Differential pattern of glycogen accumulation after protein phosphatase 1 glycogen-targeting subunit PPP1R6 overexpression, compared to PPP1R3C and PPP1R3A, in skeletal muscle cells

Background PPP1R6 is a protein phosphatase 1 glycogen-targeting subunit (PP1-GTS) abundant in skeletal muscle with an undefined metabolic control role. Here PPP1R6 effects on myotube glycogen metabolism, particle size and subcellular distribution are examined and compared with PPP1R3C/PTG and PPP1R3A/GM. Results PPP1R6 overexpression activates glycogen synthase (GS), reduces its phosphorylation at Ser-641/0 and increases the extracted and cytochemically-stained glycogen content, less than PTG but more than GM. PPP1R6 does not change glycogen phosphorylase activity. All tested PP1-GTS-cells have more glycogen particles than controls as found by electron microscopy of myotube sections. Glycogen particle size is distributed for all cell-types in a continuous range, but PPP1R6 forms smaller particles (mean diameter 14.4 nm) than PTG (36.9 nm) and GM (28.3 nm) or those in control cells (29.2 nm). Both PPP1R6- and GM-derived glycogen particles are in cytosol associated with cellular structures; PTG-derived glycogen is found in membrane- and organelle-devoid cytosolic glycogen-rich areas; and glycogen particles are dispersed in the cytosol in control cells. A tagged PPP1R6 protein at the C-terminus with EGFP shows a diffuse cytosol pattern in glucose-replete and -depleted cells and a punctuate pattern surrounding the nucleus in glucose-depleted cells, which colocates with RFP tagged with the Golgi targeting domain of β-1,4-galactosyltransferase, according to a computational prediction for PPP1R6 Golgi location. Conclusions PPP1R6 exerts a powerful glycogenic effect in cultured muscle cells, more than GM and less than PTG. PPP1R6 protein translocates from a Golgi to cytosolic location in response to glucose. The molecular size and subcellular location of myotube glycogen particles is determined by the PPP1R6, PTG and GM scaffolding

Glucose and fructose have sugar-specific effects in both liver and skeletal muscle in vivo: a role for liver fructokinase.

We examined glucose and fructose effects on serine phosphorylation levels of a range of proteins in rat liver and muscle cells. For this, healthy adult rats were subjected to either oral glucose or fructose loads. A mini-array system was utilized to determine serine phosphorylation levels of liver and skeletal muscle proteins. A glucose oral load of 125 mg/100 g body weight (G 1/2) did not induce changes in phosphorylated serines of the proteins studied. Loading with 250 mg/100 g body weight of fructose (Fr), which induced similar glycemia levels as G 1/2, significantly increased serine phosphorylation of liver cyclin D3, PI3 kinase/p85, ERK-2, PTP2 and clusterin. The G 1/2 increased serine levels of the skeletal muscle proteins cyclin H, Cdk2, IRAK, total PKC, PTP1B, c-Raf 1, Ras and the β-subunit of the insulin receptor. The Fr induced a significant increase only in muscle serine phosphorylation of PI3 kinase/p85. The incubation of isolated rat hepatocytes with 10 mM glucose for 5 min significantly increased serine phosphorylation of 31 proteins. In contrast, incubation with 10 mM fructose produced less intense effects. Incubation with 10 mM glucose plus 75 µM fructose counteracted the effects of the incubation with glucose alone, except those on Raf-1 and Ras. Less marked effects were detected in cultured muscle cells incubated with 10 mM glucose or 10 mM glucose plus 75 µM fructose. Our results suggest that glucose and fructose act as specific functional modulators through a general mechanism that involves liver-generated signals, like micromolar fructosemia, which would inform peripheral tissues of the presence of either glucose- or fructose-derived metabolites

Novel role of FATP1 in mitochondrial fatty acid oxidation in skeletal muscle cells

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the first step in long-chain fatty acid import into mitochondria, and it is believed to be rate limiting for beta-oxidation of fatty acids. However, in muscle, other proteins may collaborate with CPT1. Fatty acid translocase/CD36 (FAT/CD36) may interact with CPT1 and contribute to fatty acid import into mitochondria in muscle. Here, we demonstrate that another membrane-bound fatty acid binding protein, fatty acid transport protein 1 (FATP1), collaborates with CPT1 for fatty acid import into mitochondria. Overexpression of FATP1 using adenovirus in L6E9 myotubes increased both fatty acid oxidation and palmitate esterification into triacylglycerides. Moreover, immunocytochemistry assays in transfected L6E9 myotubes showed that FATP1 was present in mitochondria and coimmunoprecipitated with CPT1 in L6E9 myotubes and rat skeletal muscle in vivo. The cooverexpression of FATP1 and CPT1 also enhanced mitochondrial fatty acid oxidation, similar to the cooverexpression of FAT/CD36 and CPT1. However, etomoxir, an irreversible inhibitor of CPT1, blocked all these effects. These data reveal that FATP1, like FAT/CD36, is associated with mitochondria and has a role in mitochondrial oxidation of fatty acids

GNIP1 E3 ubiquitin ligase is a novel player in regulating glycogen metabolism in skeletal muscle.

