Induction of multilocus lesions by UVC-radiation in mouse L5178Y lymphoblasts

The survival, the mutant frequency and the nature of the DNA alteration responsible for the inactivation of the thymidine kinase (tk) locus were investigated in 5 strains of mouse L5178Y lymphoblasts exposed to UVC radiation. The nature of the DNA alteration was investigated in independent TK-/- mutants using Southern blot analysis. The concomitant loss of galactokinase (GK) activity in homogenates of individual TK-/- mutants was taken as an indication that the lesion inactivating the tk allele extended to the neighboring galactokinase (gk) allele. The survival of strains LY-R16 and LY-R83 was decreased to a greater extent than that of strains LY-S1, LY-SR1, and LY-3.7.2C, reflecting a deficiency in excision repair in strains derived from LY-R cells. The TK-/- mutant frequency of strain LY-R83, which is monosomic for chromosome 11 and thus hemizygous for the tk and gk genes, was only 50% of the mutant frequency of strain LY-R16 which is heterozygous for the tk gene. Moreover, a greatly reduced percentage of individual spontaneous and UVC-induced TK-/- mutants of strain LY-R83 showed loss of GK activity in comparison to the other strains. This result indicates that UVC irradiation induces intergenic mutations and that such mutants are poorly recovered in the hemizygous strain. Strain LY-3.7.2C appears to have only one active galactokinase (gk) allele, and very few TK-/- mutants of this strain showed loss of GK activity, possibly because this strain, although heterozyogous for the tk gene, is hemizygous in the region of the gk gene. Strains LY-R16 and LY-S1 are deficient in the repair of UVC- and X-radiation-induced damage, respectively, and the percentage of TK-/- mutants with intergenic mutations was higher for strain LY-R16 after UVC-radiation and for strain LY-S1 after X-radiation. These results indicate that unrepaired DNA lesions lead to an increase in intergenic mutations.     

Mutagenicity of 2-amino-N6-hydroxyadenine at the tk locus in L5178Y strains differing in repair capabilities and karyotype

The cytotoxicity and mutagenicity of 2-amino-N6-hydroxyadenine (AHA) were measured in strains of L5178Y differing in repair capabilities and karyotype. Strain LY-R83 is monosomic for chromosome 11 and is therefore hemizygous for the tk gene, while strains LY-R16 and LY-S1 are TK+/- heterozygotes. Both strain LY-R83 and LY-R16 are sensitive to UV light and are presumed to be deficient in the excision of pyrimidine dimers as shown for the parental strain, LY-R (Hagen et al., 1988; Szumiel et al., 1988). Strain LY-S1 is sensitive to the cytotoxic effects of ionizing radiation and is presumed to be defective in the repair of radiation-induced DNA double-strand breaks, as shown for the parental strain, LY-S (Evans et al., 1987a; Wlodek and Hittelman, 1987). The sensitivities of the three strains to the cytotoxic effects of AHA were similar. After a 4-hour treatment with AHA at 37 degrees C, the D37 for all three strains was approximately 35 ng/ml. The AHA-induced mutant frequency was similar for the hemizygous TK+ strain LY-R83 and the heterozygous TK +/- strain LY-R16, but was slightly higher for strain LY-S1 than for either LY-R strain at an AHA concentration of 100 ng/ml. The proportion of AHA-induced LY-S1 TK -/- mutants forming colonies with diameters less than 0.3 mm was much lower than following treatment with X radiation (24% vs. 61% for AHA and X radiation, respectively). These results indicate that the vast majority of AHA-induced TK -/- mutants harbor single gene mutations. AHA did not result in cyanide-insensitive oxygen uptake, and treatment with this compound did not induce a significant number of DNA single-strand breaks, DNA alkali labile lesions, or DNA degradation in either strain. However, two hours after AHA removal, DNA single-strand breaks and/or alkali-labile lesions, possibly due to the occurrence of DNA repair, were apparent in the DNA of both strain LY-R16 and strain LY-S1.     

