Induction by phenobarbital of the mRNA for a specific variant of rat liver microsomal cytochrome P-450

Effects of phenobarbital and valproate on the expression of cytochromes P-450 in co-cultured rat hepatocytes

We have investigated, in hepatocytes co-cultured with epithelial cells, the effects of phenobarbital and sodium valproate treatment on members of the cytochrome P-450 superfamily. The expression of the P450IIB and P450IV families was examined, using Western blotting, at 4, 7 and 14 days of co-culture either in the presence or absence of drug treatment. The amount of P450IIB was increased after exposure of the co-cultured cells to either phenobarbital or sodium valproate. In contrast, P450IV amounts were increased only by sodium valproate treatment. The maximal induction of P450IIB (17-fold) was observed at 7 and 14 days of co-culture, whereas P450IV was induced to the same extent in cells co-cultured for 4, 7 and 14 days. We also examined the expression of these two cytochrome P-450 subfamilies in hepatocytes cultured for 4 days in the absence of epithelial cells. The amounts of both P450IIB and P450IV were low or undetectable in these cells, and the induction of these proteins by either phenobarbital or valproate was less than that observed in co-cultured hepatocytes. Our results demonstrate that the co-culture system is a suitable in vitro system for examining the effects of various foreign compounds on the expression of phase I drug metabolizing enzymes.

Focus on the intermediate state: immature mRNA of cytochromes P450--methods and insights

The scattered and limited data on hnRNAs (pre-mRNAs) of cytochromes P450 (CYP) are compiled and discussed for the first time. The methods for determination and quantification of hnRNAs are compared. In most cases, CYP hnRNA levels were determined as a parameter of transcriptional activity. It is known, however, that some CYPs, in particular CYP2E1, are in addition specifically and extensively regulated by post-transcriptional processes. Obviously, these processes also influence the processing of CYP hnRNAs so that their levels cannot be considered a mere result of transcription. The underlying mechanisms of post-transcriptional CYP hnRNA and mRNA regulation are not well understood. It is our aim therefore to bring together available data on CYP hnRNA and to discuss them in the light of recent advances in knowledge concerning pre-mRNA processing and interactions between RNA and low molecular weight interfering RNAs. By doing this, we hope to drive research in a direction which appears promising in providing some long-awaited answers with respect to mechanisms of post-transcriptional CYP regulation.

Rat oesophageal cytochrome P450 (CYP) monooxygenase system: comparison to the liver and relevance in N-nitrosodiethylamine carcinogenesis

N-nitrosodiethylamine (NDEA) is able to induce tumours in the rat oesophagus. It has been suggested that this could be due to tissue specific expression of NDEA activating cytochrome P450 enzymes. We investigated this by characterizing the oesophageal monooxygenase complex of male Wistar rats and comparing it with that of the liver. Total amount of cytochrome P450, NADPH P450 reductase, cytochrome b5 and cytochrome b5 reductase of the oesophageal mucosa was approximately 7% of what was found in the liver. In addition, major differences were found in the cytochrome P450 isoenzyme composition between these organs: CYP 2B1/2B2 and CYP3A were found only in the liver, whereas CYP1A1 was constitutively expressed only in the oesophagus. Of the two well-known nitrosamine metabolizing enzymes, CYP2A3 was found only in the oesophagus whereas CYP2E1 was exclusively expressed in the liver. Catalytic studies, western blotting and RT-PCR analyses confirmed the expression of CYP2A3 in the oesophagus. CYP2A enzymes are known to be good catalysts of NDEA metabolism. Oesophageal microsomes had a K(m) for NDEA metabolism, which was about one-third of that of hepatic microsomes, but they showed similar activities when compared per nmol of total P450. NDEA activity in the oesophagus was significantly increased by coumarin (CO), which also induced oesophageal CYP2A3. Immunoinhibition of the microsomal NDEA activity showed that up to 70% of this reaction is catalysed by CYP2A3 in the oesophagus, whereas no inhibition of the hepatic NDEA activity could be achieved by the anti-CYP2A5 antibody. NDEA, but not N-nitrosodimethylamine (NDMA) inhibited the oesophageal metabolism of CO. The results of the present investigation show major differences in the enzyme composition of the oesophageal and hepatic monooxygenase complexes, and are in accordance with the hypothesis that the NDEA organotropism could, to a large extent, be due to the tissue specific expression of the activating enzymes.

