Evaluation of putative inhibitors of DNA excision repair in cultured human cells by the rapid nick translation assay

Effects of areca nut, inflorescence piper betle extracts and arecoline on cytotoxicity, total and unscheduled DNA synthesis in cultured gingival keratinocytes

Betel quid (BQ) chewing has a strong correlation with oral leukoplakia, submucous fibrosis and oral cancer. For elucidation of its pathogenesis, we investigated the effects of areca nut (AN) and inflorescence piper betle (IPB) extracts and arecoline on the growth, total DNA synthesis (TDS) and unscheduled DNA synthesis (UDS) of cultured human gingival keratinocytes (GK). Arecoline and AN extract suppressed the growth of GK over 5 days of incubation in a dose-dependent fashion. At concentrations of 100, 200 and 400 microg/ml, AN extract suppressed the growth of GK by 31%, 46% and 90%, respectively. The IPB extracts exerted less inhibitory effect on the growth of GK. IPB extract (200-400 microg/ml) decreased cell numbers by 20-40% over 5 days of incubation. Moreover, at a concentration of 0.1, 0.2 and 0.4 mM, arecoline suppressed cell growth by 44%, 77% and 96%, respectively. However, only AN extract induced TDS and UDS in cultured GK within 6 h of exposure. Induction of UDS by AN extract was concomitant with the presence of apparent intracellular vacuolization. Arecoline was also toxic to GK, but did not induce intracellular vacuolization. At a concentration range of 200-1600 microg/ml, AN extract induced TDS by 2.1- to 6.5-fold. Furthermore, at a concentration of 400-1600 microg/ml, AN extract elevated the UDS by 2.4- to 5.5-fold more than that of untreated control. On the contrary, IPB extract (200-1600 microg/ml) and arecoline (0.2-1.6 mM) inhibited the TDS and UDS of GK to a different extent. Simultaneous exposure of confluent GK to AN extract, IPB extract and arecoline for 1 to 5 days led to different degrees of cytotoxicity that was dose- and time-dependent. These results indicate that AN, IPB and arecoline take part in the pathogenesis of BQ chewing-related oral mucosal lesions, possibly through both genotoxic and non-genotoxic mechanisms.

Comparison of autoradiography, liquid scintillation counting and immunoenzymatic staining of 5-bromo-2'-deoxyuridine for measurement of unscheduled DNA synthesis and replicative DNA synthesis in rat liver

Different methods for evaluating unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) were studied in hepatocytes of F344 rats exposed in vivo to dimethylnitrosamine (DMN) or CCl4. Hepatocytes were isolated and incubated in Williams' medium E supplemented with either [3H]thymidine for autoradiography or 5-bromo-2'-deoxyuridine for immunoenzymatic staining. In the method of liquid scintillation counting, the cells were incubated with [3H]thymidine with or without hydroxyurea. The nuclear fraction was isolated and the incorporation of [3H]thymidine into nuclear DNA was determined by a liquid scintillation counter. DMN at doses of 0.625-5 mg/kg body weight induced UDS of 1.6-37.9 (0 dose; -6.9) net grains/nucleus measured by autoradiography and 337-1377 (0 dose; 177) dpm/microgram DNA in the presence of hydroxyurea measured by a liquid scintillation counter. CCl4 at doses of 50-400 mg/kg body weight induced RDS in 1.5-12.1% (0 dose; 0.12%) and 1.8-14.6% (0 dose; 0.16%) of cells with the methods of autoradiography and immunoenzymatic staining, respectively, and of 2991-24256 (0 dose; 324) dpm/microgram DNA in the absence of hydroxyurea with the method of liquid scintillation counting. Similar dose-dependent induction of UDS and RDS was observed with these methods. These results suggest that the methods of liquid scintillation counting and immunoenzymatic staining have almost the same sensitivity for measuring UDS and RDS as that of autoradiography.

