Ectopic expression of cyclin D1 prevents activation of gene transcription by myogenic basic helix-loop-helix regulators

The synaptosome-associated protein 23 (SNAP23) is necessary for proper myogenesis

Vesicle-mediated transport is necessary for maintaining cellular homeostasis and proper signaling. The synaptosome-associated protein 23 (SNAP23) is a member of the SNARE protein family and mediates the vesicle docking and membrane fusion steps of secretion during exocytosis. Skeletal muscle has been established as a secretory organ; however, the role of SNAP23 in the context of skeletal muscle development is still unknown. Here, we show that depletion of SNAP23 in C2C12 mouse myoblasts reduces their ability to differentiate into myotubes as a result of premature cell cycle exit and early activation of the myogenic transcriptional program. This effect is rescued when cells are seeded at a high density or when cultured in conditioned medium from wild type cells. Proteomic analysis of collected medium indicates that SNAP23 depletion leads to a misregulation of exocytosis, including decreased secretion of the insulin-like growth factor 1 (IGF1), a critical protein for muscle growth, development, and function. We further demonstrate that treatment of SNAP23-depleted cells with exogenous IGF1 rescues their myogenic capacity. We propose that SNAP23 mediates the secretion of specific proteins, such as IGF1, that are important for achieving proper differentiation of skeletal muscle cells during myogenesis. This work highlights the underappreciated role of skeletal muscle as a secretory organ and contributes to the understanding of factors necessary for myogenesis.

Myristic acid selectively augments β-tubulin levels in C2C12 myotubes via diacylglycerol kinase δ

Effective amelioration of type II diabetes requires therapies that increase both glucose uptake activity per cell and skeletal muscle mass. Myristic acid (14:0) increases diacylglycerol kinase (DGK) δ protein levels and enhances glucose uptake in myotubes in a DGKδ-dependent manner. However, it is still unclear whether myristic acid treatment affects skeletal muscle mass. In this study, we found that myristic acid treatment increased the protein level of β-tubulin, which constitutes microtubules and is closely related to muscle mass, in C2C12 myotubes but not in the proliferation stage in C2C12 myoblasts. However, lauric (12:0), palmitic (16:0) and oleic (18:1) acids failed to affect DGKδ and β-tubulin protein levels in C2C12 myotubes. Moreover, knockdown of DGKδ by siRNA significantly inhibited the increased protein level of β-tubulin in the presence of myristic acid, suggesting that the increase in β-tubulin protein by myristic acid depends on DGKδ. These results indicate that myristic acid selectively affects β-tubulin protein levels in C2C12 myotubes via DGKδ, suggesting that this fatty acid improves skeletal muscle mass in addition to increasing glucose uptake activity per cell.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Dlx5 and Dlx6 can antagonize cell division at the G1/S checkpoint

Background

Dlx5 and Dlx6 stimulate differentiation of diverse progenitors during embryonic development. Their actions as pro-differentiation transcription factors includes the up-regulation of differentiation markers but the extent to which differentiation may also be stimulated by regulation of the cell cycle has not been addressed.

Results

We document that expression of Dlx5 and Dlx6 antagonizes cell proliferation in a variety of cell types without inducing apoptosis or promoting cell cycle exit. Rather, a variety of evidence indicates that elevated Dlx5 and Dlx6 expression reduces the proportion of cells in S phase and affects the length of the cell cycle.

Conclusions

Antagonism of S-phase entry by Dlx5 and Dlx6 proteins likely represents a lineage-independent function to effect Dlx-mediated differentiation in multiple progenitor cell types.

SM-20 is a novel growth factor-responsive gene regulated during skeletal muscle development and differentiation

SM-20 is a novel, evolutionarily conserved "early response" gene originally cloned from a rat aortic smooth muscle cell (SMC) cDNA library. SM-20 encodes a cytoplasmic protein, which is induced by platelet-derived growth factor and angiotensin II in cultured SMC and is upregulated in intimal SMC of atherosclerotic plaques and injured arteries. We have now examined SM-20 expression during differentiation of cultured skeletal myoblasts and during skeletal myogenesis in vivo. Low levels of SM-20 mRNA and protein were expressed in proliferating mouse C2C12 myoblasts. Differentiation by serum withdrawal was associated with a marked induction of SM-20 mRNA and the expression of high levels of SM-20 antigen in myotubes. The induction was partially inhibited by blocking differentiation with bFGF or TGFbeta. Similar results were obtained with the nonfusing mouse C25 myoblast line, suggesting that SM-20 upregulation is a consequence of biochemical differentiation and is fusion independent. During mouse embryogenesis, SM-20 was first observed at 8.5E in the dermomyotomal cells of the rostral somites. SM-20 expression progressed in a rostral to caudal pattern, with highest levels seen in the muscle primordia and mature muscles. SM-20 thus represents a novel intracellular protein that is regulated during skeletal muscle differentiation and development.

Diacylglycerol kinase δ controls down-regulation of cyclin D1 for C2C12 myogenic differentiation

Diacylglycerol kinase (DGK) is a lipid-metabolizing enzyme that phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). DGKδ is highly expressed in the skeletal muscle, and a decrease in DGKδ expression increases the severity of type 2 diabetes. However, the role of DGKδ in myogenic differentiation is still unknown. The present study demonstrated that DGKδ expression was down-regulated in the early stage of C2C12 myogenic differentiation almost concurrently with a decrease in cyclin D1 expression. The knockdown of DGKδ by DGKδ-specific siRNAs significantly increased the levels of cyclin D1 expression at 48 h after C2C12 myogenic differentiation. In contrast, at the same time, the knockdown of DGKδ decreased the levels of myogenin expression and the number of myosin heavy chain (MHC)-positive cells. These results indicate that DGKδ regulates the early differentiation of C2C12 myoblasts via controlling the down-regulation of cyclin D1 expression. Moreover, the suppression of DGKδ expression increased the phosphorylation levels of conventional and novel protein kinase Cs (cnPKCs). Furthermore, DGKδ suppression increased the levels of cyclin D1 and phospho-cnPKCs even at the first 24 h of myogenic differentiation. These results suggest that DGKδ controls the down-regulation of cyclin D1 expression by attenuating the PKC signaling pathway for C2C12 myogenic differentiation.

