A family of LIM-only transcriptional coactivators: tissue-specific expression and selective activation of CREB and CREM

FHL3 is an actin-binding protein that regulates alpha-actinin-mediated actin bundling: FHL3 localizes to actin stress fibers and enhances cell spreading and stress fiber disassembly

Four and a half LIM domain (FHL) proteins are members of the LIM protein superfamily. Several FHL proteins function as co-activators of CREM/CREB transcription factors and the androgen receptor. FHL3 is highly expressed in skeletal muscle, but its function is unknown. FHL3 localized to the nucleus in C2C12 myoblasts and, following integrin engagement, exited the nucleus and localized to actin stress fibers and focal adhesions. In mature skeletal muscle FHL3 was found at the Z-line. Actin was identified as a potential FHL3 binding partner in yeast two-hybrid screening of a skeletal muscle library. FHL3 complexed with actin both in vitro and in vivo as shown by glutathione S-transferase pull-down assays and co-immunoprecipitation of recombinant and endogenous proteins. FHL3 promoted cell spreading and when overexpressed in spread C2C12 cells disrupted actin stress fibers. Increased FHL3 expression was detected in highly motile cells migrating into an artificial wound, compared with non-motile cells. The molecular mechanism by which FHL3 induced actin stress fiber disassembly was demonstrated by low speed actin co-sedimentation assays and electron microscopy. FHL3 inhibited alpha-actinin-mediated actin bundling. These studies reveal FHL3 as a significant regulator of actin cytoskeletal dynamics in skeletal myoblasts.

Activation of the coactivator four-and-a-half-LIM-only protein FHL2 and the c-fos promoter through inhibition of protein phosphatase 2A

Previous studies have demonstrated that the serine/threonine protein phosphatase 2A (PP2A) can modulate the transcriptional activity of several sequence-specific DNA-binding proteins. However, less is known about the effect of PP2A on the activities of general transcription factors and transcriptional coregulators. Here we describe that the activity of a general coactivator, the four-and-a-half-LIM-only protein 2 (FHL2), is regulated in a PP2A-dependent manner. Specific inhibition of PP2A by simian virus 40 (SV40) small t-antigen (st-ag) stimulated the intrinsic transcriptional activity of FHL2 more than 10-fold, while a st-ag mutant unable to bind PP2A had no effect. Overexpression of the B56 subunits alpha, beta, and gamma1 of PP2A impaired the induction of FHL2 by st-ag. FHL2 functioned as a coactivator for CREB-mediated transcription, and inactivation of PP2A further increased FHL2-induced CREB-directed transcription. Overexpression of FHL2 readily enhanced the transcription of the luciferase reporter gene driven by the c-fos promoter, and inhibition of PP2A further stimulated FHL2-induced transactivation of this promoter. These results suggest that dephosphorylation of the general coactivator FHL2 may represent a novel mechanism by which PP2A modulates the transcription of FHL2-responsive genes.

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.

The LIM-only protein FHL2 is a serum-inducible transcriptional coactivator of AP-1

Proteins with LIM domains have been implicated in transcriptional regulation. The four and half LIM domain (FHL) group of LIM-only proteins is composed of five members, some of which have been shown to have intrinsic activation function. Here we show that FHL2 is the only member of the family whose expression is inducible upon serum stimulation in cultured cells. Induction of FHL2 is coordinated in time with the increased levels of two early-response products, the oncoproteins Fos and Jun. FHL2 associates with both Jun and Fos, in vitro and in vivo. The FHL2-Jun interaction requires the Ser-63-Ser-73 JNK phosphoacceptor sites in c-Jun, but not their phosphorylation. FHL2 powerfully stimulates Fos- and Jun-dependent transcription, thereby acting as an inducible coactivator of AP-1 function. Moreover, we show that intracellular localization of FHL2 is controlled by signaling events and a Crm1-dependent active nuclear export mechanism. Thus, FHL2, as an inducible coactivator of AP-1, coordinately participates with Fos and Jun in the early transcriptional response to serum factors.

