Regulation of CDC6, geminin, and CDT1 in human cells that undergo polyploidization

Endo-reduplication in mouse liver after conditional mutation of ORC2 and combined mutation of ORC1 and ORC2

The six subunit Origin Recognition Complex (ORC) is essential for loading MCM2-7 at origins of DNA replication to promote initiation of DNA replication in organisms ranging from S. cerevisiae to humans. In rare instances, as in cancer cell-lines in culture with mutations in ORC1 , ORC2 or ORC5 , or in endo-reduplicating mouse hepatocytes in vivo without ORC1 , DNA replication has been observed in the virtual absence of individual ORC subunits. Although ORC1 is dispensable in the mouse liver for endo-reduplication, because of the homology of ORC1 with CDC6, it could be argued that CDC6 was substituting for ORC1 to restore functional ORC. Here, we have created mice with a conditional deletion of ORC2 , to demonstrate that mouse embryo fibroblasts require ORC2 for proliferation, but that the mouse hepatocytes can carry out DNA synthesis in vitro and endo-reduplicate in vivo , despite the deletion of ORC2 . Combining the conditional mutation of ORC1 and ORC2 revealed that the mouse liver can still carry out endo-reduplication despite the deletion of the two genes, both during normal development and after partial hepatectomy. Since endo-reduplication, like normal S phase replication, requires the presence of MCM2-7 on the chromatin, these results suggest that in primary hepatocytes there is a mechanism to load sufficient MCM2-7 to carry out effective DNA replication despite the virtual absence of two subunits of ORC.

Persistent DNA damage underlies tubular cell polyploidization and progression to chronic kidney disease in kidneys deficient in the DNA repair protein FAN1

Defective DNA repair pathways contribute to the development of chronic kidney disease (CKD) in humans. However, the molecular mechanisms underlying DNA damage-induced CKD pathogenesis are not well understood. Here, we investigated the role of tubular cell DNA damage in the pathogenesis of CKD using mice in which the DNA repair protein Fan1 was knocked out. The phenotype of these mice is orthologous to the human DNA damage syndrome, karyomegalic interstitial nephritis (KIN). Inactivation of Fan1 in kidney proximal tubule cells sensitized the kidneys to genotoxic and obstructive injury characterized by replication stress and persistent DNA damage response activity. Accumulation of DNA damage in Fan1 tubular cells induced epithelial dedifferentiation and tubular injury. Characteristic to KIN, cells with chronic DNA damage failed to complete mitosis and underwent polyploidization. In vitro and in vivo studies showed that polyploidization was caused by the overexpression of DNA replication factors CDT1 and CDC6 in FAN1 deficient cells. Mechanistically, inhibiting DNA replication with Roscovitine reduced tubular injury, blocked the development of KIN and mitigated kidney function in these Fan1 knockout mice. Thus, our data delineate a mechanistic pathway by which persistent DNA damage in the kidney tubular cells leads to kidney injury and development of CKD. Furthermore, therapeutic modulation of cell cycle activity may provide an opportunity to mitigate the DNA damage response induced CKD progression.

Megakaryocyte polyploidization: role in platelet production

Mammal megakaryocytes (MK) undergo polyploidization during their differentiation. This process leads to a marked increase in the MK size and of their cytoplasm. Contrary to division by classical mitosis, ploidization allows an economical manner to produce platelets as they arise from the fragmentation of the MK cytoplasm. The platelet production in vivo correlates to the entire MK cytoplasm mass that depends both upon the number of MKs and their size. Polyploidization occurs by several rounds of DNA replication with at the end of each round an aborted mitosis at late phase of cytokinesis. As there is also a defect in karyokinesis, MKs are giant cells with a single polylobulated nucleus with a 2^xN ploidy. However, polyploidization per se does not increase platelet production because it requires a parallel development of MK organelles such as mitochondria, granules and the demarcation membrane system. MK polyploidization is regulated by extrinsic factors, more particularly by thrombopoietin (TPO), which during a platelet stress increases first polyploidization before enhancing the MK number and by transcription factors such as RUNX1, GATA1, and FLI1 that regulate MK differentiation explaining why polyploidization and cytoplasmic maturation are intermingled. MK polyploidization is ontogenically regulated and is markedly altered in malignant myeloid disorders such as acute megakaryoblastic leukemia and myeloproliferative disorders as well as in hereditary thrombocytopenia, more particularly those involving transcription factors or signaling pathways. In addition, MKs arising from progenitors in vitro have a much lower ploidy in vitro than in vivo leading to a low yield of platelet production in vitro. Thus, it is tempting to find approaches to increase MK polyploidization in vitro. However, these approaches require molecules that are able to simultaneously increase MK polyploidization and to induce terminal differentiation. Here, we will focus on the regulation by extrinsic and intrinsic factors of MK polyploidization during development and pathological conditions.

DNA replication licensing proteins: Saints and sinners in cancer

DNA replication is all-or-none process in the cell, meaning, once the DNA replication begins it proceeds to completion. Hence, to achieve maximum control of DNA replication, eukaryotic cells employ a multi-subunit initiator protein complex known as "pre-replication complex or DNA replication licensing complex (DNA replication LC). This complex involves multiple proteins which are origin-recognition complex family proteins, cell division cycle-6, chromatin licensing and DNA replication factor 1, and minichromosome maintenance family proteins. Higher-expression of DNA replication LC proteins appears to be an early event during development of cancer since it has been a common hallmark observed in a wide variety of cancers such as oesophageal, laryngeal, pulmonary, mammary, colorectal, renal, urothelial etc. However, the exact mechanisms leading to the abnormally high expression of DNA replication LC have not been clearly deciphered. Increased expression of DNA replication LC leads to licensing and/or firing of multiple origins thereby inducing replication stress and genomic instability. Therapeutic approaches where the reduction in the activity of DNA replication LC was achieved either by siRNA or shRNA techniques, have shown increased sensitivity of cancer cell lines towards the anti-cancer drugs such as cisplatin, 5-Fluorouracil, hydroxyurea etc. Thus, the expression level of DNA replication LC within the cell determines a cell's fate thereby creating a paradox where DNA replication LC acts as both "Saint" and "Sinner". With a potential to increase sensitivity to chemotherapy drugs, DNA replication LC proteins have prospective clinical importance in fighting cancer. Hence, in this review, we will shed light on importance of DNA replication LC with an aim to use DNA replication LC in diagnosis and prognosis of cancer in patients as well as possible therapeutic targets for cancer therapy.

