The influence of the long-term chemical activation of the nuclear receptor pregnane X receptor (PXR) on liver carcinogenesis in mice

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are nuclear receptors that are highly expressed in the liver and activated by numerous chemicals. While CAR activation by its activators, such as phenobarbital (PB), induces hepatocyte proliferation and liver carcinogenesis in rodents, it remains unclear whether PXR activation drives liver cancer. To investigate the influence of PXR activation on liver carcinogenesis, we treated mice with the PXR activator pregnenolone 16α-carbonitrile (PCN) with or without PB following tumor initiation with diethylnitrosamine (DEN). After 20 weeks of treatment, preneoplastic lesions detected by immunostaining with an anti-KRT8/18 antibody were observed in PB-treated but not PCN-treated mice, and PCN cotreatment augmented the formation of preneoplastic lesions by PB. After 35 weeks of treatment, macroscopic observations indicated that PB-treated and PB/PCN-cotreated mice had increased numbers of liver tumors compared to control and PCN-treated mice. In the pathological analyses of liver sections, all the mice in the PB and PB/PCN groups developed carcinoma and/or eosinophilic adenoma, but in the PB/PCN group, the multiplicity of carcinoma and eosinophilic adenoma was significantly reduced and the size of carcinoma showed a tendency to decrease. No mouse in the control or PCN-treated group developed such tumors. Differentially expressed gene (DEG) and gene set enrichment analyses in combination with RNA sequencing suggested the increased expression of genes related to epithelial-mesenchymal transition (EMT) in mice cotreated with PCN and PB compared to those treated with PB alone. Changes in the hepatic mRNA levels of epithelial marker genes supported the results of the transcriptome analyses. In conclusion, the present results suggest that PXR activation does not promote hepatocarcinogenesis in contrast to CAR and rather attenuates CAR-mediated liver cancer development by suppressing the EMT of liver cancer cells in rodents.

PXR Functionally Interacts with NF-κB and AP-1 to Downregulate the Inflammation-Induced Expression of Chemokine CXCL2 in Mice

Pregnane X receptor (PXR) is a liver-enriched xenobiotic-responsive transcription factor. Although recent studies suggest that PXR shows anti-inflammatory effects by suppressing nuclear factor kappa B (NF-κB), the detailed mechanism remains unclear. In this study, we aimed to elucidate this mechanism. Mice were treated intraperitoneally with the PXR agonist pregnenolone 16α-carbonitrile (PCN) and/or carbon tetrachloride (CCl₄). Liver injury was evaluated, and hepatic mRNA levels were determined via quantitative reverse transcription polymerase chain reaction. Reporter assays with wild-type and mutated mouse Cxcl2 promoter-containing reporter plasmids were conducted in 293T cells. Results showed that the hepatic expression of inflammation-related genes was upregulated in CCl₄-treated mice, and PCN treatment repressed the induced expression of chemokine-encoding Ccl2 and Cxcl2 among the genes investigated. Consistently, PCN treatment suppressed the increased plasma transaminase activity and neutrophil infiltration in the liver. In reporter assays, tumor necrosis factor-α-induced Cxcl2 expression was suppressed by PXR. Although an NF-κB inhibitor or the mutation of an NF-κB-binding motif partly reduced PXR-dependent suppression, the mutation of both NF-κB and activator protein 1 (AP-1) sites abolished it. Consistently, AP-1-dependent gene transcription was suppressed by PXR with a construct containing AP-1 binding motifs. In conclusion, the present results suggest that PXR exerts anti-inflammatory effects by suppressing both NF-κB- and AP-1-dependent chemokine expression in mouse liver.

Regulation of Hepatic Long Noncoding RNAs by Pregnane X Receptor and Constitutive Androstane Receptor Agonists in Mouse Liver

Altered expression of long noncoding RNAs (lncRNAs) by environmental chemicals modulates the expression of xenobiotic biotransformation-related genes and may serve as therapeutic targets and novel biomarkers of exposure. The pregnane X receptor (PXR/NR1I2) is a critical xenobiotic-sensing nuclear receptor that regulates the expression of many drug-processing genes, and it has similar target-gene profiles and DNA-binding motifs with another xenobiotic-sensing nuclear receptor, namely, constitutive andronstrane receptor (CAR/Nr1i3). To test our hypothesis that lncRNAs are regulated by PXR in concert with protein-coding genes (PCGs) and to compare the PXR-targeted lncRNAs with CAR-targeted lncRNAs, RNA-Seq was performed from livers of adult male C57BL/6 mice treated with corn oil, the PXR agonist PCN, or the CAR agonist 1, 4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). Among 125,680 known lncRNAs, 3843 were expressed in liver, and 193 were differentially regulated by PXR (among which 40% were also regulated by CAR). Most PXR- or CAR-regulated lncRNAs were mapped to the introns and 3'-untranslated regions (UTRs) of PCGs, as well as intergenic regions. Combining the RNA-Seq data with a published PXR chromatin immunoprecipitation coupled with high-throughput sequencing; cytochrome P450 (P450; ChIP-Seq) data set, we identified 774 expressed lncRNAs with direct PXR-DNA binding sites, and 26.8% of differentially expressed lncRNAs had changes in PXR-DNA binding after PCN exposure. De novo motif analysis identified colocalization of PXR with liver receptor homolog (LRH-1), which regulates bile acid synthesis after PCN exposure. There was limited overlap of PXR binding with an epigenetic mark for transcriptional activation (histone-H3K4-di-methylation, H3K4me2) but no overlap with epigenetic marks for transcriptional silencing [H3 lysine 27 tri-methylation (H3K27me3) and DNA methylation]. Among differentially expressed lncRNAs, 264 were in proximity of PCGs, and the lncRNA-PCG pairs displayed a high coregulatory pattern by PXR and CAR activation. This study was among the first to demonstrate that lncRNAs are regulated by PXR and CAR activation and that they may be important regulators of PCGs involved in xenobiotic metabolism.

The effect of pregnane X receptor agonists on postprandial incretin hormone secretion in rats and humans

We recently showed that pregnane X receptor (PXR) agonists cause hyperglycaemia during oral glucose tolerance test (OGTT) in rats and healthy volunteers (Rifa-1 study). We now aimed to determine if the secretion of incretin hormones, especially glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP), are affected by PXR agonists since these gut-secreted hormones are major regulators of postprandial glucose metabolism. The Rifa-2 study had a one-phase, open-label design. Twelve subjects were given 600 mg of rifampicin a day for a week. OGTT with glucose, insulin, and incretin hormone measurements was performed before and after the rifampicin dosing. Incretins and insulin were analysed in previously collected rat OGTT samples after pregnenolone 16α-carbonitrile (PCN) or control treatment for 4 days. Rifampicin treatment did not affect glucose, insulin, GLP-1, GIP, glucagon, and peptide YY levels statistically significantly. Incremental AUCs (AUCincr) of glucose and insulin tended to increase (41% increase in glucose AUCincr, P = 0.21, 95% confidence interval (CI) of the difference -47, 187; 24% increase in insulin AUCincr, P = 0.084, CI of the difference -110, 1493). Glucagon AUC was increased in women (53% increase, P = 0.028) and decreased in men (19% decrease, P < 0.001) after rifampicin dosing. In combined analysis of human Rifa-1 and Rifa-2 studies, glucose AUCincr was elevated by 63% (P = 0.010) and insulin AUCincr by 37% (P = 0.011). PCN increased rat insulin level at 60 min time point but did not affect incretin and insulin AUCs statistically significantly. In conclusion, PXR agonists do not affect the secretion of incretin hormones. The regulation of glucagon secretion by PXR may be sexually dimorphic in humans. The mechanism of disrupted glucose metabolism induced by PXR activation requires further study.

Xenobiotic-induced hepatocyte proliferation associated with constitutive active/androstane receptor (CAR) or peroxisome proliferator-activated receptor α (PPARα) is enhanced by pregnane X receptor (PXR) activation in mice

Xenobiotic-responsive nuclear receptors pregnane X receptor (PXR), constitutive active/androstane receptor (CAR) and peroxisome proliferator-activated receptor α (PPARα) play pivotal roles in the metabolic functions of the liver such as xenobiotics detoxification and energy metabolism. While CAR or PPARα activation induces hepatocyte proliferation and hepatocarcinogenesis in rodent models, it remains unclear whether PXR activation also shows such effects. In the present study, we have investigated the role of PXR in the xenobiotic-induced hepatocyte proliferation with or without CAR activation by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and phenobarbital, or PPARα activation by Wy-14643 in mice. Treatment with TCPOBOP or phenobarbital increased the percentage of Ki-67-positive nuclei as well as mRNA levels of cell proliferation-related genes in livers as expected. On the other hand, treatment with the PXR activator pregnenolone 16α-carbonitrile (PCN) alone showed no such effects. Surprisingly, PCN co-treatment significantly augmented the hepatocyte proliferation induced by CAR activation with TCPOBOP or phenobarbital in wild-type mice but not in PXR-deficient mice. Intriguingly, PXR activation also augmented the hepatocyte proliferation induced by Wy-14643 treatment. Moreover, PCN treatment increased the RNA content of hepatocytes, suggesting the induction of G0/G1 transition, and reduced mRNA levels of Cdkn1b and Rbl2, encoding suppressors of cell cycle initiation. Our present findings indicate that xenobiotic-induced hepatocyte proliferation mediated by CAR or PPARα is enhanced by PXR co-activation despite that PXR activation alone does not cause the cell proliferation in mouse livers. Thus PXR may play a novel and unique role in the hepatocyte/liver hyperplasia upon exposure to xenobiotics.

