Developmental expression of rabbit cytochrome P450 CYP1A1, CYP1A2 and CYP3A6 genes. Effect of weaning and rifampicin

Effects of soy containing diet and isoflavones on cytochrome P450 enzyme expression and activity

Cytochromes P450 (CYPs) play an important role in metabolism and clearance of most clinically utilized drugs and other xenobiotics. They are important in metabolism of endogenous compounds including fatty acids, sterols, steroids and lipid-soluble vitamins. Dietary factors such as phytochemicals are capable of affecting CYP expression and activity, which may be important in diet-drug interactions and in the development of fatty liver disease, cardiovascular disease and cancer. One important diet-CYP interaction is with diets containing plant proteins, particularly soy protein. Soy diets are traditionally consumed in Asian countries and are linked to lower incidence of several cancers and of cardiovascular disease in Asian populations. Soy is also an important protein source in vegetarian and vegan diets and the sole protein source in soy infant formulas. Recent studies suggest that consumption of soy can inhibit induction of CY1 enzymes by polycyclic aromatic hydrocarbons (PAHs) which may contribute to cancer prevention. In addition, there are data to suggest that soy components promiscuously activate several nuclear receptors including PXR, PPAR and LXR resulting in increased expression of CYP3As, CYP4As and CYPs involved in metabolism of cholesterol to bile acids. Such soy-CYP interactions may alter drug pharmacokinetics and therapeutic efficacy and are associated with improved lipid homeostasis and reduced risk of cardiovascular disease. The current review summarizes results from in vitro; in vivo and clinical studies of soy-CYP interactions and examines the evidence linking the effects of soy diets on CYP expression to isoflavone phytoestrogens, particularly, genistein and daidzein that are associated with soy protein.

Population Pharmacokinetics of Rifampin in Pregnant Women with Tuberculosis and HIV Coinfection in Soweto, South Africa

Effective treatment of tuberculosis during pregnancy is essential for preventing maternal and fetal mortality, but little is known about the effects of pregnancy on the disposition of antituberculosis drugs. We explored the effects of pregnancy on the pharmacokinetics of rifampin, the key sterilizing drug in tuberculosis treatment, in Tshepiso, a prospective cohort study involving pregnant HIV-infected women with or without tuberculosis in Soweto, South Africa. Participants receiving standard first-line tuberculosis treatment underwent sparse sampling for rifampin at 37 weeks' gestation or delivery and then postpartum. Cord blood was collected when possible. A population pharmacokinetic model was developed to investigate the effects of pregnancy on rifampin pharmacokinetics. Among the 48 participants, median age and weight were 28 years and 67 kg, respectively. A one-compartment model with first-order elimination, transit compartment absorption, and allometric scaling described the data well. Pregnancy reduced rifampin clearance by 14%. The median (interquartile range) model-estimated rifampin area under the concentration-time curve over 24 h (AUC0-24) during pregnancy or intrapartum was 40.8 h · mg/liter (27.1 to 54.2 h · mg/liter) compared to 37.4 h · mg/liter (26.8 to 50.3 h · mg/liter) postpartum. The maximum concentrations were similar during pregnancy and postpartum. Rifampin was detectable in 36% (8/22) of cord blood samples, and 88% (42/48) of the women had successful treatment outcomes. There was one case of perinatal tuberculosis. In conclusion, rifampin clearance is modestly reduced during the last trimester of pregnancy. Exposures are only slightly increased, so dose adjustment during pregnancy is not needed. Rifampin was detected in cord blood samples when delivery occurred soon after dosing. The consequences of exposure to this potent inducer of metabolizing enzymes among HIV-exposed infants are unclear.

