The G1/S boundary-specific enhancer of the rat cdc2 promoter

Overexpression of regucalcin suppresses cell proliferation in cloned rat hepatoma H4-II-E cells: Involvement of intracellular signaling factors and cell cycle-related genes

The role of regucalcin, which is a regulatory protein in intracellular signaling pathway, in the regulation of cell proliferation was investigated by using the cloned rat hepatoma H4-II-E cells overexpressing regucalcin. The hepatoma cells (wild type) and stable regucalcin/pCXN2 transfectants were cultured for 72 h in a medium containing 10% fetal bovine serum (FBS) to obtain subconfluent monolayters. The proliferation of cells was significantly suppressed in transfectants cultured for 24-72 h. The proliferation of wild-type cells was significantly inhibited when the cells were cultured for 72 h in a medium containing an inhibitor of transcriptional activity or protein synthesis. Such an effect was not seen in transfectants. The presence of various inhibitors of protein kinase including PD 98059 (10(-7) or 10(-6) M), dibucaine (10(-6) M), wortmannin (10(-8) or 10(-6) M), or genistein (10(-5) M) caused a significant inhibition of the proliferation of wild-type cells. These inhibitory effects were not seen in transfectants. Staurosporine (10(-8) - 10(-7) M) significantly inhibited the proliferation of wild-type cells and transfectants. Also, the effect of vanadate (10(-5) M), an inhibitor of protein tyrosine phosphatase, or Bay K 8644 (10(-6) M), an agonist of calcium entry into cells, in inhibiting the proliferation of wild-type cells was not observed in transfectants. Moreover, the proliferation of wild-type cells was significantly inhibited in the presence of roscovitine (10(-7) or 10(-6) M) or sulforaphane (10(-7) M), which induces cell-cycle arrest. Such effect was not seen in transfectants. The inhibitory effect of sodium butyrate (8.3 x 10(-4) M) on proliferation of wild-type cells was also induced in transfectants. Gene expression in hepatoma cells cultured for 72 h with 10% FBS was determined by using reverse transcription-polymerase chain reaction (RT-PCR). The expression of p21 mRNA was significantly enhanced in transfectants, while cdc2a and chk2 mRNA expression were not significantly changed. Insulin-like growth factor-I (IGF-I) mRNA expression was significantly suppressed in transfectants. This study demonstrates that overexpression of regucalcin has a suppressive effect on cell proliferation that is partly mediated through various intracellular signaling-related factors, and that the effect may be partly involved in the change in p21 or IGF-I mRNA expression. The finding further supports that regucalcin plays an important role as a suppressor in the enhancement of cell proliferation.

Keratinocyte G2/M growth arrest by 1,25-dihydroxyvitamin D3 is caused by Cdc2 phosphorylation through Wee1 and Myt1 regulation

1,25-dihydroxyvitamin D3 (1,25[OH]2VD3) has an antiproliferative effect on keratinocyte growth, and its derivatives are used for the treatment of psoriasis. It was reported previously that 1,25[OH]2VD3 induced cell cycle arrest not only at the G0/G1 phase but also at the G2/M phase. However, the mechanism of 1,25[OH]2VD3-induced G2/M phase arrest in keratinocytes has not been fully understood. The addition of 10(-8) to 10(-6) M 1,25[OH]2VD3 to cultured normal human keratinocytes enhanced the expression of Myt1 mRNA preceding Wee1 mRNA; 10(-6) M 1,25[OH]2VD3 unregulated Myt1 mRNA from 6 h to 24 h and Wee1 mRNA from 12 to 48 h. Interestingly, the levels of phosphorylated Cdc2 were increased between 6 h and 48 h after 1,25[OH]2VD3 treatment, although the expression levels of Cdc2 mRNA and its protein production were reduced. 1,25[OH]2VD3 also decreased the expression of cyclin B1, which forms a complex with Cdc2. These data indicated that the increase of Myt1 and Wee1 induced the phosphorylation of Cdc2 leading to G2/M arrest. In conclusion, the induction of Cdc2 phosphorylation due to the increase of Wee1 and Myt1 as well as the reduction of Cdc2 and cyclin B1 are involved in 1,25[OH]2VD3-induced G2/M arrest of keratinocytes.

