The ubiquitously expressed DNA-binding protein late SV40 factor binds Ig switch regions and represses class switching to IgA

TFCP2 as a therapeutic nexus: unveiling molecular signatures in cancer

Tumor suppressor genes and proto-oncogenes comprise most of the complex genomic landscape associated with cancer, with a minimal number of genes exhibiting dual-context-dependent functions. The transcription factor cellular promoter 2 (TFCP2), a pivotal transcription factor encoded by the alpha globin transcription factor CP2 gene, is a constituent of the TFCP2/grainyhead family of transcription factors. While grainyhead members have been extensively studied for their crucial roles in developmental processes, embryogenesis, and multiple cancers, the TFCP2 subfamily has been relatively less explored. The molecular mechanisms underlying TFCP2's involvement in carcinogenesis are still unclear even though it is a desirable target for cancer treatment and a therapeutic marker. This comprehensive literature review summarizes the molecular functions of TFCP2, emphasizing its involvement in cancer pathophysiology, particularly in the epithelial-mesenchymal transition and metastasis. It highlights TFCP2's critical function as a regulatory target and explores its potential as a prognostic marker for survival and inflammation in carcinomas. Its ambiguous association with carcinomas underlines the urgent need for an in-depth understanding to facilitate the development of more efficacious targeted therapeutic modality and diagnostic tools. This study aims to elucidate the multifaceted effects of TFCP2 regulation, through a comprehensive integration of the existing knowledge in cancer therapeutics. Furthermore, the clinical relevance and the inherent challenges encountered in investigating its intricate role in cancer pathogenesis have been discussed in this review.

Expression of TSG101 protein and LSF transcription factor in HPV-positive cervical cancer cells

Our previous study demonstrated a decreased expression of tumor susceptibility gene 101 (TSG101) in cervical cancer cells. To identify the mechanism responsible for TSG101 downregulation during cervical cancer development, we analyzed the TSG101 promoter using cis-element cluster finder software. One of the transcription factors whose binding site was detected in the TSG101 promoter was late SV40 factor (LSF). The aim of this study was to analyze the TSG101 protein and LSF expression levels during cervical cancer development. Immunohistochemical analysis confirmed a previously observed decreased expression of TSG101, whereas quantitative polymerase chain reaction (qPCR) and immunohistochemistry analysis revealed high expression of LSF in cervical, precancer and cancer cells compared with human papillomavirus (HPV)-negative non-cancer samples. High expression of LSF in cervical cancer HPV-positive cells suggests that this protein may be important in the regulation of TSG101 expression, as well as in cervical carcinogenesis. The role of LSF as a mediator in cervical cancer development must be confirmed in future studies.

Antiproliferative small-molecule inhibitors of transcription factor LSF reveal oncogene addiction to LSF in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide. Despite the prevalence of HCC, there is no effective, systemic treatment. The transcription factor LSF is a promising protein target for chemotherapy; it is highly expressed in HCC patient samples and cell lines, and promotes oncogenesis in rodent xenograft models of HCC. Here, we identify small molecules that effectively inhibit LSF cellular activity. The lead compound, factor quinolinone inhibitor 1 (FQI1), inhibits LSF DNA-binding activity both in vitro, as determined by electrophoretic mobility shift assays, and in cells, as determined by ChIP. Consistent with such inhibition, FQI1 eliminates transcriptional stimulation of LSF-dependent reporter constructs. FQI1 also exhibits antiproliferative activity in multiple cell lines. In LSF-overexpressing cells, including HCC cells, cell death is rapidly induced; however, primary or immortalized hepatocytes are unaffected by treatment with FQI1. The highly concordant structure-activity relationship of a panel of 23 quinolinones strongly suggests that the growth inhibitory activity is due to a single biological target or family. Coupled with the striking agreement between the concentrations required for antiproliferative activity (GI(50)s) and for inhibition of LSF transactivation (IC(50)s), we conclude that LSF is the specific biological target of FQIs. Based on these in vitro results, we tested the efficacy of FQI1 in inhibiting HCC tumor growth in a mouse xenograft model. As a single agent, tumor growth was dramatically inhibited with no observable general tissue cytotoxicity. These findings support the further development of LSF inhibitors for cancer chemotherapy.

