Assessment of Human Pregnane X Receptor Involvement in Pesticide-Mediated Activation of CYP3A4 Gene

Assessment of foreign chemical inducibility on CYP3A4 is necessary to optimize drug therapies. The properties of chemicals such as pesticides, however, are not well investigated. In the present study, properties of various pesticides on human CYP3A4 induction have been tested using HepG2-derived cells stably expressing the CYP3A4 promoter/enhancer (3-1-10 cells) and the human pregnane X receptor (hPXR)-small interfering RNA (siRNA) system. Among the examined pesticides, 13 pesticides were observed to activate the CYP3A4 gene. Surprisingly, pyributicarb was found to increase the CYP3A4 reporter activity at 0.1 to 1 microM more strongly than typical CYP3A4 inducer rifampicin. Expression of hPXR-siRNA clearly diminished the pyributicarb-stimulated CYP3A4 reporter activity in 3-1-10 cells and decreased the endogenous CYP3A4 mRNA levels in HepG2 cells. Pyributicarb caused enhancement of CYP3A4-derived reporter activity in mouse livers introduced with hPXR by adenovirus. These results indicate pyributicarb as a potent activator of CYP3A4 gene, suggesting the existence of pesticides leading to CYP3A4 induction in our environment.

Genetic Polymorphism of Aldehyde Oxidase in Donryu Rats

One of major metabolic pathways of [(+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]-pyrimidine] (RS-8359), a selective and reversible monoamine oxidase type A inhibitor, is the aldehyde oxidase-catalyzed 2-hydroxylation at the pyrimidine ring. Donryu rats showed a dimorphic pattern for the 2-oxidation activity with about 20- to 40-fold variations in the Vmax/Km values between a low and a high activity group. The rats were classified as extensive metabolizers (EM) and poor metabolizers (PM) of RS-8359, of which ratios were approximately 1:1. One rat among the EM rats of each sex showed extremely high activity, and they were referred to as ultrarapid metabolizers. There was no significant difference in the expression levels of mRNA of aldehyde oxidase between the EM and PM rats. Analysis of nucleotide sequences showed four substitutions, of which the substitutions at 377G>A and 2604C>T caused 110Gly-Ser and 852Ala-Val amino acid changes, respectively. Amino acid residue 110 is located very near the second Fe-S center of aldehyde oxidase. Its change from nonchiral Gly to chiral Ser may result in a conformational change of aldehyde oxidase protein with the shift of isoelectric point value from 5.0 in the EM rats to 6.2 in the PM rats. The 110Gly-Ser amino acid substitution (377G>A) may be primarily responsible for the variations of aldehyde oxidase activity observed in Donryu rats, in addition to the difference of expression levels of aldehyde oxidase protein. If a new drug candidate is primarily metabolized by aldehyde oxidase, attention should be given to using a rat strain with high aldehyde oxidase activity and small individual variation.

Chenodeoxycholic acid-mediated activation of the farnesoid X receptor negatively regulates hydroxysteroid sulfotransferase

Hydroxysteroid sulfotransferase catalyzing bile acid sulfation plays an essential role in protection against lithocholic acid (LCA)-induced liver toxicity. Hepatic levels of Sult2a is up to 8-fold higher in farnesoid X receptor-null mice than in the wild-type mice. Thus, the influence of FXR ligand (chenodeoxycholic acid (CDCA) and LCA) feeding on hepatic Sult2a expression was examined in FXR-null and wild-type mice. Hepatic Sult2a protein content was elevated in FXR-null and wild-type mice fed a LCA (1% and 0.5%) diet. Treatment with 0.5% CDCA diet decreased hepatic Sult2a to 20% of the control in wild-type mice, but increased the content in FXR-null mice. Liver Sult2a1 (St2a4) mRNA levels were reduced to 26% in wild-type mice after feeding of a CDCA diet, while no decrease was observed on Sult2a1 mRNA levels in FXR-null mice after CDCA feeding. A significant inverse relationship (r(2)=0.523) was found between hepatic Sult2a protein content and small heterodimer partner (SHP) mRNA level. PCN-mediated increase in Sult2a protein levels were attenuated by CDCA feeding in wild-type mice, but not in FXR-null mice. Human SULT2A1 protein and mRNA levels were decreased in HepG2 cells treated with the FXR agonists, CDCA or GW4064 in dose-dependent manners, although SHP mRNA levels were increased. These results suggest that SULT2A is negatively regulated through CDCA-mediated FXR activation in mice and humans.

