Overexpression of the human type-1 insulin-like growth factor receptor in rat L6 myoblasts induces ligand-dependent cell proliferation and inhibition of differentiation

Exercise and Muscle Atrophy

The incidence of muscle atrophy is increasing with each passing year, which imposes a huge burden on the quality of life of patients. It is a public health issue that causes a growing concern around the world. Exercise is one of the key strategies to prevent and treat various diseases. Appropriate exercise is conducive to compensatory muscle hypertrophy, to improve muscle strength and elasticity, and to train muscle coordination, which is also beneficial to the recovery of skeletal muscle function and the regeneration of muscle cells. Sequelae of paralysis of patients with limb dyskinesia caused by muscle atrophy will be significantly alleviated after regular exercise therapy. Furthermore, exercise therapy can slow down or even reverse muscle atrophy. This article aims to introduce the characteristics of muscle atrophy and summarize the role and mechanism of exercise in the treatment of muscle atrophy in the existing studies, in order to further explore the mechanism of exercise to protect muscle atrophy and provide protection for patients with muscular atrophy.

Insulin-like Growth Factor-II Delays Early but Enhances Late Regeneration of Skeletal Muscle

This study tested whether administration of insulin-like growth factor-II (IGF-II) enhances muscle regeneration. Rat biceps femoris muscle was damaged with notexin and then IGF-II was administered for up to 7 days. Results show that the proportion of nuclei containing or surrounded by immunoreactivity to MyoD, myogenin, and developmental myosin heavy chain (dMHC) is less in the IGF-II treatment group relative to the control group on days 1 (p=0.057), 2 (p=0.034), and 3 (p=0.047), respectively. This indicates a delay in muscle precursor cell (MPC) proliferation and differentiation with IGF-II administration. This effect was not associated with decreased binding capacity of the type 1 IGF receptor, as determined by receptor autoradiography in day 1 muscle sections (NS), but was associated with inhibition of phagocytic processes. The cross-sectional area of regenerating muscle fibers was significantly greater in the IGF-II treatment group than in the control group by day 7 (p=0.0092). The enhancing effect of IGF-II on late muscle regeneration, when the main process taking place is fiber enlargement, coincides with the period in which IGF-II is normally expressed by regenerating muscle, indicating that greater endogenous production of IGF-II would be associated with improved regeneration.

The NF-ҡB modulated miR-195/497 inhibit myoblast proliferation by targeting Igf1r/Insr and cyclin genes

MicroRNAs (miRNAs) play important roles in the development of skeletal muscle. In our previous study, expression of miR-195 and miR-497 were shown to be up-regulated during muscle development in pigs. In this study, we investigated the roles of these two miRNAs in myogenesis and analyzed their transcriptional regulation. Our results showed that miR-195 and miR-497 were up-regulated during muscle development and myoblast differentiation. Moreover, miR-195/497 inhibited proliferation but not differentiation in C2C12 cells. Further investigation revealed that Igf1r, Insr, Ccnd2, and Ccne1 were directly targeted by miR-195/497 in myoblasts. In addition, we confirmed that similarly expressed miR-195 and miR-497 were negatively regulated by nuclear factor-kappaB (NF-ҡB) in both myoblasts and skeletal muscle tissue. Our data illustrated that the NF-ҡB-miR-195/497-Igf1r/Insr-Ccnd2/Ccne1 signaling pathway played important roles in the myogenesis. Our study provides novel evidence for the roles of miR-195/497 in muscle development.

An HMGA2-IGF2BP2 axis regulates myoblast proliferation and myogenesis

A group of genes that are highly and specifically expressed in proliferating skeletal myoblasts during myogenesis was identified. Expression of one of these genes, Hmga2, increases coincident with satellite cell activation, and later its expression significantly declines correlating with fusion of myoblasts into myotubes. Hmga2 knockout mice exhibit impaired muscle development and reduced myoblast proliferation, while overexpression of HMGA2 promotes myoblast growth. This perturbation in proliferation can be explained by the finding that HMGA2 directly regulates the RNA-binding protein IGF2BP2. Add-back of IGF2BP2 rescues the phenotype. IGF2BP2 in turn binds to and controls the translation of a set of mRNAs, including c-myc, Sp1, and Igf1r. These data demonstrate that the HMGA2-IGF2BP2 axis functions as a key regulator of satellite cell activation and therefore skeletal muscle development.

Separation of fast from slow anabolism by site-specific PEGylation of insulin-like growth factor I (IGF-I)

Insulin-like growth factor I (IGF-I) has important anabolic and homeostatic functions in tissues like skeletal muscle, and a decline in circulating levels is linked with catabolic conditions. Whereas IGF-I therapies for musculoskeletal disorders have been postulated, dosing issues and disruptions of the homeostasis have so far precluded clinical application. We have developed a novel IGF-I variant by site-specific addition of polyethylene glycol (PEG) to lysine 68 (PEG-IGF-I). In vitro, this modification decreased the affinity for the IGF-I and insulin receptors, presumably through decreased association rates, and slowed down the association to IGF-I-binding proteins, selectively limiting fast but maintaining sustained anabolic activity. Desirable in vivo effects of PEG-IGF-I included increased half-life and recruitment of IGF-binding proteins, thereby reducing risk of hypoglycemia. PEG-IGF-I was equipotent to IGF-I in ameliorating contraction-induced muscle injury in vivo without affecting muscle metabolism as IGF-I did. The data provide an important step in understanding the differences of IGF-I and insulin receptor contribution to the in vivo activity of IGF-I. In addition, PEG-IGF-I presents an innovative concept for IGF-I therapy in diseases with indicated muscle dysfunction.