Relationship between topoisomerase II and radiosensitivity in mouse L5178Y lymphoma strains

The cytotoxic and mutagenic effects of topoisomerase II inhibitors were measured in closely related strains of mouse lymphoma L5178Y cells differing in their sensitivity to ionizing radiation. Strain LY-S is sensitive to ionizing radiation relative to strain LY-R and is deficient in the rejoining of DNA double-strand breaks induced by this agent, whereas 2 radiation-resistant variants of strain LY-S have regained the ability to rejoin these double-strand breaks. We have found that the sensitivity of these cells to m-AMSA, VP-16, and ellipticine is correlated to their sensitivity to ionizing radiation. However, this correlation did not extend to their sensitivities to novobiocin, camptothecin, hydrogen peroxide, methyl nitrosourea and UV radiation. Thus, there appears to be a unique correlation between sensitivity to ionizing radiation and to topoisomerase II inhibitors which stabilize the cleavable complex between the enzyme and DNA. It is possible either that (1) topoisomerase II is altered in strain LY-S and that this enzyme is involved in the repair of DNA double-strand breaks or (2) strain LY-S is deficient in a reaction which is necessary for the repair of DNA double-strand breaks induced by ionizing radiation as well as the repair of DNA damage induced by these topoisomerase II inhibitors. m-AMSA, VP-16, and ellipticine were found to be highly mutagenic at the tk locus in L5178Y strains which are heterozygous for the tk gene but not in a tk hemizygous strain, indicating that these inhibitors induce multilocus lesions in DNA, as does ionizing radiation. The differences in the sensitivity of strains LY-R and LY-S to the topoisomerase II inhibitors were paralleled by differences in the induction of protein-associated DNA double-strand breaks in the 2 strains. This correlation did not extend to the radiation-resistant variants of strain LY-S, however. The variants showed resistance to the cytotoxic effects of the inhibitors relative to strain LY-S, but exhibited DNA double-strand break induction similar to that observed in strain LY-S.     

The influence of dose rate on the lethal and mutagenic effects of X-rays in proliferating L5178Y cells differing in radiation sensitivity

The lethal and mutagenic effects of ionizing radiation delivered at high (53 Gy/h) and low (0.02 Gy/h) dose rates were measured in two closely related strains of mouse lymphoma L5178Y cells differing in radiation sensitivity (LY-R and LY-S). Strain LY-R was more resistant to the lethal effects of radiation than strain LY-S when exposed at either the high or low dose rate. The survival of strain LY-R was markedly enhanced by the reduction in dose rate. The dose-rate dependence of the survival of strain LY-S was less clear, because of the biphasic nature of its survival curve following low dose-rate radiation. However, if the initial slope of the low dose-rate survival curve is compared to the slope of the high dose-rate survival curve for strain LY-S, only a slight increase in survival at the low dose rate is apparent. Although more sensitive to the lethal effects of radiation, strain LY-S was less mutable at the hypoxanthine/guanine phosphoribosyl transferase locus by both low dose-rate and high dose-rate radiation than strain LY-R. Little dose-rate dependence was exhibited by either strain with regard to the mutagenic effects of radiation. Thus, for strain LY-R, which showed marked dose-rate dependence for survival but not for mutation, the ratio of mutational to lethal lesions was much greater following exposure to low dose-rate than to high dose-rate radiation.     

The repair of potentially lethal damage and sublethal damage in strains of mouse L5178Y lymphoma cells differing in radiation sensitivity

Mouse lymphoma strains L5178Y-R (LY-R) and L5178Y-S (LY-S), which are differentially sensitive to the cytotoxic effects of ionizing radiation, were found to differ in their abilities to repair potentially lethal damage (PLD) and sublethal damage (SLD). The results showed that strain LY-R was more proficient than strain LY-S in the repair of SLD. The split dose recovery observed in strain LY-S could be accounted for by its recovery during postirradiation incubation. In contrast, SLD repair occurred in the absence of PLD repair in strain LY-R. The possibility that the repair of PLD might be completed prior to the postirradiation incubation in strain LY-R was suggested by the decreased survival observed when the cells were irradiated in a hypotonic solution. The repair of PLD and SLD in strain LY-S was temperature sensitive, occurring during postirradiation incubations between 15 and 34 degrees C, but not at 37 or 40 degrees C. This temperature sensitivity is very similar to the temperature sensitivity of the repair of pH 9.6-labile lesions in DNA in strain LY-S, as reported previously. Thus postirradiation cellular recovery processes in strain LY-S may involve the repair of pH 9.6-labile lesions in DNA. Temperature-dependent changes in the postirradiation distribution of cells throughout the cell cycle were observed which could contribute to the temperature sensitivity of the postirradiation recovery of strain LY-S.     

