Myotube growth is associated with cancer-like metabolic reprogramming and is limited by phosphoglycerate dehydrogenase.

The Warburg effect links growth and glycolysis in cancer. A key purpose of the Warburg effect is to generate glycolytic intermediates for anabolic reactions, such as nucleotides → RNA/DNA and amino acids → protein synthesis. The aim of this study was to investigate whether a similar 'glycolysis-for-anabolism' metabolic reprogramming also occurs in hypertrophying skeletal muscle. To interrogate this, we first induced C2C12 myotube hypertrophy with IGF-1. We then added 14C glucose to the differentiation medium and measured radioactivity in isolated protein and RNA to establish whether 14C had entered anabolism. We found that especially protein became radioactive, suggesting a glucose → glycolytic intermediates → non-essential amino acid(s) → protein series of reactions, the rate of which was increased by IGF-1. Next, to investigate the importance of glycolytic flux and non-essential amino acid synthesis for myotube hypertrophy, we exposed C2C12 and primary mouse myotubes to the glycolysis inhibitor 2-Deoxy-d-glucose (2DG). We found that inhibiting glycolysis lowered C2C12 and primary myotube size. Similarly, siRNA silencing of PHGDH, the key enzyme of the serine biosynthesis pathway, decreased C2C12 and primary myotube size; whereas retroviral PHGDH overexpression increased C2C12 myotube size. Together these results suggest that glycolysis is important for hypertrophying myotubes, which reprogram their metabolism to facilitate anabolism, similar to cancer cells.

Myofiber stretch induces tensile and shear deformation of muscle stem cells in their native niche.

Muscle stem cells (MuSCs) are requisite for skeletal muscle regeneration and homeostasis. Proper functioning of MuSCs, including activation, proliferation, and fate decision, is determined by an orchestrated series of events and communication between MuSCs and their niche. A multitude of biochemical stimuli are known to regulate MuSC fate and function. However, in addition to biochemical factors, it is conceivable that MuSCs are subjected to mechanical forces during muscle stretch-shortening cycles because of myofascial connections between MuSCs and myofibers. MuSCs respond to mechanical forces in vitro, but it remains to be proven whether physical forces are also exerted on MuSCs in their native niche and whether they contribute to the functioning and fate of MuSCs. MuSC deformation in their native niche resulting from mechanical loading of ex vivo myofiber bundles was visualized utilizing mT/mG double-fluorescent Cre-reporter mouse and multiphoton microscopy. MuSCs were subjected to 1 h pulsating fluid shear stress (PFSS) with a peak shear stress rate of 6.5 Pa/s. After PFSS treatment, nitric oxide, messenger RNA (mRNA) expression levels of genes involved in regulation of MuSC proliferation and differentiation, ERK 1/2, p38, and AKT activation were determined. Ex vivo stretching of extensor digitorum longus and soleus myofiber bundles caused compression as well as tensile and shear deformation of MuSCs in their niche. MuSCs responded to PFSS in vitro with increased nitric oxide production and an upward trend in iNOS mRNA levels. PFSS enhanced gene expression of c-Fos, Cdk4, and IL-6, whereas expression of Wnt1, MyoD, Myog, Wnt5a, COX2, Rspo1, Vangl2, Wnt10b, and MGF remained unchanged. ERK 1/2 and p38 MAPK signaling were also upregulated after PFSS treatment. We conclude that MuSCs in their native niche are subjected to force-induced deformations due to myofiber stretch-shortening. Moreover, MuSCs are mechanoresponsive, as evidenced by PFSS-mediated expression of factors by MuSCs known to promote proliferation.

IGF-1 Attenuates Hypoxia-Induced Atrophy but Inhibits Myoglobin Expression in C2C12 Skeletal Muscle Myotubes.