Broad but Distinct Role of Pregnane X Receptor on the Expression of Individual Cytochrome P450s in Human Hepatocytes

In the present study, we have utilized a target selective human pregnane X receptor-siRNA (hPXR-siRNA)-adenovirus expression system to examine the contribution of hPXR on the gene regulation of drug-metabolizing P450s in human hepatocytes. Introduction of the hPXR-siRNA adenoviral vector reduced the level of PXR mRNA. After infection with Ad hPXR-siRNA, the basal and ligand-activated CYP2A6, CYP2C8, CYP3A4 and CYP3A5 mRNA levels were decreased significantly in dose-dependent manners, whereas CYP2B6, CYP2C9 and CYP2C19 mRNA levels were moderately influenced after infection with Ad hPXR-siRNA. These data suggest the distinct PXR influences on the regulation of these genes. The expression of CYP1A2 and CYP2D6 mRNA were not affected by the introduction of hPXR-siRNA, suggesting that PXR plays no functional role in the expression of either of these genes. This is the first report to compare simultaneously the relative contribution of hPXR on the expression of nine forms of P450 in primary cultured human hepatocytes. Mutual sharing among nuclear receptors of their binding cis-elements becomes clear now. Thus, the present method using the combination of adenovirus-mediated hPXR-siRNA expression and human hepatocytes may offer clear information on the relative role of nuclear receptors such as hPXR on the expression of drug metabolizing genes.

Induction of human CYP2A6 is mediated by the pregnane X receptor with peroxisome proliferator-activated receptor-gamma coactivator 1alpha

CYP2A6 plays important roles in the metabolism of nicotine and some clinically used drugs. Interindividual variability in the CYP2A6 expression level in human liver might be caused by an inducible property, but the molecular mechanism of induction is unclear. Rifampicin, phenobarbital, and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime, which are activators of pregnane X receptor (PXR) and constitutive androstane receptor (CAR), induced CYP2A6 mRNA in human hepatocytes. We identified three direct repeat separated by four nucleotides (DR4)-like elements at -6698, -5476, and -4618 in the CYP2A6 gene, to which PXR and CAR could bind after dimerization with retinoid X receptor (RXR)-alpha. In luciferase assays, overexpression of PXR or CAR could not activate the transcriptional activity of CYP2A6 promoter constructs (-6754 to -1) in HepG2 cells. Cotransfection of hepatocyte nuclear factor-4alpha did not affect the transcriptional activities in the absence or presence of PXR or CAR. Interestingly, cotransfection of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) as well as PXR significantly enhanced the transcriptional activity (3.9-fold of control). By the deletion of a possible suppresser region (-4533 to -185), the effects of PXR/PGC-1alpha on the transcriptional activity were increased (6.9-fold of control). Deletion or mutation analyses revealed that two DR4-like elements at -5476 and -4618 are essential for transactivation by PXR/PGC-1alpha. Chromatin immunoprecipitation assay revealed that PXR and PGC-1alpha bind to CYP2A6 chromatin. In conclusion, we found that CYP2A6 is induced via PXR and PGC-1alpha through the DR4-like element at the distal response region. This is the first study to report the molecular mechanism of the induction of CYP2A6.

Effects of grapefruit juice on the pharmacokinetics of pitavastatin and atorvastatin

AIMS

To compare the effects of grapefruit juice (GFJ) on the pharmacokinetics of pitavastatin and atorvastatin.

METHODS

In a randomized, four-phase crossover study, eight healthy subjects consumed either GFJ or water t.i.d. for 4 days in each trial. On each final day, a single dose of 4 mg pitavastatin or 20 mg atorvastatin was administered.

RESULTS

GFJ increased the mean AUC(0-24) of atorvastatin acid by 83% (95% CI 23-144%) and that of pitavastatin acid by 13% (-3 to 29%).

CONCLUSIONS

Pitavastatin, unlike atorvastatin, appears to be scarcely affected by the CYP3A4-mediated metabolism.