APC/CCDC20 and APC/C play pivotal roles in the process of embryonic development in Artemia sinica

Anaphase Promoting Complex or Cyclosome (APC/C) is a representative E3 ubiquitin ligase, triggering the transition of metaphase to anaphase by regulating degradation and ensures the exit from mitosis. Cell division cycle 20 (CDC20) and Cell division cycle 20 related protein 1 (CDH1), as co-activators of APC/C, play significant roles in the spindle assembly checkpoint, guiding ubiquitin-mediated degradation, together with CDC23. During the embryonic development of the brine shrimp, Artemia sinica, CDC20, CDH1 and CDC23 participate in cell cycle regulation, but the specific mechanisms of their activities remain unknown. Herein, the full-length cDNAs of cdc20 and cdc23 from A. sinica were cloned. Real-time PCR analyzed the expression levels of As-cdc20 and As-cdc23. The locations of CDH1, CDC20 and CDC23 showed no tissue or organ specificity. Furthermore, western blotting showed that the levels of As-CDC20, securin, cyclin B, CDK1, CDH1, CDC14B, CDC23 and geminin proteins conformed to their complicated degradation relationships during different embryo stages. Our research revealed that As-CDC20, As-CDH1 and APC mediate the mitotic progression, downstream proteins degradation and cellular differentiation in the process of embryonic development in A. sinica.

p53 activates G₁ checkpoint following DNA damage by doxorubicin during transient mitotic arrest

Recovery from DNA damage is critical for cell survival. The serious damage is not able to be repaired during checkpoint and finally induces cell death to prevent abnormal cell growth. In this study, we demonstrated that 8N-DNA contents are accumulated via re-replication during prolonged recovery period containing serious DNA damage in mitotic cells. During the incubation for recovery, a mitotic delay and initiation of an abnormal interphase without cytokinesis were detected. Whereas a failure of cytokinesis occurred in cells with no relation with p53/p21, re-replication is an anomalous phenomenon in the mitotic DNA damage response in p53/p21 negative cells. Cells with wild-type p53 are accumulated just prior to the initiation of DNA replication through a G₁ checkpoint after mitotic DNA damage, even though p53 does not interrupt pre-RC assembly. Finally, these cells undergo cell death by apoptosis. These data suggest that p53 activates G₁ checkpoint in response to mitotic DNA damage. Without p53, cells with mitotic DNA damage undergo re-replication leading to accumulation of damage

Two nuclear export signals of Cdc6 are differentially associated with CDK-mediated phosphorylation residues for cytoplasmic translocation

Cdc6 is cleaved at residues 442 and 290 by caspase-3 during apoptosis producing p49-tCdc6 and p32-tCdc6, respectively. While p32-tCdc6 is unable to translocate into the cytoplasm, p49-tCdc6 retains cytoplasmic translocation activity, but it has a lower efficiency than wild-type Cdc6. We hypothesized that a novel nuclear export signal (NES) sequence exists between amino acids 290 and 442. Cdc6 contains a novel NES in the region of amino acids 300-315 (NES2) that shares sequence similarity with NES1 at residues 462-476. In mutant versions of Cdc6, we replaced leucine with alanine in NES1 and NES2 and co-expressed the mutant constructs with cyclin A. We observed that the cytoplasmic translocation of these mutants was reduced in comparison to wild-type Cdc6. Moreover, the cytoplasmic translocation of a mutant in which all four leucine residues were mutated to alanine was significantly inhibited in comparison to the translocation of wild-type Cdc6. The Crm1 binding activities of Cdc6 NES mutants were consistent with the efficiency of its cytoplasmic translocation. Further studies have revealed that L468 and L470 of NES1 are required for cytoplasmic translocation of Cdc6 phosphorylated at S74, while L311 and L313 of NES2 accelerate the cytoplasmic translocation of Cdc6 phosphorylated at S54. These results suggest that the two NESs of Cdc6 work cooperatively and distinctly for the cytoplasmic translocation of Cdc6 phosphorylated at S74 and S54 by cyclin A/Cdk2.

Visualizing developmentally programmed endoreplication in mammals using ubiquitin oscillators

The majority of mammalian somatic cells maintain a diploid genome. However, some mammalian cell types undergo multiple rounds of genome replication (endoreplication) as part of normal development and differentiation. For example, trophoblast giant cells (TGCs) in the placenta become polyploid through endoreduplication (bypassed mitosis), and megakaryocytes (MKCs) in the bone marrow become polyploid through endomitosis (abortive mitosis). During the normal mitotic cell cycle, geminin and Cdt1 are involved in ‘licensing’ of replication origins, which ensures that replication occurs only once in a cell cycle. Their protein accumulation is directly regulated by two E3 ubiquitin ligase activities, APCCdh1 and SCFSkp2, which oscillate reciprocally during the cell cycle. Although proteolysis-mediated, oscillatory accumulation of proteins has been documented in endoreplicating Drosophila cells, it is not known whether the ubiquitin oscillators that control normal cell cycle transitions also function during mammalian endoreplication. In this study, we used transgenic mice expressing Fucci fluorescent cell-cycle probes that report the activity of APCCdh1 and SCFSkp2. By performing long-term, high temporal-resolution Fucci imaging, we were able to visualize reciprocal activation of APCCdh1 and SCFSkp2 in differentiating TGCs and MKCs grown in our custom-designed culture wells. We found that TGCs and MKCs both skip cytokinesis, but in different ways, and that the reciprocal activation of the ubiquitin oscillators in MKCs varies with the polyploidy level. We also obtained three-dimensional reconstructions of highly polyploid TGCs in whole, fixed mouse placentas. Thus, the Fucci technique is able to reveal the spatiotemporal regulation of the endoreplicative cell cycle during differentiation.

