Molecular Actions of 1,25-Dihydroxyvitamin D3 on Genes Involved in Calcium Homeostasis

Abstract
1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] functions in vertebrate organisms as a primary regulator of calcium and phosphorus homeostasis, an activity that is achieved through direct actions on gene expression in intestine, kidney, and bone. Recent studies have identified novel genes such as TRPV5, TRPV6, and RANKL whose products are integral to the maintenance of extracellular calcium. The objective of this progress report/review is to describe our recent results that identify the mechanisms of 1,25(OH)2D3 action on the expression of TRPV6 and RANKL. A series of molecular, cellular, and in vivo studies have been conducted to define the molecular mechanisms that control the expression of TRPV6 and RANKL. Cell culture–based assays, chromatin immunoprecipitation (ChIP) and ChIP-DNA microarray (ChIP-chip) methods, and a series of molecular techniques were used to identify and characterize upstream regions of mouse and human TRPV6 and RANKL genes. We discovered that these genes were regulated by at least five separate enhancer regions. In the TRPV6 gene, these enhancers were all located within 5 kb of the transcriptional start site (TSS), and each contained one or more vitamin D regulatory elements (VDREs). In the RANKL gene, these regulatory regions span over 80 kb of upstream sequence, the most distal 76 kb from the TSS. This regulatory region is central to the regulation of RANKL expression in vitro and in vivo. Our studies identified key regulatory regions within the TRPV6 and RANKL genes that are essential for their individual expression in the intestine and bone, respectively.