Identification of a cell-type-specific and E2F-independent mechanism for repression of cdc2 transcription

CDK1 is up-regulated by temozolomide in an NF-κB dependent manner in glioblastoma

The alkylating agent, temozolomide (TMZ), is the most commonly used chemotherapeutic for the treatment of glioblastoma (GBM). The anti-glioma effect of TMZ involves a complex response that includes G2-M cell cycle arrest and cyclin-dependent kinase 1 (CDK1) activation. While CDK1 phosphorylation is a well-described consequence of TMZ treatment, we find that TMZ also robustly induces CDK1 expression. Analysis of this pathway demonstrates that CDK1 is regulated by NF-κB via a putative κB-site in its proximal promoter. CDK1 was induced in a manner dependent on mature p50 and the atypical inhibitor κB protein, BCL-3. Treatment with TMZ induced binding of NF-κB to the κB-site as assessed by gel shift analysis and chromatin immunoprecipitation. Examination of a CDK1 promoter-reporter demonstrated the functional relevance of the κB-site and underlined the requirement of p50 and BCL-3 for activation. Targeted knockdown of CDK1 or chemical inhibition with the selective CDK1 inhibitor, RO-3306, potentiated the cytotoxic effect of TMZ. These results identify CDK1 as an NF-κB target gene regulated by p50 and BCL-3 and suggest that targeting CDK1 may be a strategy to improve the efficacy of TMZ against GBM.

Arsenate-mediated G2 cell cycle arrest in U-2OS cells involves phosphorylation of human polycomb protein 2 by p38 MAPK

G2/M checkpoints ensure the proper timing of cell mitosis. We previously reported that p38 mitogen-activated protein kinase (MAPK) activation is essential for stress-induced G2 arrest in the U-2OS osteosarcoma cell line, but the molecular mechanism was obscure. Here, using the T7 phage display system, we find p38 directly binds to human polycomb protein 2 (HPC2), and arsenate-induced G2 arrest in U-2OS cell is p38- and phosphorylation of HPC2-dependent. Phosphorylation of HPC2 at threonine 495 is required for recruiting Ring1 and Rb family proteins to form the polycomb repressive complex (PRC), and PRC is required for arsenate-induced downregulation of CDC2 expression. Thus, p38 MAPK regulates cell cycle progression through phosphorylation of HPC2 to mediate transcriptional repression, providing a mechanistic link for arsenate-induced transcriptional silencing.

The aryl hydrocarbon receptor and interferon gamma generate antiviral states via transcriptional repression

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor whose activation induces the expression of numerous genes, with many effects on cells. However, AhR activation is not known to affect the replication of viruses. We show that AhR activation in macrophages causes a block to HIV-1 and HSV-1 replication. We find that AhR activation transcriptionally represses cyclin-dependent kinase (CDK)1/2 and their associated cyclins, thereby reducing SAMHD1 phosphorylation, cellular dNTP levels and both HIV-1 and HSV-1 replication. Remarkably, a different antiviral stimulus, interferon gamma (IFN-γ), that induces a largely non-overlapping set of genes, also transcriptionally represses CDK1, CDK2 and their associated cyclins, resulting in similar dNTP depletion and antiviral effects. Concordantly, the SIV Vpx protein provides complete and partial resistance to the antiviral effects of AhR and IFN-γ, respectively. Thus, distinct antiviral signaling pathways converge on CDK/cyclin repression, causing inhibition of viral DNA synthesis and replication.

AURKA Suppresses Leukemic THP-1 Cell Differentiation through Inhibition of the KDM6B Pathway

Aberrations in histone modifications are being studied in mixed-lineage leukemia (MLL)-AF9-driven acute myeloid leukemia (AML). In this study, we focused on the regulation of the differentiation of the MLL-AF9 type AML cell line THP-1. We observed that, upon phorbol 12-myristate 13-acetate (PMA) treatment, THP-1 cells differentiated into monocytes by down-regulating Aurora kinase A (AURKA), resulting in a reduction in H3S10 phosphorylation. We revealed that the AURKA inhibitor alisertib accelerates the expression of the H3K27 demethylase KDM6B, thereby dissociating AURKA and YY1 from the KDM6B promoter region. Using Flow cytometry, we found that alisertib induces THP-1 differentiation into monocytes. Furthermore, we found that treatment with the KDM6B inhibitor GSK-J4 perturbed the PMA-mediated differentiation of THP-1 cells. Thus, we discovered the mechanism of AURKA-KDM6B signaling that controls the differentiation of THP-1 cells, which has implications for biotherapy for leukemia.

Inhibition of Preadipocyte Differentiation by Germacranolides fromCalea urticifoliain 3T3-L1 Cells

The effects of germacranolides isolated from Calea urticifolia on adipocytic differentiation of 3T3-L1 cells were examined. These germacranolides inhibited adipogenesis at a concentration of 1.25-5 microM. But no inhibitory activity against cell proliferation and no nonspecific binding activity to protein were observed. These results indicate that these germacranolides are the specific inhibitors of preadipocyte differentiation.

