A major glucocorticoid-inducible P450 in rat liver is not P450 3A1

Effect of prototypical inducers on ligand activated nuclear receptor regulated drug disposition genes in rodent hepatic and intestinal cells

AIM

The aim of this study was to investigate the impact on expression of mRNA and protein by paradigm inducers/activators of nuclear receptors and their target genes in rat hepatic and intestinal cells. Furthermore, assess marked inter laboratory conflicting reports regarding species and tissue differences in expression to gain further insight and rationalise previously observed species differences between rodent and human based systems.

METHODS

Quantitative real time-polymerase chain reaction (QRT-PCR) and immunoblots were used to assess messenger RNA (mRNA) and protein expression for CYP2B2, CYP3A1, CYP3A2, CYP3A9, ABCB1a, ABCB1b, ABCC1, ABCC2, pregnane X receptor (PXR), farnesoid X receptor (FXR) and constituitive androstane receptor (CAR) in rat hepatoma cell line H411E, intestinal cells, Iec-6, and rat primary hepatocytes, in response to exposure for 18 h with prototypical inducers.

RESULTS

Dexamethasone (DEX) and pregnenolone 16alpha carbonitrile (PCN) significantly induced PXR, CYP3A9, ABCB1a and ABCB1b. However, when co-incubated, DEX appeared to restrict PCN-dependent induction. Chenodeoxycholic acid (CDCA) was the only ligand to induce FXR in all three cell types. Despite previously reported species differences between PCN and rifampicin (RIF), both compounds exhibited a similar profile of induction.

CONCLUSION

Data presented herein may explain some of the discrepancies previously reported with respect to species differences from different laboratories and have important implications for study design.

Comparison of effects of VDR versus PXR, FXR and GR ligands on the regulation of CYP3A isozymes in rat and human intestine and liver

In this study, we compared the regulation of CYP3A isozymes by the vitamin D receptor (VDR) ligand 1 alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) against ligands of the pregnane X receptor (PXR), the glucocorticoid receptor (GR) and the farnesoid X receptor (FXR) in precision-cut tissue slices of the rat jejunum, ileum, colon and liver, and human ileum and liver. In the rat, 1,25(OH)(2)D(3) strongly induced CYP3A1 mRNA, quantified by qRT-PCR, along the entire length of the intestine, induced CYP3A2 only in ileum but had no effect on CYP3A9. In contrast, the PXR/GR ligand, dexamethasone (DEX), the PXR ligand, pregnenolone-16 alpha carbonitrile (PCN), and the FXR ligand, chenodeoxycholic acid (CDCA), but not the GR ligand, budesonide (BUD), induced CYP3A1 only in the ileum, none of them influenced CYP3A2 expression, and PCN, DEX and BUD but not CDCA induced CYP3A9 in jejunum, ileum and colon. In rat liver, CYP3A1, CYP3A2 and CYP3A9 mRNA expression was unaffected by 1,25(OH)(2)D(3), whereas CDCA decreased the mRNA of all CYP3A isozymes; PCN induced CYP3A1 and CYP3A9, BUD induced CYP3A9, and DEX induced all three CYP3A isozymes. In human ileum and liver, 1,25(OH)(2)D(3) and DEX induced CYP3A4 expression, whereas CDCA induced CYP3A4 expression in liver only. In conclusion, the regulation of rat CYP3A isozymes by VDR, PXR, FXR and GR ligands differed for different segments of the rat and human intestine and liver, and the changes did not parallel expression levels of the nuclear receptors.

Species Differences in In Vitro and In Vivo Small Intestinal Metabolism of CYP3A Substrates

Intestinal first-pass metabolism has a great impact on the bioavailability of CYP3A substrates in humans, and the in vivo impact has quantitatively been evaluated using CYP3A inhibitors or inducers. In vitro and in vivo preclinical investigations for intestinal metabolism are essential in clarifying pharmacokinetic behavior in animal species and predicting the effect of intestinal metabolism in the human. In this review, we will discuss species differences in intestinal CYP3A enzymes, and CYP3A-mdediated intestinal elimination. Identical CYP3A4 enzyme is expressed in human intestine and liver, but different CYP3A enzymes in both tissues of the mouse and rat are found, that is, respective intestinal enzyme is considered as cyp3a13 and CYP3A62. There is little information on CYP3A enzymes in the monkey and dog intestine, unlike the liver. In vitro metabolic activities of midazolam and nisoldipine are higher in the human and monkey than in the rat. In vivo assessment of cyclosporine, midazolam, nifedipine, tacrolimus, and verapamil has been reported in various species (monkey, rat, mouse, and/or dog) including the human. For midazolam, the monkey shows significant in vivo intestinal metabolism, as evidenced in the human. The monkey might be an appropriate animal model for evaluating small intestinal first-pass metabolism of CYP3A substrates.

Characterization of the physiological turnover of native and inactivated cytochromes P450 3A in cultured rat hepatocytes: a role for the cytosolic AAA ATPase p97?

Mammalian hepatic cytochromes P450 (P450s) are endoplasmic reticulum (ER)-anchored hemoproteins engaged in the metabolism of numerous xeno- and endobiotics. P450s exhibit widely ranging half-lives, utilizing both autophagic-lysosomal (ALD) and ubiquitin-dependent 26S proteasomal (UPD) degradation pathways. Although suicidally inactivated hepatic CYPs 3A and "native" CYP3A4 in Saccharomyces cerevisiae are degraded via UPD, the turnover of native hepatic CYPs 3A in their physiological milieu has not been elucidated. Herein, we characterize the degradation of native, dexamethasone-inducible CYPs 3A in cultured primary rat hepatocytes, using proteasomal (MG-132 and MG-262) and ALD [NH4Cl and 3-methyladenine (3-MA)] inhibitors to examine their specific degradation route. Pulse-chase with immunoprecipitation analyses revealed a basal 52% 35S-CYP3A loss over 6 h, which was stabilized by both proteasomal inhibitors. By contrast, no corresponding CYP3A stabilization was detected with either ALD inhibitor NH4Cl or 3-MA. Furthermore, MG-262-induced CYP3A stabilization was associated with its polyubiquitylation, thereby verifying that native CYPs 3A were also degraded via UPD. To identify the specific participants in this process, cellular proteins were cross-linked in situ with paraformaldehyde (PFA) in cultured hepatocytes. Immunoblotting analyses of CYP3A immunoprecipitates after PFA-cross-linking revealed the presence of p97, a cytosolic AAA ATPase instrumental in the extraction and delivery of ubiquitylated ER proteins for proteasomal degradation. Such native CYP3A-p97 interactions were greatly magnified after CYP3A suicidal inactivation (which accelerates UPD), and/or proteasomal inhibition, and were confirmed by proteomic and confocal immunofluorescence microscopic analyses. These findings clearly reveal that native CYPs 3A undergo UPD and implicate a role for p97 in this process.

