Adaptability of the Vitamin D nuclear receptor to the synthetic ligand Gemini: remodelling the LBP with one side chain rotation

Design, Synthesis, and Biological Evaluation of New Type of Gemini Analogues with a Cyclopropane Moiety in Their Side Chain

We synthesized two new gemini analogues, UG-480 and UG-481, that incorporate a modified longer side chain containing a cyclopropane group. The evaluation of the bioactivities of the two gemini analogues indicated that the 17,20 threo (20S) compound, UG-480, is the most active one and is as active as 1,25(OH)2D3. Docking and molecular dynamics (MD) data showed that the compounds bind efficiently to vitamin D receptor (VDR) with UG-480 to form an energetically more favorable interaction with His397. Structural analysis indicated that whereas the UG-480 compound efficiently stabilizes the active VDR conformation, it induces conformational changes in the H6-H7 VDR region that are greater than those induced by the parental Gemini and that this is due to the occupancy of the secondary channel by its modified side chain.

4-Hydroxy-1α,25-Dihydroxyvitamin D3: Synthesis and Structure-Function Study

The active vitamin D metabolites, 25-hydroxyvitamin D3 (25D3) and 1,25-dihydroxyvitamin D3 (1,25D3), are produced by successive hydroxylation steps and play key roles in several cellular processes. However, alternative metabolic pathways exist, and among them, the 4-hydroxylation of 25D3 is a major one. This study aims to investigate the structure-activity relationships of 4-hydroxy derivatives of 1,25D3. Structural analysis indicates that 1,4α,25(OH)3D3 and 1,4β,25(OH)3D3 maintain the anchoring hydrogen bonds of 1,25D3 and form additional interactions, stabilizing the active conformation of VDR. In addition, 1,4α,25D3 and 1,4β,25D3 are as potent as 1,25D3 in regulating the expression of VDR target genes in rat intestinal epithelial cells and in the mouse kidney. Moreover, these two 4-hydroxy derivatives promote hypercalcemia in mice at a dose similar to that of the parent compound.

Further Studies on the Highly Active Des-C-Ring and Aromatic-D-Ring Analogues of 1α,25-Dihydroxyvitamin D3 (Calcitriol): Refinement of the Side Chain

Current efforts in the vitamin D field are directed toward the development of highly antiproliferative yet noncalcemic analogues of the natural hormone 1α,25-dihydroxyvitamin D3 (1,25D3). We have recently reported the design, synthesis, biological evaluation, and crystal structures of a series of novel analogues that both lack the steroidal C-ring and have an m-phenylene ring replacing the steroidal cyclopentane D-ring. We have now investigated the potentiating effects of incorporating selected modifications (hexafluorination and/or an internal triple bond) within the steroidal side chain in our series. An alternative synthetic strategy (Wittig-Horner approach instead of our previously used Pd-catalyzed tandem cyclization/cross-coupling) for the construction of the vitamin D triene system was found convenient for the target compounds 2, 3a, 3b, and 3c of this report. These modifications enhance vitamin D nuclear receptor (VDR) interactions and consequently VDR-associated biological properties compared to parental PG-136 compound while maintaining normal calcium levels.

Structural analysis and biological activities of C25-amino and C25-nitro vitamin D analogs

Intense synthetic efforts have been directed towards the development of noncalcemic analogs of 1,25-dihydroxyvitamin D₃. We describe here the structural analysis and biological evaluation of two derivatives of 1,25-dihydroxyvitamin D₃ with modifications limited to the replacement of the 25-hydroxyl group by a 25-amino or 25-nitro groups. Both compounds are agonists of the vitamin D receptor. They mediate biological effects similar to 1,25-dihydroxyvitamin D₃, the 25-amino derivative being the most potent one while being less calcemic than 1,25-dihydroxyvitamin D₃. The in vivo properties of the compounds make them of potential therapeutic value.

