Cloning of DNA complementary to cytochrome P-450 induced by pregnenolone-16 alpha-carbonitrile. Characterization of its mRNA, gene, and induction response

Genomic structure and regulation of the rat hepatic CYP4F1 gene by peroxisome proliferators

The rat hepatic gene CYP4F1 encodes a fatty acid omega hydroxylase P450 that metabolizes proinflammatory eicosanoids and long-chain fatty acids. We have completely sequenced the CYP4F1 gene (Accession Nos. AF200361 and AF181083), identified multiple transcription start sites, and characterized a strong core promoter region, -760/116, induced by retinoic acids and peroxisome proliferators in rat hepatoma McA-RH7777 cells. Three peroxisome proliferator responsive elements (PPRE) bind both PPARalpha/RXRalpha and HNF4alpha. Co-transfection of McA-RH7777 cells with the -760/116 reporter construct and PPARalpha/RXRalpha or HNF4alpha showed that HNF4alpha activated while PPARalpha/RXRalpha inhibited CYP4F1 promoter activity. Treating cells with Wy14,643 reversed all initial effects, indicating co-regulation of CYP4F1 gene transcription by PPARalpha/RXRalpha and HNF4alpha. Chromatin immunoprecipitation analysis of cells treated with Wy14,643 showed association of PPARalpha/RXRalpha with the active transcription of the CYP4F1 gene while in clofibrate treated rats HNF4alpha binds during gene repression, suggesting differential regulation of the CYP4F1 gene in vivo and in cell lines.

The nuclear pregnane X receptor: a key regulator of xenobiotic metabolism

The nuclear pregnane X receptor (PXR; NR1I2) is an important component of the body's adaptive defense mechanism against toxic substances including foreign chemicals (xenobiotics). PXR is activated by a large number of endogenous and exogenous chemicals including steroids, antibiotics, antimycotics, bile acids, and the herbal antidepressant St. John's wort. Elucidation of the three-dimensional structure of the PXR ligand binding domain revealed that it has a large, spherical ligand binding cavity that allows it to interact with a wide range of hydrophobic chemicals. Thus, unlike other nuclear receptors that interact selectively with their physiological ligands, PXR serves as a generalized sensor of hydrophobic toxins. PXR binds as a heterodimer with the 9-cis retinoic acid receptor (NR2B) to DNA response elements in the regulatory regions of cytochrome P450 3A monooxygenase genes and a number of other genes involved in the metabolism and elimination of xenobiotics from the body. Although PXR evolved to protect the body, its activation by a variety of prescription drugs represents the molecular basis for an important class of harmful drug-drug interactions. Thus, assays that detect PXR activity will be useful in developing safer prescription drugs.

A mechanism-based pharmacokinetic-enzyme model for cyclophosphamide autoinduction in breast cancer patients

AIMS

This study investigated the pharmacokinetics of cyclophosphamide (CP) and its main metabolite 4-hydroxycyclophosphamide (4-OH-CP) in patients with breast cancer undergoing high dose chemotherapy prior to autologous stem cell transplantation. An enzyme turn-over model was also developed to study the time course of cyclophosphamide induction.

METHODS

Fourteen patients received a combination of CP (6 g m-2 ), thiotepum (500 mg m-2 ) and carboplatin (800 mg m-2 ) as a 96 h infusion. Plasma concentrations of CP and 4-OH-CP were determined with h.p.l.c. and a pharmacokinetic and enzyme turn-over model applied to data using NONMEM.

RESULTS

CP plasma concentrations were described by a two-compartment model with a noninducible and an inducible pathway, the latter forming 4-OH-CP. In the final enzyme model, CP affects the amount of enzymes by increasing the enzyme production rate. CP concentrations decreased during the infusion with no subsequent change in 4-OH-CP concentrations. CP inducible and noninducible clearance were estimated to 1.76 l h-1 (90% C.I. 0.92-2.58) and 1.14 l h-1 (0.31-1.85), respectively. The induction resulted in an approximately doubled CP clearance through the inducible pathway at the end of treatment. The model predicted the enzyme turn-over half-life to be 24 h.

