Enhancement of DNA damage in mammalian cells upon bioreduction of the nitroimidazole-aziridines RSU-1069 and RSU-1131

Identification of proteins and cellular pathways targeted by 2-nitroimidazole hypoxic cytotoxins

Tumour hypoxia negatively impacts therapy outcomes and continues to be a major unsolved clinical problem. Nitroimidazoles are hypoxia selective compounds that become entrapped in hypoxic cells by forming drug-protein adducts. They are widely used as hypoxia diagnostics and have also shown promise as hypoxia-directed therapeutics. However, little is known about the protein targets of nitroimidazoles and the resulting effects of their modification on cancer cells. Here, we report the synthesis and applications of azidoazomycin arabinofuranoside (N3-AZA), a novel click-chemistry compatible 2-nitroimidazole, designed to facilitate (a) the LC-MS/MS-based proteomic analysis of 2-nitroimidazole targeted proteins in FaDu head and neck cancer cells, and (b) rapid and efficient labelling of hypoxic cells and tissues. Bioinformatic analysis revealed that many of the 62 target proteins we identified participate in key canonical pathways including glycolysis and HIF1A signaling that play critical roles in the cellular response to hypoxia. Critical cellular proteins such as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the detoxification enzyme glutathione S-transferase P (GSTP1) appeared as top hits, and N3-AZA adduct formation significantly reduced their enzymatic activities only under hypoxia. Therefore, GAPDH, GSTP1 and other proteins reported here may represent candidate targets to further enhance the potential for nitroimidazole-based cancer therapeutics.

Direct Comparison of Biological Effectiveness of Protons and Alpha-particles of the Same LET. III. Initial Yield of DNA Double-strand Breaks in V79 Cells

The results reported form part of a series of experiments to substantiate and extend the findings by Belli et al. (1989) that protons are more biologically effective at cell killing than alpha-particles of the same LET. The irradiations were carried out using the Variable Energy Cyclotron (VEC) at the Harwell Laboratories. V79-4 Chinese hamster cells were exposed to alpha-particles and protons with LETs of 20 and 23 keV microns-1 in the dose range 40-150 Gy. X-rays were also used for comparison. Two methods were used for measurement of initial DNA double-strand breaks: sedimentation and DNA precipitation assays. The dose-response relationships were found to be well fitted by straight lines in all cases. With the sedimentation assay a slightly lower yield of dsb was found from protons than from alpha-particles of the same LET. The yield from X-rays was not significantly different from either. The precipitation assay showed similar yields of DNA damage from both particle types but significantly higher yields from X-rays. This may reflect a difference in the type of lesions scored by the two methods. Since the initial amount of dsb does not account for the observed differences in cellular response to radiations of different qualities, it is likely that these are related to the nature of the dsb (affecting reparability) or to the occurrence of other types of molecular damage.

CI-1010 induced opening of the mitochondrial permeability transition pore precedes oxidative stress and apoptosis in SY5Y neuroblastoma cells

The hetero-bifunctional nitroimidazole radiosensitizer CI-1010, R-alpha-[[(2-bromoethyl)-amino]methyl]-2-nitro-1H-imidazole-1-ethanol monohydrobromide, causes selective irreversible apoptotic loss of retinal photoreceptor cells in vivo. The human neuroblastoma cell line, SH-SY5Y, was used as a neuronotypic model of CI-1010-mediated retinal degeneration. Exposure to CI-1010 for 24 h induced apoptosis in neuroblastoma cells, as determined by histopathological and ultrastructural analysis and by TUNEL technique. CI-1010 causes a dose-dependent decrease in cell viability in SY5Y cells, as measured by the reduction of MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Superoxide dismutase reduced loss of cell viability following CI-1010 treatment suggesting an oxidative stress-mediated mechanism of toxicity. The effects of CI-1010 on mitochondrial membrane potential and intracellular levels of reactive oxygen species were assessed in live SY5Y cells by confocal microscopy using the fluorescent dyes, tetramethylrhodamine methyl ester and 5,6-carboxy-2',7'-dihydrodichlorofluorescein diacetate. CI-1010 caused a rapid depolarization of mitochondria in SY5Y cells followed by an increase in ROS. Both CI-1010-induced mitochondrial depolarization and subsequent increases in ROS were prevented by pretreatment with either the permeability transition pore inhibitor, cyclosporin A (CsA), and by the antioxidant, alpha-tocopherol. However, CsA and alpha-tocopherol were unable to prevent apoptosis in CI-1010-treated cells, suggesting the influence of additional mechanism(s) of CI-1010-induced toxicity. This study evaluates intracellular oxidative stress associated with pore opening prior to apoptosis and provides evidence in support of a mitochondrial mechanism of CI-1010-induced neuronal cell death.

