The cyclin B2 promoter depends on NF-Y, a trimer whose CCAAT-binding activity is cell-cycle regulated

Gene expression of cyclin-dependent kinase subunit Cks2 is repressed by the tumor suppressor p53 but not by the related proteins p63 or p73

Cks2 proteins are essential components of cyclin/cyclin-dependent kinase complexes and contribute to cell cycle control. We identify Cks2 as a transcriptional target downregulated by the tumor suppressor p53. Cks2 expression was found to be repressed by p53 both at the mRNA and the protein levels. p53 downregulates transcription from the Cks2 promoter in a dose-dependent manner and in all cell types tested. This repression appears to be independent of p53 binding to the Cks2 promoter. In contrast to p53, neither p63 nor p73 proteins can repress Cks2 transcription. Thus p53, rather than its homologues p63 and p73, may contribute to control of the first metaphase/anaphase transition of mammalian meiosis by downregulation of Cks2 expression.

Expression analyses and transcriptional regulation of mouse nucleolar spindle-associated protein gene in erythroid cells: essential role of NF-Y

Nucleolar spindle-associated protein (NuSAP), a recently characterised microtubule-associated protein, appears to participate in cell cycle regulation. It has been demonstrated that NuSAP is expressed preferentially in the erythroid lineage in haematopoietic cells. To characterise its role in erythropoiesis, we examined the expression profile of the NuSAP gene. In fractionated murine erythroblasts, NuSAP mRNA was remarkably more abundant in the subset corresponding to immature erythroblasts (TER119(+)CD71(high)) than mature erythroblasts (TER119(+)CD71(low)), and it was significantly increased in TER119(+) cells from in vivo phlebotomised mice compared with control mice. Furthermore, during erythroid maturation of mouse erythroleukaemia (MEL) cells by dimethylsulfoxide, NuSAP mRNA was increased at 24-72 h. These results suggested that the NuSAP gene might contribute to the expansion of immature erythroblast pool. The regulatory mechanism of NuSAP gene was investigated using MEL cells. Sequence analysis revealed that NuSAP promoter has four CCAAT boxes, an Sp1 element, a GATA-like element, a CACCC element, a Myb element and lacks a TATA box. Promoter analyses demonstrated that duplicated CCAAT boxes located at -81/-85 and -30/-34 were essential for promoter activity. Furthermore, the promoter was trans-activated by NF-YA through these elements. These results suggest that NuSAP might play an important role in erythroid proliferation under the control of NF-Y.

Gain of function of mutant p53: the mutant p53/NF-Y protein complex reveals an aberrant transcriptional mechanism of cell cycle regulation

This article investigates the mechanistic aspects of mutant p53 "gain of function" in response to DNA damage. We show that mutant forms of p53 protein interact with NF-Y. The expression of cyclin A, cyclin B1, cdk1, and cdc25C, as well as the cdk1-associated kinase activities, is upregulated after DNA damage, provoking a mutant p53/NF-Y-dependent increase in DNA synthesis. Mutant p53 binds NF-Y target promoters and, upon DNA damage, recruits p300, leading to histone acetylation. The recruitment of mutant p53 to the CCAAT sites is severely impaired upon abrogation of NF-YA expression. Endogenous NF-Y, mutant p53, and p300 proteins form a triple complex upon DNA damage. We demonstrate that aberrant transcriptional regulation underlies the ability of mutant p53 proteins to act as oncogenic factors.

Cell type-dependent control of NF-Y activity by TGF-beta

Transforming growth factor beta (TGF-beta) is a pluripotent cytokine that regulates cell growth and differentiation in a cell type-dependent fashion. TGF-beta exerts its effects through the activation of several signaling pathways. One involves membrane proximal events that lead to nuclear translocation of members of the Smad family of transcriptional regulators. TGF-beta can also activate MAPK cascades. Here, we show that TGF-beta induces nuclear translocation of the NF-YA subunit of the transcription factor NF-Y by a process that requires activation of the ERK cascade. This results in increased binding of endogenous NF-Y to chromatin and TGF-beta-dependent transcriptional regulation of the NF-Y target gene cyclin A2. Interestingly, the kinetics of NF-YA relocalization differs between epithelial cells and fibroblasts. NIH3T3 fibroblasts show an elevated basal level of phosphorylated p38 and delayed nuclear accumulation of NF-YA after TGF-beta treatment. In contrast, MDCK cells show low basal p38 activation, higher basal ERK phosphorylation and more rapid localization of NF-YA after induction. Thus, NF-Y activation by TGF-beta1 involves ERK1/2 and potentially an interplay between MAPK pathways, thereby opening the possibility for finely tuned transcriptional regulation.

The Pole3 bidirectional unit is regulated by MYC and E2Fs

Pole3 (DPB4/YBL1/CHRAC17) is one of the subunits of the DNA polymerase e. It contains a histone-like domain required for the hererodimerization with its Pole4 (DPB3) partner. In another interaction, Pole3 heterodimerizes with YCL1/CHRAC15 and associates with the ACF1/SNF2H remodelling complex. We find that the Pol3 gene is regulated in starved NIH3T3 fibroblasts upon induction with serum, with a peak at the entry in the S phase. We characterized the Pole3 promoter, which is linked bidirectionally to C9Orf46, a gene of unknown function: it has no CCAAT nor TATA-boxes, and contains an E box and two potential E2F sites. Mutagenesis analysis points to a minimal promoter region as sufficient for activation; the E box and a neighbouring direct repeat are important for regulation. Cell-cycle regulation was reproduced in stable clones and an additional E2F site was found to be important. Chromatin immunoprecipitation analysis indicates that E2F1/4, as well as MYC, are associated with the Pole3 promoter in a phase-specific way. These data highlight coregulation of a histone-like gene with core histones upon DNA synthesis, and represent a first dissection of the interplay between two essential cell-cycle regulators on a bidirectional promoter.

Cell-specific regulation of TRBP1 promoter by NF-Y transcription factor in lymphocytes and astrocytes

HIV-1 viral production is restricted intracellularly in astrocytes compared with lymphocytes due to the limited expression of viral structural proteins. The poor translation of HIV-1 mRNA and consequent limited virion production can be restored by overexpression of TRBP proteins in the astrocytoma U251MG cells. TRBP1 and TRBP2 are double-stranded RNA binding proteins that increase HIV-1 gene expression. Both proteins are produced from a single gene that possesses two independent promoters and an alternative first exon. Endogenous expression is restricted in astrocytes due to limited TRBP promoter expression compared to lymphocytes. We examined the transcriptional regulation of TRBP1 and TRBP2 by in vivo genomic footprinting in the lymphocytic Jurkat and in the astrocytic U251MG cells. We identified one AP4 and one AP2-binding site that regulate the TRBP2 promoter in both cell types, and one Sp1 and two CCAAT-binding sites that control TRBP1 expression. Mutations in the TRBP1 promoter modulate its expression specifically in Jurkat and in U251MG. The analysis of the CCAAT-390 site by EMSA and by ChIP demonstrates that NF-Y/CBF transcription factor binds specifically to the promoter in vitro and in vivo. Furthermore, each NF-Y subunit was more highly expressed in the lymphocytic cells, compared to astrocytic cells. An NF-YA trans-dominant mutant decreased TRBP1 promoter expression fourfold in Jurkat cells, thus demonstrating the functional importance of NF-Y factors in lymphocytes. These studies suggest that the cell specifity of HIV-1 expression and replication may be regulated, in part, through the control of TRBP1 expression.

