Selective expression of specific histone H4 genes reflects distinctions in transcription factor interactions with divergent H4 promoter elements

Methylation analysis of histone 4-related gene HIST1H4F and its effect on gene expression in bladder cancer

Recently, aberrant DNA methylation of the HIST1H4F gene (encodes Histone 4 protein) has been shown in many types of cancer, which may serve as a promising biomarker for early cancer diagnosis. However, the correlation between DNA methylation of the HIST1H4F gene and its role in gene expression is unclear in bladder cancer. Therefore, the first objective of this study is to explore the DNA methylation pattern of the HIST1H4F gene and then further elucidate its effects on HIST1H4F mRNA expression in bladder cancer. To this end, the methylation pattern of the HIST1H4F gene was analyzed by pyrosequencing and the effects of the methylation profiles of this gene on HIST1H4F mRNA expression in bladder cancer were examined by qRT-PCR. Sequencing analysis revealed significantly higher methylation frequencies of the HIST1H4F gene in bladder tumor samples compared to normal samples (p < 0,0001). However, when we evaluated the correlations between hypermethylation of HIST1H4F and the clinicopathological parameters (tumor stage, tumor grade, lymph node metastasis, muscle-invasion), no significant difference was found between the groups (p > 0.05). In addition, we examined the role of hypermethylation of the HIST1H4F gene on HIST1H4F mRNA expression. We found that hypermethylation of HIST1H4F in the exon have no effect HIST1H4F mRNA expression in bladder cancer (p > 0.05). We also confirmed our finding in cultured T24 cell line which HIST1H4F gene is hypermethylated. Our results suggest that hypermethylation of the HIST1H4F seems to be a promising early diagnostic biomarker in bladder cancer patients. However, further studies are needed to determine the role of HIST1H4F hypermethylation in tumorigenesis.

Functional coupling of transcription factor HiNF-P and histone H4 gene expression during pre- and post-natal mouse development

Transcription factor Histone Nuclear Factor P (HiNF-P; gene symbol Hinfp) mediates cell cycle control of histone H4 gene expression to support the packaging of newly replicated DNA as chromatin. The HiNF-P/p220(NPAT) complex controls multiple H4 genes in established human cell lines and is critical for cell proliferation. The mouse Hinfp(LacZ) null allele causes early embryonic lethality due to a blastocyst defect. However, neither Hinfp function nor its temporal expression relative to histone H4 genes during fetal development has been explored. Here, we establish that expression of Hinfp is biologically coupled with expression of twelve functional mouse H4 genes during pre- and post-natal tissue-development. Both Hinfp and H4 genes are robustly expressed at multiple embryonic (E) days (from E5.5 to E15.5), coincident with ubiquitous LacZ staining driven by the Hinfp promoter. Five highly expressed mouse H4 genes (Hist1h4d, Histh4f, Hist1h4m and Hist2h4) account for >90% of total histone H4 mRNA throughout development. Post-natal expression of H4 genes in mice is most evident in lung, spleen, thymus and intestine, and with few exceptions (e.g., adult liver) correlates with Hinfp gene expression. Histone H4 gene expression decreases butHinfp levels remain constitutive upon cell growth inhibition in culture. The in vivo co-expression of Hinfp and histone H4 genes is consistent with the biological function of Hinfp as a principal transcriptional regulator of histone H4 gene expression during mouse development.

The histone gene transcription factor HiNF-P stabilizes its cell cycle regulatory co-activator p220NPAT

Orderly progression through the cell cycle requires the transcriptional activation of histone genes to support packaging of newly replicated DNA. Induction of human histone gene expression is mediated by a co-activation complex containing transcription factor HiNF-P and its cofactor p220NPAT. Here, using cells synchronized in S-phase and in mitosis, as well as serum-stimulated cells, we have investigated how HiNF-P is regulated during the cell cycle and examined its stability relative to p220NPAT. We find that while HiNF-P is maintained at steady-state levels throughout the cell cycle, both HiNF-P and p220NPAT are actively degraded by the proteasome pathway. Importantly, elevation of HiNF-P levels enhances the stability of its co-activator p220NPAT. The HiNF-P-dependent stabilization of p220NPAT may reinforce signaling through the cyclin E/CDK2/p220NPAT pathway and contribute to coordinate control of histone gene expression.

