A modified method of UDS detection in vitro suitable for screening the DNA-damaging effects of chemicals

Effects of arsenic on DNA damage and repair in human fetal lung fibroblasts

Previous studies suggest that arsenic may be both mutagenic and co-mutagenic. In this report, we examined the effects of sodium arsenite (As) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on unscheduled DNA synthesis (UDS) in human fetal lung fibroblasts (2BS cells) by 3H/14C double-labeling and liquid-scintillation counting techniques. Arsenic at concentrations of 1, 5 and 10 microM increased UDS value, indicating that arsenic directly damaged DNA and did not inhibit DNA repair. In addition, UDS induced by 34 microM MNNG in combination with arsenic was significantly increased by 3 microM As and not affected by 0.1, 0.5, 1.0 and 5 microM As, also indicating that arsenic did not inhibit the excision and polymerization steps of DNA repair. Based on the results and a previous study that 3 microM As is more efficient than 1 and 5 microM As in the induction of DNA-protein crosslinks, we proposed that arsenic may enhance the mutagenicity of other compounds by inducing DNA-protein crosslinks rather than inhibiting DNA repair.

Induction of cytochrome P450 isozymes in human amnion FL cells

The human epithelial FL cell line contains low levels of constitutive ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities, of which ethoxyresorufin O-deethylase could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene, but ethoxy-couarin O-deethylase and aminopyrine N-demethylase activities could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene as well as phenobarbital. Inducibility of ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities was 3.1-6.7-, 1.8-3.1- and 1.4-2.0-fold respectively. Co-treatment with 3-methylcholanthrene and norepinephrine resulted in higher ethoxyresorufin O-deethylase and aminopyrine N-demethylase activities than treatment with 3-methylcholanthrene alone. The induced ethoxyresorufin O-deethylase and ethoxycoumarin O-deethylase activities remained at a high level for 24-36 h after removal of the inducer from the medium. The metabolizing ability of induced FL cells is stronger than that of non-induced ones in a given time following induction. The results further confirm that it is feasible to use the cytochrome P450 isozyme-induced FL cell as a biological indicator in short-term tests for screening promutagens/procarcinogens.

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.

A test using cultured cells with induced mixed-function oxygenase in the unscheduled DNA synthesis assay for detecting promutagens/procarcinogens

The human FL cell line contains very low levels of constitutive AHH activity, but it could be greatly induced by NE, beta-NF and 3-MC, and induced slightly by PB. When two different types of inducer, for example, 3-MC and PB or 3-MC and NE were given in combination, an additive inductive effect was not observed. Both the constitutive and induced AHH in FL cells have characteristics of MFO, namely, NADPH-dependence and CO-sensitivity. The fact that the constitutive and induced AHH in FL cells could be inhibited by a known hydroxylase inhibitor 7,8-BF indicated that the AHH in FL cells belongs to the cytochrome P-448 dependent MFO type. After removal of inducer from the medium, the induced AHH activity remained at a high level for at least 24-36 h. By using AHH-induced FL cells in the UDS assay system for the detection of promutagens/procarcinogens, we found that AFB1 and 3-MC did not induce a UDS reaction in uninduced FL cells, while in beta-NF induced cells, 10(-6)-10(-4) M AFB1 and 10(-7)-10(-6) M 3-MC elicited a very significant UDS reaction, which was concordant with the results obtained in the UDS assay system using HeLa cells or FL cells supplemented with liver microsomes or using primary cultured hepatocytes as indicator cells. B(a)P elicited the UDS reaction at concentrations of 10(-6)-10(-3) M in beta-NF induced cells, whereas 10(-4)-10(-3) M was required in uninduced cells. The results above indicate that this new design is feasible, but further study is needed to assure its accuracy.

Cell cycle effects on the basal and DNA-damaging-agent-stimulated ADPRT activity in cultured mammalian cells

ADP-ribosyl transferase (ADPRT) is a DNA-dependent chromatin-associated enzyme which covalently attaches ADP-ribose moieties derived from NAD+ to protein acceptors to form poly(ADP-ribose). ADPRT activity is strongly stimulated by breaks in DNA, and it is suggested that its activity is required for efficient DNA excision repair. In this paper, a cell-cycle-dependent fluctuation of basal ADPRT activity was demonstrated by measuring it in permeabilized FL cells. The cell used was subjected to arginine starvation for 48 h before being released from the block by replacement of deficient medium with complete medium and cells in different proliferating stages were traced by [3H]TdR pulse labelling and obtained at different intervals after block release. The peak basal ADPRT activity appeared 4-6 h after the appearance of the peak of DNA synthesis. After treating the cells with MNNG (10(-4) M), MMS (10(-3)-10(-4) M) and 4NQO (10(-5) M) for 90 min just after release of the block, the ADPRT activity was markedly stimulated. It was further demonstrated that the effects of MNNG/4NQO and cell cycle influence on the level of poly(ADP-ribose) synthesis appear to be additive. While concerning MMS, quite a different pattern of ADPRT stimulation in the cell cycle was demonstrated, i.e., the activity of ADPRT stimulation of 10(-3) M MMS was found to be completely dependent on the basal ADPRT activity. In the cells with the highest basal ADPRT activity 12 h after block release, the MMS-induced ADPRT stimulation could not be observed. It was suggested that more than one pathway might be present in ADPRT stimulation induced by DNA-damaging chemicals, and the cells synchronized in late G1 stage might be the most suitable for demonstrating poly(ADP-ribose) synthesis after DNA damage.

Genotoxic activity of caramel on Salmonella and cultured mammalian cells

The genetic activity of 2 commercial caramel preparations, manufactured either by heating the malt sugar solution directly (non-ammoniated caramel) or by heating it with ammonia (ammoniated caramel) was studied in the Salmonella mutagenicity test and UDS assay in cultured mammalian cells. The non-ammoniated caramel was found to be mutagenic to S. typhimurium TA100, while the ammoniated one was genetically active in all the tester strains used, namely TA100, TA97 and TA98. It was also demonstrated that non-ammoniated caramel was capable of inducing UDS in cultured human amnion FL cells, but for the ammoniated one, no such activity was observed. Furthermore, based on the results obtained in the DNA synthesis inhibition assay, it was suggested that the DNA synthesis inhibition seen in our experiments with the ammoniated caramel was probably not of DNA damage in origin. These data indicate that the mutagenic fractions formed during ammoniated and non-ammoniated caramelization were quite different.