The rad18 gene of Schizosaccharomyces pombe defines a new subgroup of the SMC superfamily involved in DNA repair.

The rad18 mutant of Schizosaccharomyces pombe is very sensitive to killing by both UV and gamma radiation. We have cloned and sequenced the rad18 gene and isolated and sequenced its homolog from Saccharomyces cerevisiae, designated RHC18. The predicted Rad18 protein has all the structural properties characteristic of the SMC family of proteins, suggesting a motor function--the first implicated in DNA repair. Gene deletion shows that both rad18 and RHC18 are essential for proliferation. Genetic and biochemical analyses suggest that the product of the rad18 gene acts in a DNA repair pathway for removal of UV-induced DNA damage that is distinct from classical nucleotide excision repair. This second repair pathway involves the products of the rhp51 gene (the homolog of the RAD51 gene of S. cerevisiae) and the rad2 gene.

Loss of tumorigenicity with simultaneous changes in radiosensitivity and photosensitivity during in vitro growth of L5178Y murine lymphoma cells.

Murine leukemic lymphoblasts L5178Y-R (LY-R) undergo conversion into their L5178Y-S (LY-S) variant as a result of prolonged (5 months to 4 years) cultivation in vitro. LY-R cells are highly tumorigenic in DBA/2 mice; resistant to X-rays [D0 (mean lethal dose [reciprocal of the slope of the linear portion of dose-survival curve] ) = 0.91 grays]; and sensitive to ultraviolet radiation (D0 = 0.7 J/sq m), short (up to 60 min) heat (43 degrees) treatment, and certain potential cancer drugs. LY-S cells are practically nontumorigenic in DBA/2 mice; sensitive to X-rays (D0 = 0.56 grays); and resistant to ultraviolet radiation (D0 = 5.5 J/sq m), short heat treatment, and the drugs. The differences in sensitivity of these two cell strains to physical and chemical agents are paralleled by differences in DNA repair efficiency. Although both strains can be cloned in soft agar, LY-S cells invariably show higher plating efficiencies. In vitro mean doubling times are 12 to 15 hr and approximately 10 hr for LY-R and LY-S cells, respectively. The actual loss of tumorigenicity and changes in radio- and photosensitivity associated with conversion of LY-R cells into LY-S cells occur within a short time. This indicates that these changes (and probably other phenotypic changes mentioned above) result from a single event or from several events occurring within a relatively short time elicited by in vitro culture conditions.

Effect of amphotericin B on dipalmitoylphosphatidylcholine membranes: calorimetry, ultrasound absorption and monolayer technique studies.

Amphotericin B (AmB) is a popular drug frequently applied in the treatment of mycosis. Differential scanning calorimetry (DSC), ultrasound absorption and monomolecular layer technique were applied to study the effect of AmB on organisation of dipalmitoylphosphatidylcholine (DPPC) membranes. DSC-determined enthalpy of the main phase transition of DPPC liposomes was found to be a sensitive parameter to monitor AmB-DPPC interaction. The enthalpy of the phase transition decreases with the increase in molar fraction of AmB incorporated to membranes. The exceptionally sharp decrease in the enthalpy of the transition was observed in the membranes containing 5-7 mol% AmB. Ultrasound absorption-monitored main phase transition of DPPC is very broad under the presence of 5 mol% AmB showing destabilisation and disorganisation of a membrane structure. These findings are discussed in comparison to monomolecular layer study of two-component DPPC-AmB system. Analysis of the surface pressure-molecular area isotherms of compressing DPPC-AmB films at the air-water interface shows pronounced increase in mean molecular area at AmB concentrations corresponding to those found to destabilise DPPC membranes of liposomes. Disorganisation of lipid bilayers due to the presence of AmB in concentrations below 10 mol% with respect to lipid is discussed in terms of toxicity and side effects of this drug.

Morphological transformation of C3H 10T1/2 cells by 99mTc-cardiolite.

UNLABELLED: The induction of in vitro morphological transformation in C3H 10T1/2 cells by 99mTc-Cardiolite (contents of Cardiolite kit [hexakis(2-methoxyisobutylisonitrile) and other components] plus (99m)Tc generator eluate) was examined. METHODS: Cells were grown for 48 h in the presence of 99mTc-Cardiolite or decayed 99mTc-Cardiolite (99mTc-Cardiolite after 1 wk of storage), and cell survival and transformation were assessed by the colony-forming and focus assays, respectively. X-ray was used as a reference for radiation effects, and 20-methylcholanthrene was used as a positive control for focus formation. RESULTS: Exposure of cells to 99mTc-Cardiolite results in a transformation frequency that is not significantly different from that induced by the volume equivalent of decayed 99mTc-Cardiolite. The number of foci per viable cell increases linearly from approximately 0.17 x 10(-4) in the untreated control to 1.7 x 10(-4) at 37 kBq/mL and 30 x 10(-4) at 1100 kBq/mL 99mTc-Cardiolite or its decayed 99mTc-Cardiolite volume equivalent. Furthermore, exposure of cells to low extracellular concentrations of 99mTc-Cardiolite or decayed 99mTc-Cardiolite (cell survival, > or =88%) induces an approximately 20-fold greater number of transformants per viable cell than that observed after 0.5 Gy x-irradiation, a dose that causes the same level of toxicity. CONCLUSION: Radioactive and decayed 99mTc-Cardiolite induce morphological transformation of C3H 10T1/2 cells in vitro. The underlying mechanism does not seem to be related to the radiation effects of decaying 99mTc but to chemical(s) present in the 99mTc-Cardiolite kit.

Are lethal effects of nitracrine on L5178Y cell sublines associated with DNA-protein crosslinks?

Nitracrine (Ledakrin, 1-nitro-9-(3,3-N,N-dimethylaminopropylamino)-acridine) is of interest as a DNA intercalator and alkylator with very high cytotoxic potency, especially against hypoxic cells. DNA-DNA crosslinks [Konopa et al., Chem Biol Interact 43: 175-197, 1983; Pawlak et al., Cancer Res 44: 4289-4296, 1984] or DNA-protein crosslinks (DPCs) [Woynarowski et al., Biochem Pharmacol 38: 4095-4101, 1989; Szmigiero and Studzian, Biochim Biophys Acta 1008: 339-345, 1989] are related to the toxicity of the drug. The cytotoxic effect of and DNA damage induced by nitracrine were measured in two sublines of mouse lymphoma L5178Y, LY-R (resistant to ionizing radiation) and LY-S (sensitive to ionizing radiation). LY-R cells were more sensitive to nitracrine (D10 = 0.11 microM) than LY-S (D10 = 0.35 microM) when treated for 1 hr at 37 degrees. To a DNA-DNA crosslinking agent, mitomycin C, the comparative sensitivity was opposite. LY-R cells were more resistant to this drug than LY-S cells (D10 = 7.1 vs 2.3 microM). DNA damage induced by nitracrine was measured by the alkaline elution method and by nitrocellulose filter binding assay. Nitracrine treatment with biologically relevant concentrations (0.1-3.0 microM, 1 hr, 37 degrees) induced only DPCs. Interstrand crosslinks and DNA breaks were not detected. Nitracrine produced about two times more DPCs in LY-R cells than in LY-S cells. Both sublines removed 50% of initial lesions during 2 hr post-treatment incubation. The greater sensitivity of LY-R cells to nitracrine is thus not related to the efficiency of DNA repair, but may be a consequence of enhanced initial damage in the form of DPCs. This finding is consistent with the latter lesion being responsible for the cytotoxicity of nitracrine.

Indirect mechanisms contribute to biological effects produced by decay of DNA-incorporated iodine-125 in mammalian cells in vitro: double-strand breaks.

We have examined whether nuclear DNA can be protected from double-strand breaks (DSBs) induced by decay of the Auger-electron-emitting radionuclide 125I. Decays were accumulated at 0.3 degrees C in Chinese hamster V79 cells suspended in isotonic buffer containing 0.1 M EDTA in the presence or absence of 10% dimethyl sulfoxide (DMSO). DSBs were measured by the neutral elution method (pH 9.6) and quantified as strand scission factors. DMSO was shown to protect DNA from DSBs caused by the decay of DNA-incorporated 125I. The dose modification factor (DMF) for this radionuclide decreases as a function of 125I decays (389 to 4,100 decays, DMF = 2.5 to 1.3). Extrapolation of the curve for the DMF indicates that at approximately 15,000 decays/cell, a DMF of 1 would be obtained. Experiments using large numbers of 125I decays confirmed these extrapolations. For induction of DSBs by 137Cs gamma rays, the DMF also decreases with dose (50 to 290 Gy, DMF = 2.7 to 1.5). However, extrapolation of the curve for the DMF indicates that protection does not cease at higher doses. The data show that, at the same level of damage, DMSO can protect against gamma-ray-induced DSBs 1.35-fold more efficiently than against DSBs caused by the decay of DNA-incorporated 125I. It appears that when 125I is incorporated into DNA, chromatin structure fosters some DSB formation by an indirect mechanism(s) and that more than one DSB is generated per decaying atom.

Indirect mechanisms contribute to biological effects produced by decay of DNA-incorporated iodine-125 in mammalian cells in vitro: clonogenic survival.

We have examined whether mammalian cells in vitro can be protected against the lethal effects of irradiation by Auger electrons emitted from DNA-incorporated 125I. Chinese hamster V79 lung fibroblasts were cultivated in the presence of 5-[125I]iodo-2'-deoxyuridine (125IdU) for 18 h and resuspended in ice-cold medium in the presence or absence of 10% dimethyl sulfoxide (DMSO). DNA-incorporated 125I activity was measured and the cells were plated for survival. A portion of the cell suspensions were also stored on ice to accumulate 125I decays for 6 to 48 h, after which the cells were plated to determine survival. Storage on ice up to 48 h without radioactivity reduced plating efficiency from 67 +/- 4% (SEM) to 20 +/- 1%. DMSO had a protective effect on colony formation, as the respective cloning efficiencies were 83 +/- 3% and 72 +/- 12% at 0 and 48 h. The survival curves for 125IdU-labeled cells are exponential with D0 = 36 +/- 2 decays per cell in the absence of DMSO and 195 +/- 20 decays per cell in the presence of DMSO. Thus the dose modification factor (DMF) at 37% survival for 10% DMSO is 5.4 +/- 0.6 for DNA-incorporated 125I. In reference experiments, a DMF of 2.5 +/- 0.8 was measured for cells irradiated with 137Cs gamma rays. These results indicate that the radiotoxicity of Auger electrons from 125I decay in mammalian cells is caused mainly by an indirect mechanism(s).

Toxicity of DNA-incorporated iodine-125: quantifying the direct and indirect effects.

To understand the biophysical mechanism(s) underlying the induction of cell death by the decay of the Auger electron emitter iodine-125 in DNA, Chinese hamster V79 lung fibroblasts were labeled with 5-[(125)I]iodo-2'-deoxyuridine ((125)IdU) for two doubling times and frozen and stored at -135 degrees C in the presence of 0.26-3.0 M dimethyl sulfoxide (DMSO), which acts simultaneously as a cryoprotector and a hydroxyl radical scavenger. After the accumulation of (125)I decays, the cells were defrosted and their survival was determined. Within the range of the number of decays examined (up to 470 disintegrations per cell), the survival curves are exponential. The dependence of the D(37) on DMSO concentration is triphasic and seems to reach a plateau at approximately 1.3 M. By extrapolating to infinite DMSO concentration, we estimate the D(37) for maximal hydroxyl radical scavenging to be 411 +/- 36 disintegrations per cell. To determine the D(37) in the absence of DMSO, we extrapolate the D(37) curve to zero concentration, and a D(37) of 54 +/- 5 disintegrations per cell is obtained. The maximal dose modification factor, calculated as the ratio of the D(37) at infinite DMSO concentration (i.e. direct effects only) to the D(37) at zero DMSO concentration (i.e. direct and indirect effects), is 7.6 +/- 1.0. By inference, approximately 90% of the radiotoxic effects of DNA-incorporated (125)I are due to indirect mechanisms.

Dose-dependent transformation of cells of human fibroblast cell strain MSU-1.1 by cobalt-60 gamma radiation and characterization of the transformed cells.

Cells from an infinite-life-span near-diploid human fibroblast cell strain, MSU-1.1, were transformed after a single exposure to 60Co gamma radiation. The frequency of transformation as measured by the number of induced foci per 10(6) cells was a linear function of dose. Cells from 13 independent foci from gamma-irradiated cell populations and one from a nonirradiated cell population were isolated, clonally expanded and assayed for characteristics of malignantly transformed cells. Eight of the 13 focus-derived cell strains from the irradiated populations formed tumors in athymic mice with latent periods (time required for the tumors to reach 1 cm in diameter) of 4-27 weeks. Of these 8 cell strains, 3 were fully growth factor-independent, formed large colonies (> 120 microm in diameter) in 0.33% agarose at a high frequency (50%), and produced malignant tumors with a mean latency of 6 weeks or less at all sites injected. Four others formed colonies in agarose at a slightly lower frequency, were only partially growth factor-independent, and produced malignant tumors with a longer mean latency (7-18 weeks). The tumor-derived cell lines from these latter 4 cell strains, when tested for growth in agarose, showed markedly enhanced anchorage independence. The eighth tumorigenic focus-derived cell strain was growth factor-independent but could not produce large colonies in agarose. It produced benign tumors (fibromas) with a mean latency of 27 weeks. All 8 tumorigenic focus-derived cell strains had lost the transactivating function of the TP53 (formerly known as p53) gene. However, loss of TP53 activity was not sufficient to cause tumorigenicity since 3 of the 6 nontumorigenic focus-derived cell strains had also lost all TP53 transactivation function. The other 3, which included a cell strain from the unirradiated control, had wild-type TP53 alleles and did not form tumors. These latter results support the hypothesis that loss of TP53 transactivating function plays a role in focus formation, but does not directly cause tumorigenicity. This is in agreement with studies that demonstrate that the loss of TP53 transactivation facilitates the other changes required for tumorigenicity.