Activation of pregnane X receptor by pregnenolone 16 α-carbonitrile prevents high-fat diet-induced obesity in AKR/J mice

Pregnane X receptor (PXR) is known to function as a xenobiotic sensor to regulate xenobiotic metabolism through selective transcription of genes responsible for maintaining physiological homeostasis. Here we report that the activation of PXR by pregnenolone 16α-carbonitrile (PCN) in AKR/J mice can prevent the development of high-fat diet-induced obesity and insulin resistance. The beneficial effects of PCN treatment are seen with reduced lipogenesis and gluconeogenesis in the liver, and lack of hepatic accumulation of lipid and lipid storage in the adipose tissues. RT-PCR analysis of genes involved in gluconeogenesis, lipid metabolism and energy homeostasis reveal that PCN treatment on high-fat diet-fed mice reduces expression in the liver of G6Pase, Pepck, Cyp7a1, Cd36, L-Fabp, Srebp, and Fas genes and slightly enhances expression of Cyp27a1 and Abca1 genes. RT-PCR analysis of genes involved in adipocyte differentiation and lipid metabolism in white adipose tissue show that PCN treatment reduces expression of Pparγ2, Acc1, Cd36, but increases expression of Cpt1b and Pparα genes in mice fed with high-fat diet. Similarly, PCN treatment of animals on high-fat diet increases expression in brown adipose tissue of Pparα, Hsl, Cpt1b, and Cd36 genes, but reduces expression of Acc1 and Scd-1 genes. PXR activation by PCN in high-fat diet fed mice also increases expression of genes involved in thermogenesis in brown adipose tissue including Dio2, Pgc-1α, Pgc-1β, Cidea, and Ucp-3. These results verify the important function of PXR in lipid and energy metabolism and suggest that PXR represents a novel therapeutic target for prevention and treatment of obesity and insulin resistance.

[Effect of pravastatin on transportation of scutellarin in mouse liver and its mechanism]

This study is to investigate the transportation of scutellarin in cell and live models and study on mechanism of absorption and transport of scutellarin in mouse liver. The concentration of scutellarin in plasma and liver from control and pretreated groups was determined by high performance liquid chromatography. The uptake of scutellarin was examined in control hepatocytes group, induced hepatocytes group and induced hepatocytes plus pravastatin group. Pravastatin can affect the pharmacokinetics of scutellarin in mouse: CL is decreased while AUC is increased. The scutellarin absorption of hepatocyte induced group was higher than that of control group, but was decreased in the group with pravastatin added. The research showed that there was potential drug interaction between pravastatin and scutellarin. The drugs may compete for oatp2 mediated transport pathway consisted in the uptake of scutellarin in liver.

Pregnane X receptor as a therapeutic target to inhibit androgen activity

The androgen-androgen receptor signaling pathway plays an important role in the pathogenesis of prostate cancer. Accordingly, androgen deprivation has been the most effective endocrine therapy for hormone-dependent prostate cancer. Here, we report a novel pregnane X receptor (PXR)-mediated and metabolism-based mechanism to reduce androgenic tone. PXR is a nuclear receptor previously known as a xenobiotic receptor regulating the expression of drug metabolizing enzymes and transporters. We showed that genetic (using a PXR transgene) or pharmacological (using a PXR agonist) activation of PXR lowered androgenic activity and inhibited androgen-dependent prostate regeneration in castrated male mice that received daily injections of testosterone propionate by inducing the expression of cytochrome P450 (CYP)3As and hydroxysteroid sulfotransferase (SULT)2A1, which are enzymes important for the metabolic deactivation of androgens. In human prostate cancer cells, treatment with the PXR agonist rifampicin (RIF) inhibited androgen-dependent proliferation of LAPC-4 cells but had little effect on the growth of the androgen-independent isogenic LA99 cells. Down-regulation of PXR or SULT2A1 in LAPC-4 cells by short hairpin RNA or small interfering RNA abolished the RIF effect, indicating that the inhibitory effect of RIF on androgens was PXR and SULT2A1 dependent. In summary, we have uncovered a novel function of PXR in androgen homeostasis. PXR may represent a novel therapeutic target to lower androgen activity and may aid in the treatment and prevention of hormone-dependent prostate cancer.

Assessment of cytochrome P450 fluorometric substrates with rainbow trout and killifish exposed to dexamethasone, pregnenolone-16alpha-carbonitrile, rifampicin, and beta-naphthoflavone

Cytochrome P450s (CYPs) are important xenobiotic metabolizing proteins. While their functions are well understood in mammals, CYP function in non-mammalian vertebrate systems is much less defined, with function often inferred from mammalian data, assuming similar function across vertebrate species. In this study, we investigate whether in vivo treatment with known mammalian CYP inducers can alter the in vitro catalytic activity of fish microsomes using eleven fluorescent CYP-mediated substrates. We investigate the basal metabolism and induction potential for hepatic CYPs in two fish species, rainbow trout (Oncorhynchus mykiss) and killifish (Fundulus heteroclitus). Species differences were found in the baseline metabolism of these substrates. Killifish have significantly higher metabolic rates for all tested substrates except 7-benzyloxyquinoline and 7-benzyloxy-4-trifluoromethylcoumarin (both mammalian CYP3A substrates); significant differences were also seen between male and female killifish. Treatment with dexamethasone, pregnenolone-16alpha-carbonitrile, and rifampicin did not cause broad, measurable CYP induction in either fish species. In trout, dexamethasone (100 mg kg(-1)) significantly induced 3-cyano-7-ethoxycoumarin metabolism and rifampicin (100 mg kg(-1)) induced the dealkylation of 7-methoxyresorufin, although both were highly variable. Female killifish exposed to pregnenolone-16alpha-carbonitrile (100 mg kg(-1)) showed significantly higher metabolism of 7-pentoxyresorufin. Overall, dexamethasone, pregnenolone-16alpha-carbonitrile and rifampicin did not appear to consistently increase CYP activity in fish. Trout treated with 10 or 50 mg kg(-1) beta-naphthoflavone (BNF), a CYP1A inducer, showed significantly induced activity across almost all substrates tested, exceptions being 7-benzyloxyquinoline, 7-benzyloxy-4-trifluoromethylcoumarin and dibenzylfluorescein. 7-Methoxy-4-(aminomethyl)coumarin, a typical CYP2D substrate in mammals, was not metabolized by untreated fish liver microsomes; however, treatment with BNF significantly induced the metabolism of this substrate in trout. Induced substrate metabolism in BNF-treated microsomes was only correlated across selective substrates, suggesting that BNF induces multiple CYPs in fish liver. These include the known BNF inducible CYP1s plus a number of as yet unidentified fish CYPs. Overall, many of these catalytic assays could be valuable tools for identification of the function of specific CYP subfamilies and individual isoforms in fish.

Role for enhanced faecal excretion of bile acid in hydroxysteroid sulfotransferase-mediated protection against lithocholic acid-induced liver toxicity

The efficient clearance of toxic bile acids such as lithocholic acid (LCA) requires drug-metabolizing enzymes. We therefore assessed the influence of pregnenolone 16alpha-carbonitrile (PCN) treatment on LCA-induced hepatotoxicity and disposition of LCA metabolites using female farnesoid X receptor (FXR)-null and wild-type mice. Marked decreases in serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, and hepatic tauroLCA (TLCA) concentrations were found in LCA-fed wild-type mice co-treated with PCN. Whereas induction of Cyp3a and hydroxysteroid sulfotransferase (Sult2a) proteins was observed in FXR-null and wild-type mice, clear increases in biliary 3alpha-sulfated TLCA but not total 6alpha-hydroxy LCA (taurohyodeoxycholic acid and hyodeoxycholic acid) were only observed in PCN-treated wild-type mice. Biliary 3alpha-sulfated TLCA output rate was increased 7.2-fold, but accounts for only 4.2% of total bile acid output rate in LCA and PCN-co-treated wild-type mice. Total 3alpha-sulfated LCA (LCA and TLCA) was, however, the most abundant bile acid component in faeces suggesting that efficient faecal excretion of biliary 3alpha-sulfated TLCA through escape from enterohepatic circulation. FXR-null mice, which have constitutively high levels of the Sult2a protein, were fed a diet supplemented with 1% LCA and 0.4% dehydroepiandrosterone (DHEA), a typical Sult2a substrate/inhibitor. The faecal total 3alpha-sulfated bile acid excretion was reduced to 62% of FXR-null mice fed only the LCA diet. Hepatic TLCA concentration and serum AST activity were significantly higher in FXR-null mice fed DHEA and LCA diet than in FXR-null mice fed the LCA diet or DHEA diet. These results suggest that hepatic formation of 3alpha-sulfated TLCA is a crucial factor for protection against LCA-induced hepatotoxicity.