Cytochrome P450 system proteins reside in different regions of the endoplasmic reticulum

Cytochrome P450 (P450) function is dependent on the ability of these enzymes to successfully interact with their redox partners, NADPH-cytochrome P450 reductase (CPR) and cytochrome b5, in the endoplasmic reticulum (ER). Because the ER is heterogeneous in lipid composition, membrane microdomains with different characteristics are formed. Ordered microdomains are more tightly packed, and enriched in saturated fatty acids, sphingomyelin and cholesterol, whereas disordered regions contain higher levels of unsaturated fatty acids. The goal of the present study was to determine whether the P450 system proteins localize to different regions of the ER. The localization of CYP1A2, CYP2B4 and CYP2E1 within the ER was determined by partial membrane solubilization with Brij 98, centrifugation on a discontinuous sucrose gradient and immune blotting of the gradient fractions to identify ordered and disordered microdomains. CYP1A2 resided almost entirely in the ordered regions of the ER with CPR also localized predominantly to this region. CYP2B4 was equally distributed between the ordered and disordered domains. In contrast, CYP2E1 localized to the disordered membrane regions. Removal of cholesterol (an important constituent of ordered domains) led to the relocation of CYP1A2, CYP2B4 and CPR to the disordered regions. Interestingly, CYP1A1 and CYP1A2 localized to different membrane microdomains, despite their high degree of sequence similarity. These data demonstrate that P450 system enzymes are organized in specific membrane regions, and their localization can be affected by depletion of membrane cholesterol. The differential localization of different P450 in specific membrane regions may provide a novel mechanism for modulating P450 function.

Isolation and characterization of the CYP2D6 gene in Felidae with comparison to other mammals

The highly polymorphic CYP2D6 protein metabolizes about 25% of commonly used drugs and underlies a broad spectrum of drug responses among individuals. In contrast to extensive knowledge on the human CYP2D6 gene, little is known about the gene in non-human mammals. CYP2D6 mRNA from 23 cats (Felidae) spanning seven species were compared to available CYPD6 sequences in ten additional mammals and multiple allelic variants in humans. A relatively high mean dN/dS ratio (0.565) was observed, especially within Felidae. Pairwise dN/dS ratios were non-monotonically distributed with respect to evolutionary distance suggesting either positive selection or retention of slightly deleterious mutations. Positive selection on specific codons, most notably in regions involved in substrate recognition and membrane anchoring is supported and the possible influence of diet on specific amino acid changes in substrate binding sites is discussed.

Genetic and epigenetic regulation and expression signatures of glutathione S-transferases in developing mouse liver

The hepatic glutathione S-transferases (Gsts) are critical phase II enzymes in protecting cellular macromolecules against electrophiles and oxidative stress. Little is known about the ontogeny of Gsts and the underlying regulatory mechanisms during liver development. Therefore, in this study, the ontogeny and the regulatory mechanisms of 19 known Gst isoforms were investigated in mouse liver from 2 days before birth to postnatal day 45. With the exception of Gstm5 and MGst2 that showed a progressive decline in postnatal messenger RNA (mRNA) expression, most other Gst isoforms showed a progressive increase in postnatal mRNA expression. Two-way hierarchical clustering revealed three distinct expression patterns of these Gsts isoforms: perinatal, adolescent, and adult enriched. The expression signatures of certain Gst isoforms showed positive association with the ontogeny of critical xenobiotic-sensing transcription factors, including aryl hydrocarbon receptor, pregnane X receptor (PXR), constitutive androstane receptor, peroxisome proliferator-activated receptor alpha, and NF-E2-related factor-2. Specifically, genome-wide chromatin immunoprecipitation coupled with the next generation sequencing technology (ChIP-Seq) revealed direct PXR-binding sites to the Gsta, Gstm, Gstt, and Gstp polycistron clusters as well as to the Mgst1 gene locus. Chromatin immunoprecipitation-on-chip analysis demonstrated that DNA methylation and histone H3K27-trimethylation (H3K27me3), two-gene expression-suppressing epigenetic marks, were consistently low around the Gstz1 gene locus. In contrast, enrichment of histone H3K4-dimethylation (H3K4me2), a hallmark for gene activation, increased 60% around the Gstz1 gene locus from prenatal to the young adult period. Regression analysis revealed a strong correlation between the enrichment of H3K4me2 and Gstz1 mRNA expression (r = 0.76). In conclusion, this study characterized three distinct ontogenic expression signatures of the 19 Gst isoforms and examined some genetic and epigenetic mechanisms inducing their transcription during liver development.

