Human claspin is required for replication checkpoint control

Sister acts: coordinating DNA replication and cohesion establishment

The ring-shaped cohesin complex links sister chromatids and plays crucial roles in homologous recombination and mitotic chromosome segregation. In cycling cells, cohesin's ability to generate cohesive linkages is restricted to S phase and depends on loading and establishment factors that are intimately connected to DNA replication. Here we review how cohesin is regulated by the replication machinery, as well as recent evidence that cohesin itself influences how chromosomes are replicated.

Multiple functions of the S-phase checkpoint mediator

There is mounting evidence that replication defects are the major source of spontaneous genomic instability in cells, and that S-phase checkpoints are the principal defense against such instability. The S-phase checkpoint mediator protein Mrc1/Claspin mediates the checkpoint response to replication stress by facilitating phosphorylation of effector kinase by a sensor kinase. In this review, the multiple functions and the regulation of the S-phase checkpoint mediator are discussed.

The ubiquitous role of ubiquitin in the DNA damage response

Protein ubiquitylation has emerged as an important regulatory mechanism that impacts almost every aspect of the DNA damage response. In this review, we discuss how DNA repair and checkpoint pathways utilize the diversity offered by the ubiquitin conjugation system to modulate the response to genotoxic lesions in space and time. In particular, we will highlight recent work done on the regulation of DNA double-strand breaks signalling and repair by the RNF8/RNF168 E3 ubiquitin ligases, the Fanconi anemia pathway and the role of protein degradation in the enforcement and termination of checkpoint signalling. We also discuss the various functions of deubiquitylating enzymes in these processes along with potential avenues for exploiting the ubiquitin conjugation/deconjugation system for therapeutic purposes.

Fission yeast Swi1-Swi3 complex facilitates DNA binding of Mrc1

Replication fork protection complex Swi1-Swi3 and replication checkpoint mediator Mrc1 are required for maintenance of replication fork integrity during the course of DNA replication in the fission yeast Schizosaccharomyces pombe. These proteins play crucial roles in stabilizing stalled forks and activating replication checkpoint signaling pathways. Although they are conserved replication fork components, precise biochemical roles of these proteins are not known. Here we purified Mrc1 and Swi1-Swi3 proteins and show that these proteins bind to DNA independently but synergistically in vitro. Mrc1 binds preferentially to arrested fork or D-loop-like structures, although the affinity is relatively low, whereas the Swi1-Swi3 complex binds to double-stranded DNA with higher affinity. In the presence of a low concentration of Swi1-Swi3, Mrc1 generates a novel ternary complex and binds to various types of DNA with higher affinity. Moreover, purified Mrc1 and Swi1-Swi3 physically interact with each other, and this interaction is lost by mutations in the known DNA binding domain of Mrc1 (K235E,K236E). The interaction is also lost in a mutant form of Swi1 (E662K) that is specifically defective in polar fork arrest at a site called RTS1 and causes sensitivity to genotoxic agents, although the DNA binding affinity of Swi1-Swi3 is not affected by this mutation. As expected, the synergistic effect of the Swi1-Swi3 on DNA binding of Mrc1 is also lost by these mutations affecting the interaction between Mrc1 and Swi1-Swi3. Our results reveal an aspect of molecular interactions that may play an important role in replication pausing and fork stabilization.

Polo-like kinase 1 activated by the hepatitis B virus X protein attenuates both the DNA damage checkpoint and DNA repair resulting in partial polyploidy

Hepatitis B virus X protein (pX), implicated in hepatocarcinogenesis, induces DNA damage because of re-replication and allows propagation of damaged DNA, resulting in partial polyploidy and oncogenic transformation. The mechanism by which pX allows cells with DNA damage to continue proliferating is unknown. Herein, we show pX activates Polo-like kinase 1 (Plk1) in the G(2) phase, thereby attenuating the DNA damage checkpoint. Specifically, in the G(2) phase of pX-expressing cells, the checkpoint kinase Chk1 was inactive despite DNA damage, and protein levels of claspin, an adaptor of ataxia telangiectasia-mutated and Rad3-related protein-mediated Chk1 phosphorylation, were reduced. Pharmacologic inhibition or knockdown of Plk1 restored claspin protein levels, Chk1 activation, and p53 stabilization. Also, protein levels of DNA repair protein Mre11 were decreased in the G(2) phase of pX-expressing cells but not with Plk1 knockdown. Interestingly, in pX-expressing cells, Mre11 co-immunoprecipitated with transfected Plk1 Polo-box domain, and inhibition of Plk1 increased Mre11 stability in cycloheximide-treated cells. These results suggest that pX-activated Plk1 by down-regulating Mre11 attenuates DNA repair. Importantly, concurrent inhibition of Plk1, p53, and Mre11 increased the number of pX-expressing cells with DNA damage entering mitosis, relative to Plk1 inhibition alone. By contrast, inhibition or knockdown of Plk1 reduced pX-induced polyploidy while increasing apoptosis. We conclude Plk1, activated by pX, allows propagation of DNA damage by concurrently attenuating the DNA damage checkpoint and DNA repair, resulting in polyploidy. We propose this novel Plk1 mechanism initiates pX-mediated hepatocyte transformation.

Phosphatases in the cellular response to DNA damage

In the last fifteen years, rapid progress has been made in delineating the cellular response to DNA damage. The DNA damage response network is composed of a large number of proteins with different functions that detect and signal the presence of DNA damage in order to coordinate DNA repair with a variety of cellular processes, notably cell cycle progression. This signal, which radiates from the chromatin template, is driven primarily by phosphorylation events, mainly on serine and threonine residues. While we have accumulated detailed information about kinases and their substrates our understanding of the role of phosphatases in the DNA damage response is still preliminary. Identifying the phosphatases and their regulation will be instrumental to obtain a complete picture of the dynamics of the DNA damage response. Here we give an overview of the DNA damage response in mammalian cells and then review the data on the role of different phosphatases and discuss their biological relevance.

IKK and NF-kappaB-mediated regulation of Claspin impacts on ATR checkpoint function

In response to replication stress, Claspin mediates the phosphorylation and activation of Chk1 by ATR. Claspin is not only necessary for the propagation of the DNA-damage signal, but its destruction by the ubiquitin-proteosome pathway is required to allow the cell to continue the cell cycle allowing checkpoint recovery. Here, we demonstrate that both the NF-kappaB family of transcription factors and their upstream kinase IKK can regulate Claspin levels by controlling its mRNA expression. Furthermore, we show that c-Rel directly controls Claspin gene transcription. Disruption of IKK and specific NF-kappaB members impairs ATR-mediated checkpoint function following DNA damage. Importantly, hyperactivation of IKK results in a failure to inactivate Chk1 and impairs the recovery from the DNA checkpoint. These results uncover a novel function for IKK and NF-kappaB modulating the DNA-damage checkpoint response, allowing the cell to integrate different signalling pathways with the DNA-damage response.

