Alternatively activated macrophages do not synthesize catecholamines or contribute to adipose tissue adaptive thermogenesis

Inflammation in human adipose tissues-Shades of gray, rather than white and brown

Chronic inflammation in adipose tissues has been associated with obesity and metabolic syndrome over the years. Various studies using animal models have contributed to our knowledge on the pro- and anti- inflammatory mediators that regulate obesity. Analyses of cytokine profiles in humans have not revealed a clear scenario. Likewise, treatments targeting inflammation to control obesity and insulin resistance has not yielded promising results. In this review we summarize the data available in human obesity and discuss the possible reasons that could explain the difficulties in treating obesity and insulin resistance by targeting pro-inflammatory cytokines.

Partial depletion of CD206-positive M2-like macrophages induces proliferation of beige progenitors and enhances browning after cold stimulation

Beige adipocytes are an inducible form of thermogenic adipocytes that become interspersed within white adipose tissue (WAT) depots in response to cold exposure. Previous studies have shown that type 2 cytokines and M2 macrophages induce cold-induced browning in inguinal WAT (ingWAT) by producing catecholamines. Exactly how the conditional and partial depletion of CD206+ M2-like macrophages regulates the cold-induced browning of ingWAT, however, remains unknown. We examined the role of CD206+ M2-like macrophages in the cold-induced browning of WAT using genetically engineered CD206DTR mice, in which CD206+ M2-like macrophages were conditionally depleted. The partial depletion of CD206+ M2-like enhanced UCP1 expression in ingWAT, as shown by immunostaining, and also upregulated the expression of Ucp1 and other browning-related marker genes in ingWAT after cold exposure. A flow cytometry analysis showed that the partial depletion of CD206+ M2-like macrophages caused an increase in the number of beige progenitors in ingWAT in response to cold. Thus, we concluded that CD206+ M2-like macrophages inhibit the proliferation of beige progenitors and that the partial depletion of CD206+ M2-like macrophages releases this inhibition, thereby enhancing browning and insulin sensitivity.

Dietary n-3 long-chain polyunsaturated fatty acids upregulate energy dissipating metabolic pathways conveying anti-obesogenic effects in mice

Background

We previously reported on the anti-obesogenic and anti-inflammatory effects associated with n-3 long-chain polyunsaturated fatty acids (LCPUFA) in our diet-induced obesity (DIO) mouse model. Two isocaloric high-fat diets (HFDs; 48 kJ% fat), HFD (HF) and n-3 LCPUFA-enriched HFD (HF/n-3), and a control diet (C; 13 kJ% fat) were used. The underlying mechanisms however have largely remained unclear. Here, we assessed whether the reduced fat mass reflected n-3 LCPUFA-induced expression changes in lipid metabolism of the intestine, liver, and interscapular brown adipose tissue (iBAT), as well as increased iBAT thermogenic capacity.

Methods

For HF/n-3, saturated and monounsaturated fatty acids were partially substituted by n-3 LCPUFA eicosapentaenoic acid and docosahexaenoic acid to achieve a balanced n-6/n-3 PUFA ratio (0.84) compared to the unbalanced ratios of HF (13.5) and C (9.85). Intestine, liver and iBAT from male C57BL/6 J mice, fed defined soybean/palm oil-based diets for 12 weeks, were further analysed. Gene and protein expression analyses, immunohistochemistry and correlation analyses for metabolic interactions were performed.

Results

Compared to HF and C, our analyses suggest significantly diminished de novo lipogenesis (DNL) and/or increased hepatic and intestinal fatty acid oxidation (ω-oxidation and peroxisomal β-oxidation) in HF/n-3 mice. For iBAT, the thermogenic potential was enhanced upon HF/n-3 consistent with upregulated expression for uncoupling protein-1 and genes involved in mitochondrial biogenesis. In addition, a higher capacity for the supply and oxidation of fatty acids was observed and expression and correlation analyses indicated a coordinated regulation of energy metabolism and futile cycling of triacylglycerol (TAG). Moreover, HF/n-3 significantly increased the number of anti-inflammatory macrophages and eosinophils and significantly enhanced the levels of activated AMP-activated protein kinase α (AMPKα), peroxisome proliferator-activated receptor α (PPARα) and fibroblast growth factor 21 (FGF21).

Conclusions

Our data suggest that by targeting transcriptional regulatory pathways, AMPKα, and FGF21 as potential mediators, HF/n-3 activated less efficient pathways for energy production, such as peroxisomal β-oxidation, increased ATP consumption upon the induction of futile cycling of TAG, and additionally increased the thermogenic and oxidative potential of iBAT. Therefore, we consider n-3 LCPUFA as the potent inducer for upregulating energy dissipating metabolic pathways conveying anti-obesogenic effects in mice.

Electronic supplementary material

The online version of this article (10.1186/s12986-018-0291-x) contains supplementary material, which is available to authorized users.

Role of macrophages in depot-dependent browning of white adipose tissue

Sympathetic stimulation induces beige adipocytes in white adipose tissue (WAT), known as browning of WAT. In this study, exposure of mice to cold ambient temperature (10 °C) for 24 h induced the mRNA expression of uncoupling protein 1 (UCP1), a marker for beige adipocytes, in inguinal WAT, but not in perigonadal WAT. Thus, we examined the role of macrophages in depot-dependent WAT browning in mice. Flowcytometric analysis showed that total number of macrophages was higher in perigonadal WAT than in inguinal WAT. Cold exposure failed to change the expression of macrophage marker genes in inguinal WAT; however, it increased the mRNA expression of CD11c and tumor necrosis factor-α in perigonadal WAT, indicating that proinflammatory M1 macrophage is activated. The removal of macrophages using clodronate significantly enhanced cold-induced UCP1 mRNA expression in perigonadal WAT. These results suggest that M1 macrophages are involved in the phenotype of perigonadal WAT that hardly undergo browning.

Myeloid-specific Acat1 ablation attenuates inflammatory responses in macrophages, improves insulin sensitivity, and suppresses diet-induced obesity

Macrophages are phagocytes that play important roles in health and diseases. Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) converts cellular cholesterol to cholesteryl esters and is expressed in many cell types. Unlike global Acat1 knockout (KO), myeloid-specific Acat1 KO ( Acat1^-) does not cause overt abnormalities in mice. Here, we performed analyses in age- and sex-matched Acat1^-M/-M and wild-type mice on chow or Western diet and discovered that Acat1^-M/-M mice exhibit resistance to Western diet-induced obesity. On both chow and Western diets, Acat1^-M/-M mice display decreased adipocyte size and increased insulin sensitivity. When fed with Western diet, Acat1^-M/-M mice contain fewer infiltrating macrophages in white adipose tissue (WAT), with significantly diminished inflammatory phenotype. Without Acat1, the Ly6C^hi monocytes express reduced levels of integrin-β₁, which plays a key role in the interaction between monocytes and the inflamed endothelium. Adoptive transfer experiment showed that the appearance of leukocytes from Acat1^-M/-M mice to the inflamed WAT of wild-type mice is significantly diminished. Under Western diet, Acat1^-M/-M causes suppression of multiple proinflammatory genes in WAT. Cell culture experiments show that in RAW 264.7 macrophages, inhibiting ACAT1 with a small-molecule ACAT1-specific inhibitor reduces inflammatory responses to lipopolysaccharide. We conclude that under Western diet, blocking ACAT1 in macrophages attenuates inflammation in WAT. Other results show that Acat1^-M/-M does not compromise antiviral immune response. Our work reveals that blocking ACAT1 suppresses diet-induced obesity in part by slowing down monocyte infiltration to WAT as well as by reducing the inflammatory responses of adipose tissue macrophages.

