Radiation effects on DNA synthesis in a defined chromosomal replicon

Altered radiation responses of breast cancer cells resistant to hormonal therapy

Endocrine therapy agents (the selective estrogen receptor (ER) modulators such as tamoxifen or the selective ER down-regulators such as ICI 182,780) are key treatment regimens for hormone receptor-positive breast cancers. While these drugs are very effective in controlling ER-positive breast cancer, many tumors that initially respond well to treatment often acquire drug resistance, which is a major clinical problem. In clinical practice, hormonal therapy agents are commonly used in combination or sequence with radiation therapy. Tamoxifen treatment and radiotherapy improve both local tumor control and patient survival. However, tamoxifen treatment may render cancer cells less responsive to radiation therapy. Only a handful of data exist on the effects of radiation on cells resistant to hormonal therapy agents. These scarce data show that cells that were resistant to tamoxifen were also resistant to radiation. Yet, the existence and mechanisms of cross-resistance to endocrine therapy and radiation therapy need to be established. Here, we for the first time examined and compared radiation responses of MCF-7 breast adenocarcinoma cells (MCF-7/S0.5) and two antiestrogen resistant cell lines derived from MCF-7/S0.5: the tamoxifen resistant MCF-7/TAMR-1 and ICI 182,780 resistant MCF-7/182R-6 cell lines. Specifically, we analyzed the radiation-induced changes in the expression of genes involved in DNA damage, apoptosis, and cell cycle regulation. We found that the tamoxifen-resistant cell line in contrast to the parental and ICI 182,780-resistant cell lines displayed a significantly less radiation-induced decrease in the expression of genes involved in DNA repair. Furthermore, we show that MCF-7/TAMR-1 and MCF-7/182R-6 cells were less susceptible to radiation-induced apoptosis as compared to the parental line. These data indicate that tamoxifen-resistant breast cancer cells have a reduced sensitivity to radiation treatment. The current study may therefore serve as a roadmap to the future analysis of the mechanisms of cross-resistance between hormonal therapy and radiation.

Surviving chromosome replication: the many roles of the S-phase checkpoint pathway

Checkpoints were originally identified as signalling pathways that delay mitosis in response to DNA damage or defects in chromosome replication, allowing time for DNA repair to occur. The ATR (ataxia- and rad-related) and ATM (ataxia-mutated) protein kinases are recruited to defective replication forks or to sites of DNA damage, and are thought to initiate the DNA damage response in all eukaryotes. In addition to delaying cell cycle progression, however, the S-phase checkpoint pathway also controls chromosome replication and DNA repair pathways in a highly complex fashion, in order to preserve genome integrity. Much of our understanding of this regulation has come from studies of yeasts, in which the best-characterized targets are the stimulation of ribonucleotide reductase activity by multiple mechanisms, and the inhibition of new initiation events at later origins of DNA replication. In addition, however, the S-phase checkpoint also plays a more enigmatic and apparently critical role in preserving the functional integrity of defective replication forks, by mechanisms that are still understood poorly. This review considers some of the key experiments that have led to our current understanding of this highly complex pathway.

Damage-induced phosphorylation of Sld3 is important to block late origin firing

Origins of replication are activated throughout the S phase of the cell cycle such that some origins fire early and others fire late to ensure that each chromosome is completely replicated in a timely fashion. However, in response to DNA damage or replication fork stalling, eukaryotic cells block activation of unfired origins. Human cells derived from patients with ataxia telangiectasia are deficient in this process due to the lack of a functional ataxia telangiectasia mutated (ATM) kinase and elicit radioresistant DNA synthesis after γ-irradiation(2). This effect is conserved in budding yeast, as yeast cells lacking the related kinase Mec1 (ATM and Rad3-related (ATR in humans)) also fail to inhibit DNA synthesis in the presence of DNA damage. This intra-S-phase checkpoint actively regulates DNA synthesis by inhibiting the firing of late replicating origins, and this inhibition requires both Mec1 and the downstream checkpoint kinase Rad53 (Chk2 in humans). However, the Rad53 substrate(s) whose phosphorylation is required to mediate this function has remained unknown. Here we show that the replication initiation protein Sld3 is phosphorylated by Rad53, and that this phosphorylation, along with phosphorylation of the Cdc7 kinase regulatory subunit Dbf4, blocks late origin firing in Saccharomyces cerevisiae. Upon exposure to DNA-damaging agents, cells expressing non-phosphorylatable alleles of SLD3 and DBF4 (SLD3-m25 and dbf4-m25, respectively) proceed through the S phase faster than wild-type cells by inappropriately firing late origins of replication. SLD3-m25 dbf4-m25 cells grow poorly in the presence of the replication inhibitor hydroxyurea and accumulate multiple Rad52 foci. Moreover, SLD3-m25 dbf4-m25 cells are delayed in recovering from transient blocks to replication and subsequently arrest at the DNA damage checkpoint. These data indicate that the intra-S-phase checkpoint functions to block late origin firing in adverse conditions to prevent genomic instability and maximize cell survival.