Histone deacetylation of RB-responsive promoters: requisite for specific gene repression but dispensable for cell cycle inhibition

The retinoblastoma tumor suppressor protein (RB) is targeted for inactivation in the majority of human tumors, underscoring its critical role in attenuating cellular proliferation. RB inhibits proliferation by repressing the transcription of genes that are essential for cell cycle progression. To repress transcription, RB assembles multiprotein complexes containing chromatin-modifying enzymes, including histone deacetylases (HDACs). However, the extent to which HDACs participate in transcriptional repression and are required for RB-mediated repression has not been established. Here, we investigated the role of HDACs in RB-dependent cell cycle inhibition and transcriptional repression. We find that active RB mediates histone deacetylation on cyclin A, Cdc2, topoisomerase IIalpha, and thymidylate synthase promoters. We also demonstrate that this deacetylation is HDAC dependent, since the HDAC inhibitor trichostatin A (TSA) prevented histone deacetylation at each promoter. However, TSA treatment blocked RB repression of only a specific subset of genes, thereby demonstrating that the requirement of HDACs for RB-mediated transcriptional repression is promoter specific. The HDAC-independent repression was not associated with DNA methylation or gene silencing but was readily reversible. We show that this form of repression resulted in altered chromatin structure and was dependent on SWI/SNF chromatin remodeling activity. Importantly, we find that cell cycle inhibitory action of RB is not intrinsically dependent on the ability to recruit HDAC activity. Thus, while HDACs do play a major role in RB-mediated repression, they are dispensable for the repression of critical targets leading to cell cycle arrest.

Expression and promoter analysis of mouse mastrin gene

Human astrin is a newly identified microtubule-associated protein, which is highly expressed in the testis. Silencing of astrin has resulted in growth arrest and apoptotic cell death. In this study, we describe the cloning and genomic structure of mastrin, the mouse counterpart to astrin. The overall mouse mastrin amino-acid sequence is 66% identical to human astrin. Mastrin protein was demonstrated to localize to mitotic spindles during mitosis. Genomic clones containing mastrin gene were isolated; the gene was found to have 24 exons spanning 24kb of genomic DNA. Deletion analysis of 5(')-flanking sequences demonstrated that the first 120bp proximal to the TATA-less promoter region is necessary for minimal transcription of the mouse mastrin gene.

The E2F-Cdc2 cell-cycle pathway specifically mediates activity deprivation-induced apoptosis of postmitotic neurons

Neuronal apoptosis plays a critical role in the normal development of the mammalian brain and is thought to contribute to the pathogenesis of several neurologic disorders. However, the intracellular mechanisms underlying apoptosis of neurons remain incompletely understood. In the present study, we characterized a cell-cycle-based mechanism by which neuronal activity deprivation induces apoptosis of postmitotic neurons. Activity deprivation, but not growth factor withdrawal, was found to induce Cdc2 expression and consequent Cdc2-mediated apoptosis in granule neurons of the developing rat cerebellum. We found that activity deprivation induces cdc2 transcription in neurons via an E2F-binding element (EBE) within the cdc2 promoter. The transcription factor E2F1 that is expressed in granule neurons was found in DNA binding assays to bind to the EBE of the cdc2 gene. In chromatin immunoprecipitation analysis, endogenous E2F1 forms a complex with the promoter of the endogenous cdc2 gene in granule neurons, indicating that endogenous E2F1 is poised to activate transcription of the endogenous cdc2 gene in neurons. Consistent with this conclusion, a dominant interfering form of E2F, when expressed in granule neurons, blocked activity deprivation-induced cdc2 transcription. In other experiments, we found that the expression of E2F1 in granule neurons induces Cdc2 expression and promotes neuronal apoptosis via the activation of Cdc2. Remarkably, in contrast to inducing the E2F-mediated expression and activation of Cdc2 in granule neurons, activity deprivation fails to stimulate the expression of E2F-target genes that trigger DNA synthesis and replication. Together, our findings define a novel apoptotic mechanism whereby E2F selectively couples an activity deprivation-induced signal to cdc2 transcription in the absence of stimulating DNA synthesis and thus culminating in Cdc2-mediated apoptosis of postmitotic neurons.