The effects of accumulation of an environmentally relevant polychlorinated biphenyl mixture on cytochrome P450 and P-glycoprotein expressions in fetuses and pregnant rats

The aim of this study was to improve knowledge about transplacental transfer of an environmentally relevant PCB mixture by evaluating congener levels in livers and brains of rat dams and fetuses after maternal exposure, and correlating them to the levels of CYP450 and P-glycoprotein, involved in biotransformation and xenobiotics export, respectively. Pregnant dams were injected daily from gestation day (GD) 15 to 19 with 10mgkg(-1) of a reconstituted mixture (RM) composed of PCB138, 153, 180 and 126. Our data indicate that at GD20 RM is partitioned among maternal tissues, and that fetuses are not excluded from this distribution, evidencing a placental transfer of PCBs. Considering the ratio of maternal and fetal PCB concentrations based on lipid-weight, the amounts of congeners were 7-fold lower in fetal livers than in maternal livers and 25-30-fold higher in fetal brains than in maternal ones. Moreover, in dams the congeners were able to induce hepatic CYP450 response (total CYP450, CYP1A and CYP2B), but failed to increase P-170 expression, while in fetuses the constitutive expression of CYP450 and P-170 was not induced by treatment. Pearson Product-Moment Correlation applied to treated group data suggests that PCB accumulation in fetal livers, but not in brains, depended principally on their mothers' intoxication pattern. On the whole, these results emphasize the maternal liver and the fetal brain as depot organs for PCB sequestration and their susceptibility towards PCB toxicological risk. Moreover they highlight the lack of a coordinated response between the investigated defence mechanism.

Diets containing soy protein isolate increase hepatic CYP3A expression and inducibility in weanling male rats exposed during early development

Hepatic CYP3A enzymes were studied in weanling male Sprague-Dawley rats exposed to diets from gestational d 4 in which the sole protein source was either casein (CAS) or soy protein isolate (SPI). At age 25 d, rats were gavaged with corn oil or one of the CYP3A inducers, dexamethasone (DEX) and clotrimazole (CLT), at a dose of 50 mg/kg. Little CYP3A1 (CYP3A23), CYP3A2, or CYP3A9 mRNA was observed in CAS-fed weanling rats but CYP3A18 mRNA was readily detectable in Northern blots. In contrast, consumption of SPI without inducer treatment resulted in the expression of CYP3A1 (CYP3A23), and CYP3A2 mRNAs, expression of CYP3A apoprotein in hepatic microsomes, and a 2-fold greater turnover of the CYP3A substrate midazolam (P < 0.05). DEX induced CYP3A1, CYP3A2, and CYP3A9 (P < 0.05), but not CYP3A18 mRNA expression in rats fed both diets. Hepatic CYP3A apoprotein expression and midazolam 4-hydroxylation in SPI-fed rats was greater than that of CAS-fed rats after DEX treatment (P < 0.05). CLT also induced CYP3A2 mRNA 2-fold in rats fed both diets but CYP3A apoprotein expression in microsomes from SPI-fed CLT rats was double that of CLT-treated rats fed CAS (P < 0.05). The elevation of CYP3A apoprotein due to SPI and the CYP3A apoprotein induction by DEX and CLT treatment yielded no significant diet x inducer interaction. Analysis of heterologous nuclear RNA expression by RT-PCR using intron-specific primers for CYP3A1 revealed a 14-fold increase in RNA transcription in CAS-fed rats after treatment with DEX (P < 0.05) but no increase in rats fed SPI compared with rats fed CAS even though CYP3A1 mRNA and CYP3A apoprotein were significantly elevated. These data demonstrate that exposure to SPI during early development can increase CYP3A expression via post-transcriptional mechanisms and suggest that early soy consumption has potential effects on the metabolism of a wide variety of CYP3A substrates.