Prediction of drug-drug interactions for AUCoral of high clearance drug from in vitro data: utilization of a microtiter plate assay and a dispersion model

The purpose of this study was to propose a new method to predict in vivo drug-drug interactions (DDIs) for a high clearance drug from in vitro data. As the high clearance drug, NE-100 (N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine monohydrochloride) was used. First, approach based on I(u)/K(i) value was used for the prediction of DDIs between NE-100 and concomitant drugs. When the K(i) values (K(i-cal)) obtained from the microtiter plate (MTP) assay and the reported K(i) values (K(i-rep)) for these drugs were used to predict increases at levels of NE-100 AUC(oral) (AUC(oral) ratio), the AUC(oral) ratios from the I(u)/K(i-cal) correlated with those from the I(u)/K(i-rep). This result suggests that the K(i-cal) from the MTP assay can be used for prediction of DDIs instead of the K(i-rep) value. Second, a new approach combining the inhibition rate (R) calculated from the MTP assay and two physiological models was used to predict DDIs. When the AUC(oral) ratios of NE-100 by various drugs were predicted using the R value and the well-stirred model, the ratios were similar to those predicted using the I(u)/K(i). However, after co-administration of drugs such as quinidine, propafenone and thioridazine (potent inhibitors of CYP2D6), the NE-100 AUC(oral) ratios predicted from the dispersion model was much greater than those from well-stirred model. This result shows that application of the dispersion model to the prediction method using the R value might sensitively and precisely predict the increased levels of AUC(oral) by DDIs for high clearance drug, compared with the prediction method using I(u)/K(i) value.

Detection of a new N-oxidized metabolite of flutamide, N-[4-nitro-3-(trifluoromethyl)phenyl]hydroxylamine, in human liver microsomes and urine of prostate cancer patients

Flutamide (2-methyl-N-[4-nitro-3-(trifluoromethyl)phenyl]-propanamide), a nonsteroidal antiandrogen, is used in the treatment of prostate cancer but is occasionally associated with hepatic dysfunction. In the present study, the metabolism of flutamide including the formation of the possible reactive toxic metabolites was investigated using human liver microsomes and 10 isoforms of recombinant human cytochrome P450 (P450). 2-Hydroxyflutamide (OH-flutamide) and 4-nitro-3-(trifluoromethyl)phenylamine (FLU-1) were the main products of flutamide metabolism in human liver microsomes. The formation of OH-flutamide was markedly inhibited by ellipticine, an inhibitor of CYP1A1/1A2, and was mainly catalyzed by the recombinant CYP1A2. FLU-1 was also produced from OH-flutamide, but its metabolic rate was much less than that from flutamide. An inhibitor of carboxylesterase, bis-(p-nitrophenyl)phosphoric acid, completely inhibited the formation of FLU-1 from flutamide in human liver microsomes. A new metabolite, N-[4-nitro-3-(trifluoromethyl)phenyl]hydroxylamine (FLU-1-N-OH), was detected as a product of the reaction of FLU-1 with human liver microsomes and identified by comparison with the synthetic standard. The formation of FLU-1-N-OH was markedly inhibited by the addition of miconazole, an inhibitor of CYP3A4, and was mediated by recombinant CYP3A4. Furthermore, FLU-1-N-OH was detected mostly as the conjugates (glucuronide/sulfate) in the urine of prostate cancer patients collected for 3 h after treatment with flutamide. The formation of FLU-1-N-OH, however, did not differ between patients with and without abnormalities of hepatic functions among a total of 29 patients. The lack of an apparent association of the urinary excretion of FLU-1-N-OH and hepatic disorder may suggest the involvement of an additional unknown factor in the mechanisms of flutamide hepatotoxicity.

A Common Regulatory Region Functions Bidirectionally in Transcriptional Activation of the HumanCYP1A1andCYP1A2Genes

The human CYP1A1 and CYP1A2 genes on chromosome 15 are orientated head-to-head and are separated by a 23-kilobase (kb) intergenic spacer region. Thus, the possibility exists for sharing common regulatory elements contained in the spacer region responsible for transcriptional activation and regulation of the CYP1A1 and CYP1A2 genes. In the present study, a reporter gene construct containing -22.4 kb of the 5'-flanking region of the CYP1A2 gene was found to support beta-naphthoflavone (BNF) and 3-methylchoranthrene (3-MC)-mediated transcriptional activation. The responsive region was also functional in directing activation of the CYP1A1 promoter, indicating that the region works bidirectionally to govern transcriptional activation of both CYP1A1 and CYP1A2. To simultaneously evaluate transcriptional activation of both genes, a dual reporter vector was developed in which the spacer region was inserted between two different reporter genes, firefly luciferase and secreted alkaline phosphatase. Transient transfection of the dual reporter vector in HepG2 cells revealed increases in both reporter activities after exposure of the cells to BNF and 3-MC. Deletion studies of the spacer region indicated that a region from -464 to -1829 of the CYP1A1 gene works bidirectionally to enhance the transcriptional activation of not only CYP1A1 but also CYP1A2. In addition, a negative bidirectional regulatory region was found to exist from -18,989 to -21,992 of the CYP1A1 gene. These data established that induction of human CYP1A1 and CYP1A2 is simultaneously controlled through bidirectional and common regulatory elements.