Skeletal muscle LIM protein 1 regulates integrin-mediated myoblast adhesion, spreading, and migration

The skeletal muscle LIM protein 1 (SLIM1) is highly expressed in skeletal and cardiac muscle, and its expression is downregulated significantly in dilated human cardiomyopathy. However, the function of SLIM1 is unknown. In this study, we investigated the intracellular localization of SLIM1. Endogenous and recombinant SLIM1 localized to the nucleus, stress fibers, and focal adhesions in skeletal myoblasts plated on fibronectin, collagen, or laminin. However, after inhibition of integrin signaling either by plating on poly-l-lysine or by soluble RGD peptide, SLIM1 localized diffusely in the cytosol, with decreased nuclear expression. Disruption of the actin cytoskeleton by cytochalasin D did not inhibit nuclear localization of SLIM1 in integrin-activated cells. Green fluorescent protein-tagged SLIM1 shuttled in the nucleus of untransfected NIH 3T3 cells, in a heterokaryon fusion assay. Overexpression of SLIM1 in Sol8 myoblasts inhibited cell adhesion and promoted cell spreading and migration. These studies show SLIM1 localizes in an integrin-dependent manner to the nucleus and focal adhesions where it functions downstream of integrin activation to promote cell spreading and migration.

Identification of the LIM protein FHL2 as a coactivator of beta-catenin

Beta-catenin is a key mediator of the Wnt pathway, which plays a critical role in embryogenesis and oncogenesis. As a transcriptional activator, beta-catenin binds the transcription factors, T-cell factor and lymphoid enhancer factor, and regulates gene expression in response to Wnt signaling. Abnormal activation of beta-catenin has been linked to various types of cancer. In a yeast two-hybrid screen, we identified the four and a half of LIM-only protein 2 (FHL2) as a novel beta-catenin-interacting protein. Here we show specific interaction of FHL2 with beta-catenin, which requires the intact structure of FHL2 and armadillo repeats 1-9 of beta-catenin. FHL2 cooperated with beta-catenin to activate T-cell factor/lymphoid enhancer factor-dependent transcription from a synthetic reporter and the cyclin D1 and interleukin-8 promoters in kidney and colon cell lines. In contrast, coexpression of beta-catenin and FHL2 had no synergistic effect on androgen receptor-mediated transcription, whereas each of these two coactivators independently stimulated AR transcriptional activity. Thus, the ability of FHL2 to stimulate the trans-activating function of beta-catenin might be dependent on the promoter context. The detection of increased FHL2 expression in hepatoblastoma, a liver tumor harboring frequent beta-catenin mutations, suggests that FHL2 might enforce beta-catenin transactivation activity in cancer cells. These findings reveal a new function of the LIM coactivator FHL2 in transcriptional activation of Wnt-responsive genes.

Many facets of mammalian lanosterol 14alpha-demethylase from the evolutionarily conserved cytochrome P450 family CYP51

Lanosterol 14alpha-demethylase is a cytochrome P450 enzyme of the cholesterol biosynthetic pathway belonging to the CYP51 gene family which is the most evolutionarily conserved member of the CYP superfamily. Mammalian (human, mouse, rat, pig) CYP51 genes are unique in sharing several common characteristics: highly conserved exon/intron borders and proximal promoter structures, ubiquitous expression at the highest level in the testis, and appearance of testis-specific transcripts that arise from differential polyadenylation site usage. CYP51 protein demethylates lanosterol to form follicular fluid meiosis-activating sterol, FF-MAS, which is, besides being an intermediate of cholesterol biosynthesis, also a signaling sterol that accumulates in ovaries. CYP51 protein resides in the endoplasmatic reticulum of most cells and also in acrosomal membranes of spermatids where transport through the Golgi apparatus is suggested. While sterol regulatory element binding protein (SREBP)-dependent transcriptional regulation of CYP51 contributes to synthesis of cholesterol, the germ-cell-specific cAMP/CREMtau-dependent upregulation might contribute to increased production of MAS.

