Retinoid X receptor (RXRα) is activated by 9-cis-retinoic acid (9cRA) and regulates transcription as a homodimer or as a heterodimer with other nuclear receptors. We have previously demonstrated that β-apo-13-carotenone, an eccentric cleavage product of β-carotene, antagonizes the activation of RXRα by 9cRA in mammalian cells overexpressing this receptor. However, the molecular mechanism of β-apo-13-carotenone’s modulation on RXRα transcriptional activity is not understood and is the subject of this report. We performed transactivation assays using full length RXRα and reporter gene constructs (RXRE-Luc) transfected into COS-7 cells and luciferase activity was examined. β-apo-13-Carotenone was compared with the RXRα antagonist UVI 3003. The results showed that both β-apo-13-carotenone and UVI 3003 shifted the dose-dependent RXRα activation by 9cRA. In contrast, results of assays using a hybrid Gal4-DBD:RXRα-LBD receptor reporter cell assay that detects 9cRA-induced coactivator binding to the ligand binding domain demonstrated that UVI3003 significantly inhibited 9cRA-induced coactivator binding to RXRαLBD, but β-apo-13-carotenone did not. However, both β-apo-13-carotenone and UVI 3003 inhibited 9-cRA induction of caspase 9 gene expression in the mammary carcinoma cell line MCF7. In order to resolve this apparent contradiction we investigated the effect of β-apo-13-carotenone on the oligomeric state of purified recombinant RXRα LBD. β-apo-13-carotenone induces tetramerization of the RXRαLBD whereas UVI3003 had no effect on the oligomeric state. These observations suggest that β-apo-13-carotenone regulates RXRα transcriptional activity through inducing the formation of the “transcriptionally silent” RXRα tetramer.
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Date of publication: 16 October 2014; JBC Papers
Author information: Jian Sun (1); Sureshbabu Narayanasamy (1,2); Robert W. Curley Jr. (2); & Earl H. Harrison (1)
(1) Department of Human Sciences, The Ohio State University, Columbus, OH 43210, USA
(2) College of Pharmacy, The Ohio State University, Columbus, OH 43210, USA