Design, Synthesis, Biological Activity, and Structural Analysis of Novel Des-C-Ring and Aromatic-D-Ring Analogues of 1α,25-Dihydroxyvitamin D3

The toxic calcemic effects of the natural hormone 1α,25-dihydroxyvitamin D₃ (1,25D₃, 1,25-dihydroxycholecalciferol) in the treatment of hyperproliferative diseases demand the development of highly active and noncalcemic vitamin D analogues. We report the development of two highly active and noncalcemic analogues of 1,25D₃ that lack the C-ring and possess an m-phenylene ring that replaces the natural D-ring. The new analogues (3a, 3b) are characterized by an additional six-carbon hydroxylated side chain attached either to the aromatic nucleus or to the triene system. Both compounds were synthesized by the Pd-catalyzed tandem cyclization/cross coupling approach starting from alkyne 6 and diphenol 8. Key steps include a stereoselective Cu-assisted addition of a Grignard reagent to an aromatic alkyne and a Takai olefination of an aromatic aldehyde. The new compounds are noncalcemic and show transcriptional and antiproliferative activities similar to 1,25D₃. Structural analysis revealed that they induce a large conformational rearrangement of the vitamin D receptor around helix 6.

Vitamin D Analogs Bearing C-20 Modifications Stabilize the Agonistic Conformation of Non-Responsive Vitamin D Receptor Variants

The Vitamin D receptor (VDR) plays a key role in calcium homeostasis, as well as in cell proliferation and differentiation. Among the large number of VDR ligands that have been developed, we have previously shown that BXL-62 and Gemini-72, two C-20-modified vitamin D analogs are highly potent VDR agonists. In this study, we show that both VDR ligands restore the transcriptional activities of VDR variants unresponsive to the natural ligand and identified in patients with rickets. The elucidated mechanisms of action underlying the activities of these C-20-modified analogs emphasize the mutual adaptation of the ligand and the VDR ligand-binding pocket.

Vitamin D and Its Receptor from a Structural Perspective

The activities of 1α,25-dihydroxyvitamin D3, 1,25D₃, are mediated via its binding to the vitamin D receptor (VDR), a ligand-dependent transcription factor that belongs to the nuclear receptor superfamily. Numerous studies have demonstrated the important role of 1,25D₃ and VDR signaling in various biological processes and associated pathologies. A wealth of information about ligand recognition and mechanism of action by structural analysis of the VDR complexes is also available. The methods used in these structural studies were mainly X-ray crystallography complemented by NMR, cryo-electron microscopy and structural mass spectrometry. This review aims to provide an overview of the current knowledge of VDR structures and also to explore the recent progress in understanding the complex mechanism of action of 1,25D₃ from a structural perspective.

Advances in Vitamin D Receptor Function and Evolution Based on the 3D Structure of the Lamprey Ligand-Binding Domain

1α,25-dihydroxyvitamin D3 (1,25D₃) regulates many physiological processes in vertebrates by binding to the vitamin D receptor (VDR). Phylogenetic analysis indicates that jawless fishes are the most basal vertebrates exhibiting a VDR gene. To elucidate the mechanism driving VDR activation during evolution, we determined the crystal structure of the VDR ligand-binding domain (LBD) complex from the basal vertebratePetromyzon marinus, sea lamprey (lVDR). Comparison of three-dimensional crystal structures of the lVDR-1,25D₃ complex with higher vertebrate VDR-1,25D₃ structures suggests that 1,25D₃ binds to lVDR similarly to human VDR, but with unique features for lVDR around linker regions between H11 and H12 and between H9 and H10. These structural differences may contribute to the marked species differences in transcriptional responses. Furthermore, residue co-evolution analysis of VDR across vertebrates identifies amino acid positions in H9 and the large insertion domain VDR LBD specific as correlated.

Design, Synthesis, Evaluation and Structure of Allenic 1α,25-Dihydroxyvitamin D3 Analogs with Locked Mobility at C-17

Vitamin D receptor ligands have potential for the treatment of hyperproliferative diseases and disorders related to the immune system. However, hypercalcemic effects limit their therapeutical uses and call for the development of tissue-selective new analogs. We have designed and synthesized the first examples of 1α,25-dihydroxyvitamin D3 analogs bearing an allenic unit attached to the D ring to restrict the side-chain conformational mobility. The triene system was constructed by a Pd0 -mediated cyclization/Suzuki-Miyaura cross-coupling process in the presence of an allenic side chain. The allenic moiety was built through an orthoester-Claisen rearrangement of a propargylic alcohol. The biological activity and structure of (22S)-1α,25-dihydroxy-17,20-dien-24-homo-21-nor-vitamin D3 bound to binding domain of the vitamin D receptor, provide information concerning side-chain conformational requirements for biological activity.

