gestational diabetes mellitus, preeclampsia, endometriosis, foetal growth restriction and ovarian and endometrial cancers, evidence regarding the emerging role of adiponectin in these disorders is also discussed.

Metabolic Messengers: Adiponectin

Adiponectin is one of the most widely studied adipokines to date. First described in the mid-1990's, studying its regulation, biogenesis and physiological effects has proven to be extremely insightful and improved our understanding of the mechanisms that ensure systemic metabolic homeostasis. Here, we provide a brief overview of the current state of the field with respect to adiponectin, its history, sites and mechanisms of action, and the critical questions that will need to be addressed in the future.