FoxO1a-cyclic GMP-dependent kinase I interactions orchestrate myoblast fusion

Decreased PKG transcription mediated by PI3K/Akt/FoxO1 pathway is involved in the development of nitroglycerin tolerance

Phosphoinositide 3-kinase (PI3K)/Akt plays a pivotal role in the vascular response. The present study is to determine whether PI3K/Akt pathway in vascular smooth muscle cells is involved in nitroglycerin (NTG) tolerance and the underlying mechanism. Nitrate tolerance of porcine coronary arteries in vitro was induced by incubation of NTG (10^-5 M) for 24 h. Nitrate tolerance in vivo was obtained by subcutaneous injection of mice with NTG (20 mg kg^-1, tid, 3 days) and the aortas were used. Protein levels of total and phosphorylated Akt, forkhead box protein O1 (FoxO1), and cGMP-dependent protein kinase (PKG) were determined by western blot analysis. Isometric vessel tension was recorded by organ chamber technique. PKG mRNA was determined by real-time PCR. The cellular translocation of FoxO1 was observed by immunofluorescence. Reactive oxygen species (ROS) level was measured by DHE staining. The vascular relaxation to NTG was significantly inhibited in in vivo and in vitro NTG tolerant arteries. Meanwhile, the protein level of phosphorylated Akt at Ser473 was increased in the tolerant arteries. The attenuated relaxation and the augmented Akt-p were ameliorated by LY294002, a specific inhibitor of PI3K. The protein and mRNA expression of PKG were significantly down-regulated in NTG tolerant arteries, which were reversed by LY294002. The level of phosphorylated FoxO1 at Ser256 and its translocation from the nucleus to the cytosol were both increased in NTG tolerance and were also inhibited by LY294002. ROS production was significantly increased in NTG tolerant arteries, which was not be affected by LY294002 but inhibited by N-acetyl-L-cysteine. In conclusion, the present study suggests that PI3K/Akt in vascular smooth muscle is involved in the development of NTG tolerance via inhibiting PKG transcription and the effect is mediated by FoxO1.

FoxO1: a novel insight into its molecular mechanisms in the regulation of skeletal muscle differentiation and fiber type specification

FoxO1, a member of the forkhead transcription factor forkhead box protein O (FoxO) family, is predominantly expressed in most muscle types. FoxO1 is a key regulator of muscle growth, metabolism, cell proliferation and differentiation. In the past two decades, many researches have indicated that FoxO1 is a negative regulator of skeletal muscle differentiation while contrasting opinions consider that FoxO1 is crucial for myoblast fusion. FoxO1 is expressed much higher in fast twitch fiber enriched muscles than in slow muscles and is also closely related to muscle fiber type specification. In this review, we summarize the molecular mechanisms of FoxO1 in the regulation of skeletal muscle differentiation and fiber type specification.

Testosterone modulates FoxO3a and p53-related genes to protect C2C12 skeletal muscle cells against apoptosis

The loss of muscle mass and strength with aging, sarcopenia, is a prevalent condition among the elderly, associated with skeletal muscle dysfunction and enhanced muscle cell apoptosis. We have previously demonstrated that testosterone protects against H₂O₂-induced apoptosis in C2C12 muscle cells, at different levels: morphological, biochemical and molecular. Since we have observed that testosterone reduces p-p53 and maintains the inactive state of FoxO3a transcription factor, induced by H₂O₂, we analyzed if the hormone was exerting its antiapoptotic effect at transcriptional level, by modulating pro and antiapoptotic genes associated to them. We detected the upregulation of the proapoptotic genes Puma, PERP and Bim, and MDM2 in response to H₂O₂ at different periods of the apoptotic process, and the downregulation of the antiapoptotic gene Bcl-2, whereas testosterone was able to modulate and counteract H₂O₂ effects. Furthermore, ERK and JNK kinases have been demonstrated to be linked to FoxO3a phosphorylation and thus its subcellular distribution. This work show some transcription level components, upstream of the classical apoptotic pathway, that are activated during oxidative stress and that are points where testosterone exerts its protective action against apoptosis, exposing some of the puzzle pieces of the intricate network that aged skeletal muscle apoptosis represents.

