TGF-beta-activated Smad3 represses MEF2-dependent transcription in myogenic differentiation

Smad2 is essential for maintenance of the human and mouse primed pluripotent stem cell state

Human embryonic stem cells and mouse epiblast stem cells represent a primed pluripotent stem cell state that requires TGF-β/activin signaling. TGF-β and/or activin are commonly thought to regulate transcription through both Smad2 and Smad3. However, the different contributions of these two Smads to primed pluripotency and the downstream events that they may regulate remain poorly understood. We addressed the individual roles of Smad2 and Smad3 in the maintenance of primed pluripotency. We found that Smad2, but not Smad3, is required to maintain the undifferentiated pluripotent state. We defined a Smad2 regulatory circuit in human embryonic stem cells and mouse epiblast stem cells, in which Smad2 acts through binding to regulatory promoter sequences to activate Nanog expression while in parallel repressing autocrine bone morphogenetic protein signaling. Increased autocrine bone morphogenetic protein signaling caused by Smad2 down-regulation leads to cell differentiation toward the trophectoderm, mesoderm, and germ cell lineages. Additionally, induction of Cdx2 expression, as a result of decreased Smad2 expression, leads to repression of Oct4 expression, which, together with the decreased Nanog expression, accelerates the loss of pluripotency. These findings reveal that Smad2 is a unique integrator of transcription and signaling events and is essential for the maintenance of the mouse and human primed pluripotent stem cell state.

Krüppel-like factor 6 (KLF6) promotes cell proliferation in skeletal myoblasts in response to TGFβ/Smad3 signaling

Background

Krüppel-like factor 6 (KLF6) has been recently identified as a MEF2D target gene involved in neuronal cell survival. In addition, KLF6 and TGFβ have been shown to regulate each other’s expression in non-myogenic cell types. Since MEF2D and TGFβ also fulfill crucial roles in skeletal myogenesis, we wanted to identify whether KLF6 functions in a myogenic context.

Methods

KLF6 protein expression levels and promoter activity were analyzed using standard cellular and molecular techniques in cell culture.

Results

We found that KLF6 and MEF2D are co-localized in the nuclei of mononucleated but not multinucleated myogenic cells and, that the MEF2 cis element is a key component of the KLF6 promoter region. In addition, TGFβ potently enhanced KLF6 protein levels and this effect was repressed by pharmacological inhibition of Smad3. Interestingly, pharmacological inhibition of MEK/ERK (1/2) signaling resulted in re-activation of the differentiation program in myoblasts treated with TGFβ, which is ordinarily repressed by TGFβ treatment. Conversely, MEK/ERK (1/2) inhibition had no effect on TGFβ-induced KLF6 expression whereas Smad3 inhibition negated this effect, together supporting the existence of two separable arms of TGFβ signaling in myogenic cells. Loss of function analysis using siRNA-mediated KLF6 depletion resulted in enhanced myogenic differentiation whereas TGFβ stimulation of myoblast proliferation was reduced in KLF6 depleted cells.

Conclusions

Collectively these data implicate KLF6 in myoblast proliferation and survival in response to TGFβ with consequences for our understanding of muscle development and a variety of muscle pathologies.

Defective skeletal muscle growth in lamin A/C-deficient mice is rescued by loss of Lap2α

Mutations in lamin A/C result in a range of tissue-specific disorders collectively called laminopathies. Of these, Emery-Dreifuss and Limb-Girdle muscular dystrophy 1B mainly affect striated muscle. A useful model for understanding both laminopathies and lamin A/C function is the Lmna(-/-) mouse. We found that skeletal muscle growth and muscle satellite (stem) cell proliferation were both reduced in Lmna(-/-) mice. Lamins A and C associate with lamina-associated polypeptide 2 alpha (Lap2α) and the retinoblastoma gene product, pRb, to regulate cell cycle exit. We found Lap2α to be upregulated in Lmna(-/-) myoblasts (MBs). To specifically test the contribution of elevated Lap2α to the phenotype of Lmna(-/-) mice, we generated Lmna(-/-)Lap2α(-/-) mice. Lifespan and body mass were increased in Lmna(-/-)Lap2α(-/-) mice compared with Lmna(-/-). Importantly, the satellite cell proliferation defect was rescued, resulting in improved myogenesis. Lmna(-/-) MBs also exhibited increased levels of Smad2/3, which were abnormally distributed in the cell and failed to respond to TGFβ1 stimulation as in control cells. However, using SIS3 to inhibit signaling via Smad3 reduced cell death and augmented MB fusion. Together, our results show that perturbed Lap2α/pRb and Smad2/3 signaling are important regulatory pathways mediating defective muscle growth in Lmna(-/-) mice, and that inhibition of either pathway alone or in combination can ameliorate this deleterious phenotype.

Myostatin suppression of Akirin1 mediates glucocorticoid-induced satellite cell dysfunction

Glucocorticoids production is increased in many pathological conditions that are associated with muscle loss, but their role in causing muscle wasting is not fully understood. We have demonstrated a new mechanism of glucocorticoid-induced muscle atrophy: Dexamethasone (Dex) suppresses satellite cell function contributing to the development of muscle atrophy. Specifically, we found that Dex decreases satellite cell proliferation and differentiation in vitro and in vivo. The mechanism involved Dex-induced upregulation of myostatin and suppression of Akirin1, a promyogenic gene. When myostatin was inhibited in Dex-treated mice, Akirin1 expression increased as did satellite cell activity, muscle regeneration and muscle growth. In addition, silencing myostatin in myoblasts or satellite cells prevented Dex from suppressing Akirin1 expression and cellular proliferation and differentiation. Finally, overexpression of Akirin1 in myoblasts increased their expression of MyoD and myogenin and improved cellular proliferation and differentiation, theses improvements were no longer suppressed by Dex. We conclude that glucocorticoids stimulate myostatin which inhibits Akirin1 expression and the reparative functions of satellite cells. These responses attribute to muscle atrophy. Thus, inhibition of myostatin or increasing Akirin1 expression could lead to therapeutic strategies for improving satellite cell activation and enhancing muscle growth in diseases associated with increased glucocorticoid production.

Class IIa histone deacetylases and myocyte enhancer factor 2 proteins regulate the mesenchymal-to-epithelial transition of somatic cell reprogramming

Class IIa histone deacetylases (HDACs) and myocyte enhancer factor 2 (MEF2) proteins compose a signaling module that orchestrates lineage specification during embryogenesis. We show here that this module also regulates the generation of mouse induced pluripotent stem cells by defined transcription factors. Class IIa HDACs and MEF2 proteins rise steadily during fibroblast reprogramming to induced pluripotent stem cells. MEF2 proteins tend to block the process by inducing the expression of Tgfβ cytokines, which impairs the necessary phase of mesenchymal-to-epithelial transition (MET). Conversely, class IIa HDACs endeavor to suppress the activity of MEF2 proteins, thus enhancing the MET and colony formation efficiency. Our work highlights an unexpected role for a developmental axis in somatic cell reprogramming and provides new insight into how the MET is regulated in this context.