Chronic hypoxia is associated with muscle wasting and decreased oxidative capacity. By contrast, training under hypoxia may enhance hypertrophy and increase oxidative capacity as well as oxygen transport to the mitochondria, by increasing myoglobin (Mb) expression. The latter may be a feasible strategy to prevent atrophy under hypoxia and enhance an eventual hypertrophic response to anabolic stimulation. Mb expression may be further enhanced by lipid supplementation. We investigated individual and combined effects of hypoxia, insulin-like growth factor (IGF)-1 and lipids, in mouse skeletal muscle C2C12 myotubes. Differentiated C2C12 myotubes were cultured for 24 h under 20%, 5% and 2% oxygen with or without IGF-1 and/or lipid treatment. In culture under 20% oxygen, IGF-1 induced 51% hypertrophy. Hypertrophy was only 32% under 5% and abrogated under 2% oxygen. This was not explained by changes in expression of genes involved in contractile protein synthesis or degradation, suggesting a reduced rate of translation rather than of transcription. Myoglobin mRNA expression increased by 75% under 5% O2 but decreased by 50% upon IGF-1 treatment under 20% O2, compared to control. Inhibition of mammalian target of rapamycin (mTOR) activation using rapamycin restored Mb mRNA expression to control levels. Lipid supplementation had no effect on Mb gene expression. Thus, IGF-1-induced anabolic signaling can be a strategy to improve muscle size under mild hypoxia, but lowers Mb gene expression.

Mechanical Stimulation and IGF-1 Enhance mRNA Translation Rate in Osteoblasts Via Activation of the AKT-mTOR Pathway.

Insulin-like growth factor-1 (IGF-1) is anabolic for muscle by enhancing the rate of mRNA translation via activation of AKT and subsequent activation of the mammalian target of rapamycin complex 1 (mTOR), thereby increasing cellular protein production. IGF-1 is also anabolic for bone, but whether the mTOR pathway plays a role in the rate of bone matrix protein production by osteoblasts is unknown. We hypothesized that anabolic stimuli such as mechanical loading and IGF-1 stimulate protein synthesis in osteoblasts via activation of the AKT-mTOR pathway. MC3T3-E1 osteoblasts were either or not subjected for 1 h to mechanical loading by pulsating fluid flow (PFF) or treated with or without human recombinant IGF-1 (1-100 ng/ml) for 0.5-6 h, to determine phosphorylation of AKT and p70S6K (downstream of mTOR) by Western blot. After 4 days of culture with or without the mTOR inhibitor rapamycin, total protein, DNA, and gene expression were quantified. IGF-1 (100 ng/ml) reduced IGF-1 gene expression, although PFF enhanced IGF-1 expression. IGF-1 did not affect collagen-I gene expression. IGF-1 dose-dependently enhanced AKT and p70S6K phosphorylation at 2 and 6 h. PFF enhanced phosphorylation of AKT and p70S6K already within 1 h. Both IGF-1 and PFF enhanced total protein per cell by ∼30%, but not in the presence of rapamycin. Our results show that IGF-1 and PFF activate mTOR, thereby stimulating the rate of mRNA translation in osteoblasts. The known anabolic effect of mechanical loading and IGF-1 on bone may thus be partly explained by mTOR-mediated enhanced protein synthesis in osteoblasts.

Muscle contractile properties as an explanation of the higher mean power output in marmosets than humans during jumping.