PAX3-FOXO1 drives miR-486-5p and represses miR-221 contributing to pathogenesis of alveolar rhabdomyosarcoma

Rhabdomyosarcoma is the most common soft-tissue sarcoma in childhood and histologically resembles developing skeletal muscle. Alveolar rhabdomyosarcoma (ARMS) is an aggressive subtype with a higher rate of metastasis and poorer prognosis. The majority of ARMS tumors (80%) harbor a PAX3-FOXO1 or less commonly a PAX7-FOXO1 fusion gene. The presence of either the PAX3-FOXO1 or PAX7-FOXO1 fusion gene foretells a poorer prognosis resulting in clinical re-classification as either fusion-positive (FP-RMS) or fusion-negative RMS (FN-RMS). The PAX3/7-FOXO1 fusion genes result in the production of a rogue transcription factors that drive FP-RMS pathogenesis and block myogenic differentiation. Despite knowing the molecular driver of FP-RMS, targeted therapies have yet to make an impact for patients, highlighting the need for a greater understanding of the molecular consequences of PAX3-FOXO1 and its target genes including microRNAs. Here we show FP-RMS patient-derived xenografts and cell lines display a distinct microRNA expression pattern. We utilized both loss- and gain-of function approaches in human cell lines with knockdown of PAX3-FOXO1 in FP-RMS cell lines and expression of PAX3-FOXO1 in human myoblasts and identified microRNAs both positively and negatively regulated by the PAX3-FOXO1 fusion protein. We demonstrate PAX3-FOXO1 represses miR-221/222 that functions as a tumor suppressing microRNA through the negative regulation of CCND2, CDK6, and ERBB3. In contrast, miR-486-5p is transcriptionally activated by PAX3-FOXO1 and promotes FP-RMS proliferation, invasion, and clonogenic growth. Inhibition of miR-486-5p in FP-RMS xenografts decreased tumor growth, illustrating a proof of principle for future therapeutic intervention. Therefore, PAX3-FOXO1 regulates key microRNAs that may represent novel therapeutic vulnerabilities in FP-RMS.

Cycling through developmental decisions: how cell cycle dynamics control pluripotency, differentiation and reprogramming

A strong connection exists between the cell cycle and mechanisms required for executing cell fate decisions in a wide-range of developmental contexts. Terminal differentiation is often associated with cell cycle exit, whereas cell fate switches are frequently linked to cell cycle transitions in dividing cells. These phenomena have been investigated in the context of reprogramming, differentiation and trans-differentiation but the underpinning molecular mechanisms remain unclear. Most progress to address the connection between cell fate and the cell cycle has been made in pluripotent stem cells, in which the transition through mitosis and G1 phase is crucial for establishing a window of opportunity for pluripotency exit and the initiation of differentiation. This Review will summarize recent developments in this area and place them in a broader context that has implications for a wide range of developmental scenarios.

Stomagenesis versus myogenesis: Parallels in intrinsic and extrinsic regulation of transcription factor mediated specialized cell-type differentiation in plants and animals

Although the last common unicellular ancestor of plants and animals diverged several billion years ago, and while having developed unique developmental programs that facilitate differentiation and proliferation specific to plant and animal systems, there still exists a high degree of conservation in the logic regulating these developmental processes within these two seemingly diverse kingdoms. Stomatal differentiation in plants involves a series of orchestrated cell division events mediated by a family of closely related bHLH transcription factors (TFs) to create a pair of mature guard cells. These TFs are in turn regulated by a number of upstream signaling components that ultimately function to achieve lineage specific differentiation and organized tissue patterning on the plant epidermis. The logic involved in the specification of the myogenic differentiation program in animals is intriguingly similar to stomatal differentiation in plants: Closely-related myogenic bHLHs, known as MRFs (Myogenic Regulatory Factors) provide lineage specificity essential for cell-fate determination. These MRFs, similar to the bHLHs in plants, are regulated by several upstream signaling cascades that succinctly regulate each differentiation step, leading to the production of mature muscle fibers. This review aims at providing a perspective on the emerging parallels in the logic employed by key bHLH transcription factors and their upstream signaling components that function to precisely regulate key cell-state transition events in the stomatal as well as myogenic cell lineages.

Non-canonical functions of cell cycle cyclins and cyclin-dependent kinases

The roles of cyclins and their catalytic partners, the cyclin-dependent kinases (CDKs), as core components of the machinery that drives cell cycle progression are well established. Increasing evidence indicates that mammalian cyclins and CDKs also carry out important functions in other cellular processes, such as transcription, DNA damage repair, control of cell death, differentiation, the immune response and metabolism. Some of these non-canonical functions are performed by cyclins or CDKs, independently of their respective cell cycle partners, suggesting that there was a substantial divergence in the functions of these proteins during evolution.

Ethanol extract of Cyclolepis genistoides D. Don (palo azul) induces formation of myotubes, which involves differentiation of C2C12 myoblast cells

In this study, we examined the cell differentiation effect of an ethanol extract of Cyclolepis genistoides D. Don, a herbaceous perennial belonging to the family Asteraceae (vernacular name: palo azul). Palo azul has numerous physiological effects that contribute to the prevention of metabolic syndromes, although the mechanism remains unclear. We previously suggested that palo azul has antidiabetic activity via an adipose differentiation effect. Here, we focused on whether palo azul promoted the differentiation of myoblasts. The mouse muscle myoblast cell line C2C12 was cultured and differentiated using horse serum with or without an ethanol extract of palo azul (12.5-200 μg/mL). Quantitative real-time polymerase chain reaction was performed to evaluate differentiation markers, including insulin-like growth factor-1 and myogenin. To evaluate myotube formation, myosin heavy-chain (MHC) expression and localization were detected by immunohistochemistry. Palo azul increased the expression of the differentiation markers. Furthermore, immunohistochemistry analysis revealed increased formation of MHC myotubes after palo azul treatment along with increased diameter and fusion indices of the myotubes. The expression level of MHC was also increased. In conclusion, palo azul may increase muscle mass in the body and improve insulin resistance conditions by facilitating the formation of myotubes by promoting myocyte differentiation.

Coordinating cell proliferation and differentiation: Antagonism between cell cycle regulators and cell type-specific gene expression

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Fat deposition and accumulation in the damaged and inflamed skeletal muscle: cellular and molecular players

The skeletal muscle has the capacity to repair damage by the activation and differentiation of fiber sub-laminar satellite cells. Regeneration impairment due to reduced satellite cells number and/or functional capacity leads to fiber substitution with ectopic tissues including fat and fibrous tissue and to the loss of muscle functions. Muscle mesenchymal cells that in physiological conditions sustain or directly contribute to regeneration differentiate in adipocytes in patients with persistent damage and inflammation of the skeletal muscle. These cells comprise the fibro-adipogenic precursors, the PW1-expressing cells and some interstitial cells associated with vessels (pericytes, mesoangioblasts and myoendothelial cells). Resident fibroblasts that are responsible for collagen deposition and extracellular matrix remodeling during regeneration yield fibrotic tissue and can differentiate into adipose cells. Some authors have also proposed that satellite cells themselves could transdifferentiate into adipocytes, although recent results by lineage tracing techniques seem to put this theory to discussion. This review summarizes findings about muscle resident mesenchymal cell differentiation in adipocytes and recapitulates the molecular mediators involved in intramuscular adipose tissue deposition.

