Involvement of glucocorticoid receptor and pregnane X receptor in the regulation of mouse CYP3A44 female-predominant expression by glucocorticoid hormone.

The role of the glucocorticoid receptor (GR) and pregnane X receptor (PXR) in the regulation of female-predominant expression of mouse CYP3A44 by glucocorticoid hormones was evaluated using a primary culture of female mouse hepatocytes, as the expression was suppressed in adrenalectomized female mice, restored by dexamethasone (DEX) treatment and was not detected in male mouse livers. Glucocorticoid hormones, such as DEX, hydrocortisone, and corticosterone, 11beta-[4-dimethylamino] phenyl-17beta-hydroxy-17-[1-propynyl]estra-4,9-diene-3-one (RU486), antagonists for GR and an agonist for PXR, and rifampicin, an agonist for PXR, were chosen to investigate the relationship of GR/PXR activation and Cyp3a44 gene expression. Glucocorticoid-inducible expression of CYP3A44 was not suppressed but rather was increased by RU486. Treatment of GR expression plasmid-transfected hepatocytes with DEX concentration dependently enhanced the expression of PXR as well as CYP3A44 mRNAs. A synergistic effect of DEX at submicromolar concentrations and rifampicin is observed. Furthermore, transfection of PXR and retinoid X receptor-alpha (RXRalpha) also showed prominent induction of CYP3A44 mRNA by DEX. These results suggest that DEX plays a dual role in CYP3A44 expression: first, direct activation of the Cyp3a44 gene by the PXR-RXRalpha complex, and, second, indirect activation of the Cyp3a44 gene through the induction of PXR gene expression by the GR pathway.     

Induction of cytochrome P450 3A by paclitaxel in mice: pivotal role of the nuclear xenobiotic receptor, pregnane X receptor.

Paclitaxel, a taxane anti-microtubule agent, is known to induce CYP3A in rat and human hepatocytes. Recent studies suggest that a member of the nuclear receptor family, pregnane X Receptor (PXR), is a key regulator of the expression of CYP3A in different species. We investigated the role of PXR activation, in vitro and in vivo, in mediating Cyp3a induction by paclitaxel. Pregnenolone 16 alpha-carbonitrile (PCN), an antiglucocorticoid, was employed as a positive control for mouse PXR (mPXR) activation in vitro, and Cyp3a induction in vivo. In cell based reporter gene assays paclitaxel and PCN activated mPXR with an EC(50) of 5.6 and 0.27 microM, respectively. Employing PXR wild-type and transgenic mice lacking functional PXR (-/-), we evaluated the expression and activity of CYP3A following treatment with paclitaxel and PCN. Paclitaxel significantly induced CYP3A11 mRNA and immunoreactive CYP3A protein in PXR wild-type mice. Consistent with kinetics of CYP3A induction, the V(max) of testosterone 6 beta-hydroxylation in microsomal fraction increased 15- and 30-fold in paclitaxel- and PCN-treated mice, respectively. The Cyp3a induction response was completely abolished in paclitaxel- and PCN-treated PXR-null mice. This suggests that paclitaxel-mediated CYP3A induction in vivo requires an intact PXR-signaling mechanism. Our study validates the use of PXR activation assays in screening newer taxanes for potential drug interactions that may be related to PXR-target gene induction.     

Dehydroepiandrosterone induces human CYP2B6 through the constitutive androstane receptor.

Dehydroepiandrosterone (DHEA), the major precursor of androgens and estrogens, has several beneficial effects on the immune system, on memory function, and in modulating the effects of diabetes, obesity, and chemical carcinogenesis. Treatment of rats with DHEA influences expression of cytochrome P450 (P450) genes, including peroxisome proliferator-activated receptor alpha (PPAR alpha)- and pregnane X receptor (PXR)-mediated induction of CYP4As and CYP3A23, and suppression of CYP2C11. DHEA treatment elevated the expression and activities of CYP3A4, CYP2C9, CYP2C19, and CYP2B6 in primary cultures of human hepatocytes. Induction of CYP3A4 in human hepatocytes was consistent with studies in rats, but induction of CYP2Cs was unexpected. The role of PXR in this response was studied in transient transfection assays. DHEA activated hPXR in a concentration-dependent manner. Because CYP2B6 induction by DHEA in human hepatocytes might involve either PXR or constitutive androstane receptor (CAR) activation, we performed experiments in primary hepatocytes from CAR knockout mice and observed that CAR was required for maximal induction of Cyp2b10 by DHEA. Furthermore, CAR-mediated Cyp2b10 induction by DHEA was inhibited by the inverse agonist of CAR, androstanol (5 alpha-androstan-3 alpha-ol). Further evidence for CAR activation was provided by cytoplasmic/nuclear transfer of CAR upon DHEA treatment. Elucidation of CAR activation and subsequent induction of CYP2B6 by DHEA presented an additional mechanism by which the sterol can modify the expression of P450s. The effect of DHEA on the activation of the xenosensors PPAR alpha, PXR, and CAR, and the consequent potential for adverse drug/toxicant interactions should be considered in humans treated with this nutriceutical agent.     

