Differential induction of rat hepatic cytochrome P-448 and glutathione S-transferase B messenger RNAs by 3-methylcholanthrene

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

Induction of Cyp1a-1 and Cyp1a-2 gene expression by a reconstituted mixture of polynuclear aromatic hydrocarbons in B6C3F1 mice

The potential non-additive interactions of polynuclear aromatic hydrocarbon (PAH) mixtures as inducers of Cyp1a-1 and Cyp1a-2 gene expression were investigated in B6C3F1 mice using a reconstituted PAH mixture. The chemical composition (% by weight) of the reconstituted PAH mixture was: 2-ring PAHs--indan (0.22), naphthalene (23.8), 2-methylnaphthalene (23.2) and 1-methylnaphthalene (13.3); 3-ring PAHs--acenaphthylene (7.7), acenaphthene (0.6), dibenzofuran (0.7), fluorene (4.3), phenanthrene (10.5) and anthracene (3.4); > or = 4-ring PAHs--fluoranthene (2.4), pyrene (4.3), benz[a]anthracene (1.4), chrysene (1.5), benzo[b]fluoranthene (0.8), benzo[k]fluoranthene (0.9) and benzo[a]pyrene (0.9). The composition of the 2-, 3- and > or = 4-ring PAH fractions were based on the relative concentration of individual PAHs as noted above. The > or = 4-ring PAH fractions were based on the relative concentration of individual PAHs as noted above. The > or = 4-ring PAH fraction and reconstituted mixture induced hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity and Cyp1a-1 mRNA levels, whereas the 2- and 3-ring PAHs were only weakly active. Direct comparison of the potencies of the reconstituted mixture and > or = 4-ring PAHs showed that the Cyp1a-1 induction activity of the reconstituted mixture was due to the > or = 4-ring PAHs. The reconstituted PAH mixture and > or = 4-ring PAHs also induced Cyp1a-2 hepatic mRNA levels and microsomal methoxyresorufin O-deethylase (MROD) activity; however, their dose-response curves indicated that the reconstituted PAH mixture was more potent as a Cyp1a-2 inducer than the > or = 4 ring PAHs. The differences in potency were due to 3-ring PAHs which were found to be strong inducers of hepatic Cyp1a-2 mRNA levels and microsomal MROD activity at the lowest dose administered (37 mg/kg). The 3-ring mixture caused a maximal 29-fold increase in hepatic MROD activity at a dose of 292 mg/kg, but only 28% of maximal induction of EROD activity. Northern analysis of liver mRNA from mice treated with 3-ring PAHs showed that there was minimal induction of Cyp1a-1 mRNA levels. The 3-ring PAHs did not competitively bind to the mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor suggesting that 3-ring PAHs are a new class of Cyp1a-2 inducers which do not act through the Ah receptor.

Differential induction of glutathione S-transferase in rat aorta versus liver

Polycyclic aromatic hydrocarbons, cigarette smoke components that induce atherosclerosis in animals, require metabolic biotransformation to electrophilic intermediates to exhibit atherogenic effects. The formation of reactive metabolites depends on both rates of cytochrome P450-catalyzed oxidation and rates of detoxification through conjugation with glutathione. Thus, changes in the activity of glutathione S-transferase in vascular tissue could affect the risk of polycyclic aromatic hydrocarbon-induced atherogenesis. We compared the effects of several exogenous chemicals on levels of glutathione S-transferase in aorta and liver. Male Wistar rats were treated with 3-methylcholanthrene, a polycyclic aromatic hydrocarbon, phenobarbital and butylated hydroxytoluene, an antioxidant known to have anti-atherogenic properties. In control animals, glutathione S-transferase activity was about 20-fold greater in liver than in aorta. Subunit expression was tissue specific. GST-Yp, for example, was the most abundant subunit in aorta but was undetectable in liver. In contrast, GST-Ya was barely detectable in aorta but was abundant in liver. Each of the xenobiotics caused induction of glutathione S-transferase but the extent of induction was greater in liver than in aorta. Phenobarbital, for example, caused 300% induction in liver but only 70% induction in aorta. By western blot analysis, differences in amounts of enzyme subunits corresponded to changes in enzyme activity. Thus, exogenous chemicals differentially regulate levels of glutathione S-transferase in the aorta and liver.

