Serum response factor: transcriptional regulation of genes induced by growth factors and differentiation

Emerging role for the Serum Response Factor (SRF) as a potential therapeutic target in cancer

INTRODUCTION

The Serum Response Factor (SRF) is a transcription factor involved in three hallmarks of cancer: the promotion of cell proliferation, cell death resistance and invasion and metastasis induction. Many studies have demonstrated a leading role in the development and progression of multiple cancer types, thus highlighting the potential of SRF as a prognostic biomarker and therapeutic target, especially for cancers with poor prognosis.

AREAS COVERED

This review examines the role of SRF in several cancers in promoting cellular processes associated with cancer development and progression. SRF co-factors and signalling pathways are discussed as possible targets to inhibit SRF in a tissue and cancer-specific way. Small-molecule inhibitors of SRF, such as the CCGs series of compounds and lestaurtinib, which could be used as cancer therapeutics, are also discussed.

EXPERT OPINION

Targeting of SRF and its co-factors represents a promising therapeutic approach. Further understanding of the molecular mechanisms behind the action of SRF could provide a pipeline of novel molecular targets and therapeutic combinations for cancer. Basket clinical trials and the use of SRF immunohistochemistry as companion diagnostics will help testing of these new targets in patients.

The Function of SUMOylation and Its Critical Roles in Cardiovascular Diseases and Potential Clinical Implications

Cardiovascular disease (CVD) is a common disease caused by many factors, including atherosclerosis, congenital heart disease, heart failure, and ischemic cardiomyopathy. CVD has been regarded as one of the most common diseases and has a severe impact on the life quality of patients. The main features of CVD include high morbidity and mortality, which seriously threaten human health. SUMO proteins covalently conjugate lysine residues with a large number of substrate proteins, and SUMOylation regulates the function of target proteins and participates in cellular activities. Under certain pathological conditions, SUMOylation of proteins related to cardiovascular development and function are greatly changed. Numerous studies have suggested that SUMOylation of substrates plays critical roles in normal cardiovascular development and function. We reviewed the research progress of SUMOylation in cardiovascular development and function, and the regulation of protein SUMOylation may be applied as a potential therapeutic strategy for CVD treatment.

MKL1/miR34a/FOXP3 axis regulates cell proliferation in gastric cancer

Megakaryoblastic leukemia 1 (MKL1) was closely related to the pathogenesis of various human malignant cancers. MiR34a was reported to be closely related to cancer cell proliferation. Forkhead box protein 3 (FOXP3) was a transcription factor that played a different role in different cancer types. CDK6 was involved in cell cycle progression and was upregulated in several types of cancers. The present study investigated the effects of MKL1/miR34a/FOXP3 axis on cell proliferation in MGC803 gastric cancer cells. Our results demonstrated that overexpression of MKL1 promoted proliferation of MGC80-3 cells, MKL1 directly binding to the promoter of CDK6 to increase its expression. Knockdown of FOXP3 promoted proliferation of MGC80-3 cells and MKL1 inhibited the expression of FOXP3 via miR-34a. The finding can contribute to elucidating the regulatory mechanism involved in the cell cycle progression of gastric cancer cells and may aid in screening potential gene targets for the biological therapy of gastric cancer.

Iron deficiency upregulates Egr1 expression

Iron-deficient anemia is a prevalent disease among humans. We searched for genes regulated by iron deficiency and its regulated mechanism. cDNA microarrays were performed using Hepa1c1c7 cells treated with 100 μM desferrioxamine (DFO), an iron chelator. Early growth response 1 (Egr1) was upregulated with at least 20-fold increase within 4 h and lasted for 24 h, which was confirmed by qRT-PCR. This activation was not seen by ferric ammonium citrate (FAC). DFO increased the transcriptional activity of Egr1-luc (-604 to +160) and serum response element (SRE)-luc reporters by 2.7-folds. In addition, cycloheximide lowered DFO-induced Egr1 mRNA levels. The upregulation of Egr1 by DFO was accompanied by sustained ERK signals along with phosphorylation of Elk-1. The ERK inhibitor (PD98059) prevented the DFO-induced Egr1 mRNAs. Overexpression of Elk-1 mutant (pElk-1S383A) decreased Egr1 reporter activity. DFO lowered reactive oxygen species (ROS) production and increased caspase 3/7 activity and cell death. DFO-induced iron deficiency upregulates Egr1 in part through transcriptional activation via ERK and Elk-1 signals, which may be important in the regulation of cell death in hepatoma cells. Our study demonstrated that iron depletion controlled the expression of Egr1, which might contribute to decisions about cellular fate in response to iron deficiency.

Depletion of the transcriptional coactivators megakaryoblastic leukaemia 1 and 2 abolishes hepatocellular carcinoma xenograft growth by inducing oncogene-induced senescence

Megakaryoblastic leukaemia 1 and 2 (MKL1/2) are coactivators of the transcription factor serum response factor (SRF). Here, we provide evidence that depletion of MKL1 and 2 abolishes hepatocellular carcinoma (HCC) xenograft growth. Loss of the tumour suppressor deleted in liver cancer 1 (DLC1) and the subsequent activation of RhoA were prerequisites for MKL1/2 knockdown-mediated growth arrest. We identified oncogene-induced senescence as the molecular mechanism underlying the anti-proliferative effect of MKL1/2 knockdown. MKL1/2 depletion resulted in Ras activation, elevated p16 expression and hypophosphorylation of the retinoblastoma (Rb) protein in DLC1-deficient HCC cells. Interestingly, reconstitution of HuH7 HCC cells with DLC1 also induced senescence. Evaluation of the therapeutic efficacy of MKL1/2 knockdown in vivo revealed that systemic treatment of nude mice bearing HuH7 tumour xenografts with MKL1/2 siRNAs complexed with polyethylenimine (PEI) completely abolished tumour growth. The regression of the xenografts was associated with senescence. Importantly, PEI-complexed MKL1 siRNA alone was sufficient for complete abrogation of HCC xenograft growth. Thus, MKL1/2 represent promising novel therapeutic targets for the treatment of HCCs characterized by DLC1 loss.

Vascular smooth muscle cell differentiation-2010

Vascular smooth muscle cells have attracted considerable interest as a model for a flexible program of gene expression. This cell type arises throughout the embryo body plan via poorly understood signaling cascades that direct the expression of transcription factors and microRNAs which, in turn, orchestrate the activation of contractile genes collectively defining this cell lineage. The discovery of myocardin and its close association with serum response factor has represented a major break-through for the molecular understanding of vascular smooth muscle cell differentiation. Retinoids have been shown to improve the outcome of vessel wall remodeling following injury and have provided further insights into the molecular circuitry that defines the vascular smooth muscle cell phenotype. This review summarizes the progress to date in each of these areas of vascular smooth muscle cell biology.

