Lactate induces FGF21 expression in adipocytes through a p38-MAPK pathway

Lactate entrance into the brain facilities adipose tissue lipolysis during exercise via circulating calcitonin gene-related peptide

OBJECTIVES

We assessed the relationships between CGRP, lactate and fat regulation.

METHODS

We evaluated the effect of intracerebroventricular (i.c.v.) injection of lactate and acute exercise on brain CGRP expression, and its concentration in serum/cerebrospinal fluid (SCF) in rats.

RESULTS

Injection of lactate up-regulated CGRP expression in the cortex and CSF and activated p38-mitogen-activated protein kinases (p38-MAPK) pathway. Co-injection of lactate and sb203580, deterred lactate-induced up-regulation of CGRP in the brain and CSF. Exercise increased the CGRP expression in the brain and CSF and up-regulated fat metabolism. Inhibition of lactate entrance into the brain using alpha-cyano-4-hydroxycinnamate (4-CIN) diminished exercise-induced CGRP up-regulation in the brain and CSF. Reducing the circulating blood lactate by pre-treatment of the animals with dichloroacetate (DCA) had no effect on exercise-induced increase in CGRP expression or fat metabolism during exercise.

CONCLUSIONS

Lactate probably acts as one of a signalling molecule in the brain to regulate fat metabolism during exercise.

Associations of methylmalonic acid and depressive symptoms with mortality: a population-based study

Methylmalonic acid (MMA), a biomarker of mitochondrial dysfunction, has been reported to be associated with depression in specific populations (i.e., older adults and postpartum women). Our study aimed to investigate to what extent MMA was associated with depressive symptoms and mortality in the general population, and assess whether depressive symptoms mediate the relationship between MMA and mortality. We analyzed cross-sectional data from 8343 participants from the US National Health and Nutrition Examination Survey. MMA was measured by liquid chromatography-tandem mass spectrometry, while depressive symptoms were measured by the Patient Health Questionnaire-9. Mortality data were obtained through linkage with National Death Index records. Linear regression models were performed to assess the association between MMA and depressive symptoms. The Cox proportional hazard regression model was utilized to assess the association of MMA and depressive symptoms with mortality. Mediation analysis was conducted within the counterfactual framework. In this general population, each SD (around 0.49 μmol/L) increase in MMA was associated with a 0.03 SD (approximately 0.15 score) increase in depressive symptoms (β = 0.033, 95% CI: 0.010, 0.055, p = 0.005). Notably, this association was more pronounced in men and participants over 60 years old. Higher levels of MMA and having more depressive symptoms were associated with a higher risk of mortality. However, depressive symptoms do not mediate the relationship between MMA and mortality. Elevated MMA levels were associated with depressive symptoms and an increased risk of mortality. These findings suggest that mitochondrial dysfunction may contribute to the multifactorial etiology of depression.

Integrative analyses of whole-transcriptome sequencing reveals CeRNA regulatory network in pulmonary hypertension treated with FGF21

Noncoding RNAs have been shown to play essential roles in hypoxic pulmonary hypertension (HPH). Our preliminary data showed that HPH is attenuated by fibroblast growth factor 21 (FGF21) administration. Therefore, we further investigated the whole transcriptome RNA expression patterns and interactions in a mice HPH model treated with FGF21. By whole-transcriptome sequencing, differentially expressed mRNAs, miRNAs, lncRNAs, and circRNAs were successfully identified in normoxia (Nx) vs. hypoxia (Hx) and Hx vs. hypoxia + FGF21 (Hx + F21). Differentially expressed mRNAs, miRNAs, lncRNAs, and circRNAs regulated by hypoxia and FGF21 were selected through intersection analysis. Based on prediction databases and sequencing data, differentially co-expressed mRNAs, miRNAs, lncRNAs, and circRNAs were further screened, followed by functional enrichment analysis. MAPK signaling pathway and epigenetic modification were enriched and may play fundamental roles in the therapeutic effects of FGF21. The ceRNA regulatory network of lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA was constructed with miR-7a-5p, miR-449c-5p, miR-676-3p and miR-674-3p as the core. In addition, quantitative real-time PCR experiments were employed to verify the whole-transcriptome sequencing data. The results of luciferase reporter assays highlighted the relationship between miR-449c-5p and XR_878320.1, miR-449c-5p and Stab2, miR-449c-5p and circ_mtcp1, which suggesting that miR-449c-5p may be a key regulator of FGF21 in the treatment of PH. Taken together, this study provides potential biomarkers, pathways, and ceRNA regulatory networks in HPH treated with FGF21 and will provide an experimental basis for the clinical application of FGF21 in PH.

Glucosamine Enhancement of Learning and Memory Functions by Promoting Fibroblast Growth Factor 21 Production

Fibroblast growth factor 21 (FGF21) plays a crucial role in metabolism and brain function. Glucosamine (GLN) has been recognized for its diverse beneficial effects. This study aimed to elucidate the modulation of FGF21 production by GLN and its impact on learning and memory functions. Using both in vivo and in vitro models, we investigated the effects of GLN on mice fed with a normal diet or high-fat diet and on mouse HT22 hippocampal cells, STHdhQ7/Q7 striatal cells, and rat primary cortical neurons challenged with GLN. Our results indicated that GLN promotes learning and memory functions in mice and upregulates FGF21 expression in the hippocampus, cortex, and striatum, as well as in HT22 cells, STHdhQ7/Q7 cells, and cortical neurons. In animals receiving GLN together with an FGF21 receptor FGFR1 inhibitor (PD173074), the GLN-enhanced learning and memory functions and induction of FGF21 production in the hippocampus were significantly attenuated. While exploring the underlying molecular mechanisms, the potential involvement of NF-κB, Akt, p38, JNK, PKA, and PPARα in HT22 and NF-κB, Akt, p38, and PPARα in STHdhQ7/Q7 were noted; GLN was able to mediate the activation of p65, Akt, p38, and CREB in HT22 and p65, Akt, and p38 in STHdhQ7/Q7 cells. Our accumulated findings suggest that GLN may increase learning and memory functions by inducing FGF21 production in the brain. This induction appears to be mediated, at least in part, through GLN's activation of the NF-κB, Akt, p38, and PKA/CREB pathways.

Acute effect of exercise intensity on circulating FGF-21, FSTL-1, cathepsin B, and BDNF in young men

BACKGROUND/OBJECTIVES

Exercise intensity is potentially an important regulator of various exerkines secretion, but the optimal exercise intensity to increase and sustain exerkines levels, including FGF-21, FSTL-1, cathepsin B, and BDNF in humans, has not yet been fully elucidated. This study aimed to examine the circulating levels of FGF-21, FSTL-1, cathepsin B, and BDNF according to the exercise intensity.

METHODS

Nine young men (24.0 ± 0.4 years old) performed 4 different experimental sessions at 1-week intervals: 1) a control session (CTRL; no exercise); 2) moderate-intensity continuous exercise (MICE, 55% HRR); 3) vigorous-intensity continuous exercise (VICE, 85% HRR); and 4) high-intensity interval exercise (HIIE, 4 repetitions of a 30-s of "all out" cycling workout followed by a 4-min recovery). Blood samples were collected at 4 different time points (pre-exercise, immediately post-exercise, 30 min post-exercise, and 90 min post-exercise).

RESULTS

Serum FGF-21, FSTL-1, cathepsin B, and BDNF were higher in HIIE than in CTRL immediately post-exercise, and FSTL-1, cathepsin B, and BDNF were higher in HIIE than in MICE immediately post-exercise (P < 0.05). The AUC for FGF-21, FSTL-1, and BDNF was higher in HIIE than in CTRL, and the AUC for FGF-21 and BDNF was higher in HIIE than in MICE (P < 0.05). Furthermore, the change in blood lactate was positively correlated with the changes in all exerkines.

