Interaction of the heart-specific LIM domain protein, FHL2, with DNA-binding nuclear protein, hNP220

The roles of FHL2 in cancer

LIM domain protein 2, also known as LIM protein FHL2, is a member of the LIM-only family. Due to its LIM domain protein characteristics, FHL2 is capable of interacting with various proteins and plays a crucial role in regulating gene expression, cell growth, and signal transduction in muscle and cardiac tissue. In recent years, mounting evidence has indicated that the FHLs protein family is closely associated with the development and occurrence of human tumors. On the one hand, FHL2 acts as a tumor suppressor by down-regulating in tumor tissue and effectively inhibiting tumor development by limiting cell proliferation. On the other hand, FHL2 serves as an oncoprotein by up-regulating in tumor tissue and binding to multiple transcription factors to suppress cell apoptosis, stimulate cell proliferation and migration, and promote tumor progression. Therefore, FHL2 is considered a double-edged sword in tumors with independent and complex functions. This article reviews the role of FHL2 in tumor occurrence and development, discusses FHL2 interaction with other proteins and transcription factors, and its involvement in multiple cell signaling pathways. Finally, the clinical significance of FHL2 as a potential target in tumor therapy is examined.

Label-Free Mass Spectrometry-Based Proteomic Analysis in Lamb Tissues after Fish Oil, Carnosic Acid, and Inorganic Selenium Supplementation

Simple Summary

Advances in proteomics and bioinformatics analysis offer the potential to investigate nutrients’ influence on protein expression profiles, and consequently on biological processes, molecular functions, and cellular components. However, knowledge in this area, particular about the exact way selenium modulates protein expression, remains limited. Therefore, in this project, global differential proteomic experiments were carried out in order to identify changes in the expression of proteins in animal tissues obtained from lambs on a specific diet involving the addition of a combination of different supplements, namely, inorganic selenium compounds, fish oil, and carnosic acid. Following inorganic selenium supplementation, a protein-protein interaction network analysis of forty differentially-expressed proteins indicated two significant clusters.

Abstract

Selenium is an essential nutrient, building twenty five identified selenoproteins in humans known to perform several important biological functions. The small amount of selenium in the earth’s crust in certain regions along with the risk of deficiency in organisms have resulted in increasingly popular dietary supplementation in animals, implemented via, e.g., inorganic selenium compounds. Even though selenium is included in selenoproteins in the form of selenocysteine, the dietary effect of selenium may result in the expression of other proteins or genes. Very little is known about the expression effects modulated by selenium. The present study aimed to examine the significance of protein expression in lamb tissues obtained after dietary supplementation with selenium (sodium selenate) and two other feed additives, fish oil and carnosic acid. Label-free mass spectrometry-based proteomic analysis was successfully applied to examine the animal tissues. Protein-protein interaction network analysis of forty differently-expressed proteins following inorganic selenium supplementation indicated two significant clusters which are involved in cell adhesion, heart development, actin filament-based movement, plasma membrane repair, and establishment of organelle localization.

Protein-protein interactions of the LIM-only protein FHL2 and functional implication of the interactions relevant in cardiovascular disease

FHL2 belongs to the LIM-domain only proteins and contains four and a half LIM domains, each of which are composed of two zinc finger structures. FHL2 exhibits specific interaction with proteins exhibiting diverse functions, including transmembrane receptors, transcription factors and transcription co-regulators, enzymes, and structural proteins. The function of these proteins is regulated by FHL2, which modulates intracellular signal transduction pathways involved in a plethora of cellular tasks. The present review summarizes the current knowledge on the protein interactome of FHL2 and provides an overview of the functional implication of these interactions in apoptosis, migration, and regulation of nuclear receptor function. FHL2 was originally identified in the heart and there is extensive literature available on the role of FHL2 in the cardiovascular system, which is also summarized in this review.