Effect of Pregnane X Receptor Ligand on Pharmacokinetics of Substrates of Organic Cation Transporter Oct1 in Rats

Because rat organic cation transporter 1 (Oct1, SLC22a1) is expressed mainly in the liver and mediates drug transport, its activity may determine the hepatic handling of cationic drugs. Here, we studied the regulation mechanism of the expression of Oct1, focusing on the nuclear receptors. In vitro studies using cultured hepatocytes indicated that expression of Oct1 was up-regulated by treatment with pregnenolone-16 alpha-carbonitrile (PCN) and by overexpression of rat pregnane X receptor (PXR). In addition, isolated rat hepatocytes exhibited an increase of 1-methyl-4-phenylpyridinium (MPP(+)) uptake on treatment with PCN. When rats were subcutaneously administered PCN, an increase of biliary excretion clearance and distribution volume was observed for drugs such as MPP(+), metformin, and tetraethylammonium, although the effects on pharmacokinetic parameters were variable among the tested drugs. In addition, the expression of Oct2 in kidney was increased by treatment with PCN. Thus, PXR ligands appear to regulate the expression of organic cation transporters in rats and thereby to influence the pharmacokinetic properties of cationic drugs. Because PXR ligands include various clinically used drugs, alterations of hepatic drug handling may arise from interactions between cationic drugs that are substrates of Oct1 and ligands of PXR.

Effect of cytochrome P450 (CYP) inducers on caffeine metabolism in the rat

Our previous studies, carried out using rat cDNA-expressed cytochrome P450 (CYP) isoforms, liver microsomes and specific CYP inhibitors, showed that the 1-N- and 3-N-demethylation of caffeine at a therapeutic concentration was predominantly catalyzed by CYP1A2 and CYP2C, its 7-N-demethylation was governed by P450s of the CYP2C subfamily, while its 8-hydroxylation was specifically mediated by CYP1A2. The present study was aimed at corroborating the above-described results using another experimental model, i.e. a study of caffeine metabolism in the liver microsomes and specific CYP inducers. Animals received one of the following inducers: beta-naphthoflavone (100 mg/kg i.p. for 4 days), phenobarbital (10 mg/kg for 6 days or 100 mg/kg i.p. for 4 days), pregnenolone 16alpha-carbonitrile (100 mg/kg i.p. for 4 days) or 15% ethanol ( approximately 11 g/kg in drinking water for 6 days). Sixteen hours after the last dose of an inducer liver microsomes were prepared and the caffeine metabolism and CYP isoform activities (testosterone 2alpha-, 2beta-, 6beta-, 7alpha-, 16beta-hydroxylation and warfarin 7-hydroxylation) were investigated. beta-Naphthoflavone (mainly a CYP1A inducer and CYP2C11 inhibitor) potently accelerated the metabolism of caffeine, the effect on 7-N-demethylation being the weakest. Moreover, the influence of beta-naphthoflavone on caffeine metabolism was more potent at the substrate concentration of 100 microM than 800 microM, in particular in the case of 7-N-demethylation and 8-hydroxylation. Pregnenolone-16alpha-carbonitrile (mainly a CYP3A inducer and CYP2C11 inhibitor) moderately induced 8-hydroxylation only. Phenobarbital (an inducer of CYP2B and other CYPs and a CYP2C11 inhibitor) moderately stimulated the metabolism of caffeine, but practically did not affect 7-N-demethylation. Ethanol (mainly a CYP2E1 inducer) modestly increased the rates of the N-demethylation reactions. The presently obtained data confirm the pivotal role of CYP1A2 in the metabolism of caffeine, as well as the involvement of CYP3A in the 8-hydroxylation of caffeine and that of CYP2C11 in its 7-N-demethylation.

Hepatoprotective role of PXR activation and MRP3 in cholic acid-induced cholestasis

BACKGROUND AND PURPOSE

Activation of the pregnane X receptor (PXR) has been shown to protect against cholestatic hepatotoxicity. As PXR alters the expression of numerous hepatic bile acid transporters, we sought to delineate their potential role in hepatoprotection.

EXPERIMENTAL APPROACH

Wild-type (PXR+/+) and PXR-null (PXR-/-) mice were fed a 1% cholic acid (CA) diet with or without the PXR activator, PCN. Liver function was assessed along with the corresponding changes in hepatic gene expression.

KEY RESULTS

CA administration caused significant hepatotoxicity in PXR+/+ mice and was associated with induction of several FXR and PXR regulated genes, which encode for bile acid transport and metabolizing proteins. Compared to CA alone, co-administration of PCN to CA-fed PXR+/+ mice significantly decreased hepatotoxicity and was associated with induction of MRP3 mRNA as well as CYP3A11 mRNA and functional activity. Unexpectedly, PXR-/- mice, which expressed significantly higher basal and CA-induced levels of MRP2, MRP3, OSTalpha, OSTbeta, OATP2 and CYP3A11, were dramatically less sensitive to CA hepatotoxicity than PXR+/+ mice.

CONCLUSIONS

Protection of PXR+/+ mice against CA-induced hepatotoxicity by PCN is associated with the induction of MRP3 and CYP3A11 expression. Resistance against CA-induced hepatotoxicity in PXR-/- mice may result from higher basal and induced expression of bile acid transporters, particularly MRP3. These findings emphasize the importance of transport by MRP3 and metabolism as major protective pathways against cholestatic liver injury.

Pregnane X receptor activation ameliorates DSS-induced inflammatory bowel disease via inhibition of NF-kappaB target gene expression

Pregnane X receptor (PXR) expression was shown to be protective in inflammatory bowel disease (IBD). However, the mechanism by which PXR provides protection remains unclear. Wild-type and Pxr-null mice were treated with the PXR agonist pregnenolone-16alpha-carbonitrile or vehicle and administered 2.5% dextran sulfate sodium (DSS) in drinking water to induce IBD. Typical clinical symptoms were evaluated on a daily basis. In vivo intestinal permeability assays and proinflammatory cytokine analysis were performed. PXR agonist-treated mice were protected from DSS-induced colitis compared with vehicle-treated mice, as defined by body weight loss, diarrhea, rectal bleeding, colon length, and histology. Pregnenolone-16alpha-carbonitrile did not decrease the severity of IBD in Pxr-null mice. PXR agonist treatment did not increase epithelial barrier function but did decrease mRNA expression of several NF-kappaB target genes in a PXR-dependent manner. The present study clearly demonstrates a protective role for PXR agonist in DSS-induced IBD. The data suggest that PXR-mediated repression of NF-kappaB target genes in the colon is a critical mechanism by which PXR activation decreases the susceptibility of mice to DSS-induced IBD.

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.

Nuclear pregnane X receptor cross-talk with FoxA2 to mediate drug-induced regulation of lipid metabolism in fasting mouse liver

Upon drug activation, the nuclear pregnane X receptor (PXR) regulates not only hepatic drug but also energy metabolism. Using Pxr(-/-) mice, we have now investigated the PXR-mediated repression of lipid metabolism in the fasting livers. Treatment with PXR activator pregnenolone 16alpha-carbonitrile (PCN) down-regulated the mRNA levels of carnitine palmitoyltransferase 1A (in beta-oxidation) and mitochondrial 3-hydroxy-3-methylglutarate-CoA synthase 2 (in ketogenesis) in wild-type (Pxr(+/+)) mice only. In contrast, the stearoyl-CoA desaturase 1 (in lipogenesis) mRNA was up-regulated in the PCN-treated Pxr(+/+) mice. Reflecting these up- and down-regulations and consistent with decreased energy metabolism, the levels of hepatic triglycerides and of serum 3-hydroxybutylate were increased and decreased, respectively, in the PCN-treated Pxr(+/+) mice. Using gel shift, glutathione S-transferase pull-down and cell-based reporter assays, we then examined whether PXR could cross-talk with the insulin response forkhead factor FoxA2 to repress the transcription of the Cpt1a and Hmgcs2 genes, because FoxA2 activates these genes in fasting liver. PXR directly bound to FoxA2 and repressed its activation of the Cpt1a and Hmgcs2 promoters. Moreover, ChIP assays showed that PCN treatment attenuated the binding of FoxA2 to these promoters in fasting Pxr(+/+) but not Pxr(-/-) mice. These results are consistent with the conclusion that PCN-activated PXR represses FoxA2-mediated transcription of Ctp1a and Hmgcs2 genes in fasting liver.

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.

