Phosphorylation of the CCAAT displacement protein (CDP)/Cux transcription factor by cyclin A-Cdk1 modulates its DNA binding activity in G(2)

p200CUX1-regulated BMP8B inhibits the progression of acute myeloid leukemia via the MAPK signaling pathway

The full-length p200CUX1 protein encoded by the homology frame CUT-like protein (CUX1) plays an important role in tumors as a pro-oncogene or oncogene. However, its role and mechanism in acute myeloid leukemia remain unknown. p200CUX1 regulates several pathways, including the MAPK signaling pathway. Our data showed that p200CUX1 is lowly expressed in THP1 and U937 AML cell lines. Lentiviral overexpression of p200CUX1 reduced proliferation and promoted apoptosis and G0/G1 phase blockade, correlating with MAPK pathway suppression. Additionally, p200CUX1 regulated the expression of bone morphogenetic protein 8B (BMP8B), which is overexpressed in AML. Overexpression of p200CUX1 downregulated BMP8B expression and inhibited the MAPK pathway. Furthermore, BMP8B knockdown inhibited AML cell proliferation, enhanced apoptosis and the sensitivity of ATRA-induced cell differentiation, and blocked G0/G1 transition. Our findings demonstrate the pivotal function of the p200CUX1-BMP8B-MAPK axis in maintaining the viability of AML cells. Consequently, targeting p200CUX1 could represent a viable strategy in AML therapy.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Chemical Decorations of "MARs" Residents in Orchestrating Eukaryotic Gene Regulation

Genome organization plays a crucial role in gene regulation, orchestrating multiple cellular functions. A meshwork of proteins constituting a three-dimensional (3D) matrix helps in maintaining the genomic architecture. Sequences of DNA that are involved in tethering the chromatin to the matrix are called scaffold/matrix attachment regions (S/MARs), and the proteins that bind to these sequences and mediate tethering are termed S/MAR-binding proteins (S/MARBPs). The regulation of S/MARBPs is important for cellular functions and is altered under different conditions. Limited information is available presently to understand the structure–function relationship conclusively. Although all S/MARBPs bind to DNA, their context- and tissue-specific regulatory roles cannot be justified solely based on the available information on their structures. Conformational changes in a protein lead to changes in protein–protein interactions (PPIs) that essentially would regulate functional outcomes. A well-studied form of protein regulation is post-translational modification (PTM). It involves disulfide bond formation, cleavage of precursor proteins, and addition or removal of low-molecular-weight groups, leading to modifications like phosphorylation, methylation, SUMOylation, acetylation, PARylation, and ubiquitination. These chemical modifications lead to varied functional outcomes by mechanisms like modifying DNA–protein interactions and PPIs, altering protein function, stability, and crosstalk with other PTMs regulating subcellular localizations. S/MARBPs are reported to be regulated by PTMs, thereby contributing to gene regulation. In this review, we discuss the current understanding, scope, disease implications, and future perspectives of the diverse PTMs regulating functions of S/MARBPs.

CUX1, A Controversial Player in Tumor Development

CUX1 belongs to the homeodomain transcription factor family and is evolutionarily and functionally conserved from Drosophila to humans. In addition to the involvement in various physiological events including tissue development, cell proliferation, differentiation and migration, and DNA damage response, CUX1 has been implicated in tumorigenesis. Interestingly, CUX1 has been recently recognized as a haploinsufficient tumor suppressor, which is paradoxically overexpressed in tumor cells. While loss of heterozygosity and/or mutations of CUX1 have been frequently detected in many types of cancers, genomic amplification, and overexpression of CUX1 have also been reported in cancer tissues and are correlated with higher tumor grade and poor prognosis. Therefore, deciphering the roles of different CUX1 isoforms and in different tumor stages is required to establish a CUX1-based therapeutic strategy for cancer treatment.

Prognostic Biomarker Identification Through Integrating the Gene Signatures of Hepatocellular Carcinoma Properties

Many molecular classification and prognostic gene signatures for hepatocellular carcinoma (HCC) patients have been established based on genome-wide gene expression profiling; however, their generalizability is unclear. Herein, we systematically assessed the prognostic effects of these gene signatures and identified valuable prognostic biomarkers by integrating these gene signatures. With two independent HCC datasets (GSE14520, N = 242 and GSE54236, N = 78), 30 published gene signatures were evaluated, and 11 were significantly associated with the overall survival (OS) of postoperative HCC patients in both datasets. The random survival forest models suggested that the gene signatures were superior to clinical characteristics for predicting the prognosis of the patients. Based on the 11 gene signatures, a functional protein-protein interaction (PPI) network with 1406 nodes and 10,135 edges was established. With tissue microarrays of HCC patients (N = 60), we determined the prognostic values of the core genes in the network and found that RAD21, CDK1, and HDAC2 expression levels were negatively associated with OS for HCC patients. The multivariate Cox regression analyses suggested that CDK1 was an independent prognostic factor, which was validated in an independent case cohort (N = 78). In cellular models, inhibition of CDK1 by siRNA or a specific inhibitor, RO-3306, reduced cellular proliferation and viability for HCC cells. These results suggest that the prognostic predictive capacities of these gene signatures are reproducible and that CDK1 is a potential prognostic biomarker or therapeutic target for HCC patients.

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Eleven gene signatures have promising values for the OS prediction of HCC patients who undergo the surgical treatments.

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Prediction results for the 11 gene signatures were in moderate concordance, and genes included were functionally linked.

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CDK1 is an independent prognostic biomarker and a potential therapeutic target for HCC patients.

Various gene signatures for hepatocellular carcinoma (HCC) patients have been reported; however, their generalizability is unclear. Using two HCC patient datasets, we assessed the prognostic values of these gene signatures and identified 11 gene signatures that were associated with overall survival for postoperative HCC patients in both cohorts. Genes derived from these signatures formed a functional protein-protein interaction network with 1,406 nodes and 10,135 edges, and the expression levels of three core genes, RAD21, CDK1, and HDAC2, in the network were negatively associated with the overall survival of HCC patients. Further studies suggested that CDK1 is an independent prognostic factor and that it is a potential therapeutic target for HCC patients.

Long-range transcriptional regulation by the p110 CUX1 homeodomain protein on the ENCODE array

Background

Overexpression of the Cut homeobox 1 gene, CUX1, inversely correlates with patient survival in breast cancers. Cell-based assays and molecular studies have revealed that transcriptional regulation by CUX1 involves mostly the proteolytically processed p110 isoform. As there is no antibody specific to p110 CUX1 only, an alternate strategy must be employed to identify its targets.

