Retinoid Receptors and Therapeutic Applications of RAR/RXR Modulators

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth, differentiation, survival and death. Due to their regulatory potential, these nuclear receptors (NRs) are major drug targets for a variety of pathologies, including cancer and metabolic diseases. A large amount of RAR- and RXR-selective ligands, ranging from (partial) agonists to antagonists and inverse agonists, have been designed and the corresponding structural and functional analyses have provided deep insight into the molecular basis of ligand action. Ligands regulate, via allosteric conformational changes, the ability of these NRs to interact with different sets of coregulators, which in turn recruit enzymatically active complexes/machineries. Here, we describe strategies in the design of selective RXR and RAR modulators and review the structural mechanisms by which the diverse pharmacological classes of compounds modulate receptor functions. Finally, we discuss the perspectives for retinoid- and rexinoid-based therapies.