MMP-2 expression by fibroblasts is suppressed by the myofibroblast phenotype

During wound healing, fibroblasts transition from quiescence to a migratory state, then to a contractile myofibroblast state associated with wound closure. We found that the myofibroblast phenotype, characterized by the expression of high levels of contractile proteins, suppresses the expression of the pro-migratory gene, MMP-2. Fibroblasts cultured in a 3-D collagen lattice and allowed to develop tension showed increased contractile protein expression and decreased MMP-2 levels in comparison to a stress-released lattice. In 2-D cultures, factors that promote fibroblast contractility, including serum or TGF-β, down-regulated MMP-2. Pharmacologically inducing F-actin disassembly or reduced contractility increased MMP-2 expression, while conditions that promote F-actin assembly suppressed MMP-2 expression. In all cases, changes in MMP-2 levels were inversely related to changes in the contractile marker, smooth muscle α-actin. To determine if the mechanisms involved in contractile protein gene expression play a direct role in MMP-2 regulation, we used RNAi-mediated knock-down of the myocardin-like factors, MRTF-A and MRTF-B, which induced the down-regulation of contractile protein genes by fibroblasts under both serum-containing and serum-free conditions. In the presence of serum or TGF-β, MRTF-A/B knock-down resulted in the up-regulation of MMP-2; serum-free conditions prevented this increased expression. Together, these results indicate that, while MMP-2 expression is suppressed by F-actin formation, its up-regulation is not simply a consequence of contractile protein down-regulation.

Identifying functional single nucleotide polymorphisms in the human CArGome

Regulatory SNPs (rSNPs) reside primarily within the nonprotein coding genome and are thought to disturb normal patterns of gene expression by altering DNA binding of transcription factors. Nevertheless, despite the explosive rise in SNP association studies, there is little information as to the function of rSNPs in human disease. Serum response factor (SRF) is a widely expressed DNA-binding transcription factor that has variable affinity to at least 1,216 permutations of a 10 bp transcription factor binding site (TFBS) known as the CArG box. We developed a robust in silico bioinformatics screening method to evaluate sequences around RefSeq genes for conserved CArG boxes. Utilizing a predetermined phastCons threshold score, we identified 8,252 strand-specific CArGs within an 8 kb window around the transcription start site of 5,213 genes, including all previously defined SRF target genes. We then interrogated this CArG dataset for the presence of previously annotated common polymorphisms. We found a total of 118 unique CArG boxes harboring a SNP within the 10 bp CArG sequence and 1,130 CArG boxes with SNPs located just outside the CArG element. Gel shift and luciferase reporter assays validated SRF binding and functional activity of several new CArG boxes. Importantly, SNPs within or just outside the CArG box often resulted in altered SRF binding and activity. Collectively, these findings demonstrate a powerful approach to computationally define rSNPs in the human CArGome and provide a foundation for similar analyses of other TFBS. Such information may find utility in genetic association studies of human disease where little insight is known regarding the functionality of rSNPs.

Serum response factor controls CYLD expression via MAPK signaling pathway

Tumor suppressor gene CYLD is a deubiquitinating enzyme which negatively regulates various signaling pathways by removing the lysine 63-linked polyubiquitin chains from several specific substrates. Loss of CYLD in different types of tumors leads to either cell survival or proliferation. In this study we demonstrate that lack of CYLD expression in CYLD−/− MEFs increases proliferation rate of these cells compared to CYLD+/+ in a serum concentration dependent manner without affecting cell survival. The reduced proliferation rate in CYLD+/+ in the presence of serum was due to the binding of serum response factor (SRF) to the serum response element identified in the CYLD promoter for the up-regulation of CYLD levels. The serum regulated recruitment of SRF to the CYLD promoter was dependent on p38 mitogen-activated protein kinase (MAPK) activity. Elimination of SRF by siRNA or inhibition of p38 MAPK reduced the expression level of CYLD and increased cell proliferation. These results show that SRF acts as a positive regulator of CYLD expression, which in turn reduces the mitogenic activation of serum for aberrant proliferation of MEF cells.

Serum response factor utilizes distinct promoter- and enhancer-based mechanisms to regulate cytoskeletal gene expression in macrophages

Cells of the monocyte/macrophage lineage play essential roles in tissue homeostasis and immune responses, but mechanisms underlying the coordinated expression of cytoskeletal genes required for specialized functions of these cells, such as directed migration and phagocytosis, remain unknown. Here, using genetic and genomic approaches, we provide evidence that serum response factor (SRF) regulates both general and cell type-restricted components of the cytoskeletal gene expression program in macrophages. Genome-wide location analysis of SRF in macrophages demonstrates enrichment of SRF binding at ubiquitously expressed target gene promoters, as expected, but also reveals that the majority of SRF binding sites associated with cell type-restricted target genes are at distal inter- and intragenic locations. Most of these distal SRF binding sites are established by the prior binding of the macrophage- and the B cell-specific transcription factor PU.1 and exhibit histone modifications characteristic of enhancers. Consistent with this, representative cytoskeletal target genes associated with these elements require both SRF and PU.1 for full expression. These findings suggest that SRF uses two distinct molecular strategies to regulate programs of cytoskeletal gene expression: a promoter-based strategy for ubiquitously expressed target genes and an enhancer-based strategy at target genes that exhibit cell type-restricted patterns of expression.

In situ proteomic analysis of human breast cancer epithelial cells using laser capture microdissection: annotation by protein set enrichment analysis and gene ontology

Identification of molecular signatures that allow detection of the transition from normal breast epithelial cells to malignant invasive cells is a critical component in the development of diagnostic, therapeutic, and preventative strategies for human breast cancer. Substantial efforts have been devoted to deciphering breast cancer etiology at the genome level, but only a limited number of studies have appeared at the proteome level. In this work, we compared individual in situ proteome profiles of nonpatient matched nine noncancerous, normal breast epithelial (NBE) samples with nine estrogen receptor (ER)-positive (luminal subtype), invasive malignant breast epithelial (MBE) samples by combining laser capture microdissection (LCM) and quantitative shotgun proteomics. A total of 12,970 unique peptides were identified from the 18 samples, and 1623 proteins were selected for quantitative analysis using spectral index (SpI) as a measure of protein abundance. A total of 298 proteins were differentially expressed between NBE and MBE at 95% confidence level, and this differential expression correlated well with immunohistochemistry (IHC) results reported in the Human Protein Atlas (HPA) database. To assess pathway level patterns in the observed expression changes, we developed protein set enrichment analysis (PSEA), a modification of a well-known approach in gene expression analysis, Gene Set Enrichment Analysis (GSEA). Unlike single gene-based functional term enrichment analyses that only examines pathway overrepresentation of proteins above a given significance threshold, PSEA applies a weighted running sum statistic to the entire expression data to discover significantly enriched protein groups. Application of PSEA to the expression data in this study revealed not only well-known ER-dependent and cellular morphology-dependent protein abundance changes, but also significant alterations of downstream targets for multiple transcription factors (TFs), suggesting a role for specific gene regulatory pathways in breast tumorigenesis. A parallel GOMiner analysis revealed both confirmatory and complementary data to PSEA. The combination of the two annotation approaches yielded extensive biological feature mapping for in depth analysis of the quantitative proteomic data.