Effect of lipopolysaccharide on the xenobiotic-induced expression and activity of hepatic cytochrome P450 in mice

Infection-associated inflammation can alter the expression levels and functions of cytochrome P450s (CYPs). Cyp gene expression is regulated by the activation of several nuclear receptors, including pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR). These receptors can be activated by xenobiotics, including medicines. Here, to study the xenobiotic-induced fluctuations in CYP during inflammation, we examined the effect of lipopolysaccharide (LPS) treatment on the level of mRNAs encoding hepatic CYPs induced by xenobiotic-activated nuclear receptors, in mice. Both the mRNA induction of Cyp genes and the metabolic activities of CYP proteins were examined. LPS treatment caused a significant decrease in the induced expression of the mRNAs for Cyp3a11, 2c29, 2c55, and 1a2, but not for Cyp2b10. To assess the CYP enzymatic activities, CYP3A-mediated testosterone 6β-hydroxylation and the intrinsic clearance (CL(int)) of nifedipine in liver microsomes were measured in mice treated with the xenobiotic pregnenolone-16alpha-carbonitrile (PCN) with or without LPS administration. Both assays revealed that the CYP3A activity, which was induced by PCN, declined significantly after LPS treatment, and this decline correlated with the Cyp3a11 mRNA level. In addition, we found that the mRNAs for interleukin (IL)-1β and tumor necrosis factor (TNF) α were increased after treatment with LPS plus xenobiotics. Our findings demonstrated that LPS treatment reduces the PXR- and AhR-mediated, and possibly CAR-mediated Cyp gene expression and further suggest that these decreases are dependent on inflammatory cytokines in the liver.     

Gender-specific induction of cytochrome P450s in nonylphenol-treated FVB/NJ mice

Nonylphenol (NP) is a breakdown product of nonylphenol ethoxylates, which are used in a variety of industrial, agricultural, household cleaning, and beauty products. NP is one of the most commonly found toxicants in the United States and Europe and is considered a toxicant of concern because of its long half-life. NP is an environmental estrogen that also activates the pregnane X-receptor (PXR) and in turn induces P450s. No study to date has examined the gender-specific effects of NP on hepatic P450 expression. We provided NP at 0, 50 or 75 mg/kg/day for 7 days to male and female FVB/NJ mice and compared their P450 expression profiles. Q-PCR was performed on hepatic cDNA using primers to several CYP isoforms regulated by PXR or its relative, the constitutive androstane receptor (CAR). In female mice, NP induced Cyp2b10 and Cyp2b13, and downregulated the female-specific P450s, Cyp3a41 and Cyp3a44. In contrast, male mice treated with NP showed increased expression of Cyp2a4, Cyp2b9, and Cyp2b10. Western blots confirmed induction of Cyp2b subfamily members in both males and females. Consistent with the Q-PCR data, Western blots showed dose-dependent downregulation of Cyp3a only in females and induction of Cyp2a only in males. The overall increase in female-predominant P450s in males (Cyp2a4, 2b9) and the decrease in female-predominant P450s in females (Cyp3a41, 3a44) suggest that NP is in part feminizing the P450 profile in males and masculinizing the P450 profile in females. Testosterone hydroxylation was also altered in a gender-specific manner, as testosterone 16alpha-hydroxylase activity was only induced in NP-treated males. In contrast, NP-treated females demonstrated a greater propensity for metabolizing zoxazolamine probably due to greater Cyp2b induction in females. In conclusion, NP causes gender-specific P450 induction and therefore exposure to NP may cause distinct pharmacological and toxicological effects in males compared to females.     

Comparison of the hepatic and thyroid gland effects of sodium phenobarbital and pregnenolone-16α-carbonitrile in wild-type and constitutive androstane receptor (CAR)/pregnane X receptor (PXR) knockout rats

1. The hepatic and thyroid gland effects of the constitutive androstane receptor (CAR) activator sodium phenobarbital (NaPB) and the pregnane X receptor (PXR) activator pregnenolone-16α-carbonitrile (PCN) were examined in male Sprague-Dawley wild-type (WT) and knockout (KO) rats lacking both hepatic CAR and PXR receptors (CAR KO/PXR KO rats).

2. The treatment of WT rats for 7?d with 500?ppm NaPB in the diet and 100?mg/kg/d PCN by gavage resulted in increased relative liver weight, hepatocyte hypertrophy, increased hepatocyte replicative DNA synthesis (RDS) and induction of cytochrome P450 CYP2B and CYP3A subfamily enzymes. NaPB and PCN also induced thyroid gland follicular cell RDS and hepatic microsomal UDP-glucuronosyltransferase activity towards thyroxine as substrate. These effects were not observed in the liver and thyroid gland of CAR KO/PXR KO rats.

