Interaction of vitamin D with membrane-based signaling pathways

Many studies in different biological systems have revealed that 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) modulates signaling pathways triggered at the plasma membrane by agents such as Wnt, transforming growth factor (TGF)-β, epidermal growth factor (EGF), and others. In addition, 1α,25(OH)2D3 may affect gene expression by paracrine mechanisms that involve the regulation of cytokine or growth factor secretion by neighboring cells. Moreover, post-transcriptional and post-translational effects of 1α,25(OH)2D3 add to or overlap with its classical modulation of gene transcription rate. Together, these findings show that vitamin D receptor (VDR) cannot be considered only as a nuclear-acting, ligand-modulated transcription factor that binds to and controls the transcription of target genes. Instead, available data support the view that much of the complex biological activity of 1α,25(OH)2D3 resides in its capacity to interact with membrane-based signaling pathways and to modulate the expression and secretion of paracrine factors. Therefore, we propose that future research in the vitamin D field should focus on the interplay between 1α,25(OH)2D3 and agents that act at the plasma membrane, and on the analysis of intercellular communication. Global analyses such as RNA-Seq, transcriptomic arrays, and genome-wide ChIP are expected to dissect the interactions at the gene and molecular levels.


INTRODUCTION
The active vitamin D metabolite 1α,25-dihydroxyvitamin D 3 (1α,25(OH) 2 D 3 ) is a key regulator of gene expression in higher organisms. It modulates the activity of the vitamin D receptor (VDR), a member of the superfamily of nuclear hormone receptors that regulate gene transcription. Genome-wide chromatin immunoprecipitation studies have shown that VDR binds to hundreds of genome sites even in the absence of 1α,25(OH) 2 D 3 and that ligand binding increases and partially changes these binding sites, which depend on the cell type and the duration of treatment (Carlberg and Campbell, 2013). While a subset of VDR binding sites may be responsible for the control of gene expression (VDREs or vitamin D response elements), others might be temporary anchorage places for a population of unliganded "dormant" VDR. According to the classical view, VDR binds DNA as heterodimers with a retinoid X receptor (RXRα, β, or γ) and, upon ligand binding, changes the transcription rate of neighboring genes.
However, many genes whose expression is altered by 1α,25(OH) 2 D 3 do not contain VDREs. Putative mechanisms of this action include post-transcriptional regulation via changes in the levels of microRNAs that modulate the half-life and/or translation of their messenger RNAs (Thorne et al., 2011;Wang et al., 2011;Alvarez-Díaz et al., 2012;Kasiappan et al., 2012;Guan et al., 2013). Also, 1α,25(OH) 2 D 3 may regulate genes post-translationally via changes in the phosphorylation or other modifications of proteins which affect their stability (Lin et al., 2003;Li et al., 2004), or through changes in the level or activity of proteases that target them (Alvarez-Díaz et al., 2010).
Increasing importance has recently been accorded to another mechanism of 1α,25(OH) 2 D 3 action: the modulation of signaling pathways triggered by other agents at the plasma membrane. Indeed, a number of studies have shown that 1α,25(OH) 2 D 3 modulates the effects of growth factors and cytokines by altering either their cytosolic signaling pathways or the activity of target transcription factors in the nucleus, or even in a paracrine fashion by inhibiting their synthesis and secretion by neighboring cells.
Here we review the available data on these non-classical, alternative mechanisms by which 1α,25(OH) 2 D 3 modulates gene expression. Notably, for specific genes such as c-MYC, both direct transcriptional and indirect modes of regulation by 1α,25(OH) 2 D 3 have been described (Pan and Simpson, 1999;Pálmer et al., 2001;Toropainen et al., 2010;Salehi-Tabar et al., 2012).