The FHL2 regulation in the transcriptional circuitry of human cancers

The Four-and-a-half LIM (FHL)-only protein is a subfamily of protein members under the LIM-only protein family. These proteins are identified by their characteristic four and a half cysteinerich LIM homeodomain. Five members have been categorized into the FHL subfamily, which are FHL1, FHL2, FHL3, FHL4 and activator of CREM in testis (ACT) in human. FHL2 is amongst the most examined members within the family. Fhl2, the gene that code for the protein, is transcriptionally regulated by diverse types of transcription factors, for example, p53, serum response factor (SRF), and specificity protein 1 (Sp1). The expression of FHL2 is found in different tissues and organs and has been reported as a critical participant influencing the wide types of cancer such as breast cancer, gastrointestinal (GI) cancers, liver cancer and prostate cancer. The expression profile of FHL2 appeared to have a significant functional role in the carcinogenesis of these cancers which are mediated by different types of transcription factor including both tumor suppressors and inducers. In this review, we will first describe the molecular network governing FHL2 expression, which focus on the transcription factors conveying FHL2-initiated responses. In the second part, FHL2-linked cancers and the underlying molecular machinery will be discussed. Factors other than transcriptional regulation which may involve the cancer progression such as mutations of fhl2 and posttranslational modifications of the protein will also be mentioned.

LIM-only protein FHL2 is a positive regulator of liver X receptors in smooth muscle cells involved in lipid homeostasis

The LIM-only protein FHL2 is expressed in smooth muscle cells (SMCs) and inhibits SMC-rich-lesion formation. To further elucidate the role of FHL2 in SMCs, we compared the transcriptomes of SMCs derived from wild-type (WT) and FHL2 knockout (KO) mice. This revealed that in addition to the previously recognized involvement of FHL2 in SMC proliferation, the cholesterol synthesis and liver X receptor (LXR) pathways are altered in the absence of FHL2. Using coimmunoprecipitation experiments, we found that FHL2 interacts with the two LXR isoforms, LXRα and LXRβ. Furthermore, FHL2 strongly enhances transcriptional activity of LXR element (LXRE)-containing reporter constructs. Chromatin immunoprecipitation (ChIP) experiments on the ABCG1 promoter revealed that FHL2 enhances the association of LXRβ with DNA. In line with these observations, we observed reduced basal transcriptional LXR activity in FHL2-KO SMCs compared to WT SMCs. This was also reflected in reduced expression of LXR target genes in intact aorta and aortic SMCs of FHL2-KO mice. Functionally, the absence of FHL2 resulted in attenuated cholesterol efflux to both ApoA-1 and high-density lipoprotein (HDL), in agreement with reduced LXR signaling. Collectively, our findings demonstrate that FHL2 is a transcriptional coactivator of LXRs and points toward FHL2 being an important determinant of cholesterol metabolism in SMCs.

Identification of FHL2-regulated genes in liver by microarray and bioinformatics analysis

FHL2 is a LIM domain protein that is able to form various protein complexes and regulate gene transcription. Recent findings showed that FHL2 is a potential tumor suppressor gene that was down-regulated in hepatocellular carcinoma. In the present study, microarray profiling of gene expression was performed to identify the genes regulated by FHL2 in mouse livers. The differentially expressed genes were further analyzed by bioinformatics tools including DAVID, KEGG, and STRING. Our data illustrate that FHL2 affects genes involved in various functions including signal transduction, responses to external stimulus, cancer-related pathways, cardiovascular function and regulation of actin cytoskeleton. Moreover, a network of differentially expressed genes identified in this study and known FHL2-interacting proteins was constructed. Then, genes identified by bioinformatics tools and most functional relevant to FHL2 were selected for further validation. Finally, the differential expression of Ar, Id3, Inhbe, Alas1, Bcl6, Pparδ, Angptl4, and Erbb4 were confirmed by quantitative real-time PCR. In summary, we have established a database of genes that are potentially regulated by FHL2 and these genes should be future targets for the elucidation of functional roles of FHL2.

Epithelial expression of FHL2 is negatively associated with metastasis-free and overall survival in colorectal cancer

Background:

Four-and-a-half LIM domains protein 2 (FHL2) is a component of the focal adhesion structures and has been suggested to have a role in cancer progression. It has been shown to be overexpressed in the colorectal cancer (CRC).

