Transcriptional Activation of β-Tropomyosin Mediated by Serum Response Factor and a Novel Barx Homologue, Barx1b, in Smooth Muscle Cells

The serum response factor (SRF) is a transcriptional regulator required for mesodermal development, including heart formation and function. Previous studies have described the role of SRF in controlling expression of structural genes involved in conferring the myogenic phenotype. Recent studies by us and others have demonstrated embryonic lethal cardiovascular phenotypes in SRF-null animals, but have not directly addressed the mechanistic role of SRF in controlling broad regulatory programs in cardiac cells. In this study, we used a loss-of-function approach to delineate the role of SRF in cardiomyocyte gene expression and function. In SRF-null neonatal cardiomyocytes, we observed severe defects in the contractile apparatus, including Z-disc and stress fiber formation, as well as mislocalization and/or attenuation of sarcomeric proteins. Consistent with this, gene array and reverse transcription-PCR analyses showed down-regulation of genes encoding key cardiac transcriptional regulatory factors and proteins required for the maintenance of sarcomeric structure, function, and regulation. Chromatin immunoprecipitation analysis revealed that at least a subset of these proteins are likely regulated directly by SRF. The results presented here indicate that SRF is an essential coordinator of cardiomyocyte function due to its ability to regulate expression of numerous genes (some previously identified and at least 28 targets newly identified in this study) that are involved in multiple and disparate levels of sarcomeric function and assembly.The serum response factor (SRF) 2 is a member of the MADS (MCM1, AGAMOUS, DEFICIENS, SRF) box family of transcriptional regulatory proteins. SRF was first identified based on its ability to mediate serum and growth factor activation of the c-fos proto-oncogene (1). Subsequently, it was found that SRF and/or SRF-binding sites (CC(A/T) 6 GG), termed CArG boxes or serum response elements, regulate expression of a wide variety of inducible genes by various stimuli ranging from growth factors to changes in intracellular calcium flux (2). SRF plays a key role during early development and is required for successful gastrulation presumably due to its importance in mesoderm differentiation (3). Numerous studies also suggest a role for SRF in myogenic differentiation and cardiac development and function. Cell culture studies have shown that functional SRF-binding elements are key cis-regulatory sites in the promoters of various cardiac contractile genes, including cardiac ␣-actin, ␤-myosin heavy chain (MHC), and dystrophin (4 -7). SRF has also been implicated in the regulation of genes encoding non-contractile cardiac proteins, including the sarcoplasmic reticulum Ca 2ϩ -ATPase SERCA2 and the Na ϩ /Ca 2ϩ exchanger NCX1 (8, 9). Further support for an important role for SRF in cardiac function comes from transgenic experiments in rodent model systems demonstrating that cardiac-specific dysregulation of SRF expression can induce cardiac hypertrophy and cardiomyopathies in postnatal animals tha...

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“…5 and Table 2). These results confirm other recent studies implicating SRF in the control of tropomyosin genes (47,48).…”

Section: Discussionsupporting

confidence: 82%

Role of the Serum Response Factor in Regulating Contractile Apparatus Gene Expression and Sarcomeric Integrity in Cardiomyocytes

Balza

Misra

2006

Journal of Biological Chemistry

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“…These markers-Myh11, Cnn1, and Tpm2-are all known to be dependent upon Srf for their transcriptional activation (21,43,44,53). Although a very low level of expression of each of these three genes was evident by real-time PCR in Mkl1 Ϫ/Ϫ myoepithelial cells, each was expressed at an approximately 2.5-to 3-fold lower level compared to wild-type cells (Fig.…”

Section: Involution-related Genes Including Those Of the Stat3 Pathwmentioning

confidence: 95%

Acute Myeloid Leukemia-Associated Mkl1 (Mrtf-a) Is a Key Regulator of Mammary Gland Function