Wnt factors activate several signaling pathways upon binding to different plasma membrane receptors: the canonical or Wnt/β-catenin and the non-canonical (planar polarity, Ca 2+ . . .) pathways (Clevers and Nusse, 2012). Activation of the Wnt/β-catenin pathway by mutation of APC or AXIN tumor suppressor genes or of CTNNB1/β-catenin oncogene together with changes in the expression of a number of regulatory genes (SFRPs, DICKKOPF (DKK)s. . .) is a hallmark of most colorectal cancers and of a variable proportion of several other malignancies (Clevers and Nusse, 2012). A series of studies report that 1α,25(OH) 2 D 3 antagonizes Wnt/β-catenin signaling in colon cancer cells by several mechanisms: the reduction of transcriptionally active β-catenin/T-cell factor complexes, the induction of β-catenin relocation from the nucleus toward the adherens junctions structures at the plasma membrane, and the increase in the level of the Wnt inhibitor DKK-1 (Pálmer et al., 2001;Shah et al., 2006;Aguilera et al., 2007) (Figure 1). In this way, the pathway endpoint, i.e., the activation of β-catenin target genes, is attenuated by 1α,25(OH) 2 D 3 (Pálmer et al., 2001). Emphasizing the importance of this action, an additional indirect mechanism of Wnt/β-catenin antagonism in colon cancer has been proposed involving IL-1β, which will be reviewed in section 1α,25(OH) 2 D 3 and Cytokines. Although 1,25(OH) 2 D 3 inhibits β-catenin/TCF transcriptional activity in colon and other cancer cells, the upregulation of the Wnt/β-catenin pathway by either ligand-activated or unliganded VDR has been described in osteoblasts and keratinocytes, where it promotes bone formation and hair follicle differentiation, respectively (Larriba et al., 2013). However, the results reported in keratinocytes are controversial: while VDR enhances Wnt signaling through direct binding to Lymphocyte Enhancer-binding Factor (LEF)-1 independently of ligand and βcatenin (Luderer et al., 2011), ligand-activated VDR is believed to inhibit Wnt/β-catenin signaling (Bikle, 2011;Jiang et al., 2012).
Inhibition of Hedgehog (Hh) signaling by vitamin D compounds has also been suggested. In a study combining experiments in zebrafish, the yeast Pichia pastori and mouse fibroblasts, secreted vitamin D 3 , or its precursor 7-dehydrocholesterol (7-DHC), was shown to mediate the paracrine inhibition of Smoothened (Smo) by Patched (Ptch)1 which leads to pathway inactivation (Bijlsma et al., 2006). In the model proposed, which includes the binding of vitamin D 3 to Smo at high (micromolar) concentrations, Hh ligands activate the pathway by blocking the induction of the secretion of vitamin D 3 /7-DHC by Ptch1 (Bijlsma et al., 2006) (Figure 1). The Hh pathway is aberrantly activated in basal cell carcinoma, the most frequent human tumor type. Interestingly, 1α,25(OH) 2 D 3 inhibits proliferation and induces differentiation of mouse basal cell carcinomas and embryonal rhabdomyosarcomas with an activated Hh pathway due to Ptch1 deletion (Uhmann et al., 2011(Uhmann et al., , 2012. As in the previous study, 1α,25(OH) 2 D 3 acts at the level of Smo in a VDRindependent manner (Figure 1). Curiously, Tang et al. found that vitamin D 3 inhibits Hh and cell proliferation more effectively than 7-DHC, 25(OH)D 3 , or 1α,25(OH) 2 D 3 in murine basal cell carcinoma cells (Tang et al., 2011). Vitamin D 3 also inhibits proliferation and Hh pathway through inactivation of Smo in cultured mouse pancreatic adenocarcinoma cells, but has no antitumor activity in vivo (Brüggemann et al., 2010). A common concern in all these studies is the high concentration of vitamin D 3 required to observe the reported effects. Research in Vdr-deficient mice and in mouse skin explants has shown that lack of VDR increases the expression of several components of the Hh pathway such as Shh, Smo,Gli1,Gli2,and Ptch1,while 1α,25(OH) 2 D 3 suppresses their expression (Bikle et al., 2013) (Figure 1). However, the interaction between 1α,25(OH) 2 D 3 and Hh signaling in human skin remains to be elucidated.