Antipsychotics activate the TGFβ pathway effector SMAD3

Although effective in treating an array of neurological disorders, antipsychotics are associated with deleterious metabolic side effects. Through high-throughput screening, we previously identified phenothiazine antipsychotics as modulators of the human insulin promoter. Here, we extended our initial finding to structurally diverse typical and atypical antipsychotics. We then identified the transforming growth factor beta (TGFβ) pathway as being involved in the effect of antipsychotics on the insulin promoter, finding that antipsychotics activated SMAD3, a downstream effector of the TGFβ pathway, through a receptor distinct from the TGFβ receptor family and known neurotransmitter receptor targets of antipsychotics. Of note, antipsychotics that do not cause metabolic side effects did not activate SMAD3. In vivo relevance was demonstrated by reanalysis of gene expression data from human brains treated with antipsychotics, which showed altered expression of SMAD3 responsive genes. This work raises the possibility that antipsychotics could be designed that retain beneficial CNS activity while lacking deleterious metabolic side effects.

Load regulates bone formation and Sclerostin expression through a TGFβ-dependent mechanism

Bone continually adapts to meet changing physical and biological demands. Osteoblasts, osteoclasts, and osteocytes cooperate to integrate these physical and biochemical cues to maintain bone homeostasis. Although TGFβ acts on all three of these cell types to maintain bone homeostasis, the extent to which it participates in the adaptation of bone to mechanical load is unknown. Here, we investigated the role of the TGFβ pathway in load-induced bone formation and the regulation of Sclerostin, a mechanosensitive antagonist of bone anabolism. We found that mechanical load rapidly represses the net activity of the TGFβ pathway in osteocytes, resulting in reduced phosphorylation and activity of key downstream effectors, Smad2 and Smad3. Loss of TGFβ sensitivity compromises the anabolic response of bone to mechanical load, demonstrating that the mechanosensitive regulation of TGFβ signaling is essential for load-induced bone formation. Furthermore, sensitivity to TGFβ is required for the mechanosensitive regulation of Sclerostin, which is induced by TGFβ in a Smad3-dependent manner. Together, our results show that physical cues maintain bone homeostasis through the TGFβ pathway to regulate Sclerostin expression and the deposition of new bone.

miR-26a is required for skeletal muscle differentiation and regeneration in mice

Multiple microRNAs are known to be induced during the differentiation of myoblasts to myotubes. Yet, experiments in animals have not provided clear evidence for the requirement of most of these microRNAs in myogenic differentiation in vivo. miR-26a is induced during skeletal muscle differentiation and is predicted to target a well-known inhibitor of differentiation, the transforming growth factor β/bone morphogenetic protein (TGF-β/BMP) signaling pathway. Here we show that exogenous miR-26a promotes differentiation of myoblasts, while inhibition of miR-26a by antisense oligonucleotides or by Tough-Decoys delays differentiation. miR-26a targets the transcription factors Smad1 and Smad4, critical for the TGF-β/BMP pathway, and expression of microRNA-resistant forms of these transcription factors inhibits differentiation. Injection of antagomirs specific to miR-26a into neonatal mice derepressed both Smad expression and activity and consequently inhibited skeletal muscle differentiation. In addition, miR-26a is induced during skeletal muscle regeneration after injury. Inhibiting miR-26a in the tibialis anterior muscles through the injection of adeno-associated virus expressing a Tough-Decoy targeting miR-26a prevents Smad down-regulation and delays regeneration. These findings provide evidence for the requirement of miR-26a for skeletal muscle differentiation and regeneration in vivo.

TGF-β family signaling in stem cells

BACKGROUND

The diversity of cell types and tissue types that originate throughout development derives from the differentiation potential of embryonic stem cells and somatic stem cells. While the former are pluripotent, and thus can give rise to a full differentiation spectrum, the latter have limited differentiation potential but drive tissue remodeling. Additionally cancer tissues also have a small population of self-renewing cells with stem cell properties. These cancer stem cells may arise through dedifferentiation from non-stem cells in cancer tissues, illustrating their plasticity, and may greatly contribute to the resistance of cancers to chemotherapies.

SCOPE OF REVIEW

The capacity of the different types of stem cells for self-renewal, the establishment and maintenance of their differentiation potential, and the selection of differentiation programs are greatly defined by the interplay of signaling molecules provided by both the stem cells themselves, and their microenvironment, the niche. Here we discuss common and divergent roles of TGF-β family signaling in the regulation of embryonic, reprogrammed pluripotent, somatic, and cancer stem cells.

MAJOR CONCLUSIONS

Increasing evidence highlights the similarities between responses of normal and cancer stem cells to signaling molecules, provided or activated by their microenvironment. While TGF-β family signaling regulates stemness of normal and cancer stem cells, its effects are diverse and depend on the cell types and physiological state of the cells.

GENERAL SIGNIFICANCE

Further mechanistic studies will provide a better understanding of the roles of TGF-β family signaling in the regulation of stem cells. These basic studies may lead to the development of a new therapeutic or prognostic strategies for the treatment of cancers. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Different roles of TGF-β in the multi-lineage differentiation of stem cells

Stem cells are a population of cells that has infinite or long-term self-renewal ability and can produce various kinds of descendent cells. Transforming growth factor β (TGF-β) family is a superfamily of growth factors, including TGF-β1, TGF-β2 and TGF-β3, bone morphogenetic proteins, activin/inhibin, and some other cytokines such as nodal, which plays very important roles in regulating a wide variety of biological processes, such as cell growth, differentiation, cell death. TGF-β, a pleiotropic cytokine, has been proved to be differentially involved in the regulation of multi-lineage differentiation of stem cells, through the Smad pathway, non-Smad pathways including mitogen-activated protein kinase pathways, phosphatidylinositol-3-kinase/AKT pathways and Rho-like GTPase signaling pathways, and their cross-talks. For instance, it is generally known that TGF-β promotes the differentiation of stem cells into smooth muscle cells, immature cardiomyocytes, chondrocytes, neurocytes, hepatic stellate cells, Th17 cells, and dendritic cells. However, TGF-β inhibits the differentiation of stem cells into myotubes, adipocytes, endothelial cells, and natural killer cells. Additionally, TGF-β can provide competence for early stages of osteoblastic differentiation, but at late stages TGF-β acts as an inhibitor. The three mammalian isoforms (TGF-β1, 2 and 3) have distinct but overlapping effects on hematopoiesis. Understanding the mechanisms underlying the regulatory effect of TGF-β in the stem cell multi-lineage differentiation is of importance in stem cell biology, and will facilitate both basic research and clinical applications of stem cells. In this article, we discuss the current status and progress in our understanding of different mechanisms by which TGF-β controls multi-lineage differentiation of stem cells.