Induction of liver monooxygenases by beta-myrcene

Beta-Myrcene (MYR) is an acyclic monoterpene found in the essential oils of a variety of useful plants such as lemongrass (Cymbopogon citratus), hop, verbena, bay and others. MYR and essential oils containing this olefinic monoterpene are widely used as flavoring food additives, as fragrances in cosmetics and as scents in household products. The present study was undertaken to investigate the induction of liver monooxygenases by MYR. Female Wistar rats were treated by gavage with MYR (1000 mg/kg body weight) or corn oil (vehicle) for 1 or 3 consecutive days. Activities of ethoxycoumarin-O-deethylase (ECOD) and alkoxy-resorufin O-dealkylases (methoxy- (MROD), ethoxy- (EROD), pentoxy- (PROD) and benzyloxy-resorufin-O-dealkylation (BROD)) were determined fluorimetrically in the hepatic microsomal fraction. Exposure to MYR, either for 1 or 3 days, produced marked (13-34-fold) increases in the activities of PROD and BROD and only minor changes in ECOD, EROD and MROD. Since PROD and BROD are metabolized mainly by CYP2B isoenzymes, these results suggest that MYR induces this phenobarbital-inducible P450 subfamily. The induction of CYP2B isoenzymes was confirmed by SDS-PAGE and immunoblotting. Levels of apoproteins CYP2B1/2B2 were increased 8.2-fold after treatment with MYR (1000 mg/kg body wt, 3 days). Results from this study therefore indicate that MYR is an inducer of isoenzymes belonging to CYP2B subfamily.

Effects of the anticonvulsant, valproate, on the expression of components of the cytochrome-P-450-mediated monooxygenase system and glutathione S-transferases

It has been shown previously that the anticonvulsant agent, sodium valproate, induces certain cytochrome P-450 monooxygenase activities and decreases glutathione S-transferase activity. We have used Western blotting, RNase protection assays and Northern blot hybridization to determine the effects of valproate on the abundance of individual components of the cytochrome P-450 monooxygenase and of glutathione S-transferase subunits. Due to the short half-life of the drug in rats we have used an in vitro experimental system comprised of rat hepatocytes co-cultured with rat primitive biliary epithelial cells. Valproate was shown to be a potent inducer of two members of the cytochrome P-450 (CYP)2B subfamily, CYP2B1 and 2B2. The induction of the proteins was mediated at the level of the mRNAs, with the mRNA for CYP2B1 being more highly induced than that for CYP2B2. The drug also induced, but to a much lesser extent, two important components of the cytochrome-P-450-mediated monooxygenase system, NADPH-dependent cytochrome P-450 reductase and cytochrome b5, and their corresponding mRNAs. Thus, the effects of valproate on cytochromes P-450 and other components of the cytochrome-P450-mediated monooxygenase system mimic those of another, structurally diverse, antiepileptic drug, phenobarbital. However, in contrast to phenobarbital, which induces glutathione S-transferase subunits 1, 2, 3, 4 and 7, valproate selectively decreases the abundance of subunits 3 and/or 4. It has been shown previously that CYP2B1 is involved in the production of metabolites of valproate implicated in hepatotoxicity. The induction of this protein by valproate would thus contribute substantially to the hepatotoxic effects associated with the drug.

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.

Picrotoxin as a potent inducer of rat hepatic cytochrome P450, CYP2B1 and CYP2B2

The induction by the central stimulant picrotoxin of hepatic drug-metabolizing enzymes was studied in rats. The hepatic content of P450 and the activity of benzphetamine N-demethylation increased gradually after administration of picrotoxin dissolved in drinking water (2 mg/mL), to three-times higher levels than the initial values at the third day of treatment. The increase in benzphetamine N-demethylase activity by picrotoxin was somewhat higher than the increase produced by phenobarbital. Supporting these results, immunoblot analysis showed that CYP2B1 and 2B2 proteins in the liver microsomes were increased by picrotoxin Picrotoxinin and picrotin, which are components of the picrotoxin molecule, had the same ability to induce the hepatic activity of benzphetamine N-demethylation. The liver microsomal activities of testosterone 16 alpha- and 16 beta-hydroxylation were enhanced significantly after treatment with picrotoxinin and picrotin. However, benzo[a]pyrene 3-hydroxylation, aniline 4-hydroxylation, and testosterone hydroxylations at the 2 alpha- and 7 alpha-positions were not increased by picrotoxinin and picrotin treatment. In addition to monooxygenase, significant induction of glutathione S-transferase activity for 1-chloro-2,4-dinitrobenzene and UDP-glucuronyltransferase activity for 4-hydroxybiphenyl and 4-nitrophenol was also observed by pretreatment of picrotoxin. These results clearly indicate that picrotoxin is an inducer of phenobarbital-inducible liver enzymes.