CONCLUSIONS

The presented mechanism-based enzyme induction model where the pharmacokinetics of the inducer and the enzyme pool counterbalance each other successfully described CP autoinduction. It is reasonable to believe that CP affects its own elimination by increasing the enzyme production rate and thereby increasing the amount of enzyme by which CP is eliminated.

Nuclear receptor involvement in the regulation of rat cytochrome P450 3A23 expression

Many genes of the cytochrome P450 3A (CYP3A) subfamily, including several human and rat isoforms, are inducible by glucocorticoids. In the rat CYP3A23 gene, a 110-base pair segment of the proximal 5'-flanking region mediates dexamethasone activation. Three binding sites (DexRE-1, DexRE-2, and Site A), identified by DNase I footprinting analysis, were characterized for their relative contribution to both basal activity and dexamethasone inducibility. Site-directed mutagenesis of DexRE-1 (-144 to -169) and DexRE-2 (-118 to -136) demonstrated that each contained a core imperfect AGGTCA direct repeat, which comprised a consensus nuclear receptor binding site, and was essential for dexamethasone responsiveness but was not required for basal activity. Competition gel shift and supershift analyses revealed that both sites can bind the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor. Site A (-85 to -110) was shown to be important for both basal activity and dexamethasone responsiveness. Point mutants displayed a reduced (2-3-fold) induction response, compared with 15-fold for wild-type, which was accompanied by a 40-60% drop in basal activity. Site A was shown to bind the liver-enriched nuclear receptor hepatocyte nuclear factor 4. Our studies demonstrate that the mechanism mediating glucocorticoid-inducible transcriptional activity of CYP3A23 involves multiple binding sites for members of the nuclear receptor superfamily.

An orphan nuclear receptor activated by pregnanes defines a novel steroid signaling pathway

Steroid hormones exert profound effects on differentiation, development, and homeostasis in higher eukaryotes through interactions with nuclear receptors. We describe a novel orphan nuclear receptor, termed the pregnane X receptor (PXR), that is activated by naturally occurring steroids such as pregnenolone and progesterone, and synthetic glucocorticoids and antiglucocorticoids. PXR exists as two isoforms, PXR.1 and PXR.2, that are differentially activated by steroids. Notably, PXR.1 is efficaciously activated by pregnenolone 16alpha-carbonitrile, a glucocorticoid receptor antagonist that induces the expression of the CYP3A family of steroid hydroxylases and modulates sterol and bile acid biosynthesis in vivo. Our results provide evidence for the existence of a novel steroid hormone signaling pathway with potential implications in the regulation of steroid hormone and sterol homeostasis.

Analysis of rat cytochrome P450 isoenzyme expression using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR)

A method was developed using reverse transcriptase-polymerase chain reaction (RT-PCR) to selectively detect and qualitatively determine the levels of mRNA expression of the major isoenzymes of cytochrome P450 (P450 1A1, 1A2, 2B1/2, 2C11, 2E1, 3A1, 3A2, and 4A1) and fatty acyl-CoA oxidase (FACO) in the rat. Total liver RNA was isolated from male Sprague-Dawley rats treated with various inducers of cytochrome P450 (P450) and analyzed for the presence and relative quantities of each P450 isoenzyme mRNA using this technique. The specificity of the oligonucleotide primers used in the detection of each P450 mRNA was tested and confirmed through the simultaneous analysis of liver microsomal protein preparations for the presence of constitutive or inducible P450 apoprotein and enzyme activities using western immunoblotting and specific enzyme activity measures, respectively. This method of P450 expression analysis is proven to be highly specific and readily applicable for the assessment of P450 enzyme induction and down-regulation in the rat during routine toxicology studies when expression of the gene product is regulated by transcriptional activation and/or mRNA stabilization.