Regulation of Energy Expenditure and Brown/Beige Thermogenic Activity by Interleukins: New Roles for Old Actors

Obesity rates and the burden of metabolic associated diseases are escalating worldwide Energy burning brown and inducible beige adipocytes in human adipose tissues (ATs) have attracted considerable attention due to their therapeutic potential to counteract the deleterious metabolic effects of nutritional overload and overweight. Recent research has highlighted the relevance of resident and recruited ATs immune cell populations and their signalling mediators, cytokines, as modulators of the thermogenic activity of brown and beige ATs. In this review, we first provide an overview of the developmental, cellular and functional heterogeneity of the AT organ, as well as reported molecular switches of its heat-producing machinery. We also discuss the key contribution of various interleukins signalling pathways to energy and metabolic homeostasis and their roles in the biogenesis and function of brown and beige adipocytes. Besides local actions, attention is also drawn to their influence in the central nervous system (CNS) networks governing energy expenditure.

Understanding the Biology of Self-Renewing Macrophages

Macrophages reside in specific territories in organs, where they contribute to the development, homeostasis, and repair of tissues. Recent work has shown that the size of tissue macrophage populations has an impact on tissue functions and is determined by the balance between replenishment and elimination. Macrophage replenishment is mainly due to self-renewal of macrophages, with a secondary contribution from blood monocytes. Self-renewal is a recently discovered trait of macrophages, which can have a major impact on their physiological functions and hence on the wellbeing of the organism. In this review, I discuss our current understanding of the developmental origin of self-renewing macrophages and the mechanisms used to maintain a physiologically stable macrophage pool.

Selegiline reduces adiposity induced by high-fat, high-sucrose diet in male rats

BACKGROUND AND PURPOSE

Incidence and severity of obesity are increasing worldwide, however, efficient and safe pharmacological treatments are not yet available. Certain MAO inhibitors reduce body weight, although their effects on metabolic parameters have not been investigated. Here, we have assessed effects of a widely used, selective MAO-B inhibitor, selegiline, on metabolic parameters in a rat model of diet-induced obesity.

EXPERIMENTAL APPROACH

Male Long-Evans rats were given control (CON) or a high-fat (20%), high-sucrose (15%) diet (HFS) for 25 weeks. From week 16, animals were injected s.c. with 0.25 mg·kg^-1 selegiline (CON + S and HFS + S) or vehicle (CON, HFS) once daily. Whole body, subcutaneous and visceral fat was measured by CT, and glucose and insulin tolerance were tested. Expression of glucose transporters and chemokines was assessed by quantitative RT-PCR.

KEY RESULTS

Selegiline decreased whole body fat, subcutaneous- and visceral adiposity, measured by CT and epididymal fat weight in the HFS group, compared with HFS placebo animals, without influencing body weight. Oral glucose tolerance and insulin tolerance tests showed impaired glucose homeostasis in HFS and HFS + S groups, although insulin levels in plasma and pancreas were unchanged. HFS induced expression of Srebp-1c, Glut1 and Ccl3 in adipose tissue, which were alleviated by selegiline.

CONCLUSIONS AND IMPLICATIONS

Selegiline reduced adiposity, changes in adipose tissue energy metabolism and adipose inflammation induced by HFS diet without affecting the increased body weight, impairment of glucose homeostasis, or behaviour. These results suggest that selegiline could mitigate harmful effects of visceral adiposity.

Cold-induced protein RBM3 orchestrates neurogenesis via modulating Yap mRNA stability in cold stress

RBM3 plays a protective role in embryonic neurogenesis. This study finds that maternal cold stress affects the embryonic brain development via RBM3 and Yap. When RBM3 is knocked down or knocked out under the maternal cold stress, the embryonic neurogenesis was impaired.

In mammals, a constant body temperature is an important basis for maintaining life activities. Here, we show that when pregnant mice are subjected to cold stress, the expression of RBM3, a cold-induced protein, is increased in the embryonic brain. When RBM3 is knocked down or knocked out in cold stress, embryonic brain development is more seriously affected, exhibiting abnormal neuronal differentiation. By detecting the change in mRNA expression during maternal cold stress, we demonstrate that Yap and its downstream molecules are altered at the RNA level. By analyzing RNA-binding motif of RBM3, we find that there are seven binding sites in 3′UTR region of Yap1 mRNA. Mechanistically, RBM3 binds to Yap1-3′UTR, regulates its stability, and affects the expression of YAP1. RBM3 and YAP1 overexpression can partially rescue the brain development defect caused by RBM3 knockout in cold stress. Collectively, our data demonstrate that cold temperature affects brain development, and RBM3 acts as a key protective regulator in cold stress.

Sympathetic inputs regulate adaptive thermogenesis in brown adipose tissue through cAMP-Salt inducible kinase axis

Various physiological stimuli, such as cold environment, diet, and hormones, trigger brown adipose tissue (BAT) to produce heat through sympathetic nervous system (SNS)- and β-adrenergic receptors (βARs). The βAR stimulation increases intracellular cAMP levels through heterotrimeric G proteins and adenylate cyclases, but the processes by which cAMP modulates brown adipocyte function are not fully understood. Here we described that specific ablation of cAMP production in brown adipocytes led to reduced lipolysis, mitochondrial biogenesis, uncoupling protein 1 (Ucp1) expression, and consequently defective adaptive thermogenesis. Elevated cAMP signaling by sympathetic activation inhibited Salt-inducible kinase 2 (Sik2) through protein kinase A (PKA)-mediated phosphorylation in brown adipose tissue. Inhibition of SIKs enhanced Ucp1 expression in differentiated brown adipocytes and Sik2 knockout mice exhibited enhanced adaptive thermogenesis at thermoneutrality in an Ucp1-dependent manner. Taken together, our data indicate that suppressing Sik2 by PKA-mediated phosphorylation is a requisite for SNS-induced Ucp1 expression and adaptive thermogenesis in BAT, and targeting Sik2 may present a novel therapeutic strategy to ramp up BAT thermogenic activity in humans.

