[Show abstract][Hide abstract] ABSTRACT: Myocardin (Myocd) and Myocd-related transcription factors (MRTFs) are robust coactivators of serum response factor (SRF). RPEL motifs are monomeric globular actin (G-actin) binding elements that regulate MRTF localization and activity. However, the function of the RPEL motif in Myocd is largely unknown because of its low affinity for G-actin. Here, we demonstrated that the Myocd RPEL motif bound to actin-related protein 5 (Arp5) instead of conventional actin, resulting in a significant suppression of Myocd activity. In addition, Arp5 bound to a DNA binding domain of SRF via its C-terminal sequence and prevented the association of the Myocd-SRF complex with the promoter regions of smooth muscle genes. Well-differentiated smooth muscle cells mainly expressed a specific splicing variant of arp5; therefore, the protein level of Arp5 was markedly reduced by partial messenger RNA decay and translational suppression. In dedifferentiated smooth muscle cells, Arp5 knockdown restored the differentiated phenotype via Myocd activation. Thus, Arp5 is a key regulator of Myocd activity.
The Journal of Cell Biology 02/2014; · 10.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epithelial-msenchymal transition (EMT) is closely associated with cancer and tissue fibrosis. The nuclear accumulation of myocardin-related transcription factor A (MRTF-A/MAL/MKL1) plays a vital role in EMT. In various cells treated with CCG-1423, a novel inhibitor of Rho signaling, the nuclear accumulation of MRTF-A is inhibited. However, the molecular target of this inhibitor has not yet been identified. In this study, we investigated the mechanism of this effect of CCG-1423. The interaction between MRTF-A and importin α/β1 was inhibited by CCG-1423, but monomeric G-actin binding to MRTF-A was not inhibited. We coupled Sepharose with CCG-1423 (CCG-1423 Sepharose) to investigate this mechanism. A pull-down assay using CCG-1423 Sepharose revealed the direct binding of CCG-1423 to MRTF-A. Furthermore, we found that the N-terminal basic domain (NB) of MRTF-A, which acts as a functional nuclear localization signal (NLS) of MRTF-A, was the binding site for CCG-1423. G-actin did not bind to CCG-1423 Sepharose, but the interaction between MRTF-A and CCG-1423 Sepharose was reduced in the presence of G-actin. We attribute this result to the high binding affinity of MRTF-A for G-actin and the proximity of NB to G-actin-binding sites (RPEL motifs). Therefore, when MRTF-A forms a complex with G-actin, the binding of CCG-1423 to NB is expected to be blocked. NF-E2 related factor 2, which contains three distinct basic amino acid-rich NLSs, did not bind to CCG-1423 Sepharose, but other RPEL-containing proteins such as MRTF-B, myocardin, and Phactr1 bound to CCG-1423 Sepharose. These results suggest that the specific binding of CCG-1423 to the NLSs of RPEL-containing proteins. Our proposal to explain the inhibitory action of CCG-1423 is as follows: When the G-actin pool is depleted, CCG-1423 binds specifically to the NLS of MRTF-A/B and prevents the interaction between MRTF-A/B and importin α/β1, resulting in inhibition of the nuclear import of MRTF-A/B.
PLoS ONE 01/2014; 9(2):e89016. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myocardin-related transcription factors (MRTFs) are robust coactivators of serum response factor (SRF). MRTFs contain three copies of the RPEL motif at their N-terminus, and they bind to monomeric globular actin (G-actin). Previous studies illustrate that G-actin binding inhibits MRTF activity by preventing the MRTFs nuclear accumulation. In the living cells, the majority of G-actin is sequestered by G-actin binding proteins that prevent spontaneous actin polymerization. Here, we demonstrate that the most abundant G-actin sequestering protein thymosin-β4 (Tβ4) was involved in the regulation of subcellular localization and activity of MRTF-A. Tβ4 competed with MRTF-A for G-actin binding; thus, interfering with G-actin-MRTF-A complex formation. Tβ4 overexpression induced the MRTF-A nuclear accumulation and activation of MRTF-SRF signaling. The activation rate of MRTF-A by the Tβ4 mutant L17A, whose affinity for G-actin is very low, was lower than that by wild-type Tβ4. In contrast, the β-actin mutant 3DA, which has a lower affinity for Tβ4, more weakly suppressed MRTF-A activity than wild-type β-actin. Furthermore, ectopic Tβ4 increased the endogenous expression of SRF-dependent actin cytoskeletal genes. Thus, Tβ4 is an important MRTF regulator that controls the G-actin-MRTFs interaction.