Construction of Several Human-derived Stable Cell Lines Displaying Distinct Profiles of CYP3A4 Induction

Cell lines which stably express reporter proteins through CYP3A4 gene activation have been developed for use in predicting CYP3A4 induction. Twelve clones showing distinct profiles on chemical-induced response were isolated. Among them, two clones showing high response for CYP3A4 inducers, namely clone 3-1-10 and 3-1-20, were further evaluated for their sensitivities, reproducibilities and applicabilities to predict CYP3A4 induction in human. Clone 3-1-10 showed higher response to rifampicin than to clotrimazole, whereas clone 3-1-20 had rather higher response to clotrimazole. Optimal plating density and highly reproducible response were observed at the range of 1.65-5.0 x 10(4) cell/cm2. Clear induction responses of more than ten chemicals were observed in both cell lines. The reporter activity was further dramatically increased after an introduction of human PXR. Induction with rifampicin was, however, not much altered between the absence and presence of hPXR. The luciferase activity remained unaltered and showed little fluctuation during the culture for more than 6 months. Due to the strikingly high sensitivity and reproducibility of this system, as compared to previously published systems, these HepG2-derived cell lines showing distinct response profiles as developed in the present study will offer high advantages for chemical screening of CYP3A4 inducibility.

Regulation of drug transporters by the farnesoid X receptor in mice

The farnesoid X receptor (FXR, NR1H4) regulates bile acid and lipid homeostasis by acting as an intracellular bile acid-sensing transcription factor, resulting in altered expression of enzymes and transporters involved in bile acid synthesis and transport. Here, we quantitatively analyzed the alterations in expression levels of drug transporters, mainly organic anion-transporting polypeptides (oatp), in wild-type and FXR-null mice to evaluate the role of FXR in their expression and regulation by cholic acid. Changes in the mRNA amounts in liver, kidney, small intestine, and testis in FXR-null mice fed with or without a supplement of 0.5% cholic acid in the diet were analyzed by semiquantitative RT-PCR. In FXR-null mice, the mRNA levels of oatp1, oatp2, oatp3, and octn1 were lower than those of wild-type mice in kidney and testis, while there was no difference in liver or small intestine. Cholic acid feeding led to significantly decreased levels of expression of oatp1 and oct1 and an increased level of expression of oatp2 in wild-type mouse liver. In FXR-null mice, oatp1 and other transporters were downregulated in liver, kidney, and testis, whereas small intestine ASBT, octn2, and pept1 were upregulated. Our results suggested that FXR is involved in the transcriptional regulation of oatp and other transporters in a tissue-specific manner. Furthermore, the effect of cholic acid treatment indicates the involvement of regulatory mechanism(s) other than FXR.

Prediction of differences in in vivo oral clearance of N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl] ethylamine monohydrochloride (NE-100) between extensive and poor metabolizers from in vitro metabolic data in human liver microsomes lacking CYP2D6 activity and recombinant CYPs

1. It has previously been reported that N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) was predominantly metabolized by cytochrome P450 (CYP) 2D6 in human liver microsomes (HLM). In the present study, the contribution of CYP forms involved in the formation of the major metabolites of NE-100 in human liver lacking CYP2D6 activity (PM-HLM) has been predicted by use of in vitro kinetic data on recombinant CYPs microsomes (rCYPs). 2. In PM-HLM, NE-100 is predicted to be metabolized to N-despropyl-NE-100 (NE-098), p-hydroxy-NE-100 (NE-152) and m-hydroxyl-NE-100 (NE-163), but not to O-demethy-NE-100 (NE-125), which is a major metabolite in pooled human liver microsomes (EM-HLM). The relative activity factor approach assumed that NE-098 formation is predominantly catalysed by CYP3A4 and CYP2C9 and the NE-152+163mix (a mixture of two hydroxylated metabolites, NE-152 and NE-163) formation is only catalysed by CYP3A4. 3. The predicted contribution rates of CYP3A4 and CYP2C9 for NE-098 formation were 58.1 and 34.6%, respectively, in PM-HLM. These predicted results were strongly supported by kinetic and inhibition studies using PM-HLM. The intrinsic clearance of NE-100 predicted from rCYPs (the predicted CLint-HLM-total) corresponded to those observed from EM- and PM-HLM (the observed CLint-HLM). 4. The in vivo oral clearance (CLoral) of NE-100 in extensive metabolizers and poor metabolizers of CYP2D6 was predicted to be 50times higher in extensive metabolizers than poor metabolizers using in vitro-in vivo scaling method based on the dispersion model. These data suggest that polymorphism of CYP2D6 might greatly affect NE-100 metabolism in vivo.

