The DNA damage response during mitosis

Distinctive adaptive response to repeated exposure to hydrogen peroxide associated with upregulation of DNA repair genes and cell cycle arrest

Many environmental and physiological stresses are chronic. Thus, cells are constantly exposed to diverse types of genotoxic insults that challenge genome stability, including those that induce oxidative DNA damage. However, most in vitro studies that model cellular response to oxidative stressors employ short exposures and/or acute stress models. In this study, we tested the hypothesis that chronic and repeated exposure to a micromolar concentration of hydrogen peroxide (H₂O₂) could activate DNA damage responses, resulting in cellular adaptations. For this purpose, we developed an in vitro model in which we incubated mouse myoblast cells with a steady concentration of ~50 μM H₂O₂ for one hour daily for seven days, followed by a final challenge of a 10 or 20X higher dose of H₂O₂ (0.5 or 1 mM). We report that intermittent long-term exposure to this oxidative stimulus nearly eliminated cell toxicity and significantly decreased genotoxicity (in particular, a >5-fold decreased in double-strand breaks) resulting from subsequent acute exposure to oxidative stress. This protection was associated with cell cycle arrest in G2/M and induction of expression of nine DNA repair genes. Together, this evidence supports an adaptive response to chronic, low-level oxidative stress that results in genomic protection and up-regulated maintenance of cellular homeostasis.

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Repetitive stimulus with subtoxic H₂O₂ resulted in cellular adaptive response.

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Cells responded by abridged cytotoxicity and reduced intracellular ROS.

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Adaptive response mitigated the genotoxicity of acute oxidative insults.

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Adaption induced overexpression of selected genes of the BER, NER, MMR pathways.

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Chronic and repetitive H₂O₂ resulted in cell cycle checkpoint arrest.

The Growing Complexity of Cancer Cell Response to DNA-Damaging Agents: Caspase 3 Mediates Cell Death or Survival?

It is widely stated that wild-type p53 either mediates the activation of cell cycle checkpoints to facilitate DNA repair and promote cell survival, or orchestrates apoptotic cell death following exposure to cancer therapeutic agents. This reigning paradigm has been challenged by numerous discoveries with different human cell types, including solid tumor-derived cell lines. Thus, activation of the p53 signaling pathway by ionizing radiation and other DNA-damaging agents hinders apoptosis and triggers growth arrest (e.g., through premature senescence) in some genetic backgrounds; such growth arrested cells remain viable, secrete growth-promoting factors, and give rise to progeny with stem cell-like properties. In addition, caspase 3, which is best known for its role in the execution phase of apoptosis, has been recently reported to facilitate (rather than suppress) DNA damage-induced genomic instability and carcinogenesis. This observation is consistent with an earlier report demonstrating that caspase 3 mediates secretion of the pro-survival factor prostaglandin E₂, which in turn promotes enrichment of tumor repopulating cells. In this article, we review these and related discoveries and point out novel cancer therapeutic strategies. One of our objectives is to demonstrate the growing complexity of the DNA damage response beyond the conventional “repair and survive, or die” hypothesis.

Spontaneous tumor development in bone marrow-rescued DNA-PKcs(3A/3A) mice due to dysfunction of telomere leading strand deprotection

Phosphorylation of the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) at the Thr2609 cluster is essential for its complete function in DNA repair and tissue stem cell homeostasis. This phenomenon is demonstrated by congenital bone marrow failure occurring in DNA-PKcs^3A/3A mutant mice, which require bone marrow transplantation (BMT) to prevent early mortality. Surprisingly, an increased incidence of spontaneous tumors, especially skin cancer, was observed in adult BMT-rescued DNA-PKcs^3A/3A mice. Upon further investigation we found that spontaneous γH2AX foci occurred in DNA-PKcs^3A/3A skin biopsies and primary keratinocytes and that these foci overlapped with telomeres during mitosis, indicating impairment of telomere replication and maturation. Consistently, we observed significantly elevated frequencies of telomere fusion events in DNA-PKcs^3A/3A cells as compared to wild type and DNA-PKcs knockout cells. In addition, a previously identified DNA-PKcs Thr2609Pro mutation, found in breast cancer, also induces a similar impairment of telomere leading end maturation. Taken together, our current analyses indicate that the functional DNA-PKcs T2609 cluster is required to facilitate telomere leading strand maturation and prevention of genomic instability and cancer development.

