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

Extensive location bias of the GPCR-dependent translatome via site-selective activation of mTOR

G protein-coupled receptors (GPCRs) modulate various physiological functions by re-wiring cellular gene expression in response to extracellular signals. Control of gene expression by GPCRs has been studied almost exclusively at the transcriptional level, neglecting an extensive amount of regulation that takes place translationally. Hence, little is known about the nature and mechanisms of gene-specific post-transcriptional regulation downstream of receptor activation. Here, we apply an unbiased multiomics approach to delineate an extensive translational regulatory program initiated by the prototypical beta2-adrenergic receptor (β2-AR) and provide mechanistic insights into how these processes are orchestrated. Using ribosome profiling (Ribo-seq), we identify nearly 120 novel gene targets of adrenergic receptor activity which expression is exclusively regulated at the level of translation. We next show that all translational changes are induced selectively by endosomal β2-ARs. We further report that this proceeds through activation of the mammalian target of rapamycin (mTOR) pathway. Specifically, within the set of translational GPCR targets we discover significant enrichment of genes with 5' terminal oligopyrimidine (TOP) motifs, a gene class classically known to be translationally regulated by mTOR. We then demonstrate that endosomal β2-ARs are required for mTOR activation and subsequent mTOR-dependent TOP mRNA translation. Together, this comprehensive analysis of drug-induced translational regulation establishes a critical role for location-biased GPCR signaling in fine-tuning the cellular protein landscape.

CMKLR1 senses chemerin/resolvin E1 to control adipose thermogenesis and modulate metabolic homeostasis

Induction of beige fat for thermogenesis is a potential therapy to improve homeostasis against obesity. β3-adrenoceptor (β3-AR), a type of G protein-coupled receptor (GPCR), is believed to mediate the thermogenesis of brown fat in mice. However, β3-AR has low expression in human adipose tissue, precluding its activation as a standalone clinical modality. This study aimed at identifying a potential GPCR target to induce beige fat. We found that chemerin chemokine-like receptor 1 (CMKLR1), one of the novel GPCRs, mediated the development of beige fat via its two ligands, chemerin and resolvin E1 (RvE1). The RvE1 levels were decreased in the obese mice, and RvE1 treatment led to a substantial improvement in obese features and augmented beige fat markers. Inversely, despite sharing the same receptor as RvE1, the chemerin levels were increased in obesogenic conditions, and chemerin treatment led to an augmented obese phenotype and a decline of beige fat markers. Moreover, RvE1 and chemerin induced or restrained the development of beige fat, respectively, via the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway. We further showed that RvE1 and chemerin regulated mTORC1 signaling differentially by forming hydrogen bonds with different binding sites of CMKLR1. In conclusion, our study showed that RvE1 and chemerin affected metabolic homeostasis differentially, suggesting that selectively modulating CMKLR1 may be a potential therapeutic target for restoring metabolic homeostasis.

mTORC1 in energy expenditure: consequences for obesity

In eukaryotic cells, the mammalian target of rapamycin complex 1 (sometimes referred to as the mechanistic target of rapamycin complex 1; mTORC1) orchestrates cellular metabolism in response to environmental energy availability. As a result, at the organismal level, mTORC1 signalling regulates the intake, storage and use of energy by acting as a hub for the actions of nutrients and hormones, such as leptin and insulin, in different cell types. It is therefore unsurprising that deregulated mTORC1 signalling is associated with obesity. Strategies that increase energy expenditure offer therapeutic promise for the treatment of obesity. Here we review current evidence illustrating the critical role of mTORC1 signalling in the regulation of energy expenditure and adaptive thermogenesis through its various effects in neuronal circuits, adipose tissue and skeletal muscle. Understanding how mTORC1 signalling in one organ and cell type affects responses in other organs and cell types could be key to developing better, safer treatments targeting this pathway in obesity.

mTORC1 hyperactivation and resultant suppression of macroautophagy contribute to the induction of cardiomyocyte necroptosis by catecholamine surges

Previous studies revealed a controversial role of mechanistic target of rapamycin complex 1 (mTORC1) and mTORC1-regulated macroautophagy in isoproterenol (ISO)-induced cardiac injury. Here we investigated the role of mTORC1 and potential underlying mechanisms in ISO-induced cardiomyocyte necrosis. Two consecutive daily injections of ISO (85 mg/kg, s.c.) or vehicle control (CTL) were administered to C57BL/6J mice with or without rapamycin (RAP, 5 mg/kg, i.p.) pretreatment. Western blot analyses showed that myocardial mTORC1 signaling and the RIPK1-RIPK3-MLKL necroptotic pathway were activated, mRNA expression analyses revealed downregulation of representative TFEB target genes, and Evan's blue dye uptake assays detected increased cardiomyocyte necrosis in ISO-treated mice. However, RAP pretreatment prevented or significantly attenuated the ISO-induced cardiomyocyte necrosis, myocardial inflammation, downregulation of TFEB target genes, and activation of the RIPK1-RIPK3-MLKL pathway. LC3-II flux assays confirmed the impairment of myocardial autophagic flux in the ISO-treated mice. In cultured neonatal rat cardiomyocytes, mTORC1 signaling was also activated by ISO, and inhibition of mTORC1 by RAP attenuated ISO-induced cytotoxicity. These findings suggest that mTORC1 hyperactivation and resultant suppression of macroautophagy play a major role in the induction of cardiomyocyte necroptosis by catecholamine surges, identifying mTORC1 inhibition as a potential strategy to treat heart diseases with catecholamine surges.

ATF4 expression in thermogenic adipocytes is required for cold-induced thermogenesis in mice via FGF21-independent mechanisms

In brown adipose tissue (BAT), short-term cold exposure induces the activating transcription factor 4 (ATF4), and its downstream target fibroblast growth factor 21 (FGF21). Induction of ATF4 in BAT in response to mitochondrial stress is required for thermoregulation, partially by increasing FGF21 expression. In the present study, we tested the hypothesis that Atf4 and Fgf21 induction in BAT are both required for BAT thermogenesis under physiological stress by generating mice selectively lacking either Atf4 (ATF4 BKO) or Fgf21 (FGF21 BKO) in UCP1-expressing adipocytes. After 3 days of cold exposure, core body temperature was significantly reduced in ad-libitum-fed ATF4 BKO mice, which correlated with Fgf21 downregulation in brown and beige adipocytes, and impaired browning of white adipose tissue. Conversely, despite having reduced browning, FGF21 BKO mice had preserved core body temperature after cold exposure. Mechanistically, ATF4, but not FGF21, regulates amino acid import and metabolism in response to cold, likely contributing to BAT thermogenic capacity under ad libitum-fed conditions. Importantly, under fasting conditions, both ATF4 and FGF21 were required for thermogenesis in cold-exposed mice. Thus, ATF4 regulates BAT thermogenesis under fed conditions likely in a FGF21-independent manner, in part via increased amino acid uptake and metabolism.

Depletion of JunB increases adipocyte thermogenic capacity and ameliorates diet-induced insulin resistance

The coexistence of brown adipocytes with low and high thermogenic activity is a fundamental feature of brown adipose tissue heterogeneity and plasticity. However, the mechanisms that govern thermogenic adipocyte heterogeneity and its significance in obesity and metabolic disease remain poorly understood. Here we show that in male mice, a population of transcription factor jun-B (JunB)-enriched (JunB+) adipocytes within the brown adipose tissue exhibits lower thermogenic capacity compared to high-thermogenic adipocytes. The JunB+ adipocyte population expands in obesity. Depletion of JunB in adipocytes increases the fraction of adipocytes exhibiting high thermogenic capacity, leading to enhanced basal and cold-induced energy expenditure and protection against diet-induced obesity and insulin resistance. Mechanistically, JunB antagonizes the stimulatory effects of PPARγ coactivator-1α on high-thermogenic adipocyte formation by directly binding to the promoter of oestrogen-related receptor alpha, a PPARγ coactivator-1α downstream effector. Taken together, our study uncovers that JunB shapes thermogenic adipocyte heterogeneity, serving a critical role in maintaining systemic metabolic health.

