The homeodomain protein Barx2 promotes myogenic differentiation and is regulated by myogenic regulatory factors

Effect of Zbed6 Single-Allele Knockout on the Growth and Development of Skeletal Muscle in Mice

ZBED6, a key transcription factor, plays an important role in skeletal muscle and organ growth. ZBED6 knockout (ZBED6^-/-) leads to the upregulation of IGF2 in pig and mice muscle, thereby increasing muscle mass. However, the effects and mechanism of Zbed6 single-allele knockout (Zbed6^+/-) on mice muscle remain unknown. Here, we reported that Zbed6^+/- promotes muscle growth by a new potential target gene rather than Igf2 in mice muscle. Zbed6^+/- mice showed markedly higher muscle mass (25%) and a markedly higher muscle weight ratio (18%) than wild-type (WT) mice, coinciding with a larger muscle fiber area (28%). Despite a significant increase in muscle growth, Zbed6^+/- mice showed similar Igf2 expression with WT mice, indicating that a ZBED6-Igf2-independent regulatory pathway exists in Zbed6^+/- mice muscle. RNA-seq of muscle between the Zbed6^+/- and WT mice revealed two terms related to muscle growth. Overlapping the DEGs and C2C12 Chip-seq data of ZBED6 screened out a potential ZBED6 target gene Barx2, which may regulate muscle growth in Zbed6^+/- mice. These results may open new research directions leading to a better understanding of the integral functions of ZBED6 and provide evidence of Zbed6^+/- promoting muscle growth by regulating Barx2 in mice.

BARX2/FOXA1/HK2 axis promotes lung adenocarcinoma progression and energy metabolism reprogramming

BACKGROUND

Metabolic reprogramming is an emerging cancer feature that has recently drawn special attention since it promotes tumor cell growth and proliferation. However, the mechanism of the Warburg effect is still largely unknown. This research aimed to reveal the effects of BarH-like homeobox 2 (BARX2) in regulating tumor progression and glucose metabolism in lung adenocarcinoma (LUAD).

METHODS

Expression of BARX2 was measured by quantitative real-time polymerase chain reaction (qRT-PCR) in LUAD cell line and tissues, and the tumor-promoting function of BARX2 in LUAD cells was detected in vitro and in vivo xenograft models. The metabolic effects of BARX2 were examined by detecting glucose uptake, the production levels of lactate and pyruvate, and the extracellular acidification rate (ECAR). Chromatin immunoprecipitation (ChIP) assay and luciferase reporter gene assay were used to identify the underlying molecular mechanism of BARX2 regulation of HK2. Further studies showed that transcription factor FOXA1 directly interacts with BARX2 and promotes the transcriptional activity of BARX2.

RESULTS

BARX2 was remarkably up-regulated in LUAD tissues and positively linked to advanced clinical stage and poor prognosis. In vitro and in vivo data indicated ectopic expression of BARX2 enhanced cell proliferation and tumorigenesis, whereas BARX2 knockdown suppressed these effects. Metabolic-related experiments showed BARX2 promoted the reprogramming of glucose metabolism. Mechanistically, the BARX2/FOXA1/HK2 axis promoted LUAD progression and energy metabolism reprogramming.

CONCLUSIONS

In summary, our research first defined BARX2 as a tumor-promoting factor in LUAD and that it may act as a novel prognostic biomarker and new therapeutic target for the disease.

BarH-like homeobox 2 represses the transcription of keratin 16 and affects Ras signaling pathway to suppress nasopharyngeal carcinoma progression