CREM-dependent transcription in male germ cells controlled by a kinesin

ACT is a LIM-only protein expressed exclusively in round spermatids, where it cooperates with transcriptional activator CREM in regulating various postmeiotic genes. Targeted inactivation of CREM leads to a complete block of mouse spermiogenesis. We sought to identify the regulatory steps controlling the functional interplay between CREM and ACT. We found that ACT selectively associates with KIF17b, a kinesin highly expressed in male germ cells. The ACT-KIF17b interaction is restricted to specific stages of spermatogenesis and directly determines the intracellular localization of ACT. Sensitivity to leptomycin B indicates that KIF17b can be actively exported from the nucleus through the Crm1 receptor. Thus, a kinesin directly controls the activity of a transcriptional coactivator by a tight regulation of its intracellular localization.

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.

Oct-1 potentiates CREB-driven cyclin D1 promoter activation via a phospho-CREB- and CREB binding protein-independent mechanism

Cyclin D1, the regulatory subunit for mid-G(1) cyclin-dependent kinases, controls the expression of numerous cell cycle genes. A cyclic AMP-responsive element (CRE), located upstream of the cyclin D1 mRNA start site, integrates mitogenic signals that target the CRE-binding factor CREB, which can recruit the transcriptional coactivator CREB-binding protein (CBP). We describe an alternative mechanism for CREB-driven cyclin D1 induction that involves the ubiquitous POU domain protein Oct-1. In the breast cancer cell line MCF-7, overexpression of Oct-1 or its POU domain strongly increases transcriptional activation of cyclin D1 and GAL4 reporter genes that is specifically dependent upon CREB but independent of Oct-1 DNA binding. Gel retardation and chromatin immunoprecipitation assays confirm that POU forms a complex with CREB bound to the cyclin D1 CRE. In solution, CREB interaction with POU requires the CREB Q2 domain and, notably, occurs with CREB that is not phosphorylated on Ser 133. Accordingly, Oct-1 also potently enhances transcriptional activation mediated by a Ser133Ala CREB mutant. Oct-1/CREB synergy is not diminished by the adenovirus E1A 12S protein, a repressor of CBP coactivator function. In contrast, E1A strongly represses CBP-enhanced transactivation by CREB phosphorylated on Ser 133. Our observation that Oct-1 potentiates CREB-dependent cyclin D1 transcriptional activity independently of Ser 133 phosphorylation and E1A-sensitive coactivator function offers a new paradigm for the regulation of cyclin D1 induction by proliferative signals.

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.

The LIM-only coactivator FHL2 modulates WT1 transcriptional activity during gonadal differentiation

An essential step during sex determination is the maintenance of the Müllerian duct in females and its regression in males caused by the expression of Müllerian inhibiting substance (MIS). In testes, the Wilms' tumor suppressor and the orphan nuclear receptor SF1 cooperatively bind to the promoter and activate transcription of MIS. In the ovaries, on the other hand, the orphan nuclear receptor DAX1 binds to SF1, inhibits transactivation by WT1/SF1 and thereby suppresses the induction of MIS expression. In addition, WT1 itself is responsible for the upregulation of DAX1 transcription. So far, little is known on which protein-protein interactions or cofactors elicit the spatiotemporal control of WT1-mediated transcription. Here we demonstrate coexpression of the LIM-only coactivator FHL2 and WT1. FHL2 and WT1 functionally interact both in vitro and in vivo. The importance of this interaction is revealed by the ability of FHL2 to potentiate the synergistic induction of MIS gene expression by WT1/SF1. Moreover, FHL2 coactivates transactivation of the DAX1 promoter by WT1. Hence, we present FHL2 as a novel transcriptional coactivator of WT1. The ability to modulate both DAX1 and MIS expression might allow FHL2 to act in the molecular fine tuning of WT1-dependent control mechanisms in the reproductive organs.