Glucagon-like peptide-1 receptor activation stimulates PKA-mediated phosphorylation of Raptor and this contributes to the weight loss effect of liraglutide

The canonical target of the glucagon-like peptide-1 receptor (GLP-1R), Protein Kinase A (PKA), has been shown to stimulate mechanistic Target of Rapamycin Complex 1 (mTORC1) by phosphorylating the mTOR-regulating protein Raptor at Ser791 following β-adrenergic stimulation. The objective of these studies is to test whether GLP-1R agonists similarly stimulate mTORC1 via PKA phosphorylation of Raptor at Ser791 and whether this contributes to the weight loss effect of the therapeutic GLP-1R agonist liraglutide. We measured phosphorylation of the mTORC1 signaling target ribosomal protein S6 in Chinese Hamster Ovary cells expressing GLP-1R (CHO-Glp1r) treated with liraglutide in combination with PKA inhibitors. We also assessed liraglutide-mediated phosphorylation of the PKA substrate RRXS*/T* motif in CHO-Glp1r cells expressing Myc-tagged wild-type (WT) Raptor or a PKA-resistant (Ser791Ala) Raptor mutant. Finally, we measured the body weight response to liraglutide in WT mice and mice with a targeted knock-in of PKA-resistant Ser791Ala Raptor. Liraglutide increased phosphorylation of S6 and the PKA motif in WT Raptor in a PKA-dependent manner but failed to stimulate phosphorylation of the PKA motif in Ser791Ala Raptor in CHO-Glp1r cells. Lean Ser791Ala Raptor knock-in mice were resistant to liraglutide-induced weight loss but not setmelanotide-induced (melanocortin-4 receptor-dependent) weight loss. Diet-induced obese Ser791Ala Raptor knock-in mice were not resistant to liraglutide-induced weight loss; however, there was weight-dependent variation such that there was a tendency for obese Ser791Ala Raptor knock-in mice of lower relative body weight to be resistant to liraglutide-induced weight loss compared to weight-matched controls. Together, these findings suggest that PKA-mediated phosphorylation of Raptor at Ser791 contributes to liraglutide-induced weight loss.

Crosstalk between corepressor NRIP1 and cAMP signaling on adipocyte thermogenic programming

OBJECTIVES

Nuclear receptor interacting protein 1 (NRIP1) suppresses energy expenditure via repression of nuclear receptors, and its depletion markedly elevates uncoupled respiration in mouse and human adipocytes. We tested whether NRIP1 deficient adipocytes implanted into obese mice would enhance whole body metabolism. Since β-adrenergic signaling through cAMP strongly promotes adipocyte thermogenesis, we tested whether the effects of NRIP1 knock-out (NRIP1KO) require the cAMP pathway.

METHODS

NRIP1KO adipocytes were implanted in recipient high-fat diet (HFD) fed mice and metabolic cage studies conducted. The Nrip1 gene was disrupted by CRISPR in primary preadipocytes isolated from control vs adipose selective GsαKO (cAdGsαKO) mice prior to differentiation to adipocytes. Protein kinase A inhibitor was also used.

RESULTS

Implanting NRIP1KO adipocytes into HFD fed mice enhanced whole-body glucose tolerance by increasing insulin sensitivity, reducing adiposity, and enhancing energy expenditure in the recipients. NRIP1 depletion in both control and GsαKO adipocytes was equally effective in upregulating uncoupling protein 1 (UCP1) and adipocyte beiging, while β-adrenergic signaling by CL 316,243 was abolished in GsαKO adipocytes. Combining NRIP1KO with CL 316,243 treatment synergistically increased Ucp1 gene expression and increased the adipocyte subpopulation responsive to beiging. Estrogen-related receptor α (ERRα) was dispensable for UCP1 upregulation by NRIPKO.

CONCLUSIONS

The thermogenic effect of NRIP1 depletion in adipocytes causes systemic enhancement of energy expenditure when such adipocytes are implanted into obese mice. Furthermore, NRIP1KO acts independently but cooperatively with the cAMP pathway in mediating its effect on adipocyte beiging.

ATF4 Expression in Thermogenic Adipocytes is Required for Cold-Induced Thermogenesis in Mice via FGF21-Independent Mechanisms

In brown adipose tissue (BAT), short-term cold exposure induces the activating transcription factor 4 (ATF4), and its downstream target fibroblast growth factor 21 (FGF21). Induction of ATF4 in BAT in response to mitochondrial stress is required for thermoregulation, partially via upregulation of FGF21. In the present study, we tested the hypothesis that Atf4 and Fgf21 induction in BAT are both required for BAT thermogenesis by generating mice selectively lacking either Atf4 ( ATF4 BKO ) or Fgf21 (FGF21 BKO) in UCP1-expressing adipocytes. After 3 days of cold exposure, core body temperature was significantly reduced in ad-libitum -fed ATF4 BKO mice, which correlated with Fgf21 downregulation in brown and beige adipocytes, and impaired browning of white adipose tissue (WAT). Conversely, despite having reduced browning, FGF21 BKO mice had preserved core body temperature after cold exposure. Mechanistically, ATF4, but not FGF21, regulates amino acid import and metabolism in response to cold, likely contributing to BAT thermogenic capacity under ad libitum -fed conditions. Importantly, under fasting conditions, both ATF4 and FGF21 were required for thermogenesis in cold-exposed mice. Thus, ATF4 regulates BAT thermogenesis by activating amino acid metabolism in BAT in a FGF21-independent manner.

Amino acids contribute to adaptive thermogenesis. New insights into the mechanisms of action of recent drugs for metabolic disorders are emerging

Adaptive thermogenesis is the heat production by muscle contractions (shivering thermogenesis) or brown adipose tissue (BAT) and beige fat (non-shivering thermogenesis) in response to external stimuli, including cold exposure. BAT and beige fat communicate with peripheral organs and the brain through a variegate secretory and absorption processes - controlling adipokines, microRNAs, extracellular vesicles, and metabolites - and have received much attention as potential therapeutic targets for managing obesity-related disorders. The sympathetic nervous system and norepinephrine-releasing adipose tissue macrophages (ATM) activate uncoupling protein 1 (UCP1), expressed explicitly in brown and beige adipocytes, dissolving the electrochemical gradient and uncoupling tricarboxylic acid cycle and the electron transport chain from ATP production. Mounting evidence has attracted attention to the multiple effects of dietary and endogenously synthesised amino acids in BAT thermogenesis and metabolic phenotype in animals and humans. However, the mechanisms implicated in these processes have yet to be conclusively characterized. In the present review article, we aim to define the principal investigation areas in this context, including intestinal microbiota constitution, adipose autophagy modulation, and secretome and metabolic fluxes control, which lead to increased brown/beige thermogenesis. Finally, also based on our recent epicardial adipose tissue results, we summarise the evidence supporting the notion that the new dual and triple agonists of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon (GCG) receptor - with never before seen weight loss and insulin-sensitizing efficacy - promote thermogenic-like amino acid profiles in BAT with robust heat production and likely trigger sympathetic activation and adaptive thermogenesis by controlling amino acid metabolism and ATM expansion in BAT and beige fat.

The Role of Obesity in Breast Cancer Pathogenesis

Research has shown that obesity increases the risk for type 2 diabetes mellitus (Type 2 DM) by promoting insulin resistance, increases serum estrogen levels by the upregulation of aromatase, and promotes the release of reactive oxygen species (ROS) by macrophages. Increased circulating glucose has been shown to activate mammalian target of rapamycin (mTOR), a significant signaling pathway in breast cancer pathogenesis. Estrogen plays an instrumental role in estrogen-receptor-positive breast cancers. The role of ROS in breast cancer warrants continued investigation, in relation to both pathogenesis and treatment of breast cancer. We aim to review the role of obesity in breast cancer pathogenesis and novel therapies mediating obesity-associated breast cancer development. We explore the association between body mass index (BMI) and breast cancer incidence and the mechanisms by which oxidative stress modulates breast cancer pathogenesis. We discuss the role of glutathione, a ubiquitous antioxidant, in breast cancer therapy. Lastly, we review breast cancer therapies targeting mTOR signaling, leptin signaling, blood sugar reduction, and novel immunotherapy targets.

Salt-inducible kinase inhibition promotes the adipocyte thermogenic program and adipose tissue browning

OBJECTIVE

Norepinephrine stimulates the adipose tissue thermogenic program through a β-adrenergic receptor (βAR)-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling cascade. We discovered that a noncanonical activation of the mechanistic target of rapamycin complex 1 (mTORC1) by PKA is required for the βAR-stimulation of adipose tissue browning. However, the downstream events triggered by PKA-phosphorylated mTORC1 activation that drive this thermogenic response are not well understood.

METHODS

We used a proteomic approach of Stable Isotope Labeling by/with Amino acids in Cell culture (SILAC) to characterize the global protein phosphorylation profile in brown adipocytes treated with the βAR agonist. We identified salt-inducible kinase 3 (SIK3) as a candidate mTORC1 substrate and further tested the effect of SIK3 deficiency or SIK inhibition on the thermogenic gene expression program in brown adipocytes and in mouse adipose tissue.