MCL-1 localizes to sites of DNA damage and regulates DNA damage response

MCL-1, a pro-survival member of the BCL-2 family, was previously shown to have functions in ATR-dependent Chk1 phosphorylation following DNA damage. To further delineate these functions, we explored possible differences in DNA damage response caused by lack of MCL-1 in mouse embryo fibroblasts (MEFs). As expected, Mcl-1(-/-) MEFs had delayed Chk1 phosphorylation following etoposide treatment, compared to wild type MEFs. However, their response to hydroxyurea, which causes a G(1)/S checkpoint response, was not significantly different. In addition, appearance of gamma-H2AX was delayed in the Mcl-1(-/-) MEFs treated with etoposide. We next investigated whether MCL-1 is present, together with other DNA damage response proteins, at the sites of DNA damage. Immunoprecipitation of etoposide-treated extracts with anti-MCL-1 antibody showed association of MCL-1 with gamma-H2AX as well as NBS1. Immunofluorescent staining for MCL-1 further showed increased co-staining of MCL-1 and NBS1 following DNA damage. By using a system that creates DNA double strand breaks at specific sites in the genome, we demonstrated that MCL-1 is recruited directly adjacent to the sites of damage. Finally, in a direct demonstration of the importance of MCL-1 in allowing proper repair of DNA damage, we found that treatment for two brief exposures to etoposide , followed by periods of recovery, which mimics the clinical situation of etoposide use, resulted in greater accumulation of chromosomal abnormalities in the MEFs that lacked MCL-1. Together, these data indicate an important role for MCL-1 in coordinating DNA damage mediated checkpoint response, and have broad implications for the importance of MCL-1 in maintenance of genome integrity.

Eukaryotic chromosome DNA replication: where, when, and how?

DNA replication is central to cell proliferation. Studies in the past six decades since the proposal of a semiconservative mode of DNA replication have confirmed the high degree of conservation of the basic machinery of DNA replication from prokaryotes to eukaryotes. However, the need for replication of a substantially longer segment of DNA in coordination with various internal and external signals in eukaryotic cells has led to more complex and versatile regulatory strategies. The replication program in higher eukaryotes is under a dynamic and plastic regulation within a single cell, or within the cell population, or during development. We review here various regulatory mechanisms that control the replication program in eukaryotes and discuss future directions in this dynamic field.

The position of the amino group on the benzene ring is critical for mesalamine's improvement of replication fidelity

BACKGROUND

Individuals with ulcerative colitis are at high risk of developing colitis-associated cancer. 5-Aminosalicylate (5-ASA) protects from cancer by its antiinflammatory activity as well as by altering cell growth, inducing apoptosis, and reducing replication errors. So far neither 5-ASA's structural specificity nor its pharmacophore group have been identified. Here we compared 5-ASA with its analogs (4-ASA and 3-ASA) and its metabolite N-acetyl-5-ASA (NAc-5-ASA).

METHODS

Superoxide scavenging was analyzed by lucigenin-amplified chemiluminescence. Cell growth, cell cycle distribution, and replication fidelity at a (CA)13 microsatellite were measured in HCT116 and HT29 colon epithelial cells by MTT and flow cytometry. Nuclear protein extracts were blotted for replication protein A (RPA), claspin, p53, and p53(Ser15).

RESULTS

All compounds inhibited the growth of colon epithelial cells at a similar level and displayed potent scavenging properties, with 3-ASA being the most active, followed by 5-ASA, 4-ASA, and NAc-5-ASA. Besides 5-ASA, only 4-ASA caused an increase in the S-phase population (56%-69% and 49%-62% in HCT116 and HT29 cells, respectively). This was accompanied by nuclear recruitment of replication proteins RPA and claspin as well as phosphorylation of p53(Ser15), both of which were weaker or absent with 3-ASA or NAc-5-ASA. 5-ASA was the only compound that lowered mutations at a (CA)13 microsatellite.

CONCLUSIONS

5-ASA shares its growth inhibitory and superoxide scavenging properties with its structural analogs and metabolite, but the position of the amino group is critical for reducing replication errors.

Tipin-replication protein A interaction mediates Chk1 phosphorylation by ATR in response to genotoxic stress

Mammalian Timeless is a multifunctional protein that performs essential roles in the circadian clock, chromosome cohesion, DNA replication fork protection, and DNA replication/DNA damage checkpoint pathways. The human Timeless exists in a tight complex with a smaller protein called Tipin (Timeless-interacting protein). Here we investigated the mechanism by which the Timeless-Tipin complex functions as a mediator in the ATR-Chk1 DNA damage checkpoint pathway. We find that the Timeless-Tipin complex specifically mediates Chk1 phosphorylation by ATR in response to DNA damage and replication stress through interaction of Tipin with the 34-kDa subunit of replication protein A (RPA). The Tipin-RPA interaction stabilizes Timeless-Tipin and Tipin-Claspin complexes on RPA-coated ssDNA and in doing so promotes Claspin-mediated phosphorylation of Chk1 by ATR. Our results therefore indicate that RPA-covered ssDNA not only supports recruitment and activation of ATR but also, through Tipin and Claspin, it plays an important role in the action of ATR on its critical downstream target Chk1.

Drf1-dependent kinase interacts with Claspin through a conserved protein motif

The Dbf4/Drf1-dependent kinase (DDK) is required for the initiation of DNA replication in eukaryotes. Another protein, Claspin, mediates the activation of a cellular checkpoint response to stalled replication forks and is also a regulator of replication. In this study, we found that DDK phosphorylates Claspin in vitro and forms a nuclear complex containing Cdc7, Drf1, and Claspin in Xenopus egg extracts. In addition, purified Claspin and DDK are capable of a direct in vitro interaction. We identified a conserved binding site on Claspin required for its interaction with DDK. This site corresponds to the first of two sequence repeats in the Chk1-binding domain of Claspin. Furthermore, we have established that two amino acids in this motif, Asp(861) and Gln(866), are essential for the interaction between Claspin and DDK. We found that mutant forms of Claspin incapable of interacting with DDK are still able to associate with and activate Chk1 in response to DNA replication blockages. However, Claspin-depleted egg extracts that have been reconstituted with these mutants of Claspin undergo DNA replication more slowly. These findings suggest that the interaction of DDK with Claspin mediates a checkpoint-independent function of Claspin related to DNA replication.

