Expression of 25-Hydroxyvitamin D3-24-Hydroxylase Activity in Caco-2 Cells, Anin VitroModel of Intestinal Vitamin D Catabolism*

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Abstract
The C-24 oxidation pathway plays a major role in the degradation of vitamin D metabolites in kidney an other target tissues. The aim of the present study was to establish an intestinal cell culture system to study the mechanism 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 (1,25-(OH)2D3). Induced 24-hydroxylase activity was not inhibited by either 1,2-dianilinoethane or EDTA in the reaction mixture. 24-Hydroxylation of 25-hydroxyvitamin D3 (25OHD3) and 1,25-(OH)2D3 was detected 6 h after treatment of cells with 10-8 M 1,25-(OH)2D3, peaked at 16 h, and decreased thereafter. Treatment of cells with 10-7 M 1,25-(OH)2D3 elicited a maximal 24-hydroxylase response. Comparable time course of induction by 1,25-(OH)2D3 and 1,25-(OH)2D3-dose response curves were observed in cultured human skin fibroblasts and CaCo-2 cells. 25OHD3 was not as good an inducer of the vitamin D catabolic pathway in Caco-2 cells as 1,25-(OH)2D3. Induction of 24-hydroxylase activity by 1,25-(OH)2D3 was inhibited by pretreatment of Caco-2 cells with either actinomycin D, .alpha.-amanitin, or cycloheximide suggesting that mRNA and protein synthesis are required for induction. The present study demonstrates that 1,25-(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 1,25-(OH)2D3.

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