Revisited and revised: is RhoA always a villain in cardiac pathophysiology?

The neonatal rat ventricular myocyte model of hypertrophy has provided tremendous insight with regard to signaling pathways regulating cardiac growth and gene expression. Many mediators thus discovered have been successfully extrapolated to the in vivo setting, as assessed using genetically engineered mice and physiological interventions. Studies in neonatal rat ventricular myocytes demonstrated a role for the small G-protein RhoA and its downstream effector kinase, Rho-associated coiled-coil containing protein kinase (ROCK), in agonist-mediated hypertrophy. Transgenic expression of RhoA in the heart does not phenocopy this response, however, nor does genetic deletion of ROCK prevent hypertrophy. Pharmacologic inhibition of ROCK has effects most consistent with roles for RhoA signaling in the development of heart failure or responses to ischemic damage. Whether signals elicited downstream of RhoA promote cell death or survival and are deleterious or salutary is, however, context and cell-type dependent. The concepts discussed above are reviewed, and the hypothesis that RhoA might protect cardiomyocytes from ischemia and other insults is presented. Novel RhoA targets including phospholipid regulated and regulating enzymes (Akt, PI kinases, phospholipase C, protein kinases C and D) and serum response element-mediated transcriptional responses are considered as possible pathways through which RhoA could affect cardiomyocyte survival.

Serum response factor is essential for the proper development of skin epithelium

Mammalian epidermis is a stratified epithelium that serves as a barrier protecting the organism from mechanical stress and dehydration. Previous studies have demonstrated the importance of the actin cytoskeleton in the establishment of a functional skin epithelium. Despite what is known about the actin cytoskeleton in epithelial sheet formation, the molecules important for controlling the actin cytoskeleton during epidermal development have not been determined. Serum response factor (SRF) is a transcription factor that is considered to be an important regulator of the actin cytoskeleton. To examine the role of SRF in the developing mouse epidermis, we have employed gene targeting to ablate Srf in keratinocytes. Conditional inactivation of Srf during the embryonic timepoint leads to a defect in the organization of the epidermis. Immunohistochemical analyses demonstrated a marked loss of the filamentous actin cytoskeleton and E-cadherin localization in epidermis, as well as an aberration in the localization of tight junction proteins. Moreover, impairment of the "inside-out" epidermal barrier was shown. Srf conditional knockout keratinocytes are unable to establish proper intercellular connections or form an epithelial sheet as shown by histological examination and induced keratinocyte differentiation experiments. Our results demonstrate that Srf is essential for the actin-mediated sealing of epithelial cell-cell contacts and the development of functional stratified skin epithelium in vivo.

Serum-induced phosphorylation of the serum response factor coactivator MKL1 by the extracellular signal-regulated kinase 1/2 pathway inhibits its nuclear localization

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related coactivator of the serum response factor (SRF) transcription factor, which has an integral role in differentiation, migration, and proliferation. Serum induces RhoA-dependent translocation of MKL1 from the cytoplasm to the nucleus and also causes a rapid increase in MKL1 phosphorylation. We have mapped a serum-inducible phosphorylation site and found, surprisingly, that its mutation causes constitutive localization to the nucleus, suggesting that phosphorylation of MKL1 inhibits its serum-induced nuclear localization. The key site, serine 454, resembles a mitogen-activated protein kinase phosphorylation site, and its modification was blocked by the MEK1 inhibitor U0126, implying that extracellular signal-regulated kinase 1/2 (ERK1/2) is the serum-inducible kinase that phosphorylates MKL1. Previous results indicated that G-actin binding to MKL1 promotes its nuclear export, and we found that MKL1 phosphorylation is required for its binding to actin, explaining its effect on localization. We propose a model in which serum induction initially stimulates MKL1 nuclear localization due to a decrease in G-actin levels, but MKL1 is then downregulated by nuclear export due to ERK1/2 phosphorylation.

Serum- and glucocorticoid-inducible kinase 1 enhances zif268 expression through the mediation of SRF and CREB1 associated with spatial memory formation

Serum- and glucocorticoid-inducible kinase 1 (SGK1) has been shown to play an important role in spatial memory formation, but the molecular mechanism underlying this effect of SGK1 was not known. zif268 is an immediate early gene that is induced by water maze learning. To investigate the role of SGK1 in the regulation of zif268 expression, the dominant negative mutant of SGK1, SGK1 S422A, was infused to the hippocampal CA1 area of rats, and was found to decrease significantly the mRNA level of zif268 in both naïve animals and trained animals. SGK1 was also found to phosphorylate serum response factor (SRF) at Ser73, Ser75, and Ser99, and phosphorylate CREB1 at Ser133. Inhibition of SGK1 phosphorylation sites on SRF and CREB1 with alanine substitution significantly diminished SGK1-enhanced zif268 expression in the promoter-luciferase assay. SGK1 also phosphorylates Elk-1 and SGK1 phosphorylation of Elk-1 decreased the transcriptional activity of Elk-1. But SGK1 phosphorylation of Elk-1 did not affect SGK1-enhanced zif268 expression. Moreover, the phosphorylation of SGK1 was increased in rat CA1 area after water maze learning, accompanied by increased phosphorylation of SRF at Ser99 and increased phosphorylation of CREB1 at Ser133. All these effects were antagonized by SGK1 S422A transfection. These results together suggest that SGK1 enhances zif268 expression through the mediation of SRF and CREB1, and these signaling pathways are associated with spatial memory formation in rats.

