Role of poly(adenosine diphosphate ribose) in deoxyribonucleic acid repair in human fibroblasts

Redox dysregulation as a driver for DNA damage and its relationship to neurodegenerative diseases

Redox homeostasis refers to the balance between the production of reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), and their elimination by antioxidants. It is linked to all important cellular activities and oxidative stress is a result of imbalance between pro-oxidants and antioxidant species. Oxidative stress perturbs many cellular activities, including processes that maintain the integrity of DNA. Nucleic acids are highly reactive and therefore particularly susceptible to damage. The DNA damage response detects and repairs these DNA lesions. Efficient DNA repair processes are therefore essential for maintaining cellular viability, but they decline considerably during aging. DNA damage and deficiencies in DNA repair are increasingly described in age-related neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington's disease. Furthermore, oxidative stress has long been associated with these conditions. Moreover, both redox dysregulation and DNA damage increase significantly during aging, which is the biggest risk factor for neurodegenerative diseases. However, the links between redox dysfunction and DNA damage, and their joint contributions to pathophysiology in these conditions, are only just emerging. This review will discuss these associations and address the increasing evidence for redox dysregulation as an important and major source of DNA damage in neurodegenerative disorders. Understanding these connections may facilitate a better understanding of disease mechanisms, and ultimately lead to the design of better therapeutic strategies based on preventing both redox dysregulation and DNA damage.

Pancreatic cancer genomics: insights and opportunities for clinical translation

Pancreatic cancer is a highly lethal tumor type for which there are few viable therapeutic options. It is also caused by the accumulation of mutations in a variety of genes. These genetic alterations can be grouped into those that accumulate during pancreatic intraepithelial neoplasia (precursor lesions) and thus are present in all cells of the infiltrating carcinoma, and those that accumulate specifically within the infiltrating carcinoma during subclonal evolution, resulting in genetic heterogeneity. Despite this heterogeneity there are nonetheless commonly altered cellular functions, such as pathways controlling the cell cycle, DNA damage repair, intracellular signaling and development, which could provide for a variety of drug targets. This review aims to summarize current knowledge of the genetics and genomics of pancreatic cancer from its inception to metastatic colonization, and to provide examples of how this information can be translated into the clinical setting for therapeutic benefit and personalized medicine.

Head and neck cancer radiosensitization by the novel poly(ADP-ribose) polymerase inhibitor GPI-15427

BACKGROUND

In this study, we tested the ability of a novel poly(adenosine diphosphate ribose) polymerase (PARP) inhibitor, 10-(4-methyl-piperazin-1-ylmethyl)-2H-7-oxa-1,2-diaza-benzo[de]-anthracen-3-one (GPI-15427), to enhance the effect of radiotherapy in a xenograft model of human head and neck squamous cell carcinoma (HNSCC).

METHODS

Human xenograft HNSCC tumors were established in female nude mice: animals were treated with orally administered GPI-15427 at varied doses prior to tumor irradiation. In vitro and in vivo apoptosis analyses and neutral single-cell gel electrophoresis (comet) assay were performed, with the "tail moment" calculated to evaluate DNA double-strand break damage.

RESULTS

Orally administered GPI-15427 given before radiation therapy significantly reduced tumor volume, and cells demonstrated significantly elevated mean tail moments (indicative of DNA damage) and enhanced apoptosis both in vitro and in vivo, compared with radiation-alone and control groups.

CONCLUSIONS

Use of the PARP-1 inhibitor GPI-15427 induced significant sensitization to radiotherapy, representing a promising new treatment in the management of HNSCC.

L5178Y sublines: a look back from 40 years. Part 1: General characteristics

The aim was to review and summarize the results of studies done over the last 40 years concerning the general characteristics and response to ultraviolet C (UV-C) radiation and hydrogen peroxide of the pair of L5178Y (LY) sublines, LY-R and LY-S, that differ in their sensitivity to various DNA damaging agents. (The response of the sublines to ionizing radiation is described in the second part of the paper.) Comparison of subline karyotypes shows a number of differences in their banding patterns. The sublines differ in their ion transport, the ganglioside pattern of plasma membranes, and in the content and turnover rate of poly(adenosine diphosphoribose) polymers. Nuclear matrix proteins show a differential affinity to these polymers. A unique property of the pair of LY sublines is an inverse cross-sensitivity to X-rays and hydrogen peroxide, with cross-sensitivities to hydrogen peroxide and UV-C, as well as to UV-C and a platinum (Pt) complex (cisplatin analogue). Initial DNA damage and repair and various aspects of the cellular response of the sublines were determined in cells damaged with these agents. The higher sensitivity of LY-R cells to hydrogen peroxide, as compared with LY-S cells, is causally related to the higher content of iron ions in these cells and a less efficient anti-oxidant defence system (including a lower catalase activity). Sensitivity of LY-R cells to UV-C radiation and Pt complexes is explained by impaired excision repair (the incision step is missing).

Single-strand break repair and genetic disease

Hereditary defects in the repair of DNA damage are implicated in a variety of diseases, many of which are typified by neurological dysfunction and/or increased genetic instability and cancer. Of the different types of DNA damage that arise in cells, single-strand breaks (SSBs) are the most common, arising at a frequency of tens of thousands per cell per day from direct attack by intracellular metabolites and from spontaneous DNA decay. Here, the molecular mechanisms and organization of the DNA-repair pathways that remove SSBs are reviewed and the connection between defects in these pathways and hereditary neurodegenerative disease are discussed.

Poly (ADP-ribose) polymerase (PARP) is essential for sulfur mustard-induced DNA damage repair, but has no role in DNA ligase activation

Concurrent activation of poly (ADP-ribose) polymerase (PARP) and DNA ligase was observed in cultured human epidermal keratinocytes (HEK) exposed to the DNA alkylating compound sulfur mustard (SM), suggesting that DNA ligase activation could be due to its modification by PARP. Using HEK, intracellular 3H-labeled NAD+ (3H-adenine) was metabolically generated and then these cells were exposed to SM (1 mM). DNA ligase I isolated from these cells was not 3H-labeled, indicating that DNA ligase I is not a substrate for (ADP-ribosyl)ation by PARP. In HEK, when PARP was inhibited by 3-amino benzamide (3-AB, 2 mM), SM-activated DNA ligase had a half-life that was four-fold higher than that observed in the absence of 3-AB. These results suggest that DNA repair requires PARP, and that DNA ligase remains activated until DNA damage repair is complete. The results show that in SM-exposed HEK, DNA ligase I is activated by phosphorylation catalysed by DNA-dependent protein kinase (DNA-PK). Therefore, the role of PARP in DNA repair is other than that of DNA ligase I activation. By using the DNA ligase I phosphorylation assay and decreasing PARP chemically as well as by PARP anti-sense mRNA expression in the cells, it was confirmed that PARP does not modify DNA ligase I. In conclusion, it is proposed that PARP is essential for efficient DNA repair; however, PARP participates in DNA repair by altering the chromosomal structure to make the DNA damage site(s) accessible to the repair enzymes.

Radiosensitization by the poly(ADP-ribose) polymerase inhibitor 4-amino-1,8-naphthalimide is specific of the S phase of the cell cycle and involves arrest of DNA synthesis

Radiosensitization caused by the poly(ADP-ribose) polymerase (PARP) inhibitor 4-amino-1,8-naphthalimide (ANI) was investigated in 10 asynchronously growing rodent (V79, CHO-Xrs6, CHO-K1, PARP-1+/+ 3T3, and PARP-1-/- 3T3) or human (HeLa, MRC5VI, IMR90, M059J, and M059K) cell lines, either repair proficient or defective in DNA-PK (CHO-Xrs6 and M059J) or PARP-1 (PARP-1-/- 3T3). Pulse exposure to ANI (1-hour contact) potentiated radiation response in rodent cells except in PARP-1(-/-) 3T3 fibroblasts. In contrast, ANI did not significantly enhance radiation susceptibility in asynchronously dividing human cells; yet, single-strand break rejoining was lengthened by ca. 7-fold in all but mouse PARP-1-/- 3T3s. Circumstantial evidence suggested that radiosensitization by ANI occurs in rapidly dividing cells only. Experiments using synchronized HeLa cells consistently showed that ANI-induced radiosensitization is specific of the S phase of the cell cycle and involves stalled replication forks. Under these conditions, prolonged contact with ANI ended in the formation of de novo DNA double-strand breaks hours after irradiation, evoking collision with uncontrolled replication forks of DNA lesions whose repair was impaired by inhibition of the PARP catalytic activity. The data suggest that increased response to radiotherapy by PARP inhibitors may be achieved only in rapidly growing tumors with a high S-phase content.

Radiosensitization of human and rodent cell lines by INO-1001, a novel inhibitor of poly(ADP-ribose) polymerase

Inhibition of poly(ADP-ribose) polymerase (PARP) by a novel, potent inhibitor, INO-1001, was examined in two rodent and one human fibroblast cell lines, after single and fractionated radiation treatments. Since PARP plays a role in the early events following DNA damage and influences the effectiveness of DNA repair, its inhibition has been proposed to constitute a drug target for the development of novel radiosensitizers. We found that INO-1001 effectively inhibited PARP activity at non-cytotoxic concentrations. Combination treatment of 10 microM INO-1001 and a single dose of radiation resulted in significant radiosensitization of all three cells lines (enhancement ratios 1.4-1.6). This radioenhancement was even greater when the drug and radiation were given as fractionated treatments (enhancement ratio 8.0). Apoptosis (as evaluated by TUNEL staining) was not enhanced by the treatments, suggesting that inhibiting PARP enzyme activity by INO-1001 enhanced radiation-induced cell killing by interfering with DNA repair mechanisms, resulting in necrotic cell death. INO-1001 therefore, appears to have potential as a potent enhancer of radiation sensitivity, without any intrinsic cytotoxicity from the drug alone.

Poly(ADP-ribosyl)ation accelerates DNA repair in a pathway dependent on Cockayne syndrome B protein

Activation of poly(ADP-ribose)polymerases 1 and 2 (PARP-1 and PARP-2) is one of the earliest responses of mammalian cells to DNA damage by numerous genotoxic agents. We have analysed the influence of PARP inhibition, either achieved by over-expression of the DNA binding domain of PARP-1 or by treatment with 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone, on the repair of single-strand breaks (SSB), pyrimidine dimers and oxidative base modifications sensitive to Fpg protein (mostly 8-hydroxyguanine) in mammalian cells at very low, non-cytotoxic levels of DNA damage. The data show that the repair rates of all three types of DNA damage are significantly lower in PARP-inhibited cells. Importantly, the retardation of the repair of base modifications is not associated with accumulation of intermediates such as SSB or abasic sites. Moreover, the influence of the PARP inhibition is not observed in cells deficient in Cockayne syndrome B protein (Csb). The results indicate that PARP activation and Csb are both involved in a novel mechanism that accelerates the global repair of various types of DNA modifications.

Overexpression of the DNA-binding domain of poly(ADP-ribose) polymerase inhibits rejoining of ionizing radiation-induced DNA double-strand breaks

PURPOSE

To assess the influence of trans-dominant inhibition of poly(ADP-ribosyl)ation on the rejoining kinetics of radiation-induced DNA double-strand breaks (DSB).

MATERIALS AND METHODS

Stable transfectants of the SV40-transformed hamster cell line CO60 were used: COM3 cells contain a construct to overexpress the poly(ADP-ribose) polymerase (PARP-1) DNA-binding domain (DBD) when induced by dexamethasone, as well as a construct for the constitutive overexpression of the human glucocorticoid receptor (Hg0). COR3 are control cells containing only the Hg0 plasmid. DSB induction and rejoining in X-irradiated cells was assessed by DNA pulsed-field electrophoresis.

RESULTS

DSB induction was identical in both cell lines and independent of the presence of dexamethasone. DSB rejoining kinetics was independent of dexamethasone in COR3 cells and identical to COM3 cells without dexamethasone. However, in COM3 cells treated with dexamethasone to induce PARP-1 DBD overexpression, the fast component of the rejoining kinetic was largely reduced, and residual fragmentation increased concomitant with the increased damage fraction in slow rejoining.

