Maximal beta3-adrenergic regulation of lipolysis involves Src and epidermal growth factor receptor-dependent ERK1/2 activation

Inhibiting TRIM8 alleviates adipocyte inflammation and insulin resistance by regulating the DUSP14/MAPKs pathway

Obesity is a low-grade chronic inflammation induced by the pathological expansion of adipocytes which allows the development of obesity-associated metabolic diseases like type 2 diabetes mellitus (T2D) and non-alcoholic fatty liver disease (NAFLD). However, mechanisms regulating adipocyte inflammation remain poorly understood. Here, we observed that TRIM8 was upregulated in adipocyte inflammation and insulin resistance while DUSP14 was downregulated. TRIM8 deficiency and DUSP14 over-expression decreased the level of inflammatory cytokines, increased glucose uptake content, and improved insulin signalling transduction compared to LPS treatment alone. Conversely, silencing DUSP14 increased the expression of inflammatory cytokines. It decreased the glucose uptake content and the phosphorylation level of proteins involved in insulin signalling, further impairing insulin signalling and aggravating insulin resistance. Furthermore, The decreased level of inflammatory cytokines, increased glucose uptake, and improved insulin signalling transduction caused by TRIM8 deficiency were reversed by down-regulated DUSP14. Collectively, our findings revealed that TRIM8 can regulate adipocyte inflammation and insulin resistance by regulating the MAPKs pathway which is dependent on DUSP14.

Age-dependent decline of B3AR agonist-mediated activation of FAP UCP-1 expression in murine models of chronic rotator cuff repair

Amibegron, a β3-adrenergic receptor (B3AR) agonist, has recently been shown to provide therapeutic effects for chronic rotator cuff (RC) tears by inducing the expression of uncoupling protein 1 (UCP-1), a marker of brown fat, in fibroadipogenic progenitors (FAPs). However, it remains to be seen if these beneficial effects hold true with age and in older, more clinically relevant populations. This study seeks to understand the impacts of aging on the efficacy of amibegron to treat chronic RC tears. Young (4-month-old) and aged (33-month-old) C57BL/6 mice underwent a RC injury procedure with delayed repair (DR). Mice were equally randomized to receive amibegron or dimethyl sulfoxide (DMSO) treatments after repair. Functional ability was measured at baseline and 6-weeks after DR. Wet muscle weight and histology of injured and contralateral supraspinatus were also analyzed 6-weeks post-DR. For in vitro histology and real-time quantitative PCR experiments, FAPs were isolated from young and aged mice via fluorescence-activated cell sorting. Young and aged FAPs were treated with amibegron or DMSO either immediately after seeding (early exposure) or 8-days after seeding (late exposure). In vitro results showed that amibegron-mediated FAP UCP-1 expression decreases with age. In vivo data demonstrated that aged mice have a decreased responsiveness to amibegron and decreased propensity for intramuscular FAP UCP-1 expression. Further, delayed amibegron treatment with RC repair did not lead to improvements in muscle atrophy and functional outcomes. Our findings demonstrate that age and the timing of interventions play a critical role in FAP-targeted therapeutics for chronic injuries.

Effects of Weight Loss and Aerobic Exercise Training on Adi-Pose Tissue Zinc α2-Glycoprotein and Associated Genes in Obesity

Zinc α2-glycoprotein (ZAG) has been implicated in fatty acid metabolism and utilization and is lower in obese and higher in cachexic adults compared to those of normal weight. Previous studies suggest that ZAG binds to the beta3-adrenergic receptor (β3AR) to influence fatty acid metabolism in adipose tissue by regulating hormone sensitive lipase (HSL). The purpose of this study is to investigate the effects of a six-month weight loss (WL) or aerobic exercise (AEX) intervention on adipose tissue and skeletal muscle ZAG mRNA levels and protein expression, as well as the expression of β3AR, and HSL. Abdominal adipose tissue (AB) and gluteal adipose tissue (Glut) and vastus lateralis muscle biopsies were performed before and after WL (n = 13) or AEX (n = 13). ZAG, HSL, and β3AR expressions were determined by RT-PCR, and ZAG and HSL plasma levels by ELISA. Body weight decreased by 9.69% (p < 0.001) in WL and did not change with AEX. Maximal oxygen consumption (VO2max) increased by 7.1% (p < 0.005) after WL and by 16.69% (p < 0.001) after AEX. WL significantly decreased body weight with a reduction of percentage of fat, fat mass, fat-free mass (FFM). AEX decreased percent fat and increased VO2max, but did not change fat mass and FFM. Abdominal ZAG and HSL mRNA levels did not change significantly after WL or AEX. There were no changes in plasma ZAG, HSL and adipose tissue β3AR mRNA levels after WL and AEX. ZAG, HSL and β3AR mRNA expressions in adipose tissue are positively associated each other. Adipose tissue abdominal and gluteal HSL are negatively associated with HOMA-IR (Homeostatic Model Assessment for Insulin Resistance), and both ZAG and HSL adipose tissue are negatively associated with fasting glucose and the glucose area under the curve. Further work is needed to elucidate the role of ZAG and HSL in the propensity for weight gain and the ability of exercise to mitigate these responses.

A Proteomic Analysis of Detergent-Resistant Membranes in HIV Virological Synapse: The Involvement of Vimentin in CD4 Polarization

The cell-cell contact between HIV-1-infected and uninfected cells forms a virological synapse (VS) to allow for efficient HIV-1 transmission. Not only are HIV-1 components polarized and accumulate at cell-cell interfaces, but viral receptors and lipid raft markers are also. To better understand the nature of the HIV-1 VS, detergent-resistant membrane (DRM) fractions were isolated from an infected-uninfected cell coculture and compared to those from non-coculture samples using 2D fluorescence difference gel electrophoresis. Mass spectrometry revealed that ATP-related enzymes (ATP synthase subunit and vacuolar-type proton ATPase), protein translation factors (eukaryotic initiation factor 4A and mitochondrial elongation factor Tu), protein quality-control-related factors (protein disulfide isomerase A3 and 26S protease regulatory subunit), charged multivesicular body protein 4B, and vimentin were recruited to the VS. Membrane flotation centrifugation of the DRM fractions and confocal microscopy confirmed these findings. We further explored how vimentin contributes to the HIV-1 VS and found that vimentin supports HIV-1 transmission through the recruitment of CD4 to the cell-cell interface. Since many of the molecules identified in this study have previously been suggested to be involved in HIV-1 infection, we suggest that a 2D difference gel analysis of DRM-associated proteins may reveal the molecules that play crucial roles in HIV-1 cell-cell transmission.

The β3 adrenoceptor in proliferative retinopathies: "Cinderella" steps out of its family shadow

In the retina, hypoxic condition leads to overgrowing leaky vessels resulting in altered metabolic supply that may cause impaired visual function. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the retinal response to hypoxia by activating the transcription of numerous target genes, including vascular endothelium growth factor, which acts as a major player in retinal angiogenesis. In the present review, oxygen urge by the retina and its oxygen sensing systems including HIF-1 are discussed in respect to the role of the beta-adrenergic receptors (β-ARs) and their pharmacologic manipulation in the vascular response to hypoxia. In the β-AR family, β1- and β2-AR have long been attracting attention because their pharmacology is intensely used for human health, while β3-AR, the third and last cloned receptor is no longer increasingly emerging as an attractive target for drug discovery. Here, β3-AR, a main character in several organs including the heart, the adipose tissue and the urinary bladder, but so far a supporting actor in the retina, has been thoroughly examined in respect to its function in retinal response to hypoxia. In particular, its oxygen dependence has been taken as a key indicator of β3-AR involvement in HIF-1-mediated responses to oxygen. Hence, the possibility of β3-AR transcription by HIF-1 has been discussed from early circumstantial evidence to the recent demonstration that β3-AR acts as a novel HIF-1 target gene by playing like a putative intermediary between oxygen levels and retinal vessel proliferation. Thus, targeting β3-AR may implement the therapeutic armamentarium against neovascular pathologies of the eye.

