Protein-protein interaction of FHL2, a LIM domain protein preferentially expressed in human heart, with hCDC47

Structural domain in the Titin N2B-us region binds to FHL2 in a force-activation dependent manner

Titin N2B unique sequence (N2B-us) is a 572 amino acid sequence that acts as an elastic spring to regulate muscle passive elasticity. It is thought to lack stable tertiary structures and is a force-bearing region that is regulated by mechanical stretching. In this study, the conformation of N2B-us and its interaction with four-and-a-half LIM domain protein 2 (FHL2) are investigated using AlphaFold2 predictions and single-molecule experimental validation. Surprisingly, a stable alpha/beta structural domain is predicted and confirmed in N2B-us that can be mechanically unfolded at forces of a few piconewtons. Additionally, more than twenty FHL2 LIM domain binding sites are predicted to spread throughout N2B-us. Single-molecule manipulation experiments reveals the force-dependent binding of FHL2 to the N2B-us structural domain. These findings provide insights into the mechano-sensing functions of N2B-us and its interactions with FHL2.

The Immunohistochemical Expression of MCM-3, -5, and -7 Proteins in the Uterine Fibroids

Uterine fibroids are the most common mesenchymal uterine neoplasms; their prevalence is estimated in 40%–60% of women under 35 and in 70%–80% of women over 50 years of age. The current research aims to focus on the etiopathogenesis of uterine fibroids, the factors that affect their growth, and markers with diagnostic and prognostic properties. The MCM (minichromosome maintenance) protein family consists of peptides whose primary function is participation in the molecular mechanism of creating replication forks while regulating DNA synthesis. The aim of this work was to determine the proliferative potential of uterine fibroid cells based on the expression of the Ki-67 antigen and the MCMs—i.e., MCM-3, MCM-5, and MCM-7. In addition, the expression of estrogen (ER) and progesterone (PgR) receptors was evaluated and correlated with the expression of the abovementioned observations. Ultimately, received results were analyzed in terms of clinical and pathological data. Materials and methods: In forty-four cases of uterine fibroids, immunohistochemical reactions were performed. A tissue microarray (TMA) technique was utilized and analyzed cases were assessed in triplicate. Immunohistochemistry was performed using antibodies against Ki-67 antigen, ER, PgR, MCM-3, MCM-5, and MCM-8 on an automated staining platform. Reactions were digitalized by a histologic scanner and quantified utilizing dedicated software for nuclear analysis. Assessment was based on quantification expression of the three histiospots, each representing one case in TMA. Results: In the study group (uterine fibroids), statistically significant stronger expression of all the investigated MCMs was observed, as compared to the control group. In addition, moderate and strong positive correlations were found between all tested proliferative markers. The expression of the MCM-7 protein also correlated positively with ER and PgR. With regard to clinical and pathological data, there was a negative correlation between the expression of MCMs and the number of both pregnancies and births. Significant reductions in MCM-5 and MCM-7 expression were observed in the group of women receiving oral hormonal contraceptives, while smoking women showed an increase in MCM-7, ER, and PgR. Conclusions: Uterine fibroid cells have greater proliferative potential, as evaluated by expression of the Ki-67 antigen and MCMs, than unaltered myometrial cells of the uterine corpus. The expression of MCM-7 was found to have strong or moderate correlations in all assessed relations. In the context of the clinical data, as well evident proliferative potential of MCMs, further studies are strongly recommended.

The novel six LIM and one PET domain-containing protein Lmpt is involved in the immune response through activation of the NF-κB signalling pathway in the crustacean, Macrobrachium nipponense

