Palmitate exerts opposite effects on proliferation and differentiation of skeletal myoblasts.

The purpose of the study was to examine mechanisms controlling cell cycle progression/arrest and differentiation of mouse C2C12 myoblasts exposed to long-chain saturated fatty acid salt, palmitate. Treatment of proliferating myoblasts with palmitate (0.1 mmol/l) markedly decreased myoblast number. Cyclin A and cyclin D1 levels decreased, whereas total p21 and p21 complexed with cyclin-dependent kinase-4 (cdk4) increased in myoblasts treated with palmitate. In cells induced to differentiation addition of palmitate augmented the level of cyclin D3, the early (myogenin) and late (α-actinin, myosin heavy chain) markers of myogenesis, and caused an increase of myotube diameter. In conclusion, exposure to palmitate inhibits proliferation of myoblasts through a decrease in cyclin A and cyclin D1 levels and an increase of p21-cdk4 complex formation; however, it promotes cell cycle exit, myogenic differentiation and myotube growth.

Insulin-Like Growth Factor-I Increases Laminin, Integrin Subunits and Metalloprotease ADAM12 in Mouse Myoblasts.

The extracellular matrix (ECM) is considered a part of the myogenesis signaling mechanism. we hypothesized that insulin-like growth factor-I (IGF-I) modifies ECM during differentiation of mouse C2C12 cells. The myogenic effect of IGF-I (30 nmol/l) was manifested by increased myogenin and myosin heavy chain (MyHC) levels as well as fusion index (2.6 times over control) on the 3rd day of differentiation. IGF-I markedly augmented laminin, but not fibronectin. Cellular contents of integrin α3, α5 and ß1 during 3-day differentiation increased in the presence of IGF-I. Treatment with IGF-I increased the expression of the long form of metalloprotease ADAM12 (100 kDa) in myocytes. In conclusion: i) IGF-I caused an increase of laminin, integrin α3 and ß1 in C2C12 myogenic cells that can be secondary to stimulation of myogenesis; ii) IGF-I augmented integrin α5 and ADAM12 levels, suggesting a role of this growth factor in determination of the pool of reserve cells during myogenesis.

Transcriptional regulation of important cellular processes in skeletal myogenesis through interferon-γ.

The purpose of the present study was to investigate the effect of interferon (IFN)-γ on the transcriptomic profile of differentiating mouse C2C12 myogenic cells. Global gene expression was evaluated using whole mouse genome oligonucleotide microarrays, and the results were validated through real-time PCR. IFN-γ (1 ng/mL) increased myoblast proliferation but decreased cell respiration and myosin heavy chain content and slightly decreased the fusion index in differentiating C2C12 cell cultures. The genes upregulated through IFN-γ were involved in cell cycle; regulation of cell proliferation; programmed cell death; chemotaxis; and cytokine, growth factor, and peptidase activity, whereas the genes downregulated through IFN-γ primarily contributed to the regulation of transcription, cell-cell signaling, nitrogen compound biosynthesis, ser/thr protein kinase signaling, and regulation of the Wnt pathway. In conclusion, IFN-γ affects the expression of numerous genes associated with the regulation of several processes in myogenesis. The effects of IFN-γ on cellular transcription include (1) alteration of cytokine/growth factor expression, promoting cell proliferation and migration but inhibiting differentiation, (2) impairment of pro-myogenic transcription, (3) disruption of cell adhesion and sarcolemma/cytoskeleton organization, and (4) increased peptidase activity leading to enhanced proteolysis and apoptosis.

[Interactions of proliferation and differentiation signaling pathways in myogenesis]. / Interakcje szlaków sygnalowych proliferacji i róznicowania w miogenezie