Biochemical and Biophysical Research Communications 06/2013; · 2.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myocardin (Mycd) family members function as a transcriptional cofactor for serum response factor (SRF). Dimer formation is necessary to exhibit their function, and the coiled-coil domain (CC) plays a critical role in their dimerization. We have recently revealed a detailed molecular mechanism for their Crm1 (exportin1)-mediated nuclear export. Here, we found other unique significances of the dimerization of Mycd family members. Introduction of mutations in the CC of myocardin-related transcription factor A (MRTF-A) and truncated Mycd resulted in significant decreases in their cytoplasmic localization and increases in their nuclear localization. In accordance with such subcellular localization changes, their binding to Crm1 were reduced. These results indicate that the dimerization of Mycd family members is necessary for their Crm1-mediated nuclear export. We have recently found that the N-terminal region of Mycd consisting of 128 amino acids (Mycd N128) self-associates to Mycd via the central basic domain (CB), resulting in masking the Crm1-binding site. Such self-association of MRTF-A would be unlikely. In this study, we also revealed that the dimerization of Mycd was also necessary for this self-association. Wild-type Mycd activated SRF-mediated transcription more potently than Mycd lacking the Mycd N128 (Mycd ΔN128) did. These results suggest two possible functions of the Mycd N128: 1) stabilization of Mycd dimer to enhance SRF-mediated transcription and 2) positive regulation of the transactivation ability of Mycd. These findings provide a new insight into the functional regulation of Mycd family members.
Cell Structure and Function 04/2013; · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Myocardin (Mycd), a key factor for the smooth muscle cell differentiation, is constitutively located in the nucleus, whereas myocardin-related transcription factors A and B (MRTF-A/B) mostly reside in the cytoplasm and translocate to the nucleus in a Rho-dependent manner. Here, we investigated the nuclear export of Mycd family members. They possess two leucine (L)-rich sequences (L1 and L2): L1 in the N-terminus and L2 in the Q-rich domain. Although the L2, but not the L1, served as a Crm1-binding site of Mycd, the Crm1-mediated nuclear export did not affect its subcellular localization. Serum response factor (SRF) competitively inhibited the Mycd-Crm1 interaction. Further, such interaction was autonomously inhibited. The N-terminus of Mycd bound intramolecularly to Mycd, resulting in masking the L2. In contrast, the Crm1-binding affinity of MRTF-A was much higher than that of Mycd, because both the L1 and the L2 of MRTF-A served as the functional Crm1-binding sites and such autoinhibition observed in the Mycd-Crm1 interaction was absent in the MRTF-A-Crm1 interaction. Additionally, since the SRF-binding affinity of MRTF-A was lower than that of Mycd, the inhibitory effect by SRF on the MRTF-A-Crm1 interaction was weak. Thus, MRTF-A is much more likely to be exported from the nucleus. These differences would be one reason for a distinct subcellular localization of Mycd and MRTF-A.
Journal of Biological Chemistry 01/2013; · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Glucocorticoids (GCs) mediate the effects of stress to cause structural plasticity in brain regions such as the hippocampus, including simplification of dendrites and shrinkage of dendritic spines. However, the molecular mechanics linking stress and GCs to these effects remain largely unclear. Here, we demonstrated that corticosterone (CORT) reduces the expression levels of caldesmon (CaD), causing dendritic spines to become vulnerable. CaD regulates cell motility by modulating the actin-myosin system and actin filament stability. In cultured rat hippocampal neurons, CaD localized to dendritic spines by binding to filamentous actin (F-actin), and CaD expression levels increased during spine development. CaD stabilized the F-actin dynamics in spines, thereby enlarging the spine heads, whereas CaD knockdown decreased the spine-head size via destabilization of the F-actin dynamics. CaD was also required for chemical LTP-induced actin stabilization. The CaD expression levels were markedly decreased by exposure to CORT mediated by suppression of serum response factor-dependent transcription. High CORT levels reduced both the spine-head size and F-actin stability similarly to CaD knockdown, and overexpressing CaD abolished the detrimental effect of CORT on dendritic spine development. These results indicate that CaD enlarges the spine-head size by stabilizing F-actin dynamics, and that CaD is a critical target in the GC-induced detrimental effects on dendritic spine development.
