Growth characteristics and Ha-ras mutations of cell cultures isolated from chemically induced mouse liver tumours

On the Mechanism of Hepatocarcinogenesis of Benzodiazepines: Evidence that Diazepam and Oxazepam are CYP2B Inducers in Rats, and both CYP2B and CYP4A Inducers in Mice

The aim of this study was to evaluate diazepam and oxazepam as cytochrome P450 inducers at doses previously shown to cause liver tumors in mice but not rats. In rats, diazepam and oxazepam induced CYP2B, and were as effective as phenobarbital despite lacking phenobarbital's tumor-promoting effect in rats. In mice, diazepam and oxazepam induced both CYP2B and CYP4A at dietary doses associated with liver tumor formation. It remains to be determined why diazepam and oxazepam induce CYP4A in mice but not rats and whether this difference accounts for the apparent species difference in the tumor-promoting activity of diazepam and oxazepam.

The effect of fenbuconazole on cell proliferation and enzyme induction in the liver of female CD1 mice

Fenbuconazole, a triazole fungicide, has been associated with an increase in the incidence of liver adenomas in female mice following long-term dietary exposure. The aim of this study was to evaluate whether the mode of action for liver tumor formation by fenbuconazole is similar to that of phenobarbital. Treatment of CD1 mice with 0, 20, 60, 180 or 1300 ppm fenbuconazole for up to 4 weeks caused a dose-dependent increase in liver weight that was associated with centrilobular hepatocellular hypertrophy, cytoplasmic eosinophilia and panlobular hepatocellular vacuolation, as well as an initial increase in the cell proliferation labeling index. Fenbuconazole also caused a dose-dependent increase in liver microsomal cytochromes b(5) and P450 and the levels of immunoreactive CYP2B10 and its associated activity 7-pentoxyresorufin O-dealkylation (PROD). Treatment of mice with 1000 ppm phenobarbital elicited the same effects as treatment of mice with 1300 ppm fenbuconazole, except that phenobarbital was more effective than fenbuconazole at inducing PROD activity, even though fenbuconazole induced CYP2B10 to the same extent as did phenobarbital. This difference was attributed to the ability of fenbuconazole to bind tightly to CYP2B10 and partially mask its catalytic activity in liver microsomes, which is characteristic of several azole-containing drugs. All hepatocellular changes and induced enzyme activity returned to control levels within 4 weeks of discontinuing treatment with fenbuconazole or phenobarbital, indicating that the observed changes were fully reversible. We conclude that fenbuconazole is a phenobarbital-type inducer of mouse liver cytochrome P450, and the mode of action by which fenbuconazole induces liver adenomas in mice is similar to that of phenobarbital.

Paradoxical effects of phenobarbital on mouse hepatocarcinogenesis

Phenobarbital was the first tumor promoter for rodent liver to be associated with the 2-stage or initiation-promotion concept of carcinogenesis. In rats and mice preinitiated with genotoxic carcinogens, phenobarbital administration increases the number of hepatocellular tumors by approximately 5-fold despite its nongenotoxicity. However, in mice phenobarbital occasionally exhibits strong inhibitory effects on hepatocarcinogenesis initiated with the potent carcinogen diethylnitrosamine. Both positive and negative effects of phenobarbital on hepatocytic proliferation and apoptosis, which are mechanistically involved in the promotion stage of hepatocarcinogenesis, have been described. These complex outcomes of phenobarbital treatment and their effects on hepatocarcinogenesis in mice raise serious issues regarding extrapolation of experimental data from laboratory animals to human risk assessment. Recent work suggests that the paradoxical actions of phenobarbital on hepatocarcinogenesis can be understood by consideration of qualitative diversity in initiated lesions and differential responses to promotion stimulus.

Oncogenicity studies of piperonyl butoxide in rats and mice

1. The oncogenicity of Piperonyl butoxide (PBO) has been studied in the mouse and rat. CD-1 mice were administered PBO in the diet at target doses of 0, 30, 100 and 300 mg/kg/day for 79 weeks and Sprague-Dawley rats 0, 30, 100 and 500 mg/kg/day for 104/105 weeks. 2. At termination of the study in the mouse there was evidence of increased liver weights and an increased incidence of eosinophilic adenomas at 100 and 300 mg/kg/day in males and 300 mg/kg/day in females. 3. In rats there was increased liver weights at 100 and 500 mg/kg/day associated with hepatocyte hypertrophy in both male and female rats. There was no increased incidence of neoplasia at any site. Hypertrophy and hyperplasia of thyroid follicles was observed at 500 mg/kg/day in both sexes. 4. The observations reflect the expected changes related to the induction of the mixed function oxygenase group of enzymes. In the mouse the increased incidence of eosinophilic adenomas is not considered relevant for human risk evaluation.

A review of the hepatic tumors related to mixed-function oxidase induction in the mouse

Mixed-function oxidase (MFO) induction in the mouse liver results in a rapid and sustained centrilobular hypertrophy associated with a hyperplastic response. In many studies, the long-term sequela of prolonged exposure is an increased incidence of lesions considered to be adenomas. Studies have shown in aged control mice that the burden of adenomas usually consists of lesions with basophilic cytoplasic staining and a uniform population of hepatocyte nuclei. With long-term feeding of MFO inducers, there is an additional burden of lesions diagnosed as adenomas having a different histological appearance with increased eosinophilic cytoplasm and pleomorphic nuclei. The incidence of hepatocarcinomas usually is not modified by the increased incidence of eosinophilic adenomas. Studies into the behavior of the eosinophilic lesions show that the hepatocytes approximate in their behavior to normal and not neoplastic cells. It is suggested that these lesions should not be considered a carcinogenic response to the chemical.