Reduced differentiation potential of primary MyoD-/- myogenic cells derived from adult skeletal muscle

To gain insight into the regeneration deficit of MyoD-/- muscle, we investigated the growth and differentiation of cultured MyoD-/- myogenic cells. Primary MyoD-/- myogenic cells exhibited a stellate morphology distinct from the compact morphology of wild-type myoblasts, and expressed c-met, a receptor tyrosine kinase expressed in satellite cells. However, MyoD-/- myogenic cells did not express desmin, an intermediate filament protein typically expressed in cultured myoblasts in vitro and myogenic precursor cells in vivo. Northern analysis indicated that proliferating MyoD-/- myogenic cells expressed fourfold higher levels of Myf-5 and sixfold higher levels of PEA3, an ETS-domain transcription factor expressed in newly activated satellite cells. Under conditions that normally induce differentiation, MyoD-/- cells continued to proliferate and with delayed kinetics yielded reduced numbers of predominantly mononuclear myocytes. Northern analysis revealed delayed induction of myogenin, MRF4, and other differentiation-specific markers although p21 was upregulated normally. Expression of M-cadherin mRNA was severely decreased whereas expression of IGF-1 was markedly increased in MyoD-/- myogenic cells. Mixing of lacZ-labeled MyoD-/- cells and wild-type myoblasts revealed a strict autonomy in differentiation potential. Transfection of a MyoD-expression cassette restored cytomorphology and rescued the differentiation deficit. We interpret these data to suggest that MyoD-/- myogenic cells represent an intermediate stage between a quiescent satellite cell and a myogenic precursor cell.

Insulin-like growth factor I (IGF-I) receptor overexpression abolishes the IGF requirement for differentiation and induces a ligand-dependent transformed phenotype in C2 inducible myoblasts

Insulin-like growth factors (IGFs) stimulate both proliferation and differentiation of myogenic cell lines, and these actions are mostly mediated through the type I IGF receptor (type I IGF-R). To further investigate the role of this receptor in phenotypic characteristics of C2 murine myoblasts, we overexpressed the human type I IGF-R in the inducible clone of C2 cells, which requires IGFs in the differentiation medium to undergo terminal differentiation. Inducible myoblasts were transfected with either the eukaryotic expression vector pNTK or pNTK containing the human type I IGF-R complementary DNA, and we isolated two clones named Ind-Neo and Ind-R, respectively. Binding and autophosphorylation experiments indicate that Ind-R cells express about 10 times as much type I IGF-R compared with Ind-Neo control cells and that the transfected type I IGF-R is functional in Ind-R cells. We show that overexpression of the human type I IGF-R makes inducible myoblasts able to differentiate spontaneously, as assessed by expression of the myogenic transcription factors MyoD and myogenin, detection of the muscle-specific protein troponin T, and myotube formation. Moreover, when exposed to IGF-I, Ind-R cells lose contact inhibition, grow in the presence of a low level of growth factors and form colonies in soft agar, which is characteristic of a ligand-dependent transformed phenotype. It emerges from this study that 1) the type I IGF-R is strongly involved in the phenotypic differences between inducible and permissive cells with respect to the differentiation program; and 2) overexpression causes this receptor to act as a ligand-dependent transforming protein in muscle cells. We suggest that type I IGF-R abundance and level of activation may determine the efficiency of the autocrine mode of action of IGFs and discriminate their biological functions.

Glucocorticoid effects on the skeletal muscle differentiation program: analysis of clonal proliferation, morphological differentiation and the expression of muscle-specific and regulatory genes

We examined the effect of glucocorticoids on the proliferation and differentiation of skeletal muscle cells using the C2C12 cell line. We found that treatment with glucocorticoids enhanced muscle cell differentiation but had only minor effects on the clonal growth rate of C2C12 cells. The stimulatory effect of glucocorticoids on myogenic differentiation was reflected in the increased expression of muscle-specific genes, creatine kinase (CK) and acetylcholine receptor gamma subunit (AChR). Dexamethasone had no effect on CK and AChR mRNA stability and enhanced transcription from a CAT reporter genes containing the 3.3kb 5' flanking region of the murine CK gene (-3300MCK-CAT). Since dexamethasone did not affect the expression levels of the myogenic regulatory genes such as myoD and myogenin, the enhancement of muscle-specific transcription might reflect an increase in the functional activity of the regulatory proteins. Other possible mechanisms involved in the differentiation-enhancing effect of glucocorticoids are discussed.

