Conservation of the Chk1 checkpoint pathway in mammals: linkage of DNA damage to Cdk regulation through Cdc25

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.

Phosphatase 2A and polo kinase, two antagonistic regulators of cdc25 activation and MPF auto-amplification

The auto-catalytic activation of the cyclin-dependent kinase Cdc2 or MPF (M-phase promoting factor) is an irreversible process responsible for the entry into M phase. In Xenopus oocyte, a positive feed-back loop between Cdc2 kinase and its activating phosphatase Cdc25 allows the abrupt activation of MPF and the entry into the first meiotic division. We have studied the Cdc2/Cdc25 feed-back loop using cell-free systems derived from Xenopus prophase-arrested oocyte. Our findings support the following two-step model for MPF amplification: during the first step, Cdc25 acquires a basal catalytic activity resulting in a linear activation of Cdc2 kinase. In turn Cdc2 partially phosphorylates Cdc25 but no amplification takes place; under this condition Plx1 kinase and its activating kinase, Plkk1 are activated. However, their activity is not required for the partial phosphorylation of Cdc25. This first step occurs independently of PP2A or Suc1/Cks-dependent Cdc25/Cdc2 association. On the contrary, the second step involves the full phosphorylation and activation of Cdc25 and the initiation of the amplification loop. It depends both on PP2A inhibition and Plx1 kinase activity. Suc1-dependent Cdc25/Cdc2 interaction is required for this process.

Role of human Cds1 (Chk2) kinase in DNA damage checkpoint and its regulation by p53

In response to DNA damage, mammalian cells adopt checkpoint regulation, by phosphorylation and stabilization of p53, to delay cell cycle progression. However, most cancer cells that lack functional p53 retain an unknown checkpoint mechanism(s) by which cells are arrested at the G(2)/M phase. Here we demonstrate that a human homolog of Cds1/Rad53 kinase (hCds1) is rapidly phosphorylated and activated in response to DNA damage not only in normal cells but in cancer cells lacking functional p53. A survey of various cancer cell lines revealed that the expression level of hCds1 mRNA is inversely related to the presence of functional p53. In addition, transfection of normal human fibroblasts with SV40 T antigen or human papilloma viruses E6 or E7 causes a marked induction of hCds1 mRNA, and the introduction of functional p53 into SV40 T antigen- and E6-, but not E7-, transfected cells decreases the hCds1 level, suggesting that p53 negatively regulates the expression of hCds1. In cells without functional ataxia telangiectasia mutated (ATM) protein, phosphorylation and activation of hCds1 were observed in response to DNA damage induced by UV but not by ionizing irradiation. These results suggest that hCds1 is activated through an ATM-dependent as well as -independent pathway and that it may complement the function of p53 in DNA damage checkpoints in mammalian cells.

The physical association and phosphorylation of Cdc25C protein phosphatase by Prk

prk encodes a protein serine/threonine kinase involved in regulating M phase functions during the cell cycle. We have expressed His6-Prk and His6-Cdc25C proteins using the baculoviral vector expression system. Purified recombinant His6-Prk, but not a kinase-defective mutant His6-PrkK52R, is capable of strongly phosphorylating His6-Cdc25C in vitro. Co-immunoprecipitation and affinity column chromatography experiments demonstrate that GST-Prk and native Cdc25C interact. When co-infected with His6-Prk and His6-Cdc25C recombinant baculoviruses, sf-9 cells produce His6-Cdc25C antigen with an additional slower mobility band on denaturing polyacrylamide gels compared with cells infected with His6-Cdc25C baculovirus alone. In addition, His6-Cdc25C immunoprecipitated from sf-9 cells co-infected with His6-Prk and His6-Cdc25C baculoviruses, but not with His6-PrkK52R and His6-Cdc25C baculoviruses, contains a greatly enhanced kinase activity that phosphorylates His6-Cdc25C in vitro. Moreover, phosphopeptide mapping shows that His6-Prk phosphorylates His6-Cdc25C at two sites in vitro and that the major phosphorylation site co-migrates with the one that is phosphorylated in vivo in asynchonized cells. Further studies reveal that His6-Prk phosphorylates Cdc25C on serine216, a residue also phosphorylated by Chk1 and Chk2. Together, these observations strongly suggest that Prk's role in mitosis is at least partly mediated through direct regulation of Cdc25C.

Human cyclin A is required for mitosis until mid prophase

We have used microinjection and time-lapse video microscopy to study the role of cyclin A in mitosis. We have injected purified, active cyclin A/cyclin-dependent kinase 2 (CDK2) into synchronized cells at specific points in the cell cycle and assayed its effect on cell division. We find that cyclin A/CDK2 will drive G2 phase cells into mitosis within 30 min of microinjection, up to 4 h before control cells enter mitosis. Often this premature mitosis is abnormal; the chromosomes do not completely condense and daughter cells fuse. Remarkably, microinjecting cyclin A/CDK2 into S phase cells has no effect on progress through the following G2 phase or mitosis. In complementary experiments we have microinjected the amino terminus of p21(Cip1/Waf1/Sdi1) (p21N) into cells to inhibit cyclin A/CDK2 activity. We find that p21N will prevent S phase or G2 phase cells from entering mitosis, and will cause early prophase cells to return to interphase. These results suggest that cyclin A/CDK2 is a rate-limiting component required for entry into mitosis, and for progress through mitosis until late prophase. They also suggest that cyclin A/CDK2 may be the target of the recently described prophase checkpoint.

Caffeine inhibits the checkpoint kinase ATM

The basis of many anti-cancer therapies is the use of genotoxic agents that damage DNA and thus kill dividing cells. Agents that cause cells to override the DNA-damage checkpoint are predicted to sensitize cells to killing by genotoxic agents. They have therefore been sought as adjuncts in radiation therapy and chemotherapy. One such compound, caffeine, uncouples cell-cycle progression from the replication and repair of DNA [1] [2]. Caffeine therefore servers as a model compound in establishing the principle that agents that override DNA-damage checkpoints can be used to sensitize cells to the killing effects of genotoxic drugs [3]. But despite more than 20 years of use, the molecular mechanisms by which caffeine affects the cell cycle and checkpoint responses have not been identified. We investigated the effects of caffeine on the G2/M DNA-damage checkpoint in human cells. We report that the radiation-induced activation of the kinase Cds1 [4] (also known as Chk2 [5]) is inhibited by caffeine in vivo and that ATM kinase activity is directly inhibited by caffeine in vitro. Inhibition of ATM provides a molecular explanation of the attenuation of DNA-damage checkpoint responses and for the increased radiosensitivity of caffeine-treated cells [6] [7] [8].

