The brown fat-enriched secreted factor Nrg4 preserves metabolic homeostasis through attenuation of hepatic lipogenesis

Multifaceted Roles of Beige Fat in Energy Homeostasis Beyond UCP1

Beige adipocytes are an inducible form of thermogenic adipose cells that emerge within the white adipose tissue in response to a variety of environmental stimuli, such as chronic cold acclimation. Similar to brown adipocytes that reside in brown adipose tissue depots, beige adipocytes are also thermogenic; however, beige adipocytes possess unique, distinguishing characteristics in their developmental regulation and biological function. This review highlights recent advances in our understanding of beige adipocytes, focusing on the diverse roles of beige fat in the regulation of energy homeostasis that are independent of the canonical thermogenic pathway via uncoupling protein 1.

Cyanidin-3-O-β-glucoside regulates the activation and the secretion of adipokines from brown adipose tissue and alleviates diet induced fatty liver

AIM

Cyanidin-3-O-β-glucoside (Cy-3-G) the most abundant monomer of anthocyanins has multiple protective effects on many diseases. To date, whether Cy-3-G could regulate the function of brown adipose tissue (BAT) is still unclear and whether this regulation could influence the secretion of adipokines from BAT to prevent non-alcoholic fatty liver disease (NAFLD) indirectly remains to be explored. In this study we investigated the effect of Cy-3-G on BAT and the potential role of Cy-3-G to prevent fatty liver through regulating the secretion of BAT.

METHODS

Male C57BL/6 J mice were fed with a high fat high cholesterol (HFC) diet with or without 200 mg/kg B.W Cy-3-G for 8 weeks. In in vitro experiments, the differentiated brown adipocytes (BAC) and C3H10T1/2 clone8 cells were treated with 0.2 mM palmitate with or without Cy-3-G for 72 or 96 h. Then the culture media of C3H10T1/2 clone8 cells were collected for measuring the adipokines secretion by immunoblot assay and were applied to culture HepG2 cells or LO2 cells for 24 h. Lipid accumulation in HepG2 cells or LO2 cells were evaluated by oil red O staining.

RESULTS

Here we found that Cy-3-G regulated the activation of BAT and the expression of adipokines in BAT which were disrupted by HFC diet and alleviated diet induced fatty liver in mice. In in vitro experiments, Cy-3-G inhibited the release of adipokines including extracellular nicotinamide phosphoribosyltransferase (eNAMPT) and fibroblast growth factor 21 (FGF21) from differentiated C3H10T1/2 clone8 cells induced by palmitate, which was accompanied by a reduction of lipid accumulation in HepG2 cells and LO2 cells cultured by the corresponding collected media of C3H10T1/2 clone8 cells.

CONCLUSIONS

These results indicate that Cy-3-G can regulate the thermogenic and secretory functions of BAT. Furthermore, our data suggest that the protective effect of Cy-3-G on hepatic lipid accumulation is probably via regulating the secretion of adipokines from BAT.

The long noncoding RNA Blnc1 orchestrates homeostatic adipose tissue remodeling to preserve metabolic health

Objective

Long noncoding RNAs (lncRNAs) are emerging as powerful regulators of adipocyte differentiation and gene expression. However, their significance in adipose tissue metabolism and physiology has not been demonstrated in vivo. We previously identified Blnc1 as a conserved lncRNA regulator of brown and beige adipocyte differentiation. In this study, we investigated the physiological role of Blnc1 in thermogenesis, adipose remodeling and systemic metabolism.

Methods

We generated fat-specific Blnc1 transgenic and conditional knockout mouse strains and investigated how adipocyte Blnc1 levels are causally linked to key aspects of metabolic health following diet-induced obesity. We performed studies using cultured adipocytes to establish cell-autonomous role of Blnc1 in regulating adipocyte gene programs.

Results

Blnc1 is highly induced in both brown and white fats from obese mice. Fat-specific inactivation of Blnc1 impairs cold-induced thermogenesis and browning and exacerbates obesity-associated brown fat whitening, adipose tissue inflammation and fibrosis, leading to more severe insulin resistance and hepatic steatosis. On the contrary, transgenic expression of Blnc1 in adipose tissue elicits the opposite and beneficial metabolic effects, supporting a critical role of Blnc1 in driving adipose adaptation and homeostatic remodeling during obesity. Mechanistically, Blnc1 cell-autonomously attenuates proinflammatory cytokine signaling and promotes fuel storage in adipocytes through its protein partner Zbtb7b.

Conclusions

This study illustrates a surprisingly pleiotropic and dominant role of lncRNA in driving adaptive adipose tissue remodeling and preserving metabolic health.

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Adipocyte-specific Blnc1 inactivation accelerates BAT whitening and impairs healthy WAT expansion.

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Blnc1 transgenic mice are protected from diet-induced metabolic disorders.

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Blnc1 attenuates adipose tissue inflammation and preserves metabolic health in obesity.

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Blnc1 suppresses NF-kB signaling and TNFα-induced cytokine/chemokine release by adipocytes.

Brown adipose tissue and lipid metabolism

PURPOSE OF REVIEW

This article explores how the interplay between lipid metabolism and thermogenic adipose tissues enables proper physiological adaptation to cold environments in rodents and humans.

RECENT FINDINGS

Cold exposure triggers systemic changes in lipid metabolism, which increases fatty acid delivery to brown adipose tissue (BAT) by various routes. Next to fatty acids generated intracellularly by de-novo lipogenesis or by lipolysis at lipid droplets, brown adipocytes utilize fatty acids released by white adipose tissue (WAT) for adaptive thermogenesis. WAT-derived fatty acids are internalized directly by BAT, or indirectly after hepatic conversion to very low-density lipoproteins and acylcarnitines. In the postprandial state, chylomicrons hydrolyzed by lipoprotein lipase - activated specifically in thermogenic adipocytes - are the predominant fatty acid source. Cholesterol-enriched chylomicron remnants and HDL generated by intravascular lipolysis in BAT are cleared more rapidly by the liver, explaining the antiatherogenic effects of BAT activation. Notably, increased cholesterol flux and elevated hepatic synthesis of bile acids under cold exposure further promote BAT-dependent thermogenesis.

SUMMARY

Although pathways providing fatty acids for activated BAT have been identified, more research is needed to understand the integration of lipid metabolism in BAT, WAT and liver, and to determine the relevance of BAT for human energy metabolism.

Clinical Implications of UCP1 mRNA Expression in Human Cervical Adipose Tissue Under Physiological Conditions

OBJECTIVE

The clinical implications of human brown adipose tissue (BAT) were investigated based on the analysis of cervical adipose tissue gene expression under normal physiological conditions.

METHODS

Matched-pair specimens of adipose tissue (AT) were collected from beneath the incision plane (subcutaneous AT) and from the area surrounding the carotid sheath (carotid AT) from 60 patients undergoing thyroidectomy. The mRNA expression of BAT-associated genes in these tissues was examined, and this expression was correlated with the clinical characteristics of the subjects.

RESULTS

The UCP1 mRNA level was significantly higher in the carotid AT than in the subcutaneous AT. There was an inverse correlation between subject age and the ratio of UCP1 mRNA expression in the carotid AT relative to the subcutaneous AT, which is a measure of BAT activity (r = -0.459; P = 0.004), and there was a negative correlation between BMI and the ratio of UCP1 mRNA expression in subjects with higher BAT activity (r = -0.532; P = 0.016).

CONCLUSIONS

UCP1 was identified as the only marker of cervical BAT in humans. There was a negative correlation between obesity and BAT activity in subjects with higher BAT activity, although BAT activity decreased with age.