Transcriptional activation of the histone nuclear factor P (HiNF-P) gene by HiNF-P and its cyclin E/CDK2 responsive co-factor p220NPAT defines a novel autoregulatory loop at the G1/S phase transition

Histone nuclear factor P (HiNF-P) activates histone H4 gene transcription at the G1/S phase transition upon association with its cyclin E/CDK2 responsive co-factor p220NPAT. Here we characterize the gene regulatory pathways that control the proliferation-related expression of HiNF-P. The HiNF-P locus contains a single TATA-less 0.6 kbp promoter with multiple phylogenetically conserved transcription factor recognition motifs. Transient reporter gene assays with HiNF-P promoter deletions show that there are at least three distinct activating regions (-387/-201, -201/-100 and -100/-1) that support maximal transcription. HiNF-P gene transcription is activated by SP1 through the -100/-1 domain and repressed by E2F1 through the -201/-100 domain. The multifunctional co-regulators CBP and p300 also stimulate HiNF-P gene transcription through the -201/-1 core promoter. Importantly, the HiNF-P promoter is activated by both HiNF-P and p220NPAT. This autoregulatory activation is further enhanced by cyclin E and CDK2, while blocked by CDK inhibition (i.e., p57KIP2 p27KIP1, p21CIP). Thus, the HiNF-P gene is a key non-histone target of p220NPAT and HiNF-P. The dependence of HiNF-P gene transcription on cyclin E/CDK2/p220NPAT signaling defines a novel feed-forward loop that may sustain HiNF-P expression in proliferating cells to support the cell cycle regulated synthesis of histone H4 proteins.

HiNF-P is a bifunctional regulator of cell cycle controlled histone H4 gene transcription

Cell cycle progression beyond the G1/S phase transition requires the activation of a transcription complex containing histone nuclear factor P (HiNF-P) and nuclear protein mapped to ataxia telangiectasia (p220(NPAT)) in response to cyclin dependent kinase 2 (CDK2)/cyclin E signaling. We show here that the potent co-activating properties of HiNF-P/p220(NPAT) on the histone H4 gene promoter, which are evident in the majority of human cell types, are sporadically neutralized in distinct somatic cell lines. In cells where HiNF-P and p220(NPAT) do not activate the H4 gene promoter, HiNF-P instead represses transcription. Our data suggest that the cell type specific expression of the cyclin-dependent kinase inhibitory (CKI) protein p57(KIP2) inhibits the HiNF-P dependent activation of the histone H4 promoter. We propose that, analogous to E2F proteins and other cell cycle regulatory proteins, HiNF-P is a bifunctional transcriptional regulator that can activate or repress cell cycle controlled genes depending on the cellular context.

Establishment of histone gene regulation and cell cycle checkpoint control in human embryonic stem cells

Rapid self-renewal of human embryonic stem (ES) cells (NIH designation WA01 and WA09) is accommodated by an abbreviated cell cycle due to a reduction in the G1 phase. Thus, molecular mechanisms operative in ES cells may expedite the cellular commitment to progress into S phase to initiate replication of DNA and biosynthesis of histone proteins to form new chromatin. Here we show that the selective cell cycle regulated expression of individual histone H4 gene copies, which is typical for somatic cell types, is already firmly established in human ES cells. This early establishment of H4 gene regulation, which is E2F independent, is consistent with co-expression of the cognate transcriptional regulators HiNF-P and p220(NPAT). Human ES cells differ from somatic cells in the expression of members of the E2F family and RB-related pocket proteins (p105(RB1), p107(RBL1), and p130(RBL2/RB2)) that control expression of genes encoding enzymes for nucleotide metabolism and DNA synthesis. Human ES cells rapidly and robustly (>200-fold) induce the cyclin dependent kinase (CDK) inhibitor p21(WAF1/CIP1) upon gamma-irradiation. This DNA damage response promptly reduces histone gene expression as well as mRNA levels for HiNF-P and p220(NPAT) and causes accumulation of unprocessed histone H4 precursor RNAs. Furthermore, while E2F4, E2F5 and p130(RBL2/RB2) are the major E2F and pocket protein mRNAs in actively proliferating ES cells, expression levels of E2F5, E2F6, and p105(RB1) are most strongly elevated during cell cycle arrest in cells responding to DNA damage. Our data suggest that the brief G1 phase of ES cells is supported by a potent p21(WAF1/CIP1) related DNA damage response that functions through several mechanisms to rapidly inhibit cell cycle progression. This response may alter the E2F/pocket protein combinations that control E2F dependent genes and block H4 gene expression by inhibiting histone-specific transcription factors and processing of histone gene transcripts, as well as by destabilizing histone mRNAs.