Hydroxysafflor Yellow A Attenuates Carbon Tetrachloride-Induced Hepatic Fibrosis in Rats by Inhibiting Erk5 Signaling

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcription factor myocyte enhancer factor (MEF2) 2C plays a key role in this process. In the present study, we investigated the effect of hydroxysafflor yellow A (HSYA) on hepatic fibrosis and further investigated potential mechanisms in vivo. Sprague-Dawley rats were administered with CCl4 together with or without HYSA for 12 weeks. The effect of HYSA on hepatic fibrosis was evaluated using hematoxylin-eosin and Van Gieson staining. Messenger RNA expression was quantified by real-time polymerase chain reaction, and protein was quantified by Western blot or immunohistochemistry. Our results revealed that CCl4 treatment induced micronodular hepatic fibrosis with a pronounced deposition of collagen fibers. Treatment with HYSA resulted in a significant decrease in fibrosis, protein expression of α-SMA, and MEF-2C gene expression. This was accompanied by a decreased expression of Tβ-RI, Tβ-RII, MEKK3, MEK5, and phosphorylation of ERk5. HYSA alone had no effect on the measured parameters. Our findings demonstrate that HSYA protected, at least in part, the rat liver from CCl 4-caused fibrogenesis through inhibition of hepatic stellate cell (HSC) activation, attenuation of transforming growth factor beta (TGF-β) signaling. HSYA may become a novel and promising agent for the inhibition of hepatic fibrosis.

Testosterone improves the regeneration of old and young mouse skeletal muscle

Aging is associated with loss of muscle mass and strength, reduced satellite cell number, and lower regenerative potential. Testosterone increases muscle mass, strength, and satellite cell number in humans; however, the effects of testosterone on the regenerative potential of skeletal muscle are unclear. Here, we investigated the effect of testosterone on the skeletal muscle regeneration of young (2-month-old) and aged (24-month-old) male mice. We show that testosterone increases the number of proliferating satellite cells in regenerating "tibialis anterior" muscle of young and aged castrated mice 2 and 4 days postinjury. Testosterone supplementation increases the number and the cross-sectional area of regenerating fibers in both classes of age 4 days postinjury. Testosterone increases satellite cell activation and proliferation and the regeneration of both young and aged mouse muscle. These data suggest prospective application of androgens to improve the regenerating potential of the aged human skeletal muscle.

Inhibition of miR-29 by TGF-beta-Smad3 signaling through dual mechanisms promotes transdifferentiation of mouse myoblasts into myofibroblasts

MicroRNAs (miRNAs) are non-coding RNAs that regulate gene expression in post-transcriptional fashion, and emerging studies support their importance in regulating many biological processes, including myogenic differentiation and muscle development. miR-29 is a promoting factor during myogenesis but its full spectrum of impact on muscle cells has yet to be explored. Here we describe an analysis of miR-29 affected transcriptome in C2C12 muscle cells using a high throughput RNA-sequencing platform. The results reveal that miR-29 not only functions to promote myogenic differentiation but also suppresses the transdifferentiation of myoblasts into myofibroblasts. miR-29 inhibits the fibrogenic differentiation through down-regulating both extracellular matrix genes and cell adhesion genes. We further demonstrate that miR-29 is under negative regulation by TGF-beta (TGF-β)-Smad3 signaling via dual mechanisms of both inhibiting MyoD binding and enhancing Yin Yang 1 (YY1)-recruited Polycomb association. Together, these results identify miR-29 as a pleiotropic molecule in both myogenic and fibrogenic differentiation of muscle cells.

Mutations in protein-binding hot-spots on the hub protein Smad3 differentially affect its protein interactions and Smad3-regulated gene expression

Background

Hub proteins are connected through binding interactions to many other proteins. Smad3, a mediator of signal transduction induced by transforming growth factor beta (TGF-β), serves as a hub protein for over 50 protein-protein interactions. Different cellular responses mediated by Smad3 are the product of cell-type and context dependent Smad3-nucleated protein complexes acting in concert. Our hypothesis is that perturbation of this spectrum of protein complexes by mutation of single protein-binding hot-spots on Smad3 will have distinct consequences on Smad3-mediated responses.

Methodology/Principal Findings

We mutated 28 amino acids on the surface of the Smad3 MH2 domain and identified 22 Smad3 variants with reduced binding to subsets of 17 Smad3-binding proteins including Smad4, SARA, Ski, Smurf2 and SIP1. Mutations defective in binding to Smad4, e.g., D408H, or defective in nucleocytoplasmic shuttling, e.g., W406A, were compromised in modulating the expression levels of a Smad3-dependent reporter gene or six endogenous Smad3-responsive genes: Mmp9, IL11, Tnfaip6, Fermt1, Olfm2 and Wnt11. However, the Smad3 mutants Y226A, Y297A, W326A, K341A, and E267A had distinct differences on TGF-β signaling. For example, K341A and Y226A both reduced the Smad3-mediated activation of the reporter gene by ∼50% but K341A only reduced the TGF-β inducibilty of Olfm2 in contrast to Y226A which reduced the TGF-β inducibility of all six endogenous genes as severely as the W406A mutation. E267A had increased protein binding but reduced TGF-β inducibility because it caused higher basal levels of expression. Y297A had increased TGF-β inducibility because it caused lower Smad3-induced basal levels of gene expression.

Conclusions/Significance

Mutations in protein binding hot-spots on Smad3 reduced the binding to different subsets of interacting proteins and caused a range of quantitative changes in the expression of genes induced by Smad3. This approach should be useful for unraveling which Smad3 protein complexes are critical for specific biological responses.

Childhood rhabdomyosarcoma: recent advances and prospective views

Rhabdomyosarcoma (RMS) is a morphologically and clinically heterogeneous group of malignant tumors that resemble developing skeletal muscle and is the most common soft-tissue sarcoma in children and adolescents. The most prominent sites involve head and neck structures (~40%), genito-urinary track (~25%), and extremities (~20%). Embryonal (ERMS) and alveolar (ARMS) are the two major RMS subtypes that are distinct in their morphology and genetic make-up. The prognosis for this cancer depends strongly on tumor size, location, staging, and child's age. In general, ERMS has a more favorable outcome, whereas the mortality rate remains high in patients with ARMS, because of its aggressive and metastatic nature. Over the past two decades, researchers have made concerted efforts to delineate genetic and epigenetic changes associated with RMS pathogenesis. These molecular signatures have presented golden opportunities to design targeted therapies for treating this aggressive cancer. This article highlights recent advances in understanding the molecular pathogenesis of RMS, and addresses promising research areas for further exploration.

Inhibition of fibroblast differentiation of muscle-derived stem cells in cell implantation treatment of stress urinary incontinence

The aim of this study was to investigate the effects of transforming growth factor-β1 (TGF-β1) stimulation and the blocking of the TGF-β1/Smad3 signaling pathway by vector-mediated Smad3 shRNA on muscle-derived stem cells (MDSCs) in cell implantation treatment of stress urinary incontinence (SUI) of the rat. MDSCs were infected with the GC-shSmad3 lentivirus vector. Five days after infection, the cells were treated with TGF-β1. The expression levels of desmin (a marker of muscle differentiation) and vimentin (a marker of fibroblast differentiation) were tested by real-time PCR and Western blot. GC-shSmad3 lentivirus-infected MDSCs were injected into the bladder neck and proximal urethra of SUI rats. Urodynamic test was used to measure leak point pressure (LPP) at 2 weeks and 4 weeks after MDSC transplantation. Upregulated expression of vimentin and downregulated expression of desmin were found in MDSCs after culture with TGF-β1 in vitro. GC-shSmad3 lentivirus infection inhibited fibroblast differentiation of MDSCs but allowed muscle differentiation with desmin expression. In vivo experiments showed that GC-shSmad3 lentivirus infection could improve MDSC-mediated repairing of urethra sphincter function. In conclusion, blocking Smad3 expression inhibits the fibroblast differentiation of MDSCs induced by TGF-β1 in vitro and improves the repairing of urethral sphincter function by inhibiting the fibroblast differentiation of MDSCs in a rat model of SUI in vivo.

