Characterization of transcriptional activation and DNA-binding functions in the hinge region of the vitamin D receptor

Significance of the Vitamin D Receptor on Crosstalk with Nuclear Receptors and Regulation of Enzymes and Transporters

The vitamin D receptor (VDR), in addition to other nuclear receptors, the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), is involved in the regulation of enzymes, transporters and receptors, and therefore intimately affects drug disposition, tissue health, and the handling of endogenous and exogenous compounds. This review examines the role of 1α,25-dihydroxyvitamin D₃ or calcitriol, the natural VDR ligand, on activation of the VDR and its crosstalk with other nuclear receptors towards the regulation of enzymes and transporters, notably many of the cytochrome P450s including CYP3A4 and sulfotransferase 2A1 (SULT2A1) as well as cholesterol 7α-hydroxylase (CYP7A1). Moreover, the VDR upregulates the intestinal channel, TRPV6, for calcium absorption, LDL receptor-related protein 1 (LRP1) and receptor for advanced glycation end products (RAGE) in brain for β-amyloid peptide efflux and influx, the sodium phosphate transporters (NaP_i), the apical sodium-dependent bile acid transporter (ASBT) and organic solute transporters (OSTα-OSTβ) for bile acid absorption and efflux, respectively, the renal organic anion transporter 3 (OAT3) and several of the ATP-binding cassette protein transporters-the multidrug resistance protein 1 (MDR1) and the multidrug resistance-associated proteins (MRPs). Hence, the role of the VDR is increasingly being recognized for its therapeutic potential and pharmacologic activity, giving rise to drug-drug interactions (DDI). Therapeutically, ligand-activated VDR shows anti-inflammatory effects towards the suppression of inflammatory mediators, improves cognition by upregulating amyloid-beta (Aβ) peptide clearance in brain, and maintains phosphate, calcium, and parathyroid hormone (PTH) balance and kidney function and bone health, demonstrating the crucial roles of the VDR in disease progression and treatment of diseases.

Mitotic genome bookmarking by nuclear receptor VDR advocates transmission of cellular transcriptional memory to progeny cells

Mitosis is an essential process for the self-renewal of cells that is accompanied by dynamic changes in nuclear architecture and chromatin organization. Despite all the changes, the cell manages to re-establish all the parental epigenetic marks, post-mitotically. Recent reports suggest that some sequence-specific transcription factors remain attached to mitotic chromatin during cell division to ensure timely reactivation of a subset of transcription factors necessary to maintain cell identity. These mitotically associated factors are suggested to act as 'genome bookmarking factors' and the phenomenon is termed 'genome bookmarking'. Here, we studied this phenomenon with Vitamin D Receptor (VDR), a key regulator of calcium and phosphate homeostasis and a member of the nuclear receptor superfamily. This study, for the first time, has confirmed VDR as a mitotic bookmarking factor that may be playing a crucial role in the maintenance of cell identity and genome bookmarking. Full 'DNA binding domain (DBD)' present in VDR was identified as essential for enrichment of VDR on mitotic chromatin. Furthermore, the study also demonstrates that VDR evokes mitotic chromatin binding behaviour in its heterodimeric partner Retinoid X receptor (RXR). Interestingly, for promoting bookmarking behaviour in RXR, both DBD and/or ligand-binding domain (LBD) in conjunction with hinge region of VDR were required. Additionally, ChIP analysis showed that VDR remains associated with DR3 (direct repeat 3) region of its specific target gene promoter CYP24A1(Cytochrome P450 family 24 subfamily A member1), during mitosis. Altogether, our study illustrates a novel function of VDR in the epigenetic transmission and control of expression of target proteome for maintenance of cell identity and traits in progeny cells.

Functional effects of vitamin D: From nutrient to immunomodulator

Vitamin D can be obtained from the endogenous synthesis in the epidermis by exposure to UVB light, and from foods and supplements in the form of ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃). The main metabolite used to measure vitamin D serum status is calcidiol [25(OH)D]. However, its active metabolite calcitriol [1α,25(OH)₂D] performs pleiotropic effects in the cardiovascular, neurological, and adipose tissue as well as immune cells. Calcitriol exerts its effects through genomic mechanisms modulated by the nuclear vitamin D receptor (VDR)/retinoid X receptor (RXR) complex, to bind to vitamin D response elements (VDRE) in target genes of several cells such as activated T and B lymphocytes, neutrophils, macrophages, and dendritic cells; besides of its genomic mechanisms, VDR performs novel non-genomic mechanisms that involve its membrane expression and soluble form; highlighting that vitamin D could be an immunomodulatory nutrient that plays a key role during physiological and pathological events. Therefore, the aim of this comprehensive literature review was to describe the most relevant findings of vitamin D dietary sources, absorption, synthesis, metabolism, and factors that influence its serum status, signaling pathways, and biological effects of this immunonutrient in the health and disease.

