Previtamin D3 with a trans-Fused Decalin CD-ring Has Pronounced Genomic Activity

Abstract

1,25(OH) 2 D 3 has a central rigid CD-bicyclic ring portion to which two flexible entities, the side chain and the seco-B,Aring, are connected. The characteristic seco structure of vitamin D 3 originates from the photolytic cleavage of the provitamin 7-dehydrocholesterol. The resulting previtamin triene (previtamin D 3 ) then reversibly thermo-isomerizes to the vitamin form in its less stable 6-s-cis conformation. Rapid rotation about the C6-C7 single bond eventually yields vitamin D 3 in its more stable extended 6-s-trans conformation (2, 3). Vitamin D 3 but also its hydroxylated metabolites are in constant equilibrium with their previtamin D 3 isomers. Previous studies with analogs of 1,25(OH) 2 D 3 that are either structurally blocked in the previtamin form (4, 5) or that only slowly isomerize to the vitamin form (6) indicated that the previtamin form of 1,25(OH) 2 D 3 is a poor activator of the genomic signaling pathway with minimal effects on cell proliferation and differentiation (7). However, the previtamin D 3 form is a potent activator of the non-genomic signal transduction pathway (5-7).The aim of this study is to investigate the biological profile and the genomic signaling pathway of four different locked 19-nor-1,25(OH) 2 -(pre)-vitamin D 3 analogs, with or without a modified CD-ring. Indeed, the sigmatropic shift required for the previtamin-vitamin D conversion cannot occur in these analogs because of the deletion of C19 (4). First, the affinity to VDR was determined as well as the stability of VDR conformation after interaction with the ligand. Second, the potency of the analogs to induce interaction of the VDR-RXR heterodimer with DNA (vitamin D-responsive elements) and TIF2 (as a representative of the p160 family of coactivators; Ref. 8) was investigated. Third, the ability of 19-nor-1,25(OH) 2 -(pre)-vitamin D 3 analogs to induce VDR-dependent transcriptional activation was studied in transient transfection experiments and by determining the induction of vitamin D 24-hydroxylase (CYP 24) gene expression. Finally, docking of the analogs in the ligand binding domain (LBD) of VDR was studied by evaluation

Analogs of Calcitriol

“…This previtamin analog represents the first previtamin with potent vitamin-D-like activity which acts through the genomic signal transduction pathway as it could bind to VDR and was equipotent to 1,25(OH) 2 D 3 in inducing interaction with VDREs and the coactivator TIF-2. Mutational analysis and in silico docking suggested that this 19-nor-trans-decalin-1,25(OH) 2 -previtamin D 3 made similar contacts with the ligand-binding pocket as 1,25(OH) 2 D 3 [26].…”

“…Interestingly, the previtamin configuration of the biologically potent 19-nor-trans-decalin-1,25(OH) 2 D 3 (19-nor-trans-decalin-1,25(OH) 2 -previtamin D 3 , GAO182) was able to inhibit cell proliferation and induce cell differentiation to the same extent as 1,25(OH) 2 D 3 [26]. This previtamin analog represents the first previtamin with potent vitamin-D-like activity which acts through the genomic signal transduction pathway as it could bind to VDR and was equipotent to 1,25(OH) 2 D 3 in inducing interaction with VDREs and the coactivator TIF-2.…”

Synthesis, Structure, and Biological Activity of des‐Side Chain Analogues of 1α,25‐Dihydroxyvitamin D3 with Substituents at C18

“…The mechanistic basis for the discrepancy between receptor binding and biological activity may be attributed to changes in VDR conformation and a modified interaction of liganded VDR with co-activator molecules, which alters the structural and functional properties of the entire DNA-binding complex. [7] Interestingly, introduction of a side chain at position C18 led to a decrease in calcemic activity. Indeed, in comparison with compound 1 a, at least 600-fold higher doses of the seven-or eight-membered C18 side chain analogues could be administered on a daily basis in NMRI vitamin D-replete mice ( Table 1).…”

“…However, other mechanisms such as increased metabolic degradation, tissue-specific differences in the binding affinities of compound 1 a and its analogues for the VDR, and different signaling pathways have been suggested to contribute. [7,8] The low calcemic activity of compound 2 a together with its potent antiproliferative and prodifferentiating activity make this compound appealing for therapeutic application.…”

“…The methodology applied for the analysis of heterodimerization with RXR is described in the Supporting Information. The transactivating assay was performed as described by Verlinden et al [7] Biological activity Affinity of the analogues to hDBP and pVDR and their antiproliferative activity were determined as described previously. [8] The protocol for in vivo evaluation of calcemic activity was approved by the ethical committee for animal research at the Katholieke Universiteit Leuven and was in accordance with the NIH Guide for the Care and Use of Laboratory Animals.…”