Myocardin and ternary complex factors compete for SRF to control smooth muscle gene expression

Mechanisms responsible for the promoter-specific effects of myocardin

Understanding the mechanism of smooth muscle cell (SMC) differentiation will provide the foundation for elucidating SMC-related diseases such as atherosclerosis, restenosis, and asthma. Recent studies have demonstrated that the interaction of SRF with the co-activator myocardin is a critical determinant of smooth muscle development. It has been proposed that the specific transcriptional activation of smooth muscle-restricted genes (as opposed to other SRF-dependent genes) by myocardin results from the presence of multiple CArG boxes in smooth muscle genes that facilitate myocardin homodimer formation. This proposal was further tested in the current study. Our results show that the SMC-specific telokin promoter, which contains only a single CArG box, is strongly activated by myocardin. Furthermore, myocardin and a dimerization defective mutant myocardin induce expression of endogenous telokin but not c-fos in 10T1/2 fibroblast cells. Knocking down myocardin by small interfering RNA decreased telokin promoter activity and expression in A10 SMCs. A series of telokin and c-fos promoter chimeric and mutant reporter genes was generated to determine the mechanisms responsible for the promoter-specific effects of myocardin. Data from these experiments demonstrated that the ets binding site in the c-fos promoter partially blocks the activation of this promoter by myocardin. However, the binding of ets factors alone was not sufficient to explain the promoter-specific effects of myocardin. Elements 3' of the CArG box in the c-fos promoter act in concert with the ets binding site to block the ability of myocardin to activate the promoter. Conversely, elements 5' and 3' of the CArG box in the telokin promoter act in concert with the CArG box to facilitate myocardin stimulation of the promoter. Together these data suggest that the promoter specificity of myocardin is dependent on complex combinatorial interactions of multiple cis elements and their trans binding factors.

Requirement for serum response factor for skeletal muscle growth and maturation revealed by tissue-specific gene deletion in mice

Serum response factor (SRF) controls the transcription of muscle genes by recruiting a variety of partner proteins, including members of the myocardin family of transcriptional coactivators. Mice lacking SRF fail to form mesoderm and die before gastrulation, precluding an analysis of the roles of SRF in muscle tissues. To investigate the functions of SRF in skeletal muscle development, we conditionally deleted the Srf gene in mice by skeletal muscle-specific expression of Cre recombinase. In mice lacking skeletal muscle SRF expression, muscle fibers formed, but failed to undergo hypertrophic growth after birth. Consequently, mutant mice died during the perinatal period from severe skeletal muscle hypoplasia. The myopathic phenotype of these mutant mice resembled that of mice expressing a dominant negative mutant of a myocardin family member in skeletal muscle. These findings reveal an essential role for the partnership of SRF and myocardin-related transcription factors in the control of skeletal muscle growth and maturation in vivo.

Kruppel-like factor 4 abrogates myocardin-induced activation of smooth muscle gene expression

Platelet-derived growth factor BB (PDGF-BB) has been shown to be an extremely potent negative regulator of smooth muscle cell (SMC) differentiation. Moreover, previous studies have demonstrated that the Kruppel-like transcription factor (KLF) 4 potently represses the expression of multiple SMC genes. However, the mechanisms whereby KLF4 suppresses SMC gene expression are not known, nor is it clear whether KLF4 contributes to PDGF-BB-induced down-regulation of SMC genes. The goals of the present studies were to determine the molecular mechanisms by which KLF4 represses expression of SMC genes and whether it contributes to PDGF-BB-induced suppression of these genes. Results demonstrated that KLF4 markedly repressed both myocardin-induced activation of SMC genes and expression of myocardin. KLF4 was rapidly up-regulated in PDGF-BB-treated, cultured SMC, and a small interfering RNA to KLF4 partially blocked PDGF-BB-induced SMC gene repression. Both PDGF-BB and KLF4 markedly reduced serum response factor binding to CArG containing regions within intact chromatin. Finally, KLF4, which is normally not expressed in differentiated SMC in vivo, was rapidly up-regulated in vivo in response to vascular injury. Taken together, results indicate that KLF4 represses SMC genes by both down-regulating myocardin expression and preventing serum response factor/myocardin from associating with SMC gene promoters, and suggest that KLF4 may be a key effector of PDGF-BB and injury-induced phenotypic switching of SMC.