CONCLUSIONS

The results indicate that inhibition of cellular PARP-1 does not affect the rate-limiting step of either fast or slow DSB rejoining. Rather, it appears that absence of poly(ADP-ribosyl)ation due to dominant negative PARP-1 expression induces a shift from rapid to slow DSB rejoining and by this mechanism PARP inhibition may increase the risk of repair failures.

Role of poly(ADP-ribose) polymerase (PARP) in DNA repair in sulfur mustard-exposed normal human epidermal keratinocytes (NHEK)

We previously reported that, in normal human epidermal keratinocytes (NHEK) cultures exposed to the alkylating compound sulfur mustard (bis-(2-chloroethyl) sulfide, HD, 0.3-1 mM), there is a rapid (< or =1 h) activation (100% above unexposed control) of the DNA repair enzyme DNA ligase I (130 kD) followed by a first-order decay (1-5 h). The DNA ligase activation is accompanied by a time-dependent (0.5-4 h) and significant DNA repair. Inhibition of another putative DNA repair enzyme, poly(ADP-ribose) polymerase (PARP), by using 3-amino benzamide does not affect DNA ligase activation following HD exposure, but increases the half-life of the activated enzyme threefold. To examine the role of PARP in HD-induced DNA ligase activation and subsequent DNA repair, we conducted studies using cultured keratinocytes in which the level of PARP had been selectively lowered (> or =85%) by the use of induced expression of antisense RNA. In these cells, there was no stimulation of DNA ligase up to 3 h, and a small stimulation (ca. 30% above unexposed control at 5-6 h after HD exposure. A time-course (0.5-6 h) study of DNA repair in HD-exposed PARP-deficient keratinocytes revealed a much slower rate of repair compared with HD-exposed NHEK. The results suggest an active role of PARP in DNA ligase activation and DNA repair in mammalian cells, and also indicate that modulation of PARP-mediated mechanisms may provide a useful approach in preventing HD toxicity.

Poly(ADP-ribose) polymerase-1: what have we learned from the deficient mouse model?

Poly (ADP-ribose) polymerase (113 kDa; PARP-1) is a constitutive factor of the DNA damage surveillance network developed by the eukaryotic cell to cope with the numerous environmental and endogenous genotoxic agents. This enzyme recognizes and is activated by DNA strand breaks. This original property plays an essential role in the protection and processing of the DNA ends as they arise in DNA damage that triggers the base excision repair (BER) pathway. The generation, by homologous recombination, of three independent deficient mouse models have confirmed the caretaker function of PARP-1 in mammalian cells under genotoxic stress. Unexpectedly, the knockout strategy has revealed the instrumental role of PARP-1 in cell death after ischemia-reperfusion injury and in various inflammation process. Moreover, the residual PARP activity found in PARP-1 deficient cells has been recently attributed to a novel DNA damage-dependent poly ADP-ribose polymerase (62 kDa; PARP-2), another member of the expanding PARP family that, on the whole, appears to be involved in the genome protection. The present review summarizes the recent data obtained with the three PARP knockout mice in comparison with the chemical inhibitor approach.

Oral niacin prevents photocarcinogenesis and photoimmunosuppression in mice

Topical nicotinamide (niacinamide) has demonstrable preventive activity against photocarcinogenesis in mice. To better understand how this vitamin prevents ultraviolet (UV) carcinogenesis, we tested systemic administration of another form of the vitamin, niacin, and its capacity to elevate cutaneous nicotinamide-adenine dinucleotide (NAD) content as well as to decrease photoimmunosuppression and photocarcinogenesis. BALB/cAnNTacfBR mice were fed the AIN-76A diet supplemented with 0%, 0.1%, 0.5%, or 1.0% niacin throughout the experiment. UV irradiation consisted of five 30-minute exposures per week to banks of six FS40 Westinghouse sunlamps for 22 weeks in the carcinogenesis experiments, yielding a total cumulative dose of approximately 1.41 x 10(6) Jm-2 of UV-B radiation. Dietary supplementation with 0.1%, 0.5%, or 1.0% niacin reduced the control incidence of skin cancer from 68% to 60%, 48%, and 28%, respectively, at 26.5 weeks after the first UV treatment. Two potential mechanisms by which niacin prevents tumor formation were identified. Photoimmunosuppression, critical for photocarcinogenesis, is measured by a passive transfer assay. Syngeneic, antigenic tumor challenges grew to an average of 91.6 +/- 19.7, 79.8 +/- 11.5, 41.9 +/- 11.7, or 13.2 +/- 4.1 mm2 in naive recipients of splenocytes from UV-irradiated mice treated with 0%, 0.1%, 0.5%, or 1.0% niacin supplementation, respectively, demonstrating niacin prevention of immunosuppression. Niacin supplementation elevated skin NAD content, which is known to modulate the function of DNA strand scission surveillance proteins p53 and poly(ADP-ribose) polymerase, two proteins critical in cellular responses to UV-induced DNA damage. These results clearly demonstrate a dose-dependent preventive effect of oral niacin on photocarcinogenesis and photoimmunosuppression and establish the capacity of oral niacin to elevate skin NAD levels.

Decreased rates of replicon initiation in mammalian cells

We have designed an assay to measure the rate of initiation of DNA synthesis in Friend erythroleukemia cells and have shown that this parameter is reduced by gamma-radiation and treatment with 4'-demethyl-epipodophyllotoxin-9-(4,6-O-ethylene-beta-D-glucopyranoside) (VP-16). It is concluded, that double-strand breaks in DNA are the immediate cause for this effect. The decrease in the rate of replicon initiation is affected differently by different agents such as cis-diamminedichloroplatinum(II), cycloheximide, staurosporine, and 3-aminobenzamide. The analysis of these results indicates that the observed partial decrease of the rate of DNA initiation is most probably transmitted from the site of damage to the initiation site by one or more phosphorylation/dephosphorylation steps. It does not require de novo synthesis of protein factors, but is probably dependent on poly(ADP-ribosyl)ation of chromatin at the site of DNA breaks.

