Insulin Regulates Lipolysis and Fat Mass by Upregulating Growth/Differentiation Factor 3 in Adipose Tissue Macrophages

Interleukin-6 promotes visceral adipose tissue accumulation during aging via inhibiting fat lipolysis

BACKGROUND

Age-related visceral obesity could contribute to the development of cardiometabolic complications. The pathogenesis of visceral fat mass accumulation during the aging process remains complex and largely unknown. Interleukin-6 (IL-6) has emerged as one of the prominent inflammaging markers which are elevated in circulation during aging. However, the precise role of IL-6 in regulating age-related visceral adipose tissue accumulation remains uncertain.

RESULTS

A cross-sectional study including 77 older adults (≥65 years of age) was initially conducted. There was a significant positive association between serum IL-6 levels and visceral fat mass. We subsequently validated a modest but significant elevation in serum IL-6 levels in aged mice. Furthermore, we demonstrated that compared to wildtype control, IL-6 deficiency (IL-6 KO) significantly attenuated the accumulation of visceral adipose tissue during aging. Further metabolic characterization suggested that IL-6 deficiency resulted in improved lipid metabolism parameters and energy expenditure in aged mice. Moreover, histological examinations of adipose depots revealed that the absence of IL-6 ameliorated adipocyte hypertrophy in visceral adipose tissue of aged mice. Mechanically, the ablation of IL-6 could promote the PKA-mediated lipolysis and consequently mitigate lipid accumulation in adipose tissue in aged mice.

CONCLUSION

Our findings identify a detrimental role of IL-6 during the aging process by promoting visceral adipose tissue accumulation through inhibition of lipolysis. Therefore, strategies aimed at preventing or reducing IL-6 levels may potentially ameliorate age-related obesity and improve metabolism during aging.

Hepatic Activin E mediates liver-adipose inter-organ communication, suppressing adipose lipolysis in response to elevated serum fatty acids

OBJECTIVE

The liver is a central regulator of energy metabolism exerting its influence both through intrinsic processing of substrates such as glucose and fatty acid as well as by secreting endocrine factors, known as hepatokines, which influence metabolism in peripheral tissues. Human genome wide association studies indicate that a predicted loss-of-function variant in the Inhibin βE gene (INHBE), encoding the putative hepatokine Activin E, is associated with reduced abdominal fat mass and cardiometabolic disease risk. However, the regulation of hepatic Activin E and the influence of Activin E on adiposity and metabolic disease are not well understood. Here, we examine the relationship between hepatic Activin E and adipose metabolism, testing the hypothesis that Activin E functions as part of a liver-adipose, inter-organ feedback loop to suppress adipose tissue lipolysis in response to elevated serum fatty acids and hepatic fatty acid exposure.

METHODS

The relationship between hepatic Activin E and non-esterified fatty acids (NEFA) released from adipose lipolysis was assessed in vivo using fasted CL 316,243 treated mice and in vitro using Huh7 hepatocytes treated with fatty acids. The influence of Activin E on adipose lipolysis was examined using a combination of Inhbe knockout mice, a mouse model of hepatocyte-specific overexpression of Activin E, and mouse brown adipocytes treated with Activin E enriched media.

RESULTS

Increasing hepatocyte NEFA exposure in vivo by inducing adipose lipolysis through fasting or CL 316,243 treatment increased hepatic Inhbe expression. Similarly, incubation of Huh7 human hepatocytes with fatty acids increased expression of INHBE. Genetic ablation of Inhbe in mice increased fasting circulating NEFA and hepatic triglyceride accumulation. Treatment of mouse brown adipocytes with Activin E conditioned media and overexpression of Activin E in mice suppressed adipose lipolysis and reduced serum FFA levels, respectively. The suppressive effects of Activin E on lipolysis were lost in CRISPR-mediated ALK7 deficient cells and ALK7 kinase deficient mice. Disruption of the Activin E-ALK7 signaling axis in Inhbe KO mice reduced adiposity upon HFD feeding, but caused hepatic steatosis and insulin resistance.

CONCLUSIONS

Taken together, our data suggest that Activin E functions as part of a liver-adipose feedback loop, such that in response to increased serum free fatty acids and elevated hepatic triglyceride, Activin E is released from hepatocytes and signals in adipose through ALK7 to suppress lipolysis, thereby reducing free fatty acid efflux to the liver and preventing excessive hepatic lipid accumulation. We find that disrupting this Activin E-ALK7 inter-organ communication network by ablation of Inhbe in mice increases lipolysis and reduces adiposity, but results in elevated hepatic triglyceride and impaired insulin sensitivity. These results highlight the liver-adipose, Activin E-ALK7 signaling axis as a critical regulator of metabolic homeostasis.

Targeting activin receptor-like kinase 7 ameliorates adiposity and associated metabolic disorders

Activin receptor-like kinase 7 (ALK7) is a type I receptor in the TGF-β superfamily preferentially expressed in adipose tissue and associated with lipid metabolism. Inactivation of ALK7 signaling in mice results in increased lipolysis and resistance to both genetic and diet-induced obesity. Human genetic studies have recently revealed an association between ALK7 variants and both reduced waist to hip ratios and resistance to development of diabetes. In the present study, treatment with a neutralizing mAb against ALK7 caused a substantial loss of adipose mass and improved glucose intolerance and insulin resistance in both genetic and diet-induced mouse obesity models. The enhanced lipolysis increased fatty acid supply from adipocytes to promote fatty acid oxidation in muscle and oxygen consumption at the whole-body level. The treatment temporarily increased hepatic triglyceride levels, which resolved with long-term Ab treatment. Blocking of ALK7 signals also decreased production of its ligand, growth differentiation factor 3, by downregulating S100A8/A9 release from adipocytes and, subsequently, IL-1β release from adipose tissue macrophages. These findings support the feasibility of potential therapeutics targeting ALK7 as a treatment for obesity and diabetes.

