Insulin and insulin-like growth factor-I responsiveness and signalling mechanisms in C2C12 satellite cells: effect of differentiation and fusion

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Early-age feed restriction affects viability and gene expression of satellite cells isolated from the gastrocnemius muscle of broiler chicks

Background

Muscle growth depends on the fusion of proliferate satellite cells to existing myofibers. We reported previously that 0–14 day intermittent feeding led to persistent retardation in myofiber hypertrophy. However, how satellite cells respond to such nutritional insult has not been adequately elucidated.

Results

One-day-old broiler chicks were allocated to control (Con, ad libitum feeding), intermittent feeding (IF, feed provided on alternate days) and re-feeding (RF, 2 days ad libitum feeding after 12 days of intermittent feeding) groups. Chickens were killed on Day 15 and satellite cells were isolated. When cultured, satellite cells from the IF group demonstrated significant retardation in proliferation and differentiation potential, while RF partly restored the proliferation rate and differentiation potential of the satellite cells. Significant up-regulation of insulin like growth factor I receptor (IGF-IR) (P<0.05) and thyroid hormone receptor α (TRα) (P<0.05), and down-regulation of growth hormone receptor (GHR) (P<0.01) and IGF-I (P<0.01) mRNA expression was observed in freshly isolated IF satellite cells when compared with Con cells. In RF cells, the mRNA expression of IGF-I was higher (P<0.05) and of TRα was lower (P<0.01) than in IF cells, suggesting that RF restored the mRNA expression of TRα and IGF-I, but not of GHR and IGF-IR. The Bax/Bcl-2 ratio tended to increase in the IF group, which was reversed in the RF group (P<0.05), indicating that RF reduced the pro-apoptotic influence of IF. Moreover, no significant effect of T₃ was detected on cell survival in IF cells compared with Con (P<0.001) or RF (P<0.05) cells.

Conclusions

These data suggest that early-age feed restriction inhibits the proliferation and differentiation of satellite cells, induces changes in mRNA expression of the GH/IGF-I and thyroid hormone receptors in satellite cells, as well as blunted sensitivity of satellite cells to T₃, and that RF partially reverses these effects. Thus, a moderate nutritional strategy for feed restriction should be chosen in early chick rearing systems.

Nampt/visfatin/PBEF affects expression of myogenic regulatory factors and is regulated by interleukin-6 in chicken skeletal muscle cells

Nicotinamide phosphoribosyltransferase (Nampt/visfatin/PBEF) has been identified as a rate-limiting NAD(+) biosynthetic enzyme and an adipokine found in the circulation. Human and chicken skeletal muscles are reported to have the highest level of Nampt expression among various tissues whose functional significance remains undetermined. Expression of Nampt is regulated by interleukin-6 (IL-6), an essential cytokine for postnatal muscle growth in mammals. The objective of the current study was to characterize expression of Nampt in chicken (Gallus gallus) myogenic cells and to determine the effect of Nampt on expression of IL-6, myogenic transcription factors, and glucose uptake. We also sought to determine the effect of IL-6 on Nampt expression in chicken myogenic cells. Nampt mRNA and protein were identified in both myoblasts and myocytes, although expression did not differ between the two cell types. Treatment with recombinant human Nampt was found to decrease myoD and mrf4 expression but to increase myf5 expression in myocytes, while glucose uptake was unaffected. In response to treatment with recombinant Nampt, IL-6 expression in myocytes was increased at 24h but decreased when treated for 48 or 72 h. Forced over-expression of chicken Nampt cDNA significantly decreased myf5 expression in myoblasts. Treatment of myogenic cells with lower levels (1 ng.mL(-1)) of recombinant IL-6 increased Nampt expression, whereas a higher IL-6 concentration (100 ng.mL(-1)) decreased Nampt mRNA abundance. Collectively, these results demonstrate that Nampt, regulated in part by IL-6, alters the expression of key myogenic transcription factors and thereby may influence postnatal myogenesis.

