Requirements for adenosine triphosphate in DNA repair in isolated hepatic nuclei

Identification of ATIC as a Novel Target for Chemoradiosensitization

PURPOSE

Mutations in the gene encoding 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), a bifunctional enzyme that catalyzes the final 2 steps of the purine de novo biosynthetic pathway, were identified in a subject referred for radiation sensitivity testing. Functional studies were performed to determine whether ATIC inhibition was radiosensitizing and, if so, to elucidate the mechanism of this effect and determine whether small molecule inhibitors of ATIC could act as effective radiosensitizing agents.

METHODS AND MATERIALS

Both small interfering RNA knockdown and small molecule inhibitors were used to inactivate ATIC in cell culture. Clonogenic survival assays, the neutral comet assay, and γH2AX staining were used to assess the effects of ATIC inhibition or depletion on cellular DNA damage responses.

RESULTS

Depletion of ATIC or inhibition of its transformylase activity significantly reduced the surviving fraction of cells in clonogenic survival assays in multiple cancer cell lines. In the absence of ionizing radiation exposure, ATIC knockdown or chemical inhibition activated cell cycle checkpoints, shifting cells to the more radiosensitive G2/M phase of the cell cycle, and depleted cellular adenosine triphosphate but did not result in detectable DNA damage. Cells in which ATIC was knocked down or inhibited and then treated with ionizing radiation displayed increased numbers of DNA double-strand breaks and a delay in the repair of those breaks relative to irradiated, but otherwise untreated, controls. Supplementation of culture media with exogenous adenosine triphosphate ameliorated the DNA repair phenotypes.

CONCLUSIONS

These findings implicate ATIC as an effective, and previously unrecognized, target for chemoradiosensitization and, more broadly, suggest that purine levels in cells might have an underappreciated role in modulating the efficiency of DNA damage responses that could be exploited in radiosensitizing strategies.

Differences in DNA synthesis in vitro using isolated nuclei from regenerating livers of young and aged rats

To detect changes in DNA synthesis during ageing, we compare DNA synthesis in the livers of young and aged rats. As an intermediate between an in vivo system using intact cells and an in vitro system using purified DNA polymerases, isolated nuclei were prepared and used as the machinery for DNA synthesis. The DNA synthesizing capacity of nuclei from regenerating liver was higher than that of nuclei from normal liver and these capacities from liver and regenerating liver were lower in nuclear preparations from aged rats. DNA synthesis using isolated nuclei was stimulated by ATP and the cytoplasmic preparation. The cytoplasmic preparation from regenerating rat liver was found to stimulate DNA synthesis more than the preparation from normal liver. The activity in regenerating liver from young rats was also greater than in that from aged rats. It is well known that DNA replication is inhibited by aphidicolin and DNA repair by ddTTP. We examined the effects of aphidicolin and ddTTP on DNA synthesis using the nuclear system. Surprisingly, the inhibition by aphidicolin was 30% of total DNA synthesis using the nuclear system from young rats. On the other hand, the inhibition by ddTTP was approximately 80%. We measured the sizes of the DNA synthesized in the presence of both inhibitors. DNA synthesis was allowed to proceed for 10 min using isolated nuclei from regenerating liver of young rats and the size of the DNA was determined by sucrose density gradient centrifugation analysis. DNA products appeared in two fractions. Following a chase of 50 min in the presence or absence of aphidicolin, the short DNA product grew larger in both cases, although the amount of DNA in the presence of aphidicolin was approximately 90% that in its absence. In the same experiment using nuclei from aged rats, the amount in the presence of aphidicolin was approximately 60% that in its absence. These results suggest that DNA polymerase beta is closely related to abnormal replication when DNA polymerases alpha and delta are inhibited and that the effect of cytosol on DNA synthesis, as well as the DNA synthetic capacity of isolated nuclei, becomes lower in regenerating rat liver during ageing.

