Lipocalin 2 regulates brown fat activation via a nonadrenergic activation mechanism

The role of bone in energy metabolism: A focus on osteocalcin

Understanding the mechanisms involved in whole body glucose regulation is key for the discovery of new treatments for type 2 diabetes (T2D). Historically, glucose regulation was largely focused on responses to insulin and glucagon. Impacts of incretin-based therapies, and importance of muscle mass, are also highly relevant. Recently, bone was recognized as an endocrine organ, with several bone proteins, known as osteokines, implicated in glucose metabolism through their effects on the liver, skeletal muscle, and adipose tissue. Research efforts mostly focused on osteocalcin (OC) as a leading example. This review will provide an overview on this role of bone by discussing bone turnover markers (BTMs), the receptor activator of nuclear factor kB ligand (RANKL), osteoprotegerin (OPG), sclerostin (SCL) and lipocalin 2 (LCN2), with a focus on OC. Since 2007, some, but not all, research using mostly OC genetically modified animal models suggested undercarboxylated (uc) OC acts as a hormone involved in energy metabolism. Most data generated from in vivo, ex vivo and in vitro models, indicate that exogenous ucOC administration improves whole-body and skeletal muscle glucose metabolism. Although data in humans are generally supportive, findings are often discordant likely due to methodological differences and observational nature of that research. Overall, evidence supports the concept that bone-derived factors are involved in energy metabolism, some having beneficial effects (ucOC, OPG) others negative (RANKL, SCL), with the role of some (LCN2, other BTMs) remaining unclear. Whether the effect of osteokines on glucose regulation is clinically significant and of therapeutic value for people with insulin resistance and T2D remains to be confirmed.

Crosstalk between Lipid Metabolism and Bone Homeostasis: Exploring Intricate Signaling Relationships

Bone is a dynamic tissue reshaped by constant bone formation and bone resorption to maintain its function. The skeletal system accounts for approximately 70% of the total volume of the body, and continuous bone remodeling requires quantities of energy and material consumption. Adipose tissue is the main energy storehouse of the body and has a strong adaptive capacity to participate in the regulation of various physiological processes. Considering that obesity and metabolic syndrome have become major public health challenges, while osteoporosis and osteoporotic fractures have become other major health problems in the aging population, it would be interesting to explore these 2 diseases together. Currently, an increasing number of researchers are focusing on the interactions between multiple tissue systems, i.e., multiple organs and tissues that are functionally coordinated together and pathologically pathologically interact with each other in the body. However, there is lack of detailed reviews summarizing the effects of lipid metabolism on bone homeostasis and the interactions between adipose tissue and bone tissue. This review provides a detailed summary of recent advances in understanding how lipid molecules and adipose-derived hormones affect bone homeostasis, how bone tissue, as a metabolic organ, affects lipid metabolism, and how lipid metabolism is regulated by bone-derived cytokines.

Enhancing cardiometabolic health: unveiling the synergistic effects of high-intensity interval training with spirulina supplementation on selected adipokines, insulin resistance, and anthropometric indices in obese males

This study investigated the combined effects of 12 weeks of high-intensity interval training (HIIT) and spirulina supplementation on adipokine levels, insulin resistance, anthropometric indices, and cardiorespiratory fitness in 44 obese males (aged 25-40 years). The participants were randomly assigned to one of four groups: control (CG), supplement (SG), training (TG), or training plus supplement (TSG). The intervention involved daily administration of either spirulina or a placebo and HIIT three times a week for the training groups. Anthropometric indices, HOMA-IR, VO2peak, and circulating adipokines (asprosin and lipocalin2, omentin-1, irisin, and spexin) were measured before and after the 12-week intervention. Post-intervention analysis indicated differences between the CG and the three interventional groups for body weight, fat-free mass (FFM), percent body fat (%BF), HOMA-IR, and adipokine levels (p < 0.05). TG and SG participants had increased VO2peak (p < 0.05). Spirulina supplementation with HIIT increased VO2peak, omentin-1, irisin, and spexin, while causing decreases in lipocalin-2 and asprosin levels and improvements in body composition (weight, %fat), BMI, and HOMA-IR. Notably, the combination of spirulina and HIIT produced more significant changes in circulating adipokines and cardiometabolic health in obese males compared to either supplementation or HIIT alone (p < 0.05). These findings highlight the synergistic benefits of combining spirulina supplementation with HIIT, showcasing their potential in improving various health parameters and addressing obesity-related concerns in a comprehensive manner.

White-brown adipose tissue interplay in polycystic ovary syndrome: Therapeutic avenues

This study highlights the therapeutic potential of activating brown adipose tissue (BAT) for managing polycystic ovary syndrome (PCOS), a prevalent endocrine disorder associated with metabolic and reproductive abnormalities. BAT plays a crucial role in regulating energy expenditure and systemic insulin sensitivity, making it an attractive target for the treatment of obesity and metabolic diseases. Recent research suggests that impaired BAT function and mass may contribute to the link between metabolic disturbances and reproductive issues in PCOS. Additionally, abnormal white adipose tissue (WAT) can exacerbate these conditions by releasing adipokines and nonesterified fatty acids. In this review, we explored the impact of WAT changes on BAT function in PCOS and discussed the potential of BAT activation as a therapeutic strategy to improve PCOS symptoms. We propose that BAT activation holds promise for managing PCOS; however, further research is needed to confirm its efficacy and to develop clinically feasible methods for BAT activation.

Lipocalin 2 regulates mitochondrial phospholipidome remodeling, dynamics, and function in brown adipose tissue in male mice

Mitochondrial function is vital for energy metabolism in thermogenic adipocytes. Impaired mitochondrial bioenergetics in brown adipocytes are linked to disrupted thermogenesis and energy balance in obesity and aging. Phospholipid cardiolipin (CL) and phosphatidic acid (PA) jointly regulate mitochondrial membrane architecture and dynamics, with mitochondria-associated endoplasmic reticulum membranes (MAMs) serving as the platform for phospholipid biosynthesis and metabolism. However, little is known about the regulators of MAM phospholipid metabolism and their connection to mitochondrial function. We discover that LCN2 is a PA binding protein recruited to the MAM during inflammation and metabolic stimulation. Lcn2 deficiency disrupts mitochondrial fusion-fission balance and alters the acyl-chain composition of mitochondrial phospholipids in brown adipose tissue (BAT) of male mice. Lcn2 KO male mice exhibit an increase in the levels of CLs containing long-chain polyunsaturated fatty acids (LC-PUFA), a decrease in CLs containing monounsaturated fatty acids, resulting in mitochondrial dysfunction. This dysfunction triggers compensatory activation of peroxisomal function and the biosynthesis of LC-PUFA-containing plasmalogens in BAT. Additionally, Lcn2 deficiency alters PA production, correlating with changes in PA-regulated phospholipid-metabolizing enzymes and the mTOR signaling pathway. In conclusion, LCN2 plays a critical role in the acyl-chain remodeling of phospholipids and mitochondrial bioenergetics by regulating PA production and its function in activating signaling pathways.