A novel synthetic novobiocin analog, FM-Nov17, induces DNA damage in CML cells through generation of reactive oxygen species

OBJECTIVES

To investigate the cytotoxicity of FM-Nov17 against chronic myeloid leukemia (CML) cells, we explored its underlying mechanisms mediating the induction of DNA damage and apoptotic cell death by reactive oxygen species (ROS).

METHODS

MTT assays were used to measure the proliferation-inhibition ratio of K562 and K562/G01 cells. Flow cytometry (FCM) was used to test the level of extracellular ROS, DNA damage, cell cycle progression and apoptosis. Western blotting was used to verify the amount of protein.

RESULTS

FM-Nov17 significantly inhibited the proliferation of K562 cells, with an IC50 of 58.28±0.304μM, and K562/G01 cells, with an IC50 of 62.36±0.136μM. FM-Nov17 significantly stimulated the generation of intracellular ROS, followed by the induction of DNA damage and the activation of the ATM-p53-r-H2AX pathway and checkpoint-related signals Chk1/Chk2, which led to increased numbers of cells in the S and G2/M phases of the cell cycle. Furthermore, FM-Nov17 induced apoptotic cell death by decreasing mitochondrial membrane potential and activating caspase-3 and PARP. The above effects were all prevented by the ROS scavenger N-acetylcysteine.

CONCLUSIONS

FM-Nov17-induces DNA damage and mitochondria-dependent cellular apoptosis in CML cells. The process is mediated by the generation of ROS.

Rif1 Is Required for Resolution of Ultrafine DNA Bridges in Anaphase to Ensure Genomic Stability

Sister-chromatid disjunction in anaphase requires the resolution of DNA catenanes by topoisomerase II together with Plk1-interacting checkpoint helicase (PICH) and Bloom's helicase (BLM). We here identify Rif1 as a factor involved in the resolution of DNA catenanes that are visible as ultrafine DNA bridges (UFBs) in anaphase to which PICH and BLM localize. Rif1, which during interphase functions downstream of 53BP1 in DNA repair, is recruited to UFBs in a PICH-dependent fashion, but independently of 53BP1 or BLM. Similar to PICH and BLM, Rif1 promotes the resolution of UFBs: its depletion increases the frequency of nucleoplasmic bridges and RPA70-positive UFBs in late anaphase. Moreover, in the absence of Rif1, PICH, or BLM, more nuclear bodies with damaged DNA arise in ensuing G1 cells, when chromosome decatenation is impaired. Our data reveal a thus far unrecognized function for Rif1 in the resolution of UFBs during anaphase to protect genomic integrity.

Mutagenic potential of the isoflavone irilone in cultured V79 cells

After consumption of red clover-based dietary supplements, plasma concentrations of the isoflavone irilone (IRI) equal that of the well-investigated daidzein. Since some isoflavones are genotoxic, the potential of IRI to induce mutations was investigated. Gene mutations were determined by hypoxanthine-guanine phosphoribosyltransferase (HPRT) assay and sequencing of mutant cDNA, chromosome and genome mutations by micronucleus assay complemented by immunochemical staining of centromere proteins and microtubules in cultured V79 cells. Cell proliferation was monitored by electronic cell counting, flow cytometry and fluorescence microscopy. IRI did not affect the mutant frequency in the Hprt locus but altered the mutation spectrum by increasing the proportion of deletions and decreasing that of base pair substitutions. Induction of chromosome mutations was supported by a slight but significant increase in the number of micronucleated cells containing chromosomal fragments despite activation of three cell cycle checkpoints possibly interfering with micronuclei formation. Moreover, IRI exhibited a strong aneugenic potential characterized by disrupted mitotic spindles, mitotic arrest, and asymmetrical cell divisions leading to chromosome loss, nuclear fragmentation as well as mitotic catastrophe. Thus, IRI might be another isoflavone to be taken into account in safety assessment of dietary supplements.

Ethyl methanesulfonate induces mutations in Caenorhabditis elegans embryos at a high frequency

Mutagenesis protocols typically call for exposure of late-stage larvae or adults to a mutagen with the intention of inducing mutations in a robust germ line. Instead, ca. 16,000 CB665 [unc-58(e665)] one- to four-cell embryos of the nematode Caenorhabditis elegans were hand selected and exposed to ethyl methanesulfonate (EMS) for 50min. Twenty-one reversion mutants were recovered, of which 17 were intragenic suppressors of the e665 mutation. The mutation frequency was 6.5-fold higher than when CB665 adults were similarly mutagenized, which was predicted given that cell-cycle checkpoints are muted in C. elegans embryos. The mutation spectrum was similar to that obtained after standard EMS mutagenesis.

