Medium-term rat liver bioassay for rapid detection of carcinogens and modifiers of hepatocarcinogenesis

Potential inhibitory effects of D-allose, a rare sugar, on liver preneoplastic lesion development in F344 rat medium-term bioassay

D-allose, the C-3 epimer of d-glucose, is a monosaccharide present in minute quantities in nature and a rare sugar. The effects of D-allose on diethyl nitrosamine (DEN)-induced hepatocarcinogenesis were examined in male F344 rats by a rat medium-term bioassay based on the two-step model of hepatocarcinogenesis (experiment 1). In addition, a DNA microarray analysis was employed to clarify possible mechanisms of action of D-allose (experiment 2). The antioxidation potential of D-allose solution itself or of serum in rats treated with D-allose was also examined directly by measuring Cu(+)-reducing antioxidation power (experiment 3). Furthermore, to investigate the effects of D-allose in vivo under conditions of oxidative stress, it was administered with a choline-deficient, L-amino acid-defined diet (CDAA) in the medium-term liver carcinogenesis bioassay (experiment 4). Experiment 1 demonstrated no effects of D-allose on the development of glutathione S-transferase placental form (GST-P) positive foci in the liver. From DNA microarray analysis, several mRNA markers were found to be altered with functions related to apoptosis and cell proliferation (experiment 2), although D-allose itself and serum in vivo exhibited no antioxidation power directly (experiment 3). When D-allose was administered with the CDAA diet, decreases in the area and number of GST-P positive foci were noted with P values of 0.158 for area (%) and 0.061 for number (/cm(2)) (experiment 4). These results suggest the potential inhibitory effect of D-allose on liver carcinogenesis, particularly under oxidative stress conditions.

Framework for use of toxicity screening tools in context-based decision-making

One of the principal applications of toxicology data is to inform risk assessments and support risk management decisions that are protective of human health. Ideally, a risk assessor would have available all of the relevant information on (a) the toxicity profile of the agent of interest; (b) its interactions with living systems; and (c) the known or projected exposure scenarios: to whom, how much, by which route(s), and how often. In practice, however, complete information is seldom available. Nonetheless, decisions still must be made. Screening-level assays and tools can provide support for many aspects of the risk assessment process, as long as the limitations of the tools are understood and to the extent that the added uncertainty the tools introduce into the process can be characterized and managed. Use of these tools for decision-making may be an end in itself for risk assessment and decision-making or a preliminary step to more extensive data collection and evaluation before assessments are undertaken or completed and risk management decisions made. This paper describes a framework for the application of screening tools for human health decision-making, although with some modest modification, it could be made applicable to environmental settings as well. The framework consists of problem formulation, development of a screening strategy based on an assessment of critical data needs, and a data analysis phase that employs weight-of-evidence criteria and uncertainty analyses, and leads to context-based decisions. Criteria for determining the appropriate screening tool(s) have been identified. The choice and use of the tool(s) will depend on the question and the level of uncertainty that may be appropriate for the context in which the decision is being made. The framework is iterative, in that users may refine the question(s) as they proceed. Several case studies illustrate how the framework may be used effectively to address specific questions for any endpoint of toxicity.

Aged garlic extract inhibits development of putative preneoplastic lesions in rat hepatocarcinogenesis

A unique garlic preparation, aged garlic extract (AGE), was examined for its modifying effect on diethylnitrosamine (DEN)-induced neoplasia of the liver in male F344 rats, using the medium-term bioassay system based on the 2-step model of hepatocarcinogenesis. Carcinogenic potential was scored by comparing the numbers and areas of induced glutathione S-transferase placental form (GST-P)-positive hepatocellular foci. GST-P-positive foci were significantly decreased in rats treated with AGE at doses of 2, 5, and 10 mL/kg, i.g., 5 times per week during the promotion phase. In addition, to clarify the mechanism underlying the inhibitory effect of AGE, the effect of AGE on hepatocellular proliferation was evaluated using partially hepatectomized rats as a liver-regeneration model. The bromodeoxyuridine-labeling indices in the livers of the AGE group were significantly lower than those in the control group at 24 h, the maximum proliferation period after partial hepatectomy. These findings indicate that AGE inhibited the development of putative preneoplastic lesions in rat hepatocarcinogenesis, involving a slowing in the proliferation rate of liver cells after partial hepatectomy.