Nasopharyngeal carcinoma (NPC) refers to a malignancy initiating from the superior mucosal epithelium of the nasopharynx. Optimal therapies for NPC are still needed. In this investigation, we attempted to explore whether BarH-like homeobox 2 (BARX2), a well-known tumor suppressor, had anti-cancer properties on NPC, and the possible mechanisms. After searching for NPC-related databases, we determined BARX2 as one of the core genes in NPC. The results of RT-qPCR and immunohistochemistry or Western blot demonstrated that BARX2 was reduced in NPC patients and cells. Ectopic expression of BARX2 reverted the malignant phenotype of NPC cells. Mechanistically, BARX2 bound to the keratin 16 (KRT16) promoter to downregulate its expression. In addition, BARX2 was found to reduce the phosphorylation levels of MEK and ERK. Further KRT16 upregulation in cells overexpressing BARX2 promoted malignant aggressiveness of C666-1 and HNE3 cells and activated the Ras signaling pathway. BARX2 inhibited the growth and metastasis of tumors and suppressed the Ras signaling pathway in vivo. In conclusion, our findings indicate that BARX2 reverts malignant phenotypes of NPC cells by downregulating KRT16 in a Ras-dependent fashion. BARX2 might act as a possible therapeutic regulator for NPC.

Barx2 and Pax7 have antagonistic functions in regulation of wnt signaling and satellite cell differentiation

The canonical Wnt signaling pathway is critical for myogenesis and can induce muscle progenitors to switch from proliferation to differentiation; how Wnt signals integrate with muscle-specific regulatory factors in this process is poorly understood. We previously demonstrated that the Barx2 homeobox protein promotes differentiation in cooperation with the muscle regulatory factor (MRF) MyoD. Pax7, another important muscle homeobox factor, represses differentiation. We now identify Barx2, MyoD, and Pax7 as novel components of the Wnt effector complex, providing a new molecular pathway for regulation of muscle progenitor differentiation. Canonical Wnt signaling induces Barx2 expression in muscle progenitors and perturbation of Barx2 leads to misregulation of Wnt target genes. Barx2 activates two endogenous Wnt target promoters as well as the Wnt reporter gene TOPflash, the latter synergistically with MyoD. Moreover, Barx2 interacts with the core Wnt effectors β-catenin and T cell-factor 4 (TCF4), is recruited to TCF/lymphoid enhancer factor sites, and promotes recruitment of β-catenin. In contrast, Pax7 represses the Wnt reporter gene and antagonizes the activating effect of Barx2. Pax7 also binds β-catenin suggesting that Barx2 and Pax7 may compete for interaction with the core Wnt effector complex. Overall, the data show for the first time that Barx2, Pax7, and MRFs can act as direct transcriptional effectors of Wnt signals in myoblasts and that Barx2 and Wnt signaling participate in a regulatory loop. We propose that antagonism between Barx2 and Pax7 in regulation of Wnt signaling may help mediate the switch from myoblast proliferation to differentiation.

Low expression of BARX2 in human primary hepatocellular carcinoma correlates with metastasis and predicts poor prognosis

AIM

The homeobox gene Barx2 was recently identified as a regulator of ovarian and breast cancer; however, the expression level of BARX2 and its significance in hepatocellular carcinoma (HCC) remain unknown.

METHODS

Protein and mRNA expression levels of Barx2 were examined using Western blotting and real-time PCR respectively, in paired HCC tissue and matched adjacent non-cancerous tissue from 12 patients. The expression levels of epithelial-mesenchymal transition (EMT) markers were also detected in relation to BARX2 expression. Lastly, immunohistochemistry for BARX2 was also performed on a tissue microarray containing 231 HCC tissue samples.

RESULTS

We observed that BARX2 expression was lower in HCC tissues compared to matching adjacent non-cancerous tissue. The low expression level of BARX2 was significantly correlated with metrics of tumor size, tumor differentiation, clinical stage, metastasis and relapse. Furthermore, the patients with low BARX2 expression had adverse survival outcomes. Importantly, multivariate Cox regression analysis revealed that low BARX2 expression was an independent marker for lower overall survival (P = 0.007). Moreover, a significant negative relationship was observed between the expression of BARX2 and markers of EMT.

CONCLUSION

These findings provide evidence that the low expression level of BARX2 in HCC is significantly correlated with tumor metastasis, and that BARX2 may be an independent prognostic biomarker for patients with HCC.