Modulation of gene expression by calreticulin binding to the glucocorticoid receptor

Calreticulin is a multifunctional protein that acts as a major Ca(2+)-binding (storage) protein in the lumen of the endoplasmic reticulum. It is also found in the nucleus, suggesting that it may have a role in transcription regulation. Calreticulin has been reported to bind to the synthetic peptide KLGFFKR, which is almost identical to an amino-acid sequence in the DNA-binding domain of the superfamily of nuclear receptors. Could calreticulin interact with the DNA-binding domain of these receptors and affect their function? Here we report that the amino terminus of calreticulin interacts with the DNA-binding domain of the glucocorticoid receptor and prevents the receptor from binding to its specific glucocorticoid response element. Overexpression of calreticulin in mouse L fibroblasts inhibits glucocorticoid-response-mediated transcriptional activation of a glucocorticoid-sensitive reporter gene and of the endogenous, glucocorticoid-sensitive gene encoding cytochrome P450. Together these results indicate that calreticulin may be important in gene transcription, regulating the glucocorticoid receptor and perhaps other members of the super-family of nuclear receptors.

V79 Chinese hamster cells genetically engineered for cytochrome P450 and their use in mutagenicity and metabolism studies

V79 Chinese hamster cells are being genetically engineered for stable expression of cytochromes P450 using a SV40 early promoter containing plasmid as the eukaryotic expression vector. V79 cells lack endogenous cytochrome P450 activity. Therefore, genetically engineered V79 cell lines are defined for the cytochrome P450 isoform acquired upon cDNA mediated gene transfer. V79 cells have a longstanding tradition as indicator cells for various biological endpoints, e.g. mutation, chromosomal aberration, cytotoxicity. The genetically engineered V79 cells combine these useful biological endpoints with metabolic competence. In this sense, V79 cell lines genetically engineered for stable expression of cytochromes P450 present newly developed tools for studying and understanding metabolism related problems in toxicology and pharmacology.

Effects of marginal and severe iron deficiency on hepatic proteins in developing rats are reversible with dietary iron repletion

Transferrin, metallothionein, cytochrome P-450, and the in vitro formation of DNA-benzo[a]pyrene adducts were studied in the offspring of dams that were fed diets moderately or severely deficient in iron (Fe). The study was designed to determine whether Fe deficiency-induced alterations were reversible or if they persisted with post-weaning iron repletion. Throughout gestation and lactation the dams were fed a Control diet = 120 micrograms Fe/g diet, a Marginal Iron diet = 11 micrograms Fe/g diet, or a Low Iron diet = 7 micrograms Fe/g diet. On day 14 of lactation, 4 pups per litter were killed. On day 21, the dams were killed. Half of the remaining pups in each litter were fed their respective diets until they were killed on day 42 (Marginal Iron-Marginal Iron and Low Iron-Low Iron groups). The other half were fed the Control diet (Marginal Iron-Control and Low Iron-Control groups). The dietary intake of the Restricted Fed offspring was matched to rats in the Low Iron-Low Iron group. Offspring in the iron-deficient groups had hematocrits, hemoglobin concentrations, and liver iron levels that were lower than Controls. Day 42 offspring in the iron-deficiency groups had a lower food intake and higher liver zinc and copper levels than Controls. Day 14 Marginal and Low Iron pups had liver metallothionein levels that were lower than Controls. Day 42 Restricted Fed offspring had liver metallothionein levels that were higher than all other groups. Cytochrome P-450 levels and the in vitro formation of benzo[a]pyrene-DNA adducts were higher in Low Iron-Low Iron males than in Control males. Ethoxycoumarin O-deethylase activity was higher in day 42 Low Iron-Low Iron offspring than in Controls. These results show that the iron deficiency-induced alterations were transient, reversible with iron repletion, and in the case of cytochrome P-450 and ethoxycoumarin O-deethylase activity, dependent on the age and sex of the animal.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Divergent effects of cycloheximide on the induction of class II and class III cytochrome P450 mRNAs in cultures of adult rat hepatocytes

