The high affinity ligand binding conformation of the nuclear 1,25-dihydroxyvitamin D3 receptor is functionally linked to the transactivation domain 2 (AF-2)

Relationship between Structure and Conformational Change of the Vitamin D Receptor Ligand Binding Domain in 1α,25-Dihydroxyvitamin D3 Signaling

Vitamin D Receptor (VDR) belongs to the nuclear receptor (NR) superfamily. Whereas the structure of the ligand binding domain (LBD) of VDR has been determined in great detail, the role of its amino acid residues in stabilizing the structure and ligand triggering conformational change is still under debate. There are 13 α-helices and one β-sheet in the VDR LBD and they form a three-layer sandwich structure stabilized by 10 residues. Thirty-six amino acid residues line the ligand binding pocket (LBP) and six of these residues have hydrogen-bonds linking with the ligand. In 1α,25-dihydroxyvitamin D₃ signaling, H3 and H12 play an important role in the course of conformational change resulting in the provision of interfaces for dimerization, coactivator (CoA), corepressor (CoR), and hTAF_II 28. In this paper we provide a detailed description of the amino acid residues stabilizing the structure and taking part in conformational change of VDR LBD according to functional domains.

AT1R blockade in adverse milieus: role of SMRT and corepressor complexes

ANG II type 1 receptor blockade (AT1R-BLK) is used extensively to slow down the progression of proteinuric kidney diseases. We hypothesized that AT1R-BLK provides podocyte protection through regulation of silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and vitamin D receptor (VDR) expression under adverse milieus such as high glucose and human immunodeficiency virus infection. Both AT1R-BLK and VDR agonists (VDAs) stimulated VDR complex formation that differed not only in their composition but also in their functionality. AT1R-BLK-induced VDR complexes contained predominantly unliganded VDR, SMRT, and phosphorylated histone deacetylase 3, whereas VDA-VDR complexes were constituted by liganded VDR and CREB-binding protein/p300. AT1R-BLK-induced complexes attenuated podocyte acetyl-histone 3 levels as well as cytochrome P-450 family 24A1 expression, thus indicating their deacetylating and repressive properties. On the other hand, VDA-VDR complexes not only increased podocyte acetyl-histone 3 levels but also enhanced cytochrome P-450 family 24A1 expression, thus suggesting their acetylating and gene activation properties. AT1R-BLK- induced podocyte SMRT inhibited expression of the proapoptotic gene BAX through downregulation of Wip1 and phosphorylation of checkpoint kinase 2 in high-glucose milieu. Since SMRT-depleted podocytes lacked AT1R-BLK-mediated protection against DNA damage, it appears that SMRT is necessary for DNA repairs during AT1R-BLK. We conclude that AT1R-BLK provides podocyte protection in adverse milieus predominantly through SMRT expression and partly through unliganded VDR expression in 1,25(OH)2D-deficient states; on the other hand, AT1R-BLK contributes to liganded VDR expression in 1,25(OH)2D-sufficient states.

Small-molecule hormones: molecular mechanisms of action

Small-molecule hormones play crucial roles in the development and in the maintenance of an adult mammalian organism. On the molecular level, they regulate a plethora of biological pathways. Part of their actions depends on their transcription-regulating properties, exerted by highly specific nuclear receptors which are hormone-dependent transcription factors. Nuclear hormone receptors interact with coactivators, corepressors, basal transcription factors, and other transcription factors in order to modulate the activity of target genes in a manner that is dependent on tissue, age and developmental and pathophysiological states. The biological effect of this mechanism becomes apparent not earlier than 30-60 minutes after hormonal stimulus. In addition, small-molecule hormones modify the function of the cell by a number of nongenomic mechanisms, involving interaction with proteins localized in the plasma membrane, in the cytoplasm, as well as with proteins localized in other cellular membranes and in nonnuclear cellular compartments. The identity of such proteins is still under investigation; however, it seems that extranuclear fractions of nuclear hormone receptors commonly serve this function. A direct interaction of small-molecule hormones with membrane phospholipids and with mRNA is also postulated. In these mechanisms, the reaction to hormonal stimulus appears within seconds or minutes.

