Induction of cytochrome P-450 isoenzymes in cultured monkey hepatocytes

Substrate specificity for carnitine and glycine conjugation of branched side-chain and cyclic side-chain carboxylic acids in various experimental animals

Substrate specificities for the carnitine and glycine conjugates of branched side-chain and cyclic side-chain carboxylic acids were examined using dog, rabbit, cynomolgus monkey, and squirrel monkey hepatocytes and kidney slices. For all tested samples, the substrate specificity for carnitine or glycine conjugation showed a similar tendency to those for rat experiments reported previously, that is, the best substrate for the carnitine conjugate was cyclopropanecarboxylic acid (CPCA), while that for the glycine conjugate was benzoic acid (BA), followed by cyclohexanecarboxylic acid (CHCA). With respect to carnitine conjugation for CPCA, rat hepatocytes showed the highest ability followed by dog, rabbit, and monkey hepatocytes. Rat kidney slices showed the highest carnitine conjugation ability, followed by rabbit, dog, and monkey kidney slices, in that order. With respect to glycine conjugation for BA, rabbit hepatocytes showed the highest ability, followed by rat and monkey hepatocytes. Dog hepatocytes have no or little glycine conjugate ability for the carboxylic acids studied here. Rabbit kidney slices showed the highest glycine conjugation ability followed by rat, dog, and monkey kidney slices.

Effects of mammalian CYP3A inducers on CYP3A-related enzyme activities in grass carp (Ctenopharyngodon idellus): Possible implications for the establishment of a fish CYP3A induction model

Unexpected drug-drug interactions in fish are generally associated with the induction of CYP3A activity and may lead to the formation of drug residues and thus threaten the safety of fishery products. However, little information is available about CYP3A induction in fish. In the present study, we determined the in vivo and in vitro effects of typical mammalian CYP3A inducers (rifampicin, phenobarbital and dexamethasone) on CYP3A-related enzyme activities in a freshwater teleost, the grass carp (Ctenopharyngodon idellus). Our results showed that the response to rifampicin was similar for grass carp liver cell line (GCL), liver microsomes and the primary hepatocytes of grass carp, as indicated by the activity of aminopyrine N-demethylase (APND). When erythromycin N-demethylase (ERND) and 6beta-testosterone hydroxylase (6beta-TOH) were taken into consideration, the GCL displayed a greater capacity for conducting CYP3A metabolism and induction than the C. idellus kidney cell line (CIK). Using erythromycin and testosterone as substrates, we demonstrated that CYP3A catalysis exhibited non-Michaelis-Menten kinetics in GCL cells, and that V(max)/K(m) values were significantly increased due to rifampicin-treatment. Overall, this study may have implications for the use of GCL as a CYP3A induction model to identify physiological changes in fish as well as the similarities or differences between fish and mammals.

A novel model of acetaminophen-induced acute hepatic failure in rabbits

BACKGROUND

Few reliable and reproducible animal models of acute hepatic failure exist or conform to the criteria proposed by Terblanche and Hickman (Dig. Dis. Sci. 36: 770, 1991). In this prospective randomized study we describe the selective induction of CYP450 enzymes, depletion of glutathione, and hepatotoxic insult using acetaminophen in the development and characterization of a novel rabbit model of acute hepatic failure.

MATERIALS AND METHODS

Male New Zealand white rabbits weighing 3-5 kg were used. After preliminary dose ranging experiments, two groups of New Zealand white (n = 8 in each group) rabbits had CYP450 induction with phenobarbitone (40 mg/kg ip for 5 days) or with 20-methylcholanthrene (80 mg/kg ip). The glutathione synthetase inhibitor buthionine sulfoxime (2 mmol/kg iv) was then administered prior to acetaminophen administration (500 mg/kg sc). Clinical observations were recorded and arterial blood was sampled over 72 h.

RESULTS

Grade I-III encephalopathy occurred at 5-12, 12-25, and 28-56 h, respectively, in animals pretreated with 20-methylcholanthrene, but not in the phenobarbitone pretreated group. Mortality was 75% in the 20-methylcholanthrene group compared to 0% in the phenobarbitone group. Blood lactate (P < 0.05), prothrombin time (P < 0.005), aspartate transaminase (P < 0.005), and creatinine (P < 0.05) were higher in the 20-methylcholanthrene group compared to the phenobarbitone group. Histological changes were marked in the 20-methylcholanthrene group with massive coagulative hepatic necrosis compared to minimal histological damage in the phenobarbitone group.

CONCLUSION

The induction with 20-methylcholanthrene, glutathione depletion with buthionine sulfoxime, and subcutaneous administration of acetaminophen have led to the development of an animal model that parallels clinical, biochemical, and histological features of human hepatic failure.

