Levels of ß-klotho determine the thermogenic responsiveness of adipose tissues: involvement of the autocrine action of FGF21.

Fibroblast growth factor-21 (FGF21) is a hormonal regulator of metabolism; it promotes glucose oxidation and the thermogenic capacity of adipose tissues. The levels of ß-klotho (KLB), the co-receptor required for FGF21 action, are decreased in brown (BAT) and white (WAT) adipose tissues during obesity, diabetes, and lipodystrophy. Reduced ß-klotho levels have been proposed to account for FGF21 resistance in these conditions. In this study, we explored whether downregulation of ß-klotho affects metabolic regulation and the thermogenic responsiveness of adipose tissues using mice with total (KLB-KO) or partial (KLB-heterozygotes) ablation of ß-klotho. We herein show that KLB gene dosage was inversely associated with adiposity in mice. Upon cold exposure, impaired browning of subcutaneous WAT and milder alterations in BAT were associated with reduced KLB gene dosage in mice. Cultured brown and beige adipocytes from mice with total or partial ablation of the KLB gene showed reduced thermogenic responsiveness to ß3-adrenergic activation by treatment with CL316,243, indicating that these effects were cell-autonomous. Deficiency in FGF21 mimicked the KLB-reduction-induced impairment of thermogenic responsiveness in brown and beige adipocytes. These results indicate that the levels of KLB in adipose tissues determine their thermogenic capacity to respond to cold and/or adrenergic stimuli. Moreover, an autocrine action of FGF21 in brown and beige adipocytes may account for the ability of the KLB level to influence thermogenic responsiveness.NEW & NOTEWORTHY Reduced levels of KLB (the obligatory FGF21 co-receptor), as occurring in obesity and type 2 diabetes, reduce the thermogenic responsiveness of adipose tissues in cold-exposed mice. Impaired response to ß3-adrenergic activation in brown and beige adipocytes with reduced KLB occurs in a cell-autonomous manner involving an autocrine action of FGF21.

Parkin controls brown adipose tissue plasticity in response to adaptive thermogenesis.

Parkin is an ubiquitin-E3 ligase that acts as a key component of the cellular machinery for mitophagy. We show here that Parkin expression is reciprocally regulated in brown adipose tissue in relation to thermogenic activity. Thermogenic stimuli repress Parkin gene expression via transcriptional mechanisms that are elicited by noradrenergic and PPARα-mediated pathways that involve intracellular lipolysis in brown adipocytes. Parkin-KO mice show over-activated brown adipose tissue thermogenic activity and exhibit improved metabolic parameters, especially when fed a high-fat diet. Deacclimation, which is the return of a cold-adapted mouse to a thermoneutral temperature, dramatically induces mitophagy in brown adipocytes, with a concomitant induction of Parkin levels. We further reveal that Parkin-KO mice exhibit defects in the degradative processing of mitochondrial proteins in brown adipose tissue in response to deacclimation. These results suggest that the transcriptional control of Parkin in brown adipose tissue may contribute to modulating the mitochondrial mass and activity for adaptation to thermogenic requirements.

Lipopolysaccharide-binding protein is a negative regulator of adipose tissue browning in mice and humans.

AIMS/HYPOTHESIS: Adipocyte lipopolysaccharide-binding protein (LBP) biosynthesis is associated with obesity-induced adipose tissue dysfunction. Our purpose was to study the role of LBP in regulating the browning of adipose tissue. METHODS: Adult mice were maintained at 4°C for 3 weeks or treated with the ß3-adrenergic agonist, CL316,243, for 1 week to induce the browning of white fat. Precursor cells from brown and white adipose tissues were cultured under differentiation-inducing conditions to yield brown and beige/brite adipocytes, respectively. In vitro, Lbp was knocked down in 3T3-L1 adipocytes, and cells were treated with recombinant LBP or co-cultured in transwells with control 3T3-L1 adipocytes. Wild-type and Lbp-null mice, fed a standard or high fat diet (HFD) for 15 weeks, were also used in investigations. In humans, subcutaneous and visceral adipose tissue samples were obtained from a cohort of morbidly obese participants. RESULTS: The induction of white fat browning by exposure of mice to cold or CL316,243 treatment was strongly associated with decreased Lbp mRNA expression in white adipose tissue. The acquisition of the beige/brite phenotype in cultured cells was associated with downregulation of Lbp. Moreover, silencing of Lbp induced the expression of brown fat-related genes in adipocytes, whereas LBP treatment reversed this effect. Lbp-null mice exhibited the spontaneous induction of subcutaneous adipose tissue browning, as evidenced by a remarkable increase in Ucp1 and Dio2 gene expression and the appearance of multivacuolar adipocyte clusters. The amount of brown adipose tissue, and brown adipose tissue activity were also increased in Lbp-null mice. These changes were associated with decreased weight gain in Lbp-null mice and protection against HFD-induced inflammatory responses, as shown by reduced IL-6 levels. However, rather than improving glucose homeostasis, these effects led to glucose intolerance and insulin resistance. CONCLUSIONS/INTERPRETATION: LBP is identified as a negative regulator of the browning process, which is likely to contribute to the obesity-promoting action of LBP. The deleterious metabolic effects of LBP deletion are compatible with the concept that the appropriate regulation of inflammatory pathways is necessary for a healthy systemic metabolic profile, regardless of body weight regulation.

