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

Development of In Vitro and In Vivo Evaluation Systems for Vitamin D Derivatives and Their Application to Drug Discovery

We have developed an in vitro system to easily examine the affinity for vitamin D receptor (VDR) and CYP24A1-mediated metabolism as two methods of assessing vitamin D derivatives. Vitamin D derivatives with high VDR affinity and resistance to CYP24A1-mediated metabolism could be good therapeutic agents. This system can effectively select vitamin D derivatives with these useful properties. We have also developed an in vivo system including a Cyp27b1-gene-deficient rat (a type I rickets model), a Vdr-gene-deficient rat (a type II rickets model), and a rat with a mutant Vdr (R270L) (another type II rickets model) using a genome editing method. For Cyp27b1-gene-deficient and Vdr mutant (R270L) rats, amelioration of rickets symptoms can be used as an index of the efficacy of vitamin D derivatives. Vdr-gene-deficient rats can be used to assess the activities of vitamin D derivatives specialized for actions not mediated by VDR. One of our original vitamin D derivatives, which displays high affinity VDR binding and resistance to CYP24A1-dependent metabolism, has shown good therapeutic effects in Vdr (R270L) rats, although further analysis is needed.

Synthesis and Biological Activity of 2,22-Dimethylene Analogues of 19-Norcalcitriol and Related Compounds

Continuing our search for vitamin D analogues, we explored the modification of the steroidal side chain and inserted a methylene moiety in position C-22 together with either lengthening the side chain or introducing a ring at the terminal end. Our conformational studies confirmed that the presence of a methylene group attached to C-22 restricts the conformational flexibility of the side chain, which can result in changes in biological characteristics of a molecule. All synthesized 1α,25-dihydroxy-2,22-dimethylene-19-norvitamin D₃ analogues proved equal to calcitriol in their ability to bind to the vitamin D receptor, and most of them exert significantly higher differentiation and transcriptional activity than calcitriol. The most active compounds were characterized by the presence of an elongated side chain or 26,27-dimethylene bridge. The synthetic strategy was based on the Wittig-Horner coupling of the known A-ring phosphine oxide with the corresponding Grundmann ketones prepared from a 20-epi-Inhoffen-Lythgoe diol derived from vitamin D₂.

Synthesis of 1α,25-dihydroxyvitamin D3 analogues with α,α-difluorocycloketone at the CD-ring side chains and their biological properties in ovariectomized rats

Two novel 1α,25-dihydroxyvitamin D₃ derivatives containing a α,α-difluorocyclopentanone (3) or α,α-difluorocyclohexanone (4) moiety at the CD-ring side chains were designed, synthesized, and evaluated for their biological properties on restoring bone mass in ovariectomized (OVX) rats with established osteopenia. The synthesis of compounds 3 and 4 utilized the Wittig-Horner coupling to build up the vitamin D conjugated triene system, followed by the introduction of the cycloketone fragments at the side chain, and subsequent α,α-difluorination of the ketone by the treatment of the derived silyl enol ether with Selectfluor, as the key synthetic steps. In comparison with the natural 1α,25-dihydroxyvitamin D₃ (calcitriol; 200 ng/kg/day), oral administration of compounds 3 and 4 at the dose of 25 ng/kg/day for 6 weeks led to much improved bone mass and bone density related parameters, while maintaining normal serum calcium and serum phosphorus levels. The immunohistochemistry results showed that both compounds remarkably decreased in osteoclast number and moderately decreased in osteoblast number on trabecular bone surface. Therefore, our findings suggested that compounds 3 and 4 successfully rescue bone loss by suppression on bone turnover in OVX rat models.

Synthesis of 19-norcalcitriol analogs with pegylated alkylidene chains at C-2

The results presented in this paper constitute an extension of our synthetic efforts focused on 19-norvitamin D compounds possessing elongated 2-alkylidene substituents. Based on our previous results, molecular modeling studies, and docking experiments, we selected a novel 19-norcalcitriol analog with long chain at C-2 containing several ether moieties and terminated by 2-(pyridin-2'-yl)ethylamino fragment. It was expected that such structural modification might allow binding of transition metal by the ligand, increase solubility of the formed complexes as well as improve their affinity to the VDR. For comparison, a 19-norcalcitriol analog was also obtained with the terminal hydroxyl group at its pegylated 2-alkylidene substituent. The synthesis of the target vitamin D compounds described in this work was performed using the Wittig-Horner approach. The respective A-ring phosphine oxide was obtained starting from the D-(-)-quinic acid and then coupled with the known Grundmann ketone.

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.

Synthesis of 19-norcalcitriol analogs with alkylidene moieties at C-2 based on succinic acid and l-methionine

On the basis of the literature data, our previous research work and docking experiments, we designed novel 19-norvitamin D compounds having elongated 2-alkylidene substituents. These 19-norcalcitriol derivatives have attached 2-(3'-aminopropylidene) substituent in which the nitrogen atom bears acyl residue derived from succinic acid and l-methionine. Both compounds were obtained by the same synthetic strategy involving Julia coupling of the A-ring ketone with the known C/D-ring sulfone. In the obtained 1α,25-dihydroxy-19-norvitamin D₃ derivative, the alkylidene substituent at C-2 was further elaborated to the desired structures.

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.

