In human HL-60 promyelocytic leukemia cells, diazepinylbenzoic acid derivatives can exhibit either antagonistic or synergistic effects on the differentiation-inducing activities of natural or synthetic retinoids, the activity depending largely on the nature of the substituents on the diazepine ring. Thus, a benzolog of the retinoid antagonist LE135 (6), 4-(13H-10,11,12,13-tetrahydro-10, 10,13,13,15-pentamethyldinaphtho[2,3-b][1,2-e]diazepin-7-yl) benzoic acid (LE540, 17), exhibits a 1 order of magnitude higher antagonistic potential than the parental LE135 (6). In contrast, 4-[5H-2,3-(2,5-dimethyl-2,5-hexano)-5-methyldibenzo[b,e] [1,4]diazepin-11-yl]-benzoic acid (HX600, 7), a structural isomer of the antagonistic LE135 (6), enhanced HL-60 cell differentiation induced by RAR agonists, such as Am80 (2). This synergistic effect was further increased for a thiazepine, HX630 (29), and an azepine derivative, HX640 (30); both synergized with Am80 (2) more potently than HX600 (7). Notably, the negative and positive effects of the azepine derivatives on retinoidal actions can be related to their RAR-antagonistic and RXR-agonistic properties, respectively, in the context of the RAR-RXR heterodimer.

Mutations of a single residue in the retinoid X receptor α (RXRα) ligand‐binding pocket (LBP) generate constitutive, ligand‐binding‐competent mutants with structural and functional characteristics similar to those of agonist‐bound wild‐type RXR. Modelling of the mouse RXRαF318A LBP suggests that, like agonist binding, the mutation disrupts a cluster of van der Waals interactions that maintains helix H11 in the apo‐receptor location, thereby shifting the thermodynamic equilibrium to the holo form. Heterodimerization with some apo‐receptors (retinoic acid, thyroid hormone and vitamin D3 receptors) results in ‘silencing’ of RXRαF318A constitutive activity, which, on the other hand, efficiently contributes to synergistic transactivation within NGFI‐B‐RXR heterodimers. RAR mutants disabled for corepressor binding and/or lacking a functional AF‐2 activation domain, do not relieve RXR ‘silencing’. Not only RAR agonists, but also the RAR antagonist BMS614 induce conformational changes allowing RXR to exert constitutive (RXRαF318A) or agonist‐induced (wild‐type RXR) activity in heterodimers. Interestingly, the RXRαF318A constitutive activity generated within heterodimers in the presence of BMS614 requires the integrity of both RXR and RAR AF‐2 domains. These observations suggest that, within RXR‐RAR heterodimers, RAR can adopt a structure distinct from that of the active holo‐RAR, thus allowing RXR to become transcriptionally responsive to agonists.

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