Differential effect of phenobarbital and beta-naphthoflavone on the mRNAs coding for cytochrome P450 and NADPH cytochrome P450 reductase

Effects of phenobarbital and interleukin-6 on cytochrome P4502B1 and 2B2 in cultured rat hepatocytes

The objectives of this study were to characterize further the effects of phenobarbital (PB) on cytochrome P4502B1 and 2B2 (P4502B1/2) enzyme activity and immunoreactivity in rat hepatocytes and to investigate the mechanism(s) mediating the ability of interleukin-6 (IL-6) to inhibit this induction. PB caused a concentration-dependent increase in benzyloxyresorufin O-deethylase (BROD) activity with maximal effects (a 25-fold increase) at concentrations of 0.3 to 1 mM. The induction of BROD activity was linear over 24 hr of exposure. Immunoblot profiles of P4502B1/2 agreed with measurements of enzyme activity. In addition to inducing P4502B1/2, PB (0.75 mM) also increased the levels of P450 reductase by approximately 2-fold following a 24-hr exposure to PB. When IL-6 was added concomitantly with or up to 12 hr after the addition of PB, the PB induction of BROD activity and immunoreactivity was inhibited significantly. When 18 hr elapsed between the time of addition of PB and IL-6, the inhibitory effects of IL-6 were no longer apparent, suggesting that the actions of IL-6 were mediated by early events in the induction process. IL-6 did not affect the PB induction of P450 reductase. To determine whether IL-6 altered the degradation of P4502B1/2, hepatocytes were exposed to PB for 24 hr, then washed, and the loss of BROD activity and immunoreactivity following incubation with a protein synthesis inhibitor was measured. IL-6 did not alter the rate of loss of either enzyme activity or immunoreactivity, indicating that the effects of IL-6 could not be attributed to the enhanced degradation of P4502B1/2. Results suggest that the inhibition of PB-induced BROD activity by IL-6 is due to an action on early cellular and molecular events in the induction process.

Interaction of proteins with a cytochrome P450 2B2 gene promoter: identification of two DNA sequences that bind proteins that are enriched or activated in response to phenobarbital

Cytochromes P450 (CYPs) are of central importance in the metabolism of foreign hydrophobic compounds. Members of the CYP2B subfamily are inducible at the transcriptional level by the barbiturate, phenobarbital. Owing to the lack of a suitable phenobarbital-responsive cell line, very little is known regarding the mechanisms by which phenobarbital induces the expression of these genes. We report the use of gel retardation and DNase I footprinting to investigate the presence of regulatory protein binding sites within a CYP2B2 gene promoter. Two DNA sequences, located between -183 to -199 and -31 to -72, have been identified that bind rat liver nuclear proteins that are enriched or activated in vivo by phenobarbital. Gel retardation competition experiments demonstrated that the two sequences bound different proteins. In vitro transcription competition experiments demonstrated that the sequences and the proteins with which they interact are involved in regulating CYP2B2 gene transcription. These two DNA sequences and their cognate binding proteins may play a role in the induction of CYP2B2 gene expression in response to phenobarbital.

Purification and characterization of NADPH-cytochrome P-450 reductase from rat epidermis

NADPH-cytochrome P-450 oxidoreductase (P-450 red) transfers reducing equivalents from NADPH to cytochrome P-450 (P-450) in the monooxygenase system. Detergent solubilized proteins from the membrane fraction of neonatal rat epidermis were purified by 2',5'-ADP-agarose affinity column chromatography. The purified protein showed an apparent homogeneity on sodium dodecylsulfate-polyacrylamide gel electrophoresis and molecular weight was estimated to be 78 kDa. NADPH-cytochrome c reductase activity increased by 95-fold in the purified enzyme. Epidermal P-450 red in vitro reconstituted benzo(a)pyrene hydroxylase activity in a dose dependent manner with P-450 purified from either rat liver or epidermis. Western blot analysis demonstrated that epidermal P-450 red immunologically cross reacts to liver P-450 red. Immunohistochemical staining showed that the enzyme was predominantly localized in the epidermis. The intensity of immunohistochemical staining of rat skin sections and tissue distribution did not change in the skin treated with beta-naphthoflavone, which results in a substantial increase in P-450 1A1 activity. Quantitative assessment of P-450 red in treated and untreated epidermis also showed no change. These findings indicate that constitutive P-450 red, fully capable of supporting P-450, exists in rat epidermis, and can function in metabolism of endogenous and exogenous compounds.