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

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

Blood Sampling for Arteriovenous Difference Measurements Across Interscapular Brown Adipose Tissue in Rat.

A classic physiological approach to assess the specific uptake or release of circulating factors in organs and tissues is to measure concentration differences between venous and arterial blood. For interscapular brown adipose tissue (iBAT), the anatomic distribution of its vascularization, which drains most of the blood into Sulzer's vein, allows for local measurement of arteriovenous differences. The use of this procedure to monitor oxygen concentration changes was fundamental for the recognition of BAT as the main site of adaptive non-shivering thermogenesis. More recently, this technique has regained importance as a means to identify BAT-secreted regulatory molecules, such as fibroblast growth factor-21 and the chemokine CXCL14. In this chapter, we provide a detailed description of an optimized and feasible protocol to determine arteriovenous differences across iBAT. We include tips and practical advice for using this powerful tool to study BAT metabolism and secretory activity in rats as an experimental model.

Differential effects of dolutegravir, bictegravir and raltegravir in adipokines and inflammation markers on human adipocytes.

AIMS: To assess the potential direct effects of the integrase strand-transfer inhibitors (INsTIs) dolutegravir, bictegravir, and raltegravir, drugs used as treatment for people living with human immunodeficiency virus (PLWH), on human adipose cells. MAIN METHODS: Drugs were added to the differentiation medium of human Simpson-Golabi-Behmel syndrome (SGBS) adipose cells and morphological adipogenesis was monitored for 10 days. Also, adipocytes were exposed to drugs following differentiation (day 14). The gene expression levels of selected adipogenesis markers, adipocyte metabolism markers, adipokines, and cytokines were determined by quantitative-reverse transcription polymerase-chain reaction. The release of adiponectin and leptin into the culture medium was measured using specific enzyme-linked immunosorbent assay, and release of interleukin-6 and chemokine (CC motif) ligand-2 using Multiplex assays. KEY FINDINGS: Overall morphological adipogenesis was unaltered by INsTIs. The expression of adipogenesis marker genes (peroxisome proliferator-activated receptor-Ɣ and lipoprotein lipase) was slightly reduced in dolutegravir-treated differentiating adipocytes. Bictegravir repressed gene expression and the release of pro-inflammatory cytokines in differentiating adipocytes. Dolutegravir and raltegravir increased interleukin-6 gene expression, but only dolutegravir increased interleukin-6 release. Dolutegravir repressed adiponectin expression and release in differentiating adipocytes and had a similar but milder effect on leptin. Drug treatment of mature adipocytes reduced adiponectin gene expression in response to dolutegravir. SIGNIFICANCE: The INsTIs studied do not have a significant effect on human adipose cell differentiation but exert distinct effects on gene expression and secretion of adipokines and cytokines. These findings will help understand and manage the effects of INsTI-containing treatments on body weight and metabolic dysregulation in PLWH.

The relative deficit of GDF15 in adolescent girls with PCOS can be changed into an abundance that reduces liver fat.