Thyroid hormones and skeletal muscle--new insights and potential implications

Thyroid hormone signalling regulates crucial biological functions, including energy expenditure, thermogenesis, development and growth. The skeletal muscle is a major target of thyroid hormone signalling. The type 2 and 3 iodothyronine deiodinases (DIO2 and DIO3, respectively) have been identified in skeletal muscle. DIO2 expression is tightly regulated and catalyses outer-ring monodeiodination of the secreted prohormone tetraiodothyronine (T4) to generate the active hormone tri-iodothyronine (T3). T3 can remain in the myocyte to signal through nuclear receptors or exit the cell to mix with the extracellular pool. By contrast, DIO3 inactivates T3 through removal of an inner-ring iodine. Regulation of the expression and activity of deiodinases constitutes a cell-autonomous, pre-receptor mechanism for controlling the intracellular concentration of T3. This local control of T3 activity is crucial during the various phases of myogenesis. Here, we review the roles of T3 in skeletal muscle development and homeostasis, with a focus on the emerging local deiodinase-mediated control of T3 signalling. Moreover, we discuss these novel findings in the context of both muscle homeostasis and pathology, and examine how skeletal muscle deiodinase activity might be therapeutically harnessed to improve satellite-cell-mediated muscle repair in patients with skeletal muscle disorders, muscle atrophy or injury.

Transcriptional and post-transcriptional regulation of cGMP-dependent protein kinase (PKG-I): pathophysiological significance

The ability of the endothelium to produce nitric oxide, which induces generation of cyclic guanosine monophosphate (cGMP) that activates cGMP-dependent protein kinase (PKG-I), in vascular smooth muscle cells (VSMCs), is essential for the maintenance of vascular homeostasis. Yet, disturbance of this nitric oxide/cGMP/PKG-I pathway has been shown to play an important role in many cardiovascular diseases. In the last two decades, in vitro and in vivo models of vascular injury have shown that PKG-I is suppressed following nitric oxide, cGMP, cytokine, and growth factor stimulation. The molecular basis for these changes in PKG-I expression is still poorly understood, and they are likely to be mediated by a number of processes, including changes in gene transcription, mRNA stability, protein synthesis, or protein degradation. Emerging studies have begun to define mechanisms responsible for changes in PKG-I expression and have identified cis- and trans-acting regulatory elements, with a plausible role being attributed to post-translational control of PKG-I protein levels. This review will focus mainly on recent advances in understanding of the regulation of PKG-I expression in VSMCs, with an emphasis on the physiological and pathological significance of PKG-I down-regulation in VSMCs in certain circumstances.

The nitric oxide-cyclic GMP pathway regulates FoxO and alters dopaminergic neuron survival in Drosophila

Activation of the forkhead box transcription factor FoxO is suggested to be involved in dopaminergic (DA) neurodegeneration in a Drosophila model of Parkinson's disease (PD), in which a PD gene product LRRK2 activates FoxO through phosphorylation. In the current study that combines Drosophila genetics and biochemical analysis, we show that cyclic guanosine monophosphate (cGMP)-dependent kinase II (cGKII) also phosphorylates FoxO at the same residue as LRRK2, and Drosophila orthologues of cGKII and LRRK2, DG2/For and dLRRK, respectively, enhance the neurotoxic activity of FoxO in an additive manner. Biochemical assays using mammalian cGKII and FoxO1 reveal that cGKII enhances the transcriptional activity of FoxO1 through phosphorylation of the FoxO1 S319 site in the same manner as LRRK2. A Drosophila FoxO mutant resistant to phosphorylation by DG2 and dLRRK (dFoxO S259A corresponding to human FoxO1 S319A) suppressed the neurotoxicity and improved motor dysfunction caused by co-expression of FoxO and DG2. Nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) also increased FoxO's activity, whereas the administration of a NOS inhibitor L-NAME suppressed the loss of DA neurons in aged flies co-expressing FoxO and DG2. These results strongly suggest that the NO-FoxO axis contributes to DA neurodegeneration in LRRK2-linked PD.