Vitamin D signaling in intestinal innate immunity and homeostasis

The lumen of the gut hosts a plethora of microorganisms that participate in food assimilation, inactivation of harmful particles and in vitamin synthesis. On the other hand, enteric flora, a number of food antigens, and toxins are capable of triggering immune responses causing inflammation, which, when unresolved, may lead to chronic conditions such as inflammatory bowel disease (IBD). It is important, therefore, to contain the gut bacteria within the lumen, control microbial load and composition, as well as ensure adequate innate and adaptive immune responses to pathogenic threats. There is growing evidence that vitamin D signaling has impacts on all these aspects of intestinal physiology, contributing to healthy enteric homeostasis. VD was first discovered as the curative agent for nutritional rickets, and its classical actions are associated with calcium absorption and bone health. However, vitamin D exhibits a number of extra-skeletal effects, particularly in innate immunity. Notably, it stimulates production of pattern recognition receptors, anti-microbial peptides, and cytokines, which are at the forefront of innate immune responses. They play a role in sensing the microbiota, in preventing excessive bacterial overgrowth, and complement the actions of vitamin D signaling in enhancing intestinal barrier function. Vitamin D also favours tolerogenic rather than inflammogenic T cell differentiation and function. Compromised innate immune function and overactive adaptive immunity, as well as defective intestinal barrier function, have been associated with IBD. Importantly, observational and intervention studies support a beneficial role of vitamin D supplementation in patients with Crohn's disease, a form of IBD. This review summarizes the effects of vitamin D signaling on barrier integrity and innate and adaptive immunity in the gut, as well as on microbial load and composition. Collectively, studies to date reveal that vitamin D signaling has widespread effects on gut homeostasis, and provide a mechanistic basis for potential therapeutic benefit of vitamin D supplementation in IBD.

SMURF2 regulates bone homeostasis by disrupting SMAD3 interaction with vitamin D receptor in osteoblasts

Coordination between osteoblasts and osteoclasts is required for bone health and homeostasis. Here we show that mice deficient in SMURF2 have severe osteoporosis in vivo. This low bone mass phenotype is accompanied by a pronounced increase in osteoclast numbers, although Smurf2-deficient osteoclasts have no intrinsic alterations in activity. Smurf2-deficient osteoblasts display increased expression of RANKL, the central osteoclastogenic cytokine. Mechanistically, SMURF2 regulates RANKL expression by disrupting the interaction between SMAD3 and vitamin D receptor by altering SMAD3 ubiquitination. Selective deletion of Smurf2 in the osteoblast lineage recapitulates the phenotype of germline Smurf2-deficient mice, indicating that SMURF2 regulates osteoblast-dependent osteoclast activity rather than directly affecting the osteoclast. Our results reveal SMURF2 as an important regulator of the critical communication between osteoblasts and osteoclasts. Furthermore, the bone mass phenotype in Smurf2- and Smurf1-deficient mice is opposite, indicating that SMURF2 has a non-overlapping and, in some respects, opposite function to SMURF1.

The balance between osteoclast and osteoblast-mediated bone turnover is essential for bone health and homeostasis. Here the authors show that both germline and osteoblast-specific Smurf2-deficient mice have osteoporosis as a result of increased osteoblast RANKL production and excess osteoclastogenesis.

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.

Design and synthesis of 3-arylbenzopyran based non-steroidal vitamin-D3mimics as osteogenic agents

27benhanced osteoblast differentiation at 1 pM in mouse calvarial osteoblast cells without inherent toxicity.

Stereoselective reduction of C-20 ketone of vitamin D CD-ring and a new synthetic approach to maxacalcitol

Stereoselective reduction of C-20 ketone of vitamin D CD-ring precursor using Corey's CBS reagents, (R)-(+)-2-methyl-CBS-oxazaborolidine and (S)-(-)-2-methyl-CBS-oxazaborolidine, led to the (20S)-alcohol 5 and (20R)-epimer 4 in approximately 17:1 selectivity. A new synthetic approach to the 22-oxa-25-hydroxy Grundmann's ketone 11 was developed through the Williamson etherification of (20S)-alcohol 5 with 1-bromomethyl-2,2-dimethyloxirane, followed by regioselective reductive epoxide ring opening with LiAlH4, the removal of the silyl protecting group by TBAF, and the environmentally benign TEMPO-mediated oxidation using inexpensive Oxone as a co-oxidizing agent. The preparation of drug maxacalcitol was achieved on gram scale by the convergent Lythgoe coupling via Wittig-Horner reaction of the A-ring phosphine oxide synthon with the CD-ring fragment.

Ring-A-seco analogs of 1α,25-dihydroxy-19-norvitamin D3

The steroid hormone 1α,25-dihydroxyvitamin D3 [1α,25-(OH)2D3] is the most active metabolite of vitamin D3 which exerts its control over a multitude of biological processes related to calcium and phosphorus homeostasis, cell proliferation and differentiation, and immune regulation. Unfortunately, the therapeutic application of 1α,25-(OH)2D3 is limited by induction of hypercalcemia. The need for vitamin D compounds with selective biological profiles has stimulated the synthesis of more than three thousand analogs of 1α,25-(OH)2D3. Most of these compounds have structural modifications in the side chain and A-ring; there is also an increasing number of modifications in the CD-rings and limited number in the triene system (seco-B ring). Herein, we report the synthesis and biological evaluation of seco-A-19-nor analogs of 1α,25-dihydroxyvitamin D3, developed to study the role of ring A in the biological activity of 1α,25-(OH)2D3. Interestingly, compounds 2 and 4 show substantial ability to bind the vitamin D receptor and the former is also characterized by selective intestinal calcium transport activity. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

26- and 27-Methyl groups of 2-substituted, 19-nor-1α,25-dihydroxylated vitamin D compounds are essential for calcium mobilization in vivo

Twelve new analogs of 19-nor-1α,25-dihydroxyvitamin D36-17, were prepared by a multi-step procedure from known alcohols 18 and 19. We have examined the influence of removing two methyl groups located at C-25, as well as the 25-hydroxy group, on the biological in vitro and in vivo biological activity. Surprisingly, removal of the 26- and 27-methyl groups from either the 2α-methyl or 2-methylene-19-nor-1α,25-dihydroxyvitamin D3 reduced vitamin D receptor binding, HL-60 differentiation, and 25-hydroxylase transcription in vitro only slightly to moderately (compounds 6-13). However, these compounds were devoid of in vivo bone mobilization activity and had markedly reduced activity on intestinal calcium transport. The analogs 14-17 with a 2β-methyl substitution had little or no activity in vitro and in vivo as expected from previous work.