BRCA1 and p53 tumor suppressor molecules in Alzheimer's disease

Tumor suppressor molecules play a pivotal role in regulating DNA repair, cell proliferation, and cell death, which are also important processes in the pathogenesis of Alzheimer’s disease. Alzheimer’s disease is the most common neurodegenerative disorder, however, the precise molecular events that control the death of neuronal cells are unclear. Recently, a fundamental role for tumor suppressor molecules in regulating neurons in Alzheimer’s disease was highlighted. Generally, onset of neurodegenerative diseases including Alzheimer’s disease may be delayed with use of dietary neuro-protective agents against oxidative stresses. Studies suggest that dietary antioxidants are also beneficial for brain health in reducing disease-risk and in slowing down disease-progression. We summarize research advances in dietary regulation for the treatment of Alzheimer’s disease with a focus on its modulatory roles in BRCA1 and p53 tumor suppressor expression, in support of further therapeutic research in this field.

The apoptotic mechanism of action of the sphingosine kinase 1 selective inhibitor SKI-178 in human acute myeloid leukemia cell lines

We previously developed SKI-178 (N'-[(1E)-1-(3,4-dimethoxyphenyl)ethylidene]-3-(4-methoxxyphenyl)-1H-pyrazole-5-carbohydrazide) as a novel sphingosine kinase-1 (SphK1) selective inhibitor and, herein, sought to determine the mechanism-of-action of SKI-178-induced cell death. Using human acute myeloid leukemia (AML) cell lines as a model, we present evidence that SKI-178 induces prolonged mitosis followed by apoptotic cell death through the intrinsic apoptotic cascade. Further examination of the mechanism of action of SKI-178 implicated c-Jun NH2-terminal kinase (JNK) and cyclin-dependent protein kinase 1 (CDK1) as critical factors required for SKI-178-induced apoptosis. In cell cycle synchronized human AML cell lines, we demonstrate that entry into mitosis is required for apoptotic induction by SKI-178 and that CDK1, not JNK, is required for SKI-178-induced apoptosis. We further demonstrate that the sustained activation of CDK1 during prolonged mitosis, mediated by SKI-178, leads to the simultaneous phosphorylation of the prosurvival Bcl-2 family members, Bcl-2 and Bcl-xl, as well as the phosphorylation and subsequent degradation of Mcl-1. Moreover, multidrug resistance mediated by multidrug-resistant protein1 and/or prosurvival Bcl-2 family member overexpression did not affect the sensitivity of AML cells to SKI-178. Taken together, these findings highlight the therapeutic potential of SKI-178 targeting SphK1 as a novel therapeutic agent for the treatment of AML, including multidrug-resistant/recurrent AML subtypes.

Polo-like kinase 1 inhibits DNA damage response during mitosis

In response to genotoxic stress, cells protect their genome integrity by activation of a conserved DNA damage response (DDR) pathway that coordinates DNA repair and progression through the cell cycle. Extensive modification of the chromatin flanking the DNA lesion by ATM kinase and RNF8/RNF168 ubiquitin ligases enables recruitment of various repair factors. Among them BRCA1 and 53BP1 are required for homologous recombination and non-homologous end joining, respectively. Whereas mechanisms of DDR are relatively well understood in interphase cells, comparatively less is known about organization of DDR during mitosis. Although ATM can be activated in mitotic cells, 53BP1 is not recruited to the chromatin until cells exit mitosis. Here we report mitotic phosphorylation of 53BP1 by Plk1 and Cdk1 that impairs the ability of 53BP1 to bind the ubiquitinated H2A and to properly localize to the sites of DNA damage. Phosphorylation of 53BP1 at S1618 occurs at kinetochores and in cytosol and is restricted to mitotic cells. Interaction between 53BP1 and Plk1 depends on the activity of Cdk1. We propose that activity of Cdk1 and Plk1 allows spatiotemporally controlled suppression of 53BP1 function during mitosis.

CHK2 kinase in the DNA damage response and beyond

The serine/threonine kinase CHK2 is a key component of the DNA damage response. In human cells, following genotoxic stress, CHK2 is activated and phosphorylates >20 proteins to induce the appropriate cellular response, which, depending on the extent of damage, the cell type, and other factors, could be cell cycle checkpoint activation, induction of apoptosis or senescence, DNA repair, or tolerance of the damage. Recently, CHK2 has also been found to have cellular functions independent of the presence of nuclear DNA lesions. In particular, CHK2 participates in several molecular processes involved in DNA structure modification and cell cycle progression. In this review, we discuss the activity of CHK2 in response to DNA damage and in the maintenance of the biological functions in unstressed cells. These activities are also considered in relation to a possible role of CHK2 in tumorigenesis and, as a consequence, as a target of cancer therapy.