KDM4B protects against obesity and metabolic dysfunction

Although significant progress has been made in understanding epigenetic regulation of in vitro adipogenesis, the physiological functions of epigenetic regulators in metabolism and their roles in obesity remain largely elusive. Here, we report that KDM4B (lysine demethylase 4B) in adipose tissues plays a critical role in energy balance, oxidation, lipolysis, and thermogenesis. Loss of KDM4B in mice resulted in obesity associated with reduced energy expenditure and impaired adaptive thermogenesis. Obesity in KDM4B-deficient mice was accompanied by hyperlipidemia, insulin resistance, and pathological changes in the liver and pancreas. Adipocyte-specific deletion of Kdm4b revealed that the adipose tissues were the main sites for KDM4B antiobesity effects. KDM4B directly controlled the expression of multiple metabolic genes, including Ppargc1a and Ppara Collectively, our studies identify KDM4B as an essential epigenetic factor for the regulation of metabolic health and maintaining normal body weight in mice. KDM4B may provide a therapeutic target for treatment of obesity.

Brown and beige fat: From molecules to physiology and pathophysiology

The adipose organ portrays adipocytes of diverse tones: white, brown and beige, each type with distinct functions. Adipocytes orchestrate their adaptation and expansion to provide storage to excess nutrients, the quick mobilisation of fuel to supply peripheral functional demands, insulation, and, in their thermogenic form, heat generation to maintain core body temperature. Thermogenic adipocytes could be targets for anti-obesity and anti-diabetic therapeutic approaches aiming to restore adipose tissue functionality and increase energy dissipation. However, for thermogenic adipose tissue to become therapeutically relevant, a better understanding of its development and origins, its progenitors and their characteristics and the composition of its niche, is essential. Also crucial is the identification of stimuli and molecules promoting its specific differentiation and activation. Here we highlight the structural/cellular differences between human and rodent brown adipose tissue and discuss how obesity and metabolic complication affects brown and beige cells as well as how they could be targeted to improve their activation and improve global metabolic homeostasis. Finally, we describe the limitations of current research models and the advantages of new emerging approaches.

Resveratrol inhibits angiotensin II‑induced proliferation of A7r5 cells and decreases neointimal hyperplasia by inhibiting the CaMKII‑HDAC4 signaling pathway

Resveratrol has been reported to inhibit vascular smooth muscle cell proliferation and neointimal hyperplasia following arterial injury; however, the underlying mechanisms remain unclear. The present study was designed to investigate the effects of resveratrol on angiotensin II (AngII)‑induced proliferation of A7r5 cells and explore the molecular mechanisms responsible for the observed effects. Resveratrol inhibited cell proliferation and migration, and decreased the AngII‑induced protein expression of α‑smooth muscle actin (α‑SMA), proliferating cell nuclear antigen (PCNA) and cyclin‑dependent kinase 4 (CDK4). Resveratrol inhibited AngII‑induced activation of intracellular Ca2+/calmodulin‑dependent protein kinase II (CaMKII) and histone deacetylases 4 (HDAC4), as well as blocking AngII‑induced cell cycle progression from the G0/G1 to S‑phase. In vivo, 4‑weeks of resveratrol treatment decreased the neointima area and the neointima/media area ratio in rats following carotid balloon injury. Resveratrol also inhibited the protein expression of total and phosphorylated CaMKII and HDAC4 in the injured arteries. In conclusion, the present study demonstrated that resveratrol attenuated AngII‑induced cell proliferation and neointimal hyperplasia by inhibiting the CaMKII‑HDAC4 signaling pathway. These findings suggest that resveratrol may potentially prevent arterial restenosis.

Plasma Neuregulin 4 Levels Are Associated with Metabolic Syndrome in Patients Newly Diagnosed with Type 2 Diabetes Mellitus

Neuregulin 4 (Nrg4) has been proposed to play a role in the pathogeneses of obesity, insulin resistance, and dyslipidemia. However, information about the link between Nrg4 and metabolic syndrome (MetS) is scarce, especially in patients with newly diagnosed type 2 diabetes mellitus (nT2DM). This study aimed at investigating whether Nrg4 is associated with MetS in nT2DM patients. A total of 311 patients with nT2DM were recruited. Plasma Nrg4 concentration was determined by ELISA. Plasma Nrg4 concentration was lower in nT2DM patients with MetS than in nT2DM patients without MetS (P = 0.001). Nrg4 concentration showed negative correlations with most of the analyzed indicators of MetS. MetS was less prevalent among subjects in the highest quartile of plasma Nrg4 concentration than among those in the lowest quartile (P < 0.01). Age- and sex-adjusted plasma Nrg4 concentrations were positively correlated with concentrations of high-density lipoprotein cholesterol (HDL-C) and apolipoprotein A (both P < 0.05) and negatively correlated with triglyceride, high-sensitivity C-reactive protein (hs-CRP), and gamma-glutamyltransferase concentrations, neutrophil count, and white blood cell (WBC) count (all P < 0.05). In multivariate analysis, Nrg4 was independently associated with hs-CRP level, WBC count, and HDL-C level (P = 0.001 or P < 0.05). Multiple logistic regression analysis of MetS prediction by Nrg4 revealed an odds ratio of 0.560 (95% CI: 0.374-0.837; P < 0.01). Decreased plasma Nrg4 levels, which may be associated with augmented oxidative stress, inflammation, and dyslipidemia, might be involved in the development of MetS in nT2DM patients.

NRG1-Fc improves metabolic health via dual hepatic and central action

Neuregulins (NRGs) are emerging as an important family of signaling ligands that regulate glucose and lipid homeostasis. NRG1 lowers blood glucose levels in obese mice, whereas the brown fat-enriched secreted factor NRG4 protects mice from high-fat diet-induced insulin resistance and hepatic steatosis. However, the therapeutic potential of NRGs remains elusive, given the poor plasma half-life of the native ligands. Here, we engineered a fusion protein using human NRG1 and the Fc domain of human IgG1 (NRG1-Fc) that exhibited extended half-life in circulation and improved potency in receptor signaling. We evaluated its efficacy in improving metabolic parameters and dissected the mechanisms of action. NRG1-Fc treatment triggered potent AKT activation in the liver, lowered blood glucose, improved insulin sensitivity, and suppressed food intake in obese mice. NRG1-Fc acted as a potent secretagogue for the metabolic hormone FGF21; however, the latter was largely dispensable for its metabolic effects. NRG1-Fc directly targeted the hypothalamic POMC neurons to promote membrane depolarization and increase firing rate. Together, NRG1-Fc exhibits improved pharmacokinetic properties and exerts metabolic benefits through dual inhibition of hepatic gluconeogenesis and caloric intake.

Nonalcoholic Fatty Liver Disease Progresses into Severe NASH when Physiological Mechanisms of Tissue Homeostasis Collapse

Phenotypic modulation of NAFLD-severity by molecules derived from white (adipokines) and brown (batokines) adipose tissue may be important in inducing or protecting against the progression of the disease. Adipose tissue-derived factors can promote the progression of NAFLD towards severe histological stages (NASH-fibrosis and NASHcirrhosis). This effect can be modulated by the release of adipokines or batokines that directly trigger an inflammatory response in the liver tissue or indirectly modulate related phenotypes, such as insulin resistance. Metabolically dysfunctional adipose tissue, which is often infiltrated by macrophages and crown-like histological structures, may also show impaired production of anti-inflammatory cytokines, which may favor NAFLD progression into aggressive phenotypes by preventing its protective effects on the liver tissue.

The Common and Distinct Features of Brown and Beige Adipocytes

Two types of thermogenic fat cells, brown adipocytes and beige adipocytes, play a key role in the regulation of systemic energy homeostasis in mammals. Both brown fat and beige fat possess thermogenic properties in addition to common morphological and biochemical characteristics, including multilocular lipid droplets and cristae-dense mitochondria. Recent studies also identify features that are distinct between the two types of thermogenic fat cells, such as their developmental regulation and function. Of particular interest is the role of beige fat in the regulation of glucose homeostasis via uncoupling protein 1 (UCP1)-independent mechanisms. A better understanding of the underlying causes of these characteristics of brown and beige fat will allow us to specifically manipulate these cells to improve systemic energy metabolism and glucose homeostasis.

