Mitogen-activated protein kinase pathway is involved in the differentiation of muscle cells

Overexpression of MyoD-inducible lysosomal sialidase (neu1) inhibits myogenesis in C2C12 cells

Lysosomal sialidase, encoded by neu1, is required for the removal of terminal sialic acid residues from a variety of sialoglycoconjugates. In humans, deficiency of this enzyme results in the inborn error of metabolism sialidosis, characterized by the accumulation of sialoglycoconjugates within the nervous system and in peripheral organs. A subset of sialidosis patients present with symptoms of profound muscle dysfunction, including progressive muscular atrophy. We have previously shown that the 5' regulatory region of murine neu1 is typical of skeletal muscle-specific genes due to the presence of several E-boxes and its responsiveness to stimulation by muscle regulatory factors (MRFs) such as MyoD. Here, we report that sialidase activity is increased 6-fold during the first 24 h of differentiation of C2C12 myoblasts followed by an attenuation to pre-differentiation levels by 48 h. We demonstrate that the lysosomal sialidase promoter is highly upregulated by MyoD through a mechanism that is dependent on the MyoD chromatin remodeling domain. We also show that the sialidase promoter is repressed by activated MEK. Inappropriate overexpression of sialidase 48 h after the onset of differentiation results in downregulation of myogenin as well as myosin heavy chain expression and in a halt of the differentiation cascade. This study indicates that lysosomal sialidase is a potent regulator of the early stages of myogenesis.

The molecular scaffold kinase suppressor of Ras 1 (KSR1) regulates adipogenesis

Mitogen-activated protein kinase pathways are implicated in the regulation of cell differentiation, although their precise roles in many differentiation programs remain elusive. The Raf/MEK/extracellular signal-regulated kinase (ERK) kinase cascade has been proposed to both promote and inhibit adipogenesis. Here, we titrate expression of the molecular scaffold kinase suppressor of Ras 1 (KSR1) to regulate signaling through the Raf/MEK/ERK/p90 ribosomal S6 kinase (RSK) kinase cascade and show how it determines adipogenic potential. Deletion of KSR1 prevents adipogenesis in vitro, which can be rescued by introduction of low levels of KSR1. Appropriate levels of KSR1 coordinate ERK and RSK activation with C/EBPbeta synthesis leading to the phosphorylation and stabilization of C/EBPbeta at the precise moment it is required within the adipogenic program. Elevated levels of KSR1 expression, previously shown to enhance cell proliferation, promote high, sustained ERK activation that phosphorylates and inhibits peroxisome proliferator-activated receptor gamma, inhibiting adipogenesis. Titration of KSR1 expression reveals how a molecular scaffold can modulate the intensity and duration of signaling emanating from a single pathway to dictate cell fate.

The p38alpha/beta MAPK functions as a molecular switch to activate the quiescent satellite cell

Somatic stem cells cycle slowly or remain quiescent until required for tissue repair and maintenance. Upon muscle injury, stem cells that lie between the muscle fiber and basal lamina (satellite cells) are activated, proliferate, and eventually differentiate to repair the damaged muscle. Satellite cells in healthy muscle are quiescent, do not express MyoD family transcription factors or cell cycle regulatory genes and are insulated from the surrounding environment. Here, we report that the p38α/β family of mitogen-activated protein kinases (MAPKs) reversibly regulates the quiescent state of the skeletal muscle satellite cell. Inhibition of p38α/β MAPKs (a) promotes exit from the cell cycle, (b) prevents differentiation, and (c) insulates the cell from most external stimuli allowing the satellite cell to maintain a quiescent state. Activation of satellite cells and p38α/β MAPKs occurs concomitantly, providing further support that these MAPKs function as a molecular switch for satellite cell activation.

Dedifferentiation of adult human myoblasts induced by ciliary neurotrophic factor in vitro

Ciliary neurotrophic factor (CNTF) is primarily known for its important cellular effects within the nervous system. However, recent studies indicate that its receptor can be highly expressed in denervated skeletal muscle. Here, we investigated the direct effect of CNTF on skeletal myoblasts of adult human. Surprisingly, we found that CNTF induced the myogenic lineage-committed myoblasts at a clonal level to dedifferentiate into multipotent progenitor cells--they not only could proliferate for over 20 passages with the expression absence of myogenic specific factors Myf5 and MyoD, but they were also capable of differentiating into new phenotypes, mainly neurons, glial cells, smooth muscle cells, and adipocytes. These "progenitor cells" retained their myogenic memory and were capable of redifferentiating into myotubes. Furthermore, CNTF could activate the p44/p42 MAPK and down-regulate the expression of myogenic regulatory factors (MRFs). Finally, PD98059, a specific inhibitor of p44/p42 MAPK pathway, was able to abolish the effects of CNTF on both myoblast fate and MRF expression. Our results demonstrate the myogenic lineage-committed human myoblasts can dedifferentiate at a clonal level and CNTF is a novel regulator of skeletal myoblast dedifferentiation via p44/p42 MAPK pathway.

Leukemia inhibitory factor blocks early differentiation of skeletal muscle cells by activating ERK

Leukemia inhibitory factor (LIF) is a multifunctional cytokine belonging to the interleukin-6 family and has been shown to stimulate regeneration of injured skeletal muscle. Although LIF has been shown to stimulate muscle cell proliferation, its precise role in differentiation is unclear. Thus, we examined the effect of LIF on the differentiation of cultured C2C12 myoblast cells. In this study, we used both non-glycosylated LIF expressed in bacteria and glycosylated LIF secreted from NIH3T3 cells infected with Ad-LIF. Both non-glycosylated and glycosylated LIF blocked differentiation of myoblasts as measured by expression of myosin heavy chain and myotube formation. Treatment of myoblasts with LIF induced phosphorylation of ERK, and the LIF-induced inhibitory effect on myogenesis was blocked by pretreatment with U0126, a specific MEK inhibitor, and transient transfection with dominant negative (DN)-MEK1. In contrast, although LIF activated STAT3, the LIF-induced repression of the MCK transcriptional activity was not reversed by pretreatment with AG490, a specific Jak kinase inhibitor or transient transfection with DN-STAT3. Additionally, LIF exhibited its inhibitory effect on myogenesis only when cells were treated at earlier than 12 h after inducing differentiation. Taken together, these results suggest that LIF strongly inhibited early myogenic differentiation though activation of the ERK signaling pathway and its effect is irrespective of glycosylation.

Involvement of c-Jun N-terminal kinase activities in skeletal muscle differentiation

Previous studies on skeletal muscle differentiation showed that myogenesis is regulated by extracellular signal-regulated kinases (ERK-1/-2) and p38 mitogen activated kinase (MAPK) pathways. Present study shows that c-Jun NH2-terminal protein kinase (JNK) activities were up regulated during skeletal muscle differentiation in rat skeletal muscle L6E9 cells, as determined by Western immunoblot of differentiating cells probed with anti-phospho-JNK antibody. Inhibition of JNK activities by JNK inhibitor II drastically inhibited differentiation as determined by decreased myosin, myogenin expression and creatine kinase activity. The inhibition of the differentiation was regulated by apoptosis as determined by the detection of poly(ADP-ribose) polymerase (PARP) cleavage, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positive cells when JNK activities were inhibited. Apoptosis was accompanied by marked expression and activation of c-Jun and p53 transcription factors. Taken together, our results indicate that basal JNK activities are essential for regulating skeletal muscle differentiation, and inhibition of JNK activation affects myogenesis by apoptosis dependent on c-Jun and p53 transcription factors.

Vascular endothelial growth factor stimulates skeletal muscle regeneration in vivo

Vascular endothelial growth factor (VEGF) is a major regulator of blood vessel formation during development and in the adult organism. Recent evidence indicates that this factor also plays an important role in sustaining the proliferation and differentiation of different cell types, including progenitor cells of different tissues, including bone marrow, bone, and the central nervous system. Here we show that the delivery of the 165-aa isoform of VEGF-A cDNA using an adeno-associated virus (AAV) vector exerts a powerful effect on skeletal muscle regeneration in vivo. Following ischemia-, glycerol-, or cardiotoxin-induced damage in mouse skeletal muscle, the delivery of AAV-VEGF markedly improved muscle fiber reconstitution with a dose-dependent effect. The expression of both VEGF receptor-1 (VEGFR-1) and VEGFR-2 was upregulated both in the satellite cells of the damaged muscles and during myotube formation in vitro; the VEGF effect was mediated by the VEGFR-2, since the transfer of PlGF, a VEGF family member interacting with the VEGFR-1, was ineffective. These results are consistent with the observation that VEGF promotes the growth of myogenic fibers and protects the myogenic cells from apoptosis in vitro and prompt a therapeutic use for VEGF gene transfer in a variety of muscular disorders.