Maintenance and induction in co-cultured rat hepatocytes of components of the cytochrome P450-mediated mono-oxygenase

Hepatocytes grown in culture rapidly lose many of the cytochromes P450 (CYP) responsible for metabolizing foreign compounds. Among the proteins most readily lost are members of the CYP2B subfamily. We have investigated, by RNase protection assays, the ability of rat hepatocytes, cultured conventionally or co-cultured with rat liver epithelial cells, to maintain the expression of genes encoding members of the CYP2B subfamily, and the inducibility of this expression by phenobarbital. After 4 days of conventional hepatocyte culture CYP2B mRNAs were undetectable, but remained inducible by phenobarbital. In co-cultured hepatocytes the abundance of the mRNAs remained relatively constant from 4-14 days. After 7 days of co-culture the concentration of the mRNAs was increased 12-15-fold by phenobarbital. RNase protection assays with probes capable of distinguishing between CYP2B1 and 2B2 mRNAs demonstrated that the ratios of the abundance and inducibility of the two mRNAs were the same in co-culture as in vivo. Co-cultured hepatocytes also maintained the expression of genes coding for two other components of the cytochrome P450-mediated mono-oxygenase, namely cytochrome P450 reductase and cytochrome b5.

Quantification of cytochrome P450 reductase gene expression in human tissues

We have isolated and sequenced cDNA clones that code for a variant of human cytochrome P450 reductase. An RNase protection assay was used to quantify the corresponding mRNA in adult and fetal tissues. The results demonstrate that, in the samples analyzed, the cytochrome P450 reductase gene displays very little inter-individual variation in its expression in adult liver and is subject to little developmental or tissue-specific regulation.

Chemostat studies of microsomal enzyme induction in Saccharomyces cerevisiae

Using cells grown in a chemostat at steady state, the levels of various components of the microsomal electron transport chain of Saccharomyces cerevisiae were examined. Cytochrome P450 haemoprotein levels measured in cells grown in medium with a dissolved oxygen concentration of 15% were induced between 10- and 20-fold over levels in cells grown in medium containing 70% dissolved oxygen concentration. An increase in the dilution rate of a culture growing in medium containing 15% dissolved oxygen resulted in an increase in the residual glucose concentration of the medium. This was paralleled by an increase in the microsomal levels of cytochrome P450. The rise could not be attributed either to increases in the concentration of ethanol in the chemostat or to an increase in the proportion of energy generated using fermentative pathways. However, this effect was not observed in cells grown in an oxygen concentration of 70%. Cytochrome b5 haemoprotein levels were also induced approximately three-fold by reducing the dissolved oxygen concentration from 70% to 15%. Changes in the medium glucose concentration from 0.03% to 1.6% (w/v) had no effect on the levels of this enzyme. Conversely, levels of cytochrome P450 NADPH reductase appeared lower in cells grown in 15% as opposed to 70% dissolved oxygen concentration. Northern slot blot analysis of total RNA extracted from chemostat-grown cells, probed with a C-14 sterol demethylase cytochrome P450 gene (cytochrome P450 LIA1), revealed a pattern of message induction which matched that of the cytochrome P450 haemoprotein, indicating that control of the levels of this enzyme was at least partially transcriptional.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of a human cytochrome P-450 reductase cDNA clone and localization of the corresponding gene to chromosome 7q11.2

We have isolated and sequenced cDNA clones that code for rat and human NADPH-dependent cytochrome P-450 reductase. The cDNA coding for the human protein was used to analyse, by Southern blot hybridization, DNA isolated from a panel of 8 independent human-rodent somatic cell hybrids. The results indicate that cytochrome P-450 reductase is encoded by a single gene (POR) located on human chromosome 7(pter-q22). Analysis of human metaphase chromosomes by hybridization in situ confirmed the results and refined the localization to 7q11.2. Northern blot hybridization revealed that in human liver the expression of the gene varies by less than 3-fold between different individuals.

The effect of terpenoid compounds on cytochrome P-450 levels in rat liver

We have investigated the ability of camphor, menthol, pinene, limonene and myrcene to induce in rats members of a cytochrome P-450 sub-family termed PB P-450. These proteins have recently been designated as members of the P450IIB sub-family. None of these naturally occurring terpenoids significantly changed the total content of cytochromes P-450 or cytochrome b5. Radioimmunoassay results showed that PB P-450 was induced 6-fold by camphor and to a lesser extent by menthol and pinene. The induction was confirmed by Western blotting. It was shown by nucleic acid hybridization that induction of PB P-450 by terpenoids was mediated by an increase in the amount of the corresponding mRNA. Analysis of the denaturation of mRNA-cDNA hybrids demonstrated that the mRNA induced by the terpenoids was encoded by a member of the P450IIB sub-family. None of the terpenoids had an effect on the amount of mRNA coding for P450IA2 (a cytochrome P-450 inducible by beta-naphthoflavone and isosafrole). The results indicate that cytochromes P-450 induced by a synthetic compound, phenobarbital, may have originally evolved in response to terpenoid compounds normally present in the environment.

