BMP8B increases brown adipose tissue thermogenesis through both central and peripheral actions.

Thermogenesis in brown adipose tissue (BAT) is fundamental to energy balance and is also relevant for humans. Bone morphogenetic proteins (BMPs) regulate adipogenesis, and, here, we describe a role for BMP8B in the direct regulation of thermogenesis. BMP8B is induced by nutritional and thermogenic factors in mature BAT, increasing the response to noradrenaline through enhanced p38MAPK/CREB signaling and increased lipase activity. Bmp8b(-/-) mice exhibit impaired thermogenesis and reduced metabolic rate, causing weight gain despite hypophagia. BMP8B is also expressed in the hypothalamus, and Bmp8b(-/-) mice display altered neuropeptide levels and reduced phosphorylation of AMP-activated protein kinase (AMPK), indicating an anorexigenic state. Central BMP8B treatment increased sympathetic activation of BAT, dependent on the status of AMPK in key hypothalamic nuclei. Our results indicate that BMP8B is a thermogenic protein that regulates energy balance in partnership with hypothalamic AMPK. BMP8B may offer a mechanism to specifically increase energy dissipation by BAT.

Enhanced activation of an amino-terminally truncated isoform of the voltage-gated proton channel HVCN1 enriched in malignant B cells.

HVCN1 (Hydrogen voltage-gated channel 1) is the only mammalian voltage-gated proton channel. In human B lymphocytes, HVCN1 associates with the B-cell receptor (BCR) and is required for optimal BCR signaling and redox control. HVCN1 is expressed in malignant B cells that rely on BCR signaling, such as chronic lymphocytic leukemia (CLL) cells. However, little is known about its regulation in these cells. We found that HVCN1 was expressed in B cells as two protein isoforms. The shorter isoform (HVCN1S) was enriched in B cells from a cohort of 76 CLL patients. When overexpressed in a B-cell lymphoma line, HVCN1S responded more profoundly to protein kinase C-dependent phosphorylation. This more potent enhanced gating response was mediated by increased phosphorylation of the same residue responsible for enhanced gating in HVCN1L, Thr(29). Furthermore, the association of HVCN1S with the BCR was weaker, which resulted in its diminished internalization upon BCR stimulation. Finally, HVCN1S conferred a proliferative and migratory advantage as well as enhanced BCR-dependent signaling. Overall, our data show for the first time, to our knowledge, the existence of a shorter isoform of HVCN1 with enhanced gating that is specifically enriched in malignant B cells. The properties of HVCN1S suggest that it may contribute to the pathogenesis of BCR-dependent B-cell malignancies.

FGF21 expression and release in muscle cells: involvement of MyoD and regulation by mitochondria-driven signalling.

Although the liver is generally considered the main site of production of FGF21 (fibroblast growth factor-21), high FGF21 levels have been found to be associated with neuromuscular mitochondrial genetic diseases, and there are indications that the muscle may be a relevant site of FGF21 production under conditions of muscular mitochondrial stress. In the present study, we found that expression and release of FGF21 was associated with myogenic differentiation, and we identified MyoD as a major controller of FGF21 gene transcription. Mimicking mitochondrial dysfunction using respiratory chain/oxidative phosphorylation inhibitors resulted in enhanced expression and release of FGF21 by muscle cells. The increased production of reactive oxygen species, subsequent induction of p38 MAPK (mitogen-activated protein kinase) and activation of an ATF2 (activating transcription factor 2)-binding site at the proximal promoter region of the FGF21 gene was found to be a major mechanism linking mitochondrial dysfunction with enhanced FGF21 gene transcription in myogenic cells. The myogenic factor MyoD was required for the induction of FGF21 gene transcription by mitochondrial dysfunction, thus explaining the preferential response of muscle cells to mitochondrial dysfunction-induced FGF21 expression and secretion. FGF21 release by muscle cells in response to mitochondrial alterations may represent a physiological mechanism by which the sensing of internal energetic status by muscles results in the release of FGF21 to favour systemic metabolic adaptations.

FGF21 mediates the lipid metabolism response to amino acid starvation.