The E2F-Cdc2 cell-cycle pathway specifically mediates activity deprivation-induced apoptosis of postmitotic neurons

Neuronal apoptosis plays a critical role in the normal development of the mammalian brain and is thought to contribute to the pathogenesis of several neurologic disorders. However, the intracellular mechanisms underlying apoptosis of neurons remain incompletely understood. In the present study, we characterized a cell-cycle-based mechanism by which neuronal activity deprivation induces apoptosis of postmitotic neurons. Activity deprivation, but not growth factor withdrawal, was found to induce Cdc2 expression and consequent Cdc2-mediated apoptosis in granule neurons of the developing rat cerebellum. We found that activity deprivation induces cdc2 transcription in neurons via an E2F-binding element (EBE) within the cdc2 promoter. The transcription factor E2F1 that is expressed in granule neurons was found in DNA binding assays to bind to the EBE of the cdc2 gene. In chromatin immunoprecipitation analysis, endogenous E2F1 forms a complex with the promoter of the endogenous cdc2 gene in granule neurons, indicating that endogenous E2F1 is poised to activate transcription of the endogenous cdc2 gene in neurons. Consistent with this conclusion, a dominant interfering form of E2F, when expressed in granule neurons, blocked activity deprivation-induced cdc2 transcription. In other experiments, we found that the expression of E2F1 in granule neurons induces Cdc2 expression and promotes neuronal apoptosis via the activation of Cdc2. Remarkably, in contrast to inducing the E2F-mediated expression and activation of Cdc2 in granule neurons, activity deprivation fails to stimulate the expression of E2F-target genes that trigger DNA synthesis and replication. Together, our findings define a novel apoptotic mechanism whereby E2F selectively couples an activity deprivation-induced signal to cdc2 transcription in the absence of stimulating DNA synthesis and thus culminating in Cdc2-mediated apoptosis of postmitotic neurons.

Hyperglycemia enhances VSMC proliferation with NF-kappaB activation by angiotensin II and E2F-1 augmentation by growth factors

To clarify the mechanisms of hyperglycemia-induced proliferation of vascular smooth muscle cells (VSMC), we examined the effects of high glucose (HG) on nuclear factor (NF)-kappaB and E2F-1. Angiotensin II (Ang II) significantly enhanced DNA binding activity of NF-kappaB under HG (25.6 mM) conditions with an increase in p65 subunit of NF-kappaB, and did it slightly under normal glucose (NG; 5.6 mM) conditions. Ang II failed to induce E2F-1 expression, or its binding to the cdc2 promoter, even under HG conditions. HG greatly augmented the cdc2 inducibility of fetal calf serum (FCS), through the increase in E2F-1 activity. These data indicate that hyperglycemia contributes to abnormal proliferation of VSMC by two mechanisms; the induction of NF-kappaB activation by Ang II, which facilitates transcription of certain growth factors, and the augmentation of E2F-1 in response to growth factors.

The Rb pathway in neurogenesis

Cell division during embryogenesis plays a crucial role in the formation of the nervous system. During this developmental process, proliferating neural precursor cells commit to a neuronal fate and, as a consequence, undergo terminal mitosis and adopt a neuronal phenotype. A key cell cycle regulator, the tumor suppressor protein, retinoblastoma (Rb), is involved in both terminal mitosis and neuronal differentiation. Neural development is a complex process involving cell proliferation, cell fate determination and differentiation, as well as programmed cell death. In this review, we will examine each of these processes in turn, focussing on the role of the Rb family proteins to examine their many influences on these events.

The transcription factor E2F1 promotes dopamine-evoked neuronal apoptosis by a mechanism independent of transcriptional activation

The E2F1 transcription factor plays an important role in promoting neuronal apoptosis; however, it is not clear how E2F1 does this. Here we show that E2F1 is involved in dopamine (DA)-evoked apoptosis in cerebellar granule neurons (CGNs). E2F1 -/- CGNs and CGNs expressing an antisense E2F1 cDNA were significantly protected from DA-toxicity relative to controls. The neuronal protection was accompanied by significantly reduced caspase 3 activity. E2F1-mediated neuronal apoptosis did not require activation of gene transcription because: (1) ectopic expression of E2F1 or its mutants lacking the transactivation domain induced neuronal apoptosis, whereas an E2F1 mutant lacking the DNA-binding domain did not; (2) under all of these conditions, known E2F1 target genes including cyclin A, cdc2 and p19(ARF) were not induced; and (3) DA-evoked neuronal apoptosis was associated with up-regulated E2F1, but not transcription of its target genes. Finally, E2F1-mediated neuronal apoptosis was associated with reduced nuclear factor (NF)-kappaB DNA-binding activity. Taken together, these data suggest that E2F1 promotes DA-evoked caspase 3-dependent neuronal apoptosis by a mechanism independent of gene transactivation, and this may possibly occur through inhibition of anti-apoptotic genes including NF-kappaB.