Binding of the ERα and ARNT1 AF2 domains to exon 21 of the SRC1 isoform SRC1e is essential for estrogen- and dioxin-related transcription

Steroid receptor co-activator 1 (SRC1) is a transcriptional co-activator of numerous transcription factors involving nuclear receptors. Aryl hydrocarbon receptor nuclear translocator 1 (ARNT1) is an obligatory transcriptional partner of the aryl hydrocarbon receptor (AhR) and hypoxia inducible factor-1α (HIF-1α), as well as a co-activator of estrogen receptors (ERs). To initiate transcription, the activation function 2 (AF2) domains of estrogen-activated ERs interact with LxxLL motifs in the nuclear receptor interaction domain (NID) of SRC1. Here we describe an estrogen and LxxLL domain-independent ERα AF2 binding to SRC1e exon 21. In addition, we found an AF2 domain in exon 16 of ARNT1 that also binds to SRC1e exon 21. Surprisingly, the interaction between SRC1e exon 21 and the AF2 domain of ERα functions as a crucial enhancer of estrogen-induced transcription. The binding of ARNT1 AF2 to SRC1e exon 21 enhances the transcriptional response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but the upregulation essentially depends on two cyclin destruction boxes (D-boxes), which are also located on exon 16 of ARNT1. Our findings reveal that a binding site for ERα and ARNT1 AF2 domains in the C-terminus of SRC1e upregulates estrogen- and TCDD-related responses in mammalian cells.

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.

Ligand-dependent conformation change reflects steric structure and interactions of a vitamin D receptor/ligand complex: a fragment molecular orbital study

We used an in silico computational method to theoretically analyze important residue-ligand interactions as well as ligand conformation changes in the vitamin D receptor (VDR). The ligand used for analysis was 1alpha,25-dihydroxy-19-nor-vitamin D3 [1alpha,25-19-nor-(OH)2D3] [1,2], whose crystal structure has not been solved. To estimate amino acid residue-ligand interactions with chemical accuracy, we adopted the fragment molecular orbital (FMO) method [3,4], which is based on the nonempirical total electronic quantum calculation. The docking of the ligand to the VDR was controlled by hydrophilic and hydrophobic interactions between amino acid residues and the ligand in the ligand binding pocket (LBP) [5-8]. These molecular interactions changed when the conformation of the ligand in the VDR was changed [5,9,10]. This conformation change is important to consider in computational, in silico, approaches for analyzing the mechanism of ligand-docking to the VDR. The position of the 1alpha,25-19-nor-(OH)2D3 ligand in the VDR-LBP was related to the hydrophobic interaction that occurred between the Ile271 residue of the VDR-LBP and the ligand. The interaction between Ile271 and 1alpha,25-19-nor-(OH)2D3 was repulsive, whereas, that between Ile271 and the natural ligand, 1alpha,25-(OH)2D3, is stable. The orientation change in the isopropyl group of Ile271 affected the residue's interaction with 1alpha,25-19-nor-(OH)2D3. We also found that conformation changes in the A-ring affected electrostatic (hydrophilic) interactions between the VDR and the ligand.

The vitamin D receptor gene FokI polymorphism: functional impact on the immune system

1Alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) has important effects on the growth and function of multiple cell types. These pleiotropic effects of 1,25(OH)2D3 are mediated through binding to the vitamin D receptor (VDR). Several polymorphisms of the human VDR gene have been identified, with the FokI polymorphism resulting in VDR proteins with different structures, a long f-VDR or a shorter F-VDR. The aim of this study was to investigate the functional consequences of the FokI polymorphism in immune cells. In transfection experiments, the presence of the shorter F-VDR resulted in higher NF-kappaB- and NFAT-driven transcription as well as higher IL-12p40 promoter-driven transcription. Marginal differences were observed for AP-1-driven transcription, and no differential effects were observed for transactivation of a classical vitamin D-responsive element. Concordantly, in human monocytes and dendritic cells with a homozygous short FF VDR genotype, expression of IL-12 (mRNA and protein) was higher than in cells with a long ff VDR genotype. Additionally, lymphocytes with a short FF VDR genotype proliferated more strongly in response to phytohemagglutinin. Together, these data provide the first evidence that the VDR FokI polymorphism affects immune cell behavior, with a more active immune system for the short F-VDR, thus possibly playing a role in immune-mediated diseases.