Smad3 signaling is required for satellite cell function and myogenic differentiation of myoblasts

TGF-β and myostatin are the two most important regulators of muscle growth. Both growth factors have been shown to signal through a Smad3-dependent pathway. However to date, the role of Smad3 in muscle growth and differentiation is not investigated. Here, we demonstrate that Smad3-null mice have decreased muscle mass and pronounced skeletal muscle atrophy. Consistent with this, we also find increased protein ubiquitination and elevated levels of the ubiquitin E3 ligase MuRF1 in muscle tissue isolated from Smad3-null mice. Loss of Smad3 also led to defective satellite cell (SC) functionality. Smad3-null SCs showed reduced propensity for self-renewal, which may lead to a progressive loss of SC number. Indeed, decreased SC number was observed in skeletal muscle from Smad3-null mice showing signs of severe muscle wasting. Further in vitro analysis of primary myoblast cultures identified that Smad3-null myoblasts exhibit impaired proliferation, differentiation and fusion, resulting in the formation of atrophied myotubes. A search for the molecular mechanism revealed that loss of Smad3 results in increased myostatin expression in Smad3-null muscle and myoblasts. Given that myostatin is a negative regulator, we hypothesize that increased myostatin levels are responsible for the atrophic phenotype in Smad3-null mice. Consistent with this theory, inactivation of myostatin in Smad3-null mice rescues the muscle atrophy phenotype.

Secretome Analysis of Skeletal Myogenesis Using SILAC and Shotgun Proteomics

Myogenesis, the formation of skeletal muscle, is a multistep event that commences with myoblast proliferation, followed by cell-cycle arrest, and finally the formation of multinucleated myotubes via fusion of mononucleated myoblasts. Each step is orchestrated by well-documented intracellular factors, such as cytoplasmic signalling molecules and nuclear transcription factors. Regardless, the key step in getting a more comprehensive understanding of the regulation of myogenesis is to explore the extracellular factors that are capable of eliciting the downstream intracellular factors. This could further provide valuable insight into the acute cellular response to extrinsic cues in maintaining normal muscle development. In this paper, we survey the intracellular factors that respond to extracellular cues that are responsible for the cascades of events during myogenesis: myoblast proliferation, cell-cycle arrest of myoblasts, and differentiation of myoblasts into myotubes. This focus on extracellular perspective of muscle development illustrates our mass spectrometry-based proteomic approaches to identify differentially expressed secreted factors during skeletal myogenesis.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

TGF-beta regulates miR-206 and miR-29 to control myogenic differentiation through regulation of HDAC4

MicroRNAs (miRs) are emerging as prominent players in the regulation of many biological processes, including myogenic commitment and skeletal muscle formation. Members of the TGF-β family can influence the proliferation and myogenic differentiation of cells, although it is presently not clear what role miRNAs play in the TGF-β-mediated control of myogenic differentiation. Here, we demonstrate in the myogenic C2C12 cell line, and in primary muscle cells, that miR-206 and miR-29-two miRs that act on transcriptional events implicated in muscle differentiation are down-regulated by TGF-β. We further demonstrate that TGF-β treatment of myogenic cells is associated with increased expression of histone deacetylase 4 (HDAC4), a key inhibitor of muscle differentiation that has been identified as a target for regulation by miR-206 and miR-29. We confirmed that increased expression of miR-206 and miR-29 resulted in the translational repression of HDAC4 in the presence or absence of TGF-β via interaction with the HDAC4 3'-untranslated region. Importantly, we found that miR-206 and miR-29 can attenuate the inhibitory actions of TGF-β on myogenic differentiation. Furthermore, we present evidence that the mechanism by which miR-206 and miR-29 can inhibit the TGF-β-mediated up-regulation of HDAC4 is via the inhibition of Smad3 expression, a transducer of TGF-β signaling. These findings identify a novel mechanism of interaction between TGF-β and miR-206 and -29 in the regulation of myogenic differentiation through HDAC4.

Sca-1 is negatively regulated by TGF-beta1 in myogenic cells

Sca-1 (stem cell antigen-1) is a member of the Ly-6 family of proteins and regulates cell proliferation, differentiation, and self-renewal in multiple tissues. In skeletal muscle, Sca-1 inhibits both proliferation and differentiation of myogenic cells. Sca-1 expression is dynamically regulated during muscle regeneration, and mice lacking Sca-1 display increased fibrosis following muscle injury. Here, we show that Sca-1 expression is negatively regulated by TGF-β1 and that this inhibition is dependent on Smad3. We demonstrate that levels of TGF-β1 in skeletal muscle rapidly increase on injury and that the majority of this TGFβ1 is produced by infiltrating macrophages. Sca-1 is expressed in multiple cell types, and we demonstrate that TGF-β1 represses Sca-1 expression in T cells and other immune cell populations derived from the spleen, indicating that regulation by TGF-β1 is a general feature of Sca-1 expression in multiple cell types. Elucidation of the mechanisms by which Sca-1 expression is regulated may aid in the understanding of muscle homeostasis, potentially identifying novel therapeutic targets for muscle diseases.

Identification of retinoic acid in a high content screen for agents that overcome the anti-myogenic effect of TGF-beta-1

BACKGROUND

Transforming growth factor beta 1 (TGF-β1) is an inhibitor of muscle cell differentiation that is associated with fibrosis, poor regeneration and poor function in some diseases of muscle. When neutralizing antibodies to TGF-β1 or the angiotensin II inhibitor losartan were used to reduce TGF-β1 signaling, muscle morphology and function were restored in mouse models of Marfan Syndrome and muscular dystrophy. The goal of our studies was to identify additional agents that overcome the anti-myogenic effect of TGF-β1.

METHODOLOGY/PRINCIPAL FINDINGS

A high-content cell-based assay was developed in a 96-well plate format that detects the expression of myosin heavy chain (MHC) in C2C12 cells. The assay was used to quantify the dose-dependent responses of C2C12 cell differentiation to TGF-β1 and to the TGF-β1 Type 1 receptor kinase inhibitor, SB431542. Thirteen agents previously described as promoting C2C12 differentiation in the absence of TGF-β1 were screened in the presence of TGF-β1. Only all-trans retinoic acid and 9-cis retinoic acid allowed a maximal level of C2C12 cell differentiation in the presence of TGF-β1; the angiotensin-converting enzyme inhibitor captopril and 10 nM estrogen provided partial rescue. Vitamin D was a potent inhibitor of retinoic acid-induced myogenesis in the presence of TGF-β1. TGF-β1 inhibits myoblast differentiation through activation of Smad3; however, retinoic acid did not inhibit TGF-β1-induced activation of a Smad3-dependent reporter gene in C2C12 cells.

