RXRα acts as a carrier for TR3 nuclear export in a 9-cis retinoic acid-dependent manner in gastric cancer cells

Abstract

Retinoid X receptor (RXR) plays a crucial role in the cross talk between retinoid receptors and other hormone receptors including the orphan receptor TR3, forming different heterodimers that transduce diverse steroid/thyroid hormone signaling. Here we show that RXRα exhibits nucleocytoplasmic shuttling in MGC80-3 gastric cancer cells and that RXRα shuttling is energy-dependent through a nuclear pore complex (NPC)-mediated pathway for its import and an intact DNA binding domain-mediated pathway for its export. In the presence of its ligand 9-cis retinoic acid, RXRα was almost exclusively located in the cytoplasm. More importantly, we also show that RXRα acts as a carrier to assist translocation of TR3, which plays an important role in apoptosis. Both RXRα and TR3 colocalized in the nucleus; however, upon stimulation by 9-cis retinoic acid they cotranslocated to the cytoplasm and then localized in the mitochondria. TR3 export depends on RXRα, as in living cells GFP-TR3 alone did not result in export from the nucleus even in the presence of 9-cis retinoic acid, whereas GFP-TR3 cotransfected with RXRα was exported out of the nucleus in response to 9-cis retinoic acid. Moreover, specific reduction of RXRα levels caused by anti-sense RXRα abolished TR3 nuclear export. In contrast, specific knockdown of TR3 by antisense-TR3 or TR3-siRNA did not affect RXRα shuttling. These results indicate that RXRα is responsible for TR3 nucleocytoplasmic translocation, which is facilitated by the RXRα ligand 9-cis retinoic acid. In addition, mitochondrial TR3, but not RXRα, was critical for apoptosis, as TR3 mutants that were distributed in the mitochondria induced apoptosis in the presence or absence of 9-cis retinoic acid. These data reveal a novel aspect of RXRα function, in which it acts as a carrier for nucleocytoplasmic translocation of orphan receptors.