Determination of PCNA, cyclin D3, p27, p57 and apoptosis rate in normal and dexamethasone-induced intrauterine growth restricted rat placentas

Intrauterine growth restriction (IUGR) is a major clinical problem, which causes perinatal morbidity and mortality. One of the reasons for IUGR is abnormal placentation. In rats, fetal-placental exposure to maternally administered glucocorticoids decreases birth weight and placental weight. Proper placental development depends on the proliferation and differentiation of trophoblasts. Our knowledge about the mitotic regulators that play key roles in synchronizing these events is limited. Also the mechanisms underlying the placental growth inhibitory effects of glucocorticoids have not been elucidated. The aim of this study was to investigate the immunolocalization, mRNA and protein levels of proliferating cell nuclear antigen (PCNA), cyclin D3, p27 and p57 in normal and dexamethasone-induced IUGR Wistar rat placentas by reverse transcriptase polymerase chain reaction (RT-PCR), immunohistochemistry and Western blot. We also compared apoptotic cell numbers at the light microscopic level via terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL) and transmission electron microscopy. Glucocorticoid levels were higher in IUGR rats than in control rats after 60 and 120min of injection. We showed reduced gene and protein expressions of PCNA and cyclin D3 and increased expressions of p27 and p57 in IUGR placentas compared to control placentas. Apoptotic cell number was higher in the placentas of the IUGR group. In brief we found that maternal dexamethasone treatment led to a shift from cell proliferation to apoptosis in IUGR placentas. Dexamethasone induced placental and embryonal abnormalities which could be associated with reduced expressions of PCNA and cyclin D3, increased expressions of p27 and p57 and increased rate of apoptosis in IUGR placentas.

Double homeobox gene, Duxbl, promotes myoblast proliferation and abolishes myoblast differentiation by blocking MyoD transactivation

Homeobox genes encode transcription factors that regulate embryonic development programs including organogenesis, axis formation and limb development. Previously, we identified and cloned a mouse double homeobox gene, Duxbl, whose homeodomain exhibits the highest identity (67 %) to human DUX4, a candidate gene of facioscapulohumeral muscular dystrophy (FSHD). Duxbl proteins have been shown to be expressed in elongated myocytes and myotubes of trunk and limb muscles during embryogenesis. In this study, we found that Duxbl maintained low expression levels in various adult muscles. Duxbl proteins were induced to express in activated satellite cells and colocalized with MyoG, a myogenic differentiating marker. Furthermore, Duxbl proteins were not detected in quiescent satellite cells but detected in regenerated myocytes and colocalized with MyoD and MyoG following cardiotoxin-induced muscle injury. Ectopic Duxbl overexpressions in C2C12 myoblast cells promoted cell proliferation through mainly enhancing cyclin D1 and hyper-phosphorylated retinoblastoma protein but reducing p21 expression. However, Duxbl overexpression in C2C12 cells inhibited myogenic differentiation by decreasing MyoD downstream gene expressions, including M-cadherin, MyoG, p21 and cyclin D3 but not MyoD itself. Duxbl overexpressions also promoted cell proliferation but blocked MyoD-induced myogenic conversion in multipotent mesenchymal C3H10T1/2 cells. In addition, results of a luciferase reporter assay suggest that Duxbl negatively regulated MyoG promoter activity through the proximal two E boxes. In conclusion, these results indicate that Duxbl may play a crucial role in myogenesis and postnatal muscle regeneration by activating and proliferating satellite and myoblast cells.

Expression and methylation of FGF2, TGF-β and their downstream mediators during different developmental stages of leg muscles in chicken

A number of growth factors determine the proliferation of myoblasts and therefore the number of ultimate myofibers. The members of transforming growth factor-beta (TGF-β) family and the fibroblast growth factor 2 (FGF2) have profound effects on skeletal myoblasts proliferation in various animal systems. To investigate their involvement in different stages of avian skeletal muscle development in vivo, we detected the mRNA expression and DNA methylation profiles of TGF-β2, TGF-β3, FGF2 and their downstream mediators in leg muscles at embryonic day 10, day of hatch and day 45 posthatch, using both Arbor Acres meat-type and White Leghorn egg-type chickens. By real-time PCR, we found that the expression levels of TGF-β2, TGF-β3, Smad3 and FGF2 were significantly (P≤0.01) higher at embryonic day 10, a developmental window of abundant fetal myoblasts expansion, by comparison to day of hatch and day 45 posthatch. The methylation status of the 5' end region of these four genes was examined subsequently. A section of a CpG island in the 5' end region of FGF2 was significantly hypomethylated (P≤0.01) at embryonic day 10, compared with neonatal and postnatal stages in both stocks. Our results suggested that TGF-β2, TGF-β3, Smad3 and FGF2 may play important roles in fetal myoblasts proliferation in chicken hindlimb, and the transcription of FGF2 in this wave of myogenesis could be affected by DNA methylation in 5' flanking region. These outcomes contribute to our knowledge of the growth factors in avian myogenesis. Further investigation is needed to confirm and fully understand their functions in fetal limb myogenesis in birds.

Pax6 is required for normal cell-cycle exit and the differentiation kinetics of retinal progenitor cells

The coupling between cell-cycle exit and onset of differentiation is a common feature throughout the developing nervous system, but the mechanisms that link these processes are mostly unknown. Although the transcription factor Pax6 has been implicated in both proliferation and differentiation of multiple regions within the central nervous system (CNS), its contribution to the transition between these successive states remains elusive. To gain insight into the role of Pax6 during the transition from proliferating progenitors to differentiating precursors, we investigated cell-cycle and transcriptomic changes occurring in Pax6 (-) retinal progenitor cells (RPCs). Our analyses revealed a unique cell-cycle phenotype of the Pax6-deficient RPCs, which included a reduced number of cells in the S phase, an increased number of cells exiting the cell cycle, and delayed differentiation kinetics of Pax6 (-) precursors. These alterations were accompanied by coexpression of factors that promote (Ccnd1, Ccnd2, Ccnd3) and inhibit (P27 (kip1) and P27 (kip2) ) the cell cycle. Further characterization of the changes in transcription profile of the Pax6-deficient RPCs revealed abrogated expression of multiple factors which are known to be involved in regulating proliferation of RPCs, including the transcription factors Vsx2, Nr2e1, Plagl1 and Hedgehog signaling. These findings provide novel insight into the molecular mechanism mediating the pleiotropic activity of Pax6 in RPCs. The results further suggest that rather than conveying a linear effect on RPCs, such as promoting their proliferation and inhibiting their differentiation, Pax6 regulates multiple transcriptional networks that function simultaneously, thereby conferring the capacity to proliferate, assume multiple cell fates and execute the differentiation program into retinal lineages.