Mammalian target of rapamycin complex 1 is involved in differentiation of regenerating myofibers in vivo

This work was undertaken to provide further insight into the role of mammalian target of rapamycin complex 1 (mTORC1) in skeletal muscle regeneration, focusing on myofiber size recovery. Rats were treated or not with rapamycin, an mTORC1 inhibitor. Soleus muscles were then subjected to cryolesion and analyzed 1, 10, and 21 days later. A decrease in soleus myofiber cross-section area on post-cryolesion days 10 and 21 was accentuated by rapamycin, which was also effective in reducing protein synthesis in these freeze-injured muscles. The incidence of proliferating satellite cells during regeneration was unaltered by rapamycin, although immunolabeling for neonatal myosin heavy chain (MHC) was weaker in cryolesion+rapamycin muscles than in cryolesion-only muscles. In addition, the decline in tetanic contraction of freeze-injured muscles was accentuated by rapamycin. This study indicates that mTORC1 plays a key role in the recovery of muscle mass and the differentiation of regenerating myofibers, independently of necrosis and satellite cell proliferation mechanisms.

alpha1A-adrenoceptors activate glucose uptake in L6 muscle cells through a phospholipase C-, phosphatidylinositol-3 kinase-, and atypical protein kinase C-dependent pathway

The role of alpha1-adrenoceptor activation on glucose uptake in L6 cells was investigated. The alpha1-adrenoceptor agonist phenylephrine [pEC50 (-log10 EC50), 5.27 +/- 0.30] or cirazoline (pEC50, 5.00 +/- 0.23) increased glucose uptake in a concentration-dependent manner, as did insulin (pEC50, 7.16 +/- 0.21). The alpha2-adrenoceptor agonist clonidine was without any stimulatory effect on glucose uptake. The stimulatory effect of cirazoline was inhibited by the alpha1-adrenoceptor antagonist prazosin, but not by the beta-adrenoceptor antagonist propranolol. RT-PCR showed that the alpha1A-adrenoceptor was the sole alpha1-adrenoceptor subtype expressed in L6 cells. Cirazoline- or insulin-mediated glucose uptake was inhibited by the phosphatidylinositol-3 kinase inhibitor LY294002, suggesting a possible interaction between the alpha1-adrenoceptor and insulin pathways. Cirazoline or insulin stimulated phosphatidylinositol-3 kinase activity, but alpha1-adrenoceptor activation did not phosphorylate Akt. Both cirazoline- and insulin-mediated glucose uptake were inhibited by protein kinase C (PKC), phospholipase C, and p38 kinase inhibitors, but not by Erk1/2 inhibitors (despite both treatments being able to phosphorylate Erk1/2). Insulin and cirazoline were able to activate and phosphorylate p38 kinase. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate and the calcium ionophore A23187 produced significant increases in glucose uptake, indicating roles for PKC and calcium in glucose uptake. Down-regulation of conventional PKC isoforms inhibited glucose uptake mediated by 12-O-tetradecanoylphorbol-13-acetate, but not by insulin or cirazoline. This study demonstrates that alpha1-adrenoceptors mediate increases in glucose uptake in L6 muscle cells. This effect appears to be related to activation of phospholipase C, phosphatidylinositol-3 kinase, p38 kinase, and PKC.

Basic principles of muscle development and growth in meat-producing mammals as affected by the insulin-like growth factor (IGF) system

This presentation aims to describe how the basic events in prenatal muscle development and postnatal muscle growth are controlled by the insulin-like growth factor system (IGF). The prenatal events (myogenesis) cover the rate of proliferation, the rate and extent of fusion, and the differentiation of three myoblast populations, giving rise to primary fibers, secondary fibers, and a satellite cell population, respectively. The number of muscle fibers, a key determinant of the postnatal growth rate, is fixed late in gestation. The postnatal events contributing to myofiber hypertrophy comprise satellite cell proliferation and differentiation, and protein turnover. Muscle cell cultures produce IGFs and IGF binding proteins (IGFBPs) in various degrees depending on the origin (species, muscle type) and state of development of these cells, suggesting an autocrine/paracrine mode of action of IGF-related factors. In vivo studies and results based on cell lines or primary cell cultures show that IGF-I and IGF-II stimulate both proliferation and differentiation of myoblasts and satellite cells in a time and concentration-dependent way, via interaction with type I IGF receptors. However, IGF binding proteins (IGFBP) may either inhibit or potentiate the stimulating effects of IGFs on proliferation or differentiation. During postnatal growth in vivo or in fully differentiated muscle cells in culture, IGF-I stimulates the rate of protein synthesis and inhibits the rate of protein degradation, thereby enhancing myofiber hypertrophy. The possible roles and actions of the IGF system in regulating and determining muscle growth as affected by developmental stage and age, muscle type, feeding levels, treatment with growth hormone and selection for growth performance are discussed.