Pregnenolone and dexamethasone, modulators of cytochrome P450-3A, not increase but reduce urinary alpha-CEHC excretion in rats

In this study, the CYP3A inducer pregnenolone-16alpha-carbonitrile (PCN) and the CYP3A inhibitor ketoconazole (KCZ) were used to investigate whether the metabolism of alpha-tocopherol to its metabolite, alpha-carboxyethyl hydroxychroman (alpha-CEHC), is CYP3A-dependent in rats. In experiment 1, two groups of Wistar rats were fed for 3 wk with either a basal diet (containing 50 ppm of alpha-tocopherol) or the same diet containing 10-fold more alpha-tocopherol. In the last 3 days, each group was divided into 2 subgroups which were given a single i.p. injection of either PCN at 75 mg/kg/d (P50 & P500 groups) or DMSO (D50 & D500 groups). The liver TBARS concentration was highest in the P50 group. Two-way ANOVA analysis showed that alpha-tocopherol levels in the plasma and liver were both significantly decreased by PCN (p < 0.0001), as were alpha-CEHC levels in the urine (p = 0.0004). In experiment 2, alpha-tocopherol levels in the liver were increased and alpha-CEHC excretion in the urine decreased in the Wistar rats fed with KCZ containing diet. In experiment 3, Wistar rats administered with dexamethasone (DEX) significantly decreased alpha-tocopherol levels in the plasma and liver and alpha-CEHC levels in the urine. These data showed CYP3A is not a major contributor of the metabolism of alpha-tocopherol to alpha-CEHC. Nevertheless, vitamin E status was markedly reduced by CYP3A inducers due to increased lipid peroxidation and this would increase the consumption of alpha-tocopherol in the liver.

p53-Independent Induction of Rat Hepatic Mdm2 following Administration of Phenobarbital and Pregnenolone 16α-Carbonitrile

Murine double minute 2 (Mdm2) negatively regulates p53 by mediating its ubiquitination and proteosomal degradation, and Mdm2 is recognized as a proto-oncogene. In the present study, hepatic gene expression patterns induced by phenobarbital (PB; 100 mg/kg) and pregnenolone 16alpha-carbonitrile (PCN, 100 mg/kg) were evaluated in male and female Sprague-Dawley rats using Affymetrix Rat Genome U34A gene arrays. In addition to changes in the hepatic expression of well-characterized drug-metabolizing enzymes, an increase in Mdm2 mRNA was observed with both compounds after single or repeat dosing (5 days). However, gene array analyses did not reveal changes in other p53-dependent genes, suggesting that induction of Mdm2 occurred in a p53-independent manner. Real-time polymerase chain reaction confirmed the microarray results, as PB increased Mdm2 mRNA approximately twofold after single or repeat doses in male and female rats. PCN treatment increased Mdm2 mRNA levels up to 5- and 12-fold in male and female rats, respectively, after 5 days of dosing. Hepatic Mdm2 protein levels were increased, and immunohistochemical evaluation of rat liver demonstrated nuclear localization of Mdm2, suggesting an interaction with p53. Consequently, p53 protein levels were also decreased by approximately 35 and 50% after 5 days of PB and PCN treatment, respectively. In direct contrast to rats, PB and PCN (100 mg/kg) did not induce Mdm2 mRNA in mouse liver after 5 days of dosing. Finally, although Mdm2 in mice and humans is reported to migrate electrophoretically as two proteins with molecular weights of 76 and 90 kDa, rat Mdm2 protein was detected primarily as a 120-kDa species. Follow-up experiments indicated that rat hepatic Mdm2 was subject to posttranslational modification with small ubiquitin-modifying (SUMO) proteins. Although the molecular mechanisms controlling Mdm2 induction by PB and PCN in rats have not yet been determined, these results suggest that early effects on cell cycle regulation, response to DNA damage or cell transformation may contribute to liver tumor development.

Identification of genes controlled by the pregnane X receptor by microarray analysis of mRNAs from pregnenolone 16alpha-carbonitrile-treated rats

Mammalian liver contains a pregnane X receptor (PXR, NR1I2), which binds drugs and other xenobiotics, and stimulates (or suppresses) expression of numerous genes involved in the metabolic elimination of foreign compounds and some toxic endogenous substances. In the present study, we used microarray analysis to identify genes whose expression in rat liver was significantly altered by pregnenolone 16alpha-carbonitrile (PCN) treatment. PCN is a synthetic steroid that induces cytochrome P4503A expression and is hepatoprotective by increasing resistance to subsequent stressful insults. Significant induction was seen for 138 genes while expression of 82 genes was significantly repressed. We found induction of genes known to be induced by PCN, such as enzymes involved in drug metabolism and transport. In addition, many genes were differentially expressed whose functions concerned intracellular metabolism, transport of essential small molecules, cell cycle, and redox balance. Our results support the idea that the domain of PXR-controlled gene networks may be even more extensive than currently thought and may extend to functions apart from xenobiotic metabolism.

Phenytoin induction of the cyp2c37 gene is mediated by the constitutive androstane receptor

The CYP2C subfamily of cytochrome P450 monooxygenases is responsible for the metabolism of approximately 20% of therapeutic drugs and many endogenous compounds in humans. These enzymes can be induced by prior treatment with drugs, resulting in changes in drug efficacy. Induction of human CYP2C enzymes by xenobiotics occurs at the transcriptional level and is reported to involve the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). In the present study, we report that murine CYP2C37 mRNA is induced by phenobarbital and phenytoin. In contrast, the mouse PXR agonist 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile did not induce CYP2C37 mRNA, suggesting that PXR does not regulate this gene. The induction of CYP2C37 mRNA by phenobarbital and phenytoin is essentially abolished in CAR-null mice; thus, induction of Cyp2c37 by these xenobiotics is CAR-dependent. A functional CAR response element (CAR-RE) was identified at -2791 base pairs from the translation start site of the Cyp2c37 gene. Mutation of this CAR-RE abolished mouse CAR transactivation of a Cyp2c37 -2.9-kilobase pair luciferase reporter construct in HepG2 cells.

Regulation of mRNA expression of xenobiotic transporters by the pregnane x receptor in mouse liver, kidney, and intestine

Multiple transporter systems are involved in the disposition of xenobiotics and endogenous compounds. The pregnane X receptor (PXR) is a major chemical sensor known to activate the expression of CYP3A/Cyp3a in humans and rodents. The purpose of this study is to systematically determine whether the major xenobiotic transporters in liver, kidney, duodenum, jejunum, and ileum are induced by pregnenolone-16alpha-carbonitrile (PCN), and whether this increase is mediated by the nuclear receptor PXR. In liver, PCN induced the expression of Oatp1a4 and Mrp3 mRNA in wild-type (WT) mouse liver, but not in PXR-null mice. In kidney, PCN did not alter the expression of any drug transporter. In duodenum, PCN increased Abca1 and Mdr1a mRNA expression in WT mice, but not in PXR-null mice. In jejunum and ileum, PCN increased Mdr1a and Mrp2 mRNA, but decreased Cnt2 mRNA in WT mice, but none of these transporters was altered when PCN was administered to PXR-null mice. Therefore, PCN regulates the expression of some transporters, namely, Oatp1a4 and Mrp3 in liver, as well as Abca1, Cnt2, Mdr1a, and Mrp2 in small intestine via a PXR-mediated mechanism.

Cytochrome p450 expression of cultured rat small hepatocytes after long-term cryopreservation

Small hepatocytes (SHs) are hepatic progenitor cells that can be cryopreserved for a long time. After thawing, the cells can proliferate and, when treated with Matrigel, they can differentiate into mature hepatocytes (MHs). In this study, we investigated whether cryopreserved SHs could express cytochromes P450 (P450s), whether P450 expression was induced by appropriate inducers, and whether P450 activities were measurable. 3-Methylcholanthrene (3-MC), phenobarbital (PB), pregnenolone-16alpha-carbonitrile (PCN), and ethanol were used as inducers for CYP1A, 2B, 3A, and 2E, respectively. Immunoblot analysis indicated that cryopreserved SHs constitutively expressed CYP1A1/2, CYP2E1, and CYP3A2 as much as 26 days after plating. Significant expression of CYP1A1/2 and 3A2 in the cells treated with Matrigel was induced by 3-MC and PCN, respectively. Although Matrigel did not up-regulate the enzymatic activity of CYP1A, CYP3A and CYP2E activities increased. Induction of CYP1A and CYP3A activities by each inducer was observed in cryopreserved cells treated with Matrigel. Although the expression of CYP2B1 could be detected in subcultured SHs treated with PB, it was not detected in cryopreserved SHs. The activity of NADPH-cytochrome P450 reductase was measured in both subcultured and cryopreserved SHs, although the activities in both were approximately 30% of that of MHs. Profiles of (14)C-testosterone metabolites were examined in cultured MHs and in cryopreserved SHs by high-performance liquid chromatography. Similar peaks for testosterone metabolites in MHs and SHs were observed in the same elution time. These results indicate that, although induction of CYP3A and 2B in cryopreserved SHs is inferior to that in subcultured ones, SHs can maintain the expression and activities of P450s after long-term cryopreservation.

Expression and regulation of breast cancer resistance protein and multidrug resistance associated protein 2 in BALB/c mice

Microsomal enzyme inducers are known to influence the expression of many transporter proteins and mRNA. In this study, we examined the effects of microsomal enzyme inducers on the mRNA expression of Breast Cancer Resistance Protein (BCRP) and Multidrug Resistance Associated Protein 2 (MRP2) in BALB/c mice. mRNA expression in liver, duodenum, jejunum and ileum was examined in mice, which were treated with microsomal enzyme inducers-aryl hydrocarbon receptor (AhR) ligands 3-methylcholanthrene (3-MC) and pregnane-x-receptor (PXR) ligand pregenolone-16alpha-carbonitrile (PCN) and compared with control vehicle. The results suggested that the expression level of bcrp mRNA in the ileum was twice that in the liver, duodenum and jejunum using both semi quantitative PCR and Real-time PCR. Mrp2 mRNA was significantly increased by both PCN and 3-MC treatment. In contrast, bcrp mRNA expression was not significantly affected by these inducers. In summary, this study demonstrated that the expression of mrp2 mRNA is regulated by PCN and 3-MC, however, bcrp mRNA expression was not significantly affected by PCN and 3-MC.

