Structure of the mouse cytochrome P1-450 genomic gene

Structures and characterization of sex-specific mouse cytochrome P-450 genes as members within a large family. Duplication boundary and evolution

We characterized two female-specific testosterone 16 alpha-hydroxylase mouse cytochrome P-450 genes, 16 alpha oh-a and 16 alpha oh-b. Gene 16 alpha oh-a, consisting of nine exons, is approximately 38 kbp in size. The exon sequence of this P-450 gene is identical to cDNA pf26 nucleotide sequence [Noshiro, M., Lakso, M., Kawajiri, K. & Negishi, M. (1988) Biochemistry 27, 6434-6443], which encodes female-specific testosterone 16 alpha-hydroxylase regulated by the murine Rip locus. Gene 16 alpha oh-b, containing nine exons with the same junctions as the 16 alpha oh-a, spans at least 20 kbp, and encodes a cytochrome P-450 whose deduced amino acid sequence is 90% similar to the hydroxylase. Nucleotide sequences revealed that duplication of the two genes occurred 4-22 million years ago, and that the 5' duplication boundary is located 1336 bp upstream from the putative transcription-start site. In the flanking regions of both genes, there is a long stretch (100 bp) of CA repeats in addition to other motifs, including TATA box, glucocorticoid-response-element-core and Simian-virus-40-enhancer sequences and IgG light-chain gene promoter. We isolated many genomic DNA clones which contain exon 1 sequences, and compared their restriction maps, cross-hybridization and nucleotide sequences. The results indicate that these genomic clones represent closely related genes in the 16 alpha oh family with a minimum of 16 members, which is further divided into classes a, b and c. 16 alpha oh-a and 16 alpha oh-b belong to the first and second classes, respectively. Moreover, extensive segmental gene conversion and nonreciprocal recombination were noted among the genes, particularly among those in class b. All genes in that class contain the long ATTT repeat sequences in intron 1, which may have triggered a rapid gene conversion and/or stabilize the duplicated genes.

Heparin inhibits EcoRI endonuclease cleavage of DNA at certain EcoRI sites

Studies presented here demonstrate that heparin inhibits EcoRI endonuclease cleavage of DNA whereas related proteoglycans show no effect. The inhibition occurs at particular EcoRI sites that are near or overlap with palindromic sequences in the murine lambda 5 and Lyt-2 genes. Endogenous heparin from peritoneal mast cells co-isolates with DNA and inhibits digestion of peritoneal cell DNA at the inhibitable sites. Digestion of spleen DNA is inhibited at the same sites when commercial heparin is added prior to digestion. In both cases, the inhibition is abolished by pre-treating the DNA with heparinase. Thus, potential artifacts in restriction fragment length analyses could occur with DNA isolated either from cells that are naturally rich in heparin or from cells to which heparin has been added, e.g., as an anticoagulant.

Genes for cytochrome P-450 and their regulation

The capacity of the liver microsomal mixed-function oxidase system to metabolize a wide variety of exogenous as well as endogenous compounds reflects the participation of multiple forms of the terminal oxidase, cytochrome P-450, which have different broad, but overlapping, substrate specificities. Several of these isozymes accumulate in the liver after exposure of animals to specific inducing agents. Recent studies employing recombinant DNA techniques to investigate the genetic and evolutionary relatedness of various cytochrome P-450 isozymes as well as the molecular basis for the induction phenomenon are described. The conclusions from these investigations are presented in the context of the substantial body of data obtained from the characterization of specific cytochrome P-450 isozymes and from studies on the induction of specific isozymes or enzymatic activities during development or after treatment of animals with various inducing agents.

