Cloning and characterization of a novel CYP3A1 allelic variant: analysis of CYP3A1 and CYP3A2 sex-hormone-dependent expression reveals that the CYP3A2 gene is regulated by testosterone

Quantification of Usaramine and Its N-oxide Metabolite in Rat Plasma Using Liquid Chromatography-Tandem Mass Spectrometry

A sensitive, fast and robust liquid chromatography--tandem mass spectrometry (LC–MS-MS) method was developed and validated for the determination of usaramine (URM) and usaramine N-oxide (UNO) in rat plasma. The separation was conducted on an ACQUITY UPLC BEH C₁₈ Column (50 × 2.1 mm, 1.7 μm) and gradient eluted with mobile phase A (0.1% formic acid with 5 mM ammonium acetate in water) and B (0.1% formic acid in acetonitrile/methanol, 9/1, v/v). The method was linear over the range of 1–2,000 ng/mL for both analytes. The validated method was applied to investigate the pharmacokinetic behaviors and sex differences of URM and its N-oxide metabolite in rats. After intravenous administration of URM at 1 mg/kg, the AUC_0-t values for URM and UNO were 363 ± 65 and 172 ± 32 ng/mL*h in male rats, while 744 ± 122 and 30.7 ± 7.4 ng/mL*h in females, respectively. The clearance of URM was significantly higher in male rats than in females (2.77 ± 0.50 vs 1.35 ± 0.19 L/h/kg, P < 0.05). After oral administration of URM at 10 mg/kg, the AUC_0-t values of URM and UNO were 1,960 ± 208 and 1,637 ± 246 ng/mL*h in male rats, while 6,073 ± 488 and 300 ± 62 ng/mL*h in females, respectively. The oral bioavailability of URM in female rats (81.7%) was much higher than in males (54.0%). In conclusion, sex-based differences were observed in the pharmacokinetics, N-oxide metabolism and oral bioavailability of URM.

Change in pharmacokinetic behavior of intravenously administered midazolam due to increased CYP3A2 expression in rats treated with menthol

Menthol is used widely as a constituent of functional foods and chemical drugs. The present study investigated changes in the pharmacokinetic behavior of intravenously administered midazolam (MDZ), a probe for CYP3A, when rats were treated with menthol. The study also examined which isoforms of CYP3A1 and 3A2 were menthol-inducible and contributed to the altered disposition of midazolam. Menthol was administered intraperitoneally to rats once daily for 3 days at a dose of 10 mg/kg, while the control rats received vehicle alone. The pharmacokinetic examination of i.v. administered midazolam revealed that serum midazolam concentrations at each sampling point were lower in the menthol-treated rats than in the control rats. Regarding the pharmacokinetic parameters of the menthol-treated group, the area under the curve (AUC) was decreased significantly and, correspondingly, the elimination rate constant at terminal phase (ke) was increased significantly without significant changes in the volume of distribution at steady state (Vdss). The metabolic production of the 1'-hydroxylated and 4'-hydroxylated forms of MDZ by hepatic microsomes was significantly greater in the menthol-treated rats than in the control rats. The expression levels of mRNA and protein for hepatic CYP3A2 were more than 2.5-fold higher than the control levels when the rats were treated with menthol, whereas no changes were observed in the expression levels of CYP3A1. These results indicate that menthol enhanced the elimination clearance of midazolam by inducing hepatic CYP3A2 and that careful attention should be paid when menthol is ingested in combination with drugs that act as substrates for CYP3A.

Sex-dependent regulation of hepatic CYP3A by growth hormone: Roles of HNF6, C/EBPα, and RXRα

Sex-based differences in the pharmacological profiles of many drugs are due in part to the female-predominant expression of CYP3A4, which is the most important CYP isoform responsible for drug metabolism. Transcription factors trigger the sexually dimorphic expression of drug-metabolizing enzymes in response to sex-dependent growth hormone (GH) secretion. We investigated the roles of HNF6, C/EBPα, and RXRα in the regulation of human female-predominant CYP3A4, mouse female-specific CYP3A41, and rat male-specific CYP3A2 expression by GH secretion patterns using HepG2 cells, growth hormone receptor (GHR) knockout mice as well as rat models of orchiectomy and hypophysectomy. The constitutive expression of HNF6 and RXRα was GH-dependent, and GHR deficiency decreased HNF6/C/EBPα complex levels and increased HNF6/RXRα complex levels. Feminine GH secretion induced the binding of HNF6 and C/EBPα to the CYP3A4 and Cyp3a41 promoters and HNF6/C/EBPα complex levels was more efficiently compared with masculine pattern. Additionally, a greater inhibition of the binding of RXRα to the CYP3A4 and Cyp3a41 promoters and HNF6/RXRα complex levels was observed by feminine GH secretion, but less inhibition was observed by masculine pattern. The binding of HNF6, C/EBPα, and RXRα to the CYP3A2 promoter was not directly regulated by androgens. RXRα completely abolished the synergistic activation of the CYP3A4, Cyp3a41, and CYP3A2 promoters by HNF6 and C/EBPα. The results demonstrate that sex-dependent GH secretion patterns affect the expressions and interactions of HNF6, C/EBPα, and RXRα as well as their binding to CYP3A genes. RXRα mediates the sex-dependent influence of GH on CYP3A expression as an important signalling molecule.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

Comparative study on altered hepatic metabolism of CYP3A substrates in rats with glycerol-induced acute renal failure

To examine the mechanism accounting for the diverse alteration of hepatic metabolism of CYP3A substrates observed with renal function being severely impaired, the hepatic drug metabolizing activity was evaluated using liver microsomes prepared from rats with glycerol-induced acute renal failure (ARF). Midazolam, nifedipine and rifabutin were employed as representative CYP3A substrates. When the Michaelis-Menten parameters, K(m) and V(max) , were examined in the incubation study, the K(m) values of midazolam and nifedipine in ARF rats were shown to decrease by 50.9% and 29.9% compared with the normal value, respectively. The V(max) values of midazolam and nifedipine in ARF rats also decreased by 49.3% and 28.0%, respectively, showing that their decreased K(m) values accompanied the decreased V(max) values. The parameters of nifedipine seemed to alter to a lesser extent than those of midazolam. As for rifabutin metabolism, the decrease in the K(m) value was observed in ARF rats, but it did not accompany the decrease in the V(max) value. Then, the hepatic expressions of the CYP3A subfamily were examined with western blotting using anti-CYP3A1 and anti-CYP3A2 antibodies. It was revealed that the hepatic expression of CYP3A2 decreased, while that of CYP3A1 was unaffected. Additionally, a band signal deduced to originate from CYP3A9 was clearly detected in ARF, but not in normal rats. Considering each substrate having different specificities for CYP3A subfamily member proteins, individual alterations of hepatic CYP3A subfamily expression seem to underlie the diverse alterations of hepatic metabolism of CYP3A substrates in ARF rats.

Novel cytochrome P450-mediated ring opening of the 1,3,4-oxadiazole in setileuton, a 5-lipoxygenase inhibitor

Setileuton [4-(4-fluorophenyl)-7-[({5-[(1S)-1-hydroxy-1-(trifluoromethyl)propyl]-1,3,4-oxadiazol-2-yl}amino)methyl]-2H-1-benzopyran-2-one] is a selective inhibitor of the 5-lipoxygenase enzyme, which is under investigation for the treatment of asthma and atherosclerosis. During the development of setileuton, a metabolite (M5) was identified in incubations with rat, dog, and human liver microsomes that represented the addition of 18 Da to the 1,3,4-oxadiazole portion of the molecule. Based on mass spectral data, a ring opened structure was proposed and confirmed through comparison with a synthetic standard. The metabolic ring opening was examined in vitro in rat liver microsomes and was determined to be mediated by cytochrome P450s (P450s). Upon examination of the specific P450s involved using cDNA-expressed rat P450s, it was shown that CYP1A2 likely was the major isoform contributing to the formation of M5. Studies using stable labeled molecular oxygen and water demonstrated that the oxygen was incorporated from molecular oxygen, rather than water, and confirmed that the metabolic formation was oxidative. An alternative, comparatively slow pathway of chemical hydrolysis also was identified and described. Three potential mechanisms for the two-step metabolic ring opening of the 1,3,4-oxadizole are proposed.