Loss of Smu1 function de-represses DNA replication and over-activates ATR-dependent replication checkpoint

Smu1 is an evolutionarily conserved gene that encodes a member of the WD40-repeat protein family. Disruption of Smu1 function leads to multiple cellular defects including chromosomal instability, aberrant DNA replication and alternative RNA splicing events. In this paper, we show that Smu1 is a chromatin-bound protein that functions as a negative regulator of DNA replication. Knockdown of Smu1 gene expression promotes excessive incorporation of dNTP analogue, implicating the acceleration of DNA synthesis. Smu1-silenced cells show an excessive activation of replication checkpoint in response to ultraviolate (UV) or hydroxyurea treatment, indicating that abnormal stimulation of DNA replication leads to instability of genomic structure. Hence, we propose that Smu1 participates in the protection of genomic integrity by negatively regulating the process of DNA synthesis.

CDC6 expression is regulated by lineage-specific transcription factor GATA1

GATA1 is a hematopoietic transcription factor essential for expression of most genes encoding erythro-megakaryocytic proteins, i.e., globins and platelet glycoproteins. A role for GATA1 as a cell proliferation regulator has been proposed, as some of its bona fide targets comprise global regulators, such as c-KIT or c-MYC, or cell cycle factors, i.e., CYCLIN D or p21CIP1. In this study, we describe that GATA1 directly regulates the expression of replication licensing factor CDC6. Using reporter transactivation, electrophoretic mobility shift and chromatin immunoprecipitation assays, we show that GATA1 stimulates CDC6 transcription by binding to a canonical binding site located within a 166bp enhancer region upstream CDC6 promoter. This evolutionary conserved GATA binding site conforms to recently described chromatin occupancy rules, i.e., preferred bases within core WGATAR (TGATAA), 5' and 3' flanking bases (GGTGATAAGG) and distance to the transcription initiation site. We also found adjacent conserved binding sites for ubiquitously expressed transcription factor CP2, needed for GATA activity on CDC6 enhancer. Our results add to the growing evidence for GATA1 acting as a direct transcriptional regulator of the cell cycle machinery, thus linking cell proliferation control and specific gene expression programs during lineage differentiation.

Proposed megakaryocytic regulon of p53: the genes engaged to control cell cycle and apoptosis during megakaryocytic differentiation

During endomitosis, megakaryocytes undergo several rounds of DNA synthesis without division leading to polyploidization. In primary megakaryocytes and in the megakaryocytic cell line CHRF, loss or knock-down of p53 enhances cell cycling and inhibits apoptosis, leading to increased polyploidization. To support the hypothesis that p53 suppresses megakaryocytic polyploidization, we show that stable expression of wild-type p53 in K562 cells (a p53-null cell line) attenuates the cells' ability to undergo polyploidization during megakaryocytic differentiation due to diminished DNA synthesis and greater apoptosis. This suggested that p53's effects during megakaryopoiesis are mediated through cell cycle- and apoptosis-related target genes, possibly by arresting DNA synthesis and promoting apoptosis. To identify candidate genes through which p53 mediates these effects, gene expression was compared between p53 knock-down (p53-KD) and control CHRF cells induced to undergo terminal megakaryocytic differentiation using microarray analysis. Among substantially downregulated p53 targets in p53-KD megakaryocytes were cell cycle regulators CDKN1A (p21) and PLK2, proapoptotic FAS, TNFRSF10B, CASP8, NOTCH1, TP53INP1, TP53I3, DRAM1, ZMAT3 and PHLDA3, DNA-damage-related RRM2B and SESN1, and actin component ACTA2, while antiapoptotic CKS1B, BCL2, GTSE1, and p53 family member TP63 were upregulated in p53-KD cells. Additionally, a number of cell cycle-related, proapoptotic, and cytoskeleton-related genes with known functions in megakaryocytes but not known to carry p53-responsive elements were differentially expressed between p53-KD and control CHRF cells. Our data support a model whereby p53 expression during megakaryopoiesis serves to control polyploidization and the transition from endomitosis to apoptosis by impeding cell cycling and promoting apoptosis. Furthermore, we identify a putative p53 regulon that is proposed to orchestrate these effects.

RUNX1-induced silencing of non-muscle myosin heavy chain IIB contributes to megakaryocyte polyploidization

Megakaryocytes are unique mammalian cells that undergo polyploidization (endomitosis) during differentiation, leading to an increase in cell size and protein production that precedes platelet production. Recent evidence demonstrates that endomitosis is a consequence of a late failure in cytokinesis associated with a contractile ring defect. Here we show that the non-muscle myosin IIB heavy chain (MYH10) is expressed in immature megakaryocytes and specifically localizes in the contractile ring. MYH10 downmodulation by short hairpin RNA increases polyploidization by inhibiting the return of 4N cells to 2N, but other regulators, such as of the G1/S transition, might regulate further polyploidization of the 4N cells. Conversely, re-expression of MYH10 in the megakaryocytes prevents polyploidization and the transition of 2N to 4N cells. During polyploidization, MYH10 expression is repressed by the major megakaryocyte transcription factor RUNX1. Thus, RUNX1-mediated silencing of MYH10 is required for the switch from mitosis to endomitosis, linking polyploidization with megakaryocyte differentiation.

Geminin deletion from hematopoietic cells causes anemia and thrombocytosis in mice

HSCs maintain the circulating blood cell population. Defects in the orderly pattern of hematopoietic cell division and differentiation can lead to leukemia, myeloproliferative disorders, or marrow failure; however, the factors that control this pattern are incompletely understood. Geminin is an unstable regulatory protein that regulates the extent of DNA replication and is thought to coordinate cell division with cell differentiation. Here, we set out to determine the function of Geminin in hematopoiesis by deleting the Geminin gene (Gmnn) from mouse bone marrow cells. This severely perturbed the pattern of blood cell production in all 3 hematopoietic lineages (erythrocyte, megakaryocyte, and leukocyte). Red cell production was virtually abolished, while megakaryocyte production was greatly enhanced. Leukocyte production transiently decreased and then recovered. Stem and progenitor cell numbers were preserved, and Gmnn(–/–) HSCs successfully reconstituted hematopoiesis in irradiated mice. CD34(+) Gmnn(–/–) leukocyte precursors displayed DNA overreplication and formed extremely small granulocyte and monocyte colonies in methylcellulose. While cultured Gmnn(–/–) mega-karyocyte-erythrocyte precursors did not form erythroid colonies, they did form greater than normal numbers of megakaryocyte colonies. Gmnn(–/–) megakaryocytes and erythroblasts had normal DNA content. These data led us to postulate that Geminin regulates the relative production of erythrocytes and megakaryocytes from megakaryocyte-erythrocyte precursors by a replication-independent mechanism.