Comparison of the mouse P(1)450 gene and flanking sequences from a MOPC 41 plasmacytoma and normal liver

The murine P(1)450 gene is inducible by foreign chemicals such as dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD). The induction process appears to involve binding of TCDD to the Ah receptor, interaction of the inducer-receptor complex with chromatin, and transcriptional activation of the P(1)450 gene. P(1)450 gene expression did not occur in the MOPC 41 plasmacytoma transplanted intra-abdominally in the BALB/c mouse. By Southern blot analysis, the P(1)450 gene restriction patterns are similar between MOPC 41 DNA and normal mouse liver DNA digested with Eco RI, Bam HI, or Hind III, suggesting there is no evidence for a gross chromosomal rearrangement in the tumor P(1)450 gene. The P(1)450 gene isolated from a genomic DNA library derived from the MOPC 41 tumor lacked the first exon and two-thirds of the first intron because of a rearrangement that presumably occurred during construction of the genomic DNA library. An unknown upstream segment of 2578 bases that had recombined with the remainder of the P(1)450 gene was sequenced and found to contain several repetitive sequences on both strands. The remaining 6272 bases of the P(1)450 gene (2532 bp of exons, 1969 bp of introns and 1771 bp of 3'-flanking region) are identical between the BALB/cJ-derived MOPC 41 and normal C57BL/6N mouse liver, with the exception of one CTTT repeat in the second intron. Hence, the BALB/c and C57BL/6 inbred mice, generally regarded as among the most divergent of Mus domesticus inbred laboratory strains, possess a P(1)450 gene and flanking region that exhibit greater than 99.9% similarity in nucleotide sequence.

Identification and quantification of a messenger ribonucleic acid induced by polynuclear aromatic hydrocarbons--using a cloned human cytochrome P-450 gene

We have isolated four overlapping human genomic clones associated with the polynuclear aromatic hydrocarbon-induced form of cytochrome P-450. The form of P-450 most closely associated with polynuclear aromatic hydrocarbons induction has been defined as P1-450. These four overlapping genomic clones span a total of 31.0 X 10(3) base pairs in length with the coding sequence lying in the center of these clones. Translation in vitro of 3-methylcholanthrene-induced mRNA, selected with the human P1-450 genomic clone, detect a protein with Mr 52000, which is immunoprecipitable by the anti-(mouse P1-450) antibody. The isolated human P1-450 genomic clone hybridizes to 3-methylcholanthrene-induced mRNA from monkey liver, benzanthracene and 3-methylcholanthrene-treated human mammary tumor cells (MCF-7), but not to isosafrole-treated human cells. Upon treatment with polynuclear aromatic hydrocarbons there is a positive correlation between induced arylhydrocarbon hydroxylase (flavoprotein-linked monoxygenase) activity and the amount of mRNA that hybridizes to the isolated human genomic clone for P1-450. The size of mRNA, induced from human cells and monkey liver by polynuclear aromatic hydrocarbons, is around 3.3 X 10(3) base pairs, which is the same as the larger of two mRNA induced by polynuclear aromatic hydrocarbons in the inbred strain of mouse (C57BL/6N). Our data also showed that the isolated DNA clone can detect a mRNA size of 3.3 X 10(3) base pairs from phytohemagglutinin-activated, benzanthracene-treated human lymphocytes. Densitometer scanning indicated the presence of a 3.6-fold variation (highest-lowest) in the levels of lymphocyte P1-450 mRNA contents among six individuals studied.

Transfection by genomic DNA of cytochrome P1-450 enzymatic activity and inducibility