Human cytochrome P450-dependent differential metabolism among three 2α-substituted-1α,25-dihydroxyvitamin D(3) analogs

Our previous studies revealed that C2α-substituted-1α,25(OH)(2)D(3) analogs had unique biological activities. For example, 19-nor-2α-(3-hydroxypropyl)-1α,25(OH)(2)D(3) (MART-10), which has a high affinity for vitamin D receptor (VDR), is more bioavailable and more potent than 1α,25(OH)(2)D(3) in inhibiting cancer cell growth and invasion because of its weaker binding to vitamin D binding protein (DBP), and more resistance to CYP24A1-dependent metabolism. In this study, we examined the metabolism of MART-10 and two other 2α-substituted analogs, 2α-(3-hydroxypropoxy)-1α,25(OH)(2)D(3) (O2C3) and 2α-(3-hydroxypropyl)-1α,25(OH)(2)D(3) (O1C3) by using human liver microsomes and human P450s. We demonstrated that O2C3 was converted to 1α,2α,25(OH)(3)D(3) in human liver microsomes, whereas both O1C3 and MART-10 were hardly metabolized. The metabolism of O2C3 was significantly inhibited by ketoconazole, and the recombinant human CYP3A4 converted O2C3 to 1α,2α,25(OH)(3)D(3), which suggests that CYP3A4 is responsible for the metabolism of O2C3 in human liver. The k(cat)/K(m) values of CYP3A4 for O1C3 and MART-10 are much smaller than that for O2C3. The k(cat)/K(m) values of human CYP24A1 for the three analogs are 1% (MART-10), 3% (O2C3), and 4% (O1C3) of that for 1α,25(OH)(2)D(3), indicating that MART-10 is the most resistant to CYP24A1 hydroxylation. On the other hand, 1α,2α,25(OH)(3)D(3), the metabolite of O2C3 by CYP3A4, was metabolized by CYP24A1 via multiple pathways similar to 1α,25(OH)(2)D(3), which suggests that O2C3 can be metabolized by two sequential hydroxylations, first by CYP3A4 and then by CYP24A1 in human body. These results suggest that modification at C-2α position and C-19 demethylenation markedly change metabolic profiles and biological activities of vitamin D analogs.

A 20S combined with a 22R configuration markedly increases both in vivo and in vitro biological activity of 1α,25-dihydroxy-22-methyl-2-methylene-19-norvitamin D3

Six new analogues of 1α,25-dihydroxy-19-norvitamin D(3) (3a-4b, 5, and 6) were prepared by a convergent synthesis applying the Wittig-Horner reaction as a key step. The influence of methyl groups at C-22 on their biological activity was examined. It was established that both in vitro and in vivo activity is strongly dependent on the configuration of the stereogenic centers at C-20 and C-22. Introduction of the second methyl group at C-22 (analogues 5 and 6) generates the compounds that are slightly more potent than 1α,25-(OH)(2)D(3) in the in vitro tests but much less potent in vivo. The greatest in vitro and in vivo biological activity was achieved when the C-20 is in the S configuration and the C-22 is in the R configuration. The building blocks for the synthesis, the respective (20R,22R)-, (20R,22S)-, (20S,22R)-, and (20S,22S)-diols, were obtained by fractional crystallization of mixtures of the corresponding diastereomers. Structures and absolute configurations of the diols 21a, 21b, and 22a as well as analogues 3a, 5, and 6 were confirmed by the X-ray crystallography.

13,13-Dimethyl-des-C,D analogues of (20S)-1α,25-dihydroxy-2-methylene-19-norvitamin D₃ (2MD): total synthesis, docking to the VDR, and biological evaluation

As a continuation of our studies focused on the vitamin D compounds lacking the C,D-hydrindane system, 13,13-dimethyl-des-C,D analogues of (20S)-1α,25-dihydroxy-2-methylene-19-norvitamin D(3) (2, 2MD) were prepared by total synthesis. The known cyclohexanone 30, a precursor of the desired A-ring phosphine oxide 11, was synthesized starting with the keto acetal 13, whereas the aldehyde 12, constituting an acyclic 'upper' building block, was obtained from the isomeric esters 34, prepared previously in our laboratory. The commercial 1,4-cyclohexanedione monoethylene ketal (13) was enantioselectively α-hydroxylated utilizing the α-aminoxylation process catalyzed by l-proline, and the introduced hydroxy group was protected as a TBS, TPDPS, and SEM ether. Then the keto group in the obtained compounds 15-17 was methylenated and the allylic hydroxylation was performed with selenium dioxide and pyridine N-oxide. After separation of the isomers, the newly introduced hydroxy group was protected and the ketal group hydrolyzed to yield the corresponding protected (3R,5R)-3,5-dihydroxycyclohexanones 30-32. The esters 34, starting compounds for the C,D-fragment 12, were first α-methylated, then reduced and the resulted primary alcohols 36 were deoxygenated using the Barton-McCombie protocol. Primary hydroxy group in the obtained diether 38 was deprotected and oxidized to furnish the aldehyde 12. The Wittig-Horner coupling of the latter with the anion of the phosphine oxide 11, followed by hydroxyl deprotection furnished two isomeric 13,13-dimethyl-des-C,D analogues of 2MD (compounds 10 and 42) differing in configuration of their 7,8-double bond. Pure vitamin D analogues were isolated by HPLC and their biological activity was examined. The in vitro tests indicated that, compared to the analogue 7, unsubstituted at C-13, the synthesized vitamin D analogue 10 showed markedly improved VDR binding ability, significantly enhanced HL-60 differentiation activity as well as increased transcriptional potency. Docking simulations provided a rational explanation for the observed binding affinity of these ligands to the VDR. Biological in vivo tests proved that des-C,D compound 10 retained some intestinal activity. Its geometrical isomer 42 was devoid of any biological activity.

