p34cdc2 kinase activity is maintained upon activation of the replication checkpoint in Schizosaccharomyces pombe

G2/M cell cycle arrest and apoptosis induced by COH-203 in human promyelocytic leukemia HL-60 cells

The combretastatin A-4/oltipraz hybrid (COH), 5-(3-amino-4-methoxyphenyl)-4-(3,4,5-trimethoxyphenyl)-3H-1,2-dithiole-3-one (COH-203) is one of the COH compounds synthesized by our previous study, which has been reported to affect a number of cancer cell lines, such as SGC-7901, KB, HT-1080, HepG2, SMMC-7721 and BEL-7402. The sensitivity of human acute leukemia cell lines to COH-203, and the mechanism underlying its anti-proliferative effects remain unknown, which was investigated in the present study. In the present study, it was demonstrated that COH-203 had notable time- and dose-dependent antiproliferative effects on the human acute promyelocytic leukemia HL-60 cell line. Furthermore, COH-203 treatment resulted in cell cycle arrest at G₂/M phase in a dose-dependent manner, and subsequently induced apoptosis. Western blot analysis revealed that upregulation of cyclin B was associated with G₂/M arrest. In addition, treatment with COH-203 resulted in downregulated expression of Bcl-2. This result revealed that COH-203-induced apoptosis in HL-60 cells may occur via the mitochondrial pathway in a caspase-dependent manner.

Redundant mechanisms prevent mitotic entry following replication arrest in the absence of Cdc25 hyper-phosphorylation in fission yeast

Following replication arrest the Cdc25 phosphatase is phosphorylated and inhibited by Cds1. It has previously been reported that expressing Cdc25 where 9 putative amino-terminal Cds1 phosphorylation sites have been substituted to alanine results in bypass of the DNA replication checkpoint. However, these results were acquired by expression of the phosphorylation mutant using a multicopy expression vector in a genetic background where the DNA replication checkpoint is intact. In order to clarify these results we constructed a Cdc25(9A)-GFP native promoter integrant and examined its effect on the replication checkpoint at endogenous expression levels. In this strain the replication checkpoint operates normally, conditional on the presence of the Mik1 kinase. In response to replication arrest the Cdc25(9A)-GFP protein is degraded, suggesting the presence of a backup mechanism to eliminate the phosphatase when it cannot be inhibited through phosphorylation.

Human Raf-1 proteins associate with Rad24 and Cdc25 in cell-cycle checkpoint pathway of fission yeast, Schizosaccharomyces pombe

Raf-1 is a serine/threonine protein kinase that connects cell surface receptor signals to nuclear transcription factors. By screening Schizosaccharomyces pombe (S. pombe) cDNA library, we isolated Rad24, which is a 14-3-3 homolog that is important in the DNA damage checkpoint in S. pombe, as a Raf-1 interacting protein. The interaction found in yeast was confirmed by co-immunoprecipitation. Furthermore, Cdc25, which has been known to bind to Rad24, also associated with Raf-1 and was phosphorylated in vitro by catalytically active Raf-1. However, in the presence of Raf-1, an interaction between Rad24 and Cdc25 was inhibited in triple hybrid assay, indicating that Raf-1 inhibits the interaction between Rad24 and Cdc25. An in vitro competition assay showed that the binding of Cdc25 and of Rad24 to Raf-1 is mutually exclusive. Western blots of whole cell lysates probed with polyclonal antibodies specific for tyrosine-15-phosphorylated Cdc2 showed that overproduction of Rad24 led to the dephosphorylation of tyrosine residue on Cdc2, which is known to be activated through dephosphorylation by Cdc25 phosphatase. Unexpectedly, overexpression of catalytically inactive mutant protein of Raf-1, S624A, also caused tyrosine dephosphorylation of Cdc2. Thus, these data suggest that Raf-1 may interfere with the role of Rad24 by competing with Rad24 for binding to Cdc25 or a direct phosphorylation of Cdc25, bypassing the checkpoint pathway in DNA repair through Cdc25 activation.

