title: Critical role of helix 12 of the vitamin D(3) receptor for the partial agonism of carboxylic ester antagonists.
abstract: The carboxy-terminal alpha-helix of a nuclear receptor ligand-binding domain (LBD), helix 12, contains a critical, ligand-modulated interface for the interaction with coactivator proteins. In this study, using the example of the vitamin D receptor (VDR) and the partial antagonist ZK159222, the role of helix 12 (residues 417-427) for both antagonistic and agonistic receptor actions was investigated. Amino acid residue G423 was demonstrated to be critical for partial agonism of ZK159222, but not for the activity of the natural VDR agonist, 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)). The amount of partial agonism of ZK159222 increased when helix 12 was truncated by the last four amino acid residues (Delta424-27) and augmented even more, when in addition helix 12 of VDR's dimerization partner, retinoid X receptor (RXR), was truncated. In contrast, the low agonism of a structural derivative of ZK159222, ZK168281, was not affected comparably, whereas other close structural relatives of ZK159222 even demonstrated the same agonistic activity as that of 1alpha,25(OH)(2)D(3). The amount of agonism of ZK159222 and ZK168281 at different variations of helix 12 correlated well with VDR's ability to complex with coactivator proteins and inversely correlated with the strength of the compound's antagonistic action on 1alpha,25(OH)(2)D(3) signalling. Molecular dynamics simulations of the LBD complexed with the two antagonists could explain their different action by demonstrating a more drastic displacement of helix 12 through ZK168281 than through ZK159222. Moreover, the modelling could indicate a kink of helix 12 at amino acid residue G423, which provides the last four amino acid residues of helix 12 with a modulatory role for the partial agonism of some VDR antagonists, such as ZK159222. In conclusion, partial agonism of a VDR antagonist is lower the more it disturbs helix 12 in taking the optimal position for coactivator interaction.
authors.affiliation: Department of Biochemistry, University of Kuopio, Finland.
authors.forename: Sami
authors.initials: S
authors.lastname: Väisänen
authors.forename: Mikael
authors.initials: M
authors.lastname: Peräkylä
authors.forename: Jouni I
authors.initials: JI
authors.lastname: Kärkkäinen
authors.forename: Andreas
authors.initials: A
authors.lastname: Steinmeyer
authors.forename: Carsten
authors.initials: C
authors.lastname: Carlberg
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D002117:Calcitriol / Q000031:analogs & derivatives* / Q000494:pharmacology; D003198:Computer Simulation; D019281:Dimerization; D024202:Electrophoretic Mobility Shift Assay; D005786:Gene Expression Regulation / Q000187:drug effects; D017930:Genes, Reporter; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D018167:Receptors, Calcitriol / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D018168:Receptors, Retinoic Acid / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D047488:Retinoid X Receptors; D017384:Sequence Deletion / Q000235:genetics; D013329:Structure-Activity Relationship; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D008024:Ligands; D018167:Receptors, Calcitriol; D018168:Receptors, Retinoic Acid; D047488:Retinoid X Receptors; D014157:Transcription Factors; C417690:ZK 168281; C410228:ZK159222; D002117:Calcitriol