Results

We expressed physiological levels of a tagged-p110 CUX1 protein and performed chromatin affinity purification followed by hybridization on ENCODE and promoter arrays. Targets were validated by chromatin immunoprecipitation and transcriptional regulation by CUX1 was analyzed in expression profiling and RT-qPCR assays following CUX1 knockdown or p110 CUX1 overexpression. Approximately 47% and 14% of CUX1 binding sites were respectively mapped less than 4 Kbp, or more than 40 Kbp, away from a transcription start site. More genes exhibited changes in expression following CUX1 knockdown than p110 CUX1 overexpression. CUX1 directly activated or repressed 7.4% and 8.4% of putative targets identified on the ENCODE and promoter arrays respectively. This proportion increased to 11.2% for targets with 2 binding sites or more. Transcriptional repression was observed in a slightly higher proportion of target genes. The CUX1 consensus binding motif, ATCRAT, was found at 47.2% of the CUX1 binding sites, yet only 8.3% of the CUX1 consensus motifs present on the array were bound in vivo. The presence of a consensus binding motif did not have an impact on whether a target gene was repressed or activated. Interestingly, the distance between a binding site and a transcription start site did not significantly reduced the ability of CUX1 to regulate a target gene. Moreover, CUX1 not only was able to regulate the next adjacent gene, but also regulated the gene located beyond this one as well as the gene located further away in the opposite direction.

Conclusion

Our results demonstrate that p110 CUX1 can activate or repress transcription when bound at a distance and can regulate more than one gene on certain genomic loci.

Cutl1: a potential target for cancer therapy

CDP, a key transcription regulator encoded by Cutl1 gene, has been demonstrated to be involved in repressing or promoting expression of target genes through its specific DNA-binding, meanwhile, the activity of CDP was influenced by some types of modifications including transcriptional, posttranscriptional, translational and posttranslational modifications. In this review, we systematically analyzed the role of CDP in normal development and tumor progression, and then emphasized its interactors and downstream molecules. Eventually, we concluded that Cut1 could promote cancer progression and its down-regulating expression will be a promising strategy for cancer therapy.

Identification of CUX1 as the recurrent chromosomal band 7q22 target gene in human uterine leiomyoma

Uterine leiomyomas are benign solid tumors of mesenchymal origin which occur with an estimated incidence of up to 77% of all women of reproductive age. The majority of these tumors remains symptomless, but in about a quarter of cases they cause leiomyoma-associated symptoms including chronic pelvic pain, menorrhagia-induced anemia, and impaired fertility. As a consequence, they are the most common indication for pre-menopausal hysterectomy in the USA and Japan and annually translate into a multibillion dollar healthcare problem. Approximately 40% of these neoplasms present with recurring structural cytogenetic anomalies, including del(7)(q22), t(12;14)(q15;q24), t(1;2)(p36;p24), and anomalies affecting 6p21 and/or 10q22. Using positional cloning strategies, we and others previously identified HMGA1, HMGA2, RAD51L1, MORF, and, more recently, NCOA1 as primary target (fusion) genes associated with tumor initiation in four of these distinct cytogenetic subgroups. Despite the fact that the del(7)(q22) subgroup is the largest among leiomyomas, and was first described more than twenty years ago, the 7q22 leiomyoma target gene still awaits unequivocal identification. We here describe a positional cloning effort from two independent uterine leiomyomas, containing respectively a pericentric and a paracentric chromosomal inversion, both affecting band 7q22. We found that both chromosomal inversions target the cut-like homeobox 1 (CUX1) gene on chromosomal band 7q22.1 in a way which is functionally equivalent to the more frequently observed del(7q) cases, and which is compatible with a mono-allelic knock-out scenario, similar as was previously described for the cytogenetic subgroup showing chromosome 14q involvement.

CUX1 transcription factor is required for optimal ATM/ATR-mediated responses to DNA damage

The p110 Cut homeobox 1 (CUX1) transcription factor regulates genes involved in DNA replication and chromosome segregation. Using a genome-wide-approach, we now demonstrate that CUX1 also modulates the constitutive expression of DNA damage response genes, including ones encoding ATM and ATR, as well as proteins involved in DNA damage-induced activation of, and signaling through, these kinases. Consistently, RNAi knockdown or genetic inactivation of CUX1 reduced ATM/ATR expression and negatively impacted hallmark protective responses mediated by ATM and ATR following exposure to ionizing radiation (IR) and UV, respectively. Specifically, abrogation of CUX1 strongly reduced ATM autophosphorylation after IR, in turn causing substantial decreases in (i) levels of phospho-Chk2 and p53, (ii) γ-H2AX and Rad51 DNA damage foci and (iii) the efficiency of DNA strand break repair. Similarly remarkable reductions in ATR-dependent responses, including phosphorylation of Chk1 and H2AX, were observed post-UV. Finally, multiple cell cycle checkpoints and clonogenic survival were compromised in CUX1 knockdown cells. Our results indicate that CUX1 regulates a transcriptional program that is necessary to mount an efficient response to mutagenic insult. Thus, CUX1 ensures not only the proper duplication and segregation of the genetic material, but also the preservation of its integrity.

CUX1 transcription factors: from biochemical activities and cell-based assays to mouse models and human diseases

ChIP-chip and expression analyses indicated that CUX1 transcription factors regulate a large number of genes and microRNAs involved in multiple cellular processes. Indeed, in proliferating cells CUX1 was shown to regulate several genes involved in DNA replication, progression into S phase and later, the spindle assembly checkpoint that controls progression through mitosis. siRNA-mediated knockdown established that CUX1 is required for cell motility. Moreover, higher expression of short CUX1 isoforms, as observed in many cancers, was shown to stimulate cell migration and invasion. In parallel, elevated expression particularly in higher grade tumors of breast and pancreatic cancers implicated CUX1 in tumor initiation and progression. Indeed, transgenic mouse models demonstrated a causal role of CUX1 in cancers originating from various cell types. These studies revealed that higher CUX1 expression or activity not only stimulates cell proliferation and motility, but also promotes genetic instability. CUX1 has also been implicated in the etiology of polycystic kidney diseases, both from a transgenic approach and the analysis of CUX1 activity in multiple mouse models of this disease. Studies in neurobiology have uncovered a potential implication of CUX1 in cognitive disorders, neurodegeneration and obesity. CUX1 was shown to be expressed specifically in pyramidal neurons of the neocortex upper layers where it regulates dendrite branching, spine development, and synapse formation. In addition, modulation of CUX1 expression in neurons of the hypothalamus has been associated with changes in leptin receptor trafficking in the vicinity of the primary cilium resulting in altered leptin signaling and ultimately, eating behavior. Overall, studies in various fields have allowed the development of several cell-based assays to monitor CUX1 function and have extended the range of organs in which CUX1 plays an important role in development and tissue homeostasis.

Cut homeobox 1 causes chromosomal instability by promoting bipolar division after cytokinesis failure

Cell populations able to generate a large repertoire of genetic variants have increased potential to generate tumor cells that survive through the multiple selection steps involved in tumor progression. A mechanism for the generation of aneuploid cancer cells involves passage through a tetraploid stage. Supernumerary centrosomes, however, can lead to multipolar mitosis and cell death. Using tissue culture and transgenic mouse models of breast cancer, we report that Cut homeobox 1 (CUX1) causes chromosomal instability by activating a transcriptional program that prevents multipolar divisions and enables the survival of tetraploid cells that evolve to become genetically unstable and tumorigenic. Transcriptional targets of CUX1 involved in DNA replication and bipolar mitosis defined a gene expression signature that, across 12 breast cancer gene expression datasets, was associated with poor clinical outcome. The signature not only was higher in breast tumor subtypes of worse prognosis, like the basal-like and HER2(+) subtypes, but also identified poor outcome among estrogen receptor-positive/node-negative tumors, a subgroup considered to be at lower risk. The CUX1 signature therefore represents a unique criterion to stratify patients and provides insight into the molecular determinants of poor clinical outcome.