Drug-induced cell cycle modulation leading to cell-cycle arrest, nuclear mis-segregation, or endoreplication

BACKGROUND

Cancer cell responses to chemotherapeutic agents vary, and this may reflect different defects in DNA repair, cell-cycle checkpoints, and apoptosis control. Cytometry analysis only quantifies dye-incorporation to examine DNA content and does not reflect the biological complexity of the cell cycle in drug discovery screens.

RESULTS

Using population and time-lapse imaging analyses of cultured immortalized cells expressing a new version of the fluorescent cell-cycle indicator, Fucci (Fluorescent Ubiquitination-based Cell Cycle Indicator), we found great diversity in the cell-cycle alterations induced by two anticancer drugs. When treated with etoposide, an inhibitor of DNA topoisomerase II, HeLa and NMuMG cells halted at the G2/M checkpoint. HeLa cells remained there, but NMuMG cells then overrode the checkpoint and underwent nuclear mis-segregation or avoided the checkpoint and entered the endoreplication cycle in a drug concentration dependent manner. In contrast, an inhibitor of Cdk4 led to G1 arrest or endoreplication in NMuMG cells depending upon the initial cell-cycle phase of drug exposure.

CONCLUSIONS

Drug-induced cell cycle modulation varied not only between different cell types or following treatment with different drugs, but also between cells treated with different concentrations of the same drug or following drug addition during different phases of the cell cycle. By combining cytometry analysis with the Fucci probe, we have developed a novel assay that fully integrates the complexity of cell cycle regulation into drug discovery screens. This assay system will represent a powerful drug-discovery tool for the development of the next generation of anti-cancer therapies.

Endoreplication: polyploidy with purpose

A great many cell types are necessary for the myriad capabilities of complex, multicellular organisms. One interesting aspect of this diversity of cell type is that many cells in diploid organisms are polyploid. This is called endopolyploidy and arises from cell cycles that are often characterized as "variant," but in fact are widespread throughout nature. Endopolyploidy is essential for normal development and physiology in many different organisms. Here we review how both plants and animals use variations of the cell cycle, termed collectively as endoreplication, resulting in polyploid cells that support specific aspects of development. In addition, we discuss briefly how endoreplication occurs in response to certain physiological stresses, and how it may contribute to the development of cancer. Finally, we describe the molecular mechanisms that support the onset and progression of endoreplication.

Effect of leptin on mouse trophoblast giant cells

Leptin plays a role in both energy homeostasis and reproduction, and it is required in early pregnancy. It stimulates metalloproteinase activity in cultured human trophoblasts and invasiveness of cultured mouse trophoblasts. Our goal has been to examine mechanisms that underpin the ability of leptin to promote trophoblast invasiveness in primary cultures of mouse trophoblasts. Leptin stimulated the phosphorylation of MEK (MAP2K1) but not signal transducer and activator of transcription 3 (STAT3) in the cultures, increased the concentration of the suppressor of cytokine signaling 3 (SOCS3) protein, and upregulated metalloproteinase activity. Microarray analysis revealed that leptin stimulated select genes with roles in cell motility, including Stmn, a gene linked to invasiveness in other cell types. There was also an increase in activity of several genes associated with MAPK and RhoGTPase signaling. In addition, leptin muted expression of genes correlated with terminal differentiation of trophoblast giant cells, including ones associated with the TGFbeta signaling pathway and endoreduplication of DNA, and upregulated selected prolactin-related family members. Feulgen staining of leptin-treated cells revealed a loss of cells with low ploidy. The data suggest that leptin accelerates disappearance of non-giant cells while inhibiting terminal differentiation of committed giant cells, possibly by maintaining cells in an intermediate stage of differentiation.

The alpha1-adrenergic receptor antagonists, benoxathian and prazosin, induce apoptosis and a switch towards megakaryocytic differentiation in human erythroleukemia cells

The erythroleukemia cell lines K562 and human erythroleukemia (HEL) are established models to study erythroid and megakaryocytic differentiation in vitro. In this study, we show that the alpha1-adrenergic antagonists, benoxathian and prazosin, inhibit the proliferation and induce apoptosis in K562 and HEL cells. Furthermore, both tested substances induced the expression of the megakaryocytic marker CD41a, whereas the expression of the erythroid marker glycophorin-a was decreased or unchanged. Even though the expression of differentiation markers was similar after benoxathian and prazosin treatment in both cell lines, endomitosis of erythroleukemia cells was observed only after prazosin treatment. So far, benoxathian and prazosin are the first described extracellular ligands, which cause megakaryocytic differentiation in K562 and HEL cells. In summary, these results indicate a possible role of alpha1-adrenergic receptor signaling in the regulation of erythroid and megakaryocytic differentiation, even though the receptor dependence of the observed effects needs further investigation.

APC/CFzr/Cdh1 promotes cell cycle progression during the Drosophila endocycle

The endocycle is a commonly observed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. How the cell cycle machinery is modified to transform a mitotic cycle into endocycle has long been a matter of interest. In both plants and animals, the transition from the mitotic cycle to the endocycle requires Fzr/Cdh1, a positive regulator of the Anaphase-Promoting Complex/Cyclosome (APC/C). However, because many of its targets are transcriptionally downregulated upon entry into the endocycle, it remains unclear whether the APC/C functions beyond the mitotic/endocycle boundary. Here, we report that APC/C Fzr/Cdh1 activity is required to promote the G/S oscillation of the Drosophila endocycle. We demonstrate that compromising APC/C activity, after cells have entered the endocycle, inhibits DNA replication and results in the accumulation of multiple APC/C targets, including the mitotic cyclins and Geminin. Notably, our data suggest that the activity of APC/C Fzr/Cdh1 during the endocycle is not continuous but is cyclic, as demonstrated by the APC/C-dependent oscillation of the pre-replication complex component Orc1. Taken together, our data suggest a model in which the cyclic activity of APC/C Fzr/Cdh1 during the Drosophila endocycle is driven by the periodic inhibition of Fzr/Cdh1 by Cyclin E/Cdk2. We propose that, as is observed in mitotic cycles, during endocycles, APC/C Fzr/Cdh1 functions to reduce the levels of the mitotic cyclins and Geminin in order to facilitate the relicensing of DNA replication origins and cell cycle progression.