O-phenylphenol and its sodium and potassium salts: a toxicological assessment

Ortho-phenylphenol (OPP) and its sodium (SOPP) and potassium (POPP) salts are used as fungicides and disinfectants. Due to the widespread use of especially OPP and SOPP, the potential for consumer exposure and some "critical" findings the toxicological database is quite extensive and complex. In experimental animals toxicity after single oral and dermal administration of these compounds is low. For the skin and mucous membranes, OPP has to be considered as irritating, and SOPP and POPP as corrosive. A large number of chronic toxicity and reproduction studies did not show any indication of oestrogen-like or other endocrine effects of OPP in the mammalian organism. No teratogenic effects were observed after the administration of OPP or SOPP in rats, mice, and rabbits. In two-generation studies in rats, OPP did not affect reproduction. The available data do not suggest a relevant potential for immunotoxic properties. The administration of high dietary concentrations of OPP to mice up to 2 years induced hepatocellular changes indicative of adaptations to metabolic demands, zonal degeneration, focal hepatocellular necrosis, and/or pigmentation of the liver. Only in male mice of one study, using a strain prone to develop hepatocellular tumors at high spontaneous incidences, the incidence of hepatocellular adenomas was increased. The incidence of hepatocellular carcinomas was not affected by treatment. The urothel of the urinary bladder (at very high doses also of the renal pelvis and the papilla) is the main target tissue after the repeated oral exposure of rats. The changes initially consist of increased mitosis, followed by simple epithelial hyperplasia, developing to a papillary and/or nodular form, later on to papillomas and transitional carcinomas. Crystals or stones in the bladder do not play a decisive role in this cascade. SOPP is more effective than OPP in this respect. Male rats are much more sensitive than females. In mice, hamsters, guinea pigs, and dogs, urothelial lesions do not develop even at very high oral dose levels. The findings in rats explain why there is a large genotoxicity/mutagenicity data base not only for OPP and SOPP but also for their metabolites on nearly all kinds of endpoints/targets. The weight of evidence suggests that genotoxicity of OPP/SOPP or their metabolites does not play a decisive role for the carcinogenicity at the urothel. Among them are lack of DNA binding of OPP to the rat bladder epithelium, the differences between OPP and SOPP, between male and female rats, between rats and mice (despite roughly comparable toxicokinetics), as well as the fact that tumors develop only at dose levels inducing hyperplasias. In addition, the strong dependence of the incidence and severity of the nonneoplastic and neoplastic bladder changes on urinary pH values (modified by feeding of ammonium chloride or sodium hydrogen carbonate) is consistent with the hypothesis of a nongenotoxic mode of action. Finally, there is no correlation between the urinary concentration of OPP or its metabolites and the incidence of hyperplasias/tumors in the urinary bladder. Both tumorigenic effects in rats and male mice are considered to represent high-dose, sex- and/or species-specific phenomena, based on nongenotoxic mechanisms of action and therefore allow the conclusion that the conventional margin of safety approaches are appropriate when assessing the risk of applications of OPP and its salts.

Presence of a no-observed effect level for enhancing effects of development of the alpha-isomer of benzene hexachloride (alpha-BHC) on diethylnitrosamine-initiated hepatic foci in rats

The dose dependence of the promoting effects of the alpha-isomer of benzene hexachloride (alpha-BHC) on hepatocarcinogenesis was investigated in a medium-term rat liver bioassay (Ito test). A total of 195 F344 male rats, 6 weeks old, were given a single intraperitoneal injection of diethylnitrosamine (DEN) at the start of the experiment and subjected to two-thirds partial hepatectomy at week 3. Two weeks after the administration of DEN, alpha-BHC were fed to rats at doses of 0, 0.01, 0.1, 0.5, 1, 2, 4, 7.5, 15, 30, 60, 125 and 500 ppm in diet for 6 weeks. All surviving animals were killed at week 8, and their livers were examined immunohistochemically for detection of glutathione S-transferase placental form (GST-P)-positive foci, surrogate preneoplastic lesions. Quantitative values for numbers and areas were dose-dependently increased in rats given alpha-BHC at 0.5-500 ppm. However, those for groups treated with 0.01 and 0.1 ppm were decreased, albeit not significantly in comparison to the controls. Cytochrome P450 3A2 (CYP3A2) protein levels and activities showed a good correlation to the number and area of GST-P-positive foci. These results support evidence of hormesis and indicate a no-observed effect level for alpha-BHC promoting potentials may exist regarding rat liver carcinogenesis, which correlates with expression of CYP3A2 in the liver.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Inhibitory effect of chlorophyllin on diethylnitrosamine and phenobarbital-induced hepatocarcinogenesis in male F344 rats

Modifying effects of chlorophyllin (CHL) on the diethylnitrosamine (DEN)-phenobarbital (PB) hepatocarcinogenesis model were examined in rats. Five-week-old male F344 rats were divided into 8 groups. Groups 1 through 5 were given i.p. injections of DEN (100 mg/kg body weight) once a week for 3 weeks beginning one week after the start of the experiment, while groups 6 through 8 received vehicle treatment. Groups 1, 2, 3 and 7 received drinking water with 500 ppm PB from one week after the end of carcinogen or vehicle treatment. CHL-containing diet (2000 ppm) was given to group 2 during the initiation phase and to groups 3 and 5 during the promotion and the post-initiation phase, respectively. Group 6 was given the experimental diet alone throughout the experiment (24 weeks). Liver neoplasms were present in DEN-treated groups and PB treatment promoted liver tumorigenesis. The incidences of adenoma in groups 2 and 3 were significantly smaller than in group 1 (P<0.05 and P<0.02), although the reductions in the incidences of liver cell cancer were not significant. The average numbers of liver neoplasms/rat in group 2 were significantly smaller than in group 1 (P<0.05-P<0.005). Glutathione S-transferase placental form-positive foci were also significantly decreased by CHL treatment (P<0.05 and P<0.001). DEN and PB exposure increased liver ornithine decarboxylase activity and this increase was significantly inhibited by feeding of CHL during the initiation phase (P<0.001). These results suggest that CHL is a chemopreventive agent for liver neoplasia.