Brown and beige adipose tissues: phenotype and metabolic potential in mice and men

With the recent rediscovery of brown fat in adult humans, our outlook on adipose tissue biology has undergone a paradigm shift. While we attempt to identify, recruit, and activate classic brown fat stores in humans, identification of beige fat has also raised the possibility of browning our white fat stores. Whether such transformation of human white fat depots can be achieved to enhance the whole body oxidative potential remains to be seen. Evidence to date, however, largely points toward a major oxidative role only for classic brown fat depots, at least in rodents. White fat stores seem to provide the main fuel for sustaining thermogenesis via lipolysis. Interestingly, molecular markers consistent with both classic brown and beige fat identity can be observed in human supraclavicular depot, thereby complicating the discussion on beige fat in humans. Here, we review the recent advances made in our understanding of brown and beige fat in humans and mice. We further provide an overview of their plausible physiological relevance to whole body energy metabolism.

An essential role for neuregulin-4 in the growth and elaboration of developing neocortical pyramidal dendrites

Neuregulins, with the exception of neuregulin-4 (NRG4), have been shown to be extensively involved in many aspects of neural development and function and are implicated in several neurological disorders, including schizophrenia, depression and bipolar disorder. Here we provide the first evidence that NRG4 has a crucial function in the developing brain. We show that both the apical and basal dendrites of neocortical pyramidal neurons are markedly stunted in Nrg4^-/- neonates in vivo compared with Nrg4^+/+ littermates. Neocortical pyramidal neurons cultured from Nrg4^-/- embryos had significantly shorter and less branched neurites than those cultured from Nrg4^+/+ littermates. Recombinant NRG4 rescued the stunted phenotype of embryonic neocortical pyramidal neurons cultured from Nrg4^-/- mice. The majority of cultured wild type embryonic cortical pyramidal neurons co-expressed NRG4 and its receptor ErbB4. The difference between neocortical pyramidal dendrites of Nrg4^-/- and Nrg4^+/+ mice was less pronounced, though still significant, in juvenile mice. However, by adult stages, the pyramidal dendrite arbors of Nrg4^-/- and Nrg4^+/+ mice were similar, suggesting that compensatory changes in Nrg4^-/- mice occur with age. Our findings show that NRG4 is a major novel regulator of dendritic arborisation in the developing cerebral cortex and suggest that it exerts its effects by an autocrine/paracrine mechanism.

Uncoupling Exercise Bioenergetics From Systemic Metabolic Homeostasis by Conditional Inactivation of Baf60 in Skeletal Muscle

Impaired skeletal muscle energy metabolism is linked to the pathogenesis of insulin resistance and glucose intolerance in type 2 diabetes. The contractile and metabolic properties of myofibers exhibit a high degree of heterogeneity and plasticity. The regulatory circuitry underpinning skeletal muscle energy metabolism is critically linked to exercise endurance and systemic homeostasis. Recent work has identified the Baf60 subunits of the SWI/SNF chromatin-remodeling complex as powerful regulators of the metabolic gene programs. However, their role in integrating myofiber energy metabolism with exercise endurance and metabolic physiology remains largely unknown. In this study, we conditionally inactivated Baf60a, Baf60c, or both in mature skeletal myocytes to delineate their contribution to muscle bioenergetics and metabolic physiology. Our work revealed functional redundancy between Baf60a and Baf60c in maintaining oxidative and glycolytic metabolism in skeletal myofibers and exercise endurance. Unexpectedly, mice lacking these two factors in skeletal muscle were protected from diet-induced and age-associated metabolic disorders. Transcriptional profiling analysis identified the muscle thermogenic gene program and myokine secretion as key pathways that integrate myofiber metabolism with systemic energy balance. As such, Baf60 deficiency in skeletal muscle illustrates a surprising disconnect between exercise endurance and systemic metabolic homeostasis.

Microfluidic systems for studying dynamic function of adipocytes and adipose tissue

Recent breakthroughs in organ-on-a-chip and related technologies have highlighted the extraordinary potential for microfluidics to not only make lasting impacts in the understanding of biological systems but also to create new and important in vitro culture platforms. Adipose tissue (fat), in particular, is one that should be amenable to microfluidic mimics of its microenvironment. While the tissue was traditionally considered important only for energy storage, it is now understood to be an integral part of the endocrine system that secretes hormones and responds to various stimuli. As such, adipocyte function is central to the understanding of pathological conditions such as obesity, diabetes, and metabolic syndrome. Despite the importance of the tissue, only recently have significant strides been made in studying dynamic function of adipocytes or adipose tissues on microfluidic devices. In this critical review, we highlight new developments in the special class of microfluidic systems aimed at culture and interrogation of adipose tissue, a sub-field of microfluidics that we contend is only in its infancy. We close by reflecting on these studies as we forecast a promising future, where microfluidic technologies should be capable of mimicking the adipose tissue microenvironment and provide novel insights into its physiological roles in the normal and diseased states. Graphical abstract This critical review focuses on recent developments and challenges in applying microfluidic systems to the culture and analysis of adipocytes and adipose tissue.

The Lives and Times of Brown Adipokines

Brown adipose tissue (BAT) is responsible for adaptive non-shivering thermogenesis. Moreover, brown fat secretes regulatory factors, so-called brown adipokines, that have autocrine, paracrine, and endocrine actions. Brown adipokines are either polypeptides or nonpeptidic molecules including lipid molecules and microRNAs. The secretory properties of brown fat are essential for tissue remodeling adaptations to thermogenic necessities. The endocrine properties of brown adipokines are thought to contribute to the association between BAT activity and a healthy metabolic profile in relation to glucose and lipid homeostasis. The identification and characterization of brown adipokines may allow the discovery of circulating biomarkers of BAT activity in humans, and will lead to the development of candidate tools for therapeutic interventions in metabolic diseases.

Decreased plasma neuregulin 4 concentration is associated with increased high-sensitivity C-reactive protein in newly diagnosed type 2 diabetes mellitus patients: a cross-sectional study

AIMS

Inflammation has been reported to be involved in the pathogenesis of atherosclerosis. This principal objective of this study was to investigate if the secretion of neuregulin 4 (Nrg4), a soluble protein associated with metabolic syndrome and subclinical cardiovascular disease, is correlated with the inflammation marker high-sensitivity C-reactive protein (hs-CRP) in patients with newly diagnosed type 2 diabetes mellitus (nT2DM).

METHODS

A study group of 311 nT2DM patients was divided into three subgroups based on hs-CRP tertiles. Multiple linear regression was conducted to explore the association between plasma Nrg4 and hs-CRP levels.

RESULTS

The nT2DM patients with the highest hs-CRP levels (>2.46 mg/L) exhibited higher atherogenic coefficients and atherogenic index of plasma (AIP) levels, but lower levels of plasma Nrg4, as compared to those with the lowest hs-CRP levels (<0.63 mg/L). Plasma Nrg4 levels were inversely associated with white blood cell count, hs-CRP, and AIP and positively associated with high-density lipoprotein cholesterol (HDL-C), before and after adjustment for age, gender, body mass index (BMI), and body fat percentage (P < 0.01 or P < 0.05). hs-CRP was the factor most strongly associated with plasma Nrg4 levels.

CONCLUSIONS

These results indicate that lower plasma Nrg4 levels may be associated with elevated hs-CRP in nT2DM patients. It generates the hypothesis that decreased levels of Nrg4 may trigger the development of atherosclerosis through its proinflammatory effects. These findings need to be confirmed by further prospective studies.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

Role of brown adipose tissue in metabolic syndrome, aging, and cancer cachexia

Brown adipose tissue (BAT) plays a fundamental role in maintaining body temperature by producing heat. BAT that had been know to exist only in mammals and the human neonate has received great attention for the treatment of obesity and diabetes due to its important function in energy metabolism, ever since it is recently reported that human adults have functional BAT. In addition, beige adipocytes, brown adipocytes in white adipose tissue (WAT), have also been shown to take part in whole body metabolism. Multiple lines of evidence demonstrated that transplantation or activation of BAT or/and beige adipocytes reversed obesity and improved insulin sensitivity. Furthermore, many genes involved in BATactivation and/or the recruitment of beige cells have been found, thereby providing new promising strategies for future clinical application of BAT activation to treat obesity and metabolic diseases. This review focuses on recent advances of BAT function in the metabolic aspect and the relationship between BAT and cancer cachexia, a pathological process accompanied with decreased body weight and increased energy expenditure in cancer patients. The underlying possible mechanisms to reduce BAT mass and its activity in the elderly are also discussed.

