Tumor promoting activities of ethylphenylacetylurea and diethylacetylurea, the ring hydrolysis products of barbiturate tumor promoters phenobarbital and barbital, in rat liver and kidney initiated by N-nitrosodiethylamine

Induction of CYP2B1 mediated pentoxyresorufin O-dealkylase activity in different species, sex and tissue by prototype 2B1-inducers

The induction of CYP2B1 mediated pentoxyresorufin O-dealkylase (PROD) activity by various xenobiotics was explored in liver, kidney and lung from a variety of animal species of both sexes, in order to gain insights into the substrate specificity of induced CYPs. Marked species- and sex-related differences in the inducibility of PROD activity by tested chemicals were observed, the mouse being always more responsive when compared to hamster or rat. Induction by sodium phenobarbital (NaPB) led to a conspicuous increase in all situations, up to approximately 38-fold in female rat and mouse liver, with the exception of hamster kidney where PROD activity was only slightly affected. Unexpectedly, both sodium barbital (NaB) and phorone (PHR) moderately induce CYP2B1 isoforms in rat, the extent being highest in female kidney (PHR, 14-fold increase) and male lung (NaB, 4.5-fold). The degree of induction was maximal in the liver with some exceptions occurring in male mice where NaB induced up to 46- and 115-fold increases in lung and kidney and PHR up to 115-fold in kidney. Minimal, although significant induction of PROD activity following treatment with trans-1,2-dichloroethylene (1,2-DCE) occurred in all situations with the exception of hamster kidney and lung. Therefore, caution should be exercised when using PROD activity as specific enzymatic assay to probe CYP2B1-like induction.

Hepatic cytochrome P450 2B induction by ethyl/phenyl-substituted congeners of phenobarbital in the B6C3F1 mouse

The abilities of structural congeners of phenobarbital to induce immunoreactive hepatic cytochrome P450 2B (CYP2B) protein and associated catalytic activity (benzyloxyresorufin O-dealkylation) in the male B6C3F1 mouse were examined. Interspecies differences in inducing ability were examined through comparison of the results with induction data obtained previously with the male F344/NCr rat. The congeners were administered in the diet for 2 weeks at concentrations equimolar to 500 ppm of the prototype CYP2B inducer, phenobarbital. Of the series of compounds tested, phenobarbital was the most effective inducer of benzyloxyresorufin O-dealkylation and immunoreactive CYP2B protein, with 2-ethyl-2-phenylsuccinimide, 5-ethyl-5-phenylhydantoin, primidone, and glutethimide being only 19-42% as effective. 5-Ethyl-5-phenyloxazolidinedione and the ring-opened and decarboxylated congeners, N-(2-ethyl-2-phenylacetyl)urea and 2-ethyl-2-phenylmalonamide, displayed minimal induction of these catalytic activities. Dose-response experiments performed with 5-ethyl-5-phenylhydantoin indicated that the intrinsic CYP2B-inducing activity of this congener was as great as that of phenobarbital in the mouse, although a fourfold greater dietary concentration of this hydantoin (2000 ppm) was required to elicit a response equivalent to that caused by 500 ppm phenobarbital. When extent of induction was related to serum total xenobiotic concentration rather than to administered dietary concentration, the potencies of the two congeners were determined to be more similar (58 vs. > or = 78 microM for phenobarbital and 5-ethyl-5-phenylhydantoin, respectively).

Promoting effects of potassium dibasic phosphate on early-stage renal carcinogenesis in unilaterally nephrectomized rats treated with N-ethyl-N-hydroxyethylnitrosamine

The effects of potassium dibasic phosphate (PDP), potassium aluminum sulfate (PAS) and copper sulfate (CS) on early-stage renal carcinogenesis were investigated in unilaterally nephrectomized male Wistar rats after N-ethyl-N-hydroxyethylnitrosamine (EHEN) administration. After feeding 1,000 ppm EHEN, or basal diet for 2 weeks and removal of the left kidney at week 3, male Wistar rats were divided into 8 groups of 20 rats each. These groups received the following dietary treatments: 50,000 ppm PDP, 50,000 ppm PAS, 5,000 ppm CS or basal diet, respectively, for 18 weeks from weeks 3 to 20. The average numbers of adenomatous hyperplasias counted as preneoplastic lesions in the EHEN with 50,000 ppm PDP group were significantly higher than in the EHEN alone group or the EHEN followed by 50,000 ppm PAS or 5,000 ppm CS group. The treatment with 50,000 ppm PDP induced renal calcification and promoted the development of preneoplastic lesions in unilaterally nephrectomized rats treated with EHEN, but that with 50,000 ppm PAS or 5,000 ppm CS did not.