Meiotic DNA replication checkpoint control in fission yeast

In eukaryotes, the DNA replication checkpoint prevents entry into mitosis when DNA replication is incomplete and is crucial for maintaining genomic integrity. Much less is known about equivalent controls that operate during meiosis. Here, we show that a DNA replication checkpoint control operates during meiosis in fission yeast. The mitotic checkpoint Rad genes and the Cds1 protein kinase are required for the DNA replication checkpoint during meiosis, with Cds1 playing a more prominent role than it does during mitosis. When DNA replication is blocked, the checkpoint maintains Cdc2 tyrosine 15 phosphorylation keeping Cdc2 protein kinase activity low and preventing onset of meiosis I. Additionally, there is a second checkpoint acting during meiosis that is revealed if cells are prevented from maintaining Cdc2 tyrosine 15 phosphorylation when DNA replication is blocked. Such cells arrest with high Cdc2 protein kinase activity and separated spindle pole bodies, an arrest state similar to that observed in mitotic budding yeast cells when DNA replication is incomplete. This second checkpoint is meiosis specific and may reflect processes occurring only during meiosis such as increased recombination rates, an extended duration of nuclear division, or homolog chromosome pairing.

Cell cycle checkpoints as therapeutic targets

Most human breast tumors arise from multiple genetic changes which gradually transform differentiated and growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal breast cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell cycle arrest in response to DNA damage is an important part of the mechanism used to maintain genomic integrity. The control mechanisms that restrain cell cycle transition after DNA damage are known as cell cycle checkpoints. This review will focus on cell cycle checkpoint signaling pathways commonly mutated in human breast tumors and suggest how different components of these checkpoint pathways offer the potential for chemotherapeutic intervention.

A quantitative analysis of the kinetics of the G(2) DNA damage checkpoint system

A detailed model of the G(2) DNA damage checkpoint (G2DDC) system is presented that includes complex regulatory networks of the mitotic kinase Cdc2, phosphatase Cdc25, Wee1 kinase, and damage signal transduction pathways involving Chk1 and p53. Assumptions on the kinetic equations of the G2DDC are made, and computer simulations are carried out to demonstrate how the various subsystems operate to delay or arrest cell cycle progression. The detailed model could be used to explain various experiments relevant to G2DDC reported recently, including the nuclear export of 14-3-3-bound Cdc25, the down-regulation of cyclin B1 expression by p53, the effect of Chk1 and p53 on Cdc25 levels, and Wee1 degradation. It also is shown that, under certain conditions, p53 is necessary to sustain a G(2) arrest.

Increased susceptibility to carcinogen-induced mammary tumors in MMTV-Cdc25B transgenic mice

Cdc25 phosphatases activate cyclin-dependent kinases (Cdks) by dephosphorylating critical phospho-tyrosine and phospho-threonine residues on these proteins. Several types of studies indicate that Cdc25s can enhance cell proliferation and oncogenesis. Furthermore, overexpression of Cdc25A and/or B have been detected in several types of primary human cancers, including breast cancers. To further assess the oncogenic capacity of Cdc25B in vivo, we have generated transgenic mice that overexpress Cdc25B in the mammary epithelium, driven by the MMTV - LTR promoter. Although these mice are grossly normal for up to 18 months, the ectopic expression of Cdc25B in their mammary glands increases the susceptibility of these mice to induction of mammary tumors by the carcinogen 9,10-dimethyl-1, 2-benzanthracene (DMBA).

Rad18 is required for DNA repair and checkpoint responses in fission yeast

To survive damage to the genome, cells must respond by activating both DNA repair and checkpoint responses. Using genetic screens in the fission yeast Schizosaccharomyces pombe, we recently isolated new genes required for DNA damage checkpoint control. We show here that one of these strains defines a new allele of the previously described rad18 gene, rad18-74. rad18 is an essential gene, even in the absence of extrinsic DNA damage. It encodes a conserved protein related to the structural maintenance of chromosomes proteins. Point mutations in rad18 lead to defective DNA repair pathways responding to both UV-induced lesions and, as we show here, double-stranded breaks. Furthermore, rad18p is required to maintain cell cycle arrest in the presence of DNA damage, and failure of this leads to highly aberrant mitoses. A gene encoding a BRCT-containing protein, brc1, was isolated as an allele-specific high-copy suppressor of rad18-74. brc1 is required for mitotic fidelity and for cellular viability in strains with rad18 mutations but is not essential for DNA damage responses. Mutations in rad18 and brc1 are synthetically lethal with a topoisomerase II mutant (top2-191), indicating that these proteins play a role in chromatin organization. These studies show a role for chromatin organization in the maintenance or activation of responses to DNA damage.

Pattern formation in the absence of cell proliferation: tissue-specific regulation of cell cycle progression by string (stg) during Drosophila eye development

During Drosophila eye development, the posterior-to-anterior movement of the morphogenetic furrow coordinates cell cycle progression with the early events of pattern formation. The cdc25 phosphatase string (stg) has been proposed to contribute to the synchronization of retinal precursors anterior to the furrow by driving cells in G(2) through mitosis and into a subsequent G(1). Genetic and molecular analysis of Drop (Dr) mutations suggests that they represent novel cis-regulatory alleles of stg that inactivate expression in eye. Retinal precursors anterior to the furrow lacking stg arrest in G(2) and fail to enter mitosis, while cells within the furrow accumulate high levels of cyclins A and B. Although G(2)-arrested cells initiate normal pattern formation, the absence of stg results in retinal patterning defects due to the recruitment of extra photoreceptor cells. These results demonstrate a requirement for stg in cell cycle regulation and cell fate determination during eye development.

The G(2) checkpoint is maintained by redundant pathways

While p53 activity is critical for a DNA damage-induced G(1) checkpoint, its role in the G(2) checkpoint has not been compelling because cells lacking p53 retain the ability to arrest in G(2) following DNA damage. Comparison between normal human foreskin fibroblasts (HFFs) and HFFs in which p53 was eliminated by transduction with human papillomavirus type 16 E6 showed that treatment with adriamycin initiated arrest in G(2) with active cyclin B/CDC2 kinase, regardless of p53 status. Both E6-transduced HFFs and control (LXSN)-transduced cells maintained a prolonged arrest in G(2); however cells with functional p53 extinguished cyclin B-associated kinase activity. Down regulation was mediated by p53-dependent transcriptional repression of the CDC2 and cyclin B promoters. In contrast, cells lacking p53 showed a prolonged G(2) arrest despite high levels of cyclin B/CDC2 kinase activity, at least some of which translocated into the nucleus. Furthermore, the G(2) checkpoint became attenuated as p53-deficient cells aged in culture. Thus, at late passage, E6-transduced HFFs entered mitosis following DNA damage, whereas the age-matched parental HFFs sustained a G(2) arrest. These results indicate that normal cells have p53-independent pathways to maintain DNA damage-induced G(2) arrest, which may be augmented by p53-dependent functions, and that cells lacking p53 are at greater risk of losing the pathway that protects against aneuploidy.