Conservative mutageneic perturbations of amino acids connecting helix 12 in the 1alpha,25(OH)2-D3 receptor (VDR) to the ligand cause significant transactivational effects

The positioning of helix 12 activation domain of nuclear receptor proteins is critically important for gene regulation. Perturbations of the helix 12 by larger analogs may alter interactions with transcriptional machinery which might give rise to selectivity. To explore the topology of the ligand binding pocket and how the bound ligand conceivably gives rise to altered transcriptional efficiencies, we have targeted 4 hydrophobic residues which contact the 25-carbon of the ligand, 1alpha,25(OH)(2)-vitamin D(3), and made a series of 13 mutants. Substitution of a smaller hydrophobic residue was poorly tolerated compared to a larger one for transactivation. The larger amino acids are likely better tolerated by promoting stronger Van der Waals forces with the ligand. Valine-418 mutants demonstrated an extreme example of this observation with mutation to leucine being transactivationally unaffected with alanine being the most affected of all single mutants. V418L resulted in a 1.3-fold increase in EC(50) for 1,25-D mediated transactivation whereas V418A resulted in a 53-fold increase when compared to wildtype VDR. Importantly, this difference is not explained by ligand binding data but by differential VDR protease sensitivity implying that V418L-VDR mutation assumes a better conformational interaction surface for coactivator than V418A. Importantly, the V418 location may accommodate larger sidechains and may even enhance the interaction with specific nuclear coactivators.

Coactivation of the Human Vitamin D Receptor by the Peroxisome Proliferator-Activated Receptor γ Coactivator-1 α

The vitamin D receptor (VDR) belongs to the superfamily of steroid/thyroid hormone receptors that is activated by 1alpha,25-dihydroxyvitamin D(3). Traditional targets for 1alpha,25-dihydroxyvitamin D(3) action include tissues involved in the maintenance of calcium homeostasis and bone development and remodeling. Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha), a transcriptional coactivator that plays a role in mitochondrial biogenesis and energy metabolism, is predominantly expressed in kidney, heart, liver, and skeletal muscle. Because VDR and PGC-1alpha display an overlapping pattern of expression, we investigated the possibility that PGC-1alpha could serve as a coactivator for VDR. Transient cotransfection assays demonstrate that PGC-1alpha augments ligand-dependent VDR transcription when either full-length VDR or Gal4 DNA binding domain-VDR-ligand binding domain chimeras were analyzed. Furthermore, mammalian two-hybrid assays, coimmunoprecipitation analyses, and biochemical coactivator recruitment assays demonstrate a ligand-dependent interaction between the two proteins both in cells and in vitro. The coactivation potential of PGC-1alpha requires an intact AF-2 domain of VDR and the LXXLL motif in PGC-1alpha. Taken together, these results indicate that PGC-1alpha serves as a coactivator for VDR.

The critical role of carboxy-terminal amino acids in ligand-dependent and -independent transactivation of the constitutive androstane receptor

The mouse constitutive androstane receptor (CAR) is a unique member of the nuclear receptor superfamily, for which an inverse agonist, the testosterone metabolite 5alpha-androstan-3alpha-ol (androstanol), and an agonist, the xenobiotic 1,4-bis[2-(3, 5-dichloropyridyloxy)] benzene, are known. In this study the role of the transactivation domain 2 (AF-2) of CAR was investigated, which is formed by the seven most carboxy-terminal amino acids of the receptor. The AF-2 domain was shown to be critical for the constitutive activity by mediating a ligand-independent interaction of CAR with coactivator (CoA) proteins. In addition this domain increased and decreased contact with CoAs in the presence of agonist and inverse agonist, respectively. In analogy to classical endocrine nuclear receptors, in CAR the charge clamp between K187 (in helix 3) and E355 (within the AF-2 domain) was expected to be critical for its interaction with CoAs. However, the hydrophobic amino acids L352, L353, and I356 on the surface of the AF-2 domain were found to be more important for this protein-protein interaction. Moreover, these amino acids and C357 were shown to be involved in the response of CAR to androstanol. Interestingly, the cysteine at position 357 appears to block classical endocrine responsiveness of CAR to agonists, since mutagenesis of this amino acid both reduced CoA interaction in the absence of ligand and drastically increased inducibility by 1,4-bis[2-(3, 5-dichloropyridyloxy)] benzene. We showed that this blockade is not due to an intramolecular disulfide bridge, but is probably caused by an interaction between C357 and Y336.