RESULTS

SIK3 interacts with RAPTOR, the defining component of the mTORC1 complex, and is phosphorylated at Ser884 in a rapamycin-sensitive manner. Pharmacological SIK inhibition by a pan-SIK inhibitor (HG-9-91-01) in brown adipocytes increases basal Ucp1 gene expression and restores its expression upon blockade of either mTORC1 or PKA. Short-hairpin RNA (shRNA) knockdown of Sik3 augments, while overexpression of SIK3 suppresses, Ucp1 gene expression in brown adipocytes. The regulatory PKA phosphorylation domain of SIK3 is essential for its inhibition. CRISPR-mediated Sik3 deletion in brown adipocytes increases type IIa histone deacetylase (HDAC) activity and enhances the expression of genes involved in thermogenesis such as Ucp1, Pgc1α, and mitochondrial OXPHOS complex protein. We further show that HDAC4 interacts with PGC1α after βAR stimulation and reduces lysine acetylation in PGC1α. Finally, a SIK inhibitor well-tolerated in vivo (YKL-05-099) can stimulate the expression of thermogenesis-related genes and browning of mouse subcutaneous adipose tissue.

CONCLUSIONS

Taken together, our data reveal that SIK3, with the possible contribution of other SIKs, functions as a phosphorylation switch for β-adrenergic activation to drive the adipose tissue thermogenic program and indicates that more work to understand the role of the SIKs is warranted. Our findings also suggest that maneuvers targeting SIKs could be beneficial for obesity and related cardiometabolic disease.

Regulation of mTOR Signaling: Emerging Role of Cyclic Nucleotide-Dependent Protein Kinases and Implications for Cardiometabolic Disease

The mechanistic target of rapamycin (mTOR) kinase is a central regulator of cell growth and metabolism. It is the catalytic subunit of two distinct large protein complexes, mTOR complex 1 (mTORC1) and mTORC2. mTOR activity is subjected to tight regulation in response to external nutrition and growth factor stimulation. As an important mechanism of signaling transduction, the 'second messenger' cyclic nucleotides including cAMP and cGMP and their associated cyclic nucleotide-dependent kinases, including protein kinase A (PKA) and protein kinase G (PKG), play essential roles in mediating the intracellular action of a variety of hormones and neurotransmitters. They have also emerged as important regulators of mTOR signaling in various physiological and disease conditions. However, the mechanism by which cAMP and cGMP regulate mTOR activity is not completely understood. In this review, we will summarize the earlier work establishing the ability of cAMP to dampen mTORC1 activation in response to insulin and growth factors and then discuss our recent findings demonstrating the regulation of mTOR signaling by the PKA- and PKG-dependent signaling pathways. This signaling framework represents a new non-canonical regulation of mTOR activity that is independent of AKT and could be a novel mechanism underpinning the action of a variety of G protein-coupled receptors that are linked to the mTOR signaling network. We will further review the implications of these signaling events in the context of cardiometabolic disease, such as obesity, non-alcoholic fatty liver disease, and cardiac remodeling. The metabolic and cardiac phenotypes of mouse models with targeted deletion of Raptor and Rictor, the two essential components for mTORC1 and mTORC2, will be summarized and discussed.

Molecular and cellular regulation of thermogenic fat

Thermogenic fat, consisting of brown and beige adipocytes, dissipates energy in the form of heat, in contrast to the characteristics of white adipocytes that store energy. Increasing energy expenditure by activating brown adipocytes or inducing beige adipocytes is a potential therapeutic strategy for treating obesity and type 2 diabetes. Thus, a better understanding of the underlying mechanisms of thermogenesis provides novel therapeutic interventions for metabolic diseases. In this review, we summarize the recent advances in the molecular regulation of thermogenesis, focusing on transcription factors, epigenetic regulators, metabolites, and non-coding RNAs. We further discuss the intercellular and inter-organ crosstalk that regulate thermogenesis, considering the heterogeneity and complex tissue microenvironment of thermogenic fat.

Genetically prolonged beige fat in male mice confers long-lasting metabolic health

A potential therapeutic target to curb obesity and diabetes is thermogenic beige adipocytes. However, beige adipocytes quickly transition into white adipocytes upon removing stimuli. Here, we define the critical role of cyclin dependent kinase inhibitor 2A (Cdkn2a) as a molecular pedal for the beige-to-white transition. Beige adipocytes lacking Cdkn2a exhibit prolonged lifespan, and male mice confer long-term metabolic protection from diet-induced obesity, along with enhanced energy expenditure and improved glucose tolerance. Mechanistically, Cdkn2a promotes the expression and activity of beclin 1 (BECN1) by directly binding to its mRNA and its negative regulator BCL2 like 1 (BCL2L1), activating autophagy and accelerating the beige-to-white transition. Reactivating autophagy by pharmacological or genetic methods abolishes beige adipocyte maintenance induced by Cdkn2a ablation. Furthermore, hyperactive BECN1 alone accelerates the beige-to-white transition in mice and human. Notably, both Cdkn2a and Becn1 exhibit striking positive correlations with adiposity. Hence, blocking Cdkn2a-mediated BECN1 activity holds therapeutic potential to sustain beige adipocytes in treating obesity and related metabolic diseases.

Hepatic follistatin increases basal metabolic rate and attenuates diet-induced obesity during hepatic insulin resistance

OBJECTIVE

Body weight change and obesity follow the variance of excess energy input balanced against tightly controlled EE (energy expenditure). Since insulin resistance can reduce energy storage, we investigated whether genetic disruption of hepatic insulin signaling reduced adipose mass with increased EE.

METHODS

Insulin signaling was disrupted by genetic inactivation of Irs1 (Insulin receptor substrate 1) and Irs2 in hepatocytes of LDKO mice (Irs1L/L·Irs2L/L·CreAlb), creating a state of complete hepatic insulin resistance. We inactivated FoxO1 or the FoxO1-regulated hepatokine Fst (Follistatin) in the liver of LDKO mice by intercrossing LDKO mice with FoxO1L/L or FstL/L mice. We used DEXA (dual-energy X-ray absorptiometry) to assess total lean mass, fat mass and fat percentage, and metabolic cages to measure EE (energy expenditure) and estimate basal metabolic rate (BMR). High-fat diet was used to induce obesity.

RESULTS

Hepatic disruption of Irs1 and Irs2 (LDKO mice) attenuated HFD (high-fat diet)-induced obesity and increased whole-body EE in a FoxO1-dependent manner. Hepatic disruption of the FoxO1-regulated hepatokine Fst normalized EE in LDKO mice and restored adipose mass during HFD consumption; moreover, hepatic Fst disruption alone increased fat mass accumulation, whereas hepatic overexpression of Fst reduced HFD-induced obesity. Excess circulating Fst in overexpressing mice neutralized Mstn (Myostatin), activating mTORC1-promoted pathways of nutrient uptake and EE in skeletal muscle. Similar to Fst overexpression, direct activation of muscle mTORC1 also reduced adipose mass.

CONCLUSIONS

Thus, complete hepatic insulin resistance in LDKO mice fed a HFD revealed Fst-mediated communication between the liver and muscle, which might go unnoticed during ordinary hepatic insulin resistance as a mechanism to increase muscle EE and constrain obesity.

The adipocyte supersystem of insulin and cAMP signaling

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

mTORC1 inhibition uncouples lipolysis and thermogenesis in white adipose tissue to contribute to alcoholic liver disease

BACKGROUND

Adipose tissue thermogenic activities use fatty acids from lipolysis for heat generation. Therefore, a tight coupling between lipolysis and thermogenesis is physiologically imperative in maintaining not only body temperature but also lipids homeostasis. Adipose tissue dysfunction contributes to alcoholic liver disease (ALD). Here, studies were conducted to examine how alcohol intake affects adipose tissue thermogenic activities and whether altered adipose tissue thermogenesis contributes to ALD.

METHODS

Both the Lieber-DeCarli and the NIAAA mouse models of ALD were used. Denervation surgery in epididymal fat pads was performed. CL316,243, a selective β3-adrenoceptor agonist, SR59230A, a selective β3 adrenoceptor (ADRB3) antagonist, and rapamycin, a selective mechanistic target of rapamycin complex 1 (mTORC1) inhibitor, were administrated through i.p. injection. Adipocyte-specific Prdm16 knockout mice were subjected to alcohol-containing diet chronically.

RESULTS

Chronic alcohol consumption, which enhances adipose tissue lipolysis, inhibits thermogenic activities of beige adipocytes in inguinal white adipose tissue (WAT), leading to an uncoupling status between lipolysis and thermogenesis in WAT at both basal and ADRB3 stimulation states. CL316,243 administration exacerbates liver pathologies of ALD. Alcohol intake inhibits mTORC1 activities in WAT. In mice, mTORC1 inhibition by rapamycin inhibits the thermogenesis of iWAT, whereas enhancing WAT lipolysis. Further investigations using adipocyte-specific Prdm16 knockout mice revealed that functional deficiency of beige adipocytes aggravates liver pathologies of ALD, suggesting that the inhibitory effect of alcohol on WAT browning/thermogenesis contributes to ALD pathogenesis.