Background: Glycogenin-interacting protein 1 (GNIP1) is a tripartite motif (TRIM) protein with E3 ubiquitin ligase activity that interacts with glycogenin. These data suggest that GNIP1 could play a major role in the control of glycogen metabolism. However, direct evidence based on functional analysis remains to be obtained. Objectives: The aim of this study was 1) to define the expression pattern of glycogenin-interacting protein/ Tripartite motif containing protein 7 (GNIP/TRIM7) isoforms in humans, 2) to test their ubiquitin E3 ligase activity, and 3) to analyze the functional effects of GNIP1 on muscle glucose/glycogen metabolism both in human cultured cells and in vivo in mice. Results: We show that GNIP1 was the most abundant GNIP/TRIM7 isoform in human skeletal muscle, whereas in cardiac muscle only TRIM7 was expressed. GNIP1 and TRIM7 had autoubiquitination activity in vitro and were localized in the Golgi apparatus and cytosol respectively in LHCN-M2 myoblasts. GNIP1 overexpression increased glucose uptake in LHCN-M2 myotubes. Overexpression of GNIP1 in mouse muscle in vivo increased glycogen content, glycogen synthase (GS) activity and phospho-GSK-3α/β (Ser21/9) and phospho-Akt (Ser473) content, whereas decreased GS phosphorylation in Ser640. These modifications led to decreased blood glucose levels, lactate levels and body weight, without changing whole-body insulin or glucose tolerance in mouse. Conclusion: GNIP1 is an ubiquitin ligase with a markedly glycogenic effect in skeletal muscle

Adipose tissue glycogen accumulation is associated with obesity-linked inflammation in humans

Objective: glycogen metabolism has emerged as a mediator in the control of energy homeostasis and studies in murine models reveal that adipose tissue might contain glycogen stores. Here we investigated the physio(patho)logical role of glycogen in human adipose tissue in the context of obesity and insulin resistance. Methods: we studied glucose metabolic flux of hypoxic human adipoctyes by nuclear magnetic resonance and mass spectrometry-based metabolic approaches. Glycogen synthesis and glycogen content in response to hypoxia was analyzed in human adipocytes and macrophages. To explore the metabolic effects of enforced glycogen deposition in adipocytes and macrophages, we overexpressed PTG, the only glycogen-associated regulatory subunit (PP1-GTS) reported in murine adipocytes. Adipose tissue gene expression analysis was performed on wild type and homozygous PTG KO male mice. Finally, glycogen metabolism gene expression and glycogen accumulation was analyzed in adipose tissue, mature adipocytes and resident macrophages from lean and obese subjects with different degrees of insulin resistance in 2 independent cohorts. Results: we show that hypoxia modulates glucose metabolic flux in human adipocytes and macrophages and promotes glycogenesis. Enforced glycogen deposition by overexpression of PTG re-orients adipocyte secretion to a pro-inflammatory response linked to insulin resistance and monocyte/lymphocyte migration. Furthermore, glycogen accumulation is associated with inhibition of mTORC1 signaling and increased basal autophagy flux, correlating with greater leptin release in glycogen-loaded adipocytes. PTG-KO mice have reduced expression of key inflammatory genes in adipose tissue and PTG overexpression in M0 macrophages induces a pro-inflammatory and glycolytic M1 phenotype. Increased glycogen synthase expression correlates with glycogen deposition in subcutaneous adipose tissue of obese patients. Glycogen content in subcutaneous mature adipocytes is associated with BMI and leptin expression. Conclusion: our data establish glycogen mishandling in adipose tissue as a potential key feature of inflammatory-related metabolic stress in human obesity

Differential pattern of glycogen accumulation after protein phosphatase 1 glycogen-targeting subunit PPP1R6 overexpression, compared to PPP1R3C and PPP1R3A, in skeletal muscle cells