Hepatic neuregulin 4 signaling defines an endocrine checkpoint for steatosis-to-NASH progression

Nonalcoholic steatohepatitis (NASH) is characterized by progressive liver injury, inflammation, and fibrosis; however, the mechanisms that govern the transition from hepatic steatosis, which is relatively benign, to NASH remain poorly defined. Neuregulin 4 (Nrg4) is an adipose tissue-enriched endocrine factor that elicits beneficial metabolic effects in obesity. Here, we show that Nrg4 is a key component of an endocrine checkpoint that preserves hepatocyte health and counters diet-induced NASH in mice. Nrg4 deficiency accelerated liver injury, fibrosis, inflammation, and cell death in a mouse model of NASH. In contrast, transgenic expression of Nrg4 in adipose tissue alleviated diet-induced NASH. Nrg4 attenuated hepatocyte death in a cell-autonomous manner by blocking ubiquitination and proteasomal degradation of c-FLIPL, a negative regulator of cell death. Adeno-associated virus-mediated (AAV-mediated) rescue of hepatic c-FLIPL expression in Nrg4-deficent mice functionally restored the brake for steatosis to NASH transition. Thus, hepatic Nrg4 signaling serves as an endocrine checkpoint for steatosis-to-NASH progression by activating a cytoprotective pathway to counter stress-induced liver injury.

Intermittent fasting promotes adipose thermogenesis and metabolic homeostasis via VEGF-mediated alternative activation of macrophage

Intermittent fasting (IF), a periodic energy restriction, has been shown to provide health benefits equivalent to prolonged fasting or caloric restriction. However, our understanding of the underlying mechanisms of IF-mediated metabolic benefits is limited. Here we show that isocaloric IF improves metabolic homeostasis against diet-induced obesity and metabolic dysfunction primarily through adipose thermogenesis in mice. IF-induced metabolic benefits require fasting-mediated increases of vascular endothelial growth factor (VEGF) expression in white adipose tissue (WAT). Furthermore, periodic adipose-VEGF overexpression could recapitulate the metabolic improvement of IF in non-fasted animals. Importantly, fasting and adipose-VEGF induce alternative activation of adipose macrophage, which is critical for thermogenesis. Human adipose gene analysis further revealed a positive correlation of adipose VEGF-M2 macrophage-WAT browning axis. The present study uncovers the molecular mechanism of IF-mediated metabolic benefit and suggests that isocaloric IF can be a preventive and therapeutic approach against obesity and metabolic disorders.

Peripheral Neuropathy and Hindlimb Paralysis in a Mouse Model of Adipocyte-Specific Knockout of Lkb1

Brown adipose tissues (BAT) burn lipids to generate heat through uncoupled respiration, thus representing a powerful target to counteract lipid accumulation and obesity. The tumor suppressor liver kinase b1 (Lkb1) is a key regulator of cellular energy metabolism; and adipocyte-specific knockout of Lkb1 (Ad-Lkb1 KO) leads to the expansion of BAT, improvements in systemic metabolism and resistance to obesity in young mice. Here we report the unexpected finding that the Ad-Lkb1 KO mice develop hindlimb paralysis at mid-age. Gene expression analyses indicate that Lkb1 KO upregulates the expression of inflammatory cytokines in interscapular BAT and epineurial brown adipocytes surrounding the sciatic nerve. This is followed by peripheral neuropathy characterized by infiltration of macrophages into the sciatic nerve, axon degeneration, reduced nerve conductance, and hindlimb paralysis. Mechanistically, Lkb1 KO reduces AMPK phosphorylation and amplifies mammalian target-of-rapamycin (mTOR)-dependent inflammatory signaling specifically in BAT but not WAT. Importantly, pharmacological or genetic inhibition of mTOR ameliorates inflammation and prevents paralysis. These results demonstrate that BAT inflammation is linked to peripheral neuropathy.

BACE1-Dependent Neuregulin-1 Signaling: An Implication for Schizophrenia

Schizophrenia is a chronic psychiatric disorder with a lifetime prevalence of about 1% in the general population. Recent studies have shown that Neuregulin-1 (Nrg1) is a candidate gene for schizophrenia. At least 15 alternative splicing of NRG1 isoforms all contain an extracellular epidermal growth factor (EGF)-like domain, which is sufficient for Nrg1 biological activity including the formation of myelin sheaths and the regulation of synaptic plasticity. It is known that Nrg1 can be cleaved by β-secretase (BACE1) and the resulting N-terminal fragment (Nrg1-ntf) binds to receptor tyrosine kinase ErbB4, which activates Nrg1/ErbB4 signaling. While changes in Nrg1 expression levels in schizophrenia still remain controversial, understanding the BACE1-cleaved Nrg1-ntf and Nrg1/ErbB4 signaling in schizophrenia neuropathogenesis is essential and important. In this review paper, we included three major parts: (1) Nrg1 structure and cleavage pattern by BACE1; (2) BACE1-dependent Nrg1 cleavage associated with schizophrenia in human studies; and (3) Animal studies of Nrg1 and BACE1 mutations with behavioral observations. Our review will provide a better understanding of Nrg1 in schizophrenia and a potential strategy for using BACE1 cleavage of Nrg1 as a unique biomarker for diagnosis, as well as a new therapeutic target, of schizophrenia.

Brown and Beige Adipose Tissues in Health and Disease

Brown and beige adipocytes arise from distinct developmental origins. Brown adipose tissue (BAT) develops embryonically from precursors that also give to skeletal muscle. Beige fat develops postnatally and is highly inducible. Beige fat recruitment is mediated by multiple mechanisms, including de novo beige adipogenesis and white-to-brown adipocyte transdifferentiaiton. Beige precursors reside around vasculatures, and proliferate and differentiate into beige adipocytes. PDGFRα+Ebf2+ precursors are restricted to beige lineage cells, while another PDGFRα+ subset gives rise to beige adipocytes, white adipocytes, or fibrogenic cells. White adipocytes can be reprogramed and transdifferentiated into beige adipocytes. Brown and beige adipocytes display many similar properties, including multilocular lipid droplets, dense mitochondria, and expression of UCP1. UCP1-mediated thermogenesis is a hallmark of brown/beige adipocytes, albeit UCP1-independent thermogenesis also occurs. Development, maintenance, and activation of BAT/beige fat are guided by genetic and epigenetic programs. Numerous transcriptional factors and coactivators act coordinately to promote BAT/beige fat thermogenesis. Epigenetic reprograming influences expression of brown/beige adipocyte-selective genes. BAT/beige fat is regulated by neuronal, hormonal, and immune mechanisms. Hypothalamic thermal circuits define the temperature setpoint that guides BAT/beige fat activity. Metabolic hormones, paracrine/autocrine factors, and various immune cells also play a critical role in regulating BAT/beige fat functions. BAT and beige fat defend temperature homeostasis, and regulate body weight and glucose and lipid metabolism. Obesity is associated with brown/beige fat deficiency, and reactivation of brown/beige fat provides metabolic health benefits in some patients. Pharmacological activation of BAT/beige fat may hold promise for combating metabolic diseases. © 2017 American Physiological Society. Compr Physiol 7:1281-1306, 2017.