A structural signature motif enlightens the origin and diversification of nuclear receptors

Nuclear receptors are ligand-activated transcription factors that modulate gene regulatory networks from embryonic development to adult physiology and thus represent major targets for clinical interventions in many diseases. Most nuclear receptors function either as homodimers or as heterodimers. The dimerization is crucial for gene regulation by nuclear receptors, by extending the repertoire of binding sites in the promoters or the enhancers of target genes via combinatorial interactions. Here, we focused our attention on an unusual structural variation of the α-helix, called π-turn that is present in helix H7 of the ligand-binding domain of RXR and HNF4. By tracing back the complex evolutionary history of the π-turn, we demonstrate that it was present ancestrally and then independently lost in several nuclear receptor lineages. Importantly, the evolutionary history of the π-turn motif is parallel to the evolutionary diversification of the nuclear receptor dimerization ability from ancestral homodimers to derived heterodimers. We then carried out structural and biophysical analyses, in particular through point mutation studies of key RXR signature residues and showed that this motif plays a critical role in the network of interactions stabilizing homodimers. We further showed that the π-turn was instrumental in allowing a flexible heterodimeric interface of RXR in order to accommodate multiple interfaces with numerous partners and critical for the emergence of high affinity receptors. Altogether, our work allows to identify a functional role for the π-turn in oligomerization of nuclear receptors and reveals how this motif is linked to the emergence of a critical biological function. We conclude that the π-turn can be viewed as a structural exaptation that has contributed to enlarging the functional repertoire of nuclear receptors.

The origin of novelties is a central topic in evolutionary biology. A fundamental question is how organisms constrained by natural selection can divert from existing schemes to set up novel structures or pathways. Among the most important strategies are exaptations, which represent pre-adaptation strategies. Many examples exist in biology, at both morphological and molecular levels, such as the one reported here that focuses on an unusual structural feature called the π-turn. It is found in the structure of the most ancestral nuclear receptors RXR and HNF4. The analyses trace back the complex evolutionary history of the π-turn to more than 500 million years ago, before the Cambrian explosion and show that this feature was essential for the heterodimerization capacity of RXR. Nuclear receptor lineages that emerged later in evolution lost the π-turn. We demonstrate here that this loss in nuclear receptors that heterodimerize with RXR was critical for the emergence of high affinity receptors, such as the vitamin D and the thyroid hormone receptors. On the other hand, the conserved π-turn in RXR allowed it to accommodate multiple heterodimer interfaces with numerous partners. This structural exaptation allowed for the remarkable diversification of nuclear receptors.

Endocrine disruption of vitamin D activity by perfluoro-octanoic acid (PFOA)

Perfluoroalkyl substances (PFAS) are a class of compounds used in industry and consumer products. Perfluorooctanoic acid (PFOA) is the predominant form in human samples and has been shown to induce severe health consequences, such as neonatal mortality, neurotoxicity, and immunotoxicity. Toxicological studies indicate that PFAS accumulate in bone tissues and cause altered bone development. Epidemiological studies have reported an inverse relationship between PFAS and bone health, however the associated mechanisms are still unexplored. Here, we present computational, in silico and in vitro evidence supporting the interference of PFOA on vitamin D (VD). First, PFOA competes with calcitriol on the same binding site of the VD receptor, leading to an alteration of the structural flexibility and a 10% reduction by surface plasmon resonance analysis. Second, this interference leads to an altered response of VD-responsive genes in two cellular targets of this hormone, osteoblasts and epithelial cells of the colorectal tract. Third, mineralization in human osteoblasts is reduced upon coincubation of PFOA with VD. Finally, in a small cohort of young healthy men, PTH levels were higher in the exposed group, but VD levels were comparable. Altogether these results provide the first evidence of endocrine disruption by PFOA on VD pathway by competition on its receptor and subsequent inhibition of VD-responsive genes in target cells.

