Rat liver cytochrome P450 2B3: structure of the CYP2B3 gene and immunological identification of a constitutive P450 2B3-like protein in rat liver

Evolution of the CYP2ABFGST gene cluster in rat, and a fine-scale comparison among rodent and primate species

The evolution of gene families can be best understood by studying the modern organization and functions of family members, and by comparing parallel families in different species. In this study, the CYP2ABFGST gene cluster has been characterized in rat and compared to the syntenic clusters in mouse and human, providing an interesting example of gene family evolution. In the rat, 18 loci from six subfamilies have been identified by specifically amplifying and sequencing gene fragments from cloned DNA, and have been exactly placed on chromosome 1. The overall organization of the gene cluster in rat is relatively simple, with genes from each subfamily in tandem, and is more similar to the mouse than to the human cluster. We have reconstructed the probable structure of the CYP2ABFGST cluster in the common ancestor of primates and rodents, and inferred a model of the evolution of this gene cluster in the three species. Numerous nontandem and block duplications, inversions, and translocations have occurred entirely inside the cluster, indicating that pairing between duplicate genes is keeping the rearrangements within the cluster region. The initial tandem duplication of a CYP2 gene in an early mammalian ancestor has made this region particularly subject to such localized rearrangements. Even if duplicated genes do not have a large-scale effect on chromosomal rearrangements, on a local level clustered gene families may have contributed significantly to the genomic complexity of modern mammals.

A natural CYP2B6 TATA box polymorphism (-82T--> C) leading to enhanced transcription and relocation of the transcriptional start site

We investigated the impact of promoter polymorphisms on transcription of the human CYP2B6 gene. In total, 98 DNA samples from white persons from a previously characterized liver bank were sequenced throughout 2.3 kilobases of upstream sequence and haplotype structures were determined using additional coding sequence information. HepG2 cells and primary rat and human hepatocytes were transfected with luciferase reporter gene constructs driven by 2033 base pairs (bp) of the most frequent promoter variants. The novel haplotype *22 (-1848C--> A, -801G--> T, -750T--> C, and -82T--> C) showed 3- to 9-fold enhanced transcriptional activity in all transfected cells. Constructs containing single mutations surprisingly revealed -82T--> C, predicted to disrupt a putative TATA box, to be alone responsible for this effect. In silico analysis and electrophoretic mobility shift assay demonstrated conversion of the putative TATA box into a functional CCAAT/enhancer-binding protein binding site. Analysis of transcriptional start sites showed the mutant promoter to be transcribed from a start site located approximately 30 bp downstream of the wild-type start site, consistent with the use of a noncanonical TATA box at -55 bp. Median CYP2B6 mRNA expression and bupropion hydroxylase activity as a selective marker of CYP2B6 catalytic activity were approximately 2-fold higher in livers genotyped -82TC as in those genotyped -82TT (20.4 versus 9.8 arbitrary units, p = 0.007, and 201.8 versus 106.7 pmol/mg/min, p = 0.042, respectively). This promoter polymorphism thus contributes to CYP2B6 functional variability and represents a novel mechanism by which mutations can enhance transcription. Furthermore, a detailed interspecies comparison of CYP2B promoters and transcriptional start sites provided novel insights into evolutionary relationships.

Organization, structure and evolution of the CYP2 gene cluster on human chromosome 19

The cytochrome P450 superfamily of mixed-function oxygenases has been extensively studied due to its many critical metabolic roles, and also because it is a fascinating example of gene family evolution. The cluster of genes on human chromosome 19 from the CYP2A, 2B, and 2F subfamilies has been previously described as having a complex organization and many pseudogenes. We describe the discovery of genes from three more CYP2 subfamilies inside the cluster, and assemble a complete map of the region. We comprehensively review the organization, structure, and expression of genes from all six subfamilies. A general hypothesis for the evolution of this complex gene cluster is also presented.

Cis-acting sequences from the rat cytochrome P450 2B1 gene confer pulmonary and phenobarbital-inducible expression in transgenic mice