Lethal and mutagenic effects of radiation and alkylating agents on two strains of mouse L5178Y cells

In previous studies, the two closely related strains of L5178Y (LY) mouse lymphoma cells, LY-R and LY-S, have been shown to differ in their sensitivity to UV and ionizing radiation. Thus, in comparison to strain LY-R, strain LY-S has been found to be more sensitive to the lethal effects of ionizing radiation, more resistant to the lethal effects of UV radiation, but less mutable at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus by both UV and X-radiation. In the present work, the lethal and mutagenic effects of ethyl methanesulfonate (EMS), methyl nitrosourea (MNU) and UV radiation (254 nm) were compared in the two strains. Mutability at the Na+/K+-ATPase locus as well as the HGPRT locus was determined. As previously reported, we found strain LY-S to be more resistant than strain LY-R to the lethal effects of UV radiation. In contrast, strain LY-S was more sensitive to the cytotoxic effects of the two alkylating agents. In spite of these differences in sensitivity, we found strain LY-S to be less mutable than strain LY-R by all 3 agents at the HGPRT locus. At the Na+/K+-ATPase locus, strain LY-S was also less mutable than strain LY-R by equal concentrations of EMS and UV radiation and by equitoxic concentrations of MNU. However, the difference between the strains was much more pronounced at the HGPRT locus than at the Na+/K+-ATPase locus. We have suggested that the interaction of unrepaired lesions in strain LY-S tends to cause an excess of deletions and multilocus effects, which in turn result in a locus-dependent decrease in the recovery of viable LY-S mutant cells.     

Novobiocin treatment reverses radiation-induced alterations in higher-order DNA structure in L5178Y nucleoids

We have studied the effect of novobiocin treatment on radiation-induced damage and its repair in higher-order DNA structure in two mouse leukemia cell lines differing in their radiosensitivity, L5178Y-R (LY-R) and L5178Y-S (LY-S). We used the fluorescent halo technique to measure alterations in the superhelical density and the topological constraints of DNA in LY-R and LY-S nucleoids. The results for untreated cells show that both cell lines reached maximal DNA unwinding at the same concentration of propidium iodide (PI), whereas LY-S nucleoids were less efficient in their ability to rewind their DNA. The loop size did not differ significantly between the cell lines. Incubation of LY-R and LY-S cells with novobiocin at a concentration which does not influence survival (0.1 mM for 45 min), but inhibits DNA synthesis in LY-R cells (by 28%) to a greater extent than in LY-S cells (by 10%), also causes more DNA unwinding in LY-R nucleoids than in LY-S nucleoids. However, a decreased superhelical density was observed in nucleoids from both cell lines. Novobiocin applied before, and present during, irradiation prevents radiation-induced alterations in DNA supercoiling more efficiently in LY-R than in LY-S cells. The presence of novobiocin during the repair period increased DNA rewinding to levels not significantly different from control values in nucleoids from both cell lines.     

Labile iron pool correlates with iron content in the nucleus and the formation of oxidative DNA damage in mouse lymphoma L5178Y cell lines

Labile iron pool (LIP) constitutes a crossroad of metabolic pathways of iron-containing compounds and is midway between the cellular need for iron, its uptake and storage. In this study we investigated oxidative DNA damage in relation to the labile iron pool in a pair of mouse lymphoma L5178Y (LY) sublines (LY-R and LY-S) differing in sensitivity to hydrogen peroxide. The LY-R cells, which are hydrogen peroxide-sensitive, contain 3 times more labile iron than the hydrogen peroxide-resistant LY-S cells. Using the comet assay, we compared total DNA breakage in the studied cell lines treated with hydrogen peroxide (25 microM for 30 min at 4 degrees C). More DNA damage was found in LY-R cells than in LY-S cells. We also compared the levels of DNA lesions sensitive to specific DNA repair enzymes in both cell lines treated with H(2)O(2). The levels of endonuclease III-sensitive sites and Fapy-DNA glycosylase-sensitive sites were found to be higher in LY-R cells than in LY-S cells. Our data suggest that the sensitivity of LY-R cells to H(2)O(2) is partially caused by the higher yield of oxidative DNA damage, as compared to that in LY-S cells. The critical factor appears to be the availability of transition metal ions that take part in the OH radical-generating Fenton reaction (very likely in the form of LIP).     