Regulation of DNA replication by the S-phase DNA damage checkpoint

Cells slow replication in response to DNA damage. This slowing was the first DNA damage checkpoint response discovered and its study led to the discovery of the central checkpoint kinase, Ataxia Telangiectasia Mutated (ATM). Nonetheless, the manner by which the S-phase DNA damage checkpoint slows replication is still unclear. The checkpoint could slow bulk replication by inhibiting replication origin firing or slowing replication fork progression, and both mechanisms appear to be used. However, assays in various systems using different DNA damaging agents have produced conflicting results as to the relative importance of the two mechanisms. Furthermore, although progress has been made in elucidating the mechanism of origin regulation in vertebrates, the mechanism by which forks are slowed remains unknown. We review both past and present efforts towards determining how cells slow replication in response to damage and try to resolve apparent conflicts and discrepancies within the field. We propose that inhibition of origin firing is a global checkpoint mechanism that reduces overall DNA synthesis whenever the checkpoint is activated, whereas slowing of fork progression reflects a local checkpoint mechanism that only affects replisomes as they encounter DNA damage and therefore only affects overall replication rates in cases of high lesion density.

Checkpoint-dependent regulation of origin firing and replication fork movement in response to DNA damage in fission yeast

To elucidate the checkpoint mechanism responsible for slowing passage through S phase when fission yeast cells are treated with the DNA-damaging agent methyl methanesulfonate (MMS), we carried out two-dimensional gel analyses of replication intermediates in cells synchronized by cdc10 block (in G(1)) followed by release into synchronous S phase. The results indicated that under these conditions early-firing centromeric origins were partially delayed but late-firing telomeric origins were not delayed. Replication intermediates persisted in MMS-treated cells, suggesting that replication fork movement was inhibited. These effects were dependent on the Cds1 checkpoint kinase and were abolished in cells overexpressing the Cdc25 phosphatase, suggesting a role for the Cdc2 cyclin-dependent kinase. We conclude that both partial inhibition of the firing of a subset of origins and inhibition of replication fork movement contribute to the slowing of S phase in MMS-treated fission yeast cells.

ORC and the intra-S-phase checkpoint: a threshold regulates Rad53p activation in S phase

The intra-S-phase checkpoint in yeast responds to stalled replication forks by activating the ATM-like kinase Mec1 and the CHK2-related kinase Rad53, which in turn inhibit spindle elongation and late origin firing and lead to a stabilization of DNA polymerases at arrested forks. A mutation that destabilizes the second subunit of the Origin Recognition Complex, orc2-1, reduces the number of functional replication forks by 30% and severely compromises the activation of Rad53 by replication stress or DNA damage in S phase. We show that the restoration of the checkpoint response correlates in a dose-dependent manner with the restoration of pre-replication complex formation in G1. Other forms of DNA damage can compensate for the reduced level of fork-dependent signal in the orc2-1 mutant, yet even in wild-type cells, the amount of damage required for Rad53 activation is higher in S phase than in G2. Our data suggest the existence of an S-phase-specific threshold that may be necessary to allow cells to tolerate damage-like DNA structures present at normal replication forks.