Selective suppressions of human CYP3A forms, CYP3A5 and CYP3A7, by troglitazone in HepG2 cells

Troglitazone is an insulin sensitizer and also known as an agonist of peroxisome proliferator-activated receptor-gamma (PPARgamma). In the present study, we have studied the influence of troglitazone on CYP3A form expressions in HepG2 cells for a model of human tissue. Interestingly, constitutively expressed forms of cytochrome P450, CYP3A5 and CYP3A7, were suppressed by the pretreatment of troglitazone but not of the related thiazolidinediones, pioglitazone and rosiglitazone in this cell line. A major liver CYP3A form, CYP3A4, was not detected in this cell line with and without troglitazone treatment. The troglitazone-mediated suppressions of CYP3A5 and CYP3A7 were found to be independent of expression levels of nuclear transcriptional factors, PXR, RXRalpha and PPARgamma. These results suggest that the selectively suppressive effects of troglitazone on CYP3A5 and CYP3A7 expressions may be caused by a novel pathway.

Modulation of UDP-Glucuronosyltransferase 2B7 Function by Cytochrome P450s in Vitro: Differential Effects of CYP1A2, CYP2C9 and CYP3A4

Effects of cytochrome P450 isoforms, CYP1A2, CYP2C9 and CYP3A4, on the catalytic activity of UDP-glucuronosyltransferase 2B7 (UGT2B7) expressed in COS cell microsomal membranes were investigated using morphine as a substrate. When detergent-untreated COS cell microsomes were used as the enzyme source, the activity of morphine-3-glucuronide formation by UGT2B7 was reduced by addition of purified CYP1A2 and CYP2C9 in a concentration-dependent manner. The effect of CYP1A2 was greater than that of CYP2C9. In contrast, exogenous CYP3A4 had little effect on morphine glucuronidation activity. These results suggest that CYP1A2 and CYP2C9 have ability to modify UGT2B7 function. However, the mechanism(s) underlying the modulation of UGT2B7 function by these P450s seems to differ from that by CYP3A4.

Modulation of UDP-glucuronosyltransferase function by cytochrome P450: evidence for the alteration of UGT2B7-catalyzed glucuronidation of morphine by CYP3A4

Modulation of UDP-glucuronosyltransferase 2B7 (UGT2B7)-catalyzed morphine glucuronidation by cytochrome P450 (P450) was studied. The effects of P450 isozymes on the kinetic parameters of UGT2B7-catalyzed glucuronidation of the morphine 3-hydroxyl group were examined by simultaneous expression of UGT2B7 and either CYP3A4, -1A2, or -2C9 in COS-1 cells. Although coexpression of CYP3A4 with UGT2B7 had little effect on Vmax, the Km was increased by about 9.8-fold compared with the UGT2B7 single expression system. The other P450 isozymes (CYP1A2 and CYP2C9) had some effects on Km and Vmax values. Immunoprecipitation of UGT from solubilized human liver microsomes resulted in coprecipitation of CYP3A4 with UGT2B7. The protein-protein interaction between CYP3A4 and UGT2B7 was further confirmed by overlay assay using glutathione S-transferase-CYP3A4 fusion protein. Addition of CYP3A4 to untreated COS microsomes expressing UGT2B7 had no or minor effects on morphine glucuronidation. In contrast, the formation of morphine-3-glucuronide by detergent-treated microsomes from COS-1 cells expressing UGT2B7 was reduced by CYP3A4, whereas the formation of the 6-glucuronide was enhanced. These results strongly suggest that 1) the glucuronidation activity of UGT2B7 toward morphine is specifically modulated by interaction with CYP3A4 in microsomal membranes and that 2) CYP3A4 alters UGT2B7 regioselectivity so that the ratio of morphine activation/detoxication is increased. This study provides the first evidence that P450 is not only involved in oxidation of drugs but also modulates the function of UGTs.