The multifactorial role of ATP in repair processes and radioprotection

Endogenous ATP as a source of energy appears to play an important role associated with repair processes after ionising radiation damage in living organisms. Also exogenous ATP can play a central part in radioprotection. This paper considers interaction between the multifactorial roles of ATP in radioprotection and metabolic provision for repair processes, which can depend on its physiological regulatory function as a neuromodulator via cell membrane receptors. It is hypothesised that all these complex functions reflect an adaptive defence mechanism, against radiation insults, in which ATP plays a pivotal role to maintain homeostasis.

Effects of hyperthermia and nicotinamide on DNA repair synthesis, ADP-ribosyl transferase activity, NAD+ and ATP pools, and cytotoxicity in gamma-irradiated human mononuclear leukocytes

Effects of hyperthermia and nicotinamide on ADP-ribosyl transferase activity (ADPRT), unscheduled DNA synthesis (UDS), NAD+- and ATP-pools and cytotoxicity were investigated in gamma-irradiated human mononuclear leukocytes. A significant decrease in radiation-induced UDS after heat treatment for 45 min was found. Nicotinamide increased the UDS levels in irradiated cells, but no effect of hyperthermia on these increased UDS values was observed. In the presence of 2 mM nicotinamide radiation-induced ADPRT activity was reduced to about 50 per cent. However, hyperthermia for 45 min was found to have no effect on the enzyme activity for temperatures below 46 degrees C. Nicotinamide increased the NAD+ pool in unirradiated cells. Damaging the cells with gamma-radiation leads to a severe depletion of the NAD+ pool. The NAD+ pool is restored, however, if the cells repair for 5 h at 37 degrees C. When radiation-damaged cells were treated with hyperthermia, exogenously supplied nicotinamide could not be converted to NAD+ in sufficient amounts to prevent NAD+ depletion. These data indicate that the radiosensitizing effect of heat and nicotinamide could both be explained by effects on the enzyme ADPRT, i.e. nicotinamide by directly blocking the enzyme and hyperthermia by limiting the co-substrate (NAD+).

Toxicity, tumor promotion, and carcinogenesis in relation to excessive dosage

Excessive dosage of many toxic compounds leads to a response by those exposed cells in the form of toxic hyperplasia. This toxicity often is a result of oxidation and peroxidation products that affect multiple sites of critical importance in the cell from the membrane of DNA itself. Some of these products are both mutagenic and carcinogenic and can affect key enzymes of defense and proliferation. Chronic toxicity of this type can deplete or inactivate endogenous defense systems that would normally prevent the formation of these toxic products. Subtoxic doses will not significantly compromise cellular defense systems and are far less likely to induce the critical events associated with hyperplasia, tumor promotion, and the expression of a "carcinogenic" effect. The many new discoveries in oxidation and peroxidation toxicity open up a greater awareness of the potential problems of interpretation associated with excessive dosage in evaluation of compounds for carcinogenicity.

Possible participation of Ca2+, Mg2+-dependent endonuclease in liver DNA fragmentation after N-methyl-N-nitrosourea treatment

We examined the fragmentation of DNA treated with N-methyl-N-nitrosourea under conditions in which Ca2+, Mg2+-dependent endonuclease is active. The molecular mass of DNA found in mouse liver slices treated with methylnitrosurea in the presence of Ca2+ plus Mg2+ was 4 X 10(5) Da. Similar results were obtained with a reconstituted system containing partially purified Ca2+, Mg2+-dependent endonuclease and methylnitrosurea-treated DNA. The enzyme extensively cleaved methylnitrosurea-treated DNA, compared with non-treated DNA. The methylnitrosurea-treated nuclear proteins obtained from mouse liver nuclei had no effect on the DNA fragmentation by the enzyme. Using closed-circular DNA treated with methylnitrosurea, the enzyme produced single-strand cuts in the DNA, as was seen in non-treated, closed-circular DNA, however, the rate of hydrolysis was increased. Ca2+, Mg2+-dependent endonuclease thus warrants further investigation, with regard to the precise mechanism of extensive degradation of DNA in cells treated with carcinogenic alkylating agents.