RETRACTED ARTICLE: Quinoxaline 1,4-dioxides induce G2/M cell cycle arrest and apoptosis in human colon cancer cells

We have recently shown that quinoxaline 1,4-dioxide (QdNO) derivatives, namely 2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide (DCQ), 2-benzoyl-3-phenyl-quinoxaline 1,4-dioxide (BPQ) and 2-acetyl-3-methyl-quinoxaline 1,4-dioxide (AMQ), suppress the growth of T-84 human colon cancer cells. Here we show that the growth-suppressive effects of QdNOs are due to their ability to induce cell cycle arrest and/or apoptosis. While AMQ blocked more than 60% of cells at the G2/M phase without inducing apoptosis, DCQ caused a significant increase in apoptotic cells with no noticeable effects on the cycling of cells. Treatment with BPQ resulted in G2/M cell cycle arrest and induction of apoptosis. With regard to the effects of QdNOs on molecules that regulate apoptosis and the G2 to M transition, both BPQ and AMQ inhibited the expression of cyclin B, while DCQ significantly decreased the levels of Bcl-2 and increased Bax expression. Next, we investigated whether transforming growth factor-beta1 (TGF-beta1) and/or extracellular signal-regulated kinase (ERK) mediate the antiproliferative and apoptotic effects of QdNOs in colon cancer cells. Interestingly, the above QdNOs increased differentially total TGF beta1 mRNA expression and decreased TGF alpha mRNA and ERK phosphorylation. None of these QdNOs induced changes in TGF beta-2 mRNA expression. The addition of a specific inhibitor of MEK greatly enhanced apoptosis in cells treated with DCQ, suggesting that the inhibition of ERK phosphorylation may explain, to an extent, the apoptogenic effects of this compound. Taken together, these findings provide insights into possible molecular mechanisms of growth inhibition by QdNOs that could aid in their evaluation for anticancer therapy.

Akt phosphorylation is not sufficient for insulin-like growth factor-stimulated myogenin expression but must be accompanied by down-regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation

IGF-I has a unique biphasic effect on skeletal muscle differentiation. Initially, IGF-I inhibits expression of myogenin, a skeletal muscle-specific regulatory factor essential for myogenesis. Subsequently, IGF-I switches to stimulating expression of myogenin. The mechanisms that mediate this switch in IGF action are incompletely understood. Several laboratories have demonstrated that the phosphatidylinositol-3-kinase/Akt signaling pathway is essential for myogenic differentiation and have suggested that this pathway mediates IGF-I stimulation of myogenin mRNA expression, an early critical step in the differentiation process. These studies, however, did not address concurrent Akt and MAPK/ERK1/2 phosphorylation, the latter of which is also known to regulate myogenic differentiation. In the present study in rat L6E9 muscle cells, we have manipulated ERK1/2 phosphorylation with either an upstream inhibitor or activator and examined concurrent levels of Akt and ERK1/2 phosphorylation and of myogenin mRNA expression in response to treatment with IGF-I. We find that even in the presence of phosphorylated Akt, it is only when ERK1/2 phosphorylation is inhibited that IGF-I can stimulate myogenin mRNA expression. Thus, although Akt phosphorylation may be necessary, it is not sufficient for induction of myogenic differentiation by IGF-I and must be accompanied by a decrease in ERK1/2 phosphorylation.

FGF and PI3 kinase signaling pathways antagonistically modulate sex muscle differentiation in C. elegans

Myogenesis in vertebrate myocytes is promoted by activation of the phosphatidyl-inositol 3'-kinase (PI3 kinase) pathway and inhibited by fibroblast growth factor (FGF) signaling. We show that hyperactivation of the Caenorhabditis elegans FGF receptor, EGL-15, similarly inhibits the differentiation of the hermaphrodite sex muscles. Activation of the PI3 kinase signaling pathway can partially suppress this differentiation defect, mimicking the antagonistic relationship between these two pathways known to influence vertebrate myogenesis. When ectopically expressed in body wall muscle precursor cells, hyperactivated EGL-15 can also interfere with the proper development of the body wall musculature. Hyperactivation of EGL-15 has also revealed additional effects on a number of fundamental processes within the postembryonic muscle lineage, such as cell division polarity. These studies provide important in vivo insights into the contribution of FGF signaling events to myogenesis.

RhoA controls myoblast survival by inducing the phosphatidylinositol 3-kinase-Akt signaling pathway

The small GTPase RhoA regulates the expression of the myogenic transcription factor, MyoD, and the transcription of muscle-specific genes. We report that RhoA also affects the survival of differentiating myoblasts. Two signaling pathways, extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K)-Akt, are involved in myoblast survival. Here, we show that inhibition of RhoA prevents the phosphorylation of Akt, but does not affect the phosphorylation of ERK. Constitutive expression of an active form of Akt prevents apoptosis in myoblasts treated with the Rho inhibitor C3-transferase. These results indicate that RhoA functions to prevent myoblast death by inducing the PI3-K-Akt pathway.

Differential regulation of the phosphoinositide 3-kinase and MAP kinase pathways by hepatocyte growth factor vs. insulin-like growth factor-I in myogenic cells

Hepatocyte growth factor (HGF) promotes the proliferation of adult myoblasts and inhibits their differentiation, whereas insulin-like growth factor I (IGF-I) enhances both processes. Recent studies indicate that activation of the phosphoinositide 3'-kinase (PI3K) pathway promotes myoblast differentiation, whereas activation of the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) promotes proliferation and inhibits their differentiation. This simple model is confounded by the fact that both HGF and IGF-I have been shown to activate both pathways. In this study, we have compared the ability of HGF and IGF-I to activate PI3K and MAPK/ERK in i28 myogenic cells. We find that, although the two stimuli result in comparable recruitment of the p85alpha subunit of PI3K into complexes with tyrosine-phosphorylated proteins, the p85beta regulatory subunit and p110alpha catalytic subunit of PI3K are preferentially recruited into these complexes in response to IGF-I. In agreement with this observation, IGF-I is much more potent than HGF in stimulating phosphorylation of Akt/PKB, a protein kinase downstream of PI3K. In contrast, MAPK/ERK phosphorylation was higher in response to HGF and lasted longer, relative to IGF-I. Moreover, the specific PI3K inhibitor, Wortmannin, abolished MAPK/ERK and Elk-1 phosphorylation in HGF-treated cells, suggesting the requirement of PI3K in mediating the HGF-induced MAPK pathway. UO126, a specific MAPK pathway inhibitor, had no effect on PI3K activity or Akt phosphorylation, implying that at least in muscle cells, the MAPK/ERK pathway is not required for HGF-induced PI3K activation. These results provide a biochemical rationale for the previous observations that HGF and IGF-I have opposite effects on myogenic cells, consistent with studies linking PI3K activation to differentiation and MAPK/ERK activation to proliferation in these cells. Moreover, the finding that PI3K activity is required for HGF-induced MAPK activation suggests its additional role in proliferation, rather than exclusively in the differentiation of adult myoblasts.

Subcellular Localization Determines the Protective Effects of Activated ERK2 against Distinct Apoptogenic Stimuli in Myeloid Leukemia Cells

ERKs, mitogen-activated protein kinases, are well characterized as key mediators in the conveyance of signals that promote cell survival in cells of hemopoietic origin, a key factor in the upbringing of leukemogenesis. It is also well known that ERKs phosphorylate a wide array of substrates distributed throughout distinct cellular locations such as the nucleus, cytoplasm, and cell periphery, but the relative contribution of these compartmentalized signal components to the overall survival signal generated by activation of ERKs has yet to be established. To this end, we have utilized constitutively activated forms of ERK2, whose expression is restricted to the nucleus or to the cytoplasm, to investigate the consequences of compartmentalized activation of ERK in the survival of chronic myelogenous leukemia cells subjected to distinct apoptogenic stimuli. We show that cytoplasmic ERK2 activity protected against apoptosis caused by prolonged serum starvation, whereas ERK2 activation restricted to the nucleus antagonized apoptosis induced by the Bcr-Abl inhibitor STI571. On the other hand, neither cytoplasmic nor nuclear ERK2 activities were effective in counteracting apoptosis induced by UV light. These results demonstrate that the protective effects of ERK2 against defined apoptogenic stimuli are strictly dependent on the cellular localization where ERK activation takes place. Furthermore, we present evidence suggesting that the complex I kappa B-NF kappa B participates on ERK2-mediated survival mechanisms, in a fashion dependent on the cellular location where ERK2 is active and on the causative apoptogenic stimulus.