Characterization and functional analysis of cAMP response element modulator protein and activating transcription factor 2 (ATF2) isoforms in the human myometrium during pregnancy and labor: identification of a novel ATF2 species with potent transactivation properties

There is now extensive evidence to indicate that components of the cAMP signaling pathway are up-regulated in the human myometrium during pregnancy so as to potentiate the maintenance of uterine quiescence until term. In many tissue and cell types, increased signaling of the cAMP pathway results in profound changes in gene expression that are catalyzed via stimulation of PKA and activation of cAMP-dependent transcription factors that bind cAMP response elements (CREs) within the promoter regions of affected genes. In the myometrium, these CRE containing genes include beta2-adrenoceptor, cyclo-oxygenase 2, oxytocin receptor, and connexin-43. In preliminary investigations, we reported the differential expression of members of the cAMP bZIP protein family in the myometrium during pregnancy and labor. In this present study, we have now identified and functionally characterized these proteins with respect to myometrial gene expression. We report the identification of a 39,000 mol wt CRE response element modulator protein (CREM)tau2alpha protein having both transactivation and transrepressor properties whose expression is sequentially decreased in the myometrium during gestation and parturition. In contrast, expression of a myometrial 28,000 mol wt CREMalpha protein having only transrepressor actions progressively increased in the myometrium during pregnancy and labor. Similarly, we have isolated two ATF2 proteins of 60,000 and 28,000 mol wts, which represent full-length ATF2 and a novel small isoform of ATF2 that we have termed ATF2-small (ATF2-sm). These proteins are potent transactivators of gene expression and appear to be spatially expressed within the myometrium of the upper and lower uterine regions. The identification and functional characterization of these basic region/leucine zipper proteins in the myometrium may provide further insight into the molecular mechanisms regulating uterine activity during fetal maturation and parturition.

The transcriptional coactivator FHL2 transmits Rho signals from the cell membrane into the nucleus

GTPases of the Rho family are transducers of extracellular signals and control cellular processes such as organization of the actin cytoskeleton, motility, adhesion and gene regulation. The Rho signalling pathway is activated, for example, by bioactive sphingolipids such as sphingosine-1-phosphate (SPP) or by overexpression of Rho family members in tumorigenesis and metastases. Here, we show that stimulation of the Rho signalling pathway induces translocation of the transcriptional LIM-only coactivator FHL2 to the nucleus and subsequent activation of FHL2- and androgen receptor-dependent genes. Interestingly, prostate tumours overexpress Rho GTPases and display altered cellular localization of FHL2 concomitant with tumour dedifferentiation. SPP-induced FHL2 activation is mediated by Rho GTPases, but not by the GTPases Cdc42, Rac1 or Ras, and depends on Rho-kinase. In addition, Rho signalling influences other transcriptional coactivators, thus pointing to a general regulatory role for Rho GTPases in cofactor function. In summary, our data propose a yet undescribed signalling pathway in which the coactivator FHL2 acts as a novel molecular transmitter of the Rho signalling pathway, thereby integrating extracellular cues into altered gene expression.

Activation of extracellular signal-regulated kinases and CREB/ATF-1 mediate the expression of CCAAT/enhancer binding proteins beta and -delta in preadipocytes

The essential role of CCAAT/enhancer binding proteins (C/EBPs) beta and delta for adipocyte differentiation has been clearly established. In preadipocytes, their expression is up-regulated by the activation of leukemia inhibitory factor receptor (LIF-R) and prostacyclin receptor (IP-R) via the extracellular signal-regulated kinase (ERK) pathway and cAMP production, respectively. However, the molecular mechanisms by which LIF and prostacyclin-induced signals are propagated to the nucleus and the transcription factors mediating ERK and cAMP-induced C/EBP gene expression were unknown. Here we report that both pathways share cAMP responsive element binding protein/activation transcription factor 1 (CREB/ATF-1) as common downstream effectors. LIF-R and IP-R activation induced binding of CREB and/or ATF-1 to C/EBP promoters and CREB-dependent transcription. Expression of dominant negative forms of CREB dramatically reduced the LIF- and prostacyclin-stimulated C/EBP beta and C/EBP delta expression. Upon stimulation of the IP-R, the ERK pathway was activated in a PKA-dependent manner. ERK activation by the PKA pathway was not required for CREB/ATF-1 phosphorylation but rather was necessary for CREB-dependent up-regulation of C/EBPs expression. Our findings suggest that ERK activation is required for CREB transcriptional activity, possibly by recruitment of a coactivator.