Claspin inhibition leads to fragile site expression

Fragile sites are hot spots for sister chromatid exchanges, translocations, deletions, complex rearrangements, and gene amplification. It has been hypothesized that rearrangements at fragile sites derive from unreplicated regions resulting from stalled forks that escape the ATR replication checkpoint. In the present study, we investigated the role of the Claspin (CLSPN) gene, which codes for an adaptor protein in the ATR pathway, during DNA replication stress in human cells. We show that the inhibition of the CLSPN gene leads to both genome instability and fragile site expression. Following aphidicolin treatment, we found a transient increase of Claspin synthesis due to its requirement to checkpoint activation. However, Claspin synthesis decreased after a prolonged aphidicolin treatment. We propose that CLSPN modulation, following an extreme replication block, allows rare cells to escape checkpoint mechanisms and enter mitosis with a defect in genome assembly. Our observations provide the basis for a better understanding of cell cycle checkpoints deregulation in cancer.

Interactions of human mismatch repair proteins MutSalpha and MutLalpha with proteins of the ATR-Chk1 pathway

At clinically relevant doses, chemotherapeutic S_N1 DNA methylating agents induce an ATR-mediated checkpoint response in human cells that is dependent on functional MutSα and MutLα. Deficiency of either mismatch repair activity renders cells highly resistant to this class of drug, but the mechanisms linking mismatch repair to checkpoint activation have remained elusive. In this study we have systematically examined the interactions of human MutSα and MutLα with proteins of the ATR-Chk1 pathway using both nuclear extracts and purified proteins. Using nuclear co-immunoprecipitation, we have detected interaction of MutSα with ATR, TopBP1, Claspin, and Chk1 and interaction of MutLα with TopBP1 and Claspin. We were unable to detect interaction of MutSα or MutLα with Rad17, Rad9, or replication protein A in the extract system. Use of purified proteins confirmed direct interaction of MutSα with ATR, TopBP1, and Chk1 and of MutLα with TopBP1. MutSα-Claspin and MutLα-Claspin interactions were not demonstrable with purified proteins, suggesting that extract interactions are indirect or depend on post-translational modification. Use of a modified chromatin immunoprecipitation assay showed that proliferating cell nuclear antigen, ATR, TopBP1, and Chk1 are recruited to chromatin in a MutLα- and MutSα-dependent fashion after N-methyl-N′-nitro-N-nitrosoguanidine treatment. However, chromatin enrichment of replication protein A, Claspin, Rad17-RFC, and Rad9-Rad1-Hus1 was not detected in these experiments. Although our failure to observe enrichment of the latter activities could be due to sensitivity limitations, these observations may indicate a novel mechanism for ATR activation.

Reconstitution of human claspin-mediated phosphorylation of Chk1 by the ATR (ataxia telangiectasia-mutated and rad3-related) checkpoint kinase

ATR (ATM and Rad3-related) initiates a DNA damage signaling pathway in human cells upon DNA damage induced by UV and UV-mimetic agents and in response to inhibition of DNA replication. Genetic data with human cells and in vitro data with Xenopus egg extracts have led to the conclusion that the kinase activity of ATR toward the signal-transducing kinase Chk1 depends on the mediator protein Claspin. Here we have reconstituted a Claspin-mediated checkpoint system with purified human proteins. We find that the ATR-dependent phosphorylation of Chk1, but not p53, is strongly stimulated by Claspin. Similarly, DNA containing bulky base adducts stimulates ATR kinase activity, and Claspin acts synergistically with damaged DNA to increase phosphorylation of Chk1 by ATR. Mutations in putative phosphorylation sites in the Chk1-binding domain of Claspin abolish its ability to mediate ATR phosphorylation of Chk1. We also find that a fragment of Claspin containing the Chk1-binding domain together with a domain conserved in the yeast Mrc1 orthologs of Claspin is sufficient for its mediator activity. This in vitro system recapitulates essential components of the genetically defined ATR-signaling pathway.

The human intra-S checkpoint response to UVC-induced DNA damage

The intra-S checkpoint response to 254 nm light (UVC)-induced DNA damage appears to have dual functions to slow the rate of DNA synthesis and stabilize replication forks that become stalled at sites of UVC-induced photoproducts in DNA. These functions should provide more time for repair of damaged DNA before its replication and thereby reduce the frequencies of mutations and chromosomal aberrations in surviving cells. This review tries to summarize the history of discovery of the checkpoint, the current state of understanding of the biological features of intra-S checkpoint signaling and its mechanisms of action with a focus primarily on intra-S checkpoint responses in human cells. The differences in the intra-S checkpoint responses to UVC and ionizing radiation-induced DNA damage are emphasized. Evidence that [6-4]pyrimidine-pyrimidone photoproducts in DNA trigger the response is discussed and the relationships between cellular responses to UVC and the molecular dose of UVC-induced DNA damage are briefly summarized. The role of the intra-S checkpoint response in protecting against solar radiation carcinogenesis remains to be determined.

Prevalence and functional analysis of sequence variants in the ATR checkpoint mediator Claspin

Mutational inactivation of genes controlling the DNA-damage response contributes to cancer susceptibility within families and within the general population as well as to sporadic tumorigenesis. Claspin (CLSPN) encodes a recently recognized mediator protein essential for the ATR and CHK1-dependent checkpoint elicited by replicative stress or the presence of ssDNA. Here, we describe a study to determine whether mutational disruption of CLSPN contributes to cancer susceptibility and sporadic tumorigenesis. We resequenced CLSPN from the germline of selected cancer families with a history of breast cancer (n = 25) or a multicancer phenotype (n = 46) as well as from a panel of sporadic cancer cell lines (n = 52) derived from a variety of tumor types. Eight nonsynonymous variants, including a recurrent mutation, were identified from the germline of two cancer-prone individuals and five cancer cell lines of breast, ovarian, and hematopoietic origin. None of the variants was present within population controls. In contrast, mutations were rare within genes encoding the CLSPN-interacting protein ATR and its binding partner ATRIP. One variant of CLSPN, encoding the I783S missense mutation, was defective in its ability to mediate CHK1 phosphorylation following DNA damage and was unable to rescue sensitivity to replicative stress in CLSPN-depleted cells. Taken together, these observations raise the possibility that CLSPN may encode a component of the DNA-damage response pathway that is targeted by mutations in human cancers, suggesting the need for larger population-based studies to investigate whether CLSPN variants contribute to cancer susceptibility.