Vitamin D analogue-specific recruitment of vitamin D receptor coactivators

Synthetic ligands for the vitamin D receptor (VDR) are potential therapeutic agents for metabolic, neoplastic, and autoimmune disorders. Some of these ligands have similar or more potent antiproliferative, yet reduced hypercalcemic actions, than calcitriol. However, the mechanisms for these differential actions have not been clearly defined. We hypothesized that these gene- and tissue-specific effects may relate to ligand-directed selective recruitment of transcriptional coactivators. To identify key elements in ligand structure that facilitate VDR-coactivator interactions, the current studies assessed the ability of the VDR to recruit the coactivators GRIP1 and RAC3 following activation by a series of 20-R- and 20-S (20-epi)-modified analogues. The strength of VDR-coactivator interactions was ligand-specific and did not always correlate with ligand-receptor binding affinity. In general, the 20-epi analogues enhanced these interactions, whereas the 20-R-modified analogues were less effective than calcitriol. The 16-ene,23-yne modification and fluorinated substituents to the side-chain attenuated interaction with coactivators. The enhanced ability of the VDR to recruit GRIP1 following activation by the 20-epi analogues was consistent with potentiation of 20-epi analogue-induced transactivation of the osteocalcin gene promoter by GRIP1. Overall, the structure of the ligand side-chain as well as its orientation seemed to affect the avidity of coactivator binding. These results suggest that selective recruitment of coactivators may contribute to gene- and tissue-specific effects of vitamin D analogues.

Critical role of helix 12 of the vitamin D(3) receptor for the partial agonism of carboxylic ester antagonists

The carboxy-terminal alpha-helix of a nuclear receptor ligand-binding domain (LBD), helix 12, contains a critical, ligand-modulated interface for the interaction with coactivator proteins. In this study, using the example of the vitamin D receptor (VDR) and the partial antagonist ZK159222, the role of helix 12 (residues 417-427) for both antagonistic and agonistic receptor actions was investigated. Amino acid residue G423 was demonstrated to be critical for partial agonism of ZK159222, but not for the activity of the natural VDR agonist, 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)). The amount of partial agonism of ZK159222 increased when helix 12 was truncated by the last four amino acid residues (Delta424-27) and augmented even more, when in addition helix 12 of VDR's dimerization partner, retinoid X receptor (RXR), was truncated. In contrast, the low agonism of a structural derivative of ZK159222, ZK168281, was not affected comparably, whereas other close structural relatives of ZK159222 even demonstrated the same agonistic activity as that of 1alpha,25(OH)(2)D(3). The amount of agonism of ZK159222 and ZK168281 at different variations of helix 12 correlated well with VDR's ability to complex with coactivator proteins and inversely correlated with the strength of the compound's antagonistic action on 1alpha,25(OH)(2)D(3) signalling. Molecular dynamics simulations of the LBD complexed with the two antagonists could explain their different action by demonstrating a more drastic displacement of helix 12 through ZK168281 than through ZK159222. Moreover, the modelling could indicate a kink of helix 12 at amino acid residue G423, which provides the last four amino acid residues of helix 12 with a modulatory role for the partial agonism of some VDR antagonists, such as ZK159222. In conclusion, partial agonism of a VDR antagonist is lower the more it disturbs helix 12 in taking the optimal position for coactivator interaction.

Gene regulatory potential of 1alpha,25-dihydroxyvitamin D(3) analogues with two side chains

The nuclear hormone 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) acts through the transcription factor vitamin D receptor (VDR) via combined contact with the retinoid X receptor (RXR), coactivator proteins, and specific DNA binding sites (VDREs). Ligand-mediated conformational changes of the VDR are the core of the molecular switch of nuclear 1alpha,25(OH)(2)D(3) signalling. Studying the interaction of 1alpha,25(OH)(2)D(3) analogues with this molecular switch should allow the characterization of their potential selective biological profile. A 1alpha,25(OH)(2)D(3) analogue with two side chains (Ro27-2310 or Gemini) was found to stabilize functional VDR conformations and VDR-RXR heterodimers on a VDRE with a slightly lower sensitivity than the natural hormone. A 19-nor derivative of Gemini (Ro27-5646) showed similar sensitivity whereas 5,6-trans (Ro27-6462) 3-epi (Ro27-5840) and 1alpha-fluoro (Ro27-3752) derivatives were equal to each other, but approximately 30-times less sensitive than Gemini. A des-C,D derivative of Gemini (Ro28-1909) showed only residual activity at maximal concentrations. In contrast to 1alpha,25(OH)(2)D(3), Gemini and its derivatives showed a differential preference in stabilizing VDR conformations which was found to be modulated by DNA coactivator and corepressor proteins. An analysis of the gene regulatory potential of the VDR agonists in cellular reporter gene systems demonstrated the same ranking as in the in vitro systems, but Gemini and its 19-nor derivative were found to be more sensitive than 1alpha,25(OH)(2)D(3) which indicates that the natural hormone is selectively metabolized. This study used straightforward methods for the in vitro and ex vivo evaluation of the gene regulatory potential of 1alpha,25(OH)(2)D(3) analogues. Gemini was highlighted as an interesting drug candidate which could not be optimized through obvious chemical modifications in its A-ring. J. Cell. Biochem. Suppl. 36: 179-190, 2001.