Synergistic activity of polynuclear aromatic hydrocarbon mixtures as aryl hydrocarbon (Ah) receptor agonists

The relative potencies of benzo[a]pyrene and a complex mixture of polynuclear aromatic hydrocarbons (PAHs) produced as by-products of manufactured gas plant (MGP) residues as inducers of hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity were determined in the B6C3F1 mouse. The ED50 values for the induction response were 78 and 65 mg/kg for benzo[a]pyrene and the MGP-PAH mixture, respectively. Analysis of the MGP-PAH mixture indicated that benzo[a]pyrene and other compounds containing four or more rings and which are known to induce EROD activity were only present as trace components of this mixture. A comparison of the EROD induction potencies of benzo[a]pyrene and the MGP-PAH mixture showed that the mixture was approximately 706 times more potent than expected based on its benzo[a]pyrene content (0.17%). This induced P-450 activity could significantly increase the metabolism of the carcinogenic PAHs and thereby modulate the overall carcinogenicity of the mixture. The apparent synergistic activity of the MGP-PAH mixture was further investigated by comparing the activities of this mixture and benzo[a]pyrene for several other aryl hydrocarbon (Ah) receptor-mediated responses including (i) induction of hepatic CYP1A1 mRNA levels, (ii) transformation of the rat cytosolic Ah receptor to a complex which binds to a dioxin responsive element, (iii) induction of EROD activity and (iv) antiestrogenicity in MCF-7 human breast cancer cells, and (v) inhibition of the splenic plaque-forming cell (PFC) response to both T cell-dependent and independent antigens in B6C3F1 mice. For the EROD and CYP1A1 mRNA induction and cytosolic transformation activities and immunosuppressive effects, the MGP-PAH mixture was approximately 100-900 times more potent as an Ah receptor agonist than expected based on its benzo[a]pyrene content. The synergistic activity was lower (19-fold) for the antiestrogenic response in MCF-7 cells. The reason for the synergistic effects of the MGP-PAH mixture were not due to contamination of the mixture by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds and the results suggest that the enhanced potency of the mixture is due to unknown interactions between the individual PAHs present in the mixture.

Glutathione S-transferases: gene structure and regulation of expression

The current knowledge about the structure of GST genes and the molecular mechanisms involved in regulation of their expression are reviewed. Information derived from the study of rat and mouse GST Alpha-class, Ya genes, and a rat GST Pi-class gene seems to indicate that a single cis-regulatory element, composed of two adjacent AP-1-like binding sites in the 5'-flanking region of these GST genes, is responsible for their basal and xenobiotic-inducible activity. The identification of Fos/Jun (AP-1) complex as the trans-acting factor that binds to this element and mediates the basal and inducible expression of GST genes offers a basis for an understanding of the molecular processes involved in GST regulation. The induction of expression of Fos and Jun transcriptional regulatory proteins by a variety of extracellular stimuli is known to mediate the activation of target genes via the AP-1 binding sites. The modulation of the AP-1 activity may account for the changes induced by growth factors, hormones, chemical carcinogens, transforming oncogenes, and cellular stress-inducing agents in the pattern of GST expression. Recent observations implying reactive oxygen as the transduction signal that mediates activation of c-fos and c-jun genes are presently considered to provide an explanation for the induction of GST gene expression by chemical agents of diverse structure. The possibility that these agents may all induce conditions of oxidative stress by various pathways to activate expression of GST genes that are regulated by the AP-1 complex is discussed.

Interspecies features of hepatic cytochromes P450 IA1 and P450 IA2 in rodents

1. Antibodies to mouse liver cytochrome P3-450 (anti-P3-450) and antibodies to rat liver cytochrome P-450d (anti-P-450d-c) both inhibit the O-deethylation of 7-ethoxy-resorufin (ER) in liver microsomes of benzo(a)pyrene-induced (BP) mice but do not inhibit the O-deethylase activity in liver microsomes of BP-induced rats. 2. Anti-P3-450 and anti-P-450d-c inhibit BP hydroxylation in BP-induced mouse liver microsomes by 20%, but they do not inhibit this rection at all in BP-induced rat liver microsomes. 3. Isolated cytochrome P3-450 in a reconstituted monooxygenase system metabolized 7-ER and BP. In contrast, its homologue, cytochrome P-450d, does not metabolize these substrates. The fraction containing cytochrome P1-450 metabolized 7-ER at a low rate and BP at a rate of 3.6 nmol product/min per nmol cytochrome. 4. Western blot analysis with anti-P-450c + d revealed two bands in SDS-PAGE gels containing BP-induced mouse liver microsomes corresponding to cytochrome P1-450, 55.0 kDa, and cytochrome P3-450, 54.5 kDa. There appeared a single band (cytochrome P3-450) in interaction of mouse liver BP-microsomes with anti-P3-450 and anti-P-450d-c.