Mechanism of the anti-proliferative action of 25-hydroxy-19-nor-vitamin D(3) in human prostate cells

According to the prevailing paradigm, 1α-hydroxylation of 25-hydroxyvitamin D(3) (25(OH)D(3)) and its analogs is a pre-requisite step for their biological effects. We previously reported that 25-hydroxy-19-nor-vitamin D(3) (25(OH)-19-nor-D(3)) had anti-proliferative activity in a cell line, PZ-HPV-7, which was derived from human non-cancerous prostate tissue, and suggested that 25(OH)-19-nor-D(3) acted after 1α-hydroxylation by vitamin D 1α-hydroxylase (CYP27B1). However, metabolic studies of 25(OH)-19-nor-D(3) using recombinant CYP27B1 revealed that 25(OH)-19-nor-D(3) was rarely subjected to 1α-hydroxylation. Therefore, in this report, we attempted to clarify the mechanism of 25(OH)-19-nor-D(3) action in intact cells using PZ-HPV-7 prostate cells. After incubating the cells with 25(OH)-19-nor-D(3), eight metabolites of 24-hydroxylase (CYP24A1) were detected, whereas no products of CYP27B1 including 1α,25-dihydroxy-19-nor-vitamin D(3) (1α,25(OH)(2)-19-nor-D(3)) were found. Furthermore, the time-dependent nuclear translocation of vitamin D receptor (VDR) and the subsequent transactivation of cyp24A1 gene in the presence of 25(OH)-19-nor-D(3) were almost identical as those induced by 1α,25(OH)(2)-19-nor-D(3). These results strongly suggest that 25(OH)-19-nor-D(3) directly binds to VDR as a ligand and transports VDR into the nucleus to induce transcription of cyp24A1 gene. In addition, knock down of cyp27B1 gene did not affect the anti-proliferative activity of 25(OH)-19-nor-D(3), whereas knock down of VDR attenuated the inhibitory effect. Thus, our results clearly demonstrate that the anti-proliferative activity of 25(OH)-19-nor-D(3) is VDR dependent but 1α-hydroxylation independent, suggesting that 25(OH)D(3) analogs such as 25(OH)-19-nor-D(3) could be attractive candidates for anticancer therapy.

MSITE: a new computational tool for comparison of homological proteins in holo form

The mechanism by which nuclear receptors respond differentially to structurally distinct agonists is not a well understood process. However, it is now obvious that transcriptional activity of nuclear receptors is a function of their interactions with co-activators. Recently, we released a new computational tool, CCOMP, for comparing side chain conformations in crystal structures of homologous protein complexes. Application of the CCOMP program revealed that 20-epi-1alpha,25-(OH)2D3 changes the side chain conformation of vitamin D receptor amino acids residing mostly far away from the ligand-receptor contacts. This strongly suggests that the ligand-co-activator signaling pathway involves indirect interactions between amino acids lining the binding pocket and outer surface residues that could attract co-activators. To facilitate identification of amino acids transmitting the subtle receptor changes upon ligand/modulator binding we developed another simple tool, MSITE. The program automatically lists the nearest neighbors of a given amino acid (for example neighbors of residues that are in contact with a ligand or reorient their side chains in the presence of a co-factor) in an arbitrary number of compared complexes. Comparison of seven binary vitamin D receptor complexes holding as ligands the analogs of 1alpha,25-(OH)2D3 with inverted configuration at carbon 14 or 20, or with incorporated oxolane ring bridging carbons 20 and 23, is reported.

Interactions between 1alpha,25(OH)2D3 and residues in the ligand-binding pocket of the vitamin D receptor: a correlated fragment molecular orbital study

To provide physicochemical insight into the role of each residue in the ligand-binding pocket (LBP) of the vitamin D receptor (VDR), we evaluated the energies of the interactions between the LBP residues and 1alpha,25(OH)2D3 by using an ab initio fragment molecular orbital (FMO) method at the Møller-Plesset second-order perturbation (MP2) level. This FMO-MP2 method can be used to correctly evaluate both electrostatic and van der Waals dispersion interactions, and it affords these interaction energies separately. We deduced the nature of each interaction and determined the importance of all the LBP residues involved in ligand recognition by the VDR. We previously reported the results of alanine-scanning mutational analysis (ASMA) of all 34 non-alanine residues lining the LBP of the human VDR. The theoretical results in combination with the ASMA results enabled us to assign the role of each LBP residue. We concluded that electrostatic interactions are the major determinant of the ligand-binding activity and ligand recognition specificity and that van der Waals interactions are important for protein folding and, in turn, for cofactor binding.