Drosophila grapes/CHK1 mutants are defective in cyclin proteolysis and coordination of mitotic events

The Drosophila grapes (grp) gene, which encodes a homolog of the Schizosaccharomyces pombe Chk1 kinase, provides a cell-cycle checkpoint that delays mitosis in response to inhibition of DNA replication [1]. Grp is also required in the undisturbed early embryonic cycles: in its absence, mitotic abnormalities appear in cycle 12 and chromosomes fail to fully separate in subsequent cycles [2] [3]. In other systems, Chk1 kinase phosphorylates and suppresses the activity of Cdc25 phosphatase: the resulting failure to remove inhibitory phosphate from cyclin-dependent kinase 1 (Cdk1) prevents entry into mitosis [4] [5]. Because in Drosophila embryos Cdk1 lacks inhibitory phosphate during cycles 11-13 [6], it is not clear that known actions of Grp/Chk1 suffice in these cycles. We found that the loss of grp compromised cyclin A proteolysis and delayed mitotic disjunction of sister chromosomes. These defects occurred before previously reported grp phenotypes. We conclude that Grp activates cyclin A degradation, and functions to time the disjunction of chromosomes in the early embryo. As cyclin A destruction is required for sister chromosome separation [7], a failure in Grp-promoted cyclin destruction can also explain the mitotic phenotype. The mitotic failure described previously for cycle 12 grp embryos might be a more severe form of the phenotypes that we describe in earlier embryos and we suggest that the underlying defect is reduced degradation of cyclin A.

Inhibition of cell proliferation by lithium is associated with interference in cdc2 activation

Lithium can interfere with embryonal development in a variety of organisms. We investigated the effect of lithium on the proliferation of early embryonal cells. [3H]Thymidine incorporation of non-committed mouse P19 embryonal carcinoma cells was inhibited by lithium treatment. Similar effects were seen in a variety of other cells. This growth inhibition occurred in the G2 phase, since cells accumulated with a 4N DNA content, but the appearance of mitotic cells was blocked. Lithium could also prevent the activation of cdc2, thereby inhibiting cyclin B/cdc2 kinase activity. These data indicate that lithium might disturb embryonal development through interference in embryonal cell cycle regulation.

Cdc25B and Cdc25C differ markedly in their properties as initiators of mitosis

We have used time-lapse fluorescence microscopy to study the properties of the Cdc25B and Cdc25C phosphatases that have both been implicated as initiators of mitosis in human cells. To differentiate between the functions of the two proteins, we have microinjected expression constructs encoding Cdc25B or Cdc25C or their GFP-chimeras into synchronized tissue culture cells. This assay allows us to express the proteins at defined points in the cell cycle. We have followed the microinjected cells by time-lapse microscopy, in the presence or absence of DNA synthesis inhibitors, and assayed whether they enter mitosis prematurely or at the correct time. We find that overexpressing Cdc25B alone rapidly causes S phase and G2 phase cells to enter mitosis, whether or not DNA replication is complete, whereas overexpressing Cdc25C does not cause premature mitosis. Overexpressing Cdc25C together with cyclin B1 does shorten the G2 phase and can override the unreplicated DNA checkpoint, but much less efficiently than overexpressing Cdc25B. These results suggest that Cdc25B and Cdc25C do not respond identically to the same cell cycle checkpoints. This difference may be related to the differential localization of the proteins; Cdc25C is nuclear throughout interphase, whereas Cdc25B is nuclear in the G1 phase and cytoplasmic in the S and G2 phases. We have found that the change in subcellular localization of Cdc25B is due to nuclear export and that this is dependent on cyclin B1. Our data suggest that although both Cdc25B and Cdc25C can promote mitosis, they are likely to have distinct roles in the controlling the initiation of mitosis.

Abnormal accumulation of G2/M-phase cells from LEC strain rats after X-irradiation at S phase

The effect of X-irradiation on the progression of the cell cycle in cell lines from LEC and WKAH rats was investigated by a flow cytometer. When the cells were exposed to 5 Gy of X-rays at S phase, the proportion of S-phase cells in both cell populations decreased with incubation time and that of G2/M-phase cells was approximately 80% at 6 hr post-irradiation. At 12 hr post-irradiation, approximately 45% of the WKAH rat cells appeared in the G1 phase. However, 80-90% of LEC rat cells remained in the G2/M phase and less than 5% in the G1 phase during 6-12 hr post-irradiation. Thus, the LEC rat cells irradiated at S phase remained in the G2/M phase for at least 6 hr longer than did the WKAH rat cells.

Molecular interaction map of the mammalian cell cycle control and DNA repair systems

Eventually to understand the integrated function of the cell cycle regulatory network, we must organize the known interactions in the form of a diagram, map, and/or database. A diagram convention was designed capable of unambiguous representation of networks containing multiprotein complexes, protein modifications, and enzymes that are substrates of other enzymes. To facilitate linkage to a database, each molecular species is symbolically represented only once in each diagram. Molecular species can be located on the map by means of indexed grid coordinates. Each interaction is referenced to an annotation list where pertinent information and references can be found. Parts of the network are grouped into functional subsystems. The map shows how multiprotein complexes could assemble and function at gene promoter sites and at sites of DNA damage. It also portrays the richness of connections between the p53-Mdm2 subsystem and other parts of the network.

Mammalian Chk2 is a downstream effector of the ATM-dependent DNA damage checkpoint pathway

In response to DNA damage and replication blocks, cells activate pathways that arrest the cell cycle and induce the transcription of genes that facilitate repair. In mammals, ATM (ataxia telangiectasia mutated) kinase together with other checkpoint kinases are important components in this response. We have cloned the rat and human homologs of Saccharomyces cerevisiae Rad 53 and Schizosaccharomyces pombe Cds1, called checkpoint kinase 2 (chk2). Complementation studies suggest that Chk2 can partially replace the function of the defective checkpoint kinase in the Cds1 deficient yeast strain. Chk2 was phosphorylated and activated in response to DNA damage in an ATM dependent manner. Its activation in response to replication blocks by hydroxyurea (HU) treatment, however, was independent of ATM. Using mass spectrometry, we found that, similar to Chk1, Chk2 can phosphorylate serine 216 in Cdc25C, a site known to be involved in negative regulation of Cdc25C. These results suggest that Chk2 is a downstream effector of the ATM-dependent DNA damage checkpoint pathway. Activation of Chk2 might not only delay mitotic entry, but also increase the capacity of cultured cells to survive after treatment with gamma-radiation or with the topoisomerase-I inhibitor topotecan.

