Molecular induction by phenobarbital of a rat hepatic form of cytochrome P-450: expression of a 4-kilobase messenger RNA

Nuclease hypersensitivity of the rat cytochrome P450IA1 gene

The bovine pancreatic deoxyribonuclease I (DNAase I) hypersensitivity of the rat cytochrome P450IA1 gene was investigated. A nuclease-hypersensitive region was observed at approximately 3.2 to 5.1 kilobase pairs upstream of exon 1 in adult and fetal rat liver. This region did not necessarily correlate with gene expression following 3-methylcholanthrene induction, although it may determine the potential for inducibility of this gene.

Simple sequence DNA associated with near sequence identity of the 3'-flanking regions of rat cytochrome P450b and P450e genes

In rat liver, the two major phenobarbital (PB)-inducible cytochrome P450s, P450b (P450IIB1), and P450e (P450IIB2), are encoded by approximately 2.1-kb mRNAs showing more than 97% nucleotide sequence identity. Almost half of the sequence differences are concentrated in two short divergent segments in exon 7. An additional 4.8-kb P450b/P450e RNA, inducible by Aroclor and by PB, hybridizes with a classical P450b probe and with the 3' extension of the PB23 insert, a cloned P450b-like cDNA (Affolter et al., DNA 5, 209-218, 1986). The 4.8-kb form has now been detected in several rats under a variety of induction conditions. DNA sequencing of the 5' portion of the PB23 insert showed it is derived from a P450b gene. The 4.8-kb RNA hybridized with an oligonucleotide probe that recognizes P450b mRNA, but did not hybridize detectably with one that recognizes P450e mRNA. The 4.8-kb RNA and the PB23 insert doubtless represent P450b RNAs polyadenylated at a downstream site. DNA sequence analysis of the PB23 3' extension (which represents the 3' end of the P450b gene) and of the P450e gene 3'-flanking region demonstrated that the near identity of the P450b and P450e genes extends for at least 920 bp beyond the major polyadenylation site. This region was doubtless part of the original duplication that gave rise to the P450b and P450e genes. A short divergent segment is present in the 3'-flanking region of near sequence identity; the segment is embedded in simple sequence DNA that contains a mixed alternating pyrimidine/purine tract.

Multiple constitutive but phenobarbital-inducible rat hepatic nuclear RNA species homologous to a novel cytochrome P-450 cDNA

A cDNA clone, pPB8, representing partial information for a phenobarbital-inducible rat hepatic cytochrome P-450, immunochemically related to cytochrome P-450b and/or P-450e, hybridized to multiple hepatic nuclear RNA species. In addition to the 3.7 +/- 0.2 kb mRNA encoding this novel cytochrome P-450 isozyme, pPB8 hybridized to nuclear RNAs of 4.9 +/- 0.3, 5.4, 5.7 +/- 0.2, and 6.3 +/- 0.1 kb. These nuclear RNAs were constitutively expressed and were inducible to various extents by phenobarbital administration. The time course of induction of these nuclear RNA components suggested product-precursor relationships. A "full-length" cDNA clone, pPB8/7, synthesized from poly(A)+ RNA homologous to pPB8, detected two mRNA species of 4.6 and 1.8 kb. The 4.6 kb nuclear RNA was inducible by 3-methylcholanthrene, Aroclor 1254, and phenobarbital, while the 1.8 kb nuclear RNA was not appreciably affected. It is suggested that pPB8 and pPB8/7 were synthesized from distinct mRNAs that share homology in their 3' regions.