HES6 enhances the motility of alveolar rhabdomyosarcoma cells

HES6, a member of the hairy-enhancer-of-split family of transcription factors, plays multiple roles in myogenesis. It is a direct target of the myogenic transcription factor MyoD and has been shown to regulate the formation of the myotome in development, myoblast cell cycle exit and the organization of the actin cytoskeleton during terminal differentiation. Here we investigate the expression and function of HES6 in rhabdomyosarcoma, a soft tissue tumor which expresses myogenic genes but fails to differentiate into muscle. We show that HES6 is expressed at high levels in the subset of alveolar rhabdomyosarcomas expressing PAX/FOXO1 fusion genes (ARMSp). Knockdown of HES6 mRNA in the ARMSp cell line RH30 reduces proliferation and cell motility. This phenotype is rescued by expression of mouse Hes6 which is insensitive to HES6 siRNA. Furthermore, expression microarray analysis indicates that the HES6 knockdown is associated with a decrease in the levels of Transgelin, (TAGLN), a regulator of the actin cytoskeleton. Knockdown of TAGLN decreases cell motility, whilst TAGLN overexpression rescues the motility defect resulting from HES6 knockdown. These findings indicate HES6 contributes to the pathogenesis of ARMSp by enhancing both proliferation and cell motility.

Barx2 is expressed in satellite cells and is required for normal muscle growth and regeneration

Muscle growth and regeneration are regulated through a series of spatiotemporally dependent signaling and transcriptional cascades. Although the transcriptional program controlling myogenesis has been extensively investigated, the full repertoire of transcriptional regulators involved in this process is far from defined. Various homeodomain transcription factors have been shown to play important roles in both muscle development and muscle satellite cell-dependent repair. Here, we show that the homeodomain factor Barx2 is a new marker for embryonic and adult myoblasts and is required for normal postnatal muscle growth and repair. Barx2 is coexpressed with Pax7, which is the canonical marker of satellite cells, and is upregulated in satellite cells after muscle injury. Mice lacking the Barx2 gene show reduced postnatal muscle growth, muscle atrophy, and defective muscle repair. Moreover, loss of Barx2 delays the expression of genes that control proliferation and differentiation in regenerating muscle. Consistent with the in vivo observations, satellite cell-derived myoblasts cultured from Barx2(-/-) mice show decreased proliferation and ability to differentiate relative to those from wild-type or Barx2(+/-) mice. Barx2(-/-) myoblasts show reduced expression of the differentiation-associated factor myogenin as well as cell adhesion and matrix molecules. Finally, we find that mice lacking both Barx2 and dystrophin gene expression have severe early onset myopathy. Together, these data indicate that Barx2 is an important regulator of muscle growth and repair that acts via the control of satellite cell proliferation and differentiation.

A genome-wide association search for type 2 diabetes genes in African Americans

African Americans are disproportionately affected by type 2 diabetes (T2DM) yet few studies have examined T2DM using genome-wide association approaches in this ethnicity. The aim of this study was to identify genes associated with T2DM in the African American population. We performed a Genome Wide Association Study (GWAS) using the Affymetrix 6.0 array in 965 African-American cases with T2DM and end-stage renal disease (T2DM-ESRD) and 1029 population-based controls. The most significant SNPs (n = 550 independent loci) were genotyped in a replication cohort and 122 SNPs (n = 98 independent loci) were further tested through genotyping three additional validation cohorts followed by meta-analysis in all five cohorts totaling 3,132 cases and 3,317 controls. Twelve SNPs had evidence of association in the GWAS (P<0.0071), were directionally consistent in the Replication cohort and were associated with T2DM in subjects without nephropathy (P<0.05). Meta-analysis in all cases and controls revealed a single SNP reaching genome-wide significance (P<2.5×10(-8)). SNP rs7560163 (P = 7.0×10(-9), OR (95% CI) = 0.75 (0.67-0.84)) is located intergenically between RND3 and RBM43. Four additional loci (rs7542900, rs4659485, rs2722769 and rs7107217) were associated with T2DM (P<0.05) and reached more nominal levels of significance (P<2.5×10(-5)) in the overall analysis and may represent novel loci that contribute to T2DM. We have identified novel T2DM-susceptibility variants in the African-American population. Notably, T2DM risk was associated with the major allele and implies an interesting genetic architecture in this population. These results suggest that multiple loci underlie T2DM susceptibility in the African-American population and that these loci are distinct from those identified in other ethnic populations.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