Signals for the incorporation and orientation of cytochrome P450 in the endoplasmic reticulum membrane

Cytochrome P450b is an integral membrane protein of the rat hepatocyte endoplasmic reticulum (ER) which is cotranslationally inserted into the membrane but remains largely exposed on its cytoplasmic surface. The extreme hydrophobicity of the amino-terminal portion of P450b suggests that it not only serves to initiate the cotranslational insertion of the nascent polypeptide but that it also halts translocation of downstream portions into the lumen of the ER and anchors the mature protein in the membrane. In an in vitro system, we studied the cotranslational insertion into ER membranes of the normal P450b polypeptide and of various deletion variants and chimeric proteins that contain portion of P450b linked to segments of pregrowth hormone or bovine opsin. The results directly established that the amino-terminal 20 residues of P450b function as a combined insertion-halt-transfer signal. Evidence was also obtained that suggests that during the early stages of insertion, this signal enters the membrane in a loop configuration since, when the amino-terminal hydrophobic segment was placed immediately before a signal peptide cleavage site, cleavage by the luminally located signal peptidase took place. After entering the membrane, the P450b signal, however, appeared to be capable of reorienting within the membrane since a bovine opsin peptide segment linked to the amino terminus of the signal became translocated into the microsomal lumen. It was also found that, in addition to the amino-terminal combined insertion-halt-transfer signal, only one other segment within the P450b polypeptide, located between residues 167 and 185, could serve as a halt-transfer signal and membrane-anchoring domain. This segment was shown to prevent translocation of downstream sequences when the amino-terminal combined signal was replaced by the conventional cleavable insertion signal of a secretory protein.

Differential induction of rat hepatic glutathione S-transferase isoenzymes by hexachlorobenzene and benzyl isothiocyanate. Comparison with induction by phenobarbital and 3-methylcholanthrene

Male Wistar rats were treated with hexachlorobenzene, benzyl isothiocyanate, phenobarbital or 3-methylcholanthrene. Hepatic cytosolic glutathione S-transferase (GST) activity was determined with the substrates 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, ethacrynic acid and trans-4-phenyl-3-buten-2-one. Cytosolic glutathione peroxidase activity was measured with cumene hydroperoxide. GST activity toward 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene and ethacrynic acid was enhanced by all compounds, hexachlorobenzene and 3-methylcholanthrene causing the largest and the smallest increase respectively. Trans-4-phenyl-3-buten-2-one-conjugating activity exhibited only small changes, while peroxidase activity with cumeme hydroperoxide was not changed by any of the inducing agents. GST isoenzymes were purified on S-hexylglutathione Sepharose 6B and separated by means of FPLC-chromatofocusing, to evaluate effects on the GST isoenzyme pattern. Hexachlorobenzene and phenobarbital both caused an increase in the relative amounts of subunits 1 and 3 when compared with subunits 2 and 4 respectively. For 3-methylcholanthrene only induction of subunit 1 was observed, possibly due to the relatively low induction levels of total GST activity. In benzyl isothiocyanate-treated animals, an induction of subunit 3 was found as well as an increase in the relative amount of subunit 2. Thus, benzyl isothiocyanate behaves differently from hexachlorobenzene, phenobarbital and 3-methylcholanthrene as an inducing agent of rat hepatic glutathione S-transferases.

Expression of cytochrome P-450 and albumin genes in rat liver: effect of xenobiotics

Thioacetamide, a hepatocarcinogen and an inhibitor of heme synthesis, blocks the phenobarbitone-mediated increase in the transcription of cytochrome P-450b+e messenger RNA in rat liver. This property is also shared by CoCl2 and 3-amino-1,2,4-triazole, two other inhibitors of heme synthesis. Thus, it appears feasible that heme may serve as a positive regulator of cytochrome P-450b+e gene transcription. Thioacetamide enhances albumin messenger RNA concentration, whereas phenobarbitone decreases the same. However, these changes in albumin messenger RNA concentration are not accompanied by corresponding changes in the transcription rates. Therefore, drug-mediated changes in albumin messenger RNA concentration are due to posttranscriptional regulation. The property of thioacetamide to enhance the albumin messenger RNA concentration is not shared by CoCl2 and 3-amino-1,2,4-triazole. Therefore, heme does not appear to be a regulatory molecule mediating the reciprocal changes brought about in the concentrations of cytochrome P-450b+e and albumin messenger RNAs.