DNA damage promotes histone deacetylase 4 nuclear localization and repression of G2/M promoters, via p53 C-terminal lysines

Repression of G(2)/M promoters after DNA damage is an active mechanism that requires the p53 tumor suppressor. We have recently found that histone deacetylase 4 (HDAC4) is recruited on NF-Y-dependent repressed promoters. In this report, we describe the relationship between p53 and HDAC4 recruitment following DNA damage using immunofluorescence, chromatin immunoprecipitation, and transfection experiments. HDAC4 shuttles from the cytoplasm into the nucleus, following DNA damage, independently of the activation of p53 and becomes associated with promoters through a p53-dependent mechanism. The C-terminal lysines of p53, which are acetylated and methylated, are required for HDAC4 recruitment and transcriptional repression. Trichostatin treatment, but not HDAC4 functional inactivation, relieves the adriamycin-mediated repression of G(2)/M promoters. Our results indicate that HDAC4 is a component of the DNA damage response and that post-translational modifications of p53 are important for repression of G(2)/M genes.

Transactivation of the cyclin A promoter by bovine papillomavirus type 4 E5 protein

Bovine papillomavirus type 4 (BPV-4) E5 (formerly E8) is a 42-residue hydrophobic, membrane-localised protein that can transform NIH-3T3 cells by a poorly defined mechanism. In E5-expressing cells, the observed up-regulation of cyclin A is underpinned by transactivation of the cyclin A promoter. Here we show that E5 transactivates the minimal cell cycle-regulated cyclin A promoter in cells both stably and acutely expressing the viral protein. There are no detectable differences between control and E5 cells in protein complexes binding the E2F-like cell cycle-dependent element (CDE)/cell cycle-regulated element (CCRE) of the cyclin A promoter and E5 does not transactivate E2F reporter plasmids in an E2F-dependent manner in vivo. CCAAT box integrity and functional NF-Y complexes are required for E5-mediated transactivation and a Mr approximately 110 K CCAAT-box binding factor (p110 CBF) associates with NF-YA only in E5 cells. This suggests that E5 sets the extent of cyclin A promoter activation by a mechanism similar to other, structurally unrelated, DNA tumour virus oncoproteins but distinct from the action of serum factors and so is inconsistent with E5 acting through constitutive activation of tyrosine kinase growth factor receptors.

Discovering functional transcription-factor combinations in the human cell cycle

With the completion of full genome sequences and advancement in high-throughput technologies, in silico methods have been successfully used to integrate diverse data sources toward unraveling the combinatorial nature of transcriptional regulation. So far, almost all of these studies are restricted to lower eukaryotes such as budding yeast. We describe here a computational search for functional transcription-factor (TF) combinations using phylogenetically conserved sequences and microarray-based expression data. Taking into account both orientational and positional constraints, we investigated the overrepresentation of binding sites in the vicinity of one another and whether these combinations result in more coherent expression profiles. Without any prior biological knowledge, the search led to the discovery of several experimentally established TF associations, as well as some novel ones. In particular, we identified a regulatory module controlling cell cycle-dependent transcription of G2-M genes and expanded its functional generality. We also detected many homotypic combinations, supporting the importance of binding-site density in transcriptional regulation of higher eukaryotes.

Recruitment of nuclear factor Y to the inverted CCAAT element (ICE) by c-Jun and E1A stimulates basal transcription of the bone sialoprotein gene in osteosarcoma cells

Bone sialoprotein (BSP), a major protein in the extracellular matrix of bone, is expressed almost exclusively by bone cells and by cancer cells that have a propensity to metastasize to bone. Previous studies have shown that v-src stimulates basal transcription of bsp in osteosarcoma (ROS 17/2.8) cells by targeting the inverted CCAAT element (ICE) in the proximal promoter. To identify possible downstream effectors of Src we studied the effects of the proto-oncogene c-jun, which functions downstream of Src, on basal transcription of bsp using transient transfection assays. Increased expression of endogenous c-Jun induced by the tumor promoter 12-O-tetradecanoyl-phorbol 13-acetate and ectopic expression of c-Jun increased basal transcription of chimeric reporter constructs encompassing the proximal promoter by 1.5-3-fold in ROS 17/2.8 osteosarcoma cells, with more modest effects in a normal bone cell line, RBMC-D8. The effects of c-Jun were abrogated by mutations in the ICE box and by co-expression of dominant negative nuclear factor Y, subunit A (NF-YA). The increase in bsp transcription did not require phosphorylation of c-Jun and was not altered by trichostatin treatment or by ectopic expression of p300/CREB-binding protein (CBP) or mutated forms lacking histone acetyltransferase (HAT) activity. Similarly, ectopic expression of p300/CBP-associated factor (P/CAF), which transduces p300/CBP effects, or of HAT-defective P/CAF did not influence the c-jun effects. Surprisingly, E1A, which competes with P/CAF binding to p300/CBP, also stimulated BSP transcription through NF-Y independently of c-jun, p300/CBP, and P/CAF. Collectively, these studies show that c-Jun and E1A regulate basal transcription of bsp in osteosarcoma cells by recruiting the NF-Y transcriptional complex to the ICE box in a mechanism that is independent of p300/CBP and P/CAF HAT activities.

Selective Effects of the Anticancer Drug Yondelis (ET-743) on Cell-Cycle Promoters

Yondelis is a potent DNA-binding anticancer drug isolated from the tunicate Ecteinascidia turbinata currently undergoing phase III clinical trials. We and others have shown selective inhibition to the transcriptional induction of several genes. We tested the hypothesis that Yondelis specifically targets cell-cycle genes. Our analysis on endogenous and transfected reporter systems revealed complex patterns of transcriptional inhibition and, surprisingly, activation. Other inducible systems-the metallothionein and the CYP3A4 promoters-were little affected. We assayed whether interference of DNA binding of the common nuclear factor Y (NF-Y) activator was responsible for the observed inhibition: in vivo chromatin immunoprecipitation analysis in NIH3T3 and HCT116 cells indicates that NF-Y binding is little affected by Yondelis addition. Finally, histone acetylation was modestly affected only on Cdc2 and cyclin B2 but not on other repressed promoters. These data prove that Yondelis is not a general inhibitor of inducible genes, and its selective effects are exerted downstream from transcription factors binding and histone acetyl transferases recruitment.

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.

Direct p53 transcriptional repression: in vivo analysis of CCAAT-containing G2/M promoters

In response to DNA damage, p53 activates G(1)/S blocking and apoptotic genes through sequence-specific binding. p53 also represses genes with no target site, such as those for Cdc2 and cyclin B, key regulators of the G(2)/M transition. Like most G(2)/M promoters, they rely on multiple CCAAT boxes activated by NF-Y, whose binding to DNA is temporally regulated during the cell cycle. NF-Y associates with p53 in vitro and in vivo through the alphaC helix of NF-YC (a subunit of NF-Y) and a region close to the tetramerization domain of p53. Chromatin immunoprecipitation experiments indicated that p53 is associated with cyclin B2, CDC25C, and Cdc2 promoters in vivo before and after DNA damage, requiring DNA-bound NF-Y. Following DNA damage, p53 is rapidly acetylated at K320 and K373 to K382, histones are deacetylated, and the release of PCAF and p300 correlates with the recruitment of histone deacetylases (HDACs)-HDAC1 before HDAC4 and HDAC5-and promoter repression. HDAC recruitment requires intact NF-Y binding sites. In transfection assays, PCAF represses cyclin B2, and a nonacetylated p53 mutant shows a complete loss of repression potential, despite its abilities to bind NF-Y and to be recruited on G(2)/M promoters. These data (i) detail a strategy of direct p53 repression through associations with multiple NF-Y trimers that is independent of sequence-specific binding of p53 and that requires C-terminal acetylation, (ii) suggest that p53 is a DNA damage sentinel of the G(2)/M transition, and (iii) delineate a new role for PCAF in cell cycle control.