Inactivation of p21 by E1A leads to the induction of apoptosis in DNA-damaged cells

A major impediment to successful chemotherapy is the propensity for some tumor cells to undergo cell cycle arrest rather than apoptosis. It is well established, however, that the adenovirus E1A protein can sensitize these cells to the induction of apoptosis by anticancer agents. To further understand how E1A enhances chemosensitivity, we have made use of a human colon carcinoma cell line (HCT116) which typically undergoes cell cycle arrest in response to chemotherapeutic drugs. As seen by the analysis of E1A mutants, we show here that E1A can induce apoptosis in these cells by neutralizing the activities of the cyclin-dependent kinase inhibitor p21. E1A's ability to interact with p21 and thereby restore Cdk2 activity in DNA-damaged cells correlates with the reversal of G(1) arrest, which in turn leads to apoptosis. Analysis of E1A mutants failing to bind p300 (also called CBP) or Rb shows that they are almost identical to wild-type E1A in their ability to initially overcome a G(1) arrest in cells after DNA damage, while an E1A mutant failing to bind p21 is not. However, over time, this mutant, which can still target Rb, is far more efficient in accumulating cells with a DNA content greater than 4N but is similar to wild-type E1A and the other E1A mutants in releasing cells from a p53-mediated G(2) block following chemotherapeutic treatment. Thus, we suggest that although E1A requires the binding of p21 to create an optimum environment for apoptosis to occur in DNA-damaged cells, E1A's involvement in other pathways may be contributing to this process as well. A model is proposed to explain the implications of these findings.

Temporally coordinated assembly and disassembly of replication factories in the absence of DNA synthesis

Here we show that exposure of aphidicolin-arrested Chinese hamster ovary (CHO) cells to the protein-kinase inhibitors 2-aminopurine or caffeine results in initiation of replication at successively later-replicating chromosomal domains, loss of the capacity to synthesize DNA at earlier-replicating sites, release of Mcm2 proteins from chromatin, and redistribution of PCNA and RPA from early- to late-replicating domains in the absence of detectable elongation of replication forks. These results provide evidence that, under conditions of replicational stress, checkpoint controls not only prevent further initiation but may also be required to actively maintain the integrity of stalled replication complexes.

The catalytic subunit DNA-dependent protein kinase (DNA-PKcs) facilitates recovery from radiation-induced inhibition of DNA replication

Exposure of cells to ionizing radiation inhibits DNA replication in a dose-dependent manner. The dose response is biphasic and the initial steep component reflects inhibition of replicon initiation thought to be mediated by activation of the S-phase checkpoint. In mammalian cells, inhibition of replicon initiation requires the ataxia telagiectasia mutated ( ATM ) gene, a member of the phosphatidyl inositol kinase-like (PIKL) family of protein kinases. We studied the effect on replicon initiation of another member of the PI-3 family of protein kinases, the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) by measuring either total DNA synthesis, or size distribution of nascent DNA using alkaline sucrose gradient centrifugation. Exposure of human cells proficient in DNA-PKcs (HeLa or M059-K) to 10 Gy inhibited replicon initiation in a time-dependent manner. Inhibition was at a maximum 1 h after irradiation and recovered at later times. Similar treatment of human cells deficient in DNA-PKcs (M059-J) inhibited replicon initiation to a similar level and with similar kinetics; however, no evidence for recovery, or only limited recovery, was observed for up to 8 h after irradiation. In addition a defect was observed in the maturation of nascent DNA. Similarly, a Chinese hamster cell line deficient in DNA-PKcs (irs-20) showed little evidence for recovery of DNA replication inhibition up to 6 h after irradiation, whereas the parental CHO cells showed significant recovery and an irs-20 derivative expressing the human DNA-PKcs complete recovery within 4 h. Normal kinetics of recovery were observed in xrs-5 cells, deficient in Ku80; in 180BR cells, deficient in DNA ligase IV; as well as XR-1 cells, deficient in XRCC4, an accessory factor of DNA ligase IV. Since all these cell lines share the DNA double strand break rejoining defect of M059-J and irs20 cells, the lack of recovery of DNA replication in the latter cells may not be attributed entirely to the prolonged presence of unrepaired DNA dsb. We propose that DNA-PKcs, in addition to its functions in the rejoining of DNA dsb and in DNA replication, also operates in a pathway that in normal cells facilitates recovery of DNA replication after irradiation.