Inhibition of gene-specific repair of alkylation damage in cells depleted of poly(ADP-ribose) polymerase

The role of the enzyme poly(adenosine diphosphate-ribose) polymerase (PADPRP) in DNA repair at the level of the gene was investigated with human HeLa cells in which PADPRP antisense transcripts are inducible with dexamethasone. After such induction, the cellular content of PADPRP is reduced by 90%. DNA damage and its repair was studied in the essential dihydrofolate reductase (DHFR) gene after exposure of the cells to either ultraviolet (UV) irradiation or the alkylating agent nitrogen mustard. The expression of the antisense construct had no effect on gene-specific repair of UV-induced pyrimidine dimers. In contrast, induced antisense cells were deficient in the gene-specific repair of nitrogen mustard-induced lesions. Dexamethasone itself did not inhibit gene-specific repair in control cells. Thus, PADPRP appears to participate in the gene-specific repair of alkylation damage, but not in the repair of UV-induced pyrimidine dimers. Clonal survival assays revealed that cells depleted of PADPRP showed an increased susceptibility to nitrogen mustard, supporting the notion that repair of essential genes is critical for cellular survival.

Suppression of G1 arrest and enhancement of G2 arrest by inhibitors of poly(ADP-ribose) polymerase: possible involvement of poly(ADP-ribosyl)ation in cell cycle arrest following gamma-irradiation

Low-dose gamma-irradiation of mouse embryonic fibroblast C3D2F1 3T3-a cells caused G1 arrest along with G2 arrest and inhibition of replicative DNA synthesis. When the cells were cultured in the presence of inhibitors of poly(ADP-ribose) polymerase [EC 2.4.2.30], such as 3-aminobenzamide, benzamide and luminol, G1 arrest of C3D2F1 3T3-a cells was suppressed and enhancement of G2 arrest was observed. In contrast, 3-aminobenzoic acid, a non-inhibitory analog of 3-aminobenzamide, did not suppress G1 arrest following gamma-irradiation. These results suggest that the poly(ADP-ribosyl)ation reaction is critical for the pathway of G1 arrest and is also involved in the pathway of G2 arrest.

The function of poly (ADP-ribosylation) in DNA breakage and rejoining

Poly (ADP-ribose) polymerase has an obligatory requirement for DNA strand-breaks in order to show full enzyme activity. Exposure of cells to DNA damaging agents activates this enzyme presumably through the production of DNA strand-breaks, either directly or via cellular enzymes. Recent evidence from manipulations of the cloned cDNA of this enzyme confirm the earlier evidence, obtained using enzyme inhibitors, that this enzyme is involved in DNA excision repair, probably at or near the ligation step. A very unusual human genetic disease has provided direct evidence for a link between the enzyme activities of poly (ADP-ribose) polymerase and of DNA ligase I. There is also some evidence that this enzyme may be involved in other cases of DNA breakage and rejoining, such as homologous and non-homologous DNA recombination, for example, in sister chromatid exchanges, in DNA transfection, in the integration of retroviral proviral DNA and in variable antigen switching in African trypanosomes.

Alterations of fibroblast metabolism in early ligature-induced periodontitis in the cynomolgus monkey

Cellular and biochemical observations were made of fibroblasts harvested from ligature-induced periodontitis and treated gingivitis areas in four adult female cynomolgus monkeys (Macaca fascicularis) to define the changes that occur in the early periodontitis lesion. Compared with fibroblasts from the treated sites, fibroblasts from the diseased areas had a significantly higher rate of proliferation, produced about two-thirds the amount of total protein and collagen, and failed to respond to TGF-beta, which normally stimulates extracellular matrix formation in mesenchymal cells. The diseased cells were also deficient in the activity of poly(ADP-ribose) synthetase, an enzyme involved in the repair of DNA breaks such as occur from the insults of superoxide and other active radicals present in inflamed areas. Although the precise nature of these biochemical defects is not fully elucidated, they may have an important bearing on chronic periodontitis cases with a "downhill" course.

Cytotoxic mechanisms of 5-fluoropyrimidines. Relationships with poly(ADP-ribose) polymerase activity, DNA strand breakage and incorporation into nucleic acids

A comparative study of the cytotoxic mechanisms of 5-fluorouracil (FU) and 5-fluoro-2'-deoxyuridine (FdUrd) was carried out in Chinese hamster ovary K1 (CHO-K1) cells. The poly(ADP-ribose) polymerase (PADPRP) inhibitor, 3-aminobenzamide (3AB, 3 mM) enhanced the cytotoxicity of FU with a dose enhancement factor at 10% survival of 2. This enhancement was also evident when cells were grown in dThd-free medium, but the IC50 for FU was reduced from 50 to 35 microM. In contrast, 3AB did not enhance the cytotoxicity of FdUrd but exerted a small protective effect. The IC50 for FdUrd was reduced from 35 to 1.25 microM in dThd-free medium. A 55% reduction in NAD levels was seen within 6 hr of 5.0 microM FdUrd treatment in dThd-free medium, and this reduction persisted over 24 hr. This drop was prevented by co-incubation with 3AB, indicating that PADPRP activation was the cause of the NAD depletion. In contrast, FU treatment had little or no effect on NAD levels. Alkaline elution analysis of cells treated with up to 150 microM FU revealed no DNA strand breaks in mature DNA, but an increase in breaks in nascent DNA. Co-incubation with 3AB had little or no effect on strand break levels. FdUrd (up to 40 microM) produced a dose-dependent increase in both mature and nascent DNA strand breaks. Analysis using a "relative elution" formula demonstrated that 3AB increased the amount of FdUrd-induced strand breaks (at doses < or = 5-100 microM) in mature DNA. Whereas FU elution profiles for nascent DNA were biphasic, those for FdUrd were linear. Co-incubation with 3AB increased [3H]FU incorporation into both RNA (by 50%) and DNA (45%). 3AB also enhanced [3H]FdUrd incorporation (by 40%) into RNA but had no effect on incorporation into DNA. These data indicate that in addition to acting as an inhibitor of PADPRP, 3AB exerts other metabolic effects.