CONCLUSION

Chronic alcohol consumption induces an "uncoupling status" between lipolysis and browning/thermogenesis in WAT by inhibiting mTORC1 activation. Diminished WAT browning/thermogenesis, concomitant with enhanced lipolysis, contributes to ALD pathogenesis.

Protein kinase D1 (Prkd1) deletion in brown adipose tissue leads to altered myogenic gene expression after cold exposure, while thermogenesis remains intact

Brown adipose tissue (BAT) has in recent times been rediscovered in adult humans, and together with work from preclinical models, has shown to have the potential of providing a variety of positive metabolic benefits. These include lower plasma glucose, improved insulin sensitivity, and reduced susceptibility to obesity and its comorbidities. As such, its continued study could offer insights to therapeutically modulate this tissue to improve metabolic health. It has been reported that adipose-specific deletion of the gene for protein kinase D1 (Prkd1) in mice enhances mitochondrial respiration and improves whole-body glucose homeostasis. We sought to determine whether these effects were mediated specifically through brown adipocytes using a Prkd1 brown adipose tissue (BAT) Ucp1-Cre-specific knockout mouse model, Prkd1^BKO . We unexpectedly observed that upon both cold exposure and β₃ -AR agonist administration, Prkd1 loss in BAT did not alter canonical thermogenic gene expression or adipocyte morphology. We took an unbiased approach to assess whether other signaling pathways were affected. RNA from cold-exposed mice was subjected to RNA-Seq analysis. These studies revealed that myogenic gene expression is altered in Prkd1^BKO BAT after both acute and extended cold exposure. Given that brown adipocytes and skeletal myocytes share a common precursor cell lineage expressing myogenic factor 5 (Myf5), these data suggest that loss of Prkd1 in BAT may alter the biology of mature brown adipocytes and preadipocytes in this depot. The data presented herein clarify the role of Prkd1 in BAT thermogenesis and present new avenues for the further study of Prkd1 function in BAT.

Regulatory role of atrial natriuretic peptide in brown adipose tissue: A narrative review

Atrial natriuretic peptide (ANP) has been considered to exert an essential role as a cardiac secretory hormone in the regulation of hemodynamic homeostasis. As the research progresses, the role of ANP in the crosstalk between heart and lipid metabolism has become an interesting topic that is attracting the interest of researchers. The regulation of ANP in lipid metabolism shows favorable effects, particularly the activation of brown adipose tissue (BAT). The complex regulatory network of ANP on BAT has not been fully outlined. This narrative review critically evaluated the existing literature on the regulatory effects of ANP on BAT. In general, we have summarized the expression of ANP and its receptors in various human tissues, analyzed the progress of research on the relationship between the ANP and BAT, and described several potential pathways of ANP to BAT. Exogenous ANP, natriuretic peptide receptor C (NPRC) deficiency, cold exposure, bariatric surgery, and cardiac or renal insufficiency could all contribute to BAT expression by increasing circulating ANP levels.

Placental mesenchymal stem cells restore glucose and energy homeostasis in obesogenic adipocytes

Obesity (Ob) depicts a state of energy imbalance(s) being characterized by the accumulation of excessive fat and which predisposes to several metabolic diseases. Mesenchymal stem cells (MSCs) represent a promising option for addressing obesity and its associated metabolic co-morbidities. The present study aims at assessing the beneficial effects of human placental MSCs (P-MSCs) in mitigating Ob-associated insulin resistance (IR) and mitochondrial dysfunction both in vivo and in vitro. Under obesogenic milieu, adipocytes showed a significant reduction in glucose uptake, and impaired insulin signaling with decreased expression of UCP1 and PGC1α, suggestive of dysregulated non-shivering thermogenesis vis-a-vis mitochondrial biogenesis respectively. Furthermore, obesogenic adipocytes demonstrated impaired mitochondrial respiration and energy homeostasis evidenced by reduced oxygen consumption rate (OCR) and blunted ATP/NAD⁺/NADP⁺ production respectively. Interestingly, co-culturing adipocytes with P-MSCs activated PI3K-Akt signaling, improved glucose uptake, diminished ROS production, enhanced mitochondrial OCR, improved ATP/NAD⁺/NADP⁺ production, and promoted beiging of adipocytes evidenced by upregulated expression of PRDM16, UCP1, and PGC1α expression. In vivo, P-MSCs administration increased the peripheral blood glucose uptake and clearance, and improved insulin sensitivity and lipid profile with a coordinated increase in the ratio of ATP/ADP and NAD⁺ and NADP⁺ in the white adipose tissue (WAT), exemplified in WNIN/GR-Ob obese mutant rats. In line with in vitro findings, there was a significant reduction in adipocyte hypertrophy, increased mitochondrial staining, and thermogenesis. Our findings advocate for a therapeutic application of P-MSCs for improving glucose and energy homeostasis, i.e., probably restoring non-shivering thermogenesis towards obesity management.

DEPTOR loss impairs brown adipocyte development in vitro but has limited impacts in mice

OBJECTIVES

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that regulates growth and metabolism. In mice, activation of mTOR controls cold adaptation by promoting the recruitment and the activation of brown adipose tissue (BAT). DEP-domain containing mTOR-interacting protein (DEPTOR) interacts with mTOR to modulate its activity. Whether DEPTOR levels are modulated by cold in BAT and whether this protein regulates brown adipocyte development and thermogenic activation has never been tested.

METHODS

DEPTOR levels were measured in mouse tissues upon cold exposure and in brown preadipocytes following the induction of adipogenesis. Lentiviruses expressing short-hairpin RNA were used to repress DEPTOR expression in brown preadipocytes in vitro. Conditional deletion of DEPTOR in brown preadipocytes and in mature brown fat cells was achieved by crossing DEPTOR floxed mice with either Myf5-Cre or Ucp1-Cre^ERT2 mice. These animals were exposed to cold and extensively phenotyped.

RESULTS

DEPTOR is highly expressed in BAT and its levels are induced by chronic cold exposure, a condition that triggers BAT expansion and activation. Supporting a role for DEPTOR in brown fat cell recruitment, we found that DEPTOR is induced during brown adipocyte development and that its depletion impairs adipogenesis in vitro. This adipogenic lesion was associated with defects in both Akt activation and the expression of key adipogenic regulators. Conditional deletion of DEPTOR in brown preadipocytes or mature brown fat cells did not impact BAT recruitment and thermogenesis in mice but slightly reduced the expression of adipogenic and lipogenic genes.

CONCLUSIONS

DEPTOR is highly expressed in BAT and its levels are dynamically regulated during brown fat cell development and upon cold exposure. Although DEPTOR depletion severely represses brown fat adipogenesis in vitro, its deletion is dispensable for BAT development, recruitment, and thermogenic activation in mice.

Mechanisms of mitochondrial respiratory adaptation

Mitochondrial energetic adaptations encompass a plethora of conserved processes that maintain cell and organismal fitness and survival in the changing environment by adjusting the respiratory capacity of mitochondria. These mitochondrial responses are governed by general principles of regulatory biology exemplified by changes in gene expression, protein translation, protein complex formation, transmembrane transport, enzymatic activities and metabolite levels. These changes can promote mitochondrial biogenesis and membrane dynamics that in turn support mitochondrial respiration. The main regulatory components of mitochondrial energetic adaptation include: the transcription coactivator peroxisome proliferator-activated receptor-γ (PPARγ) coactivator 1α (PGC1α) and associated transcription factors; mTOR and endoplasmic reticulum stress signalling; TOM70-dependent mitochondrial protein import; the cristae remodelling factors, including mitochondrial contact site and cristae organizing system (MICOS) and OPA1; lipid remodelling; and the assembly and metabolite-dependent regulation of respiratory complexes. These adaptive molecular and structural mechanisms increase respiration to maintain basic processes specific to cell types and tissues. Failure to execute these regulatory responses causes cell damage and inflammation or senescence, compromising cell survival and the ability to adapt to energetically demanding conditions. Thus, mitochondrial adaptive cellular processes are important for physiological responses, including to nutrient availability, temperature and physical activity, and their failure leads to diseases associated with mitochondrial dysfunction such as metabolic and age-associated diseases and cancer.

Cooling down with Entresto. Can sacubitril/valsartan combination enhance browning more than coldness?

BACKGROUND

It is a growing importance to induce a new treatment approach to encourage weight loss but also to improve maintenance of lost weight. It has been shown that promotion of brown adipose tissue (BAT) function or acquisition of BAT characteristics in white adipose tissue (terms referred as "browning") can be protective against obesity.

