Polymorphisms of four hepatic cytochromes P-450 in twenty-eight inbred strains of rat

Thioacetamide differentially affects the expression and activity of glutathione-S-transferase in the liver of Wistar rats

Glutathione-S-transferase (GST) is a family of enzymes involved in the detoxification of toxic and carcinogenic compounds. In the present study, the effect of thioacetamide (TA), a hepatotoxic and hepatocarcinogenic compound, on the activity and expression of GST of Wistar female rats was tested. Animals were treated with a single dose of TA (250 mg/kg) for 12, 24, 48 and 72 hours. GST activity toward the broad substrate 1-chloro-2,4-dinitrobenzene was enhanced by TA. The protein level of the GST classes alpha and mu as well as the mRNA level of several GST subunits were also positively affected by the TA treatment. Female Wistar rats of the same age but from two other different colonies had their GST activity either inhibited or not affected by TA. The basal mRNA level of class alpha and class mu GST was also tested in female Wistar rats obtained from five different sources. Differences in the basal level of class alpha mRNA were observed in rats from at least three different sources, while class mu mRNA level was distinct in two groups of animals.

Differential metabolism of the pyrrolizidine alkaloid, senecionine, in fischer 344 and sprague-dawley Rats

The pyrrolizidine alkaloids (PAs), contained in a number of traditional remedies in Africa and Asia, show wide variations in metabolism between animal species but little work has been done to investigate differences between animal strains. The metabolism of the PA senecionine (SN) in Fischer 344 (F344) rats has been studied in order to compare to that found in the previously investigated Sprague-Dawley (SD) rats (Drug Metab. Dispos. 17: 387, 1989). There was no difference in the formation of (+/-) 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP, bioactivation) by hepatic microsomes from either sex of SD and F344 rats. However, hepatic microsomes from male and female F344 rats had greater activity in the N-oxidation (detoxication) of SN by 88% and 180%, respectively, when compared to that of male and female SD rats. Experiments conducted at various pH showed an optimum pH of 8.5, the optimal pH for flavin-containing monooxygenase (FMO), for SN N-oxidation by hepatic microsomes from F344 females. In F344 males, however, a bimodal pattern was obtained with activity peaks at pH 7.6 and 8.5 reflecting the possible involvement of both cytochrome P450 (CYP) and FMO. Use of specific inhibitors (SKF525A, 1-benzylimidazole and methimazole) showed that the N-oxide of SN was primarily produced by FMO in both sexes of F344 rats. In contrast, SN N-oxide formation is known to be catalyzed mainly by CYP2C11 rather than FMO in SD rats. This study, therefore, demonstrated that there were substantial differences in the formation of SN N-oxide by hepatic microsomes from F344 and SD rats and that this detoxification is catalyzed primarily by two different enzymes in the two rat strains. These findings suggest that significant variations in PA biotransformation can exist between different animal strains.

Epoxidation of arachidonic acid as an active-site probe of cytochrome P-450 2B isoforms

In the present study we determined the regioselectivity of arachidonic acid epoxidation by several members of the cytochrome P-450 2B subfamily, including rat P-450 2B1, 2B1-WM (an allelic variant of 2B1 expressed in Wistar-Munich rats), 2B2, and rabbit 2B4 and 2B5. The major products formed with all isoforms were the four regioisomeric epoxides, but each isoform produced a distinct distribution of the four epoxides. P-450 2B1 produced predominantly 14,15-epoxyeicosatrienoic acid (EET), while P-450 2B1-WM produced the 11,12-EET as the major product. P-450 2B2, 2B4, and 2B5 catalyzed the formation of all four epoxides in nearly equal amounts. The single Gly-478-->Ala substitution in the variant P-450 2B1-WM was sufficient to cause a dramatic change in the ratio of epoxides when compared with P-450 2B1. The Gly-478-->Ala mutation also changed the regioselective epoxidation of gamma-linolenic acid at the three double bonds. Four site-directed mutants of P-450 2B1 were also evaluated. The mutations included two single mutants where Ile-114 was changed to either Val or Ala and two double mutants where the Ala-478 mutation was coupled with either Val or Ala at position 114. When Ile-114 was mutated to Val, the degree of epoxidation of arachidonic acid at all four double bonds was nearly equal. However, substitution of Ile-114 with Ala, resulted in a significant reduction in the degree of epoxidation at the 14,15- and 11,12-double bonds, and the 8,9- and 5,6-EETs were the major products. When Ala was introduced at position 478 in conjunction with Val at position 114 the regioselective epoxidation of the mutant enzyme more closely resembled P-450 2B1-WM in that 11,12-EET was the major metabolite. The double mutation with Ala at both positions 114 and 478 produced a unique distribution of epoxide products with 5,6-EET as the major metabolite. The results of these studies indicate that residues 114 and 478 in the P-450 2B subfamily are important for the orientation of fatty acids in the active site.

