Inductive and repressive effects of rifampicin on rabbit liver microsomal cytochrome P-450

Induction of hepatic Cyp2b and Cyp3a subfamily enzymes by nicardipine and nifedipine in mice

1. Nicardipine (Nic) or nifedipine (Nif) was given to male and female C57BL/6J mice by a single gavage at doses of 100, 200 and 400 micromolkg(-1), and changes in the levels of mRNA and apoprotein of hepatic cytochrome P450 (P450) enzymes, including Cyp2b9, Cyp2b10, Cyp3a11 and Cyp3a41, were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. Furthermore, hepatic microsomal activities for pentoxyresorufin O-dealkylation (PROD) and nifedipine oxidation, which are mainly mediated by Cyp2b and Cyp3a subfamily enzymes, respectively, were measured. 2. Results from RT-PCR analysis revealed that Nic, but not Nif, showed a capacity for activating the Cyp3a11 gene in either sex of mice and that both chemicals could induce a male-selective activation of Cyp2b10 gene, although they had no capacity for activating the Cyp2b9 and Cyp3a41 genes in either sex. 3. Increased levels of the mRNAs of Cyp2b10 and Cyp3a11 were closely correlated with those of apoprotein and activity of the corresponding P450 subfamily enzymes. 4. The study demonstrated for the first time that Nic, but not Nif, showed the ability to induce Cyp3a11 in both sexes of mice, although both Nif and Nic led to a male-selective induction of Cyp2b10, and that Nic and Nif had no ability to induce Cyp2b9 and Cyp3a41 in either sex.

Receptor-dependent transcriptional activation of cytochrome P4503A genes: induction mechanisms, species differences and interindividual variation in man

1. The importance of CYP3A enzymes in drug metabolism and toxicology has yielded a wealth of information on the structure, function and regulation of this subfamily and recent research emphasis has been placed on the human forms, namely CYP3A4, CYP3A5, CYP3A7 and CYP3A43. 2. The current review will focus on the receptor-dependency of CYP3A regulation and includes consideration of the regulatory roles of the glucocorticoid (GR), pregnane X (PXR) and constitutive androstane (CAR) receptors. 3. Emphasis has been placed on the topics of expression and substrate specificity, assessment of induction, species differences in induction, CYP3A promoter sequences and regulation of gene expression, structural and functional aspects of receptor-mediated, CYP3A gene activation, receptor variants and interindividual variation in human CYP3A expression, the latter encompassing environmental, physiological and genetic aspects. 4. An outline of future research needs will be discussed in the context of receptor-mediated molecular mechanisms of CYP3A gene regulation and the impact on interindividual variations in CYP3A expression. 5. Taken collectively, this review highlights the importance of understanding the molecular mechanisms of CYP3A induction as a means of rationalizing human responses to many clinically used drugs, in addition to providing a mechanistically coherent platform to understand and predict interindividual variations in response and drug-drug interactions.

Intestinal cytochrome P450 and response to rifampicin in rabbits

Properties of cytochrome P450 (P450) in rabbit intestines have been investigated to assess the possibility of an experimental model for human intestinal oxidation of drugs. Significant amounts of P450 and cytochrome b5 and activities of NADPH-cytochrome P450 reductase were detected in microsomes from rabbit duodenal, jejunal, ileac and colon mucosa. All the small intestinal fractions mediated phenytoin, dextromethorphan and testosterone oxidations. Several P450 forms belonging to the CYP1A, CYP2C, CYP2D and CYP3A, but not CYP2B and CYP2E, subfamilies were detected in these tissues by Western blotting. A good correlation was observed between immunodetectable levels of CYP3A and activities of testosterone 6beta-hydroxylation. Small intestine, but not colon, CYP3A levels were increased by the pretreatment of rabbits with rifampicin (50 mg/kg for 4 days, p.o.). The extent of the increase was similar between duodena and livers. These properties of rabbit intestinal P450s were comparable to those of human intestine. These phenomena suggest the possibility that the rabbit is a beneficial in vivo model for the assessment of drug interaction occurring at the first pass of drugs ingested.

Differential induction of cytochrome P-450 isozymes by rifampicin in the Chinese hamster, Cricetus griseus

Treatment of male and female Chinese hamsters with rifampicin at intraperitoneal doses of 25 and 50 mg/kg did not increase the cytochrome P-450 content of the liver except for a 1.3-fold increase in male hamsters at a dose of 50 mg/kg. Enhancement of the activities of erythromycin N-demethylase and testosterone hydroxylases, except for 15 alpha-hydroxylation, was observed in the livers of both male and female hamsters treated with rifampicin at both doses. Western blot analysis revealed that rifampicin caused no change in the content of CYP3A subfamily proteins in the liver, whereas changes in that of CYP2A subfamily proteins were evident.

