25-hydroxyvitamin D 1α-hydroxylase gene expression : regulation by dietary P and parathyroid hormone

Abstract

Renal 25-hydroxyvitamin D la-hydroxylase (1-OHase) is a key metabolic enzyme involved in biosynthesis of 1,25-dihydroxyvitmain D3 [1,25(OH)2D3]. Decrease in renal 1-OHase activity is thought to be the primary cause of age-related alteration in vitamin D metabolism. In this study, we employed two different approaches to study the low phosphorus diet (LPD) and parathyroid hormone (PTH) regulation of renal 1 -OHase mRNA expression in vitro and in vivo at the molecular level. The first study examined the response of young (4-week-old) and adult (6-month-old) male Sprague Dawley rats to LPD (0.6 % Ca, 0.1% P) by using Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) assay. The result indicates that the response of 1-OHase expression to LPD was blunted in adult rats. The negative relationship between serum phosphorus and 1-OHase mRNA expression was also abolished in adult rats, suggesting serum P cannot mediate renal 1-OHase mRNA expression in adult rats. This study also demonstrates that the age-related alteration of vitamin D metabolism is not solely restricted to the 1-OHase expression, but also is involved in the 25-hydroxyvitamin D 24-alpha-hydroxylase (24-OHase) expression that governs the metabolic clearance of 1,25(OH)2D3. However, the mechanisms that cause the reduced 1-OHase expression remain to be determined. In the second part of the present study, we attempted to use cDNA microarray to search for possible mechanisms involved in the PTH regulation of 1-OHase in HKC-8 cell line. Several genes involved in differentiation were shown to be regulated by PTH in the renal proximal tubules. These genes might play important roles in mediating the action of PTH for performing its specific functions in the renal proximal tubules such as production of 1,25(OH)2D3