Apoptotic brown adipocytes enhance energy expenditure via extracellular inosine

Brown adipose tissue (BAT) dissipates energy1,2 and promotes cardiometabolic health3. Loss of BAT during obesity and ageing is a principal hurdle for BAT-centred obesity therapies, but not much is known about BAT apoptosis. Here, untargeted metabolomics demonstrated that apoptotic brown adipocytes release a specific pattern of metabolites with purine metabolites being highly enriched. This apoptotic secretome enhances expression of the thermogenic programme in healthy adipocytes. This effect is mediated by the purine inosine that stimulates energy expenditure in brown adipocytes by the cyclic adenosine monophosphate-protein kinase A signalling pathway. Treatment of mice with inosine increased BAT-dependent energy expenditure and induced 'browning' of white adipose tissue. Mechanistically, the equilibrative nucleoside transporter 1 (ENT1, SLC29A1) regulates inosine levels in BAT: ENT1-deficiency increases extracellular inosine levels and consequently enhances thermogenic adipocyte differentiation. In mice, pharmacological inhibition of ENT1 as well as global and adipose-specific ablation enhanced BAT activity and counteracted diet-induced obesity, respectively. In human brown adipocytes, knockdown or blockade of ENT1 increased extracellular inosine, which enhanced thermogenic capacity. Conversely, high ENT1 levels correlated with lower expression of the thermogenic marker UCP1 in human adipose tissues. Finally, the Ile216Thr loss of function mutation in human ENT1 was associated with significantly lower body mass index and 59% lower odds of obesity for individuals carrying the Thr variant. Our data identify inosine as a metabolite released during apoptosis with a 'replace me' signalling function that regulates thermogenic fat and counteracts obesity.

Fractionation of Extracts from Black Chokeberry, Cranberry, and Pomegranate to Identify Compounds That Influence Lipid Metabolism

Polyphenols show a spectrum of bioactive effects, including an influence on lipid metabolism. In this study, we performed activity-guided fractionations of black chokeberry (aronia), cranberry, and pomegranate extracts to identify the biologically active compounds. The extracts were prepared from fruit juice concentrates with the adsorbent resin Amberlite XAD-7 and were separated into a copigment and an anthocyanin fraction, followed by fractionation into a polymer and monomeric fraction by means of hexane precipitation. For further fractionation of the cranberry and pomegranate copigment fractions, high-performance countercurrent chromatography (HPCCC) was used. The compounds in each fraction were identified by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS), and the quantification was performed by ultra high-performance liquid chromatography-diode array detector (UHPLC-DAD) analyses. Each of the (sub-)fractions was tested in three in vitro assays: phosphodiesterase 3B (PDE) activity, lipid accumulation, and lipolysis in 3T3-L1 cells. The results showed that various fractions and subfractions can inhibit lipid accumulation and PDE activity as well as increase lipolysis, particularly copigments. Overall, our results indicate an influence of polyphenol-rich (sub-)fractions on the lipid metabolism.

β-Adrenergic Receptors and Adipose Tissue Metabolism: Evolution of an Old Story

The role of β-adrenergic receptors (βARs) in adipose tissue to promote lipolysis and the release of fatty acids and nonshivering thermogenesis in brown fat has been studied for so many decades that one would think there is nothing left to discover. With the rediscovery of brown fat in humans and renewed interest in UCP1 and uncoupled mitochondrial respiration, it seems that a review of adipose tissue as an organ, pivotal observations, and the investigators who made them would be instructive to understanding where the field stands now. The discovery of the β₃-adrenergic receptor was important for accurately defining the pharmacology of the adipocyte, while the clinical targeting of this receptor for obesity and metabolic disease has had its highs and lows. Many questions still remain about how βARs regulate adipocyte metabolism and the signaling molecules through which they do it.

Saponins are responsible for the anti-obesogenic activity of Acacia concinna

Acacia concinna (Willd.) DC. is a medicinal plant sourced mainly from Southeast Asia. The pods of Acacia concinna (A. concinna) are a potential candidate to treat or prevent obesity; however, these medicinal attributes have not been examined in detail. In this study, the anti-obesogenic compounds in A. concinna pods were investigated. Chromatographic separation of the pod extract guided by pancreatic lipase inhibitory activity led to the isolation of saponins. Decomposition analysis of the saponins revealed their chemical composition to be acacic acid, monoterpenes, and five types of sugars (glucose, xylose, rhamnose, quinovose, arabinose). The predicted structures of the saponins from decomposition analysis were confirmed by LC-MS analysis, showing that these saponins are mixture of various derivatives of monoterpenes and sugar units. These saponins inhibited pancreatic lipase activity strongly with an IC₅₀ of 7.9 μg/mL, and reduced lipid accumulation in 3T3-L1 adipocytes at 6.3 μg/mL. The saponins also enhanced lipolysis of 3T3-L1 adipocytes at 3.1 or 6.3 μg/mL by mediating the activity of protein kinase A and extracellular signal-regulated kinase pathways, suggesting that this mechanism is partly responsible for the observed reduction of lipid content in adipocytes. The results underline A. concinna as a potential source of the anti-obesogenic candidates for the future treatment and prevention of obesity.

Impact of Conventional and Atypical MAPKs on the Development of Metabolic Diseases

The family of mitogen-activated protein kinases (MAPKs) consists of fourteen members and has been implicated in regulation of virtually all cellular processes. MAPKs are divided into two groups, conventional and atypical MAPKs. Conventional MAPKs are further classified into four sub-families: extracellular signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK1, 2 and 3), p38 (α, β, γ, δ), and extracellular signal-regulated kinase 5 (ERK5). Four kinases, extracellular signal-regulated kinase 3, 4, and 7 (ERK3, 4 and 7) as well as Nemo-like kinase (NLK) build a group of atypical MAPKs, which are activated by different upstream mechanisms than conventional MAPKs. Early studies identified JNK1/2 and ERK1/2 as well as p38α as a central mediators of inflammation-evoked insulin resistance. These kinases have been also implicated in the development of obesity and diabetes. Recently, other members of conventional MAPKs emerged as important mediators of liver, skeletal muscle, adipose tissue, and pancreatic β-cell metabolism. Moreover, latest studies indicate that atypical members of MAPK family play a central role in the regulation of adipose tissue function. In this review, we summarize early studies on conventional MAPKs as well as recent findings implicating previously ignored members of the MAPK family. Finally, we discuss the therapeutic potential of drugs targeting specific members of the MAPK family.

A Receptor of the Immunoglobulin Superfamily Regulates Adaptive Thermogenesis

Exquisite regulation of energy homeostasis protects from nutrient deprivation but causes metabolic dysfunction upon nutrient excess. In human and murine adipose tissue, the accumulation of ligands of the receptor for advanced glycation end products (RAGE) accompanies obesity, implicating this receptor in energy metabolism. Here, we demonstrate that mice bearing global- or adipocyte-specific deletion of Ager, the gene encoding RAGE, display superior metabolic recovery after fasting, a cold challenge, or high-fat feeding. The RAGE-dependent mechanisms were traced to suppression of protein kinase A (PKA)-mediated phosphorylation of its key targets, hormone-sensitive lipase and p38 mitogen-activated protein kinase, upon β-adrenergic receptor stimulation—processes that dampen the expression and activity of uncoupling protein 1 (UCP1) and thermogenic programs. This work identifies the innate role of RAGE as a key node in the immunometabolic networks that control responses to nutrient supply and cold challenges, and it unveils opportunities to harness energy expenditure in environmental and metabolic stress.

Hurtado del Pozo et al. show that the deletion of adipocyte RAGE, whose ligands accumulate in metabolic stress, protects from obesity and cold challenges through the modulation of protein kinase A activities. This work adds RAGE to the immunometabolic networks that regulate energy expenditure in environmental and metabolic stress.