Roles of human AND-1 in chromosome transactions in S phase

Coordinated execution of DNA replication, checkpoint activation, and postreplicative chromatid cohesion is intimately related to the replication fork machinery. Human AND-1/chromosome transmission fidelity 4 is localized adjacent to replication foci and is required for efficient DNA synthesis. In S phase, AND-1 is phosphorylated in response to replication arrest in a manner dependent on checkpoint kinase, ataxia telangiectasia-mutated, ataxia telangiectasia-mutated and Rad3-related protein, and Cdc7 kinase but not on Chk1. Depletion of AND-1 increases DNA damage, delays progression of S phase, leads to accumulation of late S and/or G2 phase cells, and induces cell death in cancer cells. It also elevated UV-radioresistant DNA synthesis and caused premature recovery of replication after hydroxyurea arrest, indicating that lack of AND-1 compromises checkpoint activation. This may be partly due to the decreased levels of Chk1 protein in AND-1-depleted cells. Furthermore, AND-1 interacts with cohesin proteins Smc1, Smc3, and Rad21/Scc1, consistent with proposed roles of yeast counterparts of AND-1 in sister chromatid cohesion. Depletion of AND-1 leads to significant inhibition of homologous recombination repair of an I-SceI-driven double strand break. Based on these data, we propose that AND-1 coordinates multiple cellular events in S phase and G2 phase, such as DNA replication, checkpoint activation, sister chromatid cohesion, and DNA damage repair, thus playing a pivotal role in maintenance of genome integrity.

Network architecture of signaling from uncoupled helicase-polymerase to cell cycle checkpoints and trans-lesion DNA synthesis

When replication is blocked by a template lesion or polymerase inhibitor while helicase continues unwinding the DNA, single stranded DNA (ssDNA) accumulates and becomes coated with RPA, which then initiates signals via PCNA mono-ubiquitination to activate trans-lesion polymerases and via ATR and Chk1 to inhibit Cdk2-dependent cell cycle progression. The signals are conveyed by way of a complex network of molecular interactions. To clarify those complexities, we have constructed a molecular interaction map (MIM) using a novel hierarchical assembly procedure. Molecules were arranged on the map in hierarchical levels according to interaction step distance from the DNA region of stalled replication. The hierarchical MIM allows us to disentangle the network's interlocking pathways and loops and to suggest functionally significant features of network architecture. The MIM shows how parallel pathways and multiple feedback loops can provide failsafe and robust switch-like responses to replication stress. Within the central level of hierarchy ATR and Claspin together appear to function as a nexus that conveys signals from many sources to many destinations. We noted a division of labor between those two molecules, separating enzymatic and structural roles. In addition, the network architecture disclosed by the hierarchical map, suggested a speculative model for how molecular crowding and the granular localization of network components in the cell nucleus can facilitate function.

Human papilloma virus type16 E6 deregulates CHK1 and sensitizes human fibroblasts to environmental carcinogens independently of its effect on p53

After treatment with ultraviolet radiation (UV), human fibroblasts that express the HPV type 16 E6 oncoprotein display defects in repair of cyclobutane pyrimidine dimers, hypersensitivity to inactivation of clonogenic survival and an inability to sustain DNA replication. To determine whether these effects are specific to depletion of p53 or inactivation of its function, fibroblast lines were constructed with ectopic expression of a dominant-negative p53 allele (p53-H179Q) to inactivate function or a short-hairpin RNA (p53-RNAi) to deplete expression of p53. Only the expression of HPV16E6 sensitized fibroblasts to UV or the chemical carcinogen, benzo[a]pyrene diolepoxide I (BPDE). Carcinogen-treated cells expressing p53-H179Q or p53-RNAi were resistant to inactivation of colony formation and did not suffer replication arrest. CHK1 is a key checkpoint kinase in the response to carcinogen-induced DNA damage. Control and p53-RNAi-expressing fibroblasts displayed phosphorylation of Ser345 on CHK1 45-120 min after carcinogen treatment with a return to near baseline phosphorylation by 6 h after treatment. HPV16E6-expressing fibroblasts displayed enhanced and sustained phosphorylation of CHK1. This was associated with enhanced phosphorylation of Thr68 on CHK2 and Ser139 on H2AX, both markers of severe replication stress and DNA double strand breaks. Incubation with the phosphatase inhibitor okadaic acid produced more phosphorylation of CHK1 in UV-treated HPV16E6-expressing cells than in p53-H179Q-expressing cells suggesting that HPV16E6 may interfere with the recovery of coupled DNA replication at replication forks that are stalled at [6-4]pyrimidine-pyrimidone photoproducts and BPDE-DNA adducts. The results indicate that HPV16E6 targets a protein or proteins other than p53 to deregulate the activity of CHK1 in carcinogen-damaged cells.

Discovery of gene expression-based pharmacodynamic biomarker for a p53 context-specific anti-tumor drug Wee1 inhibitor

BACKGROUND

Wee1 is a tyrosine kinase regulating S-G2 cell cycle transition through the inactivating phosphorylation of CDC2. The inhibition of Wee1 kinase by a selective small molecule inhibitor significantly enhances the anti-tumor efficacy of DNA damaging agents, specifically in p53 negative tumors by abrogating S-G2 checkpoints, while normal cells with wild-type p53 are not severely damaged due to the intact function of the G1 checkpoint mediated by p53. Since the measurement of mRNA expression requires a very small amount of biopsy tissue and is highly quantitative, the development of a pharmacodynamic (PD) biomarker leveraging mRNA expression is eagerly anticipated in order to estimate target engagement of anti-cancer agents.

RESULTS

In order to find the Wee1 inhibition signature, mRNA expression profiling was first performed in both p53 positive and negative cancer cell lines treated with gemcitabine and a Wee1 inhibitor, MK-1775. We next carried out mRNA expression profiling of skin samples derived from xenograft models treated with the Wee1 inhibitor to identify a Wee1 inhibitor-regulatory gene set. Then, the genes that were commonly modulated in both cancer cell lines and rat skin samples were extracted as a Wee1 inhibition signature that could potentially be used as a PD biomarker independent of p53 status. The expression of the Wee1 inhibition signature was found to be regulated in a dose-dependent manner by the Wee1 inhibitor, and was significantly correlated with the inhibition level of a direct substrate, phosphorylated-CDC2. Individual genes in this Wee1 inhibition signature are known to regulate S-G2 cell cycle progression or checkpoints, which is consistent with the mode-of-action of the Wee1 inhibitor.

CONCLUSION

We report here the identification of an mRNA gene signature that was specifically changed by gemcitabine and Wee1 inhibitor combination treatment by molecular profiling. Given the common regulation of expression in both xenograft tumors and animal skin samples, the data suggest that the Wee1 inhibition gene signature might be utilized as a quantitative PD biomarker in both tumors and surrogate tissues, such as skin and hair follicles, in human clinical trials.