Peroxisom proliferator-activated receptor-γ coactivator-1α in neurodegenerative disorders: A promising therapeutic target

Neurodegenerative disorders (NDDs) are characterized by progressive loss of selectively vulnerable neuronal populations and myelin sheath, leading to behavioral and cognitive dysfunction that adversely affect the quality of life. Identifying novel therapies that attenuate the progression of NDDs would be of significance. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a widely expressed transcriptional regulator, modulates the expression of genes engaged in mitochondrial biosynthesis, metabolic regulation, and oxidative stress (OS). Emerging evidences point to the strong connection between PGC-1α and NDDs, suggesting its positive impaction on the progression of NDDs. Therefore, it is urgent to gain a deeper and broader understanding between PGC-1α and NDDs. To this end, this review presents a comprehensive overview of PGC-1α, including its basic characteristics, the post-translational modulations, as well as the interacting transcription factors. Secondly, the pathogenesis of PGC-1α in various NDDs, such as Alzheimer's (AD), Parkinson's (PD), and Huntington's disease (HD) is briefly discussed. Additionally, this study summarizes the underlying mechanisms that PGC-1α is neuroprotective in NDDs via regulating neuroinflammation, OS, and mitochondrial dysfunction. Finally, we briefly outline the shortcomings of current NDDs drug therapy, and summarize the functions and potential applications of currently available PGC-1α modulators (activator or inhibitors). Generally, this review updates our insight of the important role of PGC-1α on the development of NDDs, and provides a promising therapeutic target/ drug for the treatment of NDDs.

Bmp8a deletion leads to obesity through regulation of lipid metabolism and adipocyte differentiation

The role of bone morphogenetic proteins (BMPs) in regulating adipose has recently become a field of interest. However, the underlying mechanism of this effect has not been elucidated. Here we show that the anti-fat effect of Bmp8a is mediated by promoting fatty acid oxidation and inhibiting adipocyte differentiation. Knocking out the bmp8a gene in zebrafish results in weight gain, fatty liver, and increased fat production. The bmp8a-/- zebrafish exhibits decreased phosphorylation levels of AMPK and ACC in the liver and adipose tissues, indicating reduced fatty acid oxidation. Also, Bmp8a inhibits the differentiation of 3T3-L1 preadipocytes into mature adipocytes by activating the Smad2/3 signaling pathway, in which Smad2/3 binds to the central adipogenic factor PPARγ promoter to inhibit its transcription. In addition, lentivirus-mediated overexpression of Bmp8a in 3T3-L1 cells significantly increases NOD-like receptor, TNF, and NF-κB signaling pathways. Furthermore, NF-κB interacts with PPARγ, blocking PPARγ's activation of its target gene Fabp4, thereby inhibiting adipocyte differentiation. These data bring a signal bridge between immune regulation and adipocyte differentiation. Collectively, our findings indicate that Bmp8a plays a critical role in regulating lipid metabolism and adipogenesis, potentially providing a therapeutic approach for obesity and its comorbidities.

Novel Adipokines and Their Role in Bone Metabolism: A Narrative Review

The growing burden of obesity and osteoporosis is a major public health concern. Emerging evidence of the role of adipokines on bone metabolism has led to the discovery of novel adipokines over the last decade. Obesity is recognized as a state of adipose tissue inflammation that adversely affects bone health. Adipokines secreted from white adipose tissue (WAT) and bone marrow adipose tissue (BMAT) exerts endocrine and paracrine effects on the survival and function of osteoblasts and osteoclasts. An increase in marrow fat is implicated in osteoporosis and, hence, it is crucial to understand the complex interplay between adipocytes and bone. The objective of this review is to summarize recent advances in our understanding of the role of different adipokines on bone metabolism.

METHODS

This is a comprehensive review of the literature available in PubMED and Cochrane databases, with an emphasis on the last five years using the keywords.

RESULTS

Leptin has shown some positive effects on bone metabolism; in contrast, both adiponectin and chemerin have consistently shown a negative association with BMD. No significant association was found between resistin and BMD. Novel adipokines such as visfatin, LCN-2, Nesfatin-1, RBP-4, apelin, and vaspin have shown bone-protective and osteoanabolic properties that could be translated into therapeutic targets.

CONCLUSION

New evidence suggests the potential role of novel adipokines as biomarkers to predict osteoporosis risk, and as therapeutic targets for the treatment of osteoporosis.

Adipokines from white adipose tissue in regulation of whole body energy homeostasis

Diseases originating from altered energy homeostasis including obesity, and type 2 diabetes are rapidly increasing worldwide. Research in the last few decades on animal models and humans demonstrates that the white adipose tissue (WAT) is critical for energy balance and more than just an energy storage site. WAT orchestrates the whole-body metabolism through inter-organ crosstalk primarily mediated by cytokines named "Adipokines". The adipokines influence metabolism and fuel selection of the skeletal muscle and liver thereby fine-tuning the load on WAT itself in physiological conditions like starvation, exercise and cold. In addition, adipokine secretion is influenced by various pathological conditions like obesity, inflammation and diabetes. In this review, we have surveyed the current state of knowledge on important adipokines and their significance in regulating energy balance and metabolic diseases. Furthermore, we have summarized the interplay of pro-inflammatory and anti-inflammatory adipokines in the modulation of pathological conditions.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

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.

Thromboinflammatory Processes at the Nexus of Metabolic Dysfunction and Prostate Cancer: The Emerging Role of Periprostatic Adipose Tissue

Simple Summary

As overweight and obesity increase among the population worldwide, a parallel increase in the number of individuals diagnosed with prostate cancer was observed. There appears to be a relationship between both diseases where the increase in the mass of fat tissue can lead to inflammation. Such a state of inflammation could produce many factors that increase the aggressiveness of prostate cancer, especially if this inflammation occurred in the fat stores adjacent to the prostate. Another important observation that links obesity, fat tissue inflammation, and prostate cancer is the increased production of blood clotting factors. In this article, we attempt to explain the role of these latter factors in the effect of increased body weight on the progression of prostate cancer and propose new ways of treatment that act by affecting how these clotting factors work.

Abstract

The increased global prevalence of metabolic disorders including obesity, insulin resistance, metabolic syndrome and diabetes is mirrored by an increased incidence of prostate cancer (PCa). Ample evidence suggests that these metabolic disorders, being characterized by adipose tissue (AT) expansion and inflammation, not only present as risk factors for the development of PCa, but also drive its increased aggressiveness, enhanced progression, and metastasis. Despite the emerging molecular mechanisms linking AT dysfunction to the various hallmarks of PCa, thromboinflammatory processes implicated in the crosstalk between these diseases have not been thoroughly investigated. This is of particular importance as both diseases present states of hypercoagulability. Accumulating evidence implicates tissue factor, thrombin, and active factor X as well as other players of the coagulation cascade in the pathophysiological processes driving cancer development and progression. In this regard, it becomes pivotal to elucidate the thromboinflammatory processes occurring in the periprostatic adipose tissue (PPAT), a fundamental microenvironmental niche of the prostate. Here, we highlight key findings linking thromboinflammation and the pleiotropic effects of coagulation factors and their inhibitors in metabolic diseases, PCa, and their crosstalk. We also propose several novel therapeutic targets and therapeutic interventions possibly modulating the interaction between these pathological states.

Development of a New Index Based on Preoperative Serum Lipocalin 2 to Predict Post-LSG Weight Reduction

Background

Bariatric surgery is the most effective therapy for obesity, but targeted weight reduction is not always achieved. Serum lipocalin-2 (LCN2) is closely associated with obesity, but its impact on weight loss after surgery is unknown. We aimed to access the reliability of LCN2 levels and other parameters as effective predictors of excellent weight loss (≥ 75% excess weight loss (EWL)) 1 year after bariatric surgery.