Few studies link vitamin D with Notch signaling. Differentiation of human osteoblasts with vitamin D 3 and dexamethasone distinctly affects the expression of Notch receptor family members (Schnabel et al., 2002). In rodent osteoblasts, the transcription factor Hes-1, which is an effector of the Notch pathway, enhances the induction of SPP1/osteopontin transcription by 1α,25(OH) 2 D 3 , indicating the collaboration of 1α,25(OH) 2 D 3 and Notch pathways in bone remodeling (Shen and Christakos, 2005) (Figure 1). Transcriptomic analyses in human RWPE1 immortalized non-tumorigenic prostate cells showed the reduction of the RNA levels of the NOTCH ligands JAGGED (JAG)1, JAG2, and Delta-like (DLL)1 by 1α,25(OH) 2 D 3 (Kovalenko et al., 2010) (Figure 1). By contrast, no changes in the expression of NOTCH-1 and JAG1 were detected in cultured human keratinocytes upon 1α,25(OH) 2 D 3 treatment (Reichrath and Reichrath, 2012). As JAG1 transcription and, consequently, Notch signaling are upregulated by Wnt/β-catenin in colorectal cancer cells (Rodilla et al., 2009), the repressive effect of 1α,25(OH) 2 D 3 on the Notch pathway in this system may be secondary to the antagonism of the Wnt/β-catenin pathway (Figure 1).

INTERPLAY OF 1α,25(OH) 2 D 3 WITH AGENTS THAT TRIGGER SIGNALING PATHWAYS VIA PLASMA MEMBRANE KINASE RECEPTORS
There is mutual antagonism between 1α,25(OH) 2 D 3 and epidermal growth factor (EGF), a potent mitogen, in primary colon epithelial cells and in established colon (Caco-2) and breast (T47D) tumor cell lines. This is based on the cross-inhibition of the expression of their respective receptors, VDR and EGFR (Tong et al., 1998(Tong et al., , 1999. However, it is a cell type-dependent effect as EGF increases VDR in the rat small intestine and 1α,25(OH) 2 D 3 increases EGFR in BT-20 breast cancer cells (Bruns et al., 1989;Desprez et al., 1991). In addition, 1α,25(OH) 2 D 3 inhibits EGFR signaling by increasing the level of E-cadherin protein at the plasma membrane, which downregulates EGFR (Pálmer et al., 2001;Andl and Rustgi, 2005), and by decreasing that of SPROUTY-2, a cytosolic protein that reduces EGFR ubiquitination, internalization and degradation (Cabrita and Christofori, 2008;Barbáchano et al., 2010).
Transforming growth factor (TGF)-β has opposite roles in carcinogenesis: it inhibits proliferation of normal epithelial cells, but it later induces epithelial-mesenchymal transition, immunosuppression and metastasis (Pickup et al., 2013). 1α,25(OH) 2 D 3 induces the expression of the type I TGF-β receptor and both agents, 1α,25(OH) 2 D 3 and TGF-β, cooperate in Caco-2 cell growth inhibition (Chen et al., 2002;Pálmer et al., 2003). Moreover, Smad3, a mediator of TGF-β signaling, is a co-activator of VDR and contributes to gene regulation by 1α,25(OH) 2 D 3 FIGURE 1 | Schematic representation of the multilevel crosstalk of 1α,25(OH) 2 D 3 (1,25) with Wnt, Hedgehog, and Notch signaling pathways. For simplicity, only main components and regulators of the pathways are shown. Explanations, details, and references can be found in the text. , an effect that is abrogated by Smad7 in transfected COS-7 cells . Reinforcing the interaction between both signaling pathways, 1α,25(OH) 2 D 3 induces the expression of Smad anchor for receptor activation (SARA) (Pálmer et al., 2003), which maintains the epithelial phenotype by recruiting Smads 2/3 to the activated TGF-β receptors and regulates endocytic trafficking of EGFR and other proteins (Tang et al., 2011;Kostaras et al., 2013). Notably, a recent study of R. M. Evans' group has revealed a genome-wide overlap of VDR and Smad3 binding sites that is responsible for the abrogation by VDR ligands of the TGF-β1-mediated activation of hepatic stellate cells during liver fibrosis (Ding et al., 2013). These authors show that TGF-β1 signaling redistributes VDR-binding sites in the genome and facilitates VDR binding at Smad3 profibrotic target genes. Upon ligand activation, VDR binding at coregulated genes decreases Smad3 occupancy at these sites, causing inhibition of fibrosis (Ding et al., 2013). This is a regulatory feedback mechanism in which VDR ligands limit the fibrotic process and so ensure an appropriate non-pathological tissue response. Given the crucial roles of TGF-β in carcinogenesis, future studies should examine whether vitamin D compounds play similar roles in the maintenance of epithelial integrity opposing the onset of carcinomas.