L6 myoblast differentiation is modulated by Cdk5 via the PI3K–AKT–p70S6K signaling pathway

Cdk5 regulates myogenesis but the signaling cascade through which Cdk5 modulates this process remains to be characterized. Here, we investigated whether PI3K, Akt, p70S6K, p38 MAPK, p44/42 MAPK, and Egr-1 serve as upstream regulators of Cdk5 during L6 myoblast differentiation. Upon serum reduction, we found that besides elevated expression of Cdk5 and its activator, p35, and increased Cdk5/p35 activity, Egr-1, Akt, p70S6K, and p38 MAPK activity were upregulated in differentiating L6 cells. However, p44/42 MAPK was downregulated and SAPK/JNK was unaffected. LY294002, a PI3K inhibitor, blocked the activation of Akt and p70S6K, indicating that Akt and p70S6K activation is linked to PI3K activation. The lack of LY294002 effect on p38 MAPK suggests that p38 MAPK activation is not associated with PI3K activation. Rapamycin, a specific inhibitor of FRAP/mTOR (the upstream kinase of p70S6K), also blocked p70S6K activation, indicating the involvement of FRAP/mTOR activation. LY294002 and rapamycin also blocked the enhancement of Egr-1 level, Cdk5 activity, and myogenin expression, suggesting that upregulation of these factors is coupled to PI3K-p70S6K activation. Overexpression of dominant-negative-Akt also reduced Cdk5/p35 activity and myogenin expression, indicating that the PI3K-p70S6K-Egr-1-Cdk5 signaling cascade is linked to Akt activation. SB2023580, a p38 MAPK inhibitor, had no effect on p70S6K, Egr-1, or Cdk5 activity, suggesting that p38 MAPK activation lies in a pathway distinct from the PI3K-Akt-p70S6K-Egr-1 pathway that we identify as the upstream modulator of Cdk5 activity during L6 myoblast differentiation.

Differentiation induction of human keratinocytes by phosphatidylethanolamine-binding protein

Phosphatidylethanolamine-binding protein (PEBP) has been demonstrated to bind to Raf-1 and mitogen-activated protein kinase kinase, components of the extracellular signal-regulated protein kinase (ERK) pathway, thereby inhibiting the pathway and resulting in the suppression of cell proliferation. In the present study, we examined whether PEBP is involved in differentiation induction of human keratinocytes. PEBP expression was immunohistochemically examined in normal human skin and skin cancers with different differentiation properties. PEBP was not expressed in the basal layer of the epidermis but was expressed in the spinous and granular layers of normal skin. The protein was expressed in differentiated but not in undifferentiated carcinoma. PEBP expression was also examined in cultured normal human epidermal keratinocytes in which differentiation was induced by calcium treatment. Involucrin was used as a differentiation marker for spinous and granular cells. Northern blotting analysis indicated that both PEBP and involucrin mRNAs were enhanced 6 h after treatment with 2.0 mM CaCl(2). The protein amount of PEBP was also increased by this treatment. To investigate whether PEBP is involved in differentiation induction of keratinocytes, HaCaT keratinocytes were transfected with an expression vector. Fluorescent immunostain revealed that cells expressing PEBP exhibited enlarged and flattened cell shape, and induction of involucrin expression was demonstrated by immunoblot analysis. Although the protein amount of ERK was not altered, phosphorylated ERK levels were decreased and cell proliferation was partly inhibited by PEBP expression. These results indicate that PEBP not only inhibits cell proliferation but also induces differentiation of human keratinocytes.

Reciprocal inhibition between MyoD and STAT3 in the regulation of growth and differentiation of myoblasts

The development of myoblasts is regulated by various growth factors as well as by intrinsic muscle-specific transcriptional factors. In this study, we analyzed the roles for STAT3 in the growth and differentiation of myoblasts in terms of cell cycle regulation and interaction with MyoD using C2C12 cells. Here we found that STAT3 inhibited myogenic differentiation induced by low serum or MyoD as efficiently as the Ras/mitogen-activated protein kinase cascade. As for this mechanism, we found that STAT3 not only promoted cell cycle progression through the induction of c-myc but also inhibited MyoD activities through direct interaction. STAT3 inhibited not only DNA binding activities of MyoD but also its transcriptional activities. However, the inhibited transcriptional activities were restored by the supplement of p300/CBP and PCAF, suggesting that STAT3 might deprive MyoD of these transcriptional cofactors. In addition, we found that MyoD inhibited DNA binding activities of STAT3, thereby inhibiting STAT3-dependent cell growth and survival of Ba/F3 cells. These results suggest that the development of muscle cells is regulated by the coordination of cytokine signals and intrinsic transcription factors.

Receptor protein tyrosine phosphatase alpha signaling is involved in androgen depletion-induced neuroendocrine differentiation of androgen-sensitive LNCaP human prostate cancer cells

The neuroendocrine (NE) cells represent the third cell population in the normal prostate. Results of several clinical studies strongly indicate that the NE cell population is greatly increased in prostate carcinomas during androgen ablation therapy that correlates with hormone-refractory growth and poor prognosis. However, the mechanism of NE cell enrichment in prostate carcinoma remains an enigma. We investigated the molecular mechanism by which androgen-sensitive C-33 LNCaP human prostate cancer cells become NE-like cells in an androgen-reduced environment, mimicking clinical phenomenon. In the androgen-depleted condition, androgen-sensitive C-33 LNCaP cells gradually acquired the NE-like morphology and expressed an increased level of neuron-specific enolase (NSE), a classical marker of neuronal cells. Several NE-like subclone cells were established. Biochemical characterizations of these subclone cells showed that receptor-type protein-tyrosine phosphatase alpha (RPTPalpha) is elevated and ERK is constitutively activated, several folds higher than that in parental cells. In androgen-depleted condition, PD98059, an MEK inhibitor, could efficiently block not only the activation of ERK, but also the acquisition of the NE-like morphology and the elevation of NSE in C-33 LNCaP cells. In RPTPalpha cDNA-transfected C-33 LNCaP cells, ERK was activated and NSE was elevated. In those cells in the presence of PD98059, the ERK activation and NSE elevation were abolished, following a dose-response fashion. Additionally, in constitutively active MEK mutant cDNA-transfected C-33 LNCaP cells, ERK was activated and NSE level was elevated, and cells obtained the NE-like phenotype. Our data collectively indicated that RPTPalpha signaling via ERK is involved in the NE transdifferentiation of androgen-sensitive C-33 LNCaP human prostate cancer cells in the androgen-depleted condition.

The mitogen-activated protein kinase cascade promotes myoblast cell survival by stabilizing the cyclin-dependent kinase inhibitor, p21WAF1 protein

During myogenesis, proliferating myoblasts withdraw from the cell cycle and are either eliminated by programmed cell death or differentiate into mature myotubes. Previous studies indicate that mitogen-activated protein kinase (MAPK) activity is significantly induced with the onset of terminal differentiation of C2 myoblasts. We have investigated the part played by the MAPK pathway in the differentiation of C2 myoblasts. Specific activation of MAPK by expression of an active Raf1-estrogen receptor chimera protein reduced significantly the number of myoblasts undergoing programmed cell death in the differentiation medium. Activation of Raf1 prevented the proteolytic activation of the proapoptotic caspase 9-protein during differentiation. The antiapoptotic function of Raf1 correlated with accumulation of the p21WAF1 protein resulting from its increased stability. Antisense expression of p21 was used to determine whether the p21WAF1 protein mediated the antiapoptotic activity of Raf1. Reduction of p21WAF1 protein in muscle cells abolished the antiapoptotic activity of the MAPK pathway. We conclude that MAPK contributes to muscle differentiation by preventing apoptotic cell death of differentiating myoblasts and that this activity is mediated by stabilization of the p21WAF1 protein.