Coordinate regulation of histone mRNA metabolism and DNA replication: cyclin A/cdk1 is involved in inactivation of histone mRNA metabolism and DNA replication at the end of S phase

S phase is characterized by the replication of DNA and assembly of chromatin. This requires the synthesis of large amounts of histone proteins to package the newly replicated DNA. Histone mRNAs are the only mRNAs that do not have polyA tails, ending instead in a conserved stemloop sequence. The stemloop binding protein (SLBP) that binds the 3' end of histone mRNA is cell cycle regulated and SLBP is required in all steps of histone mRNA metabolism. Activation of cyclin E/cdk2 prior to entry into S-phase is critical for initiation of DNA replication and histone mRNA accumulation. At the end of S phase SLBP is rapidly degraded as a result of phosphorylation of SLBP by cyclin A/cdk1 and CK2 effectively shutting off histone mRNA biosynthesis. E2F1, which is required for expression of many S-phase genes, is regulated in parallel with SLBP and its degradation also requires a cyclin binding site, suggesting that it may also be regulated by the same pathway. It is likely that activation of cyclin A/cdk1 helps inhibit both DNA replication and histone mRNA accumulation, marking the end of S phase and entry into G(2)-phase.

Phosphorylation of HOX11/TLX1 on Threonine-247 during mitosis modulates expression of cyclin B1

BACKGROUND

The HOX11/TLX1 (hereafter referred to as HOX11) homeobox gene was originally identified at a t(10;14)(q24;q11) translocation breakpoint, a chromosomal abnormality observed in 5-7% of T cell acute lymphoblastic leukemias (T-ALLs). We previously reported a predisposition to aberrant spindle assembly checkpoint arrest and heightened incidences of chromosome missegregation in HOX11-overexpressing B lymphocytes following exposure to spindle poisons. The purpose of the current study was to evaluate cell cycle specific expression of HOX11.

RESULTS

Cell cycle specific expression studies revealed a phosphorylated form of HOX11 detectable only in the mitotic fraction of cells after treatment with inhibitors to arrest cells at different stages of the cell cycle. Mutational analyses revealed phosphorylation on threonine-247 (Thr247), a conserved amino acid that defines the HOX11 gene family and is integral for the association with DNA binding elements. The effect of HOX11 phosphorylation on its ability to modulate expression of the downstream target, cyclin B1, was tested. A HOX11 mutant in which Thr247 was substituted with glutamic acid (HOX11 T247E), thereby mimicking a constitutively phosphorylated HOX11 isoform, was unable to bind the cyclin B1 promoter or enhance levels of the cyclin B1 protein. Expression of the wildtype HOX11 was associated with accelerated progression through the G2/M phase of the cell cycle, impaired synchronization in prometaphase and reduced apoptosis whereas expression of the HOX11 T247E mutant restored cell cycle kinetics, the spindle checkpoint and apoptosis.

CONCLUSIONS

Our results demonstrate that the transcriptional activity of HOX11 is regulated by phosphorylation of Thr247 in a cell cycle-specific manner and that this phosphorylation modulates the expression of the target gene, cyclin B1. Since it is likely that Thr247 phosphorylation regulates DNA binding activity to multiple HOX11 target sequences, it is conceivable that phosphorylation functions to regulate the expression of HOX11 target genes involved in the control of the mitotic spindle checkpoint.

Hyperphosphorylation by cyclin B/CDK1 in mitosis resets CUX1 DNA binding clock at each cell cycle

The p110 CUX1 homeodomain protein participates in the activation of DNA replication genes in part by increasing the affinity of E2F factors for the promoters of these genes. CUX1 expression is very weak in quiescent cells and increases during G(1). Biochemical activities associated with transcriptional activation by CUX1 are potentiated by post-translational modifications in late G(1), notably a proteolytic processing event that generates p110 CUX1. Constitutive expression of p110 CUX1, as observed in some transformed cells, leads to accelerated entry into the S phase. In this study, we investigated the post-translation regulation of CUX1 during mitosis and the early G(1) phases of proliferating cells. We observed a major electrophoretic mobility shift and a complete inhibition of DNA binding during mitosis. We show that cyclin B/CDK1 interacts with CUX1 and phosphorylates it at multiple sites. Serine to alanine replacement mutations at 10 SP dipeptide sites were required to restore DNA binding in mitosis. Passage into G(1) was associated with the degradation of some p110 CUX1 proteins, and the remaining proteins were gradually dephosphorylated. Indirect immunofluorescence and subfractionation assays using a phospho-specific antibody showed that most of the phosphorylated protein remained in the cytoplasm, whereas the dephosphorylated protein was preferentially located in the nucleus. Globally, our results indicate that the hyperphosphorylation of CUX1 by cyclin B/CDK1 inhibits its DNA binding activity in mitosis and interferes with its nuclear localization following cell division and formation of the nuclear membrane, whereas dephosphorylation and de novo synthesis contribute to gradually restore CUX1 expression and activity in G(1).

p110 CUX1 homeodomain protein stimulates cell migration and invasion in part through a regulatory cascade culminating in the repression of E-cadherin and occludin

In this study, we investigated the mechanism by which the CUX1 transcription factor can stimulate cell migration and invasion. The full-length p200 CUX1 had a weaker effect than the proteolytically processed p110 isoform; moreover, treatments that affect processing similarly impacted cell migration. We conclude that the stimulatory effect of p200 CUX1 is mediated in part, if not entirely, through the generation of p110 CUX1. We established a list of putative transcriptional targets with functions related to cell motility, and we then identified those targets whose expression was directly regulated by CUX1 in a cell line whose migratory potential was strongly stimulated by CUX1. We identified 18 genes whose expression was directly modulated by p110 CUX1, and its binding to all target promoters was validated in independent chromatin immunoprecipitation assays. These genes code for regulators of Rho-GTPases, cell-cell and cell-matrix adhesion proteins, cytoskeleton-associated proteins, and markers of epithelial-to-mesenchymal transition. Interestingly, p110 CUX1 activated the expression of genes that promote cell motility and at the same time repressed genes that inhibit this process. Therefore, the role of p110 CUX1 in cell motility involves its functions in both activation and repression of transcription. This was best exemplified in the regulation of the E-cadherin gene. Indeed, we uncovered a regulatory cascade whereby p110 CUX1 binds to the snail and slug gene promoters, activates their expression, and then cooperates with these transcription factors in the repression of the E-cadherin gene, thereby causing disorganization of cell-cell junctions.

Regulation of cell proliferation and differentiation in the kidney

The mammalian cut proteins are a broadly expressed family of nuclear transcription factors related to the Drosophila protein cut. One member of the cut family, Cux1, has been shown to function as a cell cycle dependent transcription factor, regulating the expression of a number of cell cycle regulatory proteins. Cux1 expression is developmentally regulated in multiple tissues suggesting an important regulatory function. Cux1 exists as multiple isoforms that arise from proteolytic processing of a 200 kD protein or use of an alternate promoter. Several mouse models of Cux1 have been generated that suggest important roles for this gene in cell cycle regulation during hair growth, lung development and maturation, and genitourinary tract development. Moreover, the aberrant expression of Cux1 may contribute to diseases such as polycystic kidney disease and cancer. In this review, we will focus on the phenotypes observed in the five existing transgenic mouse models of Cux1, and discuss the role of Cux1 in kidney development and disease.