Comprehensive review for anticancer hybridized multitargeting HDAC inhibitors

Despite the encouraging clinical progress of chemotherapeutic agents in cancer treatment, innovation and development of new effective anticancer candidates still represents a challenging endeavor. With 15 million death every year in 2030 according to the estimates, cancer has increased rising of an alarm as a real crisis for public health and health systems worldwide. Therefore, scientist began to introduce innovative solutions to control the cancer global health problem. One of the promising strategies in this issue is the multitarget or smart hybrids having two or more pharmacophores targeting cancer. These rationalized hybrid molecules have gained great interests in cancer treatment as they are capable to simultaneously inhibit more than cancer pathway or target without drug-drug interactions and with less side effects. A prime important example of these hybrids, the HDAC hybrid inhibitors or referred as multitargeting HDAC inhibitors. The ability of HDAC inhibitors to synergistically improve the efficacy of other anti-cancer drugs and moreover, the ease of HDAC inhibitors cap group modification prompt many medicinal chemists to innovate and develop new generation of HDAC hybrid inhibitors. Notably, and during this short period, there are four HDAC inhibitor hybrids have entered different phases of clinical trials for treatment of different types of blood and solid tumors, namely; CUDC-101, CUDC-907, Tinostamustine, and Domatinostat. This review shed light on the most recent hybrids of HDACIs with one or more other cancer target pharmacophore. The designed multitarget hybrids include topoisomerase inhibitors, kinase inhibitors, nitric oxide releasers, antiandrogens, FLT3 and JAC-2 inhibitors, PDE5-inhibitors, NAMPT-inhibitors, Protease inhibitors, BRD4-inhibitors and other targets. This review may help researchers in development and discovery of new horizons in cancer treatment.

Structural Analysis of VDR Complex with ZK168281 Antagonist

Vitamin D receptor (VDR) antagonists prevent the VDR activation function helix 12 from folding into its active conformation, thus affecting coactivator recruitment and antagonizing the transcriptional regulation induced by 1α,25-dihydroxyvitamin D3. Here, we report the crystal structure of the zebrafish VDR ligand-binding domain in complex with the ZK168281 antagonist, revealing that the ligand prevents optimal folding of the C-terminal region of VDR. This interference was confirmed by hydrogen-deuterium exchange mass spectrometry (HDX-MS) in solution.

Synthesis of Gemini analogs of 19-norcalcitriol and their platinum(II) complexes

Hormonally active vitamin D₃ metabolite, calcitriol, plays an important role in calcium-phosphate homeostasis, immune system actions and cell differentiation. Although anticancer activity of calcitriol is well documented and thousands of its analogs have been synthesized, none has been approved as a potential drug against cancer. Therefore, we attempted to introduce the cytotoxic effect to the calcitriol molecule by its linking to cisplatin. Herein, we present the synthesis of vitamin D compounds, designed on the basis of molecular modeling and docking experiments to the vitamin D receptor, and characterized by the presence of significantly different two side chains attached to C-20. In this study, a new synthetic approach to Gemini analogs was developed. Preparation of the target 19-norcalcitriol compounds involved separate syntheses of several building blocks (the A-ring, C/D-rings and side-chain fragments). The convergent synthetic strategy was used to combine these components by the different coupling processes, the crucial one being Wittig-Horner reaction of the Grundmann ketone analog with the known 2-methylene A-ring phosphine oxide. Due to the nature of the constructed steroidal side chains (bidentate ligands), which allowed coordination of metal ions, the first conjugate-type platinum(II) complexes of the vitamin D analogs were also successfully prepared and characterized. The target vitamin D compounds, displaying significant affinity for a vitamin D receptor, were assessed in vitro for their anti-proliferative activities towards several cell lines.

Vitamin D and Its Synthetic Analogs

For many individuals, in particular during winter, supplementation with the secosteroid vitamin D₃ is essential for the prevention of bone disorders, muscle weakness, autoimmune diseases, and possibly also different types of cancer. Vitamin D₃ acts via its metabolite 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] as potent agonist of the transcription factor vitamin D receptor (VDR). Thus, vitamin D directly affects chromatin structure and gene regulation at thousands of genomic loci, i.e., the epigenome and transcriptome of its target tissues. Modifications of 1,25(OH)₂D₃ at its side-chain, A-ring, triene system, or C-ring, alone and in combination, as well as nonsteroidal mimics provided numerous potent VDR agonists and some antagonists. The nearly 150 crystal structures of VDR’s ligand-binding domain with various vitamin D compounds allow a detailed molecular understanding of their action. This review discusses the most important vitamin D analogs presented during the past 10 years and molecular insight derived from new structural information on the VDR protein.

Mutation goals in the vitamin D receptor predicted by computational methods

The mechanism through which nuclear receptors respond differentially to structurally distinct agonists is a poorly understood process. We present a computational method that identifies nuclear receptor amino acids that are likely involved in biological responses triggered by ligand binding. The method involves tracing how structural changes spread from the ligand binding pocket to the sites on the receptor surface, which makes it a good tool for studying allosteric effects. We employ the method to the vitamin D receptor and verify that the identified amino acids are biologically relevant using a broad range of experimental data and a genome browser. We infer that surface vitamin D receptor residues K141, R252, I260, T280, T287 and L417 are likely involved in cell differentiation and antiproliferation, whereas P122, D149, K321, E353 and Q385 are linked to carcinogenesis.