Regulation of human CYP2C18 and CYP2C19 in transgenic mice: influence of castration, testosterone, and growth hormone

The hormonal regulation of human CYP2C18 and CYP2C19, which are expressed in a male-specific manner in liver and kidney in a mouse transgenic model, was examined. The influence of prepubertal castration in male mice and testosterone treatment of female mice was investigated, as was the effect of continuous administration of growth hormone (GH) to transgenic males. Prepubertal castration of transgenic male mice suppressed the expression of CYP2C18 and CYP2C19 in liver and kidney to female levels, whereas expression was increased for the endogenous female-specific mouse hepatic genes Cyp2c37, Cyp2c38, Cyp2c39, and Cyp2c40. Testosterone treatment of female mice increased CYP2C18 and CYP2C19 expression in kidney, and to a lesser extent in liver, but was without effect in brain or small intestine, where gene expression was not gender-dependent. Continuous GH treatment of transgenic males for 7 days suppressed hepatic expression of CYP2C19 (>90% decrease) and CYP2C18 ( approximately 50% decrease) but had minimal effect on the expression of these genes in kidney, brain, or small intestine. Under these conditions, continuous GH induced all four female-specific mouse liver Cyp2c genes in males to normal female levels. These studies indicate that the human CYP2C18 and CYP2C19 genes contain regulatory elements that respond to the endogenous mouse hormonal profiles, with androgen being the primary regulator of male-specific expression in kidney, whereas the androgen-dependent pituitary GH secretory pattern is the primary regulator of male-specific expression in liver in a manner that is similar to the regulation of the endogenous gender-specific hepatic genes.

Precision-cut liver slices from rats of different ages: basal cytochrome P450-dependent monooxygenase activities and inducibility

The biotransformation capacity - of the cytochrome P450 (CYP) system for example - is lower but inducibility is more pronounced in neonates than in adults. On the other hand, both enzyme activities and inducibility decline with senescence. Precision-cut rat liver slices are widely used as an in vitro tool for the examination of drug toxicity, xenobiotic metabolism or enzyme induction. The aim of the present study was to assess whether age-related changes in CYP activities and induction observed in vivo are also mirrored in vitro in liver slices. For this purpose, different CYP model reactions were measured in precision-cut liver slices from one-day-old, 40-day-old and one-year-old rats after in vitro exposure to various inducers. Similar to the in vivo situation, basal CYP activities were distinctly lower and inducibility was much more pronounced in liver slices from neonatal than in those from adult animals. Also, enzyme activities were mostly somewhat lower in liver slices from aged rats compared to those from 40-day-old rats. However, CYP inducibility was less pronounced than with younger animals too. Thus, precision-cut rat liver slices are a suitable in vitro tool for investigating age-related changes in CYP activities and induction as well as developmental differences in drug metabolism and toxicity.

Gene expression on liver toxicity induced by administration of haloperidol in rats with severe fatty liver

Sudden deaths are often encountered in schizophrenic patients prescribed with antipsychotic drugs, and fatty liver may be more prevalent among patients with schizophrenia. The aim of this study is to investigate the adverse effects of antipsychotic drugs on fatty liver. We administered haloperidol intraperitoneally to fatty liver rats and examined the mRNA expression in the liver. Basic expressions of cytochrome P450 (CYP)1A2, CYP2C11 and CYP3A2 decreased, and response of these CYPs to haloperidol was reduced in the fatty liver. Metabolism of haloperidol was also suppressed in the fatty liver rats. Moreover, hepatic injury by administration of haloperidol was shown pathohistologically and molecular-biologically in severe fatty liver. These results suggest that fatty liver increases susceptibility to adverse effects of haloperidol, possibly leading to life-threatening events. It should be noted by clinicians that excessive dose of antipsychotic drugs may be more harmful in patients with fatty liver.

Decreased CYP3A2 expression and activity in senescent male Wistar rats: is there a role for HNF4alpha?

The effect of ageing on CYP3A2, a male specific isoform, was examined in adult (9 months) and senescent (24 months) male rats. A significant decrease (65%) of CYP3A2-related activity (midazolam oxidation) was observed in all senescent rats. Half of these rats still express CYP3A2 suggesting that decreased activities in these rats are due to post-translational modifications. The other senescent male rats did not express CYP3A2 anymore, indicating an impairment of transcription. These transcriptional modifications are due to the previously shown continuous secretion of GH in senescent male rats. GH also regulates HNF4alpha, a hepatocyte nuclear factor, essential for the basal transcriptional activation of the CYP3A2 gene. In senescent rats, a drastic reduction (76%) of HNF4alpha protein content and a decrease in DNA binding activity were observed. When these parameters were assessed in male and female rats of the same age (3 months), a higher HNF4alpha DNA binding activity and a higher HNF4alpha protein content (38%) were observed in female rats. Our results show that in male senescent rats (1) the decrease of HNF4alpha is not consistent with the continuous secretion of GH, and (2) the suppression of CYP3A2 expression is not dependent to the HNF4alpha binding activity.

Profile of territrem metabolism and cytochrome P-450 3A expression in liver microsomes from Wistar rats of both genders as a function of age

This study determined territrem metabolites after incubation of territrem A, B, or C with NADPH and liver microsomes from Wistar rat of both genders aged 2 to 76 wk. The liver microsomal cytochrome P-450 content, NADPH-cytochrome P-450 reductase activity, and CYP3A1 and CYP3A2 protein and mRNA levels were also analyzed. Male rats had significantly higher liver microsomal cytochrome P-450 content and NADPH-cytochrome P-450 reductase activities than females at 14 to 26 wk. Microsomal cytochrome P-450 content was decreased in senescence in both genders compared with postpubertal and adulthood stages. The activity of 6beta-testosterone hydroxylase in male rats, which was significantly higher than those in females at all ages, decreased after 52 wk. After 26 wk, the levels of CYP3A1 protein markely declined in both genders, which resulted in a large gender difference (male greater than female). The protein levels and mRNA of CYP3A2 were constitutively expressed in 2- to 52-wk-old male rats, but they decreased after 76 wk, and decreased in females after 6 wk. The expression of CYP3A1 or CYP3A2 in males are generally higher than in females. The metabolites of territrems MA1, MAX, MA2, MB2, MB4, and MC were measured by high-performance chromatography (HPLC). Formation of MA1, MAX, and MA2 decreased after 52 wk in males, and MAX and MA2 were not formed after 6 wk in females. The amount of MB2 formed in females was less than in males, but the amount of MC (TRC metabolites) formed in females was higher than in males. The gender differences in metabolism of TRA were related to the protein and mRNA expression of CYP3A2. The protein levels and mRNA expression of CYP3A2 and efficiency of territrems metabolism were decreased after 76 wk. The results suggested that the effects of age and gender on territrem metabolism are due to differences in CYP3A1 and CYP3A2 expression in the liver microsomes.

Modulation of hepatic and renal drug metabolizing enzyme activities in rats by subchronic administration of farnesol

Farnesol demonstrates antitumor activity in several animal models for human cancer and was being considered for development as a cancer chemopreventive agent. This study was performed to characterize the effects of minimally toxic doses of farnesol on the activity of phase I and II drug metabolizing enzymes. CD((R)) rats (20/sex/group) received daily gavage exposure to farnesol doses of 0, 500, or 1000 mg/kg/day for 28 days; 10 rats/sex/group were necropsied at the termination of farnesol exposure; remaining animals were necropsied after a 28-day recovery period. No deaths occurred during the study, and farnesol had no significant effects on body weight, food consumption, clinical signs, or hematology/coagulation parameters. Modest but statistically significant alterations in several clinical chemistry parameters were observed at the termination of farnesol exposure; all clinical pathology effects were reversed during the recovery period. At the termination of dosing, the activities of CYP1A, CYP2A1-3, CYP2B1/2, CYP2C11/12, CYP2E1, CYP3A1/2, CYP4A1-3, CYP19, glutathione reductase, NADPH/quinone oxidoreductase and UDP-glucuronosyltransferase were significantly increased in the livers of farnesol-treated rats; farnesol also increased the activity of glutathione S-transferase in the kidney. The effects of farnesol on hepatic and renal enzymes were reversed during the recovery period. At the end of the dosing period, increases in absolute and relative liver and kidney weights were seen in farnesol-treated rats. These increases may be secondary to induction of drug metabolizing enzymes, since organ weight increases were not associated with histopathologic alterations and were reversed upon discontinuation of farnesol exposure. Administration of farnesol at doses of up to 1000 mg/kg/day induced reversible increases in the activities of several hepatic and renal drug metabolizing enzymes in rats, while inducing only minimal toxicity. It is concluded that non-toxic or minimally toxic doses of farnesol could alter the metabolism, efficacy, and/or toxicity of drugs with which it is co-administered.

