A Novel Cytoplasmic Adaptor for Retinoic Acid Receptor (RAR) and Thyroid Receptor Functions as a Derepressor of RAR in the Absence of Retinoic Acid*

Abstract

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testisspecific derepressor of RAR.Nuclear receptors (NRs) 3 are ligand-dependent transcription factors that control diverse aspects of development and homeostasis by regulating expression of their target genes (1, 2). The NR superfamily, which shares a common structural organization composed of distinct domains, is classified into three groups, steroid hormone receptors, non-steroid hormone receptors, and orphan receptors. One of the major regulatory effects of NRs can be achieved by a change in subcellular location in response to various cellular factors. Steroid hormone receptors, such as glucocorticoid receptor and estrogen receptor, reside in the cytoplasm in association with heat shock protein 90 (Hsp90) but on the entry of their cognate ligands dissociate from Hsp90 and translocate to the nucleus to exert transcriptional activation (3-6). However, non-steroid hormone receptors, such as RAR and TR, reside solely in the nucleus regardless of their ligands in most mammalian cells. In the absence of ligands, these receptors transverse the nuclear membrane and associate with nuclear corepressors, such as nuclear receptor corepressor (NCoR1) or silencing mediator for retinoid and thyroid hormone receptors (SMRT or NCoR2) to mediate transcriptional repression (7,8). These corepressors then recruit histone deacetylase, resulting in histone deacetylaton, chromatin compaction, and silencing of target gene expression (9, 10). In the presence of ligands, corepressors dissociate, and instead, coactivators with histone acetyltransferase activity associate to mediate transcriptional activation (11,12). Despite significant progress in understanding nuclear regulation of RAR and TR, little is known about their regulation in the cytoplasm. A few reports have suggest...

Impaired testicular signaling of vitamin A and vitamin K contributes to the aberrant composition of the extracellular matrix in idiopathic germ cell aplasia

“…Different from most mammalian cells, where RA receptors (RARs) are nuclear regardless of their cognate ligands, in Sertoli cells RARs move from the cytoplasm to the nucleus only when RA is bound (42). Thus, to establish the in vivo response of Sertoli cells to RA, the number of Sertoli cells with nuclear expression of the RARa was counted, comparing the number of RARaþ Sertoli cells in SCOS tubules of SRpos and SRneg iNOA men to that in the seminiferous tubules still containing germ cells.…”

Vitamin A and retinoid signaling: genomic and nongenomic effects

“…Other studies demonstrated that, in Sertoli cells, RAR ␣ is a substrate for small ubiquitin-like modifi er-2 (SUMO-2) and that a dynamic process of sumoylation and desumoylation infl uences the cytosolic/nuclear localization of the receptor ( 153 ). In Sertoli cells and hepatic stellate cells, a cytosolic localization of RAR ␣ has been also observed upon interaction with the cytoplasmic adaptor for RAR and TR (CART1) protein or with cytoskeleton proteins that sequester the receptor out of nuclei ( 154,155 ). Given that sumoylation and protein interactions can be regulated by phosphorylation processes, one cannot exclude that a dynamic process of phosphorylation, sumoylation, and protein interaction infl uences the cytosolic/nuclear localization of RARs.…”

Effect of vitamin A on the relationship between maternal thyroid hormones in early pregnancy and fetal growth: A prospective cohort study

“…RA induces the expression of the thyroid hormone transporter and monocarboxylate transporter, which demonstrates cross-talk between RA signaling and thyroid hormone signaling in early development at the level of the thyroid hormone transporter ( 28 ). Although RAR and thyroid receptors do not seem to interact directly physically, they share some cofactors so that some form of competition may occur between the two ligands and their receptors ( 29 ). In conclusion, there are many levels at which vitamin A can interact with the physiology of the hypothalamo-pituitary-thyroid axis, which was also reflected by our findings, as significant positive correlations were detected with T3, FT3 and TSH, and negative correlations were detected with FT4.…”