Three-dimensional structure-function relationship of vitamin D and vitamin D receptor model

Vitamin D and polymorphisms of VDR gene in patients with systemic lupus erythematosus

The susceptibility for the development of systemic lupus erythematosus (SLE) is related to environmental, hormonal, genetic, and immunological factors. Numerous genes have been linked to the emergence of SLE, including vitamin D receptor (VDR) gene that synthesizes the receptor of vitamin D. Several polymorphisms have been described since the discovery of this gene, and their effects on VDR activity are still poorly understood. Vitamin D's biological functions are mediated by VDR. Vitamin D exerts many actions on the immune system, and several studies have suggested its role in the pathogenesis of autoimmune diseases. SLE patients have low blood levels of vitamin D, which raises the possibility of association between the deficiency of this vitamin and the onset of the disease. BsmI and FokI polymorphic variants seem to be related to the onset of the disease in Asian patients. In this article, we review the aspects related to the metabolism and immunoregulatory effects of vitamin D, VDR, and main polymorphisms involving the VDR gene and the relationship between vitamin D levels and its receptor with SLE.

The yin and yang of vitamin D receptor (VDR) signaling in neoplastic progression: operational networks and tissue-specific growth control

Substantive evidence implicates vitamin D receptor (VDR) or its natural ligand 1α,25-(OH)₂ D₃ in modulation of tumor growth. However, both human and animal studies indicate tissue-specificity of effect. Epidemiological studies show both inverse and direct relationships between serum 25(OH)D levels and common solid cancers. VDR ablation affects carcinogen-induced tumorigenesis in a tissue-specific manner in model systems. Better understanding of the tissue-specificity of vitamin D-dependent molecular networks may provide insight into selective growth control by the seco-steroid, 1α,25-(OH)₂ D₃. This commentary considers complex factors that may influence the cell- or tissue-specificity of 1α,25-(OH)₂ D₃/VDR growth effects, including local synthesis, metabolism and transport of vitamin D and its metabolites, vitamin D receptor (VDR) expression and ligand-interactions, 1α,25-(OH)₂ D₃ genomic and non-genomic actions, Ca²⁺ flux, kinase activation, VDR interactions with activating and inhibitory vitamin D responsive elements (VDREs) within target gene promoters, VDR coregulator recruitment and differential effects on key downstream growth regulatory genes. We highlight some differences of VDR growth control relevant to colonic, esophageal, prostate, pancreatic and other cancers and assess the potential for development of selective prevention or treatment strategies.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

[Vitamin D deficiency in adults: to better understand a new presentation of an old disease]

Vitamin D is synthesized in skin through a reaction mediated by sunlight, and it is metabolized to 25-hydroxyvitamin D, in liver, and in 1,25-dihydroxyvitamin D, in kidney. This last reaction has a tight feedback mechanism. 1,25-dihydroxyvitamin D is the active hormone, and its actions are mediated mainly by nuclear receptors. Its major functions are in calcium metabolism and bone mass maintenance. Hypovitaminosis D, as a disease in adult people, manifests itself with hypocalcemia and secondary hyperparathyroidism with subsequent loss of trabecular bone, thinning of cortical bone, and, eventually, a higher risk of fractures. Hypovitaminosis D is a very common condition in Europe, Africa, North America and some South American countries, such as Chile and Argentina. Measurement of serum total 25-hydroxyvitamin D concentration is the gold standard to diagnose vitamin D deficiency. Serum concentrations below 50 nmol/L are associated with an increase in parathyroid hormone concentration, and bone loss. Risk factors for vitamin D deficiency, like poor sunlight exposition, aging skin and factors that interfere with normal vitamin D metabolism, are well established. Oral vitamin D supplementation, an easy and inexpensive treatment, is needed to treat this illness.

Quantitative structure-activity relationship studies of vitamin D receptor affinity for analogues of 1alpha,25-dihydroxyvitamin D3. 1: WHIM descriptors

The weighted holistic invariant molecular (WHIM) approach has been applied to the study of the VDR affinity of 86 vitamin D analogues. A model able to describe more than 71% of the variance in the experimental activity was developed with the use of the mentioned approach. In contrast, none of three different approaches, including the use of BCUT, Galvez topological charge indices, and 2D autocorrelations descriptors, was able to explain more than 38% of the variance in the mentioned property, even with more variables in the equation.