DNA strand breakage during physiological apoptosis of the embryonic chick lens: free 3' OH end single strand breaks do not accumulate even in the presence of a cation-independent deoxyribonuclease

Epithelial cells from the lens equator differentiate into elongated fiber cells. In the final steps of differentiation, the chromatin appears quite condensed and chromatin breakdown into nucleosomes occurs. DNA breaks due to an endodeoxyribonuclease activity corresponding to at least two polypeptides of 30 and 40 kDa have been identified. To identify the nature and the developmental appearance of initial breaks, nick translation reaction was followed both biochemically and in situ in fiber and epithelial cells from chick embryonic lenses. There is no accumulation of single-strand breaks (SSB) with 3'OH ends in lens fiber cells during embryonic development. Such damage can be increased in these cells by treatment with DNAase I indicating the absence of an inhibitor of the nick translation reaction in fiber cells. However, there are indications of the presence of DNA breaks with blocked termini when the phosphatase activity of nuclease P1 is used. The presence of breaks is also indicated by the large amounts of (ADP-ribose)n found in lens fibers particularly at 11 days of embryonic development (E11) as ADP-ribosyl transferase binds to and is activated by DNA strand breaks. Incubation of lens cells in vitro, which causes nucleosomal fragmentation only in fiber cells, produces SSB with 3'OH ends in both epithelia and fibers. Incubation for short periods, observed in experiments in situ, induces SSB first in the central fiber nuclei, which are late in differentiation. This may indicate that these SSB play a physiological role. Long incubations produce larger numbers of SSB in epithelia than fibers. The SSB in the fibers may have been converted into double-strand breaks (D SB), seen as nucleosomal fragments, and therefore no longer act as substrates for nick translation. The nuclease activity responsible for SSB production is independent of divalent cations and could be implicated in lens terminal differentiation.

A novel technique for the detection of DNA single-strand breaks in human white blood cells and its combination with the unscheduled DNA synthesis assay

A modified assay for the detection of DNA single-strand breaks (SSBs) in human mononucleated white blood cells (MWBCs) based on the nick translation (NT) reaction was developed and combined with the test for unscheduled DNA synthesis (UDS). Both assays were performed on disposable 96-well filtration plates and therefore allowed rapid and sensitive examination of SSBs and UDS. Only 5-8 ml of heparinized blood is required for an eightfold determination in both assays. The uptake of radioactive nucleotide precursors was demonstrated to depend linearly upon the NT reaction time and in both assay systems on the number of investigated cells. The best results and the lowest signal to noise ratio were obtained when the NT assay was performed at 25 degrees C for 20 min. The test was standardized for 150,000 MWBCs/well and a polymerase I concentration of 20 U/ml. The same number of cells were used to measure UDS during a 4-h incubation at 37 degrees C. We observed a dose-dependent increase in SSBs after in vitro incubation with N-methyl-N-nitrosoguanidine (MNNG), with a detection limit of 50 microM when MNNG was present for 1 h and of 5 microM after 20-h incubation period. UDS in MWBCs was increased after treatment for 1 h with MNNG (200 microM) only if poly(ADP)ribose synthesis was inhibited by 3-aminobenzamide. UDS was induced by 320 microM methyl methanesulfonate, but SSBs could only be detected after inhibition of UDS by 100 microM hydroxyurea.(ABSTRACT TRUNCATED AT 250 WORDS)

Organ culture as a model for investigating the effects of antimetabolites and nucleoside transport inhibitors on rodent colonic mucosa

The in-vitro effects of hydroxyurea 5-FU and 5-FUdR have been extensively studied in experimental systems employing cell-line techniques. In this study we investigated the effects of these drugs on the levels of incorporation of labeled nucleosides into DNA in explants of intact rat colonic mucosa maintained in organ culture. The effects of the nucleoside transport inhibitors nitrobenzylthioinosine (NBMPR) and dipyridamole--which are modulators of antimetabolite cytotoxicity--on the incorporation of tritiated thymidine ([3H]TdR) into DNA were also studied. The incorporation of tritiated TdR into DNA was reduced by hydroxyurea but was not altered by either 5-FU or 5-FUdR. The levels of tritiated deoxyuridine were reduced by 5-FU and 5-FUdR in separate experiments; this is in keeping with thymidylate synthase inhibition. NBMPR and dipyridamole also reduced 3H-TdR incorporation into DNA. These results can be explained in terms of the known mechanisms of action of these drugs. This experimental model is therefore useful in assessing the effects of antimetabolites and nucleoside transport inhibitors in intact colonic mucosa.