An architectural perspective of cell-cycle control at the G1/S phase cell-cycle transition

A prominent role for the execution of cell cycle and growth regulatory mechanisms within the three-dimensional context of nuclear architecture is becoming increasingly evident. Signaling pathways and regulatory networks that govern activation and suppression of genes controlling proliferation are functionally integrated for the organization and assembly of transcriptional machinery in nuclear microenvironments. The transcriptional activation of histone genes at the G1/S phase transition (S-point) is temporarily, functionally, and spatially distinct from transcriptional mechanisms at the restriction point (R-point). The spatial distinction in R-point versus S-point control is the localization of clustered histone gene loci at cajal bodies, which is modulated during the cell cycle. Histone nuclear factor P (HiNF-P), the principal factor mediating H4 histone gene transcription, is the final link in the signaling cascade that is initiated with growth factor dependent induction of cyclin E/CDK2 kinase activity at the R-point and culminates in the NPAT-mediated activation of histone H4 genes through HiNF-P at the G1/S phase cell-cycle transition.

Characterization, localization and possible anti-inflammatory function of rat histone H4 mRNA variants

Two histone H4 mRNA variants, H4-v.1 and histogranin mRNAs, were detected in the rat genome and measured in various tissues and isolated alveolar macrophages. Medium to high levels of both mRNAs were present in the liver, adrenal glands, thymus, bone marrow and alveolar macrophages. H4-v.1 cDNA contained an open reading frame that coded for unmodified whole histone H4, whereas histogranin cDNA lacked the first ATG codon and contained an open reading frame that coded for modified (Thr89) H4-(84-102). The two genes displayed a sequence homologous (> 80%) to the open reading frame of core H4 somatic (H4s) and H4 germinal (H4g) and their variant nature was supported by the absence of histone consensus palindromic and purine-rich sequences in the proximal 3'UTR, and the presence of a polyadenylation signal in the distal 3'UTR and of specific upstream transcription factor-binding sites. H4-v.1 and histogranin transcripts, but not H4s transcript, were selectively induced by lipopolysaccharide and/or interferon gamma in alveolar macrophages. In vitro transcription/translation experiments with H4-v.1 and histogranin cDNA pCMV constructs produced peptides with the molecular mass (2 kDa) of the alternative histone H4 translation product which, like synthetic H4-(86-100) and [Thr89]H4-(86-100) or rat histogranin, inhibited lipopolysaccharide-induced prostaglandin E(2) release from rat alveolar macrophages. The synthetic peptides also inhibited the secretion of the CXC chemokine interleukin-8 (GRO/CINC-1) in response to lipopolysaccharide. The presence of H4-v.1 and histogranin mRNAs in tissues wherein immune reactions take place and the inhibitory effects of their translation products on prostaglandin E(2) and interkeukin-8 secretion by activated alveolar macrophages suggest an anti-inflammatory function.