The topoisomerase I poison camptothecin generates a Chk1-dependent DNA damage checkpoint signal in fission yeast

The protein kinase Chk1 is essential for the DNA damage checkpoint. Cells lacking Chk1 are hypersensitive to DNA-damaging agents such as UV light and gamma-irradiation because they fail to arrest the cell cycle when DNA damage is generated. Phosphorylation of Chk1 occurs after DNA damage and is dependent on the integrity of the DNA damage checkpoint pathway. We have tested whether a topoisomerase I inhibitor, camptothecin (CPT), generates DNA damage in the fission yeast Schizosaccharomyces pombe that results in Chk1 phosphorylation. We demonstrate that Chk1 is phosphorylated in response to CPT treatment in a time- and dose-dependent manner and that phosphorylation is dependent on an intact DNA damage checkpoint pathway. Furthermore, we show that cells must be actively dividing in order for CPT to generate a Chk1-responsive DNA damage signal. This observation is consistent with a model whereby the cytotoxic event caused by CPT treatment is the production of a DNA double-strand break resulting from the collision of a DNA replication fork with a trapped CPT-topoisomerase I cleavable complex. Cells lacking Chk1 are hypersensitive to CPT treatment, suggesting that the DNA damage checkpoint pathway can be an important determinant for CPT sensitivity or resistance. Finally, as a well-characterized, soluble agent that specifically causes DNA damage, CPT will allow a biochemical analysis of the checkpoint pathway that responds to DNA damage.

Four-dimensional control of the cell cycle

The cell-division cycle has to be regulated in both time and space. In the time dimension, the cell ensures that mitosis does not begin until DNA replication is completed and any damaged DNA is repaired, and that DNA replication normally follows mitosis. This is achieved by the synthesis and destruction of specific cell-cycle regulators at the right time in the cell cycle. In the spatial dimension, the cell coordinates dramatic reorganizations of the subcellular architecture at the entrance to and exit from mitosis, largely through the actions of protein kinases and phosphatases that are often localized to specific subcellular structures. Evidence is now accumulating to suggest that the spatial organization of cell-cycle regulators is also important in the temporal control of the cell cycle. Here I will focus on how the locations of the main components of the cell-cycle machinery are regulated as part of the mechanism by which the cell controls when and how it replicates and divides.

Mouse Hus1, a homolog of the Schizosaccharomyces pombe hus1+ cell cycle checkpoint gene

Cell cycle checkpoints are regulatory mechanisms that arrest the cell cycle or initiate programmed cell death when critical events such as DNA replication fail to be completed or when DNA or spindle damage occurs. In fission yeast, cell cycle checkpoint responses to DNA replication blocks and DNA damage require the hus1+ gene. Mammalian homologs of hus1+ were recently identified, and here we report a detailed analysis of mouse Hus1. An approximately 4.2-kb full-length cDNA encoding the 32-kDa mouse Hus1 protein was isolated. The genomic structure and exon-intron boundary sequences of the gene were determined, and mouse Hus1 was found to consist of nine exons. Mouse Hus1 was mapped to the proximal end of chromosome 11 and is therefore a candidate gene for the mouse mutation germ cell deficient, which maps to the same genomic region. Finally, mouse Hus1 was found to be expressed in a variety of adult tissues and at several stages of embryonic development.

Cell-cycle-dependent and ATM-independent expression of human Chk1 kinase

Checkpoint genes cause cell cycle arrest when DNA is damaged or DNA replication is blocked. Although a human homolog of Chk1 (hChk1) has recently been reported to be involved in the DNA damage checkpoint through phosphorylation of Cdc25A, B, and C, it is not known at which phase(s) of the cell cycle hChk1 functions and how hChk1 causes cell cycle arrest in response to DNA damage. In the present study, we demonstrate that in normal human fibroblasts (MJ90), hChk1 is expressed specifically at the S to M phase of the cell cycle at both the RNA and protein levels and that it is localized to the nucleus at this time. hChk1 activity, as determined by phosphorylation of Cdc25C, is readily detected at the S to M phase of the cell cycle, and DNA damage induced by UV or ionizing radiation does not enhance the expression of hChk1 or its activity. Furthermore, hChk1 exists in an active form at the S to M phase in fibroblasts derived from patients with ataxia telangiectasia (AT) which lack the functional AT mutated (ATM) gene product, suggesting that hChk1 expression is independent of functional ATM. Taken together with the findings that phosphorylation of Cdc25C on serine 216 is increased at the S to M phase, it is suggested that at this particular phase of the cell cycle, even in the absence of DNA damage, hChk1 phosphorylates Cdc25C on serine 216, which is considered to be a prerequisite for the G2/M checkpoint. Thus, hChk1 may play an important role in keeping Cdc25C prepared for responding to DNA damage by phosphorylating its serine residue at 216 during the S to M phase.

Defective regulation of Ca2+/calmodulin-dependent protein kinase II in gamma-irradiated ataxia telangiectasia fibroblasts

Recent indirect evidence suggests that a Ca2+/ calmodulin-dependent pathway, which may involve calmodulin-dependent protein kinase II (CaMKII), mediates the S-phase delay manifested by gamma-ray-exposed human fibroblasts. This pathway is severely impaired in ataxia telangiectasia (A-T) cells [Mirzayans et al. (1995) Oncogene 11, 15971. To extend these findings, we assayed CaMKII activity in irradiated normal and A-T fibroblasts. The radiation treatment induced the autonomous activity of the kinase in normal cells. In contrast, this activity was not elevated in either (i) normal cells pretreated with the selective CaMKII antagonist KN-62 or (ii) gamma-irradiated A-T cells. Moreover, A-T fibroblasts, unlike normal cells, failed to mobilize intracellular Ca2+ upon mitogenic stimulation. These findings identify a novel role for CaMKII in radiation-induced signal transduction and suggest its involvement in effecting the S-phase delay. The data also implicate ATM, the product of the gene responsible for A-T, as a key mediator of both intracellular Ca2+ mobilization and CaMKII activation in response not only to genotoxic stress but also to physiological stimuli.