'Novel alkyl side chain sulfone 1alpha,25-dihydroxyvitamin D3 analogs: a comparison of in vitro antiproliferative activities and in vivo calcemic activities'

The replacement of a t-butyl group with a trifluoromethyl group has profound effects on the biological profile of 1alpha,25-dihydroxyvitamin D(3) sulfone analogs. Investigation of whether the improved biological activities are due to steric and electronic factors of the trifluoromethyl group led to the design, synthesis and biological evaluation of two analogous alkyl sulfone molecules, methyl sulfone (AU-16-ene-25-SO(2)-CH(3)) and isopropyl sulfone (AU-16-ene-25-SO(2)-i-Pr). These alkyl sulfones are sterically comparable to, but electronically very different from a trifluoromethyl group. The syntheses, antiproliferative activities and calcemic activities of these new alkyl sulfones are presented herein. In comparing the in vitro antiproliferative profiles of the new alkyl sulfone 1alpha,25-dihydroxyvitamin D(3) analogs with the trifluoromethylsulfone and an analogous t-butyl sulfone, the activities increase in the following order: CH(3) < t-Bu approximately = i-Pr < CF(3). In contrast to the calcemic t-butyl sulfone, the novel alkyl sulfones and trifluoromethyl sulfone display desirable low calcemic levels.

Structural changes of vitamin D receptor induced by 20-epi-1alpha,25-(OH)2D3: an insight from a computational analysis

We employ a new computational tool CCOMP for the comparison of side chain (SC) conformations between crystal structures of homologous protein complexes. The program is applied to the vitamin D receptor (VDR) liganded with 1alpha,25-(OH)(2)D(3) (in 1DB1) or its 20-epi (in 1IE9) analog with an inverted C-20 configuration. This modification yields no detectable changes in the backbone configuration or ligand topology in the receptor binding cavity, yet it dramatically increases transcription, differentiation and antiproliferation activity of the VDR. We applied very stringent criteria during the comparison process. To eliminate errors arising from the different packing of investigated crystals and the thermal flexibility of atoms, we studied complexes belonging to the same space group, having a low R value (0.2) and a B-factor below 40 for compared residues. We find that 20-epi-1alpha,25-(OH)(2)D(3) changes side chain conformation of amino acids residing far away from direct ligand-VDR contacts. We further verify that a number of the reoriented residues were identified in mutational experiments as important for interaction with SRC-1, GRIP, TAFs co-activators and VDR-RXR heterodimerization. Thus, CCOMP analysis of protein complexes may be used for identifying amino acids that could serve as targets for genetic engineering, such as mutagenesis.

Structural refinement of seco-steroidal skeleton and the biological activity through nuclear receptors

1alpha,25-Dihydroxyvitamin D(3) (1) regulates a variety of biological actions through vitamin D receptor (VDR), including calcium and phosphorus homeostasis, bone remodeling, cellular proliferation and differentiation and many other functions. To enhance its potency and to study the structure/function relationship, we synthesized a series of analogs of 1 with a modification at the C-2alpha position. Introducing 2alpha-methyl, 2alpha-(3-hydroxypropyl), or 2alpha-(3-hydroxypropoxy) group increased its binding affinity for the VDR 2- to 4-fold compared to 1. The crystal structures of the VDR bound to these analogs provide a molecular explanation for the interaction between the 2alpha-substituents and water molecules exist in the VDR-ligand binding domain. Based on the accumulated knowledge in VDR agonists, we synthesized 2-substituted analogs of 'double side chain' (gemini), 19-norvitamin D(3) (MART-10), TEI-9647 (VDR antagonist), 1-alkylated vitamin D(3), 14-epi-previtamin D(3) etc. Gemini analogs showed potent HL-60 cell differentiation activity (13-38 times compared to 1), and MART-10 exhibited remarkable antiproliferative activity on PZ-HPV-7 cells even at 10(-10) M. (24S)-2alpha-(3-Hydroxypropoxy)-24-propyl-TEI-9647 showed potent VDR antagonism, and its IC(50) value was 7.4 pM against 10 nM of 1. 1alpha-Methyl-2alpha-(3-hydroxypropyl)-25-hydroxyvitamin D(3) improved the binding affinity for the mutant VDR(Arg274Leu), which causes hereditary vitamin D resistant rickets. 1alpha,25-Dihydroxy-2alpha-methyl-14-epi-previtamin D(3) showed moderate osteocalcin transcriptional activity on HOS cells. We theorize that modification at A-ring alone and in combination with functionalization of the other parts of the vitamin D molecule would provide important new information on the mechanism of vitamin D actions that could lead to the development of new therapeutic regimes for the treatment of various diseases.

Crystal structures of rat vitamin D receptor bound to adamantyl vitamin D analogs: structural basis for vitamin D receptor antagonism and partial agonism

The X-ray crystal structures of the rat VDR ligand-binding domain complexed with 19-norvitamin D compounds that contain an adamantyl substituent at the side-chain terminus, 2a (ADTT), 2b (ADNY), and 2c (ADMI4) and a coactivator peptide derived from DRIP205 are reported. These compounds show a series of partial agonistic (10-75% efficacy)/antagonistic activities. All of these complexed receptors are crystallized in the canonical active conformation, regardless of their activity profiles. The bulky adamantyl side chain does not crowd helix 12 but protrudes into the gap formed by helix 11, loop 11-12, helix 3, and loop 6-7, thereby widening the ligand binding pocket. We suggest that these structural changes destabilize the active protein conformation and reduce its contribution to equilibrium among the active and inactive conformations. The coactivator peptide traps the minor active conformation, and the equilibrium shifts to the active conformation. As a result, these ligands show partial agonistic activities.