Phenobarbital induction of cytochrome P-450 gene expression

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Rapid decrease of cytochrome P-450IIE1 in primary hepatocyte culture and its maintenance by added 4-methylpyrazole

Studies were conducted to evaluate the possible induction or the maintenance of cytochrome P-450IIE1 in primary hepatocyte cultures by the inducing agent 4-methylpyrazole. Hepatocytes were isolated from control (noninduced) rats and from rats treated in vivo with either pyrazole or 4-methylpyrazole to induce P-450IIE1. The content of P-450IIE1 was determined by Western blots with antipyrazole P-450 IgG, and catalytic activity was assessed by assays of dimethylnitrosamine demethylase activity. The treatment with 4-methylpyrazole in vivo increased the content of P-450IIE1 and dimethylnitrosamine demethylase activity sevenfold and fourfold, respectively. In cultures prepared from noninduced hepatocytes, P-450IIE1 levels fell to values of 76%, 65%, 31% and 1% of freshly isolated hepatocytes after 1, 3, 6 and 9 days in culture. A similar decrease in dimethylnitrosamine demethylase was observed during this time. In cultures prepared from induced hepatocytes, the decline in P-450IIE1 was more rapid as levels fell to 77%, 31%, 3% and 3% of initial values after 1, 3, 6 and 9 days in culture. Again, the fall in dimethylnitrosamine demethylase activity paralleled the decline in content of P-450IIE1 and was more rapid with the induced hepatocytes. With cultures prepared from noninduced or induced hepatocytes, the addition of 4-methylpyrazole in vitro did not increase the content of P-450IIE1 or the activity of dimethylnitrosamine demethylase over the initial values. However, 4-methylpyrazole appeared to stabilize the P-450IIE1 and to decrease its rate of decline in culture. In noninduced cultures, the percent remaining content of P-450IIE1 after 6 days was 31% in the absence of and 52% in the presence of 5 mol/L 4-methylpyrazole. In cultures from 4-methylpyrazole-induced hepatocytes, the percent remaining P-450IIE1 after 3 days was 31% in the absence of inducer and 59% with 4-methylpyrazole added in vitro. Similarly 4-methylpyrazole helped to prevent the rapid decline of dimethylnitrosamine demethylase activity in induced and noninduced cultures. Viability of the induced and noninduced cultures in the absence or presence of added 4-methylpyrazole was similar. Levels of mRNA for P-450IIE1 were similar for livers from control rats and from rats treated in vivo with 4-methylpyrazole. The mRNA levels rapidly declined in induced and noninduced cultures, and this decline, unlike the fall in P-450IIE1 or dimethylnitrosamine demethylase activity, could not be prevented by the addition of 4-methylpyrazole in vitro to the cultures.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of hepatic P450 induction in rat and trout (Oncorhynchus mykiss): delineation of the site of resistance of fish to phenobarbital-type inducers

1. A comprehensive approach was taken to delineate the site of refractivity of trout to phenobarbital-type (PB-type) hepatic monoxygenase (MO) inducers. 2. Model inducers beta-naphthoflavone (BNF; 3-MC-type), and PB as well as the polychlorinated biphenyl isomers, 3,4,5,3',4',5'-hexachlorobiphenyl (3,4,5-HCB; 3-MC-type) and 2,4,5,2',4',5'-hexachlorobiphenyl (2,4,5-HCB; PB-type) were used to assess MO activities, total cytochromes P450, and [35S]-methionine incorporation into de novo synthesized microsomal protein in both trout and rats. 3. In rainbow trout immunodetection of P450 isozymes and nucleic acid hybridization of rainbow trout P(1)450 mRNA using pfP(1)450-3' (trout 3-MC-inducible, P450IA1 gene) and genomic DNA using pfP(1)450-3' or pSP450-oligo (rat PB-inducible, P450IIB1 gene) cDNAs were carried out. 4. In rainbow trout, PB and 2,4,5-HCB do not increase hepatic MO activities, total cytochromes P450, de novo synthesis of microsomal protein, levels of P450 isozymes, or levels of P(1)450 mRNA. 5. Rainbow trout have, within their genome, DNA with sequence(s) similar to rat P450IIB1, but inducibility of this P450 in trout by PB-type inducers is lacking.