Glucose Sensing by Skeletal Myocytes Couples Nutrient Signaling to Systemic Homeostasis

Skeletal muscle is a major site of postprandial glucose disposal. Inadequate insulin action in skeletal myocytes contributes to hyperglycemia in diabetes. Although glucose is known to stimulate insulin secretion by β cells, whether it directly engages nutrient signaling pathways in skeletal muscle to maintain systemic glucose homeostasis remains largely unexplored. Here we identified the Baf60c-Deptor-AKT pathway as a target of muscle glucose sensing that augments insulin action in skeletal myocytes. Genetic activation of this pathway improved postprandial glucose disposal in mice, whereas its muscle-specific ablation impaired insulin action and led to postprandial glucose intolerance. Mechanistically, glucose triggers K_ATP channel-dependent calcium signaling, which promotes HDAC5 phosphorylation and nuclear exclusion, leading to Baf60c induction and insulin-independent AKT activation. This pathway is engaged by the anti-diabetic sulfonylurea drugs to exert their full glucose-lowering effects. These findings uncover an unexpected mechanism of glucose sensing in skeletal myocytes that contributes to homeostasis and therapeutic action.

Endocrine and autocrine/paracrine modulators of brown adipose tissue mass and activity as novel therapeutic strategies against obesity and type 2 diabetes

The dramatically increasing world-wide prevalence of obesity is recognized as a risk factor for the development of various diseases. The growing research on the role of adipose tissue in controlling energy homeostasis and insulin sensitivity has revealed that the promotion of brown adipose tissue (BAT) activity and the browning of white adipose tissue (WAT) leads to multiple health benefits and prevents obesity and type 2 diabetes (T2D). Inducible thermogenic adipocytes do exist in adult humans and are linked with increased energy combustion and lower body fat mass. Thus brown adipocytes are currently placed at the center of attention for novel therapeutic strategies against metabolic diseases such as obesity and diabetes. Besides the classical, norepinephrine-mediated sympathetic recruitment and activation of thermogenic adipocytes, a number of novel circulating factors have been recently identified to have a positive or negative impact on thermogenic adipocyte formation and activity. In this review their mechanism of action and the plausible therapeutic applications will be summarized and discussed.

Zbtb7b engages the long noncoding RNA Blnc1 to drive brown and beige fat development and thermogenesis

Brown and beige adipocytes convert chemical energy into heat through uncoupled respiration to defend against cold stress. Beyond thermogenesis, brown and beige fats engage other metabolic tissues via secreted factors to influence systemic energy metabolism. How the protein and long noncoding RNA (lncRNA) regulatory networks act in concert to regulate key aspects of thermogenic adipocyte biology remains largely unknown. Here we developed a genome-wide functional screen to interrogate the transcription factors and cofactors in thermogenic gene activation and identified zinc finger and BTB domain-containing 7b (Zbtb7b) as a potent driver of brown fat development and thermogenesis and cold-induced beige fat formation. Zbtb7b is required for activation of the thermogenic gene program in brown and beige adipocytes. Genetic ablation of Zbtb7b impaired cold-induced transcriptional remodeling in brown fat, rendering mice sensitive to cold temperature, and diminished browning of inguinal white fat. Proteomic analysis revealed a mechanistic link between Zbtb7b and the lncRNA regulatory pathway through which Zbtb7b recruits the brown fat lncRNA 1 (Blnc1)/heterogeneous nuclear ribonucleoprotein U (hnRNPU) ribonucleoprotein complex to activate thermogenic gene expression in adipocytes. These findings illustrate the emerging concept of a protein-lncRNA regulatory network in the control of adipose tissue biology and energy metabolism.

Growth Hormone's Effect on Adipose Tissue: Quality versus Quantity

Obesity is an excessive accumulation or expansion of adipose tissue (AT) due to an increase in either the size and/or number of its characteristic cell type, the adipocyte. As one of the most significant public health problems of our time, obesity and its associated metabolic complications have demanded that attention be given to finding effective therapeutic options aimed at reducing adiposity or the metabolic dysfunction associated with its accumulation. Growth hormone (GH) has therapeutic potential due to its potent lipolytic effect and resultant ability to reduce AT mass while preserving lean body mass. However, AT and its resident adipocytes are significantly more dynamic and elaborate than once thought and require one not to use the reduction in absolute mass as a readout of efficacy alone. Paradoxically, therapies that reduce GH action may ultimately prove to be healthier, in part because GH also possesses potent anti-insulin activities along with concerns that GH may promote the growth of certain cancers. This review will briefly summarize some of the newer complexities of AT relevant to GH action and describe the current understanding of how GH influences this tissue using data from both humans and mice. We will conclude by considering the therapeutic use of GH or GH antagonists in obesity, as well as important gaps in knowledge regarding GH and AT.

Enhancing natriuretic peptide signaling in adipose tissue, but not in muscle, protects against diet-induced obesity and insulin resistance

In addition to controlling blood pressure, cardiac natriuretic peptides (NPs) can stimulate lipolysis in adipocytes and promote the "browning" of white adipose tissue. NPs may also increase the oxidative capacity of skeletal muscle. To unravel the contribution of NP-stimulated metabolism in adipose tissue compared to that in muscle in vivo, we generated mice with tissue-specific deletion of the NP clearance receptor, NPRC, in adipose tissue (Nprc^AKO ) or in skeletal muscle (Nprc^MKO ). We showed that, similar to Nprc null mice, Nprc^AKO mice, but not Nprc^MKO mice, were resistant to obesity induced by a high-fat diet. Nprc^AKO mice exhibited increased energy expenditure, improved insulin sensitivity, and increased glucose uptake into brown fat. These mice were also protected from diet-induced hepatic steatosis and visceral fat inflammation. These findings support the conclusion that NPRC in adipose tissue is a critical regulator of energy metabolism and suggest that inhibiting this receptor may be an important avenue to explore for combating metabolic disease.

Short-chain fatty acids and inulin, but not guar gum, prevent diet-induced obesity and insulin resistance through differential mechanisms in mice

The role of dietary fibre and short-chain fatty acids (SCFA) in obesity development is controversially discussed. Here, we investigated how various types of dietary fibre and different SCFA ratios affect metabolic syndrome-related disorders. Male mice (B6) were fed high-fat diets supplemented with dietary fibres (either cellulose, inulin or guar gum) or different Ac:Pr ratios (high acetate (HAc) or propionate (HPr)) for 30 weeks. Body-fat gain and insulin resistance were greatly reduced by inulin, but not by guar gum, and completely prevented by SCFA supplementation. Only inulin and HAc increased body temperature, possibly by the induction of beige/browning markers in WAT. In addition, inulin and SCFA lowered hepatic triglycerides and improved insulin sensitivity. Both, inulin and HAc reduced hepatic fatty acid uptake, while only inulin enhanced mitochondrial capacity and only HAc suppressed lipogenesis in liver. Interestingly, HPr was accompanied by the induction of Nrg4 in BAT. Fermentable fibre supplementation increased the abundance of bifidobacteria; B. animalis was particularly stimulated by inulin and B. pseudolongum by guar gum. We conclude that in contrast to guar gum, inulin and SCFA prevent the onset of diet-induced weight gain and hepatic steatosis by different mechanisms on liver and adipose tissue metabolism.

Transcription factor Hlx controls a systematic switch from white to brown fat through Prdm16-mediated co-activation

Browning of subcutaneous white fat (iWAT) involves several reprograming events, but the underlying mechanisms are incompletely understood. Here we show that the transcription factor Hlx is selectively expressed in brown adipose tissue (BAT) and iWAT, and is translationally upregulated by β3-adrenergic signaling-mediated suppression of the translational inhibitor 4E-BP1. Hlx interacts with and is co-activated by Prdm16 to control BAT-selective gene expression and mitochondrial biogenesis. Hlx heterozygous knockout mice have defects in brown-like adipocyte formation in iWAT, and develop glucose intolerance and high fat-induced hepatic steatosis. Conversely, transgenic expression of Hlx at a physiological level drives a full program of thermogenesis and converts iWAT to brown-like fat, which improves glucose homeostasis and prevents obesity and hepatic steatosis. The adipose remodeling phenotypes are recapitulated by fat-specific injection of Hlx knockdown and overexpression viruses, respectively. Our studies establish Hlx as a powerful regulator for systematic white adipose tissue browning and offer molecular insights into the underlying transcriptional mechanism.