3. Male C57BL/6?J (WT) and CAR KO/PXR KO mice were given 1000?ppm NaPB in the diet for 7?d. In WT, but not in CAR KO/PXR KO, mice NaPB treatment resulted in liver hypertrophy and induction of hepatocyte RDS and Cyp2b enzymes.

4. These results suggest that the CAR KO/PXR KO rat and mouse models are useful experimental models for mode of action studies with rodent CAR activators.

     

Induction of CYP3A expression by dehydroepiandrosterone: involvement of the pregnane X receptor.

Dehydroepiandrosterone (DHEA) is a steroid produced by the human adrenal gland. Administration of pharmacological doses of DHEA to rats changes expression of many genes, including the cytochrome P450 family members CYP4A1 and CYP3A23. It is known that induction of CYP4A expression by DHEA requires the peroxisome proliferator-activated receptor alpha (PPAR(alpha)). In the current study, PPAR(alpha)-null mice were used to examine the role of PPAR(alpha) in expression of CYP3A. In wild-type mice, 150 mg/kg DHEA-sulfate induced Cyp4a and Cyp3a11 mRNAs by 5- and 2-fold, respectively. Induction of Cyp4a expression by DHEA-sulfate was not observed in PPAR(alpha)-null mice, whereas induction of Cyp3a11 expression by DHEA-sulfate was similar between genotypes. This suggests that PPAR(alpha) is not involved in induction of Cyp3a11 expression by DHEA. Because expression of CYP3A family members can be induced by activation of another member of the nuclear receptor superfamily, the pregnane X receptor (PXR), we examined the ability of DHEA to activate PXR. In transient transfection assays, DHEA and its metabolites androst-5-ene-3beta,17beta-diol (ADIOL), androst-5-ene-3,17-dione, and androst-4-ene-3,17-dione were activators of PXR. Maximal induction of a PXR-responsive reporter gene of approximately 3-fold was observed at concentrations of 50 to 100 microM, indicating that these steroids are relatively weak activators of PXR. Human and murine PXR exhibited different specificities for DHEA and its metabolites. ADIOL activated reporter gene expression in the presence of murine but not human PXR. Results of these studies suggest that the induction of rodent CYP3A expression upon treatment with high doses of DHEA occurs through activation of PXR.     

Coordinate regulation of xenobiotic and bile acid homeostasis by pregnane X receptor.

Identification and characterization of the pregnane X receptor (PXR) as a key regulator of cytochrome P450 3A (CYP3A) gene expression has led to an increased understanding of the molecular basis of many drug-drug interactions. Mice lacking PXR (PXR-KO) were used in the present study to delineate the role of PXR in regulating hepatomegaly and regulating the activity of CYP3A, organic anion transporting polypeptide-2 (Oatp2), and Cyp7a1 (cholesterol 7alpha-hydroxylase) gene products in vivo. Pregnenolone-16alpha-carbonitrile (PCN) produced hepatomegaly in the wild-type mice but not in the PXR-KO mice. PCN increased both the number of proliferating cell nuclear antigen immuno-positive nuclei and apparent cell size in the wild-type mice but not in the PXR-KO mice. To determine the role PXR plays in regulating CYP3A activity, 6beta-hydroxylation of testosterone and the duration of the loss of righting reflex following administration of the muscle-relaxant zoxazolamine were measured. PCN increased the level of testosterone 6beta-hydroxylation and decreased the duration of the loss of righting-reflex time following zoxazolamine administration in wild-type mice, but did not effect either of these parameters in PXR-KO mice. PCN increased the hepatic uptake of [(3)H]digoxin, an Oatp2 substrate, in wild-type mice but not in the PXR-KO mice. Similarly, PCN decreased bile acid excretion in wild-type mice but not in the PXR-KO mice. Taken together, these data demonstrate a pivotal role for PXR in the regulation of drug-induced hepatomegaly and in the metabolism (CYP3A), transport (Oatp2), biosynthesis (Cyp7a1), and excretion of xenobiotics and bile acids in vivo.     

