Role of myostatin in metabolism

Myostatin induces cyclin D1 degradation to cause cell cycle arrest through a phosphatidylinositol 3-kinase/AKT/GSK-3 beta pathway and is antagonized by insulin-like growth factor 1

Myostatin is a transforming growth factor beta superfamily member and is known as an inhibitor of skeletal muscle cell proliferation and differentiation. Exposure to myostatin induces G1 phase cell cycle arrest. In this study, we demonstrated that myostatin down-regulates Cdk4 activity via promotion of cyclin D1 degradation. Overexpression of cyclin D1 significantly blocked myostatin-induced proliferation inhibition. We further showed that phosphorylation at threonine 286 by GSK-3beta was required for myostatin-stimulated cyclin D1 nuclear export and degradation. This process is dependent upon the activin receptor IIB and the phosphatidylinositol 3-kinase/Akt pathway but not Smad3. Insulin-like growth factor 1 (IGF-1) treatment or Akt activation attenuated the myostatin-stimulated cyclin D1 degradation as well as the associated cell proliferation repression. In contrast, attenuation of IGF-1 signaling caused C2C12 cells to undergo apoptosis in response to myostatin treatment. The observation that IGF-1 treatment increases myostatin expression through a phosphatidylinositol 3-kinase pathway suggests a possible feedback regulation between IGF-1 and myostatin. These findings uncover a novel role for myostatin in the regulation of cell growth and cell death in concert with IGF-1.

Cancer cachexia: it's time for more clinical trials

Cancer cachexia (CC) is a multifactorial paraneoplastic syndrome characterized by anorexia, body weight loss, loss of adipose tissue and skeletal muscle, accounting for at least 20% of deaths in neoplastic patients. CC significantly impairs quality of life and response to anti-neoplastic therapies, increasing morbidity and mortality of cancer patients. Muscle wasting is the most important phenotypic feature of CC and the principal cause of function impairment, fatigue and respiratory complications, mainly related to a hyperactivation of muscle proteolytic pathways. Most current therapeutic strategies to counteract CC have proven to be only partially effective. In the last decade, the correction of anorexia, the inhibition of catabolic processes and the stimulation of anabolic pathways in muscle have been attempted pharmacologically with encouraging results in animal models and through preliminary clinical trials. However, data in the clinical setting are still scanty and non definitive. It is time to start prospective, randomized, controlled trials to evaluate which drugs are effective in counteracting the loss of lean of muscle mass and in improving nutritional status and quality of life in patients affected by cancer-related cachexia.

Myostatin short interfering hairpin RNA gene transfer increases skeletal muscle mass

BACKGROUND

Myostatin negatively regulates skeletal muscle growth. Myostatin knockout mice exhibit muscle hypertrophy and decreased interstitial fibrosis. We investigated whether a plasmid expressing a short hairpin interfering RNA (shRNA) against myostatin and transduced using electroporation would increase local skeletal muscle mass.

METHODS

Short interfering RNAs (siRNAs) targeting myostatin were co-transfected with a myostatin-expressing plasmid into HEK293 cells and identified for myostatin silencing by Western blot. Corresponding shRNAs were cloned into plasmid shRNA expression vectors. Myostatin or a randomer negative control shRNA plasmid was injected and electroporated into the tibialis anterior or its contralateral muscle, respectively, of nine rats that were sacrificed after 2 weeks. Six other rats received a beta-galactosidase reporter plasmid and were sacrificed at 1, 2, and 4 weeks. Uptake of plasmid was examined by beta-galactosidase expression, whereas myostatin expression was determined by real-time polymerase chain reaction (PCR) and Western blotting. Muscle fiber size was determined by histochemistry. Satellite cell proliferation was determined by PAX7 immunohistochemistry. Myosin heavy chain type II (MHCII) expression was determined by Western blot.

RESULTS

beta-Galactosidase reporter plasmid was expressed at 1 and 2 weeks but diminished by 4 weeks in tibialis anterior skeletal muscle. Myostatin shRNA reduced myostatin mRNA and protein expression by 27 and 48%, respectively. Tibialis anterior weight, fiber size, and MHCII increased by 10, 34, and 38%, respectively. Satellite cell number was increased by over 2-fold.