Nerve growth factor-induced cell cycle reentry in newborn neurons is triggered by p38MAPK-dependent E2F4 phosphorylation

Cumulative evidence indicates that activation of cyclin D-dependent kinase 4/6 (cdk4/6) represents a major trigger of cell cycle reentry and apoptosis in vertebrate neurons. We show here the existence of another mechanism triggering cell cycle reentry in differentiating chick retinal neurons (DCRNs), based on phosphorylation of E2F4 by p38(MAPK). We demonstrate that the activation of p75(NTR) by nerve growth factor (NGF) induces nuclear p38(MAPK) kinase activity, which leads to Thr phosphorylation and subsequent recruitment of E2F4 to the E2F-responsive cdc2 promoter. Inhibition of p38(MAPK), but not of cdk4/6, specifically prevents NGF-dependent cell cycle reentry and apoptosis in DCRNs. Moreover, a constitutively active form of chick E2F4 (Thr261Glu/Thr263Glu) stimulates G(1)/S transition and apoptosis, even after inhibition of p38(MAPK) activity. In contrast, a dominant-negative E2F4 form (Thr261Ala/Thr263Ala) prevents NGF-induced cell cycle reactivation and cell death in DCRNs. These results indicate that NGF-induced cell cycle reentry in neurons depends on the activation of a novel, cdk4/6-independent pathway that may participate in neurodegeneration.

MRG15 activates the cdc2 promoter via histone acetylation in human cells

Chromatin remodeling is required for transcriptional activation and repression. MRG15 (MORF4L1), a chromatin modulator, is a highly conserved protein and is present in complexes containing histone acetyltransferases (HATs) as well as histone deacetylases (HDACs). Loss of expression of MRG15 in mice and Drosophila results in embryonic lethality and fibroblast and neural stem/progenitor cells cultured from Mrg15 null mouse embryos exhibit marked proliferative defects when compared with wild type cells. To determine the role of MRG15 in cell cycle progression we performed chromatin immunoprecipitation with an antibody to MRG15 on normal human fibroblasts as they entered the cell cycle from a quiescent state, and analyzed various cell cycle gene promoters. The results demonstrated a 3-fold increase in MRG15 occupancy at the cdc2 promoter during S phase of the cell cycle and a concomitant increase in acetylated histone H4. H4 lysine 12 was acetylated at 24 h post-serum stimulation while there was no change in acetylation of lysine 16. HDAC1 and 2 were decreased at this promoter during cell cycle progression. Over-expression of MRG15 in HeLa cells activated a cdc2 promoter-reporter construct in a dose-dependent manner, whereas knockdown of MRG15 resulted in decreased promoter activity. In order to implicate HAT activity, we treated cells with the HAT inhibitor anacardic acid and determined that HAT inhibition results in loss of expression of cdc2 mRNA. Further, chromatin immunoprecipitation with Tip60 localizes the protein to the same 110bp stretch of the cdc2 promoter pulled down by MRG15. Additionally, we determined that cotransfection of MRG15 with the known associated HAT Tip60 had a cooperative effect in activating the cdc2 promoter. These results suggest that MRG15 is acting in a HAT complex involving Tip60 to modify chromatin via acetylation of histone H4 at the cdc2 promoter to activate transcription.

Function of the A-type cyclins during gametogenesis and early embryogenesis

The cyclins and their cyclin-dependent kinase partners, the Cdks, are the basic components of the machinery that regulates the passage of cells through the cell cycle. Among the cyclins, those known as the A-type cyclins are unique in that in somatic cells, they appear to function at two stages of the cell cycle, at the G1-S transition and again as the cells prepare to enter M-phase. Higher vertebrate organisms have two A-type cyclins, cyclin A1 and cyclin A2, both of which are expressed in the germ line and/or early embryo, following highly specialized patterns that suggest functions in both mitosis and meiosis. Insight into their in vivo functions has been obtained from gene targeting experiments in the mouse model. Loss of cyclin A1 results in disruption of spermatogenesis and male sterility due to cell arrest in the late diplotene stage of the meiotic cell cycle. In contrast, cyclin A2-deficiency is marked by early embryonic lethality; thus, understanding the function of cyclin A2 in the adult germ line awaits conditional mutagenesis or other approaches to knock down its expression.

E2F-1 binding affinity for pRb is not the only determinant of the E2F-1 activity

E2F-1 is the major cellular target of pRB and is regulated by pRB during cell proliferation. Interaction between pRB and E2F-1 is dependent on the phosphorylation status of pRB. Despite the fact that E2F-1 and pRB have antagonistic activities when they are overexpressed, the role of the E2F-1-pRB interaction in cell growth largely remains unknown. Ideally, it would be better to study the properties of a pRB mutant that fails to bind to E2F, but retains all other activities. To date, no pRB mutation has been characterized in sufficient detail to show that it specifically eliminates E2F binding but leaves other interactions intact. An alternative approach to this issue is to ask whether mutations that change E2F proteins binding affinity to pRB are sufficient to change cell growth in aspect of cell cycle and tumor formation. Therefore, we used the E2F-1 mutants including E2F-1/S332-7A, E2F-1/S375A, E2F-1/S403A, E2F-1/Y411A and E2F-1/L132Q that have different binding affinities for pRB to better understand the roles of the E2F-1 phosphorylation and E2F-1-pRB interaction in the cell cycle, as well as in transformation and gene expression. Data presented in this study suggests that in vivo phosphorylation at amino acids 332-337, 375 and 403 is important for the E2F-1 and pRB interaction in vivo. However, although E2F-1 mutants 332-7, 375 and 403 showed similar binding affinity to pRB, they showed different characteristics in transformation efficiency, G₀ accumulation, and target gene experiments.

