Determinants of Retinoid X Receptor Transcriptional Antagonism

The synthesis and bioactivity of the retinoid X receptor (RXR) antagonist 4-[(3'-n-butyl-5',6',7',8'-tetrahydro-5',5',8',8'-tetramethyl-2'-naphthalenyl)(cyclopropylidene)methyl]benzoic acid and several heteroatom-substituted analogues are described. Ligand design was based on the scaffold of the 3'-methyl RXR-selective agonist analogue and reports that 3'-n-propyl and longer n-alkyl groups conferred RXR antagonism. The transcriptional antagonism of the 3'-n-butyl analogue was demonstrated by its blockade of retinoic acid receptor (RAR) beta expression induced by the RXRalpha/peroxisome proliferator-activated receptor (PPAR) gamma heterodimer complexed with an RXRalpha agonist plus the PPARgamma agonist ciglitazone and the inhibition of 9-cis-RA-induced coactivator SRC-1a recruitment to RXRalpha. Receptor-ligand docking studies using full-atom flexible ligand and flexible receptor suggested that binding of the antagonist to the RXRalpha antagonist conformation was favored because the salt bridge that formed between the retinoid carboxylate and the RXRalpha helix H5 arginine-321 was far stronger than that formed on its binding to the agonist conformation. The antagonist also blocked activation of RAR subtypes alpha and beta by 9-cis-RA but not that of RARgamma.