Induction of intestinal cytochrome P450 (CYP3A) by rifampicin in beagle dogs

Both male and female beagle dogs (four dogs/sex) were orally treated with rifampicin (Rif) at the dose of 10 mg/kg/day for 7 days and an additional eight dogs (four dogs/sex) were used as a control. The inducible effect of Rif on intestinal cytochrome P450, especially CYP3A enzyme, was investigated by measuring microsomal testosterone 6beta-hydroxylation (6beta-OHT) activity, immunoblot and ELISA analysis. In male dogs, microsomal 6beta-OHT activity in the duodenum, upper, middle and lower part of the jejunum and the ileum of the control was 229, 204, 194, 129 and 57 pmol/min/mg protein, while the activity of the Rif-treated dogs significantly increased to 456, 486, 430, 192 and 138 pmol/min/mg protein, respectively. The activity of intestinal 6beta-OHT in the control and Rif-treated female dogs showed almost similar levels to those observed in the corresponding male dogs. The activity of intestinal 6beta-OHT in both control and Rif-treated dogs was specifically inhibited by anti-CYP3A12 antiserum. The apparent K(m) value for 6beta-OHT activity in all sections of the small intestine was comparable with that in the liver, and no significant changes were observed in between control and Rif-treated dogs. In both control and Rif-treated dogs, immunoblotting of intestinal microsomes with anti-CYP3A12 antiserum produced a band indistinguishable from that of purified CYP3A12 or of immunoreactive CYP3A12 in liver microsomes. A significant increase in intestinal CYP3A content by Rif treatment was quantitatively verified by the ELISA analysis and the magnitude of its increase correlated well with that of 6beta-OHT activity elevation. Furthermore, the results of immunohistochemistry using the anti-CYP3A12 antiserum indicated that CYP3A protein was specifically distributed in epithelial cells throughout the small intestine and appeared to be predominant at the apical side of villus cells. These results demonstrate that Rif induces not only hepatic CYP3A12 but also intestinal CYP3A in dogs.

Characterization of cytochrome P450 (CYP3A12) induction by rifampicin in dog liver

1. Effects of rifampicin (Rif) on the contents of cytochrome P450 (P450) enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) assessed by enzyme-linked immunosorbent assay and catalytic activities (ethoxyresorufin O-deethylase, and testosterone 6 beta-, 16 alpha- and 16 beta-hydroxylase; 6 beta-, 16 alpha- and 16 beta-OHT) in dog liver microsomes were compared between liver lobes of both the male and female dogs. 2. In the control dogs, the contents of individual P450 enzymes and their activities showed no significant differences between individual liver lobes and between the sexes. 3. Rif treatment (10 mg/kg/day, p.o. for 7 days) induced substantial increases in the content of CYP3A12 and 6 beta- and 16 beta-OHT activities, and slight increases in the content of CYP2B11 and 16 alpha-OHT activity, and their elevated levels were virtually the same between liver lobes. The magnitudes of the elevation of the CYP3A12 level and 6 beta- and 16 beta-OHT activities compared with control levels appeared to be greater in the female dogs. However, the ratios of their magnitudes (CYP3A content/6 beta-OHT activity and CYP3A content/16 beta-OHT activity) showed no differences between the sexes. 4. In both the control and Rif-treated dogs, the activities of 6 beta- and 16 beta-OHT were specifically inhibited by anti-CYP3A12 antiserum, and 16 alpha-OHT activity was specifically inhibited by anti-CYP2B11 and anti-CYP2C21 antiserum. 5. These results indicate that Rif treatment induces the expression of CYP3A12 protein, and correlates well with the elevation of its catalytic activity (6 beta- and 16 beta-OHT), and that the female dog is more responsive to Rif treatment as compared with the male.

Monooxygenation, cytochrome P4501A1 and P4501A1-mRNA in rat liver slices exposed to beta-naphthoflavone and dexamethasone in vitro

Precision-cut liver slices (0.5 mm) were incubated at 30 degrees C in a modified William's Medium E for up to 48 hrs. During the incubation, K+ and GSH/GSSG concentrations did not decrease. Cytochrome P450-dependent dealkylation rates of 7-ethoxycoumarin (ECOD), 7-allyloxycoumarin (ACOD) and 7-ethoxyresorufin (EROD) decreased to 1/3, 1/2 or did not change at all, respectively, after a 48 hrs incubation period. Exposure of the slices to 25 microM beta-naphthoflavone (beta NF) resulted in about 3 times higher monooxygenation rates. An exposure to a combination beta NF and dexamethasone (10(-6)M) caused a marked induction (6 times higher rates) after 48 hrs. Simultaneously an increase in P4501A1 content was observed. P4501A1-mRNA expression (measured by RT-PCR) was distinctly increased following beta NF exposure for 6 or 24 hrs. DMSO (0.2%) and dexamethasone alone modified monooxygenation rates, but did not have significant effects on P4501A1 content or, in the case of DMSO, P4501A1 gene expression (for dexamethasone not determined). Liver slices are a useful and simple tool for the detection of a beta NF-like induction within a few hours after preparation of the slices.