Phosphorylation motifs regulating the stability and function of myocyte enhancer factor 2A

The phosphorylation status of the myocyte enhancer factor 2 (MEF2) transcriptional regulator is a critical determinant of its tissue-specific functions. However, due to the complexity of its phosphorylation pattern in vivo, a systematic inventory of MEF2A phosphorylation sites in mammalian cells has been difficult to obtain. We employed modern affinity purification techniques, combined with mass spectrometry, to identify several novel MEF2 phosphoacceptor sites. These include an evolutionarily conserved KSP motif, which we show is important in regulating the stability and function of MEF2A. Also, an indirect pathway in which a protein kinase casein kinase 2 phosphoacceptor site is phosphorylated by activation of p38 MAPK signaling was documented. Together, these findings identify several novel aspects of MEF2 regulation that may prove important in the control of gene expression in neuronal and muscle cells.

Mitogen-activated protein kinase (MAPK) pathway activation: effects of age and acute exercise on human skeletal muscle

The purpose of this investigation was to examine the activation (phosphorylation) and total protein content of MAPK signalling cascade proteins (ERK 1/2, p90RSK, Mnk 1, eIF4E, p38 MAPK, JNK/SAPK, MKP 1) at rest and following exercise, in sedentary young and old men. Eight young (22 +/- 1 years; YM) and eight old (79 +/- 3 years; OM) men underwent a resting muscle biopsy of the vastus lateralis; they then performed a knee extensor resistance exercise session (29 contractions at approximately 70 % of max), followed by a post-exercise biopsy. Western immunoblot analysis demonstrated that the OM had higher resting phosphorylation of ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK proteins versus YM (P < 0.05). The resistance exercise bout caused an increase in phosphorylation of the ERK 1/2, p90RSK and Mnk 1 proteins (P < 0.05) in the YM. Conversely, the OM had a decrease in ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK phosphorylation (P < 0.05) after the exercise bout. Neither group showed a change in eIF4E phosphorylation. The total amount of protein expression of the MAPK signalling proteins was not different between the YM and OM, except that there was a higher (P < 0.05) MKP 1 protein content in the OM. This investigation is the first to provide evidence that MAPK proteins are differentially activated at rest and in response to a bout of resistance exercise in skeletal muscle of young and old men. These findings may have implications for other processes (e.g. transcription and translation) involved in skeletal muscle type and growth, when examining the changes occurring with ageing muscle before and after resistance exercise/training.

The differentiation of skeletal muscle cells involves a protein-tyrosine phosphatase-alpha-mediated C-Src signaling pathway

Protein-tyrosine phosphatase-alpha (PTPalpha) plays an important role in various cellular signaling events, including proliferation and differentiation. In this study, we established L6 cell lines either underexpressing or overexpressing PTPalpha by stable transfection of cells with antisense PTPalpha or with full-length wild-type human or mouse or double catalytic site Cys --> Ala mutant (DM8) PTPalpha cDNA. Expression of PTPalpha in these cell lines was determined by immunoblotting and immunofluorescence. Cells harboring antisense PTPalpha exhibited a significantly reduced growth rate and thymidine incorporation when compared with the wild-type L6 cells. In contrast, cells overexpressing PTPalpha showed more rapid (2-fold) proliferation. Myoblasts with diminished PTPalpha failed to undergo fusion and did not form myotubes in reduced serum whereas overexpression of PTPalpha promoted myogenesis 2 days earlier than wild-type L6 cells. Overexpression of phosphatase-inactive mutant PTPalpha recapitulated the phenotype of the antisense cells. The different myogenic activities of these cell lines were correlated with the expression of myogenin and creatine kinase activity. Consistent with previous reports, PTPalpha positively regulated the activity of the protein-tyrosine kinase Src. Treatment of L6 cells with PP2 or SU6656, specific inhibitors of Src family kinases, and transient transfection of dominant-inhibitory Src inhibited the formation of myotubes and expression of myogenin. Moreover, enhanced expression of PTPalpha and activation of Src was detected during myogenesis. Together, these data indicate that PTPalpha is involved in the regulation of L6 myoblast growth and skeletal muscle cell differentiation via an Src-mediated signaling pathway.

Activation of p38 MAPK induces cell cycle arrest via inhibition of Raf/ERK pathway during muscle differentiation

Cell cycle arrest is essential for initiation of muscle differentiation in myoblasts. Given the previously described essential role for p38 MAPK in myogenesis, we undertook the present study to investigate the role of p38 MAPK in the cell cycle arrest that initiates muscle differentiation. p38 MAPK activity increased during, and was required for, muscle differentiation. Inhibition of p38 MAPK stimulated Raf and ERK activities, and induced cell proliferation in differentiation medium. The concomitant inhibition of p38 MAPK and ERK, however, failed to induce differentiation or proliferation. In conclusion, inhibition of the Raf/ERK pathway and the consequent cell cycle arrest is one of the major functions of p38 MAPK during muscle differentiation.

Induction of terminal differentiation by the c-Jun dimerization protein JDP2 in C2 myoblasts and rhabdomyosarcoma cells

Muscle cell differentiation is a result of a complex interplay between transcription factors and cell signaling proteins. Proliferating myoblasts must exit from the cell cycle prior to their differentiation. The muscle regulatory factor and myocyte enhancer factor-2 protein families play a major role in promoting muscle cell differentiation. Conversely, members of the AP-1 family of transcription factors that promote cell proliferation antagonize muscle cell differentiation. Here we tested the role of the c-Jun dimerization protein JDP2 in muscle cell differentiation. Endogenous expression of JDP2 was induced in both C2C12 myoblast and rhabdomyosarcoma (RD) cells programmed to differentiate. Ectopic expression of JDP2 in C2C12 myoblast cells inhibited cell cycle progression and induced spontaneous muscle cell differentiation. Likewise, constitutive expression of JDP2 in RD cells reduced their tumorigenic characteristics and restored their ability to differentiate into myotubes. JDP2 potentiated and synergized with 12-O-tetradecanoylphorbol-13-acetate to induce muscle cell differentiation of RD cells. In addition, JDP2 induced p38 activity in both C2 and RD cells programmed to differentiate. This is the first demonstration of a single transcription factor that rescues the myogenic program in an otherwise non-differentiating cancer cell line. Our results indicate that the JDP2 protein plays a major role in promoting skeletal muscle differentiation via its involvement in cell cycle arrest and activation of the myogenic program.

PKCalpha-mediated ERK, JNK and p38 activation regulates the myogenic program in human rhabdomyosarcoma cells

We have previously suggested that PKCalpha has a role in 12-O-Tetradecanoylphorbol-13-acetate (TPA)-mediated growth arrest and myogenic differentiation in human embryonal rhabdomyosarcoma cells (RD). Here, by monitoring the signalling pathways triggered by TPA, we demonstrate that PKCalpha mediates these effects by inducing transient activation of c-Jun N-terminal protein kinases (JNKs) and sustained activation of both p38 kinase and extracellular signal-regulated kinases (ERKs) (all referred to as MAPKs). Activation of MAPKs following ectopic expression of constitutively active PKCalpha, but not its dominant-negative form, is also demonstrated. We investigated the selective contribution of MAPKs to growth arrest and myogenic differentiation by monitoring the activation of MAPK pathways, as well as by dissecting MAPK pathways using MEK1/2 inhibitor (UO126), p38 inhibitor (SB203580) and JNK and p38 agonist (anisomycin) treatments. Growth-arresting signals are triggered either by transient and sustained JNK activation (by TPA and anisomycin, respectively) or by preventing both ERK and JNK activation (UO126) and are maintained, rather than induced, by p38. We therefore suggest a key role for JNK in controlling ERK-mediated mitogenic activity. Notably, sarcomeric myosin expression is induced by both TPA and UO126 but is abrogated by the p38 inhibitor. This finding indicates a pivotal role for p38 in controlling the myogenic program. Anisomycin persistently activates p38 and JNKs but prevents myosin expression induced by TPA. In accordance with this negative role, reactivation of JNKs by anisomycin, in UO126-pre-treated cells, also prevents myosin expression. This indicates that, unlike the transient JNK activation that occurs in the TPA-mediated myogenic process, long-lasting JNK activation supports the growth-arrest state but antagonises p38-mediated myosin expression. Lastly, our results with the MEK inhibitor suggest a key role of the ERK pathway in regulating myogenic-related morphology in differentiated RD cells.