Journal of Neuroscience 10/2012; 32(42):14583-91. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To begin the process of forming neural circuits, new neurons first establish their polarity and extend their axon. Axon extension is guided and regulated by highly coordinated cytoskeletal dynamics. Here we demonstrate that in hippocampal neurons, the actin-binding protein caldesmon accumulates in distal axons, and its N-terminal interaction with myosin II enhances axon extension. In cortical neural progenitor cells, caldesmon knockdown suppresses axon extension and neuronal polarity. These results indicate that caldesmon is an important regulator of axon development.
Journal of Biological Chemistry 12/2011; 287(5):3349-56. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two members of the myocardin protein family, myocardin-related transcription factor (MRTF)-A and MRTF-B are co-activators of serum response factor (SRF). We recently reported that MRTF-A/B activates the transcription of several actin cytoskeletal/focal adhesion genes SRF dependently, thereby enhancing the formation of stress fibers and focal adhesions. Here, we showed that the levels of caldesmon and tropomyosin, both SRF/MRTF-regulated actin cytoskeletal proteins, were reduced in rat intestinal epithelial (RIE) cell lines that had been transformed with oncogenic ras (RIE-ras) or src (RIE-src) compared with their parental cell line. These cells exhibited morphological abnormalities associated with a disorganized actin cytoskeleton. The serum-stimulated nuclear translocation of MRTF-A/B was suppressed in the RIE-ras and RIE-src cells. However, the transient expression of constitutively active (CA) MRTF-A or MRTF-B reversed the reduced expression levels of caldesmon and tropomyosin and the associated morphological phenotypes. We isolated stable CA-MRTF-A-expressing cell lines from transfected RIE-ras and RIE-src cells and found that their levels of caldesmon and tropomyosin were close to those of untransformed RIE cells. Their morphologies were also normal, with a flattened cell shape and well-developed stress fibers. The CA-MRTF-A-expressing RIE-ras and RIE-src lines also showed lower invasiveness and anchorage-independent growth than their transformed parental cells, in vitro. In vivo, CA-MRTF-A expression suppressed tumor formation and reduced liver metastases. Therefore, we concluded that MRTF-A/B are potent repressors of cancer progression and metastasis and may be good targets for cancer therapy.
[Show abstract][Hide abstract] ABSTRACT: Myocardin is an important transcriptional regulator in smooth and cardiac muscle development. We noticed that the expression of myocardin was markedly downregulated in human uterine leiomyosarcoma cells. Restoration of myocardin expression induced the reexpression of smooth muscle marker proteins and the formation of well-developed actin fibers. A concomitant increase in the expression of a cyclin-dependent kinase inhibitor, p21, led to significantly reduced cell proliferation, via p21's inhibition of the G(1)-S transition. A p21 promoter-reporter assay showed that myocardin markedly increased p21's promoter activity. Furthermore, a serum response factor (SRF)-binding cis-element CArG box in the p21 promoter region was required for this myocardin effect. Chromatin immunoprecipitation and DNA-protein binding assays showed that myocardin indirectly bound to the CArG box in the p21 promoter through the interaction with SRF. Furthermore, immunohistochemistry revealed that the levels of myocardin and p21 were both lower in leiomyosarcoma samples than in normal smooth muscle tissue. Taken together, our results indicate that the downregulation of myocardin expression facilitates cell cycle progression via the reduction of p21 expression in human leimyosarcomas and suggest that myocardin could be a useful therapeutic target for this disease.
Cancer Research 01/2010; 70(2):501-11. · 9.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mammalian target of rapamycin (mTOR) is an important regulator of neuronal development and functions. Although it was reported recently that mTOR signaling is critical for neuronal polarity, the underlying mechanism remains unclear. Here, we describe the molecular pathway of mTOR-dependent axon specification, in which the collapsing response mediator protein 2 (CRMP2) and Tau are major downstream targets. The activity of mTOR effector 70-kDa ribosomal protein S6 kinase (p70S6K) specifically increases in the axon during neuronal polarity formation. The mTOR inhibitor rapamycin suppresses the translation of some neuronal polarity proteins, including CRMP2 and Tau, thereby inhibiting axon formation. In contrast, constitutively active p70S6K up-regulates the translation of these molecules, thus inducing multiple axons. Exogenous CRMP2 and Tau facilitate axon formation, even in the presence of rapamycin. In the 5'-untranslated region of Tau and CRMP2 mRNAs, we identified a 5'-terminal oligopyrimidine tract, which mediates mTOR-governed protein synthesis. The 5'-terminal oligopyrimidine tract sequences of CRMP2 and Tau mRNAs strongly contribute to the up-regulation of their translation in the axon in response to the axonal activation of the mTOR-p70S6K pathway. Taken together, we conclude that the local translation of CRMP2 and Tau, regulated by mTOR-p70S6K, is critical for the specification of neuronal polarity.