Northern hybridization analysis of RNA using diethylpyrocarbonate-treated nonfat milk

A simple and relatively inexpensive method for hybridizing RNA that is immobilized on nitrocellulose to a radioactive DNA probe is presented. The procedure, a modification of the Bovine Lacto Transfer Technique Optimizer method of Johnson et al. (1984) Gene Anal. Tech. 1, 3-8, uses nonfat milk which has been treated with the RNase inhibitor, diethylpyrocarbonate (DEPC), to saturate nonspecific binding sites on nitrocellulose paper, and to wash off unbound radioactivity. The DEPC-nonfat milk hybridization technique is faster, less costly, and more reliable than standard Northern blot hybridization conditions, yielding sharp, clear bands of RNA on autoradiographs with virtually no background reactivity.

cDNA clones for liver cytochrome P-450s from individual aroclor-treated rats: constitutive expression of a new P-450 gene related to phenobarbital-inducible forms

Differential hybridization and screening with cloned inserts was used to identify two families of cytochrome P-450 cDNA clones in libraries prepared from total liver poly(A)+RNA of individual Aroclor-treated rats. One family has cDNA inserts for the major phenobarbital-inducible P-450s, P-450b and P-450e. Two types of P-450e inserts were identified. In addition, irregular inserts were characterized from two clones (PB23 and PB24) of this group. The other family has cDNA inserts for the major 3-methylcholanthrene-inducible species, P-450c and P-450d. No coding sequence restriction site variants were detected among 26 P-450d and P-450c inserts analyzed. The restriction map of the irregular 2.2-kb PB23 insert has a P-450b-like portion, followed by a 3' extension that hybridizes to RNAs of 2.7 and 4.8 kb, which are also detectable with a classical P-450b probe. The PB23 insert and the 2.7- and 4.8-kb RNAs presumably represent 3' extensions of P-450b/P-450e mRNAs, polyadenylated at downstream sites. The 858-bp sequence of the PB24 insert encodes the carboxy-terminal portion of a P-450b/P-450e-like protein. There is approximately 20% divergence at the polypeptide level between the PB24 and P-450b/P-450e sequences; nevertheless, they share many essential features. A PB24-specific probe hybridizes to a 1.9-kb RNA species which is present in the liver of untreated rats and which is not appreciably induced by phenobarbital or Aroclor. The PB24 cDNA most likely represents a constitutive cytochrome P-450, related to phenobarbital-inducible forms.

Tissue-specific expression of rat mRNAs homologous to cytochromes P-450b and P-450e

The tissue-specific expression of cytochrome P-450b and P-450e mRNAs was examined with synthetic 18-mer oligomer probes in the liver, lung, kidney, and testis of control and inducer pretreated adult rats. RNAs homologous to the P-450e probe were detected in trace amounts in control and 3-methylcholanthrene (MC) induced livers and at high levels in livers from phenobarbital (PB) induced animals. P-450e mRNA levels were below detection limits in the other tissues examined, regardless of pretreatment. In contrast, mRNAs homologous to the P-450b oligomer were detected at low levels in control and inducer pretreated lung and testis, and at high levels in PB induced liver. No P-450b mRNAs were detected in these assays in RNA isolates from the kidney or from control or MC pretreated liver. Solution hybridization data indicated that the rat lung contained 9-12%, and the testis, 6-9%, respectively, of the levels of P-450b mRNA measured in the PB induced liver. Results from oligo(dT)-cellulose and poly(U)-affinity experiments indicated that the hepatic mRNAs for P-450b and P-450e were present predominantly in the bound, polyadenylated fraction, whereas the homologous lung and testes P-450b mRNAs predominated in the flow-thru fractions.

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.