Novel Cardiokine GDF3 Predicts Adverse Fibrotic Remodeling After Myocardial Infarction

BACKGROUND

Myocardial infarction (MI) induces a repair response that ultimately generates a stable fibrotic scar. Although the scar prevents cardiac rupture, an excessive profibrotic response impairs optimal recovery by promoting the development of noncontractile fibrotic areas. The mechanisms that lead to cardiac fibrosis are diverse and incompletely characterized. We explored whether the expansion of cardiac fibroblasts after MI can be regulated through a paracrine action of cardiac stromal cells.

METHODS

We performed a bioinformatic secretome analysis of cardiac stromal PW1⁺ cells isolated from normal and post-MI mouse hearts to identify novel secreted proteins. Functional assays were used to screen secreted proteins that promote fibroblast proliferation. The expressions of candidates were subsequently analyzed in mouse and human hearts and plasmas. The relationship between levels of circulating protein candidates and adverse post-MI cardiac remodeling was examined in a cohort of 80 patients with a first ST-segment-elevation MI and serial cardiac magnetic resonance imaging evaluations.

RESULTS

Cardiac stromal PW1⁺ cells undergo a change in paracrine behavior after MI, and the conditioned media from these cells induced a significant increase in the proliferation of fibroblasts. We identified a total of 12 candidates as secreted proteins overexpressed by cardiac PW1⁺ cells after MI. Among these factors, GDF3 (growth differentiation factor 3), a member of the TGF-β (transforming growth factor-β) family, was markedly upregulated in the ischemic hearts. Conditioned media specifically enriched with GDF3 induced fibroblast proliferation at a high level by stimulation of activin-receptor-like kinases. In line with the secretory nature of this protein, we next found that GDF3 can be detected in mice and human plasma samples, with a significant increase in the days after MI. In humans, higher GDF3 circulating levels (measured in the plasma at day 4 after MI) were significantly associated with an increased risk of adverse remodeling 6 months after MI (adjusted odds ratio, 1.76 [1.03-3.00]; P=0.037), including lower left ventricular ejection fraction and a higher proportion of akinetic segments.

CONCLUSIONS

Our findings define a mechanism for the profibrotic action of cardiac stromal cells through secreted cardiokines, such as GDF3, a candidate marker of adverse fibrotic remodeling after MI.

REGISTRATION

URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT01113268.

Nutraceuticals and the Network of Obesity Modulators

Obesity is considered an increasingly widespread disease in the world population, regardless of age and gender. Genetic but also lifestyle-dependent causes have been identified. Nutrition and physical exercise play an important role, especially in non-genetic obesity. In a three-compartment model, the body is divided into fat mass, fat-free mass and water, and obesity can be considered a condition in which the percentage of total fat mass is in excess. People with a high BMI index or overweight use self-medications, such as food supplements or teas, with the aim to prevent or treat their problem. Unfortunately, there are several obesity modulators that act both on the pathways that promote adipogenesis and those that inhibit lipolysis. Moreover, these pathways involve different tissues and organs, so it is very difficult to identify anti-obesity substances. A network of factors and cells contributes to the accumulation of fat in completely different body districts. The identification of natural anti-obesity agents should consider this network, which we would like to call "obesosome". The nutrigenomic, nutrigenetic and epigenetic contribute to making the identification of active compounds very difficult. This narrative review aims to highlight nutraceuticals that, in vitro or in vivo, showed an anti-obesity activity or were found to be useful in the control of dysfunctions which are secondary to obesity. The results suggest that it is not possible to use a single compound to treat obesity, but that the studies have to be addressed towards the identification of mixtures of nutraceuticals.

Activin receptor-like kinase 7 silencing alleviates cardiomyocyte apoptosis, cardiac fibrosis, and dysfunction in diabetic rats

Activin receptor-like kinase 7 (ALK7) is associated with lipometabolism and insulin sensitivity. Our previous study demonstrated that ALK7 participated in high glucose-induced cardiomyocyte apoptosis. The aim of our study was to investigate whether ALK7 plays an important role in modulating diabetic cardiomyopathy (DCM) and the mechanisms involved. The model of diabetes was induced in male Sprague-Dawley rats (120-140 g) by high-fat diet and intraperitoneal injections of low-dose streptozotocin (30 mg/kg). Animals were separated into four groups: control, DCM, DCM with ALK7 silencing, and DCM with vehicle control. The cardiac function was assessed by catheterization. Histopathologic analyses of collagen content and apoptosis rate, and protein analyses of ALK7, Smad2/3, Akt, Caspase3, and Bax/Bcl2 were performed. This study showed a rat model of DCM with hyperglycemia, severe insulin resistance, left ventricular dysfunction, and structural remodeling. With ALK7 silencing, the apoptotic cell death (apoptosis rate assessed by TUNEL, ratio of Bax/Bcl2 and expression of cleaved Caspase3), fibrosis areas, and Collagen I-to-III ratio decreased significantly. The insulin resistance and diastolic dysfunction were also ameliorated by ALK7 silencing. Furthermore, the depressed phosphorylation of Akt was restored while elevated phosphorylation of Smad2/3 decreased after the silencing of ALK7. The results suggest ALK7 silencing plays a protective role in DCM and may serve as a potential target for the treatment of human DCM.

The orphan ligand, activin C, signals through activin receptor-like kinase 7

Activin ligands are formed from two disulfide-linked inhibin β (Inhβ) subunit chains. They exist as homodimeric proteins, as in the case of activin A (ActA; InhβA/InhβA) or activin C (ActC; InhβC/InhβC), or as heterodimers, as with activin AC (ActAC; InhβA:InhβC). While the biological functions of ActA and activin B (ActB) have been well characterized, little is known about the biological functions of ActC or ActAC. One thought is that the InhβC chain functions to interfere with ActA production by forming less active ActAC heterodimers. Here, we assessed and characterized the signaling capacity of ligands containing the InhβC chain. ActC and ActAC activated SMAD2/3-dependent signaling via the type I receptor, activin receptor-like kinase 7 (ALK7). Relative to ActA and ActB, ActC exhibited lower affinity for the cognate activin type II receptors and was resistant to neutralization by the extracellular antagonist, follistatin. In mature murine adipocytes, which exhibit high ALK7 expression, ActC elicited a SMAD2/3 response similar to ActB, which can also signal via ALK7. Collectively, these results establish that ActC and ActAC are active ligands that exhibit a distinct signaling receptor and antagonist profile compared to other activins.

Increased Serum Levels of Growth-Differentiation Factor 3 (GDF3) and Inflammasome-Related Markers in Pregnant Women during Acute Zika Virus Infection

The systemic inflammatory response elicited by acute Zika virus (ZIKV) infection during pregnancy plays a key role in the clinical outcomes in mothers and congenitally infected offspring. The present study aimed to evaluate the serum levels of GDF-3 and inflammasome-related markers in pregnant women during acute ZIKV infection. Serum samples from pregnant (n = 18) and non-pregnant (n = 22) women with acute ZIKV infection were assessed for NLRP3, IL-1β, IL-18, and GDF3 markers through an enzyme-linked immunosorbent assay. ZIKV-negative pregnant (n = 18) and non-pregnant women (n = 15) were used as control groups. All serum markers were highly elevated in the ZIKV-infected groups in comparison with control groups (p < 0.0001). Among the ZIKV-infected groups, the serum markers were significantly augmented in the pregnant women in comparison with non-pregnant women (NLRP3 p < 0.001; IL-1β, IL-18, and GDF3 p < 0.0001). The IL-18 marker was found at significantly higher levels (p < 0.05) in the third trimester of pregnancy. Bivariate and multivariate analyses showed a strong positive correlation between GDF3 and NLRP3 markers among ZIKV-infected pregnant women (r = 0.91, p < 0.0001). The findings indicated that acute ZIKV infection during pregnancy induces the overexpression of GDF-3 and inflammasome-related markers, which may contribute to congenital disorders and harmful pregnancy outcomes.

New Insights into Adipose Tissue Macrophages in Obesity and Insulin Resistance

Obesity has become a worldwide epidemic that poses a severe threat to human health. Evidence suggests that many obesity comorbidities, such as type 2 diabetes mellitus, steatohepatitis, and cardiovascular diseases, are related to obesity-induced chronic low-grade inflammation. Macrophages are the primary immune cells involved in obesity-associated inflammation in both mice and humans. Intensive research over the past few years has yielded tremendous progress in our understanding of the additional roles of adipose tissue macrophages (ATMs) beyond classical M1/M2 polarization in obesity and related comorbidities. In this review, we first characterize the diverse subpopulations of ATMs in the context of obesity. Furthermore, we review the recent advance on the role of the extensive crosstalk between adipocytes and ATMs in obesity. Finally, we focus on the extended crosstalk within adipose tissue between perivascular mesenchymal cells and ATMs. Understanding the pathological mechanisms that underlie obesity will be critical for the development of new intervention strategies to prevent or treat this disease and its associated co-morbidities.

miR-182 targeting reprograms tumor-associated macrophages and limits breast cancer progression

The protumor roles of alternatively activated (M2) tumor-associated macrophages (TAMs) have been well established, and macrophage reprogramming is an important therapeutic goal. However, the mechanisms of TAM polarization remain incompletely understood, and effective strategies for macrophage targeting are lacking. Here, we show that miR-182 in macrophages mediates tumor-induced M2 polarization and can be targeted for therapeutic macrophage reprogramming. Constitutive miR-182 knockout in host mice and conditional knockout in macrophages impair M2-like TAMs and breast tumor development. Targeted depletion of macrophages in mice blocks the effect of miR-182 deficiency in tumor progression while reconstitution of miR-182-expressing macrophages promotes tumor growth. Mechanistically, cancer cells induce miR-182 expression in macrophages by TGFβ signaling, and miR-182 directly suppresses TLR4, leading to NFκb inactivation and M2 polarization of TAMs. Importantly, therapeutic delivery of antagomiR-182 with cationized mannan-modified extracellular vesicles effectively targets macrophages, leading to miR-182 inhibition, macrophage reprogramming, and tumor suppression in multiple breast cancer models of mice. Overall, our findings reveal a crucial TGFβ/miR-182/TLR4 axis for TAM polarization and provide rationale for RNA-based therapeutics of TAM targeting in cancer.