Methods

This retrospective study evaluated 450 patients (aged 18–65 years) with obesity at 3 months and 1 year after laparoscopic sleeve gastrectomy (LSG) surgery. Seventy-four patients who underwent LSG surgery and met the inclusion and exclusion criteria were included in this study. Serum LCN2, thyroid function, and metabolic and anthropometric parameters were assessed. Weight reduction was expressed as %EWL and percent total weight loss (%TWL) at 3 months and 1 year post surgery. Multivariable logistic regression analysis and receiver operating characteristic (ROC) curve analysis were used to evaluate predictors of ≥ 75%EWL.

Results

In our cohort, %EWL and %TWL were both strongly associated with preoperative serum LCN2 levels. The binary logistic regression analysis showed that preoperative LCN2, waist circumference, and glycated hemoglobin were independent predictors of excellent weight loss.

Conclusions

Based on these results, we determined a new P index with better predictive value for excellent weight reduction (≥ 75%EWL) 1 year after LSG surgery.

Graphical abstract

Adipose-Derived Lipid-Binding Proteins: The Good, the Bad and the Metabolic Diseases

Adipose tissue releases a large range of bioactive factors called adipokines, many of which are involved in inflammation, glucose homeostasis and lipid metabolism. Under pathological conditions such as obesity, most of the adipokines are upregulated and considered as deleterious, due to their pro-inflammatory, pro-atherosclerotic or pro-diabetic properties, while only a few are downregulated and would be designated as beneficial adipokines, thanks to their counteracting properties against the onset of comorbidities. This review focuses on six adipose-derived lipid-binding proteins that have emerged as key factors in the development of obesity and diabetes: Retinol binding protein 4 (RBP4), Fatty acid binding protein 4 (FABP4), Apolipoprotein D (APOD), Lipocalin-2 (LCN2), Lipocalin-14 (LCN14) and Apolipoprotein M (APOM). These proteins share structural homology and capacity to bind small hydrophobic molecules but display opposite effects on glucose and lipid metabolism. RBP4 and FABP4 are positively associated with metabolic syndrome, while APOD and LCN2 are ubiquitously expressed proteins with deleterious or beneficial effects, depending on their anatomical site of expression. LCN14 and APOM have been recently identified as adipokines associated with healthy metabolism. Recent findings on these lipid-binding proteins exhibiting detrimental or protective roles in human and murine metabolism and their involvement in metabolic diseases are also discussed.

Lipocalin 2 as a link between ageing, risk factor conditions and age-related brain diseases

Chronic (neuro)inflammation plays an important role in many age-related central nervous system (CNS) diseases, including Alzheimer's disease, Parkinson's disease and vascular dementia. Inflammation also characterizes many conditions that form a risk factor for these CNS disorders, such as physical inactivity, obesity and cardiovascular disease. Lipocalin 2 (Lcn2) is an inflammatory protein shown to be involved in different age-related CNS diseases, as well as risk factor conditions thereof. Lcn2 expression is increased in the periphery and the brain in different age-related CNS diseases and also their risk factor conditions. Experimental studies indicate that Lcn2 contributes to various neuropathophysiological processes of age-related CNS diseases, including exacerbated neuroinflammation, cell death and iron dysregulation, which may negatively impact cognitive function. We hypothesize that increased Lcn2 levels as a result of age-related risk factor conditions may sensitize the brain and increase the risk to develop age-related CNS diseases. In this review we first provide a comprehensive overview of the known functions of Lcn2, and its effects in the CNS. Subsequently, this review explores Lcn2 as a potential (neuro)inflammatory link between different risk factor conditions and the development of age-related CNS disorders. Altogether, evidence convincingly indicates Lcn2 as a key constituent in ageing and age-related brain diseases.

Dynamic Changes of the Bone Marrow Niche: Mesenchymal Stromal Cells and Their Progeny During Aging and Leukemia

Mesenchymal stromal cells (MSCs) are a heterogenous cell population found in a wide range of tissues in the body, known for their nutrient-producing and immunomodulatory functions. In the bone marrow (BM), these MSCs are critical for the regulation of hematopoietic stem cells (HSC) that are responsible for daily blood production and functional immunity throughout an entire organism's lifespan. Alongside other stromal cells, MSCs form a specialized microenvironment BM tissue called "niche" that tightly controls HSC self-renewal and differentiation. In addition, MSCs are crucial players in maintaining bone integrity and supply of hormonal nutrients due to their capacity to differentiate into osteoblasts and adipocytes which also contribute to cellular composition of the BM niche. However, MSCs are known to encompass a large heterogenous cell population that remains elusive and poorly defined. In this review, we focus on deciphering the BM-MSC biology through recent advances in single-cell identification of hierarchical subsets with distinct functionalities and transcriptional profiles. We also discuss the contribution of MSCs and their osteo-adipo progeny in modulating the complex direct cell-to-cell or indirect soluble factors-mediated interactions of the BM HSC niche during homeostasis, aging and myeloid malignancies. Lastly, we examine the therapeutic potential of MSCs for rejuvenation and anti-tumor remedy in clinical settings.

Neural Mechanisms of Cancer Cachexia

Simple Summary

Cancer cachexia is a devastating wasting syndrome that occurs in many illnesses, with signs and symptoms including anorexia, weight loss, cognitive impairment and fatigue. The brain is capable of exerting overarching homeostatic control of whole-body metabolism and is increasingly being recognized as an important mediator of cancer cachexia. Given the increased recognition and discovery of neural mechanisms of cancer cachexia, we sought to provide an in-depth review and update of mechanisms by which the brain initiates and propagates cancer cachexia programs. Furthermore, recent work has identified new molecular mediators of cachexia that exert their effects through their direct interaction with the brain. Therefore, this review will summarize neural mechanisms of cachexia and discuss recently identified neural mediators of cancer cachexia.

Abstract

Nearly half of cancer patients suffer from cachexia, a metabolic syndrome characterized by progressive atrophy of fat and lean body mass. This state of excess catabolism decreases quality of life, ability to tolerate treatment and eventual survival, yet no effective therapies exist. Although the central nervous system (CNS) orchestrates several manifestations of cachexia, the precise mechanisms of neural dysfunction during cachexia are still being unveiled. Herein, we summarize the cellular and molecular mechanisms of CNS dysfunction during cancer cachexia with a focus on inflammatory, autonomic and neuroendocrine processes and end with a discussion of recently identified CNS mediators of cachexia, including GDF15, LCN2 and INSL3.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Short-term protein restriction at advanced age stimulates FGF21 signalling, energy expenditure and browning of white adipose tissue

Dietary protein restriction has been demonstrated to improve metabolic health under various conditions. However, the relevance of ageing and age-related decline in metabolic flexibility on the effects of dietary protein restriction has not been addressed. Therefore, we investigated the effect of short-term dietary protein restriction on metabolic health in young and aged mice. Young adult (3 months old) and aged (18 months old) C57Bl/6J mice were subjected to a 3-month dietary protein restriction. Outcome parameters included fibroblast growth factor 21 (FGF21) levels, muscle strength, glucose tolerance, energy expenditure (EE) and transcriptomics of brown and white adipose tissue (WAT). Here, we report that a low-protein diet had beneficial effects in aged mice by reducing some aspects of age-related metabolic decline. These effects were characterized by increased plasma levels of FGF21, browning of subcutaneous WAT, increased body temperature and EE, while no changes were observed in glucose homeostasis and insulin sensitivity. Moreover, the low-protein diet used in this study was well-tolerated in aged mice indicated by the absence of adverse effects on body weight, locomotor activity and muscle performance. In conclusion, our study demonstrates that a short-term reduction in dietary protein intake can impact age-related metabolic health alongside increased FGF21 signalling, without negatively affecting muscle function. These findings highlight the potential of protein restriction as a strategy to induce EE and browning of WAT in aged individuals.