3-Methylcholanthrene-induced expression of the cytochrome P-450c gene

Transcriptional control of 3-methylcholanthrene-dependent cytochrome P-450c nuclear RNA induction was directly observed in an in vitro rat liver nuclear transcription system. Mercurated and radiolabeled ribonucleotides were incorporated into nuclear RNA transcribed in vitro, which was then isolated using thiopropyl-Sepharose 6B affinity chromatography. Dot hybridization experiments were carried out using bacteriophage M13 subclones of pRSA57 (a cDNA clone for rat serum albumin), pEB339 (a cDNA clone for rat cytochrome P-450c), and clone 46 (a cDNA clone for mouse cytochrome P1-450). The results of these studies demonstrate that 3-methylcholanthrene does not significantly influence the transcription of the rat serum albumin gene, but does increase the transcription of the cytochrome P-450c gene. Nuclear RNA precursors to the cytochrome P-450c mRNA were characterized by Northern blot analysis. Clone 46 hybridized to nuclear RNA species of 6.7 and 4.0 kb, in addition to the 3.0-kb cytochrome P-450c mRNA. pA8 (a genomic clone for rat cytochrome P-450c), hybridized to the same nuclear RNA species in addition to nuclear RNA species of 4.3, 3.4, and 2.2 kb. M13pd15 (a genomic clone containing information for the first intron of the cytochrome P-450c gene) hybridized to nuclear RNA species of 6.7 and 4.3 kb. All of these nuclear RNA species are polyadenylated. The mRNA coding for cytochrome P-450c was induced maximally in hepatic nuclei at 3 h following 3-methylcholanthrene administration. Maximal accumulation of cytochrome P-450c mRNA in hepatic cytosol has been previously shown to occur at approximately 15 h following 3-methylcholanthrene administration (Bresnick, E., Brosseau, M., Levin, W., Reik, L., Ryan, D. E., and Thomas, P. E. (1981) Proc. Natl. Acad. Sci. USA 78, 4083-4087). These data implicate a possible role of nuclear RNA transport in the regulation of induction of cytochrome P-450c, although further investigations are indicated.

Dealkylation of pentoxyresorufin: a rapid and sensitive assay for measuring induction of cytochrome(s) P-450 by phenobarbital and other xenobiotics in the rat

The O-dealkylation of pentoxyresorufin (7-pentoxyphenoxazone) by rat liver microsomes was examined. The reaction appeared highly specific for certain phenobarbital inducible forms of cytochrome P-450 and was increased 95- to 140-fold by animal pretreatment with phenobarbital (75 mg/kg/day, four ip injections) and approximately 50-fold by Aroclor 1254 (500 mg/kg, one ip injection) while animal pretreatment with 3-methylcholanthrene (50 mg/kg/day, three ip injections) resulted in less than a 2-fold increase over the rate detected in control microsomes. It was observed that this activity, in microsomes for Aroclor-pretreated rats, was dependent on O2 and was inhibited by metyrapone and SKF 525-A, indicative of cytochrome(s) P-450 mediation in the reaction. When antibodies directed against purified cytochrome(s) P-450s were employed to inhibit the pentoxyresorufin O-dealkylation reaction, antibodies to P-450PB-B greatly inhibited the reaction (greater than 90%), while antibodies to P-450PB-C or P-450PB/PCN-E had minimal effects. Assay of hepatic microsomes from rats which were pretreated with varying doses of phenobarbital (0.9-75 mg/kg/day, four ip injections) indicated that while aminopyrine-N-demethylase activity was induced only 2-fold at the maximum dose (75 mg/kg/day), pentoxyresorufin O-dealkylase activity was induced approximately 140-fold at this dose and approximately 4-fold by a dose of phenobarbital as low as 0.9 mg/kg.

Molecular cloning of steroid hydroxylases

Recombinant plasmids specific for bovine adrenocortical cytochromes P-450scc and P-45011 beta have been identified and characterized. Using these cDNA inserts as probes, it is found that tissue specificity of gene expression for these two proteins is as expected. Cytochrome P-450scc mRNA is found in adrenocortical and corpus luteum RNA while cytochrome P-45011 beta mRNA is found only in adrenocortical RNA. Neither mRNA was detected in heart, liver or kidney RNA. Cytochrome P-450scc mRNA is found to be 1.9 kb in length while cytochrome P-45011 beta mRNA is found to be 4 kb in length. Treatment of bovine adrenocortical cells with ACTH or other modulators results in increased levels of cytochrome P-450scc and cytochrome P-45011 beta mRNAs. Cytochrome P-450scc is found to be encoded by a limited number or even a single gene.