The Induction of Cytochrome P450 3A5 (CYP3A5) in the Human Liver and Intestine Is Mediated by the Xenobiotic Sensors Pregnane X Receptor (PXR) and Constitutively Activated Receptor (CAR)

Abstract

Induction of cytochrome P450 3A (CYP3A) by xenobiotics may lead to clinically relevant drug interactions. In contrast with other CYP3A family members, studies on the inducibility of CYP3A5 indicate conflicting results. We report the induction of CYP3A5 mRNA in 13 of 16 hepatocyte preparations exposed to rifampin. Furthermore, induction of CYP3A5 mRNA was observed in intestinal biopsies in three of eight probands following exposure to the antibiotic. The highest absolute levels of CYP3A5 transcripts were found following rifampin treatment in hepatocytes and intestines from carriers of CYP3A5*1 alleles. Elucidation of the mechanism involved in CYP3A5 induction revealed that constitutively activated receptor (CAR) and pregnane X receptor (PXR) transactivated the CYP3A5 promoter (؊688 to ؉49) and that the transactivation was dependent on an everted repeat separated by 6 bp (ER6-dependent). Treatment with the prototypical PXR ligand rifampin led to a 2-fold induction of the CYP3A5 promoter activity. In agreement with these observations, PXR and CAR bound specifically to the ER6 motif. Hepatic expression of PXR correlated with that of CYP3A5 mRNA levels in a bank of liver samples. Taken together, studies here revealed the presence of a functional ER6 motif in the CYP3A5 promoter located ؊100 bp upstream from the transcription start site, suggesting that CYP3A5 is inducible by mechanisms similar to those involved in CYP3A4 induction. Enhanced expression of CYP3A5 caused by exposure to inducers may phenocopy the effects of the high expression allele CYP3A5*1. In this manner, induction of CYP3A5 may contribute to the overall importance of this P450 in drug metabolism and drug interactions.Members of the human cytochrome 3A subfamily of P450 monooxygenases (CYP3A) 1 play an important role in drug metabolism because of their hepatic and intestinal abundance and because of the large substrate spectrum. CYP3A enzymes account for 50% of the total P450 in an average human liver, and these enzymes participate in the metabolism of up to 50% of the drugs currently in use (1, 2). Not surprisingly, CYP3A are frequently involved in drug-drug interactions. CYP3A inhibition by drugs such as ketoconazole or erythromycin is considered the most relevant mechanism. For example, the resulting increased levels of the co-administered nonsedating antihistamine terfenadine in the presence of ketoconazole inhibited a cardiac potassium channel, leading to serious and, in some cases, fatal cardiac arrhythmias (3). In addition, many drugs and other environmental substances increase the activity of CYP3A through transcriptional activation (induction) (4). CYP3A induction is frequently considered clinically less important than inhibition because it is expected to reduce the efficacy of co-administered CYP3A substrate drugs rather than to affect their safety. However, CYP3A inducers such as rifampin and rifabutin can reduce plasma concentrations of certain drugs up to 40-fold, effectively abolishing their efficacy (5, 6). For example, enhanced metabo...

Regulation of mRNA Expression of Xenobiotic Transporters by the Pregnane X Receptor in Mouse Liver, Kidney, and Intestine

“…However in rats, PXR agonists L-742694 and DEX increased Cyp3a1/23 and Cyp3a18 in liver, but decreased Cyp3a1/23 and Cyp3a18 in intestine (Hartley et al, 2004). In humans, PXR agonist rifampicin induced CYP3A4 and CYP3A5 expression in both human liver and intestine (Greiner et al, 1999;Burk et al, 2004). Therefore, Cyp3a/CYP3A regulation is similar in liver between rodents and humans, whereas their regulation in intestine is variable between species.…”

Discovery of Osmosensitive Transcriptional Regulation of Human Cytochrome P450 3As by the Tonicity-Responsive Enhancer Binding Protein (Nuclear Factor of Activated T Cells 5)

“…These promoter fragments introduce an NheI and a BamHI restriction site at the 5Ј-and 3Ј-ends, respectively, and were inserted into the NheI and BglII sites of the pGL3-Basic luciferase reporter gene vector, thereby destroying the 3Ј-restriction site (Goodwin et al, 1999) for subsequent reporter constructs. The 737-bp CYP3A5 promoter fragment (Ϫ688/ϩ49 from the transcription start site) (Burk et al, 2004) was generated from C 2 bbe1 genomic DNA using cloning primers (forward, 5Ј-aca gct agc aga tct atc acc aca gag tca gag ggg atg-3Ј; reverse, 5Ј-cat gga tcc gct gtt tgc tgg gct gtt tgc ctg g-3Ј), introducing an NheI-BglII tandem restriction site at the 5Ј-end and BamHI site at the 3Ј-end. This was similarly inserted into the NheI and BglII sites of pGL3-Basic vector.…”

Effect of danshen extract on the activity of CYP3A4 in healthy volunteers

“…Additionally, the PXR-and CAR-dependent induction of CYP3A4 is enhanced by GR [25,26]. Compared with CYP3A4, CYP3A5 may be a relatively minor enzyme in the human small bowel, and appears to be less sensitive to induction by PXR activators because it lacks the distal PXRresponse element cluster shown to enhance the transcription of CYP3A4 by xenobiotics [27,28].Yu et al [12] found that tanshinone IIA and cryptotanshinone were efficacious activators for human PXR, GR was also involved in the trans-activation of the CYP3A4 promoter by cryptotanshinone and tanshinone IIA, and CAR played a role in tanshinone IIA-mediated CYP3A4 induction. The in vitro study results reported are consistent with our in vivo findings here.…”