3-Aminobenzamide does not alter DNA repair in human fibroblasts through modulation of deoxynucleoside triphosphate pools

Inhibitors and activators of ADP-ribosylation reactions

ADP-ribosylation reaction, that is the transfer of the ADP-ribose moiety of NAD+ to acceptor protein, is catalyzed by two classes of ADP-ribosyltransferases, i.e., poly(ADP-ribose) synthetase and mono(ADP-ribosyl)transferases. These two types differ not only in the number of transferring ADP-ribose units but also in the acceptor amino acid(s) and protein. Their inhibitors, particularly those of poly(ADP-ribose) synthetase, have been successfully employed in studies on biological functions of the enzymes and other related fields of research. Recently, we found many potent and specific inhibitors of poly(ADP-ribose) synthetase, and broadened their chemical as well as biochemical variety. More recently, we found several potent inhibitors of arginine-specific mono(ADP-ribosyl)transferases and activators of poly(ADP-ribose) synthetase.

Nerve growth factor effects on pyridine nucleotides after oxidant injury of rat pheochromocytoma cells

Neurotrophic factors regulate neuronal survival and neurite growth in development and following injury. Oxidative stress produced in neurons as a consequence of primary injury, or during reperfusion following ischemia, may contribute to cell death. Here, the effects of nerve growth factor (NGF) on the response to H2O2 injury were examined in the PC12 rat pheochromocytoma cell line. Specifically, the effect of NGF on cell viability after H2O2 injury was measured. Pretreatment with NGF enhanced survival after H2O2 treatment, as measured by Trypan blue dye exclusion, radiolabeled amino acid incorporation, tetrazolium salt reduction, or cytoplasmic enzyme release. One early event associated with H2O2 treatment was a rapid decrease in NAD+. Although initial decreases in NAD+ levels were similar in control and NGF-treated cells, the latter recovered more rapidly and extensively. The decline in total NAD observed after NGF treatment was almost equal in magnitude to the measured increase in NADP. Inhibition of poly(ADP-ribose) polymerase also enhanced viability following H2O2 injury. Treatment with both NGF and an inhibitor of this enzyme resulted in a greater reduction of H2O2 toxicity than was observed with either agent alone. These data suggest that NGF protection is multifactorial and that a significant component of the NGF effect is due to its regulatory role in the metabolism of pyridine nucleotides.

Anticarcinogenic potential of DNA-repair modulators

Effects of compounds that inhibit repair of DNA lesions in cells have been reported frequently. The consequences include altered incidence of carcinogenicity in vivo, tumorigenic transformation of cultured cells, mutations, and increased lethality as well as sister-chromatid exchanges and chromosome aberrations. This literature is reviewed here, with major emphasis on methylxanthines (caffeine in particular) and nicotinamide analogs. Existing information is also summarized on a novel potent repair inhibitor, beta-lapachone. Compounds that inhibit both DNA replication and repair are not discussed in detail since they have been reviewed often, but miscellaneous inhibitors of repair are summarized in a table. The relatively small number of experiments performed on the anticarcinogenic effects of methyl-xanthines and nicotinamide analogs gave very conflicting results. Some investigators report decreased carcinogenicity of DNA-damaging agents when caffeine was provided, but others obtained the opposite effect. The three studies with nicotinamide analogs all reported enhanced tumorigenicity of carcinogens. The data are too few to enable firm conclusions to be drawn regarding the possibility of using repair inhibitors to prevent cancer in humans. Variations of experimental conditions, carcinogens, cells, etc. have provided conflicting results. The possibility of cancer prevention is, nevertheless, so important that further investigations with DNA-repair inhibitors, particularly with human cells, seem very well justified.