P-Glycoprotein and an Unstirred Water Layer Barring Digoxin Absorption in the Vascularly Perfused Rat Small Intestine Preparation: Induction Studies with Pregnenolone-16α-carbonitrile

Digoxin, a substrate of P-glycoprotein (Pgp) and cytochrome P450 3a (Cyp3a), was used to illustrate the inductive effects of pregnenolone-16alpha-carbonitrile (PCN), a ligand of the pregnane X receptor, on the absorption and disposition of [3H]digoxin in the vascularly perfused rat small intestine preparation. Although increased Cyp3a protein was observed with Western blotting analysis after PCN treatment, metabolism of digoxin to the digoxigenin bis-digitoxoside metabolite in the rat small intestine remained insignificant (<4% dose). PCN pretreatment significantly decreased blood perfusate [3H]digoxin concentrations for both systemic and intraluminal administrations of [3H]digoxin due to increased Pgp levels. The apical secretion by Pgp increased at 90 min with PCN treatment, from 11.2 +/- 5.1% of dose to 20.1 +/- 8.6% of dose after systemic administration of [3H]digoxin; this increase was, however, statistically insignificant (P = 0.13) because of the high variability among preparations. When the composite data for the control and PCN-treated preparations were fit to published physiologically based pharmacokinetic models: the traditional model and the segregated flow model, suboptimal parameters were obtained. The data were further fit to expanded models with a bilayer membrane compartment housing the Pgp adjacent to the apical membrane, or an unstirred water layer (UWL) external to the apical membrane. The models with the UWL yielded improved fits and reasonable parameters associated with digoxin absorption, suggesting that the UWL posed as a barrier for digoxin absorption. Similar results were obtained with the segmental models (the segmental traditional model and the segmental segregated flow model) using the UWL, when heterogeneous distributions of Pgp in the duodenum, jejunum, and ileum were considered.

Cytochrome P4503A: evidence for mRNA expression and catalytic activity in rat brain

Studies initiated to investigate the presence of cytochrome P4503A (CYP3A) isoenzymes in brain revealed constitutive mRNA and protein expression of CYP3A1 in rat brain. Western blotting studies showed that pretreatment with CYP3A inducer such as pregnenolone-16alpha -carbonitrile (PCN) significantly increased the cross reactivity comigrating with hepatic CYP3A1 and CYP3A2 in rat brain microsomes. RT-PCR studies have also shown increase in mRNA expression of CYP3A1 following pretreatment of rats with PCN. The ability of rat brain microsomes to catalyze the demethylation of erythromycin, known to be mediated by CYP3A isoenzymes in liver and significant increase in the activity of erythromycin demethylase (EMD) following pretreatment with dexamethasone or PCN have indicated that CYP3A isoenzymes expressed in brain are functionally active. Kinetic studies revealed that increase in the enzyme activity following pretreatment with PCN resulted in increase in the apparent affinity (Km) and Vmax of the reaction. Similarities in the inhibition of the constitutive and inducible brain and liver EMD activity following in vitro addition of ketoconazole, a inhibitor specific for CYP3A catalysed reactions and anti-CYP3A have further indicated that like in liver, CYP3A isoenzymes catalyse the activity of EMD in rat brain. Data also revealed regional differences in the activity of EMD in the brain. Relatively higher constitutive as well as inducible mRNA expression of CYP3A1 in hypothalamus and hippocampus, the brain regions responsive to steroid hormones have suggested that CYP3A isoenzymes may not only be involved in the process of detoxication mechanism but also in the metabolism of endogenous substrates in brain.

Pregnenolone-16alpha-carbonitrile inhibits rodent liver fibrogenesis via PXR (pregnane X receptor)-dependent and PXR-independent mechanisms

The effect of liver growth stimulation [using the rodent PXR (pregnane X receptor) activator PCN (pregnenolone-16alpha-carbonitrile)] in rats chronically treated with carbon tetrachloride to cause repeated hepatocyte necrosis and liver fibrogenesis was examined. PCN did not inhibit the hepatotoxicity of carbon tetrachloride. However, transdifferentiation of hepatic stellate cells and the extent of fibrosis caused by carbon tetrachloride treatment was significantly inhibited by PCN in vivo. In vitro, PCN directly inhibited hepatic stellate cell transdifferentiation to a profibrogenic phenotype, although the cells did not express the PXR (in contrast with hepatocytes), suggesting that PCN acts independently of the PXR. Mice with a functionally disrupted PXR gene (PXR-/-) did not respond to the antifibrogenic effects of PCN, in contrast with wild-type (PXR+/+) mice, demonstrating an antifibrogenic role for the PXR in vivo. However, PCN inhibited the transdifferentiation of PXR-/--derived mouse hepatic stellate cells in vitro, confirming that there is also a PXR-independent antifibrogenic effect of PCN through a direct interaction with hepatic stellate cells. These data suggest that the PXR is antifibrogenic in rodents in vivo and that a PXR-independent target for PXR activators exists in hepatic stellate cells that also functions to inhibit fibrosis.

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.

Potential role for human cytochrome P450 3A4 in estradiol homeostasis

Previously, a human CYP3A4-transgenic (Tg-CYP3A4) mouse line was reported to exhibit enhanced metabolism of midazolam by cytochrome P450 3A4 (CYP3A4) expressed in small intestine. Here we show that expression of CYP3A4 and murine cyp3a and cyp2b was both age and sex dependent. CYP3A4 was expressed in the livers of male and female Tg-CYP3A4 mice at 2 and 4 wk of age. Since 6 wk, CYP3A4 was undetectable in male livers, whereas it was constitutively expressed in female livers at decreased levels (3- to 5-fold). Pregnenolone 16alpha-carbonitrile markedly induced hepatic CYP3A4 expression, and the level was higher in females than males. Induction of intrinsic murine cyp3a and cyp2b was also sex dependent. Tg-CYP3A4 females were found to be deficient in lactation, leading to a markedly lower pup survival. The mammary glands of the Tg-CYP3A4 lactating mothers had underdeveloped alveoli with low milk content. Furthermore, beta-casein and whey acidic protein mRNAs were expressed at markedly lower levels in Tg-CYP3A4 pregnant and nursing mouse mammary glands compared with wild-type mice. This impaired lactation phenotype was associated with significantly reduced serum estradiol levels in Tg-CYP3A4 mice. A pharmacokinetic study revealed that the clearance of iv administrated [(3)H]estradiol was markedly enhanced in Tg-CYP3A4 mice compared with wild-type mice. These results suggest that CYP3A4 may play an important role in estradiol homeostasis. This may be of concern for treatment of pregnant and lactating women because CYP3A4 gene expression and enzymatic activity can be potentially modified by CYP3A4 inhibitors or inducers in medications, supplements, beverages, and diet.

Clofibrate and perfluorodecanoate both upregulate the expression of the pregnane X receptor but oppositely affect its ligand-dependent induction on cytochrome P450 3A23

The pregnane X receptor (PXR) interacts with a vast array of structurally dissimilar chemicals and confers induction of several major types of drug metabolizing enzymes such as cytochrome P450s (CYP). We previously reported that the expression of PXR was markedly increased in rats treated with clofibrate and perfluorodecanoic acid (PFDA). The present study was undertaken to test the hypothesis that induced expression of PXR increases PXR ligand-dependent induction on CYP3A23. Rat hepatocytes were treated with clofibrate or PFDA individually, or along with PXR ligand pregnenolone 16alpha-carbonitrile (PCN), and the levels of PXR and CYP3A23 were determined by Western blots. Both clofibrate and PFDA markedly increased the expression of PXR with PFDA being more potent, and the induction was abolished by actinomycin D, an inhibitor for mRNA synthesis. As expected, PCN alone markedly induced the expression of CYP3A23. Interestingly, co-treatment with clofibrate enhanced the induction, whereas co-treatment with PFDA suppressed it. Clofibrate and PFDA represent multi-classes of chemicals called peroxisome proliferators including many therapeutic agents and industrial pollutants. The opposing effects of clofibrate and PFDA on the PCN-induced expression of CYP3A23 suggest that peroxisome proliferators likely increase the expression of PXR but differentially alter its ligand-dependent induction. The interaction between PXR inducer and ligand provides a novel mechanism on how functionally and structurally distinct chemicals cooperatively regulate the expression of xenobiotic-metabolizing enzymes and transporters.