Coordinate control and selective expression of the full complement of replication-dependent histone H4 genes in normal and cancer cells

The replication of eukaryotic genomes necessitates the coordination of histone biosynthesis with DNA replication at the onset of S phase. The multiple histone H4 genes encode identical proteins, but their regulatory sequences differ. The contributions of these individual genes to histone H4 mRNA expression have not been described. We have determined, by real-time quantitative PCR and RNase protection, that the human histone H4 genes are not equally expressed and that a subset contributes disproportionately to the total pool of H4 mRNA. Differences in histone H4 gene expression can be attributed to observed unequal activities of the H4 gene promoters, which exhibit variations in gene regulatory elements. The overall expression pattern of the histone H4 gene complement is similar in normal and cancer cells. However, H4 genes that are moderately expressed in normal cells are sporadically silenced in tumor cells with compensation of expression by other H4 gene copies. Chromatin immunoprecipitation analyses and in vitro DNA binding assays indicated that 11 of the 15 histone H4 genes interact with the cell cycle regulatory histone nuclear factor P, which forms a complex with the cyclin E/CDK2-responsive co-regulator p220(NPAT). These 11 H4 genes account for 95% of the histone H4 mRNA pool. We conclude that the cyclin E/CDK2/p220(NPAT)/histone nuclear factor P signaling pathway is the principal regulator of histone H4 biosynthesis.

Human replication-dependent histone H3 genes are activated by a tandemly arranged pair of two CCAAT boxes

We have analysed the transcriptional regulation of the human histone H3 genes using promoter deletion series, scanning mutagenesis, specific mutagenesis and electrophoretic mobility-shift assay experiments. The promoters of five of the six examined histone H3 genes showed near-maximal activity at lengths of 133-227 bp: H3/d 198 bp, H3/h 147 bp, H3/k 133 bp, H3/m 227 bp, H3/n 140 bp (exception H3/i). To search for functional cis-elements within these regions, we performed scanning mutagenesis of the two histone H3 promoters H3/k and H3/m. Mutagenesis revealed that the functional framework of the histone H3 promoters consists of a TATA box and two tandemly arranged CCAAT boxes in relatively fixed positions. Alterations of the distance between the CCAAT boxes and of the distance between the CCAAT boxes and the TATA box resulted in significant loss of activity. In electrophoretic mobility-shift assay experiments, the factor CBF (CCAAT-binding factor)/NF-Y (nuclear factor-Y) bound to isolated CCAAT boxes of the H3/k promoter. This suggests that an initiation complex is formed on the histone H3 promoter that has a defined structure and limited flexibility, consisting of two molecules of CBF/NF-Y and further (general or specific) transcription factors.

The Yin Yang-1 (YY1) protein undergoes a DNA-replication-associated switch in localization from the cytoplasm to the nucleus at the onset of S phase

The essential Yin Yang-1 gene (YY1) encodes a ubiquitous, conserved, multifunctional zinc-finger transcription factor in animals. The YY1 protein regulates initiation, activation, or repression of transcription from a variety of genes required for cell growth, development, differentiation, or tumor suppression, as well as from genes in some retroviruses and DNA viruses. Among the specific functions attributed to YY1 is a role in cell-cycle-specific upregulation of the replication-dependent histone genes. The YY1 protein binds to the histone alpha element, a regulatory sequence found in all replication-dependent histone genes. We therefore examined the abundance, DNA-binding activity and localization of the YY1 protein throughout the cell cycle in unperturbed, shake-off-synchronized Chinese hamster ovary and HeLa cells. We found that, whereas the DNA-binding activity of YY1 increased dramatically early in S phase, the YY1 mRNA and protein levels did not. YY1 changed subcellular distribution patterns during the cell cycle, from mainly cytoplasmic at G1 to mainly nuclear at early and middle S phase, then back to primarily cytoplasmic later in S phase. Nuclear accumulation of YY1 near the G1/S boundary coincided with both an increase in YY1 DNA-binding activity and the coordinate up-regulation of the replication-dependent histone genes. The DNA synthesis inhibitor aphidicolin caused a nearly complete loss of nuclear YY1, whereas addition of caffeine or 2-aminopurine to aphidicolin-treated cells restored both DNA synthesis and YY1 localization in the nucleus. These findings reveal a mechanism by which YY1 localization is coupled to DNA synthesis and responsive to cell-cycle signaling pathways. Taken together, our results provide insight into how YY1 might participate in the cell-cycle control over a variety of nuclear events required for cell division and proliferation.

