Nuclear Import of the Retinoid X Receptor, the Vitamin D Receptor, and Their Mutual Heterodimer

Abstract

The nuclear receptor retinoid X receptor (RXR) can regulate transcription through homotetramers, homodimers, and heterodimers with other nuclear receptors such as the vitamin D receptor (VDR). The mechanisms that underlie the nuclear import of RXR, VDR, and RXR-VDR heterodimers were investigated. We show that RXR and VDR translocate into the nucleus by distinct pathways. RXR strongly bound to importin␤ and was predominantly nuclear in the absence of ligand. Importin binding and nuclear localization of RXR were modestly enhanced by its ligand, 9-cis-retinoic acid. On the other hand, VDR selectively associated with importin␣. Importin association and correspondingly nuclear import of VDR were markedly augmented by 1,25(OH) 2 D 3 . RXR-VDR dimerization inhibited the ability of RXR to bind importin␤ and to mobilize into the nucleus using its own nuclear localization signal. In contrast, VDR recruited RXR-VDR heterodimers to importin␣ and mediated nuclear import of the heterodimers in response to 1,25(OH) 2 D 3 . Hence nuclear import of RXR-VDR heterodimers is mediated preferentially by VDR and is controlled by the VDR ligand. The observations reveal a novel mechanism by which an RXR heterodimerization partner dominates the activity of the heterodimers.The retinoid X receptor (RXR) 3 is a member of the superfamily of nuclear hormone receptors that is activated by the vitamin A metabolite 9-cis-retinoic acid (9cRA). Like other nuclear receptors, RXR is comprised of several distinct functional domains: an amino-terminal domain, involved in ligand-independent basal transcriptional activity; a DNA-binding domain (DBD) containing two "zinc finger" motifs; a flexible hinge region; and a carboxyl-terminal region, termed the ligandbinding domain (LBD). The LBD contains the ligand-binding pocket as well as regions that mediate multiple protein-protein interactions including association with transcriptional co-regulators, formation of dimers, and, in the case of RXR, formation of tetramers. The LBD of nuclear receptors, including RXR, thus coordinates their liganddependent transcriptional activities (1).In the absence of its cognate ligand, RXR forms high affinity homotetramers. These tetramers are transcriptionally silent, but they rapidly dissociate upon ligand binding (2-4). Hence RXR tetramers appear to serve as an inactive storage pool from which active species can be liberated in response to 9cRA. An additional role for the RXR tetramers was recently suggested by the observations that these oligomers act as DNA architectural factors. It was thus demonstrated that binding of RXR tetramers to promoter regions that contain two RXR response elements in tandem results in a dramatic DNA looping, thereby enabling transcriptional regulation by factors placed far upstream from start sites of target genes (5). Hence by modulating DNA geometry, RXR tetramers can regulate gene expression in a manner that is responsive to 9cRA but is independent of the intrinsic transcriptional activity of the receptor (5). Although RXR can activate ...

Nuclear localization of liver X receptor α and β is differentially regulated

“…It is unknown which importin binds to NLS2 in other nuclear receptors; importin b binds to NLS1 in RXR [Yasmin et al, 2005]. We found that a conserved basic amino acid in NLS1 of both RXR (K165) and LXR b (R117) can be substituted with the neutral amino acid, glycine, without loss of nuclear localization or transcriptional activity.…”

Retinoid X receptors: common heterodimerization partners with distinct functions

“…In line with this, RXRα possess a nuclear localization signal and associates to importin , one of the cellular karyophilins binding the nuclear pore complex. When expressed as a GFP fusion protein, RXRα is mostly located in the nucleus but cytoplasmic RXR provides a piggyback transportation to the vitamin D receptor into the nucleus 49,50 . The nucleoplasmic sublocalization to the splicing factor compartment may also be another factor controlling RXR transcriptional activities 51 .…”

The Role of Vitamin D in the Prevention and Treatment of Prostate Cancer

“…Once activated, 1,25(OH) 2 D 3 (also known as calcitriol) can bind the vitamin D receptor (VDR) within the cytosol (Figure 2). Upon binding, conformational changes occur that expose the retinoid X receptor (RXR) dimerization domains and the nuclear localization domains, allowing the VDR and the RXR to heterodimerize and enter the nucleus (Yasmin et al, 2005). Nuclear receptor co-activators such as DRIP/Mediator and SRC/p160 associate with the 1,25(OH) 2 D 3 -VDR-RXR complex and regulate its transcriptional activity (Rachez & Freedman, 2000;MacDonald et al, 2001).…”