The transcriptional co-activator Prdm16 regulates browning of white adipose tissue (WAT). Here, the authors show that Prdm16 interacts with the transcription factor Hlx, which is stabilized in response to β3-adrenergic signaling, to increase thermogenic gene expression and mitochondrial biogenesis in subcutaneous WAT.

The brown-fat-secreted adipokine neuregulin 4 is decreased in gestational diabetes mellitus

AIMS

Neuregulin 4 has recently been recognized as a novel adipokine secreted by brown adipose tissue (BAT), with beneficial effects on murine insulin resistance and hepatic steatosis. Yet, thus far, neither regulation of neuregulin 4 in gestational diabetes mellitus (GDM) nor its longitudinal changes in the peripartum period have been elucidated.

METHODS

Circulating neuregulin 4 levels were measured by ELISA in 74 women with GDM and 74 healthy, gestational-age-matched controls. Also, neuregulin 4 was quantified during pregnancy and compared with postpartum levels in a follow-up study of 25 women with previous GDM and 25 healthy control women.

RESULTS

Women with GDM had lower median serum levels of the novel BAT-secreted adipokine neuregulin 4 (3.0μg/L) compared with healthy (non-GDM) pregnant controls (3.5μg/L; P=0.020), and the area under the glucose curve (AUC_Glucose) was an independent and negative predictor of circulating neuregulin 4 (P=0.033). Also, median postpartum serum concentrations of neuregulin 4 (3.2μg/L) were not significantly different from prepartum levels (2.8μg/L; P=0.328). In addition, neuregulin 4 was positively and independently associated with irisin (P=0.009), but not other BAT-secreted adipokines.

CONCLUSION/INTERPRETATION

Women with GDM have significantly lower circulating neuregulin 4 levels compared with healthy pregnant controls, and the AUC_Glucose is negatively and independently associated with neuregulin 4 during pregnancy. Neuregulin 4 is positively correlated with irisin during pregnancy, as well as in a longitudinal fashion. Future studies are now needed to better elucidate the precise pathomechanisms of the regulation of BAT-secreted adipokines during pregnancy.

Cold-induced conversion of cholesterol to bile acids in mice shapes the gut microbiome and promotes adaptive thermogenesis

Adaptive thermogenesis is an energy-demanding process that is mediated by cold-activated beige and brown adipocytes, and it entails increased uptake of carbohydrates, as well as lipoprotein-derived triglycerides and cholesterol, into these thermogenic cells. Here we report that cold exposure in mice triggers a metabolic program that orchestrates lipoprotein processing in brown adipose tissue (BAT) and hepatic conversion of cholesterol to bile acids via the alternative synthesis pathway. This process is dependent on hepatic induction of cytochrome P450, family 7, subfamily b, polypeptide 1 (CYP7B1) and results in increased plasma levels, as well as fecal excretion, of bile acids that is accompanied by distinct changes in gut microbiota and increased heat production. Genetic and pharmacological interventions that targeted the synthesis and biliary excretion of bile acids prevented the rise in fecal bile acid excretion, changed the bacterial composition of the gut and modulated thermogenic responses. These results identify bile acids as important metabolic effectors under conditions of sustained BAT activation and highlight the relevance of cholesterol metabolism by the host for diet-induced changes of the gut microbiota and energy metabolism.

Nrg4 promotes fuel oxidation and a healthy adipokine profile to ameliorate diet-induced metabolic disorders

Objective

Brown and white adipose tissue exerts pleiotropic effects on systemic energy metabolism in part by releasing endocrine factors. Neuregulin 4 (Nrg4) was recently identified as a brown fat-enriched secreted factor that ameliorates diet-induced metabolic disorders, including insulin resistance and hepatic steatosis. However, the physiological mechanisms through which Nrg4 regulates energy balance and glucose and lipid metabolism remain incompletely understood. The aims of the current study were: i) to investigate the regulation of adipose Nrg4 expression during obesity and the physiological signals involved, ii) to elucidate the mechanisms underlying Nrg4 regulation of energy balance and glucose and lipid metabolism, and iii) to explore whether Nrg4 regulates adipose tissue secretome gene expression and adipokine secretion.

Methods

We examined the correlation of adipose Nrg4 expression with obesity in a cohort of diet-induced obese mice and investigated the upstream signals that regulate Nrg4 expression. We performed metabolic cage and hyperinsulinemic-euglycemic clamp studies in Nrg4 transgenic mice to dissect the metabolic pathways regulated by Nrg4. We investigated how Nrg4 regulates hepatic lipid metabolism in the fasting state and explored the effects of Nrg4 on adipose tissue gene expression, particularly those encoding secreted factors.

Results

Adipose Nrg4 expression is inversely correlated with adiposity and regulated by pro-inflammatory and anti-inflammatory signaling. Transgenic expression of Nrg4 increases energy expenditure and augments whole body glucose metabolism. Nrg4 protects mice from diet-induced hepatic steatosis in part through activation of hepatic fatty acid oxidation and ketogenesis. Finally, Nrg4 promotes a healthy adipokine profile during obesity.

Conclusions

Nrg4 exerts pleiotropic beneficial effects on energy balance and glucose and lipid metabolism to ameliorate obesity-associated metabolic disorders. Biologic therapeutics based on Nrg4 may improve both type 2 diabetes and non-alcoholic fatty liver disease (NAFLD) in patients.

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Nrg4 is a target of pro-inflammatory and anti-inflammatory signaling in adipocytes.

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Transgenic expression of Nrg4 increased energy expenditure and glucose metabolism.

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Nrg4 stimulates hepatic fatty acid oxidation and ketogenesis during starvation.

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Nrg4 promotes a beneficial adipokine profile during obesity.

Impact of Growth Hormone on Regulation of Adipose Tissue

Increasing prevalence of obesity and obesity-related conditions worldwide has necessitated a more thorough understanding of adipose tissue (AT) and expanded the scope of research in this field. AT is now understood to be far more complex and dynamic than previously thought, which has also fueled research to reevaluate how hormones, such as growth hormone (GH), alter the tissue. In this review, we will introduce properties of AT important for understanding how GH alters the tissue, such as anatomical location of depots and adipokine output. We will provide an overview of GH structure and function and define several human conditions and cognate mouse lines with extremes in GH action that have helped shape our understanding of GH and AT. A detailed discussion of the GH/AT relationship will be included that addresses adipokine production, immune cell populations, lipid metabolism, senescence, differentiation, and fibrosis, as well as brown AT and beiging of white AT. A brief overview of how GH levels are altered in an obese state, and the efficacy of GH as a therapeutic option to manage obesity will be given. As we will reveal, the effects of GH on AT are numerous, dynamic and depot-dependent. © 2017 American Physiological Society. Compr Physiol 7:819-840, 2017.

Neuregulin 1 improves complex 2-mediated mitochondrial respiration in skeletal muscle of healthy and diabetic mice

It has been reported that neuregulin1 (NRG1) improves glucose tolerance in healthy and diabetic rodents. In vitro studies also suggest that NRG1 regulates myocyte oxidative capacity. To confirm this observation in vivo, we evaluated the effect on mitochondrial function of an 8-week treatment with NRG1 in db/db diabetic mice and C57BL/6JRJ healthy controls. NRG1 treatment improved complex 2-mediated mitochondrial respiration in the gastrocnemius of both control and diabetic mice and increased mitochondrial complex 2 subunit content by 2-fold. This effect was not associated with an increase in mitochondrial biogenesis markers. Enhanced ERBB4 phosphorylation could mediate NRG1 effects on mitochondrial function through signalling pathways, independently of ERK1/2, AKT or AMPK.

Elevated circulating Neuregulin4 level in patients with diabetes

BACKGROUND

Neuregulin4 (Nrg4) is a novel adipokine expressed in adipose tissues, enriched in brown adipose tissue, and able to improve whole-body metabolism in rodent, thus having the potential to treat obesity-associated disorders such as diabetes. However, the association between serum Nrg4 levels and diabetes risk in human remains unclear. This study was designed to examine circulating Nrg4 levels in subjects with different glucose tolerance status.

