Metabolites and analogs of 1 alpha,25-dihydroxyvitamin D-3: evaluation of actions in bone

Analogs of 1 alpha ,25-dihydroxyvitamin D-3 [1 alpha ,25(OH)(2)D-3] activat e both genomic mechanisms via the nuclear vitamin D-3 receptor (nVDR) and n ongenomic pathways via the plasma membrane vitamin D-3 receptor (pmVDR). Bo th of these pathways are normally activated by 1 alpha ,25(OH)(2)D-3, but a s 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 anal ogs to determine their potencies of action on these two distinct pathways, measuring calcium channel potentiation as an indicator of the nongenomic ac tion and measuring increases in osteocalcin mRNA and protein release and bo ne resorption as indicators of genomic action. We found that both 25(OH)- 1 6,23E-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 pa thway because double bonds in the side chain and the D ring increase the af finity for calcium channel potentiation. While the C-1 alpha -hydroxyl grou p is not necessary for potentiation of calcium channels, methyl groups at t his position can alter the affinity for calcium channel potentiation. On th e other hand, 1000 fold higher concentrations of nongenomic analogs were ne eded compared to 1 alpha ,25(OH)(2)D-3 to increase osteocalcin mRNA or prot ein 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 a t 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 bo nds in the side chain and the D ring stabilize action on the nongenomic pat hway whereas F-6 on the terminal portion of the side chain increases potenc y for nVDR. On the other hand, while the C-1 alpha -hydroxyl group is neces sary for activation of genomic events via nVDR, the activation of nongenomi c events occurs in the absence of this group. (C) 2001 Elsevier Science Inc . All rights reserved.