Adrenal cortex hypoxia modulates aldosterone production in heart failure

Plasma aldosterone concentration increases in proportion to the severity of heart failure, even during treatment with renin-angiotensin system inhibitors. This study investigated alternative regulatory mechanisms of aldosterone production that are significant in heart failure. Dahl salt-sensitive rats on a high-salt diet, a rat model of heart failure with cardio-renal syndrome, had high plasma aldosterone levels and elevated β3-adrenergic receptor expression in hypoxic zona glomerulosa cells. In H295R cells (a human adrenocortical cell line), hypoxia-induced β3-adrenergic receptor expression. Hypoxia-mediated β3-adrenergic receptor expression augmented aldosterone production by facilitating hydrolysis of lipid droplets though ERK-mediated phosphorylation of hormone-sensitive lipase, also known as cholesteryl ester hydrolase. Hypoxia also accelerated the synthesis of cholesterol esters by acyl-CoA:cholesterol acyltransferase, thereby increasing the cholesterol ester content in lipid droplets. Thus, hypoxia enhanced aldosterone production by zona glomerulosa cells via promotion of the accumulation and hydrolysis of cholesterol ester in lipid droplets. In conclusion, hypoxic zona glomerulosa cells with heart failure show enhanced aldosterone production via increased catecholamine responsiveness and activation of cholesterol trafficking, irrespective of the renin-angiotensin system.

Foxp1 controls brown/beige adipocyte differentiation and thermogenesis through regulating β3-AR desensitization

β-Adrenergic receptor (β-AR) signaling is a pathway controlling adaptive thermogenesis in brown or beige adipocytes. Here we investigate the biological roles of the transcription factor Foxp1 in brown/beige adipocyte differentiation and thermogenesis. Adipose-specific deletion of Foxp1 leads to an increase of brown adipose activity and browning program of white adipose tissues. The Foxp1-deficient mice show an augmented energy expenditure and are protected from diet-induced obesity and insulin resistance. Consistently, overexpression of Foxp1 in adipocytes impairs adaptive thermogenesis and promotes diet-induced obesity. A robust change in abundance of the β3-adrenergic receptor (β3-AR) is observed in brown/beige adipocytes from both lines of mice. Molecularly, Foxp1 directly represses β3-AR transcription and regulates its desensitization behavior. Taken together, our findings reveal Foxp1 as a master transcriptional repressor of brown/beige adipocyte differentiation and thermogenesis, and provide an important clue for its targeting and treatment of obesity.

Beta3-adrenergic receptor (b3-AR) signaling in response to cold activates adipose tissue thermogenesis. Here the authors identify the transcription factor FoxP1 as a direct negative regulator of b3-AR expression and show that loss of FoxP1 leads to enhanced development of thermogenic adipose tissue.

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

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

A Unique Role of Carboxylesterase 3 (Ces3) in β-Adrenergic Signaling-Stimulated Thermogenesis

Carboxylesterase 3 (Ces3) is a hydrolase with a wide range of activities in liver and adipose tissue. In this study, we identified Ces3 as a major lipid droplet surface-targeting protein in adipose tissue upon cold exposure by liquid chromatography-tandem mass spectrometry. To investigate the function of Ces3 in the β-adrenergic signaling-activated adipocytes, we applied WWL229, a specific Ces3 inhibitor, or genetic inhibition by siRNA to Ces3 on isoproterenol (ISO)-treated 3T3-L1 and brown adipocyte cells. We found that blockage of Ces3 by WWL229 or siRNA dramatically attenuated the ISO-induced lipolytic effect in the cells. Furthermore, Ces3 inhibition led to impaired mitochondrial function measured by Seahorse. Interestingly, Ces3 inhibition attenuated an ISO-induced thermogenic program in adipocytes by downregulating Ucp1 and Pgc1α genes via peroxisome proliferator-activated receptor γ. We further confirmed the effects of Ces3 inhibition in vivo by showing that the thermogenesis in adipose tissues was significantly attenuated in WWL229-treated or adipose tissue-specific Ces3 heterozygous knockout (Adn-Cre-Ces3^flx/wt) mice. As a result, the mice exhibited dramatically impaired ability to defend their body temperature in coldness. In conclusion, our study highlights a lipolytic signaling induced by Ces3 as a unique process to regulate thermogenesis in adipose tissue.

Everything You Always Wanted to Know about β3-AR * (* But Were Afraid to Ask)

The beta-3 adrenergic receptor (β₃-AR) is by far the least studied isotype of the beta-adrenergic sub-family. Despite its study being long hampered by the lack of suitable animal and cellular models and inter-species differences, a substantial body of literature on the subject has built up in the last three decades and the physiology of β₃-AR is unraveling quickly. As will become evident in this work, β₃-AR is emerging as an appealing target for novel pharmacological approaches in several clinical areas involving metabolic, cardiovascular, urinary, and ocular disease. In this review, we will discuss the most recent advances regarding β₃-AR signaling and function and summarize how these findings translate, or may do so, into current clinical practice highlighting β₃-AR’s great potential as a novel therapeutic target in a wide range of human conditions.

Emerging Roles of Sympathetic Nerves and Inflammation in Perivascular Adipose Tissue

Perivascular adipose tissue (PVAT) is no longer recognised as simply a structural support for the vasculature, and we now know that PVAT releases vasoactive factors which modulate vascular function. Since the discovery of this function in 1991, PVAT research is rapidly growing and the importance of PVAT function in disease is becoming increasingly clear. Obesity is associated with a plethora of vascular conditions; therefore, the study of adipocytes and their effects on the vasculature is vital. PVAT contains an adrenergic system including nerves, adrenoceptors and transporters. In obesity, the autonomic nervous system is dysfunctional; therefore, sympathetic innervation of PVAT may be the key mechanistic link between increased adiposity and vascular disease. In addition, not all obese people develop vascular disease, but a common feature amongst those that do appears to be the inflammatory cell population in PVAT. This review will discuss what is known about sympathetic innervation of PVAT, and the links between nerve activation and inflammation in obesity. In addition, we will examine the therapeutic potential of exercise in sympathetic stimulation of adipose tissue.

The weight of nutrients: kynurenine metabolites in obesity and exercise

Obesity ultimately results from an imbalance between energy intake and expenditure. However, in addition to their bioenergetic value, nutrients and their metabolites can function as important signalling molecules in energy homeostasis. Indeed, macronutrients and their metabolites can be direct regulators of metabolism through their actions on different organs. In turn, target organs can decide to use, store or transform the incoming nutrients depending on their physiological context and in coordination with other cell types. Tryptophan-kynurenine metabolites are an example of a family of compounds that can serve as systemic integrators of energy metabolism by signalling to different cell types. These include adipocytes, immune cells and muscle fibres, in addition to the well-known effects of kynurenine metabolites on the central nervous system. In the context of energy metabolism, several of the effects elicited by kynurenic acid are mediated by the G-protein-coupled receptor, GPR35. As GPR35 is expressed in tissues such as the adipose tissue, immune cells and the gastrointestinal tract, this receptor could be a potential therapeutic target for the treatment of obesity, diabetes and other metabolic diseases. In addition, metabolic disorders often coincide with states of chronic inflammation, which further highlights GPR35 as an integration node in conditions where inflammation skews metabolism. Defining the molecular interplay between different tissues in the regulation of energy homeostasis can help us understand interindividual variability in the response to nutrient intake and develop safe and efficient therapies to fight obesity and metabolic disease.

Physiology and pathophysiology of the β3-adrenergic receptor

The β₃-adrenergic receptor (β₃-AR) is an important regulator of various physiological functions, such as thermogenesis in brown adipose tissue, lipolysis in white adipose tissue, negative inotropic effect in cardiomyocyte, and relaxation in blood vessel. The activation of β₃-AR by its agonists is shown to have metabolic (antiobesity and antidiabetic) and cardiovascular effects in animal models, highlighting β₃-AR as a potential therapeutic target in the treatment of several human diseases. Moreover, a substantial number of studies performed on different populations have identified some β₃-AR polymorphic variants associated with obesity, diabetes, cardiovascular diseases, and other disorders. The clinical phenotypes and functional characteristics of these variants provide insights into potential pathophysiological roles of β₃-AR in the development of these diseases.

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.

β3 -Adrenoceptor stimulation of perivascular adipocytes leads to increased fat cell-derived NO and vascular relaxation in small arteries

Background and Purpose

In response to noradrenaline, healthy perivascular adipose tissue (PVAT) exerts an anticontractile effect on adjacent small arterial tissue. Organ bath solution transfer experiments have demonstrated the release of PVAT‐derived relaxing factors that mediate this function. The present studies were designed to investigate the mechanism responsible for the noradrenaline‐induced PVAT anticontractile effect.