Effects of the oxazolidinedione anticonvulsants trimethadione and dimethadione and the barbiturate homolog 5,5-dimethylbarbituric acid on N-nitrosodiethylamine-initiated renal and hepatic carcinogenesis in the F344/NCr rat

The oxazolidinedione anticonvulsant trimethadione (3,5,5-trimethyl-2,4-oxazolidinedione, TMO) as well as its major metabolite, dimethadione (5,5-dimethyl-2,4-oxazolidinedione, DMO), and a structural analog from the barbiturate series, 5,5-dimethylbarbituric acid (DMB), were fed to F344/NCr male rats previously given a single initiating injection of N-nitrosodiethylamine (NDEA). The known promoter, phenobarbital (5-ethyl-5-phenylbarbituric acid, PB), was employed in this study as a positive control. At dosage levels equimolar to 500 ppm PB, none of the three compounds promoted development of hepatocellular adenomas or carcinomas, in contrast to PB. The two oxazolidinedione analogs and DMB caused minimal or no induction of cytochrome P450 isozyme 2B1 (CYP2B1)-mediated alkoxyresorufin O-dealkylase activities following short-term (2 weeks) feeding to separate groups of 6-week-old male F344/NCr rats, in contrast to the dramatic induction caused by PB. Promotion of neither thyroid nor renal neoplasia was observed following prolonged feeding of any of the tested compounds, although a significantly higher frequency of premalignant renal cortical tubular lesions (dysplasias) was seen in rats exposed to TMO following NDEA initiation than in those treated with NDEA alone. These studies provide important additional data on structure/liver tumor promoting activity relationships, and yield further evidence that within this group of structurally related anticonvulsants, it is possible to separate anticonvulsant activity from tumor promoting activity in the rat liver.

A pleiotropic response to phenobarbital-type enzyme inducers in the F344/NCr rat

The effects of a number of phenobarbital-type inducers on selected drug-metabolizing enzymes in male F344/NCr rats were determined by measuring specific catalytic activities and/or by measuring the levels of RNA which hybridize with specific probes for the corresponding genes. The effects on hepatic CYP2B1 were assessed by measuring the levels of CYP2B1-specific RNA and benzyloxyresorufin O-dealkylase and testosterone 16 beta-hydroxylase activities. Levels of CYP3A were monitored by measuring the rate of hydroxylation of testosterone at the 6 beta-position. Microsomal epoxide hydrolase activity was determined by measurement of cellular RNA specific for this form and by assaying the hydrolysis of benzo[a]pyrene-4,5-oxide. UDP-glucuronyltransferase activity was assayed by measuring the glucuronidation of 3-hydroxybenz[a]anthracene. Levels of glutathione S-transferase Ya/Yc were measured by quantifying total cellular RNA coding for the proteins. When male F344/NCr rats were administered various doses of phenobarbital or dichlorodiphenyltrichloroethane (DDT), strong correlations between the induction of CYP2B1 and the induction of epoxide hydrolase or UDP-glucuronyltransferase activities were observed. Treatment of rats with barbiturates, hydantoins, halogenated pesticides such as DDT or alpha-hexachlorocyclohexane, 2,4,5,2',4',5'-hexachlorobiphenyl, CYP2B1 inhibitors such as clotrimazole or clonazepam, or such structurally-diverse compounds as 2-hexanone or diallyl sulfide resulted in induction of CYP2B1-mediated enzyme activity and induction of certain other forms of cytochrome P450, microsomal epoxide hydrolase, at least one form of UDP-glucuronyltransferase, and multiple forms of glutathione S-transferase. This suggests that, as a class, compounds which induce CYP2B1 also induce a coordinate hepatic pleiotropic response which includes induction of these other phase I and phase II drug-metabolizing enzymes.

Possible application to medium-term organ bioassays for renal carcinogenesis modifiers in rats treated with N-ethyl-N-hydroxyethylnitrosamine and unilateral nephrectomy

The effects of the renal tumor promoters; beta-cyclodextrin (beta-C), DL-serine (DL-S), basic lead acetate (LA), trisodium nitrilotriacetate monohydrate (NTA) and potassium bromate (KB), and diethylene glycol (DEG) as a negative control, on early stage of renal carcinogenesis were investigated in unilaterally nephrectomized male Wistar rats after N-ethyl-N-hydroxyethylnitrosamine (EHEN) administration. Wistar male rats were fed 1000 ppm EHEN diet for 2 weeks and the left kidney was removed at week 3, then the animals were divided into 7 groups of 15 rats each. These groups received the following treatments: 1000 ppm LA, 10000 ppm NTA or 500 ppm KB diet for 18 weeks from week 3; 45 mg/100 g body wt./day of beta-C injected sc for 7 days; 100 mg/100 g body wt. of DL-S injected sc biweekly for 6 weeks; 5% DEG in drinking water as a negative control for two days. Five rats in each group were killed at weeks 8, 12 and 20 and their kidneys were examined histologically. At week 20, the average numbers of adenomatous hyperplasias seen as preneoplastic lesions in the beta-C, DL-S, LA, NTA or KB groups were significantly higher than those in the DEG or control groups. Thus within a relatively short period of 20 weeks, promoting effects of chemicals can be detected as a significant increase of adenomatous hyperplasias in this model.