The commitment of myogenic cells in skeletal muscle differentiation requires earlier irreversible interruption of the cell cycle. At the molecular level, several key regulators of the cell cycle have been identified: cyclin-dependent kinases and their cyclins stimulate the cell cycle progress and its arrest is determined by the activity of cdk inhibitors (Cip/Kip and INK protein families) and pocket protein family: Rb, p107 and p130. The biological activity of cyclin/cdk complexes allows the successive phases of the cell cycle to occur. Myoblast specialization, differentiation and fusion require the activity of myogenic regulatory factors, which include MyoD, myogenin, Myf5 and MRF4. MyoD and Myf5 play a role in muscle cell specialization, myogenin controls the differentiation process, whereas MRF4 is involved in myotube maturation. The deregulation of the cell cycle leads to uncontrolled proliferation, which antagonizes the functions of myogenic factors and it explains the lack of differentiation-specific gene expression in dividing cells. Conversely, the myogenic factor MyoD seems to cooperate with cell cycle inhibitors leading to inhibition of cell cycle progress and commitment to the differentiation process. The hypophosphorylated form of Rb and cdk inhibitors play an important role in permanent arrest of the cell cycle in differentiated myotubes. Furthermore, cyclin/cdk complexes not only regulate cell division by phosphorylation of several substrates, but may also control other cellular processes such as signal transduction, differentiation and apoptosis. Beyond regulating the cell cycle, Cip/Kip proteins play an important role in cell death, transcription regulation, cell fate determination, cell migration and cytoskeletal dynamics. The article summarizes current knowledge concerning the interactions of intracellular signaling pathways controlling crucial stages of fetal and regenerative myogenesis.

[Systemic and local mechanisms leading to cachexia in cancer]. / Ogólnoustrojowe i miejscowe mechanizmy prowadzace do kacheksji w chorobach nowotworowych.

Cachexia is a multifactorial syndrome of atrophy of skeletal muscle and adipose tissue, resulting in progressive loss of body weight associated with low quality of life and poor prognosis in cancer. Studies on experimental animal models and observations on patients have shown that the soluble factors secreted by tumor cells and tissues of the patient can participate in regulation of the wasting process. Cachexia is often accompanied by anorexia, which is caused by predominance of signals inhibiting appetite in the hypothalamus, such as release of proopiomelanocortin and anorexigenic action of proinflammatory cytokines (IL-1α, IL-1ß, IL-6, TNF-α). Cachexia is also accompanied by extensive metabolic changes consisting of increase of resting energy expenditure and disturbance of carbohydrate, protein and lipid metabolism. Increased expression of protein uncoupling phosphorylation leads to increased thermogenesis in skeletal muscle. Tumor tissue hypoxia caused by its growth beyond blood vessels activates the transcription factor HIF-1, which results in increase in glycolysis, and leads to lactic acid accumulation and activation of the energy inefficient Cori cycle. Loss of fat tissue is caused by increase of lipolysis induced by lipid-mobilizing factor (LMF) and proinflammatory cytokines. Skeletal muscle wasting in cachexia is caused by a reduction of protein synthesis at the stage of initiation and elongation of translation and the simultaneous increase of protein degradation via ubiquitin-dependent and lysosomal pathways. The main mediators of skeletal muscle wasting in cancer are proteolysis-inducing factor (PIF), proinflammatory cytokines, and angiotensin II acting through increased levels of reactive oxygen species (ROS) and nuclear factor NF-κB activation, as well as glucocorticoid activated FOXO transcription factors and myostatin. Understanding of the complexity of the interaction of factors produced by the tumor and the patient's body may form the basis for the development of effective treatments for cachexia in cancer and other pathological conditions.

[Maternal obesity and the development of skeletal muscle in offspring--fetal origin of metabolic disorders]. / Matczyna otylosc a rozwój miesni szkieletowych u potomstwa--plodowe pochodzenie zaburzen metabolicznych.

Suboptimal fetal environments due to inadequate maternal nutrition, obesity, inflammation or gestational diabetes expose the fetus to humoral cues that alter metabolism and growth parameters leading to metabolic disturbances later in life. The fetal stage is crucial for the development of skeletal muscle, a tissue playing an important role in metabolism. Maternal obesity induces inflammation in the fetus causing modifications in the development of fetal skeletal muscle. Changes in the normal course of myogenesis may arise through several mechanisms: changes in WNT/ß-catenin signaling pathway, decreased AMPK activity evoked by TNF-α, increased activity of NF-κB in response to inflammation, which leads to a decrease in myogenic factor MyoD, and increased expression of TGF ß1. Modification in fetal development associated with maternal obesity is attributed to epigenetic changes. Polyunsaturated fatty acids supplied in the diet did affect the development of insulin-sensitive tissues during both the fetal and postnatal period. The specific phenotype of skeletal muscle fibers may play a role in the development of obesity, i.e. fiber phenotype I (slow, oxidative) may protect against obesity and insulin resistance. Exploring the mechanisms of direct impact of maternal obesity on the development of tissues in the offspring may help to reduce the occurrence of metabolic diseases in later life.