The four-and-a-half LIM-only protein family of transcription co-factors participates in various cellular processes, such as cell proliferation, cell differentiation, apoptosis, cell adhesion, migration, transcription and signal transduction. However, the knowledge of the structural characteristics and immune functions of its ancestor Lmpt, which contains six LIM domains at the C-terminus and a PET domain at the N-terminus, is limited in invertebrates, especially in crustaceans. In the present study, a novel Lmpt from oriental river prawn (Macrobrachium nipponense) was identified, and its role in the immune response was investigated. Its full-length cDNA sequence was 6407 bp, which contained a 2595 bp ORF encoding 865 amino acids, exhibiting high similarity to the structure of Lmpt derived from other invertebrates. Tissue distribution analysis revealed that MnLmpt was widely expressed in all examined tissues, and high expression levels were observed in muscle, heart and intestine in M. nipponense. After experimental challenges with bacteria and virus, the transcription levels of MnLmpt significantly fluctuated in gill and hepatopancreas, indicating that it might play a role in the innate immune response in M. nipponense. Silencing of MnLmpt by dsRNA injection in vivo could promote bacterial growth, suggesting that MnLmpt exerted an antibacterial immune function in prawn. Immunocytochemistry assay results demonstrated that MnLmpt was able to translocate from the cytoplasm to the nucleus after being stimulated with pathogens. The expression levels of NF-κB signalling cascade members, such as dorsal, relish, TAK1, TAB1, Ikkβ, and Ikkε, and AMPs, including ALF4, Cru1, and Cru2, exhibited significant downregulation in the MnLmpt silenced group. Similarly, dual-luciferase reporter assays also demonstrated that MnLmpt could stimulate an NF-κB signalling cascade. Meanwhile, all of the LIM domains of MnLmpt could trigger NF-κB signalling; however, their cumulative effect on NF-κB promoter activation was hardly observed. These results showed that MnLmpt might play a crucial role in the innate immune response in M. nipponense, and these findings paved the way for a better understanding of the immune system in crustacean species.

Expression, location and biological effects of four and a half LIM domain protein 2 (FHL2) on granulosa cells in ovine

Previous studies have shown that four and a half LIM domain protein 2 (FHL2) plays an essential role in the regulation of follicular development in mammals. Although the FHL2 genes of human and mouse have been well characterized, the expression and location of FHL2 in ovary and the biological functions of FHL2 on granulosa cells (GCs) of ovine are still not clear. In this study, full-length complementary DNA (cDNA) of FHL2 from ovine follicular GCs was amplified by real-time PCR (RT-PCR). The expression and location of FHL2 in ovary and GCs of ovine were studied by immunohistochemistry and immunofluorescence, and the biological effects of FHL2 on the cell proliferation, cell apoptosis, cell cycles and expression level of related genes of ovine GCs were also explored by overexpression or knockdown of FHL2. The results indicated that FHL2 was expressed in ovine follicular GCs and the sequence of the FHL2 cDNA was consistent with that predicted in GenBank, which did not cause an amino acid change. According to the results, FHL2 was expressed in ovine ovary and mainly located in the cytoplasm and nucleus of GCs. In addition, overexpression of FHL2 significantly reduced the cell viability, promoted the cell apoptosis and decreased the percentage of G0/G1 and S phase cells. RT-PCR showed that overexpression of FHL2 significantly increased the mRNA expression level of Bax and decreased the expression of Bcl-2 and the Bcl-2/Bax mRNA ratio compared with the control group. Besides, the knockdown of FHL2 gene in ovine GCs significantly improved the cell viability, suppressed the cell apoptosis, decreased the mRNA expression level of Caspase-3 gene, increased the Bcl-2/Bax mRNA ratio and increased the percentage of S and G2/M phase cells. Our results suggest that FHL2 may play an important role in the biological functions of GCs in ovine.

Increased expression of FHL2 promotes tumorigenesis in cervical cancer and is correlated with poor prognosis

PURPOSE

Increasing evidence demonstrates that the four and a half LIM domain (FHL) gene and its protein products have different functions in the progression of various malignancies. However, the role of FHL protein 2 (FHL2) in cervical cancer (CC) has not been fully elucidated. In this study, we investigated the prognostic value of FHL2 expression in human CC tissues and the potential molecular mechanisms through which FHL2 modulates CC cell proliferation and apoptosis.

MATERIALS AND METHODS

We measured FHL2 expression in CC cell lines and tissues by quantitative real-time polymerase chain reaction and Western blot assays. The effects of FHL2 knockdown on cell proliferation and apoptosis in two CC cell lines were examined using RNA interference, cell counting kit-8, Western blot and flow cytometry assays. Furthermore, we assessed phosphorylated protein kinase B (p-AKT) and phosphorylated mammalian target of rapamycin (p-mTOR) expression in two CC cell lines to determine whether the AKT/mTOR pathway is involved in the effects of FHL2 silencing on cell proliferation and apoptosis. Nude mice tumorigenicity experiments were also performed to evaluate the effects of FHL2 on HeLa cell growth in vivo.

