BCAA catabolism in brown fat controls energy homeostasis through SLC25A44.

Branched-chain amino acid (BCAA; valine, leucine and isoleucine) supplementation is often beneficial to energy expenditure; however, increased circulating levels of BCAA are linked to obesity and diabetes. The mechanisms of this paradox remain unclear. Here we report that, on cold exposure, brown adipose tissue (BAT) actively utilizes BCAA in the mitochondria for thermogenesis and promotes systemic BCAA clearance in mice and humans. In turn, a BAT-specific defect in BCAA catabolism attenuates systemic BCAA clearance, BAT fuel oxidation and thermogenesis, leading to diet-induced obesity and glucose intolerance. Mechanistically, active BCAA catabolism in BAT is mediated by SLC25A44, which transports BCAAs into mitochondria. Our results suggest that BAT serves as a key metabolic filter that controls BCAA clearance via SLC25A44, thereby contributing to the improvement of metabolic health

Rôle du gène suppresseur de tumeur p16INK4a dans le métabolisme hépatique des lipides au cours du jeûne

P16INK4a is a tumor suppressor protein that is a well described cell cycle regulator. Recently, genome-wide association studies (GWAS) associated the CDKN2A locus, from which p16INK4A is encoded, with increased risk for development of type 2 diabetes. A pathophysiological link between p16INK4a and hepatic glucose homeostasis has been unraveled recently, through the control of gluconeogenesis. Patients with T2D also present with disturbances in fat metabolism, associated with an increased prevalence to Non Alcoholic Fatty liver diseases (NAFLD). In this context, we investigated the role of p16INK4a in hepatic lipid metabolism in vitro using primary hepatocytes, the murin AML12 and human IHH hepatocyte cell line transfected respectively with siRNA-CDKN2A and siRNA-p16 and in vivo using p16+/+ and p16-/- mice.Transcriptomic analyses of p16+/+ and p16-/- primary hepatocytes using microarrays revealed that metabolic and PPARα signaling pathways were among the most modulated in p16 absence. Moreover, in primary hepatocytes and in hepatocyte cell lines, p16 deficiency modulates a subset of PPARα target genes associated to fatty acids oxidation (FAO). These effects were associated with an increased response to GW647, a PPAR945; agonist, and reversed by siRNA targeting PPAR45;. Investigating known PPAR945; activators and transcriptional co-activators in vitro, we found that upregulation of FAO genes expression was linked to SIRT1. AMPK is a known activator of FAO and has been shown to induce SIRT1 activation through increase of NAD/NADH ratio. Interestingly, downregulation of p16 expression in vitro led to increased AMPK phosphorylation and activation.In vitro, p16-/- primary hepatocytes demonstrated enhanced fatty acid oxidation of oleate compared to p16+/+. During fasting, enhanced FAO leads to a shift of acetyl-coA utilization from the TCA cycle to ketogenesis. Interestingly, p16-/- mice showed a