Myeloid adrenergic signaling via CaMKII forms a feedforward loop of catecholamine biosynthesis

Type 2 immune response has been shown to facilitate cold-induced thermogenesis and browning of white fat. However, whether alternatively activated macrophages produce catecholamine and substantially promote adaptive thermogenesis in adipose tissue remains controversial. Here, we show that tyrosine hydroxylase (TyrH), a rate-limiting enzyme of catecholamine biosynthesis, was expressed and phosphorylated in adipose-resident macrophages. In addition, the plasma level of adrenaline was increased by cold stress in mice, and treatment of macrophages with adrenaline stimulated phosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and TyrH. Genetic and pharmacological inhibition of CaMKII or PKA signaling diminished adrenaline-induced phosphorylation of TyrH in primary macrophages. Consistently, overexpression of constitutively active CaMKII upregulated basal TyrH phosphorylation, while suppressing the stimulatory effect of adrenaline on TyrH in macrophages. Myeloid-specific disruption of CaMKIIγ suppressed both the cold-induced production of norepinephrine and adipose UCP1 expression in vivo and the stimulatory effect of adrenaline on macrophage-dependent activation of brown adipocytes in vitro. Lack of CaMKII signaling attenuated catecholamine production mediated by cytokines IL-4 and IL-13, key inducers of type 2 immune response in primary macrophages. Taken together, these results suggest a feedforward mechanism of adrenaline in adipose-resident macrophages, and that myeloid CaMKII signaling plays an important role in catecholamine production and subsequent beige fat activation.

Common traits between the beige fat-inducing stimuli

Adipose tissues play an essential role in regulating the metabolic homeostasis and can be found in almost all parts of the body. Excessive adiposity leads to obesity and can contribute to metabolic and other disorders. Adipocytes show remarkable plasticity in their function, which can be pushed toward energy storage, or energy expenditure-a `browning' of fat. Browning is controlled by the cellular milieu of the adipose tissue, with sympathetic innervation and by immune responses as key integrators of the signals that promote browning. Here, we describe the latest contributions to our understanding of how different metabolic stimuli can shape the adipocyte function. We especially focus on the role of the gut microbiota and the negative energy balance in regulating the browning.

Eosinophils support adipocyte maturation and promote glucose tolerance in obesity

Accumulating data have indicated a fundamental role of eosinophils in regulating adipose tissue homeostasis. Here, we performed whole-genome RNA sequencing of the small intestinal tract, which suggested the presence of impaired lipid metabolism in eosinophil-deficient ΔdblGATA mice. ΔdblGATA mice fed a high-fat diet (HFD) showed reduced body fat mass, impaired enlargement of adipocytes, decreased expression of adipogenic genes, and developed glucose intolerance. HFD induced accumulation of eosinophils in the perigonadal white adipose tissue. Concordantly, adipocyte-differentiated 3T3-L1 cells promoted the migration of eosinophils through the expression of CCL11 (eotaxin-1) and likely promoted their survival through the expression of interleukin (IL)-3, IL-5, and granulocyte-macrophage colony-stimulating factor. HFD-fed ΔdblGATA mice showed increased infiltration of macrophages, CD4⁺ T-cells, and B-cells, increased expression of interferon-γ, and decreased expression of IL-4 and IL-13 in white adipose tissue. Interferon-γ treatment significantly decreased lipid deposition in adipocyte-differentiated 3T3-L1 cells, while IL-4 treatment promoted lipid accumulation. Notably, HFD-fed ΔdblGATA mice showed increased lipid storage in the liver as compared with wild-type mice. We propose that obesity promotes the infiltration of eosinophils into adipose tissue that subsequently contribute to the metabolic homeostasis by promoting adipocyte maturation.

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.

An immune-beige adipocyte communication via nicotinic acetylcholine receptor signaling

Beige adipocytes have been recently shown to regulate energy dissipation when activated, and help organisms defend against hypothermia and obesity. Prior reports indicate beige-like adipocytes exist in adult humans and may present novel opportunities to curb the global epidemic in obesity and metabolic illnesses. In an effort to identify unique features of activated beige adipocytes, we uncovered that the cholinergic receptor nicotinic alpha 2 subunit (Chrna2) is induced in subcutaneous fat during the activation of these cells, and that acetylcholine-producing immune cells within this tissue regulate this signaling pathway via paracrine mechanisms. CHRNA2 functions selectively in uncoupling protein 1 (Ucp1)⁺ beige adipocytes, increasing thermogenesis through a cAMP and PKA pathway. Furthermore, this signaling via CHRNA2 is conserved and present in human subcutaneous adipocytes. Inactivation of Chrna2 in mice compromises the cold-induced thermogenic response selectively in subcutaneous fat and exacerbates high-fat diet-induced obesity and associated metabolic disorders, indicating that even partial loss of beige fat regulation in vivo leads to detrimental consequences. Our results reveal a beige-selective immune-adipose interaction mediated through CHRNA2 and identify a novel function of nicotinic acetylcholine receptors (nAChRs) in energy metabolism. These findings may lead to identification of therapeutic targets to counteract human obesity.

PPAR-γ in innate and adaptive lung immunity

The transcription factor PPAR-γ (peroxisome proliferator-activated receptor-γ) is a key regulator of lung immunity exhibiting multiple cell type specific roles in controlling development and function of the lung immune system. It is strictly required for the generation of alveolar macrophages by controlling differentiation of fetal lung monocyte precursors. Furthermore, it plays an important role in lung allergic inflammation by licensing lung dendritic cell t helper 2 (Th2) priming capacity as well as acting as a master transcription factor for pathogenic Th2 cells. Due to this plethora of functions and its involvement in multiple pulmonary diseases including asthma and pulmonary alveolar proteinosis, understanding the role of PPAR-γ in lung immunity is an important subject of ongoing research.

Macrophages in obesity

Obesity is a worldwide public health concern yet no safe therapies are currently available. The activity of sympathetic neurons is necessary and sufficient for fat mass reduction, via norepinephrine (NE) signaling. Macrophage accumulation in the adipose tissue is thought to play the central role in the onset of obesity, yet their relation to NE has been controversial. We have identified a population of sympathetic neuron-associated macrophages (SAMs) that control obesity via the uptake and clearing of NE. Here we focus on the neuro-immune regulation of obesity by discussing the genetic, cellular and functional signatures of SAMs vis-a-vis adipose tissue macrophages (ATMs).

Toward an Understanding of How Immune Cells Control Brown and Beige Adipobiology

Immune cells were recently found to have an unexpected involvement in controlling the thermogenic activity of brown and beige adipose tissue. Here, we review how macrophages, eosinophils, type 2 innate lymphoid cells, and T lymphocytes are linked to this process. In particular, the recruitment of alternatively activated macrophages and eosinophils is associated with brown fat activation and white fat browning. Conversely, pro-inflammatory immune cell recruitment represses the thermogenic activity of brown and beige adipose tissues via cytokines that inhibit noradrenergic signaling. Macrophages also influence the noradrenergic tone by degrading norepinephrine locally and by inhibiting sympathetic innervation over time.