Human Cdc5, a regulator of mitotic entry, can act as a site-specific DNA binding protein

G(2)/M progression requires coordinated expression of many gene products, but little is known about the transcriptional regulators involved. We recently identified human Cdc5, a positive regulator of G(2)/M in mammalian cells. We also demonstrated the presence of a latent activation domain in its carboxyl terminus, suggesting that human Cdc5 regulates G(2)/M through transcriptional activation. Despite the presence of a DNA binding domain, studies by others have failed to identify a preferential binding site for Cdc5 family members. In addition, Cdc5 recently has been associated with the splicesome in several organisms, suggesting that it may not act through DNA binding. We now report the identification of a 12 bp sequence to which human Cdc5 binds specifically and with high affinity through its amino terminus. We show that this DNA-protein interaction is capable of activating transcription. We also used a selection system in yeast to identify human genomic fragments that interact with human Cdc5. Several of these contained sequences similar to the binding site. We demonstrate that these bind human Cdc5 with similar specificity and affinity. These experiments provide the first evidence that Cdc5 family members can act as site-specific DNA binding proteins, and that human Cdc5 may interact with specific, low abundance sequences in the human genome. This raises the possibility that Cdc5 proteins may participate in more than one process necessary for regulated cell division.

Subcellular compartmentalization of E2F family members is required for maintenance of the postmitotic state in terminally differentiated muscle

Maintenance of cells in a quiescent state after terminal differentiation occurs through a number of mechanisms that regulate the activity of the E2F family of transcription factors. We report here that changes in the subcellular compartmentalization of the E2F family proteins are required to prevent nuclei in terminally differentiated skeletal muscle from reentering S phase. In terminally differentiated L6 myotubes, E2F-1, E2F-3, and E2F-5 were primarily cytoplasmic, E2F-2 was nuclear, whereas E2F-4 became partitioned between both compartments. In these same cells, pRB family members, pRB, p107, and p130 were also nuclear. This compartmentalization of the E2F-1 and E2F-4 in differentiated muscle cells grown in vitro reflected their observed subcellular location in situ. We determined further that exogenous E2F-1 or E2F-4 expressed in myotubes at levels fourfold greater than endogenous proteins compartmentalized identically to their endogenous counterparts. Only when overexpressed at higher levels was inappropriate subcellular location for these proteins observed. At these levels, induction of the E2F-regulated genes, cyclins A and E, and suppression of factors associated with myogenesis, myogenin, and p21(Cip1) was observed. Only at these levels of E2F expression did nuclei in these terminally differentiated cells enter S phase. These data demonstrate that regulation of the subcellular compartmentalization of E2F-family members is required to maintain nuclei in a quiescent state in terminally differentiated cells.

Failure of cdc2 promoter activation and G(2)/M transition by ANG II and AVP in vascular smooth muscle cells

The physiological role of the vasoconstrictive hormones arginine vasopressin (AVP) and angiotensin II (ANG II) in the development of vascular hyperplasia is still unclear. We examined the effects of these hormones on cell cycle regulation of cultured rat vascular smooth muscle cells (VSMC). AVP and ANG II were able to induce G(1)/S transition and DNA synthesis in serum-starved quiescent VSMC but failed to promote further progression into G(2)/M phases. AVP and ANG II enhanced the expression and activity of cdk2, cyclin E, and proliferating cell nuclear antigen but did not induce expression of cdc2/cyclin B complex, a critical regulator of G(2)/M transition. The failure of cdc2 mRNA induction was found to be caused by a defect in cdc2 promoter activation. Binding of free E2F-1 to the cdc2 promoter did not occur in hormone-treated VSMC, which may account for the defective induction of cdc2. The absence of cdc2 promoter activation and G(2)/M transition may be important for the prevention of hyperplasia under physiological conditions but underlies the hypertrophy of VSMC.

