Megakaryoblastic Leukemia Factor-1 Transduces Cytoskeletal Signals and Induces Smooth Muscle Cell Differentiation from Undifferentiated Embryonic Stem Cells

Abstract

The SAP domain transcription factor myocardin plays a critical role in the transcriptional program regulating smooth muscle cell differentiation. In this report, we describe the capacity of myocardin to physically associate with megakaryoblastic leukemia factor-1 (MKL1) and characterize the function of MKL1 in smooth muscle cells (SMCs). The MKL1 gene is expressed in most human tissues and myocardin and MKL are co-expressed in SMCs. MKL1 and myocardin physically associate via conserved leucine zipper domains. Overexpression of MKL1 transactivates serum response factor (SRF)-dependent SMC-restricted transcriptional regulatory elements including the SM22α promoter, smooth muscle myosin heavy chain promoter/enhancer, and SM-α-actin promoter/enhancer in non-SMCs. Moreover, forced expression of MKL1 and SRF in undifferentiated SRF-/- embryonic stem cells activates multiple endogenous SMC-restricted genes at levels equivalent to, or exceeding, myocardin. Forced expression of a dominant-negative MKL1 mutant reduces myocardin-induced activation of the SMC-specific SM22α promoter. In NIH3T3 fibroblasts MKL1 localizes to the cytoplasm and translocates to the nucleus in response to serum stimulation, actin treadmilling, and RhoA signaling. In contrast, in SMCs MKL1 is observed exclusively in the nucleus regardless of serum conditions or RhoA signaling. However, when actin polymerization is disrupted MKL1 translocates from the nucleus to the cytoplasm in SMCs. Together, these data were consistent with a model wherein MKL1 transduces signals from the cytoskeleton to the nucleus in SMCs and regulates SRF-dependent SMC differentiation autonomously or in concert with myocardin. The SAP domain transcription factor myocardin plays a critical role in the transcriptional program regulating smooth muscle cell differentiation. In this report, we describe the capacity of myocardin to physically associate with megakaryoblastic leukemia factor-1 (MKL1) and characterize the function of MKL1 in smooth muscle cells (SMCs). The MKL1 gene is expressed in most human tissues and myocardin and MKL are co-expressed in SMCs. MKL1 and myocardin physically associate via conserved leucine zipper domains. Overexpression of MKL1 transactivates serum response factor (SRF)-dependent SMC-restricted transcriptional regulatory elements including the SM22α promoter, smooth muscle myosin heavy chain promoter/enhancer, and SM-α-actin promoter/enhancer in non-SMCs. Moreover, forced expression of MKL1 and SRF in undifferentiated SRF-/- embryonic stem cells activates multiple endogenous SMC-restricted genes at levels equivalent to, or exceeding, myocardin. Forced expression of a dominant-negative MKL1 mutant reduces myocardin-induced activation of the SMC-specific SM22α promoter. In NIH3T3 fibroblasts MKL1 localizes to the cytoplasm and translocates to the nucleus in response to serum stimulation, actin treadmilling, and RhoA signaling. In contrast, in SMCs MKL1 is observed exclusively in the nucleus regardless of serum conditions or RhoA signaling. However, when actin polymerization is disrupted MKL1 translocates from the nucleus to the cytoplasm in SMCs. Together, these data were consistent with a model wherein MKL1 transduces signals from the cytoskeleton to the nucleus in SMCs and regulates SRF-dependent SMC differentiation autonomously or in concert with myocardin. Smooth muscle cells (SMCs) 1The abbreviations used are: SMC, smooth muscle cell; SRF, serum response factor; TCF, ternary complex factor; SM-MHC, smooth muscle-myosin heavy chain; ES, embryonic stem; MRTF, myocardin-related transcription factor; MKL1, megakaryoblastic leukemia factor-1; HA, hemagglutinin; LZ, leucine zipper; CRE, cyclic AMP response element; RT, reverse transcriptase; ChIP, chromatin immunoprecipitation; FL-Myo, FLAG epitope-tagged myocardin; DN, dominant-negative; C(t), cycle threshold; X-Gal, 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside. are distributed in organ systems and tissues throughout the body where they regulate tonic contractile functions including modulation of vascular tone, bronchial reactivity, gastrointestinal peristalsis, and genitourinary contraction and tone. In contrast to striated muscle cells, SMCs retain the capacity to both proliferate and modulate their phenotype from contractile to synthetic during postnatal development. The plasticity and proliferative capacity of this muscle cell lineage plays a central role in physiological processes including wound healing and angiogenesis. However, modulation of SMC phenotype has also been implicated in the pathogenesis of disease processes including atherosclerosis, restenosis following coronary angioplasty/stent implantation, and asthma. Our group and others have reported that SMC differentiation and modulation of the SMC phenotype is regulated by a serum response factor (SRF)-dependent transcriptional regulatory program (for review see Refs. 1Parmacek M.S. Curr. Top. Dev. Biol. 2001; 51: 69-89Crossref PubMed Google Scholar and 2Miano J.M. J. Mol. Cell. Cardiol. 