Methods:

Here, we examined a possible prognostic value of FHL2 in CRC. Immunohistochemistry for FHL2 was performed on 296 CRCs without distant metastases at the time of surgery. Staining in the epithelial compartment was quantitatively evaluated using image analysis, and results were related to clinical variables. Antibody specificity was tested using small-interfering RNA transfection in hTERT-immortalised myofibroblasts.

Results:

Varying degrees of cytoplasmic FHL2 expression by neoplastic epithelial cells were detectable in all cases. Higher FHL2 expression in the epithelial compartment was an independent adverse prognostic factor. Multivariate Cox analysis shows that expression in the tumour invasion front (P<0.001) as well as in the centre of the tumour (P<0.001) was associated with metachronous metastases independently of the clinicopathological variables; expression in the tumour invasion front was also associated with overall survival independently of the clinicopathological variables (P<0.01).

Conclusion:

Higher FHL2 expression is involved in CRC progression and correlates with the development of metachronous metastases and overall survival, suggesting that FHL2 is an independent adverse prognostic indicator for CRC.

FHL2 interacts with CALM and is highly expressed in acute erythroid leukemia

The t(10;11)(p13;q14) translocation results in the fusion of the CALM (clathrin assembly lymphoid myeloid leukemia protein) and AF10 genes. This translocation is observed in acute myeloblastic leukemia (AML M6), acute lymphoblastic leukemia (ALL) and malignant lymphoma. Using a yeast two-hybrid screen, the four and a half LIM domain protein 2 (FHL2) was identified as a CALM interacting protein. Recently, high expression of FHL2 in breast, gastric, colon, lung as well as in prostate cancer was shown to be associated with an adverse prognosis. The interaction between CALM and FHL2 was confirmed by glutathione S-transferase-pulldown assay and co-immunoprecipitation experiments. The FHL2 interaction domain of CALM was mapped to amino acids 294–335 of CALM. The transcriptional activation capacity of FHL2 was reduced by CALM, but not by CALM/AF10, which suggests that regulation of FHL2 by CALM might be disturbed in CALM/AF10-positive leukemia. Extremely high expression of FHL2 was seen in acute erythroid leukemia (AML M6). FHL2 was also highly expressed in chronic myeloid leukemia and in AML with complex aberrant karyotype. These results suggest that FHL2 may play an important role in leukemogenesis, especially in the case of AML M6.

FHL2 protein is a novel co-repressor of nuclear receptor Nur77

The three members of the NR4A orphan nuclear receptor subfamily Nur77, Nurr1, and NOR-1, regulate a variety of biological functions including vascular disease and metabolism. In this study, we identified Four and a half LIM domains protein-2 (FHL2) as a novel interacting protein of NR4A nuclear receptors by yeast two-hybrid screen and co-immunoprecipitation studies. Each of the four LIM domains of FHL2 can bind Nur77, and both the amino-terminal domain and the DNA binding domain of Nur77 are involved in the interaction between FHL2 and Nur77. FHL2 represses Nur77 transcriptional activity in a dose-dependent manner, and short hairpin RNA-mediated knockdown of FHL2 results in increased Nur77 transcriptional activity. ChIP experiments on the enolase3 promoter revealed that FHL2 inhibits the association of Nur77 with DNA. FHL2 is highly expressed in human endothelial and smooth muscle cells, but not in monocytes or macrophages. To substantiate functional involvement of FHL2 in smooth muscle cell physiology, we demonstrated that FHL2 overexpression increases the growth of these cells, whereas FHL2 knockdown results in reduced DNA synthesis. Collectively, these studies suggest that association of FHL2 with Nur77 plays a pivotal role in vascular disease.

Regulation of adipocyte differentiation by the zinc finger protein ZNF638

Zinc finger proteins constitute the largest family of transcription regulators in eukaryotes. These factors are involved in diverse processes in many tissues, including development and differentiation. We report here the characterization of the zinc finger protein ZNF638 as a novel regulator of adipogenesis. ZNF638 is induced early during adipocyte differentiation. Ectopic expression of ZNF638 increases adipogenesis in vitro, whereas its knockdown inhibits differentiation and decreases the expression of adipocyte-specific genes. ZNF638 physically interacts and transcriptionally cooperates with CCAAT/enhancer-binding protein (C/EBP) β and C/EBPδ. This interaction leads to the expression of peroxisome proliferator-activated receptor γ, which is the key regulator of adipocyte differentiation. In summary, ZNF638 is a novel and early regulator of adipogenesis that works as a transcription cofactor of C/EBPs.

