Mitophagy controls beige adipocyte maintenance through a Parkin-dependent and UCP1-independent mechanism

Asprosin contributes to pathogenesis of obesity by adipocyte mitophagy induction to inhibit white adipose browning in mice

BACKGROUND

Asprosin (ASP) is a newly discovered adipokine secreted by white adipose tissue (WAT), which can regulate the homeostasis of glucose and lipid metabolism. However, it is not clear whether it can regulate the browning of WAT and mitophagy during the browning process. Accordingly, this study aims to investigate the effects and possible mechanisms of ASP on the browning of WAT and mitophagy in vivo and in vitro.

METHODS

In in vivo experiments, some mouse models were used including adipose tissue ASP-specific deficiency (ASP-/-), high fat diet (HFD)-induced obesity and white adipose browning; in in vitro experiments, some cell models were also established and used, including ASP-deficient 3T3-L1 preadipocyte (ASP-/-) and CL-316243 (CL, 1 µM)-induced browning. Based on these models, the browning of WAT and mitophagy were evaluated by morphology, functionality and molecular markers.

RESULTS

Our in vivo data show that adipose tissue-specific deletion of ASP contributes to weight loss in mice; supplementation of ASP inhibits the expressions of browning-related proteins including UCP1, PRDM16 and PGC1ɑ during the cold exposure-induced browning, and promotes the expressions of mitophagy-related proteins including PINK1 and Parkin under the conditions of whether normal diet (ND) or HFD. Similarly, our in vitro data also show that the deletion of ASP in 3T3-L1 cells significantly increases the expressions of the browning-related proteins and decreases the expressions of the mitophagy-related proteins.

CONCLUSIONS

These data demonstrate that ASP deletion can facilitate the browning and inhibit mitophagy in WAT. The findings will lay an experimental foundation for the development of new drugs targeting ASP and the clinical treatment of metabolic diseases related to obesity.

The autophagic regulation of rosiglitazone-promoted adipocyte browning

Objective: Browning of white adipocytes is considered an efficient approach to combat obesity. Rosiglitazone induces the thermogenetic program of white adipocytes, but the underlying mechanisms remain elusive. Methods: Expression levels of browning and autophagy flux markers were detected by real-time PCR and immunoblotting. H&E and Oil Red O staining were performed to evaluate the lipid droplets area. Nuclear protein extraction and immunoprecipitation were used to detect the proteins interaction. Results: In this study, we reported that rosiglitazone promoted adipocyte browning and inhibited autophagy. Rapamycin, an autophagy inducer, reversed adipocyte browning induced by rosiglitazone. Autophagy inhibition by rosiglitazone does not prevent mitochondrial clearance, which was considered to promote adipose whitening. Instead, autophagy inhibition increased p62 nuclear translocation and stabilized the PPARγ-RXRα heterodimer, which is an essential transcription factor for adipocyte browning. We found that rosiglitazone activated NRF2 in mature adipocytes. Inhibition of NRF2 by ML385 reversed autophagy inhibition and the pro-browning effect of rosiglitazone. Conclusion: Our study linked autophagy inhibition with rosiglitazone-promoted browning of adipocytes and provided a mechanistic insight into the pharmacological effects of rosiglitazone.

Mitochondrial heterogeneity and adaptations to cellular needs

Although it is well described that mitochondria are at the epicentre of the energy demands of a cell, it is becoming important to consider how each cell tailors its mitochondrial composition and functions to suit its particular needs beyond ATP production. Here we provide insight into mitochondrial heterogeneity throughout development as well as in tissues with specific energy demands and discuss how mitochondrial malleability contributes to cell fate determination and tissue remodelling.

Connexin43 in mesenchymal lineage cells regulates body adiposity and energy metabolism in mice

Connexin43 (Cx43) is the most abundant gap junction protein present in the mesenchymal lineage. In mature adipocytes, Cx43 mediates white adipose tissue (WAT) beiging in response to cold exposure and maintains the mitochondrial integrity of brown adipose tissue (BAT). We found that genetic deletion of Gja1 (Cx43 gene) in cells that give rise to chondro-osteogenic and adipogenic precursors driven by the Dermo1/Twist2 promoter led to lower body adiposity and partial protection against the weight gain and metabolic syndrome induced by a high-fat diet (HFD) in both sexes. These protective effects were related to increased locomotion, fuel utilization, energy expenditure, nonshivering thermogenesis, and better glucose tolerance in conditionally Gja1-ablated mice. Accordingly, Gja1-mutant mice exhibited reduced adipocyte hypertrophy, partially preserved insulin sensitivity, increased BAT lipolysis, and decreased whitening under HFD. This metabolic phenotype was not reproduced with more restricted Gja1 ablation in differentiated adipocytes, suggesting that Cx43 in adipocyte progenitors or other targeted cells restrains energy expenditures and promotes fat accumulation. These results reveal what we believe is a hitherto unknown action of Cx43 in adiposity, and offer a promising new pharmacologic target for improving metabolic balance in diabetes and obesity.

Parkin depletion prevents the age-related alterations in the FGF21 system and the decline in white adipose tissue thermogenic function in mice

Parkin is an ubiquitin-E3 ligase that is involved in cellular mitophagy and was recently shown to contribute to controlling adipose tissue thermogenic plasticity. We found that Parkin expression is induced in brown (BAT) and white (WAT) adipose tissues of aged mice. We determined the potential role of Parkin in the aging-associated decline in the thermogenic capacity of adipose tissues by analyzing subcutaneous WAT, interscapular BAT, and systemic metabolic and physiological parameters in young (5 month-old) and aged (16 month-old) mice with targeted invalidation of the Parkin (Park2) gene, and their wild-type littermates. Our data indicate that suppression of Parkin prevented adipose accretion, increased energy expenditure and improved the systemic metabolic derangements, such as insulin resistance, seen in aged mice. This was associated with maintenance of browning and reduction of the age-associated induction of inflammation in subcutaneous WAT. BAT in aged mice was much less affected by Parkin gene invalidation. Such protection was associated with a dramatic prevention of the age-associated induction of fibroblast growth factor-21 (FGF21) levels in aged Parkin-invalidated mice. This was associated with a parallel reduction in FGF21 gene expression in adipose tissues and liver in aged Parkin-invalidated mice. Additionally, Parkin invalidation prevented the protein down-regulation of β-Klotho (a key co-receptor mediating FGF21 responsiveness in tissues) in aged adipose tissues. We conclude that Parkin down-regulation leads to improved systemic metabolism in aged mice, in association with maintenance of adipose tissue browning and FGF21 system functionality.

Connexin43 in mesenchymal lineage cells regulates body adiposity and energy metabolism in mice

Connexin43 (Cx43) is the most abundant gap junction protein present in the mesenchymal lineage. In mature adipocytes, Cx43 mediates white adipose tissue (WAT) "beiging" in response to cold exposure and maintains the mitochondrial integrity of brown adipose tissue (BAT). We found that genetic deletion of Gja1 (Cx43 gene) in cells that give rise to chondro-osteogenic and adipogenic precursors driven by the Dermo1/Twist2 promoter leads to lower body adiposity and partial protection against the weight gain and metabolic syndrome induced by a high fat diet (HFD) in both sexes. These protective effects from obesogenic diet are related to increased locomotion, fuel utilization, energy expenditure, non-shivering thermogenesis, and better glucose tolerance in conditionally Gja1 ablated mice. Accordingly, Gja1 mutant mice exhibit reduced adipocyte hypertrophy, partially preserved insulin sensitivity, increased BAT lipolysis and decreased whitening under HFD. This metabolic phenotype is not reproduced with more restricted Gja1 ablation in differentiated adipocytes, suggesting that Cx43 has a hitherto unknown function in adipocyte progenitors or other targeted cells, resulting in restrained energy expenditures and fat accumulation. These results disclose an hitherto unknown action of Cx43 in adiposity, and offer a promising new pharmacologic target for improving metabolic balance in diabetes and obesity.

