Evolutionary History and Functional Characterization of the Amphibian Xenosensor CAR

Abstract

The xenosensing constitutive androstane receptor (CAR) is widely considered to have arisen in early mammals via duplication of the pregnane X receptor (PXR). We report that CAR emerged together with PXR and the vitamin D receptor from an ancestral NR1I gene already in early vertebrates, as a result of whole-genome duplications. CAR genes were subsequently lost from the fish lineage, but they are conserved in all taxa of land vertebrates. This contrasts with PXR, which is found in most fish species, whereas it is lost from Sauropsida (reptiles and birds) and plays a role unrelated to xenosensing in Xenopus. This role is fulfilled in Xenopus by CAR, which exhibits low basal activity and pronounced responsiveness to activators such as drugs and steroids, altogether resembling mammalian PXR. The constitutive activity typical for mammalian CAR emerged first in Sauropsida, and it is thus common to all fully terrestrial land vertebrates (Amniota). The constitutive activity can be achieved by humanizing just two amino acids of the Xenopus CAR. Taken together, our results provide a comprehensive reconstruction of the evolutionary history of the NR1I subfamily of nuclear receptors. They identify CAR as the more conserved and remarkably plastic NR1I xenosensor in land vertebrates. Nonmammalian CAR should help to dissect the specific functions of PXR and CAR in the metabolism of xeno- and endobiotics in humans. Xenopus CAR is a first reported amphibian xenosensor, which opens the way to toxicogenomic and bioaugmentation studies in this critically endangered taxon of land vertebrates.

Genome-wide identification, evolution and expression analysis of nuclear receptor superfamily in Nile tilapia, Oreochromis niloticus

“…Up to now, LXRβ and CAR were identified only in tetrapods (Moore et al, 2002;Reschly and Krasowski, 2006;Reschly et al, 2008;Mathäs et al, 2011). Neither has been described in fishes (Maglich et al, 2003;Bertrand et al, 2007).…”

Direct Transcriptional Regulation of Human Hepatic Cytochrome P450 3A4 (CYP3A4) by Peroxisome Proliferator–Activated Receptor Alpha (PPARα)

“…Human PPARa expression plasmid pcDNA 3-h PPARa was kindly provided by T. Tanaka, University of Tokyo, Japan (Tanaka et al, 2002); pcDhsRXRA, encoding human retinoid X receptor (RXR)a, was described elsewhere (Mathäs et al, 2012); CYP3A4 luciferase reporter gene constructs were described previously (Tegude et al, 2007). PPREX3-TK-luc construct, containing three PPREs upstream of the firefly luciferase gene under control of a minimal thymidine kinase promoter, was purchased from Addgene (Cambridge, MA) (#1015) (Kim et al, 1998).…”

An update on the constitutive androstane receptor (CAR)

“…The previous notion that CAR evolved through gene duplication of a single CAR/PXR ancestral gene has been challenged by a new view that all NR1I genes result from whole genome duplication [ 68 ]. This theory is supported by recent analysis showing that PXR/CAR duplication took place after the split of tunicates and vertebrates but before that of fish and land vertebrates [ 69 ]. In contrast to PXR , CAR genes are not found in the fish lineages but are conserved in all land vertebrates, including amphibians.…”

“…In contrast to PXR , CAR genes are not found in the fish lineages but are conserved in all land vertebrates, including amphibians. Functionally, mammalians use both PXR and CAR as xenosensors, whereas in nonmammalian land vertebrates, CAR may be the predominant xenosensing receptor [ 69 ].…”