CONCLUSIONS

This is the first demonstration that myostatin shRNA gene transfer is a potential strategy to increase muscle mass.

GRB14, GPD1, and GDF8 as potential network collaborators in weight loss-induced improvements in insulin action in human skeletal muscle

Obesity is associated with insulin resistance in skeletal muscle; accordingly, weight loss dramatically improves insulin action. We sought to identify molecular remodeling of muscle commensurate with weight loss that could explain improvements in insulin action. Muscle from morbidly obese women was studied before and after gastric bypass surgery. Gastric bypass surgery significantly reduced body mass by approximately 45% and improved insulin action. We then assessed mRNA profiles using a stringent statistical analysis (statistical concordance with three probe set algorithms), with validation in a cross-sectional study of lean (n = 8) vs. morbidly obese (n = 8) muscle. Growth factor receptor-bound protein 14 (GRB14), glycerol-3-phosphate dehydrogenase 1 (GPD1), and growth differentiation factor 8 (GDF8; myostatin) significantly decreased approximately 2.4-, 2.2-, and 2.4-fold, respectively, after weight loss (gastric bypass). Increased expression of these transcripts was associated with increased obesity in the cross-sectional group (lean vs. morbidly obese muscle). Each transcript was validated by real-time quantitative RT-PCR assays in both study groups. Using Ingenuity Pathway Analysis, we show that all three transcripts are involved in the same regulatory network including AKT1, IGF1, TNF, PPARG, and INS. These results suggest that GRB14, GPD1, and GDF8 are weight loss-responsive genes in skeletal muscle and that the observed transcriptional modulation of these would be expected to improve insulin signaling, decrease triglyceride synthesis, and increase muscle mass, respectively, with weight loss. Thus our data provide a possible regulatory pathway involved in the development of insulin resistance in the morbidly obese state, and improvement of insulin resistance with weight loss.

New and improved glucocorticoid receptor ligands

The therapeutic and prophylactic use of glucocorticoids is widespread due to their powerful anti-inflammatory, antiproliferative and immunomodulatory activity. However, long-term use of these drugs can result in severe dose-limiting side effects. One of the most critical and debilitating side effects is osteoporosis, which leads to increased risk of fractures. Glucocorticoids damage bone through several different mechanisms. The search for novel glucocorticoids that have reduced side effects in bone and other tissues is being driven by the identification of new mechanisms of action of the glucocorticoid receptor. This may facilitate the detection of new, safer therapies with efficacies equivalent to currently prescribed steroids.

The influence of growth hormone status on physical impairments, functional limitations, and health-related quality of life in adults

The availability of recombinant human GH and somatostatin analogs has resulted in widespread treatment for adults with GH deficiency (GHD) and those with GH excess (acromegaly). Despite being at opposite ends of the spectrum in terms of their GH/IGF-I axis, both of these populations experience overlapping somatic impairments. Adults with untreated GHD have low circulating levels of IGF-I that manifest as altered body composition with increased fat and reduced lean body and skeletal muscle mass. At the other end of the spectrum, adults with GH excess, who have elevated levels of IGF-I, also have altered body composition. Impairments that result from disorders of either GHD or GH excess are both associated with increased functional limitations, such as reduced ability to walk quickly for prolonged periods, and poorer health-related quality of life (HR-QoL). Adults with untreated GHD and GH excess both commonly complain of excessive fatigue that seems to be associated more with impaired aerobic than muscular performance. Several studies have documented that administration of GH or somatostatin analogs to adults with GHD or GH excess, respectively, ameliorates abnormal biochemical profile and the associated somatic impairments. However, whether these improvements translate into improved physical function in adults with GHD or GH excess remains largely unknown, and their impact on HR-QoL controversial. Review of placebo-controlled trials to date suggests that GH and somatostatin analogs have greater effects on gas exchange and aerobic performance than as anabolic agents on skeletal muscle mass and function. Future investigations should include dose-response studies to establish the optimal combination of pharmacological agents plus exercise required to improve not only biochemical markers but also physical function and HR-QoL in adults with GHD or GH excess.