DNA excision-repair defect of xeroderma pigmentosum prevents removal of a class of oxygen free radical-induced base lesions

Plasmid DNA was gamma-irradiated or treated with H2O2 in the presence of Cu2+ to generate oxygen free radical-induced lesions. Open circular DNA molecules were removed by ethidium bromide/CsCl density gradient centrifugation. The closed circular DNA fraction was treated with the Escherichia coli reagent enzymes endonuclease III (Nth protein) and Fpg protein. This treatment converted DNA molecules containing the major base lesions pyrimidine hydrates and 8-hydroxyguanine to a nicked form. Remaining closed circular DNA containing other oxygen radical-induced base lesions was used as a substrate for nucleotide excision-repair in a cell-free system. Extracts from normal human cells, but not extracts from xeroderma pigmentosum cells, catalyzed repair synthesis in this DNA. The repair defect in the latter extracts could be specifically corrected by in vitro complementation. The data suggest that accumulation of endogenous oxidative damage in cellular DNA from xeroderma pigmentosum patients contributes to the increased frequency of internal cancers and the neural degeneration occurring in serious cases of the syndrome.

Murine strain differences in acute lung injury and activation of poly(ADP-ribose) polymerase by in vitro exposure of lung slices to bleomycin

The DNA-cleaving, antitumor antibiotic bleomycin (BLM) causes pulmonary fibrosis, but the essential early events initiating the fibrotic state have not been well characterized. Thus, we have directly examined BLM-mediated pulmonary cell injury by monitoring lactate dehydrogenase (LDH) release and nuclear poly(ADP-ribose) polymerase (PAP) activity, which is stimulated by DNA breakage, using lung slices isolated from BLM-sensitive (C57B1/6) and BLM-resistant (BALB/c) mice. Lung slices were incubated continuously with or without the PAP inhibitor, 3-aminobenzamide (3-AB), and exposed to BLM for 45 min. LDH release from C57B1/6 lung slices increased 2-fold by 8.5 h after treatment with BLM. In contrast, BLM failed to enhance cumulative LDH release by BALB/c mouse lung slices. Co-incubation of C57B1/6 lung slices with 3-AB prevented BLM-induced LDH release. Nuclear PAP was activated 3- to 4-fold 1.25 h after exposure of C57B1/6 lung slices to BLM but returned to control levels by 3.75 h. Nuclear PAP was only marginally affected at these times in BALB/c lung slices. Co-incubation of C57B1/6 slices with 3-AB prevented the early increases in PAP activity. These results demonstrate that murine strain sensitivity to acute cell injury and early PAP activation by BLM in lung slices parallels the in vivo sensitivity of lungs. In addition, 3-AB suppresses PAP activation and acute cell injury in lung slices. Differential activation of PAP appears to govern murine strain variation in response to BLM and is consistent with the hypothesis that activation of PAP participates in acute pneumocyte injury, initiating the process of BLM-induced fibrosis.

Multiple effects of 3-aminobenzamide on DNA damage induced by cisplatin (DDP) in DDP-sensitive and -resistant rat ovarian tumor cell lines

To investigate the mechanisms by which 3-aminobenzamide (3AB) reverses cisplatin (DDP) resistance in a rat ovarian tumor cell line, the effects of 3AB on DDP-induced DNA damage and repair were kinetically determined over a post-exposure period of 48 h. DNA single strand breaks (SSB) occurred maximally 12 h and 24 h following DDP exposure in DDP-resistant (O-342/DDP) and -sensitive (O-342) rat ovarian tumor cells, respectively. 3AB, present during and after the exposure, significantly increased SSB formation by DDP at 24 h (P < 0.02) and 48 h (P < 0.01) in O-342/DDP cells. To a lesser extent (P > 0.05), a similar tendency was also observed in O-342 cells. Formation of DNA interstrand cross-links (ISCL) by DDP reached a maximum by 12 h in either O-342 or O-342/DDP cells, but in the resistant cells they were both much lower and more rapidly removed. 3AB decreased ISCL in the sensitive cells at 12 h and thereafter with a maximum at 24 h (P < 0.05), while in the resistant cells the same treatment decreased ISCL at 12 h, had no effect at 24 h and increased ISCL at 48 h following DDP treatment. Therefore, it is concluded that 3AB has multiple effects on DNA damage and repair induced by DDP in both cell lines and increase of DNA-ISCL by 3AB at 48 h after the exposure in O-342/DDP cells might be related to its chemosensitizing effect in this line.

DNA damage and cell death induced by 1,2-dibromo-3-chloropropane (DBCP) and structural analogs in monolayer culture of rat hepatocytes: 3-aminobenzamide inhibits the toxicity of DBCP

1,2-Dibromo-3-chloropropane (DBCP) and a number of halogenated propane analogs induced DNA damage in rat hepatocytes in vitro measured by an automated alkaline elution method. Short-term (2 hrs) cytotoxic effects of DBCP were not observed until the DBCP concentration exceeded 1 mM. The short-term cytotoxicity of all the DBCP analogs occurred in the same concentration range. Significant membrane damage, measured as cell detachment, was observed after extended exposure to lower concentrations of DBCP (100 microM) for 20 hrs. The relative, delayed cytotoxic effect of DBCP and analogs correlated with their ability to cause DNA damage. In general, the halogenated propanes with more bromines relative to chlorines were the more potent compounds. Propane analogs lacking the third halogen had little cytotoxic activity. The addition of the proposed specific poly(ADP-ribosyl)transferase inhibitor 3-aminobenzamide (3-ABA) protected against DBCP-induced cytotoxic effects and NAD+ depletion. However, 3-ABA also reduced DBCP-induced DNA damage, DBCP metabolic loss, and the formation of water soluble and covalently bound DBCP metabolites. Thus, 3-ABA may block DBCP-induced cell death by decreasing the formation of reactive DBCP-metabolites.