Acetylation-dependent interaction of SATB1 and CtBP1 mediates transcriptional repression by SATB1

Special AT-rich binding protein 1 (SATB1) acts as a global regulator of gene expression by recruiting various corepressor or coactivator complexes, thereby establishing a unique chromatin structure at its genomic targets in a context-dependent manner. Although SATB1 acts predominantly as a repressor via recruitment of histone deacetylase 1 (HDAC1) complexes, the precise mechanism of global repression is not clear. Here we report that SATB1 and C-terminal binding protein 1 (CtBP1) form a repressor complex in vivo. The interaction occurs via the CtBP1 interaction consensus motif PVPLS within the PDZ-like domain of SATB1. The acetylation of SATB1 upon LiCl and ionomycin treatments disrupts its association with CtBP1, resulting in enhanced target gene expression. Chromatin immunoprecipitation analysis indicated that the occupancy of CtBP1 and HDAC1 is gradually decreased and the occupancy of PCAF is elevated at the SATB1 binding sites within the human interleukin-2 and mouse c-Myc promoters. Moreover, gene expression profiling studies using cells in which expression of SATB1 and CtBP1 was silenced indicated commonly targeted genes that may be coordinately repressed by the SATB1-CtBP1 complex. Collectively, these results provide a mechanistic insight into the role of SATB1-CtBP1 interaction in the repression and derepression of SATB1 target genes during Wnt signaling in T cells.

CUX1 transcription factor is a downstream effector of the proteinase-activated receptor 2 (PAR2)

Proteinase-activated receptors (PARs) are G-protein-coupled receptors that have been linked to an array of cellular processes, including inflammation, migration, and proliferation. Although signal transduction downstream of PARs has been actively investigated, little is known about the mechanisms that lead to changes in transcriptional programs. Here we show that the CUX1 homeodomain protein is a downstream effector of PAR2. Treatment of epithelial and fibroblastic cells with trypsin or the PAR2-activating peptide (PAR2-AP) caused a rapid increase in CUX1 DNA binding activity. The stimulation of CUX1 was specific to PAR2 because no effect was observed with thrombin or the PAR1-AP. Using a panel of recombinant CUX1 proteins, the regulation was found to involve the cut repeat 3 (CR3) and the cut homeodomain, two DNA binding domains that are present in all CUX1 isoforms. Expression analysis in cux1(-/-) mouse embryo fibroblasts led to the identification of three genes that are regulated downstream of both PAR2 and CUX1 as follows: interleukin-1alpha, matrix metalloproteinase-10, and cyclo-oxygenase-2. p110 CUX1 was able to activate each of these genes, both in reporter assays and following the infection of cells. Moreover, the treatment of Hs578T breast tumor cells with trypsin led to a rapid recruitment of p110 CUX1 to the promoter of these genes and to a concomitant increase in their mRNA steady-state levels. Altogether, these results suggest a model whereby activation of PAR2 triggers a signaling cascade that culminates with the stimulation of p110 CUX1 DNA binding and the transcriptional activation of target genes.

Anaplasma phagocytophilum increases cathepsin L activity, thereby globally influencing neutrophil function

Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis, is an unusual obligate intracellular pathogen that persists in neutrophils. A. phagocytophilum increases the binding of a repressor, CCAAT displacement protein (CDP), to the gp91(phox) promoter, thereby diminishing the host oxidative burst. We now show that A. phagocytophilum infection also enhances the binding of CDP to the promoters of human neutrophil peptide 1 and C/EBPepsilon--molecules important for neutrophil defense and maturation--suggesting that this is a general strategy used by this pathogen to alter polymorphonuclear leukocyte function. To explore the mechanism by which A. phagocytophilum increases CDP activity, we assessed the effects of this microbe on cathepsin L, a protease that cleaves CDP into a form with increased DNA binding ability. A. phagocytophilum infection resulted in elevated cathepsin L activity and the proteolysis of CDP. Blocking the action of cathepsin L with a chemical inhibitor or small interfering RNA targeting of this molecule caused a marked reduction in the degree of A. phagocytophilum infection. These data demonstrate that increasing cathepsin L activity is a strategy used by A. phagocytophilum to alter CDP activity and thereby globally influence neutrophil function. As therapeutic options for A. phagocytophilum and related organisms are limited, these results also identify a cellular pathway that may be targeted for the treatment of A. phagocytophilum infection.

Phosphorylation of threonine 61 by cyclin a/Cdk1 triggers degradation of stem-loop binding protein at the end of S phase

Histone mRNA levels are cell cycle regulated, and a major regulatory mechanism is restriction of stem-loop binding protein (SLBP) to S phase. Degradation of SLBP at the end of S phase results in cessation of histone mRNA biosynthesis, preventing accumulation of histone mRNA until SLBP is synthesized just before entry into the next S phase. Degradation of SLBP requires an SFTTP (58 to 62) and KRKL (95 to 98) sequence, which is a putative cyclin binding site. A fusion protein with the 58-amino-acid sequence of SLBP (amino acids 51 to 108) fused to glutathione S-transferase (GST) is sufficient to mimic SLBP degradation at late S phase. Using GST-SLBP fusion proteins as a substrate, we show that cyclin A/Cdk1 phosphorylates Thr61. Furthermore, knockdown of Cdk1 by RNA interference stabilizes SLBP at the end of S phase. Phosphorylation of Thr61 is necessary for subsequent phosphorylation of Thr60 by CK2 in vitro. Inhibitors of CK2 also prevent degradation of SLBP at the end of S phase. Thus, phosphorylation of Thr61 by cyclin A/Cdk1 primes phosphorylation of Thr60 by CK2 and is responsible for initiating SLBP degradation. We conclude that the increase in cyclin A/Cdk1 activity at the end of S phase triggers degradation of SLBP at S/G(2).

p110 CUX1 cooperates with E2F transcription factors in the transcriptional activation of cell cycle-regulated genes

The transcription factor p110 CUX1 was shown to stimulate cell proliferation by accelerating entry into S phase. As p110 CUX1 can function as a transcriptional repressor or activator depending on promoter context, we investigated its mechanism of transcriptional activation using the DNA polymerase alpha gene promoter as a model system. Linker-scanning analysis revealed that a low-affinity E2F binding site is required for transcriptional activation. Moreover, coexpression with a dominant-negative mutant of DP-1 suggested that endogenous E2F factors are indeed needed for p110-mediated activation. Tandem affinity purification, coimmunoprecipitation, chromatin immunoprecipitation, and reporter assays indicated that p110 CUX1 can engage in weak protein-protein interactions with E2F1 and E2F2, stimulate their recruitment to the DNA polymerase alpha gene promoter, and cooperate with these factors in transcriptional activation. On the other hand, in vitro assays suggested that the interaction between CUX1 and E2F1 either is not direct or is regulated by posttranslational modifications. Genome-wide location analysis revealed that targets common to p110 CUX1 and E2F1 included many genes involved in cell cycle, DNA replication, and DNA repair. Comparison of the degree of enrichment for various E2F factors suggested that binding of p110 CUX1 to a promoter will favor the specific recruitment of E2F1, and to a lesser extent E2F2, over E2F3 and E2F4. Reporter assays on a subset of common targets confirmed that p110 CUX1 and E2F1 cooperate in their transcriptional activation. Overall, our results show that p110 CUX1 and E2F1 cooperate in the regulation of many cell cycle genes.