D-Mannitol Induces a Brown Fat-like Phenotype via a β3-Adrenergic Receptor-Dependent Mechanism

The presence of brown adipocytes within white adipose tissue is associated with phenotypes that exhibit improved metabolism and proper body weight maintenance. Therefore, a variety of dietary agents that facilitate the browning of white adipocytes have been investigated. In this study, we screened a natural product library comprising 133 compounds with the potential to promote the browning of white adipocytes, and found that D-mannitol induces the browning of 3T3-L1 adipocytes by enhancing the expression of brown fat-specific genes and proteins, and upregulating lipid metabolism markers. D-mannitol also increased the phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase 1 (ACC), suggesting a possible role in lipolysis and fat oxidation. Moreover, an increase in the expression of genes associated with D-mannitol-induced browning was strongly correlated with the activation of the β3-adrenergic receptor as well as AMPK, protein kinase A (PKA), and PPARγ coactivator 1α (PGC1α). D-mannitol effectively reduced the body weight of mice fed a high-fat diet, and increased the expression of β1-oxidation and energy expenditure markers, such as Cidea, carnitine palmityl transferase 1 (CPT1), uncoupling protein 1 (UCP1), PGC1α, and acyl-coenzyme A oxidase (ACOX1) in the inguinal white adipose tissue. Our findings suggest that D-mannitol plays a dual regulatory role by inducing the generation of a brown fat-like phenotype and enhancing lipid metabolism. These results indicate that D-mannitol can function as an anti-obesity supplement.

Capmatinib attenuates lipogenesis in 3T3-L1 adipocytes through an adenosine monophosphate-activated protein kinase-dependent pathway

Recently, there is a rapid increase in the incidence of obesity, a condition for which there are no effective therapeutic agents. Capmatinib (CAP), a novel mesenchymal-to-epithelial transition inhibitor, is reported to attenuate pro-inflammatory mediators and oxidative stress. In this study, the effects of CAP on lipogenesis in the adipocytes were examined. Treatment with CAP dose-dependently suppressed lipid accumulation in, and differentiation of, and increased lipolysis in, 3T3-L1 adipocytes. Additionally, CAP treatment augmented adenosine monophosphate-activated protein kinase (AMPK) phosphorylation and FNDC5 expression in the adipocytes. Transfection with si-AMPK or si-FNDC5 mitigated the CAP-induced suppression of lipogenesis and enhanced lipolysis. Furthermore, transfection with si-FNDC5 mitigated the CAP-induced phosphorylation of AMPK. These results suggest that the anti-obesity effect of CAP is mediated through the irisin/AMPK pathway and that CAP is a novel therapeutic agent for obesity.

Lipocalin-2 counteracts metabolic dysregulation in obesity and diabetes

Regulation of food intake is a recently identified endocrine function of bone that is mediated by Lipocalin-2 (LCN2). Osteoblast-secreted LCN2 suppresses appetite and decreases fat mass while improving glucose metabolism. We now show that serum LCN2 levels correlate with insulin levels and β-cell function, indices of healthy glucose metabolism, in obese mice and obese, prediabetic women. However, LCN2 serum levels also correlate with body mass index and insulin resistance in the same individuals and are increased in obese mice. To dissect this apparent discrepancy, we modulated LCN2 levels in mice. Silencing Lcn2 expression worsens metabolic dysfunction in genetic and diet-induced obese mice. Conversely, increasing circulating LCN2 levels improves metabolic parameters and promotes β-cell function in mouse models of β-cell failure acting as a growth factor necessary for β-cell adaptation to higher metabolic load. These results indicate that LCN2 up-regulation is a protective mechanism to counteract obesity-induced glucose intolerance by decreasing food intake and promoting adaptive β-cell proliferation.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

The endocrine role of bone: Novel functions of bone-derived cytokines

Bone-derived cytokines refer to various proteins and peptides that are released from the skeleton and can distribute in organisms to regulate homeostasis by targeting many organs, such as the pancreas, brain, testicles, and kidneys. In addition to providing support and movement, many studies have disclosed the novel endocrine function of bone, and bone can modulate glucose and energy metabolism as well as phosphate metabolism by versatile bone-derived cytokines. However, this specific exoskeletonfunction of bone-derived cytokines in the regulation of homeostasis and the pathological response caused by skeletal dysfunction are still not very clear, and elucidation of the above mechanisms is of great significance for understanding the pathological processes of metabolic disorders and in the search for novel therapeutic measures for maintaining organ stability and physical fitness.

Lipocalin-2 deficiency may predispose to the progression of spontaneous age-related adiposity in mice

Lipocalin-2 (Lcn2) is an innate immune protein elevated by several orders of magnitude in various inflammatory conditions including aging and obesity. Recent studies have shown that Lcn2 is secreted by adipocytes in response to inflammation and is categorized as a new adipokine cross-linking innate immunity and metabolic disorders including obesity. However, the involvement of Lcn2 and its function during the progression of obesity is largely unknown. Recently, browning of white adipose tissue (WAT) has gained attention as a therapeutic strategy to combat obesity. Herein, we have shown that treatment of mature 3T3-L1 adipocytes with recombinant Lcn2 (rec-Lcn2) resulted in the up-regulation of thermogenic and beige/brown markers (UCP1, PRDM16, ZIC-1 and TBX1) and increased mitochondrial activity. Additionally, global Lcn2 genetic knockout (Lcn2KO) mice exhibited accelerated weight gain and visceral fat deposition with age, when compared to wild type (WT) mice. Taken together, both in vitro and in vivo studies suggest that Lcn2 is a naturally occurring adipokine, and may serve as an anti-obesity agent by upregulating the thermogenic markers resulting in the browning of WAT. Therefore, Lcn2 and its downstream signaling pathways could be a potential therapeutic target for obesity.

Roles of bone-derived hormones in type 2 diabetes and cardiovascular pathophysiology

Background

Emerging evidence demonstrates that bone is an endocrine organ capable of influencing multiple physiological and pathological processes through the secretion of hormones. Recent research suggests complex crosstalk between the bone and other metabolic and cardiovascular tissues. It was uncovered that three of these bone-derived hormones—osteocalcin, lipocalin 2, and sclerostin—are involved in the endocrine regulations of cardiometabolic health and play vital roles in the pathophysiological process of developing cardiometabolic syndromes such as type 2 diabetes and cardiovascular disease. Chronic low-grade inflammation is one of the hallmarks of cardiometabolic diseases and a major contributor to disease progression. Novel evidence also implicates important roles of bone-derived hormones in the regulation of chronic inflammation.

Scope of review

In this review, we provide a detailed overview of the physiological and pathological roles of osteocalcin, lipocalin 2, and sclerostin in cardiometabolic health regulation and disease development, with a focus on the modulation of chronic inflammation.

