Comparison of 6-s-cis- and 6-s-trans-locked analogs of 1 alpha,25-dihydroxyvitamin D-3 indicates that the 6-s-cis conformation is preferred for rapid nongenomic biological responses and that neither 6-s-cis- nor 6-s-trans-locked analogs are preferred for genomic biological responses

The hormone 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3] generates biological responses via both genomic and rapid, nongenomic mechanisms. The genomic responses utilize signal transduction pathways linked to a nuclear receptor (VDRnuc) for 1 alpha;25(OH)(2)D-3, while the rapid responses are believed to utilize other signal transduction pathways that may be 1inked to a putative membrane receptor for 1 alpha,25(OH)(2)D-3. The natural seco steroid is capable of facile rotation about its 6,7 single carbon bond, which permits generation of a continuum of potential ligand shapes extending from the 6-s-cis (steroid like) to the 6-s-trans (extended). To identify the shape of conformer(s) that can serve as agonists for the genomic and rapid biological responses, we measured multiple known agonist activities of two families of chemically synthesized analogs that were either locked in the 6-s-cis (6C) or 6-s-trans (6T) conformation. We found that 6T locked analogs were inactive or significantly less active than 1 alpha,25(OH)(2)D-3 in both rapid responses (transcaltachia in perfused chick intestine, Ca-45(2+) influx in ROS 17/2.8 cells) and genomic (osteocalcin induction in MG-63 cells, differentiation of HL-60 cells, growth arrest of MCF-7 cells, promoter transfection in COS-7 cells) assays. In genomic assays, 6C locked analogs bound poorly to the VDRnuc and were significantly less effective than 1 alpha 25(OH)(2)D-3 in the same series of assays designed to measure genomic responses. In contrast, the 6C locked analogs were potent agonists of both rapid response pathways and had activities equivalent to the conformationally flexibile 1 alpha 25(OH)(2)D-3; this represents the first demonstration that 6-s-cis locked analogs can function as agonists for vitamin D responses.