The IgH locus 3' regulatory region: pulling the strings from behind

Antigen receptor gene loci are among the most complex in mammals. The IgH locus, encoding the immunoglobulin heavy chain (IgH) in B-lineage cells, undergoes major transcription-dependent DNA remodeling events, namely V(D)J recombination, Ig class-switch recombination (CSR), and somatic hypermutation (SHM). Various cis-regulatory elements (encompassing promoters, enhancers, and chromatin insulators) recruit multiple nuclear factors in order to ensure IgH locus regulation by tightly orchestrated physical and/or functional interactions. Among major IgH cis-acting regions, the large 3' regulatory region (3'RR) located at the 3' boundary of the locus includes several enhancers and harbors an intriguing quasi-palindromic structure. In this review, we report progress insights made over the past decade in order to describe in more details the structure and functions of IgH 3'RRs in mouse and human. Generation of multiple cellular, transgenic and knock-out models helped out to decipher the function of the IgH 3' regulatory elements in the context of normal and pathologic B cells. Beside its interest in physiology, the challenge of elucidating the locus-wide cross talk between distant cis-regulatory elements might provide useful insights into the mechanisms that mediate oncogene deregulation after chromosomal translocations onto the IgH locus.

Prolyl isomerase Pin1 regulates transcription factor LSF (TFCP2) by facilitating dephosphorylation at two serine-proline motifs

Transcription factor LSF is essential for cell cycle progression, being required for activating expression of the thymidylate synthase (Tyms) gene at the G1/S transition. We previously established that phosphorylation of LSF in early G1 at Ser-291 and Ser-309 inhibits its transcriptional activity and that dephosphorylation later in G1 is required for its reactivation. Here we reveal the role of prolyl cis-trans isomerase Pin1 in activating LSF, by facilitating dephosphorylation at both Ser-291 and Ser-309. We demonstrate that Pin1 binds LSF both in vitro and in vivo. Using coimmunoprecipitation assays, we identify three SP/TP motifs in LSF (at residues Ser-291, Ser-309, and Thr-329) that are required and sufficient for association with Pin1. Co-expression of Pin1 enhances LSF transactivation potential in reporter assays. The Pin1-dependent enhancement of LSF activity requires residue Thr-329 in LSF, requires both the WW and PPiase domains of Pin1, and correlates with hypophosphorylation of LSF at Ser-291 and Ser-309. These findings support a model in which the binding of Pin1 at the Thr-329-Pro-330 motif in LSF permits isomerization by Pin1 of the peptide bonds at the nearby phosphorylated SP motifs (Ser-291 and Ser-309) to the trans configuration, thereby facilitating their dephosphorylation.

The evolutionary diversification of LSF and Grainyhead transcription factors preceded the radiation of basal animal lineages

Background

The transcription factors of the LSF/Grainyhead (GRH) family are characterized by the possession of a distinctive DNA-binding domain that bears no clear relationship to other known DNA-binding domains, with the possible exception of the p53 core domain. In triploblastic animals, the LSF and GRH subfamilies have diverged extensively with respect to their biological roles, general expression patterns, and mechanism of DNA binding. For example, Grainyhead (GRH) homologs are expressed primarily in the epidermis, and they appear to play an ancient role in maintaining the epidermal barrier. By contrast, LSF homologs are more widely expressed, and they regulate general cellular functions such as cell cycle progression and survival in addition to cell-lineage specific gene expression.