CONCLUSIONS/SIGNIFICANCE

Retinoic acid alleviated the anti-myogenic effect of TGF-β1 by a Smad3-independent mechanism. With regard to the goal of improving muscle regeneration and function in individuals with muscle disease, the identification of retinoic acid is intriguing in that some retinoids are already approved for human therapy. However, retinoids also have well-described adverse effects. The quantitative, high-content assay will be useful to screen for less-toxic retinoids or combinations of agents that promote myoblast differentiation in the presence of TGF-β1.

TGF-β inhibits muscle differentiation by blocking autocrine signaling pathways initiated by IGF-II

Skeletal muscle differentiation and regeneration are regulated by interactions between exogenous hormone- and growth factor-activated signaling cascades and endogenous muscle-specific transcriptional programs. IGF-I and IGF-II can promote muscle differentiation in vitro and can enhance muscle maintenance and repair in vivo. In contrast, members of the TGF-β superfamily prominently inhibit muscle differentiation and regeneration. In this study, we have evaluated functional interactions between IGF- and TGF-β-regulated signaling pathways during skeletal muscle differentiation. In the mouse C2 muscle cell line and in human myoblasts in primary culture, addition of TGF-β1 blocked differentiation in a dose-dependent way, inhibited expression of muscle-specific mRNAs and proteins, and impaired myotube formation. TGF-β1 also diminished stimulation of IGF-II gene expression in myoblasts, decreased IGF-II secretion, and reduced IGF-I receptor activation. To test the hypothesis that TGF-β1 prevents muscle differentiation primarily by blocking IGF-II production, we examined effects of IGF analogues on TGF-β actions in myoblasts. Although both IGF-I and IGF-II restored muscle gene and protein expression, and stimulated myotube formation in the presence of TGF-β1, they did not reduce TGF-β1-stimulated signaling, as measured by no decline in phosphorylation of SMA and mothers against decapentaplegic homolog (Smad)3, or in induction of TGF-β-activated target genes, including a Smad-dependent promoter-reporter plasmid. Our results demonstrate that TGF-β disrupts an IGF-II-stimulated autocrine amplification cascade that is necessary for muscle differentiation in vitro. Because this inhibitory pathway can be overcome by exogenous IGFs, our observations point toward potential strategies to counteract disorders that reduce muscle mass and strength.

TGF-beta receptors, in a Smad-independent manner, are required for terminal skeletal muscle differentiation

Skeletal muscle differentiation is strongly inhibited by transforming growth factor type beta (TGF-beta), although muscle formation as well as regeneration normally occurs in an environment rich in this growth factor. In this study, we evaluated the role of intracellular regulatory Smads proteins as well as TGF-beta-receptors (TGF-beta-Rs) during skeletal muscle differentiation. We found a decrease of TGF-beta signaling during differentiation. This phenomenon is explained by a decline in the levels of the regulatory proteins Smad-2, -3, and -4, a decrease in the phosphorylation of Smad-2 and lost of nuclear translocation of Smad-3 and -4 in response to TGF-beta. No change in the levels and inhibitory function of Smad-7 was observed. In contrast, we found that TGF-beta-R type I (TGF-beta-RI) and type II (TGF-beta-RII) increased on the cell surface during skeletal muscle differentiation. To analyze the direct role of the serine/threonine kinase activities of TGF-beta-Rs, we used the specific inhibitor SB 431542 and the dominant-negative form of TGF-beta-RII lacking the cytoplasmic domain. The TGF-beta-Rs were important for successful muscle formation, determined by the induction of myogenin, creatine kinase activity, and myosin. Silencing of Smad-2/3 expression by specific siRNA treatments accelerated myogenin, myosin expression, and myotube formation; although when SB 431542 was present inhibition in myosin induction and myotube formation was observed, suggesting that these last steps of skeletal muscle differentiation require active TGF-beta-Rs. These results suggest that both down-regulation of Smad regulatory proteins and cell signaling through the TGF-beta receptors independent of Smad proteins are essential for skeletal muscle differentiation.

TGF-beta superfamily regulates a switch that mediates differentiation either into adipocytes or myocytes in left atrium derived pluripotent cells (LA-PCS)

Many stem cell studies have focused on the subject of cell fate and the signal molecules that modulate the regulatory switches for a given differentiation pathway. Genome-wide screens for cell fate determination signals require a cell source that differentiates purely into a single cell type. From adult rat left atrium, we established LA-PCs that differentiates into cardiac/skeletal myocytes or adipocytes with almost 100% purity. In this study, we compared gene expression profiles of undifferentiated LA-PCs with those of differentiated cells [adipocytes (Adi) or cardiac/skeletal myocytes (Myo)] to identify the signals that set the regulatory switch for adipocyte or myocyte differentiation. Microarray analysis verified the feasibility of genome-wide screening by this method. Using a pathway analysis screen, we found that members of the TGF-beta superfamily signal transduction pathways modulate the adipocyte/myocyte differentiation switch. Further analysis determined that recombinant TGF-beta inhibits adipogenesis and induces myogenesis simultaneously in a dose-dependent manner. Moreover, noggin induces differentiation into fully developed beating cardiac myocytes in vitro. These results provided new insight into the molecules that modulate the differentiation switch and validated a screening method for their identification.

The myostatin-induced E3 ubiquitin ligase RNF13 negatively regulates the proliferation of chicken myoblasts

The ubiquitin ligase RING finger protein 13 gene (RNF13) was first identified in a screen for genes whose expression is regulated by myostatin in chicken fetal myoblasts. In this study, we demonstrate that the RNF13 gene is broadly expressed in many chicken tissues. The expression of RNF13 gradually decreases during skeletal myogenesis, and myostatin up-regulates RNF13 expression at both the transcriptional and translational levels. Interestingly, ectopic expression of RNF13 inhibits cell proliferation and suppresses the expression of the myogenic genes MyoD and Caveolin-3 in muscle cells. Moreover, recently, we have reported that RNF13 is a RING-type E3 ubiquitin ligase. In this report, we provide experimental evidence to show that mutations disrupting the RING finger abolish the growth-suppressive activity of RNF13, indicating that its E3 ligase activity is required for the inhibition of cell proliferation. Taken together, our findings show that RNF13 functions as an E3 ubiquitin ligase to negatively regulate cell proliferation.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

Transcriptional induction of MMP-10 by TGF-beta, mediated by activation of MEF2A and downregulation of class IIa HDACs

Transforming growth factor (TGF)-beta regulates the expression of matrix metalloproteinases (MMPs) and components of the extracellular matrix, thereby profoundly affecting the microenvironment of cells including cancerous ones. We studied MMP-10 induction by TGF-beta in mammary epithelial cells and found that the induction was dependent on the myocyte enhancer factor (MEF)-2 transcription factor. TGF-beta upregulated the gene promoter through the MEF2 site, and knockdown of the MEF2A transcription factor negatively affected MMP-10 induction, whereas its overexpression had a positive effect on the induction. In response to TGF-beta, acetylation and concomitant binding of MEF2A to the promoter region increased, thus suggesting a critical role of MEF2A in transactivation of MMP-10 by TGF-beta. Consistent with the fact that class IIa histone deacetylases (HDACs) interact with MEF2 and suppress transcription, knockdown of HDACs increased and their overexpression inhibited MMP-10 expression. Intriguingly, TGF-beta promoted proteasome-dependent degradation of HDACs. Consistent with this, acetylation of core histones was increased around the MEF2 site of the MMP-10 promoter by TGF-beta and alleviated by overexpression of HDACs. Collectively, it is possible that TGF-beta transcriptionally upregulated MMP-10 through activation of MEF2A, concomitant with acetylation of core histones increasing around the promoter, as a consequence of degradation of the class IIa HDACs.