Antineoplastic drug resistance and DNA repair

Important aspects of the DNA repair mechanisms in mammalian, and especially human, cells are reviewed. The DNA repair processes are essential in the maintenance of the integrity of the DNA and in the defense against cancer. It has recently been discovered that the DNA repair efficiency differs in different regions of the genome and that active genes are preferentially repaired. There is mounting evidence that DNA repair processes play a role in the development of drug resistance by tumor cells. We will discuss such data as well as further approaches to clarify the relationship between DNA repair and antineoplastic drug resistance. Specifically, there is an increasing need to investigate the intragenomic heterogeneity of DNA repair and correlate the repair efficiency in specific genes to aspects of drug resistance. We also discuss the therapeutic potential of inhibiting the DNA repair processes and thereby possibly overcoming drug resistance.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.

Poly(ADP-ribose)polymerase: a perplexing participant in cellular responses to DNA breakage

Poly(ADP-ribose) polymerase is a major nuclear protein of 116 kd, coded by a gene on chromosome 1, that plays a role in cellular responses to DNA breakage. The polymerase binds to DNA at single- and double-strand breaks and synthesizes long branched chains of poly(ADP-ribose), which covalently, but transiently, modifies itself and numerous other cellular proteins and depletes cells of NAD+. This much is known, but the physiological role of the polymerization-degradation cycle is still unclear. Poly(ADP-ribosyl)ation of proteins generally inhibits their function and can dissociated chromatin proteins from DNA. Inhibition of poly(ADP-ribose) polymerase increases to toxicity of alkylating agents and some other DNA-damaging agents and increases sister-chromatid exchange frequencies. During repair of alkylation damage, inhibition of poly(ADP-ribose) polymerase makes no change in excision of damaged products. increases the total number of repair patches, accelerates the rejoining of DNA breaks, and makes variable increases or decreases in net break frequencies. The polymerization cycle consequently is a major player in the response of cells to DNA breakage, but the game it plays is yet to be explained.

Structural requirements for inhibitors of poly(ADP-ribose) polymerase

The purpose of this study was to examine the structure/activity relationships of a series of substituted benzamides as poly(ADP-ribose) polymerase inhibitors. The experimental approach has involved the use of in vitro and in vivo assays in order to gather information either on the intrinsic activity of the benzamides or on the effect of various pharmacodynamic parameters on the activity in vivo. Although some discrepancies between the data obtained in vivo and in vitro were found in this study, results seem to indicate that most powerful inhibitors were characterized by acylation of the -NH2 function in the 3 position or by substitution in this same position with hydroxy or methoxy groups. The best inhibitors were not cytotoxic under these experimental conditions. Computed calculations of molecular electrostatic potential of these molecules were also performed and a good correlation was found between the similarity index and the experimental inhibitory activity.

Isolation of Chinese hamster ovary cells with reduced poly(ADP-ribose) polymerase activity

The biological function of poly(ADP-ribose) polymerase in DNA repair, cell-cycle regulation and cellular differentiation has yet to be defined. Isolation of cells which are deficient in poly(ADP-ribose) synthesis would greatly facilitate the determination of the biological role of this enzyme. A method is described for isolating Chinese hamster ovary (CHO) cells deficient in the poly(ADP-ribose) polymerase activity by direct screening of colonies for enzyme activity. Colonies with decreased production of poly(ADP-ribose) are recovered from nylon replicas for further analysis. Using this method we have isolated a series of CHO cells which have 50% or less poly(ADP-ribose) polymerase activity. These mutants have normal generation times and are 20% more sensitive to the effects of DNA (m)ethylating agents than the parental cell. However, these mutants display normal sensitivity to gamma-rays.

Recovery of human lymphocytes damaged with gamma-radiation or enzymatically produced oxygen radicals: different effects of poly(ADP-ribosyl)polymerase inhibitors

Quiescent human lymphocytes were damaged in two different ways, both producing toxic oxygen radicals: xanthine oxidase plus hypoxanthine (XOD/HYP), or gamma-rays. Under conditions where DNA synthesis was reduced to 10-20% of control, inhibitors of poly(ADP-ribosyl)polymerase (ADPRP, an enzyme that becomes activated in the presence of DNA strand breaks) allowed lymphocytes to recover completely when the damage was caused by XOD/HYP, but they did not affect DNA synthesis of irradiated cells. However, a protective effect of ADPRP inhibitors was observed with irradiated lymphocytes receiving doses greater than or equal to 50 Gy. Unscheduled DNA synthesis was Unscheduled DNA synthesis was significantly increased when lymphocytes were damaged by high radiation doses in the presence of ADPRP inhibitors. We suggest that ionizing radiation does not stimulate poly(ADP-ribose) synthesis in lymphocytes at doses that impair lymphocyte DNA synthesis by 80-90%, while ADPRP may be involved in the repair of DNA lesions occurring at higher radiation doses.

Reconstitution of an in vitro poly(ADP-ribose) turnover system

Poly(ADP-ribose) is synthesized and degraded by poly(ADP-ribose) polymerase and glycohydrolase, respectively. We have reconstituted in vitro two turnover systems containing these two enzymes. We have measured the kinetics of NAD consumption and polymer accumulation during turnover. The combined action of the two enzymes (i.e., turnover) generates a steady state of polymer quantity. The glycohydrolase determines the time and the level at which this steady state of total polymer is reached. A major observation is that the size and calculated density of polymer bound to the total polymerase molecules is tightly regulated by the rate of polymer turnover. On the polymerase, an increase in the rate of polymer turnover does not affect the mean polymer size, but reduces the polymer density on the enzyme (i.e., the number of polymer chains per polymerase molecule). In the absence of glycohydrolase and at low histone H1 concentration (less than 1.5 micrograms/ml), poly(ADP-ribose) polymerase preferentially automodifies itself instead of modifying histone H1. In contrast, under turnover conditions, oligomer accumulation on histone H1 was greatly increased, with almost 40% of all the polymer present on H1 after 5 min of turnover. Although turnover conditions were necessary for histone H1 labelling, there was no difference between the fast and the slow turnover systems as concerns the proportion of histone H1 labelling, although the mean polymer size on histone H1 was decreased with increasing turnover rate. Due to its small size, polymer is not degraded by the glycohydrolase and accumulates on histone H1 during turnover. These data suggest that the glycohydrolase modulates the level of poly(ADP-ribosyl)action of different proteins in two ways; by degrading shorter polymers at a slower rate and probably by competing with the polymerase for polymer.