Tobacco smoke-dependent changes in cytochrome P450 1A1, 1A2, and 2E1 protein expressions in fetuses, newborns, pregnant rats, and human placenta

Tobacco smoke (TS) was described as a mixture of numerous cytochrome P450 (P450) substrates, inducers, and inhibitors. These inducers and inhibitors may modify drug clearance and xenobiotic or endogenous metabolism affecting P450s expression. In the present study, the effect of gestation and TS on: (1) cytochrome P450 CYP1A1, CYP1A2, and CYP2E1 protein expressions, and (2) cytochrome P450-linked microsomal enzyme activities, were studied in fetal rat liver, rat, and human placenta and in newborn and adult rat hepatic and extrahepatic tissues. Non-pregnant and pregnant 4-month-old female Wistar rats were exposed to TS (500, 1,000, or 1,500 mg carbon monoxide per m(3) air) in a toxicological chamber for 3 weeks (6 h daily, 5 days weekly). Human placentas were sampled from non-smoking, passive smoking, or active smoking primiparas. The efficacy of exposure was assessed by measuring urine cotinine levels. The TS-dependent inductory effect on the expression of CYP1A1 and 1A2 and related monooxygenase activities, and the inhibitory/inductory effect on CYP2E1 expression in rat tissues were observed. Pregnancy was associated with decreased levels of constitutive CYP1A1 and 2E1 in hepatic and extrahepatic tissues, TS-inducible CYP1A2 expression in the liver, and CYP1A1 expression in lungs and heart, but had no inhibitory effect on TS-inducible CYP1A1 and 2E1 expression, EROD, and P450-cooperated enzyme activities in the liver, kidney, and, in the latter case, in the heart. The presence of TS-induced CYP1A1 protein was confirmed in rat and human placenta and showed in newborn liver and lungs. CYP1A2 and 2E1 proteins were detectable in fetal rat liver. It was concluded that the expression of CYP1A1, 1A2, and 2E1, which metabolize some drugs and activate carcinogens, is controlled by age-, pregnancy-, and tissue-specific regulatory mechanisms in rats. Gestational differences in the regulation of expression of CYP1A subfamily members are not excluded. CYP1A1 and 2E1, but not CYP1A2 inductory mechanisms seem to be functional in fetal liver at day 21 of pregnancy but they appeared to be uninducible under a TS exposure. In TS-exposed pregnant females and fetuses the effects of metabolic activation of CYP1A1 and 1A2 substrates might be reduced because of lower CYP expressions or poor induction, respectively.

Developmental regulation of flavin-containing monooxygenase form 1 in the liver and kidney of fetal and neonatal rabbits

Flavin-containing monooxygenases (FMOs) comprise a multi-gene family and catalyze the oxygenation of soft nucleophilic sulfur, nitrogen, phosphorus, and selenium in xenobiotics. Previous studies have demonstrated that FMO is regulated developmentally and by the administration of certain steroid hormones. This study examined the expression of FMO form 1 in the livers and kidneys of fetal and neonatal rabbits, from day 25 of gestation through 3 weeks of age, by assaying FMO1 mRNA and protein levels, as well as catalytic activity. FMO1 mRNA and protein expression and FMO catalytic activity were present in fetal livers at the earliest time point measured (day 25 of gestation), although at levels approximately 10% of that found in adult livers. Hepatic FMO1 mRNA levels increased during and after gestation; levels were not significantly different from those measured in adult male livers. FMO1 protein content and activity rose rapidly after birth to reach 70-80% of adult levels by 3 weeks of age. The expression of FMO1 in fetal and neonatal kidneys was markedly lower than in liver. FMO1 mRNA levels never averaged more than 3.4% of adult male liver levels, but did not differ from adult kidney levels at any of the points measured. Protein levels and enzyme activity rose significantly after birth to approximately 30% of the level in adult kidneys by 3 weeks of age. The early developmental appearance of FMO1 suggests a possible role in the metabolism of xenobiotics through transplacental or lactational exposures.

Roles of cytochrome p450 in development

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.

Effects of AFB1 on CYP 1A1, 1A2 and 3A6 mRNA, and P450 expression in primary culture of rabbit hepatocytes

Although numerous studies report strong hepatic cytochrome P450 decrease during aflatoxicosis, the mechanisms involved in this decrease remain to be established. The purpose of this work is to investigate whether decreased CYP mRNA expression could explain decreased P450 expression and activity. Studies were conducted in primary cultures of rabbit hepatocytes exposed to 0.1 and 1 microM aflatoxin B1 (AFB1) incubated in the culture medium for 72 h. In order to confirm the effects of the mycotoxin, 30 microM beta-naphthoflavone or rifampicin were used as respective inductors of P450 1A1 and 1A2 or 3A6. Dose-dependent decreases of CYP mRNA expression were observed in all AFB1-treated cells; however, these decreases were not specific. Moreover, P450 expression and activity are far less decreased by the AFB1 treatment than their corresponding mRNA. Taken together, these results suggest that the specific P450 decrease observed during aflatoxicosis was not the consequence of a specific decrease of their mRNA expression.