The effect of tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs) and newer antidepressant drugs on the activity and level of rat CYP3A

The aim of the present study was to investigate the influence of tricyclic antidepressants (TADs: imipramine, amitriptyline, clomipramine, and desipramine), selective serotonin reuptake inhibitors (SSRIs: fluoxetine and sertraline) and novel antidepressant drugs (mirtazapine and nefazodone) on the activity of CYP3A measured as a rate of testosterone 2beta- and 6beta-hydroxylation. The reaction was studied in control liver microsomes in the presence of the antidepressants, as well as in microsomes of rats treated intraperitoneally (i.p.) for 1 day or 2 weeks with pharmacological doses of the drugs (imipramine, amitriptyline, clomipramine, nefazodone 10 mg kg(-1) i.p.; desipramine, fluoxetine, sertraline 5 mg kg(-1) i.p.; mirtazapine 3 mg kg(-1) i.p.), in the absence of the antidepressants in vitro. The investigated antidepressants added to control liver microsomes produced some inhibitory effects on CYP3A activity, which were very weak (most of TADs, K(i)=145-212 microM), modest (clomipramine and sertraline, K(i)=67.5 and 62 microM, respectively) or moderate (nefazodone and fluoxetine, K(i)=42 and 43 microM, respectively). Mirtazapine did not display this kind of properties. One-day exposure of rats to TADs substantially decreased the activity of CYP3A in liver microsomes, which was maintained during chronic treatment. The observed decreases in the enzyme activity were in contrast to the increased CYP3A protein level found after chronic treatment with TADs. On the other hand, sertraline increased the activity of the enzyme after its prolonged administration and its effect correlated positively with the observed elevation in CYP3A protein level. Fluoxetine, mirtazapine and nefazodone did not change the activity of CYP3A in liver microsomes after their administration to rats. Three different mechanisms of the antidepressants-CYP3A interaction are postulated: 1) a direct inhibition of CYP3A by nefazodone, SSRIs and clomipramine, shown in vitro, with the inhibitory effect of nefazodone being the strongest, but weaker than the effects of this drug on human CYP3A4; 2) in vivo inhibition of CYP3A produced by 1 day and maintained during chronic treatment with TADs, which suggests inactivation of the enzyme by reactive metabolites; 3) in vivo induction by sertraline of CYP3A produced only by chronic treatment with the antidepressant, which suggests its influence on the enzyme regulation.

Generation of specific antibodies and their use to characterize sex differences in four rat P450 3A enzymes following vehicle and pregnenolone 16alpha-carbonitrile treatment

The purpose of this study was to identify isozyme-specific antibodies and use them to determine the expression levels of four P450 3A enzymes in the livers of vehicle- and pregnenolone 16alpha-carbonitrile (PCN)-treated rats of both sexes, since previous work on mRNA levels has shown considerable sexual dimorphism. Using Western blot analysis with four isozyme-specific antibodies, we show that P450 3A1, 3A2, and 3A9 were expressed in vehicle-treated adult female rats at very low levels whereas P450 3A18 was not detected. PCN treatment of females strongly induced the expression of P450 3A1 in the livers with protein product increases of 214-, 3-, and 5-fold for P450 3A1, 3A2, and 3A9, respectively, and P450 3A18 was induced to 3.7 pmol/mg protein. In contrast, all four P450 3As were detected in livers of vehicle-treated males, in the order of 3A2 >> 3A18 > 3A9 approximately = 3A1. The protein product increases induced by PCN treatment of male rats were 92-, 3-, 6-, and 16-fold for P450 3A1, 3A2, 3A9, and 3A18, respectively.

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.

Effects of cysteine on the pharmacokinetics of intravenous torasemide in rats with protein−calorie malnutrition

Effects of cysteine on the pharmacokinetics of torasemide were investigated after intravenous administration at a dose of 2 mg/kg to control rats and rats with PCM and PCMC. Torasemide was reported to be mainly metabolized via hepatic CYP2C9 in humans, and human CYP2C9 and male rat CYP2C11 proteins have 77% homology. It has also been reported that in male rats with PCM, the CYP2C11 level decreased to approximately 20% of the control level, but the decreased CYP2C11 level in rats with PCM partially returned to the control level by oral cysteine supplementation (rats with PCMC). Hence, it could be expected that in rats with PCM, some pharmacokinetic parameters of torasemide could be significantly different compared with those in control rats and rats with PCMC; however, they could be not significantly different between control rats and rats with PCMC. This was proven by the following parameters; the AUC (1880, 4080, and 2290 microg x min/mL for control rats and rats with PCM and PCMC, respectively), terminal half-life (188, 277, and 139 min), MRT (154, 323, and 155 min), CL (1.06, 0.491, and 0.943 mL/min/kg), CL(NR) (0.992, 0.430, and 0.874 mL/min/kg), and in vitro intrinsic torasemide disappearance clearance, CL(int) (0.102, 0.0842, and 0.0997 mL/min/mg protein).

Isolation and Characterization of a New Major Intestinal CYP3A Form, CYP3A62, in the Rat

Based on information of the nucleotide sequence obtained from rat genome clones, a new CYP3A (CYP3A62) cDNA was isolated from the cDNA library of a rat liver. The CYP3A62 cDNA was 1746 base pairs (bp) in length, which included 1491 bp of an open reading frame and 93 bp and 209 bp of the respective 5'- and 3'-noncoding regions. Amino acid sequence deduced from CYP3A62 cDNA shared the highest similarity with rat CYP3A9 (79.9%) among human and rat CYP3A forms previously reported. CYP3A62 mRNA and protein were consistently detected in small intestines as well as livers. CYP3A62 was a major form in small intestines of both sexes but was a female-predominant form in livers of adult rats. CYP3A62 in both tissues of male and female rats were clearly enhanced by the treatment with dexamethasone. These expression profiles resembled those of CYP3A9. Despite clear detection of CYP3A62, no detectable levels of CYP3A1 and CYP3A2 proteins, as well as those of mRNAs, were found in the intestinal tract. Therefore, CYP3A62 may play major roles together with CYP3A9 and CYP3A18 in endogenous or exogenous detoxification at the absorption site.

Expression profiles of drug-metabolizing enzyme CYP3A and drug efflux transporter multidrug resistance 1 subfamily mRNAS in small intestine

The purpose of this study is to examine the expression profiles of CYP3A1, CYP3A2, CYP3A9, and CYP3A18 mRNAs as well as multidrug resistance (mdr)1a and mdr1b mRNAs in the liver and small intestine of normal male Wistar rats using a reverse transcription-polymerase chain reaction (PCR). In the rat liver, the PCR products for CYP3A1, CYP3A2, and CYP3A18 were readily detectable, whereas CYP3A9 was slightly and mdr1a and mdr1b barely detected. Surprisingly, no PCR products for CYP3A1 and CYP3A2 were detected in the small intestine, but those for CYP3A9, CYP3A18, and mdr1a were readily detectable, and a faint band for mdr1b was also observed. Both CYP3A9 and CYP3A18 levels were found to be high in the duodenum and decreased from the top to bottom of the gut, indicating regional differences in both CYP3A9 and CYP3A18 expression in the small intestine. In contrast, mdr1a expression increased gradually from the upper to lower intestine. Consequently, it was suggested that drug metabolism in the small intestine of normal rats was mediated by CYP3A9 and CYP3A18 rather than CYP3A1 and CYP3A2. Also, regional differences of CYP3A9, CYP3A18, and mdr1a expression were found in the small intestine. The distributions of CYP3A9 and CYP3A18 were different from the distribution of mdr1a, suggesting the cooperative action of drug clearance pathways. This information is important to drug metabolism research based on ex vivo and in vivo studies using rats.