The central role of CDE/CHR promoter elements in the regulation of cell cycle-dependent gene transcription

The cell cycle-dependent element (CDE) and the cell cycle genes homology region (CHR) control the transcription of genes with maximum expression in G(2) phase and in mitosis. Promoters of these genes are repressed by proteins binding to CDE/CHR elements in G(0) and G(1) phases. Relief from repression begins in S phase and continues into G(2) phase and mitosis. Generally, CDE sites are located four nucleotides upstream of CHR elements in TATA-less promoters of genes such as Cdc25C, Cdc2 and cyclin A. However, expression of some other genes, such as human cyclin B1 and cyclin B2, has been shown to be controlled only by a CHR lacking a functional CDE. To date, it is not fully understood which proteins bind to and control CDE/CHR-containing promoters. Recently, components of the DREAM complex were shown to be involved in CDE/CHR-dependent transcriptional regulation. In addition, the expression of genes regulated by CDE/CHR elements is mostly achieved through CCAAT-boxes, which bind heterotrimeric NF-Y proteins as well as the histone acetyltransferase p300. Importantly, many CDE/CHR promoters are downregulated by the tumor suppressor p53. In this review, we define criteria for CDE/CHR-regulated promoters and propose to distinguish two classes of CDE/CHR-regulated genes. The regulation through transcription factors potentially binding to the CDE/CHR is discussed, and recently discovered links to central pathways regulated by E2F, the pRB family and p53 are highlighted.

Krüppel-like factor 5 promotes mitosis by activating the cyclin B1/Cdc2 complex during oncogenic Ras-mediated transformation

We previously showed that the zinc finger-containing transcription factor Krüppel-like factor 5 (KLF5) is important in mediating transformation by oncogenic H-Ras through induction of cyclin D1 expression and acceleration of the G1/S transition of the cell cycle. Here we present evidence of a role for KLF5 in accelerating mitotic entry in H-Ras-transformed NIH3T3 fibroblasts. When compared with non-transformed parental NIH3T3 cells, H-Ras-transformed fibroblasts exhibit an increase in mitotic index, levels of cyclin B1 and Cdc2, and cyclin B1/Cdc2 kinase activity. Inhibition of KLF5 expression in H-Ras-transformed cells with KLF5-specific small interfering RNA (siRNA) results in a decrease in each of the aforementioned parameters, with a concomitant reduction in the transforming potential of the cells. Conversely, over-expression of KLF5 in NIH3T3 cells leads to an increase in the promoter activity of the genes encoding cyclin B1 and Cdc2. These results indicate that KLF5 accelerates mitotic entry in H-Ras-transformed cells by transcriptionally activating cyclin B1 and Cdc2, which leads to an increase in cyclin B1/Cdc2 kinase activity. Extending our previous observation that KLF5 activates cyclin D1 transcription to promote G1/S transition, our current results further support a crucial function for KLF5 in mediating cellular transformation caused by oncogenic H-Ras.

Distinct Regions of the Mouse Cyclin A1 Gene, Ccna1, Confer Male Germ-Cell Specific Expression and Enhancer Function1

The gene encoding mouse cyclin A1, Ccna1, is expressed at highest levels in late pachytene-diplotene spermatocytes, where it is required for meiotic cell division. To begin to understand the mechanisms responsible for its highly restricted pattern of expression, transgenic mouse lines carrying constructs consisting of the cyclin A1 regulatory region fused with the reporter gene lacZ were generated. Analysis of tissue-specific and testicular cell-type-specific transgene expression indicated that sequences within -1.3 kilobases (kb) of the cyclin A1 putative transcriptional start site were sufficient to direct transgene expression uniquely to late spermatocytes while maintaining repression in other tissues. However, sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site were apparently required to transcribe the reporter at levels needed for consistent X-gal staining. Comparison of the mouse, rat, and human proximal promoters revealed regions of high sequence conservation and consensus sequences both for known transcription factors, some of which are coexpressed with Ccna1, such as A-myb and Hsf2, and for elements that control expression of genes in somatic cell cycles, such as CDE, CHR, and CCAAT elements. Thus, the promoter region within 1.3 kb upstream of the putative Ccna1 transcriptional start can direct expression of lacZ to spermatocytes, while sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site may enhance expression of lacZ.