Experimental Approach

In vitro rat small arterial contractile function was assessed using wire myography in the presence and absence of PVAT and the effects of sympathomimetic stimulation on the PVAT environment explored using Western blotting and assays of organ bath buffer.

Key Results

PVAT elicited an anticontractile effect in response to noradrenaline but not phenylephrine stimulation. In arteries surrounded by intact PVAT, the β₃‐adrenoceptor agonist, CL‐316243, reduced the vasoconstrictor effect of phenylephrine but not noradrenaline. K_v7 channel inhibition using XE 991 reversed the noradrenaline‐induced anticontractile effect in exogenously applied PVAT studies. Adrenergic stimulation of PVAT with noradrenaline and CL‐316243, but not phenylephrine, was associated with increased adipocyte‐derived NO production, and the contractile response to noradrenaline was augmented following incubation of exogenous PVAT with L‐NMMA. PVAT from eNOS^−/− mice had no anticontractile effect. Assays of adipocyte cAMP demonstrated an increase with noradrenaline stimulation implicating Gα_s signalling in this process.

Conclusions and Implications

We have shown that adipocyte‐located β₃‐adrenoceptor stimulation leads to activation of Gα_s signalling pathways with increased cAMP and the release of adipocyte‐derived NO. This process is dependent upon K_v7 channel function. We conclude that adipocyte‐derived NO plays a central role in anticontractile activity when rodent PVAT is stimulated by noradrenaline.

Phosphorylation of Beta-3 adrenergic receptor at serine 247 by ERK MAP kinase drives lipolysis in obese adipocytes

OBJECTIVE

The inappropriate release of free fatty acids from obese adipose tissue stores has detrimental effects on metabolism, but key molecular mechanisms controlling FFA release from adipocytes remain undefined. Although obesity promotes systemic inflammation, we find activation of the inflammation-associated Mitogen Activated Protein kinase ERK occurs specifically in adipose tissues of obese mice, and provide evidence that adipocyte ERK activation may explain exaggerated adipose tissue lipolysis observed in obesity.

METHODS AND RESULTS

We provide genetic and pharmacological evidence that inhibition of the MEK/ERK pathway in human adipose tissue, mice, and flies all effectively limit adipocyte lipolysis. In complementary findings, we show that genetic and obesity-mediated activation of ERK enhances lipolysis, whereas adipose tissue specific knock-out of ERK2, the exclusive ERK1/2 protein in adipocytes, dramatically impairs lipolysis in explanted mouse adipose tissue. In addition, acute inhibition of MEK/ERK signaling also decreases lipolysis in adipose tissue and improves insulin sensitivity in obese mice. Mice with decreased rates of adipose tissue lipolysis in vivo caused by either MEK or ATGL pharmacological inhibition were unable to liberate sufficient White Adipose Tissue (WAT) energy stores to fuel thermogenesis from brown fat during a cold temperature challenge. To identify a molecular mechanism controlling these actions, we performed unbiased phosphoproteomic analysis of obese adipose tissue at different time points following acute pharmacological MEK/ERK inhibition. MEK/ERK inhibition decreased levels of adrenergic signaling and caused de-phosphorylation of the β3-adrenergic receptor (β3AR) on serine 247. To define the functional implications of this phosphorylation, we showed that CRISPR/Cas9 engineered cells expressing wild type β3AR exhibited β3AR phosphorylation by ERK2 and enhanced lipolysis, but this was not seen when serine 247 of β3AR was mutated to alanine.

CONCLUSION

Taken together, these data suggest that ERK activation in adipocytes and subsequent phosphorylation of the β3AR on S247 are critical regulatory steps in the enhanced adipocyte lipolysis of obesity.

Role of Sympathetic Nerves and Adipocyte Catecholamine Uptake in the Vasorelaxant Function of Perivascular Adipose Tissue

OBJECTIVE

Healthy perivascular adipose tissue (PVAT) exerts an anticontractile effect on resistance arteries which is vital in regulating arterial tone. Activation of β₃-adrenoceptors by sympathetic nerve-derived NA (noradrenaline) may be implicated in this effect and may stimulate the release of the vasodilator adiponectin from adipocytes. Understanding the mechanisms responsible is vital for determining how PVAT may modify vascular resistance in vivo.

APPROACH AND RESULTS

Electrical field stimulation profiles of healthy C57BL/6J mouse mesenteric resistance arteries were characterized using wire myography. During electrical field stimulation, PVAT elicits a reproducible anticontractile effect, which is endothelium independent. To demonstrate the release of an anticontractile factor, the solution surrounding stimulated exogenous PVAT was transferred to a PVAT-denuded vessel. Post-transfer contractility was significantly reduced confirming that stimulated PVAT releases a transferable anticontractile factor. Sympathetic denervation of PVAT using tetrodotoxin or 6-hydroxydopamine completely abolished the anticontractile effect. β₃-adrenoceptor antagonist SR59203A reduced the anticontractile effect, although the PVAT remained overall anticontractile. When the antagonist was used in combination with an OCT3 (organic cation transporter 3) inhibitor, corticosterone, the anticontractile effect was completely abolished. Application of an adiponectin receptor-1 blocking peptide significantly reduced the anticontractile effect in +PVAT arteries. When used in combination with the β₃-adrenoceptor antagonist, there was no further reduction. In adiponectin knockout mice, the anticontractile effect is absent.

CONCLUSIONS

The roles of PVAT are 2-fold. First, sympathetic stimulation in PVAT triggers the release of adiponectin via β₃-adrenoceptor activation. Second, PVAT acts as a reservoir for NA, preventing it from reaching the vessel and causing contraction.

Prior Endurance Training Enhances Beta-Adrenergic Signaling in Epidydimal Adipose from Mice Fed a High-Fat Diet

OBJECTIVE

Adipose tissue beta-adrenergic signaling is attenuated in obesity and insulin resistance. It has been previously demonstrated that prior exercise training protects against short-term, high-fat diet (HFD)-induced weight gain and glucose intolerance. This study aimed to determine whether prior exercise training results in altered beta-adrenergic and lipolytic signaling in adipose tissue when challenged with a HFD.

METHODS

Male C57BL/6J mice underwent 4 weeks of treadmill training (1 h/d, 5 d/wk). Twenty-four hours after the final bout of exercise, mice were fed a HFD (60% kcal lard) for 4 days.

RESULTS

Serum fatty acids, beta-adrenergic signaling (phosphorylated ERK, hormone-sensitive lipase, and p38), and perilipin 1 content were greater in epididymal white adipose tissue (eWAT) from previously trained mice. These changes were not evident in eWAT from trained mice prior to the HFD and were not secondary to alterations in insulin responsiveness or catecholamine concentrations. CL 316,243-mediated increases in hormone-sensitive lipase phosphorylation and fatty acid accumulation in the media were greater in adipose tissue explants from previously trained mice fed a HFD.

CONCLUSIONS

These findings suggest that previous training increases adipose tissue beta-adrenergic responsiveness to a short-term HFD. This may help to explain the protective effect of prior exercise training against the deleterious effects of a HFD.

Activation of mTORC1 is essential for β-adrenergic stimulation of adipose browning

A classic metabolic concept posits that insulin promotes energy storage and adipose expansion, while catecholamines stimulate release of adipose energy stores by hydrolysis of triglycerides through β-adrenergic receptor (βARs) and protein kinase A (PKA) signaling. Here, we have shown that a key hub in the insulin signaling pathway, activation of p70 ribosomal S6 kinase (S6K1) through mTORC1, is also triggered by PKA activation in both mouse and human adipocytes. Mice with mTORC1 impairment, either through adipocyte-specific deletion of Raptor or pharmacologic rapamycin treatment, were refractory to the well-known βAR-dependent increase of uncoupling protein UCP1 expression and expansion of beige/brite adipocytes (so-called browning) in white adipose tissue (WAT). Mechanistically, PKA directly phosphorylated mTOR and RAPTOR on unique serine residues, an effect that was independent of insulin/AKT signaling. Abrogation of the PKA site within RAPTOR disrupted βAR/mTORC1 activation of S6K1 without affecting mTORC1 activation by insulin. Conversely, a phosphomimetic RAPTOR augmented S6K1 activity. Together, these studies reveal a signaling pathway from βARs and PKA through mTORC1 that is required for adipose browning by catecholamines and provides potential therapeutic strategies to enhance energy expenditure and combat metabolic disease.