DNA double-strand breaks induced by low energy protons in V79 cells

The initial production of DNA double-strand breaks (dsb) was determined in V79 Chinese hamster cells irradiated with proton beams of 3.24, 1.50 and 0.88 MeV, corresponding to values of unrestricted LET evaluated at the cell midplane of 10.9, 20.0 and 30.5 keV/micron, respectively. X-rays were used for comparison. Dsb were measured with the low speed sedimentation technique in neutral sucrose gradients. The initial yield of dsb rose linearly with the dose and did not significantly depend on the proton LET, in contrast with the results obtained in previous studies for cell inactivation and mutation induction. Also, no significant differences for dsb induction were found between protons and X-rays. Two possible explanations, not necessarily mutually exclusive, are proposed: (1) dsb are not the only lesions involved in cellular effects; and (2) the initial number of dsb is not the only important parameter since a fundamental role is played by the degree of clustering, i.e. the association of dsb with other dsb or other types of damage.

Induction and rejoining of DNA double-strand breaks in V79-4 mammalian cells following gamma- and alpha-irradiation

The induction and rejoining of DNA double-strand breaks (dsbs) in V79-4 mammalian cells following irradiation by 60Co gamma-rays and 238Pu alpha-particles (average LET 120 keV microns-1) under aerobic conditions have been determined using both the sucrose sedimentation and filter elution techniques under non-denaturing conditions. Cellular inactivation was also determined. The dependence of the initial yield of dsbs at 277 K on dose under aerobic conditions is linear with a relative biological effectiveness (RBE) for alpha-particles of 0.85 +/- 0.14 (sedimentation) and 0.68 +/- 0.12 (elution) compared with 60Co gamma-rays. The ability of the cells to rejoin dsbs at 310K is significantly reduced for alpha-irradiations with only 30-50% rejoined for a 3-h incubation period. With low LET radiation, > 90% of the dsbs are rejoined within 3 h at a dose of 20 Gy. The RBE for cellular inactivation was determined to be 4.0 at the 1% survival level. From the cellular dimensions and the D0-value for cellular inactivation by alpha-particles, it is determined that, on average, 4.7 tracks traverse the cell nucleus per lethal lesion. Under hypoxic conditions, the RBE values for induction of dsbs and cellular inactivation (10% level) by alpha-particles are approximately 3.0 and approximately 11.8 respectively. From these findings, it is suggested that the residual DNA damage and not the initial damage is reflected in the cellular inactivation. It is inferred that the difference in repair of the various lesions is a reflection of the differences in the complexity of the clustered damage produced by these radiations.