MKK7 couples stress signalling to G2/M cell-cycle progression and cellular senescence

During the development of multicellular organisms, concerted actions of molecular signalling networks determine whether cells undergo proliferation, differentiation, death or ageing. Here we show that genetic inactivation of the stress signalling kinase, MKK7, a direct activator of JNKs in mice, results in embryonic lethality and impaired proliferation of hepatocytes. Beginning at passage 4-5, mkk7(-/-) mouse embryonic fibroblasts (MEFs) display impaired proliferation, premature senescence and G2/M cell cycle arrest. Similarly, loss of c-Jun or expression of a c-JunAA mutant in which the JNK phosphorylation sites were replaced with alanine results in a G2/M cell-cycle block. The G2/M cell-cycle kinase CDC2 was identified as a target for the MKK7-JNK-c-Jun pathway. These data show that the MKK7-JNK-c-Jun signalling pathway couples developmental and environmental cues to CDC2 expression, G2/M cell cycle progression and cellular senescence in fibroblasts.

Expression of minichromosome maintenance proteins in vascular smooth muscle cells is ERK/MAPK dependent

Proliferation of vascular smooth muscle cells (VSMC) represents a key event for the pathogenesis of postangioplasty restenosis. Minichromosome maintenance proteins (MCM) form essential components of the prereplicative complex at DNA replication origins and are regulated by E2F. The present studies were designed to investigate the signal transduction pathways controlling the expression of MCM6 and MCM7 in VSMC in response to mitogenic stimuli. MCM6 and MCM7 expression was substantially increased after stimulation with platelet-derived growth factor-BB and insulin. Pretreatment with PD98059, a specific inhibitor of the extracellular signal-regulated kinases (ERK)-mitogen-activated protein kinase (MAPK), competely inhibited the mitogen-induced MCM6 and MCM7 mRNA and protein expression, demonstrating a critical role for this pathway in transmitting transmembrane signals required for the initiation of DNA replication. The p38MAPK inhibitor SB203580, the phosphatidylinositol 3 kinase (PI3-kinase) pathway inhibitor wortmannin, and the protein kinase C pathway (PKC) inhibitor Gö 6976 did not significantly affect mitogen-induced MCM6 and MCM7 expression. Transient transfection experiments revealed that PD98059 inhibited mitogen-induced MCM6 and MCM7 transcriptional activation. In addition, blockade of ERK/MAPK signaling with PD98059 strongly inhibited phosphorylation of the retinoblastoma protein (Rb) and activity of a luciferase reporter plasmid driven by multiple E2F elements. Inhibition of mitogen-induced MCM6 and MCM7 expression by PD98059 was reversed by ectopic overexpression of E2F, indicating that ERK/MAPK signaling is required for events that occur upstream of E2F release from phosphorylated Rb. In combination, these data demonstrate that the ERK/MAPK signal transduction pathway plays a central role in regulating E2F-dependent MCM expression and DNA replication in VSMC.

Transactivation of human cdc2 promoter by adenovirus E1A

Expression of the adenovirus oncoprotein E1A 12S induces the heterotrimeric transcription factor, NF-Y. NF-Y binds to the two CCAAT motifs upstream of the transcriptional start site of the human cdc2 promoter and is required for activation of the promoter by E1A 12S in cycling cells. The observations that a number of eukaryotic cell cycle regulatory genes also contain the CCAAT motifs and NF-Y binds to them support the notion that E1A 12S could play an important role in deregulated expression of these genes through activation of NF-Y gene in cycling cells.

Rapamycin inhibits E2F-dependent expression of minichromosome maintenance proteins in vascular smooth muscle cells

Rapamycin inhibits vascular smooth muscle cell (VSMC) proliferation and rapamycin-eluting stents represent a novel therapeutic strategy for preventing postangioplasty restenosis. The precise molecular mechanism, for rapamycin-mediated inhibition of VSMC cell cycle progression and DNA replication remain to be elucidated. Minichromosome maintenance proteins (MCM) are essential regulators of DNA replication and the objective of this study was to examine the effect of rapamycin on their expression in rat aortic VSMC. Rapamycin substantially inhibited mitogen-induced MCM6 and MCM7 mRNA and protein expression in a dose-dependent fashion. Transient transfection experiments revealed that rapamycin inhibited MCM6 and MCM7 promoter activity, implicating a transcriptional mechanism. MCM6 and MCM7 transcriptional activation is regulated by E2F and activity of a luciferase reporter plasmid driven by four E2F elements was also significantly inhibited by rapamycin. The inhibitory effect of rapamycin on MCM6 and MCM7 was reversed by overexpression of E2F, indicating that their downregulation by rapamycin involves an E2F-dependent mechanism. These observations suggest that rapamycin inhibits MCM6 and MCM7 expression by blocking their E2F-dependent transactivation which may contribute importantly to the inhibition of VSMC DNA synthesis by rapamycin.

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.