The multiple roles of CUX1: insights from mouse models and cell-based assays

Cux (Cut homeobox) genes are present in all metazoans. Early reports described many phenotypes caused by cut mutations in Drosophila melanogaster. In vertebrates, CUX1 was originally characterized as the CCAAT-displacement protein (CDP). Another line of investigation revealed the presence of CUX1 within a multi-protein complex called the histone nuclear factor D (HiNF-D). Recent studies led to the identification of several CUX1 isoforms with distinct DNA binding and transcriptional properties. While the CCAAT-displacement activity was implicated in the transcriptional repression of several genes, some CUX1 isoforms were found to participate in the transcriptional activation of some genes. The expression and activity of CUX1 was shown to be regulated through the cell cycle and to be a target of TGF-beta signaling. Mechanisms of regulation include alternative transcription initiation, proteolytic processing, phosphorylation and acetylation. Cell-based assays have established a role for CUX1 in the control of cell cycle progression, cell motility and invasion. In the mouse, gene inactivation as well as over-expression in transgenic mice has revealed phenotypes in multiple organs and cell types. While some phenotypes could be explained by the presumed functions of CUX1 in the affected cells, other phenotypes invoked non-cell-autonomous effects that suggest regulatory functions with an impact on cell-cell interactions. The implication of CUX1 in cancer was suggested first from its over-expression in primary tumors and cancer cell lines and was later confirmed in mouse models.

Genome-wide location analysis and expression studies reveal a role for p110 CUX1 in the activation of DNA replication genes

Proteolytic processing of the CUX1 transcription factor generates an isoform, p110 that accelerates entry into S phase. To identify targets of p110 CUX1 that are involved in cell cycle progression, we performed genome-wide location analysis using a promoter microarray. Since there are no antibodies that specifically recognize p110, but not the full-length protein, we expressed physiological levels of a p110 isoform with two tags and purified chromatin by tandem affinity purification (ChAP). Conventional ChIP performed on synchronized populations of cells confirmed that p110 CUX1 is recruited to the promoter of cell cycle-related targets preferentially during S phase. Multiple approaches including silencing RNA (siRNA), transient infection with retroviral vectors, constitutive expression and reporter assays demonstrated that most cell cycle targets are activated whereas a few are repressed or not affected by p110 CUX1. Functional classes that were over-represented among targets included DNA replication initiation. Consistent with this finding, constitutive expression of p110 CUX1 led to a premature and more robust induction of replication genes during cell cycle progression, and stimulated the long-term replication of a plasmid bearing the oriP replicator of Epstein Barr virus (EBV).

Carboxyl-terminal proteolytic processing of CUX1 by a caspase enables transcriptional activation in proliferating cells

Proteolytic processing at the end of the G(1) phase generates a CUX1 isoform, p110, which functions either as a transcriptional activator or repressor and can accelerate entry into S phase. Here we describe a second proteolytic event that generates an isoform lacking two active repression domains in the COOH terminus. This processing event was inhibited by treatment of cells with synthetic and natural caspase inhibitors. In vitro, several caspases generated a processed isoform that co-migrated with the in vivo generated product. In cells, recombinant CUX1 proteins in which the region of cleavage was deleted or in which Asp residues were mutated to Ala, were not proteolytically processed. Importantly, this processing event was not associated with apoptosis, as assessed by terminal dUTP nick end labeling assay, cytochrome c localization, poly(ADP-ribose) polymerase cleavage, and fluorescence-activated cell sorting. Moreover, processing was observed in S phase but not in early G(1), suggesting that it is regulated through the cell cycle. The functional importance of this processing event was revealed in reporter and cell cycle assays. A recombinant, processed, CUX1 protein was a more potent transcriptional activator of several cell cycle-related genes and was able to accelerate entry into S phase, whereas mutants that could not be processed were inactive in either assay. Conversely, cells treated with the quinoline-Val Asp-2,6-difluorophenoxymethylketone caspase inhibitor proliferated more slowly and exhibited delayed S phase entry following exit from quiescence. Together, our results identify a substrate of caspases in proliferating cells and suggest a mechanism by which caspases can accelerate cell cycle progression.

Phosphorylation of SATB1, a global gene regulator, acts as a molecular switch regulating its transcriptional activity in vivo

SATB1 regulates gene expression by acting as a "docking site" for several chromatin remodeling enzymes and also by recruiting corepressors (HDACs) or coactivators (HATs) directly to promoters. However, how these contrasting effectors act at the level of SATB1 is not clear. We show here that phosphorylation by PKC acts as a switch to determine whether SATB1 interacts with HDAC1 or PCAF. Phosphorylation and dephosphorylation of SATB1 exerted opposing effects on MAR-linked reporter activity in vivo. SATB1 interacted with both CBP/p300 and PCAF HATs; however, these interactions resulted in the acetylation of the PDZ-like domain of SATB1 by PCAF but not by CBP/p300 and resulted in loss of its DNA binding activity. Using the T cell activation model, we provide mechanistic insights into how IL-2 transcription is reciprocally governed by the phosphorylation status of SATB1 and propose that a similar mechanism may dictate the ability of SATB1 to function as a global regulator.

CUTL1 is phosphorylated by protein kinase A, modulating its effects on cell proliferation and motility

CUTL1, also known as CDP (CCAAT Displacement Protein), Cut, or Cux-1, is a homeodomain transcription factor known to play an essential role in development and cell cycle progression. Previously, we identified CUTL1 as modulator of cell motility and invasiveness. Here we report that protein kinase A (PKA), known to inhibit tumor progression in various tumor types, directly phosphorylates CUTL1 at serine 1215 in NIH3T3 fibroblasts. The PKA-induced phosphorylation results in decreased DNA binding affinity of CUTL1 and diminished CUTL1-mediated cell cycle progression and cell motility. Furthermore, the expression of several CUTL1 target genes involved in proliferation and migration, such as DNA polymerase A and DKK2, was modulated by PKA-induced phosphorylation. These data identify CUTL1 as a novel target of PKA through which this protein kinase can modulate tumor cell motility and tumor progression.