Fragments of ATM which have dominant-negative or complementing activity

The ATM protein has been implicated in pathways controlling cell cycle checkpoints, radiosensitivity, genetic instability, and aging. Expression of ATM fragments containing a leucine zipper motif in a human tumor cell line abrogated the S-phase checkpoint after ionizing irradiation and enhanced radiosensitivity and chromosomal breakage. These fragments did not abrogate irradiation-induced G1 or G2 checkpoints, suggesting that cell cycle checkpoint defects alone cannot account for chromosomal instability in ataxia telangiectasia (AT) cells. Expression of the carboxy-terminal portion of ATM, which contains the PI-3 kinase domain, complemented radiosensitivity and the S-phase checkpoint and reduced chromosomal breakage after irradiation in AT cells. These observations suggest that ATM function is dependent on interactions with itself or other proteins through the leucine zipper region and that the PI-3 kinase domain contains much of the significant activity of ATM.

A p53-independent damage-sensing mechanism that functions as a checkpoint at the G1/S transition in Chinese hamster ovary cells

In response to a moderate dose of radiation, asynchronous mammalian cell populations rapidly and transiently down-regulate the rate of DNA synthesis to approximately 50% of preirradiation values. We show here that only half of the reduction in overall replication rate can be accounted for by direct inhibition of initiation at origins in S-phase cells. The other half results from the operation of a newly defined cell cycle checkpoint that functions at the G1/S transition. This checkpoint senses damage incurred at any time during the last 2 hr of G1 and effectively prevents entry into the S period. The G1/S and S-phase checkpoints are both p53-independent and, unlike the p53-mediated G1 checkpoint, respond rapidly to radiation, suggesting that they may represent major damage-sensing mechanisms connecting the replication machinery with DNA repair pathways.

Inhibition of initiation of simian virus 40 DNA replication during acute response of cells irradiated by ultraviolet light

To study the mechanism by which ultraviolet (UV) light inhibits DNA replication, we examined the effects of UV 254 nm irradiation on the replication of simian virus 40 (SV40) DNA and SV40-based plasmid in monkey cells. The study was designed to determine the relative contributions made by inhibition of replication initiation and chain elongation to the immediate inhibition of DNA replication following UV irradiation. We used two-dimensional neutral-alkaline electrophoresis to examine the behaviour of replication intermediates unambiguously. Kinetic analysis using this technique showed that initiation of replication started to decline at 15 min post-irradiation. When the pulse label incorporated in SV40 replication intermediates before irradiation was chased for 1 h, most of the label was found in mature Form I and II molecules. This indicated that replication elongation took place on damaged template. We also used a transfection technique to show that heavily irradiated plasmids replicated efficiently in unirradiated transfected cells. By the transfection technique, we observed that UV irradiation of host cells dose-dependently inhibited replication of transfected non-irradiated plasmids, suggesting that the inhibition of DNA replication is due to a global change in cellular physiology induced by UV. This change was also apparent from poor staining of the chromatin by fluorescent-DNA-binding dyes immediately after UV irradiation of intact cells. We conclude that a significant fraction of chain elongation proceeds on damaged templates and DNA replication during the acute response of cells irradiated with UV is mainly controlled by the inhibition of replication initiation.

Replication protein A binds to regulatory elements in yeast DNA repair and DNA metabolism genes

Saccharomyces cerevisiae responds to DNA damage by arresting cell cycle progression (thereby preventing the replication and segregation of damaged chromosomes) and by inducing the expression of numerous genes, some of which are involved in DNA repair, DNA replication, and DNA metabolism. Induction of the S. cerevisiae 3-methyladenine DNA glycosylase repair gene (MAG) by DNA-damaging agents requires one upstream activating sequence (UAS) and two upstream repressing sequences (URS1 and URS2) in the MAG promoter. Sequences similar to the MAG URS elements are present in at least 11 other S. cerevisiae DNA repair and metabolism genes. Replication protein A (Rpa) is known as a single-stranded-DNA-binding protein that is involved in the initiation and elongation steps of DNA replication, nucleotide excision repair, and homologous recombination. We now show that the MAG URS1 and URS2 elements form similar double-stranded, sequence-specific, DNA-protein complexes and that both complexes contain Rpa. Moreover, Rpa appears to bind the MAG URS1-like elements found upstream of 11 other DNA repair and DNA metabolism genes. These results lead us to hypothesize that Rpa may be involved in the regulation of a number of DNA repair and DNA metabolism genes.