A high-content, high-throughput siRNA screen identifies cyclin D2 as a potent regulator of muscle progenitor cell fusion and a target to enhance muscle regeneration

Cell-mediated regenerative approaches using muscle progenitor cells hold promises for the treatment of many forms of muscle disorders. Their applicability in the clinic, however, is hindered by the low levels of regeneration obtained after transplantation and the large number of cells required to achieve an effect. To better understand the mechanisms that regulate the temporal switch of replicating muscle progenitor cells into terminally differentiated cells and to develop new strategies that could enhance muscle regeneration, we have developed and performed a high-throughput screening (HTS) capable of identifying genes that play active roles during myogenesis. Secondary and tertiary screens were used to confirm the effects of RNAi in vitro and in vivo and to select for candidate hits that significantly increase regeneration into skeletal muscles. Downregulation of cyclin D2 (CCND2) was shown to dramatically enhance myogenic differentiation of muscle progenitor cells and to induce a robust regeneration after cell transplantation into skeletal muscles of dystrophin-deficient mice. Protein interaction network and pathway analysis revealed that CCND2 directly interacts with the cyclin-dependent kinase Cdk4 to inhibit phosphorylation of the retinoblastoma protein (pRb), thus blocking the activation of the myogenic switch during fusion. These studies identify CCND2 as a new key regulator of terminal differentiation in muscle progenitor cells and open new possibilities for the treatment of many forms of muscle disorders characterized by impaired regeneration and loss of muscle mass.

Small heat shock protein expression and functions during development

The expression of small heat shock proteins is tightly regulated during development in multiple organisms. As housekeeping proteins, small heat shock proteins help protect cells from apoptosis, stabilize the cytoskeleton and contribute to proteostasis. Consistently, depletion of one small heat shock protein is usually not detrimental due to a certain level of redundancy between the functions of each small heat shock protein. However, while their stress-induced expression is regulated by heat shock factors, their constitutive expression is under the control of other specific transcription factors, suggesting the existence of very specialized functions. This review focuses on the expression patterns and functions of small heat shock proteins in various organisms during development. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Down-regulation of myogenin can reverse terminal muscle cell differentiation

Certain higher vertebrates developed the ability to reverse muscle cell differentiation (dedifferentiation) as an additional mechanism to regenerate muscle. Mammals, on the other hand, show limited ability to reverse muscle cell differentiation. Myogenic Regulatory Factors (MRFs), MyoD, myogenin, Myf5 and Myf6 are basic-helix-loop-helix (bHLH) transcription factors essential towards the regulation of myogenesis.

Our current interest is to investigate whether down-regulation of MRFs in terminally differentiated mouse myotubes can induce reversal of muscle cell differentiation. Results from this work showed that reduction of myogenin levels in terminally differentiated mouse myotubes can reverse their differentiation state. Down-regulation of myogenin in terminally differentiated mouse myotubes induces cellular cleavage into mononucleated cells and cell cycle re-entry, as shown by re-initiation of DNA synthesis and increased cyclin D1 and cyclin E2 levels. Finally, we provide evidence that down-regulation of myogenin causes cell cycle re-entry (via down-regulation of MyoD) and cellularisation through separate pathways. These data reveal the important role of myogenin in maintaining terminal muscle cell differentiation and point to a novel mechanism by which muscle cells could be re-activated through its down-regulation.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Myogenesis and rhabdomyosarcoma the Jekyll and Hyde of skeletal muscle

Rhabdomyosarcoma, a neoplasm composed of skeletal myoblast-like cells, represents the most common soft tissue sarcoma in children. The application of intensive chemotherapeutics and refined surgical and radiation therapy approaches have improved survival for children with localized disease over the past 3 decades; however, these approaches have not improved the dismal outcome for children with metastatic and recurrent rhabdomyosarcoma. Elegant studies have defined the molecular mechanisms driving skeletal muscle lineage commitment and differentiation, and the machinery that couples differentiation with irreversible cell proliferation arrest. Further, detailed molecular analyses indicate that rhabdomyosarcoma cells have lost the capacity to fully differentiate when challenged to do so in experimental models. We review the intersection of normal skeletal muscle developmental biology and the molecular genetic defects in rhabdomyosarcoma with the underlying premise that understanding how the differentiation process has gone awry will lead to new treatment strategies aimed at promoting myogenic differentiation and concomitant cell cycle arrest.

Cell cyclins: triggering elements of cancer or not?

Cyclins are indispensable elements of the cell cycle and derangement of their function can lead to cancer formation. Recent studies have also revealed more mechanisms through which cyclins can express their oncogenic potential. This review focuses on the aberrant expression of G1/S cyclins and especially cyclin D and cyclin E; the pathways through which they lead to tumour formation and their involvement in different types of cancer. These elements indicate the mechanisms that could act as targets for cancer therapy.

Pleiotrophin regulates lung epithelial cell proliferation and differentiation during fetal lung development via beta-catenin and Dlk1

The role of pleiotrophin in fetal lung development was investigated. We found that pleiotrophin and its receptor, protein-tyrosine phosphatase receptor beta/zeta, were highly expressed in mesenchymal and epithelial cells of the fetal lungs, respectively. Using isolated fetal alveolar epithelial type II cells, we demonstrated that pleiotrophin promoted fetal type II cell proliferation and arrested type II cell trans-differentiation into alveolar epithelial type I cells. Pleiotrophin also increased wound healing of injured type II cell monolayer. Knockdown of pleiotrophin influenced lung branching morphogenesis in a fetal lung organ culture model. Pleiotrophin increased the tyrosine phosphorylation of beta-catenin, promoted beta-catenin translocation into the nucleus, and activated T cell factor/lymphoid enhancer factor transcription factors. Dlk1, a membrane ligand that initiates the Notch signaling pathway, was identified as a downstream target of the pleiotrophin/beta-catenin pathway by endogenous dlk1 expression, promoter assay, and chromatin immunoprecipitation. These results provide evidence that pleiotrophin regulates fetal type II cell proliferation and differentiation via integration of multiple signaling pathways including pleiotrophin, beta-catenin, and Notch pathways.

STAT3 induces muscle stem cell differentiation by interaction with myoD

Signal transducers and activators of transcription (STAT) family proteins transduce pivotal biological effects of various cytokines and hormones. STAT3 proteins are known to play a central role in the regulation of growth, differentiation, and survival of many types of cells. However, the function of STAT3 in myogenesis still remains largely unknown. We now provided direct evidence that STAT3 could induce myogenic differentiation and this effect might be mediated by interaction with MyoD--the essential transcription factor during myogenic differentiation. Furthermore, leukemia inhibitory factor (LIF) might be the upstream factor which activated JAK2/STAT3 pathway to stimulate muscle cell differentiation. Taken together, these results provide a molecular basis for further understanding of the muscle regeneration mechanism.

Interleukin-6 is an essential regulator of satellite cell-mediated skeletal muscle hypertrophy

Skeletal muscles adapt to increasing workload by augmenting their fiber size, through mechanisms that are poorly understood. This study identifies the cytokine interleukin-6 (IL-6) as an essential regulator of satellite cell (muscle stem cell)-mediated hypertrophic muscle growth. IL-6 is locally and transiently produced by growing myofibers and associated satellite cells, and genetic loss of IL-6 blunted muscle hypertrophy in vivo. IL-6 deficiency abrogated satellite cell proliferation and myonuclear accretion in the preexisting myofiber by impairing STAT3 activation and expression of its target gene cyclin D1. The growth defect was indeed muscle cell intrinsic, since IL-6 loss also affected satellite cell behavior in vitro, in a STAT3-dependent manner. Myotube-produced IL-6 further stimulated cell proliferation in a paracrine fashion. These findings unveil a role for IL-6 in hypertrophic muscle growth and provide mechanistic evidence for the contribution of satellite cells to this process.

Genetic analysis of p38 MAP kinases in myogenesis: fundamental role of p38alpha in abrogating myoblast proliferation

The p38 mitogen-activated protein kinase (MAPK) pathway plays a critical role in skeletal muscle differentiation. However, the relative contribution of the four p38 MAPKs (p38alpha, p38beta, p38gamma and p38delta) to this process is unknown. Here we show that myoblasts lacking p38alpha, but not those lacking p38beta or p38delta, are unable to differentiate and form multinucleated myotubes, whereas p38gamma-deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38alpha is caused by delayed cell-cycle exit and continuous proliferation in differentiation-promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38alpha-deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38alpha controls myogenesis by antagonizing the activation of the JNK proliferation-promoting pathway, before its direct effect on muscle differentiation-specific gene transcription. More importantly, in agreement with the defective myogenesis of cultured p38alpha(Delta/Delta) myoblasts, neonatal muscle deficient in p38alpha shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38alpha in muscle formation in vitro and in vivo.

Minireview: Cyclin D1: normal and abnormal functions

Cyclin D1 encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the retinoblastoma protein and promotes progression through the G1-S phase of the cell cycle. Amplification or overexpression of cyclin D1 plays pivotal roles in the development of a subset of human cancers including parathyroid adenoma, breast cancer, colon cancer, lymphoma, melanoma, and prostate cancer. Of the three D-type cyclins, each of which binds cyclin-dependent kinase (CDK), it is cyclin D1 overexpression that is predominantly associated with human tumorigenesis and cellular metastases. In recent years accumulating evidence suggests that in addition to its original description as a CDK-dependent regulator of the cell cycle, cyclin D1 also conveys cell cycle or CDK-independent functions. Cyclin D1 associates with, and regulates activity of, transcription factors, coactivators and corepressors that govern histone acetylation and chromatin remodeling proteins. The recent findings that cyclin D1 regulates cellular metabolism, fat cell differentiation and cellular migration have refocused attention on novel functions of cyclin D1 and their possible role in tumorigenesis. In this review, both the classic and novel functions of cyclin D1 are discussed with emphasis on the CDK-independent functions of cyclin D1.

Sharp-1/DEC2 inhibits skeletal muscle differentiation through repression of myogenic transcription factors

Skeletal muscle differentiation is regulated by the basic-helix-loop-helix (bHLH) family of transcription factors. The myogenic bHLH factors form heterodimers with the ubiquitously expressed bHLH E-proteins and bind E-box (CANNTG) sites present in the promoters of several muscle-specific genes. Our previous studies have shown that the bHLH factor Sharp-1 is expressed in skeletal muscle and interacts with MyoD and E-proteins. However, its role in regulation of myogenic differentiation remains unknown. We report here that endogenous Sharp-1 is expressed in proliferating C2C12 myoblasts and is down-regulated during myogenic differentiation. Constitutive expression of Sharp-1 in C2C12 myoblasts promotes cell cycle exit causing a decrease in cyclin D1 expression but blocks terminal differentiation. Although MyoD expression is not inhibited, the induction of differentiation-specific genes such as myogenin, MEF2C, and myosin heavy chain is impaired by Sharp-1 overexpression. We demonstrate that the interaction of Sharp-1 with MyoD and E-proteins results in reduced DNA binding and transactivation from MyoD-dependent E-box sites. Re-expression of MyoD approximately E47 rescues the differentiation defect imposed by Sharp-1, suggesting that myogenic bHLH factors function downstream of Sharp-1. Our data suggest that protein-protein interactions between Sharp-1, MyoD, and E47 resulting in interference with MyoD function underlies Sharp-1-mediated repression of myogenic differentiation.

Cyclin D3 interacts with human activating transcription factor 5 and potentiates its transcription activity

The Cyclin D3 protein is a member of the D-type cyclins. Besides serving as cell cycle regulators, D-type cyclins have been reported to be able to interact with several transcription factors and modulate their transcriptional activations. Here we report that human activating transcription factor 5 (hATF5) is a new interacting partner of Cyclin D3. The interaction was confirmed by in vivo coimmunoprecipitation and in vitro binding analysis. Neither interaction between Cyclin D1 and hATF5 nor interaction between Cyclin D2 and hATF5 was observed. Confocal microscopy analysis showed that Cyclin D3 could colocalize with hATF5 in the nuclear region. Cyclin D3 could potentiate hATF5 transcriptional activity independently of its Cdk4 partner. But Cyclin D1 and Cyclin D2 had no effect on hATF5 transcriptional activity. These data provide a new clue to understand the new role of Cyclin D3 as a transcriptional regulator.

Regulation and Function of Cyclin D2 in B Lymphocyte Subsets

Abs produced by B lymphocytes play an essential role in humoral immunity against pathogens. This response is dependent upon the extent of genome replication, which in turn allows clonal expansion of Ag-specific B cell precursors. Thus, there is considerable interest in understanding how naive B cells commit to genome replication following Ag challenge. The BCR is a key regulator of B cell growth responses in the bone marrow and the periphery. The importance of identifying BCR-coupled signaling networks and their cell cycle targets is underscored by the recognition that aberrant cell cycle control can lead to lymphoproliferative disorders or lymphoid malignancies. This review focuses on recent progress toward understanding the function of cyclin D2 in cell cycle control, and in the development of murine B lymphocytes.

Mirk/dyrk1B kinase destabilizes cyclin D1 by phosphorylation at threonine 288

The phosphorylation of cyclin D1 at threonine 286 by glycogen synthase kinase 3beta (GSK3beta) has been shown to be required for the ubiquitination and nuclear export of cyclin D1 and its subsequent degradation in the proteasome. The mutation of the nearby residue, threonine 288, to nonphosphorylatable alanine has also been shown to reduce the ubiquitination of cyclin D1, suggesting that phosphorylation at threonine 288 may also lead to degradation of cyclin D1. We now demonstrate that the G(0)/G(1)-active arginine-directed protein kinase Mirk/dyrk1B binds to cyclin D1 and phosphorylates cyclin D1 at threonine 288 in vivo and that the cyclin D1-T288A construct is more stable than wild-type cyclin D1. Transient overexpression of Mirk in nontransformed Mv1Lu lung epithelial cells blocked cells in G(0)/G(1). Depletion of endogenous Mirk by RNA interference increased cyclin D1 protein levels but not mRNA levels, indicating that Mirk destabilizes cyclin D1 protein. Destabilization was confirmed by induction of a stable Mirk transfectant of Mv1Lu cells, which blocked cell migration (Zou, Y., Lim, S., Lee, K., Deng, X., and Friedman, E. (2003) J. Biol. Chem. 278, 49573-49581), and caused a decrease in the half-life of endogenous cyclin D1, concomitant with an increase in Mirk expression. In vitro cyclin D1 was phosphorylated in an additive fashion by Mirk and GSK3beta. Mirk-phosphorylated cyclin D1 mutated at the GSK3beta phosphorylation site and was capable of phosphorylating cyclin D1 in the presence of the GSK3beta inhibitor LiCl. Mirk may function together with GSK3beta to assist cell arrest in G(0)/G(1) by destabilizing cyclin D1.