Subtype specific roles of mitogen activated protein kinases in L6E9 skeletal muscle cell differentiation

Role of mitogen activated protein kinases (MAPK) in skeletal muscle differentiation is not fully understood. We investigated subtype-specific functions and their interactions, if any, in the regulation of myogenic differentiation in L6E9 skeletal muscle cells. We show inhibition of extracellular signal-regulated kinase-1 and -2 (ERK-1/-2) and activation of p38 MAP kinase during the differentiation of L6E9 rat skeletal muscle cells under low serum condition. Inhibition of ERK-1/-2 activity dramatically enhanced differentiation as was evident from cellular morphology, expression of muscle differentiation specific marker proteins, suggesting that ERK-1/-2 activation may be inhibitory to initiation and progression of differentiation. In contrast, inhibition of p38 MAP kinase completely prevented differentiation; meaning p38 activation is required from the initiation till terminal differentiation of L6E9 cells. Moreover, inhibition of ERK-1/-2 activities enhanced the activation of p38 MAP kinase that resulted in enhancement of differentiation; whereas inhibition of p38 MAP kinase activity enhanced the ERK-1/-2 activities culminating in abrogation of differentiation. We conclude that ERK-1/-2 andp38 MAPkinase cascades oppositelyregulate each other's function(s) thereby regulating L6E9 skeletal muscle differentiation.

Raf-induced effects on the differentiation and apoptosis of skeletal myoblasts are determined by the level of Raf signaling: abrogation of apoptosis by Raf is downstream of caspase 3 activation

We examined the effect of a constitutively active Raf protein (Raf-CAAX) on the differentiation and the coincident apoptosis of skeletal myoblasts. We found that a low level of Raf signaling leads to accelerated differentiation when compared to parental myoblasts, while a higher level of Raf signaling induces a transformed morphology and abrogates both differentiation and the coincident apoptosis. Raf signaling abrogates apoptosis without blocking the activation of caspase 3 and the subsequent cleavage of caspase 3 substrates. Eliminating the signal from Raf through MEK does not restore the ability to differentiate or to undergo apoptosis in the myoblasts with a high level of Raf signal, nor does it abrogate the accelerated differentiation observed in myoblasts with lower levels of Raf signal. Constitutive signaling through MEK is required, however, to maintain a transformed morphology. These results indicate that the effect of Raf on the differentiation and apoptosis of skeletal myoblasts is dictated by the level of Raf signaling, and that Raf signaling sufficient to abrogate the apoptosis coincident with differentiation does so downstream of caspase 3 signaling.

Role of SHP-2 in fibroblast growth factor receptor-mediated suppression of myogenesis in C2C12 myoblasts

Ligand activation of the fibroblast growth factor receptor (FGFR) represses myogenesis and promotes activation of extracellular signal-regulated kinases 1 and 2 (Erks). The precise mechanism through which the FGFR transmits both of these signals in myoblasts remains unclear. The SH2 domain-containing protein tyrosine phosphatase, SHP-2, has been shown to participate in the regulation of FGFR signaling. However, no role for SHP-2 in FGFR myogenic signaling is known. In this study, we show that stimulation of C2C12 myoblasts with FGF-2 induces SHP-2 complex formation with tyrosyl-phosphorylated FGFR substrate 2 alpha (FRS-2 alpha). Both the catalytic activity and, to a much lesser extent, the Grb2 binding-tyrosyl phosphorylation sites of SHP-2 are required for maximal FGF-2-induced Erk activity and Elk-1 transactivation. When overexpressed in C2C12 myoblasts, wild-type SHP-2, but not a catalytically inactive SHP-2 mutant, potentiates the suppressive effects of FGF-2 on muscle-specific gene expression. In addition, expression of a constitutively active mutant of SHP-2 is sufficient to prevent myogenesis. The constitutively active mutant of SHP-2 induces hyper-tyrosyl phosphorylation of FRS-2 alpha but fails to stimulate or potentiate either FGF-2-induced Erk activation or Elk-1 transactivation. These data suggest that in myoblasts, SHP-2 represses myogenesis via a pathway that is independent of the Erks. We propose that SHP-2 plays a pivotal role in FGFR signaling in myoblasts via both Erk-dependent and Erk-independent pathways.

Insulin restores differentiation of Ras-transformed C2C12 myoblasts by inducing NF-kappaB through an AKT/P70S6K/p38-MAPK pathway

v-H-ras transformed C2C12 (C2Ras) myoblasts, overexpressing p21-Ras protein in the Ras-GTP active form, showed a differentiation-defective phenotype when cultured in low serum as compared with C2C12 myoblasts. Accordingly, the purpose of the present study was to delineate the signaling pathways that restore C2Ras myoblasts differentiation. Inhibition of p42/p44-MAPK with the chemical inhibitor PD98059, and activation of AKT/P70S6K and p38-MAPK with insulin, produced growth arrest (precluding the expression of PCNA, cyclin-D1 and retinoblastoma at the hyperphosphorylated state and inducing the expression of the cell cycle inhibitor p21(Cip)) and myogenesis (multinucleated myotubes formation and induction of creatine kinase, caveolin-3 and alpha-actin). Both events were accompanied by down-regulation of AP-1 and up-regulation of NF-kappaB transcriptional activities. Furthermore, inhibition of NF-kappaB transcriptional activity by the use of the proteasome inhibitor MG132 totally precluded differentiation by insulin+PD98059, demonstrating a direct role for NF-kappaB on C2Ras myogenesis. C2Ras myoblasts failed to restore differentiation when rapamycin or PD169316 were added in the presence of insulin+PD98059, indicating that the activation of both P70S6K and p38-MAPK was necessary to reach a fully differentiated phenotype. Finally, transient transfection of a constitutively active Myr-EGFP-AKT-HA construct (in the presence of PD98059) restored C2Ras myogenesis by its ability to activate P70S6K and p38-MAPK. A crosstalk between P70S6K and p38-MAPK was observed under rapamycin treatment in both insulin or active AKT induced myogenesis. Our results are delineating an AKT/P70S6K/p38-MAPK pathway involved in skeletal muscle differentiation.

Geraniol, a component of plant essential oils, sensitizes human colonic cancer cells to 5-Fluorouracil treatment

Differentiation of human colonic cancer cells at confluency has been correlated to their increased resistance to chemotherapeutic agents. The aim of this study was to determine whether blocking Caco-2 cell differentiation could sensitize the cells to 5-fluorouracil (5-FU) treatment. We show that in cells at confluency, geraniol (400 microM) prevented the formation of brush-border membranes and inhibited the expression of intestinal hydrolases (sucrase, lactase, alkaline phosphatase). The antiproliferative effect of geraniol (400 microM) together with 5-FU (5 microM) was twice that of 5-FU alone. The cytotoxicity induced by 5-FU was enhanced in the presence of geraniol, as shown by a 50% increase of lactate dehydrogenase release in the culture medium. These effects are related to enhanced intracellular accumulation of 5-FU in the presence of geraniol as shown by a 2-fold increase in intracellular 5-[6-(3)H]FU (1.5 microCi/ml). It is concluded that geraniol sensitizes colonic cancer cells to 5-FU treatment, by increasing the cytotoxicity of the drug, and that this results from the facilitated transport of 5-FU and the blockade of the morphological and functional differentiation of the cancer cells.