MAIN TEXT

Amongst numerous established environmental influences on BAT activity, cold exposure is the best interested technique due to its not only effects on of BAT depots in proliferation process but also de novo differentiation of precursor cells via β-adrenergic receptor activation. A novel combination drug, sacubitril/valsartan, has been shown to be more efficient in reducing cardiovascular events and heart failure readmission compared to conventional therapy. Also, this combination of drugs increases the postprandial lipid oxidation contributing to energy expenditure, promotes lipolysis in adipocytes and reduces body weight. To date, there is no research examining potential of combined sacubitril/valsartan use to promote browning or mechanisms in the basis of this thermogenic process.

CONCLUSION

Due to the pronounced effects of cold and sacubitril/valsartan treatment on function and metabolism of BAT, the primary goal of further research should focused on investigation of the synergistic effects of the sacubitril/valsartan treatment at low temperature environmental conditions.

Mechanisms of Male Reproductive Toxicity of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDE) are a group of flame retardants used in a variety of artificial materials. Despite being phased out in most industrial countries, they remain in the environment and human tissues due to their persistence, lipophilicity, and bioaccumulation. Populational and experimental studies demonstrate the male reproductive toxicity of PBDEs including increased incidence of genital malformations (hypospadias and cryptorchidism), altered weight of testes and other reproductive tissues, altered testes histology and transcriptome, decreased sperm production and sperm quality, altered epigenetic regulation of developmental genes in spermatozoa, and altered secretion of reproductive hormones. A broad range of mechanistic hypotheses of PBDE reproductive toxicity has been suggested. Among these hypotheses, oxidative stress, the disruption of estrogenic signaling, and mitochondria disruption are affected by PBDE concentrations much higher than concentrations found in human tissues, making them unlikely links between exposures and adverse reproductive outcomes in the general population. Robust evidence suggests that at environmentally relevant doses, PBDEs and their metabolites may affect male reproductive health via mechanisms including AR antagonism and the disruption of a complex network of metabolic signaling.

Signaling pathways in obesity: mechanisms and therapeutic interventions

Obesity is a complex, chronic disease and global public health challenge. Characterized by excessive fat accumulation in the body, obesity sharply increases the risk of several diseases, such as type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, and is linked to lower life expectancy. Although lifestyle intervention (diet and exercise) has remarkable effects on weight management, achieving long-term success at weight loss is extremely challenging, and the prevalence of obesity continues to rise worldwide. Over the past decades, the pathophysiology of obesity has been extensively investigated, and an increasing number of signal transduction pathways have been implicated in obesity, making it possible to fight obesity in a more effective and precise way. In this review, we summarize recent advances in the pathogenesis of obesity from both experimental and clinical studies, focusing on signaling pathways and their roles in the regulation of food intake, glucose homeostasis, adipogenesis, thermogenesis, and chronic inflammation. We also discuss the current anti-obesity drugs, as well as weight loss compounds in clinical trials, that target these signals. The evolving knowledge of signaling transduction may shed light on the future direction of obesity research, as we move into a new era of precision medicine.

The effects of taurine supplementation on obesity and browning of white adipose tissue in high-fat diet-fed mice

Background: In recent years, a new type of adipose tissue (beige adipose tissue) has been mentioned, unlike white adipose tissue (WAT) and brown adipose tissue (BAT). Beige cells are capable of thermogenesis like BAT. In response to various agents, beige cells can develop within WAT through a process called "browning." Therefore, the prevention of obesity and related diseases by providing WAT browning with new potential agents has been extensively studied in recent years. Taurine has many physiological functions in the body and has beneficial effects on obesity and related metabolic disorders. For this reason, we aimed to investigate whether taurine supplementation has effects on browning of WAT and attenuating obesity. Methods: Thirty-two male C57BL/6 mice were used for the study. Mice were divided into 4 groups as control, control + taurine, high fat diet (HFD) and HFD + taurine, and fed for 20 weeks. Taurine was given in drinking water (5%). Epididymal WAT samples were obtained from mice and RNA was extracted from these tissues. Expression levels of FLCN, mTOR, TFE3, PGC-1α, PGC1-1β, AMPK, S6K and UCP1 genes were measured by real-time PCR. Results: Taurine supplementation reduced HFD-induced obesity. No UCP1 expression was detected in any of the groups studied. Any of the gene expressions were not significantly different between HFD and HFD + taurine groups. Reduced PGC-1α and PGC-1β expressions were observed in both HFD and HFD + taurine groups. Conclusions: Taurine reduced the obesity in HFD fed mice, but had no effect on browning of epididymal WAT in this study.

Factors Associated with White Fat Browning: New Regulators of Lipid Metabolism

Mammalian adipose tissue can be divided into white and brown adipose tissue based on its colour, location, and cellular structure. Certain conditions, such as sympathetic nerve excitement, can induce the white adipose adipocytes into a new type of adipocytes, known as beige adipocytes. The process, leading to the conversion of white adipocytes into beige adipocytes, is called white fat browning. The dynamic balance between white and beige adipocytes is closely related to the body’s metabolic homeostasis. Studying the signal transduction pathways of the white fat browning might provide novel ideas for the treatment of obesity and alleviation of obesity-related glucose and lipid metabolism disorders. This article aimed to provide an overview of recent advances in understanding white fat browning and the role of BAT in lipid metabolism.

A Cardiac Amino-Terminal GRK2 Peptide Inhibits Maladaptive Adipocyte Hypertrophy and Insulin Resistance During Diet-Induced Obesity

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Heart disease remains the leading cause of death, in part due to increasing diabetes and obesity, though the exact mechanisms linking these disorders are not fully understood.

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In a diet-induced obesity model, we found that cardiac expression of an amino-terminal peptide of GRK2, βARKnt, preserves systemic glucose tolerance and insulin sensitivity despite normal weight gain.

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βARKnt enhanced metabolic flexibility, increased energy expenditure, protected against maladaptive visceral adipocyte hypertrophy, and induced visceral fat browning.

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βARKnt further elicited cardioprotection and increased insulin-mediated AS160 signaling during metabolic stress.

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These data point to a noncanonical cardiac regulation of systemic metabolic homeostasis that may lead to new treatment modalities for metabolic syndrome.

Heart disease remains the leading cause of death, and mortality rates positively correlate with the presence of obesity and diabetes. Despite the correlation between cardiac and metabolic dysregulation, the mechanistic pathway(s) of interorgan crosstalk still remain undefined. This study reveals that cardiac-restricted expression of an amino-terminal peptide of GRK2 (βARKnt) preserves systemic and cardiac insulin responsiveness, and protects against adipocyte maladaptive hypertrophy in a diet-induced obesity model. These data suggest a cardiac-driven mechanism to ameliorate maladaptive cardiac remodeling and improve systemic metabolic homeostasis that may lead to new treatment modalities for cardioprotection in obesity and obesity-related metabolic syndromes.

NY-ESO-1-specific redirected T cells with endogenous TCR knockdown mediate tumor response and cytokine release syndrome

Background

Because of the shortage of ideal cell surface antigens, the development of T-cell receptor (TCR)-engineered T cells (TCR-T) that target intracellular antigens such as NY-ESO-1 is a promising approach for treating patients with solid tumors. However, endogenous TCRs in vector-transduced T cells have been suggested to impair cell-surface expression of transduced TCR while generating mispaired TCRs that can become self-reactive.

Methods

We conducted a first-in-human phase I clinical trial with the TCR-transduced T-cell product (TBI-1301) in patients with NY-ESO-1-expressing solid tumors. In manufacturing TCR-T cells, we used a novel affinity-enhanced NY-ESO-1-specific TCR that was transduced by a retroviral vector that enables siRNA (small interfering RNA)-mediated silencing of endogenous TCR. The patients were divided into two cohorts. Cohort 1 was given a dose of 5×10⁸ cells (whole cells including TCR-T cells) preconditioned with 1500 mg/m² cyclophosphamide. Cohort 2 was given 5× 10⁹ cells preconditioned with 1500 mg/m² cyclophosphamide.

Results

In vitro study showed that both the CD8⁺ and CD4⁺ T fractions of TCR-T cells exhibited cytotoxic effects against NY-ESO-1-expressing tumor cells. Three patients and six patients were allocated to cohort 1 and cohort 2, respectively. Three of the six patients who received 5×10⁹ cells showed tumor response, while three patients developed early-onset cytokine release syndrome (CRS). One of the patients developed a grade 3 lung injury associated with the infiltration of the TCR-T cells. No siRNA-related adverse events other than CRS were observed. Cytokines including interleukin 6 I and monocyte chemotactic protein-1/chemokine (C-C motif) ligand (CCL2)

increased in the sera of patients with CRS. In vitro analysis showed these cytokines were not secreted from the T cells infused. A significant fraction of the manufactured T cells in patients with CRS was found to express either CD244, CD39, or both at high levels.

