EXPRESSION OF 25-HYDROXYVITAMIN-D3-24-HYDROXYLASE ACTIVITY IN CACO-2 CELLS - AN INVITRO MODEL OF INTESTINAL VITAMIN-D CATABOLISM

The C-24 oxidation pathway plays a major role in the degradation of vitamin D metabolites in kidney and other target tissues. The aim of the present study was to establish an intestinal cell culture system to study the mechanisms regulating the vitamin D catabolic pathway. 25-Hydroxyvitamin D3-24-hydroxylase (24-hydroxylase), the first enzyme in the catabolic sequence, was examined in Caco-2 cells, a human colon adenocarcinoma cell line which exhibits differentiated functions of absorbing intestinal epithelial cells. While untreated Caco-2 cells did not exhibit 24-hydroxylase activity, significant catabolic activity was induced by prior treatment of cell monolayers with 1, 25-dihydroxyvitamin D3(l, 25-(OH)2D3). Induced 24-hydroxyl-ase activity was not inhibited by either 1, 2-dianilinoethane or EDTA in the reaction mixture. 24-Hydroxylation of 25-hydroxy-vitamin D3 (250HD3) and l, 25-(OH)2D3 was detected 6 h after treatment of cells with 10-8 M l, 25-(OH)2D3peaked at 16 h, and decreased thereafter. Treatment of cells with 10-7M 1, 25-(OH))2D3 and 1, 25-(OH)2D3-dose response curves were observed in cultured human skin fibroblasts and Caco-2 cells. 250HD3was not as good an inducer of the vitamin D catabolic pathway in Caco-2 cells as l, 25-(OH)2D3. Induction of 24-hydroxylase activity by 1, 25-(OH)2D3 was inhibited by pretreatment of Caco-2 cells with either actinomycin D, α-amanitin, or cycloheximide suggesting that mRNA and protein synthesis are required for induction. The present study demonstrates that l, 25-(OH)2D3-treated Caco-2 cells express the vitamin D catabolic pathway and, therefore, constitute a useful in vitro model to study the mechanism of induction by l, 25-(OH)2D3. (Endocrinology 126: 28682875, 1990). © 1990 by The Endocrine Society.