DNA strand breaks and death of thymocytes induced by N-methyl-N-nitrosourea

DNA damage and glutathione content in radiology technicians

BACKGROUND

We wanted to investigate the effect of X-rays on mononuclear blood cells (MNCs) and red blood cells (RBCs) of radiology technicians exposed to X-rays in hospital.

METHODS

DNA damage was detected by fluorometric analysis of DNA unwinding. Glutathione levels were measured with enzymatic method in mononuclear blood cells. Glutathione content and catalase (CAT) activity of erythrocytes, and plasma malondialdehyde (MDA) levels were determined by spectrophometric methods.

RESULTS

An insignificant increase in plasma malondialdehyde levels and a significant decrease in mononuclear blood cells glutathione levels were observed in nonsmoking radiology technicians. In smoking radiology technicians, on the other hand, in addition to an increase in plasma malondialdehyde levels, DNA damage was also significantly apparent. Besides mononuclear blood cells' glutathione depletion, the glutathione content of red blood cells was also found to be decreased.

CONCLUSION

It can be suggested that smoking seems to augment the toxic effects of radiation.

Reactive oxygen species and UV-B: effect on cyanobacteria

Reactive oxygen species (ROS) are involved in the damage and response of cyanobacteria to UV-B irradiation. In cyanobacteria, there are several targets for the potentially toxic ROS such as lipids, DNA and protein. The damage to photosynthetic apparatus induces the inhibition of photosynthesis that is mediated partially by ROS. UV-B-induced oxidative stress and oxidative damage increases with irradiation time and can be reversed after long-term irradiation. This raises the interesting question of whether cyanobacteria can acclimatize to the present UV-B stress. On one hand, ROS may also act as signal molecules and mediate the genetic regulation of photosynthetic genes and the induction of antioxidant enzymes. On the other hand, the efficient defense and repair system allows cyanobacteria to recover from the oxidative damage under moderate UV-B irradiation. In addition, the following methods are discussed: the fluorogenic probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA), used to detect oxidative stress induced by UV-B; thiobarbituric acid reactive substances (TBARS), used to determine lipid peroxidation in cyanobacteria; fluorimetric analysis of DNA unwinding (FADU), used to quantify DNA strand breaks induced by ROS formation under UV-B stress.

UV-B-induced formation of reactive oxygen species and oxidative damage of the cyanobacterium Anabaena sp.: protective effects of ascorbic acid and N-acetyl-L-cysteine

Reactive oxygen species (ROS) are involved in the oxidative damage of the cyanobacterium Anabaena sp. caused by UV-B (280-315 nm) radiation. UV-B-induced overproduction of ROS as well as the oxidative stress was detected in vivo by using the ROS-sensitive probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA). Thiobarbituric acid reactive substances (TBARS) and fluorometric analysis of DNA unwinding (FADU) methods were adapted to measure lipid peroxidation and DNA strand breaks in Anabaena sp. Moderate UV-B radiation causes an increase of ROS production, enhanced lipid peroxidation and DNA strand breaks, yielding a significantly decreased survival. In contrast, the supplementation of UV-A in our work only showed a significant increase in total ROS levels and DNA strand breaks while no significant effect on lipid peroxidation, chlorophyll bleaching or survival was observed. The presence of ascorbic acid and N-acetyl-L-cysteine (NAC) reversed the oxidative stress and protected the organisms from chlorophyll bleaching and the damage of photosynthetic apparatus induced by UV-B significantly, resulting in a considerably higher survival rate. Ascorbic acid also exhibited a significant protective effect on lipid peroxidation and DNA strand breaks while NAC did not show a substantial effect. These results suggest that ascorbic acid exhibited significantly higher protective efficiency with respect to DNA strand breaks and survival than NAC while NAC appears to be especially effective in defending the photosynthetic apparatus from oxidative damage.

Different mutation frequencies and spectra among organs by N-methyl-N-nitrosourea in rpsL (strA) transgenic mice

The frequencies and spectra of N-methyl-N-nitrosourea (MNU)-induced in vivo somatic mutations were determined in rpsL (strA) transgenic mice. The wild-type rpsL gene, which exhibits a streptomycin-sensitive (Sm(S)) phenotype, was used as the rescue marker gene. Studies of mutation spectra among different organs and tissues were simplified using this system because of the short coding sequence (375 bp) of the rpsL gene. MNU administration to transgenic mice significantly elevated the mutation frequencies in various adult organs. Two distinctive patterns of mutation spectrum were observed, depending on the organs tested. Mutations derived from labile organs (spleen and thymus) were predominantly G:C to A:T transitions, as expected for MNU mutagenesis. Stable organs like the liver and brain, however, carried many fewer G:C to A:T transitions but significantly more single base deletions, of which the spectrum was very similar to that of background mutations in the rpsL transgenic mice. This spectrum difference among more and less proliferating organs was confirmed by the predominant occurrence of G:C to A:T transitions in fetal liver cells exposed to transplacental MNU treatment. In addition, most (approximately 90%) of the G:C to A:T transitions induced by MNU were detected in the first nucleotide of some 5'-G-(C or G)-3' sequences, many of which corresponded to the middle guanine residue of 5'-purine-G-(C or G)-3' sequences. It is thus suggested that at particular sites, the neighboring bases in both the 5' side and 3' side seem to influence either the susceptibility to DNA damage or the ability to repair MNU-induced lesions.

Combination of genomic DNA fingerprinting into the medaka specific-locus test system for studying environmental germ-line mutagenesis

A specific-locus test (SLT) system has been established using the medaka fish (Oryzias latipes), where recessive visible mutations (at the b, lf or gu loci) detected during early embryonic stages (TM) or after hatching (VM), and dominant lethals (DL) can be examined in the same individual F1 progeny of treated parents. It was found that approximately 90% of the F1 embryos with gamma-ray-induced specific-locus mutations were concomitantly accompanied by dominant lethals irrespective of doses and germ-cell stages at the time of exposure, suggesting that DNA alterations in such mutants might include both the marker loci and region(s) responsible for dominant lethals. In contrast, embryonic lethality of the ENU (ethylnitorosourea)-induced specific-locus mutants considerably varied among ENU concentrations as well as germ-cell stages treated. Further, synergistic effect of combined treatments with gamma-rays and ENU on induction of mutations were suggested in postmeiotic male germ cells, while in spermatogonia no synergistic effect was found. DNA alterations at the 87 arbitrarily primed polymerase chain reaction (AP-PCR) markers spread over the genome were examined for individual dominant lethal embryos from 4.75 Gy-irradiated sperm or spermatids. It was found that, 14 out of 20 dominant lethal embryos lost more than one AP-PCR markers, including multiple markers located on the identical linkage group (average genetic distances, approximately 11 cM). Also found was that frequency of loss of the AP-PCR markers in the severely malformed dominant lethal embryos was higher (approximately 4.5%) than that in the slightly malformed lethal embryos (approximately 1.6%). Here, results of these studies, including previously unpublished work, are presented to illustrate the potential usefulness of the medaka SLT system for monitoring environmental mutagens.

MNU affects mouse erythroleukemia cell differentiation at sub-cytotoxic doses

MNU is a potent carcinogen and mutagen to various tissues. Molecular events during differentiation show particular sensitivity to MNU exposure. We have investigated the mouse erythroleukemia (MEL) cell differentiation in response to DMSO and the influence of subcytotoxic doses of MNU on this process to assess the role of MNU on the course of differentiation and specific gene expression in a single cell type. Differentiation was followed by determining the extent of hemoglobinization and beta-globin gene expression, which are representative measures of red cell maturation. In this study we have shown a delay and decrease in the extent of MEL cell differentiation by MNU exposure at the time of induction to differentiate, even at sub-lethal MNU concentrations. Once the differentiation process was initiated, exposure to MNU at sub-lethal doses showed a significantly smaller effect on the molecular course of events. Pre-treatment of MEL cells with MNU before DMSO induction did not affect differentiation. The MNU-induced delay in differentiation was reflected in the delayed appearance of beta-globin transcripts during the first 12 h post induction. However, transcription could not account for reduced hemoglobinization of the MNU-treated cells at 48 and 72 h post induction.

Expression of leukocyte cell adhesion molecules on gastric carcinomas: possible involvement of LFA-3 expression in the development of distant metastases

Expression of the cell adhesion molecules ICAM-1 (CD54) and LFA-3 (CD58) was examined on primary gastric carcinomas, autologous benign mucosa and metastatic lesions. Although ICAM-1 was never observed on benign gastric epithelium, even in the presence of chronic inflammation and a strong leukocyte infiltrate, 38% (26/69) of the primary tumors expressed this molecule. ICAM-1 was restricted to differentiated tumors and correlated with the presence of leukocytes and the absence of vessel invasion. The ICAM-1 expression pattern of metastatic lesions reflected that of the primary tumor, suggesting that most tumors retain the non-inducible phenotype seen in normal mucosa while some become cytokine-sensitive. ICAM-1 expression showed no correlation with tumor relapse or survival. LFA-3 was absent from 8% (4/49) of the primary tumors and reduced (e.g., < or = 50% positive cells) in 33% (16/49). Expression of LFA-3 by more than 50% of the tumor cells correlated with cellular dedifferentiation (G3, G4), histologically detectable vessel invasion, tumor recurrence and decreased survival time. Primary tumors and metastases in draining lymph nodes demonstrated a broad range of LFA-3 expression. In contrast, distant metastases (liver and peritoneum) had uniformly high frequencies of LFA-3-positive cells, suggesting a selective advantage for these cells in the establishment of distant metastases.

Cell death due to ACNU-induced DNA fragmentation: inhibition by cycloheximide

Several anticancer drugs have recently been shown to induce cell death in a manner similar to programmed cell death or apoptosis. The purpose of this study is to explore the mode of cell death caused by ACNU, a water-soluble nitrosourea. Exposure of rat glioma cell line KEG-1 to ACNU for 2 hours resulted in oligonucleosomal DNA fragmentation, creating a 'ladder' on agarose gel electrophoresis. DNA fragmentation began 18 hours after ACNU treatment, and preceded loss of membrane integrity as evaluated by the trypan blue exclusion test. The extent of DNA fragmentation increased in a dose-dependent manner, and the cell survival rate decreased reciprocally. A translational inhibitor, cycloheximide, suppressed this DNA fragmentation and enhanced cell survival rate with partial inhibition of protein synthesis. However, a transcriptional inhibitor, actinomycin D, failed to inhibit DNA fragmentation or enhance cell survival. Cycloheximide-inhibitable DNA fragmentation was also found in the KEG-1 implanted in vivo rat model following the administration of ACNU. These findings suggest that ACNU induces cell death associates with DNA fragmentation and partially with protein synthesis.