Autosomal-dominant distal myopathy associated with a recurrent missense mutation in the gene encoding the nuclear matrix protein, matrin 3

Distal myopathies represent a heterogeneous group of inherited skeletal muscle disorders. One type of adult-onset, progressive autosomal-dominant distal myopathy, frequently associated with dysphagia and dysphonia (vocal cord and pharyngeal weakness with distal myopathy [VCPDM]), has been mapped to chromosome 5q31 in a North American pedigree. Here, we report the identification of a second large VCPDM family of Bulgarian descent and fine mapping of the critical interval. Sequencing of positional candidate genes revealed precisely the same nonconservative S85C missense mutation affecting an interspecies conserved residue in the MATR3 gene in both families. MATR3 is expressed in skeletal muscle and encodes matrin 3, a component of the nuclear matrix, which is a proteinaceous network that extends throughout the nucleus. Different disease related haplotype signatures in the two families provided evidence that two independent mutational events at the same position in MATR3 cause VCPDM. Our data establish proof of principle that the nuclear matrix is crucial for normal skeletal muscle structure and function and put VCPDM on the growing list of monogenic disorders associated with the nuclear proteome.

The Ajuba LIM domain protein is a corepressor for SNAG domain mediated repression and participates in nucleocytoplasmic Shuttling

The SNAG repression domain is comprised of a highly conserved 21-amino acid sequence, is named for its presence in the Snail/growth factor independence-1 class of zinc finger transcription factors, and is present in a variety of proto-oncogenic transcription factors and developmental regulators. The prototype SNAG domain containing oncogene, growth factor independence-1, is responsible for the development of T cell thymomas. The SNAIL proteins also encode the SNAG domain and play key roles in epithelial mesenchymal differentiation events during development and metastasis. Significantly, these oncogenic functions require a functional SNAG domain. The molecular mechanisms of SNAG domain-mediated transcriptional repression are largely unknown. Using a yeast two-hybrid strategy, we identified Ajuba, a multiple LIM domain protein that can function as a corepressor for the SNAG domain. Ajuba interacts with the SNAG domain in vitro and in vivo, colocalizes with it, and enhances SNAG-mediated transcriptional repression. Ajuba shuttles between the cytoplasm and the nucleus and may form a novel intracellular signaling system. Using an integrated reporter gene combined with chromatin immunoprecipitation, we observed rapid, SNAG-dependent assembly of a multiprotein complex that included Ajuba, SNAG, and histone modifications consistent with the repressed state. Thus, SNAG domain proteins may bind Ajuba, trapping it in the nucleus where it functions as an adapter or molecular scaffold for the assembly of macromolecular repression complexes at target promoters.

Interaction of TACC proteins with the FHL family: implications for ERK signaling

The Transforming acidic coiled coil (TACC) proteins play a conserved role in normal development and tumorigenesis through interactions with multiple complexes involved in transcription, translation, and centrosomal dynamics. However, despite significant work on the function of TACC3 in the control of centrosomal mechanics, relatively little functional data is known about the family's founding member, TACC1. From a continued analysis of clones isolated by an unbiased yeast two-hybrid assay, we now show direct physical interactions between the TACC1 and the FHL (Four and a Half LIM-only) family of proteins. The authenticity of these interactions was validated both in vitro and in cellular systems. The FHLs exhibit diverse biological roles such as the regulation of the actin cytoskeleton and are promiscuous coregulators for several transcription factors. The interaction of the endogenous TACC-FHL proteins is primarily localized to the nucleus. However, similar to FHL2, overexpression of TACC1A in HEK293 is able to sequester serum activated ERK to the cytoplasm. This has the effect of reducing the serum induced transcriptional response of the c-fos and c-jun genes. The observation that TACCs can interact with the FHLs and alter their serum induced activities raises the possibility that the TACCs participate in crosstalk between cell signaling pathways important for cancer development and tumor progression. The transforming acidic coiled coil genes are known to be important prognostic indicators for breast, ovarian and lung cancer. In this manuscript, we identify a novel interaction between the TACCs and the FHL protein family. This interaction has an affect on ERK and may in part explain the variable associations and changes in subcellular locations of each family with specific subtypes of malignancy.