Insights into gender bias: rat cytochrome P450 3A9

Some members of the CYP3A subfamily show gender-dependent expression. Using quantitative real-time polymerase chain reaction, we report that female rats have 28-fold higher CYP3A9 mRNA levels than males in liver and 3.8-fold higher in lung. Furthermore, the CYP3A9 expression profile in kidney exhibits a regio-specific distribution, i.e., a 10-fold higher expression in cortex compared with medulla. Also, we observed tissue-specific estrogen regulation of the CYP3A9 message. Estrogen treatment caused a significant up-regulation in liver and a marked down-regulation both in the cortex and medulla of the kidney. Upon ovariectomy, hepatic and brain CYP3A9 expression were reduced significantly, but a modest increase in kidney expression was observed. The effects of ovariectomy on CYP3A9 gene expression were reversed upon exogenous estrogen treatment. CYP3A protein levels and hepatic microsomal activity toward benzphetamine after various treatments showed changes parallel to CYP3A9 mRNA levels. We report for the first time that CYP3A9 levels change dramatically during the course of pregnancy.

Studies on the induction of rat hepatic CYP1A, CYP2B, CYP3A and CYP4A subfamily form mRNAs in vivo and in vitro using precision-cut rat liver slices

1. Real-time quantitative reverse transcription-polymerase chain reaction methodology (TaqMan(R)) was used to examine the induction of some selected rat hepatic cyto-chrome P450 (CYP) forms in vivo and in vitro using cultured precision-cut liver slices. 2. TaqMan primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1, CYP2B1/2, CYP3A1, CYP3A2 and CYP4A1 mRNAs. 3. To characterize the responsiveness of the rat CYP mRNA TaqMan primers and probe sets, rats were treated in vivo with a single intraperitoneal dose of 500 mg kg(-1) Aroclor 1254 (ARO) and with four daily oral doses of either 50 mg kg(-1) day(-1) dexamethasone (DEX) or 75 mg kg(-1) day(-1) methylclofenapate (MCP). Treatment with ARO produced 22 600-, 5480-, 648-, 52-, 47- and 9-fold increases in levels of CYP1A1, CYP2B1, CYP2B1/2, CYP1A2, CYP3A1 and CYP3A2 mRNA, respectively. DEX treatment produced 97-, 24-, 8- and 4-fold increases, respectively, in CYP3A1, CYP2B1, CYP2B1/2 and CYP3A2 mRNA levels, and MCP produced 339-, 126- and 25-fold increases, respectively, in CYP4A1, CYP2B1 and CYP2B1/2 mRNA levels. All three CYP inducers also increased microsomal CYP content and produced corresponding increases in CYP1A, CYP2B, CYP3A and CYP4A form marker enzyme activities. 4. Rat liver slices were cultured for 6 and 24 h in medium containing 0.1 micro M insulin and 0.1 micro M DEX, and also for 24 h in medium containing only 0.1 micro M insulin (DEX-free medium). Liver slices were cultured in control medium or in medium containing either 10 micro M beta-naphthoflavone (BNF), 10 micro g ml(-1) ARO, 500 micro M sodium phenobarbitone (NaPB), 20 micro M pregnenolone-16alpha -carbonitrile (PCN), 50 micro M Wy-14,643 (WY) or 50 micro M MCP. 5. With the exception of the effect of BNF on CYP1A1 mRNA levels, the induction of all the CYP mRNAs studied was greater after 24- than after 6-h treatment. Generally, the magnitude of induction of CYP mRNA levels was greater after 24 h in liver slices cultured in DEX-free than in DEX-supplemented medium. 6. Treatment of liver slices with BNF and ARO for 24 h in DEX-free medium produced 21- and 35-fold increases, respectively, and 38- and 37-fold increases, respectively, in CYP1A1 and CYP1A2 mRNA levels. NaPB, PCN, WY and MCP did not increase either CYP1A1 or CYP1A2 mRNA levels. 7. After 24 h, levels of CYP2B1/2 mRNA were increased 18-, 20-, 9-, 16- and 13-fold by treatment with ARO, NaPB, PCN, WY and MCP, respectively. PCN also produced 56- and 4-fold increases, respectively, in CYP3A1 and CYP3A2 mRNA levels. 8. Treatment with WY and MCP for 24 h produced 437- and 186-fold increases, respectively, in levels of CYP4A1 mRNA. None of the other CYP inducers studied had any effect on CYP4A1 mRNA levels. 9. The results demonstrate the utility of cultured precision-cut liver slices as an in vitro model system to evaluate the effects of xenobiotics on rat CYP1A, CYP2B, CYP3A and CYP4A form mRNA levels.

Effects of cysteine on the pharmacokinetics of intravenous 2-(allylthio)pyrazine, a new chemoprotective agent, in rats with protein-calorie malnutrition

The effects of cysteine on the pharmacokinetics of 2-(allylthio)pyrazine (2-AP) were investigated after intravenous administration of the drug (50 mg/kg) to control (Sprague-Dawley) rats (4-week fed on 23% casein diet), and rats with protein-calorie malnutrition (PCM, 4-week fed on 5% casein diet) and PCMC (PCM with 250 mg/kg of oral cysteine, twice daily starting from the fourth week). In rats with PCM, the area under the plasma concentration-time curve from time zero to time infinity (AUC) of 2-AP was significantly smaller than that in control rats. However, in rats with PCMC, the AUC of 2-AP was significantly greater than that in control rats and rats with PCM. This could be due to significantly greater formation of M4 in rats with PCM and significantly smaller formation of M4 in rats with PCMC than that in control rats. In rats with PCMC, some pharmacokinetic parameters of 2-AP restored fully or more than the levels of control rats. For example, in rats with PCMC, the apparent volume of distribution at steady state of 2-AP (7290, 16,600, and 7050 ml/kg for control rats, and rats with PCM and PCMC, respectively), the percentage of dose excreted in 24-h urine as unchanged 2-AP (0.242, 0.727, and 0.130%), and 'the amount' excreted in 24-h urine as M4 (100, 228, and 51%) were comparable to those in control rats. However, the AUC (739, 434, and 1240 microg/min/ml) and total body clearance (67.7, 115, and 40.2 ml/min/kg) of 2-AP were significantly greater and slower, respectively, than those in control rats. This could be at least partly due to increase in S-methyltransferase activity (to form M4) in rats with PCM and greater restoration of its activity (decrease in its activity) in rats with PCMC.