A B-myb promoter corepressor site facilitates in vivo occupation of the adjacent E2F site by p107 x E2F and p130 x E2F complexes

Transcription from the B-myb (MybL2 gene) promoter is strictly cell cycle-regulated by repression mediated through an E2F site during G(0)/early G(1). We report here the characterization of a corepressor site (downstream repression site (DRS)) required for this activity that is closely linked to the E2F site. Systematic mutagenesis of the DRS enabled a consensus to be derived, and it is notable that this sequence is compatible with cell cycle gene homology region sequences associated with cell cycle-dependent elements in the cyclin A, cdc2, and CDC25C promoters. The B-myb promoter is inappropriately active during G(0) in mouse embryo fibroblasts lacking the p107 and p130 pocket proteins, and we show that the ability of transfected p107 and p130 to re-impose repression on the promoter is dependent on the DRS. In contrast, transfected Rb was unable to repress the B-myb promoter. Consistent with the notion that Rb.E2F complexes are unable to bind the B-myb promoter E2F site in vivo, footprinting showed that this site is unoccupied in cells lacking p107 and p130. Chromatin immunoprecipitation assays showed a requirement for the DRS in recruiting p107 and p130 complexes to the B-myb promoter, indicating that in vivo the DRS governs the occupancy of the adjacent E2F site by transcriptional repressors.

Phorbol diester 12-O-tetradecanoylphorbol 13-acetate (TPA) up-regulates the expression of estrogen receptors in human THP-1 leukemia cells

In the present work, we have inspected expression of estrogen receptors (ER) and their regulation by the phorbol diester 12-O-tetradecanoylphorbol 13-acetate (TPA) in a leukemic cell line, the THP-1 cells, using multiple experimental approaches. Firstly, ligand binding assay (LBA) revealed that control (unstimulated) THP-1 cells express type I (high affinity, limited capacity) ER in the nuclear fraction only, whilst treatment of cells with TPA resulted in the appearance of type I ER in the soluble fraction as well, with the 50 ng/ml dose and the 48 h incubation time being the most effective experimental condition. A concomitant increase of type II ER was also seen in both soluble and nuclear cell fractions. Unstimulated THP-1 cells were found to be ER negative by immunocytochemistry; conversely, cells exposed to 50 ng/ml TPA for 48 h stained positively for ER, with the majority of cells having a strong nuclear staining. Scrutiny of ER mRNA expression using reverse transcriptase-polymerase chain reaction showed the presence of a wild type ER transcript in both control and TPA-treated THP-1 cells, though levels of ER mRNA were found to be comparatively higher in the latter. This combined evidence would imply that the TPA-induced differentiation of THP-1 cells is accompanied by the rise of high affinity (type I) ER, suggesting that estrogens may play a role in the regulation of macrophage activity during the inflammatory and/or the immune response.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Regulation of the G2/M transition by p53

p53 protects mammals from neoplasia by inducing apoptosis, DNA repair and cell cycle arrest in response to a variety of stresses. p53-dependent arrest of cells in the G1 phase of the cell cycle is an important component of the cellular response to stress. Here we review recent evidence that implicates p53 in controlling entry into mitosis when cells enter G2 with damaged DNA or when they are arrested in S phase due to depletion of the substrates required for DNA synthesis. Part of the mechanism by which p53 blocks cells at the G2 checkpoint involves inhibition of Cdc2, the cyclin-dependent kinase required to enter mitosis. Cdc2 is inhibited simultaneously by three transcriptional targets of p53, Gadd45, p21, and 14-3-3 sigma. Binding of Cdc2 to Cyclin B1 is required for its activity, and repression of the cyclin B1 gene by p53 also contributes to blocking entry into mitosis. p53 also represses the cdc2 gene, to help ensure that cells do not escape the initial block. Genotoxic stress also activates p53-independent pathways that inhibit Cdc2 activity, activation of the protein kinases Chk1 and Chk2 by the protein kinases Atm and Atr. Chk1 and Chk2 inhibit Cdc2 by inactivating Cdc25, the phosphatase that normally activates Cdc2. Chk1, Chk2, Atm and Atr also contribute to the activation of p53 in response to genotoxic stress and therefore play multiple roles. p53 induces transcription of the reprimo, B99, and mcg10 genes, all of which contribute to the arrest of cells in G2, but the mechanisms of cell cycle arrest by these genes is not known. Repression of the topoisomerase II gene by p53 helps to block entry into mitosis and strengthens the G2 arrest. In summary, multiple overlapping p53-dependent and p53-independent pathways regulate the G2/M transition in response to genotoxic stress.

p130/E2F4 binds to and represses the cdc2 promoter in response to p53

p53 represses the transcription of cdc2 and cyclin B1, causing loss of Cdc2 activity and G(2) arrest. Here we show that the region -22 to -2 of the cdc2 promoter called the R box is required for repression by p53 but not for basal promoter activity. The R box confers p53-dependent repression on heterologous promoters and binds to p130/E2F4 in response to overexpression of p53. R box-dependent repression requires p21/waf1, and overexpression of p21/waf1 also represses the cdc2 promoter. These observations suggest that p53 represses the cdc2 promoter by inducing p21/waf1, which inhibits cyclin-dependent kinase activity, enhancing the binding of p130 and E2F4, which together bind to and repress the cdc2 promoter.

