Differential Transactivation by Two Isoforms of the Orphan Nuclear Hormone Receptor CAR

Abstract

We have identified a new murine orphan member of the nuclear hormone receptor superfamily, termed mCAR, that is closely related to the previously described human orphan MB67, referred to here as hCAR. Like hCAR, mCAR expression is highest in liver. In addition to the most abundant mCAR1 isoform, the mCAR gene expresses a truncated mCAR2 variant that is missing the C-terminal portion of the ligand binding/dimerization domain. The mCAR gene has 8 introns, and this mCAR2 variant is generated by a splicing event that skips the 8th exon. mCAR1, like hCAR, binds as a heterodimer with the retinoid X receptor to the retinoic acid response element from the promoter of the retinoic acid receptor ␤2 isoform. Consistent with its lack of a critical heterodimerization interface, the mCAR2 variant does not bind this site. Both mCAR1 and hCAR are apparently constitutive transcriptional activators. This activity is dependent on the presence of the conserved C-terminal AF-2 transcriptional activation motif. As expected from its inability to bind DNA, the mCAR2 variant neither transactivates by itself nor inhibits transactivation by hCAR or mCAR1.The nuclear hormone receptor superfamily includes the receptors for a number of potent biological regulators, such as steroids, retinoids, and thyroid hormone. With the recent addition of nuclear prostaglandin receptors (1, 2), and an oxysterol receptor (3), there are now more than 15 genes in mammalian genomes that encode such conventional receptors. An even larger set of genes encodes the orphan receptors, which are related to the conventional receptors but do not have known ligands. Particularly since individual genes for superfamily members frequently encode more than one isoform as a consequence of either alternative promoter utilization or alternative mRNA splicing, the total number of proteins that belong to the nuclear receptor superfamily is large.

Inhibitory Cross-talk between Estrogen Receptor (ER) and Constitutively Activated Androstane Receptor (CAR)

“…The strongest evidence for this hypothesis is the loss in inhibition of ER action by CAR if the C-terminal 8 amino acids of CAR are deleted. This mutant retains the ability to bind to DNA (25) and is FIG. 8.…”

“…this mutant CAR heterodimerized with RXR and bound to the CAR/RXR binding site; however, transcriptional ability was completely abolished (25). This mutant CAR has also been reported to be expressed at levels similar to wild type (25).…”

“…This mutant CAR has also been reported to be expressed at levels similar to wild type (25). We first determined whether this mutant CAR is able to interact with the p160 coactivators.…”

Structural Determinants of Constitutive Androstane Receptor Required for Its Glucocorticoid Receptor Interacting Protein-1-mediated Nuclear Accumulation

“…CAR is unusual among nuclear receptors in having a high constitutive activity when assayed in cultured cells (5). The initial ligands identified for CAR were androstanes that were reverse agonists and down-regulated CAR activity (6).…”

Nuclear receptors CAR and PXR in the regulation of hepatic metabolism

“…Similar to PXR (also known as steroid and xenobiotic receptor SXR), CAR heterodimerizes with RXR and binds to the phenobarbital responsive enhancer module (PBREM) to induce the transcription of CYP2B genes (Baes et al 1994;Choi et al 1997;Honkakoski and Negishi 1997). Closer examination of the PBREM and the other CARregulated promoters revealed that CAR preferentially binds to DR4 (direct repeat spaced by 4 nucleotides) binding sites (Honkakoski et al 1998).…”