Lipogenic gene expression in liver is repressed in mice upon leucine deprivation. The hormone fibroblast growth factor 21 (FGF21), which is critical to the adaptive metabolic response to starvation, is also induced under amino acid deprivation. Upon leucine deprivation, we found that FGF21 is needed to repress expression of lipogenic genes in liver and white adipose tissue, and stimulate phosphorylation of hormone-sensitive lipase in white adipose tissue. The increased expression of Ucp1 in brown adipose tissue under these circumstances is also impaired in FGF21-deficient mice. Our results demonstrate the important role of FGF21 in the regulation of lipid metabolism during amino acid starvation.

Pref-1 in brown adipose tissue: specific involvement in brown adipocyte differentiation and regulatory role of C/EBPδ.

Pref-1 (pre-adipocyte factor-1) is known to play a central role in regulating white adipocyte differentiation, but the role of Pref-1 in BAT (brown adipose tissue) has not been analysed. In the present study we found that Pref-1 expression is high in fetal BAT and declines progressively after birth. However, Pref-1-null mice showed unaltered fetal development of BAT, but exhibited signs of over-activation of BAT thermogenesis in the post-natal period. In C/EBP (CCAAT/enhancer-binding protein) α-null mice, a rodent model of impaired fetal BAT differentiation, Pref-1 was dramatically overexpressed, in association with reduced expression of the Ucp1 (uncoupling protein 1) gene, a BAT-specific marker of thermogenic differentiation. In brown adipocyte cell culture models, Pref-1 was mostly expressed in pre-adipocytes and declined with brown adipocyte differentiation. The transcription factor C/EBPδ activated the Pref-1 gene transcription in brown adipocytes, through binding to the proximal promoter region. Accordingly, siRNA (small interfering RNA)-induced C/EBPδ knockdown led to reduced Pref-1 gene expression. This effect is consistent with the observed overexpression of C/EBPδ in C/EBPα-null BAT and high expression of C/EBPδ in brown pre-adipocytes. Dexamethasone treatment of brown pre-adipocytes suppressed Pref-1 down-regulation occurring throughout the brown adipocyte differentiation process, increased the expression of C/EBPδ and strongly impaired expression of the thermogenic markers UCP1 and PGC-1α [PPARγ (peroxisome-proliferator-activated receptor γ) co-activator-α]. However, it did not alter normal fat accumulation or expression of non-BAT-specific genes. Collectively, these results specifically implicate Pref-1 in controlling the thermogenic gene expression program in BAT, and identify C/EBPδ as a novel transcriptional regulator of Pref-1 gene expression that may be related to the specific role of glucocorticoids in BAT differentiation.

Thermogenic activation induces FGF21 expression and release in brown adipose tissue.

FGF21 is a novel metabolic regulator involved in the control of glucose homeostasis, insulin sensitivity, and ketogenesis. The liver has been considered the main site of production and release of FGF21 into the blood. Here, we show that, after thermogenic activation, brown adipose tissue becomes a source of systemic FGF21. This is due to a powerful cAMP-mediated pathway of regulation of FGF21 gene transcription. Norepinephrine, acting via ß-adrenergic, cAMP-mediated, mechanisms and subsequent activation of protein kinase A and p38 MAPK, induces FGF21 gene transcription and also FGF21 release in brown adipocytes. ATF2 binding to the FGF21 gene promoter mediates cAMP-dependent induction of FGF21 gene transcription. FGF21 release by brown fat in vivo was assessed directly by analyzing arteriovenous differences in FGF21 concentration across interscapular brown fat, in combination with blood flow to brown adipose tissue and assessment of FGF21 half-life. This analysis demonstrates that exposure of rats to cold induced a marked release of FGF21 by brown fat in vivo, in association with a reduction in systemic FGF21 half-life. The present findings lead to the recognition of a novel pathway of regulation the FGF21 gene and an endocrine role of brown fat, as a source of FGF21 that may be especially relevant in conditions of activation of thermogenic activity.

Peroxisome proliferator-activated receptor α (PPARα) induces PPARγ coactivator 1α (PGC-1α) gene expression and contributes to thermogenic activation of brown fat: involvement of PRDM16.