Differences in transactivation between rat CYP3A1 and human CYP3A4 genes by human pregnane X receptor

In an assay system using a human CYP3A4 reporter constructed with the promoter (+11 nt to -362 nt) and enhancer (-7.2 knt to -7.8 knt) regions including everted repeat separated by six nucleotides (ER-6) and direct repeat separated by three nucleotides (DR-3) motifs, the CYP3A4 transactivation was detected without overexpression of any nuclear receptors in rifampicin-treated HepG2 cells. Overexpression of human pregnane X receptor (hPXR) enhanced the transactivation. Rat CYP3A1 reporter constructed with the promoter region (+31 nt to -171 nt) including both DR-3 and ER-6 motifs was, however, not transactivated in rifampicin-treated cells, even after overexpression of hPXR. Although overexpression of retinoid X receptor alpha (RXRalpha) had no clear effect for both CYP3A reporters, co-expression of apolipoprotein AI regulatory protein-1 (ARP-1) with hPXR resulted in the rifampicin-induced transactivation of the CYP3A1 reporter. A truncated CYP3A4 reporter retaining the both motifs showed the rifampicin-induced transactivation by overexpression of hPXR and ARP-1, while the transactivation in hPXR-overexpressed cells was not observed. These results support the idea that a nuclear receptor other than RXRalpha may play a role in the CYP3A transactivation together with hPXR. The present study also suggests the involvement of a novel cis-element in the hPXR-mediated CYP3A4 transactivation.

Role of farnesoid X receptor in the enhancement of canalicular bile acid output and excretion of unconjugated bile acids: a mechanism for protection against cholic acid-induced liver toxicity

Mice lacking the farnesoid X receptor (FXR) involved in the maintenance of hepatic bile acid levels are highly sensitive to cholic acid-induced liver toxicity. Serum aspartate aminotransferase (AST) activity was elevated 15.7-fold after feeding a 0.25% cholic acid diet, whereas only slight increases in serum AST (1.7- and 2.5-fold) were observed in wild-type mice fed 0.25 and 1% cholic acid diet, respectively. Bile salt export pump mRNA and protein levels were increased in wild-type mice fed 1% cholic acid diet (2.1- and 3.0-fold) but were decreased in FXR-null mice fed 0.25% cholic acid diet. The bile acid output rate was 2.0- and 3.7-fold higher after feeding of 0.25 and 1.0% cholic acid diet in wild-type mice, respectively. On the other hand, no significant increase in bile acid output rate was observed in FXR-null mice fed 0.25% cholic acid diet in contrast to a significant decrease observed in mice fed a 1.0% cholic acid diet in spite of the markedly higher levels of hepatic tauro-conjugated bile acids. Unconjugated cholic acid was not detected in the bile of wild-type mice fed a control diet, but it was readily detected in wild-type mice fed 1% cholic acid diet. The ratio of biliary unconjugated cholic acid to total cholic acid (unconjugated cholic acid and tauro-conjugated cholic acid) reached 30% under conditions of hepatic taurine depletion. These results suggest that the cholic acid-induced enhancement of canalicular bile acid output rates and excretion of unconjugated bile acids are involved in adaptive responses for prevention of cholic acid-induced toxicity.

Unique properties of a renal sulfotransferase, St1d1, in dopamine metabolism

Although catecholamine sulfation is higher in the kidney than in the liver of mice, no detectable amounts of previously reported sulfotransferases (STs) such as St1a, St1b, St1c, and St1e were expressed in mouse kidney cytosols. A new sulfotransferase (St1d1) cDNA was isolated from kidney cDNA library of BALB/c strain by reverse transcription-polymerase chain reaction (RTPCR) using information from expressed sequence tags (EST) database. The cDNA sequence resembled that of cDNA reported previously (AA238910) (Sakakibara et al., 1998) but differed in two amino acids, (206)Q/K and (216)Y/F, in the deduced amino acid sequence. The St1d1 expressed had unique substrate specificities for catecholamine derivatives, which preferred their deaminated metabolites rather than their parent amines. St1d1 showed the highest activity toward 3,4-dihydroxyphenylacetic acid (230.2 +/- 2.69 nmol/mg/min) among the examined substrate. St1d1 protein was abundant in kidney, followed by liver, lung, and uterus. Furthermore, an addition of anti-St1d1 serum in the cytosolic reaction mixture resulted in complete inhibition of the sulfotransferase activity suggesting a major role of St1d1 on catecholamine sulfations. No human ST1D ortholog was detected at both mRNA and protein levels, although ST1A5 selectively catalyzing parent amine sulfation was detected in human kidney. These results indicate the functional basis of sulfation and the clear species difference on renal catecholamine metabolisms in mice and humans.

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.