The vitamin D receptor (VDR) binds to the nuclear matrix via its hinge domain: A potential mechanism for the reduction in VDR mediated transcription in mitotic cells

Vitamin D is best known for its regulation of calcium homeostasis. Vitamin D exerts its genomic actions via the vitamin D receptor (VDR). As a member of the superfamily of nuclear receptors (NR), the VDR is primarily located within the nucleus of non-dividing cells. We show here that the VDR relocates from the nucleus into the cytoplasm across all stages of cell division in CHO cells. Furthermore, we show that the VDR is transcriptionally inert during cell division. In addition, 1α, 25 dihydroxyvitamin D (1,25(OH)₂D₃) promotes VDR binding to the nuclear matrix. Finally, we assessed the structural nature of VDR binding to the nuclear matrix. Mutation of the hinge domain reduced VDR's ability to bind to the nuclear matrix and to initiate transcription in response to 1,25(OH)₂D₃. Taken together, our data suggest that the association between the VDR and the nuclear matrix accounts for the apparent cytosolic distribution as the matrix disperses within the cytoplasm when cells divide. This may also explain the dramatic reduction in VDR mediated transcription during cell division. Our data also confirm that similar to other NRs, the hinge domain of the VDR is responsible for this association.

Molecular determinants of Drosophila immunophilin FKBP39 nuclear localization

FK506-binding proteins (FKBPs) belong to a distinct class of immunophilins that interact with immunosuppressants. They use their peptidyl-prolyl isomerase (PPIase) activity to catalyze the cis-trans conversion of prolyl bonds in proteins during protein-folding events. FKBPs also act as a unique group of chaperones. The Drosophila melanogaster peptidyl-prolyl cis-trans isomerase FK506-binding protein of 39 kDa (FKBP39) is thought to act as a transcriptional modulator of gene expression in 20-hydroxyecdysone and juvenile hormone signal transduction. The aim of this study was to analyze the molecular determinants responsible for the subcellular distribution of an FKBP39-yellow fluorescent protein (YFP) fusion construct (YFP-FKBP39). We found that YFP-FKBP39 was predominantly nucleolar. To identify the nuclear localization signal (NLS), a series of YFP-tagged FKBP39 deletion mutants were prepared and examined in vivo. The identified NLS signal is located in a basic domain. Detailed mutagenesis studies revealed that residues K188 and K191 are crucial for the nuclear targeting of FKBP39 and its nucleoplasmin-like (NPL) domain contains the sequence that controls the nucleolar-specific translocation of the protein. These results show that FKBP39 possesses a specific NLS in close proximity to a putative helix-turn-helix (HTH) motif and FKBP39 may bind DNA in vivo and in vitro.

Development of a highly sensitive in vitro system to detect and discriminate between vitamin D receptor agonists and antagonists based on split-luciferase technique

Split-luciferase techniques are widely used to detect protein-protein interaction and bioactive small molecules including some hormones and vitamins. Previously, we successfully expressed chimeric proteins of luciferase and the ligand binding domain (LBD) of the vitamin D receptor (VDR), LucC-LBD-LucN in COS-7 cells. The LucC-LBD-LucN biosensor was named split-luciferase vitamin D biosensor (SLDB). This biosensor can detect and discriminate between VDR agonists and antagonists in mammalian cells. In this study, we established an in vitro screening system for VDR ligands using the SLDB proteins expressed in Escherichia coli (E. coli) cells. Our in vitro screening system using cell lysate of recombinant E. coli cells could be completed within 30min, and its activity was unchanged after 10 freeze-thaw cycles. This highly sensitive and convenient system would be quite useful to screen VDR ligands with therapeutic potential for various bone-related diseases, age-related cognitive disorders, cancer, and immune disorders. In addition, our system might be applicable to diagnostic measurement of serum concentrations of 25-hydroxyvitamin D₃ and 1α,25-dihydroxyvitamin D₃.

HDX reveals the conformational dynamics of DNA sequence specific VDR co-activator interactions

The vitamin D receptor/retinoid X receptor-α heterodimer (VDRRXRα) regulates bone mineralization via transcriptional control of osteocalcin (BGLAP) gene and is the receptor for 1α,25-dihydroxyvitamin D₃ (1,25D3). However, supra-physiological levels of 1,25D3 activates the calcium-regulating gene TRPV6 leading to hypercalcemia. An approach to attenuate this adverse effect is to develop selective VDR modulators (VDRMs) that differentially activate BGLAP but not TRPV6. Here we present structural insight for the action of a VDRM compared with agonists by employing hydrogen/deuterium exchange. Agonist binding directs crosstalk between co-receptors upon DNA binding, stabilizing the activation function 2 (AF2) surfaces of both receptors driving steroid receptor co-activator-1 (SRC1) interaction. In contrast, AF2 of VDR within VDRM:BGLAP bound heterodimer is more vulnerable for large stabilization upon SRC1 interaction compared with VDRM:TRPV6 bound heterodimer. These results reveal that the combination of ligand structure and DNA sequence tailor the transcriptional activity of VDR toward specific target genes.

The vitamin D receptor/retinoid X receptor-α heterodimer (VDRRXRα) regulates bone mineralization. Here the authors employ hydrogen/deuterium exchange (HDX) mass spectrometry to study the conformational dynamics of VDRRXRα and give mechanistic insights into how VDRRXRα controls the transcriptional activity of specific genes.

Novel screening system for high-affinity ligand of heredity vitamin D-resistant rickets-associated vitamin D receptor mutant R274L using bioluminescent sensor

Hereditary vitamin D-resistant rickets (HVDRR) is caused by mutations in the vitamin D receptor (VDR) gene. Arg274 located in the ligand binding domain (LBD) of VDR is responsible for anchoring 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂D₃) by forming a hydrogen bond with the 1α-hydroxyl group of 1α,25(OH)₂D_3. The Arg274Leu (R274L) mutation identified in patients with HVDRR causes a 1000-fold decrease in the affinity for 1α,25(OH)₂D₃, and dramatically reduces vitamin D- related gene expression. Recently, we successfully constructed fusion proteins consisting of split-luciferase and LBD of the VDR. The chimeric protein LucC-LBD-LucN, which displays the C-terminal domain of luciferase (LucC) at its N-terminus, can detect and discriminate between VDR agonists and antagonists. The LucC-LBD (R274L)-LucN was constructed to screen high-affinity ligands for the mutant VDR (R274L). Of the 33 vitamin D analogs, 5 showed much higher affinities for the mutant VDR (R274L) than 1α,25(OH)₂D₃, and 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-(OH)₂D₃ showed the highest affinity. These compounds might be potential therapeutics for HVDRR caused by the mutant VDR (R274L).

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

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

Crystal Structure of the Vitamin D Receptor Ligand-Binding Domain with Lithocholic Acids

The secondary bile acid lithocholic acid (LCA) and its derivatives act as selective modulators of the vitamin D receptor (VDR), although their structures fundamentally differ from that of the natural hormone 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3). The complexes of the ligand-binding domain of rat VDR (VDR-LBD) with LCA and its derivatives revealed that the ligands bound to the same ligand-binding pocket (LBP) of VDR-LBD that 1,25(OH)2D3 binds to, but in the opposite orientation; their A-ring was positioned at the top of the LBP, whereas their acyclic tail was located at the bottom of the LBP. However, most of the hydrophobic and hydrophilic interactions observed in the complex with 1,25(OH)2D3 were reproduced in the complexes with LCA and its derivatives. Additional interactions between VDR-LBD and the C-3 substituents of the A-ring were also observed in the complexes, probably related to the observed difference in the potency among the LCA-type ligands. Recently, zebrafish VDR has been reported to have the second LBP on the outside of the canonical LBP, although its physiological function is unclear.