Survival and DNA damage in Chinese hamster V79 cells exposed to alpha particles emitted by DNA-incorporated astatine-211.

Asynchronous Chinese hamster V79 lung fibroblasts were incubated at 37 degrees C for 30 min with the thymidine analog 5-[211At]astato-2'-deoxyuridine (211AtdU, exposure from DNA-incorporated activity) or with [211At]astatide (211At-, exposure from extracellular activity), and DNA-incorporated activity was determined. The 211AtdU content in cellular DNA increased as a function of extracellular concentration. Incorporation of 211At- was less than 1% of that of 211AtdU. After exposure, cells were frozen in the presence of 10% DMSO. One month later, survival was determined by the colony-forming assay, and DNA double-strand breaks (DSBs) were measured by the neutral elution method (pH 9.6). The survival curve for 211AtdU was biphasic (D37 = 2.8 decays per cell), reflecting killing of 211At-DNA-labeled cells and of unlabeled cells irradiated by 211At in neighboring labeled cells. The toxicity of 211At- decaying outside the cell (30-min exposure) was negligible. Analysis of the survival curve produced a D0 of 1.3 decays/cell for 211At-labeled cells. The yield of DSBs from the decay of DNA-incorporated 211At was compared with that from DNA-incorporated 125I. Each decay of 211At produced at least 10 times the number of DSBs as that obtained per 125I decay. The extreme radiotoxicity of DNA-incorporated 211AtdU seems to be associated with considerable damage to the mammalian cell genome.

Effect of UVC and araC on L5178Y-R and L5178Y-S cells. Nucleoid sedimentation.

The effects of UVC radiation (lambda = 254 nm, 85 J/m2) and/or 1-beta-D-arabino-furanosylcytosine (araC, 2 x 10(-3) M, 2 h) on two mouse lymphoma cell lines, UVC-sensitive and X-ray resistant L5178Y-R and UVC-resistant and X-ray sensitive L5178Y-S, were investigated. AraC treatment inhibited the semiconservative DNA replication to 1.4% and 3.8% in L5178Y-R and L5178Y-S cells, respectively, and decreased the sedimentation distance of nucleoids from the cells of both lines. The shortening of sedimentation distances induced by UVC and araC treatment was 8.1 mm for L5178Y-R cells and 11.8 mm for L5178Y-S, and indicated a higher number of DNA breaks in L5178Y-S cells. Assuming that such breaks are the result of the inhibition of DNA repair replication by araC, we conclude that L5178Y-S cells have a greater number of repaired sites than L5178Y-R cells.

Radiation sensitivities are not related to the sizes of DNA supercoiled domains in L5178Y-R and L5178Y-S cells.

Survival of murine lymphoblasts L5178Y-R and L5178Y-S irradiated with 60Co gamma radiation was determined. The parameters of the survival curves were D0 = 1.18 Gy, n = 1.56 and D0 = 0.55 Gy, n = 1.00 for L5178Y-R and L5178Y-S cells respectively. The sizes of DNA supercoiled domains were estimated using sedimentation of nucleoids from cells irradiated with doses from 1 to 7 Gy. These sizes were 2.44 x 10(9) and 5.13 x 10(8) Da for L5178Y-R cells and 1.30 x 10(9) and 4.07 x 10(8) Da for L5178Y-S cells. Hence, higher radiosensitivity of L5178Y-S cells was not compatible with the larger size of the DNA supercoiled domains, as suggested by Filippovich et al. (1982). We have not found any simple relation between the sizes of DNA supercoiled domains and the susceptibility of L5178Y sublines to ionizing radiation.