The muscle mass-specific mean power output (PMMS,mean) during push-off in jumping in marmosets (Callithrix jacchus) is more than twice that in humans. In the present study it was tested whether this is attributable to differences in muscle contractile properties. In biopsies of marmoset m. vastus lateralis (VL) and m. gastrocnemius medialis (GM) (N=4), fibre-type distribution was assessed using fluorescent immunohistochemistry. In single fibres from four marmoset and nine human VL biopsies, the force-velocity characteristics were determined. Marmoset VL contained almost exclusively fast muscle fibres (>99.0%), of which 63% were type IIB and 37% were hybrid fibres, fibres containing multiple myosin heavy chains. GM contained 9% type I fibres, 44% type IIB and 47% hybrid muscle fibres. The proportions of fast muscle fibres in marmoset VL and GM were substantially larger than those reported in the corresponding human muscles. The curvature of the force-velocity relationships of marmoset type IIB and hybrid fibres was substantially flatter than that of human type I, IIA, IIX and hybrid fibres, resulting in substantially higher muscle fibre mass-specific peak power (PFMS,peak). Muscle mass-specific peak power output (PMMS,peak) values of marmoset whole VL and GM, estimated from their fibre-type distributions and force-velocity characteristics, were more than twice the estimates for the corresponding human muscles. As the relative difference in estimated PMMS,peak between marmosets and humans is similar to that of PMMS,mean during push-off in jumping, it is likely that the difference in in vivo mechanical output between humans and marmosets is attributable to differences in muscle contractile properties.

Smoking cessation only partially reverses cardiac metabolic and structural remodeling in mice.

AIMS: Active cigarette smoking is a major risk factor for chronic obstructive pulmonary disease that remains elevated after cessation. Skeletal muscle dysfunction has been well documented after smoking, but little is known about cardiac adaptations to cigarette smoking. The underlying cellular and molecular cardiac adaptations, independent of confounding lifestyle factors, and time course of reversibility by smoking cessation remain unclear. We hypothesized that smoking negatively affects cardiac metabolism and induces local inflammation in mice, which do not readily reverse upon 2-week smoking cessation. METHODS: Mice were exposed to air or cigarette smoke for 14 weeks with or without 1- or 2-week smoke cessation. We measured cardiac mitochondrial respiration by high-resolution respirometry, cardiac mitochondrial density, abundance of mitochondrial supercomplexes by electrophoresis, and capillarization, fibrosis, and macrophage infiltration by immunohistology, and performed cardiac metabolome and lipidome analysis by mass spectrometry. RESULTS: Mitochondrial protein, supercomplex content, and respiration (all p < 0.03) were lower after smoking, which were largely reversed within 2-week smoking cessation. Metabolome and lipidome analyses revealed alterations in mitochondrial metabolism, a shift from fatty acid to glucose metabolism, which did not revert to control upon smoking cessation. Capillary density was not different after smoking but increased after smoking cessation (p = 0.02). Macrophage infiltration and fibrosis (p < 0.04) were higher after smoking but did not revert to control upon smoking cessation. CONCLUSIONS: While cigarette-impaired smoking-induced cardiac mitochondrial function was reversed by smoking cessation, the remaining fibrosis and macrophage infiltration may contribute to the increased risk of cardiovascular events after smoking cessation.

The NLRP3 inflammasome contributes to inflammation-induced morphological and metabolic alterations in skeletal muscle.

BACKGROUND: Systemic inflammation is associated with skeletal muscle atrophy and metabolic dysfunction. Although the nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) inflammasome contributes to cytokine production in immune cells, its role in skeletal muscle is poorly understood. Here, we studied the link between inflammation, NLRP3, muscle morphology, and metabolism in in vitro cultured C2C12 myotubes, independent of immune cell involvement. METHODS: Differentiated C2C12 myotubes were treated with lipopolysaccharide (LPS; 0, 10, and 100-200 ng/mL) to induce activation of the NLRP3 inflammasome with and without MCC950, a pharmacological inhibitor of NLRP3-induced IL-1ß production. We assessed markers of the NLRP3 inflammasome, cell diameter, reactive oxygen species, and mitochondrial function. RESULTS: NLRP3 gene expression and protein concentrations increased in a time-dependent and dose-dependent manner. Intracellular IL-1ß concentration significantly increased (P < 0.0001), but significantly less with MCC950 (P = 0.03), suggestive of moderate activation of the NLRP3 inflammasome in cultured myotubes upon LPS stimulation. LPS suppressed myotube growth after 24 h (P = 0.03), and myotubes remained smaller up to 72 h (P = 0.0009). Exposure of myotubes to IL-1ß caused similar alterations in cell morphology, and MCC950 mitigated these LPS-induced differences in cell diameter. NLRP3 appeared to co-localize with mitochondria, more so upon exposure to LPS. Mitochondrial reactive oxygen species were higher after LPS (P = 0.03), but not after addition of MCC950. Myotubes had higher glycolytic rates, and mitochondria were more fragmented upon LPS exposure, which was not altered by MCC950 supplementation. CONCLUSIONS: LPS-induced activation of the NLRP3 inflammasome in cultured myotubes contributes to morphological and metabolic alterations, likely due to its mitochondrial association.