Dissociation of CDK2 from cyclin A in response to the topoisomerase II inhibitor etoposide in v-src-transformed but not normal NIH 3T3 cells

Our previous work has demonstrated that treatment of NIH 3T3 cells with etoposide (VP16), an inhibitor of DNA topoisomerase II and widely used anticancer agent, results in G2/M-phase arrest, whereas treatment of cells transformed by v-src, v-ras, or v-raf results in an S-phase blockage. The present studies describe the mechanistic aspects of this selective S-phase arrest in the v-src-transformed cells. The S-phase arrest in these cells was found to be coupled with depletion of cyclin A-dependent kinase activity. This decrease could not be explained by changes in the overall level of cyclin A, CDK2, p27, or p21 proteins. Rather, it was associated with a time-dependent reduction of CDK2 protein complexed with cyclin A following VP16 treatment. It was further shown that the decrease of cyclin A-associated CDK2 was linked to an increase of CDK2 protein in cyclin E immunocomplexes, which suggests that CDK2 might become redistributed following treatment with VP16. Thus, oncogenic transformation by v-src can trigger separation of CDK2 protein from cyclin A in response to VP16. This might contribute to the depletion of cyclin A-dependent kinase activity and the selective S-phase arrest by VP16 in v-src-transformed cells.

Design and synthesis of a peptide derived from positions 195-244 of human cdc25C phosphatase

We have designed, synthesized and purified a 51 amino acid peptide derived from an essential domain of human cdc25C phosphatase. In vivo, differential phosphorylation of this domain regulates either the induction of mitotic processes, or the checkpoint arrest of eukaryotic cells in response to DNA damage. Peptide synthesis was achieved using the stepwise Fmoc strategy and resulted in an important yield of highly pure peptide. The final peptide was identified by amino acid analysis, electrospray mass spectrometry and nuclear magnetic resonance, which revealed that one of the two methionines within the peptide was oxidized into its sulphoxide derivative We investigated whether this 51 amino acid peptide folded into secondary structures in solution by circular dichroism and observed the formation of alpha helices in TFE. Finally, we verified that this peptide could bind to its biologically relevant 14-3-3 partner in vitro by fluorescence spectroscopy.

Cytoplasmic localization of human cdc25C during interphase requires an intact 14-3-3 binding site

cdc25C induces mitosis by activating the cdc2-cyclin B complex. The intracellular localization of cyclin B1 is regulated in a cell cycle-specific manner, and its entry into the nucleus may be required for the initiation of mitosis. To determine the cellular localization of cdc25C, monoclonal antibodies specific for cdc25C were developed and used to demonstrate that in human cells, cdc25C is retained in the cytoplasm during interphase. A deletion analysis identified a 58-amino-acid region (amino acids 201 to 258) in cdc25C that was required for the cytoplasmic localization of cdc25C. This region contained a specific binding site for 14-3-3 proteins, and mutations in cdc25C that disrupted 14-3-3 binding also disrupted the cytoplasmic localization of cdc25C during interphase. cdc25C proteins that do not contain a binding site for 14-3-3 proteins showed a pancellular localization and an increased ability to induce premature chromosome condensation. The cytoplasmic localization of cdc25C was not altered by gamma irradiation or treatment with the nuclear export inhibitor leptomycin B. These results suggest that 14-3-3 proteins may negatively regulate cdc25C function by sequestering cdc25C in the cytoplasm.

Inhibition of p34cdc2 dephosphorylation in DNA damage- and topoisomerase II inactivation-induced G2 arrests in HL-60 cells

Doxorubicin induces DNA breakage by stabilizing a cleavable topoisomerase II-DNA complex. In contrast, topoisomerase II catalytic inhibitor ICRF-193 and uncoupling inhibitor aclarubicin interfere with the cleavable complex formation. We analysed combination effects of these drugs using two-dimensional flow cytometry of DNA content and the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling assay. Both ICRF-193 and aclarubicin attenuated the cytotoxic effect of doxorubicin on HL-60 cells (85% and 46% maximum reduction, respectively), which suggested that doxorubicin exerts its cytotoxic effect at least partially through the topoisomerase II-dependent DNA cleavage. Doxorubicin and ICRF-193 both induced G2 arrest in HL-60 cells, by which they may have reduced the cytotoxic effect of vincristine. Indeed, although ICRF-193 inhibited doxorubicin-induced apoptosis, ICRF-193 and doxorubicin cooperated in arresting HL-60 cells at G2 phase. These results indicated that G2 arrest was caused not only by DNA damage but also through a DNA damage-free, topoisomerase II inactivation-induced pathway. Western blot analysis showed that both types of G2 arrest were mediated by the inhibition of p34cdc2 dephosphorylation.

Basis for the checkpoint signal specificity that regulates Chk1 and Cds1 protein kinases

Six checkpoint Rad proteins (Rad1, Rad3, Rad9, Rad17, Rad26, and Hus1) are needed to regulate checkpoint protein kinases Chk1 and Cds1 in fission yeast. Chk1 is required to prevent mitosis when DNA is damaged by ionizing radiation (IR), whereas either kinase is sufficient to prevent mitosis when DNA replication is inhibited by hydroxyurea (HU). Checkpoint Rad proteins are required for IR-induced phosphorylation of Chk1 and HU-induced activation of Cds1. IR activates Cds1 only during the DNA synthesis (S) phase, whereas HU induces Chk1 phosphorylation only in cds1 mutants. Here, we investigate the basis of the checkpoint signal specificity of Chk1 phosphorylation and Cds1 activation. We show that IR fails to induce Chk1 phosphorylation in HU-arrested cells. Release from the HU arrest following IR causes substantial Chk1 phosphorylation. These and other data indicate that Cds1 prevents Chk1 phosphorylation in HU-arrested cells, which suggests that Cds1 actively suppresses a repair process that leads to Chk1 phosphorylation. Cds1 becomes more highly concentrated in the nucleus only during the S phase of the cell cycle. This finding correlates with S-phase specificity of IR-induced activation of Cds1. However, constitutive nuclear localization of Cds1 does not enhance IR-induced activation of Cds1. This result suggests that Cds1 activation requires DNA structures or protein activities that are present only during S phase. These findings help to explain how Chk1 and Cds1 respond to different checkpoint signals.