METHODS

Age-, sex-, and body mass index-matched subjects (n = 310: 83 normal glucose tolerance [NGT], 129 prediabetes, and 96 diabetes) from the Risk Evaluation of Cancers in Chinese Diabetic Individuals: A Longitudinal study (Reaction study) were included. Serum Nrg4 was measured via enzyme-linked immunosorbent assay. Basic anthropometric parameters, fasting plasma glucose and 2-hours postload plasma glucose, hemoglobin A_1c , and serum lipid profile were also measured.

RESULTS

The serum Nrg4 levels were higher in patients with diabetes than those with NGT and prediabetes (diabetes: 396.8[65.9, 709.4], NGT: 80.1[0, 554.1], prediabetes: 168.0[32.9, 463.9] pg/mL [median (interquartile range), both P < 0.05]). The Nrg4 concentration was correlated with fasting plasma glucose. When the top versus bottom quartiles of serum Nrg4 concentrations were compared with adjustment for age and sex, an odds ratio of 3.005 was observed in diabetes prevalence, which persisted after adjusting other potential confounding variables. Other nonglucose parameters as body mass index; waist, hip, and neck circumferences; alanine aminotransferase; triglyceride; high-density lipoprotein; uric acid; and estimated glomerular filtration rate were also correlated with serum Nrg4 (P < 0.05).

CONCLUSIONS

The circulating Nrg4 level is elevated in the prediabetic and diabetic patients compared to control and is an independent risk factor associated with diabetes.

Human knockouts and phenotypic analysis in a cohort with a high rate of consanguinity

A major goal of biomedicine is to understand the function of every gene in the human genome. Loss-of-function mutations can disrupt both copies of a given gene in humans and phenotypic analysis of such 'human knockouts' can provide insight into gene function. Consanguineous unions are more likely to result in offspring carrying homozygous loss-of-function mutations. In Pakistan, consanguinity rates are notably high. Here we sequence the protein-coding regions of 10,503 adult participants in the Pakistan Risk of Myocardial Infarction Study (PROMIS), designed to understand the determinants of cardiometabolic diseases in individuals from South Asia. We identified individuals carrying homozygous predicted loss-of-function (pLoF) mutations, and performed phenotypic analysis involving more than 200 biochemical and disease traits. We enumerated 49,138 rare (<1% minor allele frequency) pLoF mutations. These pLoF mutations are estimated to knock out 1,317 genes, each in at least one participant. Homozygosity for pLoF mutations at PLA2G7 was associated with absent enzymatic activity of soluble lipoprotein-associated phospholipase A2; at CYP2F1, with higher plasma interleukin-8 concentrations; at TREH, with lower concentrations of apoB-containing lipoprotein subfractions; at either A3GALT2 or NRG4, with markedly reduced plasma insulin C-peptide concentrations; and at SLC9A3R1, with mediators of calcium and phosphate signalling. Heterozygous deficiency of APOC3 has been shown to protect against coronary heart disease; we identified APOC3 homozygous pLoF carriers in our cohort. We recruited these human knockouts and challenged them with an oral fat load. Compared with family members lacking the mutation, individuals with APOC3 knocked out displayed marked blunting of the usual post-prandial rise in plasma triglycerides. Overall, these observations provide a roadmap for a 'human knockout project', a systematic effort to understand the phenotypic consequences of complete disruption of genes in humans.

The ErbB3 receptor tyrosine kinase negatively regulates Paneth cells by PI3K-dependent suppression of Atoh1

Paneth cells (PCs), a secretory population located at the base of the intestinal crypt, support the intestinal stem cells (ISC) with growth factors and participate in innate immunity by releasing antimicrobial peptides, including lysozyme and defensins. PC dysfunction is associated with disorders such as Crohn's disease and necrotizing enterocolitis, but the specific pathways regulating PC development and function are not fully understood. Here we tested the role of the neuregulin receptor ErbB3 in control of PC differentiation and the ISC niche. Intestinal epithelial ErbB3 knockout caused precocious appearance of PCs as early as postnatal day 7, and substantially increased the number of mature PCs in adult mouse ileum. ErbB3 loss had no effect on other secretory lineages, but increased expression of the ISC marker Lgr5. ErbB3-null intestines had elevated levels of the Atoh1 transcription factor, which is required for secretory fate determination, while Atoh1⁺ cells had reduced ErbB3, suggesting reciprocal negative regulation. ErbB3-null intestinal progenitor cells showed reduced activation of the PI3K-Akt and ERK MAPK pathways. Inhibiting these pathways in HT29 cells increased levels of ATOH1 and the PC marker LYZ. Conversely, ErbB3 activation suppressed LYZ and ATOH1 in a PI3K-dependent manner. Expansion of the PC compartment in ErbB3-null intestines was accompanied with elevated ER stress and inflammation markers, raising the possibility that negative regulation of PCs by ErbB3 is necessary to maintain homeostasis. Taken together, our data suggest that ErbB3 restricts PC numbers through PI3K-mediated suppression of Atoh1 levels leading to inhibition of PC differentiation, with important implications for regulation of the ISC niche.

ErbB4 signaling stimulates pro-inflammatory macrophage apoptosis and limits colonic inflammation

Efficient clearance of pro-inflammatory macrophages from tissues after resolution of a challenge is critical to prevent prolonged inflammation. Defects in clearance can contribute to conditions such as inflammatory bowel disease, and thus may be therapeutically targetable. However, the signaling pathways that induce termination of pro-inflammatory macrophages are incompletely defined. We tested whether the ErbB4 receptor tyrosine kinase, previously not known to have role in macrophage biology, is involved in this process. In vitro, pro-inflammatory activation of cultured murine and human macrophages induced ErbB4 expression; in contrast, other ErbB family members were not induced in pro-inflammatory cells, and other innate immune lineages (dendritic cells, neutrophils) did not express detectable ErbB4 levels. Treatment of activated pro-inflammatory macrophages with the ErbB4 ligand neuregulin-4 (NRG4) induced apoptosis. ErbB4 localized to the mitochondria in these cells. Apoptosis was accompanied by loss of mitochondrial membrane potential, and was dependent upon the proteases that generate the cleaved ErbB4 intracellular domain fragment, suggesting a requirement for this fragment and mitochondrial pathway apoptosis. In vivo, ErbB4 was highly expressed on pro-inflammatory macrophages but not neutrophils during experimental DSS colitis in C57Bl/6 mice. Active inflammation in this model suppressed NRG4 expression, which may allow for macrophage persistence and ongoing inflammation. Consistent with this notion, NRG4 levels rebounded during the recovery phase, and administration of exogenous NRG4 during colitis reduced colonic macrophage numbers and ameliorated inflammation. These data define a novel role for ErbB4 in macrophage apoptosis, and outline a mechanism of feedback inhibition that may promote resolution of colitis.

Type I neuregulin1α is a novel local mediator to suppress hepatic gluconeogenesis in mice

Neuregulin1 is an epidermal growth factor (EGF)-like domain-containing protein that has multiple isoforms and functions as a local mediator in the control of various cellular functions. Here we show that type I isoform of neuregulin1 with an α-type EGF-like domain (Nrg1α) is the major isoform in mouse liver and regulates hepatic glucose production. Forced expression of Nrg1α in mouse liver enhanced systemic glucose disposal and decreased hepatic glucose production with reduced fasting blood glucose levels. Nuclear forkhead box protein O1 (FoxO1) and its downstream targets, PEPCK and G6Pase, were suppressed in liver and isolated hepatocytes by Nrg1α overexpression. In contrast, silencing of Nrg1α enhanced glucose production with increased PEPCK and G6Pase expressions in cAMP/dexamethasone-stimulated hepatocytes. Mechanistically, the recombinant α-type EGF-like domain of NRG1α (rNRG1α) stimulated the ERBB3 signalling pathway in hepatocytes, resulting in decreased nuclear FoxO1 accumulation via activation of both the AKT and ERK pathways. In addition, acute treatment with rNRG1α also suppressed elevation of blood glucose levels after both glucose and pyruvate challenge. Although a liver-specific deletion of Nrg1 gene in mice showed little effect on systemic glucose metabolism, these results suggest that NRG1α have a novel regulatory function in hepatic gluconeogenesis by regulating the ERBB3-AKT/ERK-FoxO1 cascade.