Hic-5 is a transcription coregulator that acts before and/or after glucocorticoid receptor genome occupancy in a gene-selective manner

Ligand activation and DNA-binding dictate the outcome of glucocorticoid receptor (GR)-mediated transcriptional regulation by inducing diverse receptor conformations that interact differentially with coregulators. GR recruits many coregulators via the well-characterized AF2 interaction surface in the GR ligand-binding domain, but Lin11, Isl-1, Mec-3 (LIM) domain coregulator Hic-5 (TGFB1I1) binds to the relatively uncharacterized tau2 activation domain in the hinge region of GR. Requirement of hydrogen peroxide-inducible clone-5 (Hic-5) for glucocorticoid-regulated gene expression was defined by Hic-5 depletion and global gene-expression analysis. Hic-5 depletion selectively affected both activation and repression of GR target genes, and Hic-5 served as an on/off switch for glucocorticoid regulation of many genes. For some hormone-induced genes, Hic-5 facilitated recruitment of Mediator complex. In contrast, many genes were not regulated by glucocorticoid until Hic-5 was depleted. On these genes Hic-5 prevented GR occupancy and chromatin remodeling and thereby inhibited their hormone-dependent regulation. Transcription factor binding to genomic sites is highly variable among different cell types; Hic-5 represents an alternative mechanism for regulating transcription factor-binding site selection that could apply both within a given cell type and among different cell types. Thus, Hic-5 is a versatile coregulator that acts by multiple gene-specific mechanisms that influence genomic occupancy of GR as well transcription complex assembly.

Looking at nuclear receptors from a new angle

While the structures of the DNA- and ligand-binding domains of many nuclear receptors have been determined in great detail; the mechanisms by which these domains interact and possibly 'communicate' is still under debate. The first crystal structures of receptor dimers bound to ligand, DNA and coactivator peptides provided new insights in this matter. The observed binding modes revealed exciting new interaction surfaces between the different nuclear receptor domains. Such interfaces are proposed to be the route through which allosteric signals from the DNA are passed on to the ligand-binding domain and the activating functions of the receptor. The structural determinations of DNA-bound receptor dimers in solution, however, revealed an extended structure of the receptors. Here, we discuss these apparent contradictory structural data and their possible implications for the functioning of nuclear receptors.

Selective cytotoxic action and DNA damage by calcitriol-Cu(II) interaction: putative mechanism of cancer prevention

BACKGROUND

Vitamin D is known to play an important role in cancer-prevention. One of the features associated with the onset of malignancy is the elevation of Cu (II) levels. The mode of cancer-prevention mediated by calcitriol, the biologically active form of vitamin D, remain largely unknown.

METHODS

Using exogenously added Cu (II) to stimulate a malignancy like condition in a novel cellular system of rabbit calcitriol overloaded lymphocytes, we assessed lipid peroxidation, protein carbonylation, DNA damage and consequent apoptosis. Free radical mediators were identified using free radical scavengers and the role of Cu (II) in the reaction was elucidated using chelators of redox active cellular metal ions.

RESULTS

Lipid peroxidation and protein carbonylation (markers of oxidative stress), consequent DNA fragmentation and apoptosis were observed due to calcitriol-Cu (II) interaction. Hydroxyl radicals, hydrogen peroxide and superoxide anions mediate oxidative stress produced during this interaction. Amongst cellular redox active metals, copper was found to be responsible for this reaction.

CONCLUSION

This is the first report implicating Cu (II) and calcitriol interaction as the cause of selective cytotoxic action of calcitriol against malignant cells. We show that this interaction leads to the production of oxidative stress due to free radical production and consequent DNA fragmentation, which leads to apoptosis. A putative mechanism is presented to explain this biological effect.

Crosstalk between the peroxisome proliferator-activated receptor γ (PPARγ) and the vitamin D receptor (VDR) in human breast cancer cells: PPARγ binds to VDR and inhibits 1α,25-dihydroxyvitamin D3 mediated transactivation

Heterodimerization and cross-talk between nuclear hormone receptors often occurs. For example, estrogen receptor alpha (ERα) physically binds to peroxisome proliferator-activated receptor gamma (PPARγ) and inhibits its transcriptional activity. The interaction between PPARγ and the vitamin D receptor (VDR) however, is unknown. Here, we elucidate the molecular mechanisms linking PPARγ and VDR signaling, and for the first time we show that PPARγ physically associates with VDR in human breast cancer cells. We found that overexpression of PPARγ decreased 1α,25-dihydroxyvitamin D(3) (1,25D(3)) mediated transcriptional activity of the vitamin D target gene, CYP24A1, by 49% and the activity of VDRE-luc, a vitamin D responsive reporter, by 75% in T47D human breast cancer cells. Deletion mutation experiments illustrated that helices 1 and 4 of PPARγ's hinge and ligand binding domains, respectively, governed this suppressive function. Additionally, abrogation of PPARγ's AF2 domain attenuated its repressive action on 1,25D(3) transactivation, indicating that this domain is integral in inhibiting VDR signaling. PPARγ was also found to compete with VDR for their binding partner retinoid X receptor alpha (RXRα). Overexpression of RXRα blocked PPARγ's suppressive effect on 1,25D(3) action, enhancing VDR signaling. In conclusion, these observations uncover molecular mechanisms connecting the PPARγ and VDR pathways.