Characterization of the human zinc finger protein 267 promoter: Essential role of nuclear factor Y

Liver fibrosis results from an excessive deposition of extracellular matrix proteins secreted by activated hepatic stellate cells (HSCs). The activation process is accompanied by an increased activity of various transcription factors, including zinc finger protein 267 (ZNF267). Recently, ZNF267 has been shown to modulate gene expression and to function as a transcriptional repressor. MMP-10 was identified as a target gene; its gene expression and promoter activity are inhibited by ZNF267, which might promote liver fibrogenesis through diminished matrix degradation. However, the transcriptional regulation of the ZNF267 gene is unknown. In the present study, we have cloned and characterized the human ZNF267 promoter containing a 1.5 kb fragment of the 5'-flanking region (-1414/+173). The ZNF267 gene has a TATA-less promoter with multiple transcription initiation sites. Analysis of serial 5'-deletions of luciferase reporter constructs revealed a minimal promoter between -72 and +173 bp. Mutational analysis of putative regulatory elements indicated that a CCAAT box within this region was essential for ZNF267 promoter activity. Electrophoretic mobility shift assays demonstrated that transcription factor nuclear factor Y (NF-Y) bound to the CCAAT box. In co-transfection experiments, NF-YA increased the promoter activity of ZNF267. In conclusion, our results suggest that the binding site for NF-Y is critical for ZNF267 gene regulation and, herewith, the activation of this transcriptional factor may play an important role in the activation process of HSCs and in liver fibrosis.

NF-Y behaves as a bifunctional transcription factor that can stimulate or repress the FGF-4 promoter in an enhancer-dependent manner

NF-Y is a bifunctional transcription factor capable of activating or repressing transcription. NF-Y specifically recognizes CCAAT box motifs present in many eukaryotic promoters. The mechanisms involved in regulating its activity are poorly understood. Previous studies have shown that the FGF-4 promoter is regulated positively by its CCAAT box and NF-Y in embryonal carcinoma (EC) cells where the distal enhancer of the FGF-4 gene is active. Here, we demonstrate that the CCAAT box functions as a negative cis-regulatory element when cis-regulatory elements of the FGF-4 enhancer are disrupted, or after EC cells differentiate and the FGF-4 enhancer is inactivated. We also demonstrate that NF-Y mediates the repression of the CCAAT box and that NF-Y associates with the endogenous FGF-4 gene in both EC cells and EC-differentiated cells. Importantly, we also determined that the orientation and the position of the CCAAT box are critical for its role in regulating the FGF-4 promoter. Together, these studies demonstrate that the distal enhancer of the FGF-4 gene determines whether the CCAAT box of the FGF-4 promoter functions as a positive or a negative cis-regulatory element. In addition, these studies are consistent with NF-Y playing an architectural role in its regulation of the FGF-4 promoter.

Identification of the critical cis-acting elements in the promoter of the mouse Prnd gene coding for Doppel protein

Prnd gene encodes a prion-like protein Doppel. To elucidate the transcriptional regulation of the mouse Prnd gene, we performed a study of its 5' flanking region. Serial deletion analysis of the -1863/+27 genomic fragment revealed the region -185/+27 as a core promoter. Cis-acting elements at the -182/-177 (E box) and -108/-104 (CCAAT box) positions were identified as the main activators of the Prnd expression in GC1-spg cells, with NF-Y being the important contributor. The gel-shift experiments confirmed the binding of the transcription factors to -108/-104 NF-Y consensus motif. An intron1 was found to enhance the Prnd expression in bEND.3 endothelial cells, suggesting an involvement of the endothelial-specific enhancer.

Oct-1 and nuclear factor Y bind to the SURG-1 element to direct basal and gonadotropin-releasing hormone (GnRH)-stimulated mouse GnRH receptor gene transcription

The cis-regulatory element localized to position -292/-285 of the mouse GnRH receptor (mGnRHR) gene promoter, designated Sequence Underlying Responsiveness to GnRH 1 (SURG-1), has been shown previously to contribute to stimulation of mGnRHR gene expression by GnRH. We have identified three specific protein-DNA complexes on the SURG-1 element by EMSA using nuclear extracts from the gonadotrope-derived alphaT3-1 and LbetaT2 cell lines. Serial mutagenesis and supershift assays identified nuclear factor Y (NF-Y) binding to -288/-284 and Oct-1 binding to a TAAT sequence at -290/-287. Binding of these two transcription factors was confirmed in vivo by chromatin immunoprecipitation assay and increased in response to GnRH stimulation. To define the functional significance of these sequences in the regulation of mGnRHR gene transcription, transient transfection assays were performed in alphaT3-1 cells using a 1.2-kb mGnRHR (-1164/+62) gene promoter-luciferase reporter construct with selective mutations of the Oct-1, NF-Y, and/or the previously characterized activating protein 1 (AP-1) binding site (-274/-268). Individual mutations in the Oct-1, NF-Y, and AP-1 sites decreased both basal expression and stimulation by GnRH agonist, and the combined mutation of the Oct-1 and AP-1 binding sites further reduced basal transcriptional activity and abolished GnRH stimulation. Overexpression of NF-YA increased GnRHR promoter activity, whereas expression of a dominant negative NF-YA mutant decreased activity, further supporting a role of NF-Y in regulation of mGnRHR gene transcription. In addition, knockdown of Oct-1 by small interfering RNA confirmed that Oct-1 is important for mGnRHR gene expression. In conclusion, NF-Y and Oct-1 bind to the SURG-1 element to direct basal and GnRH-stimulated expression of the mGnRHR gene.

SIRF--a novel regulator element controlling transcription from the p55Cdc/Fizzy promoter during the cell cycle

p55Cdc proteins participate in activation and timing of ubiquitin ligation by APC/C. Labeling of the substrates with ubiquitin leads to degradation of the cell cycle proteins through the proteasome in mitosis. Consistent with the phase in which the protein functions p55Cdc mRNA is expressed during the cell cycle starting in S phase with a maximum in G2/M. We analyzed the human p55Cdc promoter responsible for this expression pattern and found with SIRF (Cell-Cycle Site-Regulating p55Cdc/Fizzy-Transcription) a novel element which downregulates transcription in a cell cycle-dependent manner. Activation of gene transcription is independent of the SIRF element and NF-Y. The nucleotide sequence of SIRF is essentially identical in human, rat, and mouse p55Cdc whereas other parts of the promoter are not conserved. SIRF requires its natural promoter context for its regulatory function. With a length of 44 nucleotides this element is unusually long and may require a large protein complex for its regulation.

