Micronucleus induction by neocarzinostatin and methyl methanesulfonate in ionizing radiation--sensitive Chinese hamster V79 cell mutants

Inhibition of phosphatidylinositol-3-OH kinase/Akt signaling impairs DNA repair in glioblastoma cells following ionizing radiation

Radiation therapy is a mainstay in the treatment of glioblastomas, but these tumors are often associated with radioresistance. Activation of the phosphatidylinositol-3-OH kinase (PI3K)/Akt pathway, which occurs frequently in glioblastomas due to inactivation of the tumor suppressor phosphatase and tensin homologue (PTEN), correlates with radioresistance. To directly test the link between Akt activation and radioresistance, we utilized PTEN-deficient U251 glioblastoma cells engineered to inducibly restore PTEN upon exposure to doxycycline. These cells showed high basal levels of Akt activation (i.e. high levels of phospho-Akt), but induction of PTEN led to substantially decreased phospho-Akt and was associated with radiosensitization. To investigate whether the PTEN-induced radiosensitization was attributable to impaired sensing versus repair of DNA damage, we assessed levels of gamma-H2AX after ionizing radiation in U251 cells induced for PTEN. Initial post-radiation levels of gamma-H2AX foci were not decreased in PTEN-induced cells; however, the resolution of these foci was significantly delayed. In contrast to these results, induction of phosphatase-dead PTEN showed no appreciable effect. Finally, exposure of cells to the PI3K inhibitor LY294002 did not decrease the occurrence of gamma-H2AX foci after irradiation but did markedly delay their resolution. These results together support a direct link between Akt activation, repair of DNA damage, and radioresistance in glioblastoma. Targeting the PI3K/Akt pathway may modulate DNA repair to improve the efficacy of radiation therapy.

Evaluation of apigenin using in vitro cytochalasin blocked micronucleus assay

High doses of flavonoids are reported to be clastogenic in contrast to their potential to reduce oxidative DNA damage, retard growth of leukemia cells, obstruct cell signal transduction and induce cellular differentiation in cancers. In the present study, we evaluated apigenin, a plant-derived flavonoid in doses of 10, 33, and 100 microM per 5 ml culture using cytochalasin-B blocked micronucleus (CBMN) assay in peripheral human lymphocytes. Apigenin was found to induce micronuclei in a dose dependent manner indicating potential genotoxic hazard in humans. Hence, flavonoids may act as mutagen, pro-oxidant or as inhibitor of key enzymes to produce clastogenic effects depending upon the levels consumed as well as the physiological parameters.

Protective effect of beta-glucan extracted from Saccharomyces cerevisiae, against DNA damage and cytotoxicity in wild-type (k1) and repair-deficient (xrs5) CHO cells

A large number of functional foods, including those that contain beta-glucan, have been shown to prevent the development of cancer and other chronic diseases. The aim of the present study was to elucidate its mechanism of action, as well as to understand its effects as an antigenotoxic, anticlastogenic agent, and to determine its capacity to preserve cell viability. The investigation was carried out in the CHO-k1 and CHO-xrs5 cell lines. The cytokinesis-blocked micronucleus assay indicated that the different doses of beta-glucan examined (5, 10, 20 and 40 microg/ml) did not show clastogenic effects. In the CHO-k1 cell line, a chemopreventive effect could be observed in all the protocols tested: pre-treatment (% reduction of 35.0-57.3), simultaneous treatment (simple--5 reduction of 19.7-55.6 and with pre-incubation--of 42.7-56.4) and post-treatment (% reduction of 17.9-37.6). This finding indicates mechanisms of action involving desmutagenesis and bioantimutagenesis, albeit the latter having a lesser role. However, in the repair-deficient CHO-xrs5 cells, beta-glucan did not show a protective effect with post-treatment (% reduction of 2.96), thus supporting the involvement of bioantimutagenesis. The comet assay in CHO-k1 cells demonstrated that beta-glucan has neither a genotoxic nor an antigenotoxic effect. Cell viability tests indicated that beta-glucan preserves cell viability in both cell lines, preventing apoptotic events. These findings suggest that beta-glucan, when present in foods, could provide them with nutraceutical characteristics and act as a dietary supplement, or that beta-glucan could be used in new drug development.

Replication protein A and gamma-H2AX foci assembly is triggered by cellular response to DNA double-strand breaks

Human replication protein A (RPA p34), a crucial component of diverse DNA excision repair pathways, is implicated in DNA double-strand break (DSB) repair. To evaluate its role in DSB repair, the intranuclear dynamics of RPA was investigated after DNA damage and replication blockage in human cells. Using two different agents [ionizing radiation (IR) and hydroxyurea (HU)] to generate DSBs, we found that RPA relocated into distinct nuclear foci and colocalized with a well-known DSB binding factor, gamma-H2AX, at the sites of DNA damage in a time-dependent manner. Colocalization of RPA and gamma-H2AX foci peaked at 2 h after IR treatment and subsequently declined with increasing postrecovery times. The time course of RPA and gamma-H2AX foci association correlated well with the DSB repair activity detected by a neutral comet assay. A phosphatidylinositol-3 (PI-3) kinase inhibitor, wortmannin, completely abolished both RPA and gamma-H2AX foci formation triggered by IR. Additionally, radiosensitive ataxia telangiectasia (AT) cells harboring mutations in ATM gene product were found to be deficient in RPA and gamma-H2AX colocalization after IR. Transfection of AT cells with ATM cDNA fully restored the association of RPA foci with gamma-H2AX illustrating the requirement of ATM gene product for this process. The exact coincidence of RPA and gamma-H2AX in response to HU specifically in S-phase cells supports their role in DNA replication checkpoint control. Depletion of RPA by small interfering RNA (SiRNA) substantially elevated the frequencies of IR-induced micronuclei (MN) and apoptosis in human cells suggestive of a role for RPA in DSB repair. We propose that RPA in association with gamma-H2AX contributes to both DNA damage checkpoint control and repair in response to strand breaks and stalled replication forks in human cells.

Epstein–Barr virus latent membrane protein 1 induces micronucleus formation, represses DNA repair and enhances sensitivity to DNA-damaging agents in human epithelial cells

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is a viral oncogene and it is essential for the transformation of resting B cells by the virus. The protein acts as a ligand-less membrane receptor and triggers numerous cellular signaling pathways. Cellular transformation frequently has been associated with genomic instability. To investigate whether EBV LMP1 induces chromosomal aberrations, micronucleus (MN) formation was examined in LMP1-expressing epithelial cells. The expression of wild-type LMP1 enhanced both spontaneous and bleomycin-induced MN formation. MN formation may be induced by inactivation of DNA repair and, therefore, we investigated the effect of LMP1 on DNA repair, using a host cell reactivation (HCR) assay. In the HCR assay, LMP1 reduced the capacity for DNA repair of both NPC-TW01 (p53-wild-type) and H1299 (p53-deficient) cells. As reduction of DNA repair by LMP1 occurs in p53-wild-type and p53-deficient cells, it seems that LMP1 can repress DNA repair in a p53-independent manner. Inactivation of DNA repair may render cells sensitive to DNA-damaging agents. In this study, H1299 cells harboring LMP1 were shown to be more sensitive to UV and bleomycin than those with a vector control. Using various deletion mutants of EBV LMP1 to determine the regions of LMP1 required to enhance MN formation, inhibit DNA repair and sensitize cells to DNA-damaging agents, we found that the region a. a. 189-222 (located within the CTAR1 domain) was responsible for sensitizing cells to UV and bleomycin, as well as for enhancing MN formation and repressing DNA repair. Based on these results, we suggest that disruption of DNA repair by LMP-1 results in an accumulation of unrepaired DNA and consequent genomic instability, which may contribute to the oncogenesis of LMP1 in human epithelial cells.

Micronucleus formation in 3-day mouse embryos associated with maternal exposure to chlorpyrifos during the early preimplantation period

Chlorpyrifos, an organophosphate pesticide, was evaluated for its ability to induce cytogenetic damage in preimplantation embryos after maternal exposure. Pregnant female mice were intraperitoneally (i.p.) administered a single dose of chlorpyrifos (40 or 80 mg/kg) at 10:00 h on Day 0 of pregnancy. On Day 3 of gestation, blastocysts were collected and evaluated for gross morphology, micronucleus (MN) frequency, and cell number. A significant increase in MN frequency indicating cytogenetic damage was observed in the treatment groups in comparison to control. The MN frequency revealed a clear dose-dependent increase. There was also a significant decrease in the embryo cell number in the 80 mg/kg treated group. A simultaneous decrease in the cell number and increase in MN frequency may reflect an embryonic developmental disadvantage resulting from maternal treatment with chlorpyrifos.

Induction of micronuclei by 7H-dibenzo[c,g]carbazole and its tissue specific derivatives in Chinese hamster V79MZh1A1 cells

The clastogenicity/aneugenicity of N-heterocyclic polycyclic aromatic pollutant 7H-dibenzo[c,g]carbazole (DBC) and its two synthetic derivatives N-methyl DBC (MeDBC) and 5,9-dimethyl DBC (diMeDBC) was evaluated in the genetically engineered Chinese hamster V79 cell line V79MZh1A1 with stable expression of human cytochrome P4501A1 and in the parental V79MZ cell line without any cytochrome P450 activity. While none of the three carbazoles changed significantly the level of micronuclei in the parental V79MZ cells, a variable, but statistically significant rise of micronucleus frequencies was assessed in V79MZh1A1 cells. DBC induced dose-dependent increase in the number of micronuclei at harvest times of 24 and 48h and MeDBC at sampling time of 48h in V79MZh1A1 cells in comparison to untreated cells, however, no significant time-dependent increase in micronucleus frequencies was found. The use of the antikinetochore immunostaining revealed that DBC and MeDBC induced approximately equal levels of both kinetochore positive (C+) and kinetochore negative (C-) micronuclei. DiMeDBC, a strict hepatocarcinogen, did not manifest any effect on micronucleus induction in V79MZh1A1 cells. These studies suggest that genetically engineered Chinese hamster V79 cell lines expressing individual CYP cDNAs are a useful in vitro model for evaluation the role of particular cytochromes P450 in biotransformation of DBC and its tissue and organ specific derivatives.

Increased genomic damage in lymphocytes of patients before and after long-term maintenance hemodialysis therapy

This study investigates spontaneous genomic damage in peripheral lymphocytes of 19 patients with severe end-stage renal disease not enrolled onto a maintenance hemodialysis (MHD) program (creatinine level, 5.4 to 10.5 mg/dL) and 16 long-term MHD patients (111 to 282 months on MHD) and the possible association of genomic damage with the degree of renal insufficiency and duration of MHD. Genomic damage was assessed by evaluating the numbers of micronuclei (MN), which are cytoplasmic DNA-containing structures. The average number of MN in the control group of 23 healthy subjects was 15.3 +/- 4.7 MN/1,000 binucleate (BN) cells. The MN frequencies in the long-term MHD group were significantly greater (44.3 +/- 13.7 MN/1,000 BN) than the control frequencies. A significant increase in MN frequencies (28.2 +/- 9.4 MN/1,000 BN) was also seen in patients with advanced renal failure. The major findings of the study, high MN frequencies in long-term hemodialysis and advanced chronic renal failure patients, may result from decreased DNA repair previously shown and may contribute to the increased cancer incidence in these patients.

Discrimination between genotoxicity and cytotoxicity for the induction of DNA double-strand breaks in cells treated with aldehydes and diepoxides

The time-dependent dose-response relationships for the induction of DNA double-strand breaks (DSB) assessed by pulsed-field gel electrophoresis (PFGE) and for viability (evaluated by the MTT cytotoxicity test) were investigated in order to discriminate between genotoxic and cytotoxic mechanisms of DNA fragmentation. Cultured human lung epithelial cells (A549) were treated (i) with the aldehydes formaldehyde or glutaraldehyde and (ii) with the DNA-DNA interstrand crosslinkers melphalan, diepoxybutane or diepoxyoctane. Induction of DSB by formaldehyde and glutaraldehyde was seen only after cell viability was reduced to less than about 60% of the control values, indicating that DSB were the consequence of extragenomic damage and viability loss. Melphalan, diepoxybutane and diepoxyoctane induced DSB by a genotoxic mode with concentrations that did not affect cell survival: 8 h after treatment initiation both heat-labile crosslinks and DSB could be detected. Cells were not able to repair the crosslinks induced by diepoxybutane, the crosslinker with the shortest chain length. In contrast, with melphalan and diepoxyoctane, which have a longer crosslinking property considerable repair of crosslinks was observed. The molecular size distribution of the produced DNA fragments supported this mechanistic distinction. The DNA fragments generated by diepoxides were initially large, their concentration decreasing monotonously from 7 Mbp to less than 1 Mbp and were converted to smaller fragments by 72 h in the course of cell death. In contrast, DNA fragments induced by formaldehyde peaked below 1 Mbp, implicating activation of DNA-degrading enzymes.