Gene expression profiling in the liver of CD-1 mice to characterize the hepatotoxicity of triazole fungicides

Four triazole fungicides used in agricultural or pharmaceutical applications were examined for hepatotoxic effects in mouse liver. Besides organ weight, histopathology, and cytochrome P450 (CYP) enzyme induction, DNA microarrays were used to generate gene expression profiles and hypotheses on potential mechanisms of action for this class of chemicals. Adult male CD-1 mice were exposed daily for 14 days to fluconazole, myclobutanil, propiconazole, or triadimefon at three dose levels by oral gavage. Doses were based on previous studies that resulted in liver hypertrophy or hepatotoxicity. All four triazoles caused hepatocyte hypertrophy, and all except triadimefon increased relative liver/body weight ratios at the middle and high dose levels. CYP enzyme activities were also induced by all four triazoles at the middle and high doses as measured by the dealkylations of four alkoxyresorufins, although some differences in substrate specificity were observed. Consistent with this common histopathology and biochemistry, several CYP and xenobiotic metabolizing enzyme (XME) genes were differentially expressed in response to all four (Cyp2d26 and Cyp3a11), or three of the four (Cyp2c40, Cyp2c55, Ces2, Slco1a4) triazoles. Differential expression of numerous other CYP and XME genes discriminated between the various triazoles, consistent with differences in CYP enzyme activities, and indicative of possible differences in mechanisms of hepatotoxicity or dose response. Multiple isoforms of Cyp1a, 2b, 2c, 3a, and other CYP and XME genes regulated by the nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) were differentially expressed following triazole exposure. Based on these results, we expanded on our original hypothesis that triazole hepatotoxicity was mediated by CYP induction, to include additional XME genes, many of which are modulated by CAR and PXR.     

Nonylphenol-mediated CYP induction is PXR-dependent: The use of humanized mice and human hepatocytes suggests that hPXR is less sensitive than mouse PXR to nonylphenol treatment

Nonylphenol (NP), a by-product of alkylphenol ethoxylates, is a pervasive surfactant that activates the xenosensing nuclear receptor, the pregnane X-receptor (PXR) in transactivation assays in vitro. We are interested in determining if NP activates PXR in vivo, determining if hPXR and mPXR act similarly, and investigating the role of PXR in protecting individuals from NP. Wild-type (WT), PXR-null, and humanized PXR (hPXR) mice were treated with NP at 0, 50 or 75 mg/kg/day for one week, and cytochrome P450 (CYP) induction, liver histopathology, and serum NP concentrations were examined. WT mice treated with NP showed induction of Cyp2b, and male-specific induction of Cyp2c and Cyp3a. CYPs were not induced in PXR-null mice, demonstrating that PXR is necessary for NP-mediated CYP induction. CAR-mediated CYP induction was not observed in the PXR-null mice despite previous data demonstrating that NP is also a CAR activator. hPXR mice only showed moderate Cyp induction, suggesting that hPXR is not as sensitive to NP as mPXR in vivo. NP-mediated Cyp3a induction from three human hepatocyte donors was not significant, confirming that hPXR is not very sensitive to NP-mediated CYP induction. Lastly, mice with PXR (mPXR and hPXR) showed lower NP serum concentrations than PXR-null mice treated with NP suggesting that PXR plays a role in decreasing liver toxicity by basally regulating phase I-III detoxification enzymes that promote the metabolism and elimination of NP. In summary, PXR is required for NP-mediated CYP-induction, mPXR mediates greater CYP induction than hPXR in vivo, and the presence of PXR, especially mPXR, is associated with altered histopathology and increased clearance of NP.     

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.     

Sex-associated expression of mouse hepatic and renal CYP2B enzymes by glucocorticoid hormones

The expression of Cyp2b9 and Cyp2b10 genes was investigated in kidney, liver, and cultured hepatocytes of adult C57BL/6NCrj mice. The constitutive expression level of CYP2B mRNA in kidney was higher in female than in male mice, as it was in the liver where more CYP2B9 than CYP2B10 was expressed in the females, and more CYP2B10 was expressed in the males. After treatment with dexamethasone (Dex), induction of CYP2B10 mRNA and protein in the kidneys was far greater in male than in female mice. In contrast to Dex, phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) did not induce the expression of the Cyp2b gene in the kidneys of either sex. In the liver, PB, PCN, and DDT induced both CYP2B9 and CYP2B10 in both sexes to the same extent, whereas Dex induced only CYP2B10 and simultaneously suppressed CYP2B9. Dex-inducible expression of CYP2B mRNA was decreased by 11 beta-[4-dimethylamino]phenyl-17 beta-hydroxy-17-[1-propynyl]estra-4,9-dien-3-one (RU-486), in both the kidneys and liver from male mice, and in cultured hepatocytes. However, RU-486 itself induced the expression of CYP2B mRNA in female liver and cultured hepatocytes. Interestingly, RU-486 increased PB-inducible expression of these species in cultured hepatocytes. Gonadectomy increased the expression of CYP2B mRNA in untreated male liver, but suppressed Dex-induced expression in the kidneys of both sexes. These observations suggest that (a) there are multiple regulatory pathways in the expression of Cyp2b genes, one of which used by Dex is mediated via the glucocorticoid receptor, which is different from that used by PB, and (b) sex hormones play a role in the regulation of the sex-dependent expression of Cyp2b genes in the mouse.