An aryl hydrocarbon hydroxylase (AHH)-deficient gene A- mutant of the mouse line Hepa-1 was treated with calcium phosphate precipitates of DNA from Hepa-1, the rat line H4IIEC3, or an A- -human hybrid in which the A- mutation is complemented by the corresponding human gene. AHH+ transfectants were isolated by selection with benzo[ghi]perylene plus near UV. In addition, a gene A- mutant which also carries a mutation for hypoxanthine phosphoribosyltransferase deficiency was treated with the above genomic DNAs together with pSV2-gpt DNA, and cotransfectants were isolated after treatment with both benzo[ghi]pereylene and HAT. All transfectants and cotransfectants were inducible for AHH by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Both transfectants and cotransfectants were unstable during culture, rapidly losing AHH activity. Rat DNA-derived transfectants were probed in Southern blots with a cDNA probe to mouse cytochrome P1-450 that cross-hybridizes to the corresponding rat gene. All rat DNA-derived transfectants contained the rat P1-450 gene. In half of the transfectants, the rat gene was amplified four- to sevenfold. In one transfectant, the rat gene was truncated at the 3' end. The proportion of rat DNA in different transfectants, as determined by hybridization to a rat repetitive sequence, ranged from less than 1% to 5%. AHH activity and the rat P1-450 gene segregated together in subclones of one of the transfectants. These results demonstrate that the A gene is either the structural gene for cytochrome P1-450, or another very closely linked gene. Previous results (O. Hankinson et al., J. Biol. Chem. 260:1790-1795, 1985) favor the former alternative.

Transfection by genomic DNA of cytochrome P1-450 enzymatic activity and inducibility

An aryl hydrocarbon hydroxylase (AHH)-deficient gene A- mutant of the mouse line Hepa-1 was treated with calcium phosphate precipitates of DNA from Hepa-1, the rat line H4IIEC3, or an A- -human hybrid in which the A- mutation is complemented by the corresponding human gene. AHH+ transfectants were isolated by selection with benzo[ghi]perylene plus near UV. In addition, a gene A- mutant which also carries a mutation for hypoxanthine phosphoribosyltransferase deficiency was treated with the above genomic DNAs together with pSV2-gpt DNA, and cotransfectants were isolated after treatment with both benzo[ghi]pereylene and HAT. All transfectants and cotransfectants were inducible for AHH by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Both transfectants and cotransfectants were unstable during culture, rapidly losing AHH activity. Rat DNA-derived transfectants were probed in Southern blots with a cDNA probe to mouse cytochrome P1-450 that cross-hybridizes to the corresponding rat gene. All rat DNA-derived transfectants contained the rat P1-450 gene. In half of the transfectants, the rat gene was amplified four- to sevenfold. In one transfectant, the rat gene was truncated at the 3' end. The proportion of rat DNA in different transfectants, as determined by hybridization to a rat repetitive sequence, ranged from less than 1% to 5%. AHH activity and the rat P1-450 gene segregated together in subclones of one of the transfectants. These results demonstrate that the A gene is either the structural gene for cytochrome P1-450, or another very closely linked gene. Previous results (O. Hankinson et al., J. Biol. Chem. 260:1790-1795, 1985) favor the former alternative.

Gene structure and nucleotide sequence for rat cytochrome P-450c

Two clones from rat genomic libraries that contain the entire gene for rat cytochrome P-450c have been isolated. lambda MC4, the first clone isolated from an EcoR1 library, contained a 14-kb insert. A single 5.5-kb EcoR1 fragment from lambda MC4, the EcoR1 A fragment, hybridized to a partial cDNA clone for the 3' end of the cytochrome P-450c mRNA. This fragment was sequenced using the dideoxynucleotide chain termination methodology with recombinant M13 bacteriophage templates. Comparison of this sequence with the complete cDNA sequence of cytochrome P-450MC [Yabusaki et al. (1984) Nucleic. Acids Res. 12, 2929-2938] revealed that the EcoR1 A fragment contained the entire cytochrome P-450c gene with the exception of a 90-bp leader sequence. The gene sequence is in perfect agreement with the cDNA sequence except for two bases in exon 2. A second genomic clone, lambda MC10, which was isolated from a HaeIII library, contains the missing leading sequence as well as 5' regulatory sequences. The entire gene is about 6.1 kb in length with seven exons separated by six introns, all of the intron/exon junctions being defined by GT/AG. Amino- and carboxy-terminal information are contained in exons 2 and 7, respectively. These exons contain the highly conserved DNA sequences that have been observed in other cytochrome P-450 species. Potential regulatory sequences have been located both 5' to the gene as well as within intron I. A comparison of the coding information for cytochrome P-450c with the sequence of murine cytochrome P3-450 and rat cytochrome P-450d revealed a 70% homology in both the DNA and amino acid sequence, suggesting a common ancestral gene. Genomic blot analyses of rat DNA indicated that the 3-methylcholanthrene-inducible family of cytochrome P-450 isozymes is more limited in number compared to the phenobarbital-inducible isozymes. Cross-hybridization studies with human DNA suggest a high degree of conservation between rat cytochrome P-450c and its human homolog although gross structural differences do exist between the two genes.