General molecular biology and architecture of nuclear receptors

Nuclear receptors (NRs) regulate and coordinate multiple processes by integrating internal and external signals, thereby maintaining homeostasis in front of nutritional, behavioral and environmental challenges. NRs exhibit strong similarities in their structure and mode of action: by selective transcriptional activation or repression of cognate target genes, which can either be controlled through a direct, DNA binding-dependent mechanism or through crosstalk with other transcriptional regulators, NRs modulate the expression of gene clusters thus achieving coordinated tissue responses. Additionally, non genomic effects of NR ligands appear mediated by ill-defined mechanisms at the plasma membrane. These effects mediate potential therapeutic effects as small lipophilic molecule targets, and many efforts have been put in elucidating their precise mechanism of action and pathophysiological roles. Currently, numerous nuclear receptor ligand analogs are used in therapy or are tested in clinical trials against various diseases such as hypertriglyceridemia, atherosclerosis, diabetes, allergies and cancer and others.

Structure of the full human RXR/VDR nuclear receptor heterodimer complex with its DR3 target DNA

Transcription regulation by steroid hormones and other metabolites is mediated by nuclear receptors (NRs) such as the vitamin D and retinoid X receptors (VDR and RXR). Here, we present the cryo electron microscopy (cryo-EM) structure of the heterodimeric complex of the liganded human RXR and VDR bound to a consensus DNA response element forming a direct repeat (DR3). The cryo-EM map of the 100-kDa complex allows positioning the individual crystal structures of ligand- and DNA-binding domains (LBDs and DBDs). The LBDs are arranged perpendicular to the DNA and are located asymmetrically at the DNA 5'-end of the response element. The structure reveals that the VDR N-terminal A/B domain is located close to the DNA. The hinges of both VDR and RXR are fully visible and hold the complex in an open conformation in which co-regulators can bind. The asymmetric topology of the complex provides the structural basis for RXR being an adaptive partner within NR heterodimers, while the specific helical structure of VDR's hinge connects the 3'-bound DBD with the 5'-bound LBD and thereby serves as a conserved linker of defined length sensitive to mutational deletion.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Selective mutations in estrogen receptor alpha D-domain alters nuclear translocation and non-estrogen response element gene regulatory mechanisms

The three main mechanisms of ERα action are: 1) nuclear, genomic, direct DNA binding, 2) nuclear, genomic, "tethered"-mediated, protein-protein interactions, and 3) non-nuclear, non-genomic, rapid action responses. Reports suggest the D-domain or hinge region of ERα plays an important role in mechanisms 1 and 2 above. Studies demonstrating the functionality of the ERα hinge region have resected the full D-domain; therefore, site directed mutations were made to attribute precise sequence functionality to this domain. This study focuses on the characterization and properties of three novel site directed ERα- D-domain mutants. The Hinge 1 (H1) ERα mutant has disrupted nuclear localization, can no longer perform tethered mediated responses and has lost interaction with c-Jun, but retains estrogen response element (ERE)-mediated functions as demonstrated by confocal microscopy, reporter assays, endogenous gene expression and co-immunoprecipitation. The H2 ERα mutant is non-nuclear, but translocates to the nucleus with estradiol (E2) treatment and maintains ERE-mediated functionality. The H2+NES ERα mutant does not maintain nuclear translocation with hormone binding, no longer activates ERE-target genes, functions in ERE- or tethered-mediated luciferase assays, but does retain the non-genomic, non-nuclear, rapid action response. These studies reveal the sequence(s) in the ERα hinge region that are involved in tethered-mediated actions as well as nuclear localization and attribute important functionality to this region of the receptor. In addition, the properties of these ERα mutants will allow future studies to further dissect and characterize the three main ERα mechanisms of action and determine the mechanistic role each action has in estrogen hormone regulation.

