Metabolites and analogs of 1α,25-dihydroxyvitamin D3:: evaluation of actions in bone

Analogs of 1 alpha ,25-dihydroxyvitamin D-3 [1 alpha ,25(OH)(2)D-3] activate both genomic mechanisms via the nuclear vitamin D-3 receptor (nVDR) and nongenomic pathways via the plasma membrane vitamin D-3 receptor (pmVDR). Both of these pathways are normally activated by 1 alpha ,25(OH)(2)D-3, but as a result of synthesis of numerous analogs of 1 alpha ,25(OH)(2)D-3 these pathways can be distinguished. We used increasing doses of vitamin D-3 analogs to determine their potencies of action on these two distinct pathways, measuring calcium channel potentiation as an indicator of the nongenomic action and measuring increases in osteocalcin mRNA and protein release and bone resorption as indicators of genomic action. We found that both 25(OH)- 16,23E-diene-D-3 (R) and 1 alpha ,25(OH)(2)-16,23E-diene-D-3 (A) are 10-fold more potent than 1 alpha ,25(OH)(2)D-3 for activation of the nongenomic pathway because double bonds in the side chain and the D ring increase the affinity for calcium channel potentiation. While the C-1 alpha -hydroxyl group is not necessary for potentiation of calcium channels, methyl groups at this position can alter the affinity for calcium channel potentiation. On the other hand, 1000 fold higher concentrations of nongenomic analogs were needed compared to 1 alpha ,25(OH)(2)D-3 to increase osteocalcin mRNA or protein release. 1 alpha ,25-Dihydroxy-16-ene-23-yne-26,27-hexafluorovitamin D-3. (E) is an agent that is 10 fold more potent than 1 alpha ,25(OH)(2)D-3 at increasing osteocalcin mRNA and protein release, whereas 1 alpha ,25(OH)(2)-3-epi-D-3 increases osteocalcin mRNA and protein with a potency over 10 fold lower than 1 alpha ,25(OH)(2)D-3. These results suggest that double bonds in the side chain and the D ring stabilize action on the nongenomic pathway whereas F-6 on the terminal portion of the side chain increases potency for nVDR. On the other hand, while the C-1 alpha -hydroxyl group is necessary for activation of genomic events via nVDR, the activation of nongenomic events occurs in the absence of this group. (C) 2001 Elsevier Science Inc. All rights reserved.