Mechanisms to control rereplication and implications for cancer

Recent advances in the replication field have highlighted how the replication initiator proteins are negatively regulated by inhibitor proteins and ubiquitin-mediated degradation in mammalian cells to prevent rereplication. When these regulatory pathways go awry, uncontrolled rereplication ensues and a G2/M checkpoint is evoked to prevent cellular death. Many components of the checkpoints activated by rereplicaton are important for cancer prevention by facilitating DNA damage repair processes. The pathways that prevent rereplication themselves have also recently been implicated in preventing tumorigenesis. Studies from patient tumors, genetically altered mice, and mammalian cell culture suggest that deregulation of replication licensing proteins results in an increase in aneuploidy, chromosomal fusions, and DNA breaks. These studies provide a framework to address how regulators of replication function to maintain genomic stability.

CDC6: from DNA replication to cell cycle checkpoints and oncogenesis

Cell division cycle 6 (CDC6) is an essential regulator of DNA replication in eukaryotic cells. Its best-characterized function is the assembly of prereplicative complexes at origins of replication during the G(1) phase of the cell division cycle. However, CDC6 also plays important roles in the activation and maintenance of the checkpoint mechanisms that coordinate S phase and mitosis, and recent studies have unveiled its proto-oncogenic activity. CDC6 overexpression interferes with the expression of INK4/ARF tumor suppressor genes through a mechanism involving the epigenetic modification of chromatin at the INK4/ARF locus. In addition, CDC6 overexpression in primary cells may promote DNA hyperreplication and induce a senescence response similar to that caused by oncogene activation. These findings indicate that deregulation of CDC6 expression in human cells poses a serious risk of carcinogenesis.

Human Cdc45 is a proliferation-associated antigen

Cell division cycle protein 45 (Cdc45) plays a critical role in DNA replication to ensure that chromosomal DNA is replicated only once per cell cycle. We analysed the expression of human Cdc45 in proliferating and nonproliferating cells. Our findings show that Cdc45 protein is absent from long-term quiescent, terminally differentiated and senescent human cells, although it is present throughout the cell cycle of proliferating cells. Moreover, Cdc45 is much less abundant than the minichromosome maintenance (Mcm) proteins in human cells, supporting the concept that origin binding of Cdc45 is rate limiting for replication initiation. We also show that the Cdc45 protein level is consistently higher in human cancer-derived cells compared with primary human cells. Consequently, tumour tissue is preferentially stained using Cdc45-specific antibodies. Thus, Cdc45 expression is tightly associated with proliferating cell populations and Cdc45 seems to be a promising candidate for a novel proliferation marker in cancer cell biology.

TPA-induced multinucleation of a mesenchymal stem cell-like clone is mediated primarily by karyokinesis without cytokinesis, although cell-cell fusion also occurs

The 5F9A cell, which is a mesenchymal stem cell-like clone established from rat bone marrow substrate adherent cells, can differentiate into adipocytes and osteoblasts in vitro under the appropriate conditions. Multinucleated cells could be also induced by 12-O-tetradecanoylphorbol 13-acetate (TPA) in 5F9A cells. This effect was mediated by protein kinase C. Possible mechanisms of multinucleation by TPA were hypothesized to be either karyokinesis without cytokinesis or cell-cell fusion. By observation using time-lapse phase-contrast microscopy, we determined that the multinucleated cells were generated mainly by karyokinesis without cytokinesis. Cell fusion was studied using time-lapse photography, and confocal laser scanning microscopy using two differentially labeled cells. These techniques demonstrated that multinucleated 5F9A cells could be produced by cell fusion, albeit at a low frequency. We conclude that multinucleated 5F9A cells are formed primarily by karyokinesis without cytokinesis, although some cells are also formed by cell-cell fusion.

RB loss promotes aberrant ploidy by deregulating levels and activity of DNA replication factors

The retinoblastoma tumor suppressor (RB) is functionally inactivated in many human cancers. Classically, RB functions to repress E2F-mediated transcription and inhibit cell cycle progression. Consequently, RB ablation leads to loss of cell cycle control and aberrant expression of E2F target genes. Emerging evidence indicates a role for RB in maintenance of genomic stability. Here, mouse adult fibroblasts were utilized to demonstrate that aberrant DNA content in RB-deficient cells occurs concomitantly with an increase in levels and chromatin association of DNA replication factors. Furthermore, following exposure to nocodazole, RB-proficient cells arrest with 4 n DNA content, whereas RB-deficient cells bypass the mitotic block, continue DNA synthesis, and accumulate cells with higher ploidy and micronuclei. Under this condition, RB-deficient cells also retain high levels of tethered replication factors, MCM7 and PCNA, indicating that DNA replication occurs in these cells under nonpermissive conditions. Exogenous expression of replication factors Cdc6 or Cdt1 in RB-proficient cells does not recapitulate the RB-deficient cell phenotype. However, ectopic E2F expression in RB-proficient cells elevated ploidy and bypassed the response to nocodazole-induced cessation of DNA replication in a manner analogous to RB loss. Collectively, these results demonstrate that deregulated S phase control is a key mechanism by which RB-deficient cells acquire elevated ploidy.