Expression of PXR, CYP3A and MDR1 genes in liver of zebrafish

The Pregnane X Receptor (PXR) is a nuclear receptor involved in the transcriptional regulation of drug-metabolizing enzymes and transporters. In mammals, many xenobiotics induce the expression of cytochrome P4503A (CYP3A) and the multiple drug resistance 1 (MDR1) genes via the PXR pathway. Little attention has been given to studies about the identification and biological function of PXR homologues in non-mammalian species. Zebrafish is being widely used and accepted as model for toxicological and pharmacological studies to understand the mechanisms of human diseases and identify conserved signaling pathways. The aim of this study was to evaluate the in vivo expression of PXR, CYP3A and MDR1 genes in liver of zebrafish treated with the synthetic steroid pregnenolone 16alpha-carboninitrile (PCN), the antimycotic clotrimazole (CTZ) and the antianginal drug nifedipine (NIF). The liver of fish treated with PCN showed a 1.9-fold induction in the PXR followed by 1.8-fold induction in the CYP3A and 1.6-fold induction in the MDR1 mRNA. CTZ and NIF did not affect statistically the expression of PXR, CYP3A and MDR1. The similar pattern of mRNA expression of PXR, CYP3A and MDR1 genes found in fish treated with different PXR inducers suggests that the intrinsic association between these three genes is conserved in zebrafish.

Microarray analysis on CYPs expression in pregnant rats after treatment with pregnenolone-16alpha-carbonitrile and phenobarbital

We previously reported the protein expression profiles of nine cytochrome P450 isozymes (CYPs) in pregnant rat's liver, fetal liver, and placenta after treatment with pregnenolone-16alpha-carbonitrile (PCN), dexamethasone (DEX), or phenobarbital (PB). In this study, the gene expression of 40 CYPs and 2 orphan nuclear receptors for CYP inducers, that is, Nr1i2 (CYP3A subfamily inducible by PCN) and Nr1i3 (CYP2B subfamily inducible by PB), in pregnant rat's liver, fetal liver, and placenta was investigated at one time. Fischer 344 (F344) pregnant rats were daily treated intraperitoneally with 50 mg/kg of PCN or 80 mg/kg of PB from 13 to 16 days of gestation (DG). They were sacrificed on 17 DG, and microarray analysis using Affymetrix Rat Expression Array 230A was performed. Ten genes expression significantly increased in dam's liver in PCN group, and seven genes expression in PB group. On the other hand, four genes expression increased in fetal liver in PCN group, and three genes expression increased in PB group. Being common to dam's and fetal livers, the gene expression of Cyp3A1 (CYP3A subfamily) and cytochrome P-450e (CYP2B subfamily) increased in both PCN and PB groups. In placenta, the expression of Cyp3A1 gene was significantly induced in PB group, and it also showed a tendency to increase in PCN group. The expression of Nr1i2 gene was significantly elevated only in dam's liver of PCN group, while the expression of Nr1i3 gene showed no changes in all groups. The results of the present study of 40 CYPs gene expression mostly corresponded to our previous reports on 9 CYPs protein expression.

Histomorphological changes and cytochrome P450 isoforms expression and activities in precision-cut liver slices from neonatal rats

Precision-cut rat liver slices are a widely accepted in vitro tool for the examination of drug metabolism, enzyme induction or hepatotoxic effects of xenobiotics. After prolonged incubation, however, distinct histopathological changes and increasing losses in function are seen with liver slices from adult animals. Since tissue from neonatal animals is expected to be less vulnerable, in the present study liver slices from 1-day-old rats were examined for morphological changes and for the expression of different cytochrome P450 (CYP) isoforms after incubation for up to 24 h and after a 24 h in vitro exposure to beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) or pregnenolone 16alpha-carbonitrile (PCN). In parallel, CYP activities were assessed by different model reactions in slice homogenates and in intact slices. Histopathological changes were less pronounced in liver slices from 1-day-old rats than in those from adult animals. During the 24 h of incubation even a maturation of the tissue occurred, since the proportion of haemopoietic stem cells declined and the glycogen content of the hepatocytes increased. The CYP expression pattern after 2 and 24 h of incubation was similar to that of normal liver specimens from neonatal rats showing a moderate CYP1A1, 2B1 and 3A2 expression. The immunostaining for CYP1A1 and 2B1 was elevated after incubation with BNF. PB enhanced CYP2B1 and 3A2 expression, and DEX and PCN increased CYP3A2 immunostaining. This induction pattern was paralleled by respective effects on the corresponding model reactions. Thus, besides increased viability, slices from neonatal rats are excellently suited for the evaluation of an in vitro induction of CYP enzymes as well.

Cytoplasmic Localization of Pregnane X Receptor and Ligand-dependent Nuclear Translocation in Mouse Liver

The pregnane X receptor (PXR) plays an important role in the response to xenobiotics and endogenous toxins. We have used a specific anti-PXR antibody in the Western blotting of mouse liver nuclear extracts to show that PXR is accumulated in the nucleus after treatment with 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN), followed by an increase in Cyp3a11 mRNA. Expression of wild type PXR and various mutants as green fluorescent fusion proteins in mouse livers showed that PXR was retained in the cytoplasm from where PCN treatment translocated PXR into the nucleus. Furthermore, the xenochemical response signal, the nuclear translocation signal, and the activation function 2 domain were all required for the nuclear translocation to occur. Immunoprecipitation experiments using the hsp90 antibody demonstrated the presence of PXR in a complex with the endogenous cytoplasmic constitutive active/androstane receptor retention protein (CCRP) in HepG2 cells. Fluorescence resonance energy transfer analysis of mouse liver sections after co-expression of cyan fluorescent protein-CCRP and yellow fluorescent protein-PXR also indicated that CCRP and PXR were closely associated in vivo. Overexpression of exogenous CCRP increased the cytoplasmic level of the PXR.CCRP.hsp90 complex, whereas a decrease in endogenous CCRP by treatment with small interfering RNA for CCRP repressed the PXR-mediated reporter activity in HepG2 cells. We conclude that the CCRP mediates the retention of PXR in the cytosol and modulates the activation of PXR in response to PCN treatment.

Suppression of DHEA sulfotransferase (Sult2A1) during the acute-phase response

The acute-phase response (APR) induces alterations in lipid metabolism, and our data suggest that this is associated with suppression of type II nuclear hormone receptors that are key regulators of fatty acid, cholesterol, and bile acid metabolism. Recently, the farnesoid X receptor (FXR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) were found to regulate DHEA sulfotransferase (Sult2A1), which plays an important role in DHEA sulfation and detoxification of bile acids. Because FXR, PXR, and CAR are suppressed during the APR, we hypothesized that Sult2A1 is downregulated during the APR. To induce the APR, mice were treated with LPS, which will then trigger the release of various cytokines, and the mRNA levels of Sult2A1 and the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate synthase 2 (PAPSS2), as well as the enzyme activity of Sult2A1, were determined in the liver. We found that mRNA levels of Sult2A1 decrease in a time- and dose-dependent manner during the LPS-induced APR. Similar changes were observed in the mRNA levels of PAPSS2, the major synthase of PAPS in the liver. Moreover, hepatic Sult2A1 activity and serum levels of DHEA-sulfate (DHEA-S) were significantly decreased in LPS-treated animals. These results suggest that decreased levels or activities of FXR, PXR, and CAR during the APR could contribute to decreases in Sult2A1, resulting in decreased sulfation of DHEA and lower circulating level of DHEA-S. Finally, we found that both TNF and IL-1 caused a significant decrease in the mRNA level of Sult2A1 in Hep3B human hepatoma cells, suggesting that the proinflammatory cytokines TNF and IL-1 mediate the inhibitory effect of LPS on Sult2A1 mRNA level. Our study provides a possible mechanism by which infection and inflammation are associated with altered steroid metabolism and cholestasis.

Enhanced acetaminophen toxicity by activation of the pregnane X receptor

The pregnane X receptor (PXR) is a ligand-activated transcription factor and member of the nuclear receptor superfamily. Activation of PXR represents an important mechanism for the induction of cytochrome P450 3A (CYP3A) enzymes that can convert acetaminophen (APAP) to its toxic intermediate metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Therefore, it was hypothesized that activation of PXR plays a major role in APAP-induced hepatotoxicity. Pretreatment with the PXR activator, pregnenolone 16alpha-carbonitrile (PCN), markedly enhanced APAP-induced hepatic injury, as revealed by increased serum ALT levels and hepatic centrilobular necrosis, in wild-type but not in PXR-null mice. Further analysis showed that following PCN treatment, PXR-null mice had lower CYP3A11 expression, decreased NAPQI formation, and increased maintenance of hepatic glutathione content compared to wild-type mice. Thus, these results suggest that PXR plays a critical role in APAP-induced hepatic toxicity, probably by inducing CYP3A11 expression and hence increasing bioactivation.

