Caffeine-induced radiosensitization is independent of nonhomologous end joining of DNA double-strand breaks

Proficiency in homologous recombination repair is prerequisite for activation of G2-checkpoint at low radiation doses

Pathways of repair of DNA double strand breaks (DSBs) cooperate with DNA damage cell cycle checkpoints to safeguard genomic stability when cells are exposed to ionizing radiation (IR). It is widely accepted that checkpoints facilitate the function of DSB repair pathways. Whether DSB repair proficiency feeds back into checkpoint activation is less well investigated. Here, we study activation of the G₂-checkpoint in cells deficient in homologous recombination repair (HRR) after exposure to low IR doses (∼1 Gy) in the G₂-phase. We report that in the absence of functional HRR, activation of the G₂-checkpoint is severely impaired. This response is specific for HRR, as cells deficient in classical non-homologous end joining (c-NHEJ) develop a similar or stronger G₂-checkpoint than wild-type (WT) cells. Inhibition of ATM or ATR leaves largely unaffected residual G₂-checkpoint in HRR-deficient cells, suggesting that the G₂-checkpoint engagement of ATM/ATR is coupled to HRR. HRR-deficient cells show in G₂-phase reduced DSB-end-resection, as compared to WT-cells or c-NHEJ mutants, confirming the reported link between resection and G₂-checkpoint activation. Strikingly, at higher IR doses (≥4 Gy) HRR-deficient cells irradiated in G₂-phase activate a weak but readily detectable ATM/ATR-dependent G₂-checkpoint, whereas HRR-deficient cells irradiated in S-phase develop a stronger G₂-checkpoint than WT-cells. We conclude that HRR and the ATM/ATR-dependent G₂-checkpoint are closely intertwined in cells exposed to low IR-doses in G₂-phase, where HRR dominates; they uncouple as HRR becomes suppressed at higher IR doses. Notably, this coupling is specific for cells irradiated in G₂-phase, and cells irradiated in S-phase utilize a different mechanistic setup.

Crosstalk between the mTOR and DNA Damage Response Pathways in Fission Yeast

Cells have developed response systems to constantly monitor environmental changes and accordingly adjust growth, differentiation, and cellular stress programs. The evolutionarily conserved, nutrient-responsive, mechanistic target of rapamycin signaling (mTOR) pathway coordinates basic anabolic and catabolic cellular processes such as gene transcription, protein translation, autophagy, and metabolism, and is directly implicated in cellular and organismal aging as well as age-related diseases. mTOR mediates these processes in response to a broad range of inputs such as oxygen, amino acids, hormones, and energy levels, as well as stresses, including DNA damage. Here, we briefly summarize data relating to the interplays of the mTOR pathway with DNA damage response pathways in fission yeast, a favorite model in cell biology, and how these interactions shape cell decisions, growth, and cell-cycle progression. We, especially, comment on the roles of caffeine-mediated DNA-damage override. Understanding the biology of nutrient response, DNA damage and related pharmacological treatments can lead to the design of interventions towards improved cellular and organismal fitness, health, and survival.

The dichotomous effects of caffeine on homologous recombination in mammalian cells

This study was initiated to examine the effects of caffeine on the DNA damage response (DDR) and homologous recombination (HR) in mammalian cells. A 5 mM caffeine treatment caused the cell cycle to stall at G2/M and cells eventually underwent apoptosis. Caffeine exposure also induced a strong DDR along with subsequent activation of wildtype p53 protein. An unexpected observation was the caffeine-induced depletion of Rad51 (and Brca2) proteins. Consequently, caffeine-treated cells were expected to be inefficient in HR. However, a dichotomy in the HR response of cells to caffeine treatment was revealed. Caffeine treatment rendered cells significantly better at performing the nascent DNA synthesis that accompanies the early strand invasion steps of HR. Additionally, caffeine treatment increased chromatin accessibility and elevated the efficiency of illegitimate recombination. Conversely, the increase in nascent DNA synthesis did not translate into a higher number of gene targeting events. Thus, prolonged caffeine exposure stalls the cell cycle, induces a p53-mediated apoptotic response and a down-regulation of critical HR proteins, and for reasons discussed, stimulates early steps of HR, but not the formation of complete recombination products.