Conclusions

The trial showed that endogenous TCR-silenced and affinity-enhanced NY-ESO-1 TCR-T cells were safely administered except for grade 3 lung injury. The TCR-T cell infusion exhibited significant tumor response and early-onset CRS in patients with tumors that express NY-ESO-1 at high levels. The differentiation properties of the manufactured T cells may be prognostic for TCR-T-related CRS.

Trial registration number

NCT02366546.

Lipocalin 2 Deficiency Alters Prostaglandin Biosynthesis and mTOR Signaling Regulation of Thermogenesis and Lipid Metabolism in Adipocytes

Apart from a well-known role in the innate immune system, lipocalin 2 (Lcn2) has been recently characterized as a critical regulator of thermogenesis and lipid metabolism. However, the physiological mechanism through which Lcn2 regulates cellular metabolism and thermogenesis in adipocytes remains unknown. We found that Lcn2 expression and secretion are significantly upregulated by arachidonic acid (AA) and mTORC1 inhibition in differentiated inguinal adipocytes. AA-induced Lcn2 expression and secretion correlate with the inflammatory NFkB activation. Lcn2 deficiency leads to the upregulation of cyclooxygenase-2 (COX2) expression, as well as increased biosynthesis and secretion of prostaglandins (PGs), particularly PGE2 and PGD2, induced by AA in adipocytes. Furthermore, Lcn2 deficiency affects the mTOR signaling regulation of thermogenic gene expression, lipogenesis, and lipolysis. The loss of Lcn2 dismisses the effect of mTORC1 inhibition by rapamycin on COX2, thermogenesis genes, lipogenesis, and lipolysis, but has no impact on p70 S6Kinase-ULK1 activation in Lcn2-deficient adipocytes. We conclude that Lcn2 converges the COX2-PGE2 and mTOR signaling pathways in the regulation of thermogenesis and lipid metabolism in adipocytes.

Crosstalk between CYP2E1 and PPARα substrates and agonists modulate adipose browning and obesity

Although the functions of metabolic enzymes and nuclear receptors in controlling physiological homeostasis have been established, their crosstalk in modulating metabolic disease has not been explored. Genetic ablation of the xenobiotic-metabolizing cytochrome P450 enzyme CYP2E1 in mice markedly induced adipose browning and increased energy expenditure to improve obesity. CYP2E1 deficiency activated the expression of hepatic peroxisome proliferator-activated receptor alpha (PPARα) target genes, including fibroblast growth factor (FGF) 21, that upon release from the liver, enhanced adipose browning and energy expenditure to decrease obesity. Nineteen metabolites were increased in Cyp2e1-null mice as revealed by global untargeted metabolomics, among which four compounds, lysophosphatidylcholine and three polyunsaturated fatty acids were found to be directly metabolized by CYP2E1 and to serve as PPARα agonists, thus explaining how CYP2E1 deficiency causes hepatic PPARα activation through increasing cellular levels of endogenous PPARα agonists. Translationally, a CYP2E1 inhibitor was found to activate the PPARα–FGF21–beige adipose axis and decrease obesity in wild-type mice, but not in liver-specific Ppara-null mice. The present results establish a metabolic crosstalk between PPARα and CYP2E1 that supports the potential for a novel anti-obesity strategy of activating adipose tissue browning by targeting the CYP2E1 to modulate endogenous metabolites beyond its canonical role in xenobiotic-metabolism.

Urocortin 2 promotes hypertrophy and enhances skeletal muscle function through cAMP and insulin/IGF-1 signaling pathways

Objective

Although it is well established that urocortin 2 (Ucn2), a peptide member of the corticotrophin releasing factor (CRF) family, and its specific corticotrophin-releasing factor 2 receptor (CRF2R) are highly expressed in skeletal muscle, the role of this peptide in the regulation of skeletal muscle mass and protein metabolism remains elusive.

Methods

To elucidate the mechanisms how Ucn2 directly controls protein metabolism in skeletal muscles of normal mice, we carried out genetic tools, physiological and molecular analyses of muscles in vivo and in vitro.

Results

Here, we demonstrated that Ucn2 overexpression activated cAMP signaling and promoted an expressive muscle hypertrophy associated with higher rates of protein synthesis and activation of Akt/mTOR and ERK1/2 signaling pathways. Furthermore, Ucn2 induced a decrease in mRNA levels of atrogin-1 and in autophagic flux inferred by an increase in the protein content of LC3-I, LC3-II and p62. Accordingly, Ucn2 reduced both the transcriptional activity of FoxO in vivo and the overall protein degradation in vitro through an inhibition of lysosomal proteolytic activity. In addition, we demonstrated that Ucn2 induced a fast-to-slow fiber type shift and improved fatigue muscle resistance, an effect that was completely blocked in muscles co-transfected with mitogen-activated protein kinase phosphatase 1 (MKP-1), but not with dominant-negative Akt mutant (Aktmt).

Conclusions

These data suggest that Ucn2 triggers an anabolic and anti-catabolic response in skeletal muscle of normal mice probably through the activation of cAMP cascade and participation of Akt and ERK1/2 signaling. These findings open new perspectives in the development of therapeutic strategies to cope with the loss of muscle mass.

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Ucn2 overexpression promotes muscle growth due to an increase in protein synthesis.

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Ucn2 inhibits FoxO activity and autophagic-lysosomal system.

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Ucn2-induced skeletal muscle phenotype is dependent on Akt and ERK1/2.

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Ucn2 induces a fast-to-slow fiber type shift and improves fatigue resistance.

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The increase in muscle fatigue resistance is dependent on ERK1/2.

G-Protein Coupled Receptor Signaling and Mammalian Target of Rapamycin Complex 1 Regulation

The mammalian target of rapamycin (mTOR) senses upstream stimuli to regulate numerous cellular functions such as metabolism, growth, and autophagy. Increased activation of mTOR complex 1 (mTORC1) is typically observed in human disease and continues to be an important therapeutic target. Understanding the upstream regulators of mTORC1 will provide a crucial link in targeting hyperactivated mTORC1 in human disease. In this mini-review, we will discuss the regulation of mTORC1 by upstream stimuli, with a specific focus on G-protein coupled receptor signaling to mTORC1. SIGNIFICANCE STATEMENT: mTORC1 is a master regulator of many cellular processes and is often hyperactivated in human disease. Therefore, understanding the molecular underpinnings of G-protein coupled receptor signaling to mTORC1 will undoubtedly be beneficial for human disease.

Browning Epicardial Adipose Tissue: Friend or Foe?

The epicardial adipose tissue (EAT) is the visceral fat depot of the heart which is highly plastic and in direct contact with myocardium and coronary arteries. Because of its singular proximity with the myocardium, the adipokines and pro-inflammatory molecules secreted by this tissue may directly affect the metabolism of the heart and coronary arteries. Its accumulation, measured by recent new non-invasive imaging modalities, has been prospectively associated with the onset and progression of coronary artery disease (CAD) and atrial fibrillation in humans. Recent studies have shown that EAT exhibits beige fat-like features, and express uncoupling protein 1 (UCP-1) at both mRNA and protein levels. However, this thermogenic potential could be lost with age, obesity and CAD. Here we provide an overview of the physiological and pathophysiological relevance of EAT and further discuss whether its thermogenic properties may serve as a target for obesity therapeutic management with a specific focus on the role of immune cells in this beiging phenomenon.

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.

Adipose-tissue plasticity in health and disease

Adipose tissue, colloquially known as "fat," is an extraordinarily flexible and heterogeneous organ. While historically viewed as a passive site for energy storage, we now appreciate that adipose tissue regulates many aspects of whole-body physiology, including food intake, maintenance of energy levels, insulin sensitivity, body temperature, and immune responses. A crucial property of adipose tissue is its high degree of plasticity. Physiologic stimuli induce dramatic alterations in adipose-tissue metabolism, structure, and phenotype to meet the needs of the organism. Limitations to this plasticity cause diminished or aberrant responses to physiologic cues and drive the progression of cardiometabolic disease along with other pathological consequences of obesity.