Calcium antagonist, TMB-8, prevents the induction of adaptive response by hydrogen peroxide or X-rays in human lymphocytes.

Treatment of human lymphocytes with hydrogen peroxide (10 microM, 30 min, 37 degrees C in PBS) or with 1 cGy X-rays evoked about a 30% decrease in the frequency of micronuclei upon subsequent X-irradiation (1.5 Gy). In addition to a lower micronuclei frequency, we also found an increase in the sedimentation distance of the nucleoids, when measured 90 min (duration of the isolation procedure carried out at 4 degrees C) after the adaptive dose (hydrogen peroxide or X-rays) and preceding the challenge dose. To test whether Ca2+ is involved in the induction of the adaptive response pathway, we treated cells with the calcium chelator, EGTA. When EGTA was given at the same time as the adaptive dose, it prevented the development of the adaptive response. In addition, the calcium antagonist, TMB-8, also prevented the development of the adaptive response as it prevented the reduction of both micronuclei and increased nucleoid sedimentation. Cellular treatment with TMB-8 increased the free [Ca2+] by 40%, when given together with hydrogen peroxide. The faster sedimenting nucleoids from adapted cells were also examined by ethidium bromide titration; there was no indication of any change in supercoil density or loop size. Psi-tectorigenin, an inhibitor of phosphatidylinositol turnover, did not modify the adaptive response, indicating that inositol (1,4,5)-trisphosphate is not involved in the induction of the adaptive response, but free Ca2+ ions are.

Chemical modification of 5-[125I]iodo-2'-deoxyuridine toxicity in mammalian cells in vitro.

PURPOSE: To address the cytotoxic effects of DNA-incorporated (125)I in Chinese hamster V79 lung fibroblasts under various scavenging conditions. METHODS: The toxic effects of DNA-incorporated 5-[(125)I]iodo-2'-deoxyuridine ((125)IdUrd) were assessed by the colony-forming assay with cells incubated in medium containing serum and/or dimethyl sulphoxide (DMSO). Experiments were carried out at 0.3 or -135 degrees C. RESULTS: When (125)I decays were accumulated at 0.3 degrees C in 10% serum 0, 5 or 10% DMSO, no radioprotection was afforded by 5% DMSO, while the dose modification factor (DMF) for 10% DMSO was 2.0. For cells accumulating decays at 135 degrees C in the presence of 5 or 10% serum, DMSO was radioprotective (DMF= 1.8-1.9). D(0) obtained at each serum concentration correlated strongly (R=0.999) with the scavenging capacity of DMSO. Under these experimental conditions, 10% serum is approximately 3.6 times more protective than 5% serum. CONCLUSIONS: The contribution of indirect mechanisms to the toxicity of (125)I decaying within mammalian cell nuclear DNA can be demonstrated not only with DMSO, but also with the hydroxy radical scavengers present in serum.

Malignant transformation of human fibroblasts by ionizing radiation.

As one step in developing an assay for quantifying the induction of malignant transformation of human cells by ionizing radiation, we exposed cells from a non-tumorigenic, infinite life span, near-diploid fibroblast strain MSU-1.1 to 4.35 Gy 60Co radiation and assayed them for focus formation. The mean frequency of foci in the irradiated population was 6 x 10(-7) cells assayed. No foci were found in the control cells. Of four focus-derived cell strains studied in detail, two produced malignant tumours within 3-7 weeks. The other two did not produce tumours during the 12-month period of study. The tumours from one strain were classified as sarcomas composed exclusively of spindle-shaped cells. Tumours from the other strain were sarcomas consisting of a mixed population of round and spindle cells. Immunoprecipitation analysis of the status of the p53 gene in the focus-derived strains, using a mutant-specific anti-body (Pab240) and an antibody that recognizes both mutant and wild-type p53 protein (Pab421), showed that the tumorigenic strains were completely devoid of p53 protein. One non-tumorigenic strain expressed wild-type p53 protein, and the other expressed a lower molecular weight form of the protein. Karyotypic analysis showed that the tumour-derived cells from one tumorigenic strain had lost one copy of chromosome 6, 14, 16 and 17. The tumour-derived cells from the second strain had lost one copy of chromosome 7, 13, 14 and 17 and part of chromosome 6, as well as part of the other copy of chromosome 7 and 17. These results suggest that the common loss of one copy of chromosome 14, 17 and part of 6 plays a causal role in the malignant transformation of these cells. Furthermore, the results indicate that it will be possible to develop a system that uses near-diploid human fibroblasts to quantify radiation-induced malignant transformation.