Induction of a pleiotropic response by phenobarbital and related compounds. Response in various inbred strains of rats, response in various species and the induction of aldehyde dehydrogenase in Copenhagen rats

The ability of phenobarbital (PB) to induce a "pleiotropic response" which includes both cytochromes P450 (CYP) as well as other drug-metabolizing enzymes was investigated in mice, rabbits, hamsters, and various inbred strains of rats. PB induced similar drug-metabolizing enzymes (CYP2B, CYP3A, and epoxide hydrolase) in rats, mice, rabbits and hamsters. PB and two structural analogues (ethylphenylhydantoin and barbital) induced a variety of drug-metabolizing enzymes (CYP2B, CYP3A, CYP2A, epoxide hydrolase) in a series of inbred strains of rats. In contrast, levels of aldehyde dehydrogenase (ALDH) (propionaldehyde, NAD+) which were expressed constitutively in all strains of rats were induced by PB in only two of the eight strains (ACI, Copenhagen). Further investigations of ALDH induction by structurally diverse compounds in Copenhagen rats demonstrated a strong correlation between the induction of ALDH and other elements of the pleiotropic response (CYP2B, CYP3A, epoxide hydrolase). These results imply that induction of ALDH (propionaldehyde, NAD+) is associated with the PB pleiotropic response in Copenhagen rats.

Exocyclic-keto reductase activities for progesterone and S-warfarin in hepatic microsomes from adult male rats

Hepatic microsomes from adult male rats representing six inbred strains catalyzed quantitatively significant, NADPH dependent reductions of progesterone to the 20 beta (20R) alcohol and S-warfarin to its 11S-OH product. Microsomes from mature females and immature rats of both sexes were essentially devoid of these activities. Two strains of rat evidenced about 21% of these activities compared with the other strains and both activities were 25-81% repressed by treatment of rats with phenobarbital (PB). An excellent linear correlation was demonstrated for the two activities considering sex, age, NADPH much greater than NADH preference, PB-repression and strain differences. However, detergent latency (71%) and resistance to trypsinolysis were only observed for the keto-reductase activity with S-warfarin. Microsomes also catalyzed the reduction of progesterone to its 20 alpha-OH derivative but this activity preferred NADH greater than NADPH, was induced 2.7-fold by PB and was essentially independent of age, sex and animal strain. Furthermore, unlike the 20 beta-OH activity, this reduction was resistant to proteolytic inactivation.

Gene structure and expression of the rat cytochrome P450IIC13, a polymorphic, male-specific cytochrome in the P450IIC subfamily

The male-specific CYP2C13 gene has been isolated from two independent rat genomic libraries. This gene spans more than 50 kb and contains eight introns which are subject to the GT-AG rule. Two allelic forms of the CYP2C13 gene were identified. Determination of the exonic sequences revealed that one of them encodes cytochrome P450(+g) and the other encodes cytochrome P450(-g). Using allele-specific restriction enzyme sites, a good correlation between the genotype and the phenotype of CYP2C13 was shown. Nucleotide substitutions between the (+g) and the (-g) genes exist not only in the exons but also in the introns and the 5'-flanking region. Although five nucleotide differences were identified within 287 base pairs of the (+g) and (-g) 5'-flanking regions, the transcription initiation sites were identical. In addition to a canonical TATA box located 31 base pairs upstream of the start site of transcription, putative binding sites for the liver-enriched and liver-specific transcription factors HNF1/LF-B1/APF, HNF3, HNF4/AF-1, C/EBP, LAP, and eH-TF/TGT3 and the ubiquitous factors NF-1 and OTF-1 were identified.

Alternative splicing of mRNA encoding rat liver cytochrome P450e (P450IIB2)

Cytochrome P450e (P450IIB2) is a phenobarbital(PB)-inducible member of the rat liver P450IIB subfamily. Among P450 cDNA clones previously isolated from a cDNA library made from the liver of a single rat were several that contained P450e inserts, including PB13, PB16, and PB22. By nucleotide sequence analysis, the PB16 and PB22 inserts have now been found to contain an additional 24-bp segment not present in the PB13 insert or in previously reported P450e-coding sequences. According to the published P450e genomic sequence, the 24-bp segment is exactly at the junction of the fifth and the sixth exons and its sequence is identical to the first 24 bp of the fifth intron. Translation of this segment would add 8 amino acid residues to the P450e protein. To detect the alternatively spliced P450e mRNA, a synthetic oligodeoxyribonucleotide (oligo) corresponding to 18 of the 24 bp of the intronic sequence found in the PB16 and PB22 inserts was made. This oligo hybridized with a 2.1-kb RNA on Northern blots of liver RNA from PB- or Aroclor 1254-treated rats. Taken together, these results indicate that individual rats can possess both forms of P450e mRNA and that an alternative splicing mechanism is responsible for their formation.