Repair of gamma-ray-induced base damage in L5178Y sublines is damage type-dependent and unrelated to radiation sensitivity.

The L5178Y (LY) murine lymphoma sublines LY-R and LY-S are differentially sensitive to ionizing radiation. The high radiation sensitivity of LY-S cells is related to impaired rejoining of DNA double strand breaks. We found previously that the gamma-ray-induced base damage is higher in the more radiosensitive LY-S subline. Here, we examine the role of the repair of ionizing radiation induced base damage in relation to the radiosensitivity difference of these sublines. We used the GS/MS technique to estimate the repair rates of six types of base damage in gamma-irradiated LY cells. All modified DNA bases identified in the course of this study were typical for irradiated chromatin. The total amount of initial base damage was higher in the radiation sensitive LY-S subline than in the radiation resistant LY-R subline. The repair rates of 5-OHMeUra, 5-OHCyt, 8-OHAde were similar in both cell lines, the repair rates of FapyAde and 8-OHGua were higher in the radiosensitive LY-S cell line, whereas the repair of 5-OHUra was faster in its radioresistant counter, the LY-R. Altogether, the repair rates of the y-ray-induced DNA base damage in LY sublines are related neither to the initial amounts of the damaged bases nor to the differential lethal or mutagenic effects of ionizing radiation in these sublines.     

UV-induced mutagenesis at the hypoxanthine—guanine phosphoribosyl transferase locus in two L5178Y mouse lymphoma cell strains with different UV sensitivities

Two strains of L5178Y mouse lymphoma cells, L5178Y-R (LY-R) and L5178Y-S (LY-S), differ markedly in their sensitivity to 254 nm UV radiation (D₀ = 0.7 and 5.5 J/m²; n = 6.0 and 2.0 for LY-R and LY-S cells, respctively). In this study, the frequency o hypoxanthine-guanine-phosporibosyl-transferase-deficient mutants was determined, using 6-thioguanine (TG) as a selective agent, in populations of LY-R and LY-S cells exposed to various fluences of UV radiation. The spontaneous mutation frequency for LY-R cells was (3.7 ± 0.6) × 10^?5 TG^r mutants per viable cell, and the UV induction rate was (2.2 ± 0.8) × 10^?4 TG^r mutants per viable cell, per J/m². Both spontaneous and induced mutantion frequencies were much lower for LY-S cells. The sopntaneous mutation frequency for these cells were too low to make its measurement practicable ( < 0.0013 × 10^?5 TG^r mutants per viable cell). Mutation induction rate was (4.2 ± 2.2) × 10^?7 TG^r mutants per viable cell, per J/m². These differences in mutability do not appear to be due to gene duplication in LY-S cells, or to selective growth disadvantage of LY-S-derived TG-resistant mutants. Possible mechanisms underlying the differences in mutability of LY-R and LY-S cells are considered.     

Discrepancy between the initial DNA damage and cell survival after camptothecin treatment in two murine lymphoma L5178Y sublines

Two L5178Y (LY) murine lymphoma cell sublines, LY-R, resistant, and LY-S, sensitive, to X-irradiation display inverse cross-sensitivity to camptothecin (CPT): LY-R cells were more susceptible to this specific topoisomerase I inhibitor than LY-S cells. After 1 h incubation with CPT, the doses that inhibited growth by 50 per cent (ID₅₀) after 48 h of incubation were 0·54μM for LY-R cells and 1·25 μM for LY-S cells. Initial numbers of DNA–protein crosslinks (DPCs) measured at this level of growth inhibition were two-fold higher in LY-R (5·6 Gray-equivalents) than in LY-S cells (3·1 Gray-equivalents), which corresponds well with the greater in vitro sensitivity of Topo I from LY-R cells to CPT.^1,2 Conversely, the initial levels of single-strand DNA breaks (SSBs) and double-strand DNA breaks (DSBs) were lower in LY-R cells (4·2 Gray-equivalent SSBs and 5·8 Gray equivalent DSBs) than in LY-S cells (8·0 Gray-equivalent SSBs and 12·0 Gray-equivalent DSBs). Dissimilarity in the replication-dependent DNA damage observed after 1 h of treatment with CPT was not due to a difference in the rate of DNA synthesis between the two cell lines, but may have arisen from a substantially slower repair of DNA breaks in LY-S cells.³ Release from G₂ block by caffeine co-treatment significantly increased cell killing in the LY-S subline, and only slightly inhibited growth of LY-R cells. These results show that after CPT treatment cells arrest in G₂, allowing them time to repair the long-lived DSBs. As LY-S cells are slower in repairing the DSBs, they were more susceptible to CPT in the presence of caffeine.     