Evidence for the presence of a functional pregnane X receptor response element in the CYP3A7 promoter gene

Pregnane X Receptor (PXR) has been recently shown to regulate the inducible expression of CYP3A genes in response to xenobiotics and steroids. PXR forms a heterodimer with the retinoic acid receptor (RXR) and this complex binds to and transactivates an 18bp region containing two everted repeats TGA(A/C)CT separated by 6 nucleotides (ER6) and located at approximately -150 in the CYP3A4 promoter. In this work we have isolated and sequenced the proximal 5'-flanking region of CYP3A7 from two different human genomic libraries. In contrast to a previously reported sequence (Itoh et al., 1992), we did not observe any mutation in the 3'-half of the CYP3A7 ER6 element. Using electrophoretic mobility shift assays and cotransfection experiments we show that this element is able to bind the PXR:RXR complex and transactivates the expression of a down stream promoter in response to rifampicin, clotrimazole, and RU-486, three compounds known to specifically activate the human PXR. This is consistent with the fact that CYP3A7 mRNA is inducible in several primary cultures of human hepatocytes from different patients, as well as in two hepatocarcinoma cell lines HuH7 and HepG2, in response to these compounds. In contrast to a previous report (Blumberg et al., 1998), based on the sequence published by Itoh et al., we conclude that CYP3A7, like CYP3A4, is inducible in response to xenobiotics and presumably in a large proportion of the population.

5-Fluorocytosine antagonizes the action of sterol biosynthesis inhibitors in Candida glabrata

The concentration-dependent antagonistic interaction between 5-fluorocytosine and a sterol biosynthesis inhibitor (SBI) was studied using intact cells and cell-free extracts of Candida glabrata. 5-Fluorocytosine promoted incorporation of radioactivity into 4-desmethylsterols (P < 0.01), and enhanced the relative and absolute increases of ergosterol (P < 0.05) in C. glabrata incubated aerobically with an SBI (miconazole or amorolfine). Further aerobic incubation of C. glabrata with combinations of a nucleic acid or protein synthesis inhibitor (rifampicin or chlortetracycline) and an SBI (miconazole) promoted a similar increase in ergosterol biosynthesis. In contrast, 5-fluorocytosine reduced the incorporation of radioactivity into 4,4-dimethylsterols (P < 0.01), but had no obvious effect on the absolute ergosterol level in C. glabrata incubated statically with miconazole. In cell-free extracts of cultures previously incubated with 5-fluorocytosine, ergosterol synthesis was less sensitive to the action of miconazole. Antagonism between 5-fluorocytosine and the SBI is thus mediated by a reversal of inhibition of intracellular ergosterol synthesis. The possible mechanisms underlying antagonism between 5-fluorocytosine and SBIs that inhibit different sites of the sterol biosynthesis pathway, as well as its clinical relevance to combination therapy, are discussed.

Comparative effects of cytokines on constitutive and inducible expression of the gene encoding for the cytochrome P450 3A6 isoenzyme in cultured rabbit hepatocytes: consequences on progesterone 6beta-hydroxylation