2-Substituted-16-ene-22-thia-1alpha,25-dihydroxy-26,27-dimethyl-19-norvitamin D3 analogs: Synthesis, biological evaluation, and crystal structure

Recently, we have found that 16-ene-22-thia-26,27-dimethyl-19-norvitamin D(3) analogs 1a (n=2, 3) are 20 times more active than the natural hormone 1alpha,25-dihydroxyvitamin D(3) in terms of transcriptional activity. To further investigate the effects of the A-ring modification of 1a, b on the biological activity profile, novel 22-thia-19-norvitamin D analogs 2-11 bearing a hydroxyethoxy-, hydroxyethylidene- or methyl group at C-2 in combination with 20S- and 20R-isomers were prepared and tested for their in vitro biological activities. All of the synthesized analogs showed 0.5-140% of the activity of the natural hormone in binding to the vitamin D receptor (VDR). When compared with the transcriptional activity of C-2 or C-20 isomeric pairs of the 22-thia analogs, the 20S-isomers 2-11a were more potent than the 20R-isomers 2, 3, 8-11b, and the 2beta-hydroxyethoxy, 2E-hydroxyethylidene, and 2alpha-methyl-2beta-hydroxy-22-thia isomers showed higher potency than their corresponding counterparts. In particular, 3a exhibited an extremely higher level of potency (210-fold) than the natural hormone. To elucidate the action mode of superagonist 3a at the molecular level, we determined the crystal structures of the rat VDR-ligand-binding domain complexed with 3a or 3b in the presence of peptide containing a nuclear box motif (LxxLL) at 1.9-2.0A resolution. The crystal structures demonstrated that the 1alpha-OH, 3beta-OH, and 25-OH groups of the natural hormone and 3a were anchored by the same amino acid residues in the ligand-binding pocket, and the terminal OH moiety of the substituent at C-2 formed hydrogen bonds with Arg270 and a water molecule to create a tight water molecule network. Moreover, the methyl groups at C-26a and C-27a make additional contact with hydrophobic residues such as Leu223, Ala227, Val230, and Ala299. These hydrophilic and hydrophobic interactions in 3a may underlie the induction of superagonistic activity.

Kinetic Studies of 25-Hydroxy-19-nor-vitamin D3 and 1α,25-Dihydroxy-19-nor-vitamin D3 Hydroxylation by CYP27B1 and CYP24A1

Our previous study demonstrated that 25-hydroxy-19-nor-vitamin D(3) [25(OH)-19-nor-D(3)] inhibited the proliferation of immortalized noncancerous PZ-HPV-7 prostate cells similar to 1 alpha,25-dihydroxyvitamin D(3) [1 alpha,25(OH)(2)D(3)], suggesting that 25(OH)-19-nor-D(3) might be converted to 1 alpha,25-dihydroxy-19-nor-vitamin D(3) [1 alpha,25(OH)(2)-19-nor-D(3)] by CYP27B1 before exerting its antiproliferative activity. Using an in vitro cell-free model to study the kinetics of CYP27B1-dependent 1 alpha-hydroxylation of 25(OH)-19-nor-D(3) and 25-hydroxyvitamin D(3) [25(OH)D(3)] and CYP24A1-dependent hydroxylation of 1 alpha,25(OH)-19-nor-D(3) and 1 alpha,25(OH)(2)D(3), we found that k(cat)/K(m) for 1 alpha-hydroxylation of 25(OH)-19-nor-D(3) was less than 0.1% of that for 25(OH)D(3), and the k(cat)/K(m) value for 24-hydroxylation was not significantly different between 1 alpha,25(OH)(2)-19-nor-D(3) and 1 alpha,25(OH)(2)D(3). The data suggest a much slower formation and a similar rate of degradation of 1 alpha,25(OH)(2)-19-nor-D(3) compared with 1 alpha,25(OH)(2)D(3). We then analyzed the metabolites of 25(OH)D(3) and 25(OH)-19-nor-D(3) in PZ-HPV-7 cells by high-performance liquid chromatography. We found that a peak that comigrated with 1 alpha,25(OH)(2)D(3) was detected in cells incubated with 25(OH)D(3), whereas no 1 alpha,25(OH)(2)-19-nor-D(3) was detected in cells incubated with 25(OH)-19-nor-D(3). Thus, the present results do not support our previous hypothesis that 25(OH)-19-nor-D(3) is converted to 1 alpha,25(OH)(2)-19-nor-D(3) by CYP27B1 in prostate cells to inhibit cell proliferation. We hypothesize that 25(OH)-19-nor-D(3) by itself may have a novel mechanism to activate vitamin D receptor or it is metabolized in prostate cells to an unknown metabolite with antiproliferative activity without 1 alpha-hydroxylation. Thus, the results suggest that 25(OH)-19-nor-D(3) has potential as an attractive agent for prostate cancer therapy.