Major conclusions

Evidence supports that osteocalcin has a protective role in cardiometabolic health, and an increase of lipocalin 2 contributes to the development of cardiometabolic diseases partly via pro-inflammatory effects. The roles of sclerostin appear to be complicated: It exerts pro-adiposity and pro-insulin resistance effects in type 2 diabetes and has an anti-calcification effect during cardiovascular disease. A better understanding of the actions of these bone-derived hormones in the pathophysiology of cardiometabolic diseases will provide crucial insights to help further research develop new therapeutic strategies to treat these diseases.

Carotenoids and carotenoid conversion products in adipose tissue biology and obesity: Pre-clinical and human studies

Antiobesity activities of carotenoids and carotenoid conversion products (CCPs) have been demonstrated in pre-clinical studies, and mechanisms behind have begun to be unveiled, thus suggesting these compounds may help obesity prevention and management. The antiobesity action of carotenoids and CCPs can be traced to effects in multiple tissues, notably the adipose tissues. Key aspects of the biology of adipose tissues appear to be affected by carotenoid and CCPs, including adipogenesis, metabolic capacities for energy storage, release and inefficient oxidation, secretory function, and modulation of oxidative stress and inflammatory pathways. Here, we review the connections of carotenoids and CCPs with adipose tissue biology and obesity as revealed by cell and animal intervention studies, studies addressing the role of endogenous retinoid metabolism, and human epidemiological and intervention studies. We also consider human genetic variability influencing carotenoid and vitamin A metabolism, particularly in adipose tissues, as a potentially relevant aspect towards personalization of dietary recommendations to prevent or manage obesity and optimize metabolic health. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.

Sex-specific metabolic functions of adipose Lipocalin-2

OBJECTIVE

Lipocalin-2 (LCN2) is a secreted protein involved in innate immunity and has also been associated with several cardiometabolic traits in both mouse and human studies. However, the causal relationship of LCN2 to these traits is unclear, and most studies have examined only males.

METHODS

Using adeno-associated viral vectors we expressed LCN2 in either adipose or liver in a tissue specific manner on the background of a whole-body Lcn2 knockout or wildtype mice. Metabolic phenotypes including body weight, body composition, plasma and liver lipids, glucose homeostasis, insulin resistance, mitochondrial phenotyping, and metabolic cage studies were monitored.

RESULTS

We studied the genetics of LCN2 expression and associated clinical traits in both males and females in a panel of 100 inbred strains of mice (HMDP). The natural variation in Lcn2 expression across the HMDP exhibits high heritability, and genetic mapping suggests that it is regulated in part by Lipin1 gene variation. The correlation analyses revealed striking tissue dependent sex differences in obesity, insulin resistance, hepatic steatosis, and dyslipidemia. To understand the causal relationships, we examined the effects of expression of LCN2 selectively in liver or adipose. On a Lcn2-null background, LCN2 expression in white adipose promoted metabolic disturbances in females but not males. It acted in an autocrine/paracrine manner, resulting in mitochondrial dysfunction and an upregulation of inflammatory and fibrotic genes. On the other hand, on a null background, expression of LCN2 in liver had no discernible impact on the traits examined despite increasing the levels of circulating LCN2 more than adipose LCN2 expression. The mechanisms underlying the sex-specific action of LCN2 are unclear, but our results indicate that adipose LCN2 negatively regulates its receptor, LRP2 (or megalin), and its repressor, ERα, in a female-specific manner and that the effects of LCN2 on metabolic traits are mediated in part by LRP2.

CONCLUSIONS

Following up on our population-based studies, we demonstrate that LCN2 acts in a highly sex- and tissue-specific manner in mice. Our results have important implications for human studies, emphasizing the importance of sex and the tissue source of LCN2.

Transcription analysis of the response of the porcine adrenal cortex to a single subclinical dose of lipopolysaccharide from Salmonella Enteritidis

Lipopolysaccharide (LPS) is a bacterial endotoxin which can participate in the induction of inflammatory responses. LPS may also play a significant role in some neurodegenerative, oncological and metabolic disorders. The aim of the current study was to determine the effect of a subclinical low single dose of LPS from Salmonella Enteritidis administrated in vivo on the transcriptome of porcine adrenal cortex cells, especially gene expression levels, long non-coding RNA (lncRNA) profiles, alternative splicing events and RNA editing sites using RNA-seq technology. The subclinical dose of LPS changed the expression of 354 genes, 27 lncRNA loci and other unclassified RNAs. An analysis of alternative splicing events revealed 104 genes with differentially expressed splice junction sites, and the single nucleotide variant calling approach supported the identification of 376 canonical RNA editing candidates and 7249 allele-specific expression variants. The obtained results suggest that the RIG-I-like receptor signaling pathway, may play a more important role than the Toll-like signaling pathway after the administration of a subclinical dose of LPS. Single subclinical dose of LPS can affect the expression profiles of genes coding peptide hormones, steroidogenic enzymes and transcriptional factors, and modulate the endocrine functions of the gland.

Paradoxical role of lipocalin-2 in metabolic disorders and neurological complications

Lipocalin-2 (LCN2), also known as 24p3 and neutrophil gelatinase-associated lipocalin (NGAL), is a 25-kDa secreted protein implicated in various metabolic and inflammatory diseases. Early studies suggest the protective function of LCN2 in which it acts as a bacteriostatic agent that competes with bacteria for iron-bound siderophores. However, both detrimental and beneficial roles of LCN2 have recently been documented in metabolic and neuroinflammatory diseases. Metabolic inflammation, as observed in diabetes and obesity, has been closely associated with the upregulation of LCN2 in blood plasma and several tissues in both humans and rodents, suggesting its pro-diabetic and pro-obesogenic role. On the contrary, other studies imply an anti-diabetic and anti-obesogenic role of LCN2 whereby a deficiency in the Lcn2 gene results in the impairment of insulin sensitivity and enhances the high-fat-diet-induced expansion of fat. A similar dual role of LCN2 has also been reported in various animal models for neurological disorders. In the midst of these mixed findings, there is no experimental evidence to explain why LCN2 shows such a contrasting role in the various studies. This debate needs to be resolved (or reconciled) and an integrated view on the topic is desirable. Herein, we attempt to address this issue by reviewing the recent findings on LCN2 in metabolic disorders and assess the potential cellular or molecular mechanisms underlying the dual role of LCN2. We further discuss the possibilities and challenges of targeting LCN2 as a potential therapeutic strategy for metabolic disorders and neurological complications.

Effects of caloric restriction on the expression of lipocalin-2 and its receptor in the brown adipose tissue of high-fat diet-fed mice

Obesity causes inflammation and impairs thermogenic functions in brown adipose tissue (BAT). The adipokine lipocalin 2 (LCN2) has been implicated in inflammation and obesity. Herein, we investigated the protective effects of caloric restriction (CR) on LCN2-mediated inflammation and oxidative stress in the BAT of high-fat diet (HFD)-fed mice. Mice were fed a HFD for 20 weeks and then either continued on the HFD or subjected to CR for the next 12 weeks. CR led to the browning of the white fat-like phenotype in HFD-fed mice. Increased expressions of LCN2 and its receptor in the BAT of HFD-fed mice were significantly attenuated by CR. Additionally, HFD+CR-fed mice had fewer neutrophils and macrophages expressing LCN2 and iron-positive cells than HFD-fed mice. Further, oxidative stress and mitochondrial fission induced by a HFD were also significantly attenuated by CR. Our findings indicate that the protective effects of CR on inflammation and oxidative stress in the BAT of obese mice may be associated with regulation of LCN2.