Claspin and Chk1 regulate replication fork stability by different mechanisms

The checkpoint mediator protein Claspin facilitates the phosphorylation and activation of Chk1 by ATR and thus is required for efficient DNA replication. However, the physical association of Claspin homologues with replication factors and forks suggests that it might have additional functions in controlling DNA replication. DNA combing was used to examine the functions of Chk1 and Claspin at individual forks and to determine whether Claspin functions independently of Chk1. We find that Claspin, like Chk1, regulates fork stability and density in unperturbed cells. As expected, Chk1 regulates origin firing predominantly by controlling Cdk2-Cdc25 function. By contrast, Claspin functions independently of the Cdc25-Cdk2 pathway in mammalian cells. The findings support a model in which Claspin plays a role regulating replication fork stability that is independent of its function in mediating Chk1 phosphorylation.

Regulation of DNA damage response pathways by the cullin-RING ubiquitin ligases

Eukaryotic cells repair ultraviolet light (UV)- and chemical carcinogen-induced DNA strand-distorting damage through the nucleotide excision repair (NER) pathway. Concurrent activation of the DNA damage checkpoints is also required to arrest the cell cycle and allow time for NER action. Recent studies uncovered critical roles for ubiquitin-mediated post-translational modifications in controlling both NER and checkpoint functions. In this review, we will discuss recent progress in delineating the roles of cullin-RING E3 ubiquitin ligases in orchestrating the cellular DNA damage response through ubiquitination of NER factors, histones, and checkpoint effectors.

Nucleoside analogs: molecular mechanisms signaling cell death

Nucleoside analogs are structurally similar antimetabolites that have a broad range of action and are clinically active in both solid tumors and hematological malignancies. Many of these agents are incorporated into DNA by polymerases during normal DNA synthesis, an action that blocks further extension of the nascent strand and causes stalling of replication forks. The molecular mechanisms that sense stalled replication forks activate cell cycle checkpoints and DNA repair processes, which may contribute to drug resistance. When replication forks are not stabilized by these molecules or when subsequent DNA repair processes are overwhelmed, apoptosis is initiated either by these same DNA damage sensors or by alternative mechanisms. Recently, strategies aimed at targeting DNA damage checkpoints or DNA repair processes have demonstrated effectiveness in sensitizing cells to nucleoside analogs, thus offering a means to elude drug resistance. In addition to their DNA synthesis-directed actions many nucleoside analogs trigger apoptosis by unique mechanisms, such as causing epigenetic modifications or by direct activation of the apoptosome. A review of the cellular and molecular responses to clinically relevant agents provides an understanding of the mechanisms that cause apoptosis and may provide rationale for the development of novel therapeutic strategies.

An essential role for MCL-1 in ATR-mediated CHK1 phosphorylation

Here we report a novel role for myeloid cell leukemia 1 (Mcl-1), a Bcl-2 family member, in regulating phosphorylation and activation of DNA damage checkpoint kinase, Chk1. Increased expression of nuclear Mcl-1 and/or a previously reported short nuclear form of Mcl-1, snMcl-1, was observed in response to treatment with low concentrations of etoposide or low doses of UV irradiation. We showed that after etoposide treatment, Mcl-1 could coimmunoprecipitate with the regulatory kinase, Chk1. Chk1 is a known regulator of DNA damage response, and its phosphorylation is associated with activation of the kinase. Transient transfection with Mcl-1 resulted in an increase in the expression of phospho-Ser345 Chk1, in the absence of any evidence of DNA damage, and accumulation of cells in G2. Importantly, knockdown of Mcl-1 expression abolished Chk1 phosphorylation in response to DNA damage. Mcl-1 could induce Chk1 phosphorylation in ATM-negative (ataxia telangectasia mutated) cells, but this response was lost in ATR (AT mutated and Rad3 related)-defective cells. Low levels of UV treatment also caused transient increases in Mcl-1 levels and an ATR-dependent phosphorylation of Chk1. Together, our results strongly support an essential regulatory role for Mcl-1, perhaps acting as an adaptor protein, in controlling the ATR-mediated regulation of Chk1 phosphorylation.

A conserved proliferating cell nuclear antigen-interacting protein sequence in Chk1 is required for checkpoint function

Human checkpoint kinase 1 (Chk1) is an essential kinase required for cell cycle checkpoints and for coordination of DNA synthesis. To gain insight into the mechanisms by which Chk1 carries out these functions, we used mass spectrometry to identify previously uncharacterized interacting partners of Chk1. We describe a novel interaction between Chk1 and proliferating cell nuclear antigen (PCNA), an essential component of the replication machinery. Binding between Chk1 and PCNA was reduced in the presence of hydroxyurea, suggesting that the interaction is regulated by replication stress. A highly conserved PCNA-interacting protein (PIP) box motif was identified in Chk1. The intact PIP box is required for efficient DNA damage-induced phosphorylation and release of activated Chk1 from chromatin. We find that the PIP box of Chk1 is crucial for Chk1-mediated S-M and G(2)-M checkpoint responses. In addition, we show that mutations in the PIP box of Chk1 lead to decreased rates of replication fork progression and increased aberrant replication. These findings suggest an additional mechanism by which essential components of the DNA replication machinery interact with the replication checkpoint apparatus.

The human homolog of fission yeast Rad17 is implicated in tumor growth

The Schizosaccharomyces pombe rad17 is a checkpoint protein critical for maintenance of genomic stability. Since the loss of checkpoint control is a common feature of tumor cells, we investigated the biological function of the human homolog hRAD17. Expression of hRAD17 in a fission yeast rad17 deleted strain reduced growth of yeast colonies and caused slower progression through cell cycle. Immunoprecipitated hRad17 exhibited exonuclease activity. hRAD17 delayed growth of NIH3T3 fibroblasts transformed by the H-ras oncogene in nude mice. Our results support that hRAD17, similarly to other human genes involved in checkpoint mechanisms, plays a role in control of tumor growth.

Germline alterations in the CLSPN gene in breast cancer families

About 5-10% of breast cancer is thought to be due to an inherited disease predisposition. Currently known genes account for less than half of the hereditary cases. Claspin, a tumor suppressor protein encoded by the CLSPN gene, is involved in monitoring of replication and sensoring of DNA damage and cooperates with CHK1 and BRCA1. Association with certain cell proliferation stimulatory features has also been described. Many previously identified susceptibility factors act in similar functional pathways as claspin, suggesting possible involvement of CLSPN in heritable breast cancer susceptibility. Here we have screened affected index cases from 125 Finnish cancer families for germline defects in CLSPN using conformation sensitive gel electrophoresis (CSGE) and direct sequencing. Altogether seven different sequence changes were observed, but none of them appeared to associate with breast cancer susceptibility. To our knowledge, this is the first study reporting the mutation screening of the CLSPN gene in familial breast cancer cases.

