Optical Isomers of Atorvastatin, Rosuvastatin and Fluvastatin Enantiospecifically Activate Pregnane X Receptor PXR and Induce CYP2A6, CYP2B6 and CYP3A4 in Human Hepatocytes

Atorvastatin, fluvastatin and rosuvastatin are drugs used for treatment of hypercholesterolemia. They cause numerous drug-drug interactions by inhibiting and inducing drug-metabolizing cytochromes P450. These three statins exist in four optical forms, but they are currently used as enantiopure drugs, i.e., only one single enantiomer. There are numerous evidences that efficacy, adverse effects and toxicity of drugs may be enantiospecific. Therefore, we investigated the effects of optical isomers of atorvastatin, fluvastatin and rosuvastatin on the expression of drug-metabolizing P450s in primary human hepatocytes, using western blots and RT-PCR for measurement of proteins and mRNAs, respectively. The activity of P450 transcriptional regulators, including pregnane X receptor (PXR), aryl hydrocarbon receptor (AhR) and glucocorticoid receptor (GR), was assessed by gene reporter assays and EMSA. Transcriptional activity of AhR was not influenced by any statin tested. Basal transcriptional activity of GR was not affected by tested statins, but dexamethasone-inducible activity of GR was dose-dependently and enantioselectively inhibited by fluvastatin. Basal and ligand-inducible transcriptional activity of PXR was dose-dependently influenced by all tested statins, and the potency and efficacy between individual optical isomers varied depending on statin and optical isomer. The expression of CYP1A1 and CYP1A2 in human hepatocytes was not influenced by tested statins. All statins induced CYP2A6, CYP2B6 and CYP3A4, and the effects on CYP2C9 were rather modulatory. The effects varied between statins and enantiomers and induction potency decreased in order: atorvastatin (RR>RS = SR>SS) > fluvastatin (SR>RS = SS>RR) >> rosuvastatin (only RS active). The data presented here might be of toxicological and clinical importance.

The role of residues T248, Y249 and T422 in the function of human pregnane X receptor

The pregnane X receptor (PXR) is a key xenobiotic receptor that regulates the expression of numerous drug-metabolizing enzymes. Some posttranslational mechanisms modulate its transcriptional activity. Although several kinases have been shown to directly phosphorylate this receptor, little is known about phosphorylation sites of PXR. In the present work, we examined T248, Y249 and T422 putative phosphorylation sites determined based on in silico consensus kinase site prediction analysis. T248 and T422 residues are critical for the interaction of the PXR ligand-binding domain and the activation function-2 (AF2) domain. Site-directed mutagenesis analysis was performed to generate phospho-deficient and phospho-mimetic mutants. We examined transactivation activity of the PXR mutants in gene reporter assays, formation of PXRmutant/RXRα heterodimer, binding of PXR mutants to the CYP3A4 gene response element DR3 and CYP3A4 expression in HepG2 cells after expression of the mutants. We found that T248D mutant activated CYP3A4 transactivation constitutively regardless of the presence or absence of a ligand. Contrary, T248V mutant exhibited low basal and ligand-inducible transactivation capacity as compared to wild-type PXR. Dose-response analysis revealed reduced ligand-dependent transactivation potency of PXR Y249D mutant. Transactivation of the CYP3A4 promoter was abolished with T422A/D mutants. All PXR mutants formed heterodimer with RXRα at a similar level to that observed with wild-type PXR. The ability to bind to DNA in vitro was substantially decreased in case of T248D, T422D and T248V mutants. Our data thus indicate that phosphorylation of T248, Y249 and T422 residues may be critical for the both basal and ligand-activated function of PXR.