DNA Replication Checkpoint Prevents Precocious Chromosome Segregation by Regulating Spindle Behavior

The DNA replication checkpoint maintains replication fork integrity and prevents chromosome segregation during replication stresses. Mec1 and Rad53 (human ATM/ATR- and Chk2-like kinases, respectively) are critical effectors of this pathway in yeast. When treated with replication inhibitors, checkpoint-deficient mec1 or rad53 mutant fails to maintain replication fork integrity and proceeds to partition unreplicated chromosomes. We show that this unnatural chromosome segregation requires neither the onset of mitosis nor APC activation, cohesin cleavage, or biorientation of kinetochores. Instead, the checkpoint deficiency leads to deregulation of microtubule-associated proteins Cin8 and Stu2, which, in the absence of both chromosome cohesion and bipolar attachment of kinetochores to microtubules, induce untimely spindle elongation, causing premature chromosome separation. The checkpoint's ability to prevent nuclear division is abolished by combined deficiency of microtubule-destabilizing motor Kip3 and Mad2 functions. Thus, the DNA replication checkpoint prevents precocious chromosome segregation, not by inhibiting entry into mitosis as widely believed, but by directly regulating spindle dynamics.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

Fission yeast Fizzy-related protein srw1p is a G(1)-specific promoter of mitotic cyclin B degradation

Downregulation of cyclin-dependent kinase (Cdk)-mitotic cyclin complexes is important during cell cycle progression and in G(1) arrested cells undergoing differentiation. srw1p, a member of the Fizzy-related protein family in fission yeast, is required for the degradation of cdc13p mitotic cyclin B during G(1) arrest. Here we show that srw1p is not required for the degradation of cdc13p during mitotic exit demonstrating that there are two systems operative at different stages of the cell cycle for cdc13p degradation, and that srw1p is phosphorylated by Cdk-cdc13p only becoming dephosphorylated during G(1) arrest. We propose that this phosphorylation targets srw1p for proteolysis and inhibits its activity to promote cdc13p turnover.

DNA replication and damage checkpoints and meiotic cell cycle controls in the fission and budding yeasts

The cell cycle checkpoint mechanisms ensure the order of cell cycle events to preserve genomic integrity. Among these, the DNA-replication and DNA-damage checkpoints prevent chromosome segregation when DNA replication is inhibited or DNA is damaged. Recent studies have identified an outline of the regulatory networks for both of these controls, which apparently operate in all eukaryotes. In addition, it appears that these checkpoints have two arrest points, one is just before entry into mitosis and the other is prior to chromosome separation. The former point requires the central cell-cycle regulator Cdc2 kinase, whereas the latter involves several key regulators and substrates of the ubiquitin ligase called the anaphase promoting complex. Linkages between these cell-cycle regulators and several key checkpoint proteins are beginning to emerge. Recent findings on post-translational modifications and protein-protein interactions of the checkpoint proteins provide new insights into the checkpoint responses, although the functional significance of these biochemical properties often remains unclear. We have reviewed the molecular mechanisms acting at the DNA-replication and DNA-damage checkpoints in the fission yeast Schizosaccharomyces pombe, and the modifications of these controls during the meiotic cell cycle. We have made comparisons with the controls in fission yeast and other organisms, mainly the distantly related budding yeast.

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.

Suppressors of cdc25p overexpression identify two pathways that influence the G2/M checkpoint in fission yeast

Checkpoints maintain the order of cell-cycle events. At G2/M, a checkpoint blocks mitosis in response to damaged or unreplicated DNA. There are significant differences in the checkpoint responses to damaged DNA and unreplicated DNA, although many of the same genes are involved in both responses. To identify new genes that function specifically in the DNA replication checkpoint pathway, we searched for high-copy suppressors of overproducer of Cdc25p (OPcdc25(+)), which lacks a DNA replication checkpoint. Two classes of suppressors were isolated. One class includes a new gene encoding a putative DEAD box helicase, suppressor of uncontrolled mitosis (sum3(+)). This gene negatively regulates the cell-cycle response to stress when overexpressed and restores the checkpoint response by a mechanism that is independent of Cdc2p tyrosine phosphorylation. The second class includes chk1(+) and the two Schizosaccharomyces pombe 14-3-3 genes, rad24(+) and rad25(+), which appear to suppress the checkpoint defect by inhibiting Cdc25p. We show that rad24Delta mutants are defective in the checkpoint response to the DNA replication inhibitor hydroxyurea at 37 degrees and that cds1Delta rad24Delta mutants, like cds1Delta chk1Delta mutants, are entirely checkpoint deficient at 29 degrees. These results suggest that chk1(+) and rad24(+) may function redundantly with cds1(+) in the checkpoint response to unreplicated DNA.