Macrophages, Metabolism and Heterophagy in the Heart

The heart is a never-stopping engine that relies on a formidable pool of mitochondria to generate energy and propel pumping. Because dying cardiomyocytes cannot be replaced, this high metabolic rate creates the challenge of preserving organelle fitness and cell function for life. Here, we provide an immunologist's perspective on how the heart solves this challenge, which is in part by incorporating macrophages as an integral component of the myocardium. Cardiac macrophages surround cardiomyocytes and capture dysfunctional mitochondria that these cells eject to the milieu, effectively establishing a client cell-support cell interaction. We refer to this heterologous partnership as heterophagy. Notably, this process shares analogies with other biological systems, is essential for proteostasis and metabolic fitness of cardiomyocytes, and unveils a remarkable degree of dependence of the healthy heart on immune cells for everyday function.

Changing lanes: seasonal differences in cellular metabolism of adipocytes in grizzly bears (Ursus arctos horribilis)

Obesity is among the most prevalent of health conditions in humans leading to a multitude of metabolic pathologies such as type 2 diabetes and hyperglycemia. However, there are many wild animals that have large seasonal cycles of fat accumulation and loss that do not result in the health consequences observed in obese humans. One example is the grizzly bear (Ursus arctos horribilis) that can have body fat content > 40% that is then used as the energy source for hibernation. Previous in vitro studies found that hibernation season adipocytes exhibit insulin resistance and increased lipolysis. Yet, other aspects of cellular metabolism were not addressed, leaving this in vitro model incomplete. Thus, the current studies were performed to determine if the cellular energetic phenotype-measured via metabolic flux-of hibernating bears was retained in cultured adipocytes and to what extent that was due to serum or intrinsic cellular factors. Extracellular acidification rate and oxygen consumption rate were used to calculate proton efflux rate and total ATP defined as both ATP from glycolysis and from mitochondrial respiration. Hibernation adipocytes treated with hibernation serum produced less ATP and exhibited lower maximal respiration and glycolysis rates than active season adipocytes. These effects were reversed with serum from the opposite season. Insulin had little influence on total ATP production and lipolysis in both hibernation and active serum-treated adipocytes. Together, these results suggest that the metabolic suppression occurring in hibernation adipocytes are downstream of insulin signaling and likely due to a combined reduction in mitochondria number and/or function and glycolytic processes. Future elucidation of the serum components and the cellular mechanisms that enable alterations in mitochondrial function could provide a novel avenue for the development of treatments for human metabolic diseases.

ALK7 Inhibition Protects Osteoblast Cells Against High Glucoseinduced ROS Production via Nrf2/HO-1 Signaling Pathway

BACKGROUND

Some studies demonstrated that under high-glucose (HG) condition, osteoblasts develop oxidative stress, which will impair their normal functions. The effects of activin receptor-like kinase 7 (ALK7) silencing on HG-induced osteoblasts remained unclear.

OBJECTIVE

The aim of this study was to explore the effect of ALK7 on HG-induced osteoblasts.

METHODS

MC3T3-E1 cells were treated with different concentrations of HG (0, 50, 100, 200 and 300mg/dL), and the cell viability was detected using cell counting kit-8 (CCK-8). HG-treated MC3T3-E1 cells were transfected with siALK7 or ALK7 overexpression plasmid or siNrf2, and then the viability and apoptosis were detected by CCK-8 and flow cytometry. The levels of Reactive Oxygen Species (ROS), collagen I and calcification nodule were determined by oxidative stress kits, Enzyme-linked immunosorbent assay and Alizarin red staining. The expressions of NF-E2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1) and osteoblast-associated genes were determined by quantitative real-time PCR (qRT-PCR) and Western blot.

RESULTS

Cell viability was reduced with HG treatment. Silencing ALK7 inhibited the effect of HG on increasing cell apoptosis and ROS production, reduced cell viability, mineralized nodules, and downregulated collagen I and osteoblast-associated genes expression in MC3T3-E1 cells. ALK7 silencing activated the Nrf2/HO-1 signaling pathway by affecting expressions of HO-1 and Nrf2. ALK7 overexpression had the opposite effects. In addition, siNrf2 partially reversed the effects of ALK7 silencing on HG-induced MC3T3-E1 cells.

CONCLUSION

ALK7 silencing protected osteoblasts under HG condition possibly through activating the Nrf2/HO-1 pathway.

Lipolysis: cellular mechanisms for lipid mobilization from fat stores

The perception that intracellular lipolysis is a straightforward process that releases fatty acids from fat stores in adipose tissue to generate energy has experienced major revisions over the last two decades. The discovery of new lipolytic enzymes and coregulators, the demonstration that lipophagy and lysosomal lipolysis contribute to the degradation of cellular lipid stores and the characterization of numerous factors and signalling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels have revolutionized our understanding of lipolysis. In this review, we focus on the mechanisms that facilitate intracellular fatty-acid mobilization, drawing on canonical and noncanonical enzymatic pathways. We summarize how intracellular lipolysis affects lipid-mediated signalling, metabolic regulation and energy homeostasis in multiple organs. Finally, we examine how these processes affect pathogenesis and how lipolysis may be targeted to potentially prevent or treat various diseases.