RESULTS

We found that FHL2 was significantly upregulated in CC cell lines and tissues. According to survival curves, high FHL2 expression levels in patients were correlated with poor prognosis. Moreover, by decreasing p-AKT and p-mTOR protein levels, silencing FHL2 significantly inhibited cell proliferation and induced apoptosis. FHL2 knockdown also induced apoptosis by increasing the Bax-to-Bcl2 ratio. By contrast, FHL2 overexpression significantly promoted cell proliferation. Finally, decreased tumour growth in an in vivo animal model also demonstrated the tumour-suppressing effects of FHL2 knockdown.

CONCLUSION

Our findings indicate that FHL2 is an important prognostic factor in CC and that it plays a crucial oncoprotein role by promoting cell proliferation and inhibiting apoptosis in CC, possibly by targeting the AKT/mTOR pathway.

Expression of the Transcriptional Coregulator FHL2 in Human Breast Cancer: A Clinicopathologic Study

OBJECTIVE

Although the Four and a Half LIM domain protein 2 (FHL2) has been suggested to play an important role in tumor development, this has not been investigated in breast cancer.

METHODS

Paraffin-embedded tissues from patients (n = 85) with primary breast cancer were submitted to immunohistochemical investigation of FHL2 expression and subsequent correlation with clinicopathologic parameters and patient survival.

RESULTS

The expression of FHL2 was confined to the cytoplasm of the tumor cells. Forty (47%) of 85 samples showed weak expression of FHL2, whereas high expression was found in 45 tumors (53%). A statistically significant positive correlation was observed between FHL2 and androgen receptor expression (P = .029). Patients with tumors expressing low amounts of FHL2 were characterized by a significantly better survival compared to those with high intratumoral FHL2 expression (P = .0215, log-rank test). The additional stratification according to adjuvant tamoxifen treatment revealed a significantly improved survival rate for patients receiving tamoxifen and being diagnosed with a tumor expressing high amounts of FHL2. This might indicate that tamoxifen is at least partially capable of reversing the negative prognostic impact of high FHL2 expression. Multivariate Cox regression analysis revealed FHL2 expression as a significant independent predictor of survival.

CONCLUSION

The specific expression in tumor tissue points to an important functional role of FHL2 in human breast cancer. Our survival data indicate that the expression of FHL2 in primary breast cancer is a potentially relevant prognostic factor. Further studies are warranted to elucidate whether analysis of FHL2 expression is suitable to predict response to antihormonal treatment with tamoxifen.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

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.

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.

The four and a half LIM family members are novel interactants of the human T-cell leukemia virus type 1 Tax oncoprotein

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL). The viral regulatory protein Tax1 plays a pivotal role in T-cell transformation and ATL development. Previous studies in our laboratory, using the yeast 2-hybrid approach to screen a T-cell library for Tax1-interacting partners, identified the cellular Four and a Half Lim domain protein 3 (FHL3) as a possible Tax1-interacting candidate. FHL3 is a member of the FHL family of proteins, which function as transcriptional coactivators and cytoskeleton regulators and have a role in cancer progression and development. The aim of this study was to investigate the physical and functional interaction between Tax1 and members of the FHL family of proteins. We show that Tax1 and FHL3 interact both in vitro and in vivo. Furthermore, both FHL1 and -2 also interact with Tax1. We have demonstrated that FHL3 enhances Tax1-mediated activation of the viral long terminal repeat (LTR) without affecting basal activity and that FHL1 to -3 regulate NF-κB activation by Tax1 in a cell-specific manner. In addition, we have found that the interaction between Tax1 and FHL1 to -3 affects the localization of these proteins, leading to their redistribution in cells. Tax1 also affected FHL3 cytoskeleton function by increasing FHL3-mediated cell spreading. Overall, our results suggest that the interaction between Tax1 and the FHL family alters both the transactivating activity and the subcellular localization of Tax1 and provide new insights into molecular mechanisms that underlie the oncogenic nature of this HTLV-1 protein.