Identification of HiNF-P, a key activator of cell cycle-controlled histone H4 genes at the onset of S phase

At the G(1)/S phase cell cycle transition, multiple histone genes are expressed to ensure that newly synthesized DNA is immediately packaged as chromatin. Here we have purified and functionally characterized the critical transcription factor HiNF-P, which is required for E2F-independent activation of the histone H4 multigene family. Using chromatin immunoprecipitation analysis and ligation-mediated PCR-assisted genomic sequencing, we show that HiNF-P interacts with conserved H4 cell cycle regulatory sequences in vivo. Antisense inhibition of HiNF-P reduces endogenous histone H4 gene expression. Furthermore, we find that HiNF-P utilizes NPAT/p220, a substrate of the cyclin E/cyclin-dependent kinase 2 (CDK2) kinase complex, as a key coactivator to enhance histone H4 gene transcription. The biological role of HiNF-P is reflected by impeded cell cycle progression into S phase upon antisense-mediated reduction of HiNF-P levels. Our results establish that HiNF-P is the ultimate link in a linear signaling pathway that is initiated with the growth factor-dependent induction of cyclin E/CDK2 kinase activity at the restriction point and culminates in the activation of histone H4 genes through HiNF-P at the G(1)/S phase transition.

The cyclin E/Cdk2 substrate p220(NPAT) is required for S-phase entry, histone gene expression, and Cajal body maintenance in human somatic cells

Cyclin E/Cdk2, a central regulator of the G1/S transition, coordinates multiple cell cycle events, including DNA replication, centrosome duplication, and activation of the E2F transcriptional program. Recent studies suggest a role for cyclin E/Cdk2 in activation of histone transcription during S phase via the Cajal body-associated protein p220NPAT, and in addition, p220 can promote S-phase entry independently of histone transcriptional activation when overexpressed. Here we have examined the requirement for p220 in histone transcription, cell cycle progression, and Cajal body function through analysis of human somatic HCT116 cells engineered to contain a conditional p220 allele. p220 is required for proliferation of HCT116 cells, as assessed after expression of Cre recombinase in p220(flox/-) cells. This defect was due to an inability of these cells to transit from G0/G1 into S phase, and cell cycle arrest occurred in the presence of elevated Cdk2 kinase activity. Expression of human papillomavirus E7, but not E6, eliminated cell cycle arrest in response to p220 depletion. Optimal expression of all four core histone genes required p220, as did optimal transcription of a histone H4 promoter-luciferase construct. Basal histone H4 expression in G0/G1, although p220 dependent, occurs in the absence of detectable phosphorylation of p220 on Cdk2 sites. Cells lacking p220 displayed defects in the localization of the Cajal body component p80coilin as cells progressed from G0 to S phase in response to mitogenic signals. These finding indicate that p220 is an essential downstream component of the cyclin E/Cdk2 signaling pathway and functions to coordinate multiple elements of the G1/S transition.

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.

The tumor suppressor interferon regulatory factor 1 interferes with SP1 activation to repress the human CDK2 promoter

Cell growth control by interferons (IFNs) involves up-regulation of the tumor suppressor interferon regulatory factor 1 (IRF1). To exert its anti-proliferative effects, this factor must ultimately control transcription of several key genes that regulate cell cycle progression. Here we show that the G1/S phase-related cyclin-dependent kinase 2 (CDK2) gene is a novel proliferation-related downstream target of IRF1. We find that IRF1, but not IRF2, IRF3, or IRF7, selectively represses CDK2 gene transcription in a dose- and time-dependent manner. We delineate the IRF1-responsive repressor element between nt -68 to -31 of the CDK2 promoter. For comparison, the tumor suppressor p53 represses CDK2 promoter activity independently of IRF1 through sequences upstream of nt -68, and the CDP/cut/Cux1 homeodomain protein represses transcription down-stream of -31. Thus, IRF1 repression represents one of three distinct mechanisms to attenuate CDK2 levels. The -68/-31 segment lacks a canonical IRF responsive element but contains a single SP1 binding site. Mutation of this element abrogates SP1-dependent enhancement of CDK2 promoter activity as expected but also abolishes IRF1-mediated repression. Forced elevation of SP1 levels increases endogenous CDK2 levels, whereas IRF1 reduces both endogenous SP1 and CDK2 protein levels. Hence, IRF1 represses CDK2 gene expression by interfering with SP1-dependent transcriptional activation. Our findings establish a causal series of events that functionally connect the anti-proliferative effects of interferons with the IRF1-dependent suppression of the CDK2 gene, which encodes a key regulator of the G1/S phase transition.