Group 2 innate lymphocytes at the interface between innate and adaptive immunity

Group 2 innate lymphoid cells (ILC2) are innate immune cells that respond rapidly to their environment through soluble inflammatory mediators and cell-to-cell interactions. As tissue-resident sentinels, ILC2 help orchestrate localized type 2 immune responses. These ILC2-driven type 2 responses are now recognized in diverse immune processes, different anatomical locations, and homeostatic or pathological settings. ILC2-derived cytokines and cell surface signaling molecules function as key regulators of innate and adaptive immunity. Conversely, ILC2 are governed by their environment. As such, ILC2 form an important nexus of the immune system and may present an attractive target for immune modulation in disease.

Whole-tissue 3D imaging reveals intra-adipose sympathetic plasticity regulated by NGF-TrkA signal in cold-induced beiging

Sympathetic arborizations act as the essential efferent signals in regulating the metabolism of peripheral organs including white adipose tissues (WAT). However, whether these local neural structures would be of plastic nature, and how such plasticity might participate in specific metabolic events of WAT, remains largely uncharacterized. In this study, we exploit the new volume fluorescence-imaging technique to observe the significant, and also reversible, plasticity of intra-adipose sympathetic arborizations in mouse inguinal WAT in response to cold challenge. We demonstrate that this sympathetic plasticity depends on the cold-elicited signal of nerve growth factor (NGF) and TrkA receptor. Blockage of NGF or TrkA signaling suppresses intra-adipose sympathetic plasticity, and moreover, the cold-induced beiging process of WAT. Furthermore, we show that NGF expression in WAT depends on the catecholamine signal in cold challenge. We therefore reveal the key physiological relevance, together with the regulatory mechanism, of intra-adipose sympathetic plasticity in the WAT metabolism.

CD8+ T cells in beige adipogenesis and energy homeostasis

Although accumulation of lymphocytes in the white adipose tissue (WAT) in obesity is linked to insulin resistance, it remains unclear whether lymphocytes also participate in the regulation of energy homeostasis in the WAT. Here, we demonstrate enhanced energy dissipation in Rag1-/- mice, increased catecholaminergic input to subcutaneous WAT, and significant beige adipogenesis. Adoptive transfer experiments demonstrated that CD8+ T cell deficiency accounts for the enhanced beige adipogenesis in Rag1-/- mice. Consistently, we identified that CD8-/- mice also presented with enhanced beige adipogenesis. The inhibitory effect of CD8+ T cells on beige adipogenesis was reversed by blockade of IFN-γ. All together, our findings identify an effect of CD8+ T cells in regulating energy dissipation in lean WAT, mediated by IFN-γ modulation of the abundance of resident immune cells and of local catecholaminergic activity. Our results provide a plausible explanation for the clinical signs of metabolic dysfunction in diseases characterized by altered CD8+ T cell abundance and suggest targeting of CD8+ T cells as a promising therapeutic approach for obesity and other diseases with altered energy homeostasis.

Adipose tissue macrophages and their polarization in health and obesity

Adipose tissue is a special tissue environment due to its high lipid content. Adipose tissue macrophages (ATMs) help maintain adipose tissue homeostasis in steady state by clearing dead adipocytes. However, adipose tissue changes drastically during obesity, resulting in a state of chronic low grade inflammation and a shift in the adipose immune landscape. In this review we will discuss how these changes influence the polarization of ATMs.

Circulating molecules that control brown/beige adipocyte differentiation and thermogenic capacity

Obesity may be counteracted by increased energy expenditure. Circulating molecules act in the adipose tissue to influence brown and beige adipocyte function, differentiation, and thermogenic capacity, which in turn affects substrate utilization and impacts energy balance at the organismal level. These molecules have been envisioned as biomarkers and potential candidates for pharmacological interventions to treat obesity. Here we summarize studies that demonstrate the roles of endogenous circulating molecules of a wide variety in regulating the thermogenic potential of brown and beige fat cells. This review describes the state-of-the-art in the field and helps researchers to prioritize their targets in future studies.

Activation of hypothalamic RIP-Cre neurons promotes beiging of WAT via sympathetic nervous system

Activation of brown adipose tissue (BAT) and beige fat by cold increases energy expenditure. Although their activation is known to be differentially regulated in part by hypothalamus, the underlying neural pathways and populations remain poorly characterized. Here, we show that activation of rat-insulin-promoter-Cre (RIP-Cre) neurons in ventromedial hypothalamus (VMH) preferentially promotes recruitment of beige fat via a selective control of sympathetic nervous system (SNS) outflow to subcutaneous white adipose tissue (sWAT), but has no effect on BAT Genetic ablation of APPL2 in RIP-Cre neurons diminishes beiging in sWAT without affecting BAT, leading to cold intolerance and obesity in mice. Such defects are reversed by activation of RIP-Cre neurons, inactivation of VMH AMPK, or treatment with a β3-adrenergic receptor agonist. Hypothalamic APPL2 enhances neuronal activation in VMH RIP-Cre neurons and raphe pallidus, thereby eliciting SNS outflow to sWAT and subsequent beiging. These data suggest that beige fat can be selectively activated by VMH RIP-Cre neurons, in which the APPL2-AMPK signaling axis is crucial for this defending mechanism to cold and obesity.

Biology and function of adipose tissue macrophages, dendritic cells and B cells

The increasing incidence of obesity and its socio-economical impact is a global health issue due to its associated co-morbidities, namely diabetes and cardiovascular disease [1-5]. Obesity is characterized by an increase in adipose tissue, which promotes the recruitment of immune cells resulting in low-grade inflammation and dysfunctional metabolism. Macrophages are the most abundant immune cells in the adipose tissue of mice and humans. The adipose tissue also contains other myeloid cells (dendritic cells (DC) and neutrophils) and to a lesser extent lymphocyte populations, including T cells, B cells, Natural Killer (NK) and Natural Killer T (NKT) cells. While the majority of studies have linked adipose tissue macrophages (ATM) to the development of low-grade inflammation and co-morbidities associated with obesity, emerging evidence suggests for a role of other immune cells within the adipose tissue that may act in part by supporting macrophage homeostasis. In this review, we summarize the current knowledge of the functions ATMs, DCs and B cells possess during steady-state and obesity.

Insights into the neurochemical signature of the Innervation of Beige Fat

Objective

The potential for brown adipose tissue (BAT) to be targeted as a therapeutic option to combat obesity has been heightened by the discovery of a brown–like form of inducible “beige” adipose tissue in white fat which has overlapping structural and functional properties to “classical” BAT. The likelihood that both beige and brown fat are recruited functionally by neural mechanisms, taken together with the lack of a detailed understanding of the nature of changes in the nervous system when white adipose tissue (WAT) is transformed to brown, provides the impetus for this study. Here, we aim to identify whether there is a shift in the gene expression profile in neurons directly innervating inguinal white adipose tissue (iWAT) that has undergone “beiging” to a signature that is more similar to neurons projecting to BAT.