Peroxisome proliferator activated receptor α (PPARα) is a distinctive marker of the brown fat phenotype that has been proposed to coordinate the transcriptional activation of genes for lipid oxidation and for thermogenic uncoupling protein 1 in brown adipose tissue. Here, we investigated the involvement of PPARα in the transcriptional control of the PPARγ coactivator (PGC)-1α gene. Treatment with PPARα agonists induced PGC-1α mRNA expression in brown fat in vivo and in primary brown adipocytes. This enhancement of PGC-1α transcription was mediated by PPARα binding to a PPAR-responsive element in the distal PGC-1α gene promoter. PGC-1α gene expression was decreased in PPARα-null brown fat, both under basal conditions and in response to thermogenic activation. Moreover, PPARα- and cAMP-mediated pathways interacted to control PGC-1α transcription. PRDM16 (PRD1-BF1-RIZ1 homologous domain-containing 16) promoted PPARα induction of PGC-1α gene transcription, especially under conditions in which protein kinase A pathways were activated. This enhancement was associated with the interaction of PRDM16 with the PGC-1α promoter at the PPARα-binding site. In addition, PPARα promoted the expression of the PRDM16 gene in brown adipocytes, and activation of PPARα in human white adipocytes led to the appearance of a brown adipocyte pattern of gene expression, including induction of PGC-1α and PRDM16. Collectively, these results suggest that PPARα acts as a key component of brown fat thermogenesis by coordinately regulating lipid catabolism and thermogenic gene expression via induction of PGC-1α and PRDM16.

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha controls transcription of the Sirt3 gene, an essential component of the thermogenic brown adipocyte phenotype.

Sirt3 (silent mating type information regulation 2, homolog 3), a member of the sirtuin family of protein deacetylases with multiple actions on metabolism and gene expression is expressed in association with brown adipocyte differentiation. Using Sirt3-null brown adipocytes, we determined that Sirt3 is required for an appropriate responsiveness of cells to noradrenergic, cAMP-mediated activation of the expression of brown adipose tissue thermogenic genes. The transcriptional coactivator Pgc-1α (peroxisome proliferator-activated receptor-γ coactivator-1α) induced Sirt3 gene expression in white adipocytes and embryonic fibroblasts as part of its overall induction of a brown adipose tissue-specific pattern of gene expression. In cells lacking Sirt3, Pgc-1α failed to fully induce the expression of brown fat-specific thermogenic genes. Pgc-1α activates Sirt3 gene transcription through coactivation of the orphan nuclear receptor Err (estrogen-related receptor)-α, which bound the proximal Sirt3 gene promoter region. Errα knockdown assays indicated that Errα is required for full induction of Sirt3 gene expression in response to Pgc-1α. The present results indicate that Pgc-1α controls Sirt3 gene expression and this action is an essential component of the overall mechanisms by which Pgc-1α induces the full acquisition of a brown adipocyte differentiated phenotype.

Overexpression of mitochondrial uncoupling protein-3 does not decrease production of the reactive oxygen species, elevated by palmitate in skeletal muscle cells.

Fatty acids induced an increase in reactive oxygen species (ROS) and enhanced NF-kappaB activation in L6 myotubes differentiated in culture. Palmitate proved more effective than oleate in eliciting these effects. The induction of uncoupling protein-3 (UCP3) at levels similar to those occurring in vivo, attained through the use of an adenoviral vector, led to a reduction of mitochondrial membrane potential in L6 myotubes. However, the capacity of palmitate to increase ROS was not reduced but, quite the opposite, it was moderately enhanced due to the presence of UCP3. The presence of UCP3 in mitochondria did not modify the expression of genes encoding ROS-related enzymes, either in basal conditions or in the presence of palmitate. However, in the presence of UCP3, UCP2 mRNA expression was down-regulated in response to palmitate. We conclude that UCP3 does not act as a protective agent against palmitate-dependent induction of ROS production in differentiated skeletal muscle cells.

Thiazolidinediones and rexinoids induce peroxisome proliferator-activated receptor-coactivator (PGC)-1alpha gene transcription: an autoregulatory loop controls PGC-1alpha expression in adipocytes via peroxisome proliferator-activated receptor-gamma coactivation.