PPARα/γ synergism activates UCP1-dependent and -independent thermogenesis and improves mitochondrial dynamics in the beige adipocytes of high-fat fed mice

OBJECTIVE

The aim of this study was to investigate the role of peroxisome proliferator-activated receptor (PPAR) activation (single PPARα or PPARγ, and dual PPARα/γ) on UCP1-dependent and -independent thermogenic pathways and mitochondrial metabolism in the subcutaneous white adipose tissue of mice fed a high-fat diet.

METHODS

Male C57BL/6 mice received either a control diet (10% lipids) or a high-fat diet (HF; 50% lipids) for 12 wk. The HF group was divided to receive the treatments for 4 wk: HFγ (pioglitazone, 10 mg/kg), HFα (WY-14643, 3.5 mg/kg), and HFα/γ (tesaglitazar, 4 mg/kg).

RESULTS

The HF group was overweight, insulin resistant, and had subcutaneous white adipocyte dysfunction. Treatment with PPARα and PPARα/γ reduced body mass, mitigated insulin resistance, and induced browning with increased UCP1-dependent and -independent thermogenesis activation and improved mitochondrial metabolism to support the beige adipocyte phenotype.

CONCLUSION

PPARα and dual PPARα/γ activation recruited UCP1+ beige adipocytes and favored UCP1-independent thermogenesis, yielding body mass and insulin sensitivity normalization. Preserved mitochondrial metabolism emerges as a potential target for obesity treatment using PPAR agonists, with possible clinical applications.

Adipose tissue browning and thermogenesis under physiologically energetic challenges: a remodelled thermogenic system

Metabolic diseases such as obesity and diabetes are often thought to be caused by reduced energy expenditure, which poses a serious threat to human health. Cold exposure, exercise and caloric restriction have been shown to promote adipose tissue browning and thermogenesis. These physiological interventions increase energy expenditure and thus have emerged as promising strategies for mitigating metabolic disorders. However, that increased adipose tissue browning and thermogenesis elevate thermogenic consumption is not a reasonable explanation when humans and animals confront energetic challenges imposed by these interventions. In this review, we collected numerous results on adipose tissue browning and whitening and evaluated this bi-directional conversion of adipocytes from the perspective of energy homeostasis. Here, we propose a new interpretation of the role of adipose tissue browning under energetic challenges: increased adipose tissue browning and thermogenesis under energy challenge is not to enhance energy expenditure, but to reestablish a more economical thermogenic pattern to maintain the core body temperature. This can be achieved by enhancing the contribution of non-shivering thermogenesis (adipose tissue browning and thermogenesis) and lowering shivering thermogenesis and high intensity shivering. Consequently, the proportion of heat production in fat increases and that in skeletal muscle decreases, enabling skeletal muscle to devote more energy reserves to overcoming environmental stress.

Insights into the regulation of mitochondrial functions by protein kinase A-mediated phosphorylation

Cyclic AMP (cAMP)-protein kinase A (PKA) signaling is a highly conserved pathway in eukaryotes and plays a central role in cell signaling cascades in response to environmental changes. Elevated cAMP levels promote the activation of PKA, which phosphorylates various downstream proteins. Many cytosolic and nuclear proteins, such as metabolic enzymes and transcriptional factors, have been identified as substrates for PKA, suggesting that PKA-mediated regulation occurs predominantly in the cytosol. Mitochondrial proteins are also phosphorylated by PKA, and PKA-mediated phosphorylation of mitochondrial proteins is considered to control a variety of mitochondrial functions, including oxidative phosphorylation, protein import, morphology and quality control. In this review, we outline PKA mitochondrial substrates and summarize the regulation of mitochondrial functions through PKA-mediated phosphorylation.

Miquelianin in Folium Nelumbinis extract promotes white-to-beige fat conversion via blocking AMPK/DRP1/mitophagy and modulating gut microbiota in HFD-fed mice

The main purpose of the present study was to investigate the effect of miquelianin (quercetin 3-O-glucuronide, Q3G), one of the main flavonoids in the Folium Nelumbinis extract (FNE), on beige adipocyte formation and its underlying mechanisms. In 3T3-L1 adipocytes Q3G (12.8%)-rich FNE treatment upregulated beige-related markers such as SIRT1, COX2, PGC-1α, TFAM, and UCP1. Furthermore, Q3G enhanced mitochondrial biosynthesis and inhibited mitophagy by downregulating the expression of PINK1, PARKIN, BECLIN1 and LC-3B in 3T3-L1 cells. Moreover, in high-fat-diet (HFD)-fed mice, Q3G markedly inhibited body weight gain, reduced blood glucose/lipid levels, reduced white adipose tissues (WAT) and mitigated hepatic steatosis. Meanwhile, the induced beiging accompanied by suppressed mitophagy was also demonstrated in inguinal WAT (iWAT). Chemical intervention of AMPK activity with Compound C (Com C) and Acadesine (AICAR) revealed that AMPK/DRP1 signaling was involved in Q3G-mediated mitophagy and the beiging process. Importantly, 16S rRNA sequencing analysis showed that Q3G beneficially reshaped gut microbiota structure, specifically inhibiting unclassified_Lachnospiraceae, Faecalibaculum, Roseburia and Colidextribacter while increasing Bacteroides, Akkermansia and Mucispirillum, which may potentially facilitate WAT beiging. Collectively, our findings provide a novel biological function for Folium Nelumbinis and Q3G in the fight against obesity through activating the energy-dissipating capacity of beige fat.

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.

Pancreatic β-cell mitophagy as an adaptive response to metabolic stress and the underlying mechanism that involves lysosomal Ca2+ release

Mitophagy is an excellent example of selective autophagy that eliminates damaged or dysfunctional mitochondria, and it is crucial for the maintenance of mitochondrial integrity and function. The critical roles of autophagy in pancreatic β-cell structure and function have been clearly shown. Furthermore, morphological abnormalities and decreased function of mitochondria have been observed in autophagy-deficient β-cells, suggesting the importance of β-cell mitophagy. However, the role of authentic mitophagy in β-cell function has not been clearly demonstrated, as mice with pancreatic β-cell-specific disruption of Parkin, one of the most important players in mitophagy, did not exhibit apparent abnormalities in β-cell function or glucose homeostasis. Instead, the role of mitophagy in pancreatic β-cells has been investigated using β-cell-specific Tfeb-knockout mice (TfebΔβ-cell mice); Tfeb is a master regulator of lysosomal biogenesis or autophagy gene expression and participates in mitophagy. TfebΔβ-cell mice were unable to adaptively increase mitophagy or mitochondrial complex activity in response to high-fat diet (HFD)-induced metabolic stress. Consequently, TfebΔβ-cell mice exhibited impaired β-cell responses and further exacerbated metabolic deterioration after HFD feeding. TFEB was activated by mitochondrial or metabolic stress-induced lysosomal Ca2+ release, which led to calcineurin activation and mitophagy. After lysosomal Ca2+ release, depleted lysosomal Ca2+ stores were replenished by ER Ca2+ through ER→lysosomal Ca2+ refilling, which supplemented the low lysosomal Ca2+ capacity. The importance of mitophagy in β-cell function was also demonstrated in mice that developed β-cell dysfunction and glucose intolerance after treatment with a calcineurin inhibitor that hampered TFEB activation and mitophagy.

