Peroxisome proliferator-activated receptor {gamma} coactivator 1{alpha} (PGC-1{alpha}) promotes skeletal muscle lipid refueling in vivo by activating de novo lipogenesis and the pentose phosphate pathway

Abstract

Exercise induces a pleiotropic adaptive response in skeletal muscle, largely through peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α). PGC-1α enhances lipid oxidation and thereby provides energy for sustained muscle contraction. Its potential implication in promoting muscle refueling remains unresolved, however. Here, we investigated a possible role of elevated PGC-1α levels in skeletal muscle lipogenesis in vivo and the molecular mechanisms that underlie PGC-1α-mediated de novo lipogenesis. To this end, we studied transgenic mice with physiological overexpression of PGC-1α and human muscle biopsies pre- and post-exercise. We demonstrate that PGC-1α enhances lipogenesis in skeletal muscle through liver X receptor α-dependent activation of the fatty acid synthase (FAS) promoter and by increasing FAS activity. Using chromatin immunoprecipitation, we establish a direct interaction between PGC-1α and the liver X receptor-responsive element in the FAS promoter. Moreover, we show for the first time that increased glucose uptake and activation of the pentose phosphate pathway provide substrates for RNA synthesis and cofactors for de novo lipogenesis. Similarly, we observed increased lipogenesis and lipid levels in human muscle biopsies that were obtained post-exercise. Our findings suggest that PGC-1α coordinates lipogenesis, intramyocellular lipid accumulation, and substrate oxidation in exercised skeletal muscle in vivo.

FIGURE 1.

Enhanced lipid anabolism in EDL muscle of MPGC-1α TG animals. A–D, FAS gene expression, activity, de novo lipogenesis, and accumulation of intramyofibrillar lipids, respectively, in glycolytic muscle of MPGC-1α TG versus wild-type animals. E, relative expression of genes involved in fatty acid uptake (lipoprotein lipase (LPL), CD36, plasma membrane fatty acid-binding protein (FABPpm), and fatty acid transporter proteins (FATPs)), fatty acid activation (ACS), and esterification (acetyl-CoA:diacylglycerol acyltransferase (DGAT) and mitochondrial glycerol-3-phosphate acyltransferase (mtGPAT)). All values are expressed as means ± S.E. (n = 6–8/group). *, p < 0.05; **, p < 0.01; ***, p < 0.001.

FIGURE 2.

LXRα-dependent activation of the FAS promoter. A, FAS promoter activity following transfection of myoblasts with pcDNA (empty vector; black bars) or PGC-lα (gray bars) and in response to cotransfection with LXRα/RXRα or SREBP1c expression plasmids. Values are expressed as means ± S.E. (n = 6/group). @@@, effect of PGC-lα (p < 0.001); ###, effect of LXRα/RXRα or SREBP1c (p < 0.001); §§, interaction (p < 0.01) as assessed by two-factor analysis of variance. B, FAS promoter activity following transfection of myoblasts with pcDNA (empty vector; black bars) or PGC-lα (gray bars) and in response to silencing of LXRα or deletion of the LXR-responsive element (ΔLXRE). C, relative expression of genes involved in FAS promoter activation in muscle of MPGC-1α TG mice versus control mice. All values are expressed as means ± S.E. (n = 6–8/group). *, p < 0.05; **, p < 0.01. D, ChIP assays of mouse skeletal muscle: recruiting of PGC-lα to LXRE- and SREBP-responsive element (SRE)-binding sites in the FAS promoter of MPGC-1α TG mice and control animals, respectively. All values are expressed as means ± S.E. (n = 4/group). **, p < 0.01.

FIGURE 3.

Enhanced glucose uptake and pentose phosphate metabolism in EDL muscle of MPGC-1α TG mice. A, relative gene expression of GLUT1, GLUT4, and hexokinase 2 (HK2) in EDL muscle as measured by RT-PCR and expressed as -fold change over controls. B, glucose uptake in EDL muscle. C and D, determination of G6PDH mRNA levels and activity, respectively, in glycolytic muscle of MPGC-1α TG and control mice. All values are means ± S.E. (n = 6–8/group). *, p < 0.05.

FIGURE 4.

Increased pentose phosphate pathway products in MPGC-1α TG mice. A and B, concentrations of NADPH and NADP⁺, respectively, in glycolytic muscle of MPGC-1α TG mice versus control mice. C–E, amounts of total, reduced (GSH), and oxidized (GSSG) glutathione, respectively, extracted from glycolytic muscle. F and G, incorporation of tritium-labeled glucose into RNA and DNA, respectively, in EDL muscle of MPGC-1α TG and control mice. H, determination of ribonucleotide reductase (RiboRed) mRNA levels. All values are means ± S.E. (n = 6–8/group). *, p < 0.05.

FIGURE 5.

Higher FAS expression and IMCL in endurance athletes. A–C, FAS mRNA expression, IMCLs, and ACS mRNA expression, respectively, in human endurance athletes before and after 6 weeks of high intensity endurance exercise. D–F, relative expression of LXRα, RXRα, and RXRβ, respectively, in human muscle biopsies pre- and post-exercise. Values are expressed as means ± S.E. (n = 6). *, p < 0.05 as assessed by a paired t test.

FIGURE 6.

PGC-1α coordinates anabolic and catabolic pathways in skeletal muscle. The model integrates the findings of this study and shows the PGC-1α-mediated coordination of de novo lipogenesis, lipid accumulation, and lipid oxidation. Enzymes being activated by PGC-1α and regulating metabolic key steps are indicated in gray ovals (G6PDH, FAS, and carnitine palmitoyltransferase 1b (CPT-1b)). PGC-1α coordinates anabolic processes (lipogenesis and IMCL accumulation) and catabolic processes (β-oxidation, the Krebs cycle, and oxidative phosphorylation (OXPHOS)) in skeletal muscle. PGC-1α enhances glucose (Glu) uptake and flux through the pentose phosphate pathway. Concomitantly, NADP⁺ is reduced to NADPH, which serves as reducing agent for lipogenesis. De novo synthesized fatty acids (FA) are then stored as IMCL and serve as energy substrate during endurance exercise. By metabolizing lipids through β-oxidation, the Krebs cycle, and oxidative phosphorylation, ATP for muscle contraction is produced. The flux of glucose toward the pentose phosphate pathway also generates ribose 5-phosphate, which constitutes a structural element of ATP and other nucleotides.