Rapid and genomic biological responses are mediated by different shapes ofthe agonist steroid hormone, 1 alpha,25(OH)(2)vitamin D-3

The hormone 1 alpha,25(OH)(2)vitamin D-3 (1,25-D) produces biological respo nses via both genomic and rapid mechanisms. The genomic responses are linke d to a nuclear receptor, while the rapid responses are believed to utilize other signal transduction pathways that are likely linked to a putative cel l membrane receptor for 1,25-D. The natural seco-steroid, 1,25-D, is capabl e of facile rotation about its 6,7 single carbon bond to permit generation of a continuum of potential ligand shapes extending from the 6-s-cis (6C) t o the 6-s-trans (6T). To identify the shape of the conformer(s) that can se rve as agonists for the genomic and rapid responses, we synthesized two fam ilies of analogs that were locked in either the 6T or 6C conformation. We f ound that 6T-locked analogs were inactive or significantly less active than 1,25-D in both rapid responses (transcaltachia or the rapid stimulation of intestinal Ca2+ absorption in perfused chick intestine, stimulation of who le cell chloride currents in osteoblastic ROS 17/2.8 cells, and stimulation of phosphorylation of mitogen-activated protein kinase in promyelocytic NB 4 leukemic cells) and in genomic responses (induction of osteocalcin in hum an MG-63 osteoblastic cells). For genomic responses, the 6C-locked analogs bound poorly to the nuclear receptor and were much less potent than 1,25-D. In contrast, the BC-locked analogs were potent agonists of the three rapid responses studied and had activities equivalent to 1,25-D. These results d emonstrate that the signal transduction pathways that support rapid and gen omic responses can discriminate between different shapes of the conformatio nally flexible 1,25-D. (C) 1999 Elsevier Science Inc. All rights reserved.