The orphan Rev-erb nuclear receptors: a link between metabolism, circadian rhythm and inflammation?

Nuclear hormone receptors (NRs) function as ligand dependent DNA binding proteins that translate physiological/nutritional signals into gene regulation. Dysfunctional NR signaling leads to many disorders in reproduction, inflammation, and metabolism. The opportunity to identify novel regulatory pathways in the context of human health and disease drives the challenge to unravel the biological function of the “orphan nuclear hormone receptors”. For example, the Rev-erb (NR1D) subgroup (Rev-erbα/NR1D1 and Rev-erbβ/NR1D2) of orphan NRs are transcriptional silencers and negative regulators of RORα mediated trans-activation. The NR1D subgroup is highly enriched in peripheral tissues with onerous energy demands including skeletal muscle, brown and white adipose, brain, liver and kidney. This alludes to the involvement of this subgroup in metabolism. In this context, Rev-erbα-/- mice have a dyslipidemic phenotype. Recent studies in vascular smooth and skeletal muscle cells also suggest that the NR1D subgroup modulates inflammation by regulating IκBα/NFκB dependent gene expression. Rev-erbα has been identified as a critical regulator (and target) of circadian rhythm, a factor in blood pressure control and inflammation. Finally, two recent reports have demonstrated: (i) lithium mediated regulation of Rev-erbα stability and (ii) E75 (the Drosophila orthologue of human Rev-erbα) is tightly bound by heme, and functions as a “gas sensor” through interaction with CO/NO and interferes with the repression of DHR3 (the Drosophila orthologue of human RORα). In conclusion, the role of these receptors at the cross-roads of metabolism, inflammation, and circadian cycling underscores the importance of understanding the organ-specific function of the NR1D subgroup in homeostasis.

The possible role of myostatin in skeletal muscle atrophy and cachexia

The presence of sufficient skeletal muscle is of paramount importance for body function. Cachexia can be defined as a wasting syndrome describing the progressive loss of both adipose and skeletal muscle tissue in concert with severe injury, chronic or end-stage malignant and infectious diseases. Generally, cachexia predisposes to poor prognosis, co-morbidities and death. One signaling pathway possibly involved in muscle atrophy and cachexia is the myostatin cascade. This transforming growth factor-beta superfamily member myostatin is a strong candidate for regulating muscle mass, and is shown to inhibit muscle growth in different in vivo mammalian models. Overall, the modulation of the myostatin pathway seems interesting from the perspective of both pathology and sports medicine. Hence, myostatin signaling components and post-translational modulators are possible targets of pharmacological and other treatments against muscle loss, thus potentially contributing to the understanding and mitigation of muscle atrophies associated with inactivity, senescence and disease.

Expression of the TGF-beta family of ligands is developmentally regulated in skeletal muscle of neonatal rats

To dissect the possible role of the transforming growth factor-beta (TGF-beta) family in the regulation of skeletal muscle growth during the early postnatal period, the protein abundances of the TGF-beta family and their correlation with protein synthesis were determined in skeletal muscle of neonatal rats. To obtain direct evidence of the role of these growth factors in the regulation of protein synthesis, the TGF-beta inhibitor, follistatin, was infused into 10-d-old rats for 11 d and protein synthesis and phosphorylation of S6 kinase 1 (S6K1) and ribosomal protein (rpS6) were measured. TGF-beta2 abundance and protein synthesis in muscle decreased with development and were positively correlated. The abundances of bone morphogenetic protein 2 (BMP-2), BMP-7, and myostatin increased with development and were negatively correlated with protein synthesis. The abundances of BMP-2 and BMP-7 were positively correlated with BMP receptor IA (BMP-RIA) abundance. Activin A abundance was positively correlated with follistatin abundance and activin receptor IIB (Act-RIIB) abundance. Infusion of follistatin increased muscle protein synthesis and S6K1 and rpS6 phosphorylation. The results provide indirect and direct evidence of TGF-beta family involvement in the regulation of muscle protein synthesis during the neonatal period.