Cultured rabbit hepatocytes were used to compare the relative activities of cytokines to inhibit the constitutive or rifampicin (RIF)-induced expression of the cytochrome P450 3A6 gene (CYP3A6). Human recombinant cytokines tested were interleukin-1beta (IL-1beta) (2 U/mL), interleukin-2 (IL-2) (5,000 U/mL) and interferon-gamma (IFN-gamma) (50 U/mL). Hepatocytes were cultured in the presence or absence of 25 microM RIF for 24 hr, with or without cytokines alone or in combination. All these cytokines inhibited RIF-induced P4503A6 expression without apparent cellular toxicity. By contrast, only IFN-gamma treatment provided a significant decrease (41%) in the constitutive P4503A6 protein level. Moreover, cytokines differed in their ability to repress RIF-dependent transcriptional induction of CYP3A6: IL-1beta and IL-2 were approximately equipotent, causing an almost 40-50% suppression of CYP3A6 mRNA and protein levels, whereas IFN-gamma exerted repressive effects only on P4503A6-related erythromycin N-demethylase activity and inducible protein expression. In fact, although strongly reducing P4503A6 protein content (an approximate 70% decrease), IFN-gamma did not exhibit any influence on CYP3A6 mRNAs with the exception of its association with interleukins. All these results suggest that IL-1beta and IL-2 mainly promote a transcriptional repression mechanism, given the absence of effect of these cytokines on the basal P4503A6 level, whereas IFN-gamma exerts a post-transcriptional suppressive action on both induced and constitutive P4503A6 expression. Consequently, P4503A6-dependent progesterone 6beta-hydroxylase activity also presented a cytokine-specific pattern of inhibition, with a much greater sensitivity than P4503A6 immunoreactive protein to IL-1beta and IL-2 + IFN-gamma treatments. Thus, this study underlines the significant impact of inflammation on steroid metabolism.

Induction of cytochrome P450 activities in Drosophila melanogaster strains susceptible or resistant to insecticides

We analysed Drosophila melanogaster cytochrome P450s (P450) through the measurements of four enzymatic activities: ethoxycoumarin-O-deethylase, ethoxyresorufin-O-deethylase, lauric acid hydroxylation, and testosterone hydroxylation. We did these measurements in two Drosophila strains: one is susceptible to insecticides (Cantons) and the other is resistant to insecticides by enhanced P450 activities (RDDTR). In addition, we also treated the flies with eight chemicals (beta-naphtoflavone, benzo-alpha-pyrene, 3-methylcholanthrene, phenobarbital, aminopyrine, rifampicin, prochloraz, and clofibrate) known to induces genes from the families CYP1, CYP2, CYP3, CYP4, and CYP6. Metabolisation of all the substrates by P450 from flies microsomes was observed. The chemicals had different effects on these activities, ranging from induction to inhibition. The effects of these chemicals varied with the strains as most of them were ineffective on the RDDTR strain. The results showed that P450-dependent activities are numerous in Drosophila. Regulation features of these activities are complex. The availability of mutant strains as RDDTR should allow fundamental studies of P450 in insects.

Expression and inducibility of P450 enzymes during liver ontogeny

Several approaches, including immunoquantification of individual enzymes, profiling of substrate activities by immunoinhibition using highly specific polyclonal and monoclonal antibodies, and the estimation of corresponding mRNAs with nucleic acid probes, have been used to investigate the ontogeny of cytochromes P450 in livers of rodents and man. CYP1A1 is expressed very early in development in rodents, whereas most other enzymes either appear at or near birth (CYP2B, CYP2C23, and CYP3A) or between 2 and 4 weeks following birth (CYP2A, CYP2C6, CYP2C7, CYP2C11, CYP2C12, and CYP4A10). The constitutive expression of enzymes is subject to regulation by various transcriptional nuclear and/or hormonal factors (CYP2B and CYP2C) or in a sex-dependent manner (CYP2A, CYP2C11, CYP2C12, CYP3A, and CYP4A10). The enhanced sensitivity and specificity of immunocytochemical and in situ hybridisation studies have revealed differences, with age and xenobiotic treatment, in the intercellular expression of certain P450 enzymes of the liver. For example, in rats, the expression of CYP1A1 and 1A2 is differentially regulated at the level of the individual cell from as early as 24 hours before birth. The human foetal liver relative to rodents has a substantial level of CYP3A and also has the capacity to metabolise a greater repertoire of substrates. Evidence to date suggests that P450 enzymes in man are regulated in a manner similar to that in other animals. The balance between different individual enzymes of cytochrome P450 in foetuses and/or neonates is subject to modulation by xenobiotics, the consequences of which may lead to toxicologically compromised livers with respect to metabolic handling of certain substrates.