Results

To illuminate the early evolution of this family and reconstruct the functional divergence of LSF and GRH, we compared homologs from 18 phylogenetically diverse taxa, including four basal animals (Nematostella vectensis, Vallicula multiformis, Trichoplax adhaerens, and Amphimedon queenslandica), a choanoflagellate (Monosiga brevicollis) and several fungi. Phylogenetic and bioinformatic analyses of these sequences indicate that (1) the LSF/GRH gene family originated prior to the animal-fungal divergence, and (2) the functional diversification of the LSF and GRH subfamilies occurred prior to the divergence between sponges and eumetazoans. Aspects of the domain architecture of LSF/GRH proteins are well conserved between fungi, choanoflagellates, and metazoans, though within the Metazoa, the LSF and GRH families are clearly distinct. We failed to identify a convincing LSF/GRH homolog in the sequenced genomes of the algae Volvox carteri and Chlamydomonas reinhardtii or the amoebozoan Dictyostelium purpureum. Interestingly, the ancestral GRH locus has become split into two separate loci in the sea anemone Nematostella, with one locus encoding a DNA binding domain and the other locus encoding the dimerization domain.

Conclusions

In metazoans, LSF and GRH proteins play a number of roles that are essential to achieving and maintaining multicellularity. It is now clear that this protein family already existed in the unicellular ancestor of animals, choanoflagellates, and fungi. However, the diversification of distinct LSF and GRH subfamilies appears to be a metazoan invention. Given the conserved role of GRH in maintaining epithelial integrity in vertebrates, insects, and nematodes, it is noteworthy that the evolutionary origin of Grh appears roughly coincident with the evolutionary origin of the epithelium.

Binding of LBP-1a to specific immunoglobulin switch regions in vivo correlates with specific repression of class switch recombination

Upon stimulation of mature B cells, class switch recombination (CSR) can alter the specific immunoglobulin heavy chain constant region that is expressed. In a tissue culture cell line, we previously demonstrated that inhibition of late SV40 factor (LSF) family members enhanced IgM to IgA CSR. Here, isotype specificity of CSR regulation by LSF family members is addressed in primary mouse splenic B cells. First, we demonstrate that leader-binding protein-1a (LBP-1a) is the prevalent family member in B lymphocytes. Second, we demonstrate by ChIP that LBP-1a binds genomic sequences around mouse switch (S) regions in an isotype-specific manner, in accordance with computational predictions: binding is observed to Smu and Salpha, but not to the tested Sgamma1, regions. Importantly, binding of LBP-1a is tightly regulated, with occupancy at genomic S regions dramatically decreasing following LPS stimulation. Finally, the consequence of DNA-binding by LBP-1a is determined using bone marrow chimeric mice in which LSF/LBP-1 activity is inhibited in hematopoietic lineages. Upon in vitro stimulation of such primary B cells, CSR occurs with a higher efficiency to IgA, but not to IgG1. These results are supportive of a model whereby LBP-1a represses CSR in an isotype-specific manner via direct interaction with S regions involved in the recombination.

Phosphorylation by cyclin C/cyclin-dependent kinase 2 following mitogenic stimulation of murine fibroblasts inhibits transcriptional activity of LSF during G1 progression

Transcription factor LSF is required for progression from quiescence through the cell cycle, regulating thymidylate synthase (Tyms) expression at the G(1)/S boundary. Given the constant level of LSF protein from G(0) through S, we investigated whether LSF is regulated by phosphorylation in G(1). In vitro, LSF is phosphorylated by cyclin E/cyclin-dependent kinase 2 (CDK2), cyclin C/CDK2, and cyclin C/CDK3, predominantly on S309. Phosphorylation of LSF on S309 is maximal 1 to 2 h after mitogenic stimulation of quiescent mouse fibroblasts. This phosphorylation is mediated by cyclin C-dependent kinases, as shown by coimmunoprecipitation of LSF and cyclin C in early G(1) and by abrogation of LSF S309 phosphorylation upon suppression of cyclin C with short interfering RNA. Although mouse fibroblasts lack functional CDK3 (the partner of cyclin C in early G(1) in human cells), CDK2 compensates for this absence. By transient transfection assays, phosphorylation at S309, mediated by cyclin C overexpression, inhibits LSF transactivation. Moreover, overexpression of cyclin C and CDK3 inhibits induction of endogenous Tyms expression at the G(1)/S transition. These results identify LSF as only the second known target (in addition to pRb) of cyclin C/CDK activity during progression from quiescence to early G(1). Unexpectedly, this phosphorylation prevents induction of LSF target genes until late G(1).