The human G2 checkpoint control protein hRAD9 is a nuclear phosphoprotein that forms complexes with hRAD1 and hHUS1

Eukaryotic cells actively block entry into mitosis in the presence of DNA damage or incompletely replicated DNA. This response is mediated by signal transduction cascades called cell cycle checkpoints. We show here that the human checkpoint control protein hRAD9 physically associates with two other checkpoint control proteins, hRAD1 and hHUS1. Furthermore, hRAD1 and hHUS1 themselves interact, analogously to their fission yeast homologues Rad1 and Hus1. We also show that hRAD9 is present in multiple phosphorylation forms in vivo. These phosphorylated forms are present in tissue culture cells that have not been exposed to exogenous sources of DNA damage, but it remains possible that endogenous damage or naturally occurring replication intermediates cause the observed phosphorylation. Finally, we show that hRAD9 is a nuclear protein, indicating that in this signal transduction pathway, hRAD9 is physically proximal to the upstream (DNA damage) signal rather than to the downstream, cytoplasmic, cell cycle machinery.

Functional localization of a melanoma tumor suppressor gene to a small (< or = 2 Mb) region on 11q23

We have previously demonstrated the existence of a melanoma tumor suppressor gene(s) on the long arm of chromosome 11 through suppression of tumorigenicity assays. Although loss of heterozygosity studies also support this finding, only a large critical region (44 cM) has been identified to date on 11q22-25. To further localize a tumor suppressor gene(s) within this region, we have now generated and characterized nine melanoma microcell hybrids, each retaining an introduced fragment of 11q. Of the nine hybrids, four were suppressed for tumor formation in nude mice, while five formed tumors at the same rate as the parental melanoma cell line (UACC 903). Molecular analysis of the hybrids with 118 microsatellite markers narrowed the location of a putative suppressor gene to a small (< or =2 Mb) candidate region on 11q23 between the markers D11S1786 and D11S2077 and within the larger region frequently deleted in melanoma tumors and cell lines. While multiple tumor suppressor genes are likely to reside on 11q22-25, the presence of this region in all four suppressed hybrids supports the simplest model that a single locus is responsible for the suppressed phenotype observed in UACC 903.

The zeta isoform of 14-3-3 proteins interacts with the third intracellular loop of different alpha2-adrenergic receptor subtypes

The alpha2-adrenergic receptors (alpha2ARs) are localized to and function on the basolateral surface in polarized renal epithelial cells via a mechanism involving the third cytoplasmic loop. To identify proteins that may contribute to this retention, [35S]Met-labeled Gen10 fusion proteins with the 3i loops of the alpha2AAR (Val217-Ala377), alpha2BAR (Lys210-Trp354), and alpha2CAR (Arg248-Val363) were used as ligands in gel overlay assays. A protein doublet of approximately 30 kDa in Madin-Darby canine kidney cells or pig brain cytosol (alpha2B >/= alpha2C>> alpha2A) was identified. The interacting protein was purified by sequential DEAE and size exclusion chromatography, and subsequent microsequencing revealed that they are the zeta isoform of 14-3-3 proteins. [35S]Met-14-3-3zeta binds to all three native alpha2AR subtypes, assessed using a solid phase binding assay (alpha2A>/=alpha2B> alpha2C), and this binding depends on the presence of the 3i loops. Attenuation of the alpha2AR-14-3-3 interactions in the presence of a phosphorylated Raf-1 peptide corresponding to its 14-3-3 interacting domain (residues 251-266), but not by its non-phosphorylated counterpart, provides evidence for the functional specificity of these interactions and suggests one potential interface for the alpha2AR and 14-3-3 interactions. These studies represent the first evidence for G protein-coupled receptor interactions with 14-3-3 proteins and may provide a mechanism for receptor localization and/or coordination of signal transduction.

European multicenter study on LOH of APOC3 at 11q23 in 766 breast cancer patients: relation to clinical variables. Breast Cancer Somatic Genetics Consortium

High frequencies of loss of heterozygosity (LOH) in chromosome 11q22-qter have been observed in various malignancies, including breast cancer. Previous studies on breast carcinomas by Winqvist et al (Cancer Res 55: 2660-2664) have indicated that a survival factor gene is located in band 11q23, and that the highly informative microsatellite polymorphism at the APOC3 locus would be a suitable tool to perform more extensive LOH studies. In this European multicentre study, we have examined the occurrence of APOC3 LOH and evaluated the effect of LOH of this chromosomal subregion on the clinical behaviour of the disease in a cohort of 766 breast cancer patients in more detail. LOH for APOC3 was found in 42% of the studied tumours, but it was not found to be significantly associated with any of the studied clinical variables, including cancer-specific survival time or survival time after recurrent/metastatic disease. According to the present findings, the putative survival factor gene on 11q23 is not located close enough to the APOC3 gene, but apparently at a more proximal location.

Binding of 14-3-3 proteins and nuclear export control the intracellular localization of the mitotic inducer Cdc25

Binding of 14-3-3 proteins near the nuclear localization sequence of Xenopus Cdc25 suppresses its ability to induce entry into mitosis. We have examined the intracellular localization of green fluorescent protein (GFP)-tagged wild-type Cdc25 or a mutant (S287A) that cannot bind 14-3-3 proteins. Upon coexpression with Myc-14-3-3epsilon, GFP-Cdc25-WT was predominantly cytoplasmic, whereas GFP-Cdc25-S287A was exclusively nuclear. Leptomycin B, an inhibitor of nuclear export, elicited a prompt redistribution of GFP-Cdc25-WT to the nucleus. Mutagenesis experiments demonstrated that Cdc25 contains multiple nuclear export sequences. These studies indicate that the binding of 14-3-3 proteins and nuclear export regulate the intracellular localization of Cdc25.

Caffeine-mediated override of checkpoint controls. A requirement for rhp6 (Schizosaccharomyces pombe)

Cells exposed to inhibitors of DNA synthesis or suffering DNA damage are arrested or delayed in interphase through the action of checkpoint controls. If the arrested cell is exposed to caffeine, relatively normal cell cycle progression is resumed and, as observed in checkpoint control mutants, loss of checkpoint control activity is associated with a reduction in cell viability. To address the mechanism of caffeine's action on cell progression, fission yeast mutants that take up caffeine but are not sensitized to hydroxyurea (HU) by caffeine were selected. Mutants 788 and 1176 are point mutants of rhp6, the fission yeast homolog of the budding yeast RAD6 gene. Mutant rhp6-788 is slightly HU sensitive, radiosensitive, and exhibits normal checkpoint responses to HU, radiation, or inactivation of DNA ligase. However, the addition of caffeine does not override the associated cell cycle blocks. Both point and deletion mutations show synthetic lethality at room temperature with temperature-sensitive mutations in cyclin B (cdc13-117) or the phosphatase cdc25 (cdc25-22). These observations suggest that the rhp6 gene product, a ubiquitin-conjugating enzyme required for DNA damage repair, promotes entry to mitosis in response to caffeine treatment.