The role of genes involved in lipolysis on weight loss program in overweight and obese individuals

The ability of obese people to reduce weight in the same treatment varied. Genetic make up as well as the behavioral changes are important for the successfulness of the program. One of the most proposed genetic variations that have been reported in many intervention studies was genes that control lipolysis process. This review summarizes studies that were done showing the influence of genetic polymorphisms in lipolysis pathway and weight loss in a weight loss treatment program. Some studies had shown that certain enzymes involved in this process were related to successfulness of weight loss program. Single Nucleotide Polymorphism (SNP) in PLIN (11482G>A) and ADRB3 (Trp64Arg) are the most studied polymorphisms that have effect on weight loss intervention. However, those studies were not conclusive because of limited number of subjects used and controversies in the results. Thus, replication and confirmation on the role of those genes in weight loss are important due to their potential to be used as predictors of the results of the program.

QRFP-43 inhibits lipolysis by preventing ligand-induced complex formation between perilipin A, caveolin-1, the catalytic subunit of protein kinase and hormone-sensitive lipase in 3T3-L1 adipocytes

QRFP (RFamide) peptides are neuropeptides involved in food intake and adiposity regulation in rodents. We have previously shown that QRFP-43 (43RFa) and QRFP-26 (26RFa) inhibited isoproterenol (ISO)-induced lipolysis in adipocytes. However, the antilipolytic signaling pathways activated by QRFP peptides have not been investigated. In the present study, 3T3-L1 adipocytes were used to identify the main pathways involved in QRFP-43 decreasing ISO-induced lipolysis. Our results show that QRFP-43 reduced ISO-induced phosphorylation of perilipin A (PLIN) and hormone-sensitive lipase (HSL) on Ser660 by 43 and 25%, respectively, but increased Akt phosphorylation by 44%. However, the inhibition of phosphodiesterase 3B (PDE3B), a regulator of lipolysis activated by Akt, did not reverse the antilipolytic effect of QRFP-43. PDE3B inhibition reversed the decrease of Ser660 HSL phosphorylation associated with QRFP-43 antilipolytic effect. QRFP-43 also prevented PKC activation and ISO-induced Src kinases activation leading to the inhibition of the caveolin-1 (CAV-1) translocation on lipid droplets. Indeed, QRFP-43 attenuated phorbol 12-myristate 13-acetate-induced lipolysis and ISO-induced extracellular signal-regulated and Src kinases by 28, 37 and 48%, respectively. The attenuation of ISO-induced lipolysis by QRFP-43 was associated with a decrease of phosphorylated Ser660 HSL, PKA-catalytic (PKA-c) subunit and CAV-1 translocation on lipid droplets by 37, 50 and 46%, respectively. The decrease in ISO-induced CAV-1 and PKA-c translocation was associated with a reduction of PLIN phosphorylation by 44% in QRFP-43-treated adipocytes. These results suggest that QRFP-43 attenuated ISO-induced lipolysis by preventing the formation of an active complex on lipid droplets and the activation of Src kinases and PKC.

Brown, beige, and white: the new color code of fat and its pharmacological implications

Brown adipose tissue (BAT) was previously regarded as a special type of fat relevant only for defending hibernating animals and newborns against a cold environment. Recently, BAT has received considerable attention following its (re)discovery in humans. Using glucose tracers, multiple laboratories independently found metabolically active BAT in adults. The enormous metabolic powers of BAT in animal models could make it an attractive target for antiobesity therapies in humans. Here, we review the present knowledge on the role of BAT in energy homeostasis and metabolism, focusing on signaling pathways and potential targets for novel therapeutics. We also shine light on ongoing debates, including those about the true color of brown fat in adults, as well as on the requirements for translation of basic research on BAT into clinical medicine.

Structural alteration of cell surface heparan sulfate through the stimulation of the signaling pathway for heparan sulfate 6-O-sulfotransferase-1 in mouse fibroblast cells

Heparan sulfate (HS) is a randomly sulfated polysaccharide that is present on the cell surface and in the extracellular matrix. The sulfated structures of HS were synthesized by multiple HS sulfotransferases, thereby regulating various activities such as growth factor signaling, cell differentiation, and tumor metastasis. Therefore, if the sulfated structures of HS could be artificially controlled, those manipulations would help to understand the various functions depending on HS. However, little knowledge is currently available to realize the mechanisms controlling the expression of such enzymes. In this study, we found that the ratio of 6-O-sulfated disaccharides increased at 3 h after adrenaline stimulation in mouse fibroblast cells. Furthermore, adrenaline-induced up-regulation of HS 6-O-sulfotransferase-1 (6-OST-1) was controlled by Src-ERK1/2 signaling pathway. Finally, inhibiting the signaling pathways for 6-OST-1 intentionally suppressed the adrenaline-induced structural alteration of HS. These observations provide fundamental insights into the understanding of structural alterations in HS by extracellular cues.

Acute effect on satiety, resting energy expenditure, respiratory quotient, glucagon-like peptide-1, free fatty acids, and glycerol following consumption of a combination of bioactive food ingredients in overweight subjects

OBJECTIVE

A combination of bioactive food ingredients (capsaicinoids, epigallocatechin gallate, piperin, and l-carnitine, CBFI) may promote satiety and thermogenesis. The study was conducted in order to assess whether there is any effect on satiety, resting energy expenditure (REE), respiratory quotient, glucagon-like peptide-1 (GLP-1), free fatty acids (FFA) and glycerol release, following a standardized mixed meal with or without single consumption of a CBFI.

DESIGN

An 8-week randomized double-blind placebo-controlled trial.

SETTING

Dietetic and Metabolic Unit, Azienda di Servizi alla Persona, University of Pavia and "Villa delle Querce" Clinical Rehabilitation Institute, Rome, Italy.

PARTICIPANTS

Thirty-seven overweight adults (body mass index [BMI]: 25-35).

INTERVENTION

Nineteen overweight subjects were included in the supplemented group (14 women, 5 men; age 46.4 ± 6.4; BMI: 30.5 ± 3.3) and 18 in the placebo group (13 women, 5 men; age 40.8 ± 11.5; BMI: 30.1 ± 2.6). Satiety was assessed using 100-mm visual analogue scales (VAS) and the area under the curve was calculated.

RESULTS

All measured parameters increased significantly in comparison with baseline in response to meal, both with CBFI and with placebo. However, throughout the study day, the supplemented group experienced a significantly greater increase than the placebo group in their sensation of satiety following acute administration of the supplement.

CONCLUSION

CBFI may therefore be of great value in the treatment of overweight patients by increasing satiety and stimulating thermogenesis.

β3-Adrenoceptor activation attenuates atherosclerotic plaque formation in ApoE(-/-) mice through lowering blood lipids and glucose

AIM

To examine the effects of β3-adrenoceptor (β3-AR) activation on atherosclerotic plaque development in ApoE(-/-) mice.

METHODS

Thirty six week-old male ApoE(-/-) mice on a high-fat diet were treated with atorvastatin (10 mg·kg(-1)·d(-1), po), BRL37344 (β3-AR agonist, 1.65 or 3.30 μg/kg, ip, twice a week) or SR52390A (β3-AR antagonist, 50 μg/kg, ip, twice a week) for 12 weeks. Wild-type C57BL/6J mice receiving a normal diet were taken as healthy controls. At the end of the treatments, serum levels of triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), non-high density lipoprotein cholesterol (nHDL-C), glucose and insulin were measured. The thoracic aortas were dissected out, the area of atherosclerotic plaques and extent of fibrosis in the plaques were examined using HE and Masson's trichome staining, respectively.