Differential antimutagenicity of WR-1065 added after irradiation in L5178Y cell lines

The purpose of this study was to determine the antimutagenicity of WR-1065 added after irradiation of cells of cell lines differing in their ability to rejoin radiation-induced DNA double-strand breaks (DSBs). The postirradiation antimutagenicity of WR-1065 at the thymidine kinase locus was demonstrated for L5178Y (LY)-S1 cells that are deficient in repair of DNA DSBs. Less postirradiation antimutagenicity of WR-1065 was observed in LY-R16 and LY-SR1 cells, which are relatively efficient in DSB repair. Postirradiation treatment with WR-1065 had only a small stimulatory effect on DSB rejoining. A 3-h incubation of irradiated LY cells with WR-1065 caused slight changes in the distribution of cells in the phases of the cell cycle that differed between LY-S1 and LY-SR1 cells. Both LY-S1 and LY-SR1 cells were protected against the cytotoxic and mutagenic effects of radiation when WR-1065 was present 30 min before and during the irradiation. We conclude that the differential postirradiation effects of WR-1065 in the LY-S1 and LY-SR1 cells are not caused by differences in cellular uptake of the radioprotector or in its radical scavenging activity. Possible mechanisms for the postirradiation antimutagenicity of WR-1065 are discussed.     

The response of L5178Y lymphoma sublines to oxidative stress: antioxidant defence, iron content and nuclear translocation of the p65 subunit of NF-kappaB

We examined the response to hydrogen peroxide of two L5178Y (LY) sublines which are inversely cross-sensitive to hydrogen peroxide and X-rays: LY-R cells are radio-resistant and hydrogen peroxide-sensitive, whereas LY-S cells are radiosensitive and hydrogen peroxide-resistant. Higher initial DNA breaks and higher iron content (potentially active in the Fenton reaction) were found in the hydrogen peroxide sensitive LY-R cells than in the hydrogen peroxide resistant LY-S cells, whereas the antioxidant defence of LY-R cells was weaker. In particular, catalase activity is twofold higher in LY-S than in LY-R cells. The content of monobromobimane-reactive thiols is 54% higher in LY-S than in LY-R cells. In contrast, the activity of glutathione peroxidase (GPx) is about two times higher in LY-R than in LY-S cells; however, upon induction with selenium the activity increases 15.6-fold in LY-R cells and 50.3-fold in LY-S cells. Altogether, the sensitivity difference is related to the iron content, the amount of the initial DNA damage, as well as to the efficiency of the antioxidant defence system. Differential nuclear translocation of p65-NF-kappaB in LY sublines is due to the more efficient antioxidant defence in LY-S than in LY-R cells.     

DNA supercoiling changes in nucleoids from irradiated L5178Y-S and -R cells

DNA supercoiling ability was assayed following irradiation in two cell lines of differing radiosensitivity, L5178Y-S (LY-S) and L5178Y-R (LY-R). Cells treated with NaCl and Triton X-100 were exposed to increasing concentrations of the fluorescent, DNA-intercalating dye, propidium iodide (PI), and the diameter of the resulting fluorescent halo of DNA was measured. As the PI concentration was increased from 0.5 to 5 micrograms/ml, halo diameter increased from 20-25 to 45-55 microns due to the unwinding of the DNA supercoils. This process was similar for both cell lines under all conditions studied. As the PI concentration was increased to 50 micrograms/ml, the halo rewound to a diameter of 25-30 microns in unirradiated cells from both lines. However, following exposure to 3-12 Gy of 137Cs gamma rays, the ability of the DNA to be rewound was inhibited in a dose-dependent manner. Rewinding inhibition was greater in LY-S cells than in LY-R cells. Since the induction of DNA damage (e.g., single-strand DNA breaks) appears to be the same for both cell lines, this result implies that a similar extent of damage results in a greater loss of topological constraints on the DNA loops in LY-S. Such a change might be related to the protein composition of the nucleoid cores. One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that nucleoids from LY-S cells were missing a 55-kDa protein present in LY-R.     