Influence of rifampicin on the toxicity and the analgesic effect of acetaminophen

The influence of rifampicin on the toxicity, analgesic effect and pharmacokinetics of acetaminophen was studied in male albino mice. Repeated administration of rifampicin (50 mg/kg i.p. daily for 6 days) shortened hexobarbital sleeping time and increased liver weight, microsomal cytochrome P-450 and heme contents, NADPH-cytochrome c reductase and ethylmorphine-N-demethylase activities. Aniline hydroxylase activity was decreased and glucuronidation of p-nitrophenol was unaffected. Rifampicin pretreatment changed neither the LD50 of acetaminophen nor the hepatic glutathione level nor the glutathione depletion provoked by the toxic dose of acetaminophen (737 mg/kg p.o.). This suggests that rifampicin has no influence on the amount of acetaminophen toxic metabolites formed in the liver. Rifampicin decreased the acetaminophen analgesic effect in mice. Rifampicin decreased the Cmax, the half-time, the MRT and the AUC of acetaminophen and accelerated its clearance. The plasma concentration of acetaminophen glucuronide and acetaminophen sulfate was increased. It is assumed that the most probable mechanism by which rifampicin decreases acetaminophen analgesia is the accelerated acetaminophen elimination.

Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

Effects of dexamethasone on the hepatic cytochrome P450IIIA subfamily in two hamster strains Mesocricetus auratus and Cricetus griseus

1. Administration of dexamethasone significantly reduced the amount of hepatic cytochrome P450 in Syrian golden and Chinese hamsters, while it increased the amount in rats. 2. In contrast to rats, the induction rate of the activities of erythromycin and troleandomycin N-demethylases by dexamethasone was relatively low, while that of the testosterone 6 beta-hydroxylase was high in the two hamster strains. 3. Western blot analysis revealed that dexamethasone did not modify markedly the pattern of the P450IIIA subfamily proteins in the two hamster strains.

Rat liver microsomal progesterone metabolism: evidence for differential troleandomycin and pregnenolone 16 alpha-carbonitrile inductive effects in the cytochrome P-450 III family

The effect of Troleandomycin (TAO) and pregnenolone 16 alpha-carbonitrile (PCN) on the hepatic microsomal progesterone metabolism in the rat is evaluated. Over thirteen hydroxylated progesterone derivatives are detected, including the novel 6 beta, 21-, 6 beta, 16 alpha-, 6 beta, 16 beta- and 2,21-dihydroxy derivatives, suggesting the induction of several cytochrome P-450 isozymes. PCN treatment results overall in an augmented production of progesterone metabolites whereas TAO treatment both induces and represses specific hydroxylase activities. Progesterone metabolism with purified isozymes isolated from liver microsomes from TAO and PCN treated rats differs significantly from that observed with intact microsomes, reflecting the complexity of the induction pattern of the cytochrome P-450 III family.

Selective inactivation by 21-chlorinated steroids of rabbit liver and adrenal microsomal cytochromes P-450 involved in progesterone hydroxylation

The inactivation by 21-chlorinated steroids of rabbit liver cytochromes P-450 involved in the hydroxylation of progesterone has been investigated in intact microsomes encompassing two phenotypes of 21-hydroxylase activity, two phenotypes of 16 alpha-hydroxylase activity, and three phenotypes of 6 beta-hydroxylase activity. In liver microsomes from outbred New Zealand White male rabbits exhibiting a high content of cytochrome P-450 1, 21,21-dichloropregnenolone caused a time- and NADPH-dependent loss of 21-hydroxylase activity. This loss of activity exhibited a number of characteristics of mechanism-based inactivation, including irreversibility, saturation with increasing inhibitor concentrations, and protection by substrate, and was also documented with purified P-450 1 in a reconstituted system. 21,21-Dichloropregnenolone caused no time-dependent loss of 6 beta-hydroxylase activity in microsomes from the New Zealand White rabbits or from control or rifampicin-treated rabbits of the inbred B/J strain. In contrast, in the microsomes from the B/J rabbits, some inactivation of the 16 alpha-hydroxylase was observed (k = 0.04 min-1), regardless of the rifampicin treatment. The other two compounds tested, 21-chloropregnenolone and 21,21-dichloroprogesterone, were less effective than the dichloropregnenolone as inactivators of cytochrome P-450 1. On the other hand, 21,21-dichloroprogesterone, but not 21,21-dichloropregneolone, caused a rapid time-dependent loss of 21-hydroxylase activity in rabbit adrenal microsomes. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few forms of cytochrome P-450 may be a rational means of designing selective inhibitors of the enzyme.