The Use of Vitamin D Metabolites and Analogues in the Treatment of Chronic Kidney Disease

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are associated with abnormalities in bone and mineral metabolism, known as CKD-bone mineral disorder. CKD and ESRD cause skeletal abnormalities characterized by hyperparathyroidism, mixed uremic osteodystrophy, osteomalacia, adynamic bone disease, and frequently enhanced vascular and ectopic calcification. Hyperparathyroidism and mixed uremic osteodystrophy are the most common manifestations due to phosphate retention, reduced concentrations of 1,25-dihydroxyvitamin D, intestinal calcium absorption, and negative calcium balance. Treatment with 1-hydroxylated vitamin D analogues is useful.

Structural analysis and biological activities of BXL0124, a gemini analog of vitamin D

Gemini analogs of calcitriol, characterized by the extension of the C21-methyl group of calcitriol with a second chain, act as agonists of the vitamin D receptor (VDR). This second side chain of gemini is accommodated in a new cavity inside the VDR created by the structural rearrangement of the protein core. The resulting conformational change preserves the active state of the receptor and bestows gemini compounds with biological activities that exceed those of calcitriol. Of particular interest are gemini's anti-cancer properties, and in this study we demonstrate anti-proliferative and tumor-reducing abilities of BXL0124 and BXL0097, differing only by the presence or absence, respectively, of the methylene group on the A ring. BXL0124 acts as a more potent VDR agonist than its 19-nor counterpart by activating VDR-mediated transcription at lower concentrations. In a similar manner, BXL0124 is more active than BXL0097 in growth inhibition of breast cancer cells and reduction of tumor volume. Structural comparisons of BXL0097 and BXL0124, as their VDR complexes, explain the elevated activity of the latter.

Structural aspects of Vitamin D endocrinology

1α,25-Dihydroxvitamin D₃ (1,25(OH)₂D₃) is the hormonally active form of vitamin D₃. Its synthesis and its metabolites, their transport and elimination as well as action on transcriptional regulation involves the harmonic cooperation of diverse proteins with vitamin D binding capacities such as vitamin D binding protein (DBP), cytochrome P450 enzymes or the nuclear vitamin receptor (VDR). The genomic mechanism of 1,25(OH)₂D₃ action involves its binding to VDR that functionally acts as a heterodimer with retinoid X receptor. The crystal structures of the most important proteins for vitamin D₃, VDR, DBP, CYP2R1 and CYP24A1, have provided identification of mechanisms of actions of these proteins and those mediating VDR-regulated transcription. This review will present the structural information on recognition of the vitamin D₃ and metabolites by CYP proteins and DBP as well as the structural basis of VDR activation by 1,25(OH)₂D₃ and metabolites. Additionally, we will describe, the implications of the VDR mutants associated with hereditary vitamin D-resistant rickets (HVDRR) that display impaired function.

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

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.

Vitamin D receptor 2016: novel ligands and structural insights

INTRODUCTION

Vitamin D₃ activates via its hormonal form 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the transcription factor vitamin D receptor (VDR). VDR is expressed in most human tissues and has more than 1,000 target genes. Thus, 1α,25(OH)₂D₃ and its synthetic analogs have a broad physiological impact. The crystal structures of the VDR ligand-binding domain (LBD), and its various ligands, allows further the understanding of the receptor's molecular actions. Areas covered: We discuss the most important novel VDR ligands and the further insight derived from new structural information on VDR. Expert opinion: There is an increasing appreciation of the impact of vitamin D and its receptor VDR not only in bone biology, but also for metabolic diseases, immunological disorders, and cancer. Detailed structural analysis of the interaction of additional novel ligands with VDR highlight helices 6 and 7 of the LBD as being most critical for stabilizing the receptor for an efficient interaction with co-activator proteins, i.e. for efficient agonistic action. This permits the design of even more effective VDR agonists. In addition, chemists took more liberty in replacing major parts of the 1α,25(OH)₂D₃ molecule, such as the A- and CD-rings or the side chain, with significantly different structures, such as carboranes, and still obtained functional VDR agonists.