Induction of cytochrome P-450 by phenobarbital is mediated at the level of transcription

We have previously shown that the 43-fold induction by phenobarbital of the major phenobarbital-inducible cytochrome P-450 of rat liver microsomal membranes (PB P-450) is mediated by a 20-fold increase in the amount of its mRNA in the cytoplasm. Here we demonstrate that the induction of the mRNA can be almost entirely accounted for by an increase in the rate of transcription of genes coding for PB P-450, and involves little or no change in the rates of processing, transport or degradation of the mRNA. Phenobarbital treatment resulted in no amplification or rearrangement of PB P-450 genes.

Cloning of the CDC7 gene of Saccharomyces cerevisiae in association with centromeric DNA

The cell-division-cycle gene, CDC7, of Saccharomyces cerevisiae has been cloned in a large plasmid (pCM6) containing an insert of about 45 kb of yeast chromosomal DNA in the yeast-Escherichia coli shuttle vector, YRp7. A subclone (pCM39), having a 7-kb insert with a unique BamHI site, was capable of conferring the ts+ phenotype on a cdc7ts- strain of S. cerevisiae. When this insert was placed in the vector Y1p28, and then cut with BamHI and integrated into the ts- strain, genetic analysis showed the CDC7-complementing activity and an associated vector marker to be linked to the TRP1 locus, i.e., to chromosome IV. Analysis of subclones covering the pCM39 insert showed that pCM52, in which a 3.3-kb EcoRI-HpaI fragment with the unique BamHI site has been cloned, possessed the full complementing activity. Although the CDC7 gene lies across the BamHI site, the main part of the gene appears to be in the 2.2-kb BamHI-HpaI segment of the pCM52 insert. A Northern blot analysis showed this region to give rise to a 1.7-kb mRNA which we conclude is the transcript of CDC7. The large insert of pCM6 also contains a 1.5-kb XhoI fragment which several observations suggest is that containing the centromere (CEN4) of chromosome IV to which the CDC7 gene is closely linked. The 45-kb DNA fragment in pCM6 is believed to be an uninterrupted piece of chromosome IV.

Cloning and sequence analysis of a rat liver cDNA coding for a phenobarbital-inducible microheterogenous cytochrome P-450 variant: regulation of its messenger level by xenobiotics

A rat liver cDNA library was prepared from total polyribosomal poly(A)+ RNA extracted from phenobarbital-treated animals. A cDNA clone coding for a phenobarbital-inducible cytochrome P-450 (PB P-450) was identified by differential colony hybridization to cDNAs synthesized from liver poly(A)+RNAs isolated from phenobarbital-treated rats for positive selection and cDNAs from either untreated rats or beta-naphthoflavone-treated rats as negative controls, followed by hybrid-selected translation and analysis of the translation products by immunoprecipitation. As the cloning and screening strategies involve no prior enrichment for specific mRNAs, they also permit the identification of sequences coding for phenobarbital-induced proteins other than cytochromes P-450. This relatively straightforward approach is generally applicable to the molecular cloning of sequences coding for other inducible cytochromes P-450. Nucleic acid sequencing data indicated that the cloned PB P-450 cDNA codes for a cytochrome P-450 variant [designated P-450e(U.C.)] that is very similar, but not identical, to P-450e. Sequence analysis of the section of cDNA specifying the 3'-non-coding region of the mRNA revealed that it lacked the usual poly(A) addition site signal sequence but contained three inverted repeat structures. Solution hybridization analysis demonstrated that PB P-450 mRNA is increased 20-fold by phenobarbital treatment and decreased 3-fold by beta-naphthoflavone treatment.

Evidence that 2-allyl-2-isopropylacetamide, phenobarbital and 3,5-diethoxycarbonyl-1,4-dihydrocollidine induce the same cytochrome P450 mRNA in chick embryo liver