The p110 isoform of the CDP/Cux transcription factor accelerates entry into S phase

The CDP/Cux transcription factor was previously found to acquire distinct DNA binding and transcriptional properties following a proteolytic processing event that takes place at the G1/S transition of the cell cycle. In the present study, we have investigated the role of the CDP/Cux processed isoform, p110, in cell cycle progression. Populations of cells stably expressing p110 CDP/Cux displayed a faster division rate and reached higher saturation density than control cells carrying the empty vector. p110 CDP/Cux cells reached the next S phase faster than control cells under various experimental conditions: following cell synchronization in G0 by growth factor deprivation, synchronization in S phase by double thymidine block treatment, or enrichment in G2 by centrifugal elutriation. In each case, duration of the G1 phase was shortened by 2 to 4 h. Gene inactivation confirmed the role of CDP/Cux as an accelerator of cell cycle progression, since mouse embryo fibroblasts obtained from Cutl1z/z mutant mice displayed a longer G1 phase and proliferated more slowly than their wild-type counterparts. The delay to enter S phase persisted following immortalization by the 3T3 protocol and transformation with H-RasV12. Moreover, CDP/Cux inactivation hindered both the formation of foci on a monolayer and tumor growth in mice. At the molecular level, expression of both cyclin E2 and A2 was increased in the presence of p110 CDP/Cux and decreased in its absence. Overall, these results establish that p110 CDP/Cux functions as a cell cycle regulator that accelerates entry into S phase.

Differential regulation of CDP/Cux p110 by cyclin A/Cdk2 and cyclin A/Cdk1

Previous experiments with peptide fusion proteins suggested that cyclin A/Cdk1 and Cdk2 might exhibit similar yet distinct phosphorylation specificities. Using a physiological substrate, CDP/Cux, our study confirms this notion. Proteolytic processing of CDP/Cux by cathepsin L generates the CDP/Cux p110 isoform at the beginning of S phase. CDP/Cux p110 makes stable interactions with DNA during S phase but is inhibited in G2 following the phosphorylation of serine 1237 by cyclin A/Cdk1. In this study, we propose that differential phosphorylation by cyclin A/Cdk1 and cyclin A/Cdk2 enables CDP/Cux p110 to exert its function as a transcriptional regulator specifically during S phase. We found that like cyclin A/Cdk1, cyclin A/Cdk2 interacted efficiently with recombinant CDP/Cux proteins that contain the Cut homeodomain and an adjacent cyclin-binding motif (Cy). In contrast to cyclin A/Cdk1, however, cyclin A/Cdk2 did not efficiently phosphorylate CDP/Cux p110 on serine 1237 and did not inhibit its DNA binding activity in vitro. Accordingly, co-expression with cyclin A/Cdk2 in cells did not inhibit the DNA binding and transcriptional activities of CDP/Cux p110. To confirm that the sequence surrounding serine 1237 was responsible for the differential regulation by Cdk1 and Cdk2, we replaced 4 amino acids flanking the phosphorylation site to mimic a known Cdk2 phosphorylation site present in the Cdc6 protein. Both cyclin A/Cdk2 and Cdk1 efficiently phosphorylated the CDP/Cux(Cdc6) mutant and inhibited its DNA binding activity. Altogether our results help explain why the DNA binding activity of CDP/Cux p110 is maximal during S phase and decreases in G2 phase.

Identification of genetic loci that interact with cut during Drosophila wing-margin development

The Drosophila selector gene cut is a hierarchal regulator of external sensory organ identity and is required to pattern the sensory and nonsensory cells of the wing margin. Cut performs the latter function, in part, by maintaining expression of the secreted morphogen encoded by wingless (wg). We find that Cut is required for wing-margin sensory organ specification in addition to and independently of Wg maintenance. In addition, we performed a genetic modifier screen to identify other genes that interact with cut in the regulation of wing-margin patterning. In total, 45 genetic loci (35 gain-of-function and 10 loss-of-function loci) were identified by virtue of their ability to suppress the wing-margin defects resulting from gypsy retrotransposon-mediated insulation of the cut wing-margin enhancer. Further genetic characterization identified several subgroups of candidate cut interacting loci. One group consists of putative regulators of gypsy insulator activity. A second group is potentially required for the regulation of Cut expression and/or activity and includes longitudinals lacking, a gene that encodes a family of BTB-domain zinc-finger transcription factors. A third group, which includes a component of the Brahma chromatin remodeling complex encoded by moira, affects the level of Cut expression in two opposing ways by suppressing the gypsy-mediated ct(K) phenotype and enhancing the non-gypsy ct(53d) phenotype. This suggests that the Brahma complex modulates both enhancer-controlled transcription and gypsy-mediated gene insulation of the cut locus.

CUTL1 is a target of TGF(beta) signaling that enhances cancer cell motility and invasiveness

CUTL1, also known as CDP, Cut, or Cux-1, is a homeodomain transcriptional regulator known to be involved in development and cell cycle progression. Here we report that CUTL1 activity is associated with increased migration and invasiveness in numerous tumor cell lines, both in vitro and in vivo. Furthermore, we identify CUTL1 as a transcriptional target of transforming growth factor beta and a mediator of its promigratory effects. CUTL1 activates a transcriptional program regulating genes involved in cell motility, invasion, and extracellular matrix composition. CUTL1 expression is significantly increased in high-grade carcinomas and is inversely correlated with survival in breast cancer. This suggests that CUTL1 plays a central role in coordinating a gene expression program associated with cell motility and tumor progression.

Biochemical characterization of the mammalian Cux2 protein

The CCAAT displacement protein (CDP) and cux (Cut homeobox) genes were originally identified as the human and mouse orthologs of Drosophila melanogaster cut. More recently, vertebrates were found to possess a second cut orthologs that was generated by gene duplication: Cux2. We report the initial biochemical characterization of the Cux2 protein in tissue culture and in vitro. We generated four polyclonal antibodies that were able to recognize the human and mouse Cux2 protein but displayed little or no cross-reactivity towards CDP1 and Cux1. The expression of the Cux2 protein was convincingly detected in only one among 19 neuronal cell lines: the SH-SY5Y human neuroblastoma cell line. CDP/Cux proteins contain four DNA binding domains, three Cut repeat (CR1, CR2 and CR3) and one Cut homeodomain (HD). Purified fusion proteins containing either CR1CR2, CR2CR3HD or CR3HD exhibited similar DNA binding specificities as the corresponding domains of Cux1, but their DNA binding kinetics were much more rapid. Similarly, the full-length Cux2 protein made rapid but transient interactions with DNA. We did not observe an N-terminally processed Cux2 isoform equivalent to the Cux1 p110 isoform. Whereas Cux1 can function as a repressor or activator in a promoter-specific manner, Cux2 functioned exclusively as a transcriptional repressor in NIH3T3 cells. Overall, our results suggest that the Cux1 and Cux2 proteins carry distinct biochemical functions. Cux2 is able, like Cux1, to perform the CCAAT-displacement activity. However, Cux2 is unlikely to execute transcriptional regulatory functions that require stable interaction with DNA.