We have previously reported that when hepatocytes isolated from adult male rats are cultured in serum-free medium on matrigel, a reconstituted basement membrane gel, it is possible to elicit a stimulation of gene expression for both Class II cytochrome P450b/e and Class III cytochrome P450p by phenobarbital treatment (E.G. Schuetz et al., 1990 J. Biol. Chem. 265, 1188-1192). In the present study, an investigation of the requirement of protein synthesis for the rise in mRNAs for these cytochromes, pretreatment of the cells with cycloheximide prior to adding phenobarbital or "phenobarbital-like" inducers to the culture medium inhibited induction of P450b/e mRNA (46-90%), whereas the accumulation of P450p mRNA was enhanced (2- to 19-fold). Heme depletion did not appear to explain these observations because the inhibitory effects of cycloheximide on the induction of P450b/e mRNA were not overcome by supplementation of the medium with exogenous heme or with delta-aminolevulinic acid. Because Class IIIA P450s are regulated by gender as well as by phenobarbital, we examined the basal expression of P450p mRNA in cultures of hepatocytes derived from male rats and found that cycloheximide treatment was without effect. However, in cultures of hepatocytes isolated from female rats, where P450p mRNA is barely detectable, cycloheximide treatment greatly enhanced expression of P450p mRNA. As was observed in the cultured cells, the treatment of living female rats with cycloheximide also increased the amounts of P450p mRNA to levels comparable to those found in livers of untreated male rats. Analysis of Northern blots hybridized with oligonucleotides specific for P450PCN1(IIIA1) and P450PCN2(IIIA2), respectively, revealed that untreated male rat liver and cultures of hepatocytes prepared from these animals expressed readily detectable amounts of P450PCN1(IIIA1) mRNA. Such analyses confirmed that cycloheximide treatment selectively increased P450PCN1(IIIA1) mRNA in female rat liver, whereas the amount of mRNA for P450PCN2(IIIA2), a closely related male-specific family member, was unaffected. We conclude that the pathways for the induction of P450b/e and P450p by phenobarbital, and the pathways for the gender-specific basal expression of P450PCN1(IIIA1) and P450PCN2(IIIA2) are not the same and can be distinguished by their differential response to inhibition of ongoing protein synthesis.

The in vivo induction of rat hepatic cytochrome P450-dependent enzyme activities and their maintenance in culture

Cytochrome P450-dependent enzyme activities were measured in hepatocytes from adult male rats, induced in vivo with phenobarbitone, beta-naphthoflavone, dexamethasone or isoniazid: the stability of the induced activities in culture was also determined. Each inducer produced a characteristically different pattern of enzyme activities with dexamethasone, isoniazid and beta-naphthoflavone selectively inducing erythromycin N-demethylase, p-nitrophenol hydroxylase and ethoxyresorufin O-dealkylase respectively. In general, the induced activities were maintained for 24 hr in culture. This indicates the feasibility of an in vivo induction-hepatocyte culture system for the study of metabolism-mediated toxicity.

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Structure of 5' regions and expression of phenobarbital-inducible rabbit cytochrome P450IIC genes

To analyze phenobarbital-inducible genes in the P450IIC subfamily, fragments of rabbit genomic DNA containing portions of the P450IIC1 and P450IIC4 genes were isolated and the DNA sequences of the 5'-flanking regions and exons were determined and compared to that of the P450IIC2 gene. The sequences have consensus TATA motifs about 20 bp from the mRNA initiation sites as determined by single-strand nuclease mapping and primer extension. Sequences similar to those for proposed liver-specific regulatory factors, HNF-1 and CCAAT, are found in the P450 5'-flanking regions. A glucocorticoid response element is present in the P450IIC2 gene but is not conserved in P450IIC1 or P450IIC4. Other sequences similar to binding sites for AP-1, the octamer binding proteins, and SPH-1 are present. Except for the TATA motif, none of the potential regulatory sequences was conserved at the same location in the different genes. The P450IIC1 and P450IIC2 genes exhibited high similarity detectable by dot matrix analysis throughout the 700 bp of sequenced 5'-flanking region while the P450IIC4 gene exhibited high similarity to both P450IIC1 and P450IIC2 only for about -150 nucleotides from the RNA initiation site. Southern analysis using probes from the 5' region of the P450IIC4 gene suggests that there are at least three closely related members in the P450IIC4 subgroup in agreement with previous studies. After phenobarbital treatment, levels of P450IIC4 mRNA increased about fourfold, as measured by dot blot hybridization with a P450IIC4-specific oligonucleotide probe or by single-strand nuclease mapping of the 5' end of the mRNA, indicating that the P450IIC4 gene characterized in this study is responsive to phenobarbital. Phenobarbital treatment of rabbits increased in vitro transcription of RNA for both the P450IIC1/2 and P450IIC4 gene subgroups but only slightly and transiently for the constitutive P450IIC3 gene.