CONCLUSIONS

This study demonstrates that acute HIIE effectively increases serum FGF-21, FSTL-1, cathepsin B, and BDNF compared to MICE. Therefore, the secretion of exerkines, including FGF-21, FSTL-1, cathepsin B, and BDNF may be exercise intensity-dependent.

NRF2 is essential for adaptative browning of white adipocytes

White adipose tissue browning, defined by accelerated mitochondrial metabolism and biogenesis, is considered a promising mean to treat or prevent obesity-associated metabolic disturbances. We hypothesize that redox stress acutely leads to increased production of reactive oxygen species (ROS), which activate electrophile sensor nuclear factor erythroid 2-Related Factor 2 (NRF2) that over time results in an adaptive adipose tissue browning process. To test this, we have exploited adipocyte-specific NRF2 knockout mice and cultured adipocytes and analyzed time- and dose-dependent effect of NAC and lactate treatment on antioxidant expression and browning-like processes. We found that short-term antioxidant treatment with N-acetylcysteine (NAC) induced reductive stress as evident from increased intracellular NADH levels, increased ROS-production, reduced oxygen consumption rate (OCR), and increased NRF2 levels in white adipocytes. In contrast, and in line with our hypothesis, longer-term NAC treatment led to a NRF2-dependent browning response. Lactate treatment elicited similar effects as NAC, and mechanistically, these NRF2-dependent adipocyte browning responses in vitro were mediated by increased heme oxygenase-1 (HMOX1) activity. Moreover, this NRF2-HMOX1 axis was also important for β3-adrenergic receptor activation-induced adipose tissue browning in vivo. In conclusion, our findings show that administration of exogenous antioxidants can affect biological function not solely through ROS neutralization, but also through reductive stress. We also demonstrate that NRF2 is essential for white adipose tissue browning processes.

Physical activity and batokines

Brown and beige adipose tissue share similar functionality, being both tissues specialized in producing heat through nonshivering thermogenesis and also playing endocrine roles through the release of their secretion factors called batokines. This review elucidates the influence of physical exercise, and myokines released in response, on the regulation of thermogenic and secretory functions of these adipose tissues and discusses the similarity of batokines actions with physical exercise in the remodeling of adipose tissue. This adipose tissue remodeling promoted by autocrine and paracrine batokines or physical exercise seems to optimize its functionality associated with better health outcomes.

Skeletal muscles and gut microbiota-derived metabolites: novel modulators of adipocyte thermogenesis

Obesity occurs when overall energy intake surpasses energy expenditure. White adipose tissue is an energy storage site, whereas brown and beige adipose tissues catabolize stored energy to generate heat, which protects against obesity and obesity-associated metabolic disorders. Metabolites are substrates in metabolic reactions that act as signaling molecules, mediating communication between metabolic sites (i.e., adipose tissue, skeletal muscle, and gut microbiota). Although the effects of metabolites from peripheral organs on adipose tissue have been extensively studied, their role in regulating adipocyte thermogenesis requires further investigation. Skeletal muscles and intestinal microorganisms are important metabolic sites in the body, and their metabolites play an important role in obesity. In this review, we consolidated the latest research on skeletal muscles and gut microbiota-derived metabolites that potentially promote adipocyte thermogenesis. Skeletal muscles can release lactate, kynurenic acid, inosine, and β-aminoisobutyric acid, whereas the gut secretes bile acids, butyrate, succinate, cinnabarinic acid, urolithin A, and asparagine. These metabolites function as signaling molecules by interacting with membrane receptors or controlling intracellular enzyme activity. The mechanisms underlying the reciprocal exchange of metabolites between the adipose tissue and other metabolic organs will be a focal point in future studies on obesity. Furthermore, understanding how metabolites regulate adipocyte thermogenesis will provide a basis for establishing new therapeutic targets for obesity.

Mitochondrial respiratory complex I deficiency inhibits brown adipogenesis by limiting heme regulation of histone demethylation

Mitochondrial functions play a crucial role in determining the metabolic and thermogenic status of brown adipocytes. Increasing evidence reveals that the mitochondrial oxidative phosphorylation (OXPHOS) system plays an important role in brown adipogenesis, but the mechanistic insights are limited. Herein, we explored the potential metabolic mechanisms leading to OXPHOS regulation of brown adipogenesis in pharmacological and genetic models of mitochondrial respiratory complex I deficiency. OXPHOS deficiency inhibits brown adipogenesis through disruption of the brown adipogenic transcription circuit without affecting ATP levels. Neither blockage of calcium signaling nor antioxidant treatment can rescue the suppressed brown adipogenesis. Metabolomics analysis revealed a decrease in levels of tricarboxylic acid cycle intermediates and heme. Heme supplementation specifically enhances respiratory complex I activity without affecting complex II and partially reverses the inhibited brown adipogenesis by OXPHOS deficiency. Moreover, the regulation of brown adipogenesis by the OXPHOS-heme axis may be due to the suppressed histone methylation status by increasing histone demethylation. In summary, our findings identified a heme-sensing retrograde signaling pathway that connects mitochondrial OXPHOS to the regulation of brown adipocyte differentiation and metabolic functions.

Lactate shuttling drives the browning of white adipose tissue after burn

High levels of plasma lactate are associated with increased mortality in critically injured patients, including those with severe burns. Although lactate has long been considered a waste product of glycolysis, it was recently revealed that it acts as a potent inducer of white adipose tissue (WAT) browning, a response implicated in mediating postburn cachexia, hepatic steatosis, and sustained hypermetabolism. Despite the clinical presentation of hyperlactatemia and browning in burns, whether these two pathological responses are linked is currently unknown. Here, we report that elevated lactate plays a causal signaling role in mediating adverse outcomes after burn trauma by directly promoting WAT browning. Using WAT obtained from human burn patients and mouse models of thermal injury, we show that the induction of postburn browning is positively correlated with a shift toward lactate import and metabolism. Furthermore, daily administration of l-lactate is sufficient to augment burn-induced mortality and weight loss in vivo. At the organ level, increased lactate transport amplified the thermogenic activation of WAT and its associated wasting, thereby driving postburn hepatic lipotoxicity and dysfunction. Mechanistically, the thermogenic effects of lactate appeared to result from increased import through MCT transporters, which in turn increased intracellular redox pressure, [NADH/NAD+], and expression of the batokine, FGF21. In fact, pharmacological inhibition of MCT-mediated lactate uptake attenuated browning and improved hepatic function in mice after injury. Collectively, our findings identify a signaling role for lactate that impacts multiple aspects of postburn hypermetabolism, necessitating further investigation of this multifaceted metabolite in trauma and critical illness.NEW & NOTEWORTHY To our knowledge, this study was the first to investigate the role of lactate signaling in mediating white adipose tissue browning after burn trauma. We show that the induction of browning in both human burn patients and mice is positively correlated with a shift toward lactate import and metabolism. Daily l-lactate administration augments burn-induced mortality, browning, and hepatic lipotoxicity in vivo, whereas pharmacologically targeting lactate transport alleviates burn-induced browning and improves liver dysfunction after injury.