Remodeling of Macrophages in White Adipose Tissue under the Conditions of Obesity as well as Lipolysis

Adipose tissue macrophages (ATM) are a major source of low-grade inflammation in obesity, and yet reasons driving ATM accumulation in white adipose tissue (WAT) are not fully understood. Emerging evidence suggested that ATM underwent extensive remodeling in obesity. In addition to abundance, ATM in obesity were lipid-laden and metabolically reprogrammed, which in turn was tightly related to their functional alterations and persistence in obesity. Herein, we aimed to discuss that activation of lipid sensing signaling associated with metabolic reprogramming in ATM was indispensible for their migration, retention, or proliferation in obesity. Likewise, lipolysis also induced similar but transient ATM remodeling. Therefore, we assumed that obesity might share overlapping mechanisms with lipolysis in remodeling ATM. Formation of crown-like structures (CLS) in WAT was presumably a common event initiating ATM remodeling, with a spectrum of lipid metabolites released from adipocytes being potential signaling molecules. Moreover, adipose interlerkin-6 (IL-6) exhibited homologous alterations by obesity and lipolysis. Thus, we postulated a positive feedback loop between ATM and adipocytes via IL-6 signaling backing ATM persistence by comparison of ATM remodeling under obesity and lipolysis. An elucidation of ATM persistence could help to provide novel therapeutic targets for obesity-associated inflammation.

Astragaloside IV ameliorates fat metabolism in the liver of ageing mice through targeting mitochondrial activity

Astragaloside IV (AST) is a major bioactive compound of Radix Astragali with medical and health benefits. Previous studies have found that AST can reduce the body weights of high-fat diet fed mice. However, the effect of AST on fat metabolism of ageing mice is unclear. In this study, naturally ageing mice were administered intragastrically with AST at 30 mg/kg/day (ageing + AST-L group) and 90 mg/kg/day (ageing + AST-H group) for 16-20 months. Adult (4 months old) and ageing mice were given 1% sodium carboxyl methylcellulose as vehicle. Energy metabolism-related biological parameters of living mice were examined. Moreover, mRNA and protein levels of key enzymes/proteins involved in triglyceride (TG) lipolysis, fatty acid β-oxidation (FAO), ketone body (KB) production and mitochondrial respiratory chain were also examined after sacrifice. Results demonstrated that treatment with AST significantly reduced body weight, white fat and liver/body weight ratio of ageing mice, significantly reduced serum/hepatic TG levels, respiratory quotient, promoted fatty acid mobilization in white adipose tissue, mitochondrial FAO and KB production and mitochondrial biosynthesis/functions in the liver of ageing mice. AST also up-regulated the expression of phosphorylated AMP-activated protein kinase, acetyl-CoA carboxylase, acetyl-coenzyme A synthetase, carnitine palmitoyltransferase 1a/1b, enoyl coenzyme A hydratase-short chain, acyl-CoA dehydrogenase medium chain and mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase-2 involved in fat metabolism. These results indicated that mitochondrial activity could be the target of AST to treat abnormal fat metabolism during ageing.

Activation of GCN2 in macrophages promotes white adipose tissue browning and lipolysis under leucine deprivation

We have previously shown that leucine deprivation stimulates browning and lipolysis in white adipose tissue (WAT), which helps to treat obesity. Adipose tissue macrophages (ATMs) significantly influence WAT browning and lipolysis. However, it is unclear whether ATMs are involved in leucine deprivation-induced browning and lipolysis in WAT; the associated signals remain to be elucidated. Here, we investigated the role of ATMs and the possible mechanisms involved in WAT browning and lipolysis under leucine-deprivation conditions. In this study, macrophages were depleted in mice by injecting clodronate-liposomes (CLOD) into subcutaneous white adipose tissues. Then, mice lacking general control nonderepressible 2 kinase (GCN2), which is a sensor of amino acid starvation, specifically in Lyz2-expressing cells, were generated to investigate the changes in leucine deprivation-induced WAT browning and lipolysis. We found leucine deprivation decreased the accumulation and changed the polarization of ATMs. Ablation of macrophages by CLOD impaired WAT browning and lipolysis under leucine-deprivation conditions. Mechanistically, leucine deprivation activated GCN2 signals in macrophages. Myeloid-specific abrogation of GCN2 in mice blocked leucine deprivation-induced browning and lipolysis in WAT. Further analyses revealed that GCN2 activation in macrophages reduced the expression of monoamine oxidase A (MAOA), resulting in increased norepinephrine (NE) secretion from macrophages to adipocytes, and this resulted in enhanced WAT browning and lipolysis. Moreover, the injection of CL316,243, a β3-adrenergic receptor agonist, and inhibition of MAOA effectively increased the level of NE, leading to the enhancement of browning and lipolysis of WAT in myeloid GCN2 knockout mice under leucine deprivation. Collectively, our results demonstrate a novel function of GCN2 signals in macrophages, that is, regulating WAT browning and lipolysis under leucine deprivation. Our study provides important hints for possible treatment for obesity.