Interaction of two novel 14-epivitamin D3 analogs with vitamin D3 receptor-retinoid X receptor heterodimers on vitamin D3 responsive elements

This study provides a detailed and exact evaluation of the interactions between vitamin D3 receptor (VDR), retinoid X receptor (RXR), and vitamin D3 responsive elements (VDREs) mediated by two novel 14-epianalogs of 1,25-dihydroxyvitamin D [1,25(OH)2D3], 19-nor-14-epi-23-yne-1,25(OH)2D3 (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)2D3 (TX 527). Both analogs were more potent (14- and 75-fold, respectively) than 1,25(OH)2D3 in inhibiting cell proliferation and inducing cell differentiation. However, DNA-independent experiments indicated that both analogs had a lower affinity to VDR and that the stability of the induced VDR conformation, as measured by limited protease digestion assays, was similar (TX 527) or even weaker (TX 522) than that induced by the parent compound. However, DNA-dependent assays such as gel shift experiments revealed that those analogs were slightly more potent (3-7 times) than 1,25(OH)2D3 in enhancing binding of VDR-RXR heterodimers to a direct repeat spaced by three nucleotides (DR3) type VDRE. The functional consequences of the ligand-VDR-RXR-VDRE interactions observed in vitro were subsequently evaluated in transfection experiments. Both 14-epianalogs enhanced transcription of VDRE containing reporter constructs more efficiently than 1,25(OH)2D3 in COS-1 and MCF-7 cells regardless of the presence of ketoconazole. Transactivation activity is suggested to be a cell-specific process because maximal transcriptional induction and the half-maximal transactivation concentration for each reporter construct were different in both cell lines. The superagonistic transactivation activity closely resembled the biological potency of these analogs on the inhibition of MCF-7 cell proliferation. These data clearly indicate that superagonistic activity starts beyond the binding of the ligand-heterodimer (VDR-RXR) complex to VDRE and thus probably involves coactivator/corepressor molecules.

Central role of VDR conformations for understanding selective actions of vitamin D(3) analogues

The vitamin D(3) receptor (VDR) acts primarily as a heterodimer with the retinoid X receptor (RXR) on different types of 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) response elements (VDREs). Therefore, DNA-bound VDR-RXR heterodimers can be considered as the molecular switches of 1alpha,25(OH)(2)D(3) signalling. Functional conformations of the VDR within these molecular switches appear to be of central importance for describing the biologic actions of 1alpha,25(OH)(2)D(3) and its analogues. Moreover, VDR conformations provide a molecular basis for understanding the potential selective profile of VDR agonists, which is critical for a therapeutic application. This review discusses VDR conformations and their selective stabilization by 1alpha,25(OH)(2)D(3) and its analogues, such as EB1089 and Gemini, as a monomer in solution or as a heterodimer with RXR bound to different VDREs and complexed with coactivator or corepressor proteins.

Agonist-triggered modulation of the activated and silent state of the vitamin D(3) receptor by interaction with co-repressors and co-activators

The nuclear receptor for the hormone 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)), VDR, regulates gene expression via a ternary complex with the retinoid X receptor (RXR) and a 1alpha, 25(OH)(2)D(3) response element (VDRE). This complex mediates transcriptional repression through interaction with co-repressor proteins, such as NCoR, and transactivation through agonist-triggered contacts with co-activator proteins, such as SRC-1. This study demonstrates that the interaction of the VDR with NCoR results in a preferential stabilization of the VDR in a non-agonistic conformation (silent state), whereas within a complex with SRC-1 VDR is in its agonistic conformation (activated state). Helix 12 of the ligand-binding domain of the VDR was found to be a critical sensor for the differential stabilization of the activated and silent state of the receptor. VDR agonists that showed similar sensitivity in inducing VDR-RXR-VDRE complex formation were found to mediate a different dose-dependent release of NCoR from these complexes, which correlates with their ability to stabilize the silent state of the VDR in the presence of NCoR. Interestingly, up to 50 % of all VDR-NCoR complexes were found to be stable even in the presence of saturating agonist concentrations. This was confirmed by a quenching effect of overexpressed NCoR on agonist-induced gene activity mediated by VDR-RXR heterodimers. Taken together, co-activator and co-repressor proteins antagonize each other in stabilizing the activated and silent state of the receptor and modulate in this way the sensitivity and potency of the transcriptional activation by the ligand-responsive transcription factor VDR.

