Figure 5.

Model illustrating ligand responsiveness of retinoid receptors in different RXR dimers. Ligands interacting with a receptor partner are indicated below that partner. Level of induced transcriptional activation indicated by number of + symbols. RAR ligands RA and 4-oxo-RA activate RAR–RXR heterodimers via RAR only. 9-cis-RA activates both RARs and RXRs in RAR–RXR heterodimers, which generally leads to higher activation levels. 9-Cis-4-oxo-RA (9c4oRA) behaves as 9-cis-RA in RAR–RXR heterodimers by activating both partners. 9-cis-RA, but not 9-cis-4-oxo-RA, activates RXR homodimers. Other 9-cis-RA-responsive pathways involving heterodimers between RXR and orphan receptors (ORs) remain to be tested for 9-cis-4-oxo-RA responsiveness.