Characterization of a cDNA encoding a new member of the glucocorticoid-responsive cytochromes P450 in human liver

Adult human liver contains a form of cytochrome P450, termed HLp, that resembles the glucocorticoid-inducible cytochrome P450p in rat liver in its structure, function, and regulation and catalyzes the oxidation of such clinically important substrates as cyclosporin, nifedipine, erythromycin, and midazolam. Recent evidence, however, suggests that HLp may represent two or more closely related forms of cytochromes P450, one of which is termed P450nf. To search for additional members of the Class III human subfamily of HLp related genes, we screened a human liver cDNA library cloned in phage vector lambda gt11 with oligonucleotides and with a cDNA fragment related to HLp. We isolated a full-length cDNA (1709 nucleotides) encoding a new form of human cytochrome P450 termed HLp2. Analysis of HLp2 cDNA predicted a protein of 502 amino acids, weighing 57,294 Da 83% similar to HLp. HLp2 appears to represent a distinct gene as judged by partial sequence analysis of a cloned human gene and by hybridizations of Southern blots, under conditions of varying stringency, with a 3'-portion of HLp cDNA and with an oligonucleotide specific for HLp2. Northern blot analysis revealed that HLp/P450nf was present in all samples of liver mRNA from adult patients not treated with inducers of HLp, whereas HLp2 mRNA was undetectable in more than two-thirds. Human fetal liver RNA contained mRNA species 2.1 and 1.9 kb which hybridized with an HLp2 oligonucleotide. We conclude that HLp2 represents a third member of the Class III glucocorticoid-responsive gene family that is expressed in both fetal and adult human liver and may account for polymorphism in metabolism of clinically important drugs.

Quantitation of mRNAs specific for the mixed-function oxidase system in rat liver and extrahepatic tissues during development

Evaluation of ontogenetic expression of the cytochrome P450PCN and cytochrome P450b gene families as well as the NADPH-cytochrome P450 oxidoreductase and epoxide hydrolase genes in Holtzmann rats showed that basal levels of mRNAs encoding these enzymes could be detected in most tissues. Distinct developmental patterns of mRNA expression are evident for these four proteins in liver and extrahepatic tissues. Levels of cytochrome P450b-like mRNA were comparable in adult lung and liver, while cytochrome P450PCN-homologous mRNA exhibited low levels in lung and approximately 100-fold higher levels in liver. Cytochrome P450PCN-homologous mRNA also reached substantial levels in adult intestine, and was also present in placenta, where it increased approximately 4-fold 24 h before birth. Epoxide hydrolase mRNA was demonstrated to be highest in liver followed by kidney, lung, and intestine but was extremely low in brain. NADPH-cytochrome P450 oxidoreductase mRNA in kidney, lung, prostate, adrenal, and intestine exhibited levels comparable to that found in liver; however, the pattern of expression for oxidoreductase mRNA was unique in that levels declined at maturity in liver, kidney, and intestine but not in lung and brain. Development of mixed-function oxidase and epoxide hydrolase activities in liver was distinct from that in other tissues in that mRNAs for all four proteins rose dramatically after parturition. Testis from immature males demonstrated low levels of all the mRNAs assayed, which ranged from 20% (oxidoreductase) to less than 1% (cytochrome P450PCN and epoxide hydrolase) of the levels found in liver.

Expression of cytochrome P-450 isozymes in the liver of hypophysectomized rats. Evidence for different regulation mechanisms concerning P450IIB and P450IIIA subfamilies