Cloning, tissue distribution, and functional studies of a new cytochrome P450 3A subfamily member, CYP3A45, from rainbow trout (Oncorhynchus mykiss) intestinal ceca

In trout and mammals, the major extrahepatic expression site for CYP3A forms is in the intestine. A cDNA encoding a new CYP3A subfamily member was isolated from rainbow trout intestinal ceca by reverse transcriptase-polymerase chain reaction (RT-PCR), followed by rapid amplification of cDNA ends (RACE)-PCR. In a set of two primers for PCR, a consensus sequence in the highly conserved regions in 17 CYP3A sequences was used for one primer, and the second primer was designed based on adapter sequence ligated on the 5(') and 3(') cDNA ends. The 3(') and 5(') end nucleotide sequences of RACE-PCR products were used for the priming sites for the full-length cDNA in RT-PCR. The resulting 2615-bp cDNA contained an open reading frame of 1554 bp encoding a 518-amino acid residue protein (M(r)=59057.13, pI=6.15) with 26 amino acid differences from that of the previously cloned rainbow trout CYP3A27. The cDNA was assigned as CYP3A45 by the P450 Nomenclature Committee. The deduced amino acid sequence of rainbow trout CYP3A45 was 94% identical with trout CYP3A27, 72% with killifish CYP3A56, and 71% with both medaka CYP3A40 and killifish CYP3A30 in positional alignment comparisons. Northern blotting by a CYP3A45-specific nucleotide probe showed that the major expression site was the intestinal ceca rather than the liver in both male and female trout. Recombinant baculovirus containing a CYP3A45 cDNA (Bv-3A45) was constructed under polyhedrin promoter of the Autographa californica nuclear polyhedrosis virus and used to express CYP3A45 protein in Spodoptera frugiperda cells. The Western blot showed that the expressed CYP3A45 protein comigrated with purified LMC5 P450 and was recognized by anti-LMC5 polyclonal antibodies. The expressed CYP3A45 showed catalytic activities for the 6 beta-, 2 beta-, and 16 beta-hydroxytestosterones of 1.76, 0.193, and 0.078 nmol/min/nmol CYP3A45, respectively. In summary, a second form of CYP3A with steroid hydroxylase activity, CYP3A45, has been cloned from rainbow trout and the major site of expression was in the intestinal tissues.

Involvement of steroids and cytochromes P(450) species in the triggering of immune defenses

In vertebrates the wide variety of cytochromes P(450) (P(450)) is a key for elimination of low molecular weight xenobiotics and for the production and metabolism of steroid hormones. In contrast, xenobiotics of large molecular weight are processed and eliminated after the immune response. The suppression of immune response by native P(450)-produced glucocorticoid (GC) hormones constitutes a first link between P(450) and immunity. In the last decade, mechanisms and molecules responsible for the triggering of immune response were investigated and results showed that many tissues and organs transform native 3beta-hydroxysteroids into 7-hydroxylated metabolites that trigger immunity. Present data suggest that 7-hydroxysteroids are native anti-GCs that block the GC-induced immunosuppression. Because specific P(450) are responsible for the production of 7-hydroxylated steroids resulting into increased immunity, a second link exists between P(450) and immunity. Taken together, these findings support the proposal that P(450) are keys to all of the known defense mechanisms of vertebrates against all xenobiotic forms.

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

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

Selective induction of cytochrome P450 3A1 by dexamethasone in cultured rat hepatocytes: analysis with a novel reverse transcriptase-polymerase chain reaction assay section sign

The study of drug metabolism in cultured rat hepatocytes is hampered by the rapid loss of the expression of cytochrome P450 enzymes. Nevertheless, the activity of cytochrome P450 3A (CYP3A), one of the most important isoenzymes for drug metabolism, can be elevated by chemical inducers. In the present study, we investigated in cultured rat hepatocytes the induction of all four currently identified CYP3A isoforms by dexamethasone, and compared the results obtained in vitro with the induction profile of dexamethasone in vivo. To this end, CYP3A mRNA levels were quantified with a novel, radioactive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, and CYP3A enzymatic activity was measured by a testosterone hydroxylation assay. In the RT-PCR assay, CYP3A isoforms were co-amplified with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the presence of radioactively labeled nucleotides. This resulted in an extremely sensitive and accurate determination of CYP3A expression levels, relative to those of GAPDH. Using this RT-PCR assay, it was found that the expression of all CYP3A isoforms in rat hepatocytes, cultured on a collagen matrix, was decreased by 80-90% within one day of cultivation. After addition of dexamethasone, at one day after isolation, CYP3A1 mRNA levels were elevated to levels comparable to those in freshly isolated hepatocytes within two days. In contrast, CYP3A2, CYP3A9, and CYP3A18 mRNA levels were not affected by dexamethasone treatment, and were hardly detectable after three days of cultivation. CYP3A enzymatic activity was also induced in cultured hepatocytes (approximately 6-fold) after addition of dexamethasone. In vivo, CYP3A1 mRNA levels increased 45-fold after dexamethasone administration. However, in contrast to the situation in cultured hepatocytes, CYP3A2 and CYP3A18 were also induced, albeit to a lesser extent (4- and 7-fold elevated mRNA levels, respectively). We conclude that the selective induction of CYP3A1 in dexamethasone-treated rat hepatocytes allows the study of biotransformation reactions by CYP3A1, without interference by any of the other CYP3A isoenzymes.

Effects of physostigmine on the pharmacokinetics of intravenous parathion in rats

It was reported that the area under the plasma concentration-time curve from time zero to time infinity (AUC) of parathion was significantly smaller, and the time-averaged total body clearance (Cl) of parathion was significantly faster after intravenous administration of parathion to rats pretreated with dexamethasone than those in control rats. This was supported by significantly faster intrinsic clearance of parathion to form paraoxon in hepatic microsomal fraction of rats pretreated with dexamethasone. The above data suggested that parathion was metabolized to paraoxon by dexamethasone-inducible hepatic cytochrome P450 (CYP) 3A in rats. The purpose of this study is to explain the protective effects of physostigmine against paraoxon toxicity by suppressing CYP3A, and hence, decreasing formation of a toxic metabolite, paraoxon. The pharmacokinetic changes of parathion and paraoxon were investigated after intravenous administration of parathion, 3 mg/kg, to control Sprague-Dawley rats, and the rats pretreated with physostigmine (100 microg/kg, intraperitoneal injection 30 min before parathion administration). After a 1-min intravenous infusion of parathion to rats pretreated with physostigmine, the AUC of parathion (60.4 compared with 73.7 microg min/mL) was significantly greater, Cl of parathion (49.7 compared with 40.7 mL/min/kg) was significantly slower, and amount of paraoxon recovered from liver, mesentery and large intestine at 5 min was smaller than those in control rats. Based on in vitro rat hepatic microsomal studies, physostigmine inhibited significantly the erythromycin N-demethylase activity (1.03 compared with 0.924 nmol/mg protein/min), mainly mediated by hepatic cytochrome P450 3A in rats. The above data suggested that the formation of paraoxon was inhibited in rats pretreated with physostigmine by inhibiting CYP3A.