Thiazolidinediones (TZDs) are insulin-sensitizing drugs currently used to treat type 2 diabetes. They are activators of peroxisome proliferator-activated receptor (PPAR)-gamma, and adipose tissue constitutes a major site for their biological effects. PPAR coactivator (PGC)-1alpha is a transcriptional coactivator of PPARgamma and other transcription factors. It is involved in the control of mitochondrial biogenesis, and its activity has been linked to insulin sensitization. Here we report that PGC-1alpha gene expression in brown and white adipocytes is a direct target of TZDs via PPARgamma activation. Activators of the retinoid X receptor also induce PGC-1alpha gene expression. This is due to the presence of a PPARgamma-responsive element in the distal region of the PGC-1alpha gene promoter that binds PPARgamma/retinoid X receptor heterodimers. Moreover, there is a positive autoregulatory loop of control of the PGC-1alpha gene through coactivation of PPARgamma responsiveness to TZDs by PGC-1alpha itself. These data indicate that some of the effects of TZDs, especially promotion of mitochondrial biogenesis and oxidative pathways in adipose depots, entail PGC-1alpha up-regulation via enhanced transcription of the PGC-1alpha gene.

Defective thermoregulation, impaired lipid metabolism, but preserved adrenergic induction of gene expression in brown fat of mice lacking C/EBPbeta.

C/EBPbeta (CCAAT/enhancer-binding protein beta) is a transcriptional regulator of the UCP1 (uncoupling protein-1) gene, the specific marker gene of brown adipocytes that is responsible for their thermogenic capacity. To investigate the role of C/EBPbeta in brown fat, we studied the C/EBPbeta-null mice. When placed in the cold, C/EBPbeta(-/-) mice did not maintain body temperature. This cold-sensitive phenotype occurred, although UCP1 and PGC-1alpha (peroxisome-proliferator-activated receptor gamma co-activator-1alpha) gene expression was unaltered in brown fat of C/EBPbeta(-/-) mice. The UCP1 gene promoter was repressed by the truncated inhibitory C/EBPbeta isoform LIP (liver-enriched transcriptional inhibitory protein, the truncated inhibitory C/EBPbeta isoform). Since C/EBPbeta-null mice lack both C/EBPbeta isoforms, active LAP (liver-enriched transcriptional activatory protein, the active C/EBPbeta isoform) and LIP, the absence of LIP may have a stronger effect than the absence of LAP upon UCP1 gene expression. Gene expression for UCP2 and UCP3 was not impaired in all tissues analysed. In primary brown adipocytes from C/EBPbeta(-/-) mice, induction of gene expression by noradrenaline was preserved. In contrast, the expression of genes related to lipid storage was impaired, as was the amount of triacylglycerol mobilized after acute cold exposure in brown fat from C/EBPbeta(-/-) mice. LPL (lipoprotein lipase) activity was also impaired in brown fat, but not in other tissues of C/EBPbeta(-/-) mice. LPL protein levels were also diminished, but this effect was independent of changes in LPL mRNA, suggesting that C/EBPbeta is involved in the post-transcriptional regulation of LPL gene expression in brown fat. In summary, defective thermoregulation owing to the lack of C/EBPbeta is associated with the reduced capacity to supply fatty acids as fuels to sustain brown fat thermogenesis.

Fibroblast growth factor 21 in breast milk controls neonatal intestine function.

FGF21 is a hormonal factor with important functions in the control of metabolism. FGF21 is found in rodent and human milk. Radiolabeled FGF21 administered to lactating dams accumulates in milk and is transferred to neonatal gut. The small intestine of neonatal (but not adult) mice highly expresses ß-Klotho in the luminal area. FGF21-KO pups fed by FGF21-KO dams showed decreased expression and circulating levels of incretins (GIP and GLP-1), reduced gene expression of intestinal lactase and maltase-glucoamylase, and low levels of galactose in plasma, all associated with a mild decrease in body weight. When FGF21-KO pups were nursed by wild-type dams (expressing FGF21 in milk), intestinal peptides and digestive enzymes were up-regulated, lactase enzymatic activity was induced, and galactose levels and body weight were normalized. Neonatal intestine explants were sensitive to FGF21, as evidenced by enhanced ERK1/2 phosphorylation. Oral infusion of FGF21 into neonatal pups induced expression of intestinal hormone factors and digestive enzymes, lactase activity and lactose absorption. These findings reveal a novel role of FGF21 as a hormonal factor contributing to neonatal intestinal function via its presence in maternal milk. Appropriate signaling of FGF21 to neonate is necessary to ensure optimal digestive and endocrine function in developing intestine.

The developmental regulation of peroxisome proliferator-activated receptor-gamma coactivator-1alpha expression in the liver is partially dissociated from the control of gluconeogenesis and lipid catabolism.