Human abdominal subcutaneous-derived active beige adipocytes carrying FTO rs1421085 obesity-risk alleles exert lower thermogenic capacity

Introduction: White adipocytes store lipids, have a large lipid droplet and few mitochondria. Brown and beige adipocytes, which produce heat, are characterized by high expression of uncoupling protein (UCP) 1, multilocular lipid droplets, and large amounts of mitochondria. The rs1421085 T-to-C single-nucleotide polymorphism (SNP) of the human FTO gene interrupts a conserved motif for ARID5B repressor, resulting in adipocyte type shift from beige to white. Methods: We obtained abdominal subcutaneous adipose tissue from donors carrying FTO rs1421085 TT (risk-free) or CC (obesity-risk) genotypes, isolated and differentiated their preadipocytes into beige adipocytes (driven by the PPARγ agonist rosiglitazone for 14 days), and activated them with dibutyryl-cAMP for 4 hours. Then, either the same culture conditions were applied for additional 14 days (active beige adipocytes) or it was replaced by a white differentiation medium (inactive beige adipocytes). White adipocytes were differentiated by their medium for 28 days. Results and Discussion: RNA-sequencing was performed to investigate the gene expression pattern of adipocytes carrying different FTO alleles and found that active beige adipocytes had higher brown adipocyte content and browning capacity compared to white or inactive beige ones when the cells were obtained from risk-free TT but not from obesity-risk CC genotype carriers. Active beige adipocytes carrying FTO CC had lower thermogenic gene (e.g., UCP1, PM20D1, CIDEA) expression and thermogenesis measured by proton leak respiration as compared to TT carriers. In addition, active beige adipocytes with CC alleles exerted lower expression of ASC-1 neutral amino acid transporter (encoded by SLC7A10) and less consumption of Ala, Ser, Cys, and Gly as compared to risk-free carriers. We did not observe any influence of the FTO rs1421085 SNP on white and inactive beige adipocytes highlighting its exclusive and critical effect when adipocytes were activated for thermogenesis.

Proteolytic rewiring of mitochondria by LONP1 directs cell identity switching of adipocytes

Mitochondrial proteases are emerging as key regulators of mitochondrial plasticity and acting as both protein quality surveillance and regulatory enzymes by performing highly regulated proteolytic reactions. However, it remains unclear whether the regulated mitochondrial proteolysis is mechanistically linked to cell identity switching. Here we report that cold-responsive mitochondrial proteolysis is a prerequisite for white-to-beige adipocyte cell fate programming during adipocyte thermogenic remodelling. Thermogenic stimulation selectively promotes mitochondrial proteostasis in mature white adipocytes via the mitochondrial protease LONP1. Disruption of LONP1-dependent proteolysis substantially impairs cold- or β3 adrenergic agonist-induced white-to-beige identity switching of mature adipocytes. Mechanistically, LONP1 selectively degrades succinate dehydrogenase complex iron sulfur subunit B and ensures adequate intracellular succinate levels. This alters the histone methylation status on thermogenic genes and thereby enables adipocyte cell fate programming. Finally, augmented LONP1 expression raises succinate levels and corrects ageing-related impairments in white-to-beige adipocyte conversion and adipocyte thermogenic capacity. Together, these findings reveal that LONP1 links proteolytic surveillance to mitochondrial metabolic rewiring and directs cell identity conversion during adipocyte thermogenic remodelling.

Hyperoside prevents high-fat diet-induced obesity by increasing white fat browning and lipophagy via CDK6-TFEB pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Hypericum perforatum L. (genus Hypericum, family Hypericaceae) is a flowering plant native to Europe, North Africa and Asia, which can be used in the treatment of psychiatric disorder, cardiothoracic depression and diabetes. Crataegus pinnatifida Bunge (genus Crataegus pinnatifida Bunge, family Rosaceae) was another traditional Chinese medicine for treating hyperlipidemia. Hyperoside (Hype), a major flavonoid glycoside component of Hypericum perforatum L. and Crataegus pinnatifida Bunge, possesses multiple physiological activities, such as anti-inflammatory and antioxidant effects. However, the role of Hype on obesity and related metabolic diseases still needs to be further investigated.

AIM OF THE STUDY

We explored the effect of Hype on high-fat diet (HFD)-induced obesity and its metabolic regulation on white fat tissues.

MATERIALS AND METHODS

In vivo four-week-old male C57BL/6J mice were randomly assigned to vehicle (0.5% methycellulose) and Hype (80 mg/kg/day by gavage) group under a normal chow diet (NCD) or HFD for 8 weeks. In vitro, 3T3-L1 preadipocyte cell line and primary stromal vascular fraction (SVF) cells from inguinal white adipose tissue (iWAT) of mice were used to investigate the molecular mechanisms of Hype regulation on adipocyte energy metabolism.

RESULTS

Hype treatment in vivo promotes UCP1-dependent white to beige fat transition, increases glucose and lipid metabolism, and resists HFD-induced obesity. Meanwhile, Hype induces lipophagy, a specific autophagy that facilitates the breakdown of lipid droplets, and blocking autophagy partially reduces UCP1 expression. Mechanistically, Hype inhibited CDK6, leading to the increased nuclear translocation of TFEB, while overexpression of CDK6 partially reversed the enhancement of UCP1 by Hype.

CONCLUSIONS

Hype protects mice from HFD-induced obesity by increasing energy expenditure of white fat tissue via CDK6-TFEB pathway.

β Cell and Autophagy: What Do We Know?

Pancreatic β cells are central to glycemic regulation through insulin production. Studies show autophagy as an essential process in β cell function and fate. Autophagy is a catabolic cellular process that regulates cell homeostasis by recycling surplus or damaged cell components. Impaired autophagy results in β cell loss of function and apoptosis and, as a result, diabetes initiation and progress. It has been shown that in response to endoplasmic reticulum stress, inflammation, and high metabolic demands, autophagy affects β cell function, insulin synthesis, and secretion. This review highlights recent evidence regarding how autophagy can affect β cells' fate in the pathogenesis of diabetes. Furthermore, we discuss the role of important intrinsic and extrinsic autophagy modulators, which can lead to β cell failure.

Thermogenic adipose tissue in energy regulation and metabolic health

The ability to generate thermogenic fat could be a targeted therapy to thwart obesity and improve metabolic health. Brown and beige adipocytes are two types of thermogenic fat cells that regulate energy balance. Both adipocytes share common morphological, biochemical, and thermogenic properties. Yet, recent evidence suggests unique features exist between brown and beige adipocytes, such as their cellular origin and thermogenic regulatory processes. Beige adipocytes also appear highly plastic, responding to environmental stimuli and interconverting between beige and white adipocyte states. Additionally, beige adipocytes appear to be metabolically heterogenic and have substrate specificity. Nevertheless, obese and aged individuals cannot develop beige adipocytes in response to thermogenic fat-inducers, creating a key clinical hurdle to their therapeutic promise. Thus, elucidating the underlying developmental, molecular, and functional mechanisms that govern thermogenic fat cells will improve our understanding of systemic energy regulation and strive for new targeted therapies to generate thermogenic fat. This review will examine the recent advances in thermogenic fat biogenesis, molecular regulation, and the potential mechanisms for their failure.