Plasmin activity is required for myogenesis in vitro and skeletal muscle regeneration in vivo

Plasmin, the primary fibrinolytic enzyme, has a broad substrate spectrum and is implicated in biologic processes dependent upon proteolytic activity, such as tissue remodeling and cell migration. Active plasmin is generated from proteolytic cleavage of the zymogen plasminogen (Plg) by urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA). Here, we have investigated the role of plasmin in C2C12 myoblast fusion and differentiation in vitro, as well as in skeletal muscle regeneration in vivo, in wild-type and Plg-deficient mice. Wild-type mice completely repaired experimentally damaged skeletal muscle. In contrast, Plg(-/-) mice presented a severe regeneration defect with decreased recruitment of blood-derived monocytes and lymphocytes to the site of injury and persistent myotube degeneration. In addition, Plg-deficient mice accumulated fibrin in the degenerating muscle fibers; however, fibrinogen depletion of Plg-deficient mice resulted in a correction of the muscular regeneration defect. Because we found that uPA, but not tPA, was induced in skeletal muscle regeneration, and persistent fibrin deposition was also reproducible in uPA-deficient mice following injury, we propose that fibrinolysis by uPA-dependent plasmin activity plays a fundamental role in skeletal muscle regeneration. In summary, we identify plasmin as a critical component of the mammalian skeletal muscle regeneration process, possibly by preventing intramuscular fibrin accumulation and by contributing to the adequate inflammatory response after injury. Finally, we found that inhibition of plasmin activity with alpha2-antiplasmin resulted in decreased myoblast fusion and differentiation in vitro. Altogether, these studies demonstrate the requirement of plasmin during myogenesis in vitro and muscle regeneration in vivo.

Early stimulation and late inhibition of extracellular signal-regulated kinase 1/2 phosphorylation by IGF-I: a potential mechanism mediating the switch in IGF-I action on skeletal muscle cell differentiation

IGF-I has a unique biphasic effect on skeletal muscle cell differentiation. Initially, IGF-I inhibits differentiation and promotes proliferation of skeletal myoblasts. Subsequently, IGF-I switches to stimulating differentiation of these cells. The mechanisms responsible for this switch in IGF action remain unknown. We have examined the role of extracellular signal-regulated kinase (Erk)1/2 signaling in mediating the early inhibitory and late stimulatory effects of IGF-I on the gene expression of myogenin, a skeletal muscle-specific transcription factor essential for myogenic differentiation. We find that, concurrent with its early inhibitory and late stimulatory effects on myogenin mRNA, IGF-I has a biphasic but opposite effect on phosphorylation of Erk1/2: initially, IGF-I increases and subsequently decreases the phosphorylation of Erk1/2 in comparison to untreated cells. Cotreatment with an inhibitor of Erk1/2 activation prevents the early IGF-I-stimulation of Erk1/2 phosphorylation and partially reverses IGF-I-inhibition of myogenin mRNA. Conversely, preventing the late IGF-I-induced decrease in Erk1/2 phosphorylation blocks IGF-I-stimulation of myogenin mRNA. Our data indicate that the time-dependent, opposing effects of IGF-I on skeletal muscle cell differentiation are mediated, at least in part, by biphasic but opposite effects on activation of the Erk1/2 MAPK signaling pathway.

Muscle-specific inactivation of the IGF-I receptor induces compensatory hyperplasia in skeletal muscle

During the development of skeletal muscle, myoblasts withdraw from the cell cycle and differentiate into myotubes. The insulin-like growth factors IGF-I and IGF-II, through their cognate tyrosine kinase receptor (IGF-I receptor), are known to play a role in this process. After withdrawal of myoblasts from the cell cycle, IGF-I promotes muscle differentiation by inducing the expression or activity of myogenic regulatory factors (MyoD, myogenin) and effectors (p21). However, little is known about the intracellular mechanisms by which the IGF-I system regulates these factors during the process of myogenesis. Here we show that MKR mice, which express a dominant negative IGF-I receptor specifically in skeletal muscle, have marked muscle hypoplasia from birth to 3 weeks of age. This hypoplasia occurs concomitantly with a decrease in ERK immunoreactivity levels and decreases in MyoD and myogenin expression. BrdU immunocytochemistry showed a compensatory hyperplasia as MKR mice grew to adulthood. Interestingly, hyperplasia occurred concomitantly with an increase in p38, MyoD, myogenin, and p21 immunoreactivity levels, as well as a decrease in Twist levels. These findings suggest that regulation of these cellular elements by IGF-I may play a role in the development and differentiation of skeletal muscle in vivo.

MAP kinase converts MyoD into an instructive muscle differentiation factor in Xenopus

In amphibian development, muscle is specified in the dorsal lateral marginal zone (DLMZ) of the gastrula embryo. Two critical events specify the formation of skeletal muscle: the expression of the myogenic transcription factor, XMyoD, and the secretion of bone morphogenetic protein (BMP) antagonists by the adjacent Spemann organizer. Inhibition of BMP signaling during early gastrula stages converts XMyoD protein into an instructive differentiation factor in the DLMZ. Yet, the intracellular signaling factors connecting BMP antagonism and activation of XMyoD remain unknown. Our data show that BMP antagonism induces the activity of mitogen-activated protein kinase (MAPK), and that the activity of MAPK is necessary for muscle-specific differentiation. Treatment of gastrula-stage DLMZ explants with MAPK pathway inhibitors ventralized mesoderm and prevented muscle differentiation. Expression of XMyoD in ventral mesoderm weakly induced muscle formation; however, the coexpression of a constitutively active MEK1 with XMyoD efficiently induced muscle differentiation. Activation of the MAPK pathway did not induce the transcription of XMyoD, but increased its protein levels and transcriptional activity. Thus, MAPK activation is subsequent to BMP antagonism, and participates in the dorsalization of mesoderm by converting the XMyoD protein into a potent differentiation factor.

Insulin-like growth factor-mediated muscle differentiation: collaboration between phosphatidylinositol 3-kinase-Akt-signaling pathways and myogenin

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development in the embryo and in regeneration in the adult. To study mechanisms of IGF action in muscle, we developed a myogenic cell line that overexpresses IGF-binding protein-5. C2BP5 cells remain quiescent in low serum differentiation medium until the addition of IGF-I. Here we use this cell line to identify signaling pathways controlling IGF-mediated differentiation. Induction of myogenin by IGF-I and myotube formation were prevented by the phosphatidylinositol (PI) 3-kinase inhibitor, LY294002, even when included 2 days after growth factor addition, whereas expression of active PI 3-kinase could promote differentiation in the absence of IGF-I. Differentiation also was induced by myogenin but was blocked by LY294002. The differentiation-promoting effects of IGF-I were mimicked by a modified membrane-targeted inducible Akt-1 (iAkt), and iAkt was able to stimulate differentiation of C2 myoblasts and primary mouse myoblasts incubated with otherwise inhibitory concentrations of LY294002. These results show that an IGF-regulated PI 3-kinase-Akt pathway controls muscle differentiation by mechanisms acting both upstream and downstream of myogenin.

Rac1 inhibits myogenic differentiation by preventing the complete withdrawal of myoblasts from the cell cycle

The small GTPase protein Rac1 is involved in a wide range of biological processes, yet its role in cell differentiation is mostly unknown. Here we show that Rac1 activity is high in proliferating myoblasts and decreases during the differentiation process. To analyze the involvement of Rac1 in muscle differentiation, different forms of the protein were expressed in muscle cells. A constitutively activated form of Rac1 (Rac1Q61L) inhibited the activity of MyoD in promoting muscle differentiation, whereas a dominant negative form of Rac1 (Rac1T17N) induced the activity of MyoD in promoting muscle differentiation. Expression of Rac1T17N imposed myogenic differentiation on myoblasts growing under mitogenic conditions. In inquiring whether Rac1 affected the withdrawal of myoblasts from the cell cycle, we analyzed the expression of cyclin D1 and p21(WAF1) and the phosphorylation state of the retinoblastoma protein. According to these markers and bromodeoxyuridine incorporation, C2 myoblasts expressing Rac1T17N exited the cell cycle earlier than control C2 cells. Myoblasts expressing Rac1Q61L did not permanently withdraw from the cell cycle. An indication of the possible involvement of the mitogen-activated protein kinase (MAPK) pathway in Rac1-mediated myoblast proliferation was obtained by the use of MAPK kinase inhibitors U0126 and PD098059. These inhibitors arrested C2-Rac1Q61L cell cycling. Taken together, our results show that Rac1 activation interferes with myoblast exit from the cell cycle via or in concert with the MAPK pathway.