The regulation of IgA class switching

IgA class switching is the process whereby B cells acquire the expression of IgA, the most abundant antibody isotype in mucosal secretions. IgA class switching occurs via both T-cell-dependent and T-cell-independent pathways, and the antibody targets both pathogenic and commensal microorganisms. This Review describes recent advances indicating that innate immune recognition of microbial signatures at the epithelial-cell barrier is central to the selective induction of mucosal IgA class switching. In addition, the mechanisms of IgA class switching at follicular and extrafollicular sites within the mucosal environment are summarized. A better understanding of these mechanisms may help in the development of more effective mucosal vaccines.

The histone methyltransferase Suv39h1 increases class switch recombination specifically to IgA

Ab class (isotype) switching allows the humoral immune system to adaptively respond to different infectious organisms. Isotype switching occurs by intrachromosomal DNA recombination between switch (S) region sequences associated with C(H) region genes. Although isotype-specific transcription of unrearranged (germline) C(H) genes is required for switching, recent results suggest that isotype specificity is also determined by the sequences of downstream (acceptor) S regions. In the current study, we identify the histone methyltransferase Suv39h1 as a novel Salpha-specific factor that specifically increases IgA switching (Smu-Salpha recombination) in a transiently transfected plasmid S substrate, and demonstrate that this effect requires the histone methyltransferase activity of Suv39h1. Additionally, B cells from Suv39h1-deficient mice have an isotype-specific reduction in IgA switching with no effect on the level of germline Ialpha-Calpha transcripts. Taken together, our results suggest that Suv39h1 activity inhibits the activity of a sequence-specific DNA-binding protein that represses switch recombination to IgA.

Heterodimerization with LBP-1b is necessary for nuclear localization of LBP-1a and LBP-1c

The LBP-1 family consists of four proteins, which act as transcription factors in the formation of dimers with a member of this family. LBP-1a and LBP-1b are splicing variants from one gene, and LBP-1c and LBP-1d also arise from the alternative splicing of another gene. Investigation of subcellular localization of LBP-1 proteins fused to YFP revealed that the LBP-1b was localized in the nucleus, whereas LBP-1a and LBP-1c were exclusively localized in the cytosol. The peptide of 36 amino acids encoded by exon 6, a specific exon used only for LBP-1b, possessed the function of a nuclear localization signal (NLS). Nuclear localization of LBP-1a and LBP-1c occurred when LBP-1b was co-expressed, suggesting that heterodimerization of LBP-1a and LBP-1c with LBP-1b is important for their nuclear transport. Transiently expressed LBP-1 proteins in COS-7 cells formed speckles in the nucleus. Most speckles overlapped with the PML body. The activity of LBP-1a for accumulation in the PML body was mapped in the N-terminal region.

Mitogen-activated protein kinases regulate LSF occupancy at the human immunodeficiency virus type 1 promoter