De novo lipogenesis fuels adipocyte autophagosome and lysosome membrane dynamics

Adipocytes robustly synthesize fatty acids (FA) from carbohydrate through the de novo lipogenesis (DNL) pathway, yet surprisingly DNL contributes little to their abundant triglyceride stored in lipid droplets. This conundrum raises the hypothesis that adipocyte DNL instead enables membrane expansions to occur in processes like autophagy, which requires an abundant supply of phospholipids. We report here that adipocyte Fasn deficiency in vitro and in vivo markedly impairs autophagy, evident by autophagosome accumulation and severely compromised degradation of the autophagic substrate p62. Our data indicate the impairment occurs at the level of autophagosome-lysosome fusion, and indeed, loss of Fasn decreases certain membrane phosphoinositides necessary for autophagosome and lysosome maturation and fusion. Autophagy dependence on FA produced by Fasn is not fully alleviated by exogenous FA in cultured adipocytes, and interestingly, imaging studies reveal that Fasn colocalizes with nascent autophagosomes. Together, our studies identify DNL as a critical source of FAs to fuel autophagosome and lysosome maturation and fusion in adipocytes.

An insight into brown/beige adipose tissue whitening, a metabolic complication of obesity with the multifactorial origin

Brown adipose tissue (BAT), a thermoregulatory organ known to promote energy expenditure, has been extensively studied as a potential avenue to combat obesity. Although BAT is the opposite of white adipose tissue (WAT) which is responsible for energy storage, BAT shares thermogenic capacity with beige adipose tissue that emerges from WAT depots. This is unsurprising as both BAT and beige adipose tissue display a huge difference from WAT in terms of their secretory profile and physiological role. In obesity, the content of BAT and beige adipose tissue declines as these tissues acquire the WAT characteristics via the process called "whitening". This process has been rarely explored for its implication in obesity, whether it contributes to or exacerbates obesity. Emerging research has demonstrated that BAT/beige adipose tissue whitening is a sophisticated metabolic complication of obesity that is linked to multiple factors. The current review provides clarification on the influence of various factors such as diet, age, genetics, thermoneutrality, and chemical exposure on BAT/beige adipose tissue whitening. Moreover, the defects and mechanisms that underpin the whitening are described. Notably, the BAT/beige adipose tissue whitening can be marked by the accumulation of large unilocular lipid droplets, mitochondrial degeneration, and collapsed thermogenic capacity, by the virtue of mitochondrial dysfunction, devascularization, autophagy, and inflammation.

Adipose Tissue Remodeling in Pathophysiology

Rather than serving as a mere onlooker, adipose tissue is a complex endocrine organ and active participant in disease initiation and progression. Disruptions of biological processes operating within adipose can disturb healthy systemic physiology, the sequelae of which include metabolic disorders such as obesity and type 2 diabetes. A burgeoning interest in the field of adipose research has allowed for the elucidation of regulatory networks underlying both adipose tissue function and dysfunction. Despite this progress, few diseases are treated by targeting maladaptation in the adipose, an oft-overlooked organ. In this review, we elaborate on the distinct subtypes of adipocytes, their developmental origins and secretory roles, and the dynamic interplay at work within the tissue itself. Central to this discussion is the relationship between adipose and disease states, including obesity, cachexia, and infectious diseases, as we aim to leverage our wealth of knowledge for the development of novel and targeted therapeutics.

Mitochondrial Energy Metabolism in the Regulation of Thermogenic Brown Fats and Human Metabolic Diseases

Brown fats specialize in thermogenesis by increasing the utilization of circulating blood glucose and fatty acids. Emerging evidence suggests that brown adipose tissue (BAT) prevents the incidence of obesity-associated metabolic diseases and several types of cancers in humans. Mitochondrial energy metabolism in brown/beige adipocytes regulates both uncoupling protein 1 (UCP1)-dependent and -independent thermogenesis for cold adaptation and the utilization of excess nutrients and energy. Many studies on the quantification of human BAT indicate that mass and activity are inversely correlated with the body mass index (BMI) and visceral adiposity. Repression is caused by obesity-associated positive and negative factors that control adipocyte browning, de novo adipogenesis, mitochondrial energy metabolism, UCP1 expression and activity, and noradrenergic response. Systemic and local factors whose levels vary between lean and obese conditions include growth factors, inflammatory cytokines, neurotransmitters, and metal ions such as selenium and iron. Modulation of obesity-associated repression in human brown fats is a promising strategy to counteract obesity and related metabolic diseases through the activation of thermogenic capacity. In this review, we highlight recent advances in mitochondrial metabolism, thermogenic regulation of brown fats, and human metabolic diseases.

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.

Parkin regulates adiposity by coordinating mitophagy with mitochondrial biogenesis in white adipocytes

Parkin, an E3 ubiquitin ligase, plays an essential role in mitochondrial quality control. However, the mechanisms by which Parkin connects mitochondrial homeostasis with cellular metabolism in adipose tissue remain unclear. Here, we demonstrate that Park2 gene (encodes Parkin) deletion specifically from adipose tissue protects mice against high-fat diet and aging-induced obesity. Despite a mild reduction in mitophagy, mitochondrial DNA content and mitochondrial function are increased in Park2 deficient white adipocytes. Moreover, Park2 gene deletion elevates mitochondrial biogenesis by increasing Pgc1α protein stability through mitochondrial superoxide-activated NAD(P)H quinone dehydrogenase 1 (Nqo1). Both in vitro and in vivo studies show that Nqo1 overexpression elevates Pgc1α protein level and mitochondrial DNA content and enhances mitochondrial activity in mouse and human adipocytes. Taken together, our findings indicate that Parkin regulates mitochondrial homeostasis by balancing mitophagy and Pgc1α-mediated mitochondrial biogenesis in white adipocytes, suggesting a potential therapeutic target in adipocytes to combat obesity and obesity-associated disorders.

Loss of RUBCN/rubicon in adipocytes mediates the upregulation of autophagy to promote the fasting response

Upon fasting, adipocytes release their lipids that accumulate in the liver, thus promoting hepatic steatosis and ketone body production. However, the mechanisms underlying this process are not fully understood. In this study, we found that fasting caused a substantial decrease in the adipose levels of RUBCN/rubicon, a negative regulator of macroautophagy/autophagy, along with an increase in autophagy. Adipose-specific rubcn-knockout mice exhibited systemic fat loss that was not accelerated by fasting. Genetic inhibition of autophagy in adipocytes in fasted mice led to a reduction in fat loss, hepatic steatosis, and ketonemia. In terms of mechanism, autophagy decreased the levels of its substrates NCOA1/SRC-1 and NCOA2/TIF2, which are also coactivators of PPARG/PPARγ, leading to a fasting-induced reduction in the mRNA levels of adipogenic genes in adipocytes. Furthermore, RUBCN in adipocytes was degraded through the autophagy pathway, suggesting that autophagic degradation of RUBCN serves as a feedforward system for autophagy induction during fasting. Collectively, we propose that loss of adipose RUBCN promotes a metabolic response to fasting via increasing autophagic activity.

Mitophagy: Critical Role in Atherosclerosis Progression

Autophagy maintains intracellular homeostasis in the cardiovascular system, including in cardiomyocytes, endothelial cells (ECs), and arterial smooth muscle cells. Mitophagy, a selective autophagy that specifically removes damaged and dysfunctional mitochondria, is particularly important for cardiovascular homeostasis. Dysfunctional mitophagy contributes to cardiovascular disease, particularly atherosclerosis (AS). This review focuses on the advances of regulator mechanisms of mitophagy and its potential roles in AS. The findings are beneficial to understanding the pathological processes of atherosclerotic lesions and provide new ideas for the prevention and clinical treatment of AS.