Characterization of the DNA-binding properties of the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The DNA-binding properties of the receptor for 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) were investigated using chromatography on DNA-cellulose columns. A maximal binding of about 40% of the total receptor complex to DNA-cellulose was observed. In order to interact with DNA, the receptor must first bind TCDD. A heat-activation step followed by gel permeation chromatography using Sephadex G-25 increased the binding of the cytosolic receptor to DNA. The DNA-binding ability of the receptor was almost lost following mild proteolysis using trypsin or alpha-chymotrypsin, although these treatments did not reduce its ligand binding capacity and had no apparent effect on its size. Furthermore, pre-treatment of the DNA-cellulose column with an intercalating drug, ethidium bromide, resulted in inhibition of the binding of the TCDD-receptor complex to DNA, indicating that not only electrostatic interactions but also the configuration of DNA are of importance in receptor-DNA interactions.

Induction of drug-metabolizing enzymes: mechanisms and consequences

The activity of many enzymes that carry out biotransformation of drugs and environmental chemicals can be substantially increased by prior exposure of humans or animals to a wide variety of foreign chemicals. Increased enzyme activity is due to true enzyme induction mediated by increased synthesis of mRNAs which code for specific drug-metabolizing enzymes. Several species of cytochrome P-450 are inducible as are certain conjugating enzymes such as glutathione S-transferases, glucuronosyl transferases, and epoxide hydrolases. Induction of drug-metabolizing enzymes has been shown in several instances to alter the efficacy of some therapeutic agents. Induction of various species of cytochrome P-450 also is known to increase the rate at which potentially toxic reactive metabolic intermediates are formed from drugs or environmental chemicals. Overall, however, induction of drug-metabolizing enzymes appears to be a beneficial adaptive response for organisms living in a "chemically-hostile" world.

A comparative study of the regulation of cytochrome P-450 and glutathione transferase gene expression in rat liver

A cDNA clone for the Ya subunit of glutathione transferase from rat liver was constructed in E. coli. The clone hybridized to Ya and Yc subunit messenger RNAs. On the basis of experiments involving cell-free translation and hybridization to the cloned probe, it was shown that prototype inducers of cytochrome P-450 such as phenobarbitone and 3-methylcholanthrene as well as inhibitors such as CoCl2 and 3-amino-1,2,4-triazole enhanced the glutathione transferase (Ya+Yc) messenger RNA contents in rat liver. A comparative study with the induction of cytochrome P-450 (b+e) by phenobarbitone revealed that the drug manifested a striking increase in the nuclear pre-messenger RNAs for the cytochrome at 12 hr, but did not significantly affect the same in the case of glutathione transferase (Ya+Yc). 3-Amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone mediated increase in cytochrome P-450 (b+e) nuclear pre-messenger RNAs. These compounds did not significantly affect the glutathione transferase (Ya+Yc) nuclear pre-messenger RNA levels. The polysomal, poly (A)- containing messenger RNAs for cytochrome P-450 (b+e) increased by 12-15 fold after phenobarbitone administration, reached a maximum around 16 hr and then decreased sharply. In comparison, the increase in the case a glutathione transferase (Ya+Yc) messenger RNAs was sluggish and steady and a value of 3-4 fold was reached around 24 hr. Run-off transcription rates for cytochrome P-450 (b+e) increased by nearly 15 fold in 4 hr after phenobarbitone administration, whereas the increase for glutathione transferase (Ya+Yc) was only 2.0 fold. At 12 hr after the drug administration, the glutathione transferase (Ya+Yc) transcription rates were near normal. Administration of 3-amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone-mediated increase in the transcription of cytochrome P-450 (b+e) messenger RNAs. These compounds at best had only marginal effects on the transcription of glutathione transferase (Ya+Yc) messenger RNAs. The half-life of cytochrome P-450 (b+e) messenger RNA was estimated to be 3-4 hr, whereas that for glutathione transferase (Ya+Yc) was found to be 8-9 hr. Administration of phenobarbitone enhanced the half-life of glutathione transferase (Ya+Yc) messenger RNA by nearly two fold. It is suggested that while transcription activation may play a primary role in the induction of cytochrome P-450 (b+e), the induction of glutathione transferase (Ya+Yc) may essentially involve stabilization of the messenger RNAs.