The homeobox transcription factor Barx2 regulates plasticity of young primary myofibers

BACKGROUND

Adult mammalian muscle retains incredible plasticity. Muscle growth and repair involves the activation of undifferentiated myogenic precursors called satellite cells. In some circumstances, it has been proposed that existing myofibers may also cleave and produce a pool of proliferative cells that can re-differentiate into new fibers. Such myofiber dedifferentiation has been observed in the salamander blastema where it may occur in parallel with satellite cell activation. Moreover, ectopic expression of the homeodomain transcription factor Msx1 in differentiated C2C12 myotubes has been shown to induce their dedifferentiation. While it remains unclear whether dedifferentiation and redifferentiaton occurs endogenously in mammalian muscle, there is considerable interest in induced dedifferentiation as a possible regenerative tool.

METHODOLOGY/PRINCIPAL FINDINGS

We previously showed that the homeobox protein Barx2 promotes myoblast differentiation. Here we report that ectopic expression of Barx2 in young immature myotubes derived from cell lines and primary mouse myoblasts, caused cleavage of the syncytium and downregulation of differentiation markers. Microinjection of Barx2 cDNA into immature myotubes derived from primary cells led to cleavage and formation of mononucleated cells that were able to proliferate. However, injection of Barx2 cDNA into mature myotubes did not cause cleavage. Barx2 expression in C2C12 myotubes increased the expression of cyclin D1, which may promote cell cycle re-entry. We also observed differential muscle gene regulation by Barx2 at early and late stages of muscle differentiation which may be due to differential recruitment of transcriptional activator or repressor complexes to muscle specific genes by Barx2.

CONCLUSIONS/SIGNIFICANCE

We show that Barx2 regulates plasticity of immature myofibers and might act as a molecular switch controlling cell differentiation and proliferation.

Barx2 controls myoblast fusion and promotes MyoD-mediated activation of the smooth muscle alpha-actin gene

Remodeling of the actin cytoskeleton is a critical early step in skeletal muscle differentiation. Smooth muscle alpha-actin (SMA) is one of the earliest markers of myoblast differentiation and is important for the migration and cell shape changes that precede fusion. We have found that satellite cell-derived primary myoblasts from mice lacking the Barx2 homeobox gene show altered patterns of actin remodeling, reduced cell migration, and delayed differentiation. Consistent with the role of SMA in these processes, Barx2(-)(/)(-) myoblasts also show reduced expression of SMA mRNA and protein. The proximal SMA promoter contains binding sites for muscle regulatory factors and serum response factor as well as a conserved homeodomain binding site (HBS). We found that Barx2 binds to the HBS element and potentiates up-regulation of SMA promoter activity by MyoD. We also show that Barx2, MyoD, and serum response factor simultaneously occupy the SMA promoter in cells and that Barx2 interacts with MyoD. Overall these data indicate that Barx2 cooperates with other muscle-expressed transcription factors to regulate the early cytoskeletal remodeling events that underlie efficient myoblast differentiation.

BARX2 and estrogen receptor-alpha (ESR1) coordinately regulate the production of alternatively spliced ESR1 isoforms and control breast cancer cell growth and invasion

The estrogen receptor-alpha gene (ESR1) was previously identified as a direct target of the homeobox transcription factor BARX2 in MCF7 cells. Here, we show that BARX2 and ESR1 proteins bind to different ESR1 gene promoters and regulate the expression of alternatively spliced mRNAs that encode 66 and 46 kDa ESR1 protein isoforms. BARX2 increases the expression of both ESR1 isoforms; however, it has a greater effect on the 46 kDa isoform, leading to an increased ratio between the 46 and 66 kDa proteins. BARX2 also influences estrogen-dependent processes such as anchorage-independent growth and modulates the expression of the estrogen-responsive genes SOX5, RBM15, Dynein and Mortalin. In addition, BARX2 expression promotes cellular invasion and increases the expression of active matrix metalloproteinase-9 (MMP9). BARX2 also increases the expression of the tissue inhibitor of metalloproteinase (TIMP) genes, TIMP1 and TIMP3, in cooperation with estrogen signaling. Overall, these data indicate that BARX2 and ESR1 may coordinately regulate cell growth, survival and invasion pathways that are critical to breast cancer progression.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