Regulation of metabolic homeostasis by the TGF-β superfamily receptor ALK7

ALK7 (Activin receptor-like kinase 7) is a member of the TGF-β receptor superfamily predominantly expressed by cells and tissues involved in endocrine functions, such as neurons of the hypothalamus and pituitary, pancreatic β-cells and adipocytes. Recent studies have begun to delineate the processes regulated by ALK7 in these tissues and how these become integrated with the homeostatic regulation of mammalian metabolism. The picture emerging indicates that ALK7's primary function in metabolic regulation is to limit catabolic activities and preserve energy. Aside of the hypothalamic arcuate nucleus, the function of ALK7 elsewhere in the brain, particularly in the cerebellum, where it is abundantly expressed, remains to be elucidated. Although our understanding of the basic molecular events underlying ALK7 signaling has benefited from the vast knowledge available on TGF-β receptor mechanisms, how these connect to the physiological functions regulated by ALK7 in different cell types is still incompletely understood. Findings of missense and nonsense variants in the Acvr1c gene, encoding ALK7, of some mouse strains and human subjects indicate a tolerance to ALK7 loss of function. Recent discoveries suggest that specific inhibitors of ALK7 may have therapeutic applications in obesity and metabolic syndrome without overt adverse effects.

Galectin-3/adiponectin as a new biological indicator for assessing the risk of type 2 diabetes: a cross-sectional study in a community population

OBJECTIVE

This study aimed to explore the association between the risk of newly diagnosed type 2 diabetes and galectin-3 and adiponectin and to investigate whether their joint action shows a favorable diabetes assessment performance.

METHODS

We conducted a community-based study in 135 newly diagnosed patients with type 2 diabetes and 270 age- and sex-matched nondiabetic patients. Odds ratios and 95% confidence intervals were determined using logistic regression analysis. Receiver operating characteristic curve, decision curve analysis and calibration plot were used to explore their efficacy and clinical utility for models.

RESULTS

High quartiles of galectin-3/adiponectin (quartile 4 vs 1: OR 2.43 [95% CIs: 1.21-5.00]) showed the strongest correlation with an increased risk of type 2 diabetes in the total population, which was consistent in the older population (age≥50 years old) in adjustment models. The combination + lipids + galectin-3/adiponectin model (AUC = 0.72 [95% CIs: 0.66-0.77]) displayed better diabetes assessment performance than the other two models.

CONCLUSIONS

High galectin-3 and low adiponectin levels were associated with the high risk of diabetes, and their joint action was a superior promising factor for evaluating diabetes risk. The diabetes discriminative strength of galectin-3/adiponectin was better in the older population than the younger.

Brd4 modulates diet-induced obesity via PPARγ-dependent Gdf3 expression in adipose tissue macrophages

Macrophage-mediated inflammatory response has been implicated in the pathogenesis of obesity and insulin resistance. Brd4 has emerged as a key regulator in the innate immune response. However, the role of Brd4 in obesity-associated inflammation and insulin resistance remains uncharacterized. Here, we demonstrated that myeloid lineage-specific Brd4 knockout (Brd4-CKO) mice were protected from high-fat diet-induced (HFD-induced) obesity with less fat accumulation, higher energy expenditure, and increased lipolysis in adipose tissue. Brd4-CKO mice fed a HFD also displayed reduced local and systemic inflammation with improved insulin sensitivity. RNA-Seq of adipose tissue macrophages (ATMs) from HFD-fed WT and Brd4-CKO mice revealed that expression of antilipolytic factor Gdf3 was significantly decreased in ATMs of Brd4-CKO mice. We also found that Brd4 bound to the promoter and enhancers of Gdf3 to facilitate PPARγ-dependent Gdf3 expression in macrophages. Furthermore, Brd4-mediated expression of Gdf3 acted as a paracrine signal targeting adipocytes to suppress the expression of lipases and the associated lipolysis in cultured cells and mice. Controlling the expression of Gdf3 in ATMs could be one of the mechanisms by which Brd4 modulates lipid metabolism and diet-induced obesity. This study suggests that Brd4 could be a potential therapeutic target for obesity and insulin resistance.

Role of Macrophages in the Endocrine System

Macrophages are cells of the innate immune system that play myriad roles in the body. Macrophages are known to reside in endocrine glands, and a body of evidence now suggests that these cells interact closely with endocrine cells. Immune-endocrine interactions are important in the development of endocrine glands and their functioning during physiological states, and also become key players in pathophysiological states. Through gene expression profiling, diverse subpopulations of tissue macrophages have been discovered within endocrine organs; this has important implications for disease pathogenesis and potential pharmacotherapy. The molecular basis for the crosstalk between macrophages and endocrine cells is being unraveled, and allows the identification of multiple points for pharmacologic intervention. Macrophages in adipose tissue and pancreatic islets are key players in the process of metaflammation (metabolic inflammation) that underlies the development of insulin resistance, metabolic syndrome, diabetes mellitus, and non-alcoholic fatty liver disease. In the ovary, they play important roles in ovarian folliculogenesis and ovulation, whereas in the male reproductive tract they regulate spermatogenesis through the regulation of steroidogenesis by Leydig cells. We summarize the diverse roles played by macrophages in the endocrine system and identify potential targets for pharmacotherapy in endocrine disorders.