AKAP12alpha, an atypical serum response factor-dependent target gene

We recently identified three AKAP12 isoforms that are differentially regulated by distinct promoters. During a screen to identify molecular determinants distinguishing the activities of these promoters, we found a potential binding site for the serum response factor (SRF) in the promoter of the ubiquitously expressed AKAP12alpha isoform. SRF is an evolutionarily conserved transcription factor that governs disparate programs of gene expression linked to cellular growth and differentiation. Using a combination of reporter assays and RNA interference, we demonstrate that SRF is required for AKAP12alpha expression. SRF regulates the activity of the AKAP12alpha promoter through two conserved CArG boxes that bind SRF with different affinities. Unlike other SRF-dependent genes, AKAP12alpha is not regulated by growth or differentiation stimuli. Molecular analysis of the AKAP12alpha SRF-binding sites, or CArG boxes, indicates that sequences flanking these sites are the determinants of sensitivity to SRF-activating signals. Specifically, the AKAP12alpha CArG boxes are shielded from growth stimulation by the absence of a binding site for Ets transcription factors. Similarly, sensitivity to the differentiation-associated co-factor, myocardin, was also determined by responsive flanking sequence; however, unlike growth stimuli, sensitivity to myocardin was found to also be dependent on a consensus CArG box. Collectively, our data demonstrate that AKAP12alpha belongs to a novel class of atypical SRF-dependent target genes. Furthermore, we provide new insight into the role of flanking sequences in determining sensitivity to SRF-myocardin activity.

Biological control through regulated transcriptional coactivators

Gene activation in higher eukaryotes requires the concerted action of transcription factors and coactivator proteins. Coactivators exist in multiprotein complexes that dock on transcription factors and modify chromatin, allowing effective transcription to take place. While biological control focused at the level of the transcription factor is very common, it is now quite clear that a substantial component of gene control is directed at the expression of coactivators, involving pathways as diverse as B-cell development, smooth muscle differentiation, and hepatic gluconeogenesis. Quantitative control of coactivators allows the functional integration of multiple transcription factors and facilitates the formation of distinct biological programs. This coordination and acceleration of different steps in linked pathways has important kinetic considerations, enabling outputs of particular pathways to be increased far more than would otherwise be possible. These kinetic aspects suggest opportunities and concerns as coactivators become targets of therapeutic intervention.

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.

The ETS domain transcription factor Elk-1 regulates the expression of its partner protein, SRF

The ternary complex factors (TCF) are a subfamily of ETS domain transcription factors that bind and activate serum response elements (SREs) in the promoters of target genes in a ternary complex with a second transcription factor, serum response factor (SRF). Here, we have identified the SRF gene as a target for the TCFs, thereby providing a positive feedback loop whereby TCF activation leads to the enhancement of the expression of its partner protein SRF. The binding of the TCF Elk-1 to the SRF promoter and subsequent regulation of SRF expression occurs in a ternary complex-dependent manner. Our data therefore reveal that SRF is an important target for the ERK and Rho signaling pathways that converge on a ternary TCF-SRF complex at the SRE on the SRF promoter.

Identification of a novel serum response factor cofactor in cardiac gene regulation

The transcription factor serum response factor (SRF) plays an important role in the regulation of a variety of cardiac genes during development and during adult aging. A novel SRF cofactor, herein called p49/STRAP, for SRF-dependent transcription regulation-associated protein, was recently identified in our laboratory. This protein interacted mainly with the transcriptional activation domain of the SRF protein and was found to bind to SRF or to the complex of SRF and another cofactor, such as myocardin or Nkx2.5. The expression of p49/STRAP affected the promoter activity of SRF target genes in a non-uniform manner. For example, p49 activated MLC2v and cardiac actin promoters when it was co-transfected with SRF, but it repressed atrial natriuretic factor promoter activity, which was strongly induced by myocardin. The p49/STRAP mRNA was observed to be highly expressed in fetal, adult, and senescent human hearts, and also in hearts of young adult and old mice, suggesting that p49/STRAP may be an important SRF cofactor in the transcriptional regulation of mammalian cardiac muscle genes throughout the life span.

A G/C element mediates repression of the SM22alpha promoter within phenotypically modulated smooth muscle cells in experimental atherosclerosis

A hallmark of smooth muscle cell (SMC) phenotypic switching in atherosclerotic lesions is suppression of SMC differentiation marker gene expression. Yet little is known regarding the molecular mechanisms that control this process. Here we show that transcription of the SMC differentiation marker gene SM22alpha is reduced in atherosclerotic lesions and identify a cis regulatory element in the SM22alpha promoter required for this process. Transgenic mice carrying the SM22alpha promoter-beta-galactosidase (beta-gal) reporter transgene were crossed to apolipoprotein E (ApoE)-/- mice. Cells of the fibrous cap, intima, and underlying media showed complete loss of beta-gal activity in advanced atherosclerotic lesions. Of major significance, mutation of a G/C-rich cis element in the SM22alpha promoter prevented the decrease in SM22alpha promoter-beta-gal reporter transgene expression, including in cells that compose the fibrous cap of the lesion and in medial cells in proximity to the lesion. To begin to assess mechanisms whereby the G/C repressor element mediates suppression of SM22alpha in atherosclerosis, we tested the hypothesis that effects may be mediated by platelet-derived growth factor (PDGF)-BB-induced increases in the G/C binding transcription factor Sp1. Consistent with this hypothesis, results of studies in cultured SMCs showed that: (1) PDGF-BB increased expression of Sp1; (2) PDGF-BB and Sp1 profoundly suppressed SM22alpha promoter activity as well as smooth muscle myosin heavy chain promoter activity through mechanisms that were at least partially dependent on the G/C cis element; and (3) a short interfering RNA to Sp1 increased basal expression and attenuated PDGF-BB induced suppression of SM22alpha. Together, these results support a model whereby a G/C repressor element within the SM22alpha promoter mediates transcriptional repression of this gene within phenotypically modulated SMCs in experimental atherosclerosis and provide indirect evidence implicating PDGF-BB and Sp1 as possible mediators of these effects.