Reverse genetic analysis of the yeast RSC chromatin remodeler reveals a role for RSC3 and SNF5 homolog 1 in ploidy maintenance

The yeast “remodels the structure of chromatin” (RSC) complex is a multi-subunit “switching deficient/sucrose non-fermenting” type ATP-dependent nucleosome remodeler, with human counterparts that are well-established tumor suppressors. Using temperature-inducible degron fusions of all the essential RSC subunits, we set out to map RSC requirement as a function of the mitotic cell cycle. We found that RSC executes essential functions during G1, G2, and mitosis. Remarkably, we observed a doubling of chromosome complements when degron alleles of the RSC subunit SFH1, the yeast hSNF5 tumor suppressor ortholog, and RSC3 were combined. The requirement for simultaneous deregulation of SFH1 and RSC3 to induce these ploidy shifts was eliminated by knockout of the S-phase cyclin CLB5 and by transient depletion of replication origin licensing factor Cdc6p. Further, combination of the degron alleles of SFH1 and RSC3, with deletion alleles of each of the nine Cdc28/Cdk1-associated cyclins, revealed a strong and specific genetic interaction between the S-phase cyclin genes CLB5 and RSC3, indicating a role for Rsc3p in proper S-phase regulation. Taken together, our results implicate RSC in regulation of the G1/S-phase transition and establish a hitherto unanticipated role for RSC-mediated chromatin remodeling in ploidy maintenance.

Some molecules responsible for altering the 3-D organization of chromosomes work as complexes of more than ten different proteins, and many are conserved in fungi, plants, and animals. Two such complexes are called “remodels the structure of chromatin” (RSC) in yeast and “switching deficient/sucrose non-fermenting” (SWI/SNF) in man. SWI/SNF is known to inhibit the advent of multiple types of human cancers. Since cancer is a disease whereby cells unduly divide, we sought to define when in the yeast cell division cycle RSC executes essential functions. Using a generic method to induce inactivation of essential proteins in otherwise healthy yeast cells, we found that the RSC complex is important before chromosome replication as well as before chromosome segregation. Interestingly, combining two of the mutations we had generated caused doubling of the entire chromosome complement of yeast. As it is known that such multiplication of the cellular chromosome complements results in an increased malleability of the genetic patrimony, which itself is known to underlie some of the aggressive traits of human cancers, our discovery suggests new models as to why SWI/SNF is such a potent tumor suppressor, and this may in turn provide valuable new inroads for cancer treatment.

Oligopotent Mesenchymal Stem Cell-Like Clone Becomes Multinucleated Following Phorbol Ester, TPA Stimulation

We established a mesenchymal stem cell clone, 5F9A, from rat bone marrow substrate adherent cells by repeated limiting dilutions. The cells have a fibroblastic shape and form intimate contacts with adjacent cells with interdigitations and junctions similar to adherence and tight junctions in a semi-confluent culture. Analysis of the phenotypes of these cells by RT-PCR and FACS demonstrated that they resembled mesenchymal stem cells, and the cells could differentiate into adiopocytes and osteoblasts under appropriate conditions in vitro showing their oligopotency. Furthermore, the cells were induced to become multinuclear cells by TPA (12-o-tetradecanoylphorbol 13-acetate) stimulation.

Subcellular translocation signals regulate Geminin activity during embryonic development

BACKGROUND INFORMATION

Geminin (Gem) is a protein with roles in regulating both the fidelity of DNA replication and cell fate during embryonic development. The distribution of Gem is predominantly nuclear in cells undergoing the cell cycle. Previous studies have demonstrated that Gem performs multiple activities in the nucleus and that regulation of Gem activation requires nuclear import in at least one context. In the present study, we defined structural and mechanistic features underlying subcellular localization of Gem and tested whether regulation of the subcellular localization of Gem has an impact on its activity in cell fate specification during embryonic development.

RESULTS

We determined that nuclear localization of Gem is dependent on a bipartite NLS (nuclear localization signal) in the N-terminus of Xenopus Gem protein. This bipartite motif mapped to a Gem N-terminal region previously shown to regulate neural cell fate acquisition. Microinjection into Xenopus embryos demonstrated that import-deficient Gem was incapable of modulating ectodermal cell fate, but that this activity was rescued by fusion to a heterologous NLS. Cross-species comparison of Gem protein sequences revealed that the Xenopus bipartite signal is conserved in many non-mammalian vertebrates, but not in mammalian species assessed. Instead, we found that human Gem employs an alternative N-terminal motif to regulate the protein's nuclear localization. Finally, we found that additional mechanisms contributed to regulating the subcellular localization of Gem. These included a link to Crm1-dependent nuclear export and the observation that Cdt1, a protein in the pre-replication complex, could also mediate nuclear import of Gem.

CONCLUSIONS

We have defined new structural and regulatory features of Gem, and showed that the activity of Gem in regulating cell fate, in addition to its cell-cycle-regulatory activity, requires control of its subcellular localization. Our data suggest that rather than being constitutively nuclear, Gem may undergo nucleocytoplasmic shuttling through several mechanisms involving distinct protein motifs. The use of multiple mechanisms for modulating Gem subcellular localization is congruent with observations that Gem levels and activity must be stringently controlled during cell-cycle progression and embryonic development.

Utilizing evolutionary information and gene expression data for estimating gene networks with bayesian network models

Since microarray gene expression data do not contain sufficient information for estimating accurate gene networks, other biological information has been considered to improve the estimated networks. Recent studies have revealed that highly conserved proteins that exhibit similar expression patterns in different organisms, have almost the same function in each organism. Such conserved proteins are also known to play similar roles in terms of the regulation of genes. Therefore, this evolutionary information can be used to refine regulatory relationships among genes, which are estimated from gene expression data. We propose a statistical method for estimating gene networks from gene expression data by utilizing evolutionarily conserved relationships between genes. Our method simultaneously estimates two gene networks of two distinct organisms, with a Bayesian network model utilizing the evolutionary information so that gene expression data of one organism helps to estimate the gene network of the other. We show the effectiveness of the method through the analysis on Saccharomyces cerevisiae and Homo sapiens cell cycle gene expression data. Our method was successful in estimating gene networks that capture many known relationships as well as several unknown relationships which are likely to be novel. Supplementary information is available at http://bonsai.ims.u-tokyo.ac.jp/~tamada/bayesnet/.