Differential radiosensitization by the poly(ADP-ribose) transferase inhibitor 3-aminobenzamide in human tumor cells of varying radiosensitivity

Four newly-established human tumor cell lines, have been irradiated at dose rates of 150 and 3.2 cGy/min to compare their capacity to repair radiation damage. They included a neuroblastoma, a germ-cell carcinoma of the testis, a large cell carcinoma of the lung, and a carcinoma of the cervix. The four lines varied in their sensitivity to high dose-rate irradiation, with the neuroblastoma being most radiosensitive and the lung and cervix tumors the most radioresistant. The extent of dose sparing associated with lowering the dose rate to 3.2 cGy/min was similar in three of the lines but somewhat greater in the case of the cervix carcinoma cell line. The presence of non-toxic concentrations of the poly(ADP-ribose) transferase inhibitor, 3-aminobenzamide (3-AB), enhanced the response of 3 of the 4 tumors to irradiation; it failed to modify the sensitivity of a lung carcinoma cell line. The extent of sensitization was generally similar at high and low dose rate. Measurement of poly(ADP-ribose) transferase activity in control and irradiated cells showed the neuroblastoma cells to contain much higher initial levels than the other three lines but there were no significant differences in the extent of stimulation in enzyme levels after irradiation. Survival curves obtained at low dose-rate help define the initial slope of the acute curve and it appears that 3-AB may exert a differential effect among human tumors in modifying this component.

The cell-cycle dependent and the DNA-damaging agent-induced changes of cellular NAD content and their significance

NAD is the substrate of a novel chromatin-associated enzyme-ADP-ribosyl transferase (ADPRT). In this study, the cell-cycle dependent change in cellular NAD content was observed in a line of human amnion FL cells. It was found that the cellular NAD content of FL cells was highest in G1 and lowest in S/G2-G2. 3AB, a potent ADPRT inhibitor, can inhibit the cell cycle dependent change in cellular NAD content and also inhibit DNA synthesis in the S phase and extend the S phase. The results indicate that ADP-ribosylation may be involved in DNA replication and cell cycle progression. It was also found that the DNA-damaging agents, MNNG, MMS and 4NQO could lower cellular NAD content in a dose-dependent way. This DNA-damage-induced NAD lowering could be partially or completely prevented by the ADPRT inhibitors, 3AB or nicotinamide, which were shown to exert no influence on either the cellular NAD content of normal quiescent FL cells or the metabolic blocking agent, 2,4-DNP-induced cellular NAD lowering. The possibility of establishing a simple and specific method to detect DNA-damaging agents by measuring cellular NAD content in the presence or absence of ADPRT inhibitor is explored.

Differential radiosensitization of human tumour cells by 3-aminobenzamide and benzamide: inhibitors of poly(ADP-ribosylation)

The effect of 3-aminobenzamide (3AB) and benzamide (BZ) (inhibitors of poly(ADP-ribose) synthetase) on radiosensitivity was investigated in normal human fibroblasts and three human cell lines established from tumours with varying degrees of clinical radiocurability. The human tumour cell lines selected were: Ewing's sarcoma, a bone tumour usually considered radiocurable with moderate radiation doses; lung adenocarcinoma, a tumour considered radiocurable with high doses of radiotherapy; and osteosarcoma, a very resistant tumour which is rarely controlled by standard doses of radiotherapy. Poly(ADP-ribose) synthetase inhibitors were added to cultures 2 h prior to irradiation and removed 24 h after. Inhibitors were used at doses producing little or no toxicity in cells. In the presence of these inhibitors, a differential radiosensitization was observed. Ewing's sarcoma cells and normal human fibroblasts were sensitized to an equal extent by either 8 mM 3AB or 4 mM BZ. However, no sensitization was observed at these concentrations in the lung adenocarcinoma cells or osteosarcoma cells. The degree of radiosensitization in vitro by 3AB and BZ correlates well with the clinical radiocurability of these tumours in vivo.