Contrasting recruitment of skin-associated adipose depots during cold challenge of mouse and human

KEY POINTS

Several distinct strategies produce and conserve heat to maintain the body temperature of mammals, each associated with unique physiologies, with consequences for wellness and disease susceptibility Highly regulated properties of skin offset the total requirement for heat production  We hypothesize that the adipose component of skin is primarily responsible for modulating heat flux; here we evaluate the relative regulation of adipose depots in mouse and human, to test their recruitment to heat production and conservation We found that insulating mouse dermal white adipose tissue accumulates in response to environmentally and genetically induced cool stress; this layer is one of two adipose depots closely apposed to mouse skin, where the subcutaneous mammary gland fat pads are actively recruited to heat production In contrast, the body-wide adipose depot associated with human skin produces heat directly, potentially creating an alternative to the centrally regulated brown adipose tissue ABSTRACT: Mammalian skin impacts metabolic efficiency system-wide, controlling the rate of heat loss and consequent heat production. Here we compare the unique fat depots associated with mouse and human skin, to determine whether they have corresponding functions and regulation. For humans, we assay a skin-associated fat (SAF) body-wide depot to distinguish it from the subcutaneous fat pads characteristic of the abdomen and upper limbs. We show that the thickness of SAF is not related to general adiposity; it is much thicker (1.6-fold) in women than men, and highly subject-specific. We used molecular and cellular assays of β-adrenergic-induced lipolysis and found that dermal white adipose tissue (dWAT) in mice is resistant to lipolysis; in contrast, the body-wide human SAF depot becomes lipolytic, generating heat in response to β-adrenergic stimulation. In mice challenged to make more heat to maintain body temperature (either environmentally or genetically), there is a compensatory increase in thickness of dWAT: a corresponding β-adrenergic stimulation of human skin adipose (in vivo or in explant) depletes adipocyte lipid content. We summarize the regulation of skin-associated adipocytes by age, sex and adiposity, for both species. We conclude that the body-wide dWAT depot of mice shows unique regulation that enables it to be deployed for heat preservation; combined with the actively lipolytic subcutaneous mammary fat pads they enable thermal defence. The adipose tissue that covers human subjects produces heat directly, providing an alternative to the brown adipose tissues.

Cysteine 253 of UCP1 regulates energy expenditure and sex-dependent adipose tissue inflammation

Uncoupling protein 1 (UCP1) is a major regulator of brown and beige adipocyte energy expenditure and metabolic homeostasis. However, the widely employed UCP1 loss-of-function model has recently been shown to have a severe deficiency in the entire electron transport chain of thermogenic fat. As such, the role of UCP1 in metabolic regulation in vivo remains unclear. We recently identified cysteine-253 as a regulatory site on UCP1 that elevates protein activity upon covalent modification. Here, we examine the physiological importance of this site through the generation of a UCP1 cysteine-253-null (UCP1 C253A) mouse, a precise genetic model for selective disruption of UCP1 in vivo. UCP1 C253A mice exhibit significantly compromised thermogenic responses in both males and females but display no measurable effect on fat accumulation in an obesogenic environment. Unexpectedly, we find that a lack of C253 results in adipose tissue redox stress, which drives substantial immune cell infiltration and systemic inflammatory pathology in adipose tissues and liver of male, but not female, mice. Elevation of systemic estrogen reverses this male-specific pathology, providing a basis for protection from inflammation due to loss of UCP1 C253 in females. Together, our results establish the UCP1 C253 activation site as a regulator of acute thermogenesis and sex-dependent tissue inflammation.

Antibiotic Azithromycin inhibits brown/beige fat functionality and promotes obesity in human and rodents

Obesity, a metabolic disease caused by multiple factors, has become a global health problem. In addition to nutrient intake and sedentary lifestyle, environmental pollutants exposure has been shown to be involved in obesity epidemics. Antibiotics, a new type of environmental pollutant, have been widely used in animal husbandry, aquaculture and microorganism. However, the effects of antibiotics exposure on fat metabolism and metabolic diseases are largely unknown. Methods: We screened major types of antibiotics to examine their effects on the differentiation capacity and thermogenic functionality of brown and beige adipocytes, and found that azithromycin, one major kind of macrolide antibiotics suppressed brown and beige adipocyte functionality. We thus examined azithromycin accretion in adipose tissues of obese patients that correlates with BMI by high performance liquid chromatography-tandem mass spectrometry and systematically explore the influences of azithromycin on adiposity and metabolic performance in mice under high diet. Results: Azithromycin (macrolides) inhibits the mitochondrial and thermogenic gene programs of brown and beige adipocytes, thus disrupting their mitochondrial function and thermogenic response. Consistently, azithromycin treatment are more prone to diet-induced obesity in mice, and this was associated with impaired energy expenditure. Importantly, azithromycin is more accumulated in adipose tissue of obese patients and correlates with BMI and body weight. Mechanistically, we found that azithromycin inhibits mitochondria respiratory complex I protein levels and increases reactive oxidative species (ROS) levels, which causes damage of mitochondrial function in brown and beige adipocytes. The deleterious effects of azithromycin can be ameliorated by antioxidant N-acetyl-L-cysteine. Conclusions: Taken together, this work highlights the possible role of azithromycin in obesity epidemic and presents strategies for safe applications of antibiotics in the future.

Survivin is essential for thermogenic program and metabolic homeostasis in mice

Objective

Survivin is a member of the inhibitor of apoptosis family. Our previous study showed that survivin expression could be strongly induced by long-term, high-fat diet (HFD) exposure in vivo. It could also be induced by insulin through the PI3K/mTOR signaling pathway in vitro. Therefore, we hypothesized that under certain conditions, survivin expression might be required for adipocyte function. In the current study, we aim to further investigate the regulation of survivin expression in mature adipocytes upon various nutritional stimuli and the role of survivin using adipocyte-specific survivin knockout (SKO) mice.

Methods

SKO mice were obtained by crossing survivin^flox/flox mice with Adiponectin-Cre^+/- mice. The overall metabolic phenotype was observed under chow diet (CD) and HFD feeding conditions. The thermogenic program of mice was detected upon cold exposure. The inguinal white adipose tissue (iWAT) and brown adipose tissue (BAT) stromal vascular fraction cells were isolated and differentiated into mature adipocytes, and the effects of survivin deletion on mature adipocyte function were detected in vitro.

Results

Survivin expression in adipose tissue and adipocytes was regulated by short-term nutritional stress both in vivo and in vitro. The postnatal development of BAT was impaired in SKO mice, which resulted in drastically reduced BAT mass and decreased expression of the thermogenic protein Ucp1 in 24-week-old mice fed with CD. After HFD feeding, the iWAT and BAT mass of SKO mice were significantly decreased, causing ectopic lipid accumulation in the liver, which was associated with insulin resistance and glucose intolerance. Upon cold exposure, the expression of thermogenic genes and proteins was markedly reduced in BAT and iWAT of SKO mice, accompanied by abnormal mitochondrial structure and induced autophagy. Consistently, thermogenic program and mitochondrial oxidative phosphorylation were reduced in survivin-depleted brown and beige adipocytes in vitro.

Conclusions

Our findings showed that survivin could be regulated by nutritional stress in adipocytes and revealed a new role of survivin in maintaining normal BAT mass and positively regulating the thermogenic program and mitochondrial oxidative phosphorylation.

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Survivin expression in adipocytes is regulated by nutritional stress.

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Survivin is required for maintaining BAT mass and thermogenic program in mice.

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Survivin deletion in adipocytes impairs glucose homeostasis when exposed to HFD.

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Survivin is required for activation of thermogenic program response to cold exposure.

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Adipocyte-specific deletion of survivin induces autophagy in vivo and in vitro.

Asparagine reinforces mTORC1 signaling to boost thermogenesis and glycolysis in adipose tissues

Brown and beige fat are specialized for energy expenditure by dissipating energy from glucose and fatty acid oxidation as heat. While glucose and fatty acid metabolism have been extensively studied in thermogenic adipose tissues, the involvement of amino acids in regulating adaptive thermogenesis remains little studied. Here, we report that asparagine supplementation in brown and beige adipocytes drastically upregulated the thermogenic transcriptional program and lipogenic gene expression, so that asparagine‐fed mice showed better cold tolerance. In mice with diet‐induced obesity, the asparagine‐fed group was more responsive to β3‐adrenergic receptor agonists, manifesting in blunted body weight gain and improved glucose tolerance. Metabolomics and ¹³C‐glucose flux analysis revealed that asparagine supplement spurred glycolysis to fuel thermogenesis and lipogenesis in adipocytes. Mechanistically, asparagine stimulated the mTORC1 pathway, which promoted expression of thermogenic genes and key enzymes in glycolysis. These findings show that asparagine bioavailability affects glycolytic and thermogenic activities in adipose tissues, providing a possible nutritional strategy for improving systemic energy homeostasis.