Comparative proteomic study of liver lipid droplets and mitochondria in mice housed at different temperatures

Laboratory mice are standardly housed at around 23 °C, setting them under chronic cold stress. Metabolic changes in the liver in mice housed at thermoneutral, standard and cold temperatures remain unknown. In the present study, we isolated lipid droplets and mitochondria from their livers in a comparative proteomic study aiming to investigate the changes. According to proteomic analysis, mitochondrial tricarboxylic acid cycle (TCA cycle) and retinol metabolism are enhanced, whereas oxidative phosphorylation is not affected obviously under cold conditions, suggesting that liver mitochondria may increase TCA cycle capacity in biosynthetic pathways, as well as retinol metabolism, to help the liver to adapt. Based on proteomic and immunoblotting results, perilipin 5 and major urinary proteins are increased significantly, whereas mitochondrial pyruvate carrier is decreased dramatically under cold conditions, indicating their involvement in liver adaptation.

Adipose Tissue-Derived Signatures for Obesity and Type 2 Diabetes: Adipokines, Batokines and MicroRNAs

: Obesity is one of the main risk factors for type 2 diabetes mellitus (T2DM). It is closely related to metabolic disturbances in the adipose tissue that primarily functions as a fat reservoir. For this reason, adipose tissue is considered as the primary site for initiation and aggravation of obesity and T2DM. As a key endocrine organ, the adipose tissue communicates with other organs, such as the brain, liver, muscle, and pancreas, for the maintenance of energy homeostasis. Two different types of adipose tissues-the white adipose tissue (WAT) and brown adipose tissue (BAT)-secrete bioactive peptides and proteins, known as "adipokines" and "batokines," respectively. Some of them have beneficial anti-inflammatory effects, while others have harmful inflammatory effects. Recently, "exosomal microRNAs (miRNAs)" were identified as novel adipokines, as adipose tissue-derived exosomal miRNAs can affect other organs. In the present review, we discuss the role of adipose-derived secretory factors-adipokines, batokines, and exosomal miRNA-in obesity and T2DM. It will provide new insights into the pathophysiological mechanisms involved in disturbances of adipose-derived factors and will support the development of adipose-derived factors as potential therapeutic targets for obesity and T2DM.

Adipose Lipocalin 2 overexpression protects against age-related decline in thermogenic function of adipose tissue and metabolic deterioration

OBJECTIVES

Aging increases the risk for development of adipose tissue dysfunction, insulin resistance, dyslipidemia, and liver steatosis. Lipocalin 2 (Lcn2) deficient mice are more prone to diet-induced obesity and metabolic dysfunction, indicating a protective role for Lcn2 in younger mice. In this study, we determined whether overexpressing Lcn2 in adipose tissue can protect against age-related metabolic deterioration.

METHODS

We developed ap2-promoter-driven Lcn2 transgenic (Tg) mice and aged Lcn2 Tg mice for the metabolic assessments.

RESULTS

We found decreased adipocyte size in inguinal white adipose tissue (iWAT) from 10-month-old Lcn2 Tg mice relative to WT. This was accompanied by increased markers of adipogenesis in iWAT and attenuation of the age-related decline in AMP-activated protein kinase (AMPK) phosphorylation in adipose tissue depots. In addition to improvements in adipose tissue function, whole-body metabolic homeostasis was maintained in aged Lcn2 Tg mice. This included improved glucose tolerance and reduced serum triglycerides in older Lcn2 Tg mice relative to WT mice. Further, liver morphology and liver lipid levels were improved in older Lcn2 Tg mice, alongside a decrease in markers of liver inflammation and fibrosis.

CONCLUSIONS

We demonstrate that overexpression of Lcn2 in adipose tissue not only preserves adipose tissue function during aging but also promotes maintenance of glucose tolerance, decreases dyslipidemia, and prevents liver lipid accumulation and steatosis.

Screening of FDA-approved drugs identifies sutent as a modulator of UCP1 expression in brown adipose tissue

BACKGROUND

The pharmacological activation of thermogenesis in brown adipose tissue has long been considered promising strategies to treat obesity. However, identification of safe and effective agents remains a challenge. In this study, we addressed this challenge by developing a cellular system with a fluorescence readout, and applied in a high-throughput manner to screen for FDA-approved drugs that may activate endogenous UCP1 expression in adipocytes.

METHODS

We have generated a Ucp1-2A-GFP reporter mouse, in which GFP intensity serves as a surrogate of the endogenous expression level of UCP1 protein; and immortalized brown adipocytes were derived from this mouse model and applied in drug screening. Candidate drugs were further tested in mouse models either fed with normal chow or high fat diet to induce obesity.

FINDINGS

By using the cellular screening platform, we identified a group of FDA-approved drugs that can upregulate UCP1 expression in brown adipocyte, including previously known UCP1 activators and new candidate drugs. Further studies focusing on a previously unreported drug-sutent, revealed that sutent treatment could increase the energy expenditure and inhibit lipid synthesis in mouse adipose and liver tissues, resulting in improved metabolism and resistance to obesity.

INTERPRETATION

This study offered an easy-to-use cellular screening system for UCP1 activators, and provided a candidate list of FDA-approved drugs that can potentially treat obesity. Further study of these candidates may shed new light on the drug discovery towards obesity. FUND: National Key Research and Development Program and the Strategic Priority Research Program of the Chinese Academy of Sciences, etc. (250 words).

Lipocalin 2 regulates retinoic acid-induced activation of beige adipocytes

Lipocalin-2 (LCN2) has been previously characterized as an adipokine regulating thermogenic activation of brown adipose tissue and retinoic acid (RA)-induced thermogenesis in mice. The objective of this study was to explore the role and mechanism for LCN2 in the recruitment and retinoic acid-induced activation of brown-like or ‘beige’ adipocytes. We found LCN2 deficiency reduces key markers of thermogenesis including uncoupling protein-1 (UCP1) and peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) in inguinal white adipose tissue (iWAT) and inguinal adipocytes derived from Lcn2 −/− mice. Lcn2 −/− inguinal adipocytes have attenuated insulin-induced upregulation of thermogenic gene expression and p38 mitogen-activated protein kinase (p38MAPK) signaling pathway activation. This is accompanied by a lower basal and maximal oxidative capacity in Lcn2 −/− inguinal adipocytes, indicating mitochondrial dysfunction. Recombinant Lcn2 was able to restore insulin-induced p38MAPK phosphorylation in both WT and Lcn2 −/− inguinal adipocytes. Rosiglitazone treatment during differentiation of Lcn2 −/− adipocytes is able to recruit beige adipocytes at a normal level, however, further activation of beige adipocytes by insulin and RA is impaired in the absence of LCN2. Further, the synergistic effect of insulin and RA on UCP1 and PGC-1α expression is markedly reduced in Lcn2 −/− inguinal adipocytes. Most intriguingly, LCN2 and the retinoic acid receptor-alpha (RAR-α) are concurrently translocated to the plasma membrane of adipocytes in response to insulin, and this insulin-induced RAR-α translocation is absent in adipocytes deficient in LCN2. Our data suggest a novel LCN2-mediated pathway by which RA and insulin synergistically regulates activation of beige adipocytes via a non-genomic pathway of RA action.