Structural and functional analysis of domains mediating interaction between the bagpipe homologue, Nkx3.1 and serum response factor

Nkx3.1 is a member of the NK2 class of homeodomain proteins and is expressed in development, being an early marker of the sclerotome and prostate gland. It has been shown to be a critical factor for prostate differentiation and function. Previous studies suggested that Nkx3.1 interacts with Serum Response Factor (SRF) to transactivate the Smooth Muscle gamma-Actin (SMGA) promoter. In studies presented here, we examined the molecular mechanisms underlying the functional synergy of these factors upon SMGA transcription. We demonstrate that full length Nkx3.1 physically interacts with SRF in the absence of DNA and that these factors are able to co-associate in cellular context using a mammalian two-hybrid system. The segment of SRF responsible for Nkx3.1 interaction was mapped to a approximately 30 amino acid region (AAs 142-171) at the N-terminal segment of the MADS box. Two separate regions of Nkx3.1 were found to mediate interactions with SRF. Interestingly, recognized domains of NK2 proteins, namely the TN, homeodomain DNA binding segment, and the NK2-SD do not participate in SRF interactions. One of the Nkx3.1 SRF binding domains was mapped to the N-terminal of the protein consistent with recent studies of these proteins using NMR spectroscopy by Gelmann and colleagues (1). A second SRF binding region was mapped to amino acids C-terminal to the homeodomain. Structural predictions indicate that both of the SRF interacting segments are largely hydrophobic in character and beta-strand in structure. With co-transfection transcriptional analyses we found that interaction between SRF and Nkx3.1 as well as DNA binding by both factors was required for the observed transcriptional synergy. Thus our studies have identified novel protein-protein interacting domains within Nkx3.1 and SRF that operate in concert with their respective DNA binding domains to mediate functional transcriptional synergy of these factors to regulate SMGA gene activation.

The cytoskeleton-associated PDZ-LIM protein, ALP, acts on serum response factor activity to regulate muscle differentiation

In this report, an antisense RNA strategy has allowed us to show that disruption of ALP expression affects the expression of the muscle transcription factors myogenin and MyoD, resulting in the inhibition of muscle differentiation. Introduction of a MyoD expression construct into ALP-antisense cells is sufficient to restore the capacity of the cells to differentiate, illustrating that ALP function occurs upstream of MyoD. It is known that MyoD is under the control of serum response factor (SRF), a transcriptional regulator whose activity is modulated by actin dynamics. A dramatic reduction of actin filament bundles is observed in ALP-antisense cells and treatment of these cells with the actin-stabilizing drug jasplakinolide stimulates SRF activity and restores the capacity of the cells to differentiate. Furthermore, we show that modulation of ALP expression influences SRF activity, the level of its coactivator, MAL, and muscle differentiation. Collectively, these results suggest a critical role of ALP on muscle differentiation, likely via cytoskeletal regulation of SRF.

Acute myeloid leukemia-associated Mkl1 (Mrtf-a) is a key regulator of mammary gland function

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.

Defining the mammalian CArGome

Serum response factor (SRF) binds a 1216-fold degenerate cis element known as the CArG box. CArG boxes are found primarily in muscle- and growth-factor-associated genes although the full spectrum of functional CArG elements in the genome (the CArGome) has yet to be defined. Here we describe a genome-wide screen to further define the functional mammalian CArGome. A computational approach involving comparative genomic analyses of human and mouse orthologous genes uncovered >100 hypothetical SRF-dependent genes, including 10 previously identified SRF targets, harboring a conserved CArG element within 4000 bp of the annotated transcription start site (TSS). We PCR-cloned 89 hypothetical SRF targets and subjected each of them to at least two of several validations including luciferase reporter, gel shift, chromatin immunoprecipitation, and mRNA expression following RNAi knockdown of SRF; 60/89 (67%) of the targets were validated. Interestingly, 26 of the validated SRF target genes encode for cytoskeletal/contractile or adhesion proteins. RNAi knockdown of SRF diminishes expression of several SRF-dependent cytoskeletal genes and elicits an attending perturbation in the cytoarchitecture of both human and rodent cells. These data illustrate the power of integrating existing algorithms to interrogate the genome in a relatively unbiased fashion for cis-regulatory element discovery. In this manner, we have further expanded the mammalian CArGome with the discovery of an array of cyto-contractile genes that coordinate normal cytoskeletal homeostasis. We suggest one function of SRF is that of an ancient master regulator of the actin cytoskeleton.

Restricted inactivation of serum response factor to the cardiovascular system

Serum response factor (SRF) directs programs of gene expression linked to growth and muscle differentiation. To investigate the role of SRF in cardiovascular development, we generated mice in which SRF is knocked out in >80% of cardiomyocytes and >50% of vascular smooth muscle cells (SMC) through SM22alpha-Cre-mediated excision of SRF's promoter and first exon. Mutant mice display vascular patterning, cardiac looping, and SRF-dependent gene expression through embryonic day (e)9.5. At e10.5, attenuation in cardiac trabeculation and compact layer expansion is noted, with an attendant decrease in vascular SMC recruitment to the dorsal aorta. Ultrastructurally, cardiac sarcomeres and Z disks are highly disorganized in mutant embryos. Moreover, SRF mutant mice exhibit vascular SMC lacking organizing actin/intermediate filament bundles. These structural defects in the heart and vasculature coincide with decreases in SRF-dependent gene expression, such that by e11.5, when mutant embryos succumb to death, no SRF-dependent mRNA expression is evident. These results suggest a vital role for SRF in contractile/cytoskeletal architecture necessary for the proper assembly and function of cardiomyocytes and vascular SMC.

Expression profiling of serum inducible genes identifies a subset of SRF target genes that are MKL dependent

Background

Serum Response Factor (SRF) is a transcription factor that is required for the expression of many genes including immediate early genes, cytoskeletal genes, and muscle-specific genes. SRF is activated in response to extra-cellular signals by its association with a diverse set of co-activators in different cell types. In the case of the ubiquitously expressed immediate early genes, the two sets of SRF binding proteins that regulate its activity are the TCF family of proteins that include Elk1, SAP1 and SAP2 and the myocardin-related MKL family of proteins that include MKL1 and MKL2 (also known as MAL, MRTF-A and -B and BSAC). In response to serum or growth factors these two classes of co-activators are activated by different upstream signal transduction pathways. However, it is not clear how they differentially activate SRF target genes.

Results

In order to identify the serum-inducible SRF target genes that are specifically dependent on the MKL pathway, we have performed microarray experiments using a cell line that expresses dominant negative MKL1. This approach was used to identify SRF target genes whose activation is MKL-dependent. Twenty-eight of 150 serum-inducible genes were found to be MKL-dependent. The promoters of the serum-inducible genes were analyzed for SRF binding sites and other common regulatory elements. Putative SRF binding sites were found at a higher rate than in a mouse promoter database but were only identified in 12% of the serum-inducible promoters analyzed. Additional partial matches to the consensus SRF binding site were found at a higher than expected rate in the MKL-dependent gene promoters. The analysis for other common regulatory elements is discussed.