Mitotic DNA damage and replication checkpoints in yeast

Studies of the genetics of G2/M checkpoints in budding and fission yeasts have produced many of the defining concepts of checkpoint biology. Recent progress in the biochemistry of the checkpoint gene products is adding a mechanistic understanding to our models and identifying the components of the normal cell cycle machinery that are targeted by checkpoints.

Multiple G1 regulatory elements control the androgen-dependent proliferation of prostatic carcinoma cells

Prostatic epithelial cells and most primary prostate tumors are dependent on androgen for growth, but how androgen regulates cellular proliferation remains unsolved. Using poorly understood mechanisms, recurrent tumor cells evade the androgen requirement. We utilized androgen-dependent prostatic tumor cells to demonstrate that androgen exerts its effect on the cell cycle by influencing specific aspects of G1-S progression. Androgen depletion of these cells results in early G1 arrest, characterized by reduced cyclin-dependent kinase activity, and underphosphorylated retinoblastoma tumor suppressor protein (RB). The reduction in kinase activity was partially attributed to reduction of specific G1 cyclins and alternate regulation of cyclin-dependent kinase inhibitors. Using this information, we developed a reliable assay to assess the ability of specific G1 regulatory proteins to circumvent these controls and promote androgen-independent growth. As expected, inactivation of RB was required for progression through the cell cycle. Surprisingly, overexpression of G1 cyclins, which drives RB phosphorylation, was insufficient to promote androgen-independent cell cycle progression. Introduction of viral oncoproteins did promote G1-S progression in the absence of androgen, dependent on their ability to sequester RB and related proteins. These results provide the first evidence that multiple elements governing the G1-S transition dictate androgen-dependent growth, and the formation of androgen-independent prostatic tumors may be because of misregulation of these processes.

Tyrosine phosphorylation of cdc2 is required for the replication checkpoint in Schizosaccharomyces pombe

The DNA replication checkpoint inhibits mitosis in cells that are unable to replicate their DNA, as when nucleotide biosynthesis is inhibited by hydroxyurea. In the fission yeast Schizosaccharomyces pombe, genetic evidence suggests that this checkpoint involves the inhibition of Cdc2 activity through the phosphorylation of tyrosine-15. On the contrary, a recent biochemical study indicated that Cdc2 is in an activated state during a replication checkpoint, suggesting that phosphorylation of Cdc2 on tyrosine-15 is not part of the replication checkpoint mechanism. We have undertaken biochemical and genetic studies to resolve this controversy. We report that the DNA replication checkpoint in S. pombe is abrogated in cells that carry the allele cdc2-Y15F, expressing an unphosphorylatable form of Cdc2. Furthermore, Cdc2 isolated from replication checkpoint-arrested cells can be activated in vitro by Cdc25, the tyrosine phosphatase responsible for dephosphorylating Cdc2 in vivo, to the same extent as Cdc2 isolated from cdc25ts-blocked cells, indicating that hydroxyurea treatment causes Cdc2 activity to be maintained at a low level that is insufficient to induce mitosis. These studies show that inhibitory tyrosine-15 phosphorylation of Cdc2 is essential for the DNA replication checkpoint and suggests that Cdc25, and/or one or both of Wee1 and Mik1, the tyrosine kinases that phosphorylate Cdc2, are regulated by the replication checkpoint.

Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1+ gene is one of six fission yeast genes, termed the checkpoint rad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1+, we have demonstrated a genetic interaction between hus1+ and another checkpoint rad gene, rad1+. Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9+, suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3+ and each of the other checkpoint rad genes. Although there is no gene related to hus1+ in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.

Advancement through mitosis requires rae1 gene function in fission yeast

Growth of the rae1-1 mutant of Schizosaccharomyces pombe at restrictive temperature results in accumulation of poly(A)+ RNA in the nucleus and a cell cycle arrest at the G2/M boundary. We demonstrate here that rae1 function is required for a process other than mRNA export which is essential for advancement through mitosis. Cells lacking rae1 function arrest with elevated Cdc2p kinase levels at a step before the formation of a mitotic spindle and without separation of the spindle pole bodies. Rae1p was localized to the nuclear periphery, consistent with a role in nucleocytoplasmic trafficking, which could include protein import. We propose a model where rae1 functions in cell cycle progression through trafficking of proteins required for mitosis.