Journal of Biological Chemistry 08/2009; 284(40):27734-45. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Glucocorticoids, the most downstream effectors of the hypothalamus-pituitary-adrenal axis, are one of main mediators of the stress reaction. Indeed, exposure to high levels of stress-triggered glucocorticoids is detrimental to brain development associated with abnormal behaviors in experimental animals and the risk of psychiatric disorders in humans. Despite the wealth of this knowledge, the cellular and molecular mechanisms underlying the detrimental effects of glucocorticoids on brain development remain unclear. Here, we show that excess glucocorticoids retard the radial migration of post-mitotic neurons during the development of the cerebral cortex, and identify an actin regulatory protein, caldesmon, as the glucocorticoids' main target. The upregulation of caldesmon expression is mediated by glucocorticoid receptor-dependent transcription of the CALD1 gene encoding caldesmon. This upregulated caldesmon negatively controls the function of myosin II, leading to changes in cell shape and migration. The depletion of caldesmon in vivo impairs radial migration. The overexpression of caldesmon also causes delayed radial migration during cortical development, mimicking the excessive glucocorticoid-induced retardation of radial migration. We conclude that an appropriate range of caldesmon expression is critical for radial migration, and that its overexpression induced by excess glucocorticoid retards radial migration during cortical development. Thus, this study provides a novel insight into the underlying mechanism of glucocorticoid-related neurodevelopmental disorders.
[Show abstract][Hide abstract] ABSTRACT: Glucocorticoids (GCs) play important roles in numerous cellular processes, including growth, development, homeostasis, inhibition of inflammation, and immunosuppression. Here we found that GC-treated human lung carcinoma A549 cells exhibited the enhanced formation of the thick stress fibers and focal adhesions, resulting in suppression of cell migration. In a screen for GC-responsive genes encoding actin-interacting proteins, we identified caldesmon (CaD), which is specifically up-regulated in response to GCs. CaD is a regulatory protein involved in actomyosin-based contraction and the stability of actin filaments. We further demonstrated that the up-regulation of CaD expression was controlled by glucocorticoid receptor (GR). An activated form of GR directly bound to the two glucocorticoid-response element-like sequences in the human CALD1 promoter and transactivated the CALD1 gene, thereby up-regulating the CaD protein. Forced expression of CaD, without GC treatment, also enhanced the formation of thick stress fibers and focal adhesions and suppressed cell migration. Conversely, depletion of CaD abrogated the GC-induced phenotypes. The results of this study suggest that the GR-dependent up-regulation of CaD plays a pivotal role in regulating cell migration via the reorganization of the actin cytoskeleton.
Journal of Biological Chemistry 10/2008; 283(45):31183-96. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epithelial-mesenchymal transition (EMT) is a critical process occurring during embryonic development and in fibrosis and tumor progression. Dissociation of cell-cell contacts and remodeling of the actin cytoskeleton are major events of the EMT. Here, we show that myocardin-related transcription factors (MRTFs; also known as MAL and MKL) are critical mediators of transforming growth factor beta (TGF-beta) 1-induced EMT. In all epithelial cell lines examined here, TGF-beta1 triggers the nuclear translocation of MRTFs. Ectopic expression of constitutive-active MRTF-A induces EMT, whereas dominant-negative MRTF-A or knockdown of MRTF-A and -B prevents the TGF-beta1-induced EMT. MRTFs form complexes with Smad3. Via Smad3, the MRTF-Smad3 complexes bind to a newly identified cis-element GCCG-like motif in the promoter region of Canis familiaris and the human slug gene, which activates slug transcription and thereby dissociation of cell-cell contacts. MRTFs also increase the expression levels of actin cytoskeletal proteins via serum response factor, thereby triggering reorganization of the actin cytoskeleton. Thus, MRTFs are important mediators of TGF-beta1-induced EMT.