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

1,25-Dihydroxvitamin D3 [1,25(OH)2D3] is the hormonally active form of vitamin D. The genomic mechanism of 1,25(OH)2D3 action involves the direct binding of the 1,25(OH)2D3 activated vitamin D receptor/retinoic X receptor (VDR/RXR) heterodimeric complex to specific DNA sequences. Numerous VDR co-regulatory proteins have been identified, and genome-wide studies have shown that the actions of 1,25(OH)2D3 involve regulation of gene activity at a range of locations many kilobases from the transcription start site. The structure of the liganded VDR/RXR complex was recently characterized using cryoelectron microscopy, X-ray scattering, and hydrogen deuterium exchange. These recent technological advances will result in a more complete understanding of VDR coactivator interactions, thus facilitating cell and gene specific clinical applications. Although the identification of mechanisms mediating VDR-regulated transcription has been one focus of recent research in the field, other topics of fundamental importance include the identification and functional significance of proteins involved in the metabolism of vitamin D. CYP2R1 has been identified as the most important 25-hydroxylase, and a critical role for CYP24A1 in humans was noted in studies showing that inactivating mutations in CYP24A1 are a probable cause of idiopathic infantile hypercalcemia. In addition, studies using knockout and transgenic mice have provided new insight on the physiological role of vitamin D in classical target tissues as well as evidence of extraskeletal effects of 1,25(OH)2D3 including inhibition of cancer progression, effects on the cardiovascular system, and immunomodulatory effects in certain autoimmune diseases. Some of the mechanistic findings in mouse models have also been observed in humans. The identification of similar pathways in humans could lead to the development of new therapies to prevent and treat disease.

Structural Studies of Vitamin D Nuclear Receptor Ligand-Binding Properties

The vitamin D nuclear receptor (VDR) and its natural ligand, 1α,25-dihydroxyvitamin D3 hormone (1,25(OH)2D3, or calcitriol), classically regulate mineral homeostasis and metabolism but also much broader range of biological functions, such as cell growth, differentiation, antiproliferation, apoptosis, adaptive/innate immune responses. Being widely expressed in various tissues, VDR represents an important therapeutic target in the treatment of diverse disorders. Since ligand binding is a key step in VDR-mediated signaling, numerous 1,25(OH)2D3 analogs have been synthesized in order to selectively modulate the receptor activity. Most of the synthetic analogs have been developed by modification of a parental compound and some of them mimic 1,25(OH)2D3 scaffold without being structurally related to it. The ability of ligands that have different size and conformation to bind to VDR and to demonstrate biological effects is intriguing, and therefore, ligand-binding properties of the receptor have been extensively investigated using a variety of biochemical, biophysical, and computational methods. In this chapter, we describe different aspects of the structure-function relationship of VDR in complex with natural and synthetic ligands coming from structural analysis. With the emphasis on the binding modes of the most promising compounds, such as secosteroidal agonists and 1,25(OH)2D3 mimics, we also highlight the action of VDR antagonists and the evidence for the existence of an alternative ligand-binding site within the receptor. Additionally, we describe the crystal structures of VDR mutants associated with hereditary vitamin D-resistant rickets that display impaired ligand-binding function.

Carborane-based design of a potent vitamin D receptor agonist

The development of a promising clinical antitumor vitamin D analog possessing a side-chain o-carborane cluster that efficiently binds to VDR by unconventional dihydrogen bonding (BH···HN) is described.

The vitamin D nuclear receptor (VDR) is a potential target for cancer therapy. It is expressed in many tumors and its ligand shows anticancer actions. To combine these properties with the application of boron neutron capture therapy (BNCT), we design and synthesize a potent VDR agonist based on the skeleton of the hormone 1α,25-dihydroxyvitamin D₃ (1,25D) and an o-carborane (dicarba-o-closo-1,2-dodecaborane) at the end of its side chain. The present ligand is the first secosteroidal analog with the carborane unit that efficiently binds to VDR and functions as an agonist with 1,25D-like potency in transcriptional assay on vitamin D target genes. Moreover it exhibits similar antiproliferative and pro-differentiating activities but is significantly less hypercalcemic than 1,25D. The crystal structure of its complex with VDR ligand binding domain reveals its binding mechanism involving boron-mediated dihydrogen bonds that mimic vitamin D hydroxyl interactions. In addition to the therapeutic interest, this study establishes the basis for the design of new unconventional vitamin D analogs containing carborane moieties for specific molecular recognition, and drug research and development.