Skeletal muscle and nuclear hormone receptors: implications for cardiovascular and metabolic disease

Skeletal muscle is a major mass peripheral tissue that accounts for approximately 40% of the total body mass and a major player in energy balance. It accounts for >30% of energy expenditure, is the primary tissue of insulin stimulated glucose uptake, disposal, and storage. Furthermore, it influences metabolism via modulation of circulating and stored lipid (and cholesterol) flux. Lipid catabolism supplies up to 70% of the energy requirements for resting muscle. However, initial aerobic exercise utilizes stored muscle glycogen but as exercise continues, glucose and stored muscle triglycerides become important energy substrates. Endurance exercise increasingly depends on fatty acid oxidation (and lipid mobilization from other tissues). This underscores the importance of lipid and glucose utilization as an energy source in muscle. Consequently skeletal muscle has a significant role in insulin sensitivity, the blood lipid profile, and obesity. Moreover, caloric excess, obesity and physical inactivity lead to skeletal muscle insulin resistance, a risk factor for the development of type II diabetes. In this context skeletal muscle is an important therapeutic target in the battle against cardiovascular disease, the worlds most serious public health threat. Major risk factors for cardiovascular disease include dyslipidemia, hypertension, obesity, sedentary lifestyle, and diabetes. These risk factors are directly influenced by diet, metabolism and physical activity. Metabolism is largely regulated by nuclear hormone receptors which function as hormone regulated transcription factors that bind DNA and mediate the patho-physiological regulation of gene expression. Metabolism and activity, which directly influence cardiovascular disease risk factors, are primarily driven by skeletal muscle. Recently, many nuclear receptors expressed in skeletal muscle have been shown to improve glucose tolerance, insulin resistance, and dyslipidemia. Skeletal muscle and nuclear receptors are rapidly emerging as critical targets in the battle against cardiovascular disease risk factors. Understanding the function of nuclear receptors in skeletal muscle has enormous pharmacological utility for the treatment of cardiovascular disease. This review focuses on the molecular regulation of metabolism by nuclear receptors in skeletal muscle in the context of dyslipidemia and cardiovascular disease.

Myostatin inhibits myogenesis and promotes adipogenesis in C3H 10T(1/2) mesenchymal multipotent cells

Inactivating mutations of the mammalian myostatin gene are associated with increased muscle mass and decreased fat mass; conversely, myostatin transgenic mice that overexpress myostatin in the skeletal muscle have decreased muscle mass and increased fat mass. We investigated the effects of recombinant myostatin protein and antimyostatin antibody on myogenic and adipogenic differentiation of mesenchymal multipotent cells. Accordingly, 10T(1/2) cells were incubated with 5'-azacytidine for 3 d to induce differentiation and then treated with a recombinant protein for myostatin (Mst) carboxy terminal 113 amino acids or a polyclonal anti-Mst antibody for 3, 7, and 14 d. Cells were also cotransfected with a Mst cDNA plasmid expressing the full-length 375-amino acid protein (pcDNA-Mst375) and the silencer RNAs for either Mst (pSil-Mst) or a random sequence (pSil-RS) for 3 or 7 d, and Mst expression was determined. Adipogenesis was evaluated by quantitative image analysis of fat cells before and after oil-red-O staining, immunocytochemistry of adiponectin, and Western blot for CCAAT/enhancer binding protein-alpha. Myogenesis was estimated by quantitative image analysis-immunocytochemistry for MyoD (Myo differentiation protein), myogenin, and myosin heavy chain type II, or by Western blot for myogenin. 5'-Azacytidine-mediated differentiation induced endogenous full-length Mst expression. Recombinant Mst carboxy terminal 113 amino acids inhibited both early and late markers of myogenesis and stimulated both early and late markers of adipogenesis, whereas the antibody against Mst exerted the reverse effects. Myogenin levels at 7 d after transfection of pcDNA-Mst375 were reduced as expected and elevated by pSil-Mst, which blocked efficiently Mst375 expression. In conclusion, myostatin promotes the differentiation of multipotent mesenchymal cells into the adipogenic lineage and inhibits myogenesis.