Conclusions

These results suggest that a subset of immediate early and SRF target genes are activated by the Rho-MKL pathway. MKL may also contribute to the induction of other SRF target genes however its role is not essential, possibly due to other activation mechanisms such as MAPK phosphorylation of TCFs.

Dictyostelium discoideum developmentally regulated genes whose expression is dependent on MADS box transcription factor SrfA

The MADS box transcription factor SrfA is required for spore differentiation in Dictyostelium discoideum. srfA null strains form rounded spores that do not resist adverse environmental conditions. Five genes whose expression is dependent on SrfA have been isolated by differential hybridization. One of these genes, sigC, is identical to phg1b, previously characterized in mutants with altered adhesive properties and found to encode a nine-transmembrane-domain protein. This gene is transcribed into two mRNAs as the result of alternative splicing of two internal exons. The slower-migrating mRNA codes for a shorter protein that lacks the first transmembrane fragment and is not expressed in srfA null strains. The other four genes (sigA, sigB, sigD, and 45D) are expressed only during late developmental stages. In situ hybridization experiments showed that expression of sigA, sigB, and sigD is restricted to the sorus of developing structures. sigA codes for a homologue of malate dehydrogenase that converts pyruvate to malate to replenish the tricarboxylic acid cycle. sigB encodes a protein with significant similarity to the GP63 metalloproteinase of Leishmania, leishmanolysin. The sequence of SigD is highly similar to that of several spore coat proteins of D. discoideum, and it may play a role in that structure. The gene 45D codes for an RNA-binding protein homologue whose expression is also dependent on the GATA transcription factor stalky (StkA). The expression of sigB is also dependent on both SrfA and StkA. The expression of 45D, but not of sigA, sigB, sigC, and sigD, can be induced in srfA null cells by constitutive protein kinase A activation. Strains in which either sigA, sigB, or sigD is disrupted were isolated and found to form spores that are not detectably different from those of wild-type strains.

SRF regulates Bcl-2 expression and promotes cell survival during murine embryonic development

The transcription factor serum response factor (SRF) controls the expression of genes involved in cellular proliferation and differentiation. Interestingly, SRF also promotes cell survival by regulating the expression of antiapoptotic genes. In in vitro differentiating murine embryonic stem (ES) cells, SRF deficiency leads to increased apoptosis. Loss of SRF correlates with impaired expression of the antiapoptotic Bcl-2 and Bcl-xl genes. SRF binds the Bcl-2 promoter in vivo and activates Bcl-2 transcription. Reconstituting Bcl-2 in Srf-/- ES cells rescues these cells from apoptosis, demonstrating that SRF-dependent Bcl-2 expression is critical for ES cell survival. At the multicellular level, SRF deficiency leads to impaired cavitation and reduced Bcl-2 expression in embryoid bodies (EBs) and inappropriate apoptosis in both EBs and pregastrulation mouse embryos. Thus, our data from genetic and cellular studies uncover SRF-regulated Bcl-2 expression as a novel mechanism that is important for cell survival during early murine embryogenesis.

Involvement of serum response factor isoforms in myofibroblast differentiation during bleomycin-induced lung injury

Serum response factor (SRF) is a transcription factor essential for smooth muscle (SM) myogenesis. Its role in myofibroblast differentiation is, however, unknown. We studied the expression and the localization of SRF in bleomycin-induced pulmonary fibrosis, where myofibroblasts are abundant. We found that SRF levels were upregulated in bleomycin-exposed mouse lungs mainly due to de novo synthesis of SRFDelta5, a less myogenic SRF isoform. Before myofibroblast differentiation, SRF/SRFDelta5 was immunolocalized mostly in the cytoplasm of scattered fibroblasts at lesion sites. With the development of myofibroblasts, however, SRF/SRFDelta5 was found in myofibroblast nuclei. cDNA array analysis showed that SRFDelta5 and SRF induced expression of transforming growth factor-beta1, a critical factor in myofibroblast differentiation. This was accompanied by de novo expression of several inflammatory cell-specific mRNAs. The latter was confirmed by reverse transcriptase-polymerase chain reaction. Treatment of lung fibroblasts with tumor necrosis factor-alpha, which is produced early in the bleomycin model, induced SRFDelta5 expression and SRF/SRFDelta5 cytoplasmic accumulation, whereas addition of transforming growth factor-beta1 caused SRF/SRFDelta5 nuclear translocation followed by SM alpha-actin synthesis. Interleukin-4, another cytokine involved in myofibroblast differentiation, did not affect SRF or induce SRFDelta5 expression. Our studies therefore suggested a new mechanism whereby SRF and SRFDelta5 contribute to the emergence of myofibroblasts in lung injury and fibrosis.

Megakaryoblastic leukemia 1, a potent transcriptional coactivator for serum response factor (SRF), is required for serum induction of SRF target genes

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related transcription factor that we found strongly activated serum response element (SRE)-dependent reporter genes through its direct binding to serum response factor (SRF). The c-fos SRE is regulated by mitogen-activated protein kinase phosphorylation of ternary complex factor (TCF) but is also regulated by a RhoA-dependent pathway. The mechanism of this pathway is unclear. Since MKL1 (also known as MAL, BSAC, and MRTF-A) is broadly expressed, we assessed its role in serum induction of c-fos and other SRE-regulated genes with a dominant negative MKL1 mutant (DN-MKL1) and RNA interference (RNAi). We found that DN-MKL1 and RNAi specifically blocked SRE-dependent reporter gene activation by serum and RhoA. Complete inhibition by RNAi required the additional inhibition of the related factor MKL2 (MRTF-B), showing the redundancy of these factors. DN-MKL1 reduced the late stage of serum induction of endogenous c-fos expression, suggesting that the TCF- and RhoA-dependent pathways contribute to temporally distinct phases of c-fos expression. Furthermore, serum induction of two TCF-independent SRE target genes, SRF and vinculin, was nearly completely blocked by DN-MKL1. Finally, the RBM15-MKL1 fusion protein formed by the t(1;22) translocation of acute megakaryoblastic leukemia had a markedly increased ability to activate SRE reporter genes, suggesting that its activation of SRF target genes may contribute to leukemogenesis.