Sox9 represses alpha-sarcoglycan gene expression in early myogenic differentiation

Alpha sarcoglycan (alpha-SG) is highly expressed in differentiated striated muscle, and its disruption causes limb-girdle muscular dystrophy. Accordingly, the myogenic master regulator MyoD finely modulates its expression. However, the mechanisms preventing alpha-SG gene expression at early stages of myogenic differentiation remain unknown. In this study, we uncovered Sox9, which was not previously known to directly bind muscle gene promoters, as a negative regulator of alpha-SG gene expression. Reporter gene and chromatin immunoprecipitation assays revealed three functional Sox-binding sites that mediate alpha-SG promoter activity repression during early myogenic differentiation. In addition, we show that Sox9-mediated inhibition of alpha-SG gene expression is independent of MyoD. Moreover, we provide evidence suggesting that Smad3 enhances the repressive activity of Sox9 over alpha-SG gene expression in a transforming growth factor-beta-dependent manner. On the basis of these results, we propose that Sox9 and Smad3 are responsible for preventing precocious activation of alpha-SG gene expression during myogenic differentiation.

Regulation of myogenic differentiation by androgens: cross talk between androgen receptor/ beta-catenin and follistatin/transforming growth factor-beta signaling pathways

Androgens are important regulators of body composition and promote myogenic differentiation and inhibit adipogenesis of mesenchymal, multipotent cells. Here, we investigated the mechanisms by which androgens induce myogenic differentiation of mesenchymal multipotent cells. Incubation of mesenchymal multipotent C3H 10T1/2 cells with testosterone and dihydrotestosterone promoted nuclear translocation of androgen receptor (AR)/beta-catenin complex and physical interaction of AR, beta-catenin, and T-cell factor-4 (TCF-4). Inhibition of beta-catenin by small inhibitory RNAs significantly decreased testosterone-induced stimulation of myogenic differentiation. Overexpression of TCF-4, a molecule downstream of beta-catenin in Wnt signaling cascade, in C3H 10T1/2 cells significantly up-regulated expression of myoD and myosin heavy chain II proteins and of follistatin (Fst), which binds and antagonizes native ligands of the TGF-beta/Smad pathway. Gene array analysis of C3H 10T1/2 cells treated with testosterone revealed that testosterone up-regulated the expression of Fst and modified the expression of several signaling molecules involved in the TGF-beta/Smad pathway, including Smad7. Lowering of testosterone levels in mice by orchidectomy led to a significant decrease in Fst and Smad7 expression; conversely, testosterone supplementation in castrated mice up-regulated Fst and Smad7 mRNA expression in androgen-responsive levator ani muscle. Testosterone-induced up-regulation of MyoD and myosin heavy chain II proteins in C3H 10T1/2 cells was abolished in cells simultaneously treated with anti-Fst antibody, suggesting an essential role of Fst during testosterone regulation of myogenic differentiation. In conclusion, our data suggest the involvement of AR, beta-catenin, and TCF-4 pathway during androgen action to activate a number of Wnt target genes, including Fst, and cross communication with the Smad signaling pathway.

miRNAS in normal and diseased skeletal muscle

The last 20 years have witnessed major advances in the understanding of muscle diseases and significant inroads are being made to treat muscular dystrophy. However, no curative therapy is currently available for any of the muscular dystrophies, despite the immense progress made using several approaches and only palliative and symptomatic treatment is available for patients. The discovery of miRNAs as new and important regulators of gene expression is expected to broaden our biological understanding of the regulatory mechanism in muscle by adding another dimension of regulation to the diversity and complexity of gene-regulatory networks. As important regulators of muscle development, unravelling the regulatory circuits involved may be challenging, given that a single miRNA can regulate the expression of many mRNA targets. Although the identification of the regulatory targets of miRNAs in muscle is a challenge, it will be critical for placing them in genetic pathways and biological contexts. Therefore, combining informatics, biochemical and genetic approaches will not only expected to reveal the elucidation of the miRNA regulatory network in skeletal muscle and to bring a better knowledge on muscle tissue regulation but will also raise new opportunities for therapeutic intervention in muscular dystrophies by identifying candidate miRNAs as potential targets for clinical application.

SMAD3 functions as a transcriptional repressor of acid-sensing ion channel 3 (ASIC3) in nucleus pulposus cells of the intervertebral disc

The goal of this investigation was to study the regulation of acid-sensing ion channel (ASIC)3 expression by TGFbeta in the nucleus pulposus cells of the intervertebral disc. Analysis of human nucleus pulposus tissue indicated decreased ASIC3 and elevated TGFbeta expression in the degenerate state. In a parallel study, treatment of nucleus pulposus cells with TGFbeta resulted in decreased expression of ASIC3 mRNA and protein. Suppression of ASIC3 promoter activity was evident when the nucleus pulposus cells were treated with TGFbeta or co-transfected with the constitutively active ALK5 or a smad3 construct. On the other hand, co-transfection of dominant negative smad3 or smad7 restored ASIC3 promoter activity. We validated the role of smad3 in controlling ASIC3 expression using cells derived from smad3-null mice. ASIC3 promoter activity in the null cells was 2- to 3-fold higher than the wildtype cells. Moreover, expression of smad3 in null cells decreased ASIC3 promoter activity by almost 50%. Further studies using deletion constructs and trichostatin A treatment showed that the full-length smad3 was necessary, and the suppression involved recruitment of histone deacetylase to the promoter. To determine the mechanism, we evaluated the rat ASIC3 promoter sequence and noted the presence of two smad interacting CAGA box motifs. Gel-shift and supershift analysis indicated that smad3 protein was bound to this motif. Chromatin immunoprecipitation analysis confirmed that smad3 bound both the CAGA elements. Results of these studies clearly show that TGFbeta is highly expressed in the degenerate disc and through smad3 serves as a negative regulator of ASIC3 expression.

TGF-beta activates ERK5 in human renal epithelial cells

The role of the MAP kinase, extracellular signal-regulated kinase 5 (ERK5) remains unknown, however it is involved in cell differentiation and survival as highlighted by the embryonic lethality of the ERK5 knockout. ERK5 can be activated by growth factors and other extracellular signals. TGF-beta, a powerful controller of epithelial cell phenotype, is known to activate the MAP kinase, ERK1/2 however its effect on ERK5 remains unknown. This study demonstrates, fort the first time, ERK5 activation by TGF-beta, observed in both transformed and primary adult human PTEC; activation required ALK-5 receptor activity. In addition this work demonstrates expression of myocyte enhancer factor-2 (MEF2C) by PTEC and that TGF-beta increased the association of MEK5 with phospho-ERK5 and MEF2C. ERK5 activation by either TGF-beta or epidermal growth factor (EGF) was also inhibited by the p38 MAP kinase inhibitor, SB-202190.