Polymorphic NF-Y dependent regulation of human nicotine C-oxidase (CYP2A6)

OBJECTIVES

In humans, cytochrome P450 2A6 (CYP2A6) constitutes the principal nicotine C-oxidase. Several different polymorphic CYP2A6 gene variants are known which contribute to the highly variable expression of this enzyme among individuals. In this study we report a novel polymorphism located in the 5' flanking region (-745A > G) of the CYP2A6 gene disrupting a CCAAT box.

METHODS AND RESULTS

Electrophoretic mobility shift assays (EMSA) indicated that NF-YA is part of this nuclear protein complex. Chromatin immunoprecipitation revealed that NF-Y recognizes a region of the CYP2A6 5' flanking region located between -932 and -606. EMSA showed that out of the three CCAAT boxes in the CYP2A6 promoter, with CCAAT core sequences located between -839/-835, -748/-744, and -689/-685, only the one at -748/-744 was able to compete with the nuclear protein complex binding to the -748/-744 CCAAT box. Cotransfection experiments indicated that NF-Y acts as a positive regulatory element on CYP2A6 gene regulation. EMSA demonstrated that an NF-Y consensus oligonucleotide but not the -745A > G oligonucleotide competed efficiently with binding of the protein complex to the -748/-744 CCAAT box. Promoter activity of the -745A > G variant was significantly reduced to 78% relative to the wild-type allele in HepG2 cells transfected with luciferase reporter plasmids. Finally, haplotype analysis was carried out comprising the -745A > G variant in combination with all known CYP2A6 3' and 5' flanking single nucleotide polymorphisms: -1013A > G, -48T > G, and the CYP2A6/CYP2A7 3' flank conversion.

CONCLUSION

A new haplotype, CYP2A6*1H was identified, with allele frequencies of 3.1% in Swedish and 5.2% in Turkish populations.

Temperature may influence and regulate NF-YB expression in toad oocyte

Fully grown oocytes derived from Bufo gargarizans maintained at relatively low temperatures (4 degrees C, LTE-oocytes) acquire the competence to resume normal meiosis. In contrast, fully grown oocytes derived from toads maintained at relatively high temperatures (28 degrees C, HTE-oocytes) never acquire maturation competence. By suppression subtractive hybridization, we obtained 18 ESTs preliminarily thought to be preferentially expressed in LTE-oocytes; of these, TS1-4 shared homology with the human and mouse NF-YB genes. We cloned the full-length toad NF-YB gene by RACE and identified three alternatively spliced transcripts: tNF-YB1, tNF-YB2, and tNF-YB3 (GenBank Accession Nos. AY442015, AY442016, and AY442017, respectively). Toad NF-YB was differentially transcribed and translated in LTE-oocytes versus HTE-oocytes, likely resulting in differential CCAAT-binding and/or transcriptional activity of NF-Y. Furthermore, toad cyclin B2 was differentially transcribed at high and low temperatures. Taken together, this report of the differential expression of toad NF-YB at different temperatures is the first evidence that temperature may influence and regulate NF-YB expression.

Dynamic recruitment of NF-Y and histone acetyltransferases on cell-cycle promoters

Regulation of transcription during the cell-cycle is under the control of E2 factors (E2Fs), often in cooperation with nuclear factor Y (NF-Y), a histone-like CCAAT-binding trimer. NF-Y is paradigmatic of a constitutive, ubiquitous factor that pre-sets the promoter architecture for other regulatory proteins to access it. We analyzed the recruitment of NF-Y, E2F1/4/6, histone acetyltransferases, and histone deacetylase (HDAC) 1/3/4 to several cell-cycle promoters by chromatin immunoprecipitation assays in serum-starved and restimulated NIH3T3 cells. NF-Y binding is not constitutive but timely regulated in all promoters tested, being displaced when promoters are repressed. p300 association correlates with activation, and it is never found in the absence of NF-Y, whereas PCAF/hGCN5 is often found before NF-Y association. E2F4 and E2F6, together with HDACs, are bound to repressed promoters, including the G2/M Cyclin B2. As expected, an inverse relationship between HDACs association and histones H3/H4 acetylation is observed. Blocking cells in G1 with the cyclin-dependent kinase 2 inhibitor R-roscovitine confirms that NF-Y is bound to G1/S but not to G2/M promoters in G1. These data indicate that following the release of E2Fs/HDACs, a hierarchy of PCAF-NF-Y-p300 interactions and H3-H4 acetylations are required for activation of cell-cycle promoters.

Three CCAAT-boxes and a single cell cycle genes homology region (CHR) are the major regulating sites for transcription from the human cyclin B2 promoter

Cyclins are essential regulators of the cell division cycle. Cyclin B associates with the cyclin-dependent kinase 1 (cdc2) to form a complex which is required for cells to undergo mitosis. In mammalian cells three B-type cyclins have been characterised, cyclin B1, B2 and B3. The cell cycle-dependent synthesis of cyclin B1 and B2 has been investigated in detail displaying maximum expression in G2 which is mainly regulated on the transcriptional level. We have previously shown that this regulation of the mouse cyclin B2 promoter is controlled by a cell cycle-dependent element (CDE) and the cell cycle genes homology region (CHR). Also in a number of other genes CDE/CHR elements repress transcription in G0 and G1 and lead to relief of repression later during the cell cycle. Here, we compare human and mouse cyclin B2 promoters. Both promoters share only nine regions with nucleotide identities. Three of these sites are CCAAT-boxes spaced 33 bp apart which can bind the NF-Y transcriptional activator. NF-Y binding to the human cyclin B2 promoter could be shown by chromatin immunoprecipitation (ChIP) assays. Activation by NF-Y is responsible for more than 93% of the total promoter activity as measured by cotransfecting a plasmid coding for a dominant-negative form of NF-YA. Cell cycle-dependent repression is regulated solely through a CHR. Surprisingly, in contrast to the mouse promoter the CHR in the human cyclin B2 promoter does not rely on a CDE site in tandem with it. Together with the recently described mouse cdc25C promoter, human cyclin B2 is the second identified gene which solely requires a CHR for its cell cycle regulation.

Requirement for down-regulation of the CCAAT-binding activity of the NF-Y transcription factor during skeletal muscle differentiation

NF-Y is composed of three subunits, NF-YA, NF-YB, and NF-YC, all required for DNA binding. All subunits are expressed in proliferating skeletal muscle cells, whereas NF-YA alone is undetectable in terminally differentiated cells in vitro. By immunohistochemistry, we show that the NF-YA protein is not expressed in the nuclei of skeletal and cardiac muscle cells in vivo. By chromatin immunoprecipitation experiments, we demonstrate herein that NF-Y does not bind to the CCAAT boxes of target promoters in differentiated muscle cells. Consistent with this, the activity of these promoters is down-regulated in differentiated muscle cells. Finally, forced expression of the NF-YA protein in cells committed to differentiate leads to an impairment in the down-regulation of cyclin A, cyclin B1, and cdk1 expression and is accompanied by a delay in myogenin expression. Thus, our results indicate that the suppression of NF-Y function is of crucial importance for the inhibition of several cell cycle genes and the induction of the early muscle-specific program in postmitotic muscle cells.