Intestinal Expression and Metabolic Activity of the CYP3A Subfamily in Female Rats

The intestinal expression of the CYP3A subfamily was investigated in female rats, and the intestinal metabolism of two CYP3A substrates, testosterone and rifabutin, was examined and compared between males and females. CYP3A1/23 and CYP3A2 intestinal expression was barely detected in male and female rats. Although CYP3A9 was predominantly expressed in the female rat liver, its expression in the intestine was not different between the two sexes. The rate of testosterone 6beta-hydroxylation in the female intestine was similar to that for males. Rifabutin was also metabolized at similar rates in both intestines, although the metabolic rate was greater in the female liver. These results indicate that the intestinal drug metabolizing activity of the CYP3A subfamily is similar between males and females, and that CYP3A9 is involved in the intestinal metabolism of CYP3A substrates in both sexes.

Evaluation of 2-year-old intrasplenic fetal liver tissue transplants in rats

Liver cell transplantation into host organs like the spleen may possibly provide a temporary relief after extensive liver resection or severe liver disease or may enable treatment of an enzyme deficiency. With time, however, dedifferentiation or malignant transformation of the ectopically transplanted cells may be possible. Thus, in the present study syngenic fetal liver tissue suspensions were transplanted into the spleen of adult male rats and evaluated 2 years thereafter in comparison to orthotopic livers for histopathological changes and (as markers for preneoplastic transformation) for cytochrome P450 (P450) and glutathione S-transferase (GST) isoform expression. Because inducibility of P450 and GST isoforms may be changed in preneoplastic foci, prior to sacrifice animals were additionally treated either with beta-naphthoflavone, phenobarbital, dexamethasone, or the respective solvent. In the 2-year-old grafts more than 70% of the spleen mass was occupied by the transplant. The transplanted hepatocytes were arranged in cord-like structures. Also few bile ducts were present. Morphologically, no signs of malignancy were visible. With all rats, transplant recipients as well as controls, however, discrete nodular structures were seen in the livers. Due to age, both livers and transplants displayed only a low P450 2B1 and 3A2 and GST class alpha and mu isoform expression. No immunostaining for P450 1A1 was visible. At both sites, beta-naphthoflavone, phenobarbital, or dexamethasone treatment enhanced P450 1A1, P450 2B1 and 3A2, or P450 3A2 expression, respectively. No immunostaining for GST class pi isoforms was seen in the transplants. The livers of both transplant recipients and control rats, however, displayed GST pi-positive foci, corresponding to the nodular structures seen histomorphologically. Compared to the surrounding tissue, these foci also exhibited a more pronounced staining for GST class alpha and mu isoforms and a stronger inducibility of the P450 1A1 expression due to beta-naphthoflavone. In conclusion, in contrast to the livers, no preneoplastic foci seem to appear in the intrasplenic transplants even 2 years after transplantation. This may be due either to the protection of these transplants by the orthotopic livers or to the different humoral and nerval influences at the ectopic site.

Phylogenetic analysis of the cytochrome P450 3 (CYP3) gene family

Cytochrome P450 genes (CYP) constitute a superfamily with members known from the Bacteria, Archaea, and Eukarya. The CYP3 gene family includes the CYP3A and CYP3B subfamilies. Members of the CYP3A subfamily represent the dominant CYP forms expressed in the digestive and respiratory tracts of vertebrates. The CYP3A enzymes metabolize a wide variety of chemically diverse lipophilic organic compounds. To understand vertebrate CYP3 diversity better, we determined the killifish (Fundulus heteroclitus) CYP3A30 and CYP3A56 and the ball python (Python regius) CYP3A42 sequences. We performed phylogenetic analyses of 45 vertebrate CYP3 amino acid sequences using a Bayesian approach. Our analyses indicate that teleost, diapsid, and mammalian CYP3A genes have undergone independent diversification and that the ancestral vertebrate genome contained a single CYP3A gene. Most CYP3A diversity is the product of recent gene duplication events. There is strong support for placement of the guinea pig CYP3A genes within the rodent CYP3A diversification. The rat, mouse, and hamster CYP3A genes are mixed among several rodent CYP3A subclades, indicative of a complex history involving speciation and gene duplication.

Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

Hepatic versus extrahepatic expression of CYP3A30 and CYP3A56 in adult killifish (Fundulus heteroclitus)

Members of the CYP3A subfamily represent the largest portion of CYP proteins in liver and intestine in vertebrates. The CYP3A enzymes are involved in metabolic clearance of numerous chemically diverse compounds including toxins, carcinogens, pesticides, therapeutic drugs, dietary products and hormones. Most studies of CYP3A have been performed in mammals, whereas relatively little is known of that in non-mammalian species. We have investigated CYP3A expression in the marine and estuarine killifish (Fundulus heteroclitus). We isolated a novel CYP3A cDNA sequence, denoted CYP3A56, from killifish intestine. The CYP3A56 sequence shared 98% nucleotide and amino acid sequence identity with CYP3A30, previously isolated from killifish liver. We hypothesize that a recent gene duplication event has occurred within the killifish CYP3A subfamily. The CYP3A30 and CYP3A56 genes were co-expressed in liver, intestine, gill, kidney, spleen, brain and ovary in several individuals. The hepatic versus extrahepatic CYP3A30/56 mRNA expression was analyzed in adult killifish of both sexes, using conventional as well as real-time semi-quantitative RT-PCR. Tissues expressing CYP3A30/56 mRNA were in a descending order of magnitude: liver>intestine>>gill>spleen>kidney>brain. Furthermore, inter-individual differences (up to 18%) in CYP3A30/56 mRNA expression were evident in killifish, in particular in extrahepatic organs. For comparison, CYP3A protein expression levels were determined using polyclonal antibodies (PAb) against rainbow trout (Oncorhynchus mykiss) CYP3A. Sexually dimorphic expression of hepatic and extrahepatic CYP3A30/56 mRNA and CYP3A proteins was observed in killifish. For example, males displayed up to 2.5-fold higher CYP3A protein expression compared with females. In agreement with CYP3A30/56 mRNA analysis, highest CYP3A protein levels were observed in liver and intestine. Low CYP3A protein levels were seen in gill, kidney and spleen. Cellular localization of CYP3A protein expression was investigated using immunohistochemistry analysis and PAb against rainbow trout CYP3A. Strong CYP3A protein staining was seen in intestinal enterocytes, in gill filaments and in renal tubular epithelial cells. Moderate CYP3A staining was seen in hepatocytes in the liver, whereas mild staining was observed in hematopoietic cells in the spleen and in follicles in the ovary. Thus, similar to mammals, CYP3A expression in fish is prominent in the digestive- and respiratory tracts and may be important for the first-pass metabolism of xenobiotics. Moreover, CYP3A expression also is evident in brain and ovary in killifish, which suggests a role for CYP3A enzymes in biotransformation of xenobiotics and fine-tuning levels of steroid hormones in situ in extrahepatic organs.

Gender differences in microsomal metabolic activation of hepatotoxic clivorine in rat

The gender differences in the in vitro microsomal metabolic activation of hepatotoxic clivorine, a representative naturally occurring hepatotoxic otonecine type pyrrolizidine alkaloid, in Sprague-Dawley rats and their relation to the gender differences in susceptibility to clivorine intoxication were reported in the present study. Clivorine-induced liver damage in the male rat via metabolic activation to form the reactive pyrrolic ester followed by covalent binding to liver tissue constituents has been reported previously by our research group. The present study demonstrated, for the first time, that cytochromes p450 3A1 and 3A2, which are constitutively expressed in the male rat, might play a significant role in the metabolic activation of clivorine in the rat. Thus, in the male rat, the metabolic activation by liver microsomes to form the reactive pyrrolic ester was found as the only direct metabolic pathway of clivorine followed by subsequent formation of the toxic tissue-bound pyrroles leading to hepatotoxicity. In the case of the female rat, a less significant metabolic activation was observed, whereas the formations of two novel nonpyrrolic metabolites were determined as the predominant biotransformations. None of the four cDNA-expressed rat enzymes (cytochrome p450 2C12, 2E1, 3A1, 3A2) tested could catalyze the formation of these two new metabolites. Furthermore, the female rat (LD(50) = 114 +/- 9 mg/kg, i.p.) was found to be significantly less susceptible to clivorine intoxication than the male rat (LD(50) = 91 +/- 3 mg/kg, i.p.). Therefore, the results suggested that a significantly lower metabolic activation due to the lack of cytochrome p450 3A1 and p450 3A2 activities mainly accounted for the smaller susceptibility of the female rat to clivorine intoxication.