UV-Induced Reduction of ACVR1C Decreases SREBP1 and ACC Expression by the Suppression of SMAD2 Phosphorylation in Normal Human Epidermal Keratinocytes

Activin A receptor type 1C (ACVR1C), a type I transforming growth factor-β (TGF-β) receptor, has been implicated in sensitive skin and psoriasis and is involved in the regulation of metabolic homeostasis as well as cell proliferation and differentiation. In this study, we identified a novel role of ACVR1C in the ultraviolet (UV)-irradiation-induced reduction of epidermal lipogenesis in human skin. UV irradiation decreased ACVR1C expression and epidermal triglyceride (TG) synthesis in human skin in vivo and in primary normal human epidermal keratinocytes (NHEK) in vitro. Lipogenic genes, including genes encoding acetyl-CoA carboxylase (ACC) and sterol regulatory element binding protein-1 (SREBP1), were significantly downregulated in UV-irradiated NHEK. ACVR1C knockdown by shRNA resulted in greater decreases in SREBP1 and ACC in response to UV irradiation. Conversely, the overexpression of ACVR1C attenuated the UV-induced decreases in SREBP1 and ACC. Further mechanistic study revealed that SMAD2 phosphorylation mediated the ACVR1C-induced lipogenic gene modulation. Taken together, a decrease in ACVR1C may cause UV-induced reductions in SREBP1 and ACC as well as epidermal TG synthesis via the suppression of SMAD2 phosphorylation. ACVR1C may be a target for preventing or treating UV-induced disruptions in lipid metabolism and associated skin disorders.

4-Hydroxyisoleucine Alleviates Macrophage-Related Chronic Inflammation and Metabolic Syndrome in Mice Fed a High-Fat Diet

In obesity, macrophages and other immune cells accumulate in organs affected by insulin, leading to chronic inflammation and insulin resistance. 4-Hydroxyisoleucine (4-HIL) is a non-protein amino acid found in fenugreek seeds. 4-HIL enhances insulin sensitivity, but its mechanism is still unclear. In this study, 4-HIL intervention reduced weight gain, liver steatosis, and dyslipidemia; moreover, it increased systemic insulin sensitivity and improved insulin resistance in mice. Importantly, after administration, the accumulation of M1 like CD11c⁺ macrophages and inflammation in the liver and adipose tissue were reduced in the mice. 4-HIL also reduced the proportion of CD11c⁺ macrophages among bone marrow-derived macrophages, which were induced in vitro. These observations demonstrate a new role of 4-HIL in insulin resistance in hepatocytes and adipocytes. 4-HIL inhibits obesity-related insulin resistance by reducing inflammation and regulating the state of M1/M2 macrophages.

Heat-killed Lactobacillus plantarum L-137 attenuates obesity and associated metabolic abnormalities in C57BL/6 J mice on a high-fat diet

Heat-killed Lactobacillus plantarum L-137 (HK L-137) has anti-allergic, antitumor, and antiviral effects in mice, as well as an anti-inflammatory effect in rats with metabolic syndrome through regulation of immunity. To evaluate the influence of HK L-137 on chronic inflammation in mice with diet-induced obesity, C57BL/6 J mice were fed a normal diet (16% of energy as fat) or a high-fat diet (62% of energy as fat) with or without 0.002% HK L-137 for 4 to 20 weeks. It was found that HK L-137 supplementation alleviated weight gain and elevation of plasma glucose, cholesterol, alanine aminotransferase, and aspartate transaminase levels in mice with diet-induced obesity. Expression of several inflammation-related genes, including F4/80, CD11c, and IL-1β, in the epididymal adipose tissue of these mice was significantly downregulated by HK L-137. In addition, plasma levels of lipopolysaccharide-binding protein, a marker of endotoxemia, tended to be decreased by administration of HK L-137. These findings suggest that HK L-137 supplementation ameliorates obesity-induced metabolic abnormalities and adipose tissue inflammation, possibly through improvement of intestinal permeability.

Obesity-Linked PPARγ S273 Phosphorylation Promotes Insulin Resistance through Growth Differentiation Factor 3

The thiazolidinediones (TZDs) are ligands of PPARγ that improve insulin sensitivity, but their use is limited by significant side effects. Recently, we demonstrated a mechanism wherein TZDs improve insulin sensitivity distinct from receptor agonism and adipogenesis: reversal of obesity-linked phosphorylation of PPARγ at serine 273. However, the role of this modification hasn't been tested genetically. Here we demonstrate that mice encoding an allele of PPARγ that cannot be phosphorylated at S273 are protected from insulin resistance, without exhibiting differences in body weight or TZD-associated side effects. Indeed, hyperinsulinemic-euglycemic clamp experiments confirm insulin sensitivity. RNA-seq in these mice reveals reduced expression of Gdf3, a BMP family member. Ectopic expression of Gdf3 is sufficient to induce insulin resistance in lean, healthy mice. We find Gdf3 inhibits BMP signaling and insulin signaling in vitro. Together, these results highlight the diabetogenic role of PPARγ S273 phosphorylation and focus attention on a putative target, Gdf3.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

A review on the biology and properties of adipose tissue macrophages involved in adipose tissue physiological and pathophysiological processes

Obesity exhibits a correlation with metabolic inflammation and endoplasmic reticulum stress, promoting the progression of metabolic disease such as diabetes, hyperlipidemia, hyperuricemia and so on. Adipose tissue macrophages (ATMs) are central players in obesity-associated inflammation and metabolic diseases. Macrophages are involved in lipid and energy metabolism and mitochondrial function in adipocytes. Macrophage polarization is accompanied by metabolic shifting between glycolysis and mitochondrial oxidative phosphorylation. Here, this review focuses on macrophage metabolism linked to functional phenotypes with an emphasis on macrophage polarization in adipose tissue physiological and pathophysiological processes. In particular, the interplay between ATMs and adipocytes in energy metabolism, glycolysis, OXPHOS, iron handing and even interactions with the nervous system have been reviewed. Overall, the understanding of protective and pathogenic roles of ATMs in adipose tissue can potentially provide strategies to prevent and treat obesity-related metabolic disorders.