Effects of enzyme inducers or inhibitors on the pharmacokinetics of intravenous parathion in rats

In order to find what form of hepatic cytochrome P450 (CYP) is involved in the metabolism of parathion to form paraoxon, rats were pretreated with the enzyme inhibitors, such as SKF 525-A and ketoconazole or enzyme inducers, such as dexamethasone, isoniazid, phenobarbital, and 3-methylcholanthrene. Parathion, 3 mg/kg, was infused in 1 min via the jugular vein. In rats pretreated with SKF 525-A or ketoconazole, nonspecific CYP inhibitors, the area under the plasma concentration-time curve from time zero to time infinity (AUC) and total body clearance (Cl) of parathion were significantly greater and slower, respectively, than those in respective control rats, suggesting that parathion was metabolized by CYPs. In rats pretreated with dexamethasone (CYP3A23 inducer), the AUC was significantly smaller (41.5 compared with 52.5 microg min/mL), Cl was significantly faster (72.2 compared with 57.1 mL/min/kg), and the amounts and/or tissue-to-plasma ratios of parathion was significantly (or tended to be) smaller than those in control rats. However, the pharmacokinetic parameters of parathion were not significantly different after pretreatment with other enzyme inducers compared with respective control rats. The above data suggested that parathion was metabolized to paraoxon by dexamethasone-inducible CYP3A23, the induction of which was confirmed by Western blot analysis. This was supported by in vitro intrinsic clearance (Cl(int)) of parathion to form paraoxon in hepatic microsomal fraction; the Cl(int) in rats pretreated with dexamethasone was significantly faster (0.0900 compared with 0.0290 mL/min/mg protein) than that in control rats.

Effects of enzyme inducers and inhibitor on the pharmacokinetics of intravenous 2-(allylthio)pyrazine, a new chemoprotective agent, in rats

In order to find what types of hepatic cytochrome P450 (CYP) isozymes are involved in the metabolism of 2-(allylthio)pyrazine (2-AP) in rats, enzyme inducers, such as phenobarbital, 3-methylcholanthrene, dexamethasone, or isoniazid, and an enzyme inhibitor, such as SKF 525-A were pretreated. After 1-min intravenous administration of 2-AP, 50 mg/kg, to rats pretreated with SKF 525-A (a non-specific CYP inhibitor in rats), the plasma concentrations were significantly higher, and the area under plasma concentration-time curve from time zero to time infinity (AUC) was significantly greater (1365 compared with 1034 microg min/mL) as a result of significantly slower total body clearance (Cl) (36.6 compared with 48.3 mL/min/kg) than those in control rats, indicating that 2-AP was metabolized by CYP isozymes. After 1-min intravenous administration of 2-AP, 50 mg/kg, to rats pretreated with dexamethasone (an inducer of CYP3A in rats), phenobarbital (an inducer of CYP2B1/2, 2C6, 2C7, and 3A1/2 in rats), and 3-methylcholanthrene (an inducer of CYP1A1/2 and 2A1 in rats), the plasma concentrations were significantly lower, and AUC was significantly smaller (27, 41 and 60% decrease, respectively, compared with respective control rats) owing to faster Cl [37 (p>0.05), 70 (p<0.001), and 150% (p<0.001) increase, respectively, compared with respective control rats].

Methodologies to study the induction of rat hepatic and intestinal cytochrome P450 3A at the mRNA, protein, and catalytic activity level

Studies were conducted to characterize assays for the isolation and quantitation of rat cytochrome P450 (CYP) 3A isoforms from hepatic and intestinal tissues. Isolated intestinal microsomes were analyzed for their alkaline phosphatase activity and CYP 3A immunoreactivity. The involvement of CYP 3A in the in vitro hydroxylation of midazolam (MDZ) was also evaluated using isoform specific chemical and antibody inhibitors. The effect of glycerol (a common constituent of the microsomal reconstitution buffer) concentration on in vitro MDZ hydroxylation was also investigated. Additionally, to verify that the intestinal preparation was adequate for use in studies investigating the induction of CYP3A at the MRNA, protein, and catalytic activity within a single animal, a separate induction study was carried out with the CYP 3A inducer dexamethasone (DEX). A reverse transcription-polymerase chain reaction (RT-PCR) assay and a quantitative Western blotting method were used to reliably detect differences in CYP 3A mRNA and immunoreactivity between DEX- and vehicle (VH)-treated tissues. The in vitro hydroxylation of MDZ evaluated CYP 3A catalytic activity and identified increases in CYP 3A activity caused by DEX in comparison to VH. Collectively, these described techniques provide an experimental model to study xenobiotic induction of rat hepatic and intestinal CYP 3A from the molecular to the catalytic level in individual rats without the need for pooling of tissue.

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.

Rat pregnane X receptor: molecular cloning, tissue distribution, and xenobiotic regulation

An orphan nuclear receptor, termed the pregnane X receptor (PXR), has recently been cloned from mouse and human and defines a novel steroid signaling pathway (Cell 92, 73-82, 1998; Proc. Natl. Acad. Sci. USA 95, 12208-122313, 1998). Transient cotransfection experiments demonstrate that the PXR responds to structurally dissimilar compounds and confers the induction of cytochrome P4503A (CYP3A), a subfamily of enzymes that involve the metabolism of two-thirds of drugs and other xenobiotics. In this report, we describe the molecular cloning, tissue distribution, and xenobiotic regulation of a rat PXR designated rPXR-1. rPXR-1 exhibits a 95% sequence identity with the mouse PXR, but only 79% identity with the human PXR, providing the molecular basis that rats and mice have a similar CYP3A induction profile but differ from humans. rPXR-1 gene was expressed abundantly in liver, intestine, and, to a lesser extent, kidney, lung, and stomach. The tissue distribution and the relative abundance of rPXR-1 mRNA among these tissues resemble those of CYP3A, suggesting that PXR is important not only for induction but also for constitutive expression of these enzymes. Xenobiotics known to induce liver microsomal enzymes showed differential effects on the rPXR-1 expression as determined by Northern blot analysis. Dexamethasone, for example, increased the accumulation of rPXR-1 mRNA, whereas troleandomycin slightly suppressed it. Compounds that increase PXR expression (inducers) and compounds that interact with PXR (ligands) likely have synergistic effects on CYP3A induction, which provides a novel molecular explanation for drug-drug interactions.

Effect of arsenite on induction of CYP1A, CYP2B, and CYP3A in primary cultures of rat hepatocytes

In earlier studies, sodium arsenite treatment was shown to decrease induction of enzymatic activities associated with hepatic CYPs in rats. Here we investigated the effect of sodium arsenite on induction of CYP2B, CYP1A, and CYP3A in primary cultures of rat hepatocytes. Arsenite decreased the induction of all three families of CYP, as measured enzymatically and immunochemically. These decreases in CYPs occurred at concentrations of arsenite (2.5-10 microM) at which no toxicity was observed; however, toxicity was observed at 25 microM arsenite. With 3-methylcholanthrene as inducer, 5 microM arsenite caused a 55% decrease in CYP1A1 immunoreactive protein and enzyme activity, but only a 25% decrease in CYP1A1 mRNA. With phenobarbital (PB) as the inducer, 2.5 microM arsenite decreased CYP2B enzyme activity and immunoreactive protein 50%, with only a 25% decrease in CYP2B1 mRNA. 5 microM Arsenite decreased CYP2B enzyme activity and immunoreactive protein 80%, but decreased CYP2B1 mRNA only 50%, while CYP3A protein was decreased greater than 75% with no decrease in CYP3A23 mRNA. With dexamethasone (DEX) as inducer, 5 microM sodium arsenite caused a 50% decrease in immunoreactive CYP3A and a 30% decrease in CYP3A23 mRNA. Although arsenite-mediated increases in heme oxygenase (HO) inversely correlated with decreases in CYP2B or CYP1A activity, inclusion of heme in cultures treated with inducers of CYP1A or CYP2B did not prevent the arsenite-mediated decreases in these CYPs. Even though added heme induced HO to similar levels with and without arsenite, decreases in CYPs were only observed in the presence of arsenite. These results suggest that, in rat hepatocytes, elevated levels of HO alone are not responsible for arsenite-mediated decreases in CYP.