Complete sequence of cytochrome P450 3c cDNA and presence of two mRNA species with 3' untranslated regions of different lengths

Two cDNAs (pLM3c 4.1 and pLM3c 6.1) coding for rabbit cytochrome P450 3c were sequenced. cDNA 4.1 (1768 bp) exhibits an open reading frame from nucleotides 74 to 1576 encoding the 501 amino acid residues of the entire protein. cDNA 6.1 (189 bp) appears to encode the last 24 amino acids. Comparative amino acid sequence analysis indicated that P450 PCN1, PCN2, and HLp from rat and man, were 70, 67, and 73% homologous, respectively, to P450 3c. According to the cytochrome P450 nomenclature, the P450 3c gene is termed P450IIIA4. Comparison of the nucleotide sequences indicated that cDNA 6.1 was 100% homologous to cDNA 4.1. However, whereas a poly(A) tract started 23 nucleotides after the AATAAA consensus sequence in cDNA 6.1, cDNA 4.1 had a 3' untranslated region extending 101 bp beyond the polyadenylation signal, which lacked poly(A). This observation is consistent with the previous finding that both cDNA 4.1 and 6.1 hybridized with two distinct species of poly(A)RNA (1700 and 1850 bases) from rabbit liver. The extreme 3'-end 79-bp fragment of cDNA 4.1 therefore was isolated by subcloning in pUC12 (clone p18-Rsa I) and used to probe Northern blots of poly(A)RNA from control and rifampicin-treated rabbit liver. In contrast to cDNA 4.1 and 6.1, p18-Rsa I cDNA hybridized only with the largest (1850 bases) mRNA species. We conclude that rabbit liver contains two P450 3c mRNA species differing in the length of their 3' untranslated region.

Altered drug metabolism in parasitic diseases

While the immune system represents the main line of host defence against parasite infections, mixed function oxidase (MFO) systems (Box 1) offer the main line of defence against drugs and other biologically active substances. But, as this review shows, many parasites can exert a profound effect on the host MFO system by altering the microsomal drug-metabolizing enzymes and electron transport carriers such as cytochrome P-450. This can markedly affect the host's ability to metabolize biologically active compounds, often with adverse physiological, pharmacological and toxicological consequences. In mammals, drug metabolism occurs predominantly in the liver, and to a lesser extent in the spleen, lungs, kidneys, intestine and cerebral tissues. Thus those parasites that occupy sites in these tissues - such as amoebae, Fasciola, schistosomes and malaria - tend to be those with greatest effects on the host's ability to metabolize drugs. The effects can modify the host response to substances unrelated to the infection, and to drugs which may be administered under a chemotherapeutic regime.

Studies on the pregnenolone-16 alpha-carbonitrile-inducible form of rat liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase

Treatment of rats with pregnenolone-16 alpha-carbonitrile (PCN) markedly induces rat liver microsomal cytochrome P-450p and UDP-GT-dt1, a glucuronosyltransferase active towards the digitoxin metabolite, digitoxigenin monodigitoxoside. The present study characterizes the regulation of these two enzymes in rats treated with different xenobiotics. Like PCN, treatment of rats with dexamethasone, spironolactone, troleandomycin or erythromycin estolate markedly induced both UDP-GT-dt1 and cytochrome P-450p (measured as erythromycin demethylase and testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activities). However, compared to PCN and dexamethasone, both troleandomycin and erythromycin estolate preferentially induced cytochrome P-450p, whereas spironolactone preferentially induced UDP-GT-dt1. Treatment of rats with the polychlorinated biphenyl mixture, Aroclor 1254, increased both cytochrome P-450p and UDP-GT-dt1 activity to about 40% of that in liver microsomes from rats induced with PCN or dexamethasone. Treatment of rats with phenobarbital or chlordane caused a relatively small increase in cytochrome P-450p and UDP-GT-dt1 activity. Neither enzyme was induced by treatment of rats with 3-methylcholanthrene, rifampin or digitoxin. The induction of cytochrome P-450p and UDP-GT-dt1 by PCN followed similar dose-response curves. Although cytochrome P-450p and UDP-GT-dt1 are differentially affected by the age and the sex of rats, the enzymes responded similarly, but not identically, to xenobiotic treatment. This suggests that cytochrome P-450p and UDP-GT-dt1 are co-inducible but not coordinately regulated.

