Detection of chemical carcinogens by means of the "rat liver foci bioassay"

A medium-term, rapid rat bioassay model for the detection of carcinogenic potential of chemicals

The Ito Liver Model and the Ito Multi-organ Model are used in conjunction and constitute an efficient and rapid bioassay for the identification of both genotoxic and nongenotoxic carcinogenic chemicals. The Ito Liver Model is an 8-week bioassay system that uses the number and size of foci of hepatocytes positive for glutathione S-transferase placental form (GST-P) as the end-point marker. One hundred fifty-nine compounds were tested using the Ito Liver Model: 61 of 66 hepatocarcinogens tested positive, and 10 of 43 nonliver carcinogens were also positive. The false-positive detection of noncarcinogens was low; a single false-positive result was obtained from the 50 noncarcinogens tested. Since more than half of all known carcinogens are hepatocarcinogens in rodents, the initial 8-week bioassay is able to detect most carcinogens. The Ito Multi-organ Model is a 28-week bioassay system for the detection of carcinogens that were not identified by the Ito Liver Model. Results are evaluated by preneoplastic and neoplastic lesions in major organs. Forty-four compounds were tested using the Ito Multi-organ Model: 17 out of 17 liver carcinogens were positive, and 19 out of 22 (86%) nonliver carcinogens were positive. None of the five noncarcinogens tested positive.

A medium-term rat liver bioassay for rapid in vivo detection of carcinogenic potential of chemicals

A reliable medium-term bioassay system for rapid detection of carcinogenic potential of chemicals in the human environment has been developed. The 8-week-protocol consists of 2 stages; male F344 rats are given a single intraperitoneal injection of diethylnitrosamine (200 mg/kg) for initiation of liver carcinogenesis, followed by a 6-week test chemical treatment starting 2 weeks thereafter. Test chemicals are usually given in the diet or the drinking water and in the 2nd week of test chemical treatment, all rats are subjected to two-thirds partial hepatectomy in order to induce regenerative cell replication. The end-point marker is the glutathione S-transferase placental form (GST-P)-positive hepatic focus, the numbers and sizes of which are analyzed using an image-analyzer and expressed as values per unit liver section (1 cm2). When the yield of GST-P-positive foci is significantly enhanced (P<0.05) over the control value, a chemical is judged to possess carcinogenic or promotion potential for the liver. Among 313 chemicals already tested in this system in our laboratory, 30/31 (97%) mutagenic hepatocarcinogens and 29/33 (88%) non-mutagenic hepatocarcinogens gave positive results. Ten out of 43 (23%) agents known to be carcinogenic in organs other than the liver were also positive. It is particularly important that only one of 48 non-carcinogens gave a very weak positive result, so that the system has a very low false-positivity rate. It is now well documented that the assay system is highly effective for detecting hepatocarcinogens, bridging the gap between traditional long-term carcinogenicity tests and short-term screening assays. At the Fourth International Conference on Harmonization, our medium-term liver bioassay based on an initiation and promotion protocol was recommended in the guidelines as an acceptable alternative to the long-term rodent carcinogenicity test.

A medium-term rat liver bioassay as a rapid in vivo test for carcinogenic potential: a historical review of model development and summary of results from 291 tests

A bioassay system for rapid detection of carcinogenic agents has been developed using male Fischer 344 rats to bridge the gap between long-term carcinogenicity tests and short-term screening assays. The system, called the medium-term liver bioassay, is fundamentally based on the 2-stage hypothesis of tumor production, employing initiation by diethylnitrosamine (200 mg/kg, i.p.) in the first stage and test chemical administration during the second, in combination with two-thirds partial hepatectomy. It requires only 8 wk for animal experimentation and a further few weeks for quantitative analysis of immunohistochemically demonstrated glutathione S-transferase placental form positive hepatic foci. A total of 291 chemicals/substances have already been analyzed in our laboratory. Among 63 chemicals that were proved to be carcinogenic in the liver of rat and/or mouse, 57 (90%) gave positive results irrespective of their mutagenicity. Negative compounds include peroxisome proliferators and tamoxifen. Even nonhepatocarcinogens were positive at a rate of 24%. Eighty-six percent (12/14) of mouse liver carcinogens were also positive. On the other hand, only 2 out of 45 noncarcinogens showed very weak positivity. Thus, the efficacy of the system for hepatocarcinogens has been well established. This bioassay is increasingly regarded as an appropriate alternative test for carcinogenicity risk assessment and is practically used for a rapid evaluation of hepatocarcinogenicity of chemicals.

