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Steroid hormones serve as chemical messengers in a wide number of species and target tissues by transmitting signals that result in both genomic and nongenomic responses. Genomic responses are mediated by the formation of a ligand-receptor complex with its cognate steroid hormone nuclear receptor (NR). Nongenomic responses can be mediated at the plasma membrane by a membrane-localized NR. The focus of this Review is on the structural attributes and molecular mechanisms underlying vitamin D sterol (VDS)-vitamin D receptor (VDR) selective and stereospecific regulation of nongenomic and genomic signaling. The VDS-VDR conformational ensemble model describes how VDSs can selectively initiate or block either nongenomic or genomic biological responses by interacting with two VDR ligand-binding pockets, one kinetically favored by 1alpha,25(OH)(2)D(3) (1,25D) and the other thermodynamically favored. We describe the variables that affect the three major elements of the model: the conformational flexibility of the unliganded (apo) protein, the flexibility of the VDS, and the physicochemical selectivity of the VDR genomic pocket (VDR-GP) and alternative pocket (VDR-AP). We also discuss how these three factors collectively provide a rational explanation for the complexities of VDS regulation of cell biology and highlight the current limitations of the model.

Structural and molecular studies have shown that the vitamin D receptor (VDR) mediates 1alpha,25(OH)2-vitamin D3 gene transactivation. Recent evidence indicates that both VDR and the estrogen receptor are localized to plasma membrane caveolae and are required for initiation of nongenomic (NG) responses. Computer docking of the NG-specific 1alpha,25(OH)2-lumisterol to the VDR resulted in identification of an alternative ligand-binding pocket that partially overlaps the genomic pocket described in the experimentally determined x-ray structure. Data obtained from docking five different vitamin D sterols in the genomic and alternative pockets were used to generate a receptor conformational ensemble model, providing an explanation for how VDR and possibly the estrogen receptor can have genomic and NG functionality. The VDR model is compatible with the following: (i) NG chloride channel agonism and antagonism; (ii) variable ligand-stabilized trypsin digest banding patterns; and (iii) differential transcriptional activity, employing different VDR point mutants and 1alpha,25(OH)2-vitamin D3 analogs., (Copyright 2004 The National Academy of Sciencs of the USA)

The steroid hormone 1 alpha,25(OH)(2)-vitamin D(3) [1 alpha,25(OH)(2)D(3)] mediates through its widely distributed nuclear receptor (VDR(nuc)) regulation of gene transcription (genomic responses) and through a putative membrane receptor (VDR(mem)) a variety of rapid responses. Rapid responses studied in our laboratories include opening of voltage-gated calcium and chloride channels in ROS 17/2.8 osteoblast cells, activation of MAP-kinase in human leukemia NB4 cells and chick intestinal cells, release of insulin by rat pancreatic beta-cells, and in chick duodena transcaltachia (the rapid hormonal stimulation of intestinal Ca(2+) transport). 1 alpha,25(OH)(2)D(3) is conformationally flexible (side chain, seco B-ring and A-ring) and accordingly is able to generate a large array of different shapes to serve as ligands for available receptors (VDR(nuc) and VDR(mem)) in the vitamin D endocrine system. Our laboratories have utilized a number of conformationally restricted analogs of 1 alpha,25(OH)(2)D(3) (from a library of several hundred analogs) to evaluate the preferred shape of the ligands for rapid and genomic responses. The determination of the X-ray structure of the 1 alpha,25(OH)(2)D(3)-occupied VDR(nuc) revealed that the preferred ligand shape was a twisted 6-s-trans bowl shape [Molecular Cell 5 (2000) 173-179]. Optimal agonists for genomic responses include 1 alpha,25(OH)(2)D(3) and other side chain conformationally flexible analogs such as 20-epi-1 alpha,25(OH)(2)D(3) [approximately equal to 200-500-fold more potent than 1 alpha,25(OH)(2)D(3)] and 21-(3'-hydroxy-3-methylbutyl)-1 alpha,25(OH)(2)D(3) [an analog with two side chains] all which can achieve the preferred VDR(nuc) shape. In contrast, rapid responses require a 6-s-cis shape of the agonist ligand such as can be achieved by the natural hormone 1 alpha,25(OH)(2)D(3) or by analogs permanently locked in the 6-s-cis shape such as 1 alpha,25(OH)(2)lumisterol(3) or 1 alpha,25(OH)(2)-7-dehydrocholesterol. Additionally, we have discovered analogs that are specific in their antagonist properties for either rapid or genomic responses. Thus, 1 beta,25(OH)(2)D(3) is an antagonist of only rapid responses [via the VDR(mem)], while 23S-25-dehydro-1 alpha,25(OH)D(3)-26,23-lactone is an antagonist of only nuclear responses [via the VDR(nuc)]. In conclusion, we have presented evidence that 1 alpha,25(OH)(2)D(3) mediated rapid response and genomic response signal transduction pathways utilize differing shapes of ligand, both as agonists and antagonists.