Claspin promotes normal replication fork rates in human cells

The S phase-specific adaptor protein Claspin mediates the checkpoint response to replication stress by facilitating phosphorylation of Chk1 by ataxia-telangiectasia and Rad3-related (ATR). Evidence suggests that these components of the ATR pathway also play a critical role during physiological S phase. Chk1 is required for high rates of global replication fork progression, and Claspin interacts with the replication machinery and might therefore monitor normal DNA replication. Here, we have used DNA fiber labeling to investigate, for the first time, whether human Claspin is required for high rates of replication fork progression during normal S phase. We report that Claspin-depleted HeLa and HCT116 cells display levels of replication fork slowing similar to those observed in Chk1-depleted cells. This was also true in primary human 1BR3 fibroblasts, albeit to a lesser extent, suggesting that Claspin is a universal requirement for high replication fork rates in human cells. Interestingly, Claspin-depleted cells retained significant levels of Chk1 phosphorylation at both Ser317 and Ser345, raising the possibility that Claspin function during normal fork progression may extend beyond facilitating phosphorylation of either individual residue. Consistent with this possibility, depletion of Chk1 and Claspin together doubled the percentage of very slow forks, compared with depletion of either protein alone.

G2/M checkpoint stringency is a key parameter in the sensitivity of AML cells to genotoxic stress

Acute myeloid leukemia (AML) cells exposed to genotoxic agents arrest their cell cycle at the G2/M checkpoint and are inherently chemoresistant. To understand the mechanism of this chemoresistance, we compared the ability of immature CD34+ versus mature CD34- AML cell lines (KG1a and U937, respectively) to recover from a DNA damage-induced cell cycle checkpoint in G2. Here, we report that KG1a cells have a more stringent G2/M checkpoint response than U937 cells. We show that in both cell types, the CDC25B phosphatase participates in the G2/M checkpoint recovery and that its expression is upregulated. Furthermore, we show that CHK1 inhibition by UCN-01 in immature KG1a cells allows checkpoint exit and induces sensitivity to genotoxic agents. Similarly, UCN-01 treatment potentializes genotoxic-induced inhibition of colony formation efficiency of primary leukemic cells from AML patients. Altogether, our results demonstrate that checkpoint stringency varies during the maturation process and indicate that targeting checkpoint mechanisms might represent an attractive therapeutic opportunity for chemoresistant immature AML cells.

The ATR-mediated S phase checkpoint prevents rereplication in mammalian cells when licensing control is disrupted

DNA replication in eukaryotic cells is tightly controlled by a licensing mechanism, ensuring that each origin fires once and only once per cell cycle. We demonstrate that the ataxia telangiectasia and Rad3 related (ATR)–mediated S phase checkpoint acts as a surveillance mechanism to prevent rereplication. Thus, disruption of licensing control will not induce significant rereplication in mammalian cells when the ATR checkpoint is intact. We also demonstrate that single-stranded DNA (ssDNA) is the initial signal that activates the checkpoint when licensing control is compromised in mammalian cells. We demonstrate that uncontrolled DNA unwinding by minichromosome maintenance proteins upon Cdt1 overexpression is an important mechanism that leads to ssDNA accumulation and checkpoint activation. Furthermore, we show that replication protein A 2 and retinoblastoma protein are both downstream targets for ATR that are important for the inhibition of DNA rereplication. We reveal the molecular mechanisms by which the ATR-mediated S phase checkpoint pathway prevents DNA rereplication and thus significantly improve our understanding of how rereplication is prevented in mammalian cells.

Tipin is required for stalled replication forks to resume DNA replication after removal of aphidicolin in Xenopus egg extracts

Tipin and its interacting partner Tim1 (Timeless) form a complex at replication forks that plays an important role in the DNA damage checkpoint response. Here we identify Xenopus laevis Tipin as a substrate for cyclin E/cyclin-dependent kinases 2 that is phosphorylated in interphase and undergoes further phosphorylation upon entry into mitosis. During unperturbed DNA replication, the Tipin/Tim1 complex is bound to chromatin, and we were able to detect interactions between Tipin and the MCM helicase. Depletion of Tipin from Xenopus extracts did not significantly impair normal replication but substantially blocked the ability of stalled replication forks to recover after removal of a block imposed by aphidicolin. Tipin-depleted extracts also showed defects in the activation of Chk1 in response to aphidicolin, probably because of a failure to load the checkpoint mediator protein Claspin onto chromatin.

The Rad9-Hus1-Rad1 Checkpoint Clamp Regulates Interaction of TopBP1 with ATR

TopBP1 serves as an activator of the ATR-ATRIP complex in response to the presence of incompletely replicated or damaged DNA. This process involves binding of ATR to the ATR-activating domain of TopBP1, which is located between BRCT domains VI and VII. TopBP1 displays increased binding to ATR-ATRIP in Xenopus egg extracts containing checkpoint-inducing DNA templates. We show that an N-terminal region of TopBP1 containing BRCT repeats I-II is essential for this checkpoint-stimulated binding of TopBP1 to ATR-ATRIP. The BRCT I-II region of TopBP1 also binds specifically to the Rad9-Hus1-Rad1 (9-1-1) complex in Xenopus egg extracts. This binding occurs via the C-terminal domain of Rad9 and depends upon phosphorylation of its Ser-373 residue. Egg extracts containing either a mutant of TopBP1 lacking the BRCT I-II repeats or a mutant of Rad9 with an alanine substitution at Ser-373 are defective in checkpoint regulation. Furthermore, an isolated C-terminal fragment from Rad9 is an effective inhibitor of checkpoint signaling in egg extracts. These findings suggest that interaction of the 9-1-1 complex with the BRCT I-II region of TopBP1 is necessary for binding of ATR-ATRIP to the ATR-activating domain of TopBP1 and the ensuing activation of ATR.

Cleavage and degradation of Claspin during apoptosis by caspases and the proteasome

Apoptosis plays a crucial role in development and tissue homeostasis. Some key survival pathways, such as DNA damage checkpoints and DNA repair, have been described to be inactivated during apoptosis. Here, we describe the processing of the human checkpoint protein Claspin during apoptosis. We observed cleavage of Claspin into multiple fragments in vivo. In vitro cleavage with caspases 3 and 7 of various fragments of the protein, revealed cut sites near the N- and C-termini of the protein. Using mass spectrometry, we identified a novel caspase cleavage site in Claspin at Asp25. Importantly, in addition to cleavage by caspases, we observed a proteasome-dependent degradation of Claspin under apoptotic conditions, resulting in a reduction of the levels of both full-length Claspin and its cleavage products. This degradation was not dependent upon the DSGxxS phosphodegron motif required for SCF(beta-TrCP)-mediated ubiquitination of Claspin. Finally, downregulation of Claspin protein levels by short interfering RNA resulted in an increase in apoptotic induction both in the presence and absence of DNA damage. We conclude that Claspin has antiapoptotic activity and is degraded by two different pathways during apoptosis. The resulting disappearance of Claspin from the cells further promotes apoptosis.