Human sperm anatomy: ultrastructural localization of 1alpha,25-dihydroxyvitamin D receptor and its possible role in the human male gamete

Previous studies have suggested that 1alpha,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] has a role in reproductive function. Gonadal insufficiencies were observed as a result of 1,25(OH)(2)D(3) deficiency and in 1,25(OH)(2)D(3) receptor (VDR) null mutant mice. To study human sperm anatomy at the molecular level, we first evaluated the ultrastructural localization of VDR by immunogold electron microscopy using a monoclonal antibody against amino acids 344-424 of human VDR, in normozoospermic samples. Intriguingly, VDR was associated predominantly with the cell nucleus. In fact, it is known that VDR is a transcription factor, and that in vitamin-D-depleted animals, VDR is largely localized in the cell nucleus. To assess the significance of VDR in the male gamete, we investigated the role of 1,25(OH)(2)D(3)/VDR in sperm survival and capacitation. Our results revealed that the action of 1,25(OH)(2)D(3) depended on its concentration because although lower doses induced cholesterol efflux, protein phosphorylation and sperm survival, a higher concentration seemed to be ineffective or did not show an increased effect. These results increase our knowledge of human sperm anatomy at the molecular level and suggest that 1,25(OH)(2)D(3)/VDR may have an important role in sperm survival and the acquisition of fertilizing ability.

Identification of Biomarkers Modulated by the Rexinoid LGD1069 (Bexarotene) in Human Breast Cells Using Oligonucleotide Arrays

Retinoids have been found to be promising chemopreventive agents that play an important role in regulating cell growth, differentiation, and apoptosis. The action of retinoids is mediated by retinoid receptors (retinoic acid receptors and retinoid X receptors), which are nuclear transcription factors that, when bound to retinoids, regulate gene expression. LGD1069 is a highly selective RXR agonist that has reduced toxicity compared with retinoids. Our previous studies have shown that RXR-selective ligands (or "rexinoids"), including LGD1069, can inhibit the growth of normal and malignant breast cells and can suppress the development of breast cancer in transgenic mice. For the current study, we attempted to identify biomarkers of the chemopreventive effect of the RXR-selective retinoid LGD1069. In these experiments, we used Affymetrix microarrays to identify target genes that were modulated by LGD1069 in normal human breast cells. Affymetrix and dChip analysis identified more than 100 genes that were up-regulated or down-regulated by LGD1069 treatment. We then tested 16 of these genes in validation experiments using quantitative reverse transcription-PCR and Western blotting of independently prepared samples, and found that 15 of 16 genes were modulated in a similar manner in these validation experiments as in the microarray experiments. Genes found to be regulated include known retinoid-regulated genes, growth regulatory genes, transcription factors, and differentiation markers. We then showed that the expression of several of these rexinoid-regulated biomarkers is modulated in vivo in mammary glands from mice treated with LGD1069. These critical growth-regulating proteins will be promising targets of future agents for the prevention and treatment of breast cancer.

Characterization of the DNA-binding properties and the transactivation activity of Schistosoma mansoni nuclear receptor fushi tarazu-factor 1alpha (SmFTZ-F1alpha)

A FTZ-F1-related orphan nuclear receptor SmFTZ-F1alpha was previously identified from Schistosoma mansoni. The deduced SmFTZ-F1alpha protein contains a highly conserved DNA binding domain (DBD, C domain), a less conserved ligand binding domain (LBD, E domain) and three highly variable regions, the N-terminal A/B domain (108 aa), a large hinge region (D domain, 1027 aa) and an F domain (220 aa). Herein, we characterize the DNA binding properties and the transactivation activity of SmFTZ-F1alpha. In in vitro assays, SmFTZ-F1alpha bound as a monomer to a response element (FF1RE: TCAAGGTCA) recognized by mammalian steroidogenic factor 1 (SF-1), and to related sequences (p14: TTAAGGTCA and SmFF1a-2: CGAAGGTCA) derived from known schistosome gene promoters. Competition assays with p14 oligonucleotides containing a single mutation at each nucleotide position defined the optimum DNA sequence required for SmFTZ-F1alpha binding. The optimal consensus sequence for SmFTZ-F1alpha binding is TN(A/G)AGGTC(A/G) (N: any base). This sequence is similar but not identical to the SF-1 response element (SFRE) consensus sequence [(T/C)CAAGG(T/C)C(A/G)]. By performing yeast one-hybrid assays, the ability of SmFTZ-F1alpha to bind productively to a p14-derived 9-base pair sequence was demonstrated in vivo. The ability of the full-length SmFTZ-F1alpha to transactivate reporter gene expression was shown to be A/B domain-dependent in a yeast system. In addition, the hinge region contained an unexpected activation function (AF) domain, termed AF-3, while no transactivation activity was detected within the E/F domain. This AF-3 region (from aa 982 to aa 1110) revealed a strong autonomous transactivation activity, which was masked when it was present in the full-length SmFTZ-F1alpha. Taken together, our results suggest that SmFTZ-F1alpha possesses the characteristic DNA binding specificity of FTZ-F1 subfamily members and the capacity to transactivate a reporter gene.