In vivo expression patterns of MyoD, p21, and Rb proteins in myonuclei and satellite cells of denervated rat skeletal muscle

MyoD, a myogenic regulatory factor, is rapidly expressed in adult skeletal muscles in response to denervation. However, the function(s) of MyoD expressed in denervated muscle has not been adequately elucidated. In vitro, it directly transactivates cyclin-dependent kinase inhibitor p21 (p21) and retinoblastoma protein (Rb), a downstream target of p21. These factors then act to regulate cell cycle withdrawal and antiapoptotic cell death. Using immunohistochemical approaches, we characterized cell types expressing MyoD, p21, and Rb and the relationship among these factors in the myonucleus of denervated muscles. In addition, we quantitatively examined the time course changes and expression patterns among distinct myofiber types of MyoD, p21, and Rb during denervation. Denervation induced MyoD expression in myonuclei and satellite cell nuclei, whereas p21 and Rb were found only in myonuclei. Furthermore, coexpression of MyoD, p21, and Rb was induced in the myonucleus, and quantitative analysis of these factors determined that there was no difference among the three myofiber types. These observations suggest that MyoD may function in myonuclei in response to denervation to protect against denervation-induced apoptosis via perhaps the activation of p21 and Rb, and function of MyoD expressed in satellite cell nuclei may be negatively regulated. The present study provides a molecular basis to further understand the function of MyoD expressed in the myonuclei and satellite cell nuclei of denervated skeletal muscle.

Muscle regulatory factor MRF4 activates differentiation in rhabdomyosarcoma RD cells through a positive-acting C-terminal protein domain

Rhabdomyosarcoma (RMS) has deregulated proliferation and is blocked in the differentiation program despite Myf-5, MyoD and myogenin expression. Here we show that ectopic expression of MRF4, which is not subject to an autoregulatory pathway but regulated by the other MRFs protein family, induces growth arrest and terminal differentiation in RD cells. Deletion mapping identified a positive-acting C-terminal domain in MRF4 as the mediator of transcriptional activity, revealing a conserved motif with helix III in MyoD previously found to initiate expression of endogenous skeletal muscle genes. By using chimeric MyoD/MRF4 proteins, we observe that the C-terminal motif of MRF4 rescues MyoD activity in RD cells. Moreover, comparative induction of muscle-specific genes following activation of MyoD, through the expression of a constitutively activated MKK6 either in the absence or presence of MRF4, shows that MyoD and MRF4 can differently regulate muscle genes expression. Together, these results demonstrate that the MRF4 C-terminus functions as specification as well as activation domain in tumor cells. They provide a basis to identify gene products necessary for b-HLH-mediated differentiation versus tumor progression.

Dynamic gene expression during the onset of myoblast differentiation in vitro

Skeletal myogenesis is a well-studied differentiation process. However, despite the identification and functional characterization of the myogenic basic HLH transcription factors, molecular details are still lacking. With the advent of microarray technology, it has become possible to look at changes in gene expression profiles in a biological process on an unprecedented scale. In this study, we applied this technology to profile gene expression during the in vitro differentiation of an established myoblast cell line, C2C12. We report over 1500 genes whose expression is altered when these cells differentiate, including 624, or about 40% of the total number of genes, with unknown functions. This analysis reveals the existence of 12 groups of coordinately regulated genes that are expressed in temporal waves of gene expression prior to the transcriptional induction of myogenin. Among these are multiple families of transcription factors that are important for the process of myogenesis. In addition, the induction of the Notch signaling pathway suggests that previously unappreciated intercellular signaling occurs during myogenic differentiation. These results provide a molecular description of the skeletal myogenesis up to the activation of myogenin.

A single cdk inhibitor, p27Xic1, functions beyond cell cycle regulation to promote muscle differentiation in Xenopus

The molecular basis of the antagonism between cellular proliferation and differentiation is poorly understood. We have investigated the role of the cyclin-dependent kinase inhibitor p27(Xic1) in the co-ordination of cell cycle exit and differentiation during early myogenesis in vivo using Xenopus embryos. In this report, we demonstrate that p27(Xic1) is highly expressed in the developing myotome, that ablation of p27(Xic1) protein prevents muscle differentiation and that p27(Xic1) synergizes with the transcription factor MyoD to promote muscle differentiation. Furthermore, the ability of p27(Xic1) to promote myogenesis resides in an N-terminal domain and is separable from its cell cycle regulation function. This data demonstrates that a single cyclin-dependent kinase inhibitor, p27(Xic1), controls in vivo muscle differentiation in Xenopus and that regulation of this process by p27(Xic1) requires activities beyond cell cycle inhibition.

In vivo activation of STAT3 signaling in satellite cells and myofibers in regenerating rat skeletal muscles

Although growth factors and cytokines play critical roles in skeletal muscle regeneration, intracellular signaling molecules that are activated by these factors in regenerating muscles have been not elucidated. Several lines of evidence suggest that leukemia inhibitory factor (LIF) is an important cytokine for the proliferation and survival of myoblasts in vitro and acceleration of skeletal muscle regeneration. To elucidate the role of LIF signaling in regenerative responses of skeletal muscles, we examined the spatial and temporal activation patterns of an LIF-associated signaling molecule, the signal transducer and activator transcription 3 (STAT3) proteins in regenerating rat skeletal muscles induced by crush injury. At the early stage of regeneration, activated STAT3 proteins were first detected in the nuclei of activated satellite cells and then continued to be activated in proliferating myoblasts expressing both PCNA and MyoD proteins. When muscle regeneration progressed, STAT3 signaling was no longer activated in differentiated myoblasts and myotubes. In addition, activation of STAT3 was also detected in myonuclei within intact sarcolemmas of surviving myofibers that did not show signs of necrosis. These findings suggest that activation of STAT3 signaling is an important molecular event that induces the successful regeneration of injured skeletal muscles.