Differential regulation of NFAT and SRF by the B cell receptor via a PLCgamma-Ca(2+)-dependent pathway

NFAT and SRF are important in the regulation of proliferation and cytokine production in lymphocytes. NFAT activation by the B cell receptor (BCR) occurs via the PLCgamma-Ca(2+)-calcineurin pathway, however how the BCR activates SRF is unclear. We show here that like NFAT, BCR regulation of SRF occurs via an Src-Syk-Tec-PLCgamma-Ca(2+) (Lyn-Syk-Btk-PLCgamma-Ca(2+)) pathway. However, SRF responds to lower Ca(2+) and is less dependent on IP(3)R expression than NFAT. Ca(2+)-regulated calcineurin plays a partial role in SRF activation, in combination with diacylglycerol (DAG), while is fully required for NFAT activation. Signals from the DAG effectors protein kinase C, Ras and Rap1, and the downstream MEK-ERK pathway are required for both SRF and NFAT; however, NFAT but not SRF is dependent on JNK signals. Both SRF and NFAT were also dependent on Rac, Rho, CDC42 and actin. Finally, we show that Ca(2+) is not required for ERK activation, but instead for its association with nuclear areas of the cell. These data suggest that combinatorial assembly of signaling pathways emanating from the BCR differentially regulate NFAT and SRF, to activate gene expression.

P311 induces a TGF-beta1-independent, nonfibrogenic myofibroblast phenotype

P311, also called PTZ17, was identified by suppressive subtraction hybridization as potentially involved in smooth muscle (SM) myogenesis. P311 is an 8-kDa protein with several PEST-like motifs found in neurons and muscle. P311 transfection into two fibroblast cell lines, NIH 3T3 and C3H10 T1/2, induced phenotypic changes consistent with myofibroblast transformation, including upregulation of SM alpha-actin and SM22, induction of FGF-2, VEGF, PDGF, and PDGF receptors, upregulation of integrins alpha3 and alpha5, and increased proliferation rate. The P311-mediated changes differed, however, from the well-characterized myofibroblast in that P311 inhibited TGF-beta1, TGF-beta receptor 2, and TGF-beta1-activating MMP-2 and MMP-9, with the resultant decrease in collagen 1 and 3 expression. The effect of P311 on collagen was overcome by exogenous TGF-beta1, indicating that the cells were responsive to TGF-beta1 paracrine stimulus. In support of a role for P311 in vivo, immunohistochemical examination of human wounds showed P311 only in myofibroblasts and their activated precursors. To our knowledge, these studies are the first to implicate P311 in myofibroblast transformation, to demonstrate that transformation may occur independently of TGF-beta1, and to suggest that P311 may prevent fibrosis.

Swapping functional specificity of a MADS box protein: residues required for Arg80 regulation of arginine metabolism

Arg80 and Mcm1, two members of the MADS box family of DNA-binding proteins, regulate the metabolism of arginine in association with Arg81, the arginine sensor. In spite of the high degree of sequence conservation between the MADS box domains of the Arg80 and Mcm1 proteins (56 of 81 amino acids), these domains are not interchangeable. To determine which amino acids define the specificity of Arg80, we swapped the amino acids in each secondary-structure element of the Arg80 MADS box domain with the corresponding amino acids of Mcm1 and assayed the ability of these chimeras to regulate arginine-metabolic genes in place of the wild-type Arg80. Also performed was the converse experiment in which each variant residue in the Mcm1 MADS box domain was swapped with the corresponding residue of Arg80 in the context of an Arg80-Mcm1 fusion protein. We show that multiple regions of Arg80 are important for its function. Interestingly, the residues which have important roles in determining the specificity of Arg80 are not those which could contact the DNA but are residues that are likely to be involved in protein interactions. Many of these residues are clustered on one side of the protein, which could serve as an interface for interaction with Arg81 or Mcm1. This interface is distinct from the region used by the Mcm1 and human serum response factor MADS box proteins to interact with their cofactors. It is possible that this alternative interface is used by other MADS box proteins to interact with their cofactors.

Mechanisms controlling differential promoter-occupancy by the yeast forkhead proteins Fkh1p and Fkh2p: implications for regulating the cell cycle and differentiation

The roles of DNA and Mcm1p interactions in determining the overlapping and distinct functions of the yeast cell cycle regulatory transcription factors Fkh1p and Fkh2p were examined. Full-length recombinant Fkh1p and Fkh2p were purified and their binding to bona fide promoters examined in vitro. Each protein bound a variety of target promoters with similar specificity in vitro, consistent with the observation that these proteins bind common promoters in vivo. However, in vivo, the Fkh1p and Fkh2p occupied different target promoters to different extents, suggesting that each was primarily responsible for controlling a different set of genes. Additional in vitro studies provided a mechanistic explanation for this differential promoter-occupancy. Specifically, the Fkh2p, but not the Fkh1p, was capable of binding cooperatively with Mcm1p. The Mcm1p-Fkh2p cooperative binding was enhanced by, but did not require, the presence of a Mcm1p-binding site within a target promoter. Consistent with these data, Mcm1p was present at Fkh-controlled promoters in vivo regardless of whether they contained Mcm1p-binding sites, suggesting a role for Mcm1p at promoters not thought previously to be under Mcm1p control. Analysis of Fkh1p and Fkh2p binding to promoter targets in vivo by use of mutant strains indicated that the two proteins compete for promoter-occupancy at a number of target promoters. We postulate that Fkh1p and a stable Fkh2p/Mcm1p complex compete for binding to target promoters and that the levels and/or binding activity of Fkh1p, but not Fkh2p, are most limiting for promoter-occupancy in vivo. Interestingly, the in vitro DNA-binding assays, using a variety of promoter targets, revealed that bona fide Fkh target promoters contained two or more Fkh-binding sites that allowed the Fkh1p and Fkh2p proteins to form multiple protein-DNA complexes in vitro. Multiple Fkh-binding sites may be a distinguishing feature of bona fide Fkh promoters in yeast and other organisms.

Serum response factor cleavage by caspases 3 and 7 linked to apoptosis in human BJAB cells

Apoptosis involves the cessation of cellular processes, the breakdown of intracellular organelles, and, finally, the nonphlogistic clearance of apoptotic cells from the body. Important for these events is a family of proteases, caspases, which are activated by a proteolytic cleavage cascade and drive apoptosis by targeting key proteins within the cell. Here, we demonstrate that serum response factor (SRF), a transcription factor essential for proliferative gene expression, is cleaved by caspases and that this cleavage occurs in proliferating murine fibroblasts and can be induced in the human B-cell line BJAB. We identify the two major sites at which SRF cleavage occurs as Asp(245) and Asp(254), the caspases responsible for the cleavage and generate a mutant of SRF resistant to cleavage in BJAB cells. Investigation of the physiological and functional significance of SRF cleavage reveals that it correlates with the loss of c-fos expression, whereby neither SRF cleavage fragment retains transcriptional activity. Moreover, the expression of a noncleavable SRF in BJAB cells suppresses apoptosis induced by Fas cross-linking. These results suggest that for apoptosis to proceed, the transcriptional events promoting cell survival and proliferation, in which SRF is involved, must first be inactivated.