DNA-PKcs and ATM epistatically suppress DNA end resection and hyperactivation of ATR-dependent G2-checkpoint in S-phase irradiated cells

We previously reported that cells exposed to low doses of ionizing radiation (IR) in the G2-phase of the cell cycle activate a checkpoint that is epistatically regulated by ATM and ATR operating as an integrated module. In this module, ATR interphases exclusively with the cell cycle to implement the checkpoint, mainly using CHK1. The ATM/ATR module similarly regulates DNA end-resection at low IR-doses. Strikingly, at high IR-doses, the ATM/ATR coupling relaxes and each kinase exerts independent contributions to resection and the G2-checkpoint. DNA-PKcs links to the ATM/ATR module and defects cause hyper-resection and hyperactivation of G2-checkpoint at all doses examined. Surprisingly, our present report reveals that cells irradiated in S-phase utilize a different form of wiring between DNA-PKcs/ATM/ATR: The checkpoint activated in G2-phase is regulated exclusively by ATR/CHK1; similarly at high and low IR-doses. DNA end-resection supports ATR-activation, but inhibition of ATR leaves resection unchanged. DNA-PKcs and ATM link now epistatically to resection and their inhibition causes hyper-resection and ATR-dependent G2-checkpoint hyperactivation at all IR-doses. We propose that DNA-PKcs, ATM and ATR form a modular unit to regulate DSB processing with their crosstalk distinctly organized in S- and G2- phase, with strong dependence on DSB load only in G2-phase.

Radiation-dose-dependent functional synergisms between ATM, ATR and DNA-PKcs in checkpoint control and resection in G2-phase

Using data generated with cells exposed to ionizing-radiation (IR) in G2-phase of the cell cycle, we describe dose-dependent interactions between ATM, ATR and DNA-PKcs revealing unknown mechanistic underpinnings for two key facets of the DNA damage response: DSB end-resection and G2-checkpoint activation. At low IR-doses that induce low DSB-numbers in the genome, ATM and ATR regulate epistatically the G2-checkpoint, with ATR at the output-node, interfacing with the cell-cycle predominantly through Chk1. Strikingly, at low IR-doses, ATM and ATR epistatically regulate also resection, and inhibition of either activity fully suppresses resection. At high IR-doses that induce high DSB-numbers in the genome, the tight ATM/ATR coupling relaxes and independent outputs to G2-checkpoint and resection occur. Consequently, both kinases must be inhibited to fully suppress checkpoint activation and resection. DNA-PKcs integrates to the ATM/ATR module by regulating resection at all IR-doses, with defects in DNA-PKcs causing hyper-resection and G2-checkpoint hyper-activation. Notably, hyper-resection is absent from other c-NHEJ mutants. Thus, DNA-PKcs specifically regulates resection and adjusts the activation of the ATM/ATR module. We propose that selected DSBs are shepherd by DNA-PKcs from c-NHEJ to resection-dependent pathways for processing under the regulatory supervision of the ATM/ATR module.

Induction of H2AX phosphorylation in tumor cells by gossypol acetic acid is mediated by phosphatidylinositol 3-kinase (PI3K) family

BACKGROUND

H2AX is phosphorylated (γH2AX) by members of the phosphatidylinositol 3-kinase (PI3K) family, including Ataxia telangiectasia-mutated (ATM), ATM- and Rad3-related (ATR) and DNA-PK in response to DNA damage. Our study shows that gossypol acetic acid (GAA) alone can induce γH2AX in Human mucoepidermoid carcinoma cell line (MEC-1) in vitro. Thus, we further examined the possible mechanisms of GAA to induce γH2AX in tumor cells.

MATERIALS AND METHODS

The PI3K inhibitors caffeine and wortmannin were used in an effort to identify the kinase(s) responsible for GAA -induced γH2AX in MEC-1 cells. DNA dependent protein kinase (DNA-PK) - proficient and -deficient cells, human glioma cell lines M059K and M059J, were also used to evaluate the kinases responsible for GAA induced H2AX phosphorylation. γH2AX expression was detected by immunofluorescent microscopy. Flow cytometry assay was used to assay γH2AX and cell cycle.

RESULTS

GAA induced H2AX phosphorylation in a cell cycle-dependent manner and a significant G0/G1 phase arrest in MEC-1 cells was shown. Caffeine and wortmannin significantly inhibited GAA-induced H2AX phosphorylation in MEC-1 cells. GAA induced H2AX phosphorylation in M059K, but not in M059J. Taken together, these data suggested that GAA treatment alone could induce H2AX phosphorylation in a cell cycle dependent manner in MEC-1 and M059K, but not in M059J cells. A significant G0/G1 phase arrest was shown in MEC-1.

CONCLUSIONS

The member of PI3K family, DNA-PK, ATM and ATR are involved in the H2AX phosphorylation of MEC-1 cells.