Induction of mutations in V79-4 mammalian cells under hypoxic and aerobic conditions by the cytotoxic 2-nitroimidazole-aziridines, RSU-1069 and RSU-1131. The influence of cellular glutathione

Incubation of the 2-nitroimidazole-aziridine, RSU-1069 [1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol], and its monomethylaziridine analogue, RSU-1131 [1-(2-nitro-1-imidazolyl)-3-(1-(2-methylaziridinyl))-2-propanol], with V79-4 mammalian cells for 2 hr under aerobic or hypoxic conditions induces mutations as measured at the hypoxanthine phosphoribosyl transferase locus. The ability of these agents to induce mutations is increased by a factor of 12-14 under hypoxic conditions. The increased cytotoxicity of these agents under hypoxic conditions was confirmed following a 2 hr incubation period. Decreasing the glutathione (GSH) content of the cells with buthionine-(S,R)-sulphoximine to < 1% of the control generally results in an increase in the cytotoxicity and mutagenicity of these agents under both aerobic and hypoxic conditions. Since these agents do not modify the cellular GSH levels, it is inferred that the thiols partially detoxify through removal of a reactive metabolite of the agents, under hypoxic conditions, or removal of known DNA adducts, and not through their interaction with the agents themselves. Under aerobic conditions, the formation of mutations is consistent with the established monofunctional action of these agents whereas under hypoxic conditions the bifunctional action predominates for mutation induction, based upon the large differential aerobic:hypoxic effect. From a comparison of the number of mutations per lethal event, the effect of thiol depletion is more pronounced for cytotoxicity than for mutation induction by these agents. In summary, these agents are considered to be weak mutagens towards V79-4 cells under aerobic conditions when compared with other DNA alkylating agents, although they are more potent under anoxic conditions.

Inactivation and repair of double-stranded DNA damaged by the aziridinyl nitroimidazole RSU 1069

Incubation of RSU 1069 in the presence of biologically active double-stranded phi X174 DNA resulted in, depending on pH, ionic strength and concentration of drug, inactivation of the DNA. A variety of lesions are induced including a high number of single-strand breaks and alkali-labile lesions, which are at most partly lethal. The main inactivating damage consists probably of base damage, induced by alkylation. A considerable part of the damage induced by RSU 1069 can be repaired by the various repair enzymes of the bacterial host of the phi X174 DNA. Finally the damage (pattern) depends considerably on the ionic composition of the reaction solution, which can be explained by an equilibrium model presented in this paper.

The repair of DNA damage induced in V79 mammalian cells by the nitroimidazole-aziridine, RSU-1069. Implications for radiosensitization

The induction and repair of single (ssb) and double (dsb) strand breaks in DNA under aerobic or hypoxic conditions have been determined using sucrose sedimentation techniques following incubation of V79 mammalian cells with RSU-1069 or misonidazole, representative of a conventional 2-nitroimidazole radiosensitizer, for 1-1.5 hr at either 293 or 277 degrees K and subsequent irradiation at 277 degrees K. In all cases, the dose dependences for the induction of strand breaks are linear and consistent with an enhancement in the yield of DNA damage induced by the 2-nitroimidazoles under hypoxic conditions. With RSU-1069 at 293 degrees K, the dose dependence of ssb is displaced reflecting DNA damage induced during pre-incubation. From these dependences, it is evident that the enhanced radiosensitization by RSU-1069 may not be accounted for in terms of accumulation of the agent at DNA. From the repair studies, DNA breaks induced by RSU-1069 in the absence of radiation have been shown to persist for at least 3 hr. With a combination of RSU-1069 and radiation under hypoxic conditions, the repair timescale of the induced breaks is significantly longer and an increase in the residual yields of both ssb and dsb (at 2-3 hr) was observed when compared with the observation in the presence of misonidazole or oxygen. From these studies, it is inferred that the enhanced radiosensitization of RSU-1069 at 293 degrees K is a consequence of the formation of non-repairable DNA damage together with a modification of the repairability of the radiation-induced DNA breaks.