Calcitonin gene-related peptide exerts inhibitory effects on autophagy in the heart of mice

Calcitonin Gene-Related Peptide (CGRP) is a potent vasodilator peptide widely distributed in the central nervous system and various peripheral tissues, including cardiac muscle. However, its role in heart protein metabolism remains unknown. We examined the acute effects of CGRP on autophagy and the related signaling pathways in the heart mice and cultured neonatal cardiomyocytes. CGRP (100 μg kg^-1; s.c.) or 0.9 % saline was injected in awake male C57B16 mice, and the metabolic profile was determined up to 60 min. In fed mice, CGRP drastically increased glycemia and reduced insulinemia, an effect that was accompanied by reduced cardiac phosphorylation levels of Akt at Ser⁴⁷³ without affecting FoxO. Despite these catabolic effects, CGRP acutely inhibited autophagy as estimated by the decrease in LC3II:LC3I and autophagic flux. In addition, the fasting-induced autophagic flux in mice hearts was entirely abrogated by one single injection of CGRP. In parallel, CGRP stimulated PKA/CREB and mTORC1 signaling and increased the phosphorylation of Unc51-like kinase-1 (ULK1), an essential protein in autophagy initiation. Similar effects were observed in cardiomyocytes, in which CGRP also inhibited autophagic flux and stimulated Akt and FoxO phosphorylation. These findings suggest that CGRP in vivo acutely suppresses autophagy in the heart of fed and fasted mice, most likely through the activation of PKA/mTORC1 signaling but independent of Akt.

Increased Energy Expenditure and Protection From Diet-Induced Obesity in Mice Lacking the cGMP-Specific Phosphodiesterase PDE9

Cyclic nucleotides cAMP and cGMP are important second messengers for the regulation of adaptive thermogenesis. Their levels are controlled not only by their synthesis, but also their degradation. Since pharmacological inhibitors of cGMP-specific phosphodiesterase 9 (PDE9) can increase cGMP-dependent protein kinase signaling and uncoupling protein 1 expression in adipocytes, we sought to elucidate the role of PDE9 on energy balance and glucose homeostasis in vivo. Mice with targeted disruption of the PDE9 gene, Pde9a, were fed nutrient-matched high-fat (HFD) or low-fat diets. Pde9a -/- mice were resistant to HFD-induced obesity, exhibiting a global increase in energy expenditure, while brown adipose tissue (AT) had increased respiratory capacity and elevated expression of Ucp1 and other thermogenic genes. Reduced adiposity of HFD-fed Pde9a -/- mice was associated with improvements in glucose handling and hepatic steatosis. Cold exposure or treatment with β-adrenergic receptor agonists markedly decreased Pde9a expression in brown AT and cultured brown adipocytes, while Pde9a -/- mice exhibited a greater increase in AT browning, together suggesting that the PDE9-cGMP pathway augments classical cold-induced β-adrenergic/cAMP AT browning and energy expenditure. These findings suggest PDE9 is a previously unrecognized regulator of energy metabolism and that its inhibition may be a valuable avenue to explore for combating metabolic disease.

Adipocyte-specific mTORC2 deficiency impairs BAT and iWAT thermogenic capacity without affecting glucose uptake and energy expenditure in cold-acclimated mice

Deletion of mechanistic target of rapamycin complex 2 (mTORC2) essential component rapamycin insensitive companion of mTOR (Rictor) by a Cre recombinase under control of the broad, nonadipocyte-specific aP2/FABP4 promoter impairs thermoregulation and brown adipose tissue (BAT) glucose uptake on acute cold exposure. We investigated herein whether adipocyte-specific mTORC2 deficiency affects BAT and inguinal white adipose tissue (iWAT) signaling, metabolism, and thermogenesis in cold-acclimated mice. For this, 8-wk-old male mice bearing Rictor deletion and therefore mTORC2 deficiency in adipocytes (adiponectin-Cre) and littermates controls were either kept at thermoneutrality (30 ± 1°C) or cold-acclimated (10 ± 1°C) for 14 days and evaluated for BAT and iWAT signaling, metabolism, and thermogenesis. Cold acclimation inhibited mTORC2 in BAT and iWAT, but its residual activity is still required for the cold-induced increases in BAT adipocyte number, total UCP-1 content and mRNA levels of proliferation markers Ki67 and cyclin 1 D, and de novo lipogenesis enzymes ATP-citrate lyase and acetyl-CoA carboxylase. In iWAT, mTORC2 residual activity is partially required for the cold-induced increases in multilocular adipocytes, mitochondrial mass, and uncoupling protein 1 (UCP-1) content. Conversely, BAT mTORC1 activity and BAT and iWAT glucose uptake were upregulated by cold independently of mTORC2. Noteworthy, the impairment in BAT and iWAT total UCP-1 content and thermogenic capacity induced by adipocyte mTORC2 deficiency had no major impact on whole body energy expenditure in cold-acclimated mice due to a compensatory activation of muscle shivering. In conclusion, adipocyte mTORC2 deficiency impairs, through different mechanisms, BAT and iWAT total UCP-1 content and thermogenic capacity in cold-acclimated mice, without affecting glucose uptake and whole body energy expenditure.NEW & NOTEWORTHY BAT and iWAT mTORC2 is inhibited by cold acclimation, but its residual activity is required for cold-induced increases in total UCP-1 content and thermogenic capacity, but not glucose uptake and mTORC1 activity. The impaired BAT and iWAT total UCP-1 content and thermogenic capacity induced by adipocyte mTORC2 deficiency are compensated by activation of muscle shivering in cold-acclimated mice.

Roles of AMPK and Its Downstream Signals in Pain Regulation

Pain is an unpleasant sensory and emotional state that decreases quality of life. A metabolic sensor, adenosine monophosphate-activated protein kinase (AMPK), which is ubiquitously expressed in mammalian cells, has recently attracted interest as a new target of pain research. Abnormal AMPK expression and function in the peripheral and central nervous systems are associated with various types of pain. AMPK and its downstream kinases participate in the regulation of neuron excitability, neuroinflammation and axonal and myelin regeneration. Numerous AMPK activators have reduced pain behavior in animal models. The current understanding of pain has been deepened by AMPK research, but certain issues, such as the interactions of AMPK at each step of pain regulation, await further investigation. This review examines the roles of AMPK and its downstream kinases in neurons and non-neuronal cells, as well as their contribution to pain regulation.

Phloridzin Ameliorates Lipid Deposition in High-Fat-Diet-Fed Mice with Nonalcoholic Fatty Liver Disease via Inhibiting the mTORC1/SREBP-1c Pathway

We aimed to investigate whether phloridzin could alleviate nonalcoholic fatty liver disease (NAFLD) in mice, which was induced by feeding a high-fat diet (HFD). We initially analyzed the effect of phloridzin on alleviating HFD-induced NAFLD in C57BL/6J mice and oleic acid (OA)-stimulated human normal liver L-02 cells (L02). Then, we investigated the mechanism of phloridzin on the mTORC1/sterol-regulatory element-binding protein-1c (SREBP-1c) signaling pathway by siRNA analysis, qRT-PCR, flow cytometry, and western blot analysis in vivo and in vitro. The results revealed that phloridzin significantly inhibited the increase in body weight, alleviated abnormal lipid metabolism, and decreased lipid biosynthesis and insulin resistance. Moreover, phloridzin augmented the number of CD8⁺CD122⁺PD-1⁺ Tregs and CD4⁺FoxP3⁺ Tregs in HFD-fed C57BL/6J mice and HFD-fed aP2-SREBF1c mice and downregulated the mTORC1/SREBP-1c signaling pathway-related protein expressions in vivo and in vitro. Furthermore, phloridzin reduced the expression of SREBP-1c in SREBP-1c-RNAi-lentivirus-transfected L02 cells and reversed the SREBP-1c expression in HFD-fed aP2-SREBF1c transgenic mice. Phloridzin ameliorates lipid accumulation and insulin resistance via inhibiting the mTORC1/SREBP-1c pathways. These results indicated that phloridzin may actively ameliorate NAFLD.

Ellagic acid induces beige remodeling of white adipose tissue by controlling mitochondrial dynamics and SIRT3

To determine whether ellagic acid (EA) induces the "beige remodeling" of white adipose tissue (WAT), we treated cold-exposed mice and mouse stromal vascular fraction (SVF) cells with EA, a phytochemical abundant in fruits and vegetables, in particular berries. We then investigated the mechanism of EA in beige remodeling with a particular focus on DRP1-mediated mitochondrial fission and SIRT3. EA induced the trans-differentiation of white adipocytes to beige adipocytes by promoting the expression of UCP1 and other brown and beige adipocytes/fat factors (PRDM16, UCP1, PGC1α, CD137, and TBX1) and mitochondrial dynamics-related factors (SIRT3, NRF1, CPT1β, DRP1, and FIS1) in 3T3-L1/SVF cells, and these were confirmed in the inguinal WAT of a cold-exposed mouse model. The browning effect of EA was abolished by a potent DRP1 inhibitor Mdivi-1 or SIRT3 knockdown, suggesting that EA induces beige remodeling of WAT by regulating the mitochondrial dynamics and SIRT3.