RESULTS

Compared to wild-type mice, ApoE(-/-) mice fed on a high-fat diet exhibited prominent hyperlipidemia and insulin resistance, associated with large area of atherosclerotic plaques and great extent of fibrosis in aortas. Atorvastatin significantly decreased the serum levels of TC and nHDL-C, and reduced the plaque area and collagen content in aortas. BRL37344 significantly decreased the serum levels of TG, TC, nHDL-C, glucose and insulin, and increased HDL-C and the insulin sensitivity, and dose-dependently reduced the plaque area and collagen content in aortas. SR52390A treatment did not affect any parameters studied.

CONCLUSION

The β3-AR agonist impedes the progression of atherosclerosis in ApoE(-/-) mice, through improvement of the lipid and glucose profiles.

Nr4a1 is required for fasting-induced down-regulation of Pparγ2 in white adipose tissue

Expression of the nuclear receptor gene, Nur77 (Nr4a1), is induced in white adipose tissue (WAT) in response to β-adrenergic stimulation and fasting. Recently, Nur77 has been shown to play a gene regulatory role in the fasting response of several other major metabolic tissues. Here we investigated the effects of Nur77 on the WAT transcriptome after fasting. For this purpose, we performed gene expression profiling of WAT from wild-type and Nur77(-/-) mice submitted to prolonged fasting. Results revealed Nur77-dependent changes in expression profiles of 135 transcripts, many involved in insulin signaling, lipid and fatty acid metabolism, and glucose metabolism. Network analysis identified the deregulated genes Pparγ2 and Nur77 as central hubs and closely connected in the network, indicating overlapping biological function. We further assayed the expression level of Pparγ2 in a bigger cohort of fasted mice and found a significant Nur77-dependent down-regulation of Pparγ2 in the wild-type mice (P = 0.021, n = 10). Consistently, the expression of several known Pparγ2 targets, found among the Nur77-regulated genes (i.e. G0s2, Grp81, Fabp4, and Adipoq), were up-regulated in WAT of fasted Nur77(-/-) mice. Finally, we show with chromatin immunoprecipitation and luciferase assays that the Pparγ2 promoter is a direct target of Nurr-related 77-kDa protein (Nur77)-dependent repressive regulation and that the N-terminal domain of Nur77 is required for this regulation. In conclusion, we present data implicating Nur77 as a mediator of fasting-induced Pparγ2 regulation in WAT.

CD36 level and trafficking are determinants of lipolysis in adipocytes

CD36 has been linked to the etiology of insulin resistance and inflammation. We explored its function in regulating adipose tissue lipolysis, which influences fat accumulation by liver and muscle and overall metabolism. Knockdown of CD36 in differentiated 3T3-L1 adipocytes decreased lipolysis in response to 10 μM of the β-adrenergic agonist isoproterenol (by 42%), 10 μM of the adenyl cyclase activator forskolin (by 32%), and 500 μM of the phosphodiesterase (PDE) inhibitor isobutylmethylxanthine (by 33%). All three treatments in the knockdown adipocytes were associated with significant decreases of cAMP levels and of the hormone-sensitive lipase (HSL) and perilipin phosphorylation. An important role for PDE was supported by the lack of inhibition of the lipolysis induced by the poorly hydrolyzable dibutyryl cAMP analog. An additional contributory mechanism was diminished activation of the Src-ERK1/2 pathway. Regulation of lipolysis and lipolytic signaling by CD36 was reproduced with adipose tissue from CD36(-/-) mice. The importance of surface CD36 in this regulation was suggested by the finding that the plasma membrane-impermeable CD36 inhibitor sulfo-N-succinimidyl oleate (20 μM) decreased lipolysis. Interestingly, isoproterenol induced CD36 internalization, and this process was blocked by HSL inhibition, suggesting feedback regulation of adipocyte lipolysis via CD36 trafficking.

Perivascular adipose tissue from human systemic and coronary vessels: the emergence of a new pharmacotherapeutic target

Fat cells or adipocytes are distributed ubiquitously throughout the body and are often regarded purely as energy stores. However, recently it has become clear that these adipocytes are engine rooms producing large numbers of metabolically active substances with both endocrine and paracrine actions. White adipocytes surround almost every blood vessel in the human body and are collectively termed perivascular adipose tissue (PVAT). It is now well recognized that PVAT not only provides mechanical support for any blood vessels it invests, but also secretes vasoactive and metabolically essential cytokines known as adipokines, which regulate vascular function. The emergence of obesity as a major challenge to our healthcare systems has contributed to the growing interest in adipocyte dysfunction with a view to discovering new pharmacotherapeutic agents to help rescue compromised PVAT function. Very few PVAT studies have been carried out on human tissue. This review will discuss these and the hypotheses generated from such research, as well as highlight the most significant and clinically relevant animal studies showing the most pharmacological promise.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Characterization of murine melanocortin receptors mediating adipocyte lipolysis and examination of signalling pathways involved

The melanocortin receptors (MCRs) belong to the G-protein coupled receptors (family A). So far, 5 different subtypes have been described (MC1R-MC5R) and of these MC2R and MC5R have been proposed to act directly in adipocytes and regulate lipolysis in rodents. Using ACTH and α-melanocyte stimulating hormone (α-MSH) generated from proopiomelanocortin (POMC), as well as synthetic MSH analogues to stimulate lipolysis in murine 3T3-L1 adipocytes it is shown that MC2R and MC5R are lipolytic mediators in differentiated 3T3-L1 adipocytes. Involvement of cAMP, phosphorylated extracellular signal-regulated kinase (ERK) 1/2, protein kinase B (PKB), adenosine 5' monophosphate activated protein kinase (AMPK) and Jun-amino-terminal kinase (JNK) in MCR mediated lipolysis were studied. Interestingly, results obtained in 3T3-L1 cells suggest that lipolysis stimulated by α-MSH, NDP-α-MSH, MT-II, SHU9119 and PG-901 is mediated through MC5R in a cAMP independent manner. Finally, we identify essential differences in MCR mediated lipolysis when using 3T3-L1 cells compared to primary adipocytes.

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.

Measurement of inverse agonism in β-adrenoceptors

Increasing numbers of compounds, previously classified as antagonists, were shown to inhibit this spontaneous or constitutive receptor activity, instead of leave it unaffected as expected for a formal antagonist. In addition, some other antagonists did not have any effect by themselves, but prevented the inhibition of constitutive activity induced by thought-to-be antagonists. These thought-to-be antagonists with negative efficacy are now known as "inverse agonists." Inverse agonism at βAR has been evidenced for both subtypes in wild-type GPCRs systems and in engineered systems with high constitutive activity. It is important to mention that native systems are of particular importance for analyzing the in vivo relevance of constitutive activity because these systems have physiological expression levels of target receptors. Studies of inverse agonism of β blockers in physiological setting have also evidenced that pathophysiological conditions can affect pharmacodynamic properties of these ligands. To date, hundreds of clinically well-known drugs have been tested and classified for this property. Prominent examples include the beta-blockers propranolol, alprenolol, pindolol, and timolol used for treating hypertension, angina pectoris, and arrhythmia that act on the β₂ARs, metoprolol, and bisoprolol used for treating hypertension, coronary heart disease, and arrhythmias by acting on β₁ARs. Inverse agonists seem to be useful in the treatment of chronic disease characterized by harmful effects resulting from β₁AR and β₂AR overactivation, such as heart failure and asthma, respectively.

