Retinoids activate the RXR/SXR-mediated pathway and induce the endogenous CYP3A4 activity in Huh7 human hepatoma cells.

Steroid and xenobiotic receptor (SXR) or human pregnane X receptor (hPXR) dimerizes with retinoid X receptor (RXR) and regulates the transcription of genes encoding xenobiotic-metabolizing enzymes such as CYP3A4. Rifampin, the classical activator of CYP3A4, binds to SXR directly. It is unclear whether various natural and synthetic retinoids can regulate the expression of CYP3A4. To evaluate the effects of retinoids on the RXR/SXR-mediated pathway, transient transfection assays were performed on both CV-1 and human hepatoma Huh7 cells using a reporter construct containing multiple RXR/SXR consensus binding elements (an everted repeat with a 6-nucleotide spacer, ER-6). The results revealed that eight out of 13 retinoids screened significantly induced the RXR/SXR-mediated pathway in Huh7 cells. At an equal molar concentration, the acid forms (9-cis-RA, 13-cis-RA, and all-trans-RA) or aldehyde, the direct precursor of acid (9-cis-retinal and 13-cis-retinal), exhibited a greater or similar potency than rifampin. Depending on the ligands, RXR may serve as a silent or an active partner of SXR. Additionally, retinoids can increase CYP3A4 enzyme activity in Huh7 cells. To further evaluate the potential drug-drug interactions, which may be caused by retinoids, Huh7 cells were pretreated with 9-cis-RA and followed by acetaminophen. We showed that 9-cis-RA enhanced the covalent binding of N-acetyl-p-quinoneimine, a toxic intermediate of acetaminophen produced by phase I enzymes oxidation. This result suggested that drug-drug interaction might occur between 9-cis-RA and acetaminophen in human liver cells. Taken together, retinoids activate the RXR/SXR-mediated pathway and regulate the expression of CYP3A4. Thus, retinoids potentially can cause drug-drug interactions when they are administered with other CYP3A4 substrates.

Retinoid X receptor alpha Regulates the expression of glutathione s-transferase genes and modulates acetaminophen-glutathione conjugation in mouse liver.

Nuclear receptors, including constitutive androstane receptor, pregnane X receptor, and retinoid X receptor (RXR), modulate acetaminophen (APAP)-induced hepatotoxicity by regulating the expression of phase I cytochrome P450 (P450) genes. It has not been fully resolved, however, whether they regulate APAP detoxification at the phase II level. The aim of the current study was to evaluate the role of RXRalpha in phase II enzyme-mediated detoxification of APAP. Wild-type and hepatocyte-specific RXRalpha knockout mice were treated with a toxic dose of APAP (500 mg/kg i.p.). Mutant mice were protected from APAP-induced hepatotoxicity, even though basal liver glutathione (GSH) levels were significantly lower in mutant mice compared with those of wild-type mice. High-performance liquid chromatography analysis of APAP metabolites revealed significantly greater levels of APAP-GSH conjugates in livers and bile of mutant mice compared with those of wild-type mice. Furthermore, hepatocyte RXRalpha deficiency altered the gene expression profile of the glutathione S-transferase (Gst) family. Basal expression of 13 of 15 Gst genes studied was altered in hepatocyte-specific RXRalpha-deficient mice. This probably led to enhanced APAP-GSH conjugation and reduced accumulation of N-acetyl-p-benzoquinone imine, a toxic electrophile that is produced by biotransformation of APAP by phase I P450 enzymes. In conclusion, the data presented in this study define an RXRalpha-Gst regulatory network that controls APAP-GSH conjugation. This report reveals a potential novel strategy to enhance the detoxification of APAP or other xenobiotics by manipulating Gst activity through RXRalpha-mediated pathways.