Activation of brown adipocytes by placental growth factor

Gestational diabetes mellitus (GDM) is a type of diabetes and occurs during pregnancy. Brown adipose tissue (BAT) improves glucose homeostasis and mitigates insulin resistance, however, its activity is reduced in GDM. Placenta growth factor (PlGF) is an angiogenic factor produced by placental trophoblasts. Nevertheless, whether and how PlGF could affect BAT function in GDM are not defined. To investigate this question, 91 non-diabetic pregnant participants and 73 GDM patients were recruited to Gynaecology and Obstetrics Centre in Lu He hospital. Serum levels of PlGF were quantified by ELISA. Skin temperature was measured by far infrared thermography in the supraclavicular region where classical BATs were located. The direct effect of PlGF on BAT function was explored using the established human preadipocyte differentiation system. Thereby, we demonstrated that serum levels of PlGF were lower in GDM patients compared with controls, which was accompanied by decreased skin temperature in the supraclavicular region. By qPCR and western blot, mRNA and protein expression of UCP1 and OXPHOS were elevated in differentiated adipocytes treated with PlGF. PlGF stimulated mitochondrion transcription and increased copy number of mitochondrial. When subjected for respirometry, PlGF-treated differentiated adipocytes showed higher oxygen consumption rates than controls. PlGF induced AMPK phosphorylation and blockade of AMPK phosphorylation blunted UCP1 and OXPHOS expression in differentiated adipocytes. PlGF administration reduced cholesterol and triglyceride content in the liver and improved insulin sensitivity in db mice compared with control. In Conclusion, PlGF could activate BAT function. Downregulation of PlGF might contribute to the reduced BAT activity in GDM.

Bone: Another potential target to treat, prevent and predict diabetes

Type 2 diabetes mellitus is now a worldwide health problem with increasing prevalence. Mounting efforts have been made to treat, prevent and predict this chronic disease. In recent years, increasing evidence from mice and clinical studies suggests that bone-derived molecules modulate glucose metabolism. This review aims to summarize our current understanding of the interplay between bone and glucose metabolism and to highlight potential new means of therapeutic intervention. The first molecule recognized as a link between bone and glucose metabolism is osteocalcin (OCN), which functions in its active form, that is, undercarboxylated OCN (ucOC). ucOC acts in promoting insulin expression and secretion, facilitating insulin sensitivity, and favouring glucose and fatty acid uptake and utilization. A second bone-derived molecule, lipocalin2, functions in suppressing appetite in mice through its action on the hypothalamus. Osteocytes, the most abundant cells in bone matrix, are suggested to act on the browning of white adipose tissue and energy expenditure through secretion of bone morphogenetic protein 7 and sclerostin. The involvement of bone resorption in glucose homeostasis has also been examined. However, there is evidence indicating the implication of the receptor activator of nuclear factor κ-B ligand, neuropeptide Y, and other known and unidentified bone-derived factors that function in glucose homeostasis. We summarize recent advances and the rationale for treating, preventing and predicting diabetes by skeleton intervention.

Regulation of Energy Metabolism by Bone-Derived Hormones

Like many other organs, bone can act as an endocrine organ through the secretion of bone-specific hormones or "osteokines." At least two osteokines are implicated in the control of glucose and energy metabolism: osteocalcin (OCN) and lipocalin-2 (LCN2). OCN stimulates the production and secretion of insulin by the pancreatic β-cells, but also favors adaptation to exercise by stimulating glucose and fatty acid (FA) utilization by the muscle. Both of these OCN functions are mediated by the G-protein-coupled receptor GPRC6A. In contrast, LCN2 influences energy metabolism by activating appetite-suppressing signaling in the brain. This action of LCN2 occurs through its binding to the melanocortin 4 receptor (MC4R) in the paraventricular nucleus of the hypothalamus (PVN) and ventromedial neurons of the hypothalamus.

A20 deubiquitinase controls PGC-1α expression in the adipose tissue

BACKGROUND

Peroxisome proliferator-activated receptor γ coactivator- 1alpha (PGC-1α) plays an important role in whole body metabolism and, particularly in glucose homeostasis. Its expression is highly regulated and, small variations in tissue levels can have a major impact in a number of physiological and pathological conditions. Recent studies have shown that the ubiquitin/proteasome system plays a role in the control of PGC-1α degradation.

METHODS

Here we evaluated the interaction of PGC-1α with the protein A20, which plays a dual-role in the control of the ubiquitin/proteasome system acting as a deubiquitinase and as an E3 ligase. We employed immunoprecipitation, quantitative real-time PCR and immunofluorescence staining to evaluate PGC-1α, A20, PPARγ and ubiquitin in the adipose tissue of humans and mice.

RESULTS

In distinct sites of the adipose tissue, A20 binds to PGC-1α. At least in the subcutaneous fat of humans and mice the levels of PGC-1α decrease during obesity, while its physical association with A20 increases. The inhibition of A20 leads to a reduction of PGC-1α and PPARγ expression, suggesting that A20 acts as a protective factor against PGC-1α disposal.

CONCLUSION

We provide evidence that mechanisms regulating PGC-1α ubiquitination are potentially involved in the control of the function of this transcriptional co-activator.

Ribosomal transcription is regulated by PGC-1alpha and disturbed in Huntington's disease

PGC-1α is a versatile inducer of mitochondrial biogenesis and responsive to the changing energy demands of the cell. As mitochondrial ATP production requires proteins that derive from translation products of cytosolic ribosomes, we asked whether PGC-1α directly takes part in ribosomal biogenesis. Here, we show that a fraction of cellular PGC-1α localizes to the nucleolus, the site of ribosomal transcription by RNA polymerase I. Upon activation PGC-1α associates with the ribosomal DNA and boosts recruitment of RNA polymerase I and UBF to the rDNA promoter. This induces RNA polymerase I transcription under different stress conditions in cell culture and mouse models as well as in healthy humans and is impaired already in early stages of human Huntington’s disease. This novel molecular link between ribosomal and mitochondrial biogenesis helps to explain sarcopenia and cachexia in diseases of neurodegenerative origin.

Lipocalin-2: A Master Mediator of Intestinal and Metabolic Inflammation

Lipocalin-2 (LCN2), also known as neutrophil gelatinase-associated lipocalin (NGAL), is released by various cell types and is an attractive biomarker of inflammation, ischemia, infection, and kidney damage. Both intestinal and metabolic inflammation, as observed in obesity and related disorders, are associated with increased LCN2 synthesis. While LCN2 in the intestinal tract regulates the composition of the gut microbiota and shows anti-inflammatory activities, it also exhibits proinflammatory activities in other experimental settings. In animal models of metabolic inflammation, type 2 diabetes mellitus (T2DM), or nonalcoholic steatohepatitis (NASH), increased LCN2 expression favors inflammation via the recruitment of inflammatory cells, such as neutrophils, and the induction of proinflammatory cytokines. A better understanding of this crucial marker of innate immunity might pave the way for targeting this pathway in future therapies.