Enhancer and silencer binding proteins involved in the rat cdc2 promoter activation at the G1/S boundary

BACKGROUND

Expression of the rat cdc2 gene during G1-S phase progression is negatively and positively regulated by the silencer and enhancer elements located upstream of the basal promoter. The silencer and enhancer sequences resemble each other, but the silencer contains extra internal AG residues.

RESULTS

The cDNA clones encoding the enhancer binding proteins cdc2E1 and cdc2E2 were isolated by South-Western blotting. cdc2E1 and cdc2E2 comprise 436 and 256 amino acids and have two RNA binding domains which contain an RNP1 octamer and an RNP 2 hexamer. Both cdc2E1 and cdc2E2 bind to the double-stranded and single-stranded silencer and enhancer sequences, but their binding affinity to the enhancer was stronger than that to the silencer. Transfection of quiescent 3Y1 cells with the cdc2 promoter-luciferase constructs, followed by serum stimulation, showed that the promoter activation at the G1-S phase boundary was reduced greatly by base substitutions within the enhancer, but not within the silencer. Gel shift assays with oligonucleotides containing both the silencer and enhancer showed that formation of the large complex was greatly reduced if base-substitutions were introduced into the enhancer, but not within the silencer. The complex was supershifted completely by anti-cdc2E1 antibody and partially by anti-cdc2E2 antibody.

CONCLUSION

These results suggest that cdc2E1 and cdc2E2 preferentially form the multimeric complex at the enhancer site after the late G1 phase for activation of the cdc2 promoter.

Regulation of cdc2 gene expression by the upstream stimulatory factors (USFs)

cdc2 gene expression is under the control of multiple factors. Although E2F/DP proteins have been reported to play a central role, they cannot account for all aspects of the fine modulation of cdc2 gene expression during cell cycle and embryonic development. To characterize the transcription factors that control cdc2 gene expression during nerve cell differentiation in avians, we have previously cloned the quail cdc2 gene promoter region. We had identified an octamer (CAGGTGGC) containing an E-box, which has important activity in regulating cdc2 transcription. Using in vivo genomic footprinting experiments, we show here that this motif, currently named IG, is the target of binding proteins at different stages of neuroretina development, confirming its importance as a regulatory response element for cdc2 gene expression. A subset of Helix-Loop-Helix family of transcription factors, known as Upstream Stimulatory Factors (USFs) specifically bind to this sequence as dimers. Moreover, our results indicate that USFs transactivate the promoter of cdc2 via the IG motif. These data may help to better understand the mechanisms that control cell division in differentiating nerve cells.

Ribonucleotide reductase R2 protein is phosphorylated at serine-20 by P34cdc2 kinase

Ribonucleotide reductase is a rate-limiting enzyme in DNA synthesis and is composed of two different proteins, R1 and R2. The R2 protein appears to be rate-limiting for enzyme activity in proliferating cells, and it is phosphorylated by p34cdc2 and CDK2, mediators of cell cycle transition events. A sequence in the R2 protein at serine-20 matches a consensus sequence for p34cdc2 and CDK2 kinases. We tested the hypothesis that the serine-20 residue was the major p34cdc2 kinase site of phosphorylation. Three peptides were synthesized (from Asp-13 to Ala-28) that contained either the wild type amino acid sequence (Asp-Gln-Gln-Gln-Leu-Gln-Leu-Ser-Pro-Leu-Lys-Arg-Leu-Thr-Leu-Ala, serine peptide) or a mutation, in which the serine residue was replaced with an alanine residue (alanine peptide) or a threonine residue (threonine peptide). Only the serine peptide and threonine peptide were phosphorylated by p34cdc2 kinase. In two-dimensional phosphopeptide mapping experiments of serine peptide and Asp-N endoproteinase digested R2 protein, peptide co-migration patterns suggested that the synthetic phosphopeptide containing serine-20 was identical to the major Asp-N digested R2 phosphopeptide. To further test the hypothesis that serine-20 is the primary phosphorylated residue on R2 protein, three recombinant R2 proteins (R2-Thr, R2-Asp and R2-Ala) were generated by site-directed mutagenesis, in which the serine-20 residue was replaced with threonine, aspartic acid or alanine residues. Wild type R2 and threonine-substituted R2 proteins (R2-Thr) were phosphorylated by p34cdc2 kinase, whereas under the same experimental conditions, R2-Asp and R2-Ala phosphorylation was not detected. Furthermore, the phosphorylated amino acid residue in the R2-Thr protein was determined to be phosphothreonine. Therefore, by replacing a serine-20 residue with a threonine, the phosphorylated amino acid in R2 protein was changed to a phosphothreonine. In total, these results firmly establish that a major p34cdc2 phosphorylation site on the ribonucleotide reductase R2 protein occurs near the N-terminal end at serine-20, which is found within the sequence Ser-Pro-Leu-Lys-Arg-Leu. Comparison of ribonucleotide reductase activities between wild type and mutated forms of the R2 proteins suggested that mutation at serine-20 did not significantly affect enzyme activity.