The Involvement of the Pregnane X Receptor in Hepatic Gene Regulation during Inflammation in Mice

Inflammation and proinflammatory cytokines suppress the expression of several hepatic transporters and metabolic enzymes, often resulting in cholestatic liver disease. However, mechanism(s) of this down-regulation have not been fully elucidated. As the pregnane X receptor (PXR) is involved in inducing many of these hepatic proteins, it is possible that PXR is also involved in their down-regulation during inflammation. Thus, we compared the effect of inflammation on hepatic gene regulation in wild-type (PXR(+/+)) versus PXR-null (PXR(-/-)) mice. Treatment of PXR(+/+) but not PXR(-/-) mice with the PXR activators 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN) or 17beta-hydroxy-11beta-[4-dimethylamino phenyl]-17alpha-[1-propynyl] estra-4,9-dien-3-one (RU486) resulted in increased mRNA levels of bsep, mdr1a, mrp2, mrp3, oatp2, and cyp3a11, indicating involvement of PXR in their regulation. Significantly lower mRNA levels of bsep, mdr2, mrp2, mrp3, ntcp, oatp2, and cyp3a11 were found in endotoxin-treated PXR(+/+) mice. In endotoxin-treated PXR(-/-) mice, the extent of mrp2 suppression was significantly diminished. Changes in MRP2 expression were supported by Western blot analysis. Although interleukin (IL)-6 imposed significant decreases in the expression of bsep, mrp2, and cyp3a11 in PXR(+/+) mice, this was not observed in PXR(-/-) mice. Of note, significantly lower levels of PXR mRNA and protein were detected in endotoxin- and IL-6-treated PXR(+/+) mice. In addition, endotoxin and IL-6 were also able to suppress PCN-mediated induction of bsep, mrp2, cyp3a11, and PXR. Taken together, our results suggest that PXR plays a role in the down-regulation of several hepatic proteins during inflammation.

Simultaneous substitution of phenylalanine-305 and aspartate-318 of rat pregnane X receptor with the corresponding human residues abolishes the ability to transactivate the CYP3A23 promoter

The pregnane X receptor (PXR) is a key regulator on the expression of genes involved in the elimination of chemicals. As one of the most divergent members in the nuclear receptor family, PXR is activated in a highly species-dependent manner by certain chemicals. Pregnenolone 16alpha-carbonitrile (PCN), a glucocorticoid antagonist, efficaciously activates rodent but not human PXR. This study was undertaken to investigate the structural basis for PCN-mediated activation of rat PXR. A series of rat-human chimeric PXRs were prepared to gradually replace the ligand-binding domain of human PXR with the corresponding rat sequence at an increasing length of 20 residues. Cotransfection experiments established that region(306-326) acted as a transitional conjunction from none to full PCN responsive status. Site-directed mutagenesis study identified two residues (Phe-305 and Asp-318) that were critical in supporting PCN-mediated activation, and simultaneous substitution of both residues abolished the ability of rat PXR to transactivate the CYP3A23 promoter. In addition, substitutions on Phe-305, Asp-318, or both markedly reduced the basal transcriptional activity, and the reduction occurred with the CYP3A4 but not CYP3A23 promoter. Further study with CYP3A4 and CYP3A23 hybrid reporters demonstrated that the region harboring the distal PXR element in the CYP3A4 promoter mediated the repressive activity. PXR has been shown to interact with corepressors in the absence of ligand. The decreased responsiveness toward PCN and reduced basal transcriptional activity suggest that Phe-305 and Asp-318 are involved in both ligand-binding and corepressor interactions.

Effect of prototypical inducing agents on P-glycoprotein and CYP3A expression in mouse tissues

P-glycoprotein (P-gp) and CYP3A have considerable overlap in inducers in vitro. Characterizing P-gp induction in vivo and potential coregulation with CYP3A are important goals for predicting drug interactions. This study examined P-gp expression in mouse tissues and potential coinduction with CYP3A following oral treatment with 1 of 7 prototypical inducing agents for 5 days. P-gp expression in brain or liver was not induced by any treatment as determined by Western blot, whereas dexamethasone, pregnenolone-16alpha-carbonitrile (PCN), St. John's wort (SJW), and rifampin induced hepatic CYP3A expression. In intestine, rifampin and SJW induced P-gp expression 3.7- and 1.6-fold and CYP3A 3.5- and 2.4-fold, respectively, whereas dexamethasone and PCN induced CYP3A only. These observations suggest that P-gp in mouse small intestine is inducible by some, but not all, CYP3A inducers, whereas P-gp expression in liver or brain is not readily induced. Intriguingly, rifampin and SJW, both activators of the human pregnane X receptor (PXR), induced CYP3A in both liver and intestine but induced P-gp only in intestine, whereas PCN, an activator of murine PXR, did not induce P-gp in any tissue. Rifampin disposition was evaluated, and hepatic exposure to rifampin was comparable to intestine; in contrast, brain concentrations were low. Overall, these observations demonstrate that P-gp induction in vivo is tissue-specific; furthermore, there is a disconnect between P-gp induction and CYP3A induction that is tissue- and inducer-dependent, suggesting that PXR activation alone is insufficient for P-gp induction in vivo. Tissue-specific factors and inducer pharmacokinetic/pharmacodynamic properties may underlie these observations.

In vitro induction of cytochrome P4503A1-mRNA and testosterone hydroxylation in precision-cut liver slices from male and female rats

Cytochrome P450 (CYP) 3A is constitutively highly expressed in the liver. Thus detection of induction might be more difficult than shown for scarcely expressed CYP families. In this paper the suitability of rat liver slices to prove CYP3A inducibility was demonstrated. CYP3A dependent basal testosterone hydroxylation (TH) at positions 15beta, 6beta and 2beta was lower in liver slices from female than male rats, but was more markedly induced by 10(-6) M dexamethasone (DEX) within 24 h (mean induction factors 12.5, 18.3 and 140, respectively, for female slices and 3.7, 2.3 and 3.5, respectively, for male slices). Basal expression of CYP3A1-mRNA was stable in vitro until 24 h and did not differ between male and female rats. In liver slices from male rats this mRNA was induced about 14 fold by both DEX and pregnenolone 16alpha-carbonitrile (PCN) within 24 h. In one sample of a female rat a similar range of CYP3A1-mRNA induction was reached by DEX. Altogether, CYP3A induction can be detected more sensitively in liver slices from female than male rats, if TH rates are used as indicators. With liver slices from male rats CYP3A1-mRNA reacts more sensitively to inducers than TH.

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.

Induction of T(4) UDP-GT activity, serum thyroid stimulating hormone, and thyroid follicular cell proliferation in mice treated with microsomal enzyme inducers

The microsomal enzyme inducers phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), 3-methylcholanthrene (3MC), and Aroclor 1254 (PCB) are known to induce thyroxine (T(4)) glucuronidation and reduce serum T(4) concentrations in rats. Also, microsomal enzyme inducers that increase serum TSH (i.e., PB and PCN) also increase thyroid follicular cell proliferation in rats. Little is known about the effects of these microsomal enzyme inducers on T(4) glucuronidation, serum thyroid hormone concentrations, serum TSH, and thyroid gland growth in mice. Therefore, we tested the hypothesis that microsomal enzyme inducers induce T(4) UDP-GT activity, resulting in reduced serum T(4) concentrations, as well as increased serum TSH and thyroid follicular cell proliferation in mice. B6C3F male mice were fed a control diet or a diet containing PB (600, 1200, 1800, or 2400 ppm), PCN (250, 500, 1000, or 2000 ppm), 3MC (62.5, 125, 250, or 500 ppm), or PCB (10, 30, 100, or 300 ppm) for 21 days. All four inducers increased liver weight and hepatic microsomal UDP-GT activity toward chloramphenicol, alpha-naphthol, and T(4). PB and PCB decreased serum total T(4), but PCN and 3MC did not. Serum thyroid stimulating hormone was markedly increased by PCN and 3MC treatments, and slightly increased by PB and PCB treatments. All four microsomal enzyme inducers dramatically increased thyroid follicular cell proliferation in mice. The findings suggest that PB, PCN, 3MC, and PCB disrupt thyroid hormone homeostasis in mice.

Induction of CYP3A1 by dexamethasone and pregnenolone-16alpha-carbonitrile in pregnant rat and fetal livers and placenta

Cytochrome P450 (CYP) 3A1 was detected in the cytoplasm of giant cells in the trophoblastic region of rat placenta through pregnancy (Ejiri et al. 2001). In the present study, changes in the expression of CYP3A1 protein as well as in the histology of pregnant rat and fetal livers and placenta after treatment with dexamethasone (DEX) or pregnenolone-16alpha-carbonitrile (PCN), well-known CYP3A1 inducers, at 16 day of gestation (DG) (DEX1 and PCN1 groups: 100 mg/kg) or from 13 to 16 DG (DEX4 group: 25 mg/kg/day; PCN4 group: 50 mg/kg/day). All animals were killed at 17 DG, and Western blot and immunohistochemical analyses on CYP3A1 expression and histological examination were done. Western blot analysis revealed that PCN induced CYP3A1 more clearly than DEX and that the induction was most prominent in the fetal liver and lowest in the placenta. Except for the placenta, changes in the immunohistochemical stainability for CYP3A1 almost corresponded to those in the expression of CYP3A1 by Western blot analysis. In addition, swelling and and/or vacuolization of hepatocytes were generally observed in the mother and fetal livers showing increased expression of CYP3A1. These results suggest that DEX and PCN might pass through the placenta with no prominent induction of CYP3A1 at the placenta and be distributed to the fetal liver rapidly, resulting in high induction of CYP3A1 in the fetal liver.