Brown adipose tissue: Updates in cellular and molecular biology

Brown adipose tissue (BAT) is mainly composed of adipocytes, it is highly vascularized and innervated, and can be activated in adult humans. Brown adipocytes are responsible for performing non-shivering thermogenesis, which is exclusively mediated by uncoupling protein (UCP) -1 (a protein found in the inner mitochondrial membrane), the hallmark of BAT, responsible for the uncoupling of the proton leakage from the ATP production, therefore, generating heat (i.e. thermogenesis). Besides UCP1, other compounds are essential not only to thermogenesis, but also to the proliferation and differentiation of BAT, including peroxisome proliferator-activated receptor (PPAR) family, PPARgamma coactivator 1 (PGC1)-alpha, and PRD1-BF-1-RIZ1 homologous domain protein containing protein (PRDM) -16. The sympathetic nervous system centrally regulates thermogenesis through norepinephrine, which acts on the adrenergic receptors of BAT. This bound leads to the initialization of the many pathways that may activate thermogenesis in acute and/or chronic ways. In summary, this mini-review aims to demonstrate the latest advances in the knowledge of BAT.

Niemann-Pick type C2 deficiency impairs autophagy-lysosomal activity, mitochondrial function, and TLR signaling in adipocytes

In this study, we investigated the role and mechanism of Niemann-Pick type C (NPC)2 in regulating lysosomal activity, mitophagy, and mitochondrial function in adipocytes. We found that knocking down NPC2 impaired lysosomal activity, as evidenced by the reduced mature cathepsin B, the increased accumulation of light chain 3 (LC3) and p62, and the decreased autophagic flux. In NPC2-knockdown (kd) adipocytes, the starvation-induced conversion of LC3-I to LC3-II was abolished. More interestingly, the majority of NPC2 was found in the mitochondrial fraction, and NPC2 deficiency led to impaired autophagic flux and decreased induction of LC3-II in the mitochondrial fraction during mitochondrial stress. Moreover, cellular respiration profiling revealed that NPC2-kd adipocytes had significantly decreased basal/maximal respiration and mitochondrial gene expression compared with scrambled cells, suggesting mitochondrial dysfunction. Additionally, we found that the mitochondrial recruitment of LC3-II induced by lipopolysaccharide (LPS), but not TNFα, was blunted in NPC2-kd adipocytes. Most intriguingly, NPC2-kd selectively diminished LPS-induced NFκB and ERK1/2 phosphorylation and the expression of pro-inflammatory genes, indicating that toll-like receptor signaling activation is impaired in the absence of NPC2. Our results suggest that NPC2 is in a mitochondrially associated autophagosome and plays an important role in regulating mitophagy, mitochondrial quality control, and mitochondrial function.

PPARγ signaling and emerging opportunities for improved therapeutics

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear receptor that regulates glucose and lipid metabolism, endothelial function and inflammation. Rosiglitazone (RGZ) and other thiazolidinedione (TZD) synthetic ligands of PPARγ are insulin sensitizers that have been used for the treatment of type 2 diabetes. However, undesirable side effects including weight gain, fluid retention, bone loss, congestive heart failure, and a possible increased risk of myocardial infarction and bladder cancer, have limited the use of TZDs. Therefore, there is a need to better understand PPARγ signaling and to develop safer and more effective PPARγ-directed therapeutics. In addition to PPARγ itself, many PPARγ ligands including TZDs bind to and activate G protein-coupled receptor 40 (GPR40), also known as free fatty acid receptor 1. GPR40 signaling activates stress kinase pathways that ultimately regulate downstream PPARγ responses. Recent studies in human endothelial cells have demonstrated that RGZ activation of GPR40 is essential to the optimal propagation of PPARγ genomic signaling. RGZ/GPR40/p38 MAPK signaling induces and activates PPARγ co-activator-1α, and recruits E1A binding protein p300 to the promoters of target genes, markedly enhancing PPARγ-dependent transcription. Therefore in endothelium, GPR40 and PPARγ function as an integrated signaling pathway. However, GPR40 can also activate ERK1/2, a proinflammatory kinase that directly phosphorylates and inactivates PPARγ. Thus the role of GPR40 in PPARγ signaling may have important implications for drug development. Ligands that strongly activate PPARγ, but do not bind to or activate GPR40 may be safer than currently approved PPARγ agonists. Alternatively, biased GPR40 agonists might be sought that activate both p38 MAPK and PPARγ, but not ERK1/2, avoiding its harmful effects on PPARγ signaling, insulin resistance and inflammation. Such next generation drugs might be useful in treating not only type 2 diabetes, but also diverse chronic and acute forms of vascular inflammation such as atherosclerosis and septic shock.

Lipocalin 2, a Regulator of Retinoid Homeostasis and Retinoid-mediated Thermogenic Activation in Adipose Tissue

We have recently characterized the role of lipocalin 2 (Lcn2) as a new adipose-derived cytokine in the regulation of adaptive thermogenesis via a non-adrenergic pathway. Herein, we explored a potential non-adrenergic mechanism by which Lcn2 regulates thermogenesis and lipid metabolism. We found that Lcn2 is a retinoic acid target gene, and retinoic acid concurrently stimulated UCP1 and Lcn2 expression in adipocytes. Lcn2 KO mice exhibited a blunted effect of all-trans-retinoic acid (ATRA) on body weight and fat mass, lipid metabolism, and retinoic acid signaling pathway activation in adipose tissue under the high fat diet-induced obese condition. We further demonstrated that Lcn2 is required for the full action of ATRA on the induction of UCP1 and PGC-1α expression in brown adipocytes and the restoration of cold intolerance in Lcn2 KO mice. Interestingly, we discovered that Lcn2 KO mice have decreased levels of retinoic acid and retinol in adipose tissue. The protein levels of STRA6 responsible for retinol uptake were significantly decreased in adipose tissue. The retinol transporter RBP4 was increased in adipose tissue but decreased in the circulation, suggesting the impairment of RBP4 secretion in Lcn2 KO adipose tissue. Moreover, Lcn2 deficiency abolished the ATRA effect on RBP4 expression in adipocytes. All the data suggest that the decreased retinoid level and action are associated with impaired retinol transport and storage in adipose tissue in Lcn2 KO mice. We conclude that Lcn2 plays a critical role in regulating metabolic homeostasis of retinoids and retinoid-mediated thermogenesis in adipose tissue.

Effects and Molecular Mechanism of GST-Irisin on Lipolysis and Autocrine Function in 3T3-L1 Adipocytes

Irisin, which was recently identified as a myokine and an adipokine, transforms white adipose tissue to brown adipose tissue and has increasingly caught the attention of the medical and scientific community. However, the signaling pathway of irisin and the molecular mechanisms responsible for the lipolysis effect remain unclear. In this study, we established an efficient system for the expression and purification of GST-irisin in Escherichia coli. The biological activity of GST-irisin was verified using the cell counting kit-8 assay and by detecting the mRNA expression of uncoupling protein 1. Our data showed that GST-irisin regulates mRNA levels of lipolysis-related genes such as adipose triglyceride lipase and hormone-sensitive lipase and proteins such as the fatty acid-binding protein 4, leading to increased secretion of glycerol and decreased lipid accumulation in 3T3-L1 adipocytes. In addition, exogenous GST-irisin can increase its autocrine function in vitro by regulating the expression of fibronectin type III domain-containing protein 5. GST-irisin could regulate glucose uptake in 3T3-L1 adipocytes. Hence, we believe that recombinant GST-irisin could promote lipolysis and its secretion in vitro and can potentially prevent obesity and related metabolic diseases.