In vitro interaction of AFB1 with rabbit liver monooxygenase activities

The purpose of this study was to determine the influence of aflatoxin B1 (AFB1), incubated in vitro with rabbit liver microsomes, on some cytochrome P450-dependent monooxygenases activities. A strong competitive inhibition of the mycotoxin on aniline hydroxylation was observed. The concentration which provoked a 50% inhibition (IC50) was around 20 microM, whereas a Ki of 3 microM was determined. In contrast, only weak inhibitions of both pentoxyresorufin and ethoxyresorufin O-dealkylases (PROD and EROD) activities were obtained. They were characterized by respective IC50 of 200 and 260 microM. The inhibition was 'non competitive' for PROD activity and 'mixed' for EROD. The Ki of the reactions were respectively 177 and 510 microM. Considering the fact that AFB1 has been previously reported to decrease microsomal hepatic cytochrome P450 expression, the results obtained in this study strengthen the hypothesis that the normal metabolism of xenobiotics by the liver could be altered in AFB1 exposure.

Mouse cytochrome P450 (Cyp3a11): predominant expression in liver and capacity to activate aflatoxin B1

S1 mapping analysis for the expression of Cyp3a11 and Cyp3a13 indicated that Cyp3a11 mRNA is predominantly expressed in mouse liver, compared with that of Cyp3a13. In addition, all of six inducers, such as dexamethasone, 3-methylcholanthrene, phenobarbital, polychlorinated biphenyl, pregnenolone 16 alpha-carbonitrile, and rifampicin, increased the expression of the Cyp3a11 mRNA more extensively than that of Cyp3a13. The level of mRNAs corresponding to Cyp3a11 and Cyp3a13 reached the maximum level between 4 and 8 weeks after birth. Cyp3a11 enzyme was expressed into CR119 cells which had been established as a cell line stably expressing NADPH-cytochrome P450 reductase cDNA of guinea pigs. These transformants showed aflatoxin B1-dependent cytotoxicity in proportion to the amounts of Cyp3a11 mRNA. This cytotoxicity was enhanced by 7,8-benzoflavone, a known activator of CYP3A protein. Based on these results, we confirm that CYP3A in the mouse, which is an animal species known to be relatively insensitive to aflatoxin B1 genotoxicity, can activate this mycotoxin efficiently.

Intrahepatic cholestasis of pregnancy impairs the activities of human placental xenobiotic and steroid metabolizing enzymes in vitro

Human placental xenobiotic metabolizing and aromatase activities were measured in placentae at term obtained from pregnancies diagnosed as intrahepatic cholestasis (IC). Compared with controls, several cytochrome P450-dependent (CYP) mono-oxygenases were significantly decreased in IC placentae: 7-ethoxycoumarin O-deethylase by 75 per cent, 7-ethoxyresorufin O-deethylase by 95 per cent, aromatase activity by 37 per cent and androstenedione formation, using testosterone as a substrate, by 20 per cent. These results demonstrate that maternal intrahepatic cholestasis effectively decreases CYP dependent metabolism in human placenta in vitro which may pose a potential risk to the wellbeing of the fetus. Because aromatase activity in IC placentae is significantly lower as compared with healthy controls, safety of the drug therapies which further inhibit aromatase activity are questioned.

Translactational induction of CYP4A expression in 10.5-day neonatal rats by the hypolipidemic drug clofibrate

Lactating mothers of 7.5-day neonatal rats were injected intraperitoneally with 500 mg kg-1 clofibrate for 3 consecutive days at 24-hour intervals; 24 hours after the final injection, the maternal cytochrome P450 4A (CYP4A) mRNA levels had risen 14- and 2.5-fold above the constitutive levels of expression seen in the liver and kidney, respectively. Lactational transfer of clofibrate to the suckling 10.5-day litter was demonstrated by the 15- and 5-fold elevation observed in the neonatal hepatic and renal CYP4A mRNAs, respectively, following suckling from drug-induced mothers. A significant decrease in the relative liver weights of these neonatal pups was seen following clofibrate exposure via maternal milk, in total contrast to the normally observed increase in liver/body weight ratios of rats treated with clofibrate. Western blot analysis using a polyclonal goat anti-rat CYP4A1 antibody also demonstrated a rise in the CYP4A protein levels in both the mothers and their litters following maternal clofibrate treatment.