DNA repair,cell cycle progression and cell death following camptothecin treatment in two murine lymphoma L5178Y sublines

The processes involved in cell response to camptothecin (CPT) were investigated in two sublines of L5178Y (LY) murine lymphoma; LY-R, resistant and LY-S, sensitive to X-irradiation, which are inversely cross-sensitive to the drug. The cells were pulse-treated with 2 μM CPT for 1 h; this resulted in equal numbers of replication-related DNA double-strand breaks (DSBs) in both sublines.¹ After drug removal, at different time points up to 24 h, the levels of DSBs were measured by using field inversion gel electrophoresis (FIGE) and comet assay at neutral pH. Both methods revealed faster DSBs repair in LY-S than in LY-R cells, in contrast with X-ray-induced DSBs. This however, was followed by the appearance of secondary breaks in the former subline. The cell cycle arrest was at S/G₂ phase and comprised equal numbers of cells in LY-S and LY-R populations. In both sublines formation of giant cells took place, as well as delayed apoptosis starting about 20 h post-CPT incubation and proceeding with similar intensity. At the same time, the total number of necrotic cells appearing during post-exposure incubation in the LY-R subline exceeded that in the LY-S subline. We suggest that, beside previously documented higher susceptibility of topoisomerase I (Topo I) from LY-R cells to CPT,^2,3 a higher initial rate of replication-related DSBs repair, but not lower propensity to apoptosis, may contribute to the relative CPT resistance of LY-S versus LY-R cells. Copyright © 1998 John Wiley & Sons, Ltd.     

Modulation of the effect of camptothecin in x-irradiated L5178Y-R and L5178Y-S cells by benzamide

The L5178Y (LY) murine lymphoma subline, LY-R, is more radioresistant and more sensitive to camptothecin (CPT, inhibitor of topisomerase I) than the second subline used in our investigation, LY-S. Post-irradiation treatment with 3 μM CPT enhanced the radiosensitivity of LY-S cells (D₀ decrease from 0.52 to 0.34 Gy), but did not change it in LY-R cells. Treatment with 2 mM benzamide [BZ, inhibitor of poly(ADP-ribosylation)] before x-rays and CPT increased the radiosensitivity of LY-R cells (D₀ decrease from 1.15 to 0.52) without further modification of radiosensitivity of LY-S cells. Activity of topoisomerase I was diminished 10 min after x-irradiation (5 Gy) in LY-S, but not in LY-R cells. The data on DNA damage (fluorescent halo or comet assays) showed that the ultimate fate of the cells did not depend on the DNA damage pattern estimated immediately after treatment (e. g. the damage was greater in x-rays plus CPT than in BZ plus x-rays plus CPT treated LY-R cells, although the radiosensitivity was less). Aphidicolin (inhibitor of DNA polymerases α and δ) applied concomitantly with CPT in cells not pre-treated with BZ prevented the increase in DNA damage in LY-R cells, but was without effect in LY-S cells. Taking into account the differential inhibition by x-rays of DNA synthesis in LY sublines and its reversion by BZ in LY-S but not in LY-R cells, we conclude that the pattern of DNA damage observed by the methods applied depended on the status of DNA replication. Received: 28 November 1995 / Accepted in revised form: 20 April 1996     

Benzamide-inhibitable alterations in spectral properties of chromatin accompany DNA repair in UVC-exposed L5178Y cells

Summary We examined the response of chromatin to increasing NaCl and MgCl₂ concentrations in UVC-irradiated L5178Y (LY) R and S cells, using the spectral index method (Dixon and Burkholder 1985). We have found an alteration in chromatin properties 1 h after UVC-irradiation of repair proficient LY-S cells, but no change in repair deficient LY-R cells. The change was shown as lowered spectral index, indicating that at given Na⁺ and Mg⁺⁺ concentrations (1 or 200 mM NaCl, 0 or 0.5 mM MgCl₂) chromatin from UVC-irradiated LY-S cells was more compact than that from unirradiated ones. Benzamide treatment reversed the effect of UVC-irradiation in LY-S cells and did not change the response pattern of chromatin from LY-R cells or unirradiated LY-S cells.     