SAMM50 Regulates Thermogenesis of Beige Adipocytes Differentiated from Human Adipose-Derived Stem Cells by Balancing Mitochondrial Dynamics

Brown/beige adipocyte thermogenesis is a process that is important for energy balance. The thermogenesis of brown/beige adipocytes occurs in the mitochondria, which is modulated by the dynamic balance between mitochondrial fusion and fission. Mitophagy is also involved in mitochondrial dynamics. The sorting and assembly machinery (SAM) complex protein, SAMM50, plays a key role in mitochondrial dynamics and quality control through regulating mitophagy. However, the roles of SAMM50 in the thermogenesis of beige adipocytes remain unknown. Thus, the objective of this study was to conduct functional analyses of SAMM50. The expression of mitochondrial fusion genes was repressed by SAMM50 knockdown but was not altered by SAMM50 overexpression. These results agreed with the distribution of the fluorescence-stained mitochondria and an mtDNA copy number. In contrast, the expression of mitochondrial fission genes showed an opposite outcome. As a result, suppression by the SAMM50 shRNA inhibited the expression of thermogenic genes (UCP1, PPARGC1A, DIO2, ELOVL3, CIDEA, and CIDEC) and mitochondrial-related genes (CYCS, COX7A1, TFAM, CPT1B, and CPT2). Conversely, SAMM50 overexpression promoted the expression of the thermogenic genes and mitochondrial genes. Thus, SAMM50 links the balance between the mitochondrial dynamics and thermogenesis of beige adipocytes.

Adipocyte HIF2α functions as a thermostat via PKA Cα regulation in beige adipocytes

Thermogenic adipocytes generate heat to maintain body temperature against hypothermia in response to cold. Although tight regulation of thermogenesis is required to prevent energy sources depletion, the molecular details that tune thermogenesis are not thoroughly understood. Here, we demonstrate that adipocyte hypoxia-inducible factor α (HIFα) plays a key role in calibrating thermogenic function upon cold and re-warming. In beige adipocytes, HIFα attenuates protein kinase A (PKA) activity, leading to suppression of thermogenic activity. Mechanistically, HIF2α suppresses PKA activity by inducing miR-3085-3p expression to downregulate PKA catalytic subunit α (PKA Cα). Ablation of adipocyte HIF2α stimulates retention of beige adipocytes, accompanied by increased PKA Cα during re-warming after cold stimuli. Moreover, administration of miR-3085-3p promotes beige-to-white transition via downregulation of PKA Cα and mitochondrial abundance in adipocyte HIF2α deficient mice. Collectively, these findings suggest that HIF2α-dependent PKA regulation plays an important role as a thermostat through dynamic remodeling of beige adipocytes.

Thermogenic adipocytes maintain body temperature in response to cold, but how this is tuned during cold and re-warming is unclear. Here, the authors show HIF2α inhibits beige adipocyte retention, regulating PKA catalysis to control dynamic adipocyte remodelling.

Mitophagy Mediates the Beige to White Transition of Human Primary Subcutaneous Adipocytes Ex Vivo

Brown and beige adipocytes have multilocular lipid droplets, express uncoupling protein (UCP) 1, and promote energy expenditure. In rodents, when the stimulus of browning subsides, parkin-dependent mitophagy is activated and dormant beige adipocytes persist. In humans, however, the molecular events during the beige to white transition have not been studied in detail. In this study, human primary subcutaneous abdominal preadipocytes were differentiated to beige for 14 days, then either the beige culture conditions were applied for an additional 14 days or it was replaced by a white medium. Control white adipocytes were differentiated by their specific cocktail for 28 days. Peroxisome proliferator-activated receptor γ-driven beige differentiation resulted in increased mitochondrial biogenesis, UCP1 expression, fragmentation, and respiration as compared to white. Morphology, UCP1 content, mitochondrial fragmentation, and basal respiration of the adipocytes that underwent transition, along with the induction of mitophagy, were similar to control white adipocytes. However, white converted beige adipocytes had a stronger responsiveness to dibutyril-cAMP, which mimics adrenergic stimulus, than the control white ones. Gene expression patterns showed that the removal of mitochondria in transitioning adipocytes may involve both parkin-dependent and -independent pathways. Preventing the entry of beige adipocytes into white transition can be a feasible way to maintain elevated thermogenesis and energy expenditure.

Paligenosis: Cellular Remodeling During Tissue Repair

Complex multicellular organisms have evolved specific mechanisms to replenish cells in homeostasis and during repair. Here, we discuss how emerging technologies (e.g., single-cell RNA sequencing) challenge the concept that tissue renewal is fueled by unidirectional differentiation from a resident stem cell. We now understand that cell plasticity, i.e., cells adaptively changing differentiation state or identity, is a central tissue renewal mechanism. For example, mature cells can access an evolutionarily conserved program (paligenosis) to reenter the cell cycle and regenerate damaged tissue. Most tissues lack dedicated stem cells and rely on plasticity to regenerate lost cells. Plasticity benefits multicellular organisms, yet it also carries risks. For one, when long-lived cells undergo paligenotic, cyclical proliferation and redif-ferentiation, they can accumulate and propagate acquired mutations that activate oncogenes and increase the potential for developing cancer. Lastly, we propose a new framework for classifying patterns of cell proliferation in homeostasis and regeneration, with stem cells representing just one of the diverse methods that adult tissues employ.

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.

Identification of a Long Noncoding RNA (lncPRDM16) Inhibiting Preadipocyte Proliferation in the Chicken

Long noncoding RNAs are vital to a variety of biological and physiological processes through multiple modes of functional interaction with DNA, RNA, and proteins. In chickens, numerous lncRNAs were discovered to be important to growth or disease progression. However, the detailed molecular function and role of lncRNAs remain less explored. Here, we performed lncRNA sequencing on abdominal adipose tissues from broiler lines divergently selected for abdominal fat content, and significantly differentially expressed lncRNAs were found, including lncPRDM16, a divergently transcribed and conserved lncRNA near PRDM16. Full lengths of two transcripts of lncPRDM16 were obtained, and their genomic structures were compared. Expression dynamics of lncPRDM16 in different tissues and during preadipocyte proliferation and differentiation were profiled. Moreover, a 250-nucleotide sequence at 5'-end was found to be inevitable to the function of lncPRDM16 in inhibiting preadipocyte proliferation and regulating the promoter activities of both lncPRDM16 and PRDM16. Taken together, we identified the 5'-end functional elements of lncPRDM16 and their potential importance in inhibiting preadipocyte proliferation. Our findings provide the foundation for further exploration of lncPRDM16 function and potential improvement of chicken muscle quality.

Severe Hyperprolactinemia Promotes Brown Adipose Tissue Whitening and Aggravates High Fat Diet Induced Metabolic Imbalance

BACKGROUND

The association of high serum prolactin and increased body weight is positive but controversial, therefore we hypothesized that additional factors such as diets and the impact of prolactin on brown adipose tissue may condition its metabolic effects.

METHODS

We used LacDrd2KO females with lifelong severe hyperprolactinemia due dopamine-D2 receptor deletion from lactotropes, and slow onset of metabolic disturbances, and compared them to their respective controls (Drd2 ^loxP/loxP ). Food intake, and binge eating was evaluated. We then challenged mice with a High Fat (HFD) or a Control Diet (CD) for 8 weeks, beginning at 3 months of age, when no differences in body weight are found between genotypes. At the end of the protocol brown and white adipose tissues were weighed, and thermogenic and lipogenic markers studied, using real time PCR (Ucp1, Cidea, Pgc1a, Lpl, adiponectin, Prlr) or immunohistochemistry (UCP1). Histochemical analysis of brown adipose tissue, and glucose tolerance tests were performed.