Human immunodeficiency virus type 1 (HIV-1) establishes a persistent, nonproductive state within a small population of memory CD4(+) cells. The transcription factor LSF binds to sequences within the HIV-1 long terminal repeat (LTR) initiation region and recruits a second factor, YY1, to the LTR. These factors then cooperatively recruit histone deacetylase 1 to the LTR, resulting in inhibition of transcription. This appears to be one mechanism contributing to HIV persistence within resting CD4(+) T cells. We sought to further detail LSF binding to the HIV-1 LTR and factors that regulate LSF occupancy. We find that LSF binds the LTR as a tetramer and that binding is regulated by phosphorylation mediated by mitogen-activated protein kinases (MAPKs). In vitro, phosphorylation of LSF by Erk decreases binding to the LTR, while binding is increased by p38 phosphorylation. LSF occupancy at LTR chromatin is increased by the p38 agonist anisomycin and decreased by specific p38 inhibition. p38 inhibition also results in increased acetylation of histone H4 at the LTR nucleosome adjacent to the LSF binding site. p38 inhibition also blocked the ability of YY1 to inhibit activation of the integrated HIV promoter. Finally, HIV was recovered from the resting CD4(+) T cells of aviremic, HIV-infected donors upon treatment of these cells with specific inhibitor of p38. These data suggest that the MAPK pathway regulates LSF binding to the LTR and thereby one aspect of the regulation of HIV expression. This mechanism could be exploited as a novel therapeutic target to disrupt latent HIV infection.

Lineage-specific and ubiquitous biological roles of the mammalian transcription factor LSF

Transcriptional regulation in mammalian cells is driven by a complex interplay of multiple transcription factors that respond to signals from either external or internal stimuli. A single transcription factor can control expression of distinct sets of target genes, dependent on its state of post-translational modifications, interacting partner proteins, and the chromatin environment of the cellular genome. Furthermore, many transcription factors can act as either transcriptional repressors or activators, depending on promoter and cellular contexts [Alvarez, M., Rhodes, S.J., Bidwell, J.P., 2003. Context-dependent transcription: all politics is local. Gene 313, 43-57]. Even in this light, the versatility of LSF (Late SV40 Factor) is remarkable. A hallmark of LSF is its unusual DNA binding domain, as evidenced both by lack of homology to any other established DNA-binding domains and by its DNA recognition sequence. Although a dimer in solution, LSF requires additional multimerization with itself or partner proteins in order to interact with DNA. Transcriptionally, LSF can function as an activator or a repressor. It is a direct target of an increasing number of signal transduction pathways. Biologically, LSF plays roles in cell cycle progression and cell survival, as well as in cell lineage-specific functions, shown most strikingly to date in hematopoietic lineages. This review discusses how the unique aspects of LSF DNA-binding activity may make it particularly susceptible to regulation by signal transduction pathways and may relate to its distinct biological roles. We present current progress in elucidation of both tissue-specific and more universal cellular roles of LSF. Finally, we discuss suggestive data linking LSF to signaling by the amyloid precursor protein and to Alzheimer's disease, as well as to the regulation of latency of the human immunodeficiency virus (HIV).

DNA acrobats of the Ig class switch

Small resting B lymphocytes all start out producing IgM Abs. Upon encountering Ag, the cells become activated and make a switch from IgM to other Ig classes. This class switch serves to distribute a particular V region to different Ig C regions. Each C region mediates a specialized effector function, and so, through switching, an organism can guide its Abs to various sites. Creating the new H chain requires loop-out and deletion of DNA between switch regions. These DNA acrobatics require transcription of the switch regions, presumably so that necessary factors can gain access to the DNA. These requisite switching factors include activation-induced cytidine deaminase and components of general DNA repair, including base excision repair, mismatch repair, and double-strand break repair. Despite much recent progress, not all important factors have been discovered, especially those that may guide recombination to a particular subclass.