Nonhomologous end-joining deficiency of L5178Y-S cells is not associated with mutation in the ABCDE autophosphorylation cluster

Cells with mutated autophosphorylation sites in the ABCDE cluster of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) are defective in the repair of ionising radiation-induced DSB, but show in an in vitro test the same DNA-PK activity as the cells possessing wild type enzyme. Nevertheless, the mutated DNA-PK is able to undergo ATP-dependent autophosphorylation and inactivation. This characteristics correspond well with the phenotypic features of the L5178Y-S (LY-S) cell line that is defective in DSB repair, shows a pronounced G1 phase radiosensitivity, but in which the level of DNA-PK activity present in total cell extracts is similar to that of its radioresistant counterpart L5178Y-R (LY-R) cell line. The purpose of this work was to examine the possible alterations in the sequence encoding the cluster of autophosphorylation sites in the DNA-dependent protein kinase in LY-S cells. Despite the presence of phenotypic features indicating the possibility of such alterations, no differences were found between the sequences coding for the autophosphorylation sites in L5178Y-R and L5178Y-S cells. In conclusion, the repair defect in LY-S cells is not related to the structure of the DNA-PK autophosphorylation sites (ABCDE casette).     

Differential induction of apoptosis in x-irradiated L5178Y sublines bearing p53 mutation

We examined apoptosis and expression of p53, E2F-1, bax, bclx_L and bcl2 proteins in two L5178Y (LY) murine lymphoma sublines, LY-R and LY-S, which differ in radiosensitivity and double-strand break (DSB) repair. Both sublines are heterozygous for a p53 mutation in codon 170 that precludes the transactivation function. Accordingly, there is no G1/S arrest after irradiation.We found that there is no change in expression of E2F-1, bax, bclx_L or bcl2 proteins in both LY sublines after x-irradiation. LY-R cells do not constitutively express bcl2, whereas both sublines show high bax content. Radiation induces delayed apoptosis to a greater extent in LY-S than in LY-R cells. The apoptosis can be seen 24 h after irradiation (2 Gy) of LY-S cells, with a maximum at 48 h. LY-R cells need 5 Gy and 72 h post-irradiation incubation to show marked apoptosis (identified by the TUNEL method). The reported observations support the assumption that differential radiosensitivity of LY sublines is associated with the induction of apoptosis that is not related to transactivation by p53 and is primarily related to differential DNA repair ability. Received: 19 August 1999 / Accepted in revised form: 30 November 1999     

Mutagenicity and clastogenicity of teniposide (VM-26) in L5178Y/TK +/- -3.7.2C mouse lymphoma cells

The antitumor drug teniposide (VM-26) is a potent inducer of DNA breaks (Long et al., Cancer Res., (1985) 45, 3106), but it is only weakly mutagenic at the hprt locus in CHO cells (Singh and Gupta, Cancer Res., (1983) 43, 577). In the present study, the mutagenic and clastogenic activities of teniposide were evaluated in L5178Y/TK +/- -3.7.2C mouse lymphoma cells. Although teniposide is a weak mutagen at the hprt locus, it is a potent mutagen at the tk locus, with as little as 0.5 ng/ml producing 220 TK mutants/10(6) survivors at 96% survival (background = 100/10(6) survivors). This same dose of teniposide induced 38 aberrations per 100 metaphases (background = 7/100 cells). At 7 ng/ml, teniposide induced approximately 2700 TK mutants/10(6) survivors at approximately 10% survival. At the highest dose sampled for aberration analysis (5 ng/ml), teniposide induced 44 aberrations/100 cells. Most of the aberrations were chromosomal rather than chromatid events. As expected for a compound acting primarily by a clastogenic mechanism, most of the TK mutants were small colonies. Thus, teniposide is a potent clastogen, and it is a potent mutagen at the tk locus but not at the hprt locus. These results support the hypothesis that the location of the target gene affects the ability of the assay to detect both intragenic events and events causing functional multilocus effects. Thus, a heterozygous locus (like tk) but not a functionally hemizygous locus (like hprt) may permit the detection of mutagens that act primarily by a clastogenic mechanism. Because teniposide induces topoisomerase II-associated DNA breaks, and because there is evidence that teniposide may not interact directly with DNA, we discuss the possibility that the potent clastogenic/mutagenic activity of teniposide may be mediated by topoisomerase II.