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T he steroid hormone 1␣,25(OH) 2 -vitamin D 3 (1,25D) (Fig. 1A), other steroid hormones, retinoids, and thyroid hormones form the family of ligands for the nuclear receptor (NR) superfamily (1), the members of which produce genomic responses through selective interaction of the liganded receptor with promoters of appropriate genes and basal transcription machinery. Many of these hormones also activate rapid, nongenomic (NG), cellular signaling cascades (2, 3) (except retinoids) that range from activation of ion channels (4, 5) to promoting kinase and other cytosolic signaling cascades (6-9). Defining the structure-function requirements for 1,25D and 17␤-estradiol (E 2 ) rapid actions has been aided by the synthesis of analogs that are NG agonists like 1␣,25(OH) 2 -lumisterol (JN) (ref. 10 and Fig. 1 A) and 4-estren-3␣,17␤-diol (EST) (11) or antagonists like 1␤,25(OH) 2 -vitamin D 3 (HL) (ref. 12 and Fig. 1 A), but that are only weak genomic transactivators (13). Thus, important structural attributes of the sterol dictate its agonistic properties and subsequent genomic vs. NG signaling profile (14, 15).When 1,25D rapid signaling cascades were first discovered, it was hypothesized that the observed activities were propagated by a novel membrane protein(s) (16), because analogs JN and HL did not compete well with [ 3 H]1,25D for binding to the nuclear vitamin D receptor (VDR) (17). Recently, in studies using a VDR knockout (KO) mouse (18) and a naturally occurring human VDR mutation (19), 1,25D-mediated rapid responses were shown to require a functional VDR. Both the VDR and estrogen receptor (ER) have been found localized to the plasma membrane in caveolae (7, 20); therefore, it has been proposed that the VDR and ER propagate some NG signaling (6,9,18,21,22). However, given the poor affinity of JN and HL for the VDR, it is difficult to understand how these sterols can facilitate their activities through the VDR.Results obtained from the modeling (INSIGHT 2000.1) of JN, 1,25D, and HL in the VDR ligand-binding domain (LBD) showed that the VDR could possibly accept and form favorable nonbonding interactions with vitamin D sterols in a distinct ligand-binding pocket [an alternative ligand-binding pocket (A pocket)] from the genomic pocket (G pocket) that was previously defined by x-ray crystallography (23, 24). Our proposed A-pocket accepts ligands that differ in shape from those in the classical G pocket (3,23,24).The data from these models has led to the proposal that the VDR can function as a rapid response receptor through a conformational ensemble mechanism (3, 25) whereby the flexible 1,25D steroid hormone samples an ensemble of energetically similar protein conformations (26). In addition, the ensemble model and existence of an A pocket may provide an explanation for the observed sex-nonspecific, nongenotropic signaling through the ER␣ receptor by EST (3, 9, 11). The physiological relevance of the ensemble model and functional importance of an A pocket within the VDR is further substantiated by applying the m...

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.