The myogenic kinome: protein kinases critical to mammalian skeletal myogenesis

Myogenesis is a complex and tightly regulated process, the end result of which is the formation of a multinucleated myofibre with contractile capability. Typically, this process is described as being regulated by a coordinated transcriptional hierarchy. However, like any cellular process, myogenesis is also controlled by members of the protein kinase family, which transmit and execute signals initiated by promyogenic stimuli. In this review, we describe the various kinases involved in mammalian skeletal myogenesis: which step of myogenesis a particular kinase regulates, how it is activated (if known) and what its downstream effects are. We present a scheme of protein kinase activity, similar to that which exists for the myogenic transcription factors, to better clarify the complex signalling that underlies muscle development.

Understanding Forkhead box class O function: lessons from Drosophila melanogaster

Drosophila melanogaster is one of the most widely used model organisms. About 77% of known human disease genes have an ortholog in Drosophila, and many of the cellular signaling pathways are common between fruit flies and mammals. For example, a key signaling pathway in the regulation of growth and metabolism, the insulin/insulin-like growth factor 1 signaling pathway, is well conserved between flies and humans. Downstream effectors of this pathway are the Forkhead box class O (FOXO) family of transcription factors, with four members in mammals and a single FOXO protein in Drosophila, dFOXO. Research in Drosophila has been critical to elucidate the molecular mechanisms by which FOXO transcription factors regulate insulin signaling. In this review, we summarize the studies leading to dFOXO identification and its characterization as a central regulator of metabolism, life span, cell cycle, growth, and stress resistance.

Stabilization of cGMP-dependent protein kinase G (PKG) expression in vascular smooth muscle cells: contribution of 3'UTR of its mRNA

The type-I cGMP-dependent protein kinase (PKG-I) expression regulation is not yet completely understood. In this study, we examined the role of 3'-untranslated region (3'UTR)-PKG-I messenger RNA (mRNA) in the control of PKG-I expression in vascular smooth muscle cells (VSMCs). Using a 3'-rapid amplification of cDNA ends (RACE) for the amplification of complementary DNA (cDNA) ends, we generated and cloned a 1.2-kb-3'UTR mRNA PKG-I in pGL3 control vector downstream of the luciferase reporter gene. Serial deletions and functional studies revealed that among the deleted constructs, only the 1.2-kb-3'UTR PKG-I mRNA possesses the highest activity in transfected VSMC. Kinetic luciferase assays in the presence of actinomycin D showed that this construct stabilizes luciferase activity compared to the control vector. Sequence analysis of 3'UTR-PKG-I mRNA revealed the existence of four AU-rich regions (AU1 through AU4) in addition to a potential poly(A) site. Different riboprobes were generated either by 5'-end-labelling of designed ribonucleotides, containing individual AU-rich regions or by in vitro transcription assay using cloned 1.2-kb cDNA as a template. RNA-electrophoretic mobility shift assay (EMSA) and ultra-violet cross-linking (UV-CL) assays showed that AU1, AU3, AU4 and 1.2-kb probes were able to retard cytosolic and nuclear proteins. Taken together, these data suggest that PKG-I expression is subjected to post-transcriptional regulation in VSMC through the 3'UTR of its mRNA.

Molecular mechanisms of myoblast fusion across species

Skeletal muscle development, growth and regeneration depend on the ability of progenitor myoblasts to fuse to one another in a series of ordered steps. Whereas the cellular steps leading to the formation of a multinucleated myofiber are conserved in several model organisms, the molecular regulatory factors may vary. Understanding the common and divergent mechanisms regulating myoblast fusion in Drosophila melanogaster (fruit fly), Danio rerio (zebrafish) and Mus musculus (mouse) provides a better insight into the process of myoblast fusion than any of these models could provide alone. Deciphering the mechanisms of myoblast fusion from simpler to more complex organisms is of fundamental interest to skeletal muscle biology and may provide therapeutic avenues for various diseases that affect muscle.

Structural basis for DNA recognition by FOXO proteins

The FOXO forkhead transcription factors are involved in metabolism control, cell survival, cellular proliferation, DNA damage repair response, and stress resistance. Their transcriptional activity is regulated through a number of posttranslational modifications, including phosphorylation, acetylation and ubiquitination. The recently determined three-dimensional structures of FOXO forkhead domains bound to DNA enable to explain the structural basis for DNA recognition by FOXO proteins and its regulation. The aim of this review is to summarize the recent structural characterization of FOXO proteins, the mechanisms of DNA recognition and the role of posttranslational modifications in the regulation of FOXO DNA-binding properties. This article is part of a Special Issue entitled: PI3K-AKT-FOXO axis in cancer and aging.