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.

Synthesis and degradation of 3-methylcholanthrene-inducible cytochromes P-450 and their mRNAs in primary monolayer cultures of adult rat hepatocytes

We used primary nonproliferating cultures of adult rat hepatocytes to investigate the regulation of P-450c and P-450d, immunochemically related protein products of separate cytochromes P-450 genes that are coinduced by 3-methylcholanthrene and related compounds. In cultures of hepatocytes prepared from untreated rats and incubated in media containing 3-methylcholanthrene, beta-naphthoflavone, 3,4,3',4'-tetrachlorobiphenyl, and Aroclor 1254 (a mixture of chlorinated biphenyls) there was a 5- to 15-fold accumulation of P-450c protein (quantitated by immunoblotting), accompanied by an increased rate of P-450c synthesis (measured as incorporation of [3H]leucine into immunoprecipitable protein) and an increased amount of P-450c mRNA hybridizable to a specific cloned cDNA (p210). In contrast, there were no increases in the concentration of P-450d protein, its rate of synthesis, or the amount of P-450d mRNA hybridizable to its specific cDNA (p72). Similarly, when "preinduced" hepatocytes (isolated from rats treated with Aroclor 1254) were incubated for 4 days in culture medium, the amount of P-450c, its rate of synthesis, and the amount of P-450c mRNA remained elevated, whereas synthesis of P-450d and the amount of P-450d mRNA fell precipitously to less than 10% of the initial values despite the presence or absence of Aroclor 1254 or of isosafrole in the medium. However, the loss of P-450d protein in these cultures was almost completely prevented when isosafrole was added to the culture medium and was partially prevented when safrole, Aroclor 1254, and 3,4,5,2',4',5'-hexachlorobiphenyl, but not 3-methylcholanthrene, beta-naphthoflavone, or 3,4,3'4'-tetrachlorobiphenyl, were in the culture medium. Moreover, in similar cultures of "preinduced" hepatocytes that were pulse-labeled with [3H]leucine, the presence of isosafrole in the culture medium extended the apparent half-life for loss of radioactivity in immunoprecipitable P-450d to a value of 72 h (3-fold longer than in standard medium) but was without effect on the rate of disappearance of radiolabeled P-450c. We conclude that control of P-450d degradation is an important factor in the regulation of this hemoprotein and that induction of P-450c and P-450d proceed by separate pathways that are spontaneously divorced under standard conditions for primary culture of adult rat hepatocytes.

Transcriptional regulation of rat liver glutathione S-transferase genes by phenobarbital and 3-methylcholanthrene

The relative rates of transcription of the rat liver glutathione S-transferase Ya-Yc and Yb genes were determined in purified liver nuclei isolated at different times after phenobarbital or 3-methylcholanthrene administration. The transcriptional rates of the Ya-Yc and Yb genes were elevated approximately fivefold 8 and 6 h, respectively, after phenobarbital administration. In contrast, the transcriptional rates of the Ya-Yc genes were elevated approximately eightfold at 16 h after 3-methylcholanthrene administration, whereas the transcriptional rates of the Yb genes were elevated approximately fivefold at 6 h after the administration of this xenobiotic. The elevation in transcriptional activity of the glutathione S-transferase genes is sufficient to account for the increase in glutathione S-transferase mRNA levels determined previously by RNA blot hybridization [C. B. Pickett, C. A. Telakowski-Hopkins, G. J-F. Ding, L. Argenbright, and A. Y. H. Lu (1984) J. Biol. Chem. 259, 5182-5188]. Therefore, it appears that phenobarbital and 3-methylcholanthrene elevate the level of the rat liver glutathione S-transferases primarily by augmenting the transcriptional rates of their respective genes.