Human four-and-a-half LIM family members suppress tumor cell growth through a TGF-beta-like signaling pathway

The four-and-a-half LIM (FHL) proteins belong to a family of LIM-only proteins that regulate cell proliferation, differentiation, and apoptosis. The exact functions of each FHL protein in cancer development and progression remain unknown. Here we report that FHL1, FHL2, and FHL3 physically and functionally interact with Smad2, Smad3, and Smad4, important regulators of cancer development and progression, in a TGF-beta-independent manner. Casein kinase 1delta, but not the TGF-beta receptor, was required for the FHL-mediated TGF-beta-like responses, including increased phosphorylation of Smad2/3, interaction of Smad2/3 and Smad4, nuclear accumulation of Smad proteins, activation of the tumor suppressor gene p21, and repression of the oncogene c-myc. FHL1-3 inhibited anchorage-dependent and -independent growth of a human hepatoma cell line in vitro and tumor formation in nude mice. Further analysis of clinical samples revealed that FHL proteins are often downregulated in hepatocellular carcinomas and that this correlates with decreased TGF-beta-like responses. By establishing a link between FHL proteins and Smad proteins, this study identifies what we believe to be a novel TGF-beta-like signaling pathway and indicates that FHL proteins may be useful molecular targets for cancer therapy.

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.

Deregulated minichromosomal maintenance protein MCM7 contributes to oncogene driven tumorigenesis

Minichromosomal maintenance protein 7 (MCM7) is an essential component of the replication helicase complex (MCM2-7) required for DNA replication. Although this function is highly conserved among eukaryotes, additional functions for the MCM molecules continue to be described. Minichromosomal maintenance protein 7 is a marker for proliferation and is upregulated in a variety of tumors including neuroblastoma, prostate, cervical and hypopharyngeal carcinomas. To further investigate the general role of MCM7 in tumorigenesis, we generated a mouse model with deregulated MCM7 expression targeted to the basal layer of the epidermis using the keratin 14 (K14) promoter (K14.MCM7). When subjected to a two-stage chemical carcinogenesis protocol (dimethylbenz[alpha]anthracene (DMBA) initiation with 12-ortho-tetradecanoylphorbol-13-acetate promotion), K14.MCM7 mice showed significantly increased incidence and prevalence of tumor development relative to controls. Furthermore, within 40 weeks of treatment over 45% K14.MCM7 mice exhibited tumors that had converted to squamous cell carcinomas versus none in the control group. As predicted from previous skin carcinogenesis studies using DMBA as the initiating agent, Ras mutations where found in more than 90% of tumors isolated from K14.MCM7 mice. Whereas previous studies have shown that MCM7 is useful as a proliferation marker, our data suggest that deregulated MCM7 expression actively contributes to tumor formation, progression and malignant conversion.

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.

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.

Eukaryotic MCM proteins: beyond replication initiation

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

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.

LIM-only protein FHL3 interacts with CDC25B2 phosphatase

LIM domain proteins are important regulators of the growth, determination, and differentiation of cells. In this report, FHL3 (human four-and-a-half LIM-only protein 3) is shown to interact with human phosphatase CDC25B, a cell cycle regulator involved in the control of G2/M. We found that this interaction was specific to the CDC25B2 isoform. Deletion and point mutation studies indicated that the second LIM domain of FHL3 was essential for this interaction. FRET experiments in C2C12 cells showed that, although both proteins were colocated in the cytoplasm and the nucleus, they interacted only in the nucleus. Finally, we showed that FHL3 binding impaired neither CDC25B2 phosphatase activity nor its localization. Further work is now needed to elucidate the consequences of this interaction on myoblast fate decision and cycle control.

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.

Expression of FHL2 and cytokine messenger RNAs in human myocardium after cardiopulmonary bypass

BACKGROUND

Proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-6, and IL-8 have been implicated in myocardial injury following cardiopulmonary bypass (CPB). However, little evidence is currently available to directly confirm such a relationship. We have previously documented that a newly discovered 'four and a half LIM-only protein 2' (FHL2) is exclusively expressed in myofibres. We hypothesized that the upregulation of FHL2 is proportional to the degree of myocardial injury and investigated the myocardial expression of FHL2 together with these cytokine messenger RNAs (mRNAs) during clinical CPB.

