Zierold C, Reinholz G G, Mings J A, Prahl J M, DeLuca H F
College of Agricultural and Life Sciences, Department of Biochemistry, University of Wisconsin-Madison, 53706, USA.
Arch Biochem Biophys. 2000 Sep 15;381(2):323-7. doi: 10.1006/abbi.2000.1964.
The 24-hydroxylase is the enzyme responsible for the first step in the catabolism of 1,25-dihydroxyvitamin D3, the active form of vitamin D. This enzyme was shown to be upregulated by 1,25-dihydroxyvitamin D3 itself and downregulated by parathyroid hormone (PTH). Upregulation of 24-hydroxylase by 1,25-dihydroxyvitamin D3 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 1,25-dihydroxyvitamin D3-stimulated 24-hydroxylase mRNA and activity are repressed by PTH in AOK-B50 cells, a porcine kidney proximal tubule cell line with stably transfected opossum PTH receptors. Forskolin mimicked the effects of PTH consistent with in vivo data, and suppression by PTH was not due to changes in VDR levels. The first 1400 bp of the 24-hydroxylase promoter were not able to mediate the effects of PTH on a reporter gene. In view of the 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.
24-羟化酶是负责维生素D的活性形式1,25-二羟维生素D3分解代谢第一步的酶。该酶被证明可被1,25-二羟维生素D3自身上调，并被甲状旁腺激素（PTH）下调。1,25-二羟维生素D3对24-羟化酶的上调作用已得到表征；然而，PTH下调24-羟化酶表达的机制仍然未知。在此，我们报道了猪24-羟化酶的克隆，并表明在AOK-B50细胞（一种稳定转染了负鼠PTH受体的猪肾近端小管细胞系）中，PTH可抑制1,25-二羟维生素D3刺激的24-羟化酶mRNA和活性。 Forskolin模拟了PTH的作用，这与体内数据一致，并且PTH的抑制作用并非由于VDR水平的变化。24-羟化酶启动子的前1400 bp不能介导PTH对报告基因的作用。鉴于上述发现，我们得出结论，AOK-B50细胞是进一步研究PTH对24-羟化酶mRNA作用机制的合适模型。