1. Monooxygenase activities have been examined in rat liver to determine the effects of castration and hypophysectomy on cytochrome P-450 species. In adult males, hypophysectomy caused a decrease of total P-450 concentration, aniline hydroxylase, benzopyrene hydroxylase, benzphetamine demethylase, testosterone hydroxylase and imipramine hydroxylase and demethylase activities. The treatment of hypophysectomized animals with human growth hormone or testosterone did not restore the full activity. 2. When probed with antibodies, microsomes from hypophysectomized males and females exhibited an intense reaction with a polyclonal anti-(phenobarbital-induced P-450) which was not observed with a monoclonal antibody of anti-(phenobarbital-induced P-450). 3. These microsomal preparations also reacted with an antibody raised against a developmentally regulated P-450. No sex difference could be detected with this antibody. Furthermore, administration of human growth hormone to hypophysectomized males prevented this immunoreaction. 4. Total RNA has been prepared from the same liver; when probed with cDNAs, no changes occurred in the content in P-450 b/e, PB 24 (a constitutive member of the phenobarbital subfamily) and phenobarbital-inducible mRNA for UDP-glucuronosyltransferase. 5. In contrast, P-450 mRNA induced by pregnenolone 16 alpha-carbonitrile was modulated by hormonal manipulations: lower in females and castrated males than in intact males, increased in both sexes after hypophysectomy. Treatment of hypophysectomized males with human growth hormone abolished this rise in pregnenolone-16 alpha-carbonitrile-induced P-450 mRNA accumulation. Data collected in this study support the assumption that hypophysectomy acts differently on the regulation of various P-450 isozymes and that this regulation clearly does not involve the phenobarbital subfamily of P-450s.

Induction of cytochrome P-450 mRNA in rainbow trout: in vitro translation and immunodetection

The time course of induction of the rainbow trout microsomal hepatic monooxygenase (MO) system was examined by determination of levels of mRNA and corresponding levels of catalytic activity. Animals were pretreated with beta-naphthoflavone (beta-NF, ip, 100 mg/kg) and terminated at 0, 2, 6, 18, and 48 hr postinjection. Levels of mRNA were determined by immunoprecipitation of in vitro translation products. Levels of mRNA coding for the cytochrome P-450 LM4b isozyme were maximally increased (13-fold) at 18 hr and had decreased almost to pretreatment levels by 48 hr post-treatment. This was in contrast to the catalytic activity in which ethoxyresorufin-O-deethylase (EROD) and ethoxycoumarin-O-deethylase (ECOD) were significantly elevated at both 18 hr (25- and 5-fold, respectively) and 48 hr (46- and 8-fold, respectively). Pretreatment with beta-NF (ip, 100 mg/kg) or 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB, ip, 150 mg/kg) for 18 hr resulted in significant differences in levels of mRNA in only the beta-NF-treated group. The LM2 P-450 isozyme could not be detected by immunoprecipitation with anti-LM2 IgG in trout treated with these same inducers. The results suggest a difference between the time course of induction of the mRNA for cytochrome P-450 LM4b isozyme and the induction of catalytic activity. Under the detection system utilized, the results suggest that the phenobarbital-like inducer, 6-CB, does not induce cytochrome activity nor does it induce the mRNA for cytochrome P-450 LM4b isozyme.

Multiplicity of steroid-inducible cytochromes P-450 in rat liver microsomes

Highly purified preparations of steroid-inducible cytochromes P-450 have been isolated from liver microsomes of adult male Sprague-Dawley rats treated with pregnenolone-16 alpha-carbonitrile (PCN), phenobarbital (PB), or triacetyloleandomycin (TAO). The interrelationships among these preparations as well as their relationship to the major forms of cytochrome P-450 of this gene family identified in other laboratories have been evaluated by amino-terminal sequence analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, chromatography on DEAE-Sephacel, and Ouchterlony double-immunodiffusion. The results indicate that a cytochrome P-450 previously isolated in this laboratory and referred to as PCNa (P.E. Graves, L.S. Kaminsky, and J. Halpert (1987) Biochemistry 26, 3887-3894) is the major cytochrome P-450 induced in rats by TAO, and corresponds to P-450p (K.A. Hostetler, S.A. Wrighton, P. Kremers, and P.S. Guzelian (1987) Biochem. J. 245, 27-33) and to P450PCN1 (F.J. Gonzalez, B.J. Song, and J.P. Hardwick (1986) Mol. Cell. Biol. 6, 2969-2976). A second previously isolated cytochrome P-450 termed PCNb in this laboratory appears to be identical to PB/PCN-E (F.P. Guengerich, G.A. Dannan, S.T. Wright, M.V. Martin, and L.S. Kaminsky (1982) Biochemistry 21, 6019-6030). PCNb as well as a third cytochrome P-450 termed PCNc isolated from PB-treated rats both correspond in amino-terminal sequence to the putative protein product of the pP450PCN2/cDNA clone of Gonzalez et al. These results document at the protein level the multiplicity of steroid-inducible rat liver cytochromes P-450.