Antiproliferative, low-calcemic, fluorinated sulfone analogs of 1alpha,25-dihydroxyvitamin D3: chemical synthesis and biological evaluation

Novel fluorinated sulfone analogs of the hormone 1alpha,25-dihydroxyvitamin D(3) have been designed and synthesized in order to study the biological effects of fluorine incorporation at the terminus of the C,D-ring side chain. Although biologically active 26,27-hexafluorinated 1alpha,25-dihydroxyvitamin D(3) analogs have been synthesized previously, this investigation reports the first successful fluorinated series in which trifluoromethyl sulfone analogs present a favorable biological profile. This study shows that two new analogs featuring incorporation of a synthetically simple single trifluoromethyl sulfone group have significantly increased antiproliferative activity while causing desirably low in vivo calciuria relative to that of calcitriol. Incorporation of additional fluorines, as in a perfluorobutyl analog, results in a loss of antiproliferative activity.

Systematic solution-phase parallel synthesis of active vitamin D3 analogs with elongated side chains and their cell differentiation activities

Side-chain elongation of active vitamin D3 is acknowledged as a structural modification to enhance its cell differentiation activity; however, the comprehensive structure-activity relationship (SAR) as a result of this modification has not been reported. To clarify the SAR, we synthesized six analogs systematically elongated at the 24-position, 26,27-position, or both by methylene (normal A-ring series 1a-f) in a facile parallel solution-phase synthesis. Using parallel synthesis, we expanded the side chain-elongation study into two 19-exomethylene analog series: 19-nor-A-ring (4a-f) and 2-methylene-19-nor-A-ring (5a-f). In the 19-nor-A-ring analog series, the SAR induced by side-chain elongation was similar to the normal A-ring analog series, but in the 2-methylene-19-nor-A-ring series, the SAR was unique.

An analog of 1alpha,25-dihydroxy-19-norvitamin D3 with the 1alpha-hydroxy group fixed in the axial position lacks biological activity in vitro

The relationship between the A-ring chair conformation of vitamin D compounds and their ability to bind the vitamin D receptor (VDR) has long attracted the attention of many researchers. It was established that in the crystalline complexes of hVDRmt with the natural hormone, 1alpha,25-dihydroxyvitamin D(3) (1), and its side-chain analogs the vitamins exist in beta-chair form with an equatorial orientation of 1alpha-OH. However, with all these ligands the interconversion between both A-ring forms would be possible in solution. In an attempt to verify the conformation of vitamin D compounds required for binding the VDR we prepared analog 4, characterized by the presence of an axial 1alpha-hydroxy group. Since the additional ring connecting 3beta-oxygen and C-2 prevents A-ring conformational flexibility, the synthesized vitamin 4 can exist exclusively in the alpha-chair form. The geometrical isomer 5 with a free 3beta-OH group was also obtained. The analog 5 binds very poorly to VDR, whereas the vitamin 4 is practically devoid of binding ability. Both compounds also show very low HL-60-differentiating activity. When tested in vivo in mice the analogs 4 and 5 exhibit significant calcemic responses with analog 4 showing more activity than analog 5.

Computational analysis of the active sites in binary and ternary complexes of the vitamin D receptor

We have developed a program CCOMP that compares overlapping fragments of two protein complexes and identifies differently oriented amino acids. CCOMP initially performs a sequence alignment of the analyzed receptors, then superimposes the corresponding aligned residues, and finally calculates the root mean square deviation (RMSD) of individual atoms, every amino acid and the entire complex. Thus, amino acids important for functional differences between both complexes can be detected. Application of CCOMP to 1alpha,25-(OH)(2)D(3)-hVDR (1DB1) [Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 5491] and 1alpha,25-(OH)(2)D(3)-rVDR-peptide (1RK3) [Biochemistry 43 (2004) 4101] complexes revealed that the peptide (KNHPMLMNLLKDN) mimicking a co-activator sequence significantly changes the side chain conformation of 35 amino acids. Four of these residues (K242, I256, K260, E416) actually contact the peptide, but all of them are essential for biological activity. Only two (L309 and L400) of the 35 differently oriented amino acids contact the ligand. Interestingly, when the peptide is present (1RK3) leucine 400 shifts closer (0.7A) to the vitamin D 26-methyl group. Applying the CCOMP and DSSP programs to binary and ternary VDR complexes also resulted in establishing that seven amino acids (I238, S252, I256, L413, L415, E416, V417) exhibit significant differences in solvent accessibility and are capable of interacting with co-activators.

Vitamin D3 Derivatives with Adamantane or Lactone Ring Side Chains are Cell Type-Selective Vitamin D Receptor Modulators

The vitamin D receptor (VDR) mediates the biological actions of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], the active form of vitamin D, which regulates calcium homeostasis, immunity, cellular differentiation, and other physiological processes. We investigated the effects of three 1,25(OH)(2)D(3) derivatives on VDR function. AD47 has an adamantane ring and LAC67a and LAC67b have lactone ring substituents at the side chain position. These vitamin D derivatives bind to VDR but do not stabilize an active cofactor conformation. In a VDR transfection assay, AD47 and LAC67b act as partial agonists and all three compounds inhibit VDR activation by 1,25(OH)(2)D(3). The derivatives enhanced the heterodimerization of VDR with the retinoid X receptor, an effect unrelated to agonist/antagonist activity. AD47 and LAC67b weakly induced recruitment of the SRC-1 cofactor to VDR, and all three derivatives inhibited the recruitment of p160 family cofactors to VDR induced by 1,25(OH)(2)D(3). It is noteworthy that AD47 induced DRIP205 recruitment as effectively as 1,25(OH)(2)D(3), whereas LAC67a and LAC67b were not effective. We examined the expression of endogenous VDR target genes and the nuclear protein levels of VDR and cofactors in several cell lines, including cells derived from intestine, bone, and monocytes, and found that the vitamin D(3) derivatives act as cell type-selective VDR modulators. The data indicate that side chain modification is useful in the development of VDR antagonists and tissue-selective modulators. Further elucidation of the molecular mechanisms of action of selective VDR modulators will be essential for their clinical application.