2003; 35: 577-593Abstract Full Text Full Text PDF PubMed Scopus (488) Google Scholar)). SRF is a MADS box transcription factor that binds to a transcriptional regulatory element designated the CArG box or SRE. The functional properties of SRF and SRF binding sites were initially characterized in promoters regulating growth responsive genes including c-fos and egr-1 (3Treisman R. EMBO J. 1995; 14: 4905-4913Crossref PubMed Scopus (347) Google Scholar, 4Treisman R. Nature. 1995; 376: 468-469Crossref PubMed Scopus (31) Google Scholar, 5Johansen F.E. Prywes R. Biochim. Biophys. Acta. 1995; 1242: 1-10Crossref PubMed Scopus (105) Google Scholar). SRF-mediated activity of growth responsive genes is regulated via post-translational modifications of SRF and ternary complex factors (TCFs) in the ets family that physically associate with both SRF and specific DNA sequences (6Treisman R. Curr. Opin. Genet. Dev. 1994; 4: 96-101Crossref PubMed Scopus (620) Google Scholar). SRF activity is also modulated by intracellular signals including serum, RhoA, and actin treadmilling (7Sotiropoulos A. Gineitis J. R. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar, R. Biol. PubMed Scopus Google Scholar, J. R. Cell. 1995; Full Text PDF PubMed Scopus Google Scholar, J. Biol. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). CArG have been in SMC-restricted transcriptional regulatory elements M.S. Curr. Top. Dev. Biol. 2001; 51: 69-89Crossref PubMed Google Scholar, 2Miano J.M. J. Mol. Cell. Cardiol. 2003; 35: 577-593Abstract Full Text Full Text PDF PubMed Scopus (488) Google Scholar). of these CArG activity of these transcriptional of the CArG box in the SMC-restricted SM22α is to expression of a gene to SMCs in M.S. Mol. Cell. Biol. PubMed Scopus Google Scholar). a expressed transcription factor SRF regulate transcription of both growth responsive genes and differentiation that SRF regulates SMC differentiation via binding to the and SMC-restricted SAP domain transcription factor myocardin J. J.M. J. Mol. Cell. Cardiol. Full Text PDF PubMed Google Scholar, J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, A. 2003; PubMed Scopus Google Scholar, A. 2003; PubMed Scopus Google Scholar, 2003; PubMed Scopus Google Scholar). is a of and family of SAP that have been implicated in chromatin most R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, A. A. PubMed Scopus Google Scholar, Full Text Full Text PDF PubMed Scopus Google Scholar). Forced expression of myocardin activates SMC-restricted transcriptional regulatory elements in cells including the SM22α promoter, the smooth muscle myosin heavy chain and the SM-α-actin J. J.M. J. Mol. Cell. Cardiol. Full Text PDF PubMed Google Scholar, J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, A. 2003; PubMed Scopus Google Scholar, 2003; PubMed Scopus Google Scholar). Moreover, forced expression of myocardin and SRF in embryonic stem cells activates endogenous SMC genes including SM-MHC, and SM-α-actin J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). a in the myocardin gene at embryonic of and vascular SMCs are observed A. 2003; PubMed Scopus Google Scholar). to the role of SRF and myocardin in SMC differentiation is factor or that regulates SMC differentiation in the of myocardin in the at myocardin is expressed in the the of by in vascular SMCs the including SM-α-actin and SM22α J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). intracellular signals that regulate SMC including actin treadmilling and RhoA, modulate activity of SRF myocardin in role the myocardin-related transcription factors designated A. A. PubMed Scopus Google Scholar, J. Genet. 2001; PubMed Scopus Google Scholar, R. R. A. 2001; PubMed Scopus Google Scholar, J. Biol. Full Text Full Text PDF PubMed Scopus Google and A. A. PubMed Scopus Google Scholar, A. Prywes R. J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google in SMCs and cell the that regulate activity of myocardin and SRF in we a in the conserved of myocardin in this MKL1 in the M.S. Curr. Top. Dev. Biol. 2001; 51: 69-89Crossref PubMed Google Scholar, R. R. A. 2001; PubMed Scopus Google with megakaryoblastic leukemia in and J. Genet. 