The developmental regulation of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) gene expression was studied in mice and compared with that of marker genes of liver energy metabolism. The PGC-1alpha gene was highly expressed in fetal liver compared with that in adults and remained high in neonatal liver. The regulation of PGC-1alpha gene expression during the fetal and early neonatal periods was dissociated from that of gluconeogenic genes, i.e. the phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) genes. Only under the effects of starvation was PGC-1alpha gene expression induced in parallel to phosphoenolpyruvate carboxykinase and G6Pase mRNAs during the perinatal period. Furthermore, the PGC-1alpha gene was not regulated as part of the developmental program of gene expression associated with the maturation of hepatic gluconeogenesis, as revealed by the impaired PEPCK and G6Pase gene expression but unaltered PGC-1alpha mRNA levels in CCAAT/enhancer-binding protein-alpha-null fetus and neonates. Regulation of the PGC-1alpha gene and that of mitochondrial 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase, acyl-coenzyme A oxidase, and long-chain acyl-coenzyme dehydrogenase, marker genes of lipid catabolism, were dissociated in fetuses and neonates. The expression of lipid catabolism genes was down-regulated in fasted neonates, whereas PGC-1alpha was oppositely regulated. The independent regulation of PGC-1alpha and lipid catabolism genes was also found in peroxisome proliferator-activated receptor-alpha-null neonates, in which PGC-1alpha mRNA levels were unaffected whereas gene expression for 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase and acyl-coenzyme A oxidase was impaired. Thus, regulation of the PGC-1alpha gene is partially dissociated from the patterns of regulation of hepatic genes encoding enzymes involved in gluconeogenesis and lipid catabolism during fetal ontogeny and in response to the initiation of lactation.

Fibroblast growth factor-21 is expressed in neonatal and pheochromocytoma-induced adult human brown adipose tissue.

OBJECTIVE: In rodents, brown (BAT) and white (WAT) adipose tissues are targets and expression sites for fibroblast growth factor-21 (FGF21). In contrast, human WAT expresses negligible levels of FGF21. We examined FGF21 expression in human BAT samples, including the induced BAT found in adult patients with pheochromocytoma, and interscapular and visceral BAT from newborns. METHODS: The expression of FGF21 and uncoupling protein-1 (UCP1, a brown adipocyte marker), was determined by quantitative real-time-PCR and immunoblotting. The transcript levels of marker genes for developmentally-programmed BAT (zinc-finger-protein of the cerebellum-1, ZIC1) and inducible-BAT (cluster of differentiation-137, CD137) were also determined. RESULTS: FGF21 and UCP1 are significantly expressed in visceral adipose tissue from pheochromocytoma patients, but not in visceral fat from healthy individuals. In neonates, FGF21 and UCP1 are both expressed in visceral and interscapular fat, and their expression levels show a significant positive correlation. Marker gene expression profiles suggest that inducible BAT is present in visceral fat from pheochromocytoma patients and neonates, whereas developmentally-programmed BAT is present in neonatal interscapular fat. CONCLUSIONS: Human BAT, but not WAT, expresses FGF21. The expression of FGF21 is especially high in inducible, also called beige/brite, neonatal BAT, but it is also found in the interscapular, developmentally-programmed, BAT of neonates.

TNF-α represses ß-Klotho expression and impairs FGF21 action in adipose cells: involvement of JNK1 in the FGF21 pathway.

Fibroblast growth factor 21 (FGF21) is a member of the FGF family that reduces glycemia and ameliorates insulin resistance. Adipose tissue is a main target of FGF21 action. Obesity is associated with a chronic proinflammatory state. Here, we analyzed the role of proinflammatory signals in the FGF21 pathway in adipocytes, evaluating the effects of TNF-α on ß-Klotho and FGF receptor-1 expression and FGF21 action in adipocytes. We also determined the effects of rosiglitazone on ß-Klotho and FGF receptor-1 expression in models of proinflammatory signal induction in vitro and in vivo (high-fat diet-induced obesity). Because c-Jun NH(2)-terminal kinase 1 (JNK1) serves as a sensing juncture for inflammatory status, we also evaluated the involvement of JNK1 in the FGF21 pathway. TNF-α repressed ß-Klotho expression and impaired FGF21 action in adipocytes. Rosiglitazone prevented the reduction in ß-Klotho expression elicited by TNF-α. Moreover, ß-Klotho levels were reduced in adipose tissue from high-fat diet-induced obese mice, whereas rosiglitazone restored ß-Klotho to near-normal levels. ß-Klotho expression was increased in white fat from JNK1(-/-) mice. The absence of JNK1 increased the responsiveness of mouse embryonic fibroblast-derived adipocytes and brown adipocytes to FGF21. In conclusion, we show that proinflammatory signaling impairs ß-Klotho expression and FGF21 responsiveness in adipocytes. We also show that JNK1 activity is involved in modulating FGF21 effects in adipocytes. The impairment in the FGF21 response machinery in adipocytes and the reduction in FGF21 action in response to proinflammatory signals may play important roles in metabolic alterations in obesity and other diseases associated with enhanced inflammation.