Carbonic Anhydrase Inhibitors. DNA Cloning, Characterization, and Inhibition Studies of the Human Secretory Isoform VI, a New Target for Sulfonamide and Sulfamate Inhibitors

The secretory isozyme of human carbonic anhydrase (hCA, EC 4.2.1.1), hCA VI, has been cloned, expressed, and purified in a bacterial expression system. The kinetic parameters for the CO2 hydration reaction proved hCA VI to possess a kcat of 3.4 x 10(5) s-1 and kcat/KM of 4.9 x 10(7) M-1 s-1 (at pH 7.5 and 20 degrees C). hCA VI has a significant catalytic activity for the physiological reaction on the same order of magnitude as the ubiquitous isoform CA I or the transmembrane, tumor-associated isozyme CA IX. A series of sulfonamides and one sulfamate have been tested for their interaction with this isozyme. Simple benzenesulfonamides were rather ineffective hCA VI inhibitors, with inhibition constants in the range of 1090-6680 nM. Better inhibitors were detected among such derivatives bearing 2- or 4-amino-, 4-aminomethyl-, or 4-hydroxymethyl moieties or among halogenated sulfanilamides (KI values of 608-955 nM). Some clinically used compounds, such as acetazolamide, methazolamide, ethoxzolamide, dichlorophenamide, dorzolamide, brinzolamide, topiramate, sulpiride, and indisulam, or the orphan drug benzolamide, showed effective hCA VI inhibitory activity, with inhibition constants of 0.8-79 nM. The best inhibitors were brinzolamide and sulpiride (KI values of 0.8-0.9 nM), the latter compound being also a CA VI-selective inhibitor. The metallic taste reported as a side effect after the treatment with systemic sulfonamides may be due to the inhibition of the salivary CA VI. Some of the compounds investigated in this study might be used as additives in toothpastes for reducing the acidification produced by the relevant CO2 hydrase activity of enamel CA VI, which leads to the formation of protons and bicarbonate and may have a role in cariogenesis.

Carbonic Anhydrase Inhibitors: Hypoxia-Activatable Sulfonamides Incorporating Disulfide Bonds that Target the Tumor-Associated Isoform IX

An approach for designing bioreductive, hypoxia-activatable carbonic anhydrase (CA, EC 4.2.1.1) inhibitors targeting the tumor-associated isoforms is reported. Sulfonamides incorporating 3,3'-dithiodipropionamide/2,2'-dithiodibenzamido moieties were prepared and reduced enzymatically/chemically in conditions present in hypoxic tumors, leading to thiols. The X-ray crystal structure of the most promising compound, 4-(2-mercaptophenylcarboxamido)benzenesulfonamide, which as disulfide showed a K(I) against hCA IX of 653 nM (in reduced form of 9.1 nM), in adduct with hCA II showed the inhibitor making favorable interactions with Gln92, Val121, Phe131, Leu198, Thr199, Thr200, Pro201, and Pro202, whereas the sulfamoyl moiety was coordinated to the Zn2+ ion. The same interactions were preserved in the adduct with hCA IX, but in addition, a hydrogen bond between the SH moiety of the inhibitor and the amide nitrogen of Gln67 was evidenced, which may explain the almost 2 times more effective inhibition of the tumor-associated isozyme over the cytosolic isoform.

In silico studies using Radial Distribution Function approach for predicting affinity of 1 alpha,25-dihydroxyvitamin D(3) analogues for Vitamin D receptor

The Radial Distribution Function (RDF) approach has been applied to the study of the chick intestinal VDR affinity of 49 Vitamin D analogues. A model able to describe more than 77.5% of the variance in the experimental activity was developed with the use of the mentioned approach. In contrast, none of four different approaches, including the use of Topological, BCUT, Randić molecular profiles and Geometrical descriptors were able to explain more than 55% of the variance in the mentioned property, with the same number of variables in the equation.

Synthesis and biological activities of new 1α,25-dihydroxy-19-norvitamin D3 analogs with modifications in both the A-ring and the side chain

In a series of studies on structure-activity relationships of 2-substituted 19-norvitamin D analogs, we found that 1alpha,25-dihydroxy-19-norvitamin D3 analogs with 2beta-hydroxyethoxy or 2E-hydroxyethylidene moieties show strong binding affinity for the vitamin D receptor (VDR) as well as marked transcriptional activity. To further examine the effects of side chain structure on the activity of 2-substituted 19-norvitamin D analogs, we have synthesized new 19-norvitamin D3 analogs with modifications in both the A-ring at the C(2) position and the side chain. The side chains of these analogs contained a double bond between C(22) and C(23) or an oxygen atom at C(22). The biological activity of the analogs was evaluated in vitro. All the side chain-modified analogs were less active than 1alpha,25-dihydroxyvitamin D31e and the parent compounds 3-6e possessing a natural 20R-configuration in binding to the VDR, but, except for the (20R)-22-oxa analogs 3-6d, were significantly more potent in transcriptional activity. Of the side-chain-modified analogs 4 and 5, the 2beta-hydroxyethoxy- and 2E-hydroxyethylidene-22,24-diene-24a,26a,27a-trihomo analogs showed markedly higher transcriptional activity (25- and 17.5-fold, respectively) compared with 1e. Elongation of the side chain at the C-24, C-26, and C-27 positions and introduction of a 22,24-diene moiety strongly increased transcriptional activity, as seen in the 20-epi analogs 3-6f.