Polo-like kinase-1 in DNA damage response

Polo-like kinase-1 (Plk1) belongs to a family of serine-threonine kinases and plays a critical role in mitotic progression. Plk1 involves in the initiation of mitosis, centrosome maturation, bipolar spindle formation, and cytokinesis, well-reported as traditional functions of Plk1. In this review, we discuss the role of Plk1 during DNA damage response beyond the functions in mitotsis. When DNA is damaged in cells under various stress conditions, the checkpoint mechanism is activated to allow cells to have enough time for repair. When damage is repaired, cells progress continuously their division, which is called checkpoint recovery. If damage is too severe to repair, cells undergo apoptotic pathway. If damage is not completely repaired, cells undergo a process called checkpoint adaptation, and resume cell division cycle with damaged DNA. Plk1 targets and regulates many key factors in the process of damage response, and we deal with these subjects in this review.

Dephosphorylation enables the recruitment of 53BP1 to double-strand DNA breaks

Excluding 53BP1 from chromatin is required to attenuate the DNA damage response during mitosis, yet the functional relevance and regulation of this exclusion are unclear. Here we show that 53BP1 is phosphorylated during mitosis on two residues, T1609 and S1618, located in its well-conserved ubiquitination-dependent recruitment (UDR) motif. Phosphorylating these sites blocks the interaction of the UDR motif with mononuclesomes containing ubiquitinated histone H2A and impedes binding of 53BP1 to mitotic chromatin. Ectopic recruitment of 53BP1-T1609A/S1618A to mitotic DNA lesions was associated with significant mitotic defects that could be reversed by inhibiting nonhomologous end-joining. We also reveal that protein phosphatase complex PP4C/R3β dephosphorylates T1609 and S1618 to allow the recruitment of 53BP1 to chromatin in G1 phase. Our results identify key sites of 53BP1 phosphorylation during mitosis, identify the counteracting phosphatase complex that restores the potential for DDR during interphase, and establish the physiological importance of this regulation.

The SUMO-targeted ubiquitin ligase RNF4 localizes to etoposide-exposed mitotic chromosomes: implication for a novel DNA damage response during mitosis

RNF4, a SUMO-targeted ubiquitin ligase (STUbL), localizes to the nucleus and functions in the DNA damage response during interphase of the cell cycle. RNF4 also exists in cells undergoing mitosis, where its regulation and function remain poorly understood. Here we showed that administration of etoposide, an anticancer DNA topoisomerase II poison, to mitotic human cervical cancer HeLa cells induced SUMO-2/3-dependent localization of RNF4 to chromosomes. The FK2 antibody signals, indicative of poly/multi-ubiquitin assembly, were detected on etoposide-exposed mitotic chromosomes, whereas the signals were negligible in cells depleted for RNF4 by RNA interference. This suggests that RNF4 functions as a STUbL in the etoposide-induced damage response during mitosis. Indeed, RNF4-depletion sensitized mitotic HeLa cells to etoposide and increased cells with micronuclei. These results indicate the importance of the RNF4-mediated STUbL pathway during mitosis for the maintenance of chromosome integrity and further implicate RNF4 as a target for topo II poison-based therapy for cancer patients.

Mitosis inhibits DNA double-strand break repair to guard against telomere fusions

Mitotic cells inactivate DNA double-strand break (DSB) repair, but the rationale behind this suppression remains unknown. Here, we unravel how mitosis blocks DSB repair and determine the consequences of repair reactivation. Mitotic kinases phosphorylate the E3 ubiquitin ligase RNF8 and the nonhomologous end joining factor 53BP1 to inhibit their recruitment to DSB-flanking chromatin. Restoration of RNF8 and 53BP1 accumulation at mitotic DSB sites activates DNA repair but is, paradoxically, deleterious. Aberrantly controlled mitotic DSB repair leads to Aurora B kinase-dependent sister telomere fusions that produce dicentric chromosomes and aneuploidy, especially in the presence of exogenous genotoxic stress. We conclude that the capacity of mitotic DSB repair to destabilize the genome explains the necessity for its suppression during mitosis, principally due to the fusogenic potential of mitotic telomeres.