Characterization of the enhancer-like okadaic acid response element region of the cyclin-dependent kinase 1 (p34cdc2) promoter

Expression of the cdk1 (p34cdc2) gene is enhanced 5-10 fold as cells re-enter the cell cycle from quiescence in response to serum-refeeding or following exposure to the protein phosphatase 1/2A inhibitor okadaic acid. Transient transfection analysis of nested deletions of the human cdk1 promoter identified regions that confer sensitivity to okadaic acid on a CAT-reporter gene. Putative okadaic acid response elements (OARE) were located between nt -942 to -763 (Site I) and nt -416 to -186 (Site II) before transcription start. The Site I element has enhancer-like characteristics as activity is independent of sequence orientation. Mobility shift analysis of Site I revealed the presence of 2 high molecular weight complexes, one of which was enhanced in the presence of okadaic acid-treated cell extracts. Site I contained several sequence motifs with conserved homology to heat shock response element core sequences and homeobox protein binding sites. Site II contained a myb-binding site, a G1/S phase enhancer, and 2 retinoblastoma response elements flanking an E2F binding site. Enhancement of cdk1 expression appears dependent on 2 nonhomologous okadaic acid-sensitive promoter regions.

Induction of Sp1 in differentiating human embryonal carcinoma cells triggers transcription of the fibronectin gene

Cells of the human embryonal carcinoma line NEC14 proliferate as densely packed clusters consisting of small, polygonal stem cells and do not express a detectable level of fibronectin (FN). Upon induction of differentiation by treatment with N,N'-hexamethylene bisacetamide (HMBA), the level of FN mRNA increased steeply within 24 h and FN began to be accumulated, along with the organization of actin filaments in the cells. The FN promoter elements required for the activation were analyzed in reference to a cluster of GC boxes by using the chloramphenicol acetyltransferase (CAT) gene fused to 5' sequential-deletion derivatives of the promoter and promoters carrying base substitutions in the GC boxes. Among four GC boxes, GC boxes 2 and 3 had the greatest effect on promoter activation, and base substitutions in these GC boxes resulted in 80% reduction in promoter activity. The pattern of DNA-protein complex formation with these GC boxes changed drastically after induction of differentiation. The extract prepared from undifferentiated NEC14 cells formed fast-migrating complexes (UnD complexes), while the extract prepared from NEC14 cells treated with HMBA for 24 h formed slow-migrating complexes containing Sp1. Both complexes were formed predominantly with GC box 2. Base substitutions within the GC boxes completely abolished the formation of both UnD and Sp1 complexes. Consistent with these changes, the Sp1 level increased steeply within 24 h. Induction of Sp1 expression in NEC14 cells effectively stimulated the promoter activity of the transfected FN promoter-CAT constructs. These results indicate that activation of the FN promoter in differentiating NEC14 cells occurs by the steep induction of Sp1, which prevents an undifferentiated cell factor from binding to the Sp1 sites.