Exerkines and redox homeostasis

Exercise physiology has gained increasing interest due to its wide effects to promote health. Recent years have seen a growth in this research field also due to the finding of several circulating factors that mediate the effects of exercise. These factors, termed exerkines, are metabolites, growth factors, and cytokines secreted by main metabolic organs during exercise to regulate exercise systemic and tissue-specific effects. The metabolic effects of exerkines have been broadly explored and entail a promising target to modulate beneficial effects of exercise in health and disease. However, exerkines also have broad effects to modulate redox signaling and homeostasis in several cellular processes to improve stress response. Since redox biology is central to exercise physiology, this review summarizes current evidence for the cross-talk between redox biology and exerkines actions. The role of exerkines in redox biology entails a response to oxidative stress-induced pathological cues to improve health outcomes and to modulate exercise adaptations that integrate redox signaling.

Contribution of High-Intensity Interval Exercise in the Fasted State to Fat Browning: Potential Roles of Lactate and β-Hydroxybutyrate

PURPOSE

Fat browning contributes to energy consumption and may have metabolic benefits against obesity; however, the potential roles of lactate and β-hydroxybutyrate (β-HB) in fat browning remain unclear. We investigated the roles of a single bout of aerobic exercise that increases lactate and β-HB levels in the fasted state on the regulation of fat browning in rats and humans.

METHODS

Male Sprague-Dawley rats were exposed to 24-h fasting and/or a single bout moderate-intensity aerobic exercise (40 min): sedentary (CON), exercise (ND-EX), fasting (FAST), and exercise + fasting (F-EX). Adult men ( n = 13) were randomly assigned into control with food intake (CON), exercise with intensity at onset of blood lactate accumulation in the fasted state (F-OBLA), and high-intensity interval exercise in the fasted state (F-HIIE) until each participant expended 350 kcal of energy. For evaluating the effects of exercise intensity in rats, we conducted another set of animal experiment, including groups of sedentary fed control, fasting control, and exercise with moderate-intensity or HIIE for 40 min after a 24-h fasting.

RESULTS

Regardless of fasting, single bout of exercise increases the concentration of lactate and β-HB in rats, but the exercise in the fasted state increases the β-HB level more significantly in rats and humans. F-EX-activated fat browning (AMPK-SirT1-PGC1α pathway and PRDM16) and thermogenic factor (UCP1) in white fat of rats. In rats and humans, exercise in the fasted state increased the blood levels of fat browning-related adipomyokines. In particular, compared with F-OBLA, F-HIIE more efficiently increases free fatty acid as well as blood levels of fat browning adipomyokines in humans, which was correlated with blood levels of lactate and β-HB. In rats that performed exercise with different intensity, the higher plasma lactate and β-HB levels, and higher expression of p-AMPK, UCP1, and PRDM16 in white adipose tissue of HIIE group than those of moderate-intensity group, were observed.

CONCLUSIONS

A single bout of aerobic exercise in the fasted state significantly induced fat browning-related pathways, free fatty acid, and adipomyokines, particularly F-HIIE in human. Although further evidence for supporting our results is required in humans, aerobic exercise in the fasted state with high intensity that increase lactate and β-HB may be a modality of fat browning.

The anorectic and thermogenic effects of pharmacological lactate in male mice are confounded by treatment osmolarity and co-administered counterions

Lactate is a circulating metabolite and a signalling molecule with pleiotropic physiological effects. Studies suggest that lactate modulates energy balance by lowering food intake, inducing adipose browning and increasing whole-body thermogenesis. Yet, like many other metabolites, lactate is often commercially produced as a counterion-bound salt and typically administered in vivo through hypertonic aqueous solutions of sodium L-lactate. Most studies have not controlled for injection osmolarity and the co-injected sodium ions. Here, we show that the anorectic and thermogenic effects of exogenous sodium L-lactate in male mice are confounded by the hypertonicity of the injected solutions. Our data reveal that this is in contrast to the antiobesity effect of orally administered disodium succinate, which is uncoupled from these confounders. Further, our studies with other counterions indicate that counterions can have confounding effects beyond lactate pharmacology. Together, these findings underscore the importance of controlling for osmotic load and counterions in metabolite research.

Adipose tissue aging is regulated by an altered immune system

Adipose tissue is a widely distributed organ that plays a critical role in age-related physiological dysfunctions as an important source of chronic sterile low-grade inflammation. Adipose tissue undergoes diverse changes during aging, including fat depot redistribution, brown and beige fat decrease, functional decline of adipose progenitor and stem cells, senescent cell accumulation, and immune cell dysregulation. Specifically, inflammaging is common in aged adipose tissue. Adipose tissue inflammaging reduces adipose plasticity and pathologically contributes to adipocyte hypertrophy, fibrosis, and ultimately, adipose tissue dysfunction. Adipose tissue inflammaging also contributes to age-related diseases, such as diabetes, cardiovascular disease and cancer. There is an increased infiltration of immune cells into adipose tissue, and these infiltrating immune cells secrete proinflammatory cytokines and chemokines. Several important molecular and signaling pathways mediate the process, including JAK/STAT, NFκB and JNK, etc. The roles of immune cells in aging adipose tissue are complex, and the underlying mechanisms remain largely unclear. In this review, we summarize the consequences and causes of inflammaging in adipose tissue. We further outline the cellular/molecular mechanisms of adipose tissue inflammaging and propose potential therapeutic targets to alleviate age-related problems.

Lactate Is a Key Mediator That Links Obesity to Insulin Resistance via Modulating Cytokine Production From Adipose Tissue

Numerous evidence indicates that inflammation in adipose tissue is the primary cause of systemic insulin resistance induced by obesity. Obesity-associated changes in circulating LPS level and hypoxia/HIF-1α activation have been proposed to be involved in boosting obesity-induced inflammation. However, there is poor understanding of what triggers obesity-induced inflammation. In this study, we pinpoint lactate as a key trigger to mediate obesity-induced inflammation and systemic insulin resistance. Specific deletion of Slc16a1 that encodes MCT1, the primary lactate transporter in adipose tissues, robustly elevates blood levels of proinflammatory cytokines and aggravates systemic insulin resistance without alteration of adiposity in mice fed high-fat diet. Slc16a1 deletion in adipocytes elevates intracellular lactate level while reducing circulating lactate concentration. Mechanistically, lactate retention due to Slc16a1 deletion initiates adipocyte apoptosis and cytokine release. The locally recruited macrophages amplify the inflammation by release of proinflammatory cytokines to the circulation, leading to insulin resistance in peripheral tissues. This study, therefore, indicates that lactate within adipocytes has a key biological function linking obesity to insulin resistance, and harnessing lactate in adipocytes can be a promising strategy to break this link.

Transmembrane G protein-coupled receptor 5 signaling stimulates fibroblast growth factor 21 expression concomitant with up-regulation of the transcription factor nuclear receptor Nr4a1

Fibroblast growth factor 21 (FGF21) acts as an endocrine factor, playing important roles in the regulation of energy homeostasis, glucose and lipid metabolism. It is induced by diverse metabolic and cellular stresses, such as starvation and cold challenge, which in turn facilitate adaptation to the stress environment. The pharmacological action of FGF21 has received much attention, because the administration of FGF21 or its analogs has been shown to have an anti-obesity effect in rodent models. In the present study, we found that 3-O-acetyloleanolic acid, an active constituent isolated from the fruits of Forsythia suspensa, stimulated FGF21 production concomitant with the up-regulation of a transcription factor, nuclear receptor Nr4a1, in C2C12 myotubes. Additionally, significant increases in mFgf21 promoter activity were observed in C2C12 cells overexpressing TGR5 receptor in response to 3-O-acetyloleanolic acid treatment. Treatment with the p38 MAPK inhibitor SB203580 was effective at suppressing these stimulatory effects of 3-O-acetyloleanolic acid. Pretreatment with SB203580 also significantly repressed FGF21 mRNA abundance and FGF21 secretion in C2C12 myotubes after 3-O-acetyloleanolic acid stimulation, suggesting that p38 activation is required for the induction of FGF21 by ligand-activated TGR5 in C2C12 myotubes. These findings collectively indicated that TGR5 receptor signaling drives FGF21 expression via p38 activation, at least partly, by mediating Nr4a1 expression. Thus, the novel biological function of 3-O-acetyloleanolic acid as an agent having anti-obesity effects is likely to be mediated through the activation of TGR5 receptors.