Suppression of FHL2 expression induces cell differentiation and inhibits gastric and colon carcinogenesis

BACKGROUND AND AIMS

FHL2 (4-1/2 LIM protein 2) is an adapter and modifier in protein interactions that is expressed mainly in the heart and ovary. It functions in a cell type- or promoter-specific manner. The aims of this study were to examine its expression in gastrointestinal cancers and to determine its role in cell differentiation and tumorigenesis.

METHODS

FHL2 expression in cancerous and normal gastrointestinal cells was detected by reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry. The effect of FHL2 suppression by both antisense and siRNA methods on cell differentiation and growth were evaluated in vitro and in vivo.

RESULTS

FHL2 expression was up-regulated in gastrointestinal cancer, compared with matched normal tissues. Stable transfection of gastric cancer cell line, AGS, and colon cancer cell line, Lovo, with antisense FHL2 induced lengthened or shuttle-shape morphologic changes with long or dendritic-like cytoplasmic processes and decreased the nuclear:cytoplasmic ratio. FHL2 antisense induced expressions of carcinoembryonic antigen and E-cadherin and the maturation of F-actin. Furthermore, FHL2 antisense inhibited the transcriptions of some oncogenes including cox-2, survivin, c-jun, and hTERT, and suppressed the promoter activity of activator protein-1 and hTERT. Suppression of FHL2 inhibited serum-dependent, anchorage-dependent and -independent cell growth, and suppressed de novo tumor formation in nude mice xenograft.

CONCLUSIONS

Suppression of FHL2 induces cell differentiation and inhibits tumorigenesis. Antisense or siRNA methods targeting FHL2 is a promising strategy for treatment of gastrointestinal cancers.

Four and half lim protein 2 (FHL2) stimulates osteoblast differentiation

FHL2, a molecule that interacts with many integrins and transcription factors, was found to play an important role in osteoblast differentiation. Overexpression of FHL2 increases the accumulation of osteoblast differentiation markers and matrix mineralization, whereas FHL2 deficiency results in inhibition of osteoblast differentiation and decreased bone formation.

INTRODUCTION

Integrin-matrix interaction plays a critical role in osteoblast function. It has been shown that the cytoplasmic domains of integrin beta subunits mediate signal transduction induced by integrin-matrix interaction. We reasoned that the identification of proteins interacting with beta-cytoplasmic tails followed by analysis of the function of these proteins would enhance our understanding on integrin signaling and the roles of these proteins in osteoblast activities.

MATERIALS AND METHODS

Yeast two hybrid assay was used to identify proteins interacting with the cytoplasmic domain of integrin beta5 subunit. The association of these proteins with integrin alphavbeta5 was confirmed by confocal analysis and co-immunoprecipitation. A stable MC3T3-E1 cells line overexpressing Four and Half Lim Protein 2 (FHL2) and mouse osteoblasts deficient in FHL2 were used to study the roles of FHL2 in osteoblast differentiation and bone formation. Matrix protein expression was determined by mRNA analysis and Western blotting. Matrix mineralization was detected by Alizarin red staining. Alkaline phosphatase activity was also measured. muCT was used to determine bone histomorphometry.

RESULTS AND CONCLUSIONS

FHL2 and actin-binding proteins, palladin and filamin A, were identified as proteins interacting with beta5 cytoplasmic domain. FHL2 co-localized with alphavbeta5 at the focal adhesion sites in association with palladin and filamin A. FHL2 was also present in nuclei. Osteoblasts overexpressing FHL2 exhibited increased adhesion to and migration on matrix proteins. Conversely, FHL2 stimulation of CREB activity was dependent on integrin function because it was inhibited by Gly-Arg-Gly-Asp-Ser (GRGDS) peptide. The expression of osteoblast differentiation markers and Msx2 was upregulated, and bone matrix mineralization was increased in FHL2 overexpressing cells. In contrast, FHL2-deficient bone marrow cells and osteoblasts displayed decreased osteoblast colony formation and differentiation, respectively, compared with wildtype cells. Moreover, FHL2-deficient female mice exhibited greater bone loss than the wildtype littermates after ovariectomy. Thus, FHL2 plays an important role in osteoblast differentiation and bone formation.

