Vitamin D Receptor (VDR) Regulation of Voltage-Gated Chloride Channels by Ligands Preferring a VDR-Alternative Pocket (VDR-AP)

Abstract

We have postulated that the vitamin D receptor (VDR) contains two overlapping ligand binding sites, a genomic pocket and an alternative pocket (AP), that mediate regulation of gene transcription and rapid responses, respectively. Flexible VDR + ligand docking calculations predict that the major blood metabolite, 25(OH)-vitamin D(3) (25D3), and curcumin (CM) bind more selectively to the VDR-AP when compared with the seco-steroid hormone 1α,25(OH)(2)-vitamin D(3) (1,25D3). In VDR wild-type-transfected COS-1 cells and TM4 Sertoli cells, 1,25D3, 25D3, and CM each trigger voltage-gated, outwardly rectifying chloride channel (ORCC) currents that can be blocked by the VDR antagonist 1β,25(OH)(2)-vitamin D(3) and the chloride channel antagonist (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid). VDR mutational analysis in transfected COS-1 cells demonstrate the DNA-binding domain is not, but the ligand binding and hinge domains of the VDR are, required for 1,25D3 and 25D3 to activate the ORCC. Dose-response studies demonstrate that 25D3 and 1,25D3 are approximately equipotent in stimulating ORCC rapid responses, whereas 1 nm 1,25D3 was 1000-fold more potent than 25D3 and CM in stimulating gene expression. The VDR-AP agonist effects of 1,25D3, 25D3, and low-dose CM are lost after pretreatment of TM4 cells with VDR small interfering RNA. Collectively, these results are consistent with an essential role for the VDR-AP in initiating the signaling required for rapid opening of ORCC. The fact that 25D3 is equipotent to 1,25D3 in opening ORCC suggests that reconsideration of the ability of 25D3 to generate biological responses in vivo may be in order.

Vitamin D metabolism, sex hormones, and male reproductive function

“…The observed binding affinity is in line with a previous study showing that K d for VDR in the rat testis was 50 pM (Gensure et al 1991). Later in vitro studies also showed rapid effects when using similar 1,25(OH) 2 D 3 concentrations (50 pM-1 nM) in both immature rodent Sertoli cells and ejaculated human spermatozoa (Blomberg Jensen et al 2011, Menegaz et al 2011, Zanatta et al 2011c. Spermatozoa are highly compartmentalized cells and each compartment has different functions reflected by the spatial expression of several ion channels, transporters, etc., that are confined to a particular compartment (Publicover et al 2007).…”

The Molecular Basis for the Pharmacokinetics and Pharmacodynamics of Curcumin and Its Metabolites in Relation to Cancer

“…Curcumin is a vitamin D receptor agonist. Curcumin has recently been identified as a direct VDR agonist (Bartik et al, 2010;Menegaz et al, 2011), which, in view of the above mentioned, implies that curcumin could be used to compensate for the lack of 1a,25(OH) 2 D 3 signaling in certain cancer cells. In that respect, the range of effects that the VDR exerts depends on the structural conformation of the ligand, which defines whether the agonist binds to the genomic domain (Rochel et al, 2000) or the partially overlapping alternative binding pocket (Mizwicki et al, 2004).…”

“…Similar to the structurally flexible ligand 1a,25 (OH) 2 D 3 (Okamura et al, 1995), curcumin can associate with both VDR domains (Menegaz et al, 2011). By using radiolabeling, it was demonstrated that curcumin competes with 0.4 nM 1a,25(OH) 2 D 3 for VDR binding with a K i of 2.9 mM (Bartik et al, 2010) when assuming a K d of 10 210 M for the 1a,25(OH) 2 D 3 -VDR complex (Bouillon et al, 2008).…”

“…By using radiolabeling, it was demonstrated that curcumin competes with 0.4 nM 1a,25(OH) 2 D 3 for VDR binding with a K i of 2.9 mM (Bartik et al, 2010) when assuming a K d of 10 210 M for the 1a,25(OH) 2 D 3 -VDR complex (Bouillon et al, 2008). Molecular docking studies revealed that the keto-enol isoform of curcumin, which adopts a planar geometry, shows a strong binding affinity for the VDR alternative binding pocket (Menegaz et al, 2011). In contrast, a 'bowl-shaped' b-diketo conformation of curcumin prefers the genomic domain and thus provokes the associated genomic responses (Menegaz et al, 2011).…”

The role of adult vitamin D deficiency in cognition and brain function in mice

“…Like other neurosteroid hormones, vitamin D initiates nongenomic rapid responses via either a membrane-bound VDR (Menegaz et al, 2011) or a protein-disulfide isomerase-associated 3 protein (PDIA3) (Chen et al, 2010). The variety of signal transduction systems that are rapidly activated by vitamin D include influx of calcium; intracellular release of calcium stores; modulation of adenylate cyclase, phospholipase C, and protein kinases; and alteration of the phosphorylation states of cellular proteins (Falkenstein et al, 2000;Chen et al, 2010).…”