METHODS

Intermittent hypothermic blood cardioplegia was used in all patients. Atrial myocardial biopsies were obtained immediately at the onset and at the end of CPB in 33 consecutive patients undergoing valvular or coronary artery surgery. TNF-alpha, IL-6, and IL-8 mRNA expressions in these myocardial samples were determined by semi-quantitative reverse transcription-polymerase chain reaction. Myocardial FHL2 expression was determined by Western blot analysis. Serum levels of the MB isoenzyme of creatine kinase (CK-MB) and cardiac troponin-I (cTnI) before surgery and 24 h after the end of CPB were also measured.

RESULTS

The duration of aortic crossclamping and CPB was 70+/-33 and 99+/-37 min, respectively. No elevated myocardial TNF-alpha mRNA expression was found after CPB. IL-6 mRNA expressions were detected in 14 pairs of the myocardial biopsies and were elevated in 11 (33%) post-CPB biopsies. Similarly, IL-8 mRNA expressions were detected in 19 pairs of samples and were elevated in 14 (42%) post-CPB biopsies. Among the 17 pairs of biopsies with positive FHL2 expression, FHL2 levels were increased in 11 (33%) post-CPB samples. Moreover, the elevated FHL2 expression was associated with an increase in IL-6 (P=0.018) and IL-8 (P=0.024) mRNA expression after CPB. Postoperative CK-MB and cTnI levels were significantly higher in patients with myocardial FHL2 expressions than those without (CK-MB, 13.5+/-2.3 vs. 6.5+/-0.8 ng/ml, P=0.022; cTnI, 10.7+/-2.0 vs. 3.5+/-0.6 ng/ml, P=0.0013).

CONCLUSIONS

Our findings demonstrate for the first time that both IL-6 and IL-8 mRNAs are upregulated in human cardiac myocytes following CPB and these cytokines may be involved in myocardial ischemia-reperfusion injury, as reflected by their association with an increased expression of FHL2.

The LIM-only protein FHL2 interacts with beta-catenin and promotes differentiation of mouse myoblasts

FHL2 is a LIM-domain protein expressed in myoblasts but down-regulated in malignant rhabdomyosarcoma cells, suggesting an important role of FHL2 in muscle development. To investigate the importance of FHL2 during myoblast differentiation, we performed a yeast two-hybrid screen using a cDNA library derived from myoblasts induced for differentiation. We identified beta-catenin as a novel interaction partner of FHL2 and confirmed the specificity of association by direct in vitro binding tests and coimmunoprecipitation assays from cell lysates. Deletion analysis of both proteins revealed that the NH2-terminal part of beta-catenin is sufficient for binding in yeast, but addition of the first armadillo repeat is necessary for binding FHL2 in mammalian cells, whereas the presence of all four LIM domains of FHL2 is needed for the interaction. Expression of FHL2 counteracts beta-catenin-mediated activation of a TCF/LEF-dependent reporter gene in a dose-dependent and muscle cell-specific manner. After injection into Xenopus embryos, FHL2 inhibited the beta-catenin-induced axis duplication. C2C12 mouse myoblasts stably expressing FHL2 show increased myogenic differentiation reflected by accelerated myotube formation and expression of muscle-specific proteins. These data imply that FHL2 is a muscle-specific repressor of LEF/TCF target genes and promotes myogenic differentiation by interacting with beta-catenin.

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.

Insulin-like growth factor-binding protein 5 (IGFBP-5) interacts with a four and a half LIM protein 2 (FHL2)

Recent studies using insulin-like growth factor I (IGF-I) knockout mice demonstrate that IGF-binding protein (IGFBP)-5, an important bone formation regulator, itself is a growth factor with cellular effects not dependent on IGFs. Because IGFBP-5 contains a nuclear localization sequence that mediates transport of IGFBP-5 into the nucleus, we propose that IGFBP-5 interacts with nuclear proteins to affect transcription of genes involved in bone formation. We therefore undertook studies to identify proteins that bind to IGFBP-5 using IGFBP-5 as bait in a yeast two-hybrid screen of a U2 human osteosarcoma cDNA library. Five related clones that interacted strongly with the bait corresponded to the FHL2 gene, which contains four and a half LIM domains. Co-immunoprecipitation studies with lysates from U2 cells overexpressing FHL2 and IGFBP-5 confirmed that interaction between IGFBP-5 and FHL2 occurs in whole cells. In vitro interaction studies revealed that purified FHL2 interacted with IGFBP-5 but not with IGFBP-3, -4, or -6. Northern blot analysis showed that FHL2 was strongly expressed in human osteoblasts. Nuclear localization of both FHL2 and IGFBP-5 was evident from Western immunoblot analysis and immunofluorescence. The role of FHL2 as an intracellular mediator of the effects of IGFBP-5 and other osteoregulatory agents in osteoblasts will need to be verified in future studies.