Liver-enriched transcription factors in liver function and development. Part II: the C/EBPs and D site-binding protein in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation

In the first part of our review (see Pharmacol Rev 2002;54:129-158), we discussed the basic principles of gene transcription and the complex interactions within the network of hepatocyte nuclear factors, coactivators, ligands, and corepressors in targeted liver-specific gene expression. Now we summarize the role of basic region/leucine zipper protein family members and particularly the albumin D site-binding protein (DBP) and the CAAT/enhancer-binding proteins (C/EBPs) for their importance in liver-specific gene expression and their role in liver function and development. Specifically, regulatory networks and molecular interactions were examined in detail, and the experimental findings summarized in this review point to pivotal roles of DBP and C/EBPs in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation. These regulatory proteins are therefore of great importance in liver physiology, liver disease, and liver development. Furthermore, interpretation of the vast data generated by novel genomic platform technologies requires a thorough understanding of regulatory networks and particularly the hierarchies that govern transcription and translation of proteins as well as intracellular protein modifications. Thus, this review aims to stimulate discussions on directions of future research and particularly the identification of molecular targets for pharmacological intervention of liver disease.

The N-terminal Region of the CCAAT Displacement Protein (CDP)/Cux Transcription Factor Functions as an Autoinhibitory Domain that Modulates DNA Binding

The CCAAT displacement protein/Cut homeobox (CDP/Cux) transcription factor is expressed as multiple isoforms that may contain up to four DNA-binding domains: Cut repeats 1, 2, and 3 (CR1, CR2, CR3) and the Cut homeodomain (HD). The full-length protein, which contains all four DNA-binding domains, is surprisingly less efficient than the shorter isoforms in DNA binding. Using a panel of recombinant proteins expressed in mammalian or bacterial cells, we have identified a domain at the extreme N terminus of the protein that can inhibit DNA binding. This domain was able to inhibit the activity of full-length CDP/Cux and of proteins containing various combinations of DNA-binding domains: CR1CR2, CR3HD, or CR2CR3HD. Since inhibition of DNA binding was also observed with purified proteins obtained from bacteria, we conclude that autoinhibition does not require post-translational modification or interaction with an interacting protein but instead functions through an intramolecular mechanism. Antibodies directed against the N-terminal region were able to partially relieve inhibition. In vivo, the transition between the inactive and active states for DNA binding is likely to be governed by posttranslational modifications and/or interaction with one or more protein partners. In addition, we show that the relief of autoinhibition can be accomplished via the proteolytic processing of CDP/Cux. Altogether, these results reveal a novel mode of regulation that serves to modulate the DNA binding activity of CDP/Cux.

A novel signaling pathway mediates the inhibition of CCL3/4 expression by prostaglandin E2

In response to pathogen-associated molecular patterns, dendritic cells initiate an innate immune response characterized by expression and release of proinflammatory cytokines and chemokines. The extent of the inflammatory response is limited by various endogenous factors, including lipid mediators such as prostaglandin E(2) (PGE(2)). We described previously the inhibitory effect of PGE(2) on the expression and release of the inflammatory chemokines CCL3 and CCL4 from activated dendritic cells. In this study we describe a novel PGE(2) signaling pathway that proceeds through EP-2 --> cAMP --> EPAC --> phosphatidylinositol 3-kinase --> protein kinase B --> GSK-3 and results in increased DNA binding of the CCAAT displacement protein (CDP), a potent mammalian transcriptional repressor. The direct link between CDP and CCL3/4 transcription was established in knock-down experiments using CDP small interference RNA.

CCAAT displacement protein/cut homolog recruits G9a histone lysine methyltransferase to repress transcription

CCAAT displacement protein/cut homolog (CDP/cut) is a highly conserved homeodomain protein that contains three cut repeat sequences. CDP/cut is a transcriptional factor for many diverse cellular and viral genes that are involved in most cellular processes, including differentiation, development, and proliferation. Here, we report that CDP/cut interacts with a histone lysine methyltransferase (HKMT), G9a, in vivo and in vitro. The deletion of the cut repeats within CDP/cut abrogates the interaction with G9a. The transcriptional repressor function of CDP/cut is mediated through HKMT activity of G9a associated with CDP/cut. We show that the recruitment of G9a to the human p21(waf1/cdi1) promoter is contingent on the interaction with CDP/cut, and CDP/cut is directly associated with an increase in the methylation in vivo of Lys-9 in histone H3 within the CDP/cut-regulatory region of the p21(waf1/cdi1) promoter. The endogenous level of p21(waf1/cdi1) expression is repressed through CDP/cut and mediated by HKMT activity of G9a. Furthermore, we report the identification of G9a as a component of CDP/cut complex. G9a colocalizes with CDP/cut in the nucleus. These results indicate that G9a functions as a transcriptional corepressor in association with a CDP/cut complex. These studies now reveal the interaction of G9a with a sequence-specific transcription factor that regulates gene repression through CDP/cut.

Tumor suppressor pRB functions as a co-repressor of the CCAAT displacement protein (CDP/cut) to regulate cell cycle controlled histone H4 transcription

The CCAAT displacement protein (CDP-cut/CUTL1/cux) performs a key proliferation-related function as the DNA binding subunit of the cell cycle controlled HiNF-D complex. HiNF-D interacts with all five classes (H1, H2A, H2B, H3, and H4) of the cell-cycle dependent histone genes, which are transcriptionally and coordinately activated at the G(1)/S phase transition independent of E2F. The tumor suppressor pRB/p105 is an intrinsic component of the HiNF-D complex. However, the molecular interactions that enable CDP and pRB to form a complex and thus convey cell growth regulatory information onto histone gene promoters must be further defined. Using transient transfections, we show that CDP represses the H4 gene promoter and that pRB functions with CDP as a co-repressor. Direct physical interaction between CDP and pRB was observed in glutathione-S-transferase (GST) pull-down assays. Furthermore, interactions between these proteins were established by yeast and mammalian two-hybrid experiments and co-immunoprecipitation assays. Confocal microscopy shows that subsets of each protein are co-localized in situ. Using a series of pRB mutants, we find that the CDP/pRB interaction, similar to the E2F/pRB interaction, utilizes the A/B large pocket (LP) of pRB. Thus, several converging lines of evidence indicate that complexes between CDP and pRB repress cell cycle regulated histone gene promoters.

Development of an in vitro assay for the proteolytic processing of the CDP/Cux transcription factor

The CDP/Cux transcription factor was previously shown to be proteolytically processed at the G1/S transition. In view of characterizing and eventually identifying the protease responsible for CDP/Cux processing, we have established an in vitro proteolytic processing assay. CDP/Cux recombinant proteins expressed in mammalian or bacterial cells were efficiently processed in vitro using as a source of protease either whole cell extracts, the nuclear or the cytoplasmic fraction. Processing was found to take place optimally at a lower pH, to be insensitive to variations in salt concentration, and to be inhibited by the protease inhibitors MG132 and E64D. Interestingly, the bacterially-produced substrate was more efficiently processed than the substrate purified from mammalian cells. Moreover, processing in vitro was more efficient when CDP/Cux substrates were purified from populations of cells enriched in the S phase than in the G1 phase of the cell cycle. Altogether, these results suggest that posttranslational modifications of CDP/Cux in mammalian cells inhibits processing and contributes to the cell cycle-dependent regulation of processing. The in vitro processing assay described in this study will provide a useful tool for the purification and identification of the protease responsible for the processing of CDP/Cux.