Lactate Fluxes and Plasticity of Adipose Tissues: A Redox Perspective

Lactate, a metabolite produced when the glycolytic flux exceeds mitochondrial oxidative capacities, is now viewed as a critical regulator of metabolism by acting as both a carbon and electron carrier and a signaling molecule between cells and tissues. In recent years, increasing evidence report its key role in white, beige, and brown adipose tissue biology, and highlights new mechanisms by which lactate participates in the maintenance of whole-body energy homeostasis. Lactate displays a wide range of biological effects in adipose cells not only through its binding to the membrane receptor but also through its transport and the subsequent effect on intracellular metabolism notably on redox balance. This study explores how lactate regulates adipocyte metabolism and plasticity by balancing intracellular redox state and by regulating specific signaling pathways. We also emphasized the contribution of adipose tissues to the regulation of systemic lactate metabolism, their roles in redox homeostasis, and related putative physiopathological repercussions associated with their decline in metabolic diseases and aging.

Does an increase in serum FGF21 level predict 28-day mortality of critical patients with sepsis and ARDS?

BACKGROUND

Sepsis may be accompanied by acute respiratory distress syndrome (ARDS) in patients admitted to intensive care units (ICUs). It is essential to identify prognostic biomarkers in patients with sepsis and ARDS.

OBJECTIVE

Determine whether changes in the level of serum fibroblast growth factor 21 (FGF21) can predict the 28-day mortality of ICU patients with sepsis and ARDS.

METHODS

Consecutive sepsis patients were divided into two groups (Sepsis + ARDS and Sepsis-only), and the Sepsis + ARDS group was further classified as survivors or non-survivors. Demographic data and comorbidities were recorded. The Sequential Organ Failure Assessment (SOFA) score and serum levels of cytokines and other biomarkers were recorded 3 times after admission. Multiple Cox proportional hazards regression was used to identify risk factors associated with 28-day mortality in the Sepsis + ARDS group. Multivariate receiver operating characteristic curve analysis was used to assess the different predictive value of FGF21 and SOFA.

RESULTS

The Sepsis + ARDS group had a greater baseline SOFA score and serum levels of cytokines and other biomarkers than the Sepsis-only group; the serum level of FGF21 was almost twofold greater in the Sepsis + ARDS group (P < 0.05). Non-survivors in the Sepsis + ARDS group had an almost fourfold greater level of FGF21 than survivors in this group (P < 0.05). The serum level of FGF21 persistently increased from the baseline to the peak of shock and death in the non-survivors, but persistently decreased in survivors (P < 0.05). Changes in the serum FGF21 level between different time points were independent risk factors for mortality. No statistical difference was observed between the AUC of FGF21 and SOFA at baseline.  CONCLUSION: A large increase of serum FGF21 level from baseline is associated with 28-day mortality in ICU patients with sepsis and ARDS.

Serum fibroblast growth factor 21 levels after out of hospital cardiac arrest are associated with neurological outcome

Fibroblast growth factor (FGF) 21 is a marker associated with mitochondrial and cellular stress. Cardiac arrest causes mitochondrial stress, and we tested if FGF 21 would reflect the severity of hypoxia-reperfusion injury after cardiac arrest. We measured serum concentrations of FGF 21 in 112 patients on ICU admission and 24, 48 and 72 h after out-of-hospital cardiac arrest with shockable initial rhythm included in the COMACARE study (NCT02698917). All patients received targeted temperature management for 24 h. We defined 6-month cerebral performance category 1–2 as good and 3–5 as poor neurological outcome. We used samples from 40 non-critically ill emergency room patients as controls. We assessed group differences with the Mann Whitney U test and temporal differences with linear modeling with restricted maximum likelihood estimation. We used multivariate logistic regression to assess the independent predictive value of FGF 21 concentration for neurologic outcome. The median (inter-quartile range, IQR) FGF 21 concentration was 0.25 (0.094–0.91) ng/ml in controls, 0.79 (0.37–1.6) ng/ml in patients at ICU admission (P < 0.001 compared to controls) and peaked at 48 h [1.2 (0.46–2.5) ng/ml]. We found no association between arterial blood oxygen partial pressure and FGF 21 concentrations. We observed with linear modeling an effect of sample timepoint (F 5.6, P < 0.01), poor neurological outcome (F 6.1, P = 0.01), and their interaction (F 3.0, P = 0.03), on FGF 21 concentration. In multivariate logistic regression analysis, adjusting for relevant clinical covariates, higher average FGF 21 concentration during the first 72 h was independently associated with poor neurological outcome (odds ratio 1.60, 95% confidence interval 1.10–2.32). We conclude that post cardiac arrest patients experience cellular and mitochondrial stress, reflected as a systemic FGF 21 response. This response is higher with a more severe hypoxic injury but it is not exacerbated by hyperoxia.

Lactate fluxes mediated by the monocarboxylate transporter-1 are key determinants of the metabolic activity of beige adipocytes

Activation of energy-dissipating brown/beige adipocytes represents an attractive therapeutic strategy against metabolic disorders. While lactate is known to induce beiging through the regulation of Ucp1 gene expression, the role of lactate transporters on beige adipocytes' ongoing metabolic activity remains poorly understood. To explore the function of the lactate-transporting monocarboxylate transporters (MCTs), we used a combination of primary cell culture studies, 13C isotopic tracing, laser microdissection experiments, and in situ immunofluorescence of murine adipose fat pads. Dissecting white adipose tissue heterogeneity revealed that the MCT1 is expressed in inducible beige adipocytes as the emergence of uncoupling protein 1 after cold exposure was restricted to a subpopulation of MCT1-expressing adipocytes suggesting MCT1 as a marker of inducible beige adipocytes. We also observed that MCT1 mediates bidirectional and simultaneous inward and outward lactate fluxes, which were required for efficient utilization of glucose by beige adipocytes activated by the canonical β3-adrenergic signaling pathway. Finally, we demonstrated that significant lactate import through MCT1 occurs even when glucose is not limiting, which feeds the oxidative metabolism of beige adipocytes. These data highlight the key role of lactate fluxes in finely tuning the metabolic activity of beige adipocytes according to extracellular metabolic conditions and reinforce the emerging role of lactate metabolism in the control of energy homeostasis.

TAK1 is involved in sodium L-lactate-stimulated p38 signaling and promotes apoptosis

In the present study, we found that the phosphorylation of p38 mitogen-activated protein kinase (p38) was significantly increased in L-lactate-treated HeLa cells, which is under concentration- and time-dependent manner. The protein level of Bcl-2 was significantly reduced and Bax and C-caspase3 were significantly increased in L-lactate-treated cells. qRT-PCR analysis suggested that the expression level of apoptosis-related genes Bax, C-myc, and FasL were significantly upregulated by L-lactate treatment. In addition, p38 inhibitor SB203580 blocked the L-lactate-stimulated phosphorylation of p38 (p-p38) and apoptosis, which suggested that L-lactate-stimulated apoptosis may be related to the activation of p38. Moreover, TAK1 inhibitor Takinib reduced L-lactate-triggered phosphorylation of p38 and also apoptosis; however, ASK1 inhibitor NQDI-1 did not. Cells transfected with siRNA of TAK1(siTAK1) showed similar results with Takinib inhibitor. These results suggested that the L-lactate treatment elevated activation of p38 and apoptosis was related to TAK1. In this study, we suggested that TAK1 plays an important role in L-lactate-stimulated activation of p38 affecting apoptosis in HeLa cells.