Interaction of RSU 1069 and 1137 with DNA in vitro. Biological implications and mechanistic aspects

We have examined the capacity of the nitroimidazole aziridine antitumour drug RSU 1069 to react with DNA in vitro in order to get a better understanding of its mechanism of action. Moreover, we have utilized biologically active phi X174 DNA to investigate the biological relevance of the chemical DNA modification induced by the drug. Incubation of RSU 1069 in the presence of single-stranded phi X174 DNA resulted in extensive inactivation of the DNA, which is dependent on the concentration of drug and temperature. Only about 2% of the inactivating damage can be attributed to strand breakage. The main damage most probably consists of base damage, of which a part is non-lethal and alkali-labile which in turn can be converted into lethal lesion and subsequently into a break applying a post-incubation alkali treatment. Furthermore, from the dependence of the inactivation and also the formation of breaks on pH and ionic strength, it is concluded that the reaction most probably takes place between a protonated RSU 1069 and a negative DNA coil and that the damage pattern reflects the difference in reactivity of RSU 1069 with the phosphate groups and the bases in DNA. Comparison between RSU 1069 and its ring-open hydrolysis product RSU 1137 revealed that (lethal) damage induced in the DNA must be ascribed to the alkylating properties of the aziridine moiety.

A comparison of the techniques of alkaline filter elution and alkaline sucrose sedimentation used to assess DNA damage induced by 2-nitroimidazoles

The induction of DNA single-strand breaks (DNA-SSB) in Chinese hamster V79-379A lung fibroblasts by misonidazole or RSU-1069 under both aerobic and hypoxic conditions was examined following incubations for up to 4 hr at 310 degrees K using the technique of alkaline filter elution. Incubation with RSU-1069 induces DNA-SSB under both hypoxic and aerobic conditions, whereas incubation with misonidazole induces DNA-SSB only under hypoxia. The yield of breaks is dependent on both agent concentration and contact time. Following identical treatments with these agents, the yield of DNA-SSB (expressed in radiation dose equivalents) determined by alkaline filter elution is about one order of magnitude less than that previously determined by alkaline sucrose gradient sedimentation. In contrast to radiation induced DNA-SSB, alkaline elution is less sensitive than alkaline sucrose gradient sedimentation when determining DNA-SSB induced by RSU-1069 and misonidazole. During the filter elution assay, either increasing cell lysis from 2 to 4 hr, the pH of the lysing buffer from pH 8.7 to 12.5 or the elution buffer from pH 12.2 to 12.5 does not significantly effect the yield of DNA-SSB. Increasing the pH of the lysing or elution buffers to greater than pH 13 however results in considerable degradation of the DNA, whereby 50-85% of the total DNA passes through the filter with the lysing solution. This effect was similar for DNA from both control and chemically insulted cells. In conclusion, it is apparent that incubation with these agents results in the induction of DNA damage which is expressed as a DNA-SSB only after prolonged treatment under alkaline conditions. Further, the use of alkaline elution to study DNA-SSB damage induced chemically must be treated with caution in the light of these findings.

Assessment of the repair and damage of DNA induced by parent and reduced RSU-1069, a 2-nitroimidazole-aziridine

The cellular repair and damage of DNA induced by parent and reduced RSU-1069, a 2-nitroimidazole-aziridine, was assessed at both the molecular and cellular level. At the molecular level, after in vitro incubation with parent or reduced RSU-1069, plasmid DNA was transfected into Escherichia coli (AB1157) with subsequent selection for gene expression. For equivalent levels of DNA strand breakage following such treatment it is evident from the relative transformation frequencies that interactions with reduced RSU-1069 lead to DNA damage consistent with bifunctional action of a metabolite(s). At the cellular level, the cytoxicity of RSU-1069 was determined for a series of repair deficient mutants of E. coli under both aerobic and hypoxic conditions. The differential aerobic:hypoxic cytotoxicity ratio is approximately 3. We conclude that the repair of cellular DNA damage induced by RSU-1069 involves activation of the gene products under the control of the recA gene and not those under the control of the ada gene. The ability of cellular systems to repair damage induced by RSU-1069 may play a significant role in determining its efficiency to act as a hypoxic cell radiosensitizer and a hypoxia selective cytotoxin.