Sun¹,

Boyd

Xu³

et al. 2006

Molecular and Cellular Biology

155

147

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Transcription of immediate-early genes-as well as multiple genes affecting muscle function, cytoskeletal integrity, apoptosis control, and wound healing/angiogenesis-is regulated by serum response factor (Srf). Extracellular signals regulate Srf in part via a pathway involving megakaryoblastic leukemia 1 (Mkl1, also known as myocardin-related transcription factor A [Mrtf-a]), which coactivates Srf-responsive genes downstream of Rho GTPases. Here we investigate Mkl1 function using gene targeting and show the protein to be essential for the physiologic preparation of the mammary gland during pregnancy and the maintenance of lactation. Lack of Mkl1 causes premature involution and impairs expression of Srf-dependent genes in the mammary myoepithelial cells, which control milk ejection following oxytocin-induced contraction. Despite the importance of Srf in multiple transcriptional pathways and widespread Mkl1 expression, the spectrum of abnormalities associated with Mkl1 absence appears surprisingly restricted.Megakaryoblastic leukemia 1 (MKL1) was initially identified in acute megakaryoblastic leukemias (AMKLs) that occur in infants and young children due to its involvement in the RBM15-MKL1 fusion protein created by the t(1;22) chromosomal translocation found uniquely in these leukemias (41, 48). As a result of this translocation, the MKL1 gene (alternatively known as MAL [megakaryocytic acute leukemia], MRTF-A

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“…However, we were unable to identify the SMC-selective SRF cofactor. Interestingly, the homeodomain proteins Barx1b and Nkx3.2 and the zinc finger protein GATA6 have each been shown to form a stable, detectable ternary complex with SRF-CArG DNA (185,187), and a recent interesting study from Nishida et al (187) showed that the triad of SRF, Nkx3.2, and GATA6 cooperatively activated several SMC marker genes, including SM22␣ and caldesmon, but not c-fos. Thus this combination of factors provides a mechanism by which SMC-specific genes can be regulated selectively and independently of growth responsive genes.…”

Section: D) Posttranscriptional Modification Of Srf As Well Asmentioning

confidence: 99%

Molecular Regulation of Vascular Smooth Muscle Cell Differentiation in Development and Disease

Owens

Kumar²,

Wamhoff³

2004

Physiological Reviews

3,034

3,134

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The focus of this review is to provide an overview of the current state of knowledge of molecular mechanisms/processes that control differentiation of vascular smooth muscle cells (SMC) during normal development and maturation of the vasculature, as well as how these mechanisms/processes are altered in vascular injury or disease. A major challenge in understanding differentiation of the vascular SMC is that this cell can exhibit a wide range of different phenotypes at different stages of development, and even in adult organisms the cell is not terminally differentiated. Indeed, the SMC is capable of major changes in its phenotype in response to changes in local environmental cues including growth factors/inhibitors, mechanical influences, cell-cell and cell-matrix interactions, and various inflammatory mediators. There has been much progress in recent years to identify mechanisms that control expression of the repertoire of genes that are specific or selective for the vascular SMC and required for its differentiated function. One of the most exciting recent discoveries was the identification of the serum response factor (SRF) coactivator gene myocardin that appears to be required for expression of many SMC differentiation marker genes, and for initial differentiation of SMC during development. However, it is critical to recognize that overall control of SMC differentiation/maturation, and regulation of its responses to changing environmental cues, is extremely complex and involves the cooperative interaction of many factors and signaling pathways that are just beginning to be understood. There is also relatively recent evidence that circulating stem cell populations can give rise to smooth muscle-like cells in association with vascular injury and atherosclerotic lesion development, although the exact role and properties of these cells remain to be clearly elucidated. The goal of this review is to summarize the current state of our knowledge in this area and to attempt to identify some of the key unresolved challenges and questions that require further study.

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Transcriptional Activation of β-Tropomyosin Mediated by Serum Response Factor and a Novel Barx Homologue, Barx1b, in Smooth Muscle Cells

Cited by 23 publications

References 56 publications

Role of the Serum Response Factor in Regulating Contractile Apparatus Gene Expression and Sarcomeric Integrity in Cardiomyocytes

Role of the Serum Response Factor in Regulating Contractile Apparatus Gene Expression and Sarcomeric Integrity in Cardiomyocytes

Acute Myeloid Leukemia-Associated Mkl1 (Mrtf-a) Is a Key Regulator of Mammary Gland Function

Molecular Regulation of Vascular Smooth Muscle Cell Differentiation in Development and Disease

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