tendency to produce more ketone bodies than their control littermate after sodium octanoate injection. These findings describe a new function for p16INK4a in hepatic lipid metabolism through activation of AMPK-SIRT1-PPARα pathway.Plusieurs études génétiques d’association de gènes ont mis en évidence le locus CDKN2A, codant notamment la protéine p16INK4a (p16), un gène suppresseur de tumeur, comme étant associé au risque de développement du diabète de type 2 (T2D) et des maladies cardiovasculaires. Le T2D, caractérisé par une hyperglycémie et/ou une insulinorésistance, s’accompagne fréquemment d’une stéatose hépatique prédisposant au développement de la NASH (Non Alcoholic Steatohepatitis), et contribuant à un risque accru de complications cardiovasculaires. Nous avons montré que la déficience de p16 augmente la néoglucogenèse hépatique lors d'un jeûne suggérant un rôle de p16 dans le T2D. Cependant, le rôle de p16 dans l’homéostasie hépatique des lipides n’est à ce jour pas connu. Afin de déterminer le rôle de p16 dans le métabolisme hépatique des lipides, nous avons utilisé des hépatocytes primaires isolés de souris p16+/+ et p16-/- ainsi que les lignées d’hépatocytes murins AML12 et humains IHH transfectées respectivement avec un siRNA-CDKN2A ou siRNA-p16.Nous avons montré par l’étude transcriptomique des hépatocytes primaires de souris par puces à ADN, que l’absence de p16 module les voies métaboliques associées à PPARα et contrôle préférentiellement l’expression de certains gènes cibles de PPARα, associés au catabolisme des acides gras. _x000D_Dans les lignées cellulaires hépatocytaires, certains de ces gènes sont également modulés après diminution de l’expression de p16 par siRNA. Ces effets sont associés à une meilleure réponse à l’agoniste de PPARα, le GW647, et abolis par un siRNA ciblant PPARα. Afin d’étudier par quel(s) mécanisme(s) l’absence de p16 module l’expression des gènes cibles de PPARα, le rôle de certains de ses coactivateurs transcriptionnels a été étudié par l’utilisation d’inhibiteurs pharmacologiques ou de siRNA. De manière intéressante, nous avons pu montrer que l’absence de p16 active la voie AMPK-SIRT1 afin d’augmenter l’expression des gènes cibles de la β-oxydation et de la cétogenèse. De plus, ces effets sont indépendants du rôle de p16 dans le cycle cellulaire. In vitro, les hépatocytes primaires p16-/-, incubés avec de l’oléate radiomarqué, présentent une β-oxydation augmentée comparés aux hépatocytes primaires p16+/+. Au cours du jeûne, l’acétyl-CoA provenant de la β-oxydation est redirigé vers la production de corps cétoniques. De manière intéressante, les souris p16-/- injectées avec du sodium octanoate, un acide gras à chaîne courte préférentiellement utilisé via la cétogenèse, ont une tendance à avoir une production plus importante de corps cétoniques.Nous avons ainsi pu mettre en évidence que la déficience de p16 dans les hépatocytes favorise l’utilisation des acides gras, via l’activation de la voie SIRT1-AMPK-PPARα