Glucosamine regulation of fibroblast growth factor 21 expression in liver and adipose tissues

Obesity is associated with metabolic disorders. Fibroblast growth factor 21 (FGF21) has been recognized as important in metabolism. Glucosamine (GLN) has been demonstrated to perform diverse beneficial functions. This study aimed to reveal whether and how GLN would modulate FGF21 production in relation to metabolism. With in vivo model of normal diet (ND) and high-fat diet (HFD) mice receiving GLN injection and in vitro model of mouse AML12 liver cells and differentiated 3T3L1 adipocytes challenged with GLN, GLN appeared to improve the glucose metabolism in HFD and ND mice and to elevate FGF21 protein expression in HFD liver and to increase both FGF21 protein and mRNA levels in WAT from HFD and ND mice and it also upregulated FGF21 expression in both AML12 and differentiated 3T3L1 cells. By using inhibitors against various signaling pathways, p38, Akt, NF-κB, and PKA appeared potentially involved in GLN-mediated FGF21 production in AML12 cells; GLN was able to mediate activation of NF-κB, p38 or PKA/CREB signaling. Our accumulated findings suggest that GLN may potentially improve the metabolic performance by inducing FGF21 production in liver and adipose tissues and such induction in liver cells may act in part due to GLN induction of the NF-κB, p38 and PKA pathways.

Endocrine Mechanisms Connecting Exercise to Brown Adipose Tissue Metabolism: a Human Perspective

PURPOSE OF REVIEW

To summarize the state-of-the-art regarding the exercise-regulated endocrine signals that might modulate brown adipose tissue (BAT) activity and/or white adipose tissue (WAT) browning, or through which BAT communicates with other tissues, in humans.

RECENT FINDINGS

Exercise induces WAT browning in rodents by means of a variety of physiological mechanism. However, whether exercise induces WAT browning in humans is still unknown. Nonetheless, a number of protein hormones and metabolites, whose signaling can influence thermogenic adipocyte's metabolism, are secreted during and/or after exercise in humans from a variety of tissues and organs, such as the skeletal muscle, the adipose tissue, the liver, the adrenal glands, or the cardiac muscle. Overall, it seems plausible to hypothesize that, in humans, exercise secretes an endocrine cocktail that is likely to induce WAT browning, as it does in rodents. However, even if exercise elicits a pro-browning endocrine response, this might result in a negligible effect if blood flow is restricted in thermogenic adipocyte-rich areas during exercise, which is still to be determined. Future studies are needed to fully characterize the exercise-induced secretion (i.e., to determine the effect of the different exercise frequency, intensity, type, time, and volume) of endocrine signaling molecules that might modulate BAT activity and/or WAT browning or through which BAT communicates with other tissues, during exercise. The exercise effect on BAT metabolism and/or WAT browning could be one of the still unknown mechanisms by which exercise exerts beneficial health effects, and it might be pharmacologically mimicked.

Impact of adipokines and myokines on fat browning

Since the discovery of leptin in 1994, the adipose tissue (AT) is not just considered a passive fat storage organ but also an extremely active secretory and endocrine organ that secretes a large variety of hormones, called adipokines, involved in energy metabolism. Adipokines may not only contribute to AT dysfunction and obesity, but also in fat browning, a process that induces a phenotypic switch from energy-storing white adipocytes to thermogenic brown fat-like cells. The fat browning process and, consequently, thermogenesis can also be stimulated by physical exercise. Contracting skeletal muscle is a metabolically active tissue that participates in several endocrine functions through the production of bioactive factors, collectively termed myokines, proposed as the mediators of physical activity-induced health benefits. Myokines affect muscle mass, have profound effects on glucose and lipid metabolism, and promote browning and thermogenesis of white AT in an endocrine and/or paracrine manner. The present review focuses on the role of different myokines and adipokines in the regulation of fat browning, as well as in the potential cross-talk between AT and skeletal muscle, in order to control body weight, energy expenditure and thermogenesis.

Oral Lactate Administration Additively Enhances Endurance Training-Induced Increase in Cytochrome C Oxidase Activity in Mouse Soleus Muscle

We tested the hypothesis that oral lactate supplementation increases mitochondrial enzyme activity given the potential role of lactate for inducing mitochondrial biogenesis. In this study, mice were assigned to a saline-ingested sedentary group (S+S; n = 8), a lactate-ingested sedentary group (L+S; n = 9), a saline-ingested training group (S+T; n = 8), and a lactate-ingested training group (L+T; n = 8). Mice in the S+S and S+T groups received saline, whereas mice in the L+S and L+T groups received sodium lactate (equivalent to 5 g/kg of body weight) via oral gavage 5 days a week for 4 weeks. At 30 min after the ingestion, mice in the S+T and L+T groups performed endurance training (treadmill running, 20 m/min, 30 min, 5 days/week). At 30 min after lactate ingestion, the blood lactate level reached peak value (5.8 ± 0.4 mmol/L) in the L+S group. Immediately after the exercise, blood lactate level was significantly higher in the L+T group (9.3 ± 0.9 mmol/L) than in the S+T group (2.7 ± 0.3 mmol/L) (p < 0.01). Following a 4-week training period, a main effect of endurance training was observed in maximal citrate synthase (CS) (p < 0.01; S+T: 117 ± 3% relative to S+S, L+T: 110 ± 3%) and cytochrome c oxidase (COX) activities (p < 0.01; S+T: 126 ± 4%, L+T: 121 ± 4%) in the plantaris muscle. Similarly, there was a main effect of endurance training in maximal CS (p < 0.01; S+T: 105 ± 3%, L+T: 115 ± 2%) and COX activities (p < 0.01; S+T: 113 ± 3%, L+T: 122 ± 3%) in the soleus muscle. In addition, a main effect of oral lactate ingestion was found in maximal COX activity in the soleus (p < 0.05; L+S: 109 ± 3%, L+T: 122 ± 3%) and heart muscles (p < 0.05; L+S: 107 ± 3%, L+T: 107 ± 2.0%), but not in the plantaris muscle. Our results suggest that lactate supplementation may be beneficial for increasing mitochondrial enzyme activity in oxidative phenotype muscle.

The emerging roles of lactate as a redox substrate and signaling molecule in adipose tissues

Thermogenic (brown and beige) adipose tissues improve glucose and lipid homeostasis and therefore represent putative targets to cure obesity and related metabolic diseases including type II diabetes. Beside decades of research and the very well-described role of noradrenergic signaling, mechanisms underlying adipocytes plasticity and activation of thermogenic adipose tissues remain incompletely understood. Recent studies show that metabolites such as lactate control the oxidative capacity of thermogenic adipose tissues. Long time viewed as a metabolic waste product, lactate is now considered as an important metabolic substrate largely feeding the oxidative metabolism of many tissues, acting as a signaling molecule and as an inter-cellular and inter-tissular redox carrier. In this review, we provide an overview of the recent findings highlighting the importance of lactate in adipose tissues, from its production to its role as a browning inducer and its metabolic links with brown adipose tissue. We also discuss additional function(s) than thermogenesis ensured by brown and beige adipose tissues, i.e., their ability to dissipate high redox pressure and oxidative stress thanks to the activity of the uncoupling protein-1, helping to maintain tissue and whole organism redox homeostasis and integrity.