Increased frequency of micronuclei in the lymphocytes of patients chronically infected with hepatitis B or hepatitis C virus

In this study, we analysed the frequency of micronuclei (MN), nucleoplasmic bridges (NPBs) and nuclear buds (NBUDs) and evaluated mutagen-induced sensitivity in the lymphocytes of patients chronically infected with hepatitis B virus (HBV) or hepatitis C virus (HCV). In total, 49 patients with chronic viral hepatitis (28 HBV-infected and 21 HCV-infected patients) and 33 healthy, non-infected blood donor controls were investigated. The frequencies (‰) of MN, NPBs and NBUDs in the controls were 4.41 ± 2.15, 1.15 ± 0.97 and 2.98 ± 1.31, respectively. The frequencies of MN and NPBs were significantly increased (p < 0.0001) in the patient group (7.01 ± 3.23 and 2.76 ± 2.08, respectively) compared with the control group. When considered separately, the HBV-infected patients (7.18 ± 3.57) and HCV-infected patients (3.27 ± 2.40) each had greater numbers of MN than did the controls (p < 0.0001). The HCV-infected patients displayed high numbers of NPBs (2.09 ± 1.33) and NBUDs (4.38 ± 3.28), but only the HBV-infected patients exhibited a significant difference (NPBs = 3.27 ± 2.40, p < 0.0001 and NBUDs = 4.71 ± 2.79, p = 0.03) in comparison with the controls. Similar results were obtained for males, but not for females, when all patients or the HBV-infected group was compared with the controls. The lymphocytes of the infected patients did not exhibit sensitivity to mutagen in comparison with the lymphocytes of the controls (p = 0.06). These results showed that the lymphocytes of patients who were chronically infected with HBV or HCV presented greater chromosomal instability.

Spatially distinct functions of Clb2 in the DNA damage response

In budding yeast four mitotic cyclins (Clb1-4) cooperate in a partially redundant manner to bring about M-phase specific events, including the apical isotropic switch that ends polarized bud growth initiated at bud emergence. How exactly this morphogenetic transition is regulated by mitotic CDKs remains poorly understood. We have taken advantage of the isotropic bud growth that prevails in cells responding to DNA damage to unravel the contribution of mitotic cyclins in this cellular context. We find that clb2∆, in contrast to the other mitotic cyclin mutants, inappropriately respond to the presence of DNA damage. This aberrant response is characterized by a Cdc42- and Bni1-dependent but Cln-independent resumption of polarized bud growth after a brief period of actin depolarization. Biochemical and genetic evidence is presented that formally excludes the possibility of indirect effects due for instance to unrestrained APC activity, untimely mitotic exit or Swe1-mediated CDK inhibition. Importantly, our data demonstrate that in order to maintain the characteristic dumbbell arrest phenotype upon checkpoint activation Clb2 needs to be efficiently exported into the cytoplasm. We propose that the inhibition of mitotic cyclin destruction by the DNA damage checkpoint pathway leads to a buildup of Clb2 in the cytoplasm where this cyclin can stabilize the apical isotropic switch throughout a G 2/M checkpoint arrest. Our study also unveils an essential role of nuclear Clb2 in both survival and adaptation to the DNA damage checkpoint, illustrating a spatially distinct dual function of this mitotic cyclin in the response to DNA damage.

Ionizing radiation-induced responses in human cells with differing TP53 status

Ionizing radiation triggers diverse responses in human cells encompassing apoptosis, necrosis, stress-induced premature senescence (SIPS), autophagy, and endopolyploidy (e.g., multinucleation). Most of these responses result in loss of colony-forming ability in the clonogenic survival assay. However, not all modes of so-called clonogenic cell "death" are necessarily advantageous for therapeutic outcome in cancer radiotherapy. For example, the crosstalk between SIPS and autophagy is considered to influence the capacity of the tumor cells to maintain a prolonged state of growth inhibition that unfortunately can be succeeded by tumor regrowth and disease recurrence. Likewise, endopolyploid giant cells are able to segregate into near diploid descendants that continue mitotic activities. Herein we review the current knowledge on the roles that the p53 and p21(WAF1) tumor suppressors play in determining the fate of human fibroblasts (normal and Li-Fraumeni syndrome) and solid tumor-derived cells after exposure to ionizing radiation. In addition, we discuss the important role of WIP1, a p53-regulated oncogene, in the temporal regulation of the DNA damage response and its contribution to p53 dynamics post-irradiation. This article highlights the complexity of the DNA damage response and provides an impetus for rethinking the nature of cancer cell resistance to therapeutic agents.