RESULTS

Hyperprolactinemic mice had increased food intake and binge eating behavior. Metabolic effects induced by a HFD were exacerbated in lacDrd2KO mice. Hyperprolactinemia aggravated HFD-induced body weight gain and glucose intolerance. In brown adipose tissue pronounced cellular whitening as well as decreased expression of the thermogenic markers Ucp1 and Pgc1a were observed in response to high prolactin levels, regardless of the diet, and furthermore, hyperprolactinemia potentiated the decrease in Cidea mRNA expression induced by HFD. In subcutaneous white adipose tissue hyperprolactinemia synergistically increased tissue weight, while decreasing Prlr, Adiponectin and Lpl mRNA levels regardless of the diet.

CONCLUSIONS

Pathological hyperprolactinemia has a strong impact in brown adipose tissue, lowering thermogenic markers and evoking tissue whitening. Furthermore, it modifies lipogenic markers in subcutaneous white adipose, and aggravates HFD-induced glucose intolerance and Cidea decrease. Therefore, severe high prolactin levels may target BAT function, and furthermore represent an adjuvant player in the development of obesity induced by high fat diets.

The Emerging Roles of Autophagy in Human Diseases

Autophagy, a process of cellular self-digestion, delivers intracellular components including superfluous and dysfunctional proteins and organelles to the lysosome for degradation and recycling and is important to maintain cellular homeostasis. In recent decades, autophagy has been found to help fight against a variety of human diseases, but, at the same time, autophagy can also promote the procession of certain pathologies, which makes the connection between autophagy and diseases complex but interesting. In this review, we summarize the advances in understanding the roles of autophagy in human diseases and the therapeutic methods targeting autophagy and discuss some of the remaining questions in this field, focusing on cancer, neurodegenerative diseases, infectious diseases and metabolic disorders.

Mitochondrial Protein PGAM5 Emerges as a New Regulator in Neurological Diseases

As mitochondrial dysfunction has increasingly been implicated in neurological diseases, much of the investigation focuses on the response of the mitochondria. It appears that mitochondria can respond to external stimuli speedy fast, in seconds. Understanding how mitochondria sense the signal and communicate with cytosolic pathways are keys to understand mitochondrial regulation in diseases or in response to trauma. It was not until recently that a novel mitochondrial protein, phosphoglycerate mutase family member 5 (PGAM5) has emerged to be a new regulator of mitochondrial homeostasis. Although controversial results reveal beneficial as well as detrimental roles of PGAM5 in cancers, these findings also suggest PGAM5 may have diverse regulation on cellular physiology. Roles of PGAM5 in neuronal tissues remain to be uncovered. This review discusses current knowledge of PGAM5 in neurological diseases and provides future perspectives.

Managing risky assets - mitophagy in vivo

Mitochondria, which resemble their α-proteobacteria ancestors, are a major cellular asset, producing energy 'on the cheap' through oxidative phosphorylation. They are also a liability. Increased oxidative phosphorylation means increased oxidative stress, and damaged mitochondria incite inflammation through release of their bacteria-like macromolecules. Mitophagy (the selective macroautophagy of mitochondria) controls mitochondria quality and number to manage these risky assets. Parkin, BNIP3 and NIX were identified as being part of the first mitophagy pathways identified in mammals over a decade ago, with additional pathways, including that mediated by FUNDC1 reported more recently. Loss of Parkin or PINK1 function causes Parkinson's disease, highlighting the importance of mitophagy as a quality control mechanism in the brain. Additionally, mitophagy is induced in idiopathic Parkinson's disease and Alzheimer's disease, protects the heart and other organs against energy stress and lipotoxicity, regulates metabolism by controlling mitochondrial number in brown and beige fat, and clears mitochondria during terminal differentiation of glycolytic cells, such as red blood cells and neurons. Despite its importance in disease, mitophagy is likely dispensable under physiological conditions. This Review explores the in vivo roles of mitophagy in mammalian systems, focusing on the best studied examples - mitophagy in neurodegeneration, cardiomyopathy, metabolism, and red blood cell development - to draw out common themes.

Autophagy in major human diseases

Autophagy is a core molecular pathway for the preservation of cellular and organismal homeostasis. Pharmacological and genetic interventions impairing autophagy responses promote or aggravate disease in a plethora of experimental models. Consistently, mutations in autophagy-related processes cause severe human pathologies. Here, we review and discuss preclinical data linking autophagy dysfunction to the pathogenesis of major human disorders including cancer as well as cardiovascular, neurodegenerative, metabolic, pulmonary, renal, infectious, musculoskeletal, and ocular disorders.

The multifaceted regulation of mitophagy by endogenous metabolites

Owing to the dominant functions of mitochondria in multiple cellular metabolisms and distinct types of regulated cell death, maintaining a functional mitochondrial network is fundamental for the cellular homeostasis and body fitness in response to physiological adaptations and stressed conditions. The process of mitophagy, in which the dysfunctional or superfluous mitochondria are selectively engulfed by autophagosome and subsequently degraded in lysosome, has been well formulated as one of the major mechanisms for mitochondrial quality control. To date, the PINK1-PRKN-dependent and receptors (including proteins and lipids)-dependent pathways have been characterized to determine the mitophagy in mammalian cells. The mitophagy is highly responsive to the dynamics of endogenous metabolites, including iron-, calcium-, glycolysis-TCA-, NAD⁺-, amino acids-, fatty acids-, and cAMP-associated metabolites. Herein, we summarize the recent advances toward the molecular details of mitophagy regulation in mammalian cells. We also highlight the key regulations of mammalian mitophagy by endogenous metabolites, shed new light on the bidirectional interplay between mitophagy and cellular metabolisms, with attempting to provide a perspective insight into the nutritional intervention of metabolic disorders with mitophagy deficit.Abbreviations: acetyl-CoA: acetyl-coenzyme A; ACO1: aconitase 1; ADCYs: adenylate cyclases; AMPK: AMP-activated protein kinase; ATM: ATM serine/threonine kinase; BCL2L1: BCL2 like 1; BCL2L13: BCL2 like 13; BNIP3: BCL2 interacting protein 3; BNIP3L: BCL2 interacting protein 3 like; Ca²⁺: calcium ion; CALCOCO2: calcium binding and coiled-coil domain 2; CANX: calnexin; CO: carbon monoxide; CYCS: cytochrome c, somatic; DFP: deferiprone; DNM1L: dynamin 1 like; ER: endoplasmic reticulum; FKBP8: FKBP prolyl isomerase 8; FOXO3: forkhead box O3; FTMT: ferritin mitochondrial; FUNDC1: FUN14 domain containing 1; GABA: γ-aminobutyric acid; GSH: glutathione; HIF1A: hypoxia inducible factor 1 subunit alpha; IMMT: inner membrane mitochondrial protein; IRP1: iron regulatory protein 1; ISC: iron-sulfur cluster; ITPR2: inositol 1,4,5-trisphosphate type 2 receptor; KMO: kynurenine 3-monooxygenase; LIR: LC3 interacting region; MAM: mitochondria-associated membrane; MAP1LC3: microtubule associated protein 1 light chain 3; MFNs: mitofusins; mitophagy: mitochondrial autophagy; mPTP: mitochondrial permeability transition pore; MTOR: mechanistic target of rapamycin kinase; NAD⁺: nicotinamide adenine dinucleotide; NAM: nicotinamide; NMN: nicotinamide mononucleotide; NO: nitric oxide; NPA: Niemann-Pick type A; NR: nicotinamide riboside; NR4A1: nuclear receptor subfamily 4 group A member 1; NRF1: nuclear respiratory factor 1; OPA1: OPA1 mitochondrial dynamin like GTPase; OPTN: optineurin; PARL: presenilin associated rhomboid like; PARPs: poly(ADP-ribose) polymerases; PC: phosphatidylcholine; PHB2: prohibitin 2; PINK1: PTEN induced kinase 1; PPARG: peroxisome proliferator activated receptor gamma; PPARGC1A: PPARG coactivator 1 alpha; PRKA: protein kinase AMP-activated; PRKDC: protein kinase, DNA-activated, catalytic subunit; PRKN: parkin RBR E3 ubiquitin protein ligase; RHOT: ras homolog family member T; ROS: reactive oxygen species; SIRTs: sirtuins; STK11: serine/threonine kinase 11; TCA: tricarboxylic acid; TP53: tumor protein p53; ULK1: unc-51 like autophagy activating kinase 1; VDAC1: voltage dependent anion channel 1.