CDP/Cux stimulates transcription from the DNA polymerase alpha gene promoter

CDP/Cux (CCAAT-displacement protein/cut homeobox) contains four DNA binding domains, namely, three Cut repeats (CR1, CR2, and CR3) and a Cut homeodomain. CCAAT-displacement activity involves rapid but transient interaction with DNA. More stable DNA binding activity is up-regulated at the G(1)/S transition and was previously shown to involve an N-terminally truncated isoform, CDP/Cux p110, that is generated by proteolytic processing. CDP/Cux has been previously characterized as a transcriptional repressor. However, here we show that expression of reporter plasmids containing promoter sequences from the human DNA polymerase alpha (pol alpha), CAD, and cyclin A genes is stimulated in cotransfections with N-terminally truncated CDP/Cux proteins but not with full-length CDP/Cux. Moreover, expression of the endogenous DNA pol alpha gene was stimulated following the infection of cells with a retrovirus expressing a truncated CDP/Cux protein. Chromatin immunoprecipitation (ChIP) assays revealed that CDP/Cux was associated with the DNA pol alpha gene promoter specifically in the S phase. Using linker scanning analyses, in vitro DNA binding, and ChIP assays, we established a correlation between binding of CDP/Cux to the DNA pol alpha promoter and the stimulation of gene expression. Although we cannot exclude the possibility that stimulation of gene expression by CDP/Cux involved the repression of a repressor, our data support the notion that CDP/Cux participates in transcriptional activation. Notwithstanding its mechanism of action, these results establish CDP/Cux as an important transcriptional regulator in the S phase.

Fine tuning the transcriptional regulation of the CXCL1 chemokine

Constitutive activation of the transcription factor nuclear factor-κB (NF-κB) plays a major role in inflammatory diseases as well as cancer by inducing the endogenous expression of many proinflammatory proteins such as chemokines, and facilitating escape from apoptosis. The constitutive expression of chemokines such as CXCL1 has been correlated with growth, angiogenesis, and metastasis of cancers such as melanoma. The transcription of CXCL1 is regulated through interactions of NF-κB with other transcriptional regulatory molecules such as poly(ADP-ribose) polymerase-1 (PARP-1) and cAMP response element binding protein (CREB)-binding protein (CBP). It has been proposed that these two proteins interact with NF-κB and other enhancers to form an enhanceosome at the promoter region of CXCL1 and modulate CXCL1 transcription. In addition to these positive cofactors, a negative regulator, CAAT displacement protein (CDP), may also be involved in the transcriptional regulation of CXCL1. It has been postulated that the elevated expression of CXCL1 in melanomas is due to altered interaction between these molecules. CDP interaction with the promoter down-regulates transcription, whereas PARP and/or CBP interactions enhance transcription. Thus, elucidation of the interplay between components of the enhanceosome of this gene is important in finding more efficient and new therapies for conditions such as cancer as well as acute and chronic inflammatory diseases.

Linking cyclins to transcriptional control

Cell cycle activation is coordinated by D-type cyclins which are rate limiting and essential for the progression through the G1 phase of the cell cycle. D-type cyclins bind to and activate the cyclin-dependent kinases Cdk4 and Cdk6, which in turn phosphorylate their downstream target, the retinoblastoma protein Rb. Upon Rb phosphorylation, the E2F transcription factors activate the expression of S-phase genes and thereby induce cell cycle progression. The raise of cyclin D levels in early G1 also serves to titrate Kip/Cip proteins away from cyclinE/Cdk2 complexes, further accelerating cell cycle progression. Therefore, cyclin D plays essential roles in the response to mitogens, transmitting their signal to the Rb/E2F pathway. Surprisingly, cyclin D1-deficient animals are viable and have developmental abnormalities limited to restricted tissues, such as retina, the nervous system and breast epithelium. This observation, combined with several other studies, have raised the possibility that cyclin D1 may have new activities that are unrelated to its function as a cdk regulatory subunit and as regulator of Rb. Effectively, cyclin D has been reported to have transcriptional functions since it interacts with several transcription factors to regulate their activity. Most often, this effect does not rely on the kinase function of Cdk4, indicating that this function is probably independent of cell cycle progression. Further extending its role in gene regulation, cyclin D interacts with histone acetylases such as P/CAF or NcoA/SRC1a but also with components of the transcriptional machinery such as TAF(II)250. Therefore, these studies suggest that the functions of cyclin D might need to be reevaluated. They have established a new cdk-independent role of cyclin D1 as a transcriptional regulator, indicating that cyclin D1 can act via two different mechanisms, as a cdk activator it regulates cell cycle progression and as a transcriptional regulator, it modulates the activity of transcription factors.

Expression of N-terminally truncated isoforms of CDP/CUX is increased in human uterine leiomyomas

Genetic analyses and mRNA expression studies have implicated CUTL1 as a candidate tumor-suppressor gene in uterine leiomyomas and breast cancers. However, modulation of CDP/Cux, the protein encoded by CUTL1, does not agree with this notion. The activity of CDP/Cux, which is the DNA binding subunit of HiNF-D, was upregulated as normal cells progressed into S phase and constitutively elevated in several tumor cell lines. Activation of CDP/Cux at the G(1)/S transition involved the proteolytic processing of the protein to generate a shorter isoform. Uterine leiomyomas represent a unique reagent for molecular analysis because they are resected as homogeneous tumor tissue together with the adjacent normal myometrium and they are often very large. In the present study, proteins were isolated from 16 pairs of matched tumors and adjacent myometrium and analyzed by Western blot and electrophoretic mobility shift assays. Strikingly, in 11/16 tumors, the steady-state level of small CDP/Cux isoforms was increased compared to normal control tissue. Where tested, a corresponding increase in CDP/Cux stable DNA binding activity was observed. DNA sequencing analysis of CUTL1 cDNAs from 6 leiomyomas, including 4 with LOH of CUTL1, did not reveal any gross rearrangement or point mutations. Altogether these findings suggest that CUTL1 is probably not the tumor suppressor on 7q22. Moreover, the frequent increase in smaller CDP/Cux isoforms indicates that molecular events associated with the truncation of CDP/Cux proteins may be selected in uterine leiomyomas.

Calmodulin and cyclin D anchoring sites on the Src-suppressed C kinase substrate, SSeCKS

SSeCKS and its human orthologue, Gravin, are large scaffolding proteins that are thought to facilitate mitogenic control by anchoring key signal mediators such as protein kinase (PK) C, PKA, the plasma membrane associated isoform of alpha-1,4-galactosyltransferase (GalTase), beta2-adrenergic receptor, and cyclins. SSeCKS is also a major PKC substrate and phosphatidylserine-dependent PKC binding protein whose phosphorylation sites shares homology with a site in the MARCKS protein that encodes phosphorylation-sensitive calmodulin (CaM) binding activity. In the present study, we mapped the in vitro binding sites for CaM and cyclins on SSeCKS. Four CaM binding sites were identified by binding assays that conform to the so-called 1-5-10 motif. Notably, CaM binding was antagonized by prephosphorylation of SSeCKS by PKC. We also identified two major cyclin binding (CY) sites that overlap a major PKC phosphorylation site in SSeCKS (Ser(507/515)), and showed that cyclin D binding is attenuated if SSeCKS is prephosphorylated by PKC. These data suggest that the scaffolding activities of SSeCKS are modulated by mitogenically stimulated kinases such as PKC.