Methods

Two groups of rats, one housed at thermoneutrality (27 °C) and the other exposed to cold (8 °C) for 7 days, were killed, and their T13/L1 ganglia, stellate ganglion (T1/T2), or superior cervical ganglion (SCG, C2/3) removed. This approach yielded ganglia containing neurons that innervate either beiged white fat (8 °C for 7 days), inguinal WAT (27 °C for 7 days), BAT (both 27 °C and 8 °C for 7 days) or non-WAT (8 °C for 7 days), the latter included to isolate changes in gene expression that were more aligned with a response to cold exposure than the transformation of white to beige adipocytes. Bioinformatics analyses of RNA sequencing data was performed followed by Ingenuity Pathway Analysis (IPA) to determine differential gene expression and recruitment of biosynthetic pathways.

Results

When iWAT is “beiged” there is a significant shift in the gene expression profile of neurons in sympathetic ganglia (T13/L1) innervating this depot toward a gene neurochemical signature that is similar to the stellate ganglion projecting to BAT. Bioinformatics analyses of “beiging” related genes revealed upregulation of genes encoding neuropeptides proopiomelanocortin (POMC) and calcitonin-gene related peptide (CGRP) within ganglionic neurons. Treatment of differentiated 3T3L1 adipocytes with αMSH, one of the products cleaved from POMC, results in an elevation in lipolysis and the beiging of these cells as indicated by changes in gene expression markers of browning (Ucp1 and Ppargc1a).

Conclusion

These data indicate that, coincident with beiging, there is a shift toward a “brown-like” neurochemical signature of postganglionic neurons projecting to inguinal white fat, an increased expression of POMC, and, consistent with a causative role for this prohormone in beiging, an αMSH-mediated increase in beige gene markers in isolated adipocytes.

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RNA Seq showed shifts in neuronal gene expression following browning of white fat.

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Gene expression in ganglia projecting to white fat became brown-like with beiging.

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Bioinformatics analyses revealed neuronal gene candidates associated with beiging.

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Prominent gene candidates associated with beiging included POMC and CGRP.

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POMC cleavage product α-MSH caused beiging of cultured fat cells.

Kynurenic Acid and Gpr35 Regulate Adipose Tissue Energy Homeostasis and Inflammation

The role of tryptophan-kynurenine metabolism in psychiatric disease is well established, but remains less explored in peripheral tissues. Exercise training activates kynurenine biotransformation in skeletal muscle, which protects from neuroinflammation and leads to peripheral kynurenic acid accumulation. Here we show that kynurenic acid increases energy utilization by activating G protein-coupled receptor Gpr35, which stimulates lipid metabolism, thermogenic, and anti-inflammatory gene expression in adipose tissue. This suppresses weight gain in animals fed a high-fat diet and improves glucose tolerance. Kynurenic acid and Gpr35 enhance Pgc-1α1 expression and cellular respiration, and increase the levels of Rgs14 in adipocytes, which leads to enhanced beta-adrenergic receptor signaling. Conversely, genetic deletion of Gpr35 causes progressive weight gain and glucose intolerance, and sensitizes to the effects of high-fat diets. Finally, exercise-induced adipose tissue browning is compromised in Gpr35 knockout animals. This work uncovers kynurenine metabolism as a pathway with therapeutic potential to control energy homeostasis.

Really does temperature reduction and norepinephrine have similar effects on the energy metabolism in rat brown adipose tissue?

CONTEXT

Heat generation by brown adipose tissue (BAT) in response to temperature reduction seems to be entirely related to sympathetic nervous stimulation.

OBJECTIVE

To analyse if temperature reduction and norepinephrine may differently affect the expression of proteins related to energy metabolism in BAT.

MATERIALS AND METHODS

Isolated rats BAT was incubated with/without norepinephrine (10^-6 mol/L, 24 h at 32 °C and 37 °C).

RESULTS

In BAT, 32 °C increased the protein expression levels of carnitine palmitoyltransferase-I and -II, mitochondrial uncoupling protein-1 (UCP-1) and the expression and activity of lactate dehydrogenase. Mitochondrial F₁-ATP synthase α-chain expression was decreased at 32 °C compared to 37 °C. Norepinephrine and at 32 °C exposure, UCP-1 expression was increased but cytochrome-c oxidase and F₁-ATP synthase α-chain expression was reduced with respect to 37 °C.

DISCUSSION

Sympathetic stimulation seems not to be the only factor associated with heat generation.

CONCLUSIONS

Temperature reduction by itself exerts some different effects on the expression of proteins related to the energy metabolism than norepinephrine.

Three-Dimensional Adipose Tissue Imaging Reveals Regional Variation in Beige Fat Biogenesis and PRDM16-Dependent Sympathetic Neurite Density

While the cell-intrinsic pathways governing beige adipocyte development and phenotype have been increasingly delineated, comparatively little is known about how beige adipocytes interact with other cell types in fat. Here, we introduce a whole-tissue clearing method for adipose that permits immunolabeling and three-dimensional profiling of structures including thermogenic adipocytes and sympathetic innervation. We found that tissue architecture and sympathetic innervation differ significantly between subcutaneous and visceral depots. Subcutaneous fat demonstrates prominent regional variation in beige fat biogenesis with localization of UCP1⁺ beige adipocytes to areas with dense sympathetic neurites. We present evidence that the density of sympathetic projections is dependent on PRDM16 in adipocytes, providing another potential mechanism underlying the metabolic benefits mediated by PRDM16. This powerful imaging tool highlights the interaction of tissue components during beige fat biogenesis and reveals a previously undescribed mode of regulation of the sympathetic nervous system by adipocytes.

Aging of Brown and Beige/Brite Adipose Tissue

Brown adipose tissue aging and the concomitant loss of thermogenic capacity have been linked to an inability to maintain normal energy homeostasis in late life. Similarly, the ability of white fat to convert into brite/beige adipose tissue declines. This may ultimately exacerbate the progression of age-related metabolic pathologies, such as insulin resistance and obesity. The depletion of all types of brown adipocytes during aging is well-established and has been described in rodent models as well as humans. We here review the available literature on the potential mechanisms leading to cell-autonomous and microenvironment-related aspects of brown adipocyte dysfunction. Among these, cellular senescence, mitochondrial impairment, and deteriorating changes to the local and endocrine microenvironments have been proposed. An important goal of aging research is to develop approaches that may not only extend life expectancy but also prolong health-span. These efforts may also be aimed at maintaining metabolic health throughout life by targeting brown adipocyte function.

A Promising New Approach for the Treatment of Inflammatory Pain: Transfer of Stem Cell-Derived Tyrosine Hydroxylase-Positive Cells

OBJECTIVES

The appearance of endogenous tyrosine hydroxylase-positive cells (TH+ cells) in collagen-induced arthritis was associated with an anti-inflammatory effect. Here we investigated putative anti-inflammatory and antinociceptive effects of the transfer of induced, bone marrow stem cell-derived TH+ cells (iTH+ cells) on murine antigen-induced arthritis (AIA).