Isolation and characterization of full-length mouse cDNA and genomic clones of 3-methylcholanthrene-inducible cytochrome P1-450 and P3-450

Polysome immuno-adsorption, with immunoglobulin G directed against two 3-methylcholanthrene-induced mouse liver cytochrome P-450 proteins, was used to enrich mRNA from 3-methylcholanthrene-treated C57BL/6N mouse liver. cDNA transcribed from the P-450-enriched mRNA was then cloned into the Okayama-Berg vector. Two cDNA classes were detected upon differential screening of the clone bank with [32P]cDNA derived from 3-methylcholanthrene-induced immuno-enriched versus control mRNA. Several representatives of these two classes were judged to be near full length by comparison with their corresponding mRNA mobilities on denaturing agarose gels. A continuous reading-frame near the 5' end of one cDNA class (P1-450) corresponds to a protein having 15 of 17 residues the same as the published N-terminal sequence of rat P-450c. A continuous reading frame near the 5' end of the other class (P3-450) corresponds exactly to the first 25 amino acids of the published N-terminal sequence of rat P-450d. The P1-450 cDNA is at least 700 bp longer than the P3-450 cDNA. Heteroduplex analysis and Southern blot hybridization demonstrate that these mRNAs share approx. 1100 bp of sequence homology. Genomic P1-450 and P3-450 clones were isolated from a gene library constructed from C57BL/6N mouse liver DNA. By heteroduplex analysis with the corresponding cDNA, the P1-450 gene spans about 6 kb and the P3-450 gene about 7 kb. The intron-exon patterns are very similar, with the second and seventh exons being much larger than the other five. The 3' terminal exon of P1-450 is about 500 bp longer than that of P3-450. These data suggest that both P1-450 and P3-450 have diverged from a common ancestral gene.

Localization of cytochrome P1-450 and P3-450 genes to mouse chromosome 9

Treatment of mice with polycyclic aromatic hydrocarbons results in the induction of P1-450 and P3-450 forms of cytochrome P-450. The genes for both cytochromes have recently been cloned and shown to be coordinately regulated by the Ah receptor. The mouse analogues of P1-450 and P3-450 can be distinguished from their hamster counterparts by Southern blot analysis with Kpn I-digested DNA fragments. DNA from hamster-mouse somatic cell hybrids that have selectively lost mouse chromosomes was used in Southern blots to map the location of the two mouse genes. Chromosome segregation analysis of 12 hybrid clones demonstrated that the structural genes for both P1-450 and P3-450 can be assigned to mouse chromosome 9.

The Ah receptor: binding specificity only for foreign chemicals?

The murine Ah locus controls the induction of at least four drug-metabolizing enzymes: cytochromes P1-450, P2-450, and P3-450, and UDP-glucuronosyltransferase. The Ah gene codes for a cytosolic receptor. It is known that the induction response includes: (i) high-affinity binding of specific foreign chemicals to the Ah receptor; (ii) temperature-dependent translocation of the "activated" inducer-receptor complex into the nucleus; (iii) binding of the complex presumably to chromatin components; (iv) transcriptional activation of specific genes; (v) maximal increases in intranuclear high-molecular-weight precursor mRNA (pre-mRNA) that precede by several hours the maximal increases in cytoplasmic mRNA; (vi) translation of the mRNA principally on membrane-bound polysomes; and (vii) increases in the specific membrane-bound proteins (including architectural arrangement with other membrane-bound moieties) that reflect enhanced specific drug-metabolizing activities. It is not known how many of the other drug metabolism induction responses are also governed by receptors. The Ah locus studies have been chiefly unraveled in the mouse, due to several inbred strains having a receptor defect. In addition to "classical" pharmacologic methods (such as structure-activity studies) and standard biochemical techniques, the newer methods of recombinant DNA technology and somatic-cell genetics in culture are shown to be important in understanding the Ah receptor and its induction response. It is possible that this receptor is required for endogenous functions critical to life processes, as well as its function in the induction of drug metabolism by certain polycyclic aromatic compounds.