Differential response of excision proficient and deficient L5178Y cells to UVC irradiation and benzamide

L5178Y-R and L5178Y-S cells differ in sensitivity to UVC radiation (D0 values: 2.8 and 9.0 J m-2 respectively, exposure in Fischer's medium). The UVC sensitivity is related to the excision repair ability. Benzamide (Bz), an inhibitor of adenosine diphosphoribosyl transferase (ADPRT), does not modify the lethal effect of UVC radiation in L5178Y-R cells, whereas it sensitizes L5178Y-S cells. The content of NAD+ after irradiation decreases only in the latter cells and this decrease can be prevented by 2 mM Bz treatment. In agreement with the survival data, in L5178Y-R cells neither the proportion of abnormal cells nor the frequency of chromatid aberration are affected by 2 mM Bz treatment, in contrast with L5178Y-S cells. Bz slightly reverses inhibition of 3H-thymidine incorporation only in L5178Y-S cells, but it does not affect the proportions of cells in the different phases of the cell cycle in either cell strain after UVC exposure. These data could be taken as an indirect indication of the involvement of ADPRT in DNA repair in UVC-irradiated L5178Y-S cells. However, the increase in the number of DNA strand breaks in UVC-exposed, Bz-treated cells compared with UVC-exposed untreated cells is the same in both L5178Y strains.

Benzamide prevention of ultraviolet radiation-induced transformation as measured by anchorage-independent growth and the absence of correlation with thymidine dimer formation and DNA repair

Synchronized human fibroblasts were exposed in early S phase to increasing doses of ultraviolet (UV) irradiation in the presence and absence of an antitransforming drug, benzamide. Cellular survival, initial thymidine dimer formation and its repair, and cellular phenotypic transformation were simultaneously monitored in the presence and absence of 1 mM externally added benzamide that reaches 8 to 15 microM intracellular levels. Cellular transformation as measured by an expression of anchorage-independent growth was inhibited by nontoxic doses of benzamide. Antitransforming action of benzamide is confined to low intracellular drug concentrations, which in the case of benzamide is in the 4-9 microM range. Because of the lack of effect of benzamide on the formation of UV-induced thymidine dimers and the specific repair of these dimers, these results suggest that the processes of thymidine dimer formation and its repair are not involved in the mode of action of benzamide that influences the expression of a transformed phenotype with low malignant vigor.

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.

A possible role for altered poly(adenosine diphosphoribose)-synthesis in the sensitivity of human head and neck squamous carcinoma cells to ionizing radiation

Cytotoxicity, extent of DNA double-strand breaks, and stimulation of poly(adenosine diphosphoribose)-synthesis were measured in two established human head and neck squamous carcinoma cell lines (183A and 1483) following x-irradiation. The 1483 cell line was 15-fold more resistant to x-ray-mediated cytotoxicity than was the 183A cell line. X-ray-mediated DNA strand cleavage also differed in these two cell lines with the absolute frequency of DNA double-strand breaks in the sensitive cells 183A cells being twice that in the resistant 1483 cell line. No detectable stimulation of poly(adenosine diphosphoribose)-synthesis was measured in the sensitive 183A cells whereas a marked increase in incorporation of [3H]-nicotinamide adenine dinucleotide was readily detected following x-irradiation of the resistant 1483 cells. These findings suggest a possible role of altered poly(adenosine diphosphoribose)-synthesis in the sensitivity of human head and neck squamous carcinoma cells to ionizing radiation.

Biological effects in a chemical factory with mutagenic exposure. II. Analysis of unscheduled DNA synthesis and adenosine diphosphate ribosyl transferase, epoxide hydrolase, and glutathione transferase in resting mononuclear leukocytes

A chemical plant, where an increased number of non-Hodgkin's lymphomas and myelomas had been observed, was monitored for genotoxic effects occurring in peripheral mononuclear leukocyte samples from 76 exposed workers. Biochemical markers sensitive to DNA repair and drug metabolism were used as the indicators of genotoxic risk. Unscheduled DNA synthesis (UDS) and covalent binding induced by N-acetoxy-N-acetyl-2-aminofluorene (NA-AAF) and constitutive and gamma ray induced adenosine diphosphate ribosyl transferase (ADPRT) activities were highly and significantly elevated over the corresponding values for a control group of 48 postal workers. Microsomal and soluble epoxide hydrolases and glutathione transferase activities directed towards trans-stilbene oxide and 1-chloro-2,4-dinitrobenzene were not significantly altered in the exposed group. The exposure in this factory was complex, involving over 100 chemicals including several well known carcinogens. However, no apparent significant associations to exposure could be established.

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.

Adenosine-diphosphoribosyltransferase inhibitors can protect against or potentiate the cytotoxicity of S-phase acting drugs

We have investigated the effect of inhibitors of ADP-ribosyltransferase on the cytotoxicity of a range of S-phase acting drugs. Co-administration of 3 mM 3-aminobenzamide (3AB) potentiated the cytotoxicity of TG 2-fold, but had no effect on the cytotoxicity of HU, FdUrd or araC. Higher concentrations of benzamides (e.g. 10-20 mM 3AB) produced a G1-specific cell cycle blockade. This treatment prevented cells entering S-phase DNA synthesis and consequently protected against the cytotoxicity of the same S-phase acting drugs. Thus, using different treatment regimens with 3AB, it was possible to either potentiate or protect against the cytotoxicity of TG.

Effect of hyperthermia and doxorubicin on nucleoid sedimentation and poly (ADP-ribose) polymerase activity in L1210 cells

We report on the individual and combined effects of doxorubicin (DOX) and hyperthermia (HYP) on nucleoid sedimentation and poly (ADP-ribose) polymerase (PARP) activity of L1210 cells. The effects of HYP and DOX on nucleoid sedimentation (increased sedimentation) were similar and correlated with cell viability. No correlation of PARP activity with cell toxicity was evident; the activity of PARP was inhibited by HYP (42 degrees C; 1-3 h) and stimulated by DOX (1-10 microM; 30 min). The HYP-induced inhibition of PARP was actually ameliorated by simultaneous exposure to DOX. Although separate studies have previously suggested that chromatin alterations or the inhibition of PARP might play a role in the effect of HYP, the correlation of nucleoid changes (rather than PARP activity) with cell viability emphasizes the contribution of the former. Furthermore, the results suggest that the nucleoid technique may prove useful in screening potential treatment modalities.