Role of temperature on protein and mRNA cytochrome P450 3A (CYP3A) isozymes expression and midazolam oxidation by cultured rat precision-cut liver slices

The cytochrome P450 3A (CYP3A)-mediated midazolam oxidation was studied in rat precision-cut liver slices (PCLS) maintained for 20hr at 4, 20 and 37 degrees, and further incubated for 8hr at 37 degrees. Either at 4 or 20 degrees, midazolam was oxidised by PCLS at similar rates to that observed in freshly cut slices. Moreover, PCLS kept a regioselectivity since 4-hydroxylation was more important than 1'-hydroxylation. Conversely, PCLS totally lost their capacity to oxidise midazolam after 20hr at 37 degrees, and both CYP3A2 protein and mRNA were not detected. CYP3A1 protein was unaffected by a temperature of 37 degrees but its mRNA was totally lost. By blocking transcription with actinomycin D, the decay of both CYP3A mRNAs followed the same profile at either 20 or 37 degrees, indicating that temperature affected the CYP3A2 protein stability. Cell functionality was not involved in such an impairment since the low values of ATP, GSH and protein synthesis rates observed at 4 and 20 degrees were rapidly restored, when PCLS were further incubated at 37 degrees. The use of rat supersomes expressing either CYP3A1 or CYP3A2, strongly supported the hypothesis that 4-hydroxymidazolam was mainly formed by CYP3A2. These results suggest that: (1) CYP3A1 protein is constitutive and largely expressed in rat liver slices; (2) regioselective midazolam oxidation appears to be mainly CYP3A2 dependent; and (3) since CYP3A isoforms have similar half-lives (about 10-14hr), the loss of CYP3A2 protein at 37 degrees might be due to a selective targeting (phosphorylation ?) leading to proteolytic disposal by the proteasome.

Change in the gene expression of hepatic tamoxifen-metabolizing enzymes during the process of tamoxifen-induced hepatocarcinogenesis in female rats

Altered gene expression of the enzymes responsible for tamoxifen metabolism during the process of tamoxifen-induced hepatocarcinogenesis in female Sprague-Dawley rats was examined by the RT-PCR method. Treatment of rats with tamoxifen (20 mg/kg body/day) for 52 weeks, but not the 1 day, 2 or 12 week treatments, resulted in the formation of the liver hyperplastic nodules. The gene expression of CYP3A subfamily enzymes, especially CYP3A1, responsible for not only detoxification (N-demethylation) but also activation (alpha-hydroxylation) of tamoxifen, was increased by the tamoxifen treatments for 2 and 12 weeks, whereas after the 52 week treatment, the expression in the induced nodules returned to the control level. The gene expression of SULT2A subfamily sulfotransferases, especially HSTa, responsible for metabolic activation of alpha-hydroxytamoxifen was decreased to a level <20% of the control in the nodules, although no significant change in the expression was observed in the liver of rats treated with tamoxifen for 1 day, 2 or 12 weeks. On the other hand, the gene expression of CYP3A2 and flavin-containing monooxygenase 1 (FMO1), responsible for the N-demethylation and N-oxidation, respectively, of tamoxifen was increased in a time-dependent fashion up to the 52 week treatment. Although the gene expression of UDP-glucuronosyltransferase(s), which might be responsible for detoxification of tamoxifen, was also increased by the tamoxifen treatment for 2 or 12 weeks, it decreased to the control level in the nodules after the 52 week treatment. The present findings demonstrate that in the early stage of the formation of the liver hyperplastic nodules by tamoxifen, the genes of the enzymes responsible for not only detoxification but also activation of tamoxifen were activated, whereas in the later stage (in the nodules), the genes of the detoxification enzymes, CYP3A2 and FMO1, remained active, but those of the activation enzymes such as CYP3A1 and HSTa were suppressed.

Effect of testosterone suppression on the pharmacokinetics of a potent gnRH receptor antagonist

PURPOSE

The expression of cytochrome P450 enzymes (CYPs) in animals and humans is under complex hormonal regulation. Chronic treatment with drugs that alter sex hormone levels such as GnRH receptor agonists or antagonists may affect the expression of hormone-dependent CYPs, and as a result the pharmacokinetics of drugs metabolized by them.

METHODS

Enzyme kinetic parameters were obtained by incubating AG-045572 (0.1-30 microM) with human or rat liver microsomes, or expressed CYP3A4 and CYP3A5. The pharmacokinetics of AG-045572 (10 mg/kg i.v. or 20 mg/kg p.o.) were studied in intact male, female, castrated male and male rats pretreated with AG-045572 for 4 days.

RESULTS

AG-045572 is metabolized by CYP3A in both rats and humans. The Km values were similar in male and female human, female rat liver microsomes, and expressed CYP3A4 and CYP3A5 (0.39, 0.27, 0.28, 0.25, and 0.26 microM, respectively). The Km in male rat liver microsomes was 1.5 microM, suggesting that in male and female rats AG-045572 is metabolized by different CYP3A isozymes. The oral bioavailability of AG-045572 in intact male rats was 8%, while in female or castrated male rats it was 24%. Pretreatment of intact male rats with AG-045572 i.m. for 4 days resulted in suppression of testosterone to castrate levels, accompanied by an increase in oral bioavailability of AG-045572 to 27%. In the same experiment, the male-specific pulsatile pattern of growth hormone remained unchanged with slightly elevated baseline levels.

CONCLUSIONS

The potent GnRH receptor antagonist AG-045572 is metabolized by hormone-dependent CYP3A. As a result, suppression of testosterone by pretreatment with AG-045572 "feminized" its own pharmacokinetics.

Influence of dietary zinc deficiency during development on hepatic CYP2C11, CYP2C12, CYP3A2, CYP3A9, and CYP3A18 expression in postpubertal male rats

The present study investigated the effect of dietary zinc deficiency during the developmental period on hepatic cytochrome P450 (CYP) expression in postpubertal male rats. Twenty-one-day-old weanling male Wistar rats were randomly assigned to one of the following dietary groups: zinc-adequate (31 mg zinc/kg diet); marginal zinc-deficient (3 mg zinc/kg diet); severe zinc-deficient (1 mg zinc/kg diet); or pair-fed control for either the marginal or severe zinc-deficient group. All rats were killed at 63 days of age. Compared with the corresponding pair-fed controls, marginal zinc deficiency decreased CYP2C11-mediated testosterone 2alpha- and 16alpha-hydroxylase activities by 43 and 42%, respectively, whereas severe zinc deficiency reduced each of these activities by approximately 60%. The decrease in CYP2C11 activity was accompanied by a reduction in CYP2C11 protein and mRNA levels, as assessed by immunoblot and reverse transcription-polymerase chain reaction (RT-PCR) assays, respectively. Additional RT-PCR analysis indicated that severe zinc deficiency decreased CYP3A2 and CYP3A18 mRNA levels by 49 and 43%, respectively, whereas it increased CYP2C12 (253%) and CYP3A9 (238%) mRNA expression. Plasma testosterone concentration was decreased by 67% in the marginal zinc-deficient group when compared with the corresponding pair-fed control group. By comparison, it was below the limit of quantification (0.2 ng/mL) in the severe zinc-deficient rats. Overall, these results indicate that dietary zinc deficiency during the developmental period feminized the hepatic gene expression of the sexually dimorphic CYP2C11, CYP3A2, CYP3A18, CYP2C12, and CYP3A9 in postpubertal male rats.

Cloning, sequencing, heterologous expression, and characterization of murine cytochrome P450 3a25*(Cyp3a25), a testosterone 6beta-hydroxylase

A full-length cDNA clone encoding a novel form of the cytochrome P450 3A subfamily (Cyp3a-25) has been isolated from a mouse liver cDNA library. The sequence contained 2010 base pairs and encoded a protein with 503 amino acids. The amino acid sequence shared greater identities with rat CYP3A18 (90%) and golden hamster CYP3A10 (81%) sequences than with known mouse sequences (Cyp3a-11, Cyp3a-13, Cyp3a-16, and Cyp3a-41 [68--70%]). CYP3A25 was expressed in the Escherichia coli PCWori(+) expression vector following slight modifications of the N- and C-terminals of the cDNA. The purified CYP3A25 was recognized on an immunoblot by CYP3A1 antibody and has a molecular weight of 50 kD. CYP3A25 was catalytically active in the 6 beta-hydroxylation of testosterone and the N-demethylation of benzphetamine and erythromycin. It was demonstrated by RT-PCR that the CYP3A25 mRNA is present in both fetal and adult tissues, including liver, lung, intestines, kidney, and brain. Northern blotting demonstrated that expression is greatest in the liver and small intestine.