MicroRNA-376a sensitizes cells following DNA damage by downregulating MEPE expression

MicroRNAs (miRNAs) are a class of endogenous molecules that post-transcriptionally regulate target gene expression and play an important role in many developmental processes. Matrix extracellular phosphoglycoprotein (MEPE) is related to bone metabolism. We recently reported that MEPE protects cells from DNA damage-induced killing. The purpose of this study is to investigate whether miRNAs targeting MEPE play an important role in DNA damage response. We report in this study that miR-376a directly targets MEPE, and overexpression of miR-376a reduces the G2 arrest of the cells and sensitizes the cells to DNA damage-induced killing. These results indicate an association of MEPE gene inactivation with decreased survival after DNA damage and also provide useful information for miRNA-based drug development: a new target for sensitizing human tumor cells to radiotherapy or chemotherapy.

microRNA-143 protects cells from DNA damage-induced killing by downregulating FHIT expression

MicroRNAs (miRNAs) are posttranscriptional modulators of gene expression and play an important role in many developmental processes. Recent studies suggest roles of miRNAs in carcinogenesis. Fragile histidine triad (FHIT) gene deletion, methylation, and reduced Fhit protein expression occur in about 70% of human epithelial tumors and are clearly associated with tumor progression. Although it has been previously reported that Fhit(-/-)cells exhibit more resistance to multi-DNA damage inducers, including ionizing radiation, it remains unclear how miRNAs targeting FHIT in DNA damage response play the role. This study reports that miR-143 directly targets FHIT and that overexpression of miR-143 results in significant G2-phase arrest and protects cells from DNA damage-induced killing. These results indicate an association of FHIT gene inactivation with increased survival after DNA damage and also provide useful information for miRNA-based drug development in two directions: protect cells from DNA damage-induced killing and sensitize cells to radiation therapy.

S-phase cells are more sensitive to high-linear energy transfer radiation

PURPOSE

S-phase cells are more resistant to low-linear energy transfer (LET) ionizing radiation (IR) than nonsynchronized and G(1)-phase cells, because both nonhomologous end-joining (NHEJ) and homologous recombination repair can repair DNA double-strand breaks (DSBs) in the S phase. Although it was reported 3 decades ago that S-phase cells did not show more resistance to high-LET IR than cells in other phases, the mechanism remains unclear. We therefore attempted to study the phenotypes and elucidate the mechanism involved.

METHODS AND MATERIALS

Wild-type and NHEJ-deficient cell lines were synchronized using the double-thymidine approach. A clonogenic assay was used to detect the sensitivity of nonsynchronized, synchronized S-phase, and G(2)-phase cells to high- and low-LET IR. The amounts of Ku bound to DSBs in the high- and low-LET-irradiated cells were also examined.

RESULTS

S-phase wild-type cells (but not NHEJ-deficient cells) were more sensitive to high-LET IR than nonsynchronized and G(2)-phase cells. In addition, S-phase wild-type cells showed less efficient Ku protein binding to DSBs than nonsynchronized and G(2)-phase cells in response to high-LET IR, although all cells at all phases showed similarly efficient levels of Ku protein binding to DSBs in response to low-LET IR.

CONCLUSIONS

S-phase cells are more sensitive to high-LET IR than nonsynchronized and G(2)-phase cells, because of the following mechanism: it is more difficult for Ku protein to bind to high-LET IR-induced DNA DSBs in S-phase cells than in cells at other phases, which results in less efficient NHEJ.

1-Methylxanthine enhances the radiosensitivity of tumor cells

PURPOSE

To determine the efficacy of a caffeine derivative 1-methylxanthine (1-MTX) in increasing radiosensitivity of cancer cells and elucidate the underlying mechanisms in vitro.

MATERIALS AND METHODS

RKO human colorectal cancer cells carrying wild type protein 53 kDa (p53) were incubated with 3 mM 1-MTX for 30 min, exposed to 4 Gy ionizing radiation, and further incubated with 1-MTX for three days. The clonogenic cell death was determined, and the cell cycle distribution and apoptosis were studied with flow cytometry at different times after irradiation. The DNA double strand break (DNA DSB) was examined using phosphorylated Histone2A (gamma-H2AX) foci formation, and the expression/activity of checkpoint 2 kinase (Chk2), cell division cycle 25 (Cdc25) phosphatase and cyclin B1/Cdc2 kinase were also investigated using western blotting and in vitro kinase assays.

RESULTS

The treatment with 3 mM 1-MTX increased the radiation-induced clonogenic and apoptotic cell death. The radiation-induced phosphorylation of Chk2 and Cdc25c and the radiation-induced increase in the cyclin B1/Cdc2 kinas activity were little affected by 1-MTX. The radiation-induced G2/M arrest was only slightly shortened and the expression of radiation-induced gamma-H2AX was markedly prolonged by 1-MTX.

CONCLUSIONS

1-MTX significantly increased the radiosensitivity of RKO human colorectal cancer cells carrying wild type p53 mainly by inhibiting the repair of radiation-induced DNA DSB without causing significant alteration in radiation-induced G2/M arrest. Such a radiosensitization occurred at 1-MTX concentrations almost non-toxic to the target tumor cells.