SKI pathways inducing progression of human melanoma

The proteins SKI and SnoN are implicated in processes as diverse as differentiation, transformation and tumor progression. Until recently, SKI was solely viewed as a nuclear protein with a principal function of inhibiting TGF-beta signaling through its association with the Smad proteins. However, new studies suggest that SKI plays additional roles not only inside but also outside the nucleus. In normal melanocytes and primary non-invasive melanomas, SKI localizes predominantly in the nucleus, whereas in primary invasive melanomas SKI displays both nuclear and cytoplasmic localization. Intriguingly, metastatic melanoma tumors display nuclear and cytoplasmic or predominantly cytoplasmic SKI distribution. Cytoplasmic SKI is functional, as it associates with Smad3 and prevents its nuclear localization mediated by TGF-beta. SKI can also function as a transcriptional activator, targeting the beta -catenin pathway and activating MITF and NrCAM, two proteins involved in survival, migration and invasion. Intriguingly, SKI appears to live a dual life, one as a tumor suppressor and another as a transforming protein. Loss of one copy of mouse ski increases susceptibility to tumorigenesis in mice, whereas its overexpression is associated with cancer progression of human melanoma, esophageal, breast and colon. The molecular reasons for such dramatic change in SKI function appear to result from new acquired activities. In this review, we discuss the mechanisms by which SKI regulates crucial pathways involved in the progression of human malignant melanoma.

In Vivo HIV-1 Rev Multimerization in the Nucleolus and Cytoplasm Identified by Fluorescence Resonance Energy Transfer

Nuclear export of intron-containing human immunodeficiency virus type 1 RNA is mediated by the viral Rev protein. Rev is a nucleocytoplasmic transport protein that directly binds to its cis-acting Rev-responsive element RNA. Rev function depends on its ability to multimerize. The in vivo dynamics and the subcellular dependence of this process are still largely unexplored. To visualize and quantitatively analyze the mechanism of Rev multimeric assembly in live cells, we used high resolution in vivo fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching. By using two different dynamic FRET approaches (acceptor photobleaching and donor bleaching time measurements), we observed a strong Rev-Rev interaction in the nucleoli of living cells. Most interestingly, we could also detect Rev multimerization in the cytoplasm; however, FRET efficiency in the cytoplasm was significantly lower than in the nucleolus. By using fluorescence recovery after photobleaching, we investigated the mobility of Rev within the nucleolus. Mathematical modeling of the fluorescence recovery after photobleaching recoveries enabled us to extract relative association and dissociation constants and the diffusion coefficient of Rev in the nucleolus. Our results show that Rev multimerizes in the nucleolus of living cells, suggesting an important role of the nucleolus in nucleocytoplasmic transport.

Characterization of recombinant and natural forms of the human LIM domain-containing protein FHL2

FHL2 (Four and a Half LIM domain-containing protein 2) is a member of a small family of proteins with four LIM domains and an N-terminal half LIM domain. It is an intracellular protein thought to function as an adaptor in the formation of multi-protein complexes involved in signaling. To obtain human FHL2 in amounts allowing further characterization, we evaluated different expression systems and chose to express FHL2 with a His6 tag in insect cells using the baculovirus system. The recombinant protein was highly expressed and could be purified to >98% homogeneity as judged by SDS-PAGE analysis. Purified recombinant FHL2 was used to generate antibodies allowing detection and immunoprecipitation of FHL2 from human cells. Both recombinant and natural FHL2 were characterized by SDS-PAGE and MALDI-TOF mass spectrometry. The molecular mass of the recombinant His6-tagged protein obtained by mass spectrometry was 36,995Da, in good agreement with the apparent mass of 36kDa in SDS-PAGE and slightly higher than the 35,981Da calculated from the sequence of the construct. The measured molecular mass of natural human FHL2 was 32,742Da and the calculated mass was 32,192Da. However, the apparent molecular mass in SDS-PAGE is 41kDa, indicating that the natural protein has an abnormal electrophoretic mobility. The results show that both the recombinant and the natural proteins are post-translationally modified and indicate that such modifications may lead to an abnormal electrophoretic behavior of natural human FHL2.