IL-1beta impairs insulin-like growth factor i-induced differentiation and downstream activation signals of the insulin-like growth factor i receptor in myoblasts

Proinflammatory cytokines are elevated in disorders characterized by muscle wasting and weakness, such as inflammatory myopathies and AIDS wasting. We recently demonstrated that TNF-alpha impairs the ability of insulin-like growth factor (IGF)-I to promote protein synthesis in muscle precursor cells. In this study we extend these findings by showing that low concentrations of IL-1beta impair IGF-I-dependent differentiation of myoblasts, as assessed by expression of the muscle specific protein, myosin heavy chain. In the absence of exogenous IGF-I, IL-1beta (1 ng/ml) did not impair muscle cell development. However, in the presence of IGF-I, 100-fold lower concentrations of IL-1beta (0.01 ng/ml) significantly suppressed myoblast differentiation, protein synthesis, and myogenin expression. Increasing IL-1beta to 1 ng/ml completely blocked the anabolic actions of IGF-I in murine C(2)C(12) myoblasts. Similarly, IL-1beta inhibited IGF-I-stimulated protein synthesis in primary porcine myoblasts. IL-1beta impaired the actions of IGF-I at a point distal to the IGF receptor, and this was not due to IL-1beta-induced cell death. Instead, IL-1beta inhibited the ability of IGF-I to phosphorylate tyrosine residues on both of its downstream docking proteins, insulin receptor substrate 1 and insulin receptor substrate 2. These data establish that physiological concentrations of IL-1beta block the ability of IGF-I to promote protein synthesis, leading to reduced expression of the myogenic transcription factor, myogenin, and the subsequent development of more mature differentiated cells that express myosin heavy chain. Collectively, the results are consistent with the notion that very low concentrations of IL-1beta significantly impair myogenesis, but they are unable to do so in the absence of the growth factor IGF-I.

Metabolic and mitogenic effects of IGF-I and insulin on muscle cells of rainbow trout

The relative function of IGF-I and insulin on fish muscle metabolism and growth has been investigated by the isolation and culture at different stages (myoblasts at day 1, myocytes at day 4, and myotubes at day 10) of rainbow trout muscle cells. This in vitro model avoids interactions with endogenous peptides, which could interfere with the muscle response. In these cells, the effects of IGF-I and insulin on cell proliferation, 2-deoxyglucose (2-DG), and l-alanine uptake at different development stages, and the use of inhibitors were studied and quantified. Insulin (10-1,000 nM) and IGF-I (10-100 nM) stimulated 2-DG uptake in trout myocytes at day 4 in a similar manner (maximum of 124% for insulin and of 142% for IGF-I), and this stimulation increased when cells differentiated to myotubes (maximum for IGF-I of 193%). When incubating the cells with PD-98059 and especially cytochalasin B, a reduction in 2-DG uptake was observed, suggesting that glucose transport takes place through specific facilitative transporters. IGF-I (1-100 nM) stimulated the l-alanine uptake in myocytes at day 4 (maximum of 239%), reaching higher values of stimulation than insulin (100-1,000 nM) (maximum of 160%). This stimulation decreased when cells developed to myotubes at day 10 (118% for IGF-I and 114% for insulin). IGF-I (0.125-25 nM) had a significant effect on myoblast proliferation, measured by thymidine incorporation (maximum of 170%), and required the presence of 2-5% fetal serum (FBS) to promote thymidine uptake. On the other hand, insulin was totally ineffective in stimulating thymidine uptake. We conclude that IGF-I is more effective than insulin in stimulating glucose and alanine uptake in rainbow trout myosatellite cells and that the degree of stimulation changes when cells differentiate to myotubes. IGF-I stimulates cell proliferation in this model of muscle in vitro and insulin does not. These results indicate the important role of IGF-I on growth and metabolism of fish muscle.