Cytotoxicity of [125I]iodoHoechst 33342: contribution of scavengeable effects.

PURPOSE: The incubation of the DNA minor-groove binder [125I]iodoHoechst 33342 (125IH) with plasmid DNA leads to the production of one double-strand break (dsb) per decay, both in the presence and absence of dimethylsulfoxide (DMSO). In contrast, when 125I is incorporated into mammalian cell DNA as an iodinated pyrimidine base, DMSO decreases the dsb yield and enhances survival. Because these variations in radioprotective effects may be due either to the location of 125I vis-à-vis the DNA helix or to differences in DNA architecture, the toxicity of 125IH and its modification by DMSO were examined in mammalian cells. METHODS: Uptake and retention of 125IH in V79 cells were measured, and survival was determined after accumulation of 125I decays at 0.3 degrees C +/-10% DMSO. RESULTS: A linear-quadratic survival curve was obtained both in the absence [D37 = 114+/-36 decays/cell, alpha = (5.39 1.17) x10(-3) cell/decay] and presence [D37 = 211+/-65 decays/cell, alpha = (1.27+/-0.52) x10(-3) cell/decay] of DMSO. The dose modification factor for the linear component of the survival curve was 4.25+/-1.97, indicating the predominance of indirect mechanisms. This value is similar to that obtained with DNA-incorporated 125I (4.05+/-1.72) and for the initial slope (alpha) of 137Cs gamma-rays (4.43+/- 1.41). CONCLUSIONS: Cytotoxicity resulting from the decay of the Auger electron emitter 125I in the mammalian cell nucleus is caused mainly by indirect mechanisms.

The effect of caffeine on post-replication repair and survival in two L5178Y cell lines with different sensitivities to UV irradiation.

2 Strains of murine lymphoma L5178Y cells that varied from the point of view of sensitivity to UV irradiation (mean lethal doses: 3.6 and 8.5 J/m2 for L5178Y-R and L5178Y-S cells, respectively) also differed with respect to sensitization by caffeine. L5178Y-S cells were sensitized to UV irradiation by 0.75 mM caffeine, whereas in the same conditions L5178Y-R cells were not sensitized. Sedimentation analysis of the newly synthesized DNA indicated UV-induced gap formation in L5178Y-S cells only. The subsequent gap filling was inhibited by caffeine. Exposure to UV irradiation induced no gaps in L5178Y-R cells. However, when caffeine was added immediately after irradiation, DNA with reduced molecular weight was found in irradiated cells of both strains after a 2-h chase. On the other hand, caffeine inhibited elongation of undamaged DNA strands in neither of the 2 cell strains.

Cytotoxicity and the effect of combined treatment with X-rays of an anticancer 1-nitro-9-aminoacridine derivative in L5178Y-R and L5178Y-S cells.

The response to nitracrine, 1-nitro-9-[3'(N,N-dimethyl)aminopropyl] aminoacridine, an anticancer agent, was examined in two strains of murine lymphoma L5178Y inversely cross-sensitive to X-rays and UV light. A marked difference was found in sensitivity to the drug between L5178Y-R and L5178Y-S strain (D0 values 2.1 and 13.9 ng/ml, 3 h, 37 degrees C); survival was related to the yield of chromatid aberrations. Enhanced sensitivity of L5178Y-R cells to nitracrine as well as to UV light and an antitumor platinum complex has been ascribed to the previously found defect in post-replication repair in these cells. Combined treatment with nitracrine and X-rays under oxic conditions gave additive effects in both L5178Y strains.

Action of misonidazole on L5178Y-R and L5178Y-S cells. II. Cytotoxicity and radiosensitization under hypoxic conditions.

Misonizadole exerts an equal cytotoxic effect on L5178Y-R and L5178Y-S cells; 5 mM misonidazole sensitizes cells of both strains to a similar extent under hypoxic conditions: the effect of drug in 59% of the oxygen effect in L5178Y-R cells and 52% in L5178Y-S cells. It was previously reported that the amount of initial DNA damage determined by the alkaline unwinding technique is 1.3 times lower in irradiated hypoxic L5178Y-R cells than in L5178Y-S cells. Hence it seems that the drug ability to sensitize these cell strains does not depend on amount of initial DNA damage available for interaction. Although the examined L5178Y strains differ in radiosensitivity, their ability to repair oxygen and misonidazole-dependent cellular damage is similar.