Differential induction of peroxisomal and microsomal fatty-acid-oxidising enzymes by peroxisome proliferators in rat liver and kidney. Characterisation of a renal cytochrome P-450 and implications for peroxisome proliferation

The induction of renal fatty-acid-oxidising enzymes has been investigated following short-term exposure to a group of structurally diverse peroxisome proliferators and compared to the more extensively documented hepatic responses in the rat. There was a marked compound dependence on induction of both cytochrome P-450-IVA1-dependent omega-hydroxylation of lauric acid and enzymes of the peroxisomal fatty acid beta-oxidation pathway (measured as cyanide-insensitive palmitoyl-CoA oxidation and enoyl-CoA hydratase). Cytochrome P-450 IVA1 (or a very closely related isoenzyme in the same gene family) was a major constitutive haemoprotein in rat kidney microsomes and actively supported the omega-hydroxylation of lauric acid. This activity was induced 2-3-fold by peroxisome proliferators such as clofibrate, di-(2-ethylhexyl)phthalate, bezafibrate and nafenopin. By using a cDNA probe to the cytochrome P-450 IVA1 gene in Northern blot analysis, we have shown that increased renal and hepatic omega-hydroxylation of lauric acid, after treatment with peroxisome proliferators is a consequences of a substantial increase in the mRNA coding for this haemoprotein. In addition, programming of an in vitro rabbit reticulocyte translation system with both renal and hepatic RNA resulted in the synthesis of similar (if not identical) cytochrome-P-450-IVA1-related polypeptides. Furthermore, we have provided Western blot evidence that both rat liver and kidney microsomes contain two closely related cytochrome P-450 IVA1 polypeptides, the major one characterised by a monomeric molecular mass of 51.5 kDa (identical to authentic, purified hepatic cytochrome P-450 IVA1) and a minor one of 52 kDa. The kidney-supported fatty acid omega-hydroxylase activity was refractory to inhibition by a polyclonal antibody to liver cytochrome P-450 IVA1, which may be related to the existence of two closely related (but immunochemically distinct) fatty acid hydroxylases in this tissue. Our studies have also demonstrated that certain of the compounds tested (including clofibrate, bezafibrate and nafenopin) induced renal fatty acid beta-oxidation, mirroring the increased omega-hydroxylase activity in the endoplasmic reticulum. Our studies have also indicated that the kidney was more refractory to induction of the endoplasmic reticulum and peroxisomal fatty-acid-oxidising enzymes than the liver. Taken collectively, our data is strongly suggestive of a possible linkage of the renal fatty acid oxidative enzymes in these two organelles, a situation that also occurs in the liver. In addition, our studies have provided a possible conceptual framework that may rationalise the decreased susceptibility of the k

Cloned rainbow trout liver P(1)450 complementary DNA as a potential environmental monitor

A technique is proposed for the biological monitoring of pollutants in aquatic environments by use of a complementary DNA (cDNA) probe. The induction of hepatic cytochrome P(1)450 mRNA has been investigated utilizing pfP(1)450-3', a 3'-specific 1.5 kb cDNA clone derived from 3-methylcholanthrene-inducible mRNA of rainbow trout. A time course of induction of both the hybridizable mRNA and hepatic monooxygenase catalytic activity in rainbow trout with a known inducer in fish, beta-naphthoflavone, was studied. The cDNA probe was also shown to hybridize with induced mRNA of brook trout, scup, garter snake, painted turtle, and rat demonstrating the suitability of the probe for examining induction of mRNA in various species. The results of these experiments suggest that the cDNA probe may be useful as a biological monitoring tool for determining the presence and effects of chemical pollutants which are inducers of hepatic microsomal monooxygenase activity. The probe may have the potential to be applied as an early warning system in the monitoring of water quality.

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.

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.

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.