Background PPP1R6 is a protein phosphatase 1 glycogen-targeting subunit (PP1-GTS) abundant in skeletal muscle with an undefined metabolic control role. Here PPP1R6 effects on myotube glycogen metabolism, particle size and subcellular distribution are examined and compared with PPP1R3C/PTG and PPP1R3A/GM. Results PPP1R6 overexpression activates glycogen synthase (GS), reduces its phosphorylation at Ser-641/0 and increases the extracted and cytochemically-stained glycogen content, less than PTG but more than GM. PPP1R6 does not change glycogen phosphorylase activity. All tested PP1-GTS-cells have more glycogen particles than controls as found by electron microscopy of myotube sections. Glycogen particle size is distributed for all cell-types in a continuous range, but PPP1R6 forms smaller particles (mean diameter 14.4 nm) than PTG (36.9 nm) and GM (28.3 nm) or those in control cells (29.2 nm). Both PPP1R6- and GM-derived glycogen particles are in cytosol associated with cellular structures; PTG-derived glycogen is found in membrane- and organelle-devoid cytosolic glycogen-rich areas; and glycogen particles are dispersed in the cytosol in control cells. A tagged PPP1R6 protein at the C-terminus with EGFP shows a diffuse cytosol pattern in glucose-replete and -depleted cells and a punctuate pattern surrounding the nucleus in glucose-depleted cells, which colocates with RFP tagged with the Golgi targeting domain of β-1,4-galactosyltransferase, according to a computational prediction for PPP1R6 Golgi location. Conclusions PPP1R6 exerts a powerful glycogenic effect in cultured muscle cells, more than GM and less than PTG. PPP1R6 protein translocates from a Golgi to cytosolic location in response to glucose. The molecular size and subcellular location of myotube glycogen particles is determined by the PPP1R6, PTG and GM scaffolding

Glucose dependence of glycogen synthase activity regulation by GSK3 and MEK/ERK inhibitors and angiotensin-(1-7) action on these pathways in cultured human myotubes

Glycogen synthase (GS) is activated by glucose/glycogen depletion in skeletal muscle cells, but the contributing signaling pathways, including the chief GS regulator GSK3, have not been fully defined. The MEK/ERK pathway is known to regulate GSK3 and respond to glucose. The aim of this study was to elucidate the GSK3 and MEK/ERK pathway contribution to GS activation by glucose deprivation in cultured human myotubes. Moreover, we tested the glucose-dependence of GSK3 and MEK/ERK effects on GS and angiotensin (1-7) actions on these pathways. We show that glucose deprivation activated GS, but did not change phospho-GS (Ser640/1), GSK3β activity or activity-activating phosphorylation of ERK1/2. We then treated glucose-replete and -depleted cells with SB415286, U0126, LY294 and rapamycin to inhibit GSK3, MEK1/2, PI3K and mTOR, respectively. SB415286 activated GS and decreased the relative phospho-GS (Ser640/1) level, more in glucose-depleted than -replete cells. U0126 activated GS and reduced the phospho-GS (Ser640/1) content significantly in glucose-depleted cells, while GSK3β activity tended to increase. LY294 inactivated GS in glucose-depleted cells only, without affecting relative phospho-GS (Ser640/1) level. Rapamycin had no effect on GS activation. Angiotensin-(1-7) raised phospho-ERK1/2 but not phospho-GSK3β (Ser9) content, while it inactivated GS and increased GS phosphorylation on Ser640/1, in glucose-replete cells. In glucose-depleted cells, angiotensin-(1-7) effects on ERK1/2 and GS were reverted, while relative phospho-GSK3β (Ser9) content decreased. In conclusion, activation of GS by glucose deprivation is not due to GS Ser640/1 dephosphorylation, GSK3β or ERK1/2 regulation in cultured myotubes. However, glucose depletion enhances GS activation/Ser640/1 dephosphorylation due to both GSK3 and MEK/ERK inhibition. Angiotensin-(1-7) inactivates GS in glucose-replete cells in association with ERK1/2 activation, not with GSK3 regulation, and glucose deprivation reverts both hormone effects. Thus, the ERK1/2 pathway negatively regulates GS activity in myotubes, without involving GSK3 regulation, and as a function of the presence of glucose. © 2013 Elsevier Inc.This study was supported by the following grants: SAF2009-07559 and SAF2012-37480 from the Spanish Ministerio de Ciencia e Innovación (MCI); and SAF2012-37480 and SAF2012-30708, CIBERDEM de Diabetes y Enfermedades Metabólicas Asociadas (CB07/08/0012).Peer Reviewe

Glucose and fructose have sugar-specific effects in both liver and skeletal muscle in vivo: a role for liver fructokinase.