Characterization of tissue-specific LIM domain protein (FHL1C) which is an alternatively spliced isoform of a human LIM-only protein (FHL1)

We have cloned and characterized another alternatively spliced isoform of the human four-and-a-half LIM domain protein 1 (FHL1), designated FHL1C. FHL1C contains a single zinc finger and two tandem repeats of LIM domains at the N-terminus followed by a putative RBP-J binding region at the C-terminus. FHL1C shares the same N-terminal two-and-a-half LIM domains with FHL1 but different C-terminal protein sequences. Due to the absence of the exon 4 in FHL1C, there is a frame-shift in the 3' coding region. Sequence analysis indicated that FHL1C is the human homolog of murine KyoT2. The Northern blot and RT-PCR results revealed that FHL1 is widely expressed in human tissues, including skeletal muscle and heart at a high level, albeit as a relatively low abundance transcript in brain, placenta, lung, liver, kidney, pancreas, and testis. In contrast, FHL1C is specifically expressed in testis, skeletal muscle, and heart at a relatively low level compared with FHL1. The expression of FHL1C transcripts was also seen in aorta, left atrium, left, and right ventricles of human heart at low level. Immunoblot analysis using affinity-purified anti-FHL1C antipeptide antibodies confirmed a 20 kDa protein of FHL1C in human skeletal muscle and heart. Unlike FHL1B, which is another FHL1 isoform recently reported by our group and localized predominantly in the nucleus [Lee et al., 1999], FHL1C is localized both in the nucleus and cytoplasm of mammalian cell.

An RNA recognition motif (RRM) is required for the localization of PTB-associated splicing factor (PSF) to subnuclear speckles

Using fusions with green fluorescent protein (GFP), we have identified sequences in the polypyrimidine tract binding protein-associated splicing factor (PSF) that are involved in nuclear and subnuclear localization. Like other splicing factors, PSF localizes to the nucleus, is absent from nucleoli, and accumulates in punctate structures within the nucleus referred to as speckles. However, PSF lacks the known speckle localization domains that have been identified in other proteins. Instead, the localization of PSF to speckles is dependent on an RNA recognition motif (RRM). PSF comprises an N-terminal proline- and glutamine-rich domain, two RRMs (RRM1 and RRM2), and a C-terminal region that contains two nuclear localization signals, both of which are required for complete nuclear localization. Deletion of RRM2 led to a complete loss of speckle localization and resulted in diffuse accumulation of PSF in the nucleus, indicating that RRM2 is required for subnuclear localization. Thus, PSF appears to localize to speckles through a novel pathway that is dependent on its second RRM. Consistent with the use of a novel subnuclear targeting pathway, PSF redistributes to perinucleolar clusters upon the addition of a transcription inhibitor whereas other splicing factors display increased localization to speckles in the absence of transcription. A yeast two-hybrid screen identified four-and-a-half LIM-only protein 2 (FHL2) as a potential RRM2 interaction partner, indicating a possible role for zinc-finger or LIM domains in the localization of splicing factors to subnuclear speckles.

Translocation of a human focal adhesion LIM-only protein, FHL2, during myofibrillogenesis and identification of LIM2 as the principal determinants of FHL2 focal adhesion localization

LIM domain proteins are found to be important regulators in cell growth, cell fate determination, cell differentiation, and remodeling of the cell cytoskeleton. Human Four-and-a-half LIM-only protein 2 (FHL2) is expressed predominantly in human heart and is only slightly expressed in skeletal muscle. Since FHL2 is an abundant protein in human heart, it may play an important role in the regulation of cell differentiation and myofibrillogenesis of heart at defined subcellular compartment. Therefore, we hypothesized that FHL2 act as a multi-functional protein by the specific arrangement of the LIM domains of FHL2 and that one of the LIM domains of FHL2 can function as an anchor and localizes it into a specific subcellular compartment in a cell type specific manner to regulate myofibrillogenesis. From our results, we observed that FHL2 is localized at the focal adhesions of the C2C12, H9C2 myoblast as well as a nonmyogenic cell line, HepG2 cells. Colocalization of vinculin-CFP and FHL2-GFP at focal adhesions was also observed in cell lines. Site-directed mutagenesis, in turn, suggested that the second LIM domain-LIM2 is essential for its specific localization to focal adhesions. Moreover, FHL2 was observed along with F-actin and focal adhesion of C2C12 and H9C2 myotubes. Finally, we believe that FHL2 moves from focal adhesions and then stays at the Z-discs of terminally differentiated heart muscle.