Selective inhibition of class switching to IgG and IgE by recruitment of the HoxC4 and Oct-1 homeodomain proteins and Ku70/Ku86 to newly identified ATTT cis-elements

Immunoglobulin (Ig) class switching is central to the maturation of the antibody response as IgG, IgA, and IgE are endowed with more diverse biological effector functions than IgM. It is induced upon engagement of CD40 on B lymphocytes by CD40L expressed by activated CD4+ T cells and exposure of B cells to T cell-secreted cytokines including interleukin-4 and transforming growth factor-beta. It begins with germ line IH-CH transcription and unfolds through class switch DNA recombination (CSR). We show here that the HoxC4 and Oct-1 homeodomain proteins together with the Ku70/Ku86 heterodimer bind as a complex to newly identified switch (S) regulatory ATTT elements (SREs) in the Igamma and Iepsilon promoters and downstream regions to dampen basal germ line Igamma-Cgamma and Iepsilon-Cepsilon transcriptions and repress CSR to Cgamma and Cepsilon. This mechanism is inactive in the Calpha1/Calpha2 loci because of the lack of SREs in the Ialpha1/Ialpha2 promoters. Accordingly, in resting human IgM+IgD+ B cells, HoxC4, Oct-1, and Ku70/Ku86 can be readily identified as bound to the Igamma and Iepsilon promoters but not the Ialpha1/Ialpha2 promoters. CD40 signaling dissociates the HoxC4.Oct-1. Ku complex from the Igamma and Iepsilon promoter SREs, thereby relieving the IH-CH transcriptional repression and allowing CSR to unfold. Dissociation of HoxC4.Oct-1. Ku from DNA is hampered by CD153 engagement, a CD40-signaling inhibitor. Thus, these findings outline a HoxC4.Oct-1. Ku-dependent mechanism of selective regulation of class switching to IgG and IgE and further suggest distinct co-evolution and shared CSR activation pathways in the Cgamma and Cepsilon as opposed to the Calpha1/Calpha2 loci.

Yin Yang 1 is a lipopolysaccharide-inducible activator of the murine 3' Igh enhancer, hs3

The 3' Igh enhancers, DNase I hypersensitive site (hs) 3B and/or hs4, are required for germline transcription, and hence, class switch recombination for multiple isotypes. A number of hs3-binding transcription factors have been identified by EMSA, including octamer and NF-kappa B family members, and Pax5. We have found that the binding of the transcription factor, Yin Yang 1 (YY1), to hs3 and to the mu E1 site of the intronic enhancer, E mu, is induced in primary splenic B cells after approximately 48 h in response to LPS and other activators of class switch recombination. Transient transfection experiments in B cell lines indicate that YY1 is an activator of hs3. Interestingly, levels of YY1 expression are unchanged in resting and LPS-stimulated B cells. Mixing experiments followed by EMSA showed that a protein present in resting B cells prevented binding of YY1 to DNA. We found that recombinant retinoblastoma protein (Rb) inhibited binding of YY1 to hs3 in a dose-dependent manner, and we have identified complexes of endogenous YY1 with the Rb in resting B cells, but not in LPS-stimulated B cells. A difference in Rb phosphorylation state was also confirmed between resting (G(0)) B cells and LPS-stimulated B cells. These observations suggest that the interaction of YY1 with hypophosphorylated Rb in resting B cells prevents interaction of YY1 with DNA. After stimulation with class-switching activators, such as LPS, Rb becomes hyperphosphorylated and YY1 is released and can then bind to the hs3 enhancer and E mu.

Accessibility control and machinery of immunoglobulin class switch recombination

Immunoglobulin (Ig) class switching is a process by which B lymphocytes shift from production of IgM to other Ig classes and subclasses via Ig class switch recombination (CSR). Multiple cellular and molecular processes are involved in CSR. Induction of a given IgH germline transcription initiates CSR processes. Ig germline transcription is selectively activated and induced by specific cytokine(s) via cytokine-specific signal pathways, synergized by CD40 signaling, and optimized by the 3' Ig alpha enhancers through locus control region function. Following Ig germline transcription, the switch-region DNA undergoes conformational changes so that it can serve as an appropriate substrate for nicking and cleavage by switch recombination machinery. Finally, the double-strand breaks in donor and acceptor switch DNAs are processed, repaired, and ligated through a general nonhomologous end join pathway. CSR generates a new transcriptional unit for production of a class-switched Ig isotype.