Synergy of phenobarbital and 3-methylcholanthrene in "superinduction" of cytochrome P-450c mRNA but not enzyme activity

The combination of phenobarbital and 3-methylcholanthrene in the inductive process of the rat hepatic cytochrome P-450c gene was evaluated. Daily injections of phenobarbital (80 mg/kg, i.p.) had little or no effect on the amount of poly (A)+ RNA encoding cytochrome P-450c, whereas a single injection of 3-methylcholanthrene (25 mg/kg, i.p.) produced a significant accumulation at 15 hr in cytosolic mRNA coding for cytochrome P-450c. Four daily injections of phenobarbital followed by a single dose of 3-methylcholanthrene produced 5-24 times more poly (A)+ RNA coding for P-450c than 3-methylcholanthrene treatment alone. This superinduction of RNA transcripts was also observed for a species coding for cytochrome P-450d, which was increased 3-6 times over 3-methylcholanthrene treatment alone. However, the elevated concentration of transcripts for both the P-450d and P-450c RNA species did not result in an increase in the marker enzyme activity for cytochrome P-450c, 7-ethoxyresorufin O-deethylase. These data implicate a regulatory step in the induction of cytochrome P-450c enzyme activity which must occur at a level beyond transcription.

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.

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.

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.

A cytochrome P-450 gene family mapped to human chromosome 19

We have recently isolated a cloned cDNA coding for a cytochrome P-450 of human liver microsomal membranes, which corresponds to a major phenobarbital-inducible cytochrome P-450 of rat liver. This human cytochrome P-450 is encoded by a member of a multigene family. DNA extracted from a panel of 12 independent human-rodent somatic cell hybrids was analysed by Southern blot hybridization with the cloned cDNA. The results indicate that all components of this cytochrome P-450 gene family are located on chromosome 19. Evidence from hybrids derived from an individual carrying a balanced translocation suggests a regional localization of 19p13.2----qter. Analysis of human metaphase chromosomes by in situ hybridization localizes this cytochrome P-450 gene family further to the long arm of chromosome 19 in the region q13.1----qter. We propose the designation P450PB for this locus.

Isolation of a cDNA clone for the human muscle specific carbonic anhydrase, CAIII

The molecular cloning of cDNA for the human muscle specific carbonic anhydrase CAIII is described. The recombinant was isolated from a human muscle cDNA library prepared in the expression vector lambda gt11, and was characterized by hybridization selection and immunoprecipitation. A comparison of insert cDNA and mRNA sizes suggests that the cDNA is full length and includes extensive untranslated sequences. Preliminary sequence data have confirmed the authenticity of this clone and Southern blotting of human and rodent DNA indicates that it will be a useful probe in the analysis of somatic cell hybrids.

Phenobarbital enhances 2',5'-oligoadenylate synthesis in rat liver nuclei

Treatment of rats with phenobarbital for three days greatly increases the activity of 2,5 oligoadenylate synthetase in liver nuclei. Analysis of 2',5'-oligoadenylates synthesized in vitro showed that nuclei from both phenobarbital-treated and control rats synthesized 2',5'-oligoadenylates ranging from di- to hexamers. However, nuclei from drug treated rats showed a two fold increase in trimer and tetramer synthesis and a three-four fold increase in longer chained oligoadenylates. There was no change in the nuclear 2'-phosphodiesterase activity as the result of phenobarbital treatment, This activity remained low in nuclei from either the treated or the control rats. To our knowledge, this is the first report on phenobarbital affecting the liver 2',5'-oligoadenylate system.

Hormonal control of carbonic anhydrase III

Using radioimmunoassay, the concentration of carbonic anhydrase III (CA III) in the livers of adult male rats was found to be approximately 30 times greater than that observed in mature females. Castration of male rats led to a marked reduction in liver CA III concentrations that could be partially restored to control levels by testosterone replacement. Administration of testosterone to ovariectomized female rats induced about a 5-fold increase in liver CA III concentration. Immunoprecipitational analysis of the products of liver mRNA translation in vitro with antiserum specific for CA III showed that hormonal control of the levels of CA III in rat liver is mediated by changes in the amount of translatable CA III mRNA. Marked changes in liver CA III concentrations were also observed in developing and aging male rats. Different control mechanisms appear to operate in mouse and man.

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.