The human Tim/Tipin complex coordinates an Intra-S checkpoint response to UV that slows replication fork displacement

UV-induced DNA damage stalls DNA replication forks and activates the intra-S checkpoint to inhibit replicon initiation. In response to stalled replication forks, ATR phosphorylates and activates the transducer kinase Chk1 through interactions with the mediator proteins TopBP1, Claspin, and Timeless (Tim). Murine Tim recently was shown to form a complex with Tim-interacting protein (Tipin), and a similar complex was shown to exist in human cells. Knockdown of Tipin using small interfering RNA reduced the expression of Tim and reversed the intra-S checkpoint response to UVC. Tipin interacted with replication protein A (RPA) and RPA-coated DNA, and RPA promoted the loading of Tipin onto RPA-free DNA. Immunofluorescence analysis of spread DNA fibers showed that treating HeLa cells with 2.5 J/m(2) UVC not only inhibited the initiation of new replicons but also reduced the rate of chain elongation at active replication forks. The depletion of Tim and Tipin reversed the UV-induced inhibition of replicon initiation but affected the rate of DNA synthesis at replication forks in different ways. In undamaged cells depleted of Tim, the apparent rate of replication fork progression was 52% of the control. In contrast, Tipin depletion had little or no effect on fork progression in unirradiated cells but significantly attenuated the UV-induced inhibition of DNA chain elongation. Together, these findings indicate that the Tim-Tipin complex mediates the UV-induced intra-S checkpoint, Tim is needed to maintain DNA replication fork movement in the absence of damage, Tipin interacts with RPA on DNA and, in UV-damaged cells, Tipin slows DNA chain elongation in active replicons.

Cdc7-Dbf4 and the human S checkpoint response to UVC

The S checkpoint response to ultraviolet radiation (UVC) that inhibits replicon initiation is dependent on the ATR and Chk1 kinases. Downstream effectors of this response, however, are not well characterized. Data reported here eliminated Cdc25A degradation and inhibition of Cdk2-cyclin E as intrinsic components of the UVC-induced pathway of inhibition of replicon initiation in human cells. A sublethal dose of UVC (1 J/m(2)), which selectively inhibits replicon initiation by 50%, failed to reduce the amount of Cdc25A protein or decrease Cdk2-cyclin E kinase activity. Cdc25A degradation was observed after irradiation with cytotoxic fluences of UVC, suggesting that severe inhibition of DNA chain elongation and activation of the replication checkpoint might be responsible for the UVC-induced degradation of Cdc25A. Another proposed effector of the S checkpoint is the Cdc7-Dbf4 complex. Dbf4 interacted weakly with Chk1 in vivo but was recognized as a substrate for Chk1-dependent phosphorylation in vitro. FLAG-Dbf4 formed complexes with endogenous Cdc7, and this interaction was stable in UVC-irradiated HeLa cells. Overexpression of FLAG- or Myc-tagged Dbf4 abrogated the S checkpoint response to UVC but not ionizing radiation. These findings implicate a Dbf4-dependent kinase as a possible target of the ATR- and Chk1-dependent S checkpoint response to UVC.

HCLK2 is essential for the mammalian S-phase checkpoint and impacts on Chk1 stability

Here, we show that the human homologue of the Caenorhabditis elegans biological clock protein CLK-2 (HCLK2) associates with the S-phase checkpoint components ATR, ATRIP, claspin and Chk1. Consistent with a critical role in the S-phase checkpoint, HCLK2-depleted cells accumulate spontaneous DNA damage in S-phase, exhibit radio-resistant DNA synthesis, are impaired for damage-induced monoubiquitination of FANCD2 and fail to recruit FANCD2 and Rad51 (critical components of the Fanconi anaemia and homologous recombination pathways, respectively) to sites of replication stress. Although Thr 68 phosphorylation of the checkpoint effector kinase Chk2 remains intact in the absence of HCLK2, claspin phosphorylation and degradation of the checkpoint phosphatase Cdc25A are compromised following replication stress as a result of accelerated Chk1 degradation. ATR phosphorylation is known to both activate Chk1 and target it for proteolytic degradation, and depleting ATR or mutation of Chk1 at Ser 345 restored Chk1 protein levels in HCLK2-depleted cells. We conclude that HCLK2 promotes activation of the S-phase checkpoint and downstream repair responses by preventing unscheduled Chk1 degradation by the proteasome.

Perturbed gap-filling synthesis in nucleotide excision repair causes histone H2AX phosphorylation in human quiescent cells

Human histone H2AX is rapidly phosphorylated on serine 139 in response to DNA double-strand breaks and plays a crucial role in tethering the factors involved in DNA repair and damage signaling. Replication stress caused by hydroxyurea or UV also initiates H2AX phosphorylation in S-phase cells, although UV-induced H2AX phosphorylation in non-cycling cells has recently been observed. Here we study the UV-induced H2AX phosphorylation in human primary fibroblasts under growth-arrested conditions. This reaction absolutely depends on nucleotide excision repair (NER) and is mechanistically distinct from the replication stress-induced phosphorylation. The treatment of cytosine-beta-D-arabinofuranoside strikingly enhances the NER-dependent H2AX phosphorylation and induces the accumulation of replication protein A (RPA) and ATR-interacting protein (ATRIP) at locally UV-damaged subnuclear regions. Consistently, the phosphorylation appears to be mainly mediated by ataxia-telangiectasia mutated and Rad3-related (ATR), although Chk1 (Ser345) is not phosphorylated by the activated ATR. The cellular levels of DNA polymerases delta and epsilon and proliferating cell nuclear antigen are markedly reduced in quiescent cells. We propose a model that perturbed gap-filling synthesis following dual incision in NER generates single-strand DNA gaps and hence initiates H2AX phosphorylation by ATR with the aid of RPA and ATRIP.

Linking PCNA-dependent replication and ATR by human Claspin

Recent studies in Xenopus have identified a new checkpoint protein called Claspin that is believed to transduce the checkpoint DNA damage signals to Chk1 kinase. Here we show that the human Claspin homolog is a chromatin bound protein either in the absence or in the presence of damaged DNA, independent of its association with ATR. Furthermore, we show that human Claspin is found in complex with PCNA, an essential component of the DNA replication machinery, and is released upon DNA replication arrest. Interfering with PCNA function by overexpression of p21 mutant, impaired in its interaction with Cdks but not with PCNA, leads to ATR-dependent Chk1 activation. These findings suggest that the dissociation of Claspin-PCNA could be part of the signal leading to Chk1 activation.