Characterization of cytochrome P2-450 (20-S) mRNA. Association with the P1-450 genomic gene and differential response to the inducers 3-methylcholanthrene and isosafrole

Mouse liver cytochrome P2-450 is defined as the major isosafrole-inducible form of P-450 which is most specific for isosafrole metabolism. lambda AhP-1 represents a 15.5 X 10(3)-base-pair segment of mouse genomic DNA having the cytochrome P1-450 gene (approximately equal to 4600 base pairs) located in the middle portion. Using various subclones as probes, we investigated the differential expression of P1-450 mRNA and P2-450 mRNA induction as a function of association with the Ah locus, 3-methylcholanthrene or isosafrole dosage, tissue specificity, and developmental age. Both P1-450 (23-S) mRNA and P2-450 (20-S) mRNA induction processes are regulated by the Ah receptor. P2-450 mRNA is about 10-fold more sensitive than P1-450 mRNA to induction by either 3-methylcholanthrene or isosafrole. Phenobarbital pretreatment has no effect at all on either P1-450 mRNA or P2-450 mRNA. Whereas both P1-450 mRNA and P2-450 mRNA are induced by 3-methylcholanthrene in C57BL/6N liver, P1-450 (23-S) mRNA but not P2-450 (20-S) mRNA is induced by 3-methylcholanthrene in C57BL/6N kidney. P1-450 mRNA induction by 3-methylcholanthrene is measurable in C57BL/6N liver at day 15 of gestation, and the expression becomes enhanced with increasing age. P2-450 mRNA induction by 3-methylcholanthrene in C57BL/6N liver appears about 7 days later during development than 3-methylcholanthrene-inducible P1-450 mRNA. Both 3-methylcholanthrene-induced P1-450 mRNA and P2-450 mRNA are detectable in DBA/2N liver; their appearance is later in development, however, and at lower concentrations than that seen with C57BL/6N liver. P1-450 (23-S) mRNA and P2-450 (20-S) mRNA appear to hybridize to a common 5' fragment of the P1-450 gene.

The Ah locus: correlation of intranuclear appearance of inducer-receptor complex with induction of cytochrome P1-450 mRNA

The Ah locus regulates the induction of cytochrome P1-450 by foreign chemicals such as 3-methyl-cholanthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. The induction process is controlled by the cytosolic Ah receptor. The cytosolic and nuclear Ah receptors were studied in the liver from inbred C57BL/6N (Ahb/Ahb) mice, inbred DBA/2N (Ahd/Ahd) mice and heterozygotes (Ahb/Ahd) and homozygotes (Ahd/Ahd) derived from the (C57BL/6N X DBA/2N)F1 X DBA/2N backcross. After [3H-1,6]-2,3,7,8-tetrachlorodibenzo-p-dioxin (3H-TCDD) is given in vivo, the receptor in Ahb/Anb and Ahb/Ahd mice is detectable in the cytoplasm and nucleus; in Ahd/Ahd mice the receptor is not measurable in the cytosol, but is found in the nucleus at levels one fourth to one fifth of those in Ahb/Ahb mice. P1-450 (23S) mRNA content was estimated by Northern hybridization and by Rot analysis with a mouse P1-450 cloned cDNA. An excellent dose-response relationship (r = 0.99) was found between the amount of 3H-TCDD-Ah receptor complex appearing in the nucleus and the quantity of P1-450 mRNA induced in mice with all three possible Ah genotypes.