Downregulation of caveolin-1 enhances fusion of human BeWo choriocarcinoma cells

Background

Fusion of placental villous cytotrophoblasts with the overlying syncytiotrophoblast is essential for the maintenance of successful pregnancy, and disturbances in this process have been implicated in pathological conditions such as pre-eclampsia and intra-uterine growth retardation. Caveolin-1 has been shown to be expressed in human villous cytotrophoblast and to be downregulated during fusion into syncytiotrophoblast but it is unclear whether it plays a role in this process.

Methodology/Principal Findings

We used RNA interference to determine whether caveolin-1 plays a role in differentiation and fusion in the BeWo choriocarcinoma cell line, a model of villous cytotrophoblast fusion. Assessment of cell fusion by desmosomal protein immunostaining revealed that cells transfected with caveolin-1 siRNA showed significantly enhanced fusion in response to treatment with dibutyryl cyclic AMP compared with cells transfected with a non-silencing control. Furthermore, caveolin-1 knockdown alone was sufficient to promote spontaneous fusion. In addition, biochemical differentiation, assessed by expression of placental alkaline phosphatase, was upregulated in caveolin-1 siRNA-transfected cells, with or without dbcAMP treatment. Assessment of Akt phosphorylation showed that caveolin-1 knockdown resulted in a significant reduction in phosphorylation at Thr³⁰⁸.

Conclusions/Significance

Taken together, these results suggest that caveolin-1 regulates BeWo cell differentiation and fusion, possibly through a mechanism involving modulation of Akt activity.

Regulation of the intracellular localization of Foxo3a by stress-activated protein kinase signaling pathways in skeletal muscle cells

Muscle atrophy is a debilitating process associated with many chronic wasting diseases, like cancer, diabetes, sepsis, and renal failure. Rapid loss of muscle mass occurs mainly through the activation of protein breakdown by the ubiquitin proteasome pathway. Foxo3a transcription factor is critical for muscle atrophy, since it activates the expression of ubiquitin ligase Atrogin-1. In several models of atrophy, inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway induces nuclear import of Foxo3a through an Akt-dependent process. This study aimed to identify signaling pathways involved in the control of Foxo3a nuclear translocation in muscle cells. We observed that after nuclear import of Foxo3a by PI3K/Akt pathway inhibition, activation of stress-activated protein kinase (SAPK) pathways induced nuclear export of Foxo3a through CRM1. This mechanism involved the c-Jun NH(2)-terminal kinase (JNK) signaling pathway and was independent of Akt. Likewise, we showed that inhibition of p38 induced a massive nuclear relocalization of Foxo3a. Our results thus suggest that SAPKs are involved in the control of Foxo3a nucleocytoplasmic translocation in C2C12 cells. Moreover, activation of SAPKs decreases the expression of Atrogin-1, and stable C2C12 myotubes, in which the p38 pathway is constitutively activated, present partial protection against atrophy.

Different roles of Foxo1 and Foxo3 in the control of endothelial cell morphology

Foxo1, a member of the Foxo subfamily of forkhead box transcription factors, is known to be essential for progression of normal vascular development in the mouse embryos. In the cultures of endothelial cells derived from embryonic stem cells, Foxo1-deficient endothelial cells exhibit an abnormal morphological response to vascular endothelial growth factor-A (VEGF-A), which is characterized by a lack of cell elongation, yet the molecular mechanisms governing endothelial cell morphology under angiogenic stimulation remain unknown. Here, we report that transforming growth actor-beta also induces endothelial cell elongation in collaboration with Foxo1 and VEGF-A. Furthermore, tetracycline-regulated induction of Foxo3, another member of the Foxo subfamily, into Foxo1-null endothelial cells failed to restore abnormal morphological response to VEGF-A at an early differentiation stage. In contrast, Foxo1 and Foxo3 exerted the same function at a late differentiation stage, i.e. enhancement of VEGF responsiveness and promotion of cell elongation. Our results provide evidence that endothelial cell morphology is regulated by several mechanisms in which Foxo1 and Foxo3 express distinct functional properties depending on differentiation stages.