A survey of 178 NF-Y binding CCAAT boxes

The CCAAT box is one of the most common elements in eukaryotic promoters, found in the forward or reverse orientation. Among the various DNA binding proteins that interact with this sequence, only NF-Y (CBF, HAP2/3/4/5) has been shown to absolutely require all 5 nt. Analysis of a database with 178 bona fide NF-Y binding sites in 96 unrelated promoters confirms this need and points to specific additional flanking nucleotides (C, Pu, Pu on the 5'-side and C/G, A/G, G,A/C, G on the 3'-side) required for efficient binding. The frequency of CCAAT boxes appears to be relatively higher in TATA-less promoters, particularly in the reverse ATTGG orientation. In TATA-containing promoters the CCAAT box is preferentially located in the -80/-100 region (mean position -89) and is not found nearer to the Start site than -50. In TATA-less promoters it is usually closer to the +1 signal (at -66 on average) and is sometimes present in proximity to the Cap site. The consensus and location of NF-Y binding sites parallel almost perfectly a previous general statistical study on CCAAT boxes in 502 unrelated promoters. This is an indication that NF-Y is the major, if not the sole, CCAAT box recognizing protein and that it might serve different roles in TATA-containing and TATA-less promoters.

Activation of the rat cyclin A promoter by ATF2 and Jun family members and its suppression by ATF4

Cyclin A plays an essential role in the G1 to S phase transition in the cell cycle. The expression of cyclin A is restrained during G0 and G1, but steeply induced at the G1/S boundary. Analysis of the rat cyclin A promoter elements with the 5' sequential deletion derivatives of the promoter fused to the luciferase cDNA indicated that the ATF/CRE motif primarily determines the inducibility at G1/S. Gel shift analysis of the complex formed at the ATF/CRE site indicated that the complex was not formed with the G0/G1 cell extract, but maximally formed with the late-G1 cell extract. The complex was supershifted by anti-JunD antibody, and Western blot analysis of the immune complexes prepared with anti-JunD antibody revealed the presence of ATF2, suggesting heterodimerization of JunD with ATF2. The cyclin A promoter in a reporter plasmid was activated by nearly 10-fold in quiescent rat 3Y1 cells by cotransfection with the expression of plasmids encoding ATF2 and Jun family members. In contrast, cotransfection with the ATF4 expression plasmid suppressed the promoter activation mediated by ATF2 and Jun family members. The expression of Jun family members during G1 to S progression was induced biphasically in early and late G1 and the level of JunD increased markedly at the G1/S, while that of ATF family members was gradually increased along with the G1 to S progression. These results indicate that the cyclin A promoter activity is regulated, at least in part, by relative amounts of the ATF and Jun family members.

A mammalian homolog of fission yeast Cdc5 regulates G2 progression and mitotic entry

Progression through G2/M of the mammalian cell division cycle requires the coordinated expression of many gene products, but little is known of the transcriptional regulators involved. Schizosaccharomyces pombe Cdc5 is a putative transcription factor implicated in G2/M transit. We recently identified a cDNA encoding a putative human transcription factor, now designated human Cdc5 (hCdc5), with homology to S. pombe Cdc5. Widespread expression of hCdc5 in human tissues and homology with expressed sequences in other eukaryotes suggested an evolutionarily conserved general function. Nuclear import of hCdc5 upon serum stimulation of mammalian cells suggested a possible role in cell proliferation. We now report that overexpression of hCdc5 in mammalian cells shortened G2 and reduced cell size. A dominant negative mutant of hCdc5 lacking the carboxyl-terminal activation domain slowed G2 progression and delayed entry into mitosis. Thus, hCdc5 is the first transcriptional regulator shown to affect G2 progression and mitotic entry in mammalian cells.

Transcriptional activation of the p34cdc2 gene by cdc2 promoter binding factor/nuclear factor-Y in fetal rat ventricular myocytes

To determine how myocardial terminal differentiation is regulated by cell cycle control genes, we studied cdc2 expression in rat cardiac muscle and found that cdc2 mRNA and protein levels were reduced in neonatal compared with fetal ventricles and became undetectable in juvenile and adult ventricles. To further determine whether cdc2 downregulation is attributed to a decrease in transcription, transient expression assay was performed using the progressively truncated 6.2-, 1.8-, 1.1-, 0.7-, and 0.1-kb human cdc2 5' flanking regions. All five fragments activated reporter expression in fetal myocytes and were significantly less active in neonatal myocytes. The 0.1-kb fragment showed 65% of the activity of the 6.2-kb fragment. A protein binding site that contains an inverted CCAAT box was identified within the 0.1-kb fragment by DNase I footprint assay and named the cdc2 promoter binding factor (CPBF) site. Point mutations within the CPBF site that abolish CPBF binding significantly decreased both 0.1- and 6.2-kb promoter activities. Competition and antibody supershift assays suggested that CPBF was identical or related to the transcription factor, nuclear factor Y (NF-Y). The 0.1-kb promoter activity was suppressed by a dominant-negative NF-Y mutant in fetal myocytes. Taken together, our results demonstrate that cardiac cdc2 expression is downregulated after birth and turned off when the juvenile stage is attained. A 0.1-kb promoter fragment of cdc2 contains major information for both cdc2 transcriptional activation and suppression in fetal and neonatal myocytes, respectively. NF-Y or its related factor plays a critical role in activating the 0.1-kb cdc2 promoter.