Regulation of glutathione transferase and DT-diaphorase mRNAs in persistent hepatocyte nodules during chemical hepatocarcinogenesis

We have utilized cDNA probes and in vitro translation analysis to quantitate the levels of rat liver glutathione transferase (glutathione S-aralkyltransferase; RX:glutathione R-transferase, EC 2.5.1.18) and DT-diaphorase [NAD-(P)H:quinone-acceptor oxidoreductase, EC 1.6.99.2] mRNAs in persistent hepatocyte nodules induced by chemical carcinogens. Our results indicate that within the nodules, glutathione transferase mRNAs specific for the Ya/Yc and Yb subunits are increased 3-fold and 5-fold, respectively, over the levels observed in normal liver or in the liver tissue surrounding the nodules. Similarly, the level of DT-diaphorase mRNA is increased 5- to 7-fold within the nodules as compared to surrounding liver tissue or normal liver. When animals were administered 3-methylcholanthrene, a typical inducer of these mRNAs in normal animals, a further increase in the glutathione transferase Yb mRNA(s) and DT-diaphorase mRNA was observed in the nodules; however, the Ya/Yc mRNA levels remained unaffected. Our data indicate that during chemically induced neoplastic transformation, the mRNA levels for the Yb subunit of glutathione transferase and DT-diaphorase are increased in the nodules but still retain the capacity to be regulated by 3-methylcholanthrene. Although the glutathione transferase Ya/Yc mRNAs are also increased in the nodules, they lost their ability to be regulated by 3-methylcholanthrene. These latter data suggest that within the nodules there is a specific defect in the regulatory mechanism(s) that leads to an induction of the Ya/Yc mRNAs in normal tissue by xenobiotics.

Induction of cytochrome P-450 mRNAs quantitated by in vitro translation and immunoprecipitation

Cytochrome P-450 mRNAs were quantitated by in vitro translation of liver RNA followed by immunoprecipitation with antibodies specific for cytochromes P-450. The kinetics of cytochrome P-450 mRNA induction by 3-methylcholanthrene and by phenobarbital were examined, and differences in the types of cytochrome P-450 mRNAs induced by 3-methylcholanthrene (MC), beta-naphthoflavone (BNF), and phenobarbital (PB) in male and female rats were determined. Phenobarbital strongly induced the mRNA encoding a single peptide antigenically related to the phenobarbital-induced cytochrome P-450PB-B. Male rat liver contained 62% more translatable mRNA for this peptide than did female. 3-Methylcholanthrene and beta-naphthoflavone, but not phenobarbital, induced mRNAs encoding three peptides that were immunologically related to cytochrome P-450BNF/MC-B, which is induced by 3-methylcholanthrene. The levels of translatable mRNA coding for these peptides were twice as high in females as in males. Striking sex differences were observed in the levels of translatable mRNAs for peptides related to cytochrome P-450PB/PCN-E, which is induced by phenobarbital and by pregnenolone-16-alpha-carbonitrile. In females, only RNA preparations from the livers of phenobarbital-treated rats had significant levels of mRNAs encoding these peptides. In contrast, significant levels of these RNAs were observed even in untreated males, and the levels of these mRNAs increased markedly following phenobarbital treatment. All cytochrome P-450 inducers examined caused a 50 to 70% decrease in translatable albumin mRNA. This effect was specific for albumin mRNA, since levels of total translatable mRNA were not generally altered by these inducers. The kinetics of induction of cytochrome P-450 mRNA differed from those of induction of aryl hydrocarbon hydroxylase (AHH) activity. Translatable cytochrome P-450 mRNA was increased as early as 4 h after phenobarbital treatment, peaked between 24 and 36 h, and dropped back to control levels by 120 h. The induction of AHH lagged behind the increase in translatable mRNA, remaining at control levels well after levels of translatable mRNA began to increase but then decreasing roughly in parallel with translatable mRNA. These findings suggest that transcription was not rate limiting for regulation of PB-inducible cytochrome P-450 activity. 3-Methylcholanthrene caused parallel increases in AHH activity and translatable cytochrome P-450 mRNA, but when translatable mRNA began to decrease after about 24 h, AHH activity remained high, suggesting that this P-450 mRNA was less stable than the enzyme for which it coded.