Role of the p16INK4a tumour suppressor gene in hepatic lipid metabolism during fasting

Plusieurs études génétiques d’association de gènes ont mis en évidence le locus CDKN2A, codant notamment la protéine p16INK4a (p16), un gène suppresseur de tumeur, comme étant associé au risque de développement du diabète de type 2 (T2D) et des maladies cardiovasculaires. Le T2D, caractérisé par une hyperglycémie et/ou une insulinorésistance, s’accompagne fréquemment d’une stéatose hépatique prédisposant au développement de la NASH (Non Alcoholic Steatohepatitis), et contribuant à un risque accru de complications cardiovasculaires. Nous avons montré que la déficience de p16 augmente la néoglucogenèse hépatique lors d'un jeûne suggérant un rôle de p16 dans le T2D. Cependant, le rôle de p16 dans l’homéostasie hépatique des lipides n’est à ce jour pas connu. Afin de déterminer le rôle de p16 dans le métabolisme hépatique des lipides, nous avons utilisé des hépatocytes primaires isolés de souris p16+/+ et p16-/- ainsi que les lignées d’hépatocytes murins AML12 et humains IHH transfectées respectivement avec un siRNA-CDKN2A ou siRNA-p16.Nous avons montré par l’étude transcriptomique des hépatocytes primaires de souris par puces à ADN, que l’absence de p16 module les voies métaboliques associées à PPARα et contrôle préférentiellement l’expression de certains gènes cibles de PPARα, associés au catabolisme des acides gras. _x000D_Dans les lignées cellulaires hépatocytaires, certains de ces gènes sont également modulés après diminution de l’expression de p16 par siRNA. Ces effets sont associés à une meilleure réponse à l’agoniste de PPARα, le GW647, et abolis par un siRNA ciblant PPARα. Afin d’étudier par quel(s) mécanisme(s) l’absence de p16 module l’expression des gènes cibles de PPARα, le rôle de certains de ses coactivateurs transcriptionnels a été étudié par l’utilisation d’inhibiteurs pharmacologiques ou de siRNA. De manière intéressante, nous avons pu montrer que l’absence de p16 active la voie AMPK-SIRT1 afin d’augmenter l’expression des gènes cibles de la β-oxydation et de la cétogenèse. De plus, ces effets sont indépendants du rôle de p16 dans le cycle cellulaire. In vitro, les hépatocytes primaires p16-/-, incubés avec de l’oléate radiomarqué, présentent une β-oxydation augmentée comparés aux hépatocytes primaires p16+/+. Au cours du jeûne, l’acétyl-CoA provenant de la β-oxydation est redirigé vers la production de corps cétoniques. De manière intéressante, les souris p16-/- injectées avec du sodium octanoate, un acide gras à chaîne courte préférentiellement utilisé via la cétogenèse, ont une tendance à avoir une production plus importante de corps cétoniques.Nous avons ainsi pu mettre en évidence que la déficience de p16 dans les hépatocytes favorise l’utilisation des acides gras, via l’activation de la voie SIRT1-AMPK-PPARα.P16INK4a is a tumor suppressor protein that is a well described cell cycle regulator. Recently, genome-wide association studies (GWAS) associated the CDKN2A locus, from which p16INK4A is encoded, with increased risk for development of type 2 diabetes. A pathophysiological link between p16INK4a and hepatic glucose homeostasis has been unraveled recently, through the control of gluconeogenesis. Patients with T2D also present with disturbances in fat metabolism, associated with an increased prevalence to Non Alcoholic Fatty liver diseases (NAFLD). In this context, we investigated the role of p16INK4a in hepatic lipid metabolism in vitro using primary hepatocytes, the murin AML12 and human IHH hepatocyte cell line transfected respectively with siRNA-CDKN2A and siRNA-p16 and in vivo using p16+/+ and p16-/- mice.Transcriptomic analyses of p16+/+ and p16-/- primary hepatocytes using microarrays revealed that metabolic and PPARα signaling pathways were among the most modulated in p16 absence. Moreover, in primary hepatocytes and in hepatocyte cell lines, p16 deficiency modulates a subset of PPARα target genes associated to fatty acids oxidation (FAO). These effects were associated with an increased response to GW647, a PPAR945; agonist, and reversed by siRNA targeting PPAR45;. Investigating known PPAR945; activators and transcriptional co-activators in vitro, we found that upregulation of FAO genes expression was linked to SIRT1. AMPK is a known activator of FAO and has been shown to induce SIRT1 activation through increase of NAD/NADH ratio. Interestingly, downregulation of p16 expression in vitro led to increased AMPK phosphorylation and activation.In vitro, p16-/- primary hepatocytes demonstrated enhanced fatty acid oxidation of oleate compared to p16+/+. During fasting, enhanced FAO leads to a shift of acetyl-coA utilization from the TCA cycle to ketogenesis. Interestingly, p16-/- mice showed a tendency to produce more ketone bodies than their control littermate after sodium octanoate injection. These findings describe a new function for p16INK4a in hepatic lipid metabolism through activation of AMPK-SIRT1-PPARα pathway

CDKN2A/p16INK4a suppresses hepatic fatty acid oxidation through the AMPKα2-SIRT1-PPARα signaling pathway