2001; PubMed Scopus Google Scholar, R. R. A. 2001; PubMed Scopus Google Scholar, J. Biol. Full Text Full Text PDF PubMed Scopus Google Scholar). MKL1 has been to regulate the transcriptional activity of SRF in via in response to serum, RhoA, and actin treadmilling R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). In the in this report, we that myocardin and MKL1 physically associate via conserved leucine zipper domains. In we that MKL1 is expressed in SMCs and the regulating of MKL1 in SMCs. we MKL1 activates transcriptional regulatory elements SMC-restricted genes and activates endogenous SMC genes in undifferentiated cells via SRF-dependent expression human MKL1 by the of the human MKL1 The and expression human myocardin and SRF have been J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). The and expression and FLAG epitope-tagged human MKL1, were by the human MKL1 and and and FLAG epitope-tagged were by the human myocardin or The and expression the and of human myocardin were by myocardin expression is to the myocardin the the myocardin leucine zipper domain The the SM22α to M.S. Mol. Cell. Biol. PubMed Scopus Google Scholar). The is to the that the in and were to SRF binding M.S. Mol. Cell. Biol. PubMed Scopus Google Scholar). of the SM22α CArG binding to of the in M.S. J. Biol. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). The and the SM-α-actin and and were the of of and have been J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). of the c-fos of the of a cyclic AMP response element Biochim. Biophys. Acta. PubMed Scopus Google of the in the of to the binding domain of The expression a dominant-negative RhoA mutant and a from to the of the cells were with of the and were by growth were and of and from the and by the to the of MKL1 were and characterized transcription and were and and the and mutant were with and of were at to the and the by to and with and a were the and by M.S. J.M. J. Biol. Full Text PDF PubMed Google Scholar). In were of from cells, NIH3T3 cells, cells, and M.S. J.M. J. Biol. Full Text PDF PubMed Google Scholar). The human and MKL1 were by M.S. J.M. J. Biol. Full Text PDF PubMed Google Scholar). and cells were J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). of cells were with and to of the of the expression and of the activity and the are reported SRF-/- cells J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google were and with J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). NIH3T3 cells, cells, and SMCs were J. J. M.S. J. Biol. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). of and SRF-/- cells J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google were with and of of of of or of of from the cells with were with of reverse of specific and reverse and The following were and and and and SM-MHC, and and of the the DNA cycle conditions were by of at and and at and were in with and were to gene expression levels the 2001; Scopus Google Scholar). were the by and 2001; PubMed Scopus Google Scholar). SRF-/- cells were with or cells were in in and with and cell with and of to reverse DNA by and in to with the CArG box the binding in the SM22α and to a DNA intracellular of myocardin and MKL1 by and NIH3T3 cells with of or the expression with cells were in with the cells were in serum or with with serum the of actin polymerization the intracellular of MKL1, cells were with polymerization of actin A. M.S. Dev. PubMed Scopus Google Scholar). actin cells were with A. M.S. Dev. PubMed Scopus Google Scholar). the of RhoA the intracellular of MKL1, cells were with the expression a dominant-negative RhoA mutant The cells were with and and a A. M.S. Dev. PubMed Scopus Google Scholar). MKL1 with the regulating myocardin a of a human expression a the of the human myocardin The of myocardin this of the the conserved and and SAP that have been implicated in regulating activity of myocardin and Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). were by growth of the that to the of human A. A. PubMed Scopus Google Scholar, J. Genet. 2001; PubMed Scopus Google Scholar, R. R. A. 2001; PubMed Scopus Google Scholar). The of these were with observed in the of the and of that the human MKL1 gene of The human MKL1 a that the human MKL1 with myocardin the and with myocardin the and leucine zipper MKL1 that with the actin cytoskeleton R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google myocardin domains. In MKL1 a that is with transcriptional activation Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). that myocardin and MKL1 physically a of were FLAG epitope-tagged myocardin a a observed in with a to MKL1 observed in the both in MKL1 and in in MKL1 and observed in of MKL1 and with the the in myocardin that capacity to physically associate with MKL1, were myocardin the are in with from the myocardin mutant the in myocardin a transcriptional activation and However, of myocardin to sequences the SAP domain the leucine zipper the capacity of myocardin and to physically associate the myocardin leucine zipper