Structure activity relationship of carboxylic ester antagonists of the vitamin D(3) receptor

A 25-carboxylic ester analog of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], ZK159222 (compound 1), was recently described as a novel type of antagonist of 1alpha,25(OH)(2)D(3) signaling. In this study five derivatives of compound 1 (compounds 2-6) were selected because of their sensitivity in facilitating complex formation between the 1alpha,25(OH)(2)D(3) receptor (VDR) and the retinoid X receptor on a 1alpha,25(OH)(2)D(3) response element that was comparable to that of the natural hormone (0.2-0.9 nM). Most derivatives of compound 1 reacted as typical agonists, because they were able to promote ligand-dependent interaction of the VDR with the coactivator TIF2, stabilized the VDR preferentially in its agonistic conformation c1(LPD), and stimulated VDR-dependent gene activity with a potency similar to 1alpha,25(OH)(2)D(3). In contrast, only compound 2 showed the antagonistic profile of compound 1, which includes the incompetence to induce a VDR-TIF2 contact, the stabilization of the antagonistic conformation c2(LPD), and only a very weak and insensitive functional activity. Accordingly, only compounds 1 and 2, but not compounds 3 to 6, showed prominent antagonistic effects in cellular systems. The comparison of the structures of the compounds indicates that the essential requirements for an antagonistic function are a cyclopropyl ring at carbon 25, a hydroxy group at carbon 24, and at least a butylester. Interestingly, compound 2 was an approximately 3 times more sensitive antagonist than compound 1 and even displayed a lower residual agonistic activity. In conclusion, only a very limited number of structural variations of compound 1 are possible to keep its antagonistic profile, but the tools presented here for their in vitro evaluation allow an accurate prediction of the effects and are suited to screening for even more potent 1alpha, 25(OH)(2)D(3) antagonists.

Antagonistic action of a 25-carboxylic ester analogue of 1alpha, 25-dihydroxyvitamin D3 is mediated by a lack of ligand-induced vitamin D receptor interaction with coactivators

A 25-carboxylic ester analogue of 1alpha,25-dihydroxyvitamin D(3) (1alpha,25-(OH)(2)D(3)), ZK159222, was described as a novel type of antagonist of 1alpha,25-(OH)(2)D(3) signaling. The ligand sensitivity of ZK159222, in facilitating complex formation between 1alpha,25-(OH)(2)D(3) receptor (VDR) and the retinoid X receptor (RXR) on a 1alpha,25-(OH)(2)D(3) response element (VDRE), was approximately 7-fold lower when compared with 1alpha,25-(OH)(2)D(3). However, ZK159222 was not able to promote a ligand-dependent interaction of the VDR with the coactivator proteins SRC-1, TIF2, and RAC3, neither in solution nor in a complex with RXR on DNA. Functional analysis in HeLa and COS-7 cells demonstrated a 10-100-fold lower ligand sensitivity for ZK159222 than for 1alpha, 25-(OH)(2)D(3) and, most interestingly, a potency that was drastically reduced compared with 1alpha,25-(OH)(2)D(3). A cotreatment of 1alpha,25-(OH)(2)D(3) with a 100-fold higher concentration of ZK159222 resulted in a prominent antagonistic effect both in functional in vivo and in in vitro assays. These data suggest that the antagonistic action of ZK159222 is due to a lack of ligand-induced interaction of the VDR with coactivators with a parallel ligand sensitivity, which is sufficient for competition with the natural hormone for VDR binding.

Ligand-triggered stabilization of vitamin D receptor/retinoid X receptor heterodimer conformations on DR4-type response elements