Methylglyoxal attenuates isoproterenol-induced increase in uncoupling protein 1 expression through activation of JNK signaling pathway in beige adipocytes

Methylglyoxal (MG) is a metabolite derived from glycolysis whose levels in the blood and tissues of patients with diabetes are higher than those of healthy individuals, suggesting that MG is associated with the development of diabetic complications. However, it remains unknown whether high levels of MG are a cause or consequence of diabetes. Here, we show that MG negatively affects the expression of uncoupling protein 1 (UCP1), which is involved in thermogenesis and the regulation of systemic metabolism. Decreased Ucp1 expression is associated with obesity and type 2 diabetes. We found that MG attenuated the increase in Ucp1 expression following treatment with isoproterenol in beige adipocytes. However, MG did not affect protein kinase A signaling, the core coordinator of isoproterenol-induced Ucp1 expression. Instead, MG activated c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinases. We found that JNK inhibition, but not p38, recovered isoproterenol-stimulated Ucp1 expression under MG treatment. Altogether, these results suggest an inhibitory role of MG on the thermogenic function of beige adipocytes through the JNK signaling pathway.

miR-130b inhibits proliferation and promotes differentiation in myocytes via targeting Sp1

Muscle regeneration after damage or during myopathies requires a fine cooperation between myoblast proliferation and myogenic differentiation. A growing body of evidence suggests that microRNAs play critical roles in myocyte proliferation and differentiation transcriptionally. However, the molecular mechanisms underlying the orchestration are not fully understood. Here, we showed that miR-130b is able to repress myoblast proliferation and promote myogenic differentiation via targeting Sp1 transcription factor. Importantly, overexpression of miR-130b is capable of improving the recovery of damaged muscle in a freeze injury model. Moreover, miR-130b expression is declined in the muscle of muscular dystrophy patients. Thus, these results indicated that miR-130b may play a role in skeletal muscle regeneration and myopathy progression. Together, our findings suggest that the miR-130b/Sp1 axis may serve as a potential therapeutic target for the treatment of patients with muscle damage or severe myopathies.

Mitophagy in carcinogenesis, drug resistance and anticancer therapeutics

The mitochondrion is an organelle that plays a vital role in energy production, cytoplasmic protein degradation and cell death. Mitophagy is an autophagic procedure that specifically clears damaged mitochondria and maintains its homeostasis. Emerging evidence indicates that mitophagy is involved in many physiological processes, including cellular homeostasis, cellular differentiation and nerve protection. In this review, we describe the regulatory mechanisms of mitophagy in mammals and yeasts and highlight the recent advances relevant to its function in carcinogenesis and drug resistance. Finally, a section has been dedicated to describing the role of mitophagy in anticancer therapeutics, which is a new frontier that offers a precise and promising strategy.

Fight against fibrosis in adipose tissue remodeling

Adipose is a key tissue regulating energy homeostasis. In states of obesity, caloric intake exceeds energy expenditure, thereby accelerating lipid accumulation with ongoing extracellular matrix (ECM) remodeling. Excess deposition of lipids and expansion of adipocytes potentially decrease ECM flexibility with local hypoxia and inflammation. Hypoxia and chronic low-grade inflammation accelerate the development of adipose tissue fibrosis and related metabolic dysfunctions. Recent research investigated that some cytokines and proteins are functional in regulating energy homeostasis, meanwhile, are potential targets to fight against adipose tissue fibrosis and insulin resistance. In this review, we focused on the regulatory mechanisms and mediators in remodeling of adipose tissue fibrosis, along with their relevance to clinical manifestations.

On the potential role of globins in brown adipose tissue: a novel conceptual model and studies in myoglobin knockout mice

Myoglobin (Mb) regulates O₂ bioavailability in muscle and heart as the partial pressure of O₂ (Po₂) drops with increased tissue workload. Globin proteins also modulate cellular NO pools, "scavenging" NO at higher Po₂ and converting NO₂^- to NO as Po₂ falls. Myoglobin binding of fatty acids may also signal a role in fat metabolism. Interestingly, Mb is expressed in brown adipose tissue (BAT), but its function is unknown. Herein, we present a new conceptual model that proposes links between BAT thermogenic activation, concurrently reduced Po₂, and NO pools regulated by deoxy/oxy-globin toggling and xanthine oxidoreductase (XOR). We describe the effect of Mb knockout (Mb^-/-) on BAT phenotype [lipid droplets, mitochondrial markers uncoupling protein 1 (UCP1) and cytochrome C oxidase 4 (Cox4), transcriptomics] in male and female mice fed a high-fat diet (HFD, 45% of energy, ∼13 wk), and examine Mb expression during brown adipocyte differentiation. Interscapular BAT weights did not differ by genotype, but there was a higher prevalence of mid-large sized droplets in Mb^-/-. COX4 protein expression was significantly reduced in Mb^-/- BAT, and a suite of metabolic/NO/stress/hypoxia transcripts were lower. All of these Mb^-/--associated differences were most apparent in females. The new conceptual model, and results derived from Mb^-/- mice, suggest a role for Mb in BAT metabolic regulation, in part through sexually dimorphic systems and NO signaling. This possibility requires further validation in light of significant mouse-to-mouse variability of BAT Mb mRNA and protein abundances in wild-type mice and lower expression relative to muscle and heart.NEW & NOTEWORTHY Myoglobin confers the distinct red color to muscle and heart, serving as an oxygen-binding protein in oxidative fibers. Less attention has been paid to brown fat, a thermogenic tissue that also expresses myoglobin. In a mouse knockout model lacking myoglobin, brown fat had larger fat droplets and lower markers of mitochondrial oxidative metabolism, especially in females. Gene expression patterns suggest a role for myoglobin as an oxygen/nitric oxide-sensor that regulates cellular metabolic and signaling pathways.

Proteostasis in thermogenesis and obesity

The proper production, degradation, folding and activity of proteins, proteostasis, is essential for any cellular function. From single cell organisms to humans, selective pressures have led to the evolution of adaptive programs that ensure proteins are properly produced and disposed of when necessary. Environmental factors such as temperature, nutrient availability, pathogens as well as predators have greatly influenced the development of mechanisms such as the unfolded protein response, endoplasmic reticulum-associated protein degradation and autophagy, working together in concert to secure cellular proteostasis. In our modern society, the metabolic systems of the human body face the distinct challenge of changed diets, chronic overnutrition and sedentary lifestyles. Obesity and excess white adipose tissue accumulation are linked to a cluster of metabolic diseases and disturbed proteostasis is a common feature. Conversely, processes that promote energy expenditure such as exercise, shivering as well as non-shivering thermogenesis by brown adipose tissue (BAT) and beige adipocytes counteract metabolic dysfunction. Here we review the basic concepts of proteostasis in obesity-linked metabolic diseases and focus on adipocytes, which are critical regulators of mammalian energy metabolism.