Srf(-/-) ES cells display non-cell-autonomous impairment in mesodermal differentiation

The serum response factor (SRF) transcription factor is essential for murine embryogenesis. SRF+(-/-) embryos stop developing at the onset of gastrulation, lacking detectable mesoderm. This developmental defect may reflect cell-autonomous impairment of SRF(-/-) embryonic cells in mesoderm formation. Alternatively, it may be caused by a non-cell-autonomous defect superimposed upon inappropriate provision of mesoderm-inducing signals to primitive ectodermal cells. We demonstrate that the ability of SRF(-/-) embryonic stem (ES) cells to differentiate in vitro into mesodermal cells is indeed impaired. However, this impairment can be modulated by external, cell-independent factors. Retinoic acid, but not dimethylsulfoxide, permitted activation of the mesodermal marker gene T(Bra), which was also activated when SRF was expressed in SRF(-/-) ES cells. Embryoid bodies from SRF(-/-) ES cell aggregates also activated mesodermal marker genes, but displayed unusual morphologies and impairment in cavitation. Finally, in nude mice, Srf(-/-) ES cells readily differentiated into mesodermal cells of SRF(-/-) genotype, including cartilage, bone or muscle cells. We demonstrate that SRF contributes to mesodermal gene expression of ES cells and that SRF(-/-) ES cells display a non-cell-autonomous defect in differentiation towards mesoderm.

Activation of the c-fos enhancer by the erk MAP kinase pathway through two sequence elements: the c-fos AP-1 and p62TCF sites

The c-fos enhancer can be activated by many signaling pathways through distinct elements of the enhancer. The enhancer contains at its core the serum response element (SRE) that binds serum response factor (SRF). On the 5' side of the SRE is a site for p62TCF which binds only when SRF is bound as well. p62TCF is encoded by three ets-related genes, Elk-1, SAP1 and SAP2. Each of these factors contain a transcriptional activation domain that is activated by phosphorylation by MAP kinases. On the 3' side of the SRE is the 'c-fos AP1 site' (FAP1) whose role has been less clear. We find here that the FAP1 site contributes strongly to phorbol ester (TPA) and Erk MAP kinase activation of the c-fos enhancer and that both the p62TCF and FAP1 sites are required for effective activation of the enhancer. We further find that the FAP1 site binds ATF1 and CREB from HeLa nuclear extracts and that the phosphorylation of these factors is induced by TPA. ATF1 and CREB can be phosphorylated by Rsk2 which is a protein kinase directly activated by Erk MAP kinases. These results suggest a signaling pathway in which Erk MAP kinase activates the c-fos enhancer by direct phosphorylation of p62TCF and by activation of Rsk related kinases that phosphorylate ATF1 and CREB.

Persistent increased DNA-binding and expression of serum response factor occur with epilepsy-associated long-term plasticity changes

We have previously shown that NMDA receptor activation during status epilepticus (SE) is required to produce epilepsy in in vitro and in vivo models. As in human symptomatic epilepsy, the epilepsy in these models is permanent, suggesting that the pathological activation of NMDA receptors causes permanent plasticity changes in the brain. Ca(2+) influx through NMDA receptors is known to transiently activate a key transcription factor, serum response factor (SRF). Thus, we investigated whether this factor, in terms of its expression and ability to bind to the consensus serum response element, was altered long term in the pilocarpine model of epilepsy. In hippocampal nuclear extracts, SRF binding to DNA was significantly increased over saline-injected control rats at 24 hr and at 8 weeks after the onset of SE. This increase was shown to be the result of significantly elevated levels of SRF. DNA binding was also persistently increased in the cortical, but not in the cerebellar, extracts. Hippocampal expression of SRF was localized to neurons using immunohistochemistry. NMDA receptor activation during SE was required for these changes to take place, and the spontaneous seizures seen in epileptic rats did not appear to be responsible for the increase in SRF. The results demonstrate that SRF is persistently elevated after SE in the pilocarpine model of epilepsy and support the theory that long-term gene changes in this model occur and are associated with the long-lasting plasticity changes that are initiated during epileptogenesis.

Cellular functions of TC10, a Rho family GTPase: regulation of morphology, signal transduction and cell growth

The small Ras-related GTPase, TC10, has been classified on the basis of sequence homology to be a member of the Rho family. This family, which includes the Rho, Rac and CDC42 subfamilies, has been shown to regulate a variety of apparently diverse cellular processes such as actin cytoskeletal organization, mitogen-activated protein kinase (MAPK) cascades, cell cycle progression and transformation. In order to begin a study of TC10 biological function, we expressed wild type and various mutant forms of this protein in mammalian cells and investigated both the intracellular localization of the expressed proteins and their abilities to stimulate known Rho family-associated processes. Wild type TC10 was located predominantly in the cell membrane (apparently in the same regions as actin filaments), GTPase defective (75L) and GTP-binding defective (31N) mutants were located predominantly in cytoplasmic perinuclear regions, and a deletion mutant lacking the carboxyl terminal residues required for post-translational prenylation was located predominantly in the nucleus. The GTPase defective (constitutively active) TC10 mutant: (1) stimulated the formation of long filopodia; (2) activated c-Jun amino terminal kinase (JNK); (3) activated serum response factor (SRF)-dependent transcription; (4) activated NF-kappaB-dependent transcription; and (5) synergized with an activated Raf-kinase (Raf-CAAX) to transform NIH3T3 cells. In addition, wild type TC10 function is required for full H-Ras transforming potential. We demonstrate that an intact effector domain and carboxyl terminal prenylation signal are required for proper TC10 function and that TC10 signals to at least two separable downstream target pathways. In addition, TC10 interacted with the actin-binding and filament-forming protein, profilin, in both a two-hybrid cDNA library screen, and an in vitro binding assay. Taken together, these data support a classification of TC10 as a member of the Rho family, and in particular, suggest that TC10 functions to regulate cellular signaling to the actin cytoskeleton and processes associated with cell growth.