We examined glucose and fructose effects on serine phosphorylation levels of a range of proteins in rat liver and muscle cells. For this, healthy adult rats were subjected to either oral glucose or fructose loads. A mini-array system was utilized to determine serine phosphorylation levels of liver and skeletal muscle proteins. A glucose oral load of 125 mg/100 g body weight (G 1/2) did not induce changes in phosphorylated serines of the proteins studied. Loading with 250 mg/100 g body weight of fructose (Fr), which induced similar glycemia levels as G 1/2, significantly increased serine phosphorylation of liver cyclin D3, PI3 kinase/p85, ERK-2, PTP2 and clusterin. The G 1/2 increased serine levels of the skeletal muscle proteins cyclin H, Cdk2, IRAK, total PKC, PTP1B, c-Raf 1, Ras and the β-subunit of the insulin receptor. The Fr induced a significant increase only in muscle serine phosphorylation of PI3 kinase/p85. The incubation of isolated rat hepatocytes with 10 mM glucose for 5 min significantly increased serine phosphorylation of 31 proteins. In contrast, incubation with 10 mM fructose produced less intense effects. Incubation with 10 mM glucose plus 75 µM fructose counteracted the effects of the incubation with glucose alone, except those on Raf-1 and Ras. Less marked effects were detected in cultured muscle cells incubated with 10 mM glucose or 10 mM glucose plus 75 µM fructose. Our results suggest that glucose and fructose act as specific functional modulators through a general mechanism that involves liver-generated signals, like micromolar fructosemia, which would inform peripheral tissues of the presence of either glucose- or fructose-derived metabolites

Comparative gene expression profiling between human cultured myotubes and skeletal muscle tissue

Background: a high-sensitivity DNA microarray platform requiring nanograms of RNA input facilitates the application of transcriptome analysis to individual skeletal muscle (SM) tissue samples. Culturing myotubes from SM-biopsies enables investigating transcriptional defects and assaying therapeutic strategies. This study compares the transcriptome of aneurally cultured human SM cells versus that of tissue biopsies. Results: we used the Illumina expression BeadChips to determine the transcriptomic differences between tissue and cultured SM samples from five individuals. Changes in the expression of several genes were confirmed by QuantiGene Plex assay or reverse transcription real-time PCR. In cultured myotubes compared to the tissue, 1216 genes were regulated: 583 down and 633 up. Gene ontology analysis showed that downregulated genes were mainly associated with cytoplasm, particularly mitochondria, and involved in metabolism and the muscle-system/contraction process. Upregulated genes were predominantly related to cytoplasm, endoplasmic reticulum, and extracellular matrix. The most significantly regulated pathway was mitochondrial dysfunction. Apoptosis genes were also modulated. Among the most downregulated genes detected in this study were genes encoding metabolic proteins AMPD1, PYGM, CPT1B and UCP3, muscle-system proteins TMOD4, MYBPC1, MYOZ1 and XIRP2, the proteolytic CAPN3 and the myogenic regulator MYF6. Coordinated reduced expression of five members of the GIMAP gene family, which form a cluster on chromosome 7, was shown, and the GIMAP4-reduction was validated. Within the most upregulated group were genes encoding senescence/apoptosis-related proteins CDKN1A and KIAA1199 and potential regulatory factors HIF1A, TOP2A and CCDC80. Conclusions: cultured muscle cells display reductive metabolic and muscle-system transcriptome adaptations as observed in muscle atrophy and they activate tissue-remodeling and senescence/apoptosis processes

PGC-1α induces mitochondrial and myokine transcriptional programs and lipid droplet and glycogen accumulation in cultured human skeletal muscle cells

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) is a chief activator of mitochondrial and metabolic programs and protects against atrophy in skeletal muscle (skm). Here we tested whether PGC-1α overexpression could restructure the transcriptome and metabolism of primary cultured human skm cells, which display a phenotype that resembles the atrophic phenotype. An oligonucleotide microarray analysis was used to reveal the effects of PGC-1α on the whole transcriptome. Fifty-three different genes showed altered expression in response to PGC-1α: 42 upregulated and 11 downregulated. The main gene ontologies (GO) associated with the upregulated genes were mitochondrial components and processes and this was linked with an increase in COX activity, an indicator of mitochondrial content. Furthermore, PGC-1α enhanced mitochondrial oxidation of palmitate and lactate to CO2, but not glucose oxidation. The other most significantly associated GOs for the upregulated genes were chemotaxis and cytokine activity, and several cytokines, including IL-8/CXCL8, CXCL6, CCL5 and CCL8, were within the most highly induced genes. Indeed, PGC-1α highly increased IL-8 cell protein content. The most upregulated gene was PVALB, which is related to calcium signaling. Potential metabolic regulators of fatty acid and glucose storage were among mainly regulated genes. The mRNA and protein level of FITM1/FIT1, which enhances the formation of lipid droplets, was raised by PGC-1α, while in oleate-incubated cells PGC-1α increased the number of smaller lipid droplets and modestly triglyceride levels, compared to controls. CALM1, the calcium-modulated δ subunit of phosphorylase kinase, was downregulated by PGC-1α, while glycogen phosphorylase was inactivated and glycogen storage was increased by PGC-1α. In conclusion, of the metabolic transcriptome deficiencies of cultured skm cells, PGC-1α rescued the expression of genes encoding mitochondrial proteins and FITM1. Several myokine genes, including IL-8 and CCL5, which are known to be constitutively expressed in human skm cells, were induced by PGC-1α