The homeobox transcription factor Barx2 regulates chondrogenesis during limb development

Among the many factors involved in regulation of chondrogenesis, bone morphogenetic proteins (BMPs) and members of the Sox and homeobox transcription factor families have been shown to have crucial roles. Of these regulators, the homeobox transcription factors that function during chondrogenesis have been the least well defined. We show here that the homeobox transcription factor Barx2 is expressed in primary mesenchymal condensations, digital rays, developing joints and articular cartilage of the developing limb, suggesting that it plays a role in chondrogenesis. Using retroviruses and antisense oligonucleotides to manipulate Barx2 expression in limb bud micromass cultures, we determined that Barx2 is necessary for mesenchymal aggregation and chondrogenic differentiation. In accordance with these findings, Barx2 regulates the expression of several genes encoding cell-adhesion molecules and extracellular matrix proteins, including NCAM and collagen II (Col2a1) in the limb bud. Barx2 bound to elements within the cartilage-specific Col2a1 enhancer, and this binding was reduced by addition of Barx2 or Sox9 antibodies, or by mutation of a HMG box adjacent to the Barx2-binding element, suggesting cooperation between Barx2 and Sox proteins. Moreover, both Barx2 and Sox9 occupy Col2a1 enhancer during chondrogenesis in vivo. We also found that two members of the BMP family that are crucial for chondrogenesis, GDF5 and BMP4, regulate the pattern of Barx2 expression in developing limbs. Based on these data, we suggest that Barx2 acts downstream of BMP signaling and in concert with Sox proteins to regulate chondrogenesis.

Barx2 functions through distinct corepressor classes to regulate hair follicle remodeling

The hair-growth cycle, a complex biological system requiring coordinate alterations in gene expression and cellular behavior, provides a challenging model for investigating the interplay of specific transcriptional regulation events. Here we report that the Barx2 homeodomain factor serves as a regulator of hair follicle remodeling (catagen), and loss of Barx2 in mice causes a defect both in the initiation and progression of catagen, resulting in a protracted first catagen, and later, causing short hair in adult gene-deleted mice. Barx2 negatively regulates its own promoter, and our study highlights the role of Barx2 as a repressor in the skin that can, unexpectedly, functionally interact with two WD40-domain factors distantly related to the yeast corepressor Tup1. These two corepressors, transducin-like enhancer of split and transducin beta-like 1, function through distinct and independent interactions with Barx2 for the repression of gene targets, including the Barx2 gene itself, emphasizing the roles of complementary repression strategies in engrailed homology-1 motif-containing homeodomain factors. Together, our data suggest that the hair-remodeling defect of Barx2 mutant mice could be explained, in part, by failure to repress one or more critical target genes.

Regulation of hepatic stellate cell activation and growth by transcription factor myocyte enhancer factor 2

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcriptional regulation plays a key role in this process. We studied the role of transcription factor myocyte enhancer factor 2 (MEF2) during HSC activation.

METHODS

Culture of HSCs isolated from rat liver on plastic dishes and HSC-T6 on a basement membrane-like matrix were used as models of HSC activation and deactivation, respectively. The expression and activity of MEF2 were correlated with HSC activation. The roles of MEF2 during HSC activation were assessed in vitro and in vivo by animal models of fibrosis.

RESULTS

Early induction of MEF2 messenger RNA and protein accompanied culture-induced HSC activation. This was associated with enhanced MEF2 DNA binding and transactivation activity. p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase pathway was required for increased MEF2 activity during HSC activation. Increased MEF2 protein also correlated with fibrosis in vivo. Reversal of HSC activation was paralleled by a marked decrease in MEF2 protein and activity. Functionally, enhancing MEF2 significantly increased the expression of alpha-smooth muscle actin (alpha-SMA), activated collagen I promoter activity, and stimulated HSC proliferation. MEF2 interference RNA significantly inhibited expression of alpha-SMA, collagen alpha1(I), and proliferating cell nuclear antigen.

