Two vitamin D response elements function in the rat 1,25-dihydroxyvitