Cytogenetic toxicity of paraquat and streptonigrin in Fanconi's anemia

Elimination of young erythrocytes from blood circulation and altered erythropoietic patterns during paraquat induced anemic phase in mice

Paraquat a widely used herbicide causes a variety of toxic effects on humans and animals. The present study is focused on the interaction of paraquat with the mouse erythroid system. Administration of paraquat (10 mg/kg body weight i.p. on alternate days in C57Bl/6 mice) induced a significant fall in blood erythrocyte count on 7, 14, and 21 day time points but the erythrocyte count reverted back to normal by 28^th day indicating the emergence of refractoriness to paraquat. A marked surge in the blood reticulocyte count was observed in paraquat treated mice that also subsided by 28^th day. Young erythrocytes in circulation were randomly eliminated from blood circulation in paraquat treated mice and a significant elevation in the level of reactive oxygen species (ROS) was also observed maximally the erythrocytes of this age group. Cells representing various stages of erythroid differentiation in bone marrow and spleen were identified and enumerated flow cytometrically based on their expression of Ter119 and transferrin (CD71) receptor. Proliferative activity of erythroid cells, their relative proportion as well as their absolute numbers fell significantly in bone marrow of paraquat treated mice but all these parameters were significantly elevated in spleens of paraquat treated mice. These changes were essentially restricted to the cells belonging to the two earliest stages of erythroid differentiation. Taken together our results indicate that paraquat treatment causes a transient anemia in mice resulting from random elimination of young circulating erythrocytes as well as depressed erythropoietic activity in bone marrow. Spleen erythropoietic activity however was elevated in paraquat treated mice.

Modulation of streptonigrin cytotoxicity by nitroxide SOD mimics

Nitroxides are cell-permeable, stable radicals that react readily with paramagnetic species such as transition metals or short-lived free radicals, though not generally with diamagnetic molecules. Nitroxides can undergo one-electron selective redox reactions and thereby potentially modify the activity of cytotoxic drugs. Streptonigrin (SN) toxicity requires bioreduction to yield the semiquinone radical, and the toxicity is reportedly mediated by transition metals and oxygen-derived reactive species via redox-cycling of the semiquinone intermediate. The present study shows that (1) nitroxides protected isolated DNA and also aerated or hypoxic bacterial cells from SN toxicity; (2) H2O2 potentiated the hypoxic cytotoxicity of the drug but inhibited the damage to aerated cells; (3) pretreatment of cells with H2O2 conferred some protection, but not when the drug alone was preexposed to H2O2; and (4) desferrioxamine and 2,2-dipyridyl, though neither diethylenetriamino pentaacetate, exogenous catalase, or superoxide dismutase, decreased SN-induced cell killing. The mechanisms by which nitroxides protect from SN toxicity involve both a selective radical-radical reaction with SN semiquinone and the reoxidation of reduced cellular transition metal ions. On the other hand, H2O2 appears to exert two opposing effects: (1) facilitation of cell killing by the Fenton reaction and (2) lowering the cellular level of reducing equivalents, thus inhibiting the bioreductive activation of SN.

DNA topoisomerases and the DNA lesion in human genetic instability syndromes

Genetically determined chromosomal instability entails, among other sequelae, a condition of elevated cancer risk. Patients with the autosomal recessive disorder Fanconi's anemia (FA) often develop leukemias of the monocytic lineage together with pancytopenia, whereas the Bloom's syndrome (BS) mutation confers an early and elevated incidence of neoplasia of no particular type. Cultured cells from FA patients show spontaneously elevated rates of chromosome aberrations and a hypersensitivity to DNA cross-linking agents. Cytogenetic evaluation of cells from BS patients revealed elevated rates of sister chromatid exchanges, which were sensitive to the bromodeoxyuridine (BrdU) concentration used in the assay. Such a BrdU sensitivity was also found in cultured cells from healthy subjects exposed to the intracellular superoxide generator paraquat or to bleomycin. Skin fibroblasts from FA and BS patients show poor growth, which in the case of FA could be mitigated by lowering the oxygen concentration to 5%. Lymphoblastoid B-cell lines derived from peripheral blood samples from FA and BS patients show elevated numbers of cells arrested in the G2 phase of the cell cycle. This phenomenon could also be provoked by exposing cell lines from healthy subjects to compounds interfering with the function of DNA topoisomerase I (camptothecin) or II (m-AMSA). To test for a putative deficiency of either DNA topoisomerase, B-cell cultures from FA and BS patients were compared with cell cultures from healthy subjects regarding their sensitivity towards camptothecin and m-AMSA. No difference in sensitivity to these agents was found in patient vs. control cell lines, thus ruling out a deficiency in DNA topoisomerase I or II as the prime defect in these conditions of elevated cancer risk. The similarity between the cell cycle kinetic patterns found in untreated FA cell lines and in normal cell lines exposed to camptothecin or m-AMSA suggest that the DNA lesion in FA, presumably being caused by an oxygen-related mechanism, may interfere with DNA topoisomerase function.

Cellular responses to hematoporphyrin-induced photooxidative damage in Fanconi anemia, xeroderma pigmentosum and normal human fibroblasts

Several observations reported in the literature suggest that singlet oxygen (1O2) might play a role in the clastogenic process in Fanconi anemia (FA) cells, and that the antioxidant status of xeroderma pigmentosum (XP) may also be altered. In order to test the ability of FA and XP cells, relative to normal cells, to cope with 1O2 damage, the effects of photosensitization by hematoporphyrin (HP) have been determined (i) on host cell reactivation (HCR) of damaged infecting herpes simplex virus (HSV) or transfecting SV40 DNA, and (ii) on DNA template capability and clonogenicity of treated cells. Results showed no significant difference among the three types of cells, either for the survival of HP-photosensitized HSV, or for the yields of SV40 virus following transfection of cultures with damaged viral DNA. The treatment of cells with HP plus 365-nm light leads to a dose-dependent, homothetic reduction of 18S and 28S ribosomal RNA (rRNA) synthesis, presumably through a mechanism other than the formation of transcription termination sites. After a 24-h post-exposure incubation, the rate of rRNA synthesis was restored to higher than normal levels in all cell lines. Finally, two FA cell lines showed a higher survival to HP photosensitization than two normal cell lines. Another FA cell line and XP-A and XP-C cells were in the range of sensitivity of the two normal strains for this treatment. These results indicate that FA cells possess an antioxidant defense system at least as efficient as that of normal cells for processing 1O2-induced damage.

Superoxide dismutase activity and chromosome damage in cultured chromosome instability syndrome cells

The basal levels of superoxide dismutase (SOD) activity and chromosome aberration (CA) and sister-chromatid exchange (SCE) frequencies were examined in cultured fibroblasts or Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs). These cells were derived from patients with chromosome instability syndromes (CISs) including Bloom's syndrome (BS), Fanconi's anemia (FA) and ataxia telangiectasia (AT). Embryonal fibroblasts and LCLs from normal subjects served as controls. Although LCLs tended to exhibit a higher SOD level than fibroblasts due to an elevation of Cu/Zn-SOD activity, BS and FA fibroblasts with increased frequencies of CAs and/or SCEs showed abnormally elevated SOD activity due to the manifold increase of Mn-SOD levels compared with control cells. However, BS and AT LCLs with almost control levels of CA and SCE frequencies showed no, or a slightly elevated, SOD activity, suggesting a possible selection of such cells during EBV transformation. The observed parallelism between the SOD activity and the cytogenetic manifestation may imply an involvement of active oxygen species, especially superoxide radicals, in the increased chromosome damage of CIS cells.

Pure exogenous singlet oxygen: nonmutagenicity in bacteria

Singlet oxygen (1 delta gO2) is the lowest energy-excited state of molecular oxygen, and more reactive than the triplet ground-state molecule. Although singlet oxygen has been implicated in a variety of biological effects, including reactions with DNA or some of its components, evidence for mutagenesis by singlet oxygen has remained unclear. We have previously described a system for bacterial exposure to pure exogenous singlet oxygen that eliminates ambiguity regarding the identity of the reactive species responsible for observed results. Despite the potent toxicity of pure singlet oxygen for several different strains of bacteria, we have found no evidence for mutagenicity of singlet oxygen in 26 Salmonella typhimurium histidine-auxotrophic strains killed to 35% survival. These strains included a variety of base-pair substitution or frameshift target sequences for reversion, including targets responsive to oxidative damage and targets rich in GC base pairs. Some strains combined histidine mutations with one or more mutations affecting DNA-repair capacity. 4 strains possessing the hisG46 mutation also were not mutated when exposed to dose ranges killing less than 28% and up to 99% of the bacteria. The relative frequency of small inphase deletions was assayed in hisG428 bacteria exposed to single oxygen and found to be the same as the spontaneous level. In addition to lack of induction of mutation in these strains, the 8-azaguanine forward mutation assay yielded no evidence of mutagenesis by singlet oxygen in strains killed to 15% survival. No induction of genetic changes by singlet oxygen was seen in an assay for duplication of approximately 1/3 of the bacterial chromosome. Tests for the ability of singlet oxygen to induce lambda prophage in E. coli K12 also proved negative. These studies collectively indicate that pure singlet oxygen generated outside the bacterial cell does not react significantly with the bacterial chromosome in ways leading to base-pair substitutions, frameshift mutations, small or large deletions, large duplications, or damage that interferes with DNA replication and induces the SOS system.