Genomic organization and promoter characterization of the rat cyclin B1 gene

Cyclin B1 is a key regulatory protein involved in cellular mitosis. We have cloned 1.8kb of DNA sequence upstream of the rat cyclin B1 gene translation start site from Rattus norvegicus liver genomic DNA and a commercial rat testis genomic library. The mRNA transcription start point (tsp) was determined by primer extension and mRNA end ligation followed by RT-PCR across the ligated 3' and 5' ends. An authentic tsp was confirmed approximately 100bp upstream of the translation start site. A second potential tsp was also detected approximately 32bp downstream from the first. RT-PCR analysis of rat liver poly(A)(+) RNA using 5'-derived oligonucleotide primers indicated that the 5' end sequence was present in both the 1.6 and 2. 4kb rat liver cyclin B1 mRNA species. Like many other cyclin promoters, there was no apparent TATA box upstream of the transcription initiation sites. However, computer analysis of the promoter region identified a group of consensus transcription factor binding sites, some of which are also reported in other cyclin promoters. These include those for p53, p21, Ap-1, Ap-2, Ets-1, CAATT, E-Box and Yi. We also performed luciferase reporter assays using a set of promoter deletion constructs in human HuH-7 hepatoma and HeLa carcinoma cell lines. Our results suggest that an E-Box and/or CCAAT binding sites are important for transcription, and that there may be negative regulatory elements present between 1800 and 1100bp upstream of the translation start site.

Repression of CDK1 and other genes with CDE and CHR promoter elements during DNA damage-induced G(2)/M arrest in human cells

Entry into mitosis is controlled by the cyclin-dependent kinase CDK1 and can be delayed in response to DNA damage. In some systems, such G(2)/M arrest has been shown to reflect the stabilization of inhibitory phosphorylation sites on CDK1. In human cells, full G(2) arrest appears to involve additional mechanisms. We describe here the prolonged (>6 day) downregulation of CDK1 protein and mRNA levels following DNA damage in human cells. This silencing of gene expression is observed in primary human fibroblasts and in two cell lines with functional p53 but not in HeLa cells, where p53 is inactive. Silencing is accompanied by the accumulation of cells in G(2), when CDK1 expression is normally maximal. The response is impaired by mutations in cis-acting elements (CDE and CHR) in the CDK1 promoter, indicating that silencing occurs at the transcriptional level. These elements have previously been implicated in the repression of transcription during G(1) that is normally lifted as cells progress into S and G(2). Interestingly, we find that other genes, including those for CDC25C, cyclin A2, cyclin B1, CENP-A, and topoisomerase IIalpha, that are normally expressed preferentially in G(2) and whose promoter regions include putative CDE and CHR elements are also downregulated in response to DNA damage. These data, together with those of other groups, support the existence of a p53-dependent, DNA damage-activated pathway leading to CHR- and CDE-mediated transcriptional repression of various G(2)-specific genes. This pathway may be required for sustained periods of G(2) arrest following DNA damage.

Mechanisms of G2 arrest in response to overexpression of p53

Overexpression of p53 causes G2 arrest, attributable in part to the loss of CDC2 activity. Transcription of cdc2 and cyclin B1, determined using reporter constructs driven by the two promoters, was suppressed in response to the induction of p53. Suppression requires the regions -287 to -123 of the cyclin B1 promoter and -104 to -74 of the cdc2 promoter. p53 did not affect the inhibitory phosphorylations of CDC2 at threonine 14 or tyrosine 15 or the activity of the cyclin-dependent kinase that activates CDC2 by phosphorylating it at threonine 161. Overexpression of p53 may also interfere with the accumulation of CDC2/cyclin B1 in the nucleus, required for cells to enter mitosis. Constitutive expression of cyclin B1, alone or in combination with the constitutively active CDC2 protein T14A Y15F, did not reverse p53-dependent G2 arrest. However, targeting cyclin B1 to the nucleus in cells also expressing CDC2 T14A Y15F did overcome this arrest. It is likely that several distinct pathways contribute to p53-dependent G2 arrest.

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.

Actinobacillus actinomycetemcomitans Immunosuppressive Protein Is a Member of the Family of Cytolethal Distending Toxins Capable of Causing a G2 Arrest in Human T Cells