The synergistic effect of dimethylamino benzoylphenylurea (NSC #639829) and X-irradiation on human lung carcinoma cell lines

PURPOSE

The present study was designed to investigate the ability of N-[4-(5-bromo-2-pyrimidyloxy)-3-methylphenyl]-(dimemethylamino)-benzoylphenylurea (dimemethylamino benzoylphenylurea; BPU) to sensitize cells to radiation and to examine the relationship between phenotype versus survival, DNA damage, apoptosis, or cell cycle progression in non-small cell lung cancer (NSCLC) cell lines.

METHODS

Asynchronous cultures of three NSCLC (phenotype) lines, A549 (adenocarcinoma), NCI-H226 (squamous) and NCI-H596 (adenosquamous) were used. Cells were treated for 24 h with BPU at various concentrations (0-10 microM) to obtain drug doses for inhibiting cell survival by approximately 50% (IC50). Cells were X-irradiated without BPU or after 24 h BPU treatment at IC50. Radiation doses ranged from 0 to10 Gy. Cell survival was determined by a colony-forming ability assay. The effect of BPU on the cell cycle distribution and induction of apoptosis were measured by flow cytometry-based assays. The effect of BPU on radiation-induced DNA damage and repair was analyzed according to nuclear gammaH2AX immunofluorescence of cells exposed to X-rays alone or after BPU. Anti-gammaH2AX antibody staining, a surrogate determinant of double stranded DNA breaks, was measured using flow cytometry.

RESULTS

BPU (1.5 microM) for 24 h produced approximately 50% cell survival. BPU and X-irradiation were synergistic in the three cell lines at survival levels of 20-50%. Flow cytometry analysis of replicate experiments with BPU (1.5 microM for 24 h) showed that BPU blocked cell progression at S and/or G2/M. The incidence of apoptosis in BPU-treated versus control cells ranged from approximately 0.3 to approximately 8%. Twenty-four hour after X-irradiation cells pre-treated with BPU and X-irradiated after drug exposure showed gammaH2AX levels approximately two times higher than did the cells exposed to X-rays only.

CONCLUSIONS

The study identified BPU as a novel radiation sensitizer. The analysis of phosphorylated histone H2AX as a surrogate marker of DNA double strand breaks suggested a positive association between radiosensitization and the inhibition of X-irradiation-induced DNA damage repair by BPU.

The sensitivity of human mesenchymal stem cells to ionizing radiation

PURPOSE

Recent studies have shown that mesenchymal stem cells (MSCs) obtained from bone marrow transplantation patients originate from the host. This clinical observation suggests that MSCs in their niches could be resistant to irradiation. However, the biologic responses of bone marrow MSCs to irradiation have rarely been described in the literature.

METHODS AND MATERIALS

In this study, human bone marrow-derived, clonally expanded MSCs were used to investigate their sensitivity to irradiation in vitro, and the cellular mechanisms that may facilitate resistance to irradiation. The human lung cancer cell line A549 and the breast cancer cell line HCC1937 were used as controls for radiosensitivity; the former line has been shown to be radioresistant and the latter radiosensitive. We then examined their in vitro biologic changes and sensitivities to radiation therapy.

RESULTS

Our results suggest that MSCs are characterized as resistant to irradiation. Several cellular mechanisms were demonstrated that may facilitate resistance to irradiation: ATM protein phosphorylation, activation of cell-cycle checkpoints, double-strand break repair by homologous recombination and nonhomologous end joining (NHEJ), and the antioxidant capacity for scavenging reactive oxygen species.

CONCLUSIONS

As demonstrated, MSCs possess a better antioxidant reactive oxygen species-scavenging capacity and active double-strand break repair to facilitate their radioresistance. These findings provide a better understanding of radiation-induced biologic responses in MSCs and may lead to the development of better strategies for stem cell treatment and cancer therapy.

Inhibition of homologous recombination repair with Pentoxifylline targets G2 cells generated by radiotherapy and induces major enhancements of the toxicity of cisplatin and melphalan given after irradiation

The presentation reviews the modus operandi of the dose modifying drug Pentoxifylline and the dose enhancement factors which can be achieved in different cell types. Preclinical and clinical data show that Pentoxifylline improves the oxygenation of hypoxic tumours and enhances tumour control by irradiation. In vitro experiments demonstrate that Pentoxifylline also operates when oxygen is not limiting and produces dose modifying factors in the region of 1.2 – 2.0. This oxygen independent effect is poorly understood. In p53 mutant cells irradiation induces a G2 block which is abrogated by Pentoxifylline. The enhancement of cell kill observed when Pentoxifylline and irradiation are given together could arise from rapid entry of damaged tumour cells into mitosis and propagation of DNA lesions as the result of curtailment of repair time. Recovery ratios and repair experiments using CFGE after high dose irradiation demonstrate that Pentoxifylline inhibits repair directly and that curtailment of repair time is not the explanation. Use of the repair defective xrs1 and the parental repair competent CHO-K1 cell line shows that Pentoxifylline inhibits homologous recombination repair which operates predominantly in the G2 phase of the cell cycle. When irradiated cells residing in G2 phase are exposed to very low doses of cisplatin at a toxic dose of 5 %. (TC: 0.05) massive toxicity enhancements up to a factor of 80 are observed in melanoma, squamous carcinoma and prostate tumour cell lines. Enhancements of radiotoxicity seen when Pentoxifylline and radiation are applied together are small and do not exceed a factor of 2.0. The capacity of Pentoxifyline to inhibit homologous recombination repair has not as yet been clinically utilized. A suitable application could be in the treatment of cervical carcinoma where irradiation and cisplatin are standard modality. In vitro data also strongly suggest that regimes where irradiation is used in combination with alkylating drugs may also benefit.

Caffeine Sensitizes Nondividing Human Fibroblasts to X Rays by Inducing a High Frequency of Misrepair

Caffeine sensitizes cells to ionizing radiation, and this effect is believed to be associated with the disruption of DNA damage-responsive cell cycle checkpoints, which is controlled by ATM. Recent studies suggest that misrejoining of DSBs is one of the underlying mechanisms of AT cell hyper-radiosensitivity. In this study, we investigated the effects of caffeine and radiation on nongrowing G0 normal human fibroblast cells by determining cell survival and scoring aberrations in calyculin A-induced G2 chromosomes. Results from the cell survival study indicate that after X-ray exposure G0 cells were sensitized by 24 h treatment with caffeine. Analysis of chromosome aberrations using FISH (fluorescence in situ hybridization) revealed a high frequency of aberrant cells and color junctions in the caffeine-treated cells. Since most DNA repair in nongrowing G0 cells is believed to result from nonhomologous end joining (NHEJ), caffeine may influence the fidelity of the NHEJ pathway in irradiated G0 cells.

Inhibition of HIV-1 replication by caffeine and caffeine-related methylxanthines

Human immunodeficiency virus type I (HIV-1) DNA integration is an essential step of viral replication. We have suggested recently that this stage of HIV-1 life-cycle triggers a cellular DNA damage response and requires cellular DNA repair proteins for its completion. These include DNA-PK (DNA-dependent protein kinase), ATR (ataxia telangiectasia and Rad3-related), and, at least in some circumstances, ATM (ataxia telangiectasia mutated). Host cell proteins may constitute an attractive target for anti-HIV-1 therapeutics, since development of drug resistance against compounds targeting these cellular cofactor proteins is unlikely. In this study, we show that an inhibitor of ATR and ATM kinases, caffeine, can suppress replication of infectious HIV-1 strains, and provide evidence that caffeine exerts its inhibitory effect at the integration step of the HIV-1 life-cycle. We also demonstrate that caffeine-related methylxanthines including the clinically used compound, theophylline, act at the same step of the HIV-1 life-cycle as caffeine and efficiently inhibit HIV-1 replication in primary human cells. These data reveal the feasibility of therapeutic approaches targeting host cell proteins and further support the hypothesis that ATR and ATM proteins are involved in retroviral DNA integration.

Complex H2AX phosphorylation patterns by multiple kinases including ATM and DNA-PK in human cells exposed to ionizing radiation and treated with kinase inhibitors

In eukaryotic cells, DNA double strand breaks (DSBs) cause the prompt phosphorylation of serine 139 at the carboxy terminus of histone H2AX to generate gamma-H2AX, detectable by Western blotting or immunofluorescence. The consensus sequence at the phosphorylation site implicates the phosphatidylinositol 3-like family of protein kinases in H2AX phosphorylation. It remains open whether ATM (ataxia telangiectasia mutated) is the major H2AX kinase, or whether other members of the family, such as DNA-PK (DNA dependent protein kinase) or ATR (ATM and Rad3 related), contribute in a functionally complementary manner. To address this question, we measured global H2AX phosphorylation in cell lysates and foci formation in individual cells of either wild type or mutant (ATM or DNA-PK) genetic background. Normal global phosphorylation kinetics is observed after irradiation in cells defective either in ATM or DNA-PK alone, suggesting a complementary contribution to H2AX phosphorylation. This is further supported by the observation that initial H2AX phosphorylation is delayed when both kinases are inhibited by wortmannin, as well as when ATM is inhibited by caffeine in DNA-PK deficient cells. However, robust residual global phosphorylation is detectable under all conditions of genetic or chemical inhibition suggesting the function of additional kinases, such as ATR. Treatment with wortmannin, caffeine, or UCN-01 produces a strong DNA-PK dependent late global hyperphosphorylation of H2AX, uncoupled from DNA DSB rejoining and compatible with an inhibition of late steps in DNA DSB processing. Evaluation of gamma-H2AX foci formation confirms the major conclusions made on the basis of global H2AX phosphorylation, but also points to differences particularly several hours after exposure to IR. The results in aggregate implicate DNA-PK, ATM and possibly other kinases in H2AX phosphorylation. The functional significance and the mechanisms of coordination in space and time of these multiple inputs require further investigation.

Involvement of the Fhit gene in the ionizing radiation-activated ATR/CHK1 pathway

Fragile Histidine Triad (Fhit) gene deletion, methylation, and reduced Fhit protein expression occur in about 70% of human epithelial tumors and, in some cancers, are clearly associated with tumor progression. Specific Fhit signal pathways have not been identified, although it has been shown that Fhit overexpression leads to apoptosis in many cancer cell lines. We report in this study that Fhit-/- cells derived from gene knockout mice show much stronger S and G2 checkpoint responses than their wild type counterparts. The strong checkpoint responses are regulated by the ATR/CHK1 pathway, which contributes to the radioresistance of Fhit-/- cells. These results indicate an association of Fhit gene inactivation with increased survival after DNA damage, which is related to the over-active checkpoints regulated by the ATR/CHK1 pathway. These results also suggest the potential effects of Fhit-dependent DNA damage response on tumor progression.

The mouse genomic instability mutation chaos1 is an allele of Polq that exhibits genetic interaction with Atm

chaos1 (for chromosome aberrations occurring spontaneously 1) is a recessive mutation that was originally identified in a phenotype-based screen for chromosome instability mutants in mice. Mutant animals exhibit significantly higher frequencies of spontaneous and radiation- or mitomycin C-induced micronucleated erythrocytes, indicating a potential defect in homologous recombination or interstrand cross-link repair. The chaos1 allele was genetically associated with a missense mutation in Polq, which encodes DNA polymerase theta;. We demonstrate here that chaos1 is a mutant allele of Polq by using two genetic approaches: chaos1 mutant phenotype correction by a bacterial artificial chromosome carrying wild-type Polq and a failed complementation test between chaos1 and a Polq-disrupted allele generated by gene targeting. To investigate the potential involvement of Polq in DNA double-strand break repair, we introduced chaos1 into an Atm (for ataxia telangiectasia mutated)-deficient background. The majority ( approximately 90%) of double-homozygous mice died during the neonatal period. Surviving double mutants exhibited synergistic phenotypes such as severe growth retardation and enhanced chromosome instability. However, remarkably, double mutants had delayed onset of thymic lymphoma, significantly increasing life span. These data suggest a unique role of Polq in maintaining genomic integrity, which is probably distinctive from the major homologous recombination pathway regulated by ATM.

ATR Affecting Cell Radiosensitivity Is Dependent on Homologous Recombination Repair but Independent of Nonhomologous End Joining

ATR is one of the most important checkpoint proteins in mammalian cells responding to DNA damage. Cells defective in normal ATR activity are sensitive to ionizing radiation (IR). The mechanism by which ATR protects the cells from IR-induced killing remains unclear. DNA double-strand breaks (DSBs) induced by IR are critical lesions for cell survival. Two major DNA DSB repair pathways exist in mammalian cells: homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). We show that the doxycycline (dox)-induced ATR kinase dead (ATRkd) cells have the similar inductions and rejoining rates of DNA DSBs compared with cells without dox induction, although the dox-induced ATRkd cells are more sensitive to IR and have the deficient S and G(2) checkpoints. We also show that the dox-induced ATRkd cells have a lower HRR efficiency compared with the cells without dox induction. These results indicate that the effects of ATR on cell radiosensitivity are independent of NHEJ but are linked to HRR that may be affected by the deficient S and G(2) checkpoints.

A role for the Fanconi anemia C protein in maintaining the DNA damage-induced G2 checkpoint

Fanconi anemia (FA) is a complex, heterogeneous genetic disorder composed of at least 11 complementation groups. The FA proteins have recently been found to functionally interact with the cell cycle regulatory proteins ATM and BRCA1; however, the function of the FA proteins in cell cycle control remains incompletely understood. Here we show that the Fanconi anemia complementation group C protein (Fancc) is necessary for proper function of the DNA damage-induced G2/M checkpoint in vitro and in vivo. Despite apparently normal induction of the G2/M checkpoint after ionizing radiation, murine and human cells lacking functional FANCC did not maintain the G2 checkpoint as compared with wild-type cells. The increased rate of mitotic entry seen in Fancc-/-mouse embryo fibroblasts correlated with decreased inhibitory phosphorylation of cdc2 kinase on tyrosine 15. An increased inability to maintain the DNA damage-induced G2 checkpoint was observed in Fancc -/-; Trp53 -/-cells compared with Fancc -/-cells, indicating that Fancc and p53 cooperated to maintain the G2 checkpoint. In contrast, genetic disruption of both Fancc and Atm did not cooperate in the G2 checkpoint. These data indicate that Fancc and p53 in separate pathways converge to regulate the G2 checkpoint. Finally, fibroblasts lacking FANCD2 were found to have a G2 checkpoint phenotype similar to FANCC-deficient cells, indicating that FANCD2, which is activated by the FA complex, was also required to maintain the G2 checkpoint. Because a proper checkpoint function is critical for the maintenance of genomic stability and is intricately related to the function and integrity of the DNA repair process, these data have implications in understanding both the function of FA proteins and the mechanism of genomic instability in FA.

Selective inhibition of the DNA-dependent protein kinase (DNA-PK) by the radiosensitizing agent caffeine

Caffeine inhibits cell cycle checkpoints, sensitizes cells to ionizing radiation-induced cell killing and inhibits the protein kinase activity of two cell cycle checkpoint regulators, Ataxia-Telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR). In contrast, caffeine has been reported to have little effect on the protein kinase activity of the DNA-dependent protein kinase (DNA-PK), which is essential for the repair of DNA double-strand breaks. Previously, we reported that DNA-PK phosphorylates Thr21 of the 32 kDa subunit of replication protein A (RPA32) in response to camptothecin. In this report we demonstrate that the camptothecin-induced phosphorylation of RPA32 on Thr21 is inhibited by 2 mM caffeine. In addition, we show that caffeine inhibits immunoprecipitated and purified DNA-PK, as well as DNA-PK in cell extracts, with an IC50 of 0.2-0.6 mM. Caffeine inhibited DNA-PK activity through a mixed non-competitive mechanism with respect to ATP. In contrast, 10-fold higher concentrations of caffeine were required to inhibit DNA-PK autophosphorylation in vitro and caffeine failed to inhibit DNA-PKcs dependent double-strand break repair in vivo. These data suggest that while DNA-PK does not appear to be the target of caffeine-induced radiosensitization, caffeine cannot be used to differentiate between ATM, ATR and DNA- PK-dependent substrate phosphorylation in vivo.

Caffeine inhibits homology-directed repair of I-SceI-induced DNA double-strand breaks

We recently reported that two Chinese hamster mutants deficient in the RAD51 paralogs XRCC2 and XRCC3 show reduced radiosensitization after treatment with caffeine, thus implicating homology-directed repair (HDR) of DNA double-strand breaks (DSBs) in the mechanism of caffeine radiosensitization. Here, we investigate directly the effect of caffeine on HDR initiated by DSBs induced by a rare cutting endonuclease (I-SceI) into one of two direct DNA repeats. The results demonstrate a strong inhibition by caffeine of HDR in wild-type cells, and a substantial reduction of this effect in HDR-deficient XRCC3 mutant cells. Inhibition of HDR and cell radiosensitization to killing shows similar dependence on caffeine concentration suggesting a cause-effect relationship between these effects. UCN-01, a kinase inhibitor that effectively abrogates checkpoint activation in irradiated cells, has only a small effect on HDR, indicating that similar to radiosensitization, inhibition of checkpoint signaling is not sufficient for HDR inhibition. Recombination events occurring during treatment with caffeine are characterized by rearrangements reminiscent to those previously reported for the XRCC3 mutant, and immunofluorescence microscopy demonstrates significantly reduced formation of IR-specific RAD51 foci after caffeine treatment. In summary, our results identify inhibition of HDR as a significant contributor to caffeine radiosensitization.

Double strand break repair by homologous recombination is regulated by cell cycle-independent signaling via ATM in human glioma cells