Molecular mechanisms of lead neurotoxicity

Epidemiological studies have shown a strong relationship between the level of lead in blood and bone as assessed by performance on IQ tests and other psychometric tests. Approximately 1 out of 10 children in the United States have blood lead levels above 10 microg/dl, which has been established as the level of concern. Studies on experimental animals exposed to lead after birth have shown learning deficits at similar blood lead levels. Since learning requires the remodeling of synapses in the brain, lead may specifically affect synaptic transmission. Although the molecular targets for lead are unknown, a vast amount of evidence accumulated over many years has shown that lead disrupts processes that are regulated by calcium. Our laboratory has been studying the effect of lead on protein kinase C, a family of isozymes some of which require calcium for activity. We and others have shown that picomolar concentrations of lead can replace micromolar concentrations of calcium in a protein kinase C enzyme assay. Furthermore, lead activates protein kinase C in intact cells and induces the expression of new genes by a mechanism dependent on protein kinase C. We propose that the learning deficits caused by lead are due to events regulated by protein kinase C that most likely occur at the synapse.

Novel roles of specific isoforms of protein kinase C in activation of the c-fos serum response element

Protein kinase C (PKC) is a multigene family of enzymes consisting of at least 11 isoforms. It has been implicated in the induction of c-fos and other immediate response genes by various mitogens. The serum response element (SRE) in the c-fos promoter is necessary and sufficient for induction of transcription of c-fos by serum, growth factors, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). It forms a complex with the ternary complex factor (TCF) and with a dimer of the serum response factor (SRF). TCF is the target of several signal transduction pathways and SRF is the target of the rhoA pathway. In this study we generated dominant-negative and constitutively active mutants of PKC-alpha, PKC-delta, PKC-epsilon, and PKC-zeta to determine the roles of individual isoforms of PKC in activation of the SRE. Transient-transfection assays with NIH 3T3 cells, using an SRE-driven luciferase reporter plasmid, indicated that PKC-alpha and PKC-epsilon, but not PKC-delta or PKC-zeta, mediate SRE activation. TPA-induced activation of the SRE was partially inhibited by dominant negative c-Raf, ERK1, or ERK2, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of Elk-1. TPA-induced activation of the SRE was also partially inhibited by a dominant-negative MEKK1. Furthermore, TPA treatment of serum-starved NIH 3T3 cells led to phosphorylation of SEK1, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of c-Jun, a major substrate of JNK. Constitutively active mutants of PKC-alpha and PKC-epsilon could also induce a mutant c-fos promoter which lacks the TCF binding site, and they also induce transactivation activity of the SRF. Furthermore, rhoA-mediated SRE activation was blocked by dominant negative mutants of PKC-alpha or PKC-epsilon. Taken together, these findings indicate that PKC-alpha and PKC-epsilon can enhance the activities of at least three signaling pathways that converge on the SRE: c-Raf-MEK1-ERK-TCF, MEKK1-SEK1-JNK-TCF, and rhoA-SRF. Thus, specific isoforms of PKC may play a role in integrating networks of signal transduction pathways that control gene expression.

Serum response factor is essential for mesoderm formation during mouse embryogenesis

The transcription factor serum response factor (SRF), a phylogenetically conserved nuclear protein, mediates the rapid transcriptional response to extracellular stimuli, e.g. growth and differentiation signals. DNA- protein complexes containing SRF or its homologues function as nuclear targets of the Ras/MAPK signalling network, thereby directing gene activities associated with processes as diverse as pheromone signalling, cell-cycle progression (transitions G0-G1 and G2-M), neuronal synaptic transmission and muscle cell differentiation. So far, the activity of mammalian SRF has been studied exclusively in cultured cells. To study SRF function in a multicellular organism we generated an Srf null allele in mice. SRF-deficient embryos (Srf -/-) have a severe gastrulation defect and do not develop to term. They consist of misfolded ectodermal and endodermal cell layers, do not form a primitive streak or any detectable mesodermal cells and fail to express the developmental marker genes Bra (T), Bmp-2/4 and Shh. Activation of the SRF-regulated immediate early genes Egr-1 and c-fos, as well as the alpha-Actin gene, is severely impaired. Our study identifies SRF as a new and essential regulator of mammalian mesoderm formation. We therefore suggest that in mammals Ras/MAPK signalling contributes to mesoderm induction, as is the case in amphibia.

Interaction of ATF6 and serum response factor

Serum response factor (SRF) is a transcription factor which binds to the serum response element (SRE) in the c-fos promoter. It is required for regulated expression of the c-fos gene as well as other immediate-early genes and some tissue-specific genes. To better understand the regulation of SRF, we used a yeast interaction assay to screen a human HeLa cell cDNA library for SRF-interacting proteins. ATF6, a basic-leucine zipper protein, was isolated by binding to SRF and in particular to its transcriptional activation domain. The binding of ATF6 to SRF was also detected in vitro. An ATF6-VP16 chimera activated expression of an SRE reporter gene in HeLa cells, suggesting that ATF6 can interact with endogenous SRF. More strikingly, an antisense ATF6 construct reduced serum induction of a c-fos reporter gene, suggesting that ATF6 is involved in activation of transcription by SRF. ATF6 was previously partially cloned as a member of the ATF family. The complete cDNA of ATF6 was isolated, and its expression pattern was described.

A serum response factor-dependent transcriptional regulatory program identifies distinct smooth muscle cell sublineages

The SM22alpha promoter has been used as a model system to define the molecular mechanisms that regulate smooth muscle cell (SMC) specific gene expression during mammalian development. The SM22alpha gene is expressed exclusively in vascular and visceral SMCs during postnatal development and is transiently expressed in the heart and somites during embryogenesis. Analysis of the SM22alpha promoter in transgenic mice revealed that 280 bp of 5' flanking sequence is sufficient to restrict expression of the lacZ reporter gene to arterial SMCs and the myotomal component of the somites. DNase I footprint and electrophoretic mobility shift analyses revealed that the SM22alpha promoter contains six nuclear protein binding sites (designated smooth muscle elements [SMEs] -1 to -6, respectively), two of which bind serum response factor (SRF) (SME-1 and SME-4). Mutational analyses demonstrated that a two-nucleotide substitution that selectively eliminates SRF binding to SME-4 decreases SM22alpha promoter activity in arterial SMCs by approximately 90%. Moreover, mutations that abolish binding of SRF to SME-1 and SME-4 or mutations that eliminate each SME-3 binding activity totally abolished SM22alpha promoter activity in the arterial SMCs and somites of transgenic mice. Finally, we have shown that a multimerized copy of SME-4 (bp -190 to -110) when linked to the minimal SM22alpha promoter (bp -90 to +41) is necessary and sufficient to direct high-level transcription in an SMC lineage-restricted fashion. Taken together, these data demonstrate that distinct transcriptional regulatory programs control SM22alpha gene expression in arterial versus visceral SMCs. Moreover, these data are consistent with a model in which combinatorial interactions between SRF and other transcription factors that bind to SME-4 (and that bind directly to SRF) activate transcription of the SM22alpha gene in arterial SMCs.