Induction of sister-chromatid exchanges by the replication of 5-bromouracil-substituted DNA under conditions of nucleotide-pool imbalance

The induction of sister-chromatid exchanges (SCEs) by the replication of 5-bromouracil(BrUra)-containing DNA under conditions of nucleotide-pool imbalance was investigated. A modification of a protocol developed for the induction of mutations under these conditions (E.R. Kaufman, Mol. Cell. Biol., 4, 2449-2454, 1984) was used. To induce SCEs, Chinese hamster ovary cells were grown under non-mutagenic conditions which allowed the uniform incorporation of BrUra into their DNA at specific levels of substitution for thymine residues (25, 50 and 75% BrUra substitution). After 4 and 5 days of growth, the cells, which had incorporated BrUra into their DNA, were washed free of 5-bromodeoxyuridine (BrdUrd) and provided with fresh culture medium supplemented with various concentrations of thymidine (10 microM to 3 mM) and no BrdUrd. The cells were allowed to replicate their BrUra-containing DNA under these conditions, in the absence of BrdUrd, for two rounds of DNA synthesis to achieve sister-chromatid differentiation, and second-division metaphases were scored for SCEs. The results of these studies indicated that the SCEs observed were proportional to the level of BrUra substituted for thymine in the cellular DNA, were induced by increasing concentrations of thymidine in the culture medium during replication of the BrUra-containing DNA, correlated well with the induction of mutations to thioguanine resistance and to ouabain resistance, correlated with increases in the intracellular levels of dTTP and dGTP generated by the high concentrations of thymidine. These findings provide direct evidence for the induction of SCEs by the replication of BrUra-containing DNA and for the importance of the pools of nucleoside triphosphate precursors for DNA replication in these processes. When the effects of 3-aminobenzamide, a potent inhibitor of poly(ADP-ŕibose) synthesis, were tested, it was found that 3-aminobenzamide significantly increased SCEs, but it had no effect on mutations induced.

Disturbances in DNA precursor metabolism associated with exposure to an inhibitor of poly(ADP-ribose) synthetase

3-Aminobenzamide (3AB) is widely used as an inhibitor of poly(ADP-ribose) synthetase to study the effect of protein ribosylation on various cellular processes, but the specificity of its inhibition has not been demonstrated. We found that 3AB has a wide range of effects on DNA precursor metabolism, as determined by high-performance liquid chromatographic separation of deoxynucleosides derived from enzymatic digestion of cellular DNA. 3AB (10-20 mM) significantly reduced cell growth in human lymphoblastoid cells. Furthermore, the incorporation of [3H]deoxycytidine into DNA was significantly enhanced relative to incorporation of [3H]deoxythymidine, [3H]deoxyguanosine, and [3H]deoxyadenosine. Incorporation of fragments of [3H]glucose into the pyrimidine fraction of DNA was significantly inhibited relative to incorporation into the purine fraction. At only 1 mM, 3AB had a major inhibitory effect on the incorporation of the methyl group from [3H]methionine into deoxyguanosine, deoxyadenosine, and deoxycytidine, with 50% inhibition into deoxyguanosine and deoxyadenosine and 90% inhibition into deoxycytidine. The specificity of 3AB inhibition to poly(ADP-ribose) synthetase is therefore doubtful in view of this variety of metabolic effects, involving pyrimidine synthesis and de novo synthesis via the one-carbon pool.

Possible role of ADP-ribosylation of adenovirus core proteins in virus infection

We have investigated the role of poly(ADP)-ribosylation of adenoviral proteins in virus infection. Viral core proteins V and the precursor to protein VII were shown to be in vivo and in vitro acceptors of ADP-ribose. In vivo ADP-ribosylation was restricted to viral proteins as the histones were not labeled during the late phase of infection. The ADP-ribosylated core proteins were assembled into mature virus particles. In vitro ADP-ribosylation of adenoviral core proteins performed with purified poly(ADP-ribose) polymerase led to relaxation of the chromatin structure of both ts1 and wild type pyridine cores and pentonless particles and triggered the complete dissociation of wild type particles. A critical role for poly(ADP)-ribosylation in virus infection was confirmed by measuring the effect of the inhibitors 3-aminobenzamide and nicotinamide on virus particle yield and infectivity. Both inhibitors depressed particle yield by up to 9-fold, but infectivity was reduced by up to 10(4)-fold. These results suggest that ADP-ribosylation of adenovirus core proteins may have a role in virus decapsidation.