A novel cytochrome P450 3A isoenzyme in rat intestinal microsomes

PCR with several pairs of primers facilitates screening for new isoenzymes among highly homologous cytochrome P450s (CYPs). Combinations of two pairs of primers, which amplify N- and C-terminal coding sequences of either CYP3A1/CYP3A23 or CYP3A2 detected the presence of a previously unrecognized CYP3A in enterocyte microsomes isolated from rats. PCR, Northern blot, and immunoblotting with specific antibodies indicated that this isoenzyme is clearly distinguishable from CYP3A1, 3A23 or 3A2. Sequencing of a 285 bp coding fragment of this gene revealed 97% similarity with rat olfactory CYP3A9 (P450olf3).

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.

Environmental xenobiotics and the antihormones cyproterone acetate and spironolactone use the nuclear hormone pregnenolone X receptor to activate the CYP3A23 hormone response element

The pregnenolone X receptor (PXR), a new member of the nuclear hormone receptor superfamily, was recently demonstrated to mediate glucocorticoid agonist and antagonist activation of a hormone response element spaced by three nucleotides (DR-3) within the rat CYP3A23 promoter. Because many other steroids and xenobiotics can up-regulate CYP3A23 expression, we determined whether some of these other regulators used PXR to activate the CYP3A23 DR-3. Transient co-transfection of LLC-PK1 cells with (CYP3A23)2-tk-CAT and mouse PXR demonstrated that the organochlorine pesticides transnonachlor and chlordane and the nonplanar polychlorinated biphenyls (PCBs) each induced the CYP3A23 DR-3 element, and this activation required PXR. Additionally, this study found that PXR is activated to induce (CYP3A23)2-tk-CAT by antihormones of several steroid classes including the antimineralocorticoid spironolactone and the antiandrogen cyproterone acetate. These studies reveal that PXR is involved in the induction of CYP3A23 by pharmacologically and structurally distinct steroids and xenobiotics. Moreover, PXR-mediated PCB activation of the (CYP3A23)2-tk-CAT may serve as a rapid assay for effects of nonplanar PCBs.

Change of 7alpha-hydroxy-dehydroepiandrosterone levels in serum of mice treated by cytochrome P450-modifying agents

Dehydroepiandrosterone (DHEA) is 7alpha-hydroxylated in liver, brain and other organs of murine and in other species. Several works suggest that the 7alpha-hydroxy-DHEA produced may be one of the native antiglucocorticoids, and compounds modifying its production may prove useful in investigation of 7alpha-hydroxy-DHEA production and effects. After treatment of mice with dexamethasone, phenobarbital, trilostane, melatonin or metyrapone, we have used gas chromatography-mass spectrometry with negative ion detection for measurement of 7alpha-hydroxy-DHEA levels in serum of control and treated animals. The 7alpha-hydroxylating rates of liver and brain microsomes from the same animals were also measured. Results showed that serum levels of 7alpha-hydroxy-DHEA were significantly increased after treatment by all compounds except metyrapone. Significantly increased 7alpha-hydroxy-DHEA levels were directly related with significantly increased 7alpha-hydroxylation yields in liver and not in brain. In contrast, metyrapone decreased 7alpha-hydroxylation in liver and brain. These findings indicate that in brain and in liver, different enzyme systems may be responsible for production of 7alpha-hydroxy-DHEA and that treatment-induced modifications of circulating 7alpha-hydroxy-DHEA levels are mainly due to change of 7alpha-hydroxylating rates in liver.

Differential inducibility of specific mRNA corresponding to five CYP3A isoforms in female rat liver by RU486 and food deprivation: comparison with protein abundance and enzymic activities

The induction of cytochrome P450 3A (CYP3A) protein and mRNA by RU486 [17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-1-pro pyl-estra-4,9-dien-3-one] treatment and food deprivation in female rat liver was studied using Western blotting and competitive reverse transcription-polymerase chain reaction (RT-PCR). CYP3A apoprotein levels increased in response to food deprivation and to RU486 treatment, and the combination of RU486 treatment plus food deprivation had an apparent additive effect. Food deprivation and RU486 treatment also caused increases in CYP3A1, CYP3A18, and CYP3A23 mRNA, and the combined effects of these treatments on each of these mRNA forms were synergistic. CYP3A2 mRNA was not detected in any of the treatment groups, and there was a lack of concordance between CYP3A9 mRNA levels and the specific messages corresponding to the other CYP3A isoforms. CYP3A9 mRNA levels were highest in food-deprived animals, whereas RU486 inhibited CYP3A9 mRNA expression and suppressed the induction effect of food deprivation. Food deprivation and RU486 treatment each separately caused increased microsomal diazepam C3-hydroxylase activity, and the combined effects of these treatments on this monooxygenase were additive. In contrast, the [N-methyl-14C]erythromycin demethylase activity of the fasted, RU486-treated group of rats did not differ from that of the untreated group, and kinetic analyses revealed that both groups of animals exhibited similar Km and Vmax values. These results suggest that CYP3A9 may be primarily responsible for erythromycin N-demethylation and that the isoforms induced by the combination of fasting and RU486 administration are CYP3A1, CYP3A23, and, to a lesser extent, CYP3A18.

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.

Comparative induction of CYP3A and CYP2B in rat liver by 3-benzoylpyridine and metyrapone

3-Benzoylpyridine (3BP) is a major metabolite of HGG-12, and oxime that has been synthesized as a potential antidote to the toxic effects of soman and other anticholinesterases. Structural similarities exist between 3BP, the cytochrome P450 (CYP)-inducer metyrapone (MET) and other 3-substituted pyridines that interact with CYPs. The present study evaluated the regulatory effects of 3BP on CYP expression in rat liver. Both 3BP and MET (100 mg/kg) increased total hepatic microsomal holo-CYP content significantly 24 h after administration to male rats. Pronounced increases in activities mediated by CYP2B (androstenedione 16 beta-hydroxylation and 7-pentylresorufin O-depentylation) were produced by 3BP and MET, which correlated with respective 9- and 14-fold increases in CYP2B immunoreactive protein. In addition, both agents slightly increased rates of microsomal CYP3A-dependent steroid 6 beta-hydroxylation, troleandomycin metabolite complex formation and total CYP3A immunoreactive protein. Induction of the dexamethasone-inducible CYP3A23 mRNA to 4.5- and 2.5-fold of control was detected in liver of MET- and 3BP-induced rats; CYP3A2 mRNA levels were unchanged. Analogous in vitro studies revealed that MET was a preferential inhibitor of CYP3A-mediated steroid 6 beta-hydroxylation activity, but 3BP was inactive against constitutive steroid hydroxylase CYPs. These findings indicate that the structurally related 3BP and MET elicit similar induction effects on CYPs 2B and 3A23 in rat liver after in vivo administration, but differential inhibitory effects of the chemicals on CYP activity in vitro. Recent reports have implicated a microsomal binding site in the induction of CYP3A1/3A23 in rat liver. In light of the present findings, substituted pyridines like 3BP may be useful tools in structure-activity studies to evaluate the physicochemical requirements for binding to this protein.