Identification of genes involved in epithelial-mesenchymal transition and tumor progression

The adenovirus E1A12S gene product (WT12S) immortalizes epithelial cells and they retain their differentiated characteristics, but certain mutants cannot do the latter. Characterization of mutant immortalized epithelial cells indicated that they had undergone epithelial mesenchymal transition (EMT). Coexpression of V12ras with WT12S leads to benign tumors, but to malignant tumors with 12S mutants. Since EMT is critical for tumor progression, identification of the molecular mechanisms involved should elucidate novel therapeutic targets. To this end, representational difference analysis (RDA) was used to identify cDNAs upregulated in the mutant cell line. Thirty-five differentially expressed mRNAs were identified and classified into several functional categories, including nine novel cDNAs. Among the 26 known cDNAs, extracellular matrix and related proteins made up the largest group of differentially expressed genes, followed by growth factors and receptors and transcription factors. There was also an ion transporter, a cytoskeletal protein, glycosylation and amidinotransferase enzymes and proteins with unknown functions. Some of the known genes have previously been associated with EMT and/or tumor progression and thus served to validate the system to obtain the desired target genes, while other cDNAs are newly linked with dedifferentiation/malignancy. Array analyses indicated that the cDNAs were specifically upregulated in invasive or metastatic tumors, especially of breast, uterus and lung, suggesting their involvement in the progression of these tumors.

Transcriptional regulation by the phosphorylation-dependent factor CREB

The transcription factor CREB -- for 'cyclic AMP response element-binding protein' -- functions in glucose homeostasis, growth-factor-dependent cell survival, and has been implicated in learning and memory. CREB is phosphorylated in response to various signals, but how is specificity achieved in these signalling pathways?

Transcriptional cascades during spermatogenesis: pivotal role of CREM and ACT

The gene CREM plays key physiological and developmental roles within the hypothalamic--pituitary--gonadal axis. We have previously shown that CREM is highly expressed in male postmeiotic cells. Spermiogenesis is a complex process by which postmeiotic male germ cells differentiate into mature spermatozoa. CREM regulates the expression of a number of post-meiotic genes involved in the process of spermiogenesis. Using homologous recombination we have generated CREM-mutant mice that display a complete block at the first step of spermiogenesis. The molecular mechanism by which CREM elicits its regulatory function involves ACT (Activator of CREM in Testis), a testis-specific coactivator constituted by a repeat of four and half LIM domains. ACT is coordinately expressed with CREM, associates with it and confers a powerful transcriptional activation function. It is able to bypass the classical requirement of CREM phosphorylation and recruiting of CBP.

Spermiogenesis deficiency in mice lacking the Trf2 gene

The discovery of TATA-binding protein-related factors (TRFs) has suggested alternative mechanisms for gene-specific transcriptional regulation and raised interest in their biological functions. In contrast to recent observations of an embryonic lethal phenotype for TRF2 inactivation in Caenorhabditis elegans and Xenopus laevis, we found that Trf2-deficient mice are viable. However, Trf2-/- mice are sterile because of a severe defect in spermiogenesis. Postmeiotic round spermatids advance at most to step 7 of differentiation but fail to progress to the elongated form, and gene-specific transcription deficiencies were identified. We speculate that mammals may have evolved more specialized TRF2 functions in the testis that involve transcriptional regulation of genes essential for spermiogenesis.

Cloning and expression of activator of CREM in testis in human testicular tissue

Activator of cAMP-responsive element modulator (CREM) in testis (ACT) has recently been found in the mouse testis where it activates CREM, a transcription factor essential for the differentiation of spermatids into mature spermatozoa. The importance of CREM in human spermatogenesis prompted us to examine whether ACT was also present in the human testis. Western blot analysis, performed with an anti-mouse ACT serum, showed the presence of a single immunoreactive band of a size similar to murine ACT. A library screening resulted in the isolation and characterization of the complete cDNA which showed 88% homology with the mouse counterpart. The human ACT gene is composed of five coding exons, being the first untranslated, and the mRNA spans 835 nucleotides coding for a 284 amino acid protein. Expression studies by RT-PCR confirmed that ACT is present in normal human testis. The human ACT gene is localized on the chromosome 6.