International audienceIn addition to their well-known role in the control of cellular proliferation and cancer, cell cycle regulators are increasingly identified as important metabolic modulators. Several GWAS have identified SNPs near CDKN2A, the locus encoding for p16INK4a (p16), associated with elevated risk for cardiovascular diseases and type-2 diabetes development, two pathologies associated with impaired hepatic lipid metabolism. Although p16 was recently shown to control hepatic glucose homeostasis, it is unknown whether p16 also controls hepatic lipid metabolism. Using a combination of in vivo and in vitro approaches, we found that p16 modulates fasting-induced hepatic fatty acid oxidation (FAO) and lipid droplet accumulation. In primary hepatocytes, p16-deficiency was associated with elevated expression of genes involved in fatty acid catabolism. These transcriptional changes led to increased FAO and were associated with enhanced activation of PPARα through a mechanism requiring the catalytic AMPKα2 subunit and SIRT1, two known activators of PPARα. By contrast, p16 overexpression was associated with triglyceride accumulation and increased lipid droplet numbers in vitro, and decreased ketogenesis and hepatic mitochondrial activity in vivo. Finally, gene expression analysis of liver samples from obese patients revealed a negative correlation between CDKN2A expression and PPARA and its target genes. Our findings demonstrate that p16 represses hepatic lipid catabolism during fasting and may thus participate in the preservation of metabolic flexibility

Hepatic PPARα is critical in the metabolic adaptation to sepsis.

BACKGROUND & AIMS: Although the role of inflammation to combat infection is known, the contribution of metabolic changes in response to sepsis is poorly understood. Sepsis induces the release of lipid mediators, many of which activate nuclear receptors such as the peroxisome proliferator-activated receptor (PPAR)α, which controls both lipid metabolism and inflammation. We aimed to elucidate the previously unknown role of hepatic PPARα in the response to sepsis. METHODS: Sepsis was induced by intraperitoneal injection of Escherichia coli in different models of cell-specific Ppara-deficiency and their controls. The systemic and hepatic metabolic response was analyzed using biochemical, transcriptomic and functional assays. PPARα expression was analyzed in livers from elective surgery and critically ill patients and correlated with hepatic gene expression and blood parameters. RESULTS: Both whole body and non-hematopoietic Ppara-deficiency in mice decreased survival upon bacterial infection. Livers of septic Ppara-deficient mice displayed an impaired metabolic shift from glucose to lipid utilization resulting in more severe hypoglycemia, impaired induction of hyperketonemia and increased steatosis due to lower expression of genes involved in fatty acid catabolism and ketogenesis. Hepatocyte-specific deletion of PPARα impaired the metabolic response to sepsis and was sufficient to decrease survival upon bacterial infection. Hepatic PPARA expression was lower in critically ill patients and correlated positively with expression of lipid metabolism genes, but not with systemic inflammatory markers. CONCLUSION: During sepsis, Ppara-deficiency in hepatocytes is deleterious as it impairs the adaptive metabolic shift from glucose to FA utilization. Metabolic control by PPARα in hepatocytes plays a key role in the host defense against infection. LAY SUMMARY: As the main cause of death in critically ill patients, sepsis remains a major health issue lacking efficacious therapies. While current clinical literature suggests an important role for inflammation, metabolic aspects of sepsis have mostly been overlooked. Here, we show that mice with an impaired metabolic response, due to deficiency of the nuclear receptor PPARα in the liver, exhibit enhanced mortality upon bacterial infection despite a similar inflammatory response, suggesting that metabolic interventions may be a viable strategy for improving sepsis outcomes.status: publishe

BCAA catabolism in brown fat controls energy homeostasis through SLC25A44.

Branched-chain amino acid (BCAA; valine, leucine and isoleucine) supplementation is often beneficial to energy expenditure; however, increased circulating levels of BCAA are linked to obesity and diabetes. The mechanisms of this paradox remain unclear. Here we report that, on cold exposure, brown adipose tissue (BAT) actively utilizes BCAA in the mitochondria for thermogenesis and promotes systemic BCAA clearance in mice and humans. In turn, a BAT-specific defect in BCAA catabolism attenuates systemic BCAA clearance, BAT fuel oxidation and thermogenesis, leading to diet-induced obesity and glucose intolerance. Mechanistically, active BCAA catabolism in BAT is mediated by SLC25A44, which transports BCAAs into mitochondria. Our results suggest that BAT serves as a key metabolic filter that controls BCAA clearance via SLC25A44, thereby contributing to the improvement of metabolic health