A vitamin D receptor selectively activated by gemini analogs reveals ligand dependent and independent effects

The bioactive form of vitamin D [1,25(OH)2D3] regulates mineral and bone homeostasis and exerts potent anti-inflammatory and antiproliferative properties through binding to the vitamin D receptor (VDR). The 3D structures of the VDR ligand-binding domain with 1,25(OH)2D3 or gemini analogs unveiled the molecular mechanism underlying ligand recognition. On the basis of structure-function correlations, we generated a point-mutated VDR (VDR(gem)) that is unresponsive to 1,25(OH)2D3, but the activity of which is efficiently induced by the gemini ligands. Moreover, we show that many VDR target genes are repressed by unliganded VDR(gem) and that mineral ion and bone homeostasis are more impaired in VDR(gem) mice than in VDR null mice, demonstrating that mutations abolishing VDR ligand binding result in more severe skeletal defects than VDR null mutations. As gemini ligands induce VDR(gem) transcriptional activity in mice and normalize their serum calcium levels, VDR(gem) is a powerful tool to further unravel both liganded and unliganded VDR signaling.

Structural considerations of vitamin D signaling

Crystal structures represent the static picture in the life of a molecule giving a sneak preview what it might be in reality. Hence, it is very hard to extrapolate from these photos toward dynamic processes such as transcriptional regulation. Mechanistically VDR may be considered as molecular machine able to perform ligand-, DNA- and protein recognition, and interaction in a multi-task manner. Taking this into account the functional net effect will be the combination of all these processes. The long awaited answer to explain the differences in physiological effects for various ligands was one of the biggest disappointment that crystal structures provided since no substantial distinction could be made for the conformation of the active VDR-ligand complexes. This may have come from the limitation on the complexity of the available ligand-VDR structures. The recent studies with full length VDR-RXRα showed somewhat more comprehensive perspective for the 3D organization and possible function of the VDR-RXRα-cofactor complex. In addition to in vitro approaches, also computational tools had been introduced with the aim to get understanding on the mechanic and dynamic properties of the VDR complexes with some success. Using these methods and based on measurable descriptors such as pocket size and positions of side chains it is possible to note subtle differences between the structures. The meaning of these differences has not been fully understood yet but the possibility of a “butterfly effect” may have more extreme consequences in terms of VDR signaling. In this review, the three functional aspects (ligand-, DNA- and protein recognition, and binding) will be discussed with respect to available data as well as possible implication and questions that may be important to address in the future.

Structure-activity relationship of nonsecosteroidal vitamin D receptor modulators

The vitamin D receptor (VDR), a receptor for the secosteroid 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a promising drug target in the treatment of bone and mineral disorders, cancer, autoimmune disease, infection, and cardiovascular disease. Indeed, approximately 100 nonsecosteroidal VDR modulators (VDRMs) have been developed. Analysis of X-ray crystal structures reveals: (i) nonsecosteroidal VDRMs bind to VDR in a position similar to 1,25(OH)2D3; (ii) hydrogen bond interactions between ligands and VDR are the most important for VDR binding; (iii) hydrophobic interactions and CH-π interactions in aromatic ligands are also important for VDR binding; and (iv) exchange of C-O-C linkage to C-CH2-C linkage in VDRMs increases transactivation activity, probably as a result of an entropic effect of solvation/desolvation of molecules. Several VDRMs have better therapeutic efficacy when compared to 1,25(OH)2D3 in experimental models of cancer and osteoporosis with less induction of hypercalcemia, a major potential adverse effect in the clinical application of VDR ligands.

Structural insights into the molecular mechanism of vitamin D receptor activation by lithocholic acid involving a new mode of ligand recognition

The vitamin D receptor (VDR), an endocrine nuclear receptor for 1α,25-dihydroxyvitamin D3, acts also as a bile acid sensor by binding lithocholic acid (LCA). The crystal structure of the zebrafish VDR ligand binding domain in complex with LCA and the SRC-2 coactivator peptide reveals the binding of two LCA molecules by VDR. One LCA binds to the canonical ligand-binding pocket, and the second one, which is not fully buried, is anchored to a site located on the VDR surface. Despite the low affinity of the alternative site, the binding of the second molecule promotes stabilization of the active receptor conformation. Biological activity assays, structural analysis, and molecular dynamics simulations indicate that the recognition of two ligand molecules is crucial for VDR agonism by LCA. The unique binding mode of LCA provides clues for the development of new chemical compounds that target alternative binding sites for therapeutic applications.