Self-augmentation effect of male-specific products on sexually differentiated progesterone metabolism in adult male rat liver microsomes

It is well known that several 3-keto-4-ene steroids such as progesterone and testosterone are metabolized in a gender-specific or -predominant manner by adult rat liver microsomes. In the male, these steroids are primarily metabolized into two oxidized (16alpha-hydroxyl and 6beta-hydroxyl) products mainly by the respective, male-specific cytochrome P450 subforms, CYP2C11 and CYP3A2, while they are primarily metabolized into the 5alpha-reduced products by female-predominant 5alpha-reductase in the female. These sexually differentiated enzyme activities are largely regulated at the transcription level under endocrine control. In the present study, we show that unlabeled 16alpha-hydroxyprogesterone and 6beta-hydroxyprogesterone inhibited the 5alpha-reductive [(3)H]progesterone metabolism by adult male rat liver microsomes without significantly inhibiting the CYP2C11 and CYP3A2 activities producing themselves, whereas 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnane-3,20-dione not only stimulated the 5alpha-reductive metabolism producing themselves but also inhibited the male-specific oxidative metabolism. This finding compels us to propose a novel hypothesis that adult male rat liver microsomes may possess a self-augmentation system regulated by the male-specific products on sexually differentiated steroid metabolism, besides regulation by gene expressions of the related enzymes.

A comparison of the ontogeny of enterocytic and hepatic cytochromes P450 3A in the rat

Enzymes of the cytochrome P450 3A (CYP3A) sub-family are abundant in adult liver and gut and contribute significantly to the first-pass metabolism of many orally administered drugs. The development of CYP3A enzymes with regard to their expression and activity in enterocytic and hepatic microsomes from 1-day-old through to adult male and female rats has been studied. Microsomes were prepared by calcium precipitation. Enzyme expression was assessed semi-quantitatively by Western blotting using rat polyclonal CYP3A2 and 2C11 antibodies and peptide antibodies specific to rat CYPs 3A1, 3A2, 2C12, and 2C13. The formation of 6beta-hydroxytestosterone (6OHT), determined by HPLC, was used as a measure of enzyme activity. Formation of 6OHT by enterocytic microsomes was similar for males and females and showed a sharp increase at weaning. This pattern was mirrored by levels of immunoquantifiable CYP3A2 (CYP3A9), but CYP3A1 followed a more gradual development. CYPs 2C11, 2C12, or 2C13 were not detected in gut microsomes. In contrast, CYPs 3A1, 3A2, 2C11, 2C12, and 2C13 were all expressed in hepatic microsomes. There was no surge in hepatic enzyme expression or hepatic 6OHT formation at weaning, and a marked sex difference in 6OHT formation was apparent from day 25. The surge in gut activity at weaning may be a protective mechanism against ingested toxins.

Antimutagenic effects of wheat bran diet through modification of xenobiotic metabolising enzymes

Diets containing wheat bran (WB) protect against cancers of the colon or breast in rats, and may be beneficial in humans. In a previous study of rats treated with the carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), inclusion of 10% wheat bran in the diet led to an apparent reduction in IQ metabolites but not of intact IQ in plasma. In the present study, male Wistar rats were fed diets containing 0, 10 or 20% wheat bran, and effects on xenobiotic metabolising enzymes compared. Wheat bran-supplementation showed differential effects on phase I enzymes, significantly increasing the activity of hepatic cytochrome P450 isozyme CYP3A2, but slightly reducing the activity of CYP1A1/2. The activities of both hepatic phase II detoxification enzymes glutathione-S-transferase and glucuronosyl transferase were also reduced. Western blotting revealed similar effects on expression of the proteins. Interestingly, the expression of xenobiotic metabolising enzymes (XME) in the colon appeared to be modulated independently of hepatic XME. Although the wheat bran-supplemented diet still led to an increased expression of CYP3A, it now slightly increased CYP1A in the colon. However, 20% wheat bran significantly increased the expression of both glutathione transferase isozymes, GST A1 & A2, in the colon. Natures Gold (NG) is a commercial wheat bran derivative which is lower than wheat bran in dietary fibre, but enriched in vitamins, minerals and various phytochemicals. Dietary supplementation with 20% Natures Gold led to similar trends as seen in wheat bran-fed rats, but more potent effects in both hepatic and colonic enzymes. The significance of these changes for activation of carcinogens to mutagenic metabolites was investigated using the Salmonella/mammalian microsome mutagenicity test. The activation of IQ and benzo[a]pyrene, but not cyclophosphamide, to a mutagen by hepatic S9 from wheat bran-fed or Natures Gold-fed rats was significantly reduced compared with S9 from animals on a diet lacking wheat bran. We suggest that modulation of xenobiotic metabolising enzymes may be an important component of cancer protection by wheat bran, and this effect may relate to micronutrients or cancer-protective non-nutrient phytochemicals rather more than to dietary fibre.

Cytochrome P450 3A9 catalyzes the metabolism of progesterone and other steroid hormones

The catalytic requirements and the role of P450 3A9, a female-specific isoform of CYP3A from rat brain, in the metabolism of several steroid hormones were studied using recombinant P450 3A9 protein. The optimal steroid hormone hydroxylase activities of P450 3A9 required cholate but not cytochrome b5. P450 3A9 was active in the hydroxylation reactions of testosterone, androstenedione, progesterone and dehydroepiandrosterone (DHEA). No activity of P450 3A9 toward cortisol was detectable under our reconstitution conditions. Among all the steroid hormones examined, female-specific P450 3A9 seemed to catalyze most efficiently the metabolism of progesterone, one of the major female hormones, to form three mono-hydroxylated products, 6beta-, 16alpha-, and 21-hydroxyprogesterone. Our data also showed that P450 3A9 can catalyze the formation of a dihydroxy product, 4-pregnen-6beta, 21-diol-3, 20-dione, from progesterone with a turnover number, 1.3 nmol/min/nmol P450. Based on the Vmax/Km values for P450 3A9 using either 21-hydroxprogesterone or 6beta-hydroxyprogesterone as a substrate, 4-pregnen-6beta, 21-diol-3, 20-dione may be formed either by 6beta-hydroxylation of 21-hydroxprogesterone or 21-hydroxylation of 6beta-hydroxyprogesterone. As a major isoform of CYP3A expressed in rat brain, the activities of P450 3A9 toward two major neurosteroids, progesterone and DHEA suggested a possible role for P450 3A9 in the metabolism of neurosteroids.

Growth hormone regulation and developmental expression of rat hepatic CYP3A18, CYP3A9, and CYP3A2

The present study investigated the role of growth hormone (GH) in hepatic CYP3A18 and CYP3A9 expression in prepubertal and adult male rats. For comparison, the effects of GH on CYP3A2 expression were also measured. Initial experiments demonstrated that CYP3A18 mRNA levels were greater during puberty and adulthood than during the prepubertal period, CYP3A9 mRNA was not expressed until puberty and its expression increased in adulthood, and CYP3A2 mRNA levels were relatively constant from prepuberty to adult life. Hypophysectomy, which results in the loss of multiple pituitary factors including GH, increased CYP3A2 and CYP3A18 mRNA expression 3- to 4-fold, but it did not affect CYP3A9 mRNA levels or CYP3A-mediated testosterone 2beta- or 6beta-hydroxylase activity in adult rats. GH administered as twice daily s.c. injections (0.12 microg/g body weight) to hypophysectomized or intact adult rats did not affect CYP3A18 or CYP3A9 mRNA expression. The same treatment decreased CYP3A2 mRNA and protein and testosterone 2beta- and 6beta-hydroxylase activity levels in intact but not hypophysectomized rats. However, in intact prepubertal rats, intermittent GH administration decreased CYP3A18 and CYP3A2 mRNA levels, but a higher dosage (3.6 microg/g) was required to suppress CYP3A2. Overall, the present study demonstrated that: (a) the constitutive expression of CYP3A18, CYP3A9, and CYP3A2 does not require the presence of GH, (b) CYP3A18 is more sensitive than CYP3A9 to GH modulation in adult rats; and (c) CYP3A2 is less sensitive to the suppressive influence of GH during the prepubertal period than during adult life.