Nuclear receptors integrate an incoming signal in the form of a nuclear hormone by undergoing a conformational change that results via co-activator proteins in an activation of the basal transcriptional machinery. The vitamin D(3) receptor is the nuclear receptor for 1alpha,25-dihydroxyvitamin D(3 )(1alpha,25(OH)(2)D(3)) and is known to function as a heterodimer with the retinoid X receptor on DR3-type 1alpha,25(OH)(2)D(3) response elements. Here, it could be demonstrated that DR4-type response elements are at least as effective as DR3-type 1alpha,25(OH)(2)D(3) response elements. Gel shift clipping analysis showed that vitamin D(3) receptor-retinoid X receptor heterodimers form in response to 1alpha, 25(OH)(2)D(3) and retinoid X receptor ligands, the pan-agonist 9-cis retinoic acid (9cRA) and the retinoid X receptor-selective retinoid CD2425, different conformations on the DR4-type element of the rat Pit-1 gene. Interestingly, on this response element the heterodimeric complexes of retinoid X receptor with the thyroid hormone receptor, the retinoic acid receptor and the benzoate ester receptor also displayed characteristic individual ligand-dependent complex formation. On the level of complex formation, utilizing DNA affinity and functional assays, only vitamin D(3) receptor-retinoid X receptor heterodimers showed a synergistic interaction of both ligands. However, the sensitivity of vitamin D(3) receptor-retinoid X receptor heterodimers to 1alpha,25(OH)(2)D(3) was found to be much higher than to retinoid X receptor ligands. Taken together, this study demonstrates a unique interaction potential of vitamin D(3) receptor and retinoid X receptor but also establishes DR4-type response elements as multi-functional DNA binding sites with a potential to integrate various hormone signalling pathways.

Steroid-induced conformational changes of rat glucocorticoid receptor cause altered trypsin cleavage of the putative helix 6 in the ligand binding domain

Steroid-induced changes in receptor protein conformation constitute a logical means of translating the variations in steroid structures into the observed array of whole cell biological activities. One conformational change in the rat glucocorticoid receptor (GR) can be readily discerned by following the ability of trypsin digestion to afford a 16-kDa fragment. This fragment is seen after proteolysis of steroid-free receptors but disappears in digests of either glucocorticoid- or antiglucocorticoid-bound receptors. The location of this cleavage site has now been located unambiguously as R651, in helix 6 of the ligand binding domain, by a combination of point mutagenesis, arginine specific protease digestion, and radiochemical sequencing. This 16-kDa species, corresponding to amino acids 652-795, was non-covalently associated with another, approximately 17-kDa species that was determined to be amino acids 518-651 after a comparison of co-immunoprecipitated fragments from wild type and two chimeric receptors. These assignments revise our earlier report of amino acids 537-673 being the 16-kDa fragment and suggest that sequences of the entire ligand binding domain are required for high affinity and specificity binding. This was supported by the observation that trypsin digestion of the steroid-free R651A mutant GR gave rise to the 30-kDa meroreceptor (amino acids 518-795), which displayed wild type affinity. This 30-kDa species is thus the smallest non-associated fragment of GR possessing wild type steroid binding affinity. This suggests that other GR regions do not influence steroid binding affinity. The above results are reminiscent of those observed for the estrogen receptor. However, unlike the estrogen receptor or the more closely related progesterone receptor, the precise proteolytic cleavage points of both the steroid-free and -bound GR fall within regions that are predicted, on the basis of X-ray crystal structures of related receptors, to be alpha-helical and resistant to proteolysis. Thus, the tertiary structure of the GR ligand binding domain may be distinctly different from that of estrogen and progesterone receptors.

Hormone-dependent translocation of vitamin D receptors is linked to transactivation

Vitamin D receptor (VDR) acts as a transcription factor mediating genomic actions of calcitriol. Our earlier studies suggested that calcitriol induces translocation of cytoplasmic VDR, but the physiologic relevance of this finding remained uncertain. Previous studies demonstrated that the activation function 2 domain (AF-2) plays an essential role in VDR transactivation. To elucidate hormone-dependent VDR translocation and its role, we constructed green fluorescent protein (GFP) chimeras with full-length VDR (VDR-GFP), AF-2-truncated VDR (AF-2del-VDR-GFP), and ligand-binding domain (LBD)-truncated VDR (LBDdel-VDR-GFP). COS-7 cells were transiently transfected with these constructs. Western blot analysis, fluorescent microscopy, and transactivation assays showed that the generated chimeras are expressed and fluoresce and that VDR-GFP is transcriptionally active. After hormone treatment, cytoplasmic VDR-GFP translocated to the nucleus in a concentration-, time-, temperature-, and analog-specific manner. Hormone dose-response relationships for translocation and for transactivation were similar. Truncation of LBD and truncation of AF-2 each abolished hormone-dependent translocation and transactivation. Our data confirm a hormone-dependent VDR translocation, demonstrate that an intact AF-2 domain is required for this translocation, and indicate that translocation is part of the receptor activation process.