Regulation of gene expression in adult rat hepatocytes cultured on a basement membrane matrix

Freshly isolated adult rat hepatocytes, when cultured on type I collagen (commercially available as Vitrogen), assume a polygonal shape, form a stable monolayer within 24 hours, but lose the capacity to express some liver-specific functions over time in culture. We incubated hepatocytes in a serum-free medium on a reconstituted basement membrane gel, "matrigel" (prepared from an extract of extracellular matrix of the murine Engelbreth-Holm-Swarm sarcoma), and observed that the cells adhered firmly, remained rounded as single cells or clusters, and maintained liver-specific gene expression for more than 1 week in vitro. Hepatocytes on matrigel secreted substantially higher amounts of albumin, transferrin, haptoglobin, and hemopexin, Northern blot analyses of extracted cellular RNA, expressed increased amounts of mRNA for the liver-specific protein albumin (as compared with cells on vitrogen). In cultures treated with phenobarbital, cytochrome P-450b, and cytochrome P-450e, mRNAs and proteins were barely detectable in cells on Vitrogen but were induced to levels similar to those in the liver in vivo in matrigel cultures. Likewise, the use of matrigel greatly enhanced the induction of mRNA and protein for P-450c by 3-methylcholanthrene and for P-450p by steroidal and nonsteroidal inducers. However, neither substratum permitted induction of P-450d by 3-methylcholanthrene, suggesting that the effects of matrigel are selective even for expression in liver of members of the superfamily of cytochrome P-450 genes. Within 5 days in cultures on Vitrogen, hepatocytes expressed detectable amounts of fetal liver aldolase activity and also mRNA for vimentin and type I collagen, each considered a phenotypic change reflecting hepatocyte "dedifferentiation." None of these was present in cells on matrigel. Responsiveness to mitogenic stimuli, as judged by incorporation of 3H-thymidine into DNA, was also decreased in hepatocytes cultured on matrigel. Finally, there was a remarkable increase in the levels of both matrices during the first 2 days in culture. However, the continuously cytoskeleton mRNA over time in culture than did the rounded cells on matrigel. We conclude that hepatocytes cultured on matrigel, as opposed to the standard collagen, exhibit remarkably enhanced expression of many liver-specific functions.

Human P450PCN1: sequence, chromosome localization, and direct evidence through cDNA expression that P450PCN1 is nifedipine oxidase

P450PCN protein levels and nifedipine oxidase activities were quantitated in 12 human livers and were shown to be highly correlated. Antibody against rat P450PCN1 completely inhibited all nifedipine oxidase activity in three human liver samples. These results suggest that a human P450 related to rat P450PCN1 is the major form of P450 catalyzing nifedipine oxidation. The cDNA for a human P450, designated phPCN1, was isolated from a human liver lambda gt11 cDNA library, and sequenced completely. The deduced amino acid sequence is 77% similar to rat P450PCN1. By use of the adenovirus- and SV40-based expression vecotr p91023(B), the phP450PCN1 cDNA was expressed in COS cells and had high nifedipine oxidase activity, providing conclusive evidence that this P450 is the primary enzyme associated with metabolism and inactivation of this important drug. Using somatic cell hybrids, the P450PCN gene was localized to human chromosome 7.

Molecular regulation of ethanol-inducible cytochrome P450-IIEI in hamsters

Liver polysomal poly(A)+ RNA, isolated from hamsters treated with ethanol or pyrazole, was translated in vitro to determine the effect of these compounds on specific mRNA encoding P450-IIEI, an ethanol-inducible P450 isozyme. As assessed by immunoprecipitation of translation products, ethanol and pyrazole increased hepatic P450-IIEI mRNA levels by 160% and 45%, respectively, when compared to controls. In liver microsomes from the same animals, ethanol and pyrazole caused a two-fold increase in microsomal P450-IIEI protein and a two- to three-fold enhancement of microsomal ethanol oxidation and p-nitrophenol hydroxylation. Our results show that the induction of P450-IIEI protein in hamsters by ethanol and pyrazole, an "ethanol-like" inducer, is accompanied by an increase in translatable P450-IIEI mRNA.