Novel crosstalk to BMP signalling: cGMP-dependent kinase I modulates BMP receptor and Smad activity

Integration of multiple signals into the canonical BMP/Smad pathway poses a big challenge during the course of embryogenesis and tissue homeostasis. Here, we show that cyclic guanosine 3',5'-monophosphate (cGMP)-dependent kinase I (cGKI) modulates BMP receptors and Smads, providing a novel mechanism enhancing BMP signalling. cGKI, a key mediator of vasodilation and hypertension diseases, interacts with and phosphorylates the BMP type II receptor (BMPRII). In response to BMP-2, cGKI then dissociates from the receptors, associates with activated Smads, and undergoes nuclear translocation. In the nucleus, cGKI binds with Smad1 and the general transcription factor TFII-I to promoters of BMP target genes such as Id1 to enhance transcriptional activation. Accordingly, cGKI has a dual function in BMP signalling: (1) it modulates BMP receptor/Smad activity at the plasma membrane and (2) after redistribution to the nucleus, it further regulates transcription as a nuclear co-factor for Smads. Consequently, cellular defects caused by mutations in BMPRII, found in pulmonary arterial hypertension patients, were compensated through cGKI, supporting the positive action of cGKI on BMP-induced Smad signalling downstream of the receptors.

Rho/Rho-associated kinase signal regulates myogenic differentiation via myocardin-related transcription factor-A/Smad-dependent transcription of the Id3 gene

RhoA is known to be involved in myogenic differentiation, but whether it acts as a positive or negative regulator is controversial. To resolve this issue, we investigated the differentiation stage-specific roles of RhoA and its effector, Rho-associated kinase, using C2C12 myoblasts. We found that proliferating myoblasts show high levels of RhoA and serum-response factor activities and strong expression of the downstream target of RhoA, myocardin-related transcription factor-A (MRTF-A or MAL); these activities and expression are markedly lower in differentiating myocytes. We further demonstrated that, in proliferating myoblasts, an increase in MRTF-A, which forms a complex with Smad1/4, strikingly activates the expression level of the Id3 gene; the Id3 gene product is a potent inhibitor of myogenic differentiation. Finally, we found that during differentiation, one of the forkhead transcription factors translocates into the nucleus and suppresses Id3 expression by preventing the association of the MRTF-A-Smad complex with the Id3 promoter, which leads to the enhancement of myogenic differentiation. We conclude that RhoA/Rho-associated kinase signaling plays positive and negative roles in myogenic differentiation, mediated by MRTF-A/Smad-dependent transcription of the Id3 gene in a differentiation stage-specific manner.

Structure/function relationships underlying regulation of FOXO transcription factors

The FOXO subgroup of forkhead transcription factors plays a central role in cell-cycle control, differentiation, metabolism control, stress response and apoptosis. Therefore, the function of these important molecules is tightly controlled by a wide range of protein-protein interactions and posttranslational modifications including phosphorylation, acetylation and ubiquitination. The mechanisms by which these processes regulate FOXO activity are mostly elusive. This review focuses on recent advances in structural studies of forkhead transcription factors and the insights they provide into the mechanism of DNA recognition. On the basis of these data, we discuss structural aspects of protein-protein interactions and posttranslational modifications that target the forkhead domain and the nuclear localization signal of FOXO proteins.

Tissue expression of porcine FoxO1 and its negative regulation during primary preadipocyte differentiation