Interactions between p300 and multiple NF-Y trimers govern cyclin B2 promoter function

The CCAAT box is one of the most common elements in eukaryotic promoters and is activated by NF-Y, a conserved trimeric transcription factor with histone-like subunits. Usually one CCAAT element is present in promoters at positions between -60 and -100, but an emerging class of promoters harbor multiple NF-Y sites. In the triple CCAAT-containing cyclin B2 cell-cycle promoter, all CCAAT boxes, independently from their NF-Y affinities, are important for function. We investigated the relationships between NF-Y and p300. Chromatin immunoprecipitation analysis found that NF-Y and p300 are bound to the cyclin B2 promoter in vivo and that their binding is regulated during the cell cycle, positively correlating with promoter function. Cotransfection experiments determined that the coactivator acts on all CCAAT boxes and requires a precise spacing between the three elements. We established the order of in vitro binding of the three NF-Y complexes and find decreasing affinities from the most distal Y1 to the proximal Y3 site. Binding of two or three NF-Y trimers with or without p300 is not cooperative, but association with the Y1 and Y2 sites is extremely stable. p300 favors the binding of NF-Y to the weak Y3 proximal site, provided that a correct distance between the three CCAAT is respected. Our data indicate that the precise spacing of multiple CCAAT boxes is crucial for coactivator function. Transient association to a weak site might be a point of regulation during the cell cycle and a general theme of multiple CCAAT box promoters.

Cyclin B1 transcription is enhanced by the p300 coactivator and regulated during the cell cycle by a CHR-dependent repression mechanism

Cyclin B is a central regulator of transition from the G(2) phase of the cell cycle to mitosis. In mammalian cells two B-type cyclins have been characterised, cyclin B1 and B2. Both are expressed with a maximum in G(2) and their synthesis is mainly regulated on the transcriptional level. We show that a single cell cycle genes homology region, lacking a functional cell cycle-dependent element in tandem with it, contributes most of the cell cycle-dependent transcription from the cyclin B1 promoter. The coactivator p300 binds to the cyclin B1 promoter and synergises with the transcription factor NF-Y in activating transcription of cyclin B1.

Nuclear factor-Y binding to the topoisomerase IIalpha promoter is inhibited by both the p53 tumor suppressor and anticancer drugs

Expression of the human DNA topoisomerase IIalpha (topo IIalpha) gene is positively regulated by the binding of the nuclear factor Y (NF-Y) transcription factor to four of five inverted CCAAT boxes (ICBs) located in its promoter. We have demonstrated previously that expression of the p53 tumor suppressor inhibits human topo IIalpha promoter activity in murine (10)1 cells. In this report, we demonstrate that the inhibition of topo IIalpha gene expression by wild-type p53 correlates with the decreased binding of the transcription factor NF-Y to the first four ICBs of the topo IIalpha promoter. The expression of mutant p53 does not affect the binding of NF-Y. In NIH3T3 cells, we show that topo II-targeted drugs inhibit the binding of NF-Y to ICB sites in the topo IIalpha promoter. This effect is seen not only with drugs that result in DNA strand breaks but also with drugs that inhibit the catalytic activity of topo II, and even with the mitotic spindle inhibitor, vinblastine. Further experiments with p53-null (10)1 cells treated with these same drugs also demonstrate decreased NF-Y binding to the topo IIalpha ICBs. The data presented points to the existence of both p53-dependent and -independent mechanisms for regulating NF-Y binding to ICBs in the topo IIalpha promoter and thus the modulation of topo IIalpha gene expression.

DNA damage-induced inhibition of securin expression is mediated by p53

Tumor suppressor p53 induces the cellular response to DNA damage mainly by regulating expression of its downstream target genes. The human securin is an anaphase inhibitor, preventing premature chromosome separation through inhibition of separase activity. It is also known as the product of the human pituitary tumor-transforming gene, pttg, a proto-oncogene. Here we report that the expression of human securin is suppressed in cells treated with the DNA-damaging drugs doxorubicin and bleomycin. This suppression requires functional p53. Analysis of the human securin promoter reveals that DNA-binding sites for Sp1 and NF-Y are both required for activation of securin expression; however, only the NF-Y site is essential for the suppression by p53. Our study indicates that securin is a p53 target gene and may play a role in p53-mediated cellular response to DNA damage.

Functional interaction of nuclear factor y and sp1 is required for activation of the epstein-barr virus C promoter

Two Epstein-Barr virus (EBV) latent cycle promoters, Wp and Cp, are activated sequentially during virus-induced transformation of primary B lymphocytes. Immediately postinfection, viral transcription initiates from Wp, leading to expression of EBV nuclear antigen 2 (EBNA2) and EBNA5. Within 36 h, there is a switch in promoter usage from Wp to the upstream Cp, which leads to expression of EBNA1 to EBNA6. EBNA2 appears to be required for the Wp-to-Cp switch, but the switching mechanism is not fully understood at the molecular level. In a previous investigation we showed that there is an EBNA2-independent activity of reporter constructs containing deletion fragments of Cp in B-lymphoid cell lines, and we demonstrated that Cp activity is highly dependent on several cellular transcription factors, including nuclear factor Y (NF-Y) and Sp1. In the present work, we analyzed the effect of NF-Y on Cp activity in greater detail. We demonstrate that (i) a dominant negative analogue of NF-Y abolishes Cp activity, (ii) NF-Y and Sp1 costimulate Cp, and (iii) the oriPI-EBNA1-induced transactivation of Cp requires concomitant expression of NF-Y and Sp1, although additional factors seem necessary for optimal activation. Furthermore, using the lymphoblastoid cell line EREB2-5, in which EBNA2 function is regulated by estrogen, we demonstrate that inactivation of EBNA2 results in decreased expression of NF-Y and down-regulation of Cp. On reconstitution of the EBNA2 function, the cells enter the cell cycle, NF-Y levels increase, and a concomitant Wp-to-Cp switch occurs. Taken together, our results suggest that NF-Y is essential for Cp activation and that up-regulation of NF-Y may contribute to a successful Wp-to-Cp switch during B-cell transformation.

Characterization of the human ephrin-A4 promoter

Expression of the ephrin-A4 ligand, a family member of ligands binding the Eph receptor tyrosine kinases, is induced after an antigen-receptor stimulation of lymphocytes. To understand the transcription regulation of the ephrin-A4 gene, its promoter was identified and regulating elements were characterized. The ephrin-A4 promoter contains cis elements directing the cell-specific expression. By deletion studies, three specific regions, which were contributing to the transcription activity in lymphoid cells, were localized. In one of these regions, an inverted CCAAT box was identified and shown to bind the transcription activator nuclear factor-Y (NF-Y). The importance of NF-Y binding for the ephrin-A4 promoter activity is shown by a total abrogation of promoter activity after destruction of its binding site. NF-Y binding and activity are also crucially dependent on the integrity of the surrounding sequence. In addition, electrophoretic mobility-shift assay and serial-mutation analysis of the two remaining regulating regions revealed cis regulatory elements contributing to the transcription activity of the ephrin-A4 promoter.