A food contaminant ochratoxin A suppresses pregnane X receptor (PXR)-mediated CYP3A4 induction in primary cultures of human hepatocytes

Ochratoxin A (OCHA) is a mycotoxin, which can be found in food such as coffee, wine, cereals, meat, nuts. Since it is absorbed via gastrointestinal tract, it is reasonable to anticipate that the liver will be the first organ to which OCHA comes into the contact before systemic circulation. Many xenobiotics are metabolically modified after the passage of the liver to biologically more active substances, sometimes with more harmful activity. Promoting own metabolism is often achieved via transcriptional regulation of biotransformation enzymes through ligand-activated transcription factors. Pregnane X receptor (PXR) belongs to such a group of regulators and it was demonstrated to be activated by many compounds of synthetic as well as natural origin. Our intention was to investigate if OCHA is capable of activating the PXR with consequent induction of PXR-regulated CYP3A4 gene. We found that OCHA does not activate PXR but displays antagonist-like behavior when combined with rifampicin (RIF) in gene reporter assay in human embryonal kidney cells (Hek293T). It was very weak inducer of CYP3A4 mRNA in primary cultures of human hepatocytes and it antagonized RIF-mediated CYP3A4 induction of mRNA as well as protein. In addition, it caused the decline of PXR protein as well as mRNA which was faster than that with actinomycin D, a transcription inhibitor. Since we found that OCHA induced the expression of miR-148a, which was described to regulate PXR expression, we conclude that antagonist-like behavior of OCHA is not due to the antagonism itself but due to the downregulation of PXR gene expression. Herein we provide important findings which bring a piece of puzzle into the understanding of mechanism of toxic action of ochratoxin A.

Differential effects of the enantiomers of tamsulosin and tolterodine on P-glycoprotein and cytochrome P450 3A4

The pregnane X receptor (PXR) is a transcription factor regulating P-glycoprotein (P-gp; ABCB1)-mediated transport and cytochrome P450 3A4 (CYP3A4)-mediated metabolism of xenobiotics thereby affecting the pharmacokinetics of many drugs and potentially modulating clinical efficacy. Thus, pharmacokinetic drug-drug interactions can arise from PXR activation. Here, we examined whether the selective α1-adrenoreceptor blocker tamsulosin or the antagonist of muscarinic receptors tolterodine affect PXR-mediated regulation of CYP3A4 and of P-gp at the messenger RNA (mRNA) and protein level in an enantiomer-specific way. In addition, the effect of tamsulosin and tolterodine on P-gp activity was evaluated. We used quantitative real-time PCR, gene reporter assay, western blotting, rhodamine efflux assay, and calcein assay for determination of expression, activity, and inhibition of P-glycoprotein. The studied compounds significantly and concentration-dependently increased PXR activity in the ABCB1-driven luciferase-based reporter gene assay. We observed much stronger induction of ABCB1 mRNA by S-tamsulosin as compared to the R or racemic form. R or racemic form of tolterodine and R-tamsulosin concentration-dependently increased P-gp protein expression; the latter also enhanced P-gp efflux function in a rhodamine-based efflux assay. R-tamsulosin and all forms of tolderodine slightly inhibited P-gp. The effect on CYP3A4 expression followed the same pattern but was much weaker. Taken together, tamsulosin and tolterodine are demonstrated to interfere with P-gp and CYP3A4 regulation in an enantiomer-specific way.

Investigation of Orlistat effects on PXR activation and CYP3A4 expression in primary human hepatocytes and human intestinal LS174T cells

Drugs for weight loss have been in use for nearly hundred years. Orlistat (Xenical) is a non-centrally acting anti-obesity drug that inactivates gastric and intestinal lipases, thus, preventing absorption of dietary triglycerides. There are reports indicating that Orlistat reduces bioavailability of Cyclosporin to a clinically relevant degree. Since Cyclosporin is metabolized by cytochrome P450 CYP3A4, we examined whether interaction between Orlistat and Cyclosporin involves induction of CYP3A4. Human Caucasian colon adenocarcinoma cells LS174T and primary cultures of human hepatocytes were used, as in vitro models of intestinal and hepatic cells, respectively. Treatment of LS174T cells for 24h with Orlistat (1-100mg/L) did not cause induction of CYP3A4 mRNA levels as compared to control cells while Orlistat (100mg/L) slightly induced CYP3A4 mRNA in human hepatocytes. Rifampicin, a model CYP3A4 inducer, significantly induced CYP3A4 mRNA in both types of cells. The level of CYP3A4 protein in human hepatocytes was increased by Orlistat after 48h, while rifampicin strongly induced CYP3A4 protein level. In addition, Orlistat moderately dose-independently activated pregnane X receptor (PXR) in LS174T cells transiently transfected with p3A4-luc reporter construct containing the basal promoter (-362/+53) with proximal PXR response element and the distal xenobiotic responsive enhancer module (-7836/-7208) of the CYP3A4 gene 5'-flanking region. In conclusion, we report here that Orlistat is weak PXR activator and CYP3A4 inducer in human hepatocytes, but it has no effect on CYP3A4 in intestinal cells, implying no role of CYP3A4 induction in the interaction between Orlistat and Cyclosporin in absorption process.

