1 alpha,25(OH)2-vitamin D3 analog structure-function assessment of intestinal nuclear receptor occupancy with...

1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] has been shown to generate biological responses via both genomic and nongenomic pathways. In previous studies concerning the mechanism of 1 alpha,25(OH)2D3-stimulated rapid intestinal transport of Ca2+, a process termed transcaltachia, we assessed the ability of seven analogs of 1 alpha,25(OH)2D3 to initiate transcaltachia in the vitamin D-replete chick and bind in vitro to the classical nuclear chick intestinal receptor for 1 alpha,25(OH)2D3 (VDR). Two groups of analogs were found, one able to stimulate transcaltachia and the other able to bind to the VDR. In the present study, we have examined both the genomic effect of these analogs in the intestine of vitamin D-deficient chicks and their in vitro binding to vitamin D binding protein. It was found that analogs known to bind effectively to the nuclear receptor in vitro could achieve a significant occupancy of the VDR in vivo and stimulated calbindin-D28K messenger RNA and protein synthesis. In contrast, those analogs that were good agonists of transcaltachia were ineffective either in occupying the VDR in vitro or in stimulating in vivo calbindin-D28K messenger RNA and protein synthesis. These differences are consistent with our hypothesis that there may be two receptors for 1,25-(OH)2D3 with differing ligand binding domains, one being the classical nuclear VDR and the second a membrane receptor associated with transcaltachia.