Inhibition of cytochrome P450 by furanocoumarins in grapefruit juice and herbal medicines

Furanocoumarins (psoralens) exist in various plants and some of them are used to cure skin diseases. These chemicals draw attentions recently because of their abilities to arouse drug interaction through inhibition of cytochrome P450. Grapefruit juice is a well-known example for food-drug interaction. But in vitro and in vivo studies have shown that the causative components are mainly furanocoumarin derivatives with geranyloxy side chains. In vitro experiments confirmed that furanocoumarins from grapefruit juice are both competitive and mechanism-based inhibitors of CYP3A4. Although the inhibition appeared to be stronger in the dimers than that in the monomers, all contribute comprehensively to the grapefruit juice-drug interaction. Further experiments with other furanocoumarins and related citrus fruits or umbelliferous herbal medicines indicate that drug interaction might also occur with stuffs other than grapefruit juice, especially with traditional medicine.

Grapefruit juice intake does not enhance but rather protects against aflatoxin B1-induced liver DNA damage through a reduction in hepatic CYP3A activity

Influence of grapefruit juice intake on aflatoxin B1 (AFB1)-induced liver DNA damage was examined using a Comet assay in F344 rats given 5 mg/kg AFB1 by gavage. Rats allowed free access to grapefruit juice for 5 days prior to AFB1 administration resulted in clearly reduced DNA damage in liver, to 65% of the level in rats that did not receive grapefruit juice. Furthermore, rats treated with grapefruit juice extract (100 mg/kg per os) for 5 days prior to AFB1 treatment also reduced the DNA damage to 74% of the level in rats that did not receive grapefruit juice. No significant differences in the portal blood and liver concentrations of AFB1 were observed between grapefruit juice intake rats and the controls. In an Ames assay with AFB1 using Salmonella typhimurium TA98, lower numbers of revertant colonies were detected with hepatic microsomes prepared from rats administered grapefruit juice, compared with those from control rats. Microsomal testosterone 6beta-hydroxylation was also lower with rats given grapefruit juice than with control rats. Immunoblot analyses showed a significant decrease in hepatic CYP3A content, but not CYP1A and CYP2C content, in microsomes of grapefruit juice-treated rats than in non-treated rats. No significant difference in hepatic glutathione S-transferase (GST) activity and glutathione content was observed in the two groups. GSTA5 protein was not detected in hepatic cytosol of the two groups. In microsomal systems, grapefruit juice extract inhibited AFB1-induced mutagenesis in the presence of a microsomal activation system from livers of humans as well as rats. These results suggest that grapefruit juice intake suppresses AFB1-induced liver DNA damage through inactivation of the metabolic activation potency for AFB1 in rat liver.

In vitro inhibition of human small intestinal and liver microsomal astemizole O-demethylation: different contribution of CYP2J2 in the small intestine and liver

1. The effects of chemical agents on the metabolism of the antihistamine drug astemizole were investigated to evaluate drug-drug interactions. 2. Chemical inhibitors of astemizole O-demethylation were screened using the small intestinal and liver microsomes from rabbit as an animal model for the first-pass metabolism of humans. In the rabbit small intestine, astemizole O-demethylation was clearly inhibited by ebastine, arachidonic acid, alpha-naphthoflavone, ketoconazole, tranylcypromine, troglitazone and terfenadine. 3. In humans, these inhibitors also reduced microsomal astemizole O-demethylation in both the small intestine and liver. However, the inhibition rate of almost all these chemicals were clearly greater in the small intestine than in the liver. Thus, a different contribution of cytochrome p450 in each tissue is suggested. 4. All the chemicals inhibited astemizole O-demethylation in recombinant CYP2J2 microsomes. The results suggest that CYP2J2 is involved in astemizole O-demethylation in both the human small intestine and liver; however, the contribution in the liver is lower than in the small intestine. The effects of the CYP2J2 inhibitors during first-pass metabolism may be more important in the small intestine than in the liver. Since all the inhibition profiles of astemizole O-demethylation were different in the liver and small intestine, involvement of another p450 in astemizole O-demethylation in human liver may be speculated. 5. In the rabbit microsomal systems, the same metabolites found in humans were qualitatively detected and the inhibition profiles of the chemical agents in the microsomes resembled that of humans.