Role for cellular Src kinase in myoblast proliferation

In this study, a role for cellular Src in muscle cell proliferation and differentiation was investigated. Pharmacological inhibition of Src-class kinases repressed proliferation and promoted differentiation of the C2C12 muscle cell line, even when the cells were cultured under growth-inducing conditions of high serum. Pharmacological inhibition of Src-class kinases also affected cellular components that regulate proliferation and differentiation in muscle; cyclin D1 levels were reduced while, myogenin was increased. Suppression of cyclin D1 and enhancement of myogenin levels also occurred upon expression of a dominant negative Src, corroborating a role for Src kinases in regulating proliferation and differentiation. Inhibition of Src-family kinases also blocked fibroblast growth factor (FGF) induced proliferation but, notably, did not reverse the effect of FGF to inhibit differentiation. Evidence for the Src-class kinase Src in myoblast mitogenesis was obtained by determining the pattern of protein expression and activity for this kinase. Under all conditions examined, Src's expression and enzymatic activity were high in cultures of myoblasts and down-regulated during differentiation. Importantly, Src's activity was rapidly stimulated by mitogen-containing serum and attenuated when myoblasts were switched to low serum-containing differentiation medium. These data indicate that Src is important for maintaining muscle cell proliferation.

Synergism between INK4a/ARF inactivation and aberrant HGF/SF signaling in rhabdomyosarcomagenesis

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in children, yet molecular events associated with the genesis and progression of this potentially fatal disease are largely unknown. For the molecules and pathways that have been implicated, genetic validation has been impeded by lack of a mouse model of RMS. Here we show that simultaneous loss of Ink4a/Arf function and disruption of c-Met signaling in Ink4a/Arf(-/-) mice transgenic for hepatocyte growth factor/scatter factor (HGF/SF) induces RMS with extremely high penetrance and short latency. In cultured myoblasts, c-Met activation and Ink4a/Arf loss suppress myogenesis in an additive fashion. Our data indicate that human c-MET and INK4a/ARF, situated at the nexus of pathways regulating myogenic growth and differentiation, represent critical targets in RMS pathogenesis. The marked synergism in mice between aberrant c-Met signaling and Ink4a/Arf inactivation, lesions individually implicated in human RMS, suggests a therapeutic combination to combat this devastating childhood cancer.

Cyclin D1 represses the basic helix-loop-helix transcription factor, BETA2/NeuroD

Expression of the hormone secretin in enteroendocrine cells is restricted to the nondividing villus compartment of the intestine, implying that terminal differentiation is linked to cell cycle arrest and that differentiation is repressed in actively proliferating cells. We have shown previously that the basic helix-loop-helix protein, BETA2/NeuroD, induces cell cycle withdrawal in addition to increasing secretin gene expression. A number of transcription factors important for differentiation are repressed by D cyclins. Repression by D cyclins appears to be independent of its effects on the cell cycle. We show that cyclin D1 represses BETA2/NeuroD-dependent transcription of the secretin gene. Examination of cyclin box mutants shows that repression is unrelated to Cdk4 activation. Although cyclin D1 and BETA2 associate in vivo, they do not directly interact. Cyclin D1 may be recruited to BETA2 by binding to the C-terminal domain of the p300 coactivator, downstream from the BETA2-binding site. In the small intestine, cyclin D1 expression occurs only in the actively proliferating crypts of Lieberkuhn but not in villi. Thus repression by cyclin D1 may serve to prevent secretin gene transcription from occurring in relatively immature epithelial progenitor cells.

Persistent expression of cyclin D1 disrupts normal photoreceptor differentiation and retina development

The differentiation of neuronal cells in the developing mammalian retina is closely coupled to cell cycle arrest and proceeds in a highly organized manner. Cyclin D1, which regulates cell proliferation in many cells, also drives the proliferation of photoreceptor progenitors. In the mouse retina, cyclin D1 protein normally decreases as photoreceptors mature. To study the importance of the down-regulation of cyclin D1 during photoreceptor development, we generated a transgenic mouse in which cyclin D1 was persistently expressed in developing photoreceptor cells. We observed numerous abnormalities in both photoreceptors and other nonphotoreceptor cells in the retina of these transgenic mice. In particular, we observed delayed opsin expression in developing photoreceptors and alterations in their number and morphology in the mature retina. These alterations were accompanied by disorganization of the inner nuclear and plexiform layers. The expression of cyclin D1 caused excess photoreceptor cell proliferation and apoptosis. Loss of the p53 tumor suppressor gene decreased cyclin D1-induced apoptosis and led to microscopic hyperplasia in the retina. These findings are distinct from other mouse models in which the retinoblastoma gene pathway is disrupted and suggest that the IRBP-cyclin D1 mouse model may recapitulate an early step in the development of retinoblastoma.

Recruitment of (β)-catenin to cadherin-mediated intercellular adhesions is involved in myogenic induction

Cadherin-mediated cell adhesion is involved in muscle differentiation from early stages of myogenic induction to late stages of myoblast interaction and fusion. (β)-Catenin is a major constituent of cadherin-based adherens junctions and also serves as a signal transduction molecule that regulates gene expression during development. In this study, we explored the involvement of (β)-catenin in myogenic differentiation. We show here that shortly after a switch from growth to differentiation medium, (β)-catenin translocates to cell-cell junctions and its levels increase. We further show that elevation of (β)-catenin levels, induced either by inhibition of its breakdown, using LiCl, or by its overexpression, suppresses the formation of adherens junctions, resulting in a sharp decline in myogenin expression and an arrest of myogenic progression. Recruitment of (β)-catenin to adherens junctions after transfection with N-cadherin restores myogenin expression in the transfected cells. These results suggest that increased cadherin-mediated adhesion and translocation of (β)-catenin to adherens junctions are involved in activating the early steps of myogenic differentiation.

Identification of the E2A gene products as regulatory targets of the G1 cyclin-dependent kinases

The E2A gene products, E12 and E47, are multifunctional transcription factors that as homodimers regulate B cell development, growth, and survival. In this report, the E2A gene products are shown to be targets for regulation by the G1 cyclin-dependent kinases. Two novel G1 cyclin-dependent kinase sites are identified on the N-terminal domain of E12/E47. One site displays homology to a preferential D-type cyclin-dependent kinase site (serine 780) on the retinoblastoma susceptibility gene product (pRB) and, consistent with this homology, is more efficiently phosphorylated by cyclin D1-CDK4 than by the other cyclin-dependent kinases (CDK) that were tested. The second kinase site is phosphorylated by both cyclin D1-CDK4 and cyclin A/E-CDK2 complexes. Mutation studies indicated that phosphorylation of the cyclin D1-CDK4 site, or more potently, of both the cyclin D1-CDK4 and cyclin A/E-CDK2 sites, negatively regulates the growth suppressor function associated with the N-terminal domain of E12/E47. Transient expression studies showed that ectopic expression of cyclin D1 or E negatively regulates sequence-specific activation of gene transcription by E12/E47. Analysis of site mutants, however, indicated that inhibition of E12/E47 transcriptional activity did not require the N-terminal G1 cyclin-dependent kinase sites. Together, the results suggest that the growth suppressor and transcriptional activator functions of E12/E47 are targets for regulation by G1 cyclin-dependent kinases but that the mechanisms of regulation for each function are distinct.