Biological characteristics of megakaryocytes: Specific lineage commitment and associated disorders

Megakayocytes (Megakaryocytes) are among the rarest type of haematopoietic cells and highly specialized precursors for platelets. Normal mature megakaryocytes are, perhaps, the largest cells in the marrow. In contrast, their annucleated platelets progeny are the smallest subcellular fragments in the circulation, which in spite of their size, play crucial roles in thrombostasis and haemostasis. Megakaryopoiesis involves complicated and multi-step biological processes. Research over the last decade has resulted in certain important new discoveries, such as the specific megakaryocyte-forming haematopoietic stem cell (HSC) subpopulation, thrombopoietin (Tpo), formation and release of platelets, etc. Substantial understanding of the specific lineage commitment, differentiation, and the molecular regulatory mechanisms of megakaryopoiesis has also been achieved. Despite existing controversies and questions, megakaryopoiesis remains an exciting field in biomedical research. Certain recent biological findings as well as future research in megakaryopoiesis are summarised in this article. Certain pathological changes associated with megakaryocytes, such as immune thrombocytopenia purpura (ITP), acute megakayoblastic leukaemia, etc., are also discussed in this article.

CDKs promote DNA replication origin licensing in human cells by protecting Cdc6 from APC/C-dependent proteolysis

Cyclin-dependent kinases (CDKs) restrict DNA replication origin firing to once per cell cycle by preventing the assembly of prereplicative complexes (pre-RCs; licensing) outside of G1 phase. Paradoxically, under certain circumstances, CDKs such as cyclin E-cdk2 are also required to promote licensing. Here, we show that CDK phosphorylation of the essential licensing factor Cdc6 stabilizes it by preventing its association with the anaphase promoting complex/cyclosome (APC/C). APC/C-dependent Cdc6 proteolysis prevents pre-RC assembly in quiescent cells and, when cells reenter the cell cycle from quiescence, CDK-dependent Cdc6 stabilization allows Cdc6 to accumulate before the licensing inhibitors geminin and cyclin A which are also APC/C substrates. This novel mechanism for regulating protein stability establishes a window of time prior to S phase when pre-RCs can assemble which we propose represents a critical function of cyclin E.

p53-Dependent regulation of Cdc6 protein stability controls cellular proliferation

Activation of tumor suppressor p53 in response to genotoxic stress imposes cellular growth arrest or apoptosis. We identified Cdc6, a licensing factor of the prereplication complex, as a novel target of the p53 pathway. We show that activation of p53 by DNA damage results in enhanced Cdc6 destruction by the anaphase-promoting complex. This destruction is triggered by inhibition of CDK2-mediated CDC6 phosphorylation at serine 54. Conversely, suppression of p53 expression results in stabilization of Cdc6. We demonstrate that loss of p53 results in more replicating cells, an effect that can be reversed by reducing Cdc6 protein levels. Collectively, our data suggest that initiation of DNA replication is regulated by p53 through Cdc6 protein stability.

Geminin-Cdt1 balance is critical for genetic stability

A cell limits its DNA replication activity to once per cell division cycle to maintain its genomic integrity. Studies in a variety of organisms are elucidating how these controls are exercised. Key amongst these is the regulation of replication initiator proteins such as Cdt1. Cdt1 is present in cells in G1 phase where it is required for initiation of replication. Once origins have fired, Cdt1 is either exported out of the nucleus or degraded, thereby preventing another round of replication. Higher eukaryotes have evolved another redundant mechanism, an inhibitor called geminin, to restrain Cdt1 activity. Studies in multiple organisms have shown that unregulated Cdt1 activity stimulates overreplication of the genome. Interestingly, the same seems to be true when geminin is depleted. The imbalance in the activities of these proteins causes the activation of key checkpoint proteins, the ATM/ATR kinases and the tumor suppressor, p53. This review proposes that a balance between Cdt1 and geminin is important for maintaining genomic stability.

Anaphase-promoting complex-dependent proteolysis of cell cycle regulators and genomic instability of cancer cells

Genomic instability can be found in most cancer cells. Cell proliferation is under tight control to ensure accurate DNA replication and chromosome segregation. Cyclin-dependent kinases (Cdks) and their activating subunits, the cyclins, are the driving forces of the cell division cycle. Regulation of cyclin oscillation by ubiquitin-dependent proteolysis thereby has a central role in cell cycle regulation. The anaphase-promoting complex (APC) is a specific ubiquitin ligase and is essential for chromosome segregation, exit from mitosis and a stable subsequent G1 phase allowing cell differentiation or accurate DNA replication in the following S phase. The APC is activated by the regulatory subunits Cdc20 (APC(Cdc20)) and Cdh1 (APC(Cdh1)) to target securin, mitotic cyclins and other cell cycle regulatory proteins for proteasomal degradation. This review is focused on the role of APC-dependent proteolysis in cell cycle regulation and how its deregulation may lead to genomic instability of cancer cells.

A novel E2 box-GATA element modulates Cdc6 transcription during human cells polyploidization

Cdc6 is a key regulator of the strict alternation of S and M phases during the mitotic cell cycle. In mammalian and plant cells that physiologically become polyploid, cdc6 is transcriptionally and post-translationally regulated. We have recently reported that Cdc6 levels are maintained in megakaryoblastic HEL cells, but severely downregulated by ectopic expression of transcriptional repressor Drosophila melanogaster escargot. Here, we show that cdc6 promoter activity is upregulated during megakaryocytic differentiation of HEL endoreplicating cells, and that Escargot interferes with such activation. Transactivation experiments showed that a 1.7 kb region located at 2800 upstream cdc6 transcription initiation site behaved as a potent enhancer in endoreplicating cells only. This activity was mainly dependent on a novel cis-regulatory element composed by an E2 box overlapping a GATA motif. Ectopic Escargot could bind this regulatory element in vitro and endogenous GATA-1 and E2A formed specific complexes in megakaryoblastic cells as well as in primary megakaryocytes. Chromatin Immunoprecipitation analysis revealed that both transcription factors were occupying the E2 box/GATA site in vivo. Altogether, these data suggest that cdc6 expression could be actively maintained during megakaryocytic differentiation through transcriptional mechanisms involving specific cis- and trans-regulatory elements.