Sexually dimorphic expression of rat CYP3A9 and CYP3A18 genes is regulated by growth hormone

The cDNAs for two CYP3A genes were isolated from the livers of rats using an RT-PCR approach with CYP3A subfamily-specific primers. Sequence analysis revealed these cDNAs to be identical to CYP3A9, which had previously been isolated from rat brain and nasal epithelium and the recently described CYP3A18. The hepatic expression of both genes was sexually dimorphic. Thus CYP3A18 mRNA levels were 25-fold higher in male livers compared to females, while CYP3A9 showed a reverse pattern with 6-fold higher expression in the liver of females. Exposure of male rats to the female pattern of growth hormone secretion led to an increase in hepatic CYP3A9 mRNA expression and suppressed expression of CYP3A18. These findings indicate that the CYP3A subfamily in rats has both male- and female-specific isoforms which are regulated by growth hormone in a manner similar to some other sexually dimorphic cytochrome P450s.

Characterization of a pretranscriptional pathway for induction by phenobarbital of cytochrome P450 3A23 in primary cultures of adult rat hepatocytes

Our laboratory has proposed that phenobarbital (PB), a typical lipophilic agent that induces some members of the supergene family of liver microsomal cytochromes P450 (e.g., CYP2B1/2 and CYP3A23), acts through a complex process inhibitable by the presence of growth hormone (GH), the absence of some components of the extracellular matrix, or a disrupted cytoskeleton. To verify that these manipulations of the culture environment block specific steps in the PB induction pathway rather than simply exerting nonspecific or toxic effects on CYP2B1/2 gene transcription, we have now examined PB induction of CYP3A23, a gene known to also be transcriptionally activated by dexamethasone (DEX) through a "nonclassical" pathway apparently involving the glucocorticoid receptor. We found that in primary cultures of adult rat hepatocytes treated with PB, induction of CYP3A23 mRNA, just as we reported for induction of CYP2B1/2 mRNA, required the use of Matrigel (a reconstituted basement membrane) and was blocked by the presence of cytoskeletal inhibitors (colchicine or cytochalasins) or of physiologic concentrations of GH in the culture medium. Moreover, PB induction of CYP3A23 and of CYP2B1/2 mRNAs was greatly diminished by inhibitors of cAMP-dependent protein kinase (PKA). In striking contrast, induction of CYP3A23 mRNA by DEX was unaffected by any of these alterations of the culture conditions that block its induction by PB. We conclude that the effects of extracellular matrix, GH, disruption of the cytoskeleton, and activation of cAMP-dependent protein kinase, pharmacologically define multiple, pretranscriptional steps in the pathway(s) for PB induction of liver cytochromes P450.

Ethylbenzene modulates the expression of different cytochrome P-450 isozymes by discrete multistep processes

Ethylbenzene (EB) treatment to male Holtzman rats was shown to alter the expression of cytochrome P-450s 1A1, 2B, 2C11, 2E1, and 3A, with several isozymes exhibiting complex multiphasic induction patterns when treated for 1 and 3 days with the alkylbenzene. Male rats were treated with daily i.p. injections of EB for either one or three days, and the effects on P-450 dependent activities, P-450 immunoreactive protein levels and their corresponding mRNA levels were measured. Although levels of P-450 2B, 2C11, 2E1, and 3A were all modulated by EB treatment, each exhibited different temporal characteristics. P-450 2B1/2B2 were induced after a single EB exposure and continued to be elevated after EB treatment for 3 days. However, P-450 2B1 and 2B2 mRNA levels were elevated about 50-fold after a single injection, and returned to control values after continued EB administration. P-450 2C11 expression was decreased to about 45% of controls after either single or repeated EB exposure with corresponding changes being observed in the levels of 2C11 mRNA. P-450 2E1 was induced by EB according to a complex multistep induction pattern. Both P-450 2E1 protein and RNA levels were increased 2-4-fold after a single EB treatment but returned to control values after continued administration. P-450 3A-dependent testosterone 2beta-hydroxylation and P-450 3A immunoreactive protein levels were both increased about 3-fold after a single EB treatment, whereas levels were only elevated 2-fold after EB treatment for 3 days. In contrast, P-450 3A2 mRNA was unaffected by a single EB injection but was increased 3.5-fold with repeated administration. Changes in P-450 3A1/2 were similar to those observed with P-450 3A2, whereas changes in P-450 3A1/23 and 3A23 mRNAs were not detectable. These data indicate that while EB can influence the expression of several P-450 isozymes, the hydrocarbon appears to alter P-450 expression by acting at different regulatory steps.

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.

Metabolite complex formation of orphenadrine with cytochrome P450. Involvement of CYP2C11 and CYP3A isozymes

Expression and inhibition of cytochrome P450 (CYP) isozymes capable of forming an orphenadrine metabolite complex were studied in microsomes of untreated and inducer-treated male and female rats. High levels of complex-forming isozymes were found in microsomes of untreated male as compared to female rats. Treatment of male rats with several P450 inducers did not considerably increase the extent of in vitro complex formation. In female rats, however, phenobarbital or dexamethasone treatments led to pronounced induction. The isozyme specificity of complex formation was investigated by several approaches including: 1. inhibition by orphenadrine of isozyme-specific P450 activities, such as hydroxylation of testosterone, O-dealkylation of pentoxy-and ethoxyresorufin and complex formation with triacetyloleandomycin (TAO), 2. inhibition of orphenadrine complex formation by metyrapone, TAO, and cimetidine, and 3. correlation of complex levels with immunochemically, enzymatically, or spectroscopically determined amounts of P450 isozymes. Our data suggest that CYP2C11, a CYP3A isozyme and an unidentified P450 species are involved in complex formation with orphenadrine, but exclude the involvement of CYP1A1/2 and CYP2B1/2. The capability of CYP2C11 to form a metabolite complex with orphenadrine is strongly suggested for the following reasons: 1. Efficient inhibition of testosterone 2 alpha- and 16 alpha-hydroxylation by complex formation with orphenadrine in microsomes of untreated male rats, 2. high expression of orphenadrine-complexing isozymes in untreated male compared to female rats, 3. specific inhibition of in vitro complex formation by cimetidine, 4. suppression of complex-forming isozymes by 3-methylcholanthrene and beta-naphthoflavone, and 5. concomitant induction of complex-forming isozymes, immunodetectable CYP2C11, and testosterone 2 alpha-hydroxylase by stanozolol. That at least one, but not all, CYP3A isozymes is involved in complex formation is concluded from inhibition experiments with TAO that show that orphenadrine complexation can be significantly inhibited in microsomes of dexamethasone-treated, but not in microsomes of untreated rats. Furthermore, complex formation with TAO is not inhibited by orphenadrine in microsomes of phenobarbital (PB)-treated rats. In PB-treated female rats, a further unidentified complex-forming isozyme can be detected that is not inhibited by complex formation with TAO.