Tanycytes Regulate Lipid Homeostasis by Sensing Free Fatty Acids and Signaling to Key Hypothalamic Neuronal Populations via FGF21 Secretion

The hypothalamus plays a key role in the detection of energy substrates to regulate energy homeostasis. Tanycytes, the hypothalamic ependymo-glia, are located at a privileged position to integrate multiple peripheral inputs. We observed that tanycytes produce and secrete Fgf21 and are located close to Fgf21-sensitive neurons. Fasting, likely via the increase in circulating fatty acids, regulates this central Fgf21 production. Tanycytes store palmitate in lipid droplets and oxidize it, leading to the activation of a reactive oxygen species (ROS)/p38-MAPK signaling pathway, which is essential for tanycytic Fgf21 expression upon palmitate exposure. Tanycytic Fgf21 deletion triggers an increase in lipolysis, likely due to impaired inhibition of key neurons during fasting. Mice deleted for tanycytic Fgf21 exhibit increased energy expenditure and a reduction in fat mass gain, reminiscent of a browning phenotype. Our results suggest that tanycytes sense free fatty acids to maintain body lipid homeostasis through Fgf21 signaling within the hypothalamus.

LncRNA UCA1/miR-143 miR-216b/HK2/MAPK signaling pathway is involved in the regulation of endothelial cell proliferation via the modulation of glycolysis in melanoma

Long noncoding RNAs (lncRNAs) and microRNAs (miRNAs/miRs) are noncoding RNAs that function as regulators of tumor suppressors and oncogenes. The aim of the present study was to investigate the potential mechanism associated with the involvement of urothelial cancer associated 1 (UCA1) in melanoma. Reverse transcription-quantitative polymerase chain reaction and western blot analysis were performed in order to determine the expression levels of UCA1, miR-143, miR-216b, and hexokinase 2 (HK2) in the melanoma and control groups, as well as the influence of UCA1, miR-143, and miR-216b on the expression of HK2, and the effect of lactate and UCA1 on the phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK). Bioinformatics algorithm analysis and a luciferase assay were performed in order to predict miRNA targets. In addition, an MTT assay was performed in order to determine the effect of lactate and UCA1 expression on cell proliferation. A total of 39 participants, consisting of 18 patients with melanoma and 21 healthy control subjects, were included in the present study. The present study demonstrated that the expression levels of UCA1 mRNA, and HK2 mRNA and protein were enhanced in patients with melanoma compared with healthy controls; whereas the expression levels of miR-143 and miR-216b mRNA were suppressed in patients with melanoma compared with healthy controls. Furthermore, it was revealed that UCA1 negatively modulated the expression of miR-143 and miR-216b, and that miR-143 and miR-216b directly targeted the HK2 protein by binding to the HK2 3′ untranslated region (UTR). In addition, it was demonstrated that miR-143 and miR-216 suppressed the luciferase activity exhibited by wild-type HK2 3′-UTR. Furthermore, it was revealed that transfection with UCA1 small interfering RNA, and miR-143 and miR-216b mimics markedly suppressed HK2 mRNA and protein expression levels as well as lactate levels in human umbilical vein endothelial cells; however, O2 consumption was revealed to be enhanced post transfection. By contrast, transfection with UCA1 enhanced HK2 mRNA and protein expression levels as well as lactate production; however, O2 consumption was revealed to be suppressed post transfection. Lactate-induced phosphorylation of p38 MAPK was revealed to occur in a concentration-dependent manner, and UCA1 enhanced the phosphorylation level of p38 MAPK via the inhibition of miR-143 and miR-216b expression. Lactate and UCA1 were demonstrated to enhance cell proliferation. In conclusion, the present study demonstrated that the lncRNA UCA1/miR-143 miR-216b/HK2/lactic acid/MAPK axis may be involved in the pathogenesis of melanoma via the modulation of endothelial cells, and thus, lncRNA UCA1 may serve as a potential therapeutic target for melanoma treatment.

Adipose Organ Development and Remodeling

During the last decades, research on adipose tissues has spread in parallel with the extension of obesity. Several observations converged on the idea that adipose tissues are organized in a large organ with endocrine and plastic properties. Two parenchymal components: white (WATs) and brown adipose tissues (BATs) are contained in subcutaneous and visceral compartments. Although both have endocrine properties, their function differs: WAT store lipids to allow intervals between meals, BAT burns lipids for thermogenesis. In spite of these opposite functions, they share the ability for reciprocal reversible transdifferentiation to tackle special physiologic needs. Thus, chronic need for thermogenesis induces browning and chronic positive energy balance induce whitening. Lineage tracing and data from explant studies strongly suggest other remodeling properties of this organ. During pregnancy and lactation breast WAT transdifferentiates into milk-secreting glands, composed by cells with abundant cytoplasmic lipids (pink adipocytes) and in the postlactation period pink adipocytes transdifferentiate back into WAT and BAT. The plastic properties of mature adipocytes are supported also by a liposecretion process in vitro where adult cell in culture transdifferentiate to differentiated fibroblast-like elements able to give rise to different phenotypes (rainbow adipocytes). In addition, the inflammasome system is activated in stressed adipocytes from obese adipose tissue. These adipocytes die and debris are reabsorbed by macrophages inducing a chronic low-grade inflammation, potentially contributing to insulin resistance and T2 diabetes. Thus, the plastic properties of this organ could open new therapeutic perspectives in the obesity-related metabolic disease and in breast pathologies. © 2018 American Physiological Society. Compr Physiol 8:1357-1431, 2018.

A2A Receptor Activation Attenuates Hypertensive Cardiac Remodeling via Promoting Brown Adipose Tissue-Derived FGF21

Adipocytes play important roles in regulating cardiovascular health and disease. However, the molecular mechanism underlying the endocrine role of brown adipose tissue (BAT) in pathological cardiac remodeling remains unknown. Herein we show that adenosine A_2A receptor (A_2AR) knockout (A_2ARKO) causes interscapular BAT (iBAT) dysfunction, leading to accelerated cardiac remodeling in hypertension compared with wild-type (WT) mice. Surgical iBAT depletion induces dramatic cardiac remodeling in WT but not in A_2ARKO hypertensive mice. AMPK/PGC1α signaling-induced fibroblast growth factor 21 (FGF21) in brown adipocytes is required for A_2AR-mediated inhibition of hypertensive cardiac remodeling. Recombinant FGF21 administration improves cardiac remodeling in iBAT-depleted hypertensive mice. More importantly, brown adipocyte-specific A_2ARKO inhibits FGF21 production and accelerates cardiac damage in hypertension. Consistently, brown adipocyte-specific FGF21 knockout abolishes the effects of A_2AR agonism in attenuating hypertensive cardiac remodeling. Our findings reveal a distinctive endocrine role of BAT in hypertensive cardiac remodeling via activating A_2AR/FGF21 pathway.

The LXCXE Retinoblastoma Protein-Binding Motif of FOG-2 Regulates Adipogenesis

GATA transcription factors and their FOG cofactors play a key role in tissue-specific development and differentiation, from worms to humans. Mammals have six GATA and two FOG factors. We recently demonstrated that interactions between retinoblastoma protein (pRb) and GATA-1 are crucial for erythroid proliferation and differentiation. We show here that the LXCXE pRb-binding site of FOG-2 is involved in adipogenesis. Unlike GATA-1, which inhibits cell division, FOG-2 promotes proliferation. Mice with a knockin of a Fog2 gene bearing a mutated LXCXE pRb-binding site are resistant to obesity and display higher rates of white-to-brown fat conversion. Thus, each component of the GATA/FOG complex (GATA-1 and FOG-2) is involved in pRb/E2F regulation, but these molecules have markedly different roles in the control of tissue homeostasis.