METHODS

Bone marrow-derived stem cells were differentiated into iTH+ cells. These cells were transferred to mice immunized against methylated bovine serum albumin (mBSA) 2 days before AIA was induced by injection of mBSA into one knee joint. In AIA control mice and iTH+-treated mice the severity of AIA, pain-related behavior, humoral and cellular responses, and the invasion of macrophages into the dorsal root ganglia were assessed.

RESULTS

The intravenous transfer of iTH+ cells before AIA induction did not cause a sustained suppression of AIA severity but significantly reduced inflammation-evoked pain-related behavior. The iTH+ cells used for transfer exhibited enormous production of interleukin-4. A major difference between AIA control mice and iTH+-treated AIA mice was a massive invasion of the dorsal root ganglia by iNOS-negative, arginine 1-positive macrophages corresponding to an M2 phenotype. The differences in other cellular and humoral immune parameters such as release of cytokines from stimulated lymphocytes between AIA control mice and iTH+-treated mice were small.

CONCLUSIONS

The transfer of iTH+ cells may cause a long-lasting reduction of arthritis-induced pain even if it does not ameliorate inflammation. The invasion of M2 macrophages into the dorsal root ganglia is likely to be an important mechanism of antinociception.

Macrophage polarization and meta-inflammation

Chronic overnutrition and obesity induces low-grade inflammation throughout the body. Termed "meta-inflammation," this chronic state of inflammation is mediated by macrophages located within the colon, liver, muscle, and adipose tissue. A sentinel orchestrator of immune activity and homeostasis, macrophages adopt variable states of activation as a function of time and environmental cues. Meta-inflammation phenotypically skews these polarization states and has been linked to numerous metabolic disorders. The past decade has revealed several key regulators of macrophage polarization, including the signal transducer and activator of transcription family, the peroxisome proliferator-activated receptor gamma, the CCAAT-enhancer-binding proteins (C/EBP) family, and the interferon regulatory factors. Recent studies have also suggested that microRNAs and long noncoding RNA influence macrophage polarization. The pathogenic alteration of macrophage polarization in meta-inflammation is regulated by both extracellular and intracellular cues, resulting in distinct secretome profiles. Meta-inflammation-altered macrophage polarization has been linked to insulin insensitivity, atherosclerosis, inflammatory bowel disease, cancer, and autoimmunity. Thus, further mechanistic exploration into the skewing of macrophage polarization promises to have profound impacts on improving global health.

Play It Again, SAM: Macrophages Control Peripheral Fat Metabolism

Macrophages and other immune cells are increasingly recognized to have unique and nontraditional functions in various tissues of the body. In a recent issue of Nature Medicine, Pirzgalska et al. [1] characterized a unique set of tissue-specialized macrophages that modulate the connection between the nervous system and subcutaneous fat.

Elevating adipose eosinophils in obese mice to physiologically normal levels does not rescue metabolic impairments

Objective

Obesity is a metabolic disorder that has reached epidemic proportions worldwide and leads to increased risk for diabetes, cardiovascular disease, asthma, certain cancers, and various other diseases. Obesity and its comorbidities are associated with impaired adipose tissue (AT) function. In the last decade, eosinophils have been identified as regulators of proper AT function. Our study aimed to determine whether normalizing the number of AT eosinophils in obese mice, to those of lean healthy mice, would reduce obesity and/or improve metabolic fitness.

Methods

C57BL/6J mice fed a high fat diet (HFD) were simultaneously given recombinant interleukin-5 (rIL5) for 8 weeks to increase AT eosinophils. Metabolic fitness was tested by evaluating weight gain, AT inflammation, glucose, lipid, and mixed-meal tolerance, AT insulin signaling, energy substrate utilization, energy expenditure, and white AT beiging capacity.

Results

Eosinophils were increased ∼3-fold in AT of obese HFD-fed mice treated with rIL5, and thus were restored to levels observed in lean healthy mice. However, there were no significant differences in rIL5-treated mice among the above listed comprehensive set of metabolic assays, despite the increased AT eosinophils.

Conclusions

We have shown that restoring obese AT eosinophils to lean healthy levels is not sufficient to allow for improvement in any of a range of metabolic features otherwise impaired in obesity. Thus, the mechanisms that identified eosinophils as positive regulators of AT function, and therefore systemic health, are more complex than initially understood and will require further study to fully elucidate.

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Adipose tissue eosinophils declined with high fat diet induced weight gain.

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Recombinant interleukin 5 treatment restored adipose eosinophils during obesity.

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Restoring adipose eosinophils didn't reduce weight gain or adipose mass.

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Restoring adipose eosinophils didn't rescue glucose tolerance or insulin signaling.

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Restoring adipose eosinophils didn't alter energy expenditure or beiging capacity.

Metabolic Regulation of Innate Lymphoid Cell-Mediated Tissue Protection-Linking the Nutritional State to Barrier Immunity

Innate lymphoid cells (ILC) are a recently described group of tissue-resident immune cells that play essential roles in maintaining and protecting the tissue barrier against invading pathogens. Extensive research has revealed that ILC-mediated immune responses are controlled by dietary components and metabolites. An additional role of ILC as important direct regulators of host metabolism and glucose tolerance is emerging. This suggests that ILC may act as key dietary sensors integrating nutritional and metabolic stress to facilitate both maintenance of barrier sites and a coordinated immune response protecting these tissues. In this respect, investigations have begun to determine how different ILC responses are metabolically fueled and the impact of nutrient availability on the regulation of ILC function. Here, we discuss the current literature concerning dietary and metabolic control of ILC. In particular, we address whether the dietary and metabolic control of ILC and their simultaneous influence on host metabolism may function as a coordinated program of barrier defense.

Obesity-promoting and anti-thermogenic effects of neutrophil gelatinase-associated lipocalin in mice

Neutrophil gelatinase-associated lipocalin (NGAL, lipocalin 2 or LCN2) is an iron carrier protein whose circulating level is increased by kidney injury, bacterial infection and obesity, but its metabolic consequence remains elusive. To study physiological role of LCN2 in energy homeostasis, we challenged female Lcn2 knockout (KO) and wild-type (WT) mice with high fat diet (HFD) or cold exposure. Under normal diet, physical constitutions of Lcn2 KO and WT mice were indistinguishable. During HFD treatment, Lcn2 KO mice exhibited larger brown adipose tissues (BAT), consumed more oxygen, ate more food and gained less body weights as compared to WT mice. When exposed to 4 °C, KO mice showed higher body temperature and more intense ¹⁸F-fluorodeoxyglucose uptake in BAT, which were cancelled by β3 adrenergic receptor blocker or iron-loaded (but not iron-free) LCN2 administration. These findings suggest that circulating LCN2 possesses obesity-promoting and anti-thermogenic effects through inhibition of BAT activity in an iron-dependent manner.

Fatty acid oxidation is required for active and quiescent brown adipose tissue maintenance and thermogenic programing

OBJECTIVE

To determine the role of fatty acid oxidation on the cellular, molecular, and physiologic response of brown adipose tissue to disparate paradigms of chronic thermogenic stimulation.