Construction and characterization of hepatocyte nuclear factor HNF4alpha1 over-expressing cell line derived from human hepatoma HepG2 cells

Cancer cell lines derived from hepatocytes have an altered phenotype and they lack hepatocyte-specific functions. It is at least partly due to the under-expression of transcription factors such as hepatocyte nuclear factor 4α (HNF4α), steroid receptor co-activator 1 (SRC1) etc. Recently, a strategy of transient transfection of human hepatic cells with HNF4α revealed improved hepatospecific functions, including the expression of drug-metabolizing enzymes. In the current study we established a human cell line derived from HepG2 cells stably transfected with human HNF4α, and we examined this line for hepatospecific markers. Of the 9 clones analyzed, we found an increased secretion of fibrinogen (9 clones), albumin (5 clones) and plasminogen (3 clones), while secretion of alpha1-antitrypsin was not changed. The expression of pregnane X receptor (PXR) and aryl hydrocarbon receptor (AhR) proteins but not mRNAs was slightly increased. TCDD-dependent induction of CYP1A1 mRNA and protein was augmented in 50% of clones, but there was no correlation between the CYP1A1 inducibility and expression levels of AhR and HNF4α. Induction of CYP3A4 mRNA by rifampicin was about 1.5-2.5 fold (clones 2, 4, 6, 7) and it was not significantly different from CYP3A4 mRNA induction in parent HepG2. The basal expression of CYP3A4 protein was increased in all clones, but rifampicin-induced expression of CYP3A4 protein was in all clones lower than in parent HepG2. Overall, the stable over-expression of HNF4α in HepG2 cells restores some of the hepatospecific functions, but it has a minor effect on the expression of xenobiotic-metabolizing enzymes and their regulators.

Construction and characterization of peroxisome proliferator-activated receptor-gamma co-activator 1 alpha (PGC-1α over-expressing cell line derived from human hepatocyte carcinoma HepG2 cells)

AIMS

The aim was develop stable human cell line stable over-expressing transcription co-activator peroxisome proliferator-activated receptor gamma co-activator 1α (PGC-1α) with restored hepatospecific functions and increased expression of major xenobiotic metabolizing enzymes.

METHODS

Six clones of HepG2-PGC-1α and one control clone HepG2-pcDNA3 were isolated and analyzed for secretion of hepatospecific markers, fibrinogen, albumin and alpha1-antitrypsin. Expression levels of protein and mRNA of hepatocyte nuclear factor (HNF4α), pregnane X receptor (PXR) and aryl hydrocarbon receptor (AhR) were determined. We measured basal and ligand inducible expression of CYP1A1 and CYP3A4.

RESULTS

Stably transfected cell line HepG2-PGC-1α derived from HepG2 cells over-expressing PGC-1α displayed increased secretion of fibrinogen, but not albumin or alpha1-antitrypsin compared to parent HepG2 cells. We found increased levels of HNF4α, PXR and AhR proteins but not their mRNAs in HepG2-PGC1 cells. Basal expression of CYP3A4 protein in HepG2-PGC-1α cells was increased but rifampicin-inducible expression of CYP3A4 protein was lowered in comparison with parent HepG2 cells. Induction of CYP3A4 mRNA varied between 1.3 - 1.9 fold in individual clones. Expression of TCDD-inducible CYP1A1 protein was lower in HepG2-PGC-1α cells than in parent HepG2 cells. Induction of CYP1A1 mRNA by TCDD in HepG2-PGC-1α cells was comparable with that in parent HepG2 cells and ranged between 103 - 198 fold.

CONCLUSION

Stable expression of PGC-1α in HepG2 cells restores several hepatospecific functions, such as secretion of fibrinogen, expression of HNF4α1 and xenoreceptors PXR and AhR. However, the expression and induction of key drug-metabolizing enzymes (CYP1A1 and CYP3A4) were not improved.