Structure-function studies of 1,25-dihydroxyvitamin D3 and the vitamin D endocrine system. 1,25-dihydroxy-pentadeuterio-previtamin D3 (as a 6-s-cis analog) stimulates nongenomic but not genomic biological responses

The hormone 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) generates biological responses via both genomic and nongenomic mechanisms. This article addresses activity differences between the 6-s-trans (extended) and the 6-s-cis (steroid-like) conformation of 1,25-(OH)2D3 to initiate these responses. Because of facile interconversion of the 6-s-trans and 6-s-cis conformers of 1,25-(OH)2D3, kinetically competent amounts of both conformers exist to interact with any potential receptors for 1,25-(OH)2D3. We have chemically synthesized 1,25-(OH)2-9,14,19,19,19-pentadeuterio-pre-D3 (1,25-(OH)2-d5-pre-D3), an analog of the 6-s-cis conformation of 1,25-(OH)2D3. We found that 1,25-(OH)2-d5-pre-D3 and 1,25-(OH)2D3 were equivalently active in two nongenomic systems (transcaltachia as measured in the perfused chick intestine and 45Ca2+ uptake through voltage-gated Ca2+ channels in ROS 17/2.8 cells). 1,25-(OH)2-d5-pre-D3 was significantly less active both in binding in vitro to the plasma vitamin D-binding protein (7%) and to the chick (10%) and pig (4%) intestinal nuclear 1,25-(OH)2D3 receptors generating genomic biological responses in vivo (induction of plasma levels of osteocalcin, < 5%) or in cultured cells (inhibition of HL-60 cell differentiation, < 5%; inhibition of MG-63 proliferation, < 2%; and induction of osteocalcin, < 2%). These results suggest that the genomic and nongenomic responses are mediated by separate receptors. Further, the 6-s-cis form (steroid-like conformation) of the natural hormone, 1,25-(OH)2D3, may be selectively responsible for its nongenomic function(s).