1α,25-(OH)2-Vitamin D3Analogs with Minimalin VivoCalcemic Activity Can Stimulate Significant Transepithelial Calcium Transport and mRNA Expressionin Vitro

Several 1α,25-(OH)2-vitamin D3(1α,25-(OH)2-D3) analogs have significant antiproliferative effectsin vitrobut do not elevate serum calciumin vivo.We tested whether the lack of a calcemic response of a vitamin D analogin vivois due to its inability to stimulate intestinal calcium absorption by examining the effect of several such compounds on transepithelial calcium transport in the human colonic carcinoma cell line Caco-2. The relative stimulations of calcium transport by the four A-ring diastereomers of 1α,25-(OH)2-D3(1α,3β) and a 3β-bromoacetate analog (1α,3β-BrAc) of the vitamin following 48-h treatment of cells at 10 nMwere 1α,3β (=100%), 1α,3α (+45.2%), 1β,3β (−15.6%), 1β,3α (+6.5%), and 1α,3β-BrAc (+50.6%). This was similar to the reported affinity of these compounds for the vitamin D receptor (VDR) and suggests that VDR binding predicts calcium transport. In contrast, three noncalcemic, sidechain- or D-ring-modified analogs of vitamin D, 1α,25-(OH)2-16-ene-D3, 1α,25-(OH)2-16-ene-23-yne-D3, and 1α,25,28-(OH)3-D2(at 10 nMfor 48 h), showed a different relationship between VDR affinity (150, 60, and 63% of 1α,25-(OH)2-D3, respectively) and calcium transport (74.1, 126, and 10%, respectively). Elevated calcium transport was accompanied by higher 24-hydroxylase and calbindin D9kmRNA levels. The data demonstrate that although some vitamin D compounds cannot stimulate calcium transport due to an inability to interact with the VDR (e.g., 1β isomers), other factors, e.g., differential cellular metabolism, may account for variations in biological responsein vivoto various vitamin D analogs.