Positive and negative control of Ucp1 gene transcription and the role of β-adrenergic signaling networks

Adrenergic receptor signaling in adipocytes controls not only the hydrolysis of triglycerides as fuel for other organs but is also a driver of brown adipocyte thermogenesis and energy consumption. As the appearance of these mitochondria-rich, thermogenically active cells in 'white' adipocyte depots is correlated with resistance to overnutrition and glucose intolerance, the molecular basis of their genesis and metabolic activity needs to be understood. β-adrenergic receptors regulate the enzymatic machinery for lipolysis and fuel utilization. They also coordinately stimulate the transcription of genes that support the specific functions of white and brown adipocytes. They accomplish this through the activation of a network of signaling pathways that include cAMP-dependent protein kinase and members of the mitogen-activated protein kinase family. In brown adipocytes, these kinases control the transcription of nuclear factors such as peroxisome proliferator-activated receptor-γ coactivator-1s, as well as other molecules discovered to respond to adrenergic signals, to increase mitochondrial biogenesis and uncoupling protein-1 (UCP1) expression. However, it is also important to understand the mechanisms that may actively repress these energy-wasting processes. Toward that end, we provide evidence for an important role for the nuclear receptor LXRα as a cAMP- and oxysterol-dependent transcriptional repressor of the Ucp1 gene. Adipocytes from LXRα-null mice have increased expression of most 'markers' of brown adipocytes, increased mitochondrial mass and uncoupled respiration. These studies reveal potential new targets and directions for controlling the relative levels of white versus brown adipocytes as a means of metabolic fuel utilization in the struggle against obesity and related metabolic diseases.

β-Adrenoceptor Signaling Networks in Adipocytes for Recruiting Stored Fat and Energy Expenditure

THE ADIPOCYTE IS LIKE A BANK: a place to store excess (caloric) cash in times of plenty, and from which one can withdraw savings during "lean times." The β-adrenoceptors (βAR) are the gateways to this mobilization of fat to be consumed in other tissues. This review discusses the βAR signaling pathway(s) in white and brown adipocytes. Studies in rodent models show that brown adipocytes nestled with white fat depots correlate with and are considered a key enabling factor in resistance to diet-induced obesity. Since it is now recognized that adult humans have brown adipocytes, knowing the steps in these signaling pathways may provide the opportunity to manipulate adipocytes to be net consumers of energy.

Bombyx adipokinetic hormone receptor activates extracellular signal-regulated kinase 1 and 2 via G protein-dependent PKA and PKC but β-arrestin-independent pathways

Neuropeptides of the adipokinetic hormone (AKH) family are among the best studied hormone peptides. They play important roles in insect hemolymph sugar homeostasis, larval lipolysis, and storage-fat mobilization. Mechanistic investigations have shown that, upon AKH stimulation, adipokinetic hormone receptor (AKHR) couples to a Gs protein and enhances adenylate cyclase activity, leading to intracellular cAMP accumulation. However, the underlying molecular mechanism by which this signaling pathway connects to extracellular signal-regulated kinase 1/2 (ERK1/2) remains to be elucidated. Using HEK293 cells stably or transiently expressing AKHR, we demonstrated that activation of AKHR elicited transient phosphorylation of ERK1/2. Our investigation indicated that AKHR-mediated activation of ERK1/2 was significantly inhibited by H-89 (protein kinase A inhibitor), Go6983, and GF109203X (protein kinase C inhibitors) but not by U73122 (PLC inhibitor) or FIPI (PLD inhibitor). Moreover, AKHR-induced ERK1/2 phosphorylation was blocked by the calcium chelators EGTA and BAPTA-AM. Furthermore, ERK1/2 activation in both transiently and stably AKHR-expressing HEK293 cells was found to be sensitive to pretreatment of pertussis toxin, whereas AKHR-mediated ERK1/2 activation was insensitive to siRNA-induced knockdown of β-arrestins and to pretreatment of inhibitors of EGFR, Src, and PI3K. On the basis of our data, we propose that activated AKHR signals to ERK1/2 primarily via PKA- and calcium-involved PKC-dependent pathways. Our current study provides the first in-depth study defining the mechanisms of AKH-mediated ERK activation through the Bombyx AKHR.

Acute stimulation of white adipocyte respiration by PKA-induced lipolysis

OBJECTIVE

We examined the effect of β-adrenergic receptor (βAR) activation and cAMP-elevating agents on respiration and mitochondrial uncoupling in human adipocytes and probed the underlying molecular mechanisms.

RESEARCH DESIGN AND METHODS

Oxygen consumption rate (OCR, aerobic respiration) and extracellular acidification rate (ECAR, anaerobic respiration) were examined in response to isoproterenol (ISO), forskolin (FSK), and dibutyryl-cAMP (DB), coupled with measurements of mitochondrial depolarization, lipolysis, kinase activities, and gene targeting or knock-down approaches.

RESULTS

ISO, FSK, or DB rapidly increased oxidative and glycolytic respiration together with mitochondrial depolarization in human and mouse white adipocytes. The increase in OCR was oligomycin-insensitive and contingent on cAMP-dependent protein kinase A (PKA)-induced lipolysis. This increased respiration and the uncoupling were blocked by inhibiting the mitochondrial permeability transition pore (PTP) and its regulator, BAX. Interestingly, compared with lean individuals, adipocytes from obese subjects exhibited reduced OCR and uncoupling capacity in response to ISO.

CONCLUSIONS

Lipolysis stimulated by βAR activation or other maneuvers that increase cAMP levels in white adipocytes acutely induces mitochondrial uncoupling and cellular energetics, which are amplified in the absence of scavenging BSA. The increase in OCR is dependent on PKA-induced lipolysis and is mediated by the PTP and BAX. Because this effect is reduced with obesity, further exploration of this uncoupling mechanism will be needed to determine its cause and consequences.

Anti-obesity effect of Morus bombycis root extract: anti-lipase activity and lipolytic effect

AIM OF THE STUDY

This study evaluated anti-obesity effect of the ethanolic extract of Morus bombycis root on lipase activity and lipolysis in adipocytes and adipose tissues.

MATERIALS AND METHODS

Lipase (triacylgycerol acylhydrolase, EC 3.1.1.3) activity was determined by measuring the hydrolysis of p-nitrophenyl butyrate to p-nitrophenol at 405 nm. Lipolytic effects were assayed in fully differentiated 3T3-L1 adipocytes and adipose tissues. In vitro, phosphodiesterase (PDE) activity was also measured.

RESULTS

Morus bombycis root extract exhibited strong anti-lipase activity, with an IC50 value of 2.07 microg/mL.In differentiated adipocytes and adipose tissues, the extract increased lipolytic effects such as decreased intracellular triglyceride and the release of glycerol. Further, the extract inhibited PDE activity in a dose-dependent manner.

CONCLUSION

The present study suggests that Morus bombycis root extract might be of therapeutic interest with respect to the treatment of obesity.

Vimentin is a functional partner of hormone sensitive lipase and facilitates lipolysis

Lipolysis involves a number of components including signaling pathways, droplet-associated proteins, and lipases such as hormone-sensitive lipase (HSL). We used surface enhanced laser desorption/ionization time-of-flight mass spectroscopy to identify cellular proteins that might interact with HSL and potentially influence lipolysis. Using recombinant HSL as bait on protein chips, clusters of proteins of 14.7-18.9, 25.8-26.8, 36.1, 44.3-49.1, and 53.7 kDa were identified that interact with HSL, particularly when lysates were examined from beta-agonist treated mouse adipocytes. The ability to detect these interacting proteins was markedly diminished when the adipocytes were treated with insulin. A very similar pattern of proteins was identified when anti-HSL IgG was used as the bait. Following immunocapture, the identification of the prominent 53.7 kDa protein was carried out by tryptic digestion and MS analysis and determined to be vimentin. The interaction of HSL with vimentin, and its hormonal dependence, was confirmed by coimmunoprecipitation. beta-Agonist stimulated lipolysis and the rate of HSL translocation were impaired in vimentin null adipocytes, even though normal amounts of lipases and droplet-associated proteins are expressed. The current studies provide evidence that vimentin participates in lipolysis through direct, hormonally regulated interactions with HSL.