Stretch activation of GTP-binding proteins in C2C12 myoblasts

Mechanical stimulation has been proposed as a fundamental determinant of muscle physiology. The mechanotransduction of strain and strain rate in C2C12 myoblasts were investigated utilizing a radiolabeled GTP analogue to detect stretch-induced GTP-binding protein activation. Cyclic uniaxial strains of 10% and 20% at a strain rate of 20% s(-1) rapidly (within 1 min) activated a 25-kDa GTPase (183 +/- 17% and 186 +/- 19%, respectively), while 2% strain failed to elicit a response (109 +/- 11%) relative to controls. One, five, and sixty cycles of 10% strain elicited 187 +/- 20%, 183 +/- 17%, and 276 +/- 38% increases in activation. A single 10% stretch at 20% s(-1), but not 0.3% s(-1), resulted in activation. Insulin activated the same 25-kDa band in a dose-dependent manner. Western blot analysis revealed a panel of GTP-binding proteins in C2C12 myoblasts, and tentatively identified the 25-kDa GTPase as rab5. In separate experiments, a 40-kDa protein tentatively identified as Galpha(i) was activated (240 +/- 16%) by 10% strain at 1 Hz for 15 min. These results demonstrate the rapid activation of GTP-binding proteins by mechanical strain in myoblasts in both a strain magnitude- and strain rate-dependent manner.

Growth factor array fabrication using a color ink jet printer

We have developed a novel method for growth factor analysis using a commercial color ink jet printer to fabricate substrata patterned with growth factors. We prepared substrata with insulin printed in a simple pattern or containing multiple areas of varying quantities of printed insulin. When we cultured the mouse myoblast cell line, C2C12, on the insulin-patterned substrata, the cells were grown in the same pattern with the insulin-printed pattern. Cell culture with the latter substrata demonstrated that quantity control of insulin deposition by a color ink jet printer is possible. For further applications, we developed substrata with insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF) spotted in 16 different areas in varying combinations and concentrations (growth factor array). With this growth factor array, C2C12 cells were cultured, and the onset of muscle cell differentiation was monitored for the expression of the myogenic regulator myogenin. The ratio of cells expressing myogenin varied with the doses of IGF-I and bFGF in the sections, demonstrating a feasibility of growth factor array fabrication by a color ink jet printer. Since a printer manipulates several colors, this method can be easily applied to multivariate analyses of growth factors and attachment factors affecting cell growth and differentiation. This method may provide a powerful tool for cell biology and tissue engineering, especially for stem cell research in investigating unknown conditions for differentiation.

Overexpression of chemokine-like factor 2 promotes the proliferation and survival of C2C12 skeletal muscle cells

Chemokine-like factor 1 (CKLF1) is a novel cytokine first cloned from U937 cells. It contains different splicing forms and has chemotactic effects on a wide spectrum of cells both in vitro and in vivo; it can also stimulate the regeneration of skeletal muscle cells in vivo, but the mechanism remains unclear. To probe the myogenesis function of CKLF2, which is the largest isoform of CKLFs, C2C12 murine myoblasts were stably transfected with human CKLF2 eukaryotic expression vector. Compared with control vector transfected C2C12 cells, CKLF2 overexpression causes accelerated myoblast proliferation as determined by cell counting and [(3)H]TdR incorporation assays. In addition, CKLF2 overexpression also promotes cell differentiation, which was determined by higher expression levels of myogenin, creatine kinase, myosin and the accelerated myoblast fusion. Further analysis also indicates that CKLF2 could activate the transcription activity of the bHLH/MyoD and MEF2 families. Finally, DNA synthesis and myotube formation could also be promoted by growing C2C12 cells in conditioned media from CKLF2-transfected cells. These findings strongly suggest a role for human CKLF2 in regulation of skeletal muscle myogenesis.

Different roles of the IGF-I Ec peptide (MGF) and mature IGF-I in myoblast proliferation and differentiation

The physiological function of a recently cloned splice variant of insulin-like growth factor-I (IGF-I; mechano growth factor (MGF)) was studied using an in vitro cell model. Unlike mature IGF-I, the distinct E domain of MGF inhibits terminal differentiation whilst increasing myoblast proliferation. Blocking the IGF-I receptor with a specific antibody indicated that the function of MGF E domain is mediated via a different receptor. The results provide a basis for localized tissue adaptation and helps explain why loss of muscle mass occurs in the elderly and in dystrophic muscle in which MGF production is markedly affected.