Multifunctional and potent roles of the 3-hydroxypropoxy group provide eldecalcitol's benefit in osteoporosis treatment

Eldecalcitol (1α,25-dihydroxy-2β-(3-hydroxypropoxy)vitamin D3, [developing code: ED-71]), a new osteoporosis treatment drug that was recently approved in Japan, is a best-in-class drug in the class of calcitriol (1α,25-dihydroxyvitamin D3) and its prodrug alfacalcidol (1α-hydroxyvitamin D3), which have been used to treat osteoporosis for 30 years. In a comparative Phase III clinical study with alfacalcidol in osteoporosis patients, eldecalcitol demonstrated superior efficacy in the endpoints of increment of bone mineral density and reduction of bone fracture with equivalent safety to alfacalcidol. Eldecalcitol was discovered by searching synthetic analogs of calcitriol and alfacalcidol, and its main structural characteristic is having the 3-hydroxypropoxy group at the 2β-position. This review discusses why introducing the group leads to excellent efficacy and safety in osteoporosis treatment and elucidates the functional roles of the 3-hydroxypropoxy group. Briefly, the functional roles of the group are, first, realizing the metabolism switching in which eldecalcitol shows resistance to CYP24A1 and is metabolized in the liver; second, increasing the affinity to the serum carrier protein and prolonging the half-life to 53h; and third, stabilizing the eldecalcitol-receptor complex. Taken together, these functional roles of the 3-hydroxypropoxy group are beneficial in osteoporosis treatment. This review attempts to give a detailed account of the mode of action of eldecalcitol by clarifying these multifunctional roles of the 3-hydroxypropoxy group from the medicinal chemist's perspective.

Correlation between secosteroid-induced vitamin D receptor activity in melanoma cells and computer-modeled receptor binding strength

To define the interaction of novel secosteroids produced by the action of cytochrome P450scc with vitamin D receptor (VDR), we used a human melanoma line overexpressing VDR fused with enhanced green fluorescent protein (EGFP) and tested the ligand induced translocation of VDR from the cytoplasm to the nucleus. Hydroxyderivatives of vitamin D(3) with a full length (D(3)) side chain and hydroxy-secosteroids with a shortened side chain (pD) stimulated VDR translocation and inhibited proliferation, however, with different potencies. In general the D(3) were more potent than pD analogues. Molecular modeling of the binding of the secosteroids to the VDR genomic binding pocket (G-pocket) correlated well with the experimental data for VDR translocation. In contrast, docking scores for the non-genomic binding site of the VDR were poor. In conclusion, both the length of the side chain and the number and position of hydroxyl groups affect the activation of VDR by novel secosteroids.

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.

Butyl pocket formation in the vitamin D receptor strongly affects the agonistic or antagonistic behavior of ligands

Previously, we reported that 22S-butyl-25,26,27-trinor-1α,24-dihydroxyvitamin D(3)2 represents a new class of antagonist for the vitamin D receptor (VDR). The crystal structure of the ligand-binding domain (LBD) of VDR complexed with 2 showed the formation of a butyl pocket to accommodate the 22-butyl group and insufficient interactions between ligand 2 and the C-terminus of VDR. Here, we designed and synthesized new analogues 5a-c and evaluated their biological activities to probe whether agonistic activity is recovered when the analogue restores interactions with the C-terminus of VDR. Analogues 5a-c exhibited full agonistic activity in transactivation. Interestingly, 5c, which bears a 24-diethyl group, completely recovered agonistic activity, although 3c and 4c act as an antagonist and a weak agonist, respectively. The crystal structures of VDR-LBD complexed with 3a, 4a, 5a, and 5c were solved, and the results confirmed that butyl pocket formation in VDR strongly affects the agonistic or antagonistic behaviors of ligands.

Structural analysis of nuclear receptors: from isolated domains to integral proteins

Nuclear receptors (NRs) are ligand dependent transcription factors that regulate gene expression. A number of in depth structure-function relationship studies have been performed, in particular with drug design perspectives. Recent structural results concerning integral receptors in diverse functional states, obtained using a combination of different methods, now allow a better understanding of the mechanisms involved in molecular regulation. The structural data highlight the importance of DNA sequences for binding selectivity and the role of promoter response elements in the spatial organization of the protein domains into functional complexes. The solution structures of several heterodimer complexes reveal how the DNA directs the positioning of coactivators. In the case of PPARγ-RXRα the comparison with the crystal structure reveals two different conformational states that illustrate the flexibility of the receptors. The results shed light on the dynamics of the molecular recognition process.

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.