Effect of varying the exposure and 3H-thymidine labeling period upon the outcome of the primary hepatocyte DNA repair assay

Evaluation studies on the in vitro rat hepatocyte micronucleus assay

Based on a previous study with 8 chemicals (Müller et al., 1993) the applicability of the in vitro rat hepatocyte micronucleus assay was evaluated by testing a further 21 compounds of different chemical classes. The obtained results are in good agreement with the known genotoxic profiles of about 90% of the in total tested compounds. Several known mutagens and carcinogens, i.e., alkylating agents, aromatic amines, nitrosamines, nitro compounds, cross-linking agents, and pyrrolizidine alkaloids gave clear positive results in this assay, whereas all of the tested non-carcinogens were negative. The hepatocyte micronucleus assay was shown to distinguish between carcinogenic/non-carcinogenic isomers, such as 2- and 4-acetylaminofluorene (AAF) and 2- and 1-nitropropane (NP). Furthermore, the non-genotoxic nature of several hepatocarcinogens, i.e., the peroxisome proliferating agents fenofibrate, nafenopin, Wy-14,643, diethyl(hexyl)phthalate (DEHP), and the sedative phenobarbital, could be confirmed in this assay. The hepatocarcinogen coumarin exerted mitogenic but no mutagenic properties in the rat hepatocyte micronucleus assay. This compound may act as a liver tumor promoter. Benzo[a]pyrene (B[a]P) and 7,12-dimethylbenzanthacene (DMBA), both belonging to the group of known carcinogenic and mutagenic polycyclic aromatic hydrocarbons, failed to induce micronucleus formation in rat hepatocytes. The high susceptibility of in vitro proliferating hepatocytes to mitotic inhibition, exerted by the strong cytotoxic actions of these compounds, seems to be responsible for these negative results. A strongly reduced mitotic activity can prevent the formation of micronuclei, even when clastogenic effects may have occurred. In the present stage, the in vitro rat hepatocyte micronucleus assay cannot be recommended for screening genotoxicity testing. It should rather be used for special purposes, e.g., when liver-specific mutagenic effects are expected.

Validation of a flow cytometric in vitro DNA repair (UDS) assay in rat hepatocytes

An in vitro flow cytometric (FCM) DNA repair assay has been developed and validated by comparison to conventional autoradiography (ARG). Both assays measure unscheduled DNA synthesis (UDS). Cultures of hepatocytes from young male Sprague-Dawley rats were exposed to a battery of 26 chemicals plus bromodeoxyuridine (BrdUrd) or 3H-thymidine (3H-dT) for 18-20 h before harvest. Selection of test chemicals was based upon both their genotoxicity classifications and carcinogenicity bioassay results in male rats. DNA repair in chemically treated cultures was detected flow cytometrically by measuring the uptake of BrdUrd in non-replicating (G1, G2, mitotic and 4C) cells. Intracellular levels of incorporated BrdUrd were visualized by immunochemical labeling with fluorescein isothiocyanate (FITC), and total cellular DNA content was simultaneously estimated by counterstaining samples with the nucleic acid intercalator, propidium iodide (PI). Information was obtained from 10(4) cells/sample. Since repairing cells incorporate significantly less BrdUrd per unit of time than replicating cells, low intensity BrdUrd-FITC fluorescent signals from repairing cells are readily discriminated from high intensity signals from replicating cells when displayed on linear univariate histograms. Further distinction between repairing and replicating cells was achieved by displaying the DNA contents of all cells on linear bivariate histograms. Thus, repairing cells were resolved without subjecting these cultures to agents which suppress replicative synthesis (e.g., hydroxyurea). Results from these concurrent FCM and ARG investigations include the following: (1) conclusions (DNA repair positive or negative) were in agreement, with one exception, cinnamyl anthranilate, for which cytotoxic doses produced a positive FCM response, but lack of intact hepatocytes in parallel ARG preparations prevented analysis; (2) similar sensitivities for most of the positive chemicals were reported; (3) a high correlation (85%) exists between the reported genotoxicity classification and these DNA repair results in the absence of overt cytotoxicity; (4) a poor correlation exists between these DNA repair results and hepatocarcinogenesis (only 4/11 liver carcinogens tested positive) or overall carcinogenesis in the male rat (only 9/21 carcinogens tested positive). This FCM assay provides a rapid, sensitive, safe and reliable means of identifying agents which induce DNA repair in mammalian cells.

A short-term test for nucleotoxicity that uses chick embryo cells treated in vitro and in vivo--physico-chemical and biochemical investigations

1. In vitro, some nucleotoxic or potentially nucleotoxic agents were tested with freshly isolated liver and brain cells from chick embryos as well as with thymic cells of the rat. 2. In vivo, chicken embryos were exposed to X-rays, methyl methanesulfonate, methyl nitrosourea, triethylene thiophosphoramide, and dimethylnitrosamine. 3. The toxic effects were determined by viscometry of alkaline cell lysates, nucleoid sedimentation, scheduled (SDS) and unscheduled (UDS) DNA synthesis and/or RNA synthesis. 4. The dose-effect curves obtained in vitro show that directly acting genotoxic agents are detected by the embryonic cells with equal or comparable sensitivity as by mammalian cells. 5. In vivo, genotoxicity is reflected by a decrease in alkaline lysate viscosity, nucleoid sedimentation and SDS and an increase in UDS. 6. From the present results it is suggested that chick embryo cells offer a simple, rapid and inexpensive short-term nucleotoxicity test for directly acting agents, the main disadvantage being the innate inability to biotransform indirectly acting agents.

Characterization of the in vitro unscheduled DNA synthesis assay in primary lung cells of the rat

The in vitro unscheduled DNA synthesis (UDS) assay has been evaluated in rat primary lung cells with known genotoxicants. The autoradiographic method was employed to detect UDS in both alveolar macrophages and primary pulmonary cells. Data of a time course study revealed that a high radioactive labeling of DNA repair was achieved after a 16-h incubation with [3H]thymidine. Coupled with low serum (1%), hydroxyurea at the concentration of 20 mM inhibited regular DNA synthesis in primary lung cells in a satisfactory manner (81-88% inhibition). With this protocol, a dose-related increase in UDS was induced by N-methyl-N'-nitro-N-nitrosoguanidine and 2-aminoanthracene in both rat alveolar macrophages and primary lung cells. The results suggest that primary rat lung cells in culture possess DNA-repair ability and that the UDS assay may be useful for assessing the pulmonary genotoxic effect of chemicals in this cell system.

Structure-activity relationships in the rat hepatocyte DNA-repair test for 300 chemicals

312 chemicals/mixtures were tested for genotoxicity in the rat hepatocyte/DNA-repair test. A variety of structure-activity relationships was evident. Of the 309 pure chemicals, 142 were positive. Of these, 43 were judged by IARC to have sufficient or limited evidence of carcinogenicity and none of the remainder was a proven noncarcinogen. Among the 167 negative chemicals, 44 were carcinogens. Some of these are known to be genotoxic in other systems, but based on several lines of evidence, many are considered to be epigenetic carcinogens that lack the ability to react with DNA and rather lead to neoplasia by nongenotoxic mechanisms.