Specific cytochrome P450 enzymes show tissue-specific induction, and different regulatory units for expression of these enzymes have been identified. The regulation of the phenobarbital (PB)-inducible P450 genes has been relatively well characterized in terms of PB induction, but less so with regard to tissue-specific expression. CYP2B2 is not expressed in the rat lung, whereas cytochrome P450 2B1 (CYP2B1) is a dominating enzyme in the same tissue. The constitutive expression of CYP2B1 and CYP2B2 in liver is low, but inducible by PB, whereas the pulmonary expression of CYP2B1 is not induced by PB. This indicates utilization of different regulating mechanisms in the two organs. A gene construct consisting of the structural gene for LacZ coupled to a 1.3-kb 5' fragment of the rat CYP2B1 gene was used to generate transgenic mice in order to further elucidate the mechanism behind tissue-specific expression and PB induction of the CYP2B1 gene. Using reverse transcriptase-polymerase chain reaction on total RNA extracted from lung and liver tissue, a lung-specific transcription of the transgene was observed. Transcription of the construct was also observed in livers from PB-treated transgenic animals. By histochemical staining of lung sections with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal), we demonstrated expression at the protein level in bronchiolar cells. In conclusion, our results revealed that the region extending to -1. 3 kb in the 5' flanking region of the CYP2B1 gene included sequences that could partly account for the lung-specific transcription of CYP2B1 and the hepatic induction of CYP2B1 transcription by PB.

In vitro metabolism of chlorotriazines: characterization of simazine, atrazine, and propazine metabolism using liver microsomes from rats treated with various cytochrome P450 inducers

The in vitro metabolism of chlorotriazines, simazine (SIZ), atrazine (ATZ), and propazine (PRZ) was studied using control, 3-methylcholanthrene-, phenobarbital-, pyridine-, dexamethasone-, and clofibrate-treated rat liver microsomes. The metabolites were determined by HPLC. The principal reactions by cytochrome P450 (P450) system were N-monodealkylation and isopropylhydroxylation in all rat liver microsomes. As a result, 2-chloro-4-ethylamino-6-amino-1,3,5-triazine (M1) (SIZ-M1 for SIZ and ATZ-M1 for ATZ) and 2-chloro-4-amino-6-isopropylamino-1,3, 5-triazine (M2) (ATZ-M2 for ATZ and PRZ-M2 for PRZ), 2-chloro-4-ethylamino-6-(1-hydroxyisopropylamino)-1,3,5-triazine (M3) (ATZ-M3 for ATZ), and 2-chloro-4-isopropylamino-6-(1-hydroxyisopropylamino)-1,3,5-triazi ne (M4) (PRZ-M4 for PRZ) were detected as the metabolites. N-bidealkylation and 2-hydroxylation were not found in this system. The formation rates of SIZ-M1, ATZ-M1, ATZ-M2, and PRZ-M2 were markedly induced by 3-methylcholanthrene, phenobarbital, and pyridine. On the other hand, the formation rates of ATZ-M3 and PRZ-M4 were significantly induced by phenobarbital, pyridine, and/or clofibrate, but not by 3-methylcholanthrene. The enzyme kinetics of chlorotriazine metabolism were examined by mean of Eadie-Hofstee analyses. Although there was no remarkable difference of Km for the products in chlorotriazine metabolism among the microsomes tested, the Vmax and Clint (Vmax/Km) for the products in chlorotriazine metabolism are affected by P450 inducers, except for dexamethasone. The formation rates of SIZ-M1, ATZ-M1, ATZ-M2, and PRZ-M2 were significantly correlated with 7-ethoxyresorufin O-deethylase, acetanilide 4-hydroxylase, 7-ethoxycoumarin O-deethylase, 4-nitrophenol 2-hydroxylase, and testosterone 7alpha-hydroxylase activities and CYP1A1/2 level, whereas the formation rates of ATZ-M3 and PRZ-M4 were significantly correlated with testosterone 16beta-hydroxylase, bufuralol 1'-hydroxylase, and 4-nitrophenol 2-hydroxylase activities and CYP2B1/2 level. These results suggest that the inducibility in metabolism of SIZ, ATZ, and PRZ is different between N-monodealkylation and isopropylhydroxylation and that the N-monodealkylation and isopropylhydroxylation are induced by CYP1A1/2, CYP2B1/2, and CYP2B1/2, respectively.

New proteins in the rat CYP2B subfamily: presence in liver microsomes of the constitutive CYP2B3 protein and the phenobarbital-inducible protein product of alternatively spliced CYP2B2 mRNA