mutant a myocardin in the leucine zipper observed in and of the myocardin mutant expressed in to that the myocardin mutant the capacity to SRF when SRF to the CArG in the SM22α promoter, were by SRF-/- cells with expression or FLAG epitope-tagged mutant with and in cells with the and the expression and with the CArG However, in SRF-/- cells with expression FLAG epitope-tagged myocardin or FLAG epitope-tagged mutant and and with a to the that the mutant the capacity to SRF to specific CArG and these that myocardin and MKL1 physically associate and this is the leucine zipper domain of myocardin. of MKL1 the of MKL1 gene expression in human tissues with a human MKL1 to from a of human tissues the human MKL1 to a of in the human and and However, in contrast to the myocardin gene that is expressed in the and SMCs J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, Cell. 2001; Full Text Full Text PDF PubMed Scopus Google MKL1 gene expression in both tissues including the and and tissues including muscle and levels of MKL1 gene expression with the most MKL1 observed in the from the human and to the cell lineage of MKL1 gene expression with MKL1 is expressed in were a MKL1 and from a of and cell including cells, NIH3T3 cells, SMCs from the SMCs from the and of SMCs with the of MKL1 observed in human a to the of MKL1 A. A. PubMed Scopus Google observed in cells, NIH3T3 cells, cells, and SMCs. data were consistent with gene these data by expression of MKL1 in tissues and SMCs J. Genet. 2001; PubMed Scopus Google Scholar, A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). MKL1 SMC-restricted in MKL1 transactivates the SMC-specific SM22α in cells were with the expression and the the gene the transcriptional of the SM22α M.S. Mol. Cell. Biol. PubMed Scopus Google Scholar). Forced expression of MKL1 in these cells in a in activity with cells with the and the expression The of with that observed in cells with expression myocardin and J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). In contrast, forced expression of MKL1 to the the transcriptional of the SM22α in and that SRF binding The capacity of MKL1 to the specific CArG box elements their in the SMC-specific SM22α and the c-fos were by cells with and the transcriptional of a to of the or of the c-fos Forced expression of MKL1 the a in activity observed in cells with the In contrast, MKL1 to of a forced expression of MKL1 transactivates SMC transcriptional regulatory elements in NIH3T3 cells were with the expression and the transcriptional of the SM-α-actin promoter/enhancer and the promoter/enhancer that have been to gene expression to SMCs in PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). a in observed in NIH3T3 cells with the MKL1 expression and the a in activity observed in NIH3T3 cells with and these data that MKL1 is a transcriptional of CArG transcriptional regulatory elements in SMC-specific genes and J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). The capacity to these SMC-specific elements to the specific of the CArG forced expression of MKL1 of the SM22α the c-fos SRE. these that of CArG elements is capacity to physically associate with myocardin is expressed in or NIH3T3 MKL1 of the SM22α and MKL1 in the transcriptional activation are function in a dominant-negative J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). MKL1 and myocardin function a transcriptional cells were with the the expression MKL1 and or of expression dominant-negative MKL1 mutant or dominant-negative myocardin mutant Forced expression of MKL1 the activity by that observed in cells with the and However, activity in a when with of dominant-negative MKL1 and with the dominant-negative activity and Moreover, myocardin-induced of the in a in of cells with and data that MKL1 and myocardin function in a transcriptional SMC-restricted transcriptional regulatory elements and that dominant-negative MKL1 SM22α activity by with the myocardin binding to SRF by with myocardin. Forced of MKL1 SMC-restricted in forced expression of MKL1 expression of endogenous genes SMC and to the of SMC genes is both SRF and SRF-/- cells were with and from and the expression of SMC genes and by were to gene expression levels the 2001; Scopus Google Scholar). In SRF-/- cells with the or expression myocardin and MKL1 gene and levels observed in the SRF-/- cells with the with J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google in this of forced expression of myocardin SRF in SRF-/- cells a in SM22α a in a in SM-α-actin and a in levels observed in cells with the SRF forced expression of MKL1 SRF expression of these SMC-restricted genes in SRF-/- cells to levels equivalent to, or exceeding, with myocardin SM22α by by SM-α-actin by and by levels observed in cells with SRF forced expression of MKL1 in SRF-/- cells