Regulation of the porcine 1,25-dihydroxyvitamin D-3-24-hydroxylase (CYP24)by 1,25-dihydroxyvitamin D-3 and parathyroid hormone in AOK-B50 cells

The 24-hydroxylase is the enzyme responsible for the first step in the cata bolism of 1,25-dihydroxyvitamin D-3, the active form of vitamin D. This enz yme was shown to be upregulated by 1,25-dihydroxyvitamin D-3 itself and dow nregulated by parathyroid hormone (PTH). Upregulation of 24-hydroxylase by 1,25-dihydroxyvitamin D-3 has been characterized; however, the mechanism by which PTH acts to downregulate 24-hydroxylase expression remains unknown. Here we report the cloning of the porcine 24-hydroxylase, and show that 1,2 5-dihydroxyvitamin D-3-stimulated 24-hydroxylase mRNA and activity are repr essed by PTH in AOK-B50 cells, a porcine kidney proximal tubule cell line w ith stably transfected opossum PTH receptors. Forskolin mimicked the effect s of PTH consistent with in vivo data, and suppression by PTH was not due t o changes in VDR levels, The first 1400 bp of the 24-hydroxylase promoter w ere not able to mediate the effects of PTH on a reporter gene. In view of t he above findings we concluded that AOK-B50 cells are a suitable model for further studying the mechanism of action of PTH on 24-hydroxylase mRNA. (C) 2000 Academic Press.