Overexpression of the replication licensing regulators hCdt1 and hCdc6 characterizes a subset of non-small-cell lung carcinomas: synergistic effect with mutant p53 on tumor growth and chromosomal instability--evidence of E2F-1 transcriptional control over hCdt1

Replication licensing ensures once per cell cycle replication and is essential for genome stability. Overexpression of two key licensing factors, Cdc6 and Cdt1, leads to overreplication and chromosomal instability (CIN) in lower eukaryotes and recently in human cell lines. In this report, we analyzed hCdt1, hCdc6, and hGeminin, the hCdt1 inhibitor expression, in a series of non-small-cell lung carcinomas, and investigated for putative relations with G(1)/S phase regulators, tumor kinetics, and ploidy. This is the first study of these fundamental licensing elements in primary human lung carcinomas. We herein demonstrate elevated levels (more than fourfold) of hCdt1 and hCdc6 in 43% and 50% of neoplasms, respectively, whereas aberrant expression of hGeminin was observed in 49% of cases (underexpression, 12%; overexpression, 37%). hCdt1 expression positively correlated with hCdc6 and E2F-1 levels (P = 0.001 and P = 0.048, respectively). Supportive of the observed link between E2F-1 and hCdt1, we provide evidence that E2F-1 up-regulates the hCdt1 promoter in cultured mammalian cells. Interestingly, hGeminin overexpression was statistically related to increased hCdt1 levels (P = 0.025). Regarding the kinetic and ploidy status of hCdt1- and/or hCdc6-overexpressing tumors, p53-mutant cases exhibited significantly increased tumor growth values (Growth Index; GI) and aneuploidy/CIN compared to those bearing intact p53 (P = 0.008 for GI, P = 0.001 for CIN). The significance of these results was underscored by the fact that the latter parameters were independent of p53 within the hCdt1-hCdc6 normally expressing cases. Cumulatively, the above suggest a synergistic effect between hCdt1-hCdc6 overexpression and mutant-p53 over tumor growth and CIN in non-small-cell lung carcinomas.

Identification of BRCA1-IRIS, a BRCA1 locus product

Breast cancer is the most common malignancy among women, and mutations in the BRCA genes produce increased susceptibility to these malignancies in certain families. Here we identify BRCA1-IRIS as a 1,399-amino-acid BRCA1 gene product encoded by an uninterrupted open reading frame that extends from codon 1 of the known BRCA1 open reading frame to a termination point 34 triplets into intron 11. Unlike full-length BRCA1 (p220), BRCA1-IRIS is exclusively chromatin-associated, fails to interact with BARD1 in vivo or in vitro and exhibits unique nuclear immunostaining. Unlike BRCA1FL (or p220), BRCA1-IRIS also co-immunoprecipitated with DNA-replication-licensing proteins and with known replication initiation sites. Suppression of BRCA1-IRIS expression hindered the normal departure of geminin from pre-replication complexes, and depressed the rate of cellular DNA replication and possibly initiation-related synthesis. In contrast, BRCA1-IRIS overexpression stimulated DNA replication. These data imply that endogenous BRCA1-IRIS positively influences the DNA replication initiation machinery.

DNA replication licensing affects cell proliferation or endoreplication in a cell type-specific manner

In eukaryotic cells, the function of DNA replication licensing components (Cdc6 and Cdt1, among others) is crucial for cell proliferation and genome stability. However, little is known about their role in whole organisms and whether licensing control interfaces with differentiation and developmental programs. Here, we study Arabidopsis thaliana CDT1, its regulation, and the consequences of overriding licensing control. The availability of AtCDT1 is strictly regulated at two levels: (1) at the transcription level, by E2F and growth-arresting signals, and (2) posttranscriptionally, by CDK phosphorylation, a step that is required for its proteasome-mediated degradation. We also show that CDC6 and CDT1 are key targets for the coordination of cell proliferation, differentiation, and development. Indeed, altered CDT1 or CDC6 levels have cell type-specific effects in developing Arabidopsis plants: in leaf cells competent to divide, cell proliferation is stimulated, whereas in cells programmed to undergo differentiation-associated endoreplication rounds, extra endocycles are triggered. Thus, we propose that DNA replication licensing control is critical for the proper maintenance of proliferative potential, developmental programs, and morphogenetic patterns.

Bovine papillomavirus replicative helicase E1 is a target of the ubiquitin ligase APC

The papillomavirus E1 replicative helicase is essential for replication and maintenance of extrachromosomal viral genomes in infected cells. We previously found that the bovine papillomavirus E1 protein is a substrate of the ubiquitin-dependent proteolytic pathway. Here we show that E1 is targeted for degradation by the anaphase-promoting complex (APC). Inhibition of APC activity by the specific inhibitor Emi1 or point mutations in the D-box and KEN-box motifs of E1 stabilize the protein and increase viral DNA replication in both a cell-free system and in living cells. These findings involve APC as the ubiquitin ligase that controls E1 levels to maintain a constant low copy number of the viral genome during latent infection.

Caspase-3-mediated cleavage of Cdc6 induces nuclear localization of p49-truncated Cdc6 and apoptosis

We show that Cdc6, an essential initiation factor for DNA replication, undergoes caspase-3-mediated cleavage in the early stages of apoptosis in HeLa cells and SK-HEP-1 cells induced by etoposide, paclitaxel, ginsenoside Rh2, or tumor necrosis factor-related apoptosis-inducing ligand. The cleavage occurs at the SEVD442/G motif and generates an N-terminal truncated Cdc6 fragment (p49-tCdc6) that lacks the carboxy-terminal nuclear export sequence. Cdc6 is known to be phosphorylated by cyclin A-cyclin dependent kinase 2 (Cdk2), an event that promotes its exit from the nucleus and probably blocks it from initiating inappropriate DNA replication. In contrast, p49-tCdc6 translocation to the cytoplasm is markedly reduced under the up-regulated conditions of Cdk2 activity, which is possibly due to the loss of nuclear export sequence. Thus, truncation of Cdc6 results in an increased nuclear retention of p49-tCdc6 that could act as a dominant negative inhibitor of DNA replication and its accumulation in the nucleus could promote apoptosis. Supporting this is that the ectopic expression of p49-tCdc6 not only promotes apoptosis of etoposide-induced HeLa cells but also induces apoptosis in untreated cells. Thus, the caspase-mediated cleavage of Cdc6 creates a truncated Cdc6 fragment that is retained in the nucleus and induces apoptosis.