Myostatin is increased and complexes with amyloid-beta within sporadic inclusion-body myositis muscle fibers

Myostatin is a negative regulator of muscle mass and strength. Sporadic inclusion-body myositis (s-IBM) is the most common degenerative muscle disease of older persons and is characterized by pronounced muscle wasting. s-IBM is of unknown etiology and pathogenesis, and it lacks definitive treatment. We have now demonstrated in samples from 12 s-IBM biopsies that: (1) by light and electron microscopic immunocytochemistry, myostatin/myostatin precursor is accumulated within muscle fibers and co-localized with amyloid-beta (Abeta); (2) by immunoblots, both myostatin and myostatin precursor are increased; and (3) by immunoprecipitation, myostatin precursor complexes with Abeta. Our study suggests that myostatin/myostatin precursor, either alone, or bound to Abeta, may play a novel role in the pathogenesis of s-IBM.

Muscle growth patterns and regulation during fish ontogeny

In fish, the skeletal muscle of the trunk and the tail derives from the somites which form in the paraxial mesoderm in a rostro-caudal sequence. The development of the fish myotome begins with the onset of myogenic regulatory factors expression and continues with the formation of a distinct superficial layer of slow muscle fibres that covers a bulk of fast muscle fibres located in the deep portion of the myotome. Muscle fibres of the slow-twitch lineage originate in fish embryos from adaxial cells, a distinct subpopulation of the paraxial mesoderm that flanks the notochord. During the early maturation of the somite these adaxial cells migrate away from the notochord towards the lateral part of the somite where they form the superficial slow fibres. Lateral presomitic cells that remain deep in the myotome differentiate into fast muscle fibres. Morphogens of the hedgehog family secreted by the notochord have a pivotal role in inducing the slow-twitch lineage. In late embryos, additional fibres are added from discrete germinal zones situated at the ventral and dorsal extremes of the developing myotome. This regionalised process has been termed "stratified hyperplasia." In fish which grow to a large final size this is followed by a mosaic hyperplastic process that leads to the formation of new fibres throughout the whole myotome. Current knowledge about the endocrine and autocrine factors that potentially regulate the proliferation and the differentiation of muscle cells within the embryonic and larval fish myotome is reviewed.

Alterations of protein metabolism in acromegaly

PURPOSE OF REVIEW

Growth hormone is a powerful anabolic hormone necessary for normal growth, but its importance in maintaining the cellular and protein mass in adult life is still unclear. However, it is viewed as a drug capable of combating the tissue loss and some metabolic derangements of aging. Growth hormone excess causes acromegaly, a disease characterized by overgrowth of some tissues and multiple metabolic abnormalities. The purpose of this article is to review recent knowledge in acromegaly considering it as a model for clarifying aspects of growth hormone action on body composition, protein dynamics and molecular mechanisms in adult life.

RECENT FINDINGS

Acromegaly induces well-documented changes in body fat (decreased), and bone density and water retention (increased), but there are less-clear changes in protein and body cell-mass accretion. Recent studies related insulin resistance to glucose metabolism to accelerated fat oxidation and described the reversibility of such alterations after surgical or pharmacologic therapy. Less attention was paid to changes in protein metabolism. Acromegalics are profoundly insulin-resistant to the antiproteolytic action of insulin, but amino acids are channelled towards protein synthesis because they are still normally spared from oxidation by insulin. This insulin resistance persists months after the surgical cure of acromegaly when glucose metabolism is already normalized. Recent studies suggested that increased use of fat for fuel by growth hormone may also promote protein anabolism and reduce amino acid oxidation.

SUMMARY

Despite important advances in understanding molecular mechanisms in acromegaly, the specific effects on body cell and protein mass and the specific modulation of local protein dynamics remain poorly defined.