Structure–activity relationship studies on vitamin D lactam derivatives as vitamin D receptor antagonist

Synthetic Chemical Probes That Dissect Vitamin D Activities

Vitamin D₃ metabolites are capable of controlling gene expression in mammalian cells through two independent pathways: vitamin D receptor (VDR) and sterol regulatory element-binding protein (SREBP) pathways. In the present study, we dissect the complex biological activity of vitamin D by designing synthetic vitamin D₃ analogs specific for VDR or SREBP pathway, i.e., a VDR activator that lacks SREBP inhibitory activity, or an SREBP inhibitor devoid of VDR activity. These synthetic vitamin D probes permitted identification of one of the vitamin D-responsive genes, Soat1, as an SREBP-suppressed gene. The chemical probes developed in the present study may prove useful in dissecting the intricate interplay of vitamin D actions, thereby providing insights into how vitamin D target genes are regulated.

Synthetic studies of (23S,25R)-1α,25-dihydroxyvitamin D3 26,23-lactone (calcitriol lactone) and its derivatives

(23S,25R)-1α,25-Dihydroxyvitamin D₃ 26,23-lactone (calcitriol lactone) is a major metabolite of 1α,25-dihydroxyvitamin D₃ that binds to vitamin D receptor (VDR) and exhibits various biological activities. This lactone and its derivatives are considered to have potential as drug candidates to treat VDR-related diseases, but their biological activities have not yet been fully characterized, mainly because of their limited availability by chemical synthesis. This review deals with synthetic studies of calcitriol lactone, and its derivatives, i.e., methylene lactones (TEI-9647 and its derivatives) and calcitriol lactams (DLAMs). We also discuss their biological activities, VDR-binding affinity and structure-activity relationships.

Regulation of the vitamin D receptor by vitamin D lactam derivatives

The active metabolite of vitamin D3 , 1α,25-dihydroxyvitamin D3 , acts as a ligand for the vitamin D receptor (VDR) and activates VDR-mediated gene expression. Recently, we characterized 1α,25-dihydroxyvitamin D3 -26,23-lactams (DLAMs), which mimic vitamin D3 metabolites, as noncalcemic VDR ligands that barely activate the receptor. In this study, we present structural insights onto the regulation of VDR function by DLAMs. X-ray crystallographic analysis revealed that DLAMs induced a large conformational change in the loop region between helices H6 and H7 in the VDR ligand-binding domain. Our structural analysis suggests that targeting of the loop region may be a new mode of VDR regulation.

Inhibitors for the Vitamin D Receptor-Coregulator Interaction

The vitamin D receptor (VDR) belongs to the superfamily of nuclear receptors and is activated by the endogenous ligand 1,25-dihydroxyvitamin D3. The genomic effects mediated by VDR consist of the activation and repression of gene transcription, which includes the formation of multiprotein complexes with coregulator proteins. Coregulators bind many nuclear receptors and can be categorized according to their role as coactivators (gene activation) or corepressors (gene repression). Herein, different approaches to develop compounds that modulate the interaction between VDR and coregulators are summarized. This includes coregulator peptides that were identified by creating phage display libraries. Subsequent modification of these peptides including the introduction of a tether or nonhydrolyzable bonds resulted in the first direct VDR-coregulator inhibitors. Later, small molecules that inhibit VDR-coregulator inhibitors were identified using rational drug design and high-throughput screening. Early on, allosteric inhibition of VDR-coregulator interactions was achieved with VDR antagonists that change the conformation of VDR and modulate the interactions with coregulators. A detailed discussion of their dual agonist/antagonist effects is given as well as a summary of their biological effects in cell-based assays and in vivo studies.

Peroxisome proliferation-activated receptor δ agonist GW0742 interacts weakly with multiple nuclear receptors, including the vitamin D receptor

A high-throughput screening campaign was conducted to identify small molecules with the ability to inhibit the interaction between the vitamin D receptor (VDR) and steroid receptor coactivator 2. These inhibitors represent novel molecular probes for modulating gene regulation mediated by VDR. Peroxisome proliferator-activated receptor (PPAR) δ agonist GW0742 was among the identified VDR-coactivator inhibitors and has been characterized herein as a pan nuclear receptor antagonist at concentrations of > 12.1 μM. The highest antagonist activity for GW0742 was found for VDR and the androgen receptor. Surprisingly, GW0742 behaved as a PPAR agonist and antagonist, activating transcription at lower concentrations and inhibiting this effect at higher concentrations. A unique spectroscopic property of GW0742 was identified as well. In the presence of rhodamine-derived molecules, GW0742 increased the fluorescence intensity and level of fluorescence polarization at an excitation wavelength of 595 nm and an emission wavelength of 615 nm in a dose-dependent manner. The GW0742-inhibited NR-coactivator binding resulted in a reduced level of expression of five different NR target genes in LNCaP cells in the presence of agonist. Especially VDR target genes CYP24A1, IGFBP-3, and TRPV6 were negatively regulated by GW0742. GW0742 is the first VDR ligand inhibitor lacking the secosteroid structure of VDR ligand antagonists. Nevertheless, the VDR-meditated downstream process of cell differentiation was antagonized by GW0742 in HL-60 cells that were pretreated with the endogenous VDR agonist 1,25-dihydroxyvitamin D3.

Discovery of the first irreversible small molecule inhibitors of the interaction between the vitamin D receptor and coactivators

The vitamin D receptor (VDR) is a nuclear hormone receptor that regulates cell proliferation, cell differentiation, and calcium homeostasis. The receptor is activated by vitamin D analogues that induce the disruption of VDR-corepressor binding and promote VDR-coactivator interactions. The interactions between VDR and coregulators are essential for VDR-mediated transcription. Small molecule inhibition of VDR-coregulator binding represents an alternative method to the traditional ligand-based approach in order to modulate the expression of VDR target genes. A high throughput fluorescence polarization screen that quantifies the inhibition of binding between VDR and a fluorescently labeled steroid receptor coactivator 2 peptide was applied to discover the new small molecule VDR-coactivator inhibitors, 3-indolylmethanamines. Structure-activity relationship studies with 3-indolylmethanamine analogues were used to determine their mode of VDR-binding and to produce the first VDR-selective and irreversible VDR-coactivator inhibitors with the ability to regulate the transcription of the human VDR target gene TRPV6.

An o-aminoanilide analogue of 1α,25-dihydroxyvitamin D(3) functions as a strong vitamin D receptor antagonist

Vitamin D receptor (VDR) antagonists have therapeutic potential in treatment of allergic conditions and hypercalcemia driven by granulomatous diseases. We have identified an o-aminoanilide analogue of the hormonal form of vitamin D with a dienyl side chain that functions as a strong VDR antagonist. Modeling studies indicate that antagonism arises from side chain rigidity, when compared to a more flexible saturated analogue, which interferes with H12 folding/alignment.