The Forkhead transcription factor O 1 (FoxO1) gene plays an important role in the integration of hormone-activated signaling pathways with the complex transcriptional cascade that promotes clonal cell line differentiation. However, tissue expression of porcine FoxO1 and its function during porcine preadipocyte differentiation remained poorly understood. In the present study, we investigated tissue expressions of FoxO1 in pig by real time quantitative RT-PCR (qRT-PCR) and western blotting, and explored its role in porcine preadipocytes differentiation by RNA interference technique and qRT-PCR. FoxO1 gene expressions were highly in subcutaneous adipose and visceral adipose tissues, and higher in piglets than those in adults (P < 0.05). We showed that expression of endogenous FoxO1 in preadipocytes transfected with pBS/U6-siFoxO1-1748 expression vector was inhibited efficiently. After reducing expression of FoxO1, glycerol-3-phosphate dehydrogenase (GPDH) activity and triglyceride (TG) content increased from day 1 to 9, and the time-course expressions of several key adipogenic genes mRNA, including peroxisome proliferator-activated receptor gamma on day 3, 5, and 7, adipocyte fatty acid binding protein on day 1, 3, and 5, and sirtuin1 on day 1, 3, and 5, were increased significantly (P < 0.05). Lipoprotein lipase was unrelevant to FoxO1. By using insulin-like growth factor-I treating, expression of FoxO1 reduced at day 3 and 5 (P < 0.05), and significant differentiation of porcine preadipocyte with increasing number of filled-lipid cell and size of lipid droplets, GPDH activity and TG content were promoted. These results suggested that porcine FoxO1 gene took part in the regulation of adipose and was a negative transcription regulation factor in preadipocyte differentiation.

[Expression of FoxO1 mRNA in muscle tissue of Bamei, Landrace and Landrace x Bamei]

Total RNA was extracted from soleus muscle, gastrocnemius muscle and extensor digitorum longus in hind leg of the 6-month-old male Bamei, Landrace, and the crossbred pigs (LandracexBamei). The primers of pig FoxO1 were designed and synthesized according to its homologous sequences of human, chimpanzee and rat. The optimal reaction condition and system of PCR was sieved to clone pig FoxO1 gene. Subsequently, the differential expression of FoxO1 mRNA was investigated in different types of skeletal muscle from Bamei, Landrace, and the crossbred pigs by SQ RT-PCR. The expression level of beta-actin was determined as a loading control. The results showed that the expression level of FoxO1 was different in various economic pig breeds and skeletal muscle types. The expression of FoxO1 in skeletal muscle of Bamei and the crossbred pigs was significantly higher than that of Landrace (Plt;0.01). Moreover, the expression of FoxO1 was lowest in soleus muscle, which mainly contains typemuscle fiber, and most abundant in extensor digitorum longus, which mainly contains typeb muscle fiber. This suggests that FoxO1 expression level has the negative correlation with the con-tent of typemuscle fiber. Skeletal muscle development of different economic types of pig breeds may be closely related to the regulation of FoxO1 gene.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

A cysteine-rich LIM-only protein mediates regulation of smooth muscle-specific gene expression by cGMP-dependent protein kinase

Vascular smooth muscle cells (VSMCs) undergo phenotypic modulation, changing from a differentiated, contractile to a de-differentiated, synthetic phenotype; the change is associated with decreased expression of smooth muscle (SM)-specific genes and loss of cGMP-dependent protein kinase (PKG), but transfection of PKG into de-differentiated VSMCs restores SM-specific gene expression. We show that small interference RNA-mediated down-regulation or pharmacologic inhibition of PKG reduced SM-specific gene expression in differentiated VSMCs and provide a mechanism for cGMP/PKG regulation of SM-specific genes involving the cysteine-rich LIM-only protein CRP4. PKG associated with CRP4 and phosphorylated the protein in intact cells. CRP4 had no intrinsic transcriptional activity, but exhibited adaptor function, because it acted synergistically with serum response factor (SRF) and GATA6 to activate the SM-alpha-actin promoter. cGMP stimulation of the promoter required PKG and CRP4 co-expression with SRF and GATA6. A phosphorylation-deficient mutant CRP4 and a CRP4 deletion mutant deficient in PKG binding did not support cGMP/PKG stimulation of the SM-alpha-actin promoter. In the presence of wild-type but not mutant CRP4, cGMP/PKG enhanced SRF binding to a probe encoding the distal SM-alpha-actin promoter CArG (CC(AT)(6)GG) element. CRP4 and SRF associated with CArG elements of endogenous SM-specific genes in intact chromatin. Small interference RNA-mediated down-regulation of CRP4 prevented the positive effects of cGMP/PKG on SM-specific gene expression. In the presence of CRP4, cGMP/PKG increased SRF- and GATA6-dependent expression of endogenous SM-specific genes in pluripotent 10T1/2 cells. Thus, CRP4 mediates cGMP/PKG stimulation of SM-specific gene expression, and PKG plays an important role in regulating the phenotype of VSMCs.