Inhibition of DNA repair and the enhancement of cytotoxicity of alkylating agents

DNA damage was evaluated by flow cytometric (FCM) analysis of cells treated with L-phenylalanine mustard (L-PAM) and stained with anti-DNA monoclonal antibody (MAb) F7-26. DNA damage was rapidly repaired, as indicated by the loss of DNA immunoreactivity after removal of L-PAM. Two types of drug combinations were found to inhibit DNA repair. Combinations containing inhibitors of DNA polymerase (ara-C, aphidicolin) or these inhibitors and hydroxyurea inhibited DNA repair in A2780/PAM and A549 cells. The inhibition of DNA repair by combinations of DNA-damaging agents thioTEPA or cisplatin and DNA polymerase inhibitors is a novel observation based on the specificity of DNA damage assay with MAb F7-26. Combinations containing thioTEPA or cisplatin inhibited DNA repair in A549 but not in A2780/PAM cells. Drug combinations which inhibited DNA repair also significantly enhanced cell killing by L-PAM. Cell survival in cultures treated with L-PAM and efficient inhibitors was 2 to 3 orders of magnitude lower than was expected for additive survival. ThioTEPA and cisplatin play a dual role in combination chemotherapy by inducing DNA damage and inhibiting repair of DNA damage. FCM analysis of DNA repair may be a useful component of drug evaluation and could be applied to determine cell-type specific sensitivity to inhibitors of DNA repair.

In vivo short-term assays of repair and replication of rat liver DNA

A short-term in vivo method for assay of repair and replication of rat liver DNA has been developed, by which possible hepatocarcinogens could be identified in a few days. F344 rats were treated orally with two genotoxic hepatocarcinogens, dimethylnitrosamine (DMN) and 2-acetylaminofluorene (2AAF), or a nongenotoxic hepatocarcinogen, carbon tetrachloride (CCl4). Then at suitable times after treatment, their hepatocytes were isolated by a two-step collagenase perfusion technique in situ and incubated with [3H]dThd with or without hydroxyurea, which inhibits DNA replication. Their nuclear DNA was then extracted and the incorporation of [3H]dThd into nuclear DNA was determined in a liquid scintillation counter. Unscheduled DNA synthesis (DNA repair), induced by DMN at doses of 2.5-10 mg/kg body weight and by 2AAF at doses of 12.5-50 mg/kg body weight, could be detected 2 h and 4 h after their administration as an increase of DNA synthesis of up to 5.8-fold and 6.0-fold, respectively, in the presence of hydroxyurea. Replicative DNA synthesis, induced by CCl4 at a dose of 200 mg/kg body weight, could be detected 48 h after its administration as a 23-fold increase of DNA synthesis in the absence of hydroxyurea and was inhibited approximately 97%-99% by hydroxyurea. Replicative DNA synthesis induced by 2AAF at a dose of 25 mg/kg body weight 16 h after its administration could be detected as a 6.8-fold increase of DNA synthesis in the absence of hydroxyurea. These results show that unscheduled and replicative DNA synthesis can be clearly distinguished by simultaneous measurements of the incorporation of [3H]dThd into nuclear DNA in the presence and absence of hydroxyurea.

Differential effects of 5-azacytidine and 5-azadeoxycytidine on cytotoxicity, DNA-strand breaking and repair of X-ray-induced DNA damage in HeLa cells

The cytotoxicity, DNA-strand breaking ability, and effects on repair of X-ray-induced DNA damage by short treatments with 5-azacytidine (azaCyd) and 5-azadeoxycytidine (azadCyd) were examined in HeLa cells. azaCyd was shown to be an effective inhibitor of the repair of X-ray-induced DNA single-strand breaks whereas azadCyd did not have this effect. At high doses, both compounds also induced DNA damage by themselves. The cytotoxicity, inhibition of repair, and drug-induced DNA damage associated with azaCyd treatment were all reversed by the concurrent addition to the cells of cytidine or uridine but not by thymidine, deoxycytidine or deoxyuridine. Cytotoxicity and drug-induced strand breaks associated with azadCyd treatment were reversed to varying degrees with all nucleosides and deoxynucleosides. These results support the notion that these two antileukemic cytidine analogs may have different mechanisms of action in exerting their antiproliferative activity.