The cyclin E/Cdk2 substrate and Cajal body component p220(NPAT) activates histone transcription through a novel LisH-like domain

p220(NPAT) is a substrate of cyclin E/Cdk2 that localizes in nuclear organelles called Cajal bodies in a cell cycle-regulated manner. In normal diploid fibroblasts, p220 is concentrated in two Cajal bodies tethered to histone gene clusters at chromosome 6p21 during G(1), S, and G(2) phases and two additional Cajal bodies tethered to histone genes at 1q21 during S, and G(2) phases. Overexpression of p220 in U2OS cells can promote the G(1)/S transition and can also promote transcription from histone H2B and H4 luciferase reporter constructs. How p220 expression induces these activities and whether the two activities are related are unknown. In this study, we developed a "lox-scanning" mutagenesis approach to identify functional domains in p220. We identified two distinct functional regions of p220. The C-terminal half of the protein contains multiple elements that are required for its ability to induce S phase in transfected cells. In contrast, sequences at the N terminus appear to be critical for activation of histone H4 and H2B reporter constructs. We identified an approximately 30-amino-acid motif at the N terminus of p220 that has the characteristics of a LisH motif. LisH motifs are found in a large number of proteins in the database but are of unknown function. Point mutations in conserved residues in the LisH motif of p220 block histone H4 transcriptional activity without affecting localization in Cajal bodies or phosphorylation on Cdk2 phosphorylation sites. These studies indicate that the ability of p220 to promote S phase is independent of its ability to promote histone H4 transcription and suggests that p220 may link cyclin E/Cdk2 to multiple independent downstream functions.

Maintenance of open chromatin and selective genomic occupancy at the cell cycle-regulated histone H4 promoter during differentiation of HL-60 promyelocytic leukemia cells

During the shutdown of proliferation and onset of differentiation of HL-60 promyelocytic leukemia cells, expression of the cell cycle-dependent histone genes is downregulated at the level of transcription. To address the mechanism by which this regulation occurs, we examined the chromatin structure of the histone H4/n (FO108, H4FN) gene locus. Micrococcal nuclease, DNase I, and restriction enzymes show similar cleavage sites and levels of sensitivity at the H4/n locus in both proliferating and differentiated HL-60 cells. In contrast, differentiation-related activation of the cyclin-dependent kinase inhibitor p21(cip1/WAF1) gene is accompanied by increased nuclease hypersensitivity. Chromatin immunoprecipitation assays of the H4/n gene reveal that acetylated histones H3 and H4 are maintained at the same levels in proliferating and postproliferative cells. Thus, the chromatin of the H4/n locus remains in an open state even after transcription ceases. Using ligation-mediated PCR to visualize genomic DNase I footprints at single-nucleotide resolution, we find that protein occupancy at the site II cell cycle element is selectively diminished in differentiated cells while the site I element remains occupied. Decreased occupancy of site II is reflected by loss of the site II binding protein HiNF-P. We conclude that H4 gene transcription during differentiation is downregulated by modulating protein interaction at the site II cell cycle element and that retention of an open chromatin conformation may be associated with site I occupancy.

Genetic ablation of the CDP/Cux protein C terminus results in hair cycle defects and reduced male fertility