To investigate double strand break (DSB) repair and signaling in human glioma cells, we stably transfected human U87 (ATM(+), p53(+)) glioma cells with a plasmid having a single I-SceI site within an inactive green fluorescent protein (GFP) expression cassette, allowing for the detection of homologous recombination repair (HRR) by GFP expression. HRR and nonhomologous end joining (NHEJ) were also determined by PCR. DSB repair was first detected at 12 h postinfection with an adenovirus expressing I-SceI with repair reaching plateau levels between 24 and 48 h. Within this time frame, NHEJ predominated over HRR in the range of 3-50-fold. To assess the involvement of ATM in DSB repair, we first examined whether ATM was associated with the DSB. Chromatin immunoprecipitation showed that ATM was present at the site of the DSB as early as 18 h postinfection. In cells treated with caffeine, an inhibitor of ATM, HRR was reduced, whereas NHEJ was not. In support of this finding, GFP flow cytometry demonstrated that caffeine reduced HRR by 90% under conditions when ATM kinase activity was inhibited. Dominant-negative ATM expressed from adenovirus inhibited HRR by 45%, also having little to no effect on NHEJ. Furthermore, HRR was inhibited by caffeine in serum-starved cells arrested in G(0)/G(1), suggesting that ATM is also important for HRR outside of the S and G(2) cell cycle phases. Altogether, these results demonstrate that HRR contributes substantially to DSB repair in human glioma cells, and, importantly, ATM plays a critical role in regulating HRR but not NHEJ throughout the cell cycle.

Inhibition of DNA repair by Pentoxifylline and related methylxanthine derivatives

The methylxanthine drug Pentoxifylline is reviewed for new properties which have emerged only relatively recently and for which clinical applications can be expected. After a summary on the established systemic effects of Pentoxifylline on the microcirculation and reduction of tumour anoxia, the role of the drug in the treatment of vasoocclusive disorders, cerebral ischemia, infectious diseases, septic shock and acute respiratory distress, the review focuses on another level of drug action which is based on in vitro observations in a variety of cell lines. Pentoxifylline and the related drug Caffeine are known radiosensitizers especially in p53 mutant cells. The explanation that the drug abrogates the G2 block and shortens repair in G2 by promoting early entry into mitosis is not anymore tenable because enhancement of radiotoxicity requires presence of the drug during irradiation and fails when the drug is added after irradiation at the G2 maximum. Repair assays by measurement of recovery ratios and by delayed plating experiments indeed strongly suggested a role in repair which is now confirmed for Pentoxifylline by constant field gel electrophoresis (CFGE) measurements and for Pentoxifylline and for Caffeine by use of a variety of repair mutants. The picture now emerging shows that Caffeine and Pentoxifylline inhibit homologous recombination by targeting members of the PIK kinase family (ATM and ATR) which facilitate repair in G2. Pentoxifylline induced repair inhibition between irradiation dose fractions to counter interfraction repair has been successfully applied in a model for stereotactic surgery. Another realistic avenue of application of Pentoxifylline in tumour therapy comes from experiments which show that repair events in G2 can be targeted directly by addition of cytotoxic drugs and Pentoxifylline at the G2 maximum. Under these conditions massive dose enhancement factors of up to 80 have been observed suggesting that it may be possible to realise dramatic improvements to tumour growth control in the clinic.

An overactivated ATR/CHK1 pathway is responsible for the prolonged G2 accumulation in irradiated AT cells

Induction of checkpoint responses in G1, S, and G2 phases of the cell cycle after exposure of cells to ionizing radiation (IR) is essential for maintaining genomic integrity. Ataxia telangiectasia mutated (ATM) plays a key role in initiating this response in all three phases of the cell cycle. However, cells lacking functional ATM exhibit a prolonged G2 arrest after IR, suggesting regulation by an ATM-independent checkpoint response. The mechanism for this ataxia telangiectasia (AT)-independent G2-checkpoint response remains unknown. We report here that the G2 checkpoint in irradiated human AT cells derives from an overactivation of the ATR/CHK1 pathway. Chk1 small interfering RNA abolishes the IR-induced prolonged G2 checkpoint and radiosensitizes AT cells to killing. These results link the activation of ATR/CHK1 with the prolonged G2 arrest in AT cells and show that activation of this G2 checkpoint contributes to the survival of AT cells.

Caffeine could not efficiently sensitize homologous recombination repair-deficient cells to ionizing radiation-induced killing

Caffeine inhibits ATM and ATR, two important checkpoint regulators, abolishes ionizing radiation-induced checkpoint response, and radiosensitizes cells. Radiation-induced DNA double-strand breaks (DSBs) are repaired by two major processes, homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). It remains unclear which repair process, HRR or NHEJ, is affected when the checkpoint responses are abolished by caffeine. In this study we observed the effect of caffeine on gene-targeted DT40 chicken lymphoblast cells. We show that caffeine efficiently abolishes S- and G(2)-phase checkpoint responses after irradiation in all cell lines tested and greatly radiosensitizes wild-type and ATM(-/-) cells, the partially checkpoint-deficient cells. However, caffeine has a much smaller radiosensitizing effect on RAD54(-/-) cells and has no effect on RAD51-deficient cells. RAD51 and RAD54 are the important factors for HRR. Our results indicate that the checkpoint responses abolished by caffeine (S and G(2)) mainly affect HRR, which results in cell radiosensitization.