to expression of these SMC genes that activation of SMC genes in undifferentiated cells is SRF-dependent forced expression of MKL1 SRF myocardin forced expression of myocardin SRF to MKL1 in these undifferentiated Together, these data forced expression of MKL1 expression of multiple SRF-dependent SMC genes in undifferentiated Moreover, these data that these factors function of in this The of SMC in in forced expression of MKL1 multiple SMC-restricted transcriptional regulatory elements in and NIH3T3 cells that MKL1 and myocardin is transcriptional activation of these SMC-specific transcriptional regulatory the capacity of myocardin to or physically associate with MKL1 is to expression of endogenous SMC genes in undifferentiated cells, SRF-/- cells were with the expression a myocardin mutant a leucine zipper domain The of SM22α and gene expression by in this mutant with MKL1 and Moreover, that the mutant the to physically associate with SRF to the CArG box the SM22α with the J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google forced expression of myocardin SRF in SM22α and a in in SRF-/- cells In contrast, forced expression of and SRF to expression of SM22α and in undifferentiated SRF-/- cells levels observed in cells with the SRF data that the leucine zipper domain is myocardin-induced expression of SMC genes in undifferentiated cells and that or of myocardin and MKL1 are myocardin-induced expression of SMC MKL1 to the in MKL1 translocates from the cytoplasm to the nucleus in response to serum stimulation, actin treadmilling, and RhoA R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). the intracellular of MKL1 in and NIH3T3 fibroblasts with MKL1 were with that the with R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google in NIH3T3 cells MKL1 exclusively to the cytoplasm conditions and serum MKL1 the nucleus in both and vascular MKL1 observed exclusively in the nucleus of SMCs In contrast, in a of myocardin observed exclusively in the nucleus of and NIH3T3 cells SMCs actin polymerization RhoA regulates the intracellular of MKL1 in of SMCs and SMCs in and serum, were to the actin polymerization in SMCs (7Sotiropoulos A. Gineitis J. R. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). in NIH3T3 cells SRF via the by MKL1 R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). when SMCs in or conditions were to MKL1 from the nucleus to the cytoplasm with this in SMCs with actin and activates SRF in R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google MKL1 observed in the nucleus MKL1 observed in the cytoplasm of NIH3T3 cells with the RhoA expression However, in SMCs with expression the dominant-negative mutant MKL1 observed in the SMC nucleus data that the regulating the intracellular of MKL1 are regulated in SMCs and non-SMCs. In these data that expression of MKL1 in the nucleus of SMCs of serum In contrast to striated muscle cell SMCs retain the capacity to proliferate and modulate their phenotype during postnatal in response to a of and intracellular and Our group and others have that the and SMC transcription factor myocardin plays a critical role in regulating expression of genes SMC contractile J. J.M. J. Mol. Cell. Cardiol. Full Text PDF PubMed Google Scholar, J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, A. 2003; PubMed Scopus Google Scholar, A. 2003; PubMed Scopus Google Scholar, 2003; PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). However, is signals are from the cytoplasm to the nucleus to regulate myocardin-induced SMC differentiation. the of myocardin-induced SMC we a in to that physically associate with myocardin. MKL1, has been to RhoA and signals from the cytoplasm to the nucleus in R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, A. A. PubMed Scopus Google Scholar, J. Genet. 2001; PubMed Scopus Google Scholar, R. R. A. 2001; PubMed Scopus Google Scholar, J. Biol. Full Text Full Text PDF PubMed Scopus Google Scholar, A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). In this is that RhoA and signals also SMC differentiation J. Biol. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). In this report, we have myocardin and MKL1 physically the human MKL1 gene is expressed in forced expression of MKL1 activates multiple SMC transcriptional regulatory elements at levels equivalent to or myocardin; of MKL1 in undifferentiated embryonic stem cells expression of genes SMC differentiation in SRF-dependent and the of MKL1 in SMCs and to intracellular signals including serum and Together, these data a model wherein MKL1 transduces signals from the SMC cytoskeleton to the nucleus in concert with SRF and activates SMC differentiation MKL1 is a of the SAP domain family of of conserved SAP domain R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, A. A. PubMed Scopus