A prime concern of young patients with Polycystic Ovary Syndrome (PCOS) is the control of body adiposity, given their tendency to gain weight and/or their difficulty to lose weight. Circulating growth-and-differentiation factor-15 (GDF15) facilitates the control of body weight via receptors in the brainstem. C-reactive protein (CRP) and insulin are endogenous GDF15 secretagogues. We hypothesised that PCOS in non-obese adolescents is characterised by low concentrations of circulating GDF15, when judged by the degree of CRP and insulin drive. GDF15 was added as a post-hoc endpoint of two previously reported, randomised studies in non-obese adolescent girls with PCOS (N = 58; 60% normal weight; 40% overweight) who received either an oral oestroprogestogen contraceptive (OC), or a low-dose combination of spironolactone-pioglitazone-metformin (SPIOMET) for 1 year; subsequently, all girls remained untreated for 1 year. Adolescent girls with regular menses (N = 20) served as healthy controls. Circulating GDF15, CRP and fasting insulin were assessed prior to treatment, and halfway the on- and post-treatment years. Pre-treatment, the absolute GDF15 concentrations were normal in PCOS girls, but their relative levels were markedly low, in view of the augmented CRP and insulin drives. OC treatment was accompanied by a near-doubling of circulating GDF15 (on average, from 296 to 507 pg/mL) and CRP, so that the relative GDF15 levels remained low. SPIOMET treatment was accompanied by a 3.4-fold rise of circulating GDF15 (on average, from 308 to 1045 pg/mL) and by a concomitant lowering of CRP and insulin concentrations towards normal, so that the relative GDF15 levels became markedly abundant. Post-OC, the relatively low GDF15 levels persisted; post-SPIOMET, the circulating concentrations of GDF15, CRP and insulin were all normal. BMI remained stable in both treatment groups. Only SPIOMET was accompanied by a reduction of hepato-visceral fat (by MRI) towards normal. In conclusion, early PCOS was found to be characterised by a relative GDF15 deficit that may partly explain the difficulties that young patients experience to control their body adiposity. This relative GDF15 deficit persisted during and after OC treatment. In contrast, SPIOMET treatment was accompanied by an absolute and a relative abundance of GDF15, and followed by normal GDF15, CRP and insulin concentrations. The present findings strengthen the rationale to raise the concentrations of circulating GDF15 in early PCOS, for example with a SPIOMET-like intervention that attenuates low-grade inflammation, insulin resistance and ectopic adiposity, without necessarily lowering body weight.Clinical trial registries: ISRCTN29234515 and ISRCTN11062950.

The role of autophagy in brown and beige adipose tissue plasticity.

Since the rediscovery of active brown and beige adipose tissues in humans a decade ago, great efforts have been made to identify the mechanisms underlying the activation and inactivation of these tissues, with the hope of designing potential strategies to fight against obesity and associated metabolic disorders such as type 2 diabetes. Active brown/beige fat increases the energy expenditure and is associated with reduced hyperglycemia and hyperlipidemia, whereas its atrophy and inactivation have been associated with obesity and aging. Autophagy, which is the process by which intracellular components are degraded within the lysosomes, has recently emerged as an important regulatory mechanism of brown/beige fat plasticity. Studies have shown that autophagy participates in the intracellular remodeling events that occur during brown/beige adipogenesis, thermogenic activation, and inactivation. The autophagic degradation of mitochondria appears to be important for the inactivation of brown fat and the transition from beige-to-white adipose tissue. Moreover, autophagic dysregulation in adipose tissues has been associated with obesity. Thus, understanding the regulatory mechanisms that control autophagy in the physiology and pathophysiology of adipose tissues might suggest novel treatments against obesity and its associated metabolic diseases.

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

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

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

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

Brown Adipocytes Secrete GDF15 in Response to Thermogenic Activation.

OBJECTIVE: Transcriptomic analysis of gene expression in brown adipose tissue (BAT) from mice in response to cold revealed strong induction of growth and differentiation factor 15 (GDF15). This study aimed to characterize GDF15 as a brown adipokine released in response to thermogenic activation and to determine its target functions. METHODS: GDF15 expression was measured in adipose tissues from mice in response to physiological and pharmacological modulators of thermogenesis. Brown and beige cell cultures were used to dissect the mechanisms regulating GDF15 expression. Brown adipocyte cellular models of fibroblast growth factor 21 and ß-klotho invalidation were employed to identify the autocrine regulators of GDF15. RAW 264.7 macrophages were used to explore the targeting of GDF15 released by brown adipocytes. RESULTS: Cold exposure of mice strongly induced GDF15 expression in BAT. Norepinephrine and cyclic adenosine monophosphate induced GDF15 expression and release by cells through protein kinase A-mediated mechanisms. Noradrenergic regulation of GDF15 required the active fibroblast growth factor 21 pathway in brown adipocytes. GDF15 released by brown adipocytes targeted macrophages and downregulated the expression of proinflammatory genes. CONCLUSIONS: GDF15 is a brown adipokine released by brown and beige cells in response to thermogenic activity. GDF15 released by BAT targets macrophages and may mediate downregulation of local inflammatory pathways.