Biological activity and conformational analysis of C20 and C14 epimers of CD-ring modified trans-decalin 1alpha,25-dihydroxyvitamin D analogs

In the context of our ongoing study of vitamin D structure-function relationships and in an attempt to obtain a better dissociation of their prodifferentiating (HL-60) and/or antiproliferative (MCF-7) activities and their calcemic activity, further 20-epi and 14-epi modifications were made to three trans-decalin CD-ring analogs of 1,25-dihydroxyvitamin D(3), the hormonally active metabolite of vitamin D(3), possessing a natural 20R side chain and featuring additional structural modifications in the seco-B-ring and in the A-ring. Following a previously observed trend and in agreement with the conformational analysis results, all three 20-epi derivatives show substantially lower biological activities, opposite to what is usually observed for analogs having the natural CD-ring. The 14-epi modification (cis-decalins) has little effect on the biological activity of the ynediene type and the saturated derivative, but results in an approximate 10-fold reduction in activity of the previtamin derivative. No better dissociation of the prodifferentiating and/or antiproliferative activities and the calcemic activity was achieved.

Novel synthesis of 2-substituted 19-norvitamin D A-ring phosphine oxide from D-glucose as a building block

19-norvitamin D A-ring phosphine oxide 5 was synthesized by a new sequence mode starting from D-glucose as a chiral template. Transformation of the pyranoside ring into the A-ring carbocycle was achieved by the Pd-catalyzed Ferrier rearrangement. The phosphine oxide 5 was obtained in an 18% overall yield by this novel cost-effective method.

Affinity labeling of the nuclear vitamin D receptor with nonsteroidal alkylating agents

Synthesis of an affinity alkylating non steroidal mimic of 1alpha,25-dihydroxyvitamin D(3) and its radiolabeled counterpart is presented. We also report the affinity labeling of the VDR-ligand binding domain (VDR-LBD) with this analogue.

Structure-function relationships of vitamin D including ligand recognition by the vitamin D receptor

First, the general structure and function of nuclear receptors (NRs) are described briefly to help our understanding of the mechanism of action of vitamin D mediated by the vitamin D receptor (VDR), a member of the NRs. Then we discuss the structure-function relationship (SFR) of vitamin D on the basis of ligand structures and the interaction of the ligand with the VDR. The SFR of vitamin D side chain analogs is discussed extensively in terms of our active space group concept, which was derived from conformational analyses of the side chains of vitamin D analogs and from studies with conformationally restricted 22-methyl-1,25-(OH)(2)D(3) isomers. The mobile area of the side chain of vitamin D can be grouped into five regions (E, G, EA, EG, and F), and the SFR has been analyzed in terms of these spatial regions. The SFR of ligand/VDR interaction is discussed on the basis of the crystal structure of VDR-LBD(delta 165-215), docking of various vitamin D ligands into the ligand binding pocket (LBP) of the VDR, and functional analysis of amino acids lining the LBP. Finally, we discuss total SFR, combining the results of the two approaches, and future aspects of structure-based design of vitamin D analogs.

Synthesis of new 18-substituted analogues of calcitriol using a photochemical remote functionalization

A novel convergent synthetic approach to new analogues of calcitriol modified at the C-18 position is reported. The key step in the synthesis is the 20-hydroxyl-directed photochemical iodination of the 18-methyl group in the presence of (diacetoxyiodo)benzene. Using this methodology, two new analogues of calcitriol were prepared: the first contains a hydroxylated alkyl side chain attached at C-18 with the natural side chain replaced by an isopropylidene group; the second is a conformationally locked analogue due to an extra oxacycle between the C-18 and C-20 positions.

Rational design, synthesis, and biological activity of novel conformationally restricted vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3)

Two new vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3) (3 and 4), were rationally designed on the basis of the active space group concept previously proposed by us. The 22R ethyl group of 3 restricts the mobility of the side chain to active space regions, whereas the 22S ethyl group of 4 confines the side chain to an inactive region. The double bond at C(23) further restricts the side chain flexibility. These compounds (3 and 4) were synthesized using ortho ester Claisen rearrangement as the key step. As expected, the 22R isomer 3 has nearly 100 times higher efficacy than 1,25-dihydroxyvitamin D(3) (1) in cell differentiation, although its affinity for the vitamin D receptor (VDR) was one-seventh of that of 1. The 22S isomer 4 has significantly lower efficacy than 3. A docking study in combination with site-directed mutation analysis revealed that two carbon elongated side chain analogue 3 could be fitted in the ligand binding pocket of the VDR by adopting a stable conformation.