Google Scholar, Full Text Full Text PDF PubMed Scopus Google Scholar). SAP a of intracellular functions including chromatin and transcriptional R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, A. A. PubMed Scopus Google Scholar, Full Text Full Text PDF PubMed Scopus Google Scholar). SAP the of MKL1 is most to that of myocardin and A. A. PubMed Scopus Google Scholar). However, the cell of expression and intracellular of these factors The myocardin gene is expressed exclusively in and smooth muscle cells Cell. 2001; Full Text Full Text PDF PubMed Scopus Google of expression in vascular SMCs is complex in the J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). is expressed in a that the and SMCs A. A. PubMed Scopus Google Scholar, A. Prywes R. J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). In contrast, the MKL1 gene is expressed in a MKL1 is observed in postnatal human and tissues In MKL1 is in SMCs and SMC are consistent with the of A. A. PubMed Scopus Google observed throughout the at observed the levels of MKL1 gene expression in a of cells, and cells of the and during The of MKL1 gene expression is consistent with to a function consistent with capacity to RhoA and signals from the cytoplasm to the nucleus R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). MKL1, is a transcriptional of expression of a of SRF-dependent SMC genes in undifferentiated embryonic stem The that forced expression of MKL1 in undifferentiated embryonic stem cells activates the of genes SMC differentiation of myocardin is expressed in undifferentiated that the of myocardin and MKL1 is of SMC gene expression in cells in However, the that of the leucine zipper myocardin capacity to expression of SMC genes in undifferentiated cells this mutant capacity to SRF to that of myocardin is functional activity in data were also to the that during embryonic in the the myocardin gene is expressed in the cells the the of by in SMC including SM-α-actin and SM22α are expressed in these cells J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). to that at this of a transcriptional program vascular SMC differentiation J. M.S. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). The that MKL1 expression of SMC in undifferentiated cells that MKL1 a critical role in The that MKL1 is observed exclusively in the nucleus of SMCs and to to the cytoplasm when RhoA is that the plays and role in SMCs (7Sotiropoulos A. Gineitis J. R. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). with this when disrupted in SMCs by to the actin MKL1 to the data were consistent with a model wherein a is via MKL1 from the SMC cytoskeleton to the nucleus in a and In this is that SMCs in actin in the cells to tonic contraction in the and in tissues PubMed Scopus Google Scholar). The SMC cytoskeleton in to the contractile phenotype of vascular and SMCs by signals via MKL1 to SRF and the of SRF-dependent SMC model is by that including RhoA, and of the actin and activity of the and the in SMCs J. Biol. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). Moreover, during of the coronary SMC differentiation of these cells their actin cytoskeleton from to is a J. Dev. Biol. 2001; PubMed Scopus Google Scholar). model the capacity of vascular SMCs to modulate their phenotype from contractile to synthetic during and in response to vascular The capacity of MKL1 and to of genes in by the capacity of to and specific and CArG in transcriptional regulatory elements regulating expression of genes including c-fos and egr-1 R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). In SMC-restricted transcriptional regulatory elements CArG that the of to the transcription of SMC In MKL1 and are by intracellular the of MKL1 and binding to genes and growth responsive genes are of (7Sotiropoulos A. Gineitis J. R. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar, R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). in signals via MKL1 regulate the of SRF-dependent genes contractile and intracellular signals regulate activity to or expression of with this expression of a MKL1 box mutant that endogenous MKL1 in the cytoplasm activity of a SRF activation of the c-fos and egr-1 genes (7Sotiropoulos A. Gineitis J. R. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar, R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). In expression of MKL1 box activity of a R. Cell. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). Moreover, forced expression of a dominant-negative MKL1 mutant serum of serum of a c-fos A. Prywes R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar). body of in SRF a and signals The data to this model and that MKL1 plays a critical role in this RhoA and signals from the cytoplasm to the nucleus in SMCs to regulate the expression of genes SMC-restricted and and of