Accumulation of wild-type p53 protein upon gamma-irradiation induces a G2 arrest-dependent immunoglobulin kappa light chain gene expression

The exposure of cells to DNA-damaging agents leads to the accumulation of wild-type p53 protein. Furthermore, overexpression of the wild-type p53, mediated by transfection of p53-coding cDNA, induced cells to undergo apoptosis or cell differentiation. In this study we found that the gamma-irradiation that caused the accumulation of wild-type p53 in 70Z/3 pre-B cells induced, in addition to apoptosis, cell differentiation. This was manifested by the expression of the kappa light chain immunoglobulin gene that coincided with the accumulation of cells at the G2 phase. Overexpression of mutant p53 in 70Z/3 cells interferes with both differentiation and accumulation of cells at the G2 phase, as well as with apoptosis, which were induced by gamma-irradiation. Furthermore, the increment in the wild-type p53 protein level following gamma-irradiation was disrupted in the mutant p53 overproducer-derived cell lines. This suggests that mutant p53 may exert a dominant negative effect in all of these activities. Data presented here show that while p53-induced apoptosis is associated with the G1 checkpoint, p53-mediated differentiation, which may be an additional pathway to escape the fixation of genetic errors, may be associated with the G2 growth arrest phase.

Cell cycle regulation in response to DNA damage in mammalian cells: a historical perspective

Cell cycle delay has long been known to occur in mammalian cells after exposure to DNA-damaging agents. It has been hypothesized that the function of this delay is to provide additional time for repair of DNA before the cell enters critical periods of the cell cycle, such as DNA synthesis in S phase or chromosome condensation in G2 phase. Recent evidence that p53 protein is involved in the delay in G1 in response to ionizing radiation has heightened interest in the importance of cell cycle delay, because mutations in p53 are commonly found in human cancer cells. Because mammalian cells defective in p53 protein show increased genomic instability, it is tempting to speculate that the instability is due to increased chromosome damage resulting from the lack of a G1 delay. Although this appears at first glance to be a highly plausible explanation, a review of the research performed on cell cycle regulation and DNA damage in mammalian cells provides little evidence to support this hypothesis. Studies involving cells treated with caffeine, cells from humans with the genetic disease ataxia telangiectasia, and cells that are deficient in p53 show no correlation between G1 delay and increased cell killing or chromosome damage in response to ionizing radiation. Instead, G1 delay appears to be only one aspect of a complex cellular response to DNA damage that also includes delays in S phase and G2 phase, apoptosis and chromosome repair. The exact mechanism of the genomic instability associated with p53, and its relationship to the failure to repair DNA before progression through the cell cycle, remains to be determined.

Mimosine, a novel inhibitor of DNA replication, binds to a 50 kDa protein in Chinese hamster cells

We recently demonstrated that the plant amino acid, mimosine, is an extremely efficacious inhibitor of DNA replication in mammalian cells [P. A. Dijkwel and J. L. Hamlin (1992) Mol. Cell. Biol. 12, 3715-3722; P. J. Mosca et al. (1992) Mol. Cell. Biol. 12, 4375-4383]. Several of its properties further suggested that mimosine might target initiation at origins of replication, which would make it a unique and very useful inhibitor for studying the regulation of DNA synthesis. However, mimosine is known to chelate iron, a cofactor for ribonucleotide reductase. Thus, the possibility arose that mimosine functions in vivo simply by lowering intracellular deoxyribonucleotide pools. In the present study, we show that, in fact, it is possible to override mimosine inhibition in vivo by adding excess iron; however, copper, which is not a substitute for iron in ribonucleotide reductase, is equally effective. Evidence is presented that mimosine functions instead by binding to an intracellular protein. We show that radiolabeled mimosine can be specifically cross-linked to a 50 kDa polypeptide (termed p50) in vitro. Binding to p50 is virtually undetectable in CHO cells selected for resistance to 1 mM mimosine, arguing that p50 is the biologically relevant target. p50 is not associated with the cellular membrane fraction and, hence, is probably not a channel protein. Furthermore, the binding activity does not vary markedly as a function of cell cycle position, arguing that p50 is not a cyclin. Finally, both iron and copper are able to reverse the mimosine-p50 interaction in vitro, probably explaining why both metal ions are able to overcome mimosine's inhibitory effect on DNA synthesis in vivo.