Beta(3)-adrenergic signaling acutely down regulates adipose triglyceride lipase in brown adipocytes

Mice exposed to cold rely upon brown adipose tissue (BAT)-mediated nonshivering thermogenesis to generate body heat using dietary glucose and lipids from the liver and white adipose tissue. In this report, we investigate how cold exposure affects the PI3 K/Akt signaling cascade and the expression of genes involved in lipid metabolism and trafficking in BAT. Cold exposure at an early time point led to the activation of the PI3 K/Akt, insulin-like signaling cascade followed by a transient decrease in adipose triglyceride lipase (ATGL) gene and protein expression in BAT. To further investigate how cold exposure-induced signaling altered ATGL expression, cultured primary brown adipocytes were treated with the beta(3)-adrenergic receptor (beta(3)AR) agonist CL 316,243 (CL) resulting in activation of PI3 K/Akt, ERK 1/2, and p38 signaling pathways and significantly decreased ATGL protein levels. ATGL protein levels decreased significantly 30 min post CL treatment suggesting protein degradation. Inhibition of PKA signaling by H89 rescued ATGL levels. The effects of PKA signaling on ATGL were shown to be independent of relevant pathways downstream of PKA such as PI3 K/Akt, ERK 1/2, and p38. However, CL treatment in 3T3-L1 adipocytes did not decrease ATGL protein and mRNA expression, suggesting a distinct response in WAT to beta3-adrenergic agonism. Transitory effects, possibly attributed to acute Akt activation during the early recruitment phase, were noted as well as stable changes in gene expression which may be attributed to beta3-adrenergic signaling in BAT.

Cross-regulation between beta 1- and beta 3-adrenoceptors following chronic beta-adrenergic stimulation in neonatal rat cardiomyocytes

BACKGROUND AND PURPOSE

We have previously shown that beta-adrenoceptors continuously stimulated with noradrenaline induces an increase in beta(3)-adrenoceptors (G alpha(i)PCRs) and a decrease in beta(1)-adrenoceptors (G alpha(s)PCRs) at functional, genomic and protein levels. This compensatory modification induced by noradrenaline is probably one of the consequences of cardiac depression observed in heart disease. Therefore, we investigated further the interaction between beta(1)- and beta(3)-adrenoceptors in neonatal rat cardiomyocytes.

EXPERIMENTAL APPROACH

Functional studies were performed by cyclic adenosine monophosphate (cAMP) accumulation assays in cells untreated or treated with dobutamine and ICI 118551 (beta(1)-adrenoceptor) or CL-3162436243 (beta(3)-adrenoceptor) for 24 h in the presence or absence of protein kinase inhibitors. Beta-adrenoceptor and protein kinase expression was monitored by quantitative reverse transcription-polymerase chain reaction (RT-PCR) and by Western blotting, respectively.

KEY RESULTS

Chronic beta(1)- or beta(3)-adrenoceptor stimulation reduced beta(1)-adrenoceptor-mediated cAMP accumulation in association with a decrease in beta(1)-adrenoceptor mRNA and protein levels through protein kinase C (PKC), phosphoinositide 3-kinase (PI3K) and p38 mitogen-activated protein kinase (p38MAPK) activation. In contrast, both treatments induced an increase in beta(3)-adrenoceptor expression and beta(3)-adrenoceptor-inhibited forskolin response through PKC, extracellular-signal-regulated kinases 1 and 2 (ERK1/2) and p38MAPK phosphorylation, although no beta(3)-adrenoceptor response was observed in untreated cells. ERK1/2 and p38MAPK were activated by both treatments. The modulation of beta(1)- or beta(3)-adrenoceptor function did not require stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) although chronic beta(1)-adrenoceptor stimulation activated SAPK/JNK. Beta(3)-adrenoceptor treatment activated Akt although PI3K was not involved in beta(3)-adrenoceptor up-regulation.

CONCLUSION AND IMPLICATIONS

We show for the first time that chronic beta(1)- or beta(3)-adrenoceptor stimulation leads to the modulation of beta(1)- and beta(3)-adrenoceptors by a cross-regulation involving PKC, PI3K p38MAPK and MEK/ERK1/2 pathway, and through protein kinase A when beta(1)-adrenoceptors are chronically activated.

Anti-lipase and lipolytic activities of ursolic acid isolated from the roots of Actinidia arguta

The aim of this study was to investigate the anti-obestic effects of ursolic acid isolated from the roots of Actinidia arguta, as well as the mechanism of action of this compound. This was conducted by testing whether ursolic acid inhibited the elevation of the rat plasma triacylglycerol levels after oral administration of a lipid emulsion containing corn oil in rats. Ursolic acid prevented the elevation of plasma triacylglycerol levels 2 h after oral administration of the lipid emulsion at a dose of 100 mg/kg. Furthermore, ursolic acid inhibited phosphodiesterase activity in vitro with an IC(50) of 51.21 muM and enhanced lipolysis in rat fat cells. We suggest that the inhibitory effects of ursolic acid, isolated from the roots of A. arguta, on obesity, might be attributable to the inhibition of lipid absorption through the inhibition of pancreatic lipase and by enhancing lipolysis in fat cells.

Signaling pathways mediating beta3-adrenergic receptor-induced production of interleukin-6 in adipocytes

The beta(3)-adrenergic receptor (beta(3)AR) is an essential regulator of metabolic and endocrine functions. A major cellular and clinically significant consequence of beta(3)AR activation is the substantial elevation in interleukin-6 (IL-6) levels. Although the beta(3)AR-dependent regulation of IL-6 expression is well established, the cellular pathways underlying this regulation have not been characterized. Using a novel method of homogenous reporters, we assessed the pattern of activation of 43 transcription factors in response to the specific beta(3)AR agonist CL316243 in adipocytes, cells that exhibit the highest expression of beta(3)ARs. We observed a unique and robust activation of the CRE-response element, suggesting that IL-6 transcription is regulated via the G(s)-protein/cAMP/protein kinase A (PKA) but not nuclear factor kappa B (NF-kappaB) pathway. However, pretreatment of adipocytes with pharmacologic inhibitors of PKA pathway failed to block beta(3)AR-mediated IL-6 up-regulation. Additionally, stimulation of adipocytes with the exchange protein directly activated by cAMP (Epac) agonist did not induce IL-6 expression. Instead, the beta(3)AR-mediated transcription of IL-6 required activation of both the p38 and PKC pathways. Western blot analysis further showed that transcription factors CREB and ATF-2 but not ATF-1 were activated in a p38- and PKC-dependent manner. Collectively, our results suggest that while stimulation of the beta(3)AR leads to a specific activation of CRE-dependent transcription, there are several independent cellular pathways that converge at the level of CRE-response element activation, and in the case of IL-6 this activation is mediated by p38 and PKC but not PKA pathways.

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.

Bacterial endotoxin stimulates adipose lipolysis via toll-like receptor 4 and extracellular signal-regulated kinase pathway

Bacterial endotoxin/lipopolysaccharide elicits inflammatory responses and also elevates circulating levels of free fatty acids (FFAs) and impairs insulin sensitivity. Serum FFA elevation in acute endotoxemia has long been thought to be due to endotoxin dysregulating lipid disposal and counterregulatory hormones and cytokines. Here, we investigated the direct lipolysis effect of endotoxin in rodents and in isolated primary adipocytes. Endotoxin increases lipolysis in vivo in adipose tissues, elevates circulating FFA level, induces insulin resistance in rats, and directly stimulates chronic lipolysis in vitro in adipocytes. The lipolytic action of endotoxin is mediated via its lipid A moiety and is blocked by anti-endotoxin peptides. Neither adipocytokine secretion nor nuclear factor-kappaB activation is involved in endotoxin-induced lipolysis. Different from catecholamine, endotoxin stimulates lipolysis without elevating cAMP production and activating protein kinase A and protein kinase C. Instead, endotoxin induces phosphorylation of Raf-1, MEK1/2, and ERK1/2. Upon inhibition of ERK1/2 but not JNK and p38 MAPK, endotoxin-stimulated lipolysis ceases. Endotoxin causes perilipin down-regulation and phosphorylation and increases the activity and protein levels of hormone-sensitive lipase and adipose triglyceride lipase but does not induce hormone-sensitive lipase translocation to intracellular lipid droplets. In TLR4 (Toll-like receptor 4)-deficient mice and adipocytes, endotoxin fails to increase in vivo and in vitro lipolysis. These findings suggest that endotoxin stimulates lipolysis via TLR4 and ERK1/2 signaling in adipocytes. The lipolytic action of endotoxin liberates FFA efflux from adipocytes to the bloodstream, which is a possible basis for systemic FFA elevation and insulin resistance in endotoxemia or Gram-negative bacterial infection.