Differential effects of Ras signaling through NFkappaB on skeletal myogenesis

Oncogenic Ras (H-Ras G12V) inhibits skeletal myogenesis through multiple signaling pathways. Previously, we demonstrated that the major downstream effectors of Ras (i.e., MEK/MAPK, RalGDS and Rac/Rho) play a minor, if any, role in the differentiation-defective phenotype of Ras myoblasts. Recently, NFkappaB, another Ras signaling target, has been shown to inhibit myogenesis presumably by stimulating cyclin D1 accumulation and cell cycle progression. In this study, we address the involvement of NFkappaB activation in the Ras-induced inhibition of myogenesis. Using H-Ras G12V and three G12V effector-loop variants, we detect high levels of NFkappaB transcriptional activity in C3H10T1/2-MyoD cells treated with differentiation medium. Myogenesis is blocked by all Ras proteins tested, yet only in the case of H-Ras G12V are cyclin D1 levels increased and cell cycle progression maintained. Expression of IkappaBalpha SR, an inhibitor of NFkappaB, does not reverse the differentiation-defective phenotype of Ras expressing cultures, but does induce differentiation in cultures treated with tumor necrosis factor (TNFalpha) or in cultures expressing the RelA/p65 subunit of NFkappaB. These data confirm that NFkappaB is a target of Ras and suggest that the cellular actions of NFkappaB require additional signals that are discriminated by the Ras effector-loop variants. Results with IkappaBalpha SR convincingly demonstrate that H-Ras G12V does not rely on NFkappaB activity to block myogenesis, an observation that continues to implicate another unidentified signaling pathway(s) in the inhibition of skeletal myogenesis by Ras.

NF-kappaB and cell-cycle regulation: the cyclin connection

The cyclins are a family of proteins that are centrally involved in cell cycle regulation and which are structurally identified by conserved "cyclin box" regions. They are regulatory subunits of holoenzyme cyclin-dependent kinase (CDK) complexes controlling progression through cell cycle checkpoints by phosphorylating and inactivating target substrates. CDK activity is controlled by cyclin abundance and subcellular location and by the activity of two families of inhibitors, the cyclin-dependent kinase inhibitors (CKI). Many hormones and growth factors influence cell growth through signal transduction pathways that modify the activity of the cyclins. Dysregulated cyclin activity in transformed cells contributes to accelerated cell cycle progression and may arise because of dysregulated activity in pathways that control the abundance of a cyclin or because of loss-of-function mutations in inhibitory proteins.Analysis of transformed cells and cells undergoing mitogen-stimulated growth implicate proteins of the NF-kappaB family in cell cycle regulation, through actions on the CDK/CKI system. The mammalian members of this family are Rel-A (p65), NF-kappaB(1) (p50; p105), NF-kappaB(2) (p52; p100), c-Rel and Rel-B. These proteins are structurally identified by an amino-terminal region of about 300 amino acids, known as the Rel-homology domain. They exist in cytoplasmic complexes with inhibitory proteins of the IkappaB family, and translocate to the nucleus to act as transcription factors when activated. NF-kappaB pathway activation occurs during transformation induced by a number of classical oncogenes, including Bcr/Abl, Ras and Rac, and is necessary for full transforming potential. The avian viral oncogene, v-Rel is an NF-kappaB protein. The best explored link between NF-kappaB activation and cell cycle progression involves cyclin D(1), a cyclin which is expressed relatively early in the cell cycle and which is crucial to commitment to DNA synthesis. This review examines the interactions between NF-kappaB signaling and the CDK/CKI system in cell cycle progression in normal and transformed cells. The growth-promoting actions of NF-kappaB factors are accompanied, in some instances, by inhibition of cellular differentiation and by inhibition of programmed cell death, which involve related response pathways and which contribute to the overall increase in mass of undifferentiated tissue.

Regulation of MyoD function in the dividing myoblast

Proliferating myoblasts express MyoD, yet no phenotypic markers are activated as long as mitogen levels are sufficient to keep the cells dividing. Depending upon mitogen levels, a decision is made in G1 that commits the myoblast to either continue to divide or to exit from the cell cycle and activate terminal differentiation. Ectopic expression of MyoD under the control of the RSV or CMV promoters causes 10T1/2 cells to rapidly exit the cell cycle and differentiate as single myocytes, even in growth medium, whereas expression of MyoD under the weaker SV40 promoter is compatible with proliferation. Co-expression of MyoD and cyclin D1, but not cyclins A, B, E or D3, blocks transactivation of a MyoD responsive reporter. Similarly, transfection of myoblasts with the cyclin-dependent kinase (cdk) inhibitors p16 and p21 supports some muscle-specific gene expression even in growth medium. Taken altogether, these results suggest cell cycle progression negatively regulates myocyte differentiation, possibly through a mechanism involving the D1 responsive cdks. We review evidence coupling growth status, the cell cycle and myogenesis. We describe a novel mitogen-sensitive mechanism that involves the cyclin D1-dependent direct interaction between the G1 cdks and MyoD in the dividing myoblast, which regulates MyoD function in a mitogen-sensitive manner.

Coordinating cell proliferation and differentiation

Cell proliferation and differentiation are highly coordinated processes during development. Recent studies have revealed that this coordination may result from dual functions residing in the central regulators of proliferation, allowing them to also regulate differentiation. Studies have also shown that some terminally differentiated cells can be made to divide beyond their normal capacity.

Erythropoietin stimulates proliferation and interferes with differentiation of myoblasts

Erythropoietin (Epo) is required for the production of mature red blood cells. The requirement for Epo and its receptor (EpoR) for normal heart development and the response of vascular endothelium and cells of neural origin to Epo provide evidence that the function of Epo as a growth factor or cytokine to protect cells from apoptosis extends beyond the hematopoietic lineage. We now report that the EpoR is expressed on myoblasts and can mediate a biological response of these cells to treatment with Epo. Primary murine satellite cells and myoblast C2C12 cells, both of which express endogenous EpoR, exhibit a proliferative response to Epo and a marked decrease in terminal differentiation to form myotubes. We also observed that Epo stimulation activates Jak2/Stat5 signal transduction and increases cytoplasmic calcium, which is dependent on tyrosine phosphorylation. In erythroid progenitor cells, Epo stimulates induction of transcription factor GATA-1 and EpoR; in C2C12 cells, GATA-3 and EpoR expression are induced. The decrease in differentiation of C2C12 cells is concomitant with an increase in Myf-5 and MyoD expression and inhibition of myogenin induction during differentiation, altering the pattern of expression of the MyoD family of transcription factors during muscle differentiation. These data suggest that, rather than acting in an instructive or specific mode for differentiation, Epo can stimulate proliferation of myoblasts to expand the progenitor population during differentiation and may have a potential role in muscle development or repair.