1α,25(OH) 2 D 3 and TGF-β interact also in bone. Curiously, in rat (UMR 106 and ROS 17/2.8) and human (MG-63) osteoblastic cells TGF-β increases VDR expression but inhibits the stimulation of osteocalcin and osteopontin transcription and RNA levels by 1α,25(OH) 2 D 3 (Staal et al., 1994). TGF-β exerts this inhibitory effect by reducing the binding of VDR-RXR complexes to VDREs localized in the promoter of these genes without affecting the nuclear availability of VDR at least in ROS 17/2.8 cells (Staal et al., 1996). In contrast to the stimulation of osteocalcin synthesis in human and rat cells, 1α,25(OH) 2 D 3 decreases osteocalcin production in mouse fetal long bone cultures and neonatal osteoblastic MC3T3 cells while stimulating bone resorption (Staal et al., 1998). This bone resorption action of 1α,25(OH) 2 D 3 is dose-dependently inhibited by TGF-β (Staal et al., 1998).
In concordance with its regulatory role in the organism, 1α,25(OH) 2 D 3 favors physiological and homeostatic angiogenesis but inhibits angiogenesis in pathological conditions. Thus, 1α,25(OH) 2 D 3 promotes myogenic differentiation of C2C12 cells by increasing the expression of two key angiogenic factors: vascular endothelial growth factor (VEGF) and fibroblast growth factor-1 (Garcia et al., 2013). In addition, 1α,25(OH) 2 D 3 stimulates pro-angiogenic properties of endothelial progenitor cells by increasing VEGF levels (Grundmann et al., 2012). 1α,25(OH) 2 D 3 also upregulates VEGF expression in osteoblast-like cells but not in breast cancer cells (Schlaeppi et al., 1997). Likewise, ED-71, a vitamin D analog, enhances VEGF expression and promotes angiogenesis in a murine bone marrow ablation model (Okuda et al., 2007). Indeed, increased production of VEGF in vascular smooth muscle cells results from the activation of a VDRE present in the VEGF gene promoter (Cardus et al., 2009). By contrast, 1α,25(OH) 2 D 3 downregulates hypoxia-inducible factor (HIF)-1 and VEGF protein expression in several human colon, prostate and breast cancer cell lines (Ben-Shoshan et al., 2007), decreases VEGF production by human lumbar annulus cells (Gruber et al., 2008), and protects against diabetic retinopathy in rats by inhibiting VEGF expression in the retina (Ren et al., 2012).
1α,25(OH) 2 D 3 also modulates the activity of signaling pathways mediated by other types of plasma membrane receptors such us G protein-coupled receptors. Shen et al. found that 1α,25(OH) 2 D 3 suppresses the expression of parathyroid hormone-related protein (PTHrP) in prostate cancer cells via a negative VDRE localized within the non-coding region of the gene, thus antagonizing the induction of cell proliferation and of the expression of the pro-invasive integrin α 6 β 4 exerted by PTHrP signaling (Shen et al., 2007).