Cellular and molecular regulation of muscle growth and development in meat animals1,2

Although in vivo and in vitro studies have established that anabolic steroids, transforming growth factor-beta (TGF-beta), and myostatin affect muscle growth in meat-producing animals, their mechanisms of action are not completely understood. Anabolic steroids have been widely used as growth promoters in feedlot cattle for over 50 yr. A growing body of evidence suggests that increased muscle levels of IGF-I and increased muscle satellite cell numbers play a role in anabolic steroid enhanced muscle growth. In contrast to anabolic steroids, the members of the TGF-beta-myostatin family suppress muscle growth in vivo and suppress both proliferation and differentiation of cultured myogenic cells. Recent evidence suggests that IGFBP-3 and IGFBP-5 play a role in mediating the proliferation-suppressing actions of both TGF-beta and myostatin on cultured myogenic cells. Consequently, this review will focus on the roles of IGF-I and IGFBP in the cellular and molecular mechanisms of action of anabolic steroids and TGF-beta and myostatin, respectively.

Transforming growth factor-beta-regulated miR-24 promotes skeletal muscle differentiation

MicroRNAs (miRNAs) have recently been proposed as a versatile class of molecules involved in regulation of a variety of biological processes. However, the role of miRNAs in TGF-beta-regulated biological processes is poorly addressed. In this study, we found that miR-24 was upregulated during myoblast differentiation and could be inhibited by TGF-beta1. Using both a reporter assay and Northern blot analysis, we showed that TGF-beta1 repressed miR-24 transcription which was dependent on the presence of Smad3 and a Smads binding site in the promoter region of miR-24. TGF-beta1 was unable to inhibit miR-24 expression in Smad3-deficient myoblasts, which exhibited accelerated myogenesis. Knockdown of miR-24 led to reduced expression of myogenic differentiation markers in C2C12 cells, while ectopic expression of miR-24 enhanced differentiation, and partially rescued inhibited myogenesis by TGF-beta1. This is the first study demonstrating a critical role for miRNAs in modulating TGF-beta-dependent inhibition of myogenesis, and provides a novel mechanism of the genetic regulation of TGF-beta signaling during skeletal muscle differentiation.

Superficial, nodular, and morpheiform basal-cell carcinomas exhibit distinct gene expression profiles

Basal-cell carcinoma (BCC), the most common neoplasm in humans, occurs in a variety of morphological presentations. The mechanisms of BCC development downstream of the initial genetic mutations are not well understood, and different BCC morphological presentations might exhibit distinct gene expression patterns. We investigated superficial (n=8), nodular (n=8), and morpheiform (n=7) BCCs using 21K cDNA microarrays. Global gene expression profiles between respective BCC subtypes, and as compared with normal skin (n=8), were statistically defined by significance analysis of microarrays (SAM). Thirty-seven genes were subsequently validated by quantitative reverse transcriptase-PCR analysis using an expanded set of 31 BCCs. Gene ontology analysis indicated that gene expression patterns of BCC subtypes in multiple biological processes showed significant variation, particularly in genes associated with the mitogen-activated protein kinase (MAPK) pathway. Notably, genes involved in response to DNA-damage stimulus were uniquely upregulated in morpheiform BCCs. Our results indicate a relative similarity in gene expression between nodular and superficial BCC subtypes. In contrast, morpheiform BCCs are more diverse, with gene expression patterns consistent with their more "invasive" phenotype. These data may help us understand the complex behavior of BCC subtypes and may eventually lead to new therapeutic strategies.

Transforming growth factor-beta and myostatin signaling in skeletal muscle

The superfamily of transforming growth factor-beta (TGF-beta) cytokines has been shown to have profound effects on cellular proliferation, differentiation, and growth. Recently, there have been major advances in our understanding of the signaling pathway(s) conveying TGF-beta signals to the nucleus to ultimately control gene expression. One tissue that is potently influenced by TGF-beta superfamily signaling is skeletal muscle. Skeletal muscle ontogeny and postnatal physiology have proven to be exquisitely sensitive to the TGF-beta superfamily cytokine milieu in various animal systems from mice to humans. Recently, major strides have been made in understanding the role of TGF-beta and its closely related family member, myostatin, in these processes. In this overview, we will review recent advances in our understanding of the TGF-beta and myostatin signaling pathways and, in particular, focus on the implications of this signaling pathway for skeletal muscle development, physiology, and pathology.

A novel role of transforming growth factor beta1 in transcriptional repression of human cholesterol 7alpha-hydroxylase gene

BACKGROUND & AIMS

Inhibition of cholesterol 7alpha-hydroxylase (CYP7A1) by bile acids and inflammatory cytokines provides an important mechanism to protect hepatocytes from bile acid toxicity during cholestasis. Transforming growth factor beta1 (TGFbeta1) released by hepatic stellate cells during chronic liver injury plays a critical role in liver inflammation and fibrogenesis. The objective of this study is to investigate the role of TGFbeta1 in hepatic bile acid synthesis.

METHODS

mRNA expressions in primary human hepatocytes and HepG2 cells were measured by quantitative real-time polymerase chain reaction. Reporter assay, glutathione-S-transferase pull-down assay, adenovirus-mediated gene transduction, and chromatin immunoprecipitation assay were used to study the mechanism of TGFbeta1 regulation of CYP7A1 gene transcription.

RESULTS

TGFbeta1 inhibited the mRNA expression of CYP7A1 and bile acid synthesis in HepG2 cells and primary human hepatocytes. Mothers against decapentaplegic homolog (Smad3) inhibited both CYP7A1 promoter activity and mRNA expression by inhibiting DNA-binding activity of hepatocyte nuclear factor 4alpha (HNF4alpha). The histone deacetylase (HDAC) inhibitor Tricostatin A partially blocked the TGFbeta1 inhibition of CYP7A1 mRNA expression, whereas TGFbeta1 decreased histone 3 acetylation in the CYP7A1 chromatin. TGFbeta1 treatment and adenovirus Smad3 reduced HNF4alpha binding but increased the recruitment of Smad3, HDAC1, and a repressor mSin3A to the CYP7A1 chromatin.

