{beta}-apo-13-Carotenone Regulates Retinoid X Receptor Transcriptional Activity Through Tetramerization of the Receptor [Lipids]

October 16th, 2014 by Sun, J., Narayanasamy, S., Curley, R. W., Harrison, E. H.

Retinoid X receptor (RXRa) 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 beta-apo-13-carotenone, an eccentric cleavage product of bea-carotene, antagonizes the activation of RXRa by 9cRA in mammalian cells overexpressing this receptor. However, the molecular mechanism of beta-apo-13-carotenone modulation on RXRa transcriptional activity is not understood and is the subject of this report. We performed transactivation assays using full length RXRa and reporter gene constructs (RXRE-Luc) transfected into COS-7 cells and luciferase activity was examined. beta-apo-13-Carotenone was compared with the RXRα antagonist UVI 3003. The results showed that both beta-apo-13-carotenone and UVI 3003 shifted the dose-dependent RXRa activation by 9cRA. In contrast, results of assays using a hybrid Gal4-DBD:RXRa-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 beta-apo-13-carotenone did not. However, both beta-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 beta-apo-13-carotenone on the oligomeric state of purified recombinant RXRa LBD. beta-apo-13-carotenone induces tetramerization of the RXRaLBD whereas UVI3003 had no effect on the oligomeric state. These observations suggest that beta-apo-13-carotenone regulates RXRa transcriptional activity through inducing the formation of the transcriptionally silent RXRa tetramer.