Protective role of hydroxysteroid sulfotransferase in lithocholic acid-induced liver toxicity

Supplement of 1% lithocholic acid (LCA) in the diet for 5-9 days resulted in elevated levels of the marker for liver damage aspartate aminotransferase and alkaline phosphatase activities in both farnesoid X receptor (FXR)-null and wild-type female mice. The levels were clearly higher in wild-type mice than in FXR-null mice, despite the diminished expression of a bile salt export pump in the latter. Consistent with liver toxicity marker activities, serum and liver levels of bile acids, particularly LCA and taurolithocholic acid, were clearly higher in wild-type mice than in FXR-null mice after 1% LCA supplement. Marked increases in hepatic sulfating activity for LCA (5.5-fold) and hydroxysteroid sulfotransferase (St) 2a (5.8-fold) were detected in liver of FXR-null mice. A 7.4-fold higher 3alpha-sulfated bile acid concentration was observed in bile of FXR-null mice fed an LCA diet compared with that of wild-type mice. Liver St2a content was inversely correlated with levels of alkaline phosphatase. In contrast, microsomal LCA 6beta-hydroxylation was not increased and was in fact lower in FXR-null mice compared in wild-type mice. Clear decreases in mRNA encoding sodium taurocholate cotransporting polypeptide, organic anion transporting polypeptide 1, and liver-specific organic anion transporter-1 function in bile acid import were detected in LCA-fed mice. These transporter levels are higher in FXR-null mice than wild-type mice after 1% LCA supplement. No obvious changes were detected in the Mrp2, Mrp3, and Mrp4 mRNAs. These results indicate hydroxysteroid sulfotransferase-mediated LCA sulfation as a major pathway for protection against LCA-induced liver damage. Furthermore, Northern blot analysis using FXR-null, pregnane X receptor-null, and FXR-pregnane X receptor double-null mice suggests a repressive role of these nuclear receptors on basal St2a expression.

Thalidomide-induced suppression of embryo fibroblast proliferation requires CYP1A1-mediated activation

An enzyme involved in the metabolic activation of thalidomide has been investigated using embryo fibroblast proliferation as a marker. Thalidomide (30 microM) induced-suppression of embryo fibroblast proliferation was detected in the presence of liver microsomes from rabbit but not from mouse. The addition of a selective inhibitor of CYP1A, alpha-naphthoflavone (4 microM), or furafylline (4 microM), to the incubation mixture abolished the thalidomide-induced suppression. Furthermore, addition of anti-rat CYP1A1 antibody also resulted in inhibition of suppression. The thalidomide-induced suppression was also observed with the microsomal system from human HepG2 cells pretreated with 3-methylcholanthrene (10 microM) but not from those pretreated with the vehicle. Both CYP1A1 and CYP1A2 proteins were detected in the rabbit liver microsomes by immunoblot analyses, but only CYP1A2 protein was detected in the mouse liver microsomes. In addition, CYP1A1 protein was detected in microsomes from HepG2 cells pretreated with 3-methylcholanthrene but not with the vehicle. These results strongly suggest the involvement of CYP1A1 in the thalidomide-induced suppression of embryo fibroblast proliferation.

Metabolism of N,N-Dipropyl-2-[4-Methoxy-3-(2-Phenyl-Ethoxy)-Phenyl]-Ethyl-Amine-Monohydrochloride (NE-100), A Novel Sigma Ligand: Contribution of Cytochrome P450 Forms Involved in the Formation of Individual Metabolites in Human Liver and Small Intestine

In the present study, human cytochrome P450 (CYP) forms involved in producing the primary metabolites of NE-100 were identified. Major metabolites of NE-100 in human liver microsomes (HLM) were N-depropylation of NE-100 (NE-098), p-hydroxylation of phenyl group of NE-100 (NE-152), m-hydroxylation of phenyl group of NE-100 (NE-163) and O-demethylation of NE-100 (NE-125). Judging from the correlation and inhibition studies, NE-125 and NE-152+163mix formations were predominantly mediated by CYP2D6 and NE-098 formation was mediated by multiple CYP forms at a low NE-100 concentration (0.1 microM) in the HLM. According to relative activity factor (RAF) approaches, all these reactions were predominantly catalyzed by CYP2D6 at a substrate concentration assuming a plasma level of NE-100 (K(m)>>S) in case of the human liver. Depending on the increase in NE-100 concentrations, the rate of contribution for NE-098 and NE-152+163mix formations increased in CYP3A4, although the predominant contribution of CYP2D6 for NE-125 formation did not change. In human intestinal microsomes (HIM), NE-100 was mainly metabolized to NE-098 and NE-152+163mix by CYP3A4. The intrinsic clearance for their formations in HIM was 3.2 and 14.9 times less than those in HLM, respectively, and no formation of NE-125 was observed in HIM. These results strongly suggest that CYP2D6 is the predominant form for NE-100 metabolism in the human liver in in vivo conditions (K(m)>>S) and the liver plays a more important role than does the small intestine in the first pass metabolism.