The Journal of Cell Biology 01/2008; 179(5):1027-42. · 10.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: RhoA is a crucial regulator of stress fiber and focal adhesion formation through the activation of actin nucleation and polymerization. It also regulates the nuclear translocation of myocardin-related transcription factor-A and -B (MRTF-A/B, MAL or MKL 1/2), which are co-activators of serum response factor (SRF). In dominant-negative MRTF-A (DN-MRTF-A)-expressing NIH 3T3 cell lines, the expressions of several cytoskeletal/focal adhesion genes were down-regulated, and the formation of stress fiber and focal adhesion was severely diminished. MRTF-A/B-knockdown cells also exhibited such cytoskeletal defects. In reporter assays, both RhoA and MRTF-A enhanced promoter activities of these genes in a CArG-box-dependent manner, and DN-MRTF-A inhibited the RhoA-mediated activation of these promoters. In dominant-negative RhoA (RhoA-N19)-expressing NIH 3T3 cell lines, the nuclear translocation of MRTF-A/B was predominantly prevented, resulting in the reduced expression of cytoskeletal/focal adhesion proteins. Further, constitutive-active MRTF-A/B increased the expression of endogenous cytoskeletal/focal adhesion proteins, and thereby rescued the defective phenotype of stress fibers and focal adhesions in RhoA-N19 expressing cells. These results indicate that MRTF-A/B act as pivotal mediators of stress fiber and focal adhesion formation via the transcriptional regulation of a subset of cytoskeletal/focal adhesion genes.
Experimental Cell Research 11/2007; 313(16):3432-45. · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The podosome and invadopodium are dynamic cell-adhesion structures that degrade the extracellular matrix (ECM) and promote cell invasion. We recently reported that the actin-binding protein caldesmon is a pivotal regulator of podosome formation. Here, we analyzed the caldesmon's involvement in podosome/invadopodium-mediated invasion by transformed and cancer cells. The ectopic expression of caldesmon reduced the number of podosomes/invadopodia and decreased the ECM degradation activity, resulting in the suppression of cell invasion. Conversely, the depletion of caldesmon facilitated the formation of podosomes/invadopodia and cell invasion. Taken together, our results indicate that caldesmon acts as a potent repressor of cancer cell invasion.
[Show abstract][Hide abstract] ABSTRACT: Podosomes are dynamic cell adhesion structures that degrade the extracellular matrix, permitting extracellular matrix remodeling. Accumulating evidence suggests that actin and its associated proteins play a crucial role in podosome dynamics. Caldesmon is localized to the podosomes, and its expression is down-regulated in transformed and cancer cells. Here we studied the regulatory mode of caldesmon in podosome formation in Rous sarcoma virus-transformed fibroblasts. Exogenous expression analyses revealed that caldesmon represses podosome formation triggered by the N-WASP-Arp2/3 pathway. Conversely, depletion of caldesmon by RNA interference induces numerous small-sized podosomes with high dynamics. Caldesmon competes with the Arp2/3 complex for actin binding and thereby inhibits podosome formation. p21-activated kinases (PAK)1 and 2 are also repressors of podosome formation via phosphorylation of caldesmon. Consequently, phosphorylation of caldesmon by PAK1/2 enhances this regulatory mode of caldesmon. Taken together, we conclude that in Rous sarcoma virus-transformed cells, changes in the balance between PAK1/2-regulated caldesmon and the Arp2/3 complex govern the formation of podosomes.
Journal of Biological Chemistry 04/2007; 282(11):8454-63. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A 70-year-old man presenting with a chief complaint of tongue swelling had been diagnosed with prostate cancer 1 year earlier. He had been on an oral angiotensin-converting enzyme inhibitor (ACE) inhibitor for hypertension for 20 years. Two months before the first of 4 episodes of tongue swelling within a period of 40 days, he had been prescribed oral estramustine phosphate (EMP) for the prostate cancer. He was admitted to our hospital for the evaluation after massive swelling of the tongue and epiglottis which necessitated tracheotomy. Food allergies, allergic reactions to environmental factors, and hereditary angioneurotic edema were excluded. Massive swelling of the tongue and epiglottis disappeared completely after EMP was discontinued. We concluded that angioedema was induced by EMP used concurrently with the ACE inhibitor.
Journal of investigational allergology & clinical immunology: official organ of the International Association of Asthmology (INTERASMA) and Sociedad Latinoamericana de Alergia e Inmunología 02/2006; 16(6):388-90. · 1.89 Impact Factor