Variable sister-chromatid exchange response in human lymphocytes exposed in vitro to gossypol acetic acid

Gossypol has potential for widespread use as a male oral antifertility agent in humans since it appears to be highly efficacious, with reversible spermatostatic effects and minimal side effects. Furthermore, it is both inexpensive and readily available. Therefore, a thorough understanding of gossypol's genotoxic potential is critical. Although genotoxicity studies have produced conflicting reports, increased sister-chromatid exchange (SCE) and DNA-strand breaks have been reported in human cells exposed to gossypol in vitro. In the present study, SCE was examined in purified human lymphocytes and whole blood cultures exposed to gossypol acetic acid at various concentrations in serum-free medium. A small but statistically significant increase in SCE was observed in pooled analysis of 7 donors in whole blood cultures exposed to 0.70 microM gossypol acetic acid (p less than 0.02). Individual analyses revealed only one donor with a significant SCE response (p less than 0.001). In subsequent experiments, exposure at higher doses had no effect on SCE frequencies. A small but significant increase in SCE was observed in ficoll/hypaque purified lymphocytes exposed to 0.07 and 0.70 microM gossypol acetic acid. Interpretation of SCE data with variable response is discussed.

DNA-protein crosslinking in normal human skin fibroblasts exposed to solar ultraviolet wavelengths

Three normal human skin fibroblast cell lines were exposed to the simulated solar UV radiation produced by a fluorescent sunlamp under conditions in which the wavelength components shorter than either 295, 305 or 315 nm were excluded. The level of DNA-protein crosslinks (DPC) was then measured in those cells using the alkaline elution technique either immediately after irradiation or following a 24 h incubation. In each case, cells were exposed to fluences that induce similar levels of DPC. For cells exposed to 10 kJ m(-2) of sunlamp UV > 295 nm, the level of DPC exhibited a 2-5-fold increase following incubation. In contrast, 40-100% of the DPC were removed upon incubation of cells irradiated with either 100 kJ m(-2) of sunlamp UV > 305 nm or 150 kJ m(-2) of sunlamp UV > 315 nm. A major difference between the effects induced by these wavelength regions is that, in addition to DPC, a very high level of pyrimidine dimers is also produced by sunlamp UV > 295 nm, whereas much lower dimer yields result from treatment with either sunlamp UV > 305 nm or sunlamp UV > 315 nm. A potential role for type II DNA topoisomerase in the formation of these DPC resulting from either the change in conformational structure caused by the presence of a high level of dimers or an involvement of this enzyme in dimer excision repair is discussed.

DNA repair precedes replicative synthesis during early germination in maize

DNA synthesis was studied during germination by following the rate of incorporation of radioactive thymidine into high molecular weight DNA. A peak of DNA synthesis was observed between the 8th and the 12th hour, i.e. before the beginning of the semi-conservative replication of genomic DNA, accompanied by an increase in the DNA content of the embryo. By the use of nucleoid sedimentation and nick-translation it was shown that, during the first hours of germination, extensive repair occurs of the DNA single-strand breaks present in the dry embryo. As a result, the DNA of the 16-h-germinated embryo acquires the conformation typical of that of the root meristemic cells active in transcription and replication.In addition we have shown that cytoplasmic organelle (most probably mitochondrial) DNA synthesis is very active during the prereplicative state which confirms earlier microscopic data on mitochondrial biogenesis during early germination.

Dominant mutation in mouse cells associated with resistance to Hoechst 33258 dye, but sensitivity to ultraviolet light and DNA base-damaging compounds

A spontaneous derivative of murine L tk- cells has been isolated which has gained a resistance to the cytostatic/lethal effects of high concentrations of Hoechst 33258. The resistant clone HoeR-415 was at least 20-fold more resistant to the dye (D10 dose). HoeR-415 cells have a normal response to X-rays and mitomycin-C and colchicine but were found to show a small sensitivity to UV light, 4NQO, and EMS (1.4, 1.6, and 1.6-fold lower D10 doses, respectively). HoeR-415 cells do not show an increased mutability by EMS. The HoeR phenotype was found to be codominant in hybrids. In order to explain these various characteristics, we suggest that the HoeR-415 mutation may result in an altered topoisomerase activity. Consistent with this we find HoeR-415 cells have an increased sensitivity to novobiocin.