Common angiotensin receptor blockers may directly modulate the immune system via VDR, PPAR and CCR2b

Background

There have been indications that common Angiotensin Receptor Blockers (ARBs) may be exerting anti-inflammatory actions by directly modulating the immune system. We decided to use molecular modelling to rapidly assess which of the potential targets might justify the expense of detailed laboratory validation. We first studied the VDR nuclear receptor, which is activated by the secosteroid hormone 1,25-dihydroxyvitamin-D. This receptor mediates the expression of regulators as ubiquitous as GnRH (Gonadatrophin hormone releasing hormone) and the Parathyroid Hormone (PTH). Additionally we examined Peroxisome Proliferator-Activated Receptor Gamma (PPARgamma), which affects the function of phagocytic cells, and the C-CChemokine Receptor, type 2b, (CCR2b), which recruits monocytes to the site of inflammatory immune challenge.

Results

Telmisartan was predicted to strongly antagonize (Ki≈0.04nmol) the VDR. The ARBs Olmesartan, Irbesartan and Valsartan (Ki≈10 nmol) are likely to be useful VDR antagonists at typical in-vivo concentrations. Candesartan (Ki≈30 nmol) and Losartan (Ki≈70 nmol) may also usefully inhibit the VDR. Telmisartan is a strong modulator of PPARgamma (Ki≈0.3 nmol), while Losartan (Ki≈3 nmol), Irbesartan (Ki≈6 nmol), Olmesartan and Valsartan (Ki≈12 nmol) also seem likely to have significant PPAR modulatory activity. Olmesartan andIrbesartan (Ki≈9 nmol) additionally act as antagonists of a theoretical modelof CCR2b. Initial validation of this CCR2b model was performed, and a proposed model for the AngiotensinII Type1 receptor (AT2R1) has been presented.

Conclusion

Molecular modeling has proven valuable to generate testable hypotheses concerning receptor/ligand binding and is an important tool in drug design. ARBs were designed to act as antagonists for AT2R1, and it was not surprising to discover their affinity for the structurally similar CCR2b. However, this study also found evidence that ARBs modulate the activation of two key nuclear receptors-VDR and PPARgamma. If our simulations are confirmed by experiment, it is possible that ARBs may become useful as potent anti-inflammatory agents, in addition to their current indication as cardiovascular drugs.

Crystal structure of the human LRH-1 DBD-DNA complex reveals Ftz-F1 domain positioning is required for receptor activity

The DNA-binding and ligand-binding functions of nuclear receptors are localized to independent domains separated by a flexible hinge. The DNA-binding domain (DBD) of the human liver receptor homologue-1 (hLRH-1), which controls genes central to development and metabolic homeostasis, interacts with monomeric DNA response elements and contains an Ftz-F1 motif that is unique to the NR5A nuclear receptor subfamily. Here, we present the 2.2A resolution crystal structure of the hLRH-1 DBD in complex with duplex DNA, and elucidate the sequence-specific DNA contacts essential for the ability of LRH-1 to bind to DNA as a monomer. We show that the unique Ftz-F1 domain folds into a novel helix that packs against the DBD but does not contact DNA. Mutations expected to disrupt the positioning of the Ftz-F1 helix do not eliminate DNA binding but reduce the transcriptional activity of full-length LRH-1 significantly. Moreover, we find that altering the Ftz-F1 helix positioning eliminates the enhancement of LRH-1-mediated transcription by the coactivator GRIP1, an action that is associated primarily with the distantly located ligand-binding domain (LBD). Taken together, these results indicate that subtle structural changes in a nuclear receptor DBD can exert long-range functional effects on the LBD of a receptor, and significantly impact transcriptional regulation.