Transcriptional regulation of myeloid differentiation primary response (MyD) genes during myeloid differentiation is mediated by nuclear factor Y

To understand the molecular mechanism by which interleukin-6 (IL-6) regulates myeloid differentiation primary response (MyD) genes at the onset of M1 myeloid differentiation, we used JunB as a representative MyD gene to isolate and characterize IL-6 responsive elements. An IL-6 responsive element was localized between -65 and -52 of the JunB promoter (-65/-52 IL-6RE). By using antibody and oligonucleotide competition assays in electrophoretic mobility shift assay experiments, we have shown that the heterotrimeric transcription nuclear factor Y (NF-Y) complex binds to this element. A dominant-negative form of NF-YA, ectopically expressed in M1 cells, blocked NF-Y binding to the -65/-52 IL-6RE and reduced induction of JunB by IL-6. Furthermore, inhibition of NF-Y binding also reduced MyD gene induction by IL-6 and dampened the IL-6-induced M1 differentiation program. These findings are consistent with the observation that most MyD genes contain intact NF-Y binding motifs in their promoter regions. In contrast to M1 cells, during myeloid differentiation of bone marrow (BM), there was induction of NF-Y binding to the -65/-52 IL-6RE. This induced binding can be attributed to the observed induction of NF-YA protein expression and may reflect the molecular mechanism that couples proliferation to terminal differentiation of normal myeloblasts. Similar to M1 cells, blocking NF-Y binding in BM resulted in a reduction in mature macrophages. It can be concluded that NF-Y plays a role in the transcriptional regulation of MyD genes and is required for optimum myeloid differentiation.

Expression of p53 and its homologues in primary and recurrent squamous cell carcinomas of the head and neck

The tumour-suppressor protein p53 belongs to a family that includes 2 structurally related proteins, p63 and p73. Because of their structural homology, it has been hypothesized that both homologues serve as "spare mechanisms" in p53 mutations to regulate the cell cycle by inducing apoptosis. We investigated the mutational and protein expression status of p53 in correlation to its homologues, p73 and p63, in primary and recurrent squamous cell carcinomas of the head and neck (HNSCC) and corresponding nonneoplastic mucosa. Expression and mutation of p53 and its homologues p63 (including the 2 major isotypes TAp63 and DeltaNp63) and p73 was examined by direct DNA sequencing and immunohistochemistry in 29 primary and 39 recurrent (secondary) HNSCCs after microdissection. Our results were correlated with pathohistologic stage and grade. p53 mutations were detected in 32/68 (47%) carcinomas of 17 patients, with a discordant mutation pattern of primary and consecutive tumours in all cases. Positive immunostaining for p63 was found in 55/68 (81%) carcinomas of 29 patients. Immunohistochemistry revealed p73 protein expression in 32/68 (47%) tumours. In normal mucosa, p63 and p73 were expressed in 40/68 (59%) and 12/68 (18%) cases, respectively. We failed to detect specific mutations of p73 or p63 in primary and recurrent carcinoma of the head and neck. p73 and p63 were rarely mutated in HNSCC, but both were expressed in a subset of tumours. The lack of correlation between p73/p63 and p53 protein expression suggests that neither p73 nor p63 can replace p53 when it is mutated.

A single cell cycle genes homology region (CHR) controls cell cycle-dependent transcription of the cdc25C phosphatase gene and is able to cooperate with E2F or Sp1/3 sites

The cdc25C phosphatase participates in regulating transition from the G2 phase of the cell cycle to mitosis by dephosphorylating cyclin-dependent kinase 1. The tumor suppressor p53 down-regulates expression of cdc25C as part of G2/M checkpoint control. Transcription of cdc25C oscillates during the cell cycle with no expression in resting cells and maximum transcription in G2. We had identified earlier a new mechanism of cell cycle-dependent transcription that is regulated by a cell cycle-dependent element (CDE) in conjunction with a cell cycle genes homology region (CHR). The human cdc25C gene was the first example. CDE/CHR tandem elements have since been found in promoters of many cell cycle genes. Here we show that the mouse cdc25C gene is regulated by a CHR but does not hold a CDE. Therefore, it is the first identified gene with CHR-dependent transcriptional regulation during the cell cycle not relying on a CDE located upstream of it. The CHR leads to repression of cdc25C transcription early in the cell cycle and directs a release of this repression in G2. Furthermore, we find that this CHR can cooperate in cell cycle-dependent transcription with elements placed directly upstream of it binding E2F, Sp1 or Sp3 transcription factors.

Regulation and possible function of beta-catenin in human monocytes

In this study, we demonstrate that adherence factors, serum constituents, LPS, and zymosan are capable of inducing a cellular accumulation of beta-catenin in human monocytes. Whereas adherence-dependent accumulation of beta-catenin can be blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase, accumulation induced by the remaining stimuli cannot be prevented by inhibition of phosphatidylinositol 3-kinase, implying the involvement of beta-catenin in other not yet described signal transduction pathways. A role of beta-catenin in adherence-dependent processes by interacting with classical cadherins can be excluded as we could not detect cadherins in monocytes. To test whether it is possible that beta-catenin interacts with LEF/TCF (lymphoid enhancer factor/T cell factor) transcription factors, we studied the expression of this protein family. TCF-4 was identified as the LEF/TCF transcription factor present in human monocytes. However, neither cellular induction of beta-catenin nor cotransfection experiments with beta-catenin conducted in the monocytic cell line THP-1 resulted in the activation of a LEF/TCF-dependent promoter, suggesting the requirement of additional signals. Concurrent with this suggestion, we found that LPS and zymosan, two physiological inducers of beta-catenin, caused an increase in the expression of genes that are positively regulated by beta-catenin.

Transcriptional regulation of the mismatch repair gene hMLH1

We have characterized the promoter region of the human gene coding for the MLH1 mismatch repair protein. The total transcriptional activity of the hMLH1 promoter is driven by two positive cis-elements included between nucleotides -300 and -220. The upstream element is a canonical CCAAT box, and it is recognized by the heterotrimeric transcription factor NF-Y. On the other hand, the downstream element is recognized by a nuclear factor of about 120 kDa. Variations in hMLH1 intracellular levels may influence the surveillance of the genome integrity. The identification of the two elements may shad some light on the regulation of the transcriptional regulation of hMLH1 expression.

p53 and its homologues, p63 and p73, induce a replicative senescence through inactivation of NF-Y transcription factor

Recent studies have identified two p53 homologues, p63 and p73. They activate p53-responsive promoters and induce apoptosis when overexpressed in certain human tumors. Here, we report that p63, like p53 and p73, induces replicative senescence when expressed in a tetracycline-regulated manner in EJ cells lacking a functional p53. In addition to transcription activation of p53-responsive genes, we found that p63 and p73 repress transcription of the cdk1 and cyclin B genes, both of which are irreversibly repressed in senescent human fibroblast. In transient transfection assay, p63 and p73 repress the cdk1 promoter regardless of the presence of a dominant negative mutant form of p53. Furthermore, we found that DNA binding activity of NF-Y transcription factor, which is essential for transcription of the cdk1 and cyclin B genes and inactivated in senescent fibroblast, is significantly decreased by expression of either of p53, p63, or p73. Since NF-Y binds to many promoters besides the cdk1 and cyclin B promoters, inactivation of NF-Y by p53 family genes may be a general mechanism for transcription repression in replicative senescence.