Murine CDP/Cux, a homologue of the Drosophila Cut homeoprotein, modulates the promoter activity of cell cycle-related and cell-type-specific genes. CDP/Cux interacts with histone gene promoters as the DNA binding subunit of a large nuclear complex (HiNF-D). CDP/Cux is a ubiquitous protein containing four conserved DNA binding domains: three Cut repeats and a homeodomain. In this study, we analyzed genetically targeted mice (Cutl1(tm2Ejn), referred to as Delta C) that express a mutant CDP/Cux protein with a deletion of the C terminus, including the homeodomain. In comparison to the wild-type protein, indirect immunofluorescence showed that the mutant protein exhibited significantly reduced nuclear localization. Consistent with these data, DNA binding activity of HiNF-D was lost in nuclear extracts derived from mouse embryonic fibroblasts (MEFs) or adult tissues of homozygous mutant (Delta C(-/-)) mice, indicating the functional loss of CDP/Cux protein in the nucleus. No significant difference in growth characteristics or total histone H4 mRNA levels was observed between wild-type and Delta C(-/-) MEFs in culture. However, specific histone genes (H4.1 and H1) containing CDP/Cux binding sites have reduced expression levels in homozygous mutant MEFs. Stringent control of growth and differentiation appears to be compromised in vivo. Homozygous mutant mice have stunted growth (20 to 50% weight reduction), a high postnatal death rate of 60 to 70%, sparse abnormal coat hair, and severely reduced fertility. The deregulated hair cycle and severely diminished fertility in Cutl1(tm2Ejn/tm2Ejn) mice suggest that CDP/Cux is required for the developmental control of dermal and reproductive functions.

The cell cycle control element of histone H4 gene transcription is maximally responsive to interferon regulatory factor pairs IRF-1/IRF-3 and IRF-1/IRF-7

Interferon regulatory factors (IRFs) are transcriptional mediators of interferon-responsive signaling pathways that are involved in antiviral defense, immune response, and cell growth regulation. To investigate the role of IRF proteins in the regulation of histone H4 gene transcription, we compared the transcriptional contributions of IRF-1, IRF-2, IRF-3, and IRF-7 using transient transfection assays with H4 promoter/luciferase (Luc) reporter genes. These IRF proteins up-regulate reporter gene expression but IRF-1, IRF-3, and IRF-7 are more potent activators of the H4 promoter than IRF-2. Forced expression of different IRF combinations reveals that IRF-2 reduces IRF-1 or IRF-3 dependent activation, but does not affect IRF-7 function. Thus, IRF-2 may have a dual function in histone H4 gene transcription by acting as a weak activator at low dosage and a competitive inhibitor of other strongly activating IRFs at high levels. IRF-1/IRF-3 and IRF-1/IRF-7 pairs each mediate the highest levels of site II-dependent promoter activity and can up-regulate transcription by 120-150-fold. We also find that interferon gamma up-regulates IRF-1 and site II-dependent promoter activity. This up-regulation is not observed when the IRF site is mutated or if cells are preloaded with IRF-1. Our results indicate that IRF-1, IRF-2, IRF-3, and IRF-7 can all regulate histone H4 gene expression. The pairwise utilization of distinct IRF factors provides a flexible transcriptional mechanism for integration of diverse growth-related signaling pathways.

Molecular characterization of celtix-1, a bromodomain protein interacting with the transcription factor interferon regulatory factor 2

Transcriptional control at the G1/S-phase transition of the cell cycle requires functional interactions of multimeric promoter regulatory complexes that contain DNA binding proteins, transcriptional cofactors, and/or chromatin modifying enzymes. Transcriptional regulation of the human histone H4/n gene (FO108) is mediated by Interferon Regulatory Factor-2 (IRF-2), as well as other histone gene promoter factors. To identify proteins that interact with cell-cycle regulatory factors, we performed yeast two-hybrid analysis with IRF-2 and identified a novel human protein termed Celtix-1 which binds to IRF-2. Celtix-1 contains several phylogenetically conserved domains, including a bromodomain, which is found in a number of transcriptional cofactors. Using a panel of IRF-2 deletion mutants in yeast two-hybrid assays, we established that Celtix-1 contacts the C-terminus of IRF-2. Celtix-1 directly interacts with IRF-2 based on binding studies with glutathione S-transferase (GST)/IRF-2 fusion proteins, and immunofluorescence studies suggest that Celtix-1 and IRF-2 associate in situ. Celtix-1 is distributed throughout the nucleus in a heterodisperse pattern. A subset of Celtix-1 colocalizes with the hyperacetylated forms of histones H3 and H4, as well as with the hyperphosphorylated, transcriptionally active form of RNA polymerase II. We conclude that the bromodomain protein Celtix-1 is a novel IRF-2 interacting protein that associates with transcriptionally active chromatin in situ.