Electron Density of Adipose Tissues Determined by Phase-Contrast Computed Tomography Provides a Measure for Mitochondrial Density and Fat Content

Phase-contrast computed tomography (PCCT) is an X-ray-based imaging method measuring differences in the refractive index during tissue passage. While conventional X-ray techniques rely on the absorption of radiation due to differing tissue-specific attenuation coefficients, PCCT enables the determination of the electron density (ED). By the analysis of respective phantoms and ex vivo specimens, we identified the components responsible for different electron densities in murine adipose tissue depots to be cellular fat and mitochondrial content, two parameters typically different between white adipose tissue (WAT) and brown adipose tissue (BAT). Brown adipocytes provide mammals with a means of non-shivering thermogenesis to defend normothermia in a cold environment. Brown adipocytes are found in dedicated BAT depots and interspersed within white fat depots, a cell type referred to as brite (brown in white) adipocyte. Localization and quantification of brown and brite adipocytes in situ allows an estimate of depot thermogenic capacity and potential contribution to maximal metabolic rate in the cold. We utilized PCCT to infer the composition of white, brite, and brown adipose tissue from ED of individual depots. As proof of principle, we imaged mice 10, 20, and 30 days of age. During this period, several WAT depots are known to undergo transient browning. Based on ED, classical WAT and BAT could be clearly distinguished. Retroperitoneal and inguinal WAT depots increased transiently in ED during the known remodeling from white to brite/brown and back to white. We systematically analyzed 18 anatomically defined adipose tissue locations and identified changes in fat content and mitochondrial density that imply an orchestrated pattern of simultaneous browning and whitening on the organismic level. Taken together, PCCT provides a three-dimensional imaging technique to visualize ED of tissues in situ. Within the adipose organ, ED provides a measure of mitochondrial density and fat content. Depending on experimental setting, these constitute surrogate markers of cellular distribution of white, brite, and brown adipocytes and thereby an estimate of thermogenic capacity.

Early increases in serum FGF21 levels predict mortality of septic patients

BACKGROUND

Potential prognostic biomarkers for patients with sepsis have yet to be identified. The present study evaluated the prognostic value of fibroblast growth factor 21 (FGF21) levels in patients with sepsis.

METHODS

A total of 120 consecutive Chinese patients with sepsis were prospectively included, and serum levels of FGF21 and biomarkers such as interleukin (IL)-6, tumor necrosis factor alpha (TNF-α), IL-10, procalcitonin (PCT), C-reactive protein (CRP), and lactate (LAC) were measured within 24 h after intensive care unit admission. The demographic and clinical characteristics including underlying diseases, Sequential Organ Failure Assessment (△SOFA), and acute physiology and chronic health evaluation II (APACHE II) scores were recorded. Patients were categorized into survival and non-survival groups according to the 28-day mortality. Correlations between FGF21, serum indicators, severity score and 28-day mortality were analyzed, and Cox regression analysis was performed to identify prognostic factors. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal cut-off of FGF21 for survival prediction.

RESULTS

Non-survivors had significantly higher levels of FGF21, IL-6, TNF-α, IL-10, PCT, CRP, and LAC as well as higher SOFA and APACHE II scores compared with the survivors. FGF21 levels were positively correlated with age, waist circumference, levels of IL-6, IL-10, TNF- α, PCT, CRP, and LAC, △SOFA and APACHE II scores. ROC curves showed that FGF21 had a high sensitivity of 81.3% and specificity of 89.8% for predicting 28-day mortality. Patients with a FGF21 levels <3200 pg/ml had a significantly better survival rate than those with levels >3200 pg/ml, and thus, FGF21 was an independent prognostic factor for survival.

CONCLUSION

FGF21 could serve as a new prognostic biomarker for sepsis survival.

Stereoselective effects of lactate enantiomers on the enhancement of 3T3-L1 adipocyte differentiation

Lactate contains a chiral carbon and thus has two optical isomers-d-lactate and l-lactate. l-Lactate is the predominant form that is produced by the body and can be delivered to the organs. On the other hand, gut microbiota produce both isomers, which can then flow into the body. Although both d-lactate and l-lactate can contribute to energy metabolism, their potential roles in adipocyte differentiation remain to be elucidated. Here, we investigated the effects of l-lactate and d-lactate on the differentiation of 3T3-L1 preadipocytes. Both lactate enantiomers were demonstrated to enhance triglyceride accumulation by stimulating the early phase of adipocyte differentiation. Notably, d-lactate was more potent than l-lactate in inducing triglyceride accumulation. The degree of triglyceride accumulation induced by l-lactate was similar to that induced by pyruvate. d-Lactate was more potent than l-lactate in increasing the activity of glycerol-3-phosphate dehydrogenase. Both lactate enantiomers did not affect cell viability. Moreover, both enantiomers upregulated the expression of peroxisome proliferator-activated receptor γ, CCAAT/enhancer-binding protein (C/EBP) α, sterol regulatory element-binding protein-1c, and fatty acid synthase, with d-lactate exerting stronger effects than l-lactate. By contrast, lactate did not influence the expression of C/EBPβ and C/EBPδ. d-Lactate significantly increased and l-lactate tended to increase p38 MAPK phosphorylation, and the p38 MAPK inhibitor SB203580 inhibited the stimulation of adipocyte differentiation by d-lactate and l-lactate. These findings showed that both lactate enantiomers stimulate preadipocyte differentiation, with d-lactate showing more potent effects than l-lactate. In addition, our study demonstrated that d-lactate and l-lactate exert different effects on physiological events.

Omega-3 fatty acids and adipose tissue biology

This review provides evidence for the importance of white and brown adipose tissue (i.e. WAT and BAT) function for the maintenance of healthy metabolic phenotype and its preservation in response to omega-3 polyunsaturated fatty acids (omega-3 PUFA), namely in the context of diseased states linked to aberrant accumulation of body fat, systemic low-grade inflammation, dyslipidemia and insulin resistance. More specifically, the review deals with (i) the concept of immunometabolism, i.e. how adipose-resident immune cells and adipocytes affect each other and define the immune-metabolic interface; and (ii) the characteristic features of "healthy adipocytes" in WAT, which are relatively small fat cells endowed with a high capacity for mitochondrial oxidative phosphorylation, triacylglycerol/fatty acid (TAG/FA) cycling and de novo lipogenesis (DNL). The intrinsic metabolic features of WAT and their flexible regulations, reflecting the presence of "healthy adipocytes", provide beneficial local and systemic effects, including (i) protection against in situ endoplasmic reticulum stress and related inflammatory response during activation of adipocyte lipolysis; (ii) prevention of ectopic fat accumulation and dyslipidemia caused by increased hepatic VLDL synthesis, as well as prevention of lipotoxic damage of insulin signaling in extra-adipose tissues; and also (iii) increased synthesis of anti-inflammatory and insulin-sensitizing lipid mediators with pro-resolving properties, including the branched fatty acid esters of hydroxy fatty acids (FAHFAs), also depending on the activity of DNL in WAT. The "healthy adipocytes" phenotype can be induced in WAT of obese mice in response to various stimuli including dietary omega-3 PUFA, especially when combined with moderate calorie restriction, and possibly also with other life style (e.g. physical activity) or pharmacological (e.g. thiazolidinediones) interventions. While omega-3 PUFA could exert beneficial systemic effects by improving immunometabolism of WAT without a concomitant induction of BAT, it is currently not clear whether the metabolic effects of the combined intervention using omega-3 PUFA and calorie restriction or thiazolidinediones depend also on the activation of BAT function and/or the induction of brite/beige adipocytes in WAT. It remains to be established why omega-3 PUFA intervention in type 2 diabetic subjects does not improve insulin sensitivity and glucose homeostasis despite inducing various anti-inflammatory mediators in WAT, including the recently discovered docosahexaenoyl esters of hydroxy linoleic acid, the lipokines from the FAHFA family, as well as several endocannabinoid-related anti-inflammatory lipids. To answer the question whether and to which extent omega-3 PUFA supplementation could promote the formation of "healthy adipocytes" in WAT of human subjects, namely in the obese insulin-resistant patients, represents a challenging task that is of great importance for the treatment of some serious non-communicable diseases.