A measure of the mitotic index: studies of the abundance and half-life of p34cdc2 in cultured cells and normal and neoplastic tissues

BACKGROUND

The cdc2 gene encodes a protein kinase, p34cdc2, that is essential for mitosis, and is present at high levels in dividing cells. Classical studies of the levels of this protein in dividing and resting cells used antibodies that cross-react with other members of the CDK family, in particular with CDK2. We have therefore re-examined the abundance of p34cdc2 in a variety of tissues and cell lines, using a highly specific, epitope-mapped monoclonal antibody that does not react with CDK2.

RESULTS

We observed high levels of p34cdc2 in proliferating cells, especially those in neoplastic tissues. Cells that have withdrawn from the cell cycle have low or undetectable levels. At the end of mitosis, the level of p34cdc2 declines, with simple first-order kinetics, with a half-life which is never less than 6h and is more typically about 18h. The persistence of p34cdc2 after the last cell division is comparable to that of PCNA, a commonly used marker of proliferation.

CONCLUSIONS

The immunochemical detection of p34cdc2 provides an accurate, reliable and meaningful measure of the proliferative activity of cells in tissues. We suggest that p34cdc2 should be considered as the most authentic molecular marker of the mitotic index.

Transcriptional activation of the cdc2 gene is associated with Fas-induced apoptosis of human hematopoietic cells

Apoptosis has recently been hypothesized to be the result of aberrant cell cycle control. In this study, we have investigated the role of cell cycle-regulatory elements in Fas-induced apoptosis of hematopoietic cells. When HL-60 cells were treated with anti-Fas antibody, rapid activation of growth-associated histone H1 kinase was observed without any change in cell cycle distribution. This was accompanied by the increase in cdc2 mRNA expression and Cdc2 kinase activity. Up-regulation of cdc2 mRNA was similarly induced in BCL-2-overexpressing HL-60 subline by anti-Fas treatment independently of the appearance of apoptotic phenotypes. Fas-induced apoptosis was completely inhibited by butyrolactone I, a specific inhibitor of Cdc2 kinase. Moreover, the same phenomenon was observed during Fas-induced but not spontaneous apoptosis of postmitotic granulocytes. Finally, we have found that "Fas-responsive element" was located between nucleotides -730 and -552 of the cdc2 promoter and was responsive for transcriptional activation of the cdc2 gene during Fas-induced apoptosis. These results indicate that aberrant activation of Cdc2 is associated with Fas-induced apoptosis of hematopoietic cells, and that the mechanism of cdc2 transcription during Fas-induced apoptosis is different from that in normal cell cycle control.

Positive regulation of cdc2 gene activity by protein phosphatase type 2A

Several lines of evidence indicate that serine/threonine protein phosphatases may act as negative regulators of cellular growth. For example, treatment of cells with the tumor-promoter okadaic acid, an inhibitor of certain types of these phosphatases, resulted in the increased expression of several proto-oncogenes, indicating a negative role of the respective phosphatases in gene regulation. However, it was puzzling to find that okadaic acid-treated cells, even in the presence of highly expressed proto-oncogenes, did not proliferate, but were arrested at certain points of the cell cycle. To further analyze this discrepancy, we investigated the involvement of protein phosphatases in the control of other cell cycle regulatory genes, such as cdc2 which encodes an essential cell cycle regulatory kinase. We found that cdc2 gene expression was blocked by okadaic acid, but stimulated by protein phosphatase 2A. Protein phosphatase 2A is shown to be a positive regulator of cdc2 gene activity and to be required for cdc2 expression. Thus, our findings identify protein phosphatase 2A as a positive regulator of a major cell cycle regulatory gene and therefore suggest a stimulatory role of this enzyme in this aspect of cellular growth control.