Differential glucocorticoid responses of CYP3A23 and CYP3A2 are mediated by selective binding of orphan nuclear receptors

CYP3A2 and CYP3A23 are two cytochrome P450 genes in rat that are differentially regulated in both their constitutive activities and their responsiveness to glucocorticoids, the prototypic CYP3A inducers. CYP3A2 displays 20-25% of the response to glucocorticoids as CYP3A23 despite extensive sequence homology in their 5'-regulatory regions. Promoter deletion analyses revealed that the CYP3A2 -57 to -168 region, homologous to the CYP3A23 dexamethasone-responsive region, mediated its low level activation. When this region was analyzed by DNase I footprinting, three binding sites were shown to correspond to the functional elements described for CYP3A23: DexRE-1, DexRE-2, and Site A (J. M. Huss and C. B. Kasper (1998) J. Biol. Chem. 273: 16155-16162). The CYP3A2 DexRE-2 and Site A elements bear two mismatches each from the CYP3A23 elements but displayed similar binding patterns in footprinting and gel-shift analyses as their CYP3A23 counterparts. The region containing 3A2DexRE-1 has six mismatches and displayed unique footprinting and gel-shift patterns compared to 3A23DexRE-1. Functional assays revealed that four mismatches within the DexRE-1 and DexRE-2 elements accounted for the differential inducibility of the two isoforms. We propose that the reduced responsiveness of CYP3A2 is the result of preferential binding of COUP-TF at the CYP3A2 DexRE-1 site. In contrast, CYP3A23 DexRE-1 associates with an accessory factor(s) that acts in concert with downstream sites to mediate the strong glucocorticoid induction response observed for CYP3A23. Site A mismatches did not influence induction magnitude but were responsible for basal activity differences. Higher CYP3A23 basal activity appears to be due to an E-box in 3A23SiteA that interacts with USF1, a ubiquitous bHLH/leucine zipper transcription factor. This site is disrupted in the corresponding 3A2SiteA. Hence, 4 nucleotide mismatches within two elements account for the difference in glucocorticoid induction, and a single mismatch is responsible for the fivefold difference in the basal activities of CYP3A2 and CYP3A23.

Altered expression and glucocorticoid-inducibility of hepatic CYP3A and CYP2B enzymes in male rats fed diets containing soy protein isolate

Hepatic CYP3A and CYP2B enzymes were studied in male Sprague-Dawley rats derived from 5-7 litters fed diets in which the protein source was either casein or soy protein isolate. At age 65 d, rats were gavaged with corn oil (vehicle) or 50 mg/kg dexamethasone. Hepatic expression of CYP3A and CYP2B1 mRNA, apoprotein and associated monooxygenase activities were measured. Consumption of soy diets significantly increased monooxygenase activity toward the following: the CYP3A substrates erythromycin and ethylmorphine N-demethylase; corticosterone and testosterone 6beta-hydroxylase; and apoprotein and mRNA expression of CYP3A2 (P < 0.05). Dexamethasone significantly induced turnover of erythromycin and testosterone, expression of CYP3A apoprotein, and expression of CYP3A1 and CYP3A2 mRNA (P < 0.05). In addition, significant diet-inducer interactions were observed in the expression of CYP3A apoprotein and activities toward ethylmorphine, corticosterone and testosterone (P < 0.05). Significant diet-inducer interactions were also observed on CYP2B1-dependent pentoxyresorufin O-depentylase activity (P < 0.05). However, although dexamethasone significantly induced CYP2B1 expression at the apoprotein and mRNA level (P < 0.05), no significant diet effects were observed. These data suggest potential effects of soy consumption on the metabolism of a wide variety of CYP3A and CYP2B1 substrates, especially in situations involving coexposure to CYP inducers.

Metabolism of cyclosporine by cytochromes P450 3A9 and 3A4

The ability of P450 3A9 to transform cyclosporine was studied and compared to that of human P450 3A4. Purified P450 3A4 and P450 3A9 proteins were reconstituted in a system containing potassium phosphate buffer, lipids, NADPH-P450 reductase, and glutathione with NADPH added to initiate the reaction. Cyclosporine was added alone and with or without the inhibitors, ketoconazole or troleandomycin. High performance liquid chromatography with ultraviolet (HPLC/UV) techniques were used to analyze for cyclosporine metabolites. Both P450 3A4 and P450 3A9 transformed cyclosporine to three metabolites: AM1, AM9, and AM4n. P450 3A4 predominantly formed AM1 (63% of metabolites formed) while P450 3A9 formed AM4n (59% of metabolites formed). Ketoconazole (0.5 microM) completely inhibited P450 3A9 catalyzed formation of AM1 and AM9 and reduced AM4n formation to 28% of control. AM4n, AM1, and AM9 formation catalyzed by P450 3A4 was reduced to 50%, 30%, and 10% of control, respectively, by 0.5 microM ketoconazole. Troleandomycin (> 10 microM) inhibited the formation of AM4n by P450 3A4 and P450 3A9 to 60-70% of control, while the production of AM1 by P450 3A4 was increased to 120% of control and the production of AM1 by P450 3A9 was inhibited to 50% of control. Inhibition of P450 3A4 by troleandomycin (> 10 microM) reduced the formation of AM9 to 40% of control, but only reduced P450 3A9 formation of AM9 to 80% of control. This study shows that rat P450 3A9 is capable of transforming cyclosporine to multiple metabolites similar to those generated by human P450 3A4.

Structure and expression of the rat CYP3A1 gene: isolation of the gene (P450/6betaB) and characterization of the recombinant protein

A P450 gene (P450/6betaB) of the CYP3A subfamily was isolated from a rat genomic library. Nucleotide sequencing of the exons revealed a high similarity with P450PCN1 cDNA (Gonzalez et al. (1985), J. Biol. Chem. 260, 7345-7441), but differed in 41 nucleotides, resulting in 11 changes and 2 deletions of amino acid residues. The P450/6betaB spanned about 30 kbp and consisted of 13 exons, and was in exon number and size identical with CYP3A2 gene except in the 6th exon, which was shorter than that of CYP3A2. 6beta-B mRNA, which may be transcribed from P450/6betaB, was detected on Northern blotting and by reverse transcription-polymerase chain reaction (RT-PCR). Profiles of the developmental change and induction by a treatment with several chemicals were very similar to those of P450PCN1 mRNA reported previously. P450PCN1 mRNA and gene, however, were not detected by PCR in rats. To determine whether P450/6betaB encodes an active protein, a cDNA was isolated and expressed. Expression of 6beta-B cDNA in COS-1 cells was carried out and revealed that the recombinant protein comigrated with purified P4506beta-4 previously identified as CYP3A1. The recombinant 6beta-B protein showed similar turnover rate and regioselectivity for testosterone with purified P4506beta-4 by the simultaneous addition of NADPH-cytochrome P450 reductase and cytochrome b5. These data suggest that P450/6betaB encodes an active P450 form corresponding to CYP3A1 and P450PCN1 reported previously does not exist in rats.

P450-catalyzed in-chain desaturation of valproic acid: isoform selectivity and mechanism of formation of Delta 3-valproic acid generated by baculovirus-expressed CYP3A1

The mechanism of formation of the in-chain, unsaturated fatty acid metabolite, Delta3-valproic acid (Delta3-VPA) by rat liver microsomes was examined. Microsomal rates of formation of Delta3-VPA were below quantifiable limits in reactions catalyzed by control female rat liver microsomes, but were induced more than 20-fold following pretreatment with triacetyloleandomycin and pregnenolone-16alpha-carbonitrile. Microsomal incubations conducted with 3-hydroxy-VPA or [2-2H1]VPA demonstrated that Delta3-VPA did not arise by dehydration of preformed alcohol nor was it reversibly isomerized to Delta2-VPA. CYP3A1 expression was optimized in the baculovirus expression vector system, and infected insect cell membranes which were supplemented with P450 reductase catalyzed formation of 3-OH-, 4-OH-, 5-OH-, Delta3-, and Delta4-VPA in ratios of 160:35:6:3:1. Intramolecular deuterium isotope effects on metabolite formation, determined with cDNA-expressed CYP3A1 and either [3,3-2H2]VPA or [4,4-2H2]VPA, yielded kH/kD values for Delta3-VPA of 2.00 +/- 0.06 and 2.36 +/- 0.08, respectively. These values were significantly lower than the isotope effects observed in the same incubations for 3-OH-VPA formation from 3,3-D2-VPA (kH/kD = 6.04 +/- 0.08), or for 4-OH- and Delta4-VPA formation from 4, 4-D2-VPA (kH/kD > 5). Collectively, these data demonstrate the existence of a microsomal P450-dependent in-chain fatty acid desaturase system distinct from the well-documented cytochrome b5-linked CoA desaturases and suggest further that CYP3A1-dependent formation of Delta3-VPA arises via nonselective, initial hydrogen atom abstraction from either the C-3 or the C-4 position.