1α,25(OH) 2 D 3 AND CYTOKINES
The anti-inflammatory and immunomodulatory actions, and thus some of the anticancer and antimicrobial effects of 1α,25(OH) 2 D 3 , are mediated by the regulation of cytokine production and/or through the control of their receptors or downstream signaling pathways. Globally, 1α,25(OH) 2 D 3 contributes to the autocrine and paracrine control of innate and adaptative immune responses (Adorini and Penna, 2008).
1α,25(OH) 2 D 3 also changes the expression of target genes in immune cells by repressing crucial transcription factors such as nuclear factor kappa B (NFkB) and signaling pathways such as Janus kinase-signal transducer and activator of transcription (JAK-STAT) (Yu et al., 1995;Muthian et al., 2006;Geldmeyer-Hilt et al., 2011). 1α,25(OH) 2 D 3 also represses NFκB activity in fibroblasts and adipocytes (Harant et al., 1998;Mutt et al., 2012), and fibroblasts lacking VDR have increased NFκB activity (Sun et al., 2006). Direct (increase in IκBα expression and reduction of nuclear translocation of p65) and indirect (upregulation of IGFBP3 and clusterin) mechanisms contribute to the inhibition of NFκB activation . D. Feldman's group has reported that, in addition to inhibiting NFκB, the antiinflammatory effects of 1α,25(OH) 2 D 3 in prostate cancer cells include the reduction of pro-inflammatory prostaglandins (PG) production via suppression of ciclooxygenase-2, downregulation of PG receptors, and upregulation of 15-hydroxyprostaglandin dehydrogenase, which inactivates PGs . Moreover, 1α,25(OH) 2 D 3 decreases the synthesis of proinflammatory IL-6 through the inactivation of p38 kinase due to the upregulation of the mitogen kinase phosphatase (MKP)5 and the blockade of TNF-α . In Jurkat cells, the repression of IL-2 gene by 1α,25(OH) 2 D 3 is at least partially due to the blockade of NFATp/AP-1 complex formation at a positive regulatory NFAT-1 site, which is bound by VDR-RXR heterodimers (Alroy et al., 1995).
In human osteoblasts, 1α,25(OH) 2 D 3 completely overrules the inhibitory effect of IFN-β on mineralization. This dominant effect on osteoblast differentiation and bone formation is reflected in the downregulation of IFN-related and -regulated genes by 1α,25(OH) 2 D 3 (Woeckel et al., 2012). Concomitantly, 1α,25(OH) 2 D 3 also induces activin A, a strong inhibitor of mineralization, and represses follistatin, the natural antagonist of activin A, to ensure a fine-tuned regulation of the mineralization process (Woeckel et al., 2013b).
Recent findings have underscored the complexity of 1α,25(OH) 2 D 3 action and its role in the antimicrobial response as part of innate and adaptative immunity. Thus, activation of macrophage Toll-like receptors (TLRs) by intracellular bacteria such as Mycobacterium tuberculosis upregulates VDR and CYP27B1 genes that allow the induction of the antimicrobial peptide cathelicidin by 1α,25(OH) 2 D 3 (Liu et al., 2006). In monocytes, TLR activation triggers induction of defensin β4 (DEFB4) gene requiring the cooperation between IL-1β and 1α,25(OH) 2 D 3 , which is explained by the presence of one VDRE and two IL-1β-activatable NFkB sites in the DEFB4 promoter (Liu et al., 2009). In addition, 1α,25(OH) 2 D 3 is required for the antimicrobial effect of IFN-γ in human macrophages (Fabri et al., 2011). Moreover, by inducing the expression of TLR2 and CD14 receptors and cathelicidin, 1α,25(OH) 2 D 3 mediates the effect of TGF-β favoring the response to microbial infection and wound injury by keratinocytes (Schauber et al., 2007). These findings show also unexpected cooperation of 1α,25(OH) 2 D 3 with agents (IL-1β, TGF-β, IFN-γ) that are antagonistic in other cell types.