TGF-ß Regulates Collagen Type I Expression in Myoblasts and Myotubes via Transient Ctgf and Fgf-2 Expression.

Transforming Growth Factor ß (TGF-ß) is involved in fibrosis as well as the regulation of muscle mass, and contributes to the progressive pathology of muscle wasting disorders. However, little is known regarding the time-dependent signalling of TGF-ß in myoblasts and myotubes, as well as how TGF-ß affects collagen type I expression and the phenotypes of these cells. Here, we assessed effects of TGF-ß on gene expression in C2C12 myoblasts and myotubes after 1, 3, 9, 24 and 48 h treatment. In myoblasts, various myogenic genes were repressed after 9, 24 and 48 h, while in myotubes only a reduction in Myh3 expression was observed. In both myoblasts and myotubes, TGF-ß acutely induced the expression of a subset of genes involved in fibrosis, such as Ctgf and Fgf-2, which was subsequently followed by increased expression of Col1a1. Knockdown of Ctgf and Fgf-2 resulted in a lower Col1a1 expression level. Furthermore, the effects of TGF-ß on myogenic and fibrotic gene expression were more pronounced than those of myostatin, and knockdown of TGF-ß type I receptor Tgfbr1, but not receptor Acvr1b, resulted in a reduction in Ctgf and Col1a1 expression. These results indicate that, during muscle regeneration, TGF-ß induces fibrosis via Tgfbr1 by stimulating the autocrine signalling of Ctgf and Fgf-2.

Comparison of human retinal pigment epithelium gene expression in macula and periphery highlights potential topographic differences in Bruch's membrane.

PURPOSE: To describe gene expression differences between healthy, young human retinal pigment epithelium (RPE) cells from the macular area and RPE cells from two locations in the retinal periphery. METHODS: RPE cells from six human donor eyes, ages 17-36, without histopathological abnormalities, were dissected by laser and isolated from cryosections. Total RNA was isolated, amplified, and hybridized to a custom made oligonucleotide array containing 22,000 genes. Bioinformatic analysis was carried out using the computer programs Rosetta Resolver and the webtools EASE/David and GoStat. Confirmatory real time PCR (RT-PCR) and immunohistochemistry were performed according to standard protocols. RESULTS: Microarray and statistical analysis yielded 438 genes that were differentially expressed between macular RPE, and at least one out of two peripheral RPE locations. Out of these genes, 33 that showed fold changes of four, or higher, were selected for RT-PCR confirmation. For 17 genes (51%), a significant differential expression was found, while 11 additional genes (33%) showed a similar trend. Immuno-staining of one target (WFDC1) confirmed its differential expression on the protein level. Functional annotation and overrepresentation analysis independently defined extracellular matrix (ECM) genes as a statistically overrepresented class of genes in our RPE dataset. In total, 33 ECM genes were differentially expressed between macular and peripheral RPE regions. A subset of proteins corresponding to these genes is known to be present in Bruch's membrane. CONCLUSIONS: Our data showed that consistent topographical gene expression differences in the human RPE constitute around 1-5% of the RPE transcriptome. These changes may underlie topographical differences in RPE physiology, and pathology and may reflect local differences in the molecular composition and turnover of Bruch's membrane.