Aging is associated with increased FGF21 levels but unaltered FGF21 responsiveness in adipose tissue.

Fibroblast growth factor 21 (FGF21) has been proposed to be an antiaging hormone on the basis of experimental studies in rodent models. However, circulating FGF21 levels are increased with aging in rodents and humans. Moreover, despite the metabolic health-promoting effects of FGF21, the levels of this hormone are increased under conditions such as obesity and diabetes, an apparent incongruity that has been attributed to altered tissue responsiveness to FGF21. Here, we investigated serum FGF21 levels and expression of genes encoding components of the FGF21-response molecular machinery in adipose tissue from healthy elderly individuals (≥70 years old) and young controls. Serum FGF21 levels were increased in elderly individuals and were positively correlated with insulinemia and HOMA-IR, indices of mildly deteriorated glucose homeostasis. Levels of ß-Klotho, the coreceptor required for cellular responsiveness to FGF21, were increased in subcutaneous adipose tissue from elderly individuals relative to those from young controls, whereas FGF receptor-1 levels were unaltered. Moreover, total ERK1/2 protein levels were decreased in elderly individuals in association with an increase in the ERK1/2 phosphorylation ratio relative to young controls. Adipose explants from aged and young mice respond similarly to FGF21 "ex vivo". Thus, in contrast to what is observed in obesity and diabetes, high levels of FGF21 in healthy aging are not associated with repressed FGF21-responsiveness machinery in adipose tissue. The lack of evidence for impaired FGF21 responsiveness in adipose tissue establishes a distinction between alterations in the FGF21 endocrine system in aging and chronic metabolic pathologies.

CXCL14, a Brown Adipokine that Mediates Brown-Fat-to-Macrophage Communication in Thermogenic Adaptation.

The beneficial effects of brown adipose tissue (BAT) are attributed to its capacity to oxidize metabolites and produce heat, but recent data suggest that secretory properties of BAT may also be involved. Here, we identify the chemokine CXCL14 (C-X-C motif chemokine ligand-14) as a novel regulatory factor secreted by BAT in response to thermogenic activation. We found that the CXCL14 released by brown adipocytes recruited alternatively activated (M2) macrophages. Cxcl14-null mice exposed to cold showed impaired BAT activity and low recruitment of macrophages, mainly of the M2 phenotype, into BAT. CXCL14 promoted the browning of white fat and ameliorated glucose/insulin homeostasis in high-fat-diet-induced obese mice. Impairment of type 2 cytokine signaling, as seen in Stat6-null mice, blunts the action of CXCL14, promoting adipose tissue browning. We propose that active BAT is a source of CXCL14, which concertedly promotes adaptive thermogenesis via M2 macrophage recruitment, BAT activation, and the browning of white fat.

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.

The lipid sensor GPR120 promotes brown fat activation and FGF21 release from adipocytes.

The thermogenic activity of brown adipose tissue (BAT) and browning of white adipose tissue are important components of energy expenditure. Here we show that GPR120, a receptor for polyunsaturated fatty acids, promotes brown fat activation. Using RNA-seq to analyse mouse BAT transcriptome, we find that the gene encoding GPR120 is induced by thermogenic activation. We further show that GPR120 activation induces BAT activity and promotes the browning of white fat in mice, whereas GRP120-null mice show impaired cold-induced browning. Omega-3 polyunsaturated fatty acids induce brown and beige adipocyte differentiation and thermogenic activation, and these effects require GPR120. GPR120 activation induces the release of fibroblast growth factor-21 (FGF21) by brown and beige adipocytes, and increases blood FGF21 levels. The effects of GPR120 activation on BAT activation and browning are impaired in FGF21-null mice and cells. Thus, the lipid sensor GPR120 activates brown fat via a mechanism that involves induction of FGF21.