Complementary DNA and amino acid sequence of rat liver microsomal, xenobiotic epoxide hydrolase

The coding nucleotide sequence for rat liver microsomal, xenobiotic epoxide hydrolase was determined from two overlapping cDNA clones, which together contain 1750 nucleotides complementary to epoxide hydrolase mRNA. The single open reading frame of 1365 nucleotides codes for a 455 amino acid polypeptide with a molecular weight of 52,581. The deduced amino acid composition agrees well with those determined by direct amino acid analysis of the rat protein, and the amino acid sequence is 81% identical to that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and comparison to codon usage for NADPH-cytochrome P-450 oxidoreductase and cytochromes P-450b, P-450d, and P-450PCN, suggest that epoxide hydrolase is more conserved than cytochromes P-450b and P-450PCN; comparison of the extent of sequence conservation for 12 homologous proteins between the rat and rabbit, including cytochrome P-450b, supports this hypothesis, and indicates that much of epoxide hydrolase is constrained to maintain its hydrophobic character, consistent with its intramembranous location. The predicted membrane topology of epoxide hydrolase delineates 6 membrane-spanning segments, less than the 8 or 10 predicted for two cytochrome P-450 isozymes; the lower number of membrane-spanning segments predicted for epoxide hydrolase correlates with its lesser dependence on the membrane for maintenance of its tertiary structure and catalytic activity.

Induction of different isozymes of cytochrome P-450 and of microsomal epoxide hydrolase in rat liver by 2-acetylaminofluorene and structurally related compounds

The amounts of five different forms of cytochrome P-450 and of microsomal epoxide hydrolase were determined immunochemically in rat liver microsomes before and after treatment of the animals with 2-acetylaminofluorene and 15 structurally related compounds. The amount of cytochrome P-450c was found to be increased about 60-fold after treatment with 2-aminofluorene and 3-aminofluorene. Administration of 1-aminofluorene, 4-aminofluorene, 2-acetylaminofluorene and nitrofluorene increased this isozyme about 15-19 times. 2-Aminofluorene was found to inhibit the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to a cytoplasmic receptor 50% at a concentration of 3.12 microM, while no such inhibition could be detected with 2-acetylaminofluorene. Induction of ethoxyresorufin O-deethylase activity was found to be highly correlated (+0.95) with the induction of cytochrome P-450c. Also correlated with the induction of this form was the amount of cytochrome P-450d (+0.84), which could be maximally increased about fourfold. Cytochromes P-450b + e were induced by 2-acetylaminofluorene, 4-acetylaminofluorene and fluorene (about tenfold), while 4-aminofluorene and 4-acetylaminofluorene were found to elevate cytochrome P-450PB/PCN-E about threefold. Microsomal epoxide hydrolase was induced by many of the compounds tested, with 2,7-diaminofluorene, 2,7-diacetylaminofluorene, 2-acetylaminofluorene and 2-(N-hydroxy)acetylaminofluorene being the most potent. No correlation of the induction of this enzyme with the induction of any isozyme of cytochrome P-450 was observed.

Expression of rat liver cytochrome P-450MC cDNA in Saccharomyces cerevisiae

Rat liver cytochrome P-450MC cDNA was inserted between the ADH1 promoter and terminator regions of the yeast expression vector pAAH5. On introduction of the resulting recombinant plasmid pAMC1, Saccharomyces cerevisiae cells synthesized up to 8 X 10(5) molecules per cell of the cytochrome P-450MC protein, most of which was localized in yeast microsomes. Approximately half of the synthesized cytochrome contained heme in the enzyme molecule. These formed a functional electron-transport chain in the microsomes which exhibited aryl hydrocarbon hydroxylase activity toward benzo[a]pyrene.