The in vivo turnover of rat liver microsomal epoxide hydrolase and both the apoprotein and heme moieties of specific cytochrome P-450 isozymes

The in vivo turnover rates of liver microsomal epoxide hydrolase and both the heme and apoprotein moieties of cytochromes P-450a, P-450b + P-450e, and P-450c have been determined by following the decay in specific radioactivity from 2 to 96 h after simultaneous injections of NaH14CO3 and 3H-labeled delta-aminolevulinic acid to Aroclor 1254-treated rats. Total liver microsomal protein was characterized by an apparent biphasic exponential decay in specific radioactivity, with half-lives of 5-9 and 82 h for the fast- and slow-phase components, respectively. Most (approximately 90%) of the rapidly turning over microsomal protein fraction was immunologically distinct from membrane-associated serum protein, and thus appeared to represent integral membrane proteins. The existence of two distinct populations of cytochrome P-450a was suggested by the apparent biphasic turnover of both the heme and apoprotein moieties of the holoenzyme. The half-lives of the apoprotein were estimated to be 12 and 52 h for the fast- and slow-phase components, respectively, and 7 and 34 h for the heme moiety. The turnover of cytochromes P-450b + P-450e was identical to that of cytochrome P-450c, with half-lives of 37 and 28 h for the apoprotein and heme moieties, respectively. In all cases, the shorter half-lives of the heme component compared to the protein component were statistically significant. In contrast to the cytochrome P-450 isozymes, epoxide hydrolase (t1/2 = 132 h) turned over slower than the "average" microsomal protein (t1/2 = 82 h). The differential rates of degradation of these major integral membrane proteins during both the rapid and slow phases of total microsomal protein turnover argue against the concepts of unit membrane degradation and unidirectional membrane flow of liver endoplasmic reticulum.

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.

A rapid one-step purification of NADPH-cytochrome c (P-450) reductase from rat liver microsomes

NADPH-cytochrome c (P-450) reductase from liver microsomes of phenobarbital-treated rats has been purified in a single step by affinity chromatography on agarose-hexane-adenosine 2',5'-diphosphate. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, enzyme assay, and radioimmunoassay the protein obtained by this single step procedure is as pure as that isolated by multicolumn procedures.

The effect of phenobarbital on the transcriptional activity of liver

The effect of phenobarbital on the transcriptional activity of liver was studied by measuring the synthesis of RNA by suspensions of hepatocytes isolated from rats treated with phenobarbital for various time periods. The absolute rates of RNA synthesis by isolated hepatocytes were determined by measuring the incorporation of [3H]orotic acid into RNA as UMP and the specific radioactivity of the UTP pool. The specific radioactivity of the UTP extracted from hepatocytes isolated from phenobarbital-treated rats was consistently lower than that of the UTP pool of hepatocytes from untreated rats. Phenobarbital treatment increased the rate of RNA synthesis 10-fold over that observed for hepatocytes from untreated rats. The maximum rate of RNA synthesis was observed 16-18 h after phenobarbital administration. Phenobarbital treatment also affected the nuclear-cytoplasmic transport of RNA by isolated hepatocytes. Immediately after phenobarbital treatment, the transport of RNA decreased; however, 24 h after phenobarbital administration, the transport of RNA was increased 4-fold. An increase in the synthesis of RNA in vivo by liver was found 18 h after phenobarbital treatment, and the incubation of suspensions of hepatocytes with various concentrations of phenobarbital increased RNA synthesis significantly.

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.

Complexes of cytochrome P450 with metyrapone. A convenient method for the quantitative analysis of phenobarbital-inducible cytochrome P450 in rat liver microsomes

Phenobarbital-inducible cytochrome P450 variants have a higher affinity (app. Ks = 0.6 microM). The difference in affinity has enabled us to quantitate, by metyrapone-induced difference spectral analysis, phenobarbital-inducible cytochromes P450 in liver microsomal membranes. The result obtained by this simple, rapid technique compare well with those found by more sophisticated techniques such as radioimmunoassay and immunoprecipitation.

Synthesis of rat muscle carbonic anhydrase III in a cell-free translation system

Carbonic anhydrase III (CA III) was identified in the products of rat soleus muscle RNA translation in vitro by both a radioimmunoassay and a specific immunoprecipitation technique followed by SDS--polyacrylamide gel electrophoresis analysis of the precipitated antigen. The primary translation product has the same Mr-value as the native isoenzyme. CA III mRNA was found to represent approximately 0.55% of the total mRNA present in rat soleus muscle.