Cell cycle regulation of poly(ADP-ribose) synthetase in FR3T3 cells

The properties of poly(ADP-ribose) synthetase were studied throughout the cell cycle progression of non-synchronized rat FR3T3 fibroblasts using an immunological and biochemical approach. Cells in the various phases of the cell cycle were sorted from an asynchronously growing population by using flow cytofluorometry. G1, S and G2 + M fractions were used for enzymatic assays in the presence of saturating concentrations of DNAase I for the analysis of total poly(ADP-ribose) synthetase; maximal enzyme activity was found in the G2 + M phase. Purified IgG, specific for the FR3T3 poly(ADP-ribose) synthetase were used for the labelling of endogenous synthetase in order to quantify the enzyme immunologically. Localization of nuclear immunofluorescence was observed and analysis of poly(ADP-ribose) synthetase content throughout the cell cycle were carried out using double fluorescent staining and cytofluorometry. Poly(ADP-ribose) synthetase content as measured immunologically was found to increase from G1 to S and G2 + M phases. Quiescent cells showed a lower content as measured immunologically of poly(ADP-ribose) synthetase than cells in the G1 phase. In exponentially growing cells, the ratio between enzyme activity of poly(ADP-ribose) synthetase over the amount of enzyme measured immunologically was found to be higher in the G2 + M phase. These results show that a cell-cycle specific event activates poly(ADP-ribose) synthetase in the G2 + M phase.

3-Aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase, is a stimulator, not an inhibitor, of DNA repair

An inhibitor of poly(ADP-ribose) synthesis, 3-aminobenzamide (3AB), at low concentrations (0.01-0.1 mM) was found to reduce strand-break frequencies and increase repair replication in human lymphoid cells damaged by methyl methanesulfonate. A concentration of 0.1 mM 3AB was adequate to produce a maximum effect on strand-break frequencies and repair replication. This evidence, together with our previous measurements, demonstrates that 3AB cannot be regarded as an inhibitor of DNA repair; rather, it actually accelerates the ligation of DNA repair patches. Previous considerations of 3AB as a repair inhibitor may have derived from the use of excessive concentrations above 1 mM that may have stimulated additional damage and from the use of ethyl alcohol as a solvent for 3AB. Interpretations of the role of single-strand breaks and poly(ADP-ribose) in DNA repair, differentiation, and gene activity may need reevaluation because they have frequently been based on an erroneous notion of 3AB as a repair inhibitor, when its mode of action is, in fact, more complex.

Induction of B-cell differentiation antigens in interferon- or phorbol ester-treated Daudi cells is impaired by inhibitors of ADP-ribosyltransferase

Treatment of the Daudi Burkitt lymphoma-derived cell line with human interferon alpha, which inhibits cell proliferation in this system, induces differentiation of these B-lymphoid cells into cells with a plasmacytoid phenotype. This differentiation, quantified by the appearance of surface antigens characteristic of mature plasma cells, is impaired by addition to the culture medium of the ADP-ribosyltransferase (ADPRT; EC 2.4.2.30) inhibitors 3-methoxybenzamide or 3-aminobenzamide. These agents also protect the cells against the inhibition of proliferation induced by low doses of interferon alpha. In contrast, the large inhibition of thymidine incorporation into DNA caused by interferon treatment is not affected by the ADPRT inhibitors. The phorbol ester phorbol 12-tetradecanoate 13-acetate induces the same plasma cell surface antigens that are induced by interferon treatment, and this effect is also impaired by the ADPRT inhibitors. These results suggest that interferons and phorbol esters share a mechanism of action that requires ADPRT activity. Protection of the cells against the antiproliferative effect of interferons by the ADPRT inhibitors suggests that growth inhibition may be a consequence of cell differentiation. In contrast, the inhibition of thymidine incorporation alone is not sufficient for the cessation of cell proliferation and is not a true reflection of the rate of DNA synthesis.

Suppression of nuclear ADP-ribosyltransferase activity in regenerating rat liver by 5-azacytidine and its relevance to the nuclear methylating activities

A single administration of 5-azacytidine (5-ACR) to partially hepatectomized rats 24 h following operation resulted in a dose-dependent reduction of nuclear ADP-ribosyltransferase (ADPRT) activity in the liver, when assayed after the nuclei were isolated 22 h after injection. No such a suppression by 5-ACR was observed in the liver of intact rats. Cytidine, a known agent which prevents the incorporation of 5-ACR into DNA, abolished the suppression of ADPRT, when it was given in combination with 5-ACR. The 5-ACR suppressed nuclei from regenerating liver showed no decreased DNA methylating activity, as estimated from the rate of radiolabel transfer from [methyl-3H]SAM to the bulk DNA. The methylation of nuclear RNA and protein was markedly reduced. These results suggest that the incorporation of 5-ACR into nucleic acids inactivates chromatin-bound ADPRT without inhibition of DNA methylation.

Cellular responses to ionizing radiation: effects of interrupting DNA repair with chemical agents

This review is concerned with the influence of different classes of chemical agents on cellular repair of DNA damage induced by ionizing radiation. Single-strand break rejoining is little affected by inhibitors of DNA synthesis; however, such inhibitors do lead to a persistence of double-strand breaks in the DNA, and this correlates with an enhancement of chromosome aberrations and cell killing. Experiments with antagonists of topoisomerase II suggest an intriguing role for this DNA unwinding enzyme in double-strand break repair. Interference with poly(ADP-ribose) synthesis, by means of the inhibitor 3-aminobenzamide, does not have a clear-cut effect on recovery from ionizing radiation damage. Various substances (for example, caffeine and trypsin) affect DNA repair via a modulation of the cell cycle, altering the time available to the cell for repairing potentially lethal DNA damage before such damage is 'fixed' by the process of DNA replication. Finally, disturbing cellular energy metabolism, and depressing the level of ATP, can inhibit the repair of radiation damage.