Expression of five forms of microsomal cytochrome P-450 in primary cultures of rabbit hepatocytes treated with various classes of inducers

In order to investigate the expression of five different forms of microsomal cytochrome P-450 including P-450 2 (P450IIBI), 3b (P450IIC3), 3c (P450IIIA4), 4 and 6 (P450IA2 and A1), hepatocytes were isolated from untreated rabbit and maintained in primary monolayer cultures in serum free modified Waymouth medium in the absence and in the presence of various classes of inducers including phenobarbital (PB), rifampicin (RIF), dexamethasone (DEX) and B-naphthoflavone (BNF). In untreated cultures the level of the various forms of P-450, determined by immunoblot with the use of specific antibodies, generally declined with time but at markedly different rates. In cultures treated with the inducers decline of the various forms was either unaffected, reduced, or even reversed, so that 96 hr after plating some of these forms appeared to be induced several-fold with respect to the untreated cultures. The forms 2 and 3c were co-induced by PB, RIF or DEX; as in vivo, BNF induced forms 4 and 6. Induction of forms 2, 3c, 4 and 6 was accompanied by stimulation of related monooxygenase activities, benzphetamine demethylase, progesterone 6B hydroxylase and benzpyrene hydroxylase and ethoxyresorufin deethylase, respectively. In all cases, induction was accompanied by an increased rate of de novo synthesis of the protein, determined by radio-immunoprecipitation assay with the use of specific antibodies on [3H]-Leu labeled cell lysate. Both induction and increased de novo synthesis were time- and inducer concentration-dependent. In cultures treated with RIF or BNF de novo synthesis of P-450 3c or of P-450 4 and 6 was correlated with the level of their specific mRNA quantitated from northern blots probed with either pLM3c-4.1 or pLM6.1, two plasmids containing inserted cDNA coding for P-450 3c or P-450 6, respectively. We conclude from these experiments that rabbit hepatocytes in primary monolayer cultures represent suitable models for studying regulation induction and pharmacological implications of the microsomal cytochromes P-450.

Intraspecies individuality for the metabolism of steroids

A variety of regulatory factors contribute to differences in the rates of 6 beta-hydroxylation, 16 alpha-hydroxylation and 21-hydroxylation of progesterone as catalysed by liver microsomes prepared from individual rabbits. It is likely that the 6 beta-hydroxylation of progesterone is catalysed primarily by cytochrome P-450 3c, an enzyme that exhibits allosteric activation by alpha-napthoflavone, and by a form of P-450 3b, 6 beta+, that is expressed in some rabbits in an autosomal dominant manner. The mechanism of activation for P-450 3c appears to reflect an effector mediated increase of the affinity of the enzyme for substrate as judged by substrate binding studies. A second form of P-450 3b, 6 beta-, catalyses a major portion of hepatic progesterone 16 alpha-hydroxylation and exhibits activation by a variety of C21 steroids of which 5 beta-pregnane-3 beta,20 alpha-diol is the most efficacious. P-450 1, which catalyses the 21-hydroxylation of progesterone, is expressed at 10-fold higher levels in the 21H phenotype than the 21L phenotype, and the former is inherited as an autosomal dominant characteristic. A cDNA encoding a P-450 1-related gene product exhibits a predicted amino acid sequence that is 95% homologous to that of P-450 1. The P-450 1-related gene product is expressed in liver to a similar degree in both 21H and 21L rabbits.

Cloning of a cDNA coding for P-450 LM3c from rabbit liver microsomes and regulation of its expression

Liver cytochromes P-450 LM3c in the rabbit and P-450p in the rat are two related forms, inducible by macrolide antibiotics such as triacetyloleandomycin (TAO) and glucocorticoids such as dexamethasone. We prepared a cDNA library from TAO induced rabbit liver mRNA and characterized a cDNA (pLM3c-4.1) that hybridized to pDex 3.22, a cDNA complementary to cytochrome P-450p mRNA. Northern blots of liver poly(A)RNA from untreated or TAO, erythromycin and rifampicin treated animals, revealed two mRNA species of approximately 1700 and 1850 nucleotides in length, that hybridized to LM3c cDNA and to pDEX 3.22. The level of both mRNAs was increased five fold over control by macrolide antibiotics but unaffected by both phenobarbital and B-naphthoflavone. After 5 days of TAO treatment LM3c mRNA had increased 5 fold while LM3c protein had increased 25 fold. However, the rate of P-450 LM3c gene transcription measured in isolated liver nuclei remained unchanged throughout five days of TAO treatment. We conclude that TAO may induce cytochrome P-450 LM3c by post-transcriptional effects.

Isolation and partial characterization of a rifampicin induced rabbit liver microsomal cytochrome P-450

Rifampicin administration to New Zealand male rabbits increased the concentration of an LM3 form of cytochrome P-450 to up to 30% of the microsomal P-450 concentration. This enzyme was purified to electrophoretic homogeneity with a yield of 8% of the original total microsomal P-450 concentration. Isolated as a low spin hemoprotein in its substrate free oxidized form, it displays in its reduced CO-complexed form an absorption maximum at 449 nm. Immunological assays, as well as activity measurements, in particular its stereospecific progesterone hydroxylation in the 6 beta-position, show a relationship between LM3,Rif and LM3c (from untreated rabbits).