Peroxisome proliferator-activated receptors-α and -γ, and cAMP-mediated pathways, control retinol-binding protein-4 gene expression in brown adipose tissue.

Retinol binding protein-4 (RBP4) is a serum protein involved in the transport of vitamin A. It is known to be produced by the liver and white adipose tissue. RBP4 release by white fat has been proposed to induce insulin resistance. We analyzed the regulation and production of RBP4 in brown adipose tissue. RBP4 gene expression is induced in brown fat from mice exposed to cold or treated with peroxisome proliferator-activated receptor (PPAR) agonists. In brown adipocytes in culture, norepinephrine, cAMP, and activators of PPARγ and PPARα induced RBP4 gene expression and RBP4 protein release. The induction of RBP4 gene expression by norepinephrine required intact PPAR-dependent pathways, as evidenced by impaired response of the RBP4 gene expression to norepinephrine in PPARα-null brown adipocytes or in the presence of inhibitors of PPARγ and PPARα. PPARγ and norepinephrine can also induce the RBP4 gene in white adipocytes, and overexpression of PPARα confers regulation by this PPAR subtype to white adipocytes. The RBP4 gene promoter transcription is activated by cAMP, PPARα, and PPARγ. This is mediated by a PPAR-responsive element capable of binding PPARα and PPARγ and required also for activation by cAMP. The induction of the RBP4 gene expression by norepinephrine in brown adipocytes is protein synthesis dependent and requires PPARγ-coactivator-1-α, which acts as a norepinephine-induced coactivator of PPAR on the RBP4 gene. We conclude that PPARγ- and PPARα-mediated signaling controls RBP4 gene expression and releases in brown adipose tissue, and thermogenic activation induces RBP4 gene expression in brown fat through mechanisms involving PPARγ-coactivator-1-α coactivation of PPAR signaling.

Hepatic FGF21 expression is induced at birth via PPARalpha in response to milk intake and contributes to thermogenic activation of neonatal brown fat.

Plasma FGF21 levels and hepatic FGF21 gene expression increase dramatically after birth in mice. This induction is initiated by suckling, requires lipid intake, is impaired in PPARalpha null neonates, and is mimicked by treatment with the PPARalpha activator, Wy14,643. Neonates exhibit reduced FGF21 expression in response to fasting, in contrast to the upregulation occurring in adults. Changes in FGF21 expression due to suckling or nutritional manipulations were associated with circulating free fatty acid and ketone body levels. We mimicked the FGF21 postnatal rise by injecting FGF21 into fasting neonates, and found that this enhanced the expression of genes involved in thermogenesis within brown fat, and increased body temperature. Brown adipocytes treated with FGF21 exhibited increased expression of thermogenic genes, higher total and uncoupled respiration, and enhanced glucose oxidation. We propose that the induction of FGF21 production by the liver mediates direct activation of brown fat thermogenesis during the fetal-to-neonatal transition.

PPARdelta, but not PPARalpha, activates PGC-1alpha gene transcription in muscle.

PGC-1alpha induces mitochondrial biogenesis in muscle and its activity has been related to insulin sensitization. Here, we report that fibrates induce PGC-1alpha gene expression in muscle both in vivo and in vitro. However, only activation via PPARdelta but not PPARalpha underlies this effect. PPARdelta induces PGC-1alpha gene transcription through a PPAR-response element in the PGC-1alpha promoter. Moreover, PGC-1alpha coactivates the PPARdelta-responsiveness of its own gene. A further positive autoregulatory loop of control relies on the induction of PPARdelta expression by PGC-1alpha. These data point to a distinct value of PPARdelta rather than PPARalpha agonists in the improvement of oxidative metabolism in muscle.