Selective potent restriction of P450b- but not P450e-dependent 7,12-dimethylbenz[a]anthracene metabolism by the microsomal environment

The prototypic members of the rat liver cytochrome P450IIB subfamily, P450b and P450e, differ by only 13 amino acids and yet purified P450b is considerably more active than P450e for all known substrates. A unique regioselectivity difference between cytochromes P450b and P450e for the metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) and a genetic deficiency in P450e expression in the Marshall (M520/N) rat strain have been exploited to determine the microsomal contributions of the respective forms toward the metabolism of DMBA. The total contribution to metabolism by each isozyme has been assessed based on the sensitivity to rabbit anti-P450b/e IgG and comparison with microsomal P450b and P450e content as measured by Western blots. Liver microsomes from untreated M520/N rats do not express detectable levels of P450e but express P450b at a level that is 2-fold higher than that of P450e in liver microsomes from untreated F344 rats (50 pmol/mg). However, only 4% of the constitutive DMBA metabolizing activity of liver microsomes from the M520/N rat strain could be inhibited by anti-P450b/e IgG. A 30-fold induction of hepatic P450b by phenobarbital (PB) was also completely ineffective in increasing P450b-dependent DMBA metabolism. PB treatment had no appreciable effect on either the levels of expression of P450b protein or P450b-dependent DMBA metabolism, in M520/N lung and adrenal microsomes. In contrast, PB treatment of F344 rats considerably increased P450b/e-dependent metabolism by liver, lung, and adrenal microsomes. The regioselectivity of the anti-P450b/e-sensitive metabolism (predominantly 12-methyl hydroxylation), however, indicated a much greater contribution from P450e than P450b in every tissue examined despite a several fold higher expression of P450b than of P450e. P450b was expressed constitutively in lung microsomes from both strains but again failed to exhibit appreciable DMBA metabolizing activity. Based on these activities and microsomal P450b contents, P450b consistently exhibited turnover numbers (0.02-0.15 nmol/nmol P450b/min) that were at least 10-fold lower than those of pure P450b. In contrast, the calculated turnover numbers for microsomal P450e were consistently comparable to those of pure P450e (approximately 1 nmol/nmol P450e/min).

Comparison of microsomal drug-metabolizing enzymes in 14 rat inbred strains

Drug metabolic capacity in liver microsomes of 14 rat inbred strains was investigated. Cytochrome P-450 content as well as the following enzyme activities were measured: NADPH cyt. c(P-450) reductase (Red.), aminopyrine N-demethylase (APDM), ethoxycoumarin O-deethylase (ECOD), 1-naphthol: UDP-glucuronosyltransferase (NGT) and hydrolysis of acetylsalicylic acid (ASA; measured at pH 5.5 and pH 7.4). All enzymes measured were found to exhibit statistically significant inter-strain differences. In males the enzyme activities varied over a 7.3-fold (ECOD) to 1.4-fold (cytochrome P-450) range. Other inter-strain differences were generally larger than 2-fold: ASA-hydrolysis at pH 5.5 and 7.4 (3.9- and 3.3-fold variation, respectively), NGT and Red. (2.1-fold variation) and APDM (1.8-fold variation). In females similar, but somewhat smaller inter-strain differences were observed. Correlations between different enzyme activities were generally poor (correlation coefficients r less than 0.7). An exception was the correlation between ASA-hydrolysis at pH 5.5 and pH 7.4 (r = 0.79). We conclude that ASA hydrolysis at pH 5.5 and 7.4 is mediated by the same enzyme or by coregulated enzymes and that all other activities are mediated by different or differentially regulated enzymes. Based on analysis of variance and subsequent inter-strain comparisons, all strains appear to express a unique profile of liver microsomal drug metabolism. No two strains are identical with respect to all activities measured. We suggest that differences between inbred rat strains and particularly the difference in balance between different enzymes in various strains can be used advantageously in pharmacological and toxicological experiments.

Hepatic expression of rat P450 mRNA assessed by in situ hybridization to oligomer probes

Cell-specific chemical toxicities may be influenced by P450-catalyzed biotransformation reactions. We have undertaken an analysis of P450 expression in isolated rat liver sections to assess better the cellular distribution of P450 gene products. Discriminatory 18-mer oligodeoxynucleotides directed to the phenobarbital (PB) inducible P450s, P450IIB1 (P450b) and P450IIB2 (P450e), were employed as probes for in situ hybridization experiments. With these techniques we demonstrate that P450b and P450e mRNAs are each expressed in the hepatic lobule with similar spatial distribution. In animals pretreated with PB, only cells within the immediate periportal region were refractory to induction. Based on autoradiographic grain densities, responsive hepatocytes accumulated P450b mRNA at levels exceeding that for P450e. We employed in situ hybridization methodology in combination with Northern blot analyses to compare the expression of these mRNAs in two rat strains, Sprague-Dawley and Marshall 520/N (the latter being deficient in the synthesis of P450e isozyme; Rampersaud and Walz, 1987). These strains provided valuable comparative models, demonstrating the specificity and sensitivity of the oligomer probes. The continued development of in situ hybridization methodologies, especially when used in conjunction with synthetic oligomer probes, will permit a detailed analysis of P450 expression in different tissues, under a variety of chemical exposures, and may be a valuable adjuvant to the prediction of target organ toxicities.