CONCLUSIONS

The studies provide the first evidence for the presence of MEF2 in the liver and show that MEF2 regulates multiple aspects of HSC activation. These studies show a novel role of MEF2 as a key nuclear mediator that may participate in the pathologic process of liver fibrogenesis in vivo.

Homeobox genes in pulmonary vascular development and disease

Determining how the pulmonary vascular system is formed, maintained, or disrupted during development and disease represents a major challenge in contemporary lung biology. Whereas it is appreciated that cellular proliferation, differentiation, migration, and apoptosis need to be carefully controlled in order to attain pulmonary vascular homeostasis, knowledge of the underlying cellular and molecular mechanisms involved remains surprisingly limited. Because homeobox genes represent master regulators of organogenesis and tissue patterning, it is likely that these transcription factors play a critical role in the formation of blood vessels within the lung, as well as in pathologic states in which the highly ordered structure of the pulmonary vascular tree is compromised. The aim of this review is to discuss some of the known functions of homeobox genes in the vasculature, and to extrapolate these findings to their potential roles in developing and diseased pulmonary vessels.

Identification of novel binding elements and gene targets for the homeodomain protein BARX2

BARX2 is a homeobox transcription factor that influences cellular differentiation in various developmental contexts. To begin to identify the gene targets that mediate its effects, chromatin immunoprecipitation (ChIP) was used to isolate BARX2 binding sites from the human MCF7 breast cancer cell line. Cloning and sequencing of BARX2-ChIP-derived DNA fragments identified 60 potential BARX2 target loci that were proximal to or within introns of genes involved in cytoskeletal organization, cell adhesion, growth factor signaling, transcriptional regulation, and RNA metabolism. The sequences of over half of the fragments showed homology with the mouse genome, and several sequences could be mapped to orthologous human and mouse genes. Binding of BARX2 to 21 genomic loci examined was confirmed quantitatively by replicate ChIP assays. A combination of sequence analysis and electrophoretic mobility shift assays revealed homeodomain binding sites within several fragments that bind to BARX2 in vitro. The majority of BARX2 binding fragments tested (14/19), also affected transcription in luciferase reporter gene assays. Mutation analyses of three fragments showed that their transcriptional activities required the HBS, and suggested that BARX2 regulates gene expression by binding to DNA elements containing paired TAAT motifs that are separated by a poly(T) sequence. Inhibition of BARX2 expression in MCF7 cells led to reduced expression of eight genes associated with BARX2 binding sites, indicating that BARX2 directly regulates their expression. The data suggest that BARX2 can coordinate the expression of a network of genes that influence the growth of MCF7 cells.

Megakaryoblastic leukemia-1/2, a transcriptional co-activator of serum response factor, is required for skeletal myogenic differentiation

Serum response factor (SRF) is required for the expression of a wide variety of muscle-specific genes that are expressed upon differentiation and is thus required for both striated and smooth muscle differentiation in addition to its role in regulating growth factor-inducible genes. A heart and smooth muscle-specific SRF co-activator, myocardin, has been shown to be required for cardiac development and smooth muscle differentiation. However, no such co-factors of SRF have been identified in the skeletal myogenic differentiation program. Myocardin and the related transcription factor megakaryoblastic leukemia-1 (MKL1/MAL/MRTF-A) can strongly potentiate the activity of SRF. Here we report the cloning of the third member of the myocardin/MKL family in humans, MKL2. MKL2 binds to and activates SRF similar to myocardin and MKL1. To determine the role of these factors in skeletal myogenic differentiation we used a dominant negative MKL2 to show that the MKL family of proteins is required for skeletal myogenic differentiation. Expression of the dominant negative protein in C2C12 skeletal myoblasts blocked the differentiation-induced expression of the SRF target genes skeletal alpha-actin and alpha-myosin heavy chain and blocked differentiation of the myoblasts to myotubes in vitro. C2C12 cells express both MKL1 and MKL2, but not myocardin, implicating MKL1 and/or MKL2 in the requirement for skeletal myogenic differentiation. MKL1 was predominantly cytoplasmic in C2C12 cells, with a small amount in the nucleus, however, no movement of MKL1 to the nucleus was observed upon differentiation.