Measurement of unscheduled DNA synthesis and S-phase synthesis in rodent hepatocytes following in vivo treatment: testing of 24 compounds.

The in vivo-in vitro hepatocyte DNA repair assay has been shown to be useful for studying genotoxic hepatocarcinogens. In addition, measurement of S-phase synthesis (SPS) provides an indirect indicator of hepatocellular proliferation, which may be an important mechanism in rodent carcinogenesis. This assay was used to examine 24 chemicals for their ability to induce unscheduled DNA synthesis (UDS) or SPS in Fischer-344 rats or B6C3F1 mice following in vivo treatment. Hepatocytes were isolated by liver perfusion and incubated with 3H-thymidine following in vivo treatment by gavage. UDS was measured by quantitative autoradiography as net grains/nucleus (NG). Controls from both sexes of both species yielded less than 0.0 NG. Chemicals chosen for testing were from the National Toxicology Program (NTP) genetic toxicology testing program and most were also evaluated in long-term animal studies conducted by the NTP. 11-Aminoundecanoic acid, benzyl acetate, bis(2-chloro-1-methylethyl)ether (BCMEE), C.I. Solvent Yellow 14, cinnamaldehyde, cinnamyl anthranilate, dichloromethane, dichlorvos, glutaraldehyde, 4,4'-methylenedianiline (MDA), 4-nitrotoluene, 4,4'-oxydianiline, a polybrominated biphenyl mixture (PBB), reserpine, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, trichloroethylene, and 2,6-xylidine all failed to induce UDS in rats and/or mice. Dinitrotoluene and Michler's Ketone induced positive UDS response in rat, while N-nitrosodiethanolamine and selenium sulfide induced equivocal UDS results in mouse and rat, respectively. BCMEE, bromoform, chloroform, PBB, 1,1,2-trichloroethane, and trichloroethylene were all potent inducers of SPS in mouse liver, while C.I. Solvent Yellow 14, and 1,1,2,2-tetrachloroethane yielded equivocal SPS results in rat and mouse, respectively. These results indicate that most of the test compounds do not induce UDS in the liver; however, the significant S-phase responses induced by many of these compounds, especially the halogenated solvents, may be an important mechanism in their hepatocarcinogenicity.

Examination of genotoxicity, toxicity and morphologic alterations in hepatocytes following in vivo or in vitro exposure to methapyrilene.

The antihistamine methapyrilene (MP) was widely used as a component of cold, allergy and sleep-aid medications in the 1970s until it was identified as a potent rat liver carcinogen. MP does not induce positive responses in most short-term genotoxicity assays, which suggests that it is carcinogenic by a non-genotoxic mechanism. We have evaluated the potential of MP to induce unscheduled DNA synthesis (UDS), a genetic end point and S-phase synthesis (SPS), and indicator of cell proliferation, in Fischer-344 (F344) rat and B6C3F1 mouse liver. We also examined the response of MP in hepatocytes from two species treated in vitro. MP failed to induce UDS in rat or mouse liver following in vivo treatment, or in hepatocytes from rat and adult human treated in vitro. Control rats and mice yielded less than 0.3% of cells in S-phase (%S). In contrast, MP induced significant elevations in SPS both in male F344 rat (6.3%S) and female B6C3F1 mice (1.4%S). In the male rat, sorbitol dehydrogenase (SDH), bilirubin, serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvate transaminase (SGPT) showed elevations of 9-, 10-. 17- and 28-fold over controls respectively, indicating that significant hepatotoxicity was induced by MP. This was confirmed by histopathologic examination, which revealed significant periportal and focal necrosis followed by an increased presence of mitotic figures. These results indicate that MP is not genotoxic in rat liver, but is a potent inducer of hepatic cell proliferation by inducing toxicity and subsequent regeneration, which may be an important mechanism of hepatocarcinogenesis.

Induction of hepatic cell proliferation and unscheduled DNA synthesis in mouse hepatocytes following in vivo treatment.

We have modified the in vivo-in vitro hepatocyte DNA repair assay for measurement of unscheduled DNA synthesis (UDS) and hepatic cell proliferation in B6C3F1 mice. Dimethylnitrosamine and methylmethane sulfonate produced significant increases in UDS in both rats and mice. 2-Acetylaminofluorene induced a significant increase in UDS in rats, but not in mice. The mouse hepatocarcinogens, carbon tetrachloride, trichloroethylene, polybrominated biphenyls and 2,6-dichloro-p-phenylenediamine all failed to induce UDS in male and female mice, but all induced significant elevations in hepatic cell proliferation. Increased cell turnover in the liver may therefore be an important mechanism in hepatocarcinogenicity in the mouse.