Proliferation precedes differentiation in IGF-I-stimulated myogenesis

The insulin-like growth factors (IGFs) have dramatic and complex effects on the growth of many tissues and have been implicated in both the proliferation and differentiation of skeletal muscle cells. A detailed analysis of gene expression was performed in L6E9 myoblast cultures treated with IGF-I to dissect the early events leading to the stimulation of myogenic differentiation by this growth factor. A time course of transcript accumulation in confluent L6E9 myoblasts treated with defined media containing IGF-I revealed an initial transient decrease in myogenic factors, accompanied by an increase in cell cycle markers and cell proliferation. This pattern was reversed at later time points, when the subsequent activation of myogenic factors resulted in a net increase in structural gene expression and larger myotubes. The data presented here support the hypothesis that IGF-I activates proliferation first, and subsequently stimulates events leading to the expression of muscle-specific genes in myogenic cell cultures.

Overexpression of the type-1 insulin-like growth factor receptor increases ligand-dependent proliferation and differentiation in bovine skeletal myogenic cultures

Previous studies demonstrated that overexpression of the type-1 insulin-like growth factor (IGF) receptor (IGF-1R) in skeletal myogenic cell lines increased proliferation and differentiation responses to IGF. However, it was unclear if such manipulations in primary, untransformed skeletal myogenic cells would result in modulation of these responses, which may be more stringently regulated in primary cells than in myogenic cell lines. In this study, low passage untransformed fetal bovine myogenic cultures were infected with a replication-deficient retroviral expression vector (LISN) coding for the human IGF-1R or with a control retroviral vector (LNL6). Bovine myogenic cultures infected with the LISN vector (Bov-LISN) displayed ten times more IGF-1Rs than controls (Bov-LNL6). Bov-LISN myogenic cultures exhibited elevated rates of IGF-I-stimulated proliferation and increased rates of terminal differentiation which were reduced to control levels by the anti-human IGF-1R antibody alpha IR3. These findings indicate overexpression of the IGF-1R can enhance IGF sensitivity and thereby modify the proliferation and differentiation behavior of untransformed low passage myoblasts. Such manipulations may be useful to increase muscle mass in clinical or agricultural applications.

Constitutive insulin-like growth factor-II expression interferes with the enterocyte-like differentiation of CaCo-2 cells

In this study we have examined the role of insulin-like growth factor-II (IGF-II) in the differentiation of the CaCo-2 human colon carcinoma cell line. We have shown previously that IGF-II is an autocrine growth factor for CaCo-2 cells. IGF-II expression is high in proliferating, undifferentiated CaCo-2 cells and markedly decreases when cells become confluent and start to differentiate. To evaluate whether differentiation of CaCo-2 cells depends on an IGF-II related pathway, we treated cells with a blocking antibody to the IGF-I receptor that mediates most IGF-II biological effects. Treatment of preconfluent CaCo-2 cells with this antibody decreased by 40% autonomous cell proliferation and induced differentiation as shown by an increase in sucrase isomaltase activity and apolipoprotein A-I (apoA-I) mRNA levels. To examine the significance of autocrine IGF-II production in CaCo-2 cell differentiation, we generated stable CaCo-2 cell lines that constitutively express rat IGF-II under the control of a Rous sarcoma virus promoter. Sustained expression of IGF-II resulted in: (a) increased proliferative rate; (b) high IGF-I receptor number, even after reaching confluence; (c) increased capability of anchorage-independent growth; (d) inhibition of the expression of apoA-I and SI mRNAs. Analysis of several independent IGF-II-transfected clones showed an inverse correlation between IGF-II mRNA levels and expression of the differentiation markers, the cells expressing the higher levels of the transfected IGF-II being the less differentiated ones. Our data suggest that perturbation of IGF-II-mediated cell proliferation interferes with the enterocyte-like differentiation pathway of CaCo-2 cells.

Type-1 insulin-like growth factor receptor overexpression produces dual effects on myoblast proliferation and differentiation

Using a retroviral vector, we developed a line of C2 mouse skeletal myoblasts, C2-LISN, which expressed high levels of the human type-1 insulin-like growth factor (IGF) receptor. When switched to low serum medium, C2-LISN myoblasts underwent terminal differentiation extremely rapidly compared to control C2 myoblasts. In high serum conditions which were not permissive for differentiation, C2-LISN myoblasts expressed ten-fold higher levels of the myogenic transcription factor myogenin than did control C2 myoblasts. When cultured in low serum medium with both transforming growth factor-beta (TGF-beta) and high concentrations of IGF-I, C2-LISN myoblasts failed to differentiate and grew to very high saturation densities, forming multilayers. Upon removal of TGF-beta, multilayered C2-LISN myoblasts differentiated within 2 days. These results demonstrate that overexpression of the type-1 IGF receptor can amplify signals which stimulate myogenic differentiation. Overexpressed type-1 IGF receptors can also mediate strong mitogenic signals if differentiation is inhibited by TGF-beta. The C2-LISN myoblast cell line may be a useful model to investigate the intracellular pathways which stimulate myogenic differentiation. Additionally, overexpression of the type-1 IGF receptor could provide a strategy to expand populations of differentiation-competent myoblasts for experimental or clinical applications.