CONCLUSIONS

This study provides the first evidence that TGFbeta1 represses CYP7A1 gene transcription in human hepatocytes by a mechanism involving Smad3-dependent inhibition of HNF4alpha and HDAC remodeling of CYP7A1 chromatin. The TGFbeta1/Smad3 signaling may reduce bile acid synthesis in the liver and prevent hepatocyte injury in cholestatic liver disease.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

c-Ski activates MyoD in the nucleus of myoblastic cells through suppression of histone deacetylases

c-Ski, originally identified as an oncogene product, induces myogenic differentiation in nonmyogenic fibroblasts through transcriptional activation of muscle regulatory factors. Although c-Ski does not bind to DNA directly, it binds to DNA through interaction with Smad proteins and regulates signaling activities of transforming growth factor-beta (TGF-beta). In the present study, we show that c-Ski activates the myogenin promoter independently of regulation of endogenous TGF-beta signaling. Expression of myogenin is regulated by a transcription factor complex containing proteins of the MyoD family and the myocyte enhancer factor 2 (MEF2) family. c-Ski acts on the MyoD-MEF2 complex and modulates the activity of MyoD in myogenin promoter regulation. Interestingly, histone deacetylase (HDAC) inhibitors up-regulated basal activity of transcription from a MyoD-responsive reporter, although c-Ski failed to further augment this transcription in the presence of HDAC inhibitors. c-Ski is observed both in the cytoplasm and in the nucleus, but its nuclear localization is required for myogenic differentiation. We conclude that c-Ski induces myogenic differentiation through acting on MyoD and inhibiting HDAC activity in the nucleus of myogenic cells.

Effects of hypoxia on proliferation and differentiation of myoblasts

Oxygen is an environmental and developmental signal regulator, and its role is involved in energy homeostasis, development and process of differentiation. Myoblasts persist in skeletal muscle as satellite cells, which possess capability of self-renewing and differentiation into mature myofiber. Myoblasts play a critical role in postnatal muscle regeneration after injury as well as maintaining myofibers' function. Though oxygen is vital to nearly all forms of life, studies focused on investigating the effects of oxygen level on proliferation and differentiation of myoblasts are few. Lower oxygen concentration is more close to the level of oxygen in physiological and pathological environment in vivo. So physiological environment is actually optimum condition for myogenesis. It is significant for understanding repair and regeneration of skeletal muscle to study on effects of hypoxia on myogenesis. HIF-1 signaling pathway was involved in these processes as well as other signaling pathways would be, and accordingly, deep studying and further revealing the signaling pathways involved in mechanism will provide evidences or references for looking for novel targets for stem cells therapy and drug treatment.

The hedgehog regulated oncogenes Gli1 and Gli2 block myoblast differentiation by inhibiting MyoD-mediated transcriptional activation

The mechanism by which activation of the Hedgehog (Hh) pathway modulates differentiation and promotes oncogenesis in specific tissues is poorly understood. We therefore, analysed rhabdomyosarcomas from mice that were haploinsufficient for the Hh-binding protein, Hip1, or for the Hh receptor, Patched 1 (Ptch1). Transfection of the Hh-regulated transcription factor Gli1, which is expressed in a subset of mouse and human rhabdomyosarcomas, suppressed differentiation of myogenic rhabdomyosarcoma lines generated from Hip1+/- and Ptch1+/- mice. The closely related factor, Gli2, had similar effects. Gli1 and Gli2 inhibited myogenesis by repressing the capacity of MyoD to activate transcription. Deletion analysis of Gli1 indicated that multiple domains of Gli1 are required for efficient inhibition of MyoD. Gli1 reduced the ability of MyoD to heterodimerize with E12 and bind DNA, providing one mechanism whereby the Gli proteins modulate the activity of MyoD. This novel activity of Gli proteins provides new insights into how Hh signaling modulates terminal differentiation through inhibition of tissue-specific factors such as MyoD. This mechanism may contribute to the broad role of Hh signaling and the Gli proteins in differentiation decisions and cancer formation.

The MH1 domain of Smad3 interacts with Pax6 and represses autoregulation of the Pax6 P1 promoter

Pax6 transcription is under the control of two main promoters (P0 and P1), and these are autoregulated by Pax6. Additionally, Pax6 expression is under the control of the TGFβ superfamily, although the precise mechanisms of such regulation are not understood. The effect of TGFβ on Pax6 expression was studied in the FHL124 lens epithelial cell line and was found to cause up to a 50% reduction in Pax6 mRNA levels within 24 h. Analysis of luciferase reporters showed that Pax6 autoregulation of the P1 promoter, and its induction of a synthetic promoter encoding six paired domain-binding sites, were significantly repressed by both an activated TGFβ receptor and TGFβ ligand stimulation. Subsequently, a novel Pax6 binding site in P1 was shown to be necessary for autoregulation, indicating a direct influence of Pax6 protein on P1. In transfected cells, and endogenously in FHL124 cells, Pax6 co-immunoprecipitated with Smad3 following TGFβ receptor activation, while in GST pull-down experiments, the MH1 domain of Smad3 was observed binding the RED sub-domain of the Pax6 paired domain. Finally, in DNA adsorption assays, activated Smad3 inhibited Pax6 from binding the consensus paired domain recognition sequence. We hypothesize that the Pax6 autoregulatory loop is targeted for repression by the TGFβ/Smad pathway, and conclude that this involves diminished paired domain DNA-binding function resulting from a ligand-dependant interaction between Pax6 and Smad3.

TGFbeta1 regulation of vimentin gene expression during differentiation of the C2C12 skeletal myogenic cell line requires Smads, AP-1 and Sp1 family members

Vimentin exhibits a complex pattern of developmental and tissue-specific expression regulated by such growth factors as TGFbeta1, PDGF, FGF, EGF and cytokines. Vimentin is expressed in the more migratory, mesenchymal cell and its expression is often down-regulated to make way for tissue-specific intermediate filaments proteins such as desmin in muscle. Here, we suggest a mechanism to explain how TGFbeta1 contributes to the up-regulation of vimentin expression while blocking myogenesis. TGFbeta1 binds to serine/threonine kinase receptors resulting in the phosphorylation of Smad2 and Smad3, followed by formation of a heteromeric complex with Smad4. The translocation of this complex to the nucleus modulates transcription of selected genes such as vimentin. However, the vimentin gene lacks a consensus TGFbeta1 response element. By transient transfection analysis of vimentin's various promoter elements fused to the CAT reporter gene, we have determined that tandem AP-1 sites surrounded by GC-boxes are required for TGFbeta1 induction. Mutations within this region eliminated the ability of Smad3 to induce reporter gene expression. DNA precipitation and ChIP assays suggest that c-Jun, c-Fos, Smad3 and Sp1/Sp3 interact over this region, but this interaction changes during myogenesis with TGFbeta1 induction.

Oct-3/4 dose dependently regulates specification of embryonic stem cells toward a cardiac lineage and early heart development

The transcriptional mechanisms underlying lineage specification and differentiation of embryonic stem (ES) cells remain elusive. Oct-3/4 (POU5f1) is one of the earliest transcription factors expressed in the embryo. Both the pluripotency and the fate of ES cells depend upon a tight control of Oct-3/4 expression. We report that transgene- or TGFbeta-induced increase in Oct-3/4 mRNA and protein levels in undifferentiated ES cells and at early stages of differentiation triggers expression of mesodermal and cardiac specific genes through Smad2/4. cDNA antisense- and siRNA-mediated inhibition of upregulation of Oct-3/4 in ES cells prevent their specification toward the mesoderm and their differentiation into cardiomyocytes. Similarly, Oct-3/4 siRNA injected in the inner cell mass of blastocysts impairs cardiogenesis in early embryos. Thus, quantitative Oct-3/4 expression is regulated by a morphogen, pointing to a pivotal and physiological function of the POU factor in mesodermal and cardiac commitments of ES cells and of the epiblast.