Rat liver DT-diaphorase: regulation of functional mRNA levels by 3-methylcholanthrene, trans-stilbene oxide, and phenobarbital

Total liver poly(A+)-RNA isolated from untreated, and 3-methylcholanthrene-, trans-stilbene oxide-, and phenobarbital-treated rats has been translated in the rabbit reticulocyte lysate system in order to determine the effect of these xenobiotics on the level of translationally active DT-diaphorase mRNA. The in vitro translation systems were subjected to immunoprecipitation with rabbit IgG raised against purified DT-diaphorase and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The identity of the radiolabeled, immunoprecipitated product as DT-diaphorase was confirmed by limited peptide mapping using Staphylococcus aureus V-8 protease. These quantitation results demonstrate that 3-methylcholanthrene leads to an eight-fold elevation in functional DT-diaphorase mRNA at 8 h after a single administration of 3-methylcholanthrene; whereas, trans-stilbene oxide and phenobarbital produced only a modest elevation, two- to three-fold, in the functional DT-diaphorase mRNA level. These data indicate that the increase in the level of DT-diaphorase after 3-methylcholanthrene administration noted previously [B. Höjeberg, K. Blomberg, S. Stenberg, and C. Lind (1981) Arch. Biochem. Biophys. 207, 205-216] can be totally accounted for by an elevation in the mRNA level specific for this protein.

Isolation and characterization of a DNA sequence complementary to rat liver glutathione S-transferase B mRNA

Total rat liver poly(A+)-RNA has been isolated from phenobarbital-treated rats and fractionated on sucrose gradients to enrich for glutathione S-transferase B mRNA. Poly(A+)-RNA fractions were assayed for glutathione S-transferase B mRNA activity by in vitro translation and those fractions enriched in glutathione S-transferase B mRNA were used as a template for cDNA synthesis. The cDNA was cloned into the PstI site of pBR322 by G-C tailing. Bacterial clones harboring inserts complementary to glutathione S-transferase mRNA were identified by colony hybridization using a [32P]cDNA probe reverse transcribed from poly(A+)-RNA enriched significantly in glutathione S-transferase B mRNA and by hybrid-select translation. Two recombinant clones, pGTB6 and pGTB15 hybrid-selected the mRNAs specific for the Ya and Yc subunits, indicating these two mRNAs share significant sequence homology. Radiolabeled pGTB6 was utilized in RNA gel-blot experiments to determine that the size of glutathione S-transferase B mRNA is 980 nucleotides and the degree of induction of the mRNA in response to 3-methylcholanthrene administration is threefold.

The induction of microsomal electron transport enzymes

Liver endoplasmic reticulum contains as NADPH-dependent electron transport complex where the family of hemeproteins, termed cytochrome P-450, serve as catalysts for the oxidation of a variety of different organic chemicals. The content and inventory of the types of cytochrome P-450 is readily modified following in vivo treatment of animals with 'inducing agents' such as barbiturates, steroids and polycyclic hydrocarbons. Recent studies have applied the methods of molecular biology to evaluate changes in the transcription and translation of genomic information occurring concomitant with the initiation of synthesis of various types of cytochrome P-450. The ability to isolate unique cytochrome P-450 proteins and to prepare specific antibodies now permits the study of in vitro translation of mRNA and the preparation of specific cDNAs. The present review summarizes the historic background leading to current concepts of cytochrome P-450 induction and describes recent advances in our knowledge of the regulation of cytochrome P-450 synthesis in the liver.

ACTH-mediated induction of synthesis and activity of cytochrome P-450s and related enzymes in cultured bovine adrenocortical cells

1. Cytochromes P-450sec and P-450(11) beta, adrenodoxin and adrenodoxin reductase, mitochondrial components of the adrenocortical steroid hydroxylase pathway, are synthesized as higher molecular weight precursors; cytochrome P-450C-21, a microsomal component of this pathway, is synthesized as the mature form. 2. Synthesis of the above mitochondrial components is induced by ACTH in a co-ordinated fashion. Synthesis of cytochrome P-450C-21 and NADPH-cytochrome P-450 reductase is also induced by ACTH, however, the induction of these microsomal components is not co-ordinated with that of the mitochondrial components. 3. Following treatment of cultured cells with ACTH, the pattern of glucocorticoid output changes from approximately equal amounts of cortisol and corticosterone to predominately cortisol within 24 h. This change results from a large induction of cytochrome P-450(17) alpha activity in response to ACTH. 4. Bovine adrenocortical cells in culture become refractory to continued treatment with ACTH. This refractoriness is manifested in terms of steroid output; synthesis of cytochromes P-450scc, P-450(11) beta and P-450C-21, adrenodoxin and adrenodoxin reductase; and activities of cholesterol side-chain cleavage, 11 beta-hydroxylase and 17 alpha-hydroxylase.