METHODS

Mice with an adipose-specific loss of Carnitine Palmitoyltransferase 2 (Cpt2^A-/-), that lack mitochondrial long chain fatty acid β-oxidation, were subjected to environmental and pharmacologic interventions known to promote thermogenic programming in adipose tissue.

RESULTS

Chronic administration of β3-adrenergic (CL-316243) or thyroid hormone (GC-1) agonists induced a loss of BAT morphology and UCP1 expression in Cpt2^A-/- mice. Fatty acid oxidation was also required for the browning of white adipose tissue (WAT) and the induction of UCP1 in WAT. In contrast, chronic cold (15 °C) stimulation induced UCP1 and thermogenic programming in both control and Cpt2^A-/- adipose tissue albeit to a lesser extent in Cpt2^A-/- mice. However, thermoneutral housing also induced the loss of UCP1 and BAT morphology in Cpt2^A-/- mice. Therefore, adipose fatty acid oxidation is required for both the acute agonist-induced activation of BAT and the maintenance of quiescent BAT. Consistent with this data, Cpt2^A-/- BAT exhibited increased macrophage infiltration, inflammation and fibrosis irrespective of BAT activation. Finally, obese Cpt2^A-/- mice housed at thermoneutrality exhibited a loss of interscapular BAT and were refractory to β3-adrenergic-induced energy expenditure and weight loss.

CONCLUSION

Mitochondrial long chain fatty acid β-oxidation is critical for the maintenance of the brown adipocyte phenotype both during times of activation and quiescence.

Obesity Alters B Cell and Macrophage Populations in Brown Adipose Tissue

OBJECTIVE

The prevalence of obesity continues to rise, and it is understood that regulation of white adipose tissue (WAT) function is important to systemic metabolic homeostasis. Immune cells play a central role in the maintenance of WAT, and their compositions change in number and inflammatory phenotype with the progression of obesity. Because of its energy-burning capabilities, brown adipose tissue (BAT) has become a focus of obesity research. Although novel studies have focused on the function of brown adipocytes in thermogenesis, the tissue as a whole has not been immunologically characterized.

METHODS

BAT immune cell populations were analyzed by flow cytometry and immunohistochemistry in mice with diet-induced obesity (3, 8, or 16 weeks of diet) and in aged mice (1, 6-7, and 10-15 months).

RESULTS

The data confirmed the presence of macrophages and eosinophils, as previously reported, and showed that 20% to 30% of the immune cells in BAT were B cells. The number of B cells and eosinophils increased with diet-induced obesity, whereas macrophages decreased. There was no change in number of any immune cell quantified with age.

CONCLUSIONS

These studies reveal a novel finding of B220 + B cells in BAT and show that BAT immune cell populations change in response to diet-induced obesity.

Growth Differentiation Factor 15 Mediates Systemic Glucose Regulatory Action of T-Helper Type 2 Cytokines

T-helper type 2 (Th2) cytokines, including interleukin (IL)-13 and IL-4, produced in adipose tissue, are critical regulators of intra-adipose and systemic lipid and glucose metabolism. Furthermore, IL-13 is a potential therapy for insulin resistance in obese mouse models. Here, we examined mediators produced by adipocytes that are responsible for regulating systemic glucose homeostasis in response to Th2 cytokines. We used RNA sequencing data analysis of cultured adipocytes to screen factors secreted in response to recombinant IL-13. Recombinant IL-13 induced expression of growth differentiation factor 15 (GDF15) via the Janus kinase-activated STAT6 pathway. In vivo administration of α-galactosylceramide or IL-33 increased IL-4 and IL-13 production, thereby increasing GDF15 levels in adipose tissue and in plasma of mice; however, these responses were abrogated in STAT6 knockout mice. Moreover, administration of recombinant IL-13 to wild-type mice fed a high-fat diet (HFD) improved glucose intolerance; this was not the case for GDF15 knockout mice fed the HFD. Taken together, these data suggest that GDF15 is required for IL-13-induced improvement of glucose intolerance in mice fed an HFD. Thus, beneficial effects of Th2 cytokines on systemic glucose metabolism and insulin sensitivity are mediated by GDF15. These findings open up a potential pharmacological route for reversing insulin resistance associated with obesity.

Interactions between Innate Lymphoid Cells and Cells of the Innate and Adaptive Immune System

Type 2 innate lymphoid cells (ILC2s) are a major source of cytokines, which are also produced by Th2 cells and several cell types of the innate immune system. Work over the past few years indicates that ILC2s play a central role in regulating type 2 immune responses against allergens and helminths. ILC2s can interact with a variety of cells types of the innate and adaptive immune system by cell–cell contacts or by communication via soluble factors. In this review, we provide an overview about recent advances in our understanding how ILC2s orchestrate type 2 immune responses with focus on direct interactions between ILC2s and other cells of the immune system.

Macrophage VLDLR mediates obesity-induced insulin resistance with adipose tissue inflammation

Obesity is closely associated with increased adipose tissue macrophages (ATMs), which contribute to systemic insulin resistance and altered lipid metabolism by creating a pro-inflammatory environment. Very low-density lipoprotein receptor (VLDLR) is involved in lipoprotein uptake and storage. However, whether lipid uptake via VLDLR in macrophages affects obesity-induced inflammatory responses and insulin resistance is not well understood. Here we show that elevated VLDLR expression in ATMs promotes adipose tissue inflammation and glucose intolerance in obese mice. In macrophages, VLDL treatment upregulates intracellular levels of C16:0 ceramides in a VLDLR-dependent manner, which potentiates pro-inflammatory responses and promotes M1-like macrophage polarization. Adoptive transfer of VLDLR knockout bone marrow to wild-type mice relieves adipose tissue inflammation and improves insulin resistance in diet-induced obese mice. These findings suggest that increased VLDL-VLDLR signaling in ATMs aggravates adipose tissue inflammation and insulin resistance in obesity.

VLDLR regulates cellular lipoprotein uptake and storage. Here, the authors show that VLDLR, expressed on adipose tissue macrophages, is upregulated in obesity and promotes adipose tissue inflammation by upregulating ceramide production and facilitating M1-like macrophage polarization.

Foundations of Immunometabolism and Implications for Metabolic Health and Disease

Highly ordered interactions between immune and metabolic responses are evolutionarily conserved and paramount for tissue and organismal health. Disruption of these interactions underlies the emergence of many pathologies, particularly chronic non-communicable diseases such as obesity and diabetes. Here, we examine decades of research identifying the complex immunometabolic signaling networks and the cellular and molecular events that occur in the setting of altered nutrient and energy exposures and offer a historical perspective. Furthermore, we describe recent advances such as the discovery that a broad complement of immune cells play a role in immunometabolism and the emerging evidence that nutrients and metabolites modulate inflammatory pathways. Lastly, we discuss how this work may eventually lead to tangible therapeutic advancements to promote health.