The rat CYP2B gene subfamily includes CYP2B1, CYP2B2 and CYP2B3. Translation of an alternatively spliced hepatic CYP2B2 mRNA would generate a CYP2B2 variant, CYP2B2v, having eight additional amino acid residues inserted between CYP2B2 positions 274 and 275. The presence of CYP2B3 and CYP2B2v in rat liver has yet to be demonstrated. cDNA expression vectors were obtained for CYP2B1, CYP2B2, CYP2B3 and CYP2B2v. All four proteins react with an anti-CYP2B1 antibody and can be resolved by SDS-PAGE. A CYP2B3-specific polyclonal antibody raised against an undecapeptide (SPVDPNTIDMT) from near the C-terminus of CYP2B3 detected a constitutive protein on immunoblots of rat liver microsomes, thus demonstrating that the CYP2B3 mRNA is translated in the liver. Similarly, a CYP2B2v-specific polyclonal antibody was raised against a peptide containing the eight additional amino acid residues (VSPAWMRE) predicted to be present in the CYP2B2v protein. It detected a phenobarbital- and Aroclor 1254-inducible protein in rat liver microsomes. Microsomes of Ad293 cells expressing cDNAs for CYP2B2 and CYP2B2v were used to metabolize 7,12-dimethylbenz[a]anthracene (DMBA), and the metabolites produced were compared with those generated by microsomes of cells expressing CYP2B1 cDNA. CYP2B2v had activity similar to that of CYP2B2 for DMBA metabolism. Both CYP2B2 forms preferentially catalyzed 12-hydroxylation, whereas CYP2B1 preferred 7-hydroxylation and exhibited turnover that was strongly suppressed as previously reported. These results demonstrate the existence in rat liver of two new CYP2B proteins: CYP2B3, the major constitutive CYP2B form, and CYP2B2v, which represents a rare case of non-aberrant alternative splicing among xenobiotic-metabolizing P450s.

Identification of CYP2B14P and CYP2B16P, two apparent pseudogenes in the rat cytochrome P450 2B (CYP2B) subfamily

Cytochrome P450 2B3 (CYP2B3) is a member of the CYP2B subfamily and is present constitutively in rat liver. During cloning of the CYP2B3 gene, several variant genomic inserts were isolated, exoncontaining fragments of which were identified by hybridizing to CYP2B3 cDNA or exonic probes. Nucleotide sequence analysis of one set of variant inserts showed them to carry exon 1 and the 5'-flanking region of a gene designated CYP2B14P. The last codon of CYP2B14P exon 1 is a TAG translation stop codon, and thus CYP2B14P is a pseudogene. Three other inserts covering 34 kb of genomic sequence together carried 9 exons and the 5'-and 3'-flanking regions of the gene designated CYP2B16P. The 5'-splice site of CYP2B16P intron 1 is inactivated by the replacement of G by T in the normally invariant GT dinucleotide. Thus, CYP2B16P also has the characteristics of a pseudogene. The identification of CYP2B14P and CYP2B16P brings to at least seven the number of genes or pseudogenes shown by nucleotide sequence analysis to belong to the rat CYP2B subfamily.

Chromatographic separation and behavior of microsomal cytochrome P450 and cytochrome b5

The methods used for separation of the multiple mammalian cytochrome P450 enzymes by liquid chromatography are reviewed. In addition to the chromatographic techniques, preparation and handling of samples and prefractionation procedures are considered. Conditions that affect stability and chromatographic resolution of cytochromes P450 are also discussed. Special emphasis is put on useful methods which are not routinely used for P450 separation, such as immobilized metal affinity or hydrophobic-interaction chromatography. Applications of low- and high-pressure methods with regard to preparative and analytical separations are compared. It is shown that high- and medium-pressure ion-exchange chromatography are suitable tools for separation of closely related P450 enzymes, especially when specific detection methods are available. In addition to fractionation of cytochromes P450, the isolation and chromatographic behavior of cytochrome b5 is discussed.

Characterization of phenobarbital-inducible mouse Cyp2b10 gene transcription in primary hepatocytes

The mouse phenobarbital (PB)-inducible Cyp2b10 gene promoter has been isolated and sequenced, and control of its expression has been characterized. The 1405-base pair (bp) Cyp2bl0 promoter sequence is 83% identical to the corresponding region from the rat CYP2B2 gene. In addition to the lack of CA repeats, differences include insertion of 42 base pairs (-123/-82 bp) into the middle of a consensus sequence to the so-called "Barbie box." In this report, we have developed a primary mouse hepatocyte culture system in which endogenous 2B10 mRNA as well as Cyp2b10-driven CAT activity were induced by PB and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), but not by the 3-chloro derivative of TCPOBOP. Deletion analysis of the Cyp2b10 promoter identified a basal transcription element at -64/-34 bp and a negative element at -971/-775 bp. Sequences contained within the -1404/-971 bp region are responsible for the induced CAT activity. DNase I protection and gel shift assays detected five major protein binding sites within the -1404/-971 bp fragment, one of which shared high sequence identity with a portion of a regulatory element in CYP2B2 gene (Trottier, E., Belzil, A., Stoltz, C., and Anderson, A. (1995) Gene 158, 263-268). Our results indicate that sequences important for PB-induced transcription of Cyp2b10 gene are located in the distal promoter.