Ras/MAP kinase pathway is associated with the control of myotube formation but not myofibril assembly in quail myoblasts transformed with Rous sarcoma virus

Tyrosine kinase activity of v-Src from Rous sarcoma virus (RSV) inhibits the differentiation of quail myoblasts. To clarify the inhibitory mechanism, we focused on the signaling pathways from v-Src. When the activation of the Ras/MAP (mitogen-activated protein) kinase pathway was inhibited by a dominant-negative mutant of Ras or PD98059, a specific inhibitor of p42 MAP kinase kinase, differentiation was restored; muscle specific proteins were expressed and myotubes formed even under active conditions of v-Src. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (P13-kinase), showed no effects on the inhibition by v-Src. These findings suggest that v-Src activates the Ras/MAP kinase signaling pathway, but not the P13-kinase pathway, and inhibits the differentiation. However, the myotubes derived from the dominant-negative Ras did not form actin fibers, suggesting that myofibril assembly is regulated by other pathway(s) from v-Src.

Colony-stimulating factor-1 (CSF-1) receptor-mediated macrophage differentiation in myeloid cells: a role for tyrosine 559-dependent protein phosphatase 2A (PP2A) activity

M1 myeloid cells transfected with the wild-type (WT) colony-stimulating factor-1 (CSF-1) receptor (CSF-1R; M1/WT cells) undergo CSF-1-dependent macrophage differentiation. By mutation studies, we have provided prior evidence that tyrosine 559 in the CSF-1R cytoplasmic domain governs the Src-dependent differentiation pathway. Further components of this pathway were then sought. We report that the extent of CSF-1-mediated tyrosine phosphorylation of protein phosphatase 2A (PP2A), and the associated loss of its activity were reduced in M1 cells transfected with the CSF-1R with a tyrosine-to-phenylalanine mutation at position 559 (M1/559 cells), compared with the corresponding responses in CSF-1-treated M1/WT cells. This evidence for an involvement of a reduction in PP2A activity in the differentiation process was supported by the restoration of the defect in the CSF-1-mediated differentiation of M1/559 cells by the addition of the PP2A inhibitor, okadaic acid. It was also found that the degree of activation of extracellular-signal-regulated kinase (ERK) activities by CSF-1 was reduced in M1/559 cells, suggesting their involvement in the differentiation process. These data suggest that PP2A and ERK form part of the Src-dependent signal-transduction cascade governing CSF-1-mediated macrophage differentiation in M1 cells.

Effects of concentric and eccentric contractions on phosphorylation of MAPK(erk1/2) and MAPK(p38) in isolated rat skeletal muscle

1. Exercise and contractions of isolated skeletal muscle induce phosphorylation of mitogen-activated protein kinases (MAPKs) by undefined mechanisms. The aim of the present study was to determine exercise-related triggering factors for the increased phosphorylation of MAPKs in isolated rat extensor digitorum longus (EDL) muscle. 2. Concentric or eccentric contractions, or mild or severe passive stretches were used to discriminate between effects of metabolic/ionic and mechanical alterations on phosphorylation of two MAPKs: extracellular signal-regulated kinase 1 and 2 (MAPK(erk1/2)) and stress-activated protein kinase p38 (MAPK(p38)). 3. Concentric contractions induced a 5-fold increase in MAPK(erk1/2) phosphorylation. Application of the antioxidants N-acetylcysteine (20 mM) or dithiothreitol (5 mM) suppressed concentric contraction-induced increase in MAPK(erk1/2) phosphorylation. Mild passive stretches of the muscle increased MAPK(erk1/2) phosphorylation by 1.8-fold, whereas the combination of acidosis and passive stretches resulted in a 2.8-fold increase. Neither concentric contractions, nor mild stretches nor acidosis significantly affected phosphorylation of MAPK(p38). 4. High force applied upon muscle by means of either eccentric contractions or severe passive stretches resulted in 5.7- and 9.5-fold increases of phosphorylated MAPK(erk1/2), respectively, whereas phosphorylation of MAPK(p38) increased by 7.6- and 1.9-fold (not significant), respectively. 5. We conclude that in isolated rat skeletal muscle an increase in phosphorylation of both MAPK(erk1/2) and MAPK(p38) is induced by mechanical alterations, whereas contraction-related metabolic/ionic changes (reactive oxygen species and acidosis) cause increased phosphorylation of MAPK(erk1/2) only. Thus, contraction-induced phosphorylation can be explained by the combined action of increased production of reactive oxygen species, acidification and mechanical perturbations for MAPK(erk1/2) and by high mechanical stress for MAPK(p38).

Activated MEK1 binds the nuclear MyoD transcriptional complex to repress transactivation

To elucidate the mechanism through which MAPK signaling regulates the MyoD family of transcription factors, we investigated the role of the signaling intermediate MEK1 in myogenesis. Transfection of activated MEK1 strongly repressed gene activation and myogenic conversion by the MyoD family. This repression was not mediated by direct phosphorylation of MyoD or by changes in MyoD stability or subcellular distribution. Deletion mapping revealed that MEK1-mediated repression required the MyoD amino-terminal transactivation domain. Moreover, activated MEK1 was nuclearly localized and bound a complex containing MyoD in a manner that is dependent on the presence of the MyoD amino terminus. Together, these data demonstrate that MEK1 signaling has a strong negative effect on MyoD activity via a novel mechanism involving binding of MEK1 to the nuclear MyoD transcriptional complex.

Proteomic analysis of macrophage differentiation. p46/52(Shc) Tyrosine phosphorylation is required for CSF-1-mediated macrophage differentiation

Macrophage colony stimulating factor (M-CSF or CSF-1) acts to regulate the development and function of cells of the macrophage lineage. Murine myeloid FDC-P1 cells transfected with the CSF-1 receptor (FD/WT) adopt a macrophage-like morphology when cultured in CSF-1. This process is abrogated in FDC-P1 cells transfected with the CSF-1 receptor with a tyrosine to phenyalanine substitution at position 807 (FD/807), suggesting that a molecular interaction critical to differentiation signaling is lost (Bourette, R. P., Myles, G. M., Carlberg, K., Chen, A. R., and Rohrschneider, L. R. (1995) Cell Growth Differ. 6, 631--645). A detailed examination of lysates of CSF-1-treated FD/807 cells by two-dimensional SDS-polyacrylamide gel electrophoresis (PAGE) revealed a number of proteins whose degree of tyrosine phosphorylation was modulated by the Y807F mutation. Included in this category were three phosphorylated proteins that co-migrated with p46/52(Shc). Immunoprecipitation, Western blotting, and in vitro binding studies suggest that they are indeed p46/52(Shc). A key regulator of differentiation in a number of cell systems, ERK was observed to exhibit an activity that correlated with the relative degree of differentiation induced by CSF-1 in the two cell types. Transfection of cells with a non-tyrosine-phosphorylatable form of p46/52(Shc) prevented the normally observed CSF-1-mediated macrophage differentiation as determined by adoption of macrophage-like morphology and expression of the monocyte/macrophage lineage cell surface marker, Mac-1. These results are the first to suggest that p46/52(Shc) may play a role in CSF-1-induced macrophage differentiation. Additionally, a number of proteins were identified by two-dimensional SDS-PAGE whose degree of tyrosine phosphorylation is also modulated by the Y807F substitution. This group of molecules may contain novel signaling molecules important in macrophage differentiation.

Isolation of hyperactive mutants of the MAPK p38/Hog1 that are independent of MAPK kinase activation

Mitogen-activated protein kinases (MAPKs) play pivotal roles in growth, development, differentiation, and apoptosis. The exact role of a given MAPK in these processes is not fully understood. This question could be addressed using active forms of these enzymes that are independent of external stimulation and upstream regulation. Yet, such molecules are not available. MAPK activation requires dual phosphorylation, on neighboring Tyr and Thr residues, catalyzed by MAPK kinases (MAPKKs). It is not known how to force MAPK activation independent of MAPKK phosphorylation. Here we describe a series of nine hyperactive (catalytically and biologically), MAPKK-independent variants of the MAPK Hog1. Each of the active molecules contains just a single point mutation. Six mutations are in the conserved L16 domain of the protein. The active Hog1 mutants were obtained through a novel genetic screen that could be applied for isolation of active MAPKs of other families. Equivalent mutations, introduced to the human p38alpha, rendered the enzyme active even when produced in Escherichia coli, showing that the mutations increased the intrinsic catalytic activity of p38. It implies that the activating mutations could be directly used for production of active forms of MAPKs from yeasts to humans and could open the way to revealing their biological functions.