Adipocyte cannabinoid receptor CB1 regulates energy homeostasis and alternatively activated macrophages

Dysregulated adipocyte physiology leads to imbalanced energy storage, obesity, and associated diseases, imposing a costly burden on current health care. Cannabinoid receptor type-1 (CB1) plays a crucial role in controlling energy metabolism through central and peripheral mechanisms. In this work, adipocyte-specific inducible deletion of the CB1 gene (Ati-CB1-KO) was sufficient to protect adult mice from diet-induced obesity and associated metabolic alterations and to reverse the phenotype in already obese mice. Compared with controls, Ati-CB1-KO mice showed decreased body weight, reduced total adiposity, improved insulin sensitivity, enhanced energy expenditure, and fat depot-specific cellular remodeling toward lowered energy storage capacity and browning of white adipocytes. These changes were associated with an increase in alternatively activated macrophages concomitant with enhanced sympathetic tone in adipose tissue. Remarkably, these alterations preceded the appearance of differences in body weight, highlighting the causal relation between the loss of CB1 and the triggering of metabolic reprogramming in adipose tissues. Finally, the lean phenotype of Ati-CB1-KO mice and the increase in alternatively activated macrophages in adipose tissue were also present at thermoneutral conditions. Our data provide compelling evidence for a crosstalk among adipocytes, immune cells, and the sympathetic nervous system (SNS), wherein CB1 plays a key regulatory role.

Human ADMC-Derived Adipocyte Thermogenic Capacity Is Regulated by IL-4 Receptor

Type two innate immune system is anti-inflammatory and may play an important role as the means whereby “browning” is induced in subcutaneous adipocytes. It was shown that IL-4 may influence the fate of adipose cell precursors by promoting differentiation towards more thermogenic adipocytes in mice. Here, we investigated the influence of IL-4 and IL-4 receptor, a type two immune cytokine pathway, on the metabolic activity and thermogenic potential of human adipocytes differentiated from adipose-derived mesenchymal stem cells (ADMSCs) obtained from subcutaneous samples of healthy women undergoing abdominoplasty. Western blot analysis, qPCR, and biochemical analyses were performed 10 days after ADMSC differentiation into mature adipocytes was induced. IL-4 receptor was expressed in both precursor and differentiated adipocytes, and IL-4 treatment increased phosphorylation Y641 of signal transducer and activator of transcription 6 (STAT6) in both cell types. IL-4 treatment also increased expression of thermogenic proteins PGC-1α, UCP-1, and CITED1. In addition, IL-4 increased the secretion of adiponectin, leptin, and FGF21 and promoted lipolysis in differentiated adipocytes. In conclusion, IL-4 may directly modulate differentiation of human adipocytes towards a beige phenotype acting through IL-4 receptors on both adipose precursors and differentiated human adipocytes, metabolic effect that must be considered in some antiallergic drugs.

Sympathetic neuron-associated macrophages contribute to obesity by importing and metabolizing norepinephrine

The cellular mechanism(s) linking macrophages to norepinephrine (NE)-mediated regulation of thermogenesis have been a topic of debate. Here we identify sympathetic neuron-associated macrophages (SAMs) as a population of cells that mediate clearance of NE via expression of solute carrier family 6 member 2 (SLC6A2), an NE transporter, and monoamine oxidase A (MAOA), a degradation enzyme. Optogenetic activation of the sympathetic nervous system (SNS) upregulates NE uptake by SAMs and shifts the SAM profile to a more proinflammatory state. NE uptake by SAMs is prevented by genetic deletion of Slc6a2 or inhibition of the encoded transporter. We also observed an increased proportion of SAMs in the SNS of two mouse models of obesity. Genetic ablation of Slc6a2 in SAMs increases brown adipose tissue (BAT) content, causes browning of white fat, increases thermogenesis, and leads to substantial and sustained weight loss in obese mice. We further show that this pathway is conserved, as human sympathetic ganglia also contain SAMs expressing the analogous molecular machinery for NE clearance, which thus constitutes a potential target for obesity treatment.

Intestinal SIRT3 overexpression in mice improves whole body glucose homeostasis independent of body weight

OBJECTIVE

Intestinal metabolism might play a greater role in regulating whole body metabolism than previously believed. We aimed to enhance enterocyte metabolism in mice and investigate if it plays a role in diet-induced obesity (DIO) and its comorbidities.

METHODS

Using the cre-loxP system, we overexpressed the mitochondrial NAD+ dependent protein deacetylase SIRT3 in enterocytes of mice (iSIRT3 mice). We chronically fed iSIRT3 mice and floxed-SIRT3 control (S3fl) mice a low-fat, control diet (CD) or a high-fat diet (HFD) and then phenotyped the mice.

RESULTS

There were no genotype differences in any of the parameters tested when the mice were fed CD. Also, iSIRT3 mice were equally susceptible to the development of DIO as S3fl mice when fed HFD. They were, however, better able than S3fl mice to regulate their blood glucose levels in response to exogenous insulin and glucose, indicating that they were protected from developing insulin resistance. This improved glucose homeostasis was accompanied by an increase in enterocyte metabolic activity and an upregulation of ketogenic gene expression in the small intestine.

CONCLUSION

Enhancing enterocyte oxidative metabolism can improve whole body glucose homeostasis.

Second messenger signaling mechanisms of the brown adipocyte thermogenic program: an integrative perspective

β-adrenergic receptors (βARs) are well established for conveying the signal from catecholamines to adipocytes. Acting through the second messenger cyclic adenosine monophosphate (cAMP) they stimulate lipolysis and also increase the activity of brown adipocytes and the 'browning' of adipocytes within white fat depots (so-called 'brite' or 'beige' adipocytes). Brown adipose tissue mitochondria are enriched with uncoupling protein 1 (UCP1), which is a regulated proton channel that allows the dissipation of chemical energy in the form of heat. The discovery of functional brown adipocytes in humans and inducible brown-like ('beige' or 'brite') adipocytes in rodents have suggested that recruitment and activation of these thermogenic adipocytes could be a promising strategy to increase energy expenditure for obesity therapy. More recently, the cardiac natriuretic peptides and their second messenger cyclic guanosine monophosphate (cGMP) have gained attention as a parallel signaling pathway in adipocytes, with some unique features. In this review, we begin with some important historical work that touches upon the regulation of brown adipocyte development and physiology. We then provide a synopsis of some recent advances in the signaling cascades from β-adrenergic agonists and natriuretic peptides to drive thermogenic gene expression in the adipocytes and how these two pathways converge at a number of unexpected points. Finally, moving from the physiologic hormonal signaling, we discuss yet another level of control downstream of these signals: the growing appreciation of the emerging roles of non-coding RNAs as important regulators of brown adipocyte formation and function. In this review, we discuss new developments in our understanding of the signaling mechanisms and factors including new secreted proteins and novel non-coding RNAs that control the function as well as the plasticity of the brown/beige adipose tissue as it responds to the energy needs and environmental conditions of the organism.