Nuclear receptor involvement in the regulation of rat cytochrome P450 3A23 expression

Many genes of the cytochrome P450 3A (CYP3A) subfamily, including several human and rat isoforms, are inducible by glucocorticoids. In the rat CYP3A23 gene, a 110-base pair segment of the proximal 5'-flanking region mediates dexamethasone activation. Three binding sites (DexRE-1, DexRE-2, and Site A), identified by DNase I footprinting analysis, were characterized for their relative contribution to both basal activity and dexamethasone inducibility. Site-directed mutagenesis of DexRE-1 (-144 to -169) and DexRE-2 (-118 to -136) demonstrated that each contained a core imperfect AGGTCA direct repeat, which comprised a consensus nuclear receptor binding site, and was essential for dexamethasone responsiveness but was not required for basal activity. Competition gel shift and supershift analyses revealed that both sites can bind the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor. Site A (-85 to -110) was shown to be important for both basal activity and dexamethasone responsiveness. Point mutants displayed a reduced (2-3-fold) induction response, compared with 15-fold for wild-type, which was accompanied by a 40-60% drop in basal activity. Site A was shown to bind the liver-enriched nuclear receptor hepatocyte nuclear factor 4. Our studies demonstrate that the mechanism mediating glucocorticoid-inducible transcriptional activity of CYP3A23 involves multiple binding sites for members of the nuclear receptor superfamily.

cDNA cloning and expression of a novel CYP3A from the Syrian hamster, CYP3A31

A clone, encoding a cytochrome P450 protein consisting of 501 amino acids, was isolated from a cDNA library constructed from mRNA of Syrian hamster liver. The deduced amino acid sequence of this clone showed a high homology (65 to 81%) with other mammalian CYP3As and hence, this novel isozyme was named CYP3A31. By Northern blotting, using an oligonucleotide specific to CYP3A31, the mRNA for this isozyme was shown to be expressed constitutively in liver and induced by treatment with phenobarbital but repressed by 3-methylcholanthrene or dexamethasone treatments. The increase in mRNA expression by phenobarbital and decrease by dexamethasone corresponded to changes in CYP3A protein as analysed by Western blotting. These indicate that CYP3A31 might constitute one of the major CYP3A isozymes in the hamster.

Developmental changes in the constitutive and inducible expression of cytochrome P450 3A2

Using a CYP3A2-specific oligonucleotide and an antipeptide antibody raised against the C terminus of CYP3A2 (VINGA) it is demonstrated that metyrapone administration to adult (12 weeks old) but not immature (3 weeks old) male Sprague Dawley rats induces the hepatic expression of CYP3A2 mRNA and protein. The constitutively expressed level of CYP3A2 protein in adult male rats is markedly lower than the levels expressed in immature rats as determined using the anti-VINGA antibody, in contrast to previous reports using antibodies that do not discriminate between CYP3A forms. Hepatic microsomal CYP3A2 protein expression, examined between 3 and 15 weeks of age, is extinguished between 9 and 12 weeks of age in contrast to immunoreactive CYP3A protein (determined using a nonselective antibody) and CYP3A-dependent androstenedione 6beta-hydroxylase activity. These data suggest that the regulation of the induction of CYP3A2 is developmentally controlled and that the major expressed adult form(s) of constitutively expressed CYP3A is not CYP3A2.

Regulation of CYP3A9 gene expression by estrogen and catalytic studies using cytochrome P450 3A9 expressed in Escherichia coli

Sexual dimorphism in the expression of CYP3A9, a novel form of CYP3A from rat brain, is shown for the first time in rat brain as well as in rat liver. CYP3A9 expression is female specific in rat liver as judged by its 10-fold higher expression in females than in males. CYP3A9 gene expression was inducible by estrogen treatment both in male and in female rats. Ovariectomy of adult female rats elicited a drastic reduction on the mRNA level of CYP3A9 which could be fully restored by estrogen replacement. These results suggest that estrogen may play an important role in the female-specific expression of the CYP3A9 gene. P450 3A9 recombinant protein was expressed in Escherichia coli by means of the pCWOri+ expression vector and the MALLLAVF amino terminal sequence modification. This construct gave a high level of expression (130 nmol P450 3A9/liter culture) and the recombinant protein of the modified P450 3A9 was purified to electrophoretic homogeneity with a specific content of 10.1 nmol P450/mg protein from solubilized fractions through two chromatographic steps. The purified P450 3A9 protein was active in the metabolism of imipramine, erythromycin, benzphetamine, and ethylmorphine as well as 17beta-estradiol in a reconstituted system containing lipid and rat NADPH-P450 reductase. Of special interest is the finding that P450 3A9 can catalyze the formation of desipramine with a turnover number of 4.9 nmol/min/nmol P450, suggesting the possible involvement of this isoform in the metabolism of imipramine in brain. Optimal reconstitution conditions for P450 3A9 activities required a lipid mixture (1:1:1 mixture of L-alpha-dilauroyl phosphatidylcholine, L-alpha-dioleoyl phosphatidylcholine, and phosphatidylserine) and GSH.

Strain differences in age-associated change in testosterone 6β-hydroxylation in Wistar and Dark Agouti rats

This study examines strain differences in testosterone (T)-hydroxylations between Wistar and Dark Agouti (DA) rats of both genders. The DA rat, an animal model, is a poor metabolizer of such drugs as debrisoquine, which are metabolized by cytochrome P450 (CYP) 2D. T-16α-, 2α-hydroxylations, which are linked to CYP2C11, were catalyzed at similar rates by the microsomes of both strains. In contrast, the liver microsomes from mature male DA rats catalyzed T-6β-hydroxylation, the CYP3A mediated activity, at higher rates (∼ 2-fold) than Wistar rat liver microsomes did. There was no difference between immature male DA and Wistar rats for T-6β-hydroxylation, indicating that the activity in male DA rat increases with maturation. Polyclonal antibodies raised against rat liver microsomal CYP3A2 and a CYP3A inhibitor, troleandomycin (TAO), effectively inhibited T-6β-hydroxylation by liver microsomes from both strains of rats. The level of T-6β- hydroxylation activity correlated well with the amount of CYP3A protein in the microsomes in mature as well as in immature male and female Wistar and DA rats. Northern blot analysis repeatedly indicated that the cellular contents of CYP3A2 mRNA are slightly (∼ 20%) higher in the liver of mature DA rats than in that of mature Wistar rats. These results indicate that the increased levels of CYP3A are responsible for the increased T-6β-hydroxylation activity and protein in DA rat.

Critical amino-terminal segments in insertion of rat liver cytochrome P450 3A1 into the endoplasmic reticulum membrane

An in vitro transcription-translation assay was used to study the membrane topology of rat liver cytochrome P450 3A1. N-terminus deletion mutants were constructed to assess the importance of N-terminal regions in the stable incorporation of the protein into the microsomal membranes. Wild-type nascent cytochrome P450 bound to microsomes as an integral membrane protein through its hydrophobic N-terminal segments, uncleaved by signal peptidase. Deletion of the most N-terminal hydrophobic segment (positions 7-26) had a dramatic effect on endoplasmic reticulum membrane integration. Confirming the essential role of this stretch in P450 3A1 membrane targeting, proteolysis-resistant membrane-associated peptides were observed in all the in vitro translated mutants containing that segment. It is concluded that the membrane topogenesis of P450 3A1 is determined mainly by the amino-terminal hydrophobic segment.