Protective effect of rifampicin against acute liver injury induced by carbon tetrachloride in mice

Rifampicin conferred significant protection against carbon tetrachloride (CCl4)-induced liver injury. Serum alanine transaminase (ALT) and aspartate transaminase (AST) activities were not markedly altered and only hepatocellular fatty degeneration was found in mice pretreated with rifampicin (200 mg/kg), whereas severe centrilobular necrosis was observed and serum ALT and AST activities were as high as 281 and 271 I.U./l, respectively, in the control group following administration of CCl4 (400 microliters/kg). The contents and activities of microsomal drug-metabolizing enzymes in rifampicin-pretreated animals were also much higher than those of the controls. CCl4-mediated malondialdehyde (MDA) formation was increased in rifampicin-treated liver microsomes, demonstrating that rifampicin was capable of increasing the NADPH-dependent metabolism of CCl4 catalyzed by P-450 2E1 to produce free radicals. However, MDA formation was obviously depressed by rifampicin at varying concentrations from 2 to 32 x 10(-6) M in an in vitro cytochrome P-450 (P-450) enzyme system. On the other hand, NADPH oxidation in the metabolism of CCl4 and aniline hydroxylation were not suppressed in the presence of rifampicin in this systems, suggesting that rifampicin did not influence the biotransformation of CCl4 by P-450 2E1 in vitro. Therefore, the protective effect of rifampicin against CCl4 hepatotoxicity appeared to result from the direct inhibition of lipid peroxidation generated by CCl4-derived free radicals.

A novel form of mouse cytochrome P450 3A (Cyp3a-16). Its cDNA cloning and expression in fetal liver

A complementary DNA clone coding for a novel form of cytochrome P450, Cyp3a-16, in mouse fetal livers was isolated and completely sequenced. This clone encoded a polypeptide of 504 deduced amino acids and showed 87.3% and 66.6% amino acid identities with mouse Cyp3a-11 and Cyp3a-13, respectively. Cyp3a-16 transcript was detectable before birth and remarkably diminished five weeks after birth in mice. We conclude that Cyp3a-16 is a fetal- and puberty-specific cytochrome P450 in mice.

Characterization of the rabbit CYP1A1 and CYP1A2 genes: developmental and dioxin-inducible expression of rabbit liver P4501A1 and P4501A2

In adult rabbits, the CYP1A1 and CYP1A2 genes are expressed constitutively. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads to elevations in both CYP1A1 and CYP1A2 gene products (S. T. Okino et al., 1985, Proc. Natl. Acad. Sci. USA 82, 5310-5314). In this report, we have characterized the rabbit CYP1A1 and CYP1A2 genes, and analyzed the pattern of expression of these genes in neonatal animals following exposure to TCDD. Genomic clones encoding the entire rabbit CYP1A1 and CYP1A2 genes were characterized. Restriction enzyme analysis and partial DNA sequence analysis identified the seven exons for the CYP1A1 and CYP1A2 genes. Primer extension analysis using mRNA from TCDD-treated neonatal rabbits helped confirm the start of transcription for the CYP1A genes. The length of the noncoding first exon of the CYP1A1 gene was 74 bases, compared to 90 and 88 bases for the human and rodent CYP1A1 genes. The length of the noncoding CYP1A2 gene first exon was 53 bases, similar to its counterpart in human and rodents. DNA sequence analysis of the 5' regulatory regions and comparison to the rodent and human CYP1 genes demonstrated that the rabbit CYP1A1 and CYP1A2 genes were most similar to their human orthologs. The 5' region of the CYP1A1 gene contained several consensus dioxin (Ah)-receptor responsive elements (XREs), while no functional XRE sequences were identified in the CYP1A2 gene. When expression of the two genes were monitored, a small amount of constitutive P4501A1 mRNA was detected in neonatal rabbits from the ages of 1 to 17 days, while P4501A2 mRNA levels could not be observed until 8-12 days postpartum. In response to TCDD treatment, P4501A1 mRNA levels were inducible at all neonatal time points, while P4501A2 mRNA levels could not be induced until the animals were 3-5 days postpartum. While the dioxin Ah-receptor most likely plays a major role in the induction of these genes by TCDD, early expression of the CYP1A1 and CYP1A2 genes is differentially regulated in a developmental fashion.