Insulin-like growth factor-I in muscle metabolism and myotherapies

The critical anabolic and trophic role of signaling by insulin-like growth factors (IGF) I and II via the type-I IGF receptor (IGF-IR) is reviewed throughout the life of skeletal myocytes. The proliferative effects of IGF-IR stimulation, both during embryogenesis and during satellite cell proliferation following denervation or muscle injury, are mediated primarily through activation of mitogen-activated protein kinases. Signaling through phosphatidylinositol 3-kinase is essential to muscle protein synthesis and glucose uptake and may contribute to the observed resilience of mature muscle to programmed cell death. Degeneration or inhibition of the GH--IGF-I axis by aging, cachexia, sepsis, diabetes, drugs, and disuse all enhance muscle catabolism, and opposition of these effects by IGF-I may form the basis of effective myotherapy.

Immobilization depresses insulin signaling in skeletal muscle

Prolonged immobilization depresses insulin-induced glucose transport in skeletal muscle and leads to a catabolic state in the affected areas, with resultant muscle wasting. To elucidate the altered intracellular mechanisms involved in the insulin resistance, we examined insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit (IR-beta) and insulin receptor substrate (IRS)-1 and activation of its further downstream molecule, phosphatidylinositol 3-kinase (PI 3-K), after unilateral hindlimb immobilization in the rat. The contralateral hindlimb served as control. After 7 days of immobilization of the rat, insulin was injected into the portal vein, and tibialis anterior muscles on both sides were extracted. Immobilization reduced insulin-stimulated tyrosine phosphorylation of IR-beta and IRS-1. Insulin-stimulated binding of IRS-1 to p85, the regulatory subunit of PI 3-K, and IRS-1-associated PI 3-K activity were also decreased in the immobilized hindlimb. Although IR-beta and p85 protein levels were unchanged, IRS-1 protein expression was downregulated by immobilization. Thus prolonged immobilization may cause depression of insulin-stimulated glucose transport in skeletal muscle by altering insulin action at multiple points, including the tyrosine phosphorylation, protein expression, and activation of essential components of insulin signaling pathways.

Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during differentiation were studied in two myoblast cell lines, L6 and H9C2. Addition of insulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were localized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. Addition of an inhibitor of mitogen-activated protein kinase, PD 98059, did not affect differentiation but blocked nuclear translocation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamycin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothionein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein occurs during the early stage of differentiation of myoblasts to myotubes and can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early stage of differentiation.

Human rhabdomyosarcoma cells retain insulin-regulated glucose transport activity through glucose transporter 1

We evaluated the expression of glucose transporter (glut) isoforms and its function in RD cells, human rhabdomyosarcoma, which retain the potential to differentiate into muscle. Gluts 1, 3, and 4 were expressed in RD cells, as detected by reverse-transcription polymerase chain reaction and immunocytochemistry. Supraphysiological concentration (1 microM) of insulin treatment increased 2-deoxy glucose transport by up to 1.68-fold together with concomitant tyrosine phosphorylation of the insulin receptor beta subunit and of insulin receptor substrate 1. Suppression of glut 1 mRNA by 38% by antisense oligonucleotide transfection led to a reduction of basal and insulin-stimulated 2-deoxy glucose transport by 38 and 55%, respectively. Suppression of gluts 3 and 4 by antisense oligonucleotide transfection did not affect both basal and insulin-stimulated 2-deoxy glucose transport. Thus, glut 1 accounts for the major part of basal and insulin-stimulated glucose transport in RD cells. Next, we transfected expression vectors carrying human gluts 1 and 4 cDNAs into RD cells to add further support for the role of glut 1 in glucose transport. Overexpression of glut 1 stimulated basal and insulin-stimulated 2-deoxy glucose transport by 1.66- and 1.43-fold, respectively. Glut 4 overexpression did not affect basal and insulin-stimulated 2-deoxy glucose transport. Western blot analysis using glut 1 antibody showed that glut 1 was redistributed from intracellular membrane to plasma membrane. These observations support the notion that RD cells, with the potential to differentiate into muscle, retain insulin responsiveness. As human muscle cell lines are not available at this point, RD cells can serve as a useful alternative to human muscle for studies related to insulin signal transduction and glucose transport.