Cell cycle dependent subcellular distribution of Cdc25B subtypes

The dual specificity phosphatase and oncogene Cdc25B has been implicated in the G2/M cell cycle checkpoint, but the mode by which it is regulated remains poorly understood. Regional subcellular redistribution of proteins represents a unique potential regulatory mechanism. Thus, we examined in live cells the subcellular localization characteristics of Cdc25B2 and Cdc25B3 fused to green fluorescent protein. Cdc25B2 partitioned primarily in the cytoplasm during G1 and progressively migrated to the nucleus as cells transited from S to G2/M phase. In contrast, Cdc25B3 maintained a homogeneously staining diffuse phenotype irrespective of cell cycle phase. Treatment of the Cdc25B2-green fluorescent protein stable transfectants with vanadate inhibited the cell cycle dependency of intracellular distribution, while okadaic acid had little effect except in G1, suggesting regulation by at least one phosphorylation-dependent pathway. The DNA topoisomerase II poison and DNA damaging agent, etoposide, inhibited nuclear localization of Cdc25B2 in S phase, possibly by invoking a sequestration cascade. Thus, differences in the spatial distribution of Cdc25B subtypes exist within cells and the 41 amino acid insert in the N-terminus of the Cdc25B3 splice variant encodes an important inhibitory determinant for such regulation. The subcellular redistribution of Cdc25B2 could be functionally important for G2/M checkpoint regulation.

Maintenance of G2 arrest in the Xenopus oocyte: a role for 14-3-3-mediated inhibition of Cdc25 nuclear import

Cdc2-cyclin B1 in the G2-arrested Xenopus oocyte is held inactive by phosphorylation of Cdc2 at two negative regulatory sites, Thr14 and Tyr15. Upon treatment with progesterone, these sites are dephosphorylated by the dual specificity phosphatase, Cdc25, leading to Cdc2-cyclin B1 activation. Whereas maintenance of the G2 arrest depends upon preventing Cdc25-induced Cdc2 dephosphorylation, the mechanisms responsible for keeping Cdc25 in check in these cells have not yet been described. Here we report that Cdc25 in the G2-arrested oocyte is bound to 14-3-3 proteins and that progesterone treatment abrogates this binding. We demonstrate that Cdc25, apparently statically localized in the cytoplasm, is actually capable of shuttling in and out of the oocyte nucleus. Binding of 14-3-3 protein markedly reduces the nuclear import rate of Cdc25, allowing nuclear export mediated by a nuclear export sequence present in the N-terminus of Cdc25 to predominate. If 14-3-3 binding to Cdc25 is prevented while nuclear export is inhibited, the coordinate nuclear accumulation of Cdc25 and Cdc2-cyclin B1 facilitates their mutual activation, thereby promoting oocyte maturation.

The emerging p53 gene family

Perturbation of p53 protein function is a common, if not universal, finding in human cancer. Tumor suppression by p53 is due, at least in part, to its ability to activate transcription of certain genes involved in cell cycle control and apoptosis (programmed cell death). Two additional members of the mammalian p53 family, p73 and p51, which is also known as p40, p63, KET, or p73L, were recently identified. Both of these proteins share substantial sequence homology with p53 and can, at least when overproduced, activate p53-responsive promoters and induce apoptosis. Nonetheless, data on differences between these proteins and p53 are emerging. For example, p73 is not induced by DNA damage and is not targeted for inactivation by viral oncoproteins such as simian virus 40 (SV40) T antigen, adenovirus E1B 55K, and human papillomavirus E6. In contrast to p53, neither p73 nor p51 appears to be frequently mutated in human cancers on the basis of the limited studies reported to date. Finally, unlike p53, cells produce multiple p73 and p51 isoforms as a result of alternative splicing, and production of p73 and p51 appears to be restricted to certain tissues. Additional studies are required to determine the role, if any, that p73 and p51 play in cell growth control and carcinogenesis.

Cdc25 inhibited in vivo and in vitro by checkpoint kinases Cds1 and Chk1

In the fission yeast Schizosaccharomyces pombe, the protein kinase Cds1 is activated by the S-M replication checkpoint that prevents mitosis when DNA is incompletely replicated. Cds1 is proposed to regulate Wee1 and Mik1, two tyrosine kinases that inhibit the mitotic kinase Cdc2. Here, we present evidence from in vivo and in vitro studies, which indicates that Cds1 also inhibits Cdc25, the phosphatase that activates Cdc2. In an in vivo assay that measures the rate at which Cdc25 catalyzes mitosis, Cds1 contributed to a mitotic delay imposed by the S-M replication checkpoint. Cds1 also inhibited Cdc25-dependent activation of Cdc2 in vitro. Chk1, a protein kinase that is required for the G2-M damage checkpoint that prevents mitosis while DNA is being repaired, also inhibited Cdc25 in the in vitro assay. In vitro, Cds1 and Chk1 phosphorylated Cdc25 predominantly on serine-99. The Cdc25 alanine-99 mutation partially impaired the S-M replication and G2-M damage checkpoints in vivo. Thus, Cds1 and Chk1 seem to act in different checkpoint responses to regulate Cdc25 by similar mechanisms.

Phosphorylation-dependent interactions between enzymes of plant metabolism and 14-3-3 proteins

Far-Western overlays of soluble extracts of cauliflower revealed many proteins that bound to digoxygenin (DIG)-labelled 14-3-3 proteins. Binding to DIG-14-3-3s was prevented by prior dephosphorylation of the extract proteins or by competition with 14-3-3-binding phosphopeptides, indicating that the 14-3-3 proteins bind to phosphorylated sites. The proteins that bound to the DIG-14-3-3s were also immunoprecipitated from extracts with anti-14-3-3 antibodies, demonstrating that they were bound to endogenous plant 14-3-3 proteins. 14-3-3-binding proteins were purified from cauliflower extracts, in sufficient quantity for amino acid sequence analysis, by affinity chromatography on immobilised 14-3-3 proteins and specific elution with a 14-3-3-binding phosphopeptide. Purified 14-3-3-binding proteins included sucrose-phosphate synthase, trehalose-6-phosphate synthase, glutamine synthetases, a protein (LIM17) that has been implicated in early floral development, an approximately 20 kDa protein whose mRNA is induced by NaCl, and a calcium-dependent protein kinase that was capable of phosphorylating and rendering nitrate reductase (NR) sensitive to inhibition by 14-3-3 proteins. In contrast to the phosphorylated NR-14-3-3 complex which is activated by dissociation with 14-3-3-binding phosphopeptides, the total sugar-phosphate synthase activity in plant extracts was inhibited by up to 40% by a 14-3-3-binding phosphopeptide and the phosphopeptide-inhibited activity was reactivated by adding excess 14-3-3 proteins. Thus, 14-3-3 proteins are implicated in regulating several aspects of primary N and C metabolism. The procedures described here will be valuable for determining how the phosphorylation and 14-3-3-binding status of defined target proteins change in response to extracellular stimuli.

