Allosteric regulation of the discriminative responsiveness of retinoic acid receptor to natural and synthetic ligands by retinoid X receptor and DNA

Transcriptional activation by retinoids is mediated through two families of nuclear receptors, all-trans-retinoic acid (RARs) and 9-cis retinoic acid receptors (RXRs), Conformationally restricted retinoids are used to achieve selective activation of RAR isotype alpha, beta or gamma, which reduces si de effects in therapeutical applications, Synthetic retinoids mimic some of all-trans retinoic acid biological effects in vivo but interact differentl y with the ligand binding domain of RAR alpha and induce distinct structura l transitions of the receptor, In this report, we demonstrate that RAR-sele ctive ligands have distinct quantitative activation properties which are re flected by their abilities to promote interaction of DNA-bound human RXR al pha (hRXR alpha)-hRAR alpha heterodimers with the nuclear receptor coactiva tor (NCoA) SRC-1 in vitro. The hormone response element core motifs spacing defined the relative affinity of liganded heterodimers for two NCoAs, SRC- 1 and RIP140, hRXR alpha activating function 2 was critical to confer hRAR alpha full responsiveness but not differential sensitivity of hRAR alpha to natural or synthetic retinoids. We also provide evidence showing that lysi nes located in helices 3 and 4, which define part of hRAR alpha NCoA bindin g surface, contribute differently to (i) the transcriptional activity and ( ii) the interaction of RXR-RAR heterodimers with SRC-1, when challenged by either natural or RAR-selective retinoids, Thus, ligand structure, DNA, and RXR exert allosteric regulations on hRAR alpha conformation organized as a DNA-bound heterodimer. We suggest that the use of physically distinct NCoA binding interfaces may be important in controlling specific genes by confo rmationally restricted ligands.