Effects of metyrapone on expression of CYPs 2C11, 3A2, and other 3A genes in rat hepatocytes cultured on matrigel

Hepatocytes cultured on matrigel express many liver-specific functions, but the levels and activities of the predominant male-specific rat hepatic CYPs, 3A2 and 2C11, decline rapidly in culture. Metyrapone maintains the level of total cytochrome P450 of rat hepatocytes in primary culture, but the mechanism underlying this effect has not been completely elucidated. The present study sought to determine whether metyrapone acts solely to stabilise CYP proteins in rat hepatocytes cultured on matrigel, or whether it also influences mRNA levels of the encoding genes. Metyrapone maintained the level of total cytochrome P450 in cultured hepatocytes so that values were > 200% of those found in untreated control cells 24 hr after isolation. At this time, CYP3A2-mediated testosterone 6 beta-hydroxylation was approximately 7-fold higher in hepatocytes cultured in the presence of metyrapone than in control cells, and CYP2C11-dependent testosterone 2 alpha- and 16 alpha-hydroxylation activities were between 2 and 3-fold greater. The results inferred from catalytic activities were supported by immunoquantitation of CYP3A and 2C11 proteins. The trend of increased CYP protein levels in metyrapone-treated cells continued throughout the 48-hr culture period. In control cells, CYP3A2 and 2C11 mRNA levels fell abruptly in culture to reach values at 24 hr that were < 30% of those in freshly isolated cells; addition of metyrapone failed to arrest this fall. However, treatment of cells with metyrapone considerably elevated levels of one or more CYP3A subfamily mRNA species, as detected by a riboprobe based on the cDNA for CYP3A1 ("CYP3A1-like mRNA') that were demonstrated, by another riboprobe, not to be CYP3A2 or RNCYP3AM. RT-PCR of mRNA prepared from cultured hepatocytes, followed by restriction mapping of the cloned cDNAs was used to characterise the CYP3A induced by metyrapone. This revealed that elevated levels of the CYP3A1-like mRNA were attributable to induction of RL33/cDEX mRNA; there were no CYP3A1 cDNAs isolated from these cells. These data are interpreted as indicating that metyrapone stabilises the expression of cytochrome P450 in culture by both pre- and posttranslational mechanisms. The particular mechanism employed is gene-specific, whereby even the highly homologous genes CYP3A2, RL33/cDEX and, possibly, RNCYP3AM are subject to different types of regulation in the presence of metyrapone.

Effects of food deprivation and adrenalectomy on CYP3A induction by RU486 in female rats

We have studied the effects of food deprivation and adrenalectomy on the induction by RU486 of female rat liver microsomal CYP3A apoprotein, erythromycin N-demethylase and diazepam C3-hydroxylase activities. RU486 was a potent inducer of CYP3A apoprotein in intact animals and food deprivation enhanced this response. Food deprivation alone caused only weak CYP3A apoprotein induction suggesting a synergistic interaction in the regulation of protein expression. These results were reflected in the measurements of diazepam C3-hydroxylase activity. This confirms diazepam C3-hydroxylase as a useful and easily measured index of CYP3A monooxygenase content in female rat liver microsomes. Erythromycin N-demethylase did not show concordance with this pattern; this monooxygenase was much more strongly induced by food deprivation alone than by RU486 administration and, in addition, adrenalectomy abolished the induction response to food deprivation. The lack of correspondence between the apoprotein and erythromycin N-demethylase results suggests that non-CYP3A or novel, hitherto uncharacterized CYP3A isoforms may contribute to erythromycin N-demethylation in female rats. The close agreement between the results for CYP3A apoprotein and diazepam C3-hydroxylase indicates that although RU486 possesses a terminal acetylenic moeity it does not, at the dosages used here, cause mechanism-based inactivation of the CYP3A monooxygenase protein it induces. Current studies are directed to characterizing the particular CYP3A isoform(s) whose production is stimulated by RU486.

Dexamethasone responsiveness of a major glucocorticoid-inducible CYP3A gene is mediated by elements unrelated to a glucocorticoid receptor binding motif

Elements responsible for dexamethasone responsiveness of CYP3A23, a major glucocorticoid-inducible member of the CYP3A gene family, have been identified. DNase I footprint analysis of the proximal promoter region revealed three protected sites (sites A, B, and C) within the sequence defined by -167 to -60. Mutational analysis demonstrated that both sites B and C were necessary for maximum glucocorticoid responsiveness and functioned in a cooperative manner. Interestingly, neither site contained a glucocorticoid responsive element. Embedded in site C was an imperfect direct repeat (5'-AACTCAAAGGAGGTCA-3'), showing homology to an AGGTCA steroid receptor motif, typically recognized by the estrogen receptor family, while site B contained an ATGAACT direct repeat; these core sequences were designated dexamethasone response elements 1 and 2 (DexRE-1 and -2), respectively. Neither element has previously been associated with a glucocorticoid-activated transcriptional response. Conversion of the DexRE-1 to either a perfect thyroid hormone or vitamin D3 responsive element further enhanced induction by dexamethasone. Gel-shift analysis demonstrated that glucocorticoid receptor did not associate with either DexRE-1 or -2; hence, glucocorticoid receptor does not directly mediate glucocorticoid induction of CYP3A23. These unusual features suggest an alternate pathway through which glucocorticoids exert their effects.

Impact of tamoxifen on peripubertal androgen imprinting of rat hepatic cytochrome P450 2C11, cytochrome P450 3A2, and steroid 5 alpha-reductase

Expression of sex-dependent rat hepatic cytochromes P450 and steroid 5 alpha-reductase is regulated mainly by the sex-specific pattern of growth hormone (GH) secretion and is subject to androgen imprinting. Since tamoxifen suppresses GH pulse amplitude and nadir levels, we investigated the effect of tamoxifen on peripubertal testosterone imprinting of hepatic CYP2C11, CYP3A2, CYP2A1, and steroid 5 alpha-reductase. Prepubertal tamoxifen administration (5 mg once daily s.c. on days 28 and 29 of age) to non-ovariectomized female Sprague-Dawley rats did not affect hepatic microsomal CYP2C11-dependent testosterone 2 alpha-hydroxylase, CYP3A-mediated testosterone 6 beta-hydroxylase, CYP2A1-dependent testosterone 7 alpha-hydroxylase, or steroid 5 alpha-reductase activity in adult rats. Testosterone treatment (5 mumol/kg, s.c., once daily) of intact female rats during either puberty (days 35-49 of age) or adult life (days 69-77 of age) had no effect on these enzyme activities in adult (78-day-old) female rats, but the same treatment given during both of these periods induced the male-specific testosterone 2 alpha- and 6 beta-hydroxylase activities and suppressed the female-predominant testosterone 7 alpha-hydroxylase and steroid 5 alpha-reductase activities, indicating that peripubertal testosterone administration imprints the adult androgen responsiveness but not the basal levels of these enzyme activities in non-ovariectomized female rats. However, peripubertal androgen imprinting of the basal levels of testosterone 2 alpha-hydroxylase and steroid 5 alpha-reductase activities was observed in female rats administered tamoxifen prepubertally. Tamoxifen pretreatment also enhanced testosterone imprinting of the adult androgen responsiveness of testosterone 2 alpha- and 6 beta-hydroxylase and steroid 5 alpha-reductase activities. The enhanced testosterone hydroxylase activities were, however, not associated with an increase in microsomal NADPH-cytochrome P450 reductase activity, but were accompanied by elevated hepatic CYP2C11 and CYP3A2 protein levels. Overall, the present study indicates that prepubertal tamoxifen administration does not interfere with the normal sex differentiation of the gender-dependent hepatic cytochromes P450 and steroid 5 alpha-reductase, but this drug modulates peripubertal androgen imprinting of CYP2C11, CYP3A2, and steroid 5 alpha-reductase in adult female rats.

Hormonal regulation of the zonated expression of cytochrome P-450 3A in rat liver

Most cytochrome P-450 enzymes are expressed characteristically in a zonated pattern in the liver. The factors responsible for this heterogenous expression are largely unknown. Here we report how growth hormone and tri-iodothyronine regulate the steroid-hydroxylating cytochrome P-450 (CYP) 3A forms, which are constitutively expressed mainly in the perivenous (downstream) liver region. By comparing cell lysates obtained from the periportal and perivenous acinar regions we observed that the elevated CYP3A expression observed after hypophysectomy was due mainly to a dramatic increase in the normally silent periportal region. This effect was particularly strong in females. Treatment with growth hormone re-established the perivenous expression pattern, a finding corroborated by immunohistochemical analysis of liver sections. Analysis of periportal and perivenous mRNA by reverse-transcriptase PCR demonstrated that in males the changes in CYP3A2 mRNA paralleled the changes at the protein level. In females, CYP3A2 mRNA was detected only after hypophysectomy, and the zonal protein changes seemed to be governed by changes in CYP3A1 mRNA levels. Treatment of hypophysectomized animals with tri-iodothyronine also suppressed the expression of CYP3A, both in males and females. However, this occurred almost exclusively in the periportal region. This was observed both at the protein level, as determined by immunoblotting and immunohistochemically, and at the CYP3A1 and 3A2 mRNA level. These results indicate that growth hormone and thyroid hormone regulate the expression of CYP3A genes zone-specifically by suppressing their transcription in the periportal (upstream) region of the liver.