Regulating the onset of mitosis

In eukaryotes, G2/M progression is mediated by activation of mitosis promoting factor (MPF). To ensure faithful chromosome segregation, the activity of key mitotic inducers and inhibitors are coupled with chromosome replication, spindle pole duplication, morphogenesis, and DNA damage. Evidence gathered in the past two years has underscored the importance of positioning MPF and its regulators in the proper place at the proper time to ensure orderly progression through the G2/M transition. Altering the spatial organization of G2/M regulators also contributes to prevention of mitosis following DNA damage.

A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

GADD45 induction of a G2/M cell cycle checkpoint

G1/S and G2/M cell cycle checkpoints maintain genomic stability in eukaryotes in response to genotoxic stress. We report here both genetic and functional evidence of a Gadd45-mediated G2/M checkpoint in human and murine cells. Increased expression of Gadd45 via microinjection of an expression vector into primary human fibroblasts arrests the cells at the G2/M boundary with a phenotype of MPM2 immunopositivity, 4n DNA content and, in 15% of the cells, centrosome separation. The Gadd45-mediated G2/M arrest depends on wild-type p53, because no arrest was observed either in p53-null Li-Fraumeni fibroblasts or in normal fibroblasts coexpressed with p53 mutants. Increased expression of cyclin B1 and Cdc25C inhibited the Gadd45-mediated G2/M arrest in human fibroblasts, indicating that the mechanism of Gadd45-mediated G2/M checkpoint is at least in part through modulation of the activity of the G2-specific kinase, cyclin B1/p34(cdc2). Genetic and physiological evidence of a Gadd45-mediated G2/M checkpoint was obtained by using GADD45-deficient human or murine cells. Human cells with endogenous Gadd45 expression reduced by antisense GADD45 expression have an impaired G2/M checkpoint after exposure to either ultraviolet radiation or methyl methanesulfonate but are still able to undergo G2 arrest after ionizing radiation. Lymphocytes from gadd45-knockout mice (gadd45 -/-) also retained a G2/M checkpoint initiated by ionizing radiation and failed to arrest at G2/M after exposure to ultraviolet radiation. Therefore, the mammalian genome is protected by a multiplicity of G2/M checkpoints in response to specific types of DNA damage.

The Drosophila ATM homologue Mei-41 has an essential checkpoint function at the midblastula transition

BACKGROUND

Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required.

RESULTS

We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT.

CONCLUSIONS

The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.

Calcium/calmodulin-dependent phosphorylation and activation of human Cdc25-C at the G2/M phase transition in HeLa cells

The human tyrosine phosphatase (p54(cdc25-c)) is activated by phosphorylation at mitosis entry. The phosphorylated p54(cdc25-c) in turn activates the p34-cyclin B protein kinase and triggers mitosis. Although the active p34-cyclin B protein kinase can itself phosphorylate and activate p54(cdc25-c), we have investigated the possibility that other kinases may initially trigger the phosphorylation and activation of p54(cdc25-c). We have examined the effects of the calcium/calmodulin-dependent protein kinase (CaM kinase II) on p54(cdc25-c). Our in vitro experiments show that CaM kinase II can phosphorylate p54(cdc25-c) and increase its phosphatase activity by 2.5-3-fold. Treatment of a synchronous population of HeLa cells with KN-93 (a water-soluble inhibitor of CaM kinase II) or the microinjection of AC3-I (a specific peptide inhibitor of CaM kinase II) results in a cell cycle block in G2 phase. In the KN-93-arrested cells, p54(cdc25-c) is not phosphorylated, p34(cdc2) remains tyrosine phosphorylated, and there is no increase in histone H1 kinase activity. Our data suggest that a calcium-calmodulin-dependent step may be involved in the initial activation of p54(cdc25-c).

Association of Chk1 with 14-3-3 proteins is stimulated by DNA damage

The protein kinase Chk1 is required for cell cycle arrest in response to DNA damage. We have found that the 14-3-3 proteins Rad24 and Rad25 physically interact with Chk1 in fission yeast. Association of Chk1 with 14-3-3 proteins is stimulated in response to DNA damage. DNA damage results in phosphorylation of Chk1 and the 14-3-3 proteins bind preferentially to the phosphorylated form. Genetic analysis has independently implicated both Rad24 and Rad25 in the DNA-damage checkpoint pathway. We suggest that DNA damage-dependent association of phosphorylated Chk1 with 14-3-3 proteins mediates an important step along the DNA-damage checkpoint pathway, perhaps by directing Chk1 to a particular substrate or to a particular location within the cell. An additional role for 14-3-3 proteins in the DNA-damage checkpoint has been suggested based on the observation that human Chk1 can phosphorylate Cdc25C in vitro creating a 14-3-3 binding site. Our results suggest that in fission yeast the interaction between the 14-3-3 proteins and Cdc25 does not require Chk1 function and is unaffected by DNA damage, in sharp contrast to the interaction between the 14-3-3 proteins and Chk1.

hRAD17, a structural homolog of the Schizosaccharomyces pombe RAD17 cell cycle checkpoint gene, stimulates p53 accumulation

The RAD17 gene product of S. Pombe is an essential component of the checkpoint control pathway which responds to both DNA damage and disruption of replication. We have identified a human cDNA that encodes a polypeptide which is structurally conserved with the S. Pombe Rad17 protein. The human gene, designated hRAD17, predicts an encoded protein of 590 amino acids and a molecular weight of 69 kD. Amino acid sequence alignment revealed that hRadl7 has 28.3% and 52.5% similarity with the S. Pombe Rad17 protein, and 21.8% identity and 45.8% similarity to the budding yeast cell cycle checkpoint protein, Rad 24. When introduced into the S. Pombe rad17 mutant, hRAD17 was able to partially revert its hydroxyurea and ionizing radiation hypersensitivity, but not its UV hypersensitivity. Permanent overexpression of the hRAD17 gene in human fibrosarcoma cells resulted in p53 activation and a significant reduction of S- and G2/M-phase cells accompanied by an accumulation of the G1-phase population, suggesting that hRAD17 may have a role in cell cycle checkpoint control. Immunostaining of HT-1080 cells transiently transfected with a hRAD17 construct confirmed the nuclear accumulation of p53, which mimics the induction caused by DNA damage. Using FISH analysis, we have mapped the hRAD17 locus to human chromosome 5q11.2.