Expression of cytochromes P450 3A in mouse lung: effects of dexamethasone and pregnenolone 16alpha-carbonitrile

Expression of cytochromes P450 3A (CYP3A) has been reported in the lung, but its regulation has received little attention. In the present study, we assessed lung levels of Cyp3a mRNA, protein and activity in control mice and in mice treated with either dexamethasone (DEX), pregnenolone 16alpha-carbonitrile (PCN) or a mixture of DEX+PCN. Lung expression of the pregnane X receptor (PXR) was also investigated. Constitutive levels of Cyp3a mRNA were found in the lung from control mice by polymerase chain reaction after reverse transcription of total RNA (RT-PCR). These levels were significantly increased (2.0-fold, P<0.05) in mice treated with DEX and further enhanced (2.7-fold increase, P<0.01) in mice treated with DEX+PCN. In control mice, basal levels of Cyp3a protein and activity were also found, as assessed by western blot and measure of testosterone 6beta-hydroxylation, respectively. In mice treated with DEX or DEX+PCN, changes in Cyp3a protein and activity exhibited the same pattern as those in Cyp3a mRNA. In contrast, PCN alone failed to trigger consistent increases in lung Cyp3a mRNA, protein and activity. PXR mRNA was not detected in the lung from control or PCN-treated mice by RT-PCR, but was found at significant levels in the lungs from mice treated with DEX or DEX+PCN. Our results show that expression of Cyp3a is upregulated by glucocorticoids in mouse lung, and that this effect is potentiated by antiglucocorticoids. This potentiation may involve PXR, expression of which is induced in the lung of glucocorticoid-treated mice.

Expression of rat Multidrug Resistance Protein 2 (Mrp2) in male and female rats during normal and pregnenolone-16alpha-carbonitrile (PCN)-induced postnatal ontogeny

The normal maturation of biliary organic anion excretion in newborn rats can be enhanced by microsomal enzyme-inducing chemical treatment, yet the mechanism for this phenomenon is not known. Multidrug Resistance Protein 2 (Mrp2) is a biliary efflux transporter that is inducible by select microsomal enzyme-inducing chemicals. Thus, the aims of this study were to compare the normal and pregnenolone-16alpha-carbonitrile (PCN)-induced postnatal ontogeny of Mrp2 in male and female rats. Mrp2 protein increased in an age-dependent manner in both sexes between 0 and 90 days of age. At birth, Mrp2 protein in both male and female rats was the same, approximately 70% of adult levels. Mrp2 protein in both sexes reached maximal expression levels that were higher than adult levels (male: days 25-40; female: day 45), then decreased to adult levels, at which age Mrp2 protein expression in male and female rats was the same. Second, male and female rats of various ages were treated with PCN (75 mg/kg, ip) or corn oil for 4 days, after which livers were removed and analyzed for Mrp2 protein and mRNA expression. PCN accelerated the expression of Mrp2 protein in male and female rats as early as 10 days of age, whereas, PCN did not affect male and female Mrp2 mRNA ontogeny. These data suggest that PCN increased Mrp2 protein by a sex-independent posttranscriptional mechanism.

In vitro induction of cytochrome P450 2B1- and 3A1-mRNA and enzyme immunostaining in cryopreserved precision-cut rat liver slices

With the exception of cytochrome P450 (CYP) 1A1 and its mRNA, in vitro induction of other CYP forms has not been demonstrated in cryopreserved liver slices until now. Therefore precision-cut rat liver slices were cultured after cryopreservation and thawing in William's medium E for up to 24 h in the presence of inducers to demonstrate CYP2B1- and CYP3A1-mRNA induction. CYP-mRNA expression was determined by competitive RT-PCR. Exposure to 100 microM phenobarbital caused a more than 20-fold increase in CYP2B1-mRNA expression within 24 h, reaching concentrations comparable with those of PB-exposed fresh rat liver slices. Exposure to 1 microM pregnenolone 16 alpha-carbonitrile enhanced CYP3A1-mRNA expression by more than 30-fold within 24 h. This is in the same range, although with higher variability, as detected with fresh liver slices. In spite of considerable variability among the thawed slices, the induction factors are high enough for a sensitive detection of an induction at mRNA level. Additionally, immunostaining of respective CYP-forms was performed in sections of few samples, indicating CYP increase in viable cells of cryopreserved slices.

Gene expression profiling of drug metabolism and toxicology markers using a low-density DNA microarray

DNA microarrays are useful tools to study changes of gene expression in response to a treatment with drugs. Here, we describe the optimization of conditions for the cDNA synthesis and hybridization protocols to be used for a low-density DNA microarray called 'Rat HepatoChips.' This DNA microarray with 59 carefully selected genes could be used to study changes in gene expression levels due to a treatment with xenobiotic. These 59 genes (including 8 housekeeping genes) have been selected among potential toxic markers involved in basic cellular processes and drug metabolism related genes. Using the optimized conditions, the results were shown to be reproducible, with 6% variation between the duplicated spots and 10% between arrays. Conditions were optimized to allow quantification with a dynamic range of four log units. In order to demonstrate the major advantage of these tool for studying gene expression, samples of control rat liver were compared with those of animals dosed with phenobarbital (PB) or pregnenolone-16 alpha-carbonitrile (PCN), two compounds well known to induce cytochrome P450 isoforms of 2B and 3A subfamilies, respectively. This microarray has shown that other genes apart from the corresponding CYP P450 genes have been changed due to PB and PCN treatment. Apoptosis-related genes have shown to be changed due to PB and PCN treatment, which confirms results from previous work.

Ectopic expression of MHC class II genes (RT1.B(I) beta/alpha) in rat hepatocytes in vivo and in culture can be elicited by treatment with the pregnane X receptor agonists pregnenolone 16 alpha-carbonitrile and dexamethasone

The synthetic steroid, pregnenolone-16-alpha-carbonitrile (PCN), has served for decades as a probe for a postulated series of hepatic defenses activated under situations of environmental "stress". PCN, an antiglucocorticoid, and also such glucocorticoids as dexamethasone (Dex) appear to stimulate hepatic metabolism and elimination of xenobiotics by binding to the nuclear pregnane X receptor (PXR) which then interacts with a distinct DNA response element associated with induction of cytochrome P450 3A genes. To explore the full domain of genes controlled by PCN/PXR, we used differential display to detect rat liver mRNA species selectively induced by PCN or by Dex. Sequence analysis identified one of many PCN induced cDNA fragments as RT1.B(I)beta, a member of the major histocompatability class II (MHC) gene family usually found only in antigen presenting cells. Northern blot analysis of RNA from rat liver or from cultured hepatocytes confirmed that amounts of RT1.B(I)beta mRNA and also of its companion gene, RT1.B(I)alpha mRNA, became readily detectable within 3-6 hours following treatment with PCN or Dex, whereas no induction was observed in spleen RNA. Induction by PCN of RT1.B(I)beta immunoreactive protein was localized to the hepatocytes as judged by immunofluorescence. We conclude that ectopic expression of MHC II genes, an unprecedented effect of steroids or drugs, is rapidly evoked by PCN acting on the liver, directly. The concept of a set of genes coordinately controlled to maintain homeostasis in parenchymal tissues during toxic stress must now be extended to include the immune system.

Use of dominant negative nuclear receptors to study xenobiotic-inducible gene expression in primary cultured hepatocytes

INTRODUCTION

To determine the feasibility of using dominant negative nuclear receptors to dissect the regulation of inducible gene expression in primary cultured hepatocytes, a series of dominant negative nuclear receptor expression plasmids were designed with truncated AF-2 subdomains.

METHODS

Plasmids expressing dominant negative or wild-type constitutive androstane receptor (CAR), pregnane X receptor (PXR), farnesoid X receptor (FXR), liver X receptor (LXR), or peroxisome proliferator-activated receptor alpha (PPARalpha) were transiently cotransfected into primary cultured rat hepatocytes, together with an appropriate reporter plasmid.

RESULTS

Treatment with prototypic inducers, 10(-4) M phenobarbital (CAR activator), 10(-5) M pregnenolone 16alpha-carbonitrile (PXR activator), 3x10(-5) M chenodeoxycholate (FXR activator), or 10(-4) M ciprofibrate (PPARalpha activator), significantly activated expression from the corresponding reporter plasmid. Treatment with 22(R)-hydroxycholesterol (LXR activator) only weakly activated the LXR-responsive reporter, while pregnenolone 16alpha-carbonitrile treatment significantly activated this reporter. Cotransfection with wild-type LXRalpha strongly enhanced 22(R)-hydroxycholesterol-inducible expression from the LXR-responsive reporter. Cotransfection of hepatocyte cultures with each of the dominant negative nuclear receptor plasmids significantly inhibited inducible expression of the corresponding reporter while, with one exception (LXRalpha), cotransfection with the wild-type receptor moderately enhanced or had little effect on reporter expression. When each dominant negative nuclear receptor was cross-examined against all inducer-reporter pairs, effects on multiple inducer-reporter pairs were frequently observed. However, in general, only cotransfection with the appropriate dominant negative inhibited inducible reporter expression to a greater extent than did cotransfection with the corresponding wild-type receptor.

DISCUSSION

We suggest that the application of dominant negative nuclear receptors has utility in transient transfection studies aimed at discerning the regulatory role of individual nuclear receptor transcription factors in inducible hepatic gene expression, provided that appropriate controls are employed.

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.