APPL1 knockdown blocks adipogenic differentiation and promotes adipocyte lipolysis

Adipocyte dysfunction is closely associated with the development of obesity, insulin resistance, and type 2 diabetes. In addition to having a positive effect on adiponectin pathway and insulin signaling through direct and/or indirect mechanisms, adapter protein APPL1 has also been reported to regulate body weight, brown fat tissues thermogenesis, and body fat distribution in diabetic individuals. However, there is dearth of data on the specific role of APPL1 on adipogenic differentiation and adipocyte lipolysis. In this study, APPL1's function in adipocyte differentiation and adipocyte lipolysis was evaluated, and the possible mechanisms were investigated. We found that APPL1 knockdown (KD) impeded differentiation of 3T3-L1 preadipocytes into mature 3T3-L1 adipocytes and enhanced basal and insulin-suppressed lipolysis in mature 3T3-L1 adipocytes. APPL1 KD cells presented a reduced autophagic activity in 3T3-L1 preadipocytes and mature 3T3-L1 adipocytes. In 3T3-L1 preadipocytes, APPL1 KD reduced PPARγ protein levels, which was prevented by administration with proteasome inhibitor MG132. Furthermore, APPL1 KD-reduced autophagic activity in mature 3T3-L1 adipocytes was markedly restored by inhibition of PKA, accompanied with prevention of APPL1-induced lipolysis. In addition, APPL1 KD caused insulin resistance in mature 3T3-L1 adipocytes. Unexpectedly, we found that APPL1 overexpression did not appear to play a role in adipogenic differentiation and adipocyte lipolysis. Our results confirmed that APPL1 KD inhibits adipogenic differentiation by suppressing autophagy and enhances adipocyte lipolysis through activating PKA respectively. These findings may deepen our understanding of APPL1 function, especially its regulation on adipocyte biology.

GDF3 Protects Mice against Sepsis-Induced Cardiac Dysfunction and Mortality by Suppression of Macrophage Pro-Inflammatory Phenotype

Macrophages are critical for regulation of inflammatory response during endotoxemia and septic shock. However, the mediators underlying their regulatory function remain obscure. Growth differentiation factor 3 (GDF3), a member of transforming growth factor beta (TGF-β) superfamily, has been implicated in inflammatory response. Nonetheless, the role of GDF3 in macrophage-regulated endotoxemia/sepsis is unknown. Here, we show that serum GDF3 levels in septic patients are elevated and strongly correlate with severity of sepsis and 28-day mortality. Interestingly, macrophages treated with recombinant GDF3 protein (rGDF3) exhibit greatly reduced production of pro-inflammatory cytokines, comparing to controls upon endotoxin challenge. Moreover, acute administration of rGDF3 to endotoxin-treated mice suppresses macrophage infiltration to the heart, attenuates systemic and cardiac inflammation with less pro-inflammatory macrophages (M1) and more anti-inflammatory macrophages (M2), as well as prolongs mouse survival. Mechanistically, GDF3 is able to activate Smad2/Smad3 phosphorylation, and consequently inhibits the expression of nod-like receptor protein-3 (NLRP3) in macrophages. Accordingly, blockade of Smad2/Smad3 phosphorylation with SB431542 significantly offsets rGDF3-mediated anti-inflammatory effects. Taken together, this study uncovers that GDF3, as a novel sepsis-associated factor, may have a dual role in the pathophysiology of sepsis. Acute administration of rGDF3 into endotoxic shock mice could increase survival outcome and improve cardiac function through anti-inflammatory response by suppression of M1 macrophage phenotype. However, constitutive high levels of GDF3 in human sepsis patients are associated with lethality, suggesting that GDF3 may promote macrophage polarization toward M2 phenotype which could lead to immunosuppression.

Anti-Lipolysis Induced by Insulin in Diverse Pathophysiologic Conditions of Adipose Tissue

As an important energy reservoir, adipose tissue maintains lipid balance and regulates energy metabolism. When the body requires energy, adipocytes provide fatty acids to peripheral tissues through lipolysis. Insulin plays an important role in regulating normal fatty acid levels by inhibiting lipolysis. When the morphology of adipose tissue is abnormal, its microenvironment changes and the lipid metabolic balance is disrupted, which seriously impairs insulin sensitivity. As the most sensitive organ to respond to insulin, lipolysis levels in adipose tissue are affected by impaired insulin function, which results in serious metabolic diseases. However, the specific underlying mechanisms of this process have not yet been fully elucidated, and further study is required. The purpose of this review is to discuss the effects of adipose tissue on the anti-lipolysis process triggered by insulin under different conditions. In particular, the functional changes of this process respond to inconsonantly morphological changes of adipose tissue.

Gaining Weight: Insulin-Eating Islet Macrophages

The role of the immune system in homeostasis of pancreatic β cells in type 2 diabetes mellitus is poorly characterized. In a recent issue of Cell Metabolism, Ying et al. (2018) report that two subpopulations of macrophages expand during obesity to impair β cell function.