Phosphatase type 2A-dependent and -independent pathways for ATR phosphorylation of Chk1

ATM and Rad3-related (ATR) is a regulatory kinase that, when activated by hydroxyurea, UV, or human immunodeficiency virus-1 Vpr, causes cell cycle arrest through Chk1-Ser(345) phosphorylation. We demonstrate here that of these three agents only Vpr requires protein phosphatase type 2A (PP2A) to activate ATR for Chk1-Ser(345) phosphorylation. A requirement for PP2A by Vpr was first shown with the PP2A-specific inhibitor okadaic acid, which reduced Vpr-induced G(2) arrest and Cdk1-Tyr(15) phosphorylation. Using small interference RNA to down-regulate specific subunits of PP2A indicated that the catalytic beta-isoform PP2A(Cbeta) and the A regulatory alpha-isoform PP2A(Aalpha) are involved in the G(2) induction, and these downregulations decreased the Vpr-induced, ATR-dependent phosphorylations of Cdk1-Tyr(15) and Chk1-Ser(345). In contrast, the same down-regulations had no effect on hydroxyurea- or UV-activated ATR-dependent Chk1-Ser(345) phosphorylation. Vpr and hydroxyurea/UV all induce ATR-mediated gammaH2AX-Ser(139) phosphorylation and foci formation, but down-regulation of PP2A(Aalpha) or PP2A(Cbeta) did not decrease gammaH2AX-Ser(139) phosphorylation by any of these agents or foci formation by Vpr. Conversely, H2AX down-regulation had little effect on PP2A(Aalpha/Cbeta)-mediated G(2) arrest and Chk1-Ser(345) phosphorylation by Vpr. The expression of vpr increases the amount and phosphorylation of Claspin, an activator of Chk1 phosphorylation. Down-regulation of either PP2A(Cbeta) or PP2A(Aalpha) had little effect on Claspin phosphorylation, but the amount of Claspin was reduced. Claspin may then be one of the phosphoproteins through which PP2A(Aalpha/Cbeta) affects Chk1 phosphorylation when ATR is activated by human immunodeficiency virus-1 Vpr.

Human Tim/Timeless-interacting Protein, Tipin, Is Required for Efficient Progression of S Phase and DNA Replication Checkpoint

Tipin was originally isolated as a protein interacting with Timeless/Tim1/Tim (Tim), which is known to be involved in both circadian rhythm and cell cycle checkpoint regulation. The endogenous Tim and Tipin proteins in human cells, interacting through the N-terminal segment of each molecule, form a complex throughout the cell cycle. Tipin and Tim are expressed in the interphase nuclei mostly at constant levels during the cell cycle, and small fractions are recovered in the chromatin-enriched fractions during S phase. Depletion of endogenous Tipin results in reduced growth rate, and this may be due in part to inefficient progression of S phase and DNA synthesis. Knockdown of Tipin induces radioresistant DNA synthesis and inhibits phosphorylation of Chk1 kinase caused by replication stress, as was observed with that of Tim. Knockdown of Tipin or Tim results in reduced protein level and relocation to the cytoplasm of the respective binding partner, suggesting that the complex formation may be required for stabilization and nuclear accumulation of both proteins. Furthermore, both Tipin and Tim may facilitate the accumulation of Claspin in the nuclei under replication stress, whereas nuclear localization of Tipin and Tim is unaffected by Claspin. Our results indicate that mammalian Tipin is a checkpoint mediator that cooperates with Tim and may regulate the nuclear relocation of Claspin in response to replication checkpoint.

Evaluation of claspin as a proliferation marker in human cancer and normal tissues

Claspin is a nuclear protein involved in DNA replication and the DNA damage response. Its structural and functional properties suggest that it may represent a potentially useful proliferation marker. To this end, a monoclonal antibody was generated and the expression of claspin was investigated in normal fibroblasts and various cancer cell lines, as well as in tumour and normal tissues from patients with primary epithelial carcinomas. Immunoblotting analysis confirmed the specificity of the antibody, while immunohistochemistry demonstrated its applicability in archival material. In normal cells and tissues, claspin expression was weak, whereas increased levels were observed in cancer cell lines and tumour specimens. Claspin staining correlated strongly with Ki67 staining in both normal (p < 0.001) and tumour tissues (p < 0.001). However, the labelling index (LI) of claspin was consistently lower than that of Ki67, suggesting that claspin expression may be limited to a narrower part of the cell cycle. Co-localization assays with cyclin A and cell synchronization experiments indicated that claspin expression coincides with the S phase. Interestingly, the relative increase of the claspin LI in tumour samples compared with normal tissues was significantly higher (14-fold) than that of the Ki67 LI (five-fold), suggesting that claspin may be a more sensitive marker of aberrant proliferation.

5-ASA affects cell cycle progression in colorectal cells by reversibly activating a replication checkpoint

BACKGROUND & AIMS

Individuals with inflammatory bowel disease are at risk of developing colorectal cancer (CRC). Epidemiologic, animal, and laboratory studies suggest that 5-amino-salicylic acid (5-ASA) protects from the development of CRC by altering cell cycle progression and by inducing apoptosis. Our previous results indicate that 5-ASA improves replication fidelity in colorectal cells, an effect that is active in reducing mutations. In this study, we hypothesized that 5-ASA restrains cell cycle progression by activating checkpoint pathways in colorectal cell lines, which would prevent tumor development and improve genomic stability.

METHODS

CRC cells with different genetic backgrounds such as HT29, HCT116, HCT116(p53-/-), HCT116+chr3, and LoVo were treated with 5-ASA for 2-96 hours. Cell cycle progression, phosphorylation, and DNA binding of cell cycle checkpoint proteins were analyzed.

RESULTS

We found that 5-ASA at concentrations between 10 and 40 mmol/L affects cell cycle progression by inducing cells to accumulate in the S phase. This effect was independent of the hMLH1, hMSH2, and p53 status because it was observed to a similar extent in all cell lines under investigation. Moreover, wash-out experiments demonstrated reversibility within 48 hours. Although p53 did not have a causative role, p53 Ser15 was strongly phosphorylated. Proteins involved in the ATM-and-Rad3-related kinase (ATR)-dependent S-phase checkpoint response (Chk1 and Rad17) were also phosphorylated but not ataxia telengectasia mutated kinase.

CONCLUSIONS

Our data demonstrate that 5-ASA causes cells to reversibly accumulate in S phase and activate an ATR-dependent checkpoint. The activation of replication checkpoint may slow down DNA replication and improve DNA replication fidelity, which increases the maintenance of genomic stability and counteracts carcinogenesis.