Abstract
An important focus of structure-function studies of synthetic ligands for the vitamin D receptor (VDR) concerns the chiral center at carbon 20 of the steroid side chain; 20-epi analogues are 100-10, 000 times more potent transcriptionally than the natural hormone 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3). We have compared the binding properties of three pairs of analogues either with a natural (N) or 20-epi (E) orientation. In intact cells, 45-60% of VDR.N-analogue complexes, but only 5-20% of VDR.E-analogue complexes, dissociated over a 3-h interval. The two groups of ligands induced distinct changes in VDR conformation as revealed by protease clipping assays. Mapping of ligand-VDR binding activity by deletions indicated that amino acids 420-427 were important for high affinity of VDR.N-analogue complexes, but not for VDR.E-analogue complexes. Site-directed mutagenesis revealed that residues 421 and 422 were essential for 1alpha,25-(OH)2D3-induced conformational changes, high affinity of 1alpha,25-(OH)2D3 for VDR, and transcriptional activity, but not for binding of its 20-epi analogue. In contrast, deletion of residues 396-427 abolished binding of 1alpha,25-(OH)2D3, but binding of its 20-epi analogue was still detectable. The results suggest that the ligand-binding domain of VDR has multiple and different contact sites for the two families of side chain-modified ligands, resulting in VDR.ligand complexes with different half-lives and transcriptional activities.
Highlights
The action of the vitamin D receptor (VDR),1 like that of other nuclear receptors, is dependent primarily on interaction with its biologically active ligand, 1␣,25-dihydroxyvitamin D3 (1–3)
To further study the role of the chemistry and stereochemistry of the side chain in VDR actions, we examined the transcriptional activity of three E-analogues and their N
We demonstrated for the first time that the E-analogues can activate VDR differently from the natural ligand and hypothesized that the E-analogues interacted with VDR at contact points not used by the natural hormone
Summary
The action of the vitamin D receptor (VDR),1 like that of other nuclear receptors, is dependent primarily on interaction with its biologically active ligand, 1␣,25-dihydroxyvitamin D3 (1–3). To assess exchange of unlabeled 1␣,25-(OH)2D3 or its analogues with 1␣,25-(OH)2[3H]D3, monolayers of VDR-transfected COS-1 cells were washed three times in PBS and incubated for 1 h with ligand in serum-free medium.
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