We have previously shown that Actinobacillus actinomycetecomitans produces an immunosuppressive factor (ISF) capable of impairing human lymphocyte function by perturbing cell cycle progression. We now report that ISF is the product of the cdtB gene, one of three genes encoding the family of cytolethal distending toxins (Cdt). The ISF polypeptide exhibits ≥95% identity with Hemophilus ducreyi CdtB protein and ≤60% homology with Escherichia coli or Campylobacter jejuni CdtB. Pretreatment of PHA-activated lymphocytes with 5–25 ng ISF results in G2 arrest of CD4+ and CD8+ T cells. Similarly, treatment of HeLa cells results in G2 arrest and cell elongation and distension. However, lymphocytes are at least 5 times more sensitive to ISF than HeLa cells and do not undergo the elongation and distension that characterizes interactions of Cdts with cell lines. ISF-treated lymphocytes express normal cyclin A and B1 levels, but contain reduced levels of cell cycle-dependent kinase-1 (Cdk1). Additionally, the majority of Cdk1 is in the hyperphosphorylated, inactive, form. In contrast, PHA-induced G2 cells contain elevated levels of the hypophosphorylated, active Cdk1. Failure of ISF-treated cells to dephosphorylate Cdk1 is not associated with decreased availability of Cdc25. These studies suggest that the CdtB protein alone is capable of inducing G2 arrest in lymphocytes and cell cycle arrest, elongation, and distension of HeLa cells. Our studies also suggest that lymphocytes may be primary targets for A. actinomycetemcomitans CdtB (ISF) and possibly for other Cdt family members as well. Thus, Cdts may function to impair host immunity and contribute to the pathogenesis of disease associated with Cdt-producing organisms.

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.

Characterization of the chicken CTCF genomic locus, and initial study of the cell cycle-regulated promoter of the gene

CTCF is a multifunctional transcription factor encoded by a novel candidate tumor suppressor gene (Filippova, G. N., Lindblom, A., Meinke, L. J., Klenova, E. M., Neiman, P. E., Collins, S. J., Doggett, N. D., and Lobanenkov, V. V. (1998) Genes Chromosomes Cancer 22, 26-36). We characterized genomic organization of the chicken CTCF (chCTCF) gene, and studied the chCTCF promoter. Genomic locus of chCTCF contains a GC-rich untranslated exon separated from seven coding exons by a long intron. The 2-kilobase pair region upstream of the major transcription start site contains a CpG island marked by a "Not-knot" that includes sequence motifs characteristic of a TATA-less promoter of housekeeping genes. When fused upstream of a reporter chloramphenicol acetyltransferase gene, it acts as a strong transcriptional promoter in transient transfection experiments. The minimal 180-base pair chCTCF promoter region that is fully sufficient to confer high level transcriptional activity to the reporter contains high affinity binding element for the transcription factor YY1. This element is strictly conserved in chicken, mouse, and human CTCF genes. Mutations in the core nucleotides of the YY1 element reduce transcriptional activity of the minimal chCTCF promoter, indicating that the conserved YY1-binding sequence is critical for transcriptional regulation of vertebrate CTCF genes. We also noted in the chCTCF promoter several elements previously characterized in cell cycle-regulated genes, including the "cell cycle-dependent element" and "cell cycle gene homology region" motifs shown to be important for S/G2-specific up-regulation of cdc25C, cdc2, cyclin A, and Plk (polo-like kinase) gene promoters. Presence of the cell cycle-dependent element/cell cycle gene homology region element suggested that chCTCF expression may be cell cycle-regulated. We show that both levels of the endogenous chCTCF mRNA, and the activity of the stably transfected chCTCF promoter constructs, increase in S/G2 cells.

Regulation of the cell cycle at the G2/M boundary in metastatic melanoma cells by 12-O-tetradecanoyl phorbol-13-acetate (TPA) by blocking p34cdc2 kinase activity

12-O-Tetradecanoyl phorbol-13-acetate (TPA) inhibits the growth of most malignant melanoma cells but stimulates the growth of normal human melanocytes. We previously showed that addition of TPA inhibits the growth of the human metastatic melanoma cell line, Demel, by blocking cells at both the G1/S and G2/M cell cycle transitions (D. L. Coppock et al., 1992, Cell Growth Differ. 3, 485-494). To examine the G2/M transition, we developed a method to synchronize the cells in early S phase using Lovastatin and mevalonate, followed by treatment with hydroxyurea (HU). TPA (30 nM) was effective in blocking cells from entering mitosis and reentering G1 when added up to the end of G2. These cells arrested in G2. Examination of the levels of cyclins A and B1 demonstrated that the levels of these cyclins were not limiting for entrance into M. However, the addition of TPA blocked the increase in p34(cdc2)/cyclin B1 kinase activity. In cells treated with TPA, most p34(cdc2) was found in the slowly migrating forms on Western blots, which contained increased levels of phosphotyrosine. In addition, the level of the cyclin-dependent kinase inhibitor p21(Cip1/Waf1), but not of p27(Kip1), was increased. We examined the expression of protein kinase C (PKC) isoforms in Demel cells using Western blots to understand which types were involved in the G2 arrest. Demel cells expressed the PKC alpha, betaI, betaII, delta, epsilon, iota/lambda, zeta, and mu isozymes. PKC eta and PKC theta were not detected. Addition of TPA did not completely down regulate any PKC isozymes over a 12-h period in these synchronized cells. PKC alpha, betaI, betaII, delta, and epsilon isozymes were translocated to the membrane fraction from the cytosolic fraction when treated with TPA. PKC delta appeared as a doublet and the addition of TPA shifted a majority to the slower migrating form. The level of PKC mu was constant; however, a slow mobility form was observed in TPA-treated cells. This reduced mobility was at least partially due to phosphorylation. Thus, the arrest of growth in G2 appears to be due to the inhibition of the p34(cdc2) kinase activity which is associated with the increased expression of p21(Cip1/Waf1) and increased phosphorylation on tyrosine of p34(cdc2). This arrest, in turn, is associated with a shift of PKC isozymes PKC alpha, PKC betaI, PKC betaII, PKC delta, PKC epsilon, and PKC mu to the membrane fraction which is induced by addition of TPA.

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.

Activated lymphocytes increase expression of 5-lipoxygenase and its activating protein in THP-1 cells

The aim of this study was to investigate the regulation of the 5-lipoxygenase pathway of arachidonic acid metabolism by lymphocytes using the monocyte-like cell line, THP-1. When THP-1 cells were incubated over 4-7 days in 10% supernatant from lectin-activated human lymphocytes, their capacity to synthesize 5-lipoxygenase products was significantly increased. In contrast, the supernatant from nonactivated lymphocytes had no effect. The increase in capacity to synthesize 5-lipoxygenase products was mimicked by the addition of either granulocyte macrophage colony-stimulating factor (GM-CSF) or interleukin-3. These increases in synthetic capacity reflected increased enzymatic activity. Increased immunoreactive protein and mRNA for the enzymes 5-lipoxygenase and 5-lipoxygenase-activating protein were also found in cells conditioned with activated lymphocyte supernatants. Furthermore, the increase in mRNA for both enzymes was not blocked by cycloheximide, suggesting that the effect on steady-state mRNA levels does not require the synthesis of new protein. The increase in mRNA could be reproduced by GM-CSF. We conclude that lymphocytes can regulate the expression of 5-lipoxygenase in THP-1 cells over a period of days via the release of soluble factors.

The mitogenic effect of parathyroid hormone is associated with E2F-dependent activation of cyclin-dependent kinase 1 (cdc2) in osteoblast precursors

Injections of parathyroid hormone (PTH) have been reported to stimulate skeletal accretion through increased bone formation in several species, and osteoblast proliferation is a critical component of bone formation. However, the biological mechanisms of PTH-stimulated bone cell proliferation are largely unknown. In this study, we demonstrated that PTH stimulates proliferation of the osteoblast precursor cell line, TE-85, in association with increasing cdc2 protein levels and its kinase activity. cdc2 antisense oligonucleotides blocked PTH-induced DNA synthesis and cell cycle progression. Analysis of the time course of PTH-stimulated cdc2 message levels demonstrated that cdc2 mRNA levels were increased 1.5- to 4-fold between 3-18 h following release from cell synchronization. Transfections of TE-85 cells with a series of cdc2 promoter-luciferase deletion constructs revealed PTH stimulation of the cdc2 promoter. Promoter constructs containing a mutation in the E2F binding site were not stimulated by PTH. Gel mobility shift assays demonstrated increased free E2F levels in TE-85 nuclear extracts in response to PTH. Furthermore, the ratios of hyperphosphorylated to hypophosphorylated forms of Rb protein were increased by PTH treatment. These results demonstrate that PTH-stimulated cdc2 expression was associated with TE-85 cell proliferation and that the mechanism of stimulating cdc2 gene expression involved increasing the levels of free E2F.

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.

In vivo structure of the human cdc2 promoter: release of a p130-E2F-4 complex from sequences immediately upstream of the transcription initiation site coincides with induction of cdc2 expression

In quiescent cells, cdc2 mRNA is almost undetectable. Stimulation of cells to reenter the cell cycle results in induction of cdc2 expression, beginning at the G1-to-S transition and reaching maximum levels during late S and G2 phases. To investigate cdc2 transcriptional regulation throughout cell cycle progression, we monitored protein-DNA interactions by in vivo footprinting along 800 bp of the human cdc2 promoter in quiescent fibroblasts and at different time points following serum stimulation. We found 11 in vivo protein-binding sites, but no protein binding was observed at a high-affinity E2F site that had previously been implicated in cdc2 regulation. Nine of the identified in vivo binding sites (among them were two inverted CCAAT boxes, two Sp1 sites, and one ets-2 site) bind transcription factors constitutively throughout the cell cycle. However, at two elements located at positions -60 and -20 relative to the transcription start site, the binding pattern changes significantly as the cells are entering S phase. A G0- and G1-specific protein complex disappears at the -20 element at the beginning of S phase. This sequence deviates at one base position from known E2F consensus binding sites. We found that the major E2F activity in human fibroblasts contains E2F-4 and p130. The -20 element of the cdc2 gene specifically interacts with a subset of E2F-4-p130 complexes present in G0 cells but does not interact with S-phase-specific E2F complexes. Transient-transfection experiments with wild-type and mutant cdc2 promoter constructs indicate that the -20 element is involved in suppressing cdc2 activity in quiescent cells. We suggest that the presence of the p130-E2F-4 complex in G0/G1 blocks access of components of the basal transcription machinery or prevents transaction by the constitutively bound upstream activator proteins.