Expression of the p53 homologues p63 and p73 in multiple simultaneous gastric cancer

The tumour-suppressor protein p53 has recently been shown to belong to a family that includes two structurally related proteins, p63 and p73. This study investigated the status of p53 and its two homologues in multiple simultaneous gastric carcinomas. Expression and mutation of p53, p73 and p63 including the two major isotypes TAp63 and black triangleNp63, were examined by direct DNA-sequencing, in situ hybridization, western blotting and immunohistochemistry in 68 gastric carcinomas of 32 patients. The results obtained were correlated with pathohistological stage (according to UICC(16)) and several other histopathological factors and finally with patient survival. p53 mutations were detected in 23/68 carcinomas (34%) from 18 patients with a discordant mutation pattern. Independently of p53 mutation status, p73 transcripts and protein expression were found in 33/68 carcinomas from 24 patients. p63 positivity was found in 21 patients; 25 out of 68 tumours expressed p63. The number of cells containing p63 and their distribution depend on the degree of tumour differentiation. High grade carcinomas of the diffuse type exhibited a significantly higher p63 expression. In intestinal metaplasia and atrophic gastritis, an increase of TAp63 and black triangleNp63 staining was also observed. Specific mutations of p73 or p63 causing amino acid substitutions were not identified. Neither p53, p73 nor p63 were related to prognosis. p73 and p63 have rarely been found to be mutated in gastric carcinomas, but both proteins were expressed in only a subset of tumours. The status of these p53 homologues was discordant in all patients with multiple simultaneous gastric carcinomas. The increased expression of p63 (TAp63 and black triangleNp63) in less well differentiated gastric carcinomas may indicate that p63 can act to promote neoplastic growth in the gastric epithelium.

NF-Y binding is required for transactivation of neuronal aromatic L-amino acid decarboxylase gene promoter by the POU-domain protein Brn-2

We have previously characterized binding sites for the NF-Y transcription factor (-71/-52) and Brn-2 POU-domain protein (-92/-71) in the neuronal promoter of the human aromatic L-amino acid decarboxylase gene [Mol. Brain Res. 56 (1998) 227]. We have now explored the functional role of these binding sites in transfected SK-N-BE neuroblastoma cells. Mutations of the NF-Y site that abolish binding depressed expression of a luciferase reporter gene up to 25-fold. The overexpression of a dominant negative mutant of NF-YA subunit depressed expression by 60%. Promoter activity was increased by the overexpression of Brn-2. Mutations or deletion of the binding site of Brn-2 did not suppress transcriptional activation by overexpressed Brn-2, while promoters defective in NF-Y binding were not transactivated by Brn-2. A GST-pulldown experiment showed that recombinant human Brn-2 protein weakly interacts with recombinant NF-Y outside of DNA. Cooperative binding of recombinant NF-Y and GST--Brn-2 proteins on the neuronal promoter was evidenced by an electrophoretic mobility shift assay. The POU-domain of Brn-2 was sufficient for such interaction. The results thus suggest that the activation of the neuronal promoter of the aromatic L-amino acid decarboxylase gene requires a direct interaction between the ubiquitous NF-Y factor and a cell-specific POU-domain protein. The NF-Y, but not the Brn-2 binding site, is essential for the recruitment of the NF-Y/Brn-2 complex on the promoter.

Mechanism for the transcriptional repression by c-Myc on PDGF (β)-receptor

c-Myc plays a key role in the cell cycle dependent control of the PDGF (β)-receptor mRNA. The mouse platelet-derived growth factor (PDGF) (β)-receptor promoter contains a CCAAT motif, and NF-Y plays an essential role in its transcription. Coexpression of c-Myc represses PDGF (β)-receptor luciferase reporter activity, and the CCAAT motif in the promoter is indispensable for this repression. Here we show that c-Myc binds NF-Y subunits, YB and YC, by immunoprecipitation from cotransfected COS-1 cells. The in vitro-translated c-Myc also binds the glutathione S-transferase (GST)-NF-YB fusion protein and GST-NF-YC, but not GST-NF-YA. The most C-terminal region of HAP domains of NF-YB and NF-YC, and the Myc homology boxes, but not the C-terminal bHLHZip domain, are indispensable for the coimmunoprecipitation, and also for the repression of PDGF (β)-receptor. c-Myc binds NF-Y complex without affecting the efficiency of NF-Y binding to DNA. However, the expression of Myc represses the transcriptional activation of NF-YC when fused to the GAL4 DNA binding domain. Furthermore, this repression was seen only when Myc homology boxes are present, and NF-YC contains the c-Myc binding region.

Cell cycle regulation of the murine cdc25B promoter: essential role for nuclear factor-Y and a proximal repressor element

Expression of the cdc25B gene is up-regulated late during cell cycle progression (S/G(2)). We have cloned the murine cdc25B promoter to identify elements involved in transcriptional regulation. A detailed structure-function analysis led to the identification of several elements that are located upstream of a canonical Inr motif at the site of transcription initiation and are involved in transcriptional activation and regulation. Activation of the promoter is largely mediated by NF-Y and Sp1/3 interacting with one and four proximal binding sites, respectively. In addition, NF-Y plays an essential role in cell cycle regulation in conjunction with a repressor element (cell cycle-regulated repressor) located approximately 30 nucleotides upstream of the putative Inr element and overlapping a consensus TATA motif. The cell cycle-regulated repressor is unrelated to the previously described cell cycle-regulated repressor elements. Taken together, our observations suggest that expression of the cdc25B gene is controlled through a novel mechanism of cell cycle-regulated transcription.

NF-Y mediates the transcriptional inhibition of the cyclin B1, cyclin B2, and cdc25C promoters upon induced G2 arrest

During normal cell cycles, the function of mitotic cyclin-cdk1 complexes, as well as of cdc25C phosphatase, is required for G2 phase progression. Accordingly, the G2 arrest induced by DNA damage is associated with a down-regulation of mitotic cyclins, cdk1, and cdc25C phosphatase expression. We found that the promoter activity of these genes is repressed in the G2 arrest induced by DNA damage. We asked whether the CCAAT-binding NF-Y modulates mitotic cyclins, cdk1, and cdc25C gene transcription during this type of G2 arrest. In our experimental conditions, the integrity of the CCAAT boxes of cyclin B1, cyclin B2, and cdc25C promoters, as well as the presence of a functional NF-Y complex, is strictly required for the transcriptional inhibition of these promoters. Furthermore, a dominant-negative p53 protein, impairing doxorubicin-induced G2 arrest, prevents transcriptional down-regulation of the mitotic cyclins, cdk1, and cdc25C genes. We conclude that, as already demonstrated for cdk1, NF-Y mediates the transcriptional inhibition of the mitotic cyclins and the cdc25C genes during p53-dependent G2 arrest induced by DNA damage. These data suggest a transcriptional regulatory role of NF-Y in the G2 checkpoint after DNA damage.

The tumour suppressor protein p53 can repress transcription of cyclin B

The tumour suppressor protein p53 has functions in controlling the G(1)/S and G(2)/M transitions. Central regulators for progression from G(2) to mitosis are B-type cyclins complexed with cdc2 kinase. In mammals two cyclin B proteins are found, cyclin B1 and B2. We show that upon treatment of HepG2 cells with 5-fluorouracil or methotrexate, p53 levels increase while concentrations of cyclin B2 mRNA, measured by RT-PCR with the LightCycler system, are reduced. In DLD-1 colorectal adenocarcinoma cells (DLD-1-tet-off-p53) cyclin B1 and B2 mRNA levels drop after expression of wild-type p53 but not after induction of a DNA binding-deficient mutant of p53. Analysis of the cyclin B2 promoter reveals specific repression of this gene by p53. Transfection of wild-type p53 into SaOS-2 cells shuts off transcription from a cyclin B2 promoter-luciferase construct whereas a p53 mutant protein does not. The cyclin B2 promoter does not contain a consensus p53 binding site. Most of the p53-dependent transcriptional responsiveness resides in its 226 bp core promoter. Taken together with earlier observations on p53-dependent transcription of cyclin B1, our results suggest that one way of regulating G(2) arrest may be a reduction in cyclin B levels through p53-dependent transcriptional repression.