Lack of tumor-promoting effects of flavonoids: studies on rat liver preneoplastic foci and on in vivo and in vitro gap junctional intercellular communication

Possible tumor-promoting activity of four flavonoids, quercetin (QC), tangeretin (TG), flavone (FO), and flavanone (FN), was examined in a rat liver short-term carcinogenesis assay as well as with in vivo and in vitro assays of inhibition of gap junctional intercellular communication (GJIC). Rat hepatocarcinogenesis was induced by aflatoxin B1 treatment followed by a selection phase (2-acetylaminofluorene treatment and partial hepatectomy), then treatment with or without test chemicals (in vivo studies of antipromotion were not performed). Using glutathione S-transferase placental form (GST-P)-positive foci, we compared the effects of flavonoids (at 1,000 ppm in the diet) with the effects of phenobarbital (PB) on the occurrence of liver preneoplastic lesions. In addition, we studied the effects of flavonoids on GJIC in the livers derived from these experiments and in two types of cultured cells. No significant difference in the number and area of GST-P-positive foci was found after one or three months of treatment between any flavonoid group and control group. In the positive control group, PB markedly increased the numbers and areas of preneoplastic lesions at three months. Whereas PB also decreased by 60% the average size of lucifer yellow dye spread in slices of liver parenchyma free of preneoplastic lesions among the different flavonoids, only TG decreased the dye transfer in vivo: by 30% at one month and 50% at three months. With the dye transfer assay applied to a rat liver epithelial cell line (REL) and the Chinese hamster V79 metabolic cooperation assay, none of the tested flavonoids (< or = 25 microM) inhibited GJIC. Conversely, protective properties were seen for some of the compounds in antipromotion in vitro studies, because TG and FN enhanced the dye transfer in REL cells and FO, TG, and QC partly prevented the inhibition of metabolic cooperation by 12-O-tetradecanoylphorbol-13-acetate. Thus, taken together, our results suggest that QC, FO, and FN do not show tumor-promoting activity. Concerning TG, some discrepancies in the in vivo data are observed. Some of them (GJIC inhibition in liver slices) are probably more relevant to promotion of hepatocarcinogenesis.

Hepatocyte growth factor enhancement of preneoplastic hepatic foci development in rats treated with diethylnitrosamine and N-ethyl-N-hydroxyethylnitrosamine

Effects of hepatocyte growth factor were investigated in a two-stage rat liver carcinogenesis protocol. Male F344 rats were first treated with diethylnitrosamine (200 mg/kg, i.p.) and then, starting two weeks later, with N-ethyl-N-hydroxyethylnitrosamine (EHEN) for 6 weeks at a dose of 0.01% in drinking water. Hepatocyte growth factor, which was injected i.v. at a dose of 200 micrograms/kg body weight one (at week 3) or two times (at weeks 3 and 4) during EHEN administration, significantly increased the development of preneoplastic glutathione S-transferase placental form-positive foci. Although the observed effects of hepatocyte growth factor were weaker than that of the two-thirds partial hepatectomy (PH) performed at week 3, the present results suggest that the enhancing effects of PH performed during the promotion stage may be largely mediated through induction of hepatocyte growth factor.

Effect of ingestion of 20 pesticides in combination at acceptable daily intake levels on rat liver carcinogenesis

Exposure of agricultural workers and the general population to pesticides is a major concern, and possible summation or synergistic effects deserves particular attention. In this study, however, combined dietary administration of 19 organophosphorus compounds and one organochlorine pesticide, each at acceptable daily intake (ADI) levels, did not enhance rat liver preneoplastic lesion development initiated by diethylnitrosamine. In contrast, a mixture of 100 times ADI significantly increased the number and area of lesions. The results thus provide direct support for the present safety factor approach to the quantitative hazard evaluation of pesticides.

C-raf expression in early rat liver tumorigenesis after promotion with polychlorinated biphenyls or phenobarbital

1. The expression of c-raf protooncogene in early stages of chemically induced rat liver tumorigenesis was studied in weanling female and adult male Sprague-Dawley rats. After initiation with diethylnitrosamine, promotion by polychlorinated biphenyls (PCBs) or phenobarbital (PB) was studied in the female. Male rats were promoted with PCBs only. 2. The incidence of enzyme-altered foci was evaluated histochemically by demonstrating a deficiency in adenosine-5'-triphosphatase and the emergence of gamma-glutamyl-transpeptidase. C-raf expression was measured in liver tissue containing preneoplastic foci, and in small (< 3 mm in diameter) and large (> 3 mm in diameter) neoplastic nodules up to 36 weeks. 3. Foci numbers amounted to 60-70 per cm2 liver section with both histochemical markers and both promoters in female rats. In male rats foci numbers were about 20-40 per cm2 liver section with both markers and with PCBs as promoting agents. Foci area developed more rapidly in female rats. 4. Small and large nodules were found in females during the entire observation period with both promoting agents, PCBs being more effective than PB. C-raf expression in nodules was increased up to 10-fold in PCB-treated animals compared with untreated controls. No dependence on the size of the nodules was seen. In male rats nodule incidence was very low and c-raf induction was marginal. 5. In conclusion, c-raf proto-oncogene expression correlated with the incidence of foci and nodules, female rats being more sensitive than males.

Lack of promoting activity of 7-methoxy-2-nitro-naphtho-[2,1-b]furan (R7000) in a medium-term rat liver bioassay

Among the nitro-naphthofurans, 7-methoxy-2-nitro-naphtho[2,1-b]furan (R7000) has proved to be a very potent mutagen that causes sarcomas at the subcutaneous injection site and carcinomas in the forestomach after p.o. administration. In the present study, possible promoting activity for diethylnitrosamine (200 mg/kg, i.p.)-initiated liver carcinogenesis was assessed in rats. R7000 dissolved in 5% ethanol solution at doses of 10 or 50 mg/l given as the drinking water for 6 weeks did not enhance development of pre-neoplastic glutathione S-transferase placental form positive foci. Thus, it was concluded that R7000 may not be carcinogenic for the rat liver under the present experimental conditions.