Measurement of unscheduled DNA synthesis in rat kidney cells following in vivo treatment with genotoxic agents.

The kidney is a key target tissue in animal and human carcinogenesis, yet there are no established short-term tests for studying the genotoxicity of chemicals in the kidney. We have developed an assay for the measurement of chemically induced DNA repair as unscheduled DNA synthesis (UDS) in isolated rat kidney cells following in vivo treatment. Male Fischer-344 rats were injected intraperitoneally with chemicals dissolved in saline or corn oil. After various treatment times, the kidneys were perfused with a collagenase/trypsin solution (CTS), minced into small pieces, and stirred in CTS at 37 degrees C for 1 hr to dissociate cells. Cultures contain a high proportion of epithelial cells from the proximal and distal tubules. Cultures were incubated for 16-18 hr with 3H-thymidine in Williams' Medium E supplemented with 20% fetal bovine serum. UDS was measured by quantitative autoradiography as net grains/nucleus (NG). The percentage of cells in repair (% IR) was defined as the percentage of cells with greater than or equal to 3 NG. Saline- or corn oil-injected controls consistently produced -3 to -5 NG with less than 1% IR. The time course of DNA repair following treatment with the direct-acting mutagen methylmethane sulfonate (MMS) or the renal carcinogen azaserine showed a peak response at 2 hr after treatment. Azaserine showed a rapid decline in UDS at 12 and 24 hr, whereas MMS exhibited a relatively high UDS level at 24 hr. The renal carcinogens methylazoxymethanol acetate, N-methyl-N-nitrosourea, and streptozotocin all yielded strong positive UDS responses. The liver and intestinal carcinogen 1, 1-dimethylhydrazine at doses up to 50 mg/kg was cytotoxic to kidney cells, but induced less than 0 NG. Treatment with 1,2-dimethylhydrazine, which induces kidney tumors in mice but not rats, also induced less than 0 NG. Treatment with o-anisidine, a weak renal carcinogen, did not induce UDS in the kidney, suggesting that it may be acting as a tumor promoter. These results demonstrate the usefulness of this assay for the detection and study of a variety of genotoxic kidney carcinogens.

Detection of genotoxic carcinogens in the in vivo-in vitro hepatocyte DNA repair assay.

The in vivo-in vitro hepatocyte DNA repair assay has been shown to be useful in the evaluation of the carcinogenic potential of chemicals. The purpose of this study was to apply this assay to determining the genotoxicity of the compounds from a wide variety of structural classes. Male Fischer-334 rats were treated by gavage or ip injection with compounds dissolved in either corn oil, water, or dimethyl sulfoxide (DMSO). At selected times after treatment, hepatocytes were isolated by liver perfusion and cultured with 3H-thymidine, Unscheduled DNA synthesis (UDS) was measured by quantitative autoradiography as net grains/nucleus (NG); greater than or equal to 5 NG was considered positive. Water, corn oil, or DMSO controls produced -3 to -6 NG with less than or equal to 6% of the cells in repair. All genotoxic hepatocarcinogens tested produced strong positive responses of greater than 15 NG including dimethylnitrosamine, 2-acetylaminofluorene (2-AAF), azoxymethane, 1,2-dimethylhydrazine, benzidine, aflatoxin B1, 2,6-dinitrotoluene, and 2,4-diaminotoluene. The noncarcinogen, 2,6-diaminotoluene, was negative. The mutagen and rat brain carcinogen methyl methanesulfonate (MMS) and the rat pancreatic carcinogen azaserine were also positive. The carcinogens benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene yielded from -2 to -4 NG. This negative response is consistent with their lack of carcinogenic activity in rat liver. MMS produced the greatest amount of UDS 2 hr after treatment whereas 2-AAF did not induce its maximum response until 12 hr post-treatment. The potent hepatotoxin carbon tetrachloride induced a 40-fold elevation in DNA replication 48 hr after a 400 mg/kg dose, but no UDS was observed at 2, 12, 24, or 48 hr post-treatment. The weak hepatocarcinogen safrole induced no UDS suggesting that it is either nongenotoxic or is metabolized to an active form at an extremely slow rate following a single administration. These results demonstrate that this assay is valuable for the detection and study of a variety of genotoxic carcinogens.