A Selective Look at Autophagy in Pancreatic β-Cells

Insulin-producing pancreatic β-cells are central to glucose homeostasis, and their failure is a principal driver of diabetes development. To preserve optimal health β-cells must withstand both intrinsic and extrinsic stressors, ranging from inflammation to increased peripheral insulin demand, in addition to maintaining insulin biosynthesis and secretory machinery. Autophagy is increasingly being appreciated as a critical β-cell quality control system vital for glycemic control. Here we focus on the underappreciated, yet crucial, roles for selective and organelle-specific forms of autophagy as mediators of β-cell health. We examine the unique molecular players underlying each distinct form of autophagy in β-cells, including selective autophagy of mitochondria, insulin granules, lipid, intracellular amyloid aggregates, endoplasmic reticulum, and peroxisomes. We also describe how defects in selective autophagy pathways contribute to the development of diabetes. As all forms of autophagy are not the same, a refined view of β-cell selective autophagy may inform new approaches to defend against the various insults leading to β-cell failure in diabetes.

The cellular and functional complexity of thermogenic fat

Brown and beige adipocytes are mitochondria-enriched cells capable of dissipating energy in the form of heat. These thermogenic fat cells were originally considered to function solely in heat generation through the action of the mitochondrial protein uncoupling protein 1 (UCP1). In recent years, significant advances have been made in our understanding of the ontogeny, bioenergetics and physiological functions of thermogenic fat. Distinct subtypes of thermogenic adipocytes have been identified with unique developmental origins, which have been increasingly dissected in cellular and molecular detail. Moreover, several UCP1-independent thermogenic mechanisms have been described, expanding the role of these cells in energy homeostasis. Recent studies have also delineated roles for these cells beyond the regulation of thermogenesis, including as dynamic secretory cells and as a metabolic sink. This Review presents our current understanding of thermogenic adipocytes with an emphasis on their development, biological functions and roles in systemic physiology.

Thermogenic Fat: Development, Physiological Function, and Therapeutic Potential

The concerning worldwide increase of obesity and chronic metabolic diseases, such as T2D, dyslipidemia, and cardiovascular disease, motivates further investigations into preventive and alternative therapeutic approaches. Over the past decade, there has been growing evidence that the formation and activation of thermogenic adipocytes (brown and beige) may serve as therapy to treat obesity and its associated diseases owing to its capacity to increase energy expenditure and to modulate circulating lipids and glucose levels. Thus, understanding the molecular mechanism of brown and beige adipocytes formation and activation will facilitate the development of strategies to combat metabolic disorders. Here, we provide a comprehensive overview of pathways and players involved in the development of brown and beige fat, as well as the role of thermogenic adipocytes in energy homeostasis and metabolism. Furthermore, we discuss the alterations in brown and beige adipose tissue function during obesity and explore the therapeutic potential of thermogenic activation to treat metabolic syndrome.

Sesamol promotes browning of white adipocytes to ameliorate obesity by inducing mitochondrial biogenesis and inhibition mitophagy via β3-AR/PKA signaling pathway

Background

Obesity is defined as an imbalance between energy intake and expenditure, and it is a serious risk factor of non-communicable diseases. Recently many studies have shown that promoting browning of white adipose tissue (WAT) to increase energy consumption has a great therapeutic potential for obesity. Sesamol, a lignan from sesame oil, had shown potential beneficial functions on obesity treatment.

Objective

In this study, we used C57BL/6J mice and 3T3-L1 adipocytes to investigate the effects and the fundamental mechanisms of sesamol in enhancing the browning of white adipocytes to ameliorate obesity.

Methods

Sixteen-week-old C57BL/6J male mice were fed high-fat diet (HFD) for 8 weeks to establish the obesity models. Half of the obese mice were administered with sesamol (100 mg/kg body weight [b.w.]/day [d] by gavage for another 8 weeks. Triacylglycerol (TG) and total cholesterol assay kits were used to quantify serum TG and total cholesterol (TC). Oil red O staining was used to detect lipid droplet in vitro. Mito-Tracker Green was used to detect the mitochondrial content. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used to detect the levels of beige-specific genes. Immunoblotting was used to detect the proteins involved in beige adipocytes formation.

Results

Sesamol decreased the content of body fat and suppressed lipid accumulation in HFD-induced obese mice. In addition, sesamol significantly upregulated uncoupling protein-1 (UCP1) protein in adipose tissue. Further research found that sesamol also significantly activated the browning program in mature 3T3-L1 adipocytes, manifested by the increase in beige-specific genes and proteins. Moreover, sesamol greatly increased mitochondrial biogenesis, as proved by the upregulated protein levels of mitochondrial biogenesis, and the inhibition of the proteins associated with mitophagy. Furthermore, β3-adrenergic receptor (β3-AR), protein kinase A-C (PKA-C) and Phospho-protein kinase A (p-PKA) substrate were elevated by sesamol, and these effects were abolished by the pretreatment of antagonists β3-AR.

Conclusion

Sesamol promoted browning of white adipocytes by inducing mitochondrial biogenesis and inhibiting mitophagy through the β3-AR/PKA pathway. This preclinical data promised the potential to consider sesamol as a metabolic modulator of HFD-induced obesity.

TFEB deficiency attenuates mitochondrial degradation upon brown adipose tissue whitening at thermoneutrality

OBJECTIVE

Brown adipose tissue (BAT) thermogenesis offers the potential to improve metabolic health in mice and humans. However, humans predominantly live under thermoneutral conditions, leading to BAT whitening, a reduction in BAT mitochondrial content and metabolic activity. Recent studies have established mitophagy as a major driver of mitochondrial degradation in the whitening of thermogenic brite/beige adipocytes, yet the pathways mediating mitochondrial breakdown in whitening of classical BAT remain largely elusive. The transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy belonging to the MiT family of transcription factors, is the only member of this family that is upregulated during whitening, pointing toward a role of TFEB in whitening-associated mitochondrial breakdown.

METHODS

We generated brown adipocyte-specific TFEB knockout mice, and induced BAT whitening by thermoneutral housing. We characterized gene and protein expression patterns, BAT metabolic activity, systemic metabolism, and mitochondrial localization using in vivo and in vitro approaches.

RESULTS

Under low thermogenic activation conditions, deletion of TFEB preserves mitochondrial mass independently of mitochondriogenesis in BAT and primary brown adipocytes. However, this does not translate into elevated thermogenic capacity or protection from diet-induced obesity. Autophagosomal/lysosomal marker levels are altered in TFEB-deficient BAT and primary adipocytes, and lysosomal markers co-localize and co-purify with mitochondria in TFEB-deficient BAT, indicating trapping of mitochondria in late stages of mitophagy.

CONCLUSION

We identify TFEB as a driver of BAT whitening, mediating mitochondrial degradation via the autophagosomal and lysosomal machinery. This study provides proof of concept that interfering with the mitochondrial degradation machinery can increase mitochondrial mass in classical BAT under human-relevant conditions. However, it must be considered that interfering with autophagy may result in accumulation of non-functional mitochondria. Future studies targeting earlier steps of mitophagy or target recognition are therefore warranted.