Mutant actins that stabilise F-actin use distinct mechanisms to activate the SRF coactivator MAL

Nuclear accumulation of the serum response factor coactivator MAL/MKL1 is controlled by its interaction with G-actin, which results in its retention in the cytoplasm in cells with low Rho activity. We previously identified actin mutants whose expression promotes MAL nuclear accumulation via an unknown mechanism. Here, we show that actin interacts directly with MAL in vitro with high affinity. We identify a further activating mutation, G15S, which stabilises F-actin, as do the activating actins S14C and V159N. The three mutants share several biochemical properties, but can be distinguished by their ability to bind cofilin, ATP and MAL. MAL interaction with actin S14C is essentially undetectable, and that with actin V159N is weakened. In contrast, actin G15S interacts more strongly with MAL than the wild-type protein. Strikingly, the nuclear accumulation of MAL induced by overexpression of actin S14C is substantially dependent on Rho activity and actin treadmilling, while that induced by actin G15S expression is not. We propose a model in which actin G15S acts directly to promote MAL nuclear entry.

A myocardin-related transcription factor regulates activity of serum response factor in Drosophila

Serum response factor (SRF) regulates genes involved in cell proliferation, migration, cytoskeletal organization, and myogenesis. Myocardin and myocardin-related transcription factors (MRTFs) act as powerful transcriptional coactivators of SRF in mammalian cells. We describe an MRTF from Drosophila, called DMRTF, which shares high homology with the functional domains of mammalian myocardin and MRTFs. DMRTF forms a ternary complex with and stimulates the activity of Drosophila SRF, which has been implicated in branching of the tracheal (respiratory) system and formation of wing interveins. A loss-of-function mutation introduced into the DMRTF locus by homologous recombination results in abnormalities in tracheal branching similar to those in embryos lacking SRF. Misexpression in wing imaginal discs of a dominant negative DMRTF mutant also causes a diminution of wing interveins, whereas overexpression of DMRTF results in excess intervein tissue, abnormalities reminiscent of SRF loss- and gain-of-function phenotypes, respectively. Overexpression of these DMRTF mutants in mesoderm and in the tracheal system also perturbs mesoderm cell migration and tracheal branching, respectively. We conclude that the interaction of MRTFs with SRF represents an ancient protein partnership involved in cytoplasmic outgrowth and cell migration during development.

Stretch of the vascular wall induces smooth muscle differentiation by promoting actin polymerization

Stretch of the vascular wall by the intraluminal blood pressure stimulates protein synthesis and contributes to the maintenance of the smooth muscle contractile phenotype. The expression of most smooth muscle specific genes has been shown to be regulated by serum response factor and stimulated by increased actin polymerization. Hence we hypothesized that stretch-induced differentiation is promoted by actin polymerization. Intact mouse portal veins were cultured under longitudinal stress and compared with unstretched controls. In unstretched veins the rates of synthesis of several proteins associated with the contractile/cytoskeletal system (alpha-actin, calponin, SM22alpha, tropomyosin, and desmin) were dramatically lower than in stretched veins, whereas other proteins (beta-actin and heat shock proteins) were synthesized at similar rates. The cytoskeletal proteins gamma-actin and vimentin were weakly stretch-sensitive. Inhibition of Rho-associated kinase by culture of stretched veins with Y-27632 produced similar but weaker effects compared with the absence of mechanical stress. Induction of actin polymerization by jasplakinolide increased SM22alpha synthesis in unstretched veins to the level in stretched veins. Stretch stimulated Rho activity and phosphorylation of the actin-severing protein cofilin-2, although both effects were slow in onset (Rho-GTP, >15 min; cofilin-P, >1 h). Cofilin-2 phosphorylation of stretched veins was inhibited by Y-27632. The F/G-actin ratio after 24 h of culture was significantly greater in stretched than in unstretched veins, as shown by both ultracentrifugation and confocal imaging with phalloidin/DNase I labeling. The results show that stretch of the vascular wall stimulates increased actin polymerization, activating synthesis of smooth muscle-specific proteins. The effect is partially, but probably not completely, mediated via Rho-associated kinase and cofilin downstream of Rho.