A CDE/CHR tandem element regulates cell cycle-dependent repression of cyclin B2 transcription

Cyclin B is an important regulator of progression through the cell division cycle. The oscillating appearance of cyclin B1 and B2 proteins during the cell cycle is in part due to fluctuating mRNA levels. We had identified earlier a tandem promoter element named cell cycle-dependent element (CDE) and cell cycle genes homology region (CHR) which regulates cell cycle-dependent transcription of cdc25C, cyclin A and cdc2. Here we describe that cyclin B2 transcription is repressed through a novel CDE/CHR element in resting and G(1) cells. By relief of this repression in S and G(2) oscillating expression of cyclin B2 mRNA is achieved during the cell cycle.

Isolation and characterization of the human A-myb promoter: regulation by NF-Y and Sp1

The A-myb transcription factor shows a restricted tissue distribution and is cell cycle regulated. Furthermore its deregulation has profound effects on the growth and/or differentiation of the cells in which it is normally expressed. We have therefore characterized its promoter. A 12 kb genomic clone was isolated that comprises the first exon, part of the first intron as well as upstream regulatory sequences. Multiple transcription start sites have been identified which operate in both B lymphocytes and epithelial cells and the upsteam region was shown to have promoter, activity. The boundaries of the minimal promoter region (-183-14), of a positive upstream (-538-183) and a negative downstream regulatory region (NRE) (+83+374) have been defined. The NRE is promoter- and orientation-independent but position specific. The A-myb minimal promoter is GC-rich, does not contain any TATA box but has a functional CCAAT box. The CCAAT box and minimal promoter is highly conserved in the corresponding murine sequence. The CCAAT box efficiently binds the NF-Y complex and its mutation decreases basal promoter activity by 50%. Two Sp1 binding sites are present upstream from the CCAAT box which can bind Spl and contribute to A-myb promoter activity by 70 and 30%, respectively. The two Sp1 sites and CCAAT box together contribute to over 80% of A-myb basal promoter activity and are therefore the major regulatory elements. Finally, we show that the promoter is cell cycle regulated and that the SP1 and CCAAT elements are required for S phase induction.

Interference of transcriptional activation by the antineoplastic drug ecteinascidin-743

Ecteinascidin-743 (ET-743) is a tetrahydroisoquinoline alkaloid isolated from the tunicate Ecteinascidia turbinata currently under phase II clinical trials for its potent anticancer activity. ET-743 binds DNA in the minor groove and forms covalent adducts with some sequence specificity. It selectively inhibits in vitro binding of the CCAAT box factor NF-Y. In this study, we assayed ET-743 function in vivo on the HSP70 promoter. On heat induction, the drug blocks transcription rapidly at pharmacological concentrations and in a CCAAT-dependent manner, whereas the activity of the CCAAT-less simian virus 40 promoter is not affected. The effect is exerted at the mRNA level. The distamycin-like alkylating tallimustine is inactive in these assays. Binding of NF-Y and of the heat-shock factor is normal in ET-743-treated cells. Run-on analysis of several endogenous genes further proves that the drug has rapid, profound, and selective negative effects on transcription. Thus, this marine-derived compound is a promoter-specific, transcription-interfering agent.

Nuclear factor Y (NF-Y) and cellular senescence

NF-Y, also termed CBF, is a major CCAAT-binding transcription factor that specifically recognizes the consensus sequence 5'-CTGATTGGYYRR-3 or 5'-YYRRCCAATCAG-3' (Y = pyrimidines and R = purines) present in the promoter region of many constitutive, inducible, and cell-cycle-dependent eukaryotic genes. The functional NF-Y is a heterotrimeric protein, consisting of three different subunits, A, B, and C. Each of the three subunits contains two or three distinct protein-interacting domains for trimer formation and for interacting with other nuclear proteins. Only the trimeric NF-Y, and not the individual subunit, possess DNA-binding activity. The transcriptional activity of NF-Y can be regulated by differential expression, alternative splicing, protein-protein interactions, and cellular redox potential. The regulation of thymidine kinase (TK) and dihydrofolate reductase (DHFR) genes in human diploid fibroblasts serves as an example of how NF-Y may have a role in replicative senescence by regulating age-dependent G1/S genes.

The molecular biology of the CCAAT-binding factor NF-Y

Protein coding genes are transcribed by Polymerase II, under the control of short discrete DNA elements in promoters and enhancers, recognized with high efficiency and specificity by trans-acting factors and by general transcription proteins (Tjian and Maniatis, 1994). The former regulate specific genes or set of genes, usually in a tissue-, developmental-, cell-cycle or stimuli-dependent way; the latter are involved in the activation of all promoters, as a whole multi-subunit holoenzyme (Parvis and Young, 1998). A limited set of elements, such as the GC and CCAAT-boxes, are present in a very high number of promoters. The whole process is further complicated by the need to operate in the context of higher order chromatin structures (Workman and Kingston, 1998). This review focuses on the CCAAT sequence and on the NF-Y protein, also known as CBF, which binds to it.

Expression of p73 and its relation to histopathology and prognosis in hepatocellular carcinoma

BACKGROUND

The protein p73, the first identified homologue of the tumor suppressor gene p53 (also known as TP53), has been shown to induce apoptosis (programmed cell death), but its function in tumor development has not been established. This study was undertaken to investigate the expression of p73 in liver tissue of patients with hepatocellular carcinoma (HCC) and to determine whether this expression has any impact on prognosis.

METHODS

In situ hybridization and immunohistochemistry for the detection of p73 RNA transcripts and protein, respectively, were performed in tissues from 193 patients with curatively (R0-) resected HCC. Patients receiving liver transplantation were excluded. The results obtained were analyzed with respect to their association with pathohistologic stage, Edmondson grade, p53 expression status and several histopathologic factors of possible prognostic value, and, finally, with patient survival.

RESULTS

RNA transcripts encoding p73 were detected by in situ hybridization in tumor cells but not in stromal, endothelial, or inflammatory cells or in cholangiocytes. Transcripts were also found occasionally in non-neoplastic hepatocytes. By immunohistochemistry, we detected p73 protein in 61 (32%) of the 193 carcinomas examined. Positive immunohistochemical staining was confined to the cell nucleus. Univariate survival analysis showed that p73 expression status was statistically significantly related to prognosis (two-sided P<.0001). Patients with p73-positive tumors had a poorer prognosis than those with p73-negative carcinomas. Multivariate Cox survival analysis identified the age of the patient, p73 expression status, co-existing cirrhosis, and Edmondson grade as independent prognostic factors.

CONCLUSION

The protein p73 is overexpressed by a subset of HCCs and could serve as a useful indicator of prognosis in patients with this disease.