Effects of serum deprivation, insulin and dexamethasone on polysome percentages in C2C12 myoblasts and differentiating myoblasts

An increase in the rate of protein synthesis in living cells can be achieved by regulating the quantity of mRNA, ribosomes, and enzymes available for translation or by regulating the efficiency at which existing components are used. Efficiency can be measured by comparing the number of ribosomes actively engaged in the synthesis of protein (polysomes) to the pool of free ribosomes. The objective of this study was to determine the percentage of ribosomes found as polysomes in C2C12 cells deprived of serum or exposed to insulin or dexamethasone 24 h before and after being stimulated to differentiate. Individual 60 mm culture dishes were exposed to serum-free control medium, medium containing serum, insulin, or dexamethasone for a period of 1 h or 2 h and then quickly frozen. The ribosomes and polysomes from these cells were separated by ultracentrifugation on 15 to 60% sucrose gradients and the absorbance across the gradient at 254 nm was recorded. Polysome percentages were determined as the area under the polysome peak divided by the total area under the curve. Serum deprivation caused a 12% decline in the percentage of ribosomes found as polysomes (P < 0.01). Dexamethasone caused a quadratic decline (P < 0.05) in polysome percentage, while insulin yielded a quadratic increase (P < 0.05). Protein synthesis assays measuring 3H-tyrosine uptake showed similar responses. These changes occurred in the absence of any differences in total RNA concentration. It was concluded that differentiation and the absence of serum in the media reduced the rate of recruitment of ribosomes for protein synthesis. Insulin increased ribosome recruitment which was also observed by a similar increase in incorporation of radio-labeled tyrosine.

Induction of Akt-2 correlates with differentiation in Sol8 muscle cells

Phosphatidylinositol 3-kinase is involved in the regulation of muscle cell differentiation. The serine/threonine kinase Akt has been implicated in the signaling pathway downstream of PI3-kinase. Here we demonstrate that differentiation of Sol8 skeletal muscle cells is associated with a marked increase in endogenous Akt-2 protein. Myogenesis was induced by three different conditions: cell confluence, low serum or treatment with insulin or insulin-like growth factor-I. Differentiation by cell confluence resulted in an increase in the endogenous protein content and activation of Akt-2. Low serum conditions induced a dramatic raise in Akt-2 protein levels which correlates with the induction of the muscle cell differentiation marker myogenin. Treatment of Sol8 cells with the PI3-kinase inhibitor LY294002 prevented the expression of myogenin as effectively as the increase in Akt-2 content induced by low-serum conditions. Similarly, differentiation of Sol8 cells stimulated by 50 nM insulin or 10 nM IGF-I markedly increased Akt-2 protein levels. These results and the recent observation that active Akt translocates to the cell nucleus (J. Biol. Chem. 272, 30491-30497; 31515-31524, 1997) suggests that Akt-2 might play a crucial role in the initiation of the genetic program responsible for muscle cell differentiation.

A role for phospholipase D activation in the lipid signalling cascade generated by bradykinin and thrombin in C2C12 myoblasts

In the present study evidence is provided for a rapid activation of lipid signalling pathways induced by thrombin and bradykinin (BK) in C2C12 myoblasts. Both agonists were able to increase [3H]inositol phosphates (InsP), 1,2-[3H]diacylglycerol (DAG) and [3H]phosphatidic acid (PtdOH) levels. In particular [3H]PtdOH values were rapidly increased and maintained at significantly high values at prolonged times of incubation. BK and thrombin were able to activate phospholipase D (PLD) in vivo as demonstrated by the accumulation of [3H]phosphatidylethanol (PtdEtOH) through the transphoshatidylation reaction catalyzed by the enzyme in the presence of ethanol. The observation that ethanol could significantly reduce [3H]PtdOH formation in myoblasts stimulated with BK and thrombin indicates that stimulation of PLD has a major role. The two agonists appear to stimulate PLD activity through a common molecular mechanism, involving the activation of protein kinase C (PKC). In addition, BK and thrombin appear able to activate DAG kinase at early times of incubation and also this pathway may contribute to determine the increase in [3H]PtdOH levels. This is the first report which describes activation of lipid signalling pathways by BK and thrombin in myoblast cells and it is possible that these early signals may have an important role in mediating the biological effects of the two agonists.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.