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.

In vivo induction of cytochrome P450 CYP3A expression in rat leukocytes using various inducers

Although the induction of cytochromes P450 3A (CYP3A) is relatively well characterized in liver, its inducibility in an easily available tissue such as the peripheral leukocytes is not known. The purpose of this study was, therefore, to determine if CYP3A is inducible in vivo in peripheral leukocytes. Microsomes from rat leukocytes and liver were examined for CYP3A protein expression using Western blotting with a rabbit polyclonal antibody against rat CYP3A. Although CYP3A was not detected in control leukocytes, in vivo treatment with known CYP3A inducers (dexamethasone, clotrimazole, phenobarbital, pregnenolone-16 alpha-carbonitrile) resulted in CYP3A leukocyte levels of 0.2-0.8 pmol/mg protein. This leukocyte induction was approximately 1000-fold lower than in induced liver. Interestingly, there was an apparent linear relationship between leukocyte and liver CYP3A contents (r2 = 0.748, n = 29). These results not only demonstrate for the first time that CYP3A is inducible in rat leukocytes after in vivo treatment with various CYP3A inducers, but also suggest that peripheral leukocytes could be used to assess induction in vivo.

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.

Lack of effect of methotrexate on the expression of constitutive hepatic cytochromes P450 in the male rat

1. The effects of methotrexate (MTX) on the expression of selected constitutive cytochrome P450 (CYP) isozymes in the liver of male rats at the catalytic activity and mRNA levels were examined. 2. Male rats received a single intraperitoneal injection of MTX (4 mg/kg) or vehicle and were killed, 1, 2, 7 or 14 days following drug administration. 3. Hepatic microsomes were used for determination of total CYP content, NADPH-CYP reductase activity, aminopyrine N-demethylase activity, and androstenedione (AD) hydroxylation activity; total RNA was also isolated from liver and was used for hybridization analysis of CYP isozyme expression at the mRNA level. 4. MTX did not affect any of the following parameters at any time-point in comparison with the corresponding vehicle control: body weight, liver weight, hepatic microsomal protein content, total CYP content, NADPH-CYP reductase activity, aminopyrine N-demethylase activity, AD 6 beta- and 7 alpha-hydroxylase activity, and CYP3A2 mRNA content. 5. The major male-specific CYP isozyme, 2C11, was down-regulated by MTX treatment as revealed by a marginal (25%), but statistically significant decrease in AD 16 alpha-hydroxylase activity at day 14 and a marginal (18%), but statistically significant decrease in CYP2C11 mRNA content at day 14. 6. In comparison with other antineoplastic drugs that have been examined, MTX appears to possess a lesser capacity for modulation of hepatic CYP enzymes.

A novel cis-acting element in a liver cytochrome P450 3A gene confers synergistic induction by glucocorticoids plus antiglucocorticoids

The induction by dexamethasone of rat liver CYP3A1 differs from classical glucocorticoid gene regulation in part because both glucocorticoids and antiglucocorticoids such as pregnenolone 16 alpha-carbonitrile (PCN) induce CYP3A1 through transcriptional gene activation. In the present study, we transiently expressed in primary cultures of rat hepatocytes plasmids consisting of CYP3A1 5'-flanking sequences fused to a chloramphenicol acetyltransferase reporter plasmid. Deletional analysis identified a 78-base pair (bp) element located approximately 135 bp upstream of the transcriptional start site that was inducible by treatment of the cultures with dexamethasone or PCN and was induced synergistically by dexamethasone plus PCN. Nuclear extract from control rat liver protected two regions within the 78-bp sequence against digestion with DNase I. The same two regions were protected when nuclear extracts from dexamethasone-treated animals were used. Analysis of both of the "footprints" (FP1 and FP2) failed to reveal a classical sequence for the glucocorticoid-responsive element. A 33-bp element that includes FP1 sequences inserted into the chloramphenicol acetyltransferase reporter plasmid and transiently expressed in rat hepatocytes conferred a profile of dexamethasone and PCN induction similar to that of the 78-bp element. However, an Escherichia coli expressed glucocorticoid receptor protein failed to protect sequences within FP1 in DNase I footprinting experiments and failed to change its mobility in gel shift assays. Moreover, as judged by the gel shift assay, the specific protein binding to this fragment was the same whether nuclear extracts from the liver of untreated or dexamethasone-treated rats were used. We conclude that the activation of CYP3A1 gene transcription by glucocorticoids may involve proteins already bound to the controlling element in the CYP3A1 gene through a mechanism in which GR in the presence of hormone does not bind directly to CYP3A1 DNA.

Differential regulation of the cytochrome P450 3A1 gene transcription by dexamethasone in immature and adult rat liver

We have previously shown that the in vivo induction of cytochrome P450 3A1 by dexamethasone occurs through a sharp and early transcriptional activation in the immature rat liver that is drastically impaired in adults [Telhada, M. B., Pereira, T. M. & Lechner, M. C. (1992) Arch. Biochem. Biophys. 298, 714-725]. In the present study we investigate the relative importance of cytochrome P450 3A1 gene transcription on the adaptive response to the synthetic glucocorticoid dexamethasone, by measuring the time-course run-on transcription rate and concomitant mRNA accumulation in the male rat liver at two different ontological developmental stages. The primary (direct) or secondary (dependent on protein neo-synthesis) nature of the in vivo inductive response to dexamethasone and to pregnenolone 16 alpha-carbonitrile, is further investigated by inhibiting translation by cycloheximide pretreatment. The induction of cytochrome P450 3A1 gene transcription by the anti-glucocorticoid pregnenolone 16 alpha-carbonitrile is demonstrated to occur through a secondary mechanism, requiring ongoing protein biosynthesis, regardless of the developmental stage of the animals. Conversely, a significant developmentally controlled change is observed in the inductive response of the cytochrome P450 3A1 gene to dexamethasone, characterized by a markedly delayed transcriptional activation in the adult rat liver (90 day old) as compared to the immature rat liver (21 day old). This is consistent with the net primary response of the cytochrome P450 3A1 gene to dexamethasone demonstrated in this study to occur in the immature rat liver and almost lost at the adult stage, when protein neo-synthesis becomes essential for the inductive response. Our results demonstrate (a) a difference in the mechanisms underlying induction of the cytochrome P450 3A1 gene by the glucocorticoid agonist dexamethasone and by the antagonist pregnenolone 16 alpha-carbonitrile, and (b) an important change in the mechanisms of the inductive response to dexamethasone, associated with the immature/adult liver phenotype transition. This indicates the participation of specific labile transcription factors in the induction of cytochrome P450 3A1 gene by the synthetic glucocorticoids.