The induction of cytochrome P450 in chick embryo liver has been studied using three different porphyrinogenic drugs, 2-allyl-2-isopropylacetamide, 3,5-diethoxycarbonyl-1,4-dihydrocollidine and phenobarbital. Pulse-labelling studies have shown that for each drug the cytochrome P450 synthesized either in ovo or in a wheat germ translation system reacted immunologically with antibody raised against the purified 2-allyl-2-isopropylacetamide-induced enzyme (Mr = 50000). To investigate whether this is due to the three drugs inducing the same protein or different proteins with common immunological determinants, nucleic acid hybridization studies have been carried out using a recently characterised 2-allyl-2-isopropylacetamide-induced cytochrome P450 cloned cDNA probe [Brooker, J. D. et al. (1982) Eur. J. Biochem. 129, 325-333]. It has been shown that the mRNA induced by each drug hybridizes with this probe and all are of similar size. The melting profile of the mRNA . cDNA hybrids indicates that the mRNAs induced by the three drugs have at least 98% homology with the cDNA probe. Restriction endonuclease digestions of total chick embryo genomal DNA and a chick cytochrome P450 genomal clone indicates that the cytochrome P450 gene homologous with the cDNA probe is represented in the genome only once. These results strongly suggest that the three drugs cause increased levels of the same cytochrome P450 mRNA, possibly due to enhanced expression of the same gene. Results are also presented which show that other cytochrome-P450-inducing drugs, 3-methylcholanthrene, beta-naphthoflavone or pregnenolone-16 alpha-carbonitrile do not increase the level of the 2-allyl-2-isopropylacetamide-inducible mRNA but rather reduce it to a level which was lower than that of the untreated controls.

At least six forms of extremely homologous cytochromes P-450 in rat liver are encoded at two closely linked genetic loci

A subpopulation of phenobarbital-induced cytochrome P-450 in rat liver has been shown to consist of four closely related forms of the enzyme that appeared to be strain-related. In the present study, polypeptides composing this family were analyzed by two-dimensional electrophoresis of hepatic microsomes from 64 individual phenobarbital-treated rats. The animals surveyed included both sexes from four inbred and five outbred strains/colonies and F1 progenies from 10 crosses. Two new members of this polypeptide family were identified on the basis of their unique electrophoretic behavior and peptide maps. Eight phenotypes were observed that consisted of two to four member polypeptides. The six closely related cytochromes P-450 were found to be encoded at two genetic loci with at least four alleles at the P-450b locus and at least two alleles at the P-450e locus. Most colonies of outbred strains were characterized by polymorphism at one or both of these loci, and in no case did they contain unique alleles. Analyses of parents and their F1 progenies indicated that the P-450b and P-450e loci are closely linked on the same autosome and are expressed codominantly. Furthermore, the products of these loci appear to be coordinately regulated. The extreme homology between P-450b and P-450e genes, their high degree of polymorphism, and their close linkage suggest that they are subject to the same genetic mechanisms that maintain these features in other multigene families.

Quantification of NADPH: cytochrome P-450 reductase in liver microsomes by a specific radioimmunoassay technique

We have developed a specific radioimmunoassay to quantify NADPH: cytochrome P-450 reductase. The assay is based on the use of 125I-labelled NADPH: cytochrome P-450 reductase as the radiolabelled antigen and can detect quantities of this protein in amounts as low as 30 pg. The results of the radioimmunoassay demonstrates that the 2.7-fold increase in enzyme activity in rat liver microsomal membranes after phenobarbital treatment is due to increased amounts of the protein. beta-Naphthoflavone treatment, however, did not alter the activity or the quantity of this enzyme in microsomes. The quantification of NADPH: cytochrome P-450 reductase in the microsomes isolated from control and phenobarbital- and beta-naphthoflavone-treated animals permits the calculation of the ratio of this protein to that of total cytochromes P-450. A molar ratio of 15:1 (cytochromes P-450/NADPH: cytochrome P-450 reductase) was calculated for control and phenobarbital-treated animals. This ratio increased to 21:1 after beta-naphthoflavone treatment. Thus the molar ratio of these proteins in liver microsomes can vary with exposure of the animals to particular xenobiotics.

Induction and repression of the major phenobarbital-induced cytochrome P-450 measured by radioimmunoassay

Two independent radioimmunoassay techniques for the major phenobarbital-inducible cytochrome P-450 (PB P-450) of rat liver microsomal membranes are described. The first technique employs as the source of radiolabelled antigen the products of translation in vitro labelled with [35S]methionine. The second technique employs purified antigen labelled with 125I and is quicker, less expensive and more precise. Both assays are highly specific for PB P-450 and can detect quantities of this variant as small as 1 ng. This is several orders of magnitude more sensitive than any method described previously for the quantification of cytochromes P-450, and consequently the technique is particularly well suited for the quantification of so-called constitutive cytochrome P-450 variants that are present in very low amounts. The results of the radioimmunoassays demonstrate that the apparent 2.6-fold induction of total cytochromes P-450 after phenobarbital treatment is due to a 43-fold increase in Pb P-450. Although beta-naphthoflavone increases the total content of cytochrome P-450 of microsomal membranes 1.4-fold, it actually causes a 55% decrease in the amount of PB P-450. Thus different xenobiotics can have differential effects on the expression of the genes for specific cytochrome P-450 variants.