INTERPLAY OF 1α,25(OH) 2 D 3 /VDR WITH TRANSCRIPTION FACTORS
Liganded or unliganded VDR interacts with or regulates the expression of a number of transcription factors that are downstream effectors of different signaling pathways (Table 1). An interesting example is the upregulation by 1α,25(OH) 2 D 3 of CDKN1B/p27 Kip1 , a cell cycle regulator gene which lacks VDREs. 1α,25(OH) 2 D 3 was first shown to induce CDKN1B transcription by stimulating the binding of Sp1 and NF-Y transcription factors to the CDKN1B promoter in the myelomonocytic U937 cell line (Inoue et al., 1999). Later, direct VDR-Sp1 interaction at the promoter Sp1 sites was described as responsible for this effect (Huang et al., 2004). In addition to the enhancement of transcription, 1α,25(OH) 2 D 3 increases the stability of p27 Kip1 protein by repressing p45 Skp2 , an F-box protein, through the induction of VDR-Sp1 complexes that together with histone deacetylase 1 are recruited to Sp1 sites at the p45 Skp2 gene promoter (Lin et al., 2003;Li et al., 2004;Huang and Hung, 2006).
The granulocyte-macrophage colony-stimulating factor (GM-CSF) gene is another example of unusual regulation by 1α,25(OH) 2 D 3 . Ligand-activated VDR represses GM-CSF through a composite DNA element recognized by Jun-Fos heterodimers (AP-1) and nuclear factor of activated T-cells (NFAT)1 (Towers et al., 1999). In the absence of RXR, VDR binds to c-Jun and stabilizes AP-1 bound to DNA, which outcompetes NFAT1 and decreases GM-CSF transcription. In Caco-2 cells, 1α,25(OH) 2 D 3 stimulates AP-1 via activation of protein kinase C-α, ERK and JNK leading to cell differentiation (Chen et al., 1999). In renal cells, 1α,25(OH) 2 D 3 suppresses renin gene expression by blocking the cyclic AMP response element (CRE) through direct binding of VDR to CRE-binding protein (CREB) and so, inhibiting the binding of CREB to the CRE (Yuan et al., 2007). By a complex mechanism, 1α,25(OH) 2 D 3 also regulates several Forkhead box (FOX) transcription factors. Ligandactivated VDR binds FOXO3a and FOXO4 together with their regulators, sirtuin 1 deacetylase and protein phosphatase 1, inducing deacetylation and dephosphorylation of FOXO proteins, thereby activating these . In the case of the p53 tumor suppressor protein a mutual regulation takes place: while mutated p53 interacts physically with VDR and changes VDR-target genes, converting 1α,25(OH) 2 D 3 from a pro-apoptotic into an anti-apoptotic agent (Stambolsky et al., 2010), 1α,25(OH) 2 D 3 activates the promoter of Mdm2 in a p53-dependent fashion promoting the expression of this negative regulator of p53 protein stability and function .