The role of the T-box for the function of the vitamin D receptor on different types of response elements

The nuclear hormone 1alpha,25-dihydroxyvitamin D3(VD) mainly functions through a heterodimer formed between the VD receptor (VDR) and the retinoid X receptor (RXR). This transcription factor complex specifically recognizes DNA sequences, referred to as VD response elements (VDREs), that are formed by two hexameric core binding motifs arranged either as direct repeats spaced by 3 nt (DR3) or inverted palindromes with nine intervening nucleotides (IP9). Gel shift clipping assays provided the first evidence that VDR-RXR heterodimers form different conformations on these two types of VDREs. Since the T-box within the C-terminal extension of the receptor DNA binding domain (DBD) was previously shown to form a dimerization interface with the partner receptor DBD when bound to DR-type response elements, all six amino acid residues of the VDR T-box were investigated for their role in VDR-RXR heterodimer complex formation on DR3- and IP9-type VDREs. Interestingly, the residue Phe93 (F93) was found to be critical on both types of VDREs, whereas the role of the residue Ile94 (I94) was found to depend on ionic strength of the binding reaction and the nature of the VDRE. However, under physiological conditions I94 was also shown to be critical on both VDRE types. The monitored differences between the two VDR-containing protein-DNA complexes helps in an understanding of the differential action of the nuclear hormone VD and its therapeutically important analogues.

Metabolism of the vitamin D3 analogue EB1089 alters receptor complex formation and reduces promoter selectivity

1. 1alpha,25-dihydroxyvitamin3 (VD) is a nuclear hormone that has important cell regulatory functions but also a strong calcemic effect. EB1089 is a potent antiproliferative VD analogue, which has a modified side chain resulting in increased metabolic stability and a selective functional profile. Since EB1089 is considered for potential systemic application, it will be investigated to what extent its recently identified metabolites (hydroxylated at positions C26 and C26a) contribute to biological profile of the VD analogue. 2. Limited protease digestion analysis demonstrated that EB1089 is able to stabilize the high affinity ligand binding conformation of the VDR, starting at concentrations of 0.1 nM and affecting up to 80% of all receptor molecules. The metabolites EB1445 and EB1470 showed to be 100 fold less potent than EB1089, whereas the remaining three metabolites (EB1435, EB1436 and EB1446) showed a clearly reduced ability to stabilize the high affinity ligand binding conformation. Interestingly, at pharmacological concentrations all EB1089 metabolites stabilized a second, apparently lower affinity conformation to a much higher extent than EB1089. 3. In reporter gene assays all metabolites showed lower potency than EB1089. Moreover, the preference of EB1089 for activation of VDR binding to sites formed by inverted palindromic arrangements spaced by nine nucleotide (IP9-type VD response elements) appeared to be reduced (with EB1445 and EB1470) or completely lost (with EB1435, EB1436 and EB1446). The ranking of EB1089 and its metabolites that was obtained by limited protease digestion and reporter gene assays was confirmed by an analysis of their antiproliferative effect in breast cancer cells. . The potency and selectivity of the EB1089 metabolites in mediating gene regulatory effects was found to be drastically reduced in comparison to the parent compound suggesting that the contribution of the metabolites to the biological effect of EB1089 is minor. However, the compounds showed to be interesting tools for understanding the selective biological profile of EB1089.

Functional conformations of the nuclear 1alpha,25-dihydroxyvitamin D3 receptor

The nuclear hormone 1alpha,25-dihydroxyvitamin D3 (VD) has important cell regulatory functions. Various synthetic VD analogues are under investigation to identify candidates with an improved therapeutic profile against hyperproliferative diseases. VD directly activates the transcription factor VD receptor (VDR), which in turn stimulates the expression of a cascade of primary and secondary VD-responsive genes. The activation of the VDR through binding of its natural and synthetic ligands is linked to a conformational change presenting the interface with co-activator proteins, referred to as the (trans)activation function 2 (AF-2) domain. Multiple conformations of the VDR might be the key to understanding a selective action of VD analogues. The method of limited protease digestion was used here to characterize up to three different functional VDR conformations stabilized individually by VD and its analogues. The relative potency of VDR ligands can be quantified in the interaction with these VDR conformations by determination of a functional dissociation constant, where a two-concentration-point comparison has already provided important information. In this way seven amino acid residues in the AF-2 domain have been analysed as potential ligand contact points. Interestingly, residues Phe-422 and Val-418 seem to interact with all tested VDR ligands, whereas VD analogues such as the anti-psoriatic drug MC903 displayed additional contact points within the AF-2 domain. Taken together, limited protease digestion is a powerful method for studying functional VDR conformations and seems to be very appropriate for screening VD analogues.