Regulation of the levels of small heat-shock proteins during differentiation of C2C12 cells

Levels of the small heat-shock proteins (sHSPs) HSP27 and alphaB-crystallin during differentiation of mouse C2C12 cells were determined using specific immunoassays. Increases of these proteins were about 3-fold and 10-fold, respectively. Under the same conditions, however, the level of HSP70 in C2C12 cells barely increased, indicating selective accumulation of HSP27 and alphaB-crystallin with differentiation. While expression of mRNA for alphaB-crystallin was also markedly increased and that for HSP27 was but to a lesser extent, mRNA for HSP70 could barely be detected during differentiation. Activation of the heat-shock factor was not observed, in contrast to the case with heat-stressed undifferentiated cells. Various inhibitors of protein kinases affected the differentiation and the associated increase of sHSPs. Rapamycin, an inhibitor of p70 S6 kinase, completely inhibited the differentiation and suppressed the accumulation of HSP27 and alphaB-crystallin. SB203580, an inhibitor of p38 MAP kinase, also inhibited differentiation, but the accumulation of alphaB-crystallin was rather enhanced. PD98059, an inhibitor of MAP kinase kinase, significantly increased expression of a differentiation marker for muscle cells, creatine kinase M isozyme, as well as accumulation of alphaB-crystallin. These results suggest that accumulation of sHSPs during differentiation of C2C12 cells is regulated in a complex manner.

Alterations of MAPK activities associated with intestinal cell differentiation

Three distinct groups of mitogen-activated protein kinases (MAPKs) have been identified in mammalian cells (i.e., ERK, JNK, and p38) which play an important role in the differentiation and apoptosis of various cells. The purpose of our present study was to determine MAPK activity and levels associated with sodium butyrate (NaBT)-mediated differentiation and apoptosis in the human colon cancer cell lines Caco-2 and HT29. Intestinal alkaline phosphatase (IAP) activity, a marker of intestinal differentiation, was increased at 48 h after NaBT treatment followed by cell death at 72 h. ERK activity was decreased in differentiated Caco-2 cells either induced with NaBT or allowed to differentiate spontaneously and in HT29 cells treated with NaBT. The combination of the MEK inhibitor, PD98059, with NaBT further increased IAP activity and cell death compared with NaBT alone. In contrast to ERK, JNK1 activity and c-Jun phosphorylation was increased 8 h after NaBT treatment suggesting a role for the JNK pathway in intestinal cell differentiation and apoptosis. p38 activity was increased at 24 and 48 h after NaBT treatment. Taken together, our results suggest that alterations in MAPKs (i.e., ERK inhibition and JNK induction) contribute to the differentiation and apoptotic pathways in intestinal cells.

SHP-2 complex formation with the SHP-2 substrate-1 during C2C12 myogenesis

Myogenesis is a highly ordered process that involves the expression of muscle-specific genes, cell-cell recognition and multinucleated myotube formation. Although protein tyrosine kinases have figured prominently in myogenesis, the involvement of tyrosine phosphatases in this process is unknown. SHP-2 is an SH2 domain-containing tyrosine phosphatase, which positively regulates growth and differentiation. We show that in C2C12 myoblasts, SHP-2 becomes upregulated early on during myogenesis and associates with a 120 kDa tyrosyl-phosphorylated complex. We have identified that the 120 kDa complex consists of the SHP-2 substrate-1 (SHPS-1) and the Grb2-associated binder-1 (Gab-1). SHPS-1, but not Gab-1, undergoes tyrosyl phosphorylation and association with SHP-2 during myogenesis, the kinetics of which correlate with the expression of MyoD. Either constitutive expression or inducible activation of MyoD in 10T½ fibroblasts promotes SHPS-1 tyrosyl phosphorylation and its association with SHP-2. It has been shown that p38 mitogen-activated protein kinase (MAPK) activity is required for the expression/activation of MyoD and MyoD-responsive genes. Inhibition of p38 MAPK by SB203580 in differentiating C2C12 myoblasts blocks MyoD expression, SHPS-1 tyrosyl phosphorylation and the association of SHPS-1 with SHP-2. These data suggest that SHPS-1/SHP-2 complex formation is an integral signaling component of skeletal muscle differentiation.

Overexpression of MnSOD protects murine fibrosarcoma cells (FSa-II) from apoptosis and promotes a differentiation program upon treatment with 5-azacytidine: involvement of MAPK and NFkappaB pathways

Stable transfection of neomycin and human manganese superoxide dismutase (MnSOD2) expression plasmids into a murine fibrosarcoma cell line (FSa-II) was previously done in our laboratory. Treatment with 10 microM 5-azacytidine induced apoptosis in the control cell line (NEO), whereas the MnSOD-overexpressing cell line (SOD-H) demonstrated differentiated-appearing morphology. The levels of the myogenic transcription factor, MyoD, and the muscle-specific marker, alpha-actin, were increased over time with 5-azacytidine treatment in the SOD-H cell line. Nuclear transcription factor NFkappaB was activated in the SOD-H cell line, whereas inhibition of NFkappaB activation reduced the levels of MyoD and alpha-actin. Members of mitogen-activated protein kinase pathway and the Raf1/MEK/ERK cascade were shown to play a positive role in this event. Overexpression of MnSOD not only can protect cells from the toxic effects of 5-azacytidine, but can also promote the fibrosarcoma cells to enter a differentiation program.

The small muscle-specific protein Csl modifies cell shape and promotes myocyte fusion in an insulin-like growth factor 1-dependent manner

We have isolated a murine cDNA encoding a 9-kD protein, Chisel (Csl), in a screen for transcriptional targets of the cardiac homeodomain factor Nkx2-5. Csl transcripts were detected in atria and ventricles of the heart and in all skeletal muscles and smooth muscles of the stomach and pulmonary veins. Csl protein was distributed throughout the cytoplasm in fetal muscles, although costameric and M-line localization to the muscle cytoskeleton became obvious after further maturation. Targeted disruption of Csl showed no overt muscle phenotype. However, ectopic expression in C2C12 myoblasts induced formation of lamellipodia in which Csl protein became tethered to membrane ruffles. Migration of these cells was retarded in a monolayer wound repair assay. Csl-expressing myoblasts differentiated and fused normally, although in the presence of insulin-like growth factor (IGF)-1 they showed dramatically enhanced fusion, leading to formation of large dysmorphogenic "myosacs." The activities of transcription factors nuclear factor of activated T cells (NFAT) and myocyte enhancer-binding factor (MEF)2, were also enhanced in an IGF-1 signaling-dependent manner. The dynamic cytoskeletal localization of Csl and its dominant effects on cell shape and behavior and transcription factor activity suggest that Csl plays a role in the regulatory network through which muscle cells coordinate their structural and functional states during growth, adaptation, and repair.

MAPK/ERK overrides the apoptotic signaling from Fas, TNF, and TRAIL receptors

The tumor necrosis factor (TNF), Fas, and TNF-related apoptosis-inducing ligand (TRAIL) receptors (R) are highly specific physiological mediators of apoptotic signaling. We observed earlier that a number of FasR-insensitive cell lines could redirect the proapoptotic signal to an anti-apoptotic ERK1/2 signal resulting in inhibition of caspase activation. Here we determine that similar mechanisms are operational in regulating the apoptotic signaling of other death receptors. Activation of the FasR, TNF-R1, and TRAIL-R, respectively, rapidly induced subsequent ERK1/2 activation, an event independent from caspase activity. Whereas inhibition of the death receptor-mediated ERK1/2 activation was sufficient to sensitize the cells to apoptotic signaling from FasR and TRAIL-R, cells were still protected from apoptotic TNF-R1 signaling. The latter seemed to be due to the strong activation of the anti-apoptotic factor NF-kappaB, which remained inactive in FasR or TRAIL-R signaling. However, when the cells were sensitized with cycloheximide, which is sufficient to sensitize the cells also to apoptosis by TNF-R1 stimulation, we noticed that adenovirus-mediated expression of constitutively active MKK1 could rescue the cells from apoptosis induced by the respective receptors by preventing caspase-8 activation. Taken together, our results show that ERK1/2 has a dominant protecting effect over apoptotic signaling from the death receptors. This protection, which is independent of newly synthesized proteins, acts in all cases by suppressing activation of the caspase effector machinery.