Analysis of the adaptor function of the LIM domain-containing protein FHL2 using an affinity chromatography approach

Containing four LIM domains and an N-terminal half LIM domain, FHL2 has been predicted to have an adaptor function in the formation of higher order molecular complexes in the nucleus and the cytoplasm of cells. We expressed recombinant FHL2 in insect cells using the baculovirus system and used it to isolate direct or indirect interaction partners from the cytosolic fraction of fibroblasts by affinity chromatography. These were identified by their peptide mass fingerprints using MALDI-TOF mass spectrometry. Cytoskeleton-associated proteins present among the bound proteins were shown to co-localise with FHL2 in cell lamellipodia by indirect immunofluorescence staining.

Extracellular signal-regulated kinase 2 interacts with and is negatively regulated by the LIM-only protein FHL2 in cardiomyocytes

The mitogen-activated protein kinase (MAPK) signaling pathway regulates diverse biologic functions including cell growth, differentiation, proliferation, and apoptosis. The extracellular signal-regulated kinases (ERKs) constitute one branch of the MAPK pathway that has been implicated in the regulation of cardiac differentiated growth, although the downstream mechanisms whereby ERK signaling affects this process are not well characterized. Here we performed a yeast two-hybrid screen with ERK2 bait and a cardiac cDNA library to identify novel proteins involved in regulating ERK signaling in cardiomyocytes. This screen identified the LIM-only factor FHL2 as an ERK interacting protein in both yeast and mammalian cells. In vivo, FHL2 and ERK2 colocalized in the cytoplasm at the level of the Z-line, and interestingly, FHL2 interacted more efficiently with the activated form of ERK2 than with the dephosphorylated form. ERK2 also interacted with FHL1 and FHL3 but not with the muscle LIM protein. Moreover, at least two LIM domains in FHL2 were required to mediate efficient interaction with ERK2. The interaction between ERK2 and FHL2 did not influence ERK1/2 activation, nor was FHL2 directly phosphorylated by ERK2. However, FHL2 inhibited the ability of activated ERK2 to reside within the nucleus, thus blocking ERK-dependent transcriptional responsiveness of ELK-1, GATA4, and the atrial natriuretic factor promoter. Finally, FHL2 partially antagonized the cardiac hypertrophic response induced by activated MEK-1, GATA4, and phenylephrine agonist stimulation. Collectively, these results suggest that FHL2 serves a repressor function in cardiomyocytes through its ability to inhibit ERK1/2 transcriptional coupling.

Analysis ofSOX10 mutations identified in Waardenburg-Hirschsprung patients: Differential effects on target gene regulation

SOX10 is a member of the SOX gene family related by homology to the high-mobility group (HMG) box region of the testis-determining gene SRY. Mutations of the transcription factor gene SOX10 lead to Waardenburg-Hirschsprung syndrome (Waardenburg-Shah syndrome, WS4) in humans. A number of SOX10 mutations have been identified in WS4 patients who suffer from different extents of intestinal aganglionosis, pigmentation, and hearing abnormalities. Some patients also exhibit signs of myelination deficiency in the central and peripheral nervous systems. Although the molecular bases for the wide range of symptoms displayed by the patients are still not clearly understood, a few target genes for SOX10 have been identified. We have analyzed the impact of six different SOX10 mutations on the activation of SOX10 target genes by yeast one-hybrid and mammalian cell transfection assays. To investigate the transactivation activities of the mutant proteins, three different SOX target binding sites were introduced into luciferase reporter gene constructs and examined in our series of transfection assays: consensus HMG domain protein binding sites; SOX10 binding sites identified in the RET promoter; and Sox10 binding sites identified in the P0 promoter. We found that the same mutation could have different transactivation activities when tested with different target binding sites and in different cell lines. The differential transactivation activities of the SOX10 mutants appeared to correlate with the intestinal and/or neurological symptoms presented in the patients. Among the six mutant SOX10 proteins tested, much reduced transactivation activities were observed when tested on the SOX10 binding sites from the RET promoter. Of the two similar mutations X467K and 1400del12, only the 1400del12 mutant protein exhibited an increase of transactivation through the P0 promoter. While the lack of normal SOX10 mediated activation of RET transcription may lead to intestinal aganglionosis, overexpression of genes coding for structural myelin proteins such as P0 due to mutant SOX10 may explain the dysmyelination phenotype observed in the patients with an additional neurological disorder.