DRAL is a p53-responsive gene whose four and a half LIM domain protein product induces apoptosis

DRAL is a four and a half LIM domain protein identified because of its differential expression between normal human myoblasts and the malignant counterparts, rhabdomyosarcoma cells. In the current study, we demonstrate that transcription of the DRAL gene can be stimulated by p53, since transient expression of functional p53 in rhabdomyosarcoma cells as well as stimulation of endogenous p53 by ionizing radiation in wild-type cells enhances DRAL mRNA levels. In support of these observations, five potential p53 target sites could be identified in the promoter region of the human DRAL gene. To obtain insight into the possible functions of DRAL, ectopic expression experiments were performed. Interestingly, DRAL expression efficiently triggered apoptosis in three cell lines of different origin to the extent that no cells could be generated that stably overexpressed this protein. However, transient transfection experiments as well as immunofluorescence staining of the endogenous protein allowed for the localization of DRAL in different cellular compartments, namely cytoplasm, nucleus, focal contacts, as well as Z-discs and to a lesser extent the M-bands in cardiac myofibrils. These data suggest that downregulation of DRAL might be involved in tumor development. Furthermore, DRAL expression might be important for heart function.

The LIM-only protein DRAL/FHL2 binds to the cytoplasmic domain of several alpha and beta integrin chains and is recruited to adhesion complexes

LIM proteins contain one or more double zinc finger structures (LIM domains) mediating specific contacts between proteins that participate in the formation of multiprotein complexes. We report that the LIM-only protein DRAL/FHL2, with four and a half LIM domains, can associate with alpha(3A), alpha(3B), alpha(7A), and several beta integrin subunits as shown in yeast two-hybrid assays as well as after overexpression in human cells. The amino acid sequence immediately following the conserved membrane-proximal region in the integrin alpha subunits or the C-terminal region with the conserved NXXY motif of the integrin beta subunits are critical for binding DRAL/FHL2. Furthermore, the DRAL/FHL2 associates with itself and with other molecules that bind to the cytoplasmic domain of integrin alpha subunits. Deletion analysis of DRAL/FHL2 revealed that particular LIM domains or LIM domain combinations bind the different proteins. These results, together with the fact that full-length DRAL/FHL2 is found in cell adhesion complexes, suggest that it is an adaptor/docking protein involved in integrin signaling pathways.

Alzheimer's disease-associated presenilin 2 interacts with DRAL, an LIM-domain protein

Using the yeast two-hybrid system, we screened for proteins interacting with presenilin 2 (PS2) and cloned DRAL. DRAL is an LIM-only protein containing four LIM domains and an N-terminal half LIM domain. Previously DRAL has been cloned as a co-activator of the androgen receptor and as a protein interacting with a DNA replication regulatory protein, hCDC47. Our yeast two-hybrid assay showed that DRAL interacted with a hydrophilic loop region (amino acids 269-298) in the endoproteolytic N-terminal fragment of PS2, but not that of PS1, although the region 269-298 of PS2 and the corresponding PS1 sequence differ by only three amino acids. Each point mutation within this region, R275A, T280A, Q282A, R284A, N285A, P287T, I288L, F289A and S296A, in PS2 abolished the binding. This suggests that DRAL recognizes the PS2 structure specifically. The in vitro interaction was confirmed by affinity column assay and the physiological interactions between endogenous PS2 and DRAL by co-immunoprecipitation from human lung fibroblast MRC5 cells. Furthermore, in PS2-overexpressing HEK293 cells, we found an increase in the amount of DRAL in the membrane fraction and an increase in the amount of DRAL that was co-immunoprecipitated with PS2. The potential role of DRAL in the cellular signaling suggests that DRAL functions as an adaptor protein that links PS2 to an intracellular signaling.