Multiple interplays between 1α,25(OH) 2 D 3 /VDR and other nuclear receptor ligands have been described. Among them, crosstalk between liganded VDR and peroxisome proliferatoractivated receptor (PPAR)-α/δ in melanoma cells (Sertznig et al., 2009a,b) that may involve the stimulation of PPAR-δ expression by 1α,25(OH) 2 D 3 (Dunlop et al., 2005). A synergistic action of 1α,25(OH) 2 D 3 and rosiglitazone, a PPAR-γ ligand, has been shown during osteoblast-mediated mineralization (Woeckel et al., 2013a), while in human T47D breast cancer cells PPARγ binds VDR and represses its transcriptional activity, possibly also by competing for RXR heterodimerization (Alimirah et al., 2012). Titration out of common co-activators, but not of RXR, may be the mechanism by which ligand-bound VDR represses retinoic acid receptor (RAR) transactivation in GH4C1 pituitary cells (Jiménez-Lara and Aranda, 1999). The relation between 1α,25(OH) 2 D 3 and retinoic acid is however complex, as cooperative effects on target genes and cellular outcome (proliferation inhibition and differentiation) have been described in other systems (Tavera-Mendoza et al., 2006;Anand et al., 2008;Ng et al., 2010). As for estrogen receptor (ER), D. Feldman's group has shown that 1α,25(OH) 2 D 3 exerts a multilevel protective effect against breast cancer that includes the inhibition of estrogen synthesis through the direct and indirect repression of aromatase (CYP19) and the downregulation of ER-α expression through two VDREs in its promoter region Swami et al., 2013). Likewise, there is a complex and unresolved relationship between 1α,25(OH) 2 D 3 and androgen receptor (AR) synthesis and signaling. 1α,25(OH) 2 D 3 induces AR in LNCaP cells (Zhao et al., 1999) while AR reduces VDR transcriptional activity (Ting et al., 2005), perhaps in some cells by a mechanism mediated by prohibitin (Mooso et al., 2010). In addition, 1α,25(OH) 2 D 3 inhibits glucuronidation and so, inactivation of androgen in prostate cancer cells through the repression of UDP-glucuronosyltransferases (UGT) 2B15 and 2B17, which is counterintuitive given the growth promoting action of androgen and the antiproliferative effect of 1α,25(OH) 2 D 3 in prostate cancer cells (Kaeding et al., 2008). In human bladder, 1α,25(OH) 2 D 3 and analogs inhibit cell proliferation promoted by androgen and keratinocyte growth factor and induce apoptosis at least in part by repressing Bcl-2 expression (Crescioli et al., 2005).
Pituitary transcription factor (Pit)-1 activates growth hormone and prolactin genes in the anterior pituitary and also in breast cancer cells (Seoane and Pérez-Fernández, 2006). In MCF7 cells, VDR homodimers bind the PIT-1 promoter and inhibit its expression in the presence of 1α,25(OH) 2 D 3 without involvement of RXR (Seoane and Pérez-Fernández, 2006).

CONCLUSIONS
The available evidence shows that the classical view of VDR only as a nuclear-acting ligand-modulated transcription factor that regulates the rate of transcription of those genes to which it binds is outdated. Instead, VDR and its ligand constitute a multilevel main regulator of gene expression in higher cells acting directly or indirectly, and via a variety of different mechanisms, on many signaling pathways. Some of them are triggered from the plasma membrane by paracrine or endocrine agents, and 1α,25(OH) 2 D 3 interacts at different levels: membrane receptors, cytosolic signaling molecules or effector nuclear transcription factors. In most cases 1α,25(OH) 2 D 3 action is mediated by nuclear VDR but in a few others this is unclear and non-canonical VDR-independent or extranuclear effects have been proposed. Available studies show that 1α,25(OH) 2 D 3 and these signaling pathways interact variably and with distinct outcomes in a cell/tissue-specific fashion and sometimes also differentially between normal and malignant cells.

PERSPECTIVES
The increasingly recognized importance of its non-cell autonomous actions has widened the scope of the study of 1α,25(OH) 2 D 3 . On the one hand, an in-depth study of the interplay between 1α,25(OH) 2 D 3 and other agents, which seems to be cell-specific in terms of biological outcome, is necessary to elucidate the possibilities of combined therapies using vitamin D compounds and inhibitors or activators of a variety of signaling pathways. On the other hand, several of these interactions take place at the intercellular level. By using high-throughput techniques and genome-wide analyses, we expect to be able to identify secreted paracrine and intracellular mediators of the interaction between 1α,25(OH) 2 D 3 and other signaling pathways responsible for the regulatory actions of 1α,25(OH) 2 D 3 in the organism. Future research should aim to discern how vitamin D compounds modulate tissue and organ physiology and how they may be used to treat pathological processes such as infections, autoimmune disorders, or cancer.