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.

To read the full article click HERE.

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

Tagged With: