The unique tryptophan residue of the vitamin D receptor is critical for ligand binding and transcriptional activation

Design, synthesis, and anti-liver fibrosis activity of novel non-steroidal vitamin D receptor agonists based on open-ring steroid scaffold

Vitamin D receptor (VDR) has emerged as a crucial target for the treatment of hepatic fibrosis, a condition characterized by excessive deposition of extracellular matrix (ECM) components leading to impaired liver function. Activation of VDR has been shown to inhibit the transformation of hepatic stellate cells (HSCs), which play a key role in the development of liver fibrosis, thus reducing ECM production. In this study, a series of 37 non-steroidal VDR agonists with novel scaffold were designed and synthesized utilizing the scaffold hopping strategy. Over one-third of these compounds demonstrated significant VDR affinity and agonistic activity. Among them, compound E15 exhibited the highest VDR agonistic activity, showing promising results in vitro by effectively inhibiting HSC activation. Further in vivo assessments of E15 in a carbon tetrachloride-induced murine model of liver fibrosis demonstrated significant anti-fibrotic activity. Histological analyses revealed a reduction in lesions, inflammatory cell infiltration, and collagen deposition. Concurrently, blood biochemical assays indicated decreased hepatic fibrosis markers and improved serum liver function indices. Notably, E15 achieved these therapeutic effects without inducing hypercalcemia, a common adverse effect associated with VDR agonists such as calcipotriol. These findings underscore the potential of E15 as a potent and safe therapeutic agent for the treatment of liver fibrosis.

Design, synthesis and anti-hepatic fibrosis activity of novel diphenyl vitamin D receptor agonists

Hepatic fibrosis poses a significant threat to human health due to excessive extracellular matrix (ECM) deposition leading to liver function damage. Ligand-activated vitamin D receptor (VDR) has been identified as an effective target for hepatic fibrosis, reducing ECM by inhibiting hepatic stellate cell (HSC) activation. Here, a series of novel diphenyl VDR agonists have been rationally designed and synthesized. Among these, compounds 15b, 16i, and 28m showed better transcriptional activity compared to sw-22, which was previously reported to be a potent non-secosteroidal VDR modulator. Moreover, these compounds exhibited outstanding efficacy to inhibit collagen deposition in vitro. In models of CCl₄-induced and bile duct ligation-induced hepatic fibrosis, compound 16i showed the most significant therapeutic effect by ultrasound imaging and histological examination. Moreover, 16i was able to repair liver tissue by reducing the expression levels of fibrosis genes and serum liver function indexes without causing hypercalcemia in mice. In conclusion, compound 16i is a potent VDR agonist with significant anti-hepatic fibrosis action both in vitro and in vivo.

Down-regulation of COX-2 activity by 1α,25(OH)2D3 is VDR dependent in endothelial cells transformed by Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor

Our previous reports showed that 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) has antiproliferative actions in endothelial cells stably expressing viral G protein-coupled receptor (vGPCR) associated with the pathogenesis of Kaposi's sarcoma. It has been reported that COX-2 enzyme, involved in the tumorigenesis of many types of cancers, is induced by vGPCR. Therefore, we investigated whether COX-2 down-regulation is part of the growth inhibitory effects of 1α,25(OH)2D3. Proliferation was measured in presence of COX-2 inhibitor Celecoxib (10-20 μM) revealing a decreased in vGPCR cell number, displaying typically apoptotic features in a dose dependent manner similarly to 1α,25(OH)2D3. In addition, the reduced cell viability observed with 20 μM Celecoxib was enhanced in presence of 1α,25(OH)2D3. Remarkably, although COX-2 mRNA and protein levels were up-regulated after 1α,25(OH)2D3 treatment, COX-2 enzymatic activity was reduced in a VDR-dependent manner. Furthermore, an interaction between COX-2 and VDR was revealed through GST pull-down and computational analysis. Additionally, high-affinity prostanoid receptors (EP3 and EP4) were found down-regulated by 1α,25(OH)2D3. Altogether, these results suggest a down-regulation of COX-2 activity and of prostanoid receptors as part of the antineoplastic mechanism of 1α,25(OH)2D3 in endothelial cells transformed by vGPCR.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) is caused by mutations in the VDR gene, and its inheritance is autosomal recessive. In this report, we aimed to confirm whether HVDRR is occasionally inherited as a dominant trait. An 18-month-old Japanese boy was evaluated for short stature and bowlegs. His father had been treated for rickets during childhood, and his paternal grandfather had bowlegs. We diagnosed him with HVDRR based on laboratory data and radiographic evidence of rickets. Sequence analyses of VDR were performed, and the functional consequences of the detected mutations were analyzed for transcriptional activity, ligand binding, and interaction with the retinoid X receptor, cofactors, and the vitamin D response element (VDRE). A novel mutation (Q400LfsX7) and a reported variant (R370H) were identified in the patient. Heterozygous Q400LfsX7 was detected in his father, and heterozygous R370H was detected in his healthy mother. Functional studies revealed that the transcriptional activity of Q400LfsX7-VDR was markedly disturbed. The mutant had a dominant-negative effect on wild-type-VDR, and the ligand binding affinity of Q400LfsX7-VDR was completely impaired. Interestingly, Q400LfsX7-VDR had a strong interaction with corepressor NCoR and could interact with VDRE without the ligand. R370H-VDR was functionally similar to wild-type-VDR. In conclusion, we found a dominant-negative mutant of VDR causing dominantly inherited HVDRR through a constitutive corepressor interaction, a mechanism similar to that in dominantly inherited thyroid hormone receptor mutations. Our report together with a reported pedigree suggested a distinct inheritance of HVDRR and enriched our understanding of VDR abnormalities.

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We report the pedigree of dominantly inherited hereditary vitamin D resistant rickets.

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A novel VDR mutation (Q400LfsX7) was identified in the pedigree.

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The mutant had a dominant-negative effect on the wild type VDR and interacted strongly with corepressor NCoR.

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The mechanism for dominantly inheritance was similar to that in dominantly inherited thyroid hormone receptor mutations.

A human vitamin D receptor mutant activated by cholecalciferol

The human vitamin D receptor (hVDR) is a member of the nuclear receptor superfamily, involved in calcium and phosphate homeostasis; hence implicated in a number of diseases, such as Rickets and Osteoporosis. This receptor binds 1α,25-dihydroxyvitamin D(3) (also referred to as 1,25(OH)(2)D(3)) and other known ligands, such as lithocholic acid. Specific interactions between the receptor and ligand are crucial for the function and activation of this receptor, as implied by the single point mutation, H305Q, causing symptoms of Type II Rickets. In this work, further understanding of the significant and essential interactions between the ligand and the receptor was deciphered, through a combination of rational and random mutagenesis. A hVDR mutant, H305F, was engineered with increased sensitivity towards lithocholic acid, with an EC(50) value of 10 μM and 40±14 fold activation in mammalian cell assays, while maintaining wild-type activity with 1,25(OH)(2)D(3). Furthermore, via random mutagenesis, a hVDR mutant, H305F/H397Y, was discovered to bind a novel small molecule, cholecalciferol, a precursor in the 1α,25-dihydroxyvitamin D(3) biosynthetic pathway, which does not activate wild-type hVDR. This variant, H305F/H397Y, binds and activates in response to cholecalciferol concentrations as low as 100 nM, with an EC(50) value of 300 nM and 70±11 fold activation in mammalian cell assays. In silico docking analysis of the variant displays a dramatic conformational shift of cholecalciferol in the ligand binding pocket in comparison to the docked analysis of cholecalciferol with wild-type hVDR. This shift is hypothesized to be due to the introduction of two bulkier residues, suggesting that the addition of these bulkier residues introduces molecular interactions between the ligand and receptor, leading to activation with cholecalciferol.

[Ethical aspects of the fluoridation of water, salt, and milk]

The article discusses two ethical aspects of the fluoridation of water, salt, and milk. First, it considers whether fluoridation contradicts the right of self-determination. Second, it discusses the chances and risks of fluoridation. The answer to the first question depends on whether people can choose other options. Freedom of choice is not simply the right to choose between different options. It is a right which defends the moral integrity of persons. Nobody should be coerced to eat or drink something which he or she rejects morally. In the political sphere, personal rights of persons can be restricted if and only if it is necessary, if there is a public interest, and if the restriction of the right is reasonable. Regarding fluoridation, even in the best risk-chance scenario, some persons have to expect a net harm. Therefore, the reasoning in favor of fluoridation has to have a specific purpose. The proclaimed reasoning is that fluoridation will benefit the worst off and is therefore a demand of justice. But this argument fails as there are other options to benefit the worst off. Even in the best risk-chance scenario, only one option is morally permissible: the fluoridation of salt, which respects the freedom of choice.

NMR assignments of tryptophan residue in apo and holo LBD-rVDR

Binding sites in the full-length, ligand-binding domain of rat vitamin D receptor (LBD-rVDR) for an active hormone derived from vitamin D (1alpha,25-dihydroxyvitamin D(3)) and three of its C-2 substituted analogs were compared by nuclear magnetic resonance (NMR) spectroscopy. Specific residue labeled with [UL]-(15)N(2) Trp allowed assignment of the side-chain H(epsilon1) and N(epsilon1) resonances of the single tryptophan residue at position 282 in LBD-rVDR. Comparison of (1)H[(15)N] Heteronuclear Single Quantum Correlation (HSQC) spectra of apo and holo LBD-rVDR revealed that the position of the Trp282 H(epsilon1) and N(epsilon1) signals are sensitive to the presence of the ligand in the receptor cavity. Binding of the ligands to LBD-rVDR results in a shift of both Trp H(epsilon1) and N(epsilon1) resonances to lower frequencies. The results indicate that the interaction between the ligands and Trp282 is not responsible for differences in calcemic activity observed in vitamin D analogs.

Nitrated and Oxidized Products of a Single Tryptophan Residue in Human Cu,Zn-Superoxide Dismutase Treated with Either Peroxynitrite-Carbon Dioxide or Myeloperoxidase-Hydrogen Peroxide-Nitrite

We reported previously that a single tryptophan residue, Trp32, in human Cu,Zn-superoxide dismutase is specifically modified by peroxynitrite-CO2 [Yamakura et al. (2001) Biochim. Biophys. Acta 1548, 38-46]. In this study, we modified Cu,Zn-superoxide dismutase by using a combination of myeloperoxidase, hydrogen peroxide, and nitrite. The modified enzyme showed no loss of copper and zinc, and 15% less enzymatic activity. Trp32 was the only significant amino acid lost. After trypsin digestion of the modified SOD with peroxynitrite-CO2 and the myeloperoxidase system, six newly appearing peptides containing tryptophan derivatives were observed on microLC-ESI-Q-TOF mass analyses and HPLC with a photodiode-array detector. The derivatives of the tryptophan residue exhibiting mass increases of 4, 16 (2 peaks), 32, 45 (major), and 45 Da (minor) were identified as kynurenine, oxindole-3-alanine and its derivatives, dihydroxytryptophan, 6-nitrotryptophan and 5-nitrotryptophan, respectively. We further identified 6-nitrotryptophan from the 1H-NMR spectrum for the pronase-digested product and calculated the yield of 6-nitrotryptophan as being about 30% for each of the modification methods. The tryptophan residue in the modified human Cu,Zn-superoxide dismutase gave the same spectra for the products including 6-nitrotryptophan as the major nitrated product with the two different modification systems.

Small exterior hydrophobic cluster contributes to conformational stability and steroid binding in ketosteroid isomerase from Pseudomonas putida biotype B

A structural motif called the small exterior hydrophobic cluster (SEHC) has been proposed to explain the stabilizing effect mediated by solvent-exposed hydrophobic residues; however, little is known about its biological roles. Unusually, in Delta(5)-3-ketosteroid isomerase from Pseudomonas putida biotype B (KSI-PI) Trp92 is exposed to solvent on the protein surface, forming a SEHC with the side-chains of Leu125 and Val127. In order to identify the role of the SEHC in KSI-PI, mutants of those amino acids associated with the SEHC were prepared. The W92A, L125A/V127A, and W92A/L125A/V127A mutations largely decreased the conformational stability, while the L125F/V127F mutation slightly increased the stability, indicating that hydrophobic packing by the SEHC is important in maintaining stability. The crystal structure of W92A revealed that the decreased stability caused by the removal of the bulky side-chain of Trp92 could be attributed to the destabilization of the surface hydrophobic layer consisting of a solvent-exposed beta-sheet. Consistent with the structural data, the binding affinities for three different steroids showed that the surface hydrophobic layer stabilized by SEHC is required for KSI-PI to efficiently recognize hydrophobic steroids. Unfolding kinetics based on analysis of the Phi(U) value also indicated that the SEHC in the native state was resistant to the unfolding process, despite its solvent-exposed site. Taken together, our results demonstrate that the SEHC plays a key role in the structural integrity that is needed for KSI-PI to stabilize the hydrophobic surface conformation and thereby contributes both to the overall conformational stability and to the binding of hydrophobic steroids in water solution.

Structural determinants for vitamin D receptor response to endocrine and xenobiotic signals

The vitamin D receptor (VDR), initially identified as a nuclear receptor for 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], regulates calcium metabolism, cellular proliferation and differentiation, immune responses, and other physiological processes. Recently, secondary bile acids such as lithocholic acid (LCA) were identified as endogenous VDR agonists. To identify structural determinants required for VDR activation by 1alpha,25(OH)2D3 and LCA, we generated VDR mutants predicted to modulate ligand response based on sequence homology to pregnane X receptor, another bile acid-responsive nuclear receptor. In both vitamin D response element activation and mammalian two-hybrid assays, we found that VDR-S278V is activated by 1alpha,25(OH)2D3 but not by LCA, whereas VDR-S237M can respond to LCA but not to 1alpha,25(OH)2D3. Competitive ligand binding analysis reveals that LCA, but not 1alpha,25(OH)2D3, effectively binds to VDR-S237M and both 1alpha,25(OH)2D3 and LCA bind to VDR-S278V. We propose a docking model for LCA binding to VDR that is supported by mutagenesis data. Comparative analysis of the VDR-LCA and VDR-1alpha,25(OH)2D3 structure-activity relationships should be useful in the development of bile acid-derived synthetic VDR ligands that selectively target VDR function in cancer and immune disorders without inducing adverse hypercalcemic effects.

Tryptophan missense mutation in the ligand-binding domain of the vitamin D receptor causes severe resistance to 1,25-dihydroxyvitamin D

In this study, two related young children, brother and sister, exhibited severe vitamin D-resistant rickets without alopecia. Sequence analysis of the total vitamin D receptor (VDR) cDNA from skin fibroblasts revealed a substitution of the unique tryptophan of the VDR by arginine at amino acid 286 (W286R). Cultured skin fibroblasts of the two patients expressed normal-size VDR protein (immunocytochemistry and Western blotting) and normal length VDR mRNA (Northern blotting). But, these fibroblasts, as well as COS-7 cells transfected with the W286R mutant, failed to bind 3H 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The tryptophan substitution did not affect VDR trafficking toward the nucleus but abolished the 24-hydroxylase gene response to 1,25(OH)2D3, even at 10(-6) M concentrations. In conclusion, this case report of a new family with hereditary vitamin D-resistant rickets (HVDRR) emphasizes the crucial role of the VDR tryptophan for ligand binding and for transactivation of 1,25(OH)2D3 target genes. It clearly shows the clinical significance of this VDR amino acid for calcium homeostasis and bone mineralization. This observation suggests further that the presence of a stable VDR-bound ligand may not be obligatory for normal hair follicle development.

Mapping the unique activation function 3 in the progesterone B-receptor upstream segment. Two LXXLL motifs and a tryptophan residue are required for activity

Progesterone receptors (PR) contain three activation functions (AFs) that together define the extent to which they regulate transcription. AF1 and AF2 are common to the two isoforms of PR, PR-A and PR-B, whereas AF3 lies within the N-terminal 164 amino acids unique to PR-B, termed the "B-upstream segment" (BUS). To define the BUS regions that contribute to AF3 function, we generated a series of deletion and amino acid substitution mutants and tested them in three backgrounds as follows: BUS alone fused to the PR DNA binding domain (BUS-DBD), the entire PR-B N terminus linked to its DBD (NT-B), and full-length PR-B. Analyses of these mutants identified two regions in BUS whose loss reduces AF3 activity by more than 90%. These are associated with amino acids 54-90 (R1) and 120-154 (R2). R1 contains a consensus (55)LXXLL(59) motif (L1) identical to ones found in nuclear receptor co-activators. R2 is adjacent to a second nuclear receptor box (L2) at (115)LXXLL(119) and contains a conserved tryptophan (Trp-140). Their mutation completely disrupts AF3 activity in a promoter and cell type-independent manner. Critical mutations elicited similar effects on all three B-receptor backgrounds. This underscores the probability that these mutations alter a process linking BUS structure to the function of full-length PR-B in a fundamental way.

Ligand affinity, homodimerization, and ligand-induced secondary structural change of the human vitamin d receptor

The intrinsic tryptophan fluorescence signal of the full-length nuclear receptor hVDR was used to directly determine the dissociation constants, K(d), of two ligands yielding K(d) = 32 nM for 1alpha,25(OH)(2)D(3) and K(d) = 322 nM for 25(OH)D(3). Ligand binding was accompanied by a conformational change in the alpha-helical part of hVDR as revealed by CD spectroscopy. In addition, the presence of calcitriol was found to be a necessary prerequisite for the homodimerisation of hVDR which was monitored using fluorescence anisotropy. We conclude that the observed ligand-induced structural change of hVDR is conditional for dimerisation of the protein.