Use of FGF-21 as a Biomarker of Mitochondrial Disease in Clinical Practice

Recent work has suggested that fibroblast growth factor-21 (FGF-21) is a useful biomarker of mitochondrial disease (MD). We routinely measured FGF-21 levels on patients who were investigated at our centre for MD and evaluated its diagnostic performance based on detailed genetic and other laboratory findings. Patients’ FGF-21 results were assessed by the use of age-adjusted z-scores based on normalised FGF-21 values from a healthy population. One hundred and fifty five patients were investigated. One hundred and four of these patients had molecular evidence for MD, 27 were deemed to have disorders other than MD (non-MD), and 24 had possible MD. Patients with defects in mitochondrial DNA (mtDNA) maintenance (n = 32) and mtDNA rearrangements (n = 17) had the highest median FGF-21 among the MD group. Other MD patients harbouring mtDNA point mutations (n = 40) or mutations in other autosomal genes (n = 7) and those with partially characterised MD had lower FGF-21 levels. The area under the receiver operating characteristic curve for distinguishing MD from non-MD patients was 0.69. No correlation between FGF-21 and creatinine, creatine kinase, or cardio-skeletal myopathy score was found. FGF-21 was significantly associated with plasma lactate and ocular myopathy. Although FGF-21 was found to have a low sensitivity for detecting MD, at a z-score of 2.8, its specificity was above 90%. We suggest that a high serum concentration of FGF-21 would be clinically useful in MD, especially in adult patients with chronic progressive external ophthalmoplegia, and may enable bypassing muscle biopsy and directly opting for genetic analysis. Availability of its assay has thus modified our diagnostic pathway.

Hormonal factors in the control of the browning of white adipose tissue

Adipose tissue has been historically classified into anabolic white adipose tissue (WAT) and catabolic brown adipose tissue (BAT). Recent studies have revealed the plasticity of WAT, where white adipocytes can be induced into 'brown-like' heat-producing adipocytes (BRITE or beige adipocytes). Recruiting and activating BRITE adipocytes in WAT (so-called 'browning') is believed to provide new avenues for the treatment of obesity-related diseases. A number of hormonal factors have been found to regulate BRITE adipose development and activity through autocrine, paracrine and systemic mechanisms. In this mini-review we will discuss the impact of these factors on the browning process, especially those hormonal factors identified with direct effects on white adipocytes.

Amelioration of late-onset hepatic steatosis in IDH2-deficient mice

Nonalcoholic fatty liver disease (NAFLD) has a high prevalence in the general population and can evolve into nonalcoholic steatohepatosis (NASH), cirrhosis, and complications such as liver failure and hepatocellular carcinoma. Recently, we reported that mitochondrial NADP⁺-dependent isocitrate dehydrogenase, encoded by the IDH2, plays an important role in the regulation of redox balance and oxidative stress levels, which are tightly associated with intermediary metabolism and energy production. In the present study, we showed that in mice targeted disruption of IDH2 attenuates age-associated hepatic steatosis by the activation of p38/cJun NH2-terminal kinase (JNK) and p53, presumably induced by the elevation of mitochondrial reactive oxygen species (ROS), which in turn resulted in the suppression of hepatic lipogenesis and inflammation via the upregulation of fibroblast growth factor 21 (FGF21) and the inhibition of NFκB signaling pathways. Our finding uncovers a new mechanism involved in hepatocellular steatosis and IDH2 may be a valuable therapeutic target for the management of NAFLD.

Fibroblast growth factor 21 and its novel association with oxidative stress

Fibroblast growth factor 21 (FGF21) is an endocrine-member of the FGF family. It is synthesized mainly in the liver, but it is also expressed in adipose tissue, skeletal muscle, and many other organs. It has a key role in glucose and lipid metabolism, as well as in energy balance. FGF21 concentration in plasma is increased in patients with obesity, insulin resistance, and metabolic syndrome. Recent findings suggest that such increment protects tissue from an increased oxidative stress environment. Different types of physical stress, such as strenuous exercising, lactation, diabetic nephropathy, cardiovascular disease, and critical illnesses, also increase FGF21 circulating concentration. FGF21 is now considered a stress-responsive hormone in humans. The discovery of an essential response element in the FGF21 gene, for the activating transcription factor 4 (ATF4), involved in the regulation of oxidative stress, and its relation with genes such as NRF2, TBP-2, UCP3, SOD2, ERK, and p38, places FGF21 as a key regulator of the oxidative stress cell response. Its role in chronic diseases and its involvement in the treatment and follow-up of these diseases has been recently the target of new studies. The diminished oxidative stress through FGF21 pathways observed with anti-diabetic therapy is another clue of the new insights of this hormone.

Lactate as a Signaling Molecule That Regulates Exercise-Induced Adaptations

Lactate (or its protonated form: lactic acid) has been studied by many exercise scientists. The lactate paradigm has been in constant change since lactate was first discovered in 1780. For many years, it was unfairly seen as primarily responsible for muscular fatigue during exercise and a waste product of glycolysis. The status of lactate has slowly changed to an energy source, and in the last two decades new evidence suggests that lactate may play a much bigger role than was previously believed: many adaptations to exercise may be mediated in some way by lactate. The mechanisms behind these adaptations are yet to be understood. The aim of this review is to present the state of lactate science, focusing on how this molecule may mediate exercise-induced adaptations.

Hormonal and nutritional signalling in the control of brown and beige adipose tissue activation and recruitment

Recent research has revealed that the activity of adipose tissue (BAT) in adult humans is higher than previously thought, and that obese patients show abnormally low levels of brown fat activity. Studies in experimental animals have shown that BAT is a site of energy expenditure, and that BAT activity protects against obesity and associated metabolic diseases. The action of the sympathetic nervous activity on BAT depots is considered the main regulator of BAT activity in rodent models and possibly also in humans. However, recent research has revealed the existence of additional hormonal factors, produced by distinct peripheral tissues or present in the diet, that influence the amount and activity of BAT. These hormonal factors may act on BAT directly, but also indirectly by targeting the brain and determining the intensity of sympathetic action upon BAT. Identification and characterization of novel factors that control BAT may provide clues for the development of new strategies to treat obesity and metabolic diseases.

What induces watts in WAT?

Excess calories stored in white adipose tissue (WAT) could be reduced either through the activation of brown adipose tissue (BAT) or the development of brown-like cells ("beige" or "brite") in WAT, a process named "browning." Calorie dissipation in brown and beige adipocytes might rely on the induction of uncoupling protein 1 (UCP1), which is absent in white fat cells. Any increase in UCP1 is commonly considered as the trademark of energy expenditure. The intracellular events involved in the recruitment process of beige precursors were extensively studied lately, as were the effectors, hormones, cytokines, nutrients and drugs able to modulate the route of browning and theoretically affect fat mass in rodents and in humans. The aim of this review is to update the characterization of the extracellular effectors that induce UCP1 in WAT and potentially provoke calorie dissipation. The potential influence of metabolic cycling in energy expenditure is also questioned.