C6ORF32 is upregulated during muscle cell differentiation and induces the formation of cellular filopodia

We have identified a gene by microarray analysis that is located on chromosome 6 (c6orf32), whose expression is increased during human fetal myoblast differentiation. The protein encoded by c6orf32 is expressed both in myogenic and non-myogenic primary cells isolated from 18-week old human fetal skeletal muscle. Immunofluorescent staining indicated that C6ORF32 localizes to the cellular cytoskeleton and filopodia, and often displays polarized expression within the cell. mRNA knockdown experiments in the C2C12 murine myoblast cell line demonstrated that cells lacking c6orf32 exhibit a myogenic differentiation defect, characterized by a decrease in the expression of myogenin and myosin heavy chain (MHC) proteins, whereas MyoD1 was unaltered. In contrast, overexpression of c6orf32 in C2C12 or HEK293 cells (a non-muscle cell line) promoted formation of long membrane protrusions (filopodia). Analysis of serial deletion mutants demonstrated that amino acids 55-113 of C6ORF32 are likely involved in filopodia formation. These results indicate that C6ORF32 is a novel protein likely to play multiple functions, including promoting myogenic cell differentiation, cytoskeletal rearrangement and filopodia formation.

Regulation of cGMP-dependent protein kinase expression by Rho and Kruppel-like transcription factor-4

Type I cGMP-dependent protein kinase (PKG I) plays a major role in vascular homeostasis by mediating smooth muscle relaxation in response to nitric oxide, but little is known about the regulation of PKG I expression in smooth muscle cells. We found opposing effects of RhoA and Rac1 on cellular PKG I expression: (i) cell density-dependent changes in PKG I expression varied directly with Rac1 activity and inversely with RhoA activity; (ii) RhoA activation by calpeptin suppressed PKG I, whereas RhoA down-regulation by small interfering RNA increased PKG I expression; and (iii) PKG I promoter activity was suppressed in cells expressing active RhoA or Rho-kinase but was enhanced in cells expressing active Rac1 or a dominant negative RhoA. Sp1 consensus sequences in the PKG I promoter were required for Rho regulation and bound nuclear proteins in a cell density-dependent manner, including the Krüppel-like factor 4 (KLF4). KLF4 was identified as a major trans-acting factor at two proximal Sp1 sites; active RhoA suppressed KLF4 DNA binding and trans-activation potential on the PKG I promoter. Experiments with actin-binding agents suggested that RhoA could regulate KLF4 via its ability to induce actin polymerization. Regulation of PKG I expression by RhoA may explain decreased PKG I levels in vascular smooth muscle cells found in some models of hypertension and vascular injury.

FOXO1a acts as a selective tumor suppressor in alveolar rhabdomyosarcoma

Rhabdomyosarcoma (RMS), the most common pediatric soft-tissue sarcoma, has two major histological subtypes: embryonal RMS (ERMS), which has a favorable prognosis, and alveolar RMS (ARMS), which has a poor outcome. Although both forms of RMS express muscle cell–specific markers, only ARMS cells express PAX3-FOXO1a or PAX7-FOXO1a chimeric proteins. In mice, Pax3 and Pax7 play key roles in muscle cell development and differentiation, and FoxO1a regulates myoblast differentiation and fusion; thus, the aberrant regulation of these proteins may contribute to the development of ARMS. In this paper, we report that FOXO1a is not expressed in primary ARMS tumors or ARMS-derived tumor cell lines and that restoration of FOXO1a expression in ARMS cells is sufficient to induce cell cycle arrest and apoptosis. Strikingly, the effects of FOXO1a are selective, as enforced expression of FOXO1a in ERMS-derived tumor cell lines had no effect. Furthermore, FOXO1a induced apoptosis in ARMS by directly activating the transcription of caspase-3. We conclude that FOXO1a is a potent and specific tumor suppressor in ARMS, suggesting that agents that restore or augment FOXO1a activity may be effective as ARMS therapeutics.