The LIM protein FHL3 binds basic Krüppel-like factor/Krüppel-like factor 3 and its co-repressor C-terminal-binding protein 2

The ability of DNA-binding transcription factors to recruit specific cofactors is central to the mechanism by which they regulate gene expression. BKLF/KLF3, a member of the Krüppel-like factor family of zinc finger proteins, is a potent transcriptional repressor that recruits a CtBP co-repressor. We show here that BKLF also recruits the four and a half LIM domain protein FHL3. Different but closely linked regions of BKLF mediate contact with CtBP2 and FHL3. We present evidence that CtBP2 also interacts with FHL3 and demonstrate that the three proteins co-elute in gel filtration experiments. CtBP and FHL proteins have been implicated in both nuclear and cytoplasmic functions, but expression of BKLF promotes the nuclear accumulation of both FHL3 and CtBP2. FHL proteins have been shown to act predominantly as co-activators of transcription. However, we find FHL3 can repress transcription. We suggest that LIM proteins like FHL3 are important in assembling specific repression or activation complexes, depending on conditions such as cofactor availability and promoter context.

Subcellular targeting of metabolic enzymes to titin in heart muscle may be mediated by DRAL/FHL-2

During sarcomere contraction skeletal and cardiac muscle cells consume large amounts of energy. To satisfy this demand, metabolic enzymes are associated with distinct regions of the sarcomeres in the I-band and in the M-band, where they help to maintain high local concentrations of ATP. To date, the mechanism by which metabolic enzymes are coupled to the sarcomere has not been elucidated. Here, we show that the four and a half LIM-only protein DRAL/FHL-2 mediates targeting of the metabolic enzymes creatine kinase, adenylate kinase and phosphofructokinase by interaction with the elastic filament protein titin in cardiomyocytes. Using yeast two-hybrid assays, colocalisation experiments, co-immunoprecipitation and protein pull-down assays, we show that DRAL/FHL-2 is bound to two distinct sites on titin. One binding site is situated in the N2B region, a cardiac-specific insertion in the I-band part of titin, and the other is located in the is2 region of M-band titin. We also show that DRAL/FHL-2 binds to the metabolic enzymes creatine kinase, adenylate kinase and phosphofructokinase and might target these enzymes to the N2B and is2 regions in titin. We propose that DRAL/FHL-2 acts as a specific adaptor protein to couple metabolic enzymes to sites of high energy consumption in the cardiac sarcomere.

The LIM-only protein DRAL/FHL2 interacts with and is a corepressor for the promyelocytic leukemia zinc finger protein

Members of the four-and-a-half-LIM domain (FHL) protein family, which are expressed in a tissue- and stage-specific manner, have been reported previously to function as transcriptional coactivators. One of these is the p53-inducible protein DRAL/FHL2 (where DRAL is down-regulated in rhabdomyosarcoma LIM domain protein). In this work, we identified potential binding partners for DRAL/FHL2 using an inducible yeast two-hybrid system. We present evidence of a functional interaction between the promyelocytic leukemia zinc finger protein (PLZF) and DRAL/FHL2. PLZF is a sequence-specific transcriptional repressor whose function relies on recruitment of corepressors that form part of the histone deacetylase complex involved in chromatin remodeling. DRAL/FHL2 interacts specifically with PLZF in vitro and in vivo and augments transcriptional repression mediated by PLZF. This is the first reported incidence of a bona fide FHL protein-mediated corepression and supports the notion of these proteins having a role as coregulators of tissue-specific gene expression.