1 alpha,25-(OH)2-vitamin D3 analogs with minimal in vivo calcemic activity can stimulate significant transepithelial calcium transport and mRNA expression in vitro

Abstract

Several 1 alpha,25-(OH)2-vitamin D3 (1 alpha,25-(OH)2-D3) analogs have significant antiproliferative effects in vitro but do not elevate serum calcium in vivo. We tested whether the lack of a calcemic response of a vitamin D analog in vivo is 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 1 alpha, 25-(OH)2-D3 (1 alpha,3 beta) and a 3 beta-bromoacetate analog (1 alpha,3 beta-BrAc) of the vitamin following 48-h treatment of cells at 10 nM were 1 alpha,3 beta (=100%), 1 alpha,3 alpha (+45.2%), 1 beta,3 beta (-15.6%), 1 beta,3 alpha (+6.5%), and 1 alpha,3 beta-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, 1 alpha,25-(OH)2-16-ene-D3, 1 alpha,25-(OH)2-16-ene-23-yne-D3, and 1 alpha,25,28-(OH)3-D2 (at 10 nM for 48 h), showed a different relationship between VDR affinity (150, 60, and 63% of 1 alpha, 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 D9k mRNA 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., 1 beta isomers), other factors, e.g., differential cellular metabolism, may account for variations in biological response in vivo to various vitamin D analogs.