Brown adipose tissue as a secretory organ

Brown adipose tissue (BAT) is the main site of adaptive thermogenesis and experimental studies have associated BAT activity with protection against obesity and metabolic diseases, such as type 2 diabetes mellitus and dyslipidaemia. Active BAT is present in adult humans and its activity is impaired in patients with obesity. The ability of BAT to protect against chronic metabolic disease has traditionally been attributed to its capacity to utilize glucose and lipids for thermogenesis. However, BAT might also have a secretory role, which could contribute to the systemic consequences of BAT activity. Several BAT-derived molecules that act in a paracrine or autocrine manner have been identified. Most of these factors promote hypertrophy and hyperplasia of BAT, vascularization, innervation and blood flow, processes that are all associated with BAT recruitment when thermogenic activity is enhanced. Additionally, BAT can release regulatory molecules that act on other tissues and organs. This secretory capacity of BAT is thought to be involved in the beneficial effects of BAT transplantation in rodents. Fibroblast growth factor 21, IL-6 and neuregulin 4 are among the first BAT-derived endocrine factors to be identified. In this Review, we discuss the current understanding of the regulatory molecules (the so-called brown adipokines or batokines) that are released by BAT that influence systemic metabolism and convey the beneficial metabolic effects of BAT activation. The identification of such adipokines might also direct drug discovery approaches for managing obesity and its associated chronic metabolic diseases.

Circulating neuregulin 4 levels are inversely associated with subclinical cardiovascular disease in obese adults

Neuregulin 4 (Nrg4) has been identified as a new secreted adipokine that may protect against development of obesity and metabolic disorders. However, information is not available regarding the association between circulating Nrg4 and subclinical atherosclerosis in humans. We measured serum Nrg4 in 485 obese adult subjects (aged 40 years or older) who had the measurement of carotid intima-media thickness (CIMT) recruited from the community. Individuals with increased CIMT and carotid plaque had lower levels of circulating Nrg4 than controls (p < 0.05). The risks of increased CIMT and atherosclerotic plaque were significantly decreased by 28% and 31% [OR (95% CI): 0.72 (0.53–0.98) and 0.69 (0.50–0.96), respectively], adjusting for age, sex, current smoking, alcohol consumption, physical activity, BMI, systolic BP, fasting glucose, total cholesterol, HDL-c, HOMA-IR, and body fat. Importantly, individuals in the lowest quartile of serum Nrg4 were 3.70 times (p < 0.001) more likely to have increased CIMT and 2.06 times (p < 0.05) more likely to have atherosclerotic plaque than those in the highest quartile in multivariable logistic regression analyses. These findings suggest that circulating Nrg4 concentrations are inversely associated with subclinical atherosclerosis in obese adults, and indicating that circulating Nrg4 might play a role in identifying patients at high risk for CVD.

Conserved function of the long noncoding RNA Blnc1 in brown adipocyte differentiation

OBJECTIVE

Long noncoding RNAs (lncRNAs) are emerging as important regulators of diverse biological processes. Recent work has demonstrated that the inducible lncRNA Blnc1 stimulates thermogenic gene expression during brown and beige adipocyte differentiation. However, whether Blnc1 is functionally conserved in humans has not been explored. In addition, the molecular basis of the Blnc1 ribonucleoprotein complex in thermogenic gene induction remains incompletely understood. The aims of the current study were to: i) investigate functional conservation of Blnc1 in mice and humans and ii) elucidate the molecular mechanisms by which Blnc1 controls the thermogenic gene program in brown adipocytes.

METHODS

Full-length human Blnc1 was cloned and examined for its ability to stimulate brown adipocyte differentiation. Different truncation mutants of Blnc1 were generated to identify functional RNA domains responsible for thermogenic gene induction. RNA-protein interaction studies were performed to delineate the molecular features of the Blnc1 ribonucleoprotein complex.

RESULTS

Blnc1 is highly conserved in mice and humans at the sequence and function levels, both capable of stimulating brown adipocyte gene expression. A conserved RNA domain was identified to be required and sufficient for the biological activity of Blnc1. We identified hnRNPU as an RNA-binding protein that facilitates the assembly and augments the transcriptional function of the Blnc1/EBF2 ribonucleoprotein complex.

CONCLUSIONS

Blnc1 is a conserved lncRNA that promotes thermogenic gene expression in brown adipocytes through formation of the Blnc1/hnRNPU/EBF2 ribonucleoprotein complex.

Control of brown and beige fat development

Brown and beige adipocytes expend chemical energy to produce heat and are therefore important in regulating body temperature and body weight. Brown adipocytes develop in discrete and relatively homogenous depots of brown adipose tissue, whereas beige adipocytes are induced to develop in white adipose tissue in response to certain stimuli - notably, exposure to cold. Fate-mapping analyses have identified progenitor populations that give rise to brown and beige fat cells, and have revealed unanticipated cell-lineage relationships between vascular smooth muscle cells and beige adipocytes, and between skeletal muscle cells and brown fat. In addition, non-adipocyte cells in adipose tissue, including neurons, blood vessel-associated cells and immune cells, have crucial roles in regulating the differentiation and function of brown and beige fat.

Association of circulating neuregulin 4 with metabolic syndrome in obese adults: a cross-sectional study

BACKGROUND

Neuregulin 4 (Nrg4) is a secreted adipokine recently identified as playing an important role in modulating systemic energy metabolism and the development of obesity-associated disorders. However, information is not available regarding the association between circulating Nrg4 and risk of metabolic syndrome (MetS) in humans.

METHODS

We measured serum Nrg4 in 1212 obese adult subjects (aged 40 years or older), with a waist circumference greater than 90 cm for men or 80 cm for women, recruited from the community.

RESULTS

MetS subjects had lower levels of circulating Nrg4 than healthy controls (P < 0.01). The prevalence of MetS was higher in subjects with lower levels of circulating Nrg4 compared to those with higher values (67.3 % vs. 57.4 %, P < 0.05). Likewise, subjects with low levels of circulating Nrg4 had high prevalence of raised fasting glucose and blood pressure, but there was no association with raised triglycerides and reduced HDL-c. In multivariable logistic regression analyses, increased serum Nrg4 was significantly associated with reduced risk of MetS (OR: 0.603; 95 % CI, 0.439-0.828; P = 0.002), adjusting for age, gender, current smoking, alcohol consumption, physical activity, BMI, systolic blood pressure, fasting glucose, triglyceride, HDL-c, HOMA-IR, and body fat mass; however, such associations with serum Nrg4 were not noted for each component of MetS.

CONCLUSIONS

These findings indicate that circulating Nrg4 concentrations are inversely associated with risk of MetS in obese Chinese adults, suggesting that circulating Nrg4 concentrations may be a protective factor in the development of MetS.

Unravelling the adiponectin paradox: novel roles of adiponectin in the regulation of cardiovascular disease

Adipose tissue (AT) has recently been identified as a dynamic endocrine organ secreting a wide range of adipokines. Adiponectin is one such hormone, exerting endocrine and paracrine effects on the cardiovascular system. At a cellular and molecular level, adiponectin has anti-inflammatory, antioxidant and anti-apoptotic roles, thereby mitigating key mechanisms underlying cardiovascular disease (CVD) pathogenesis. However, adiponectin expression in human AT as well as its circulating levels are increased in advanced CVD states, and it is actually considered by many as a 'rescue hormone'. Due to the complex mechanisms regulating adiponectin's biosynthesis in the human AT, measurement of its levels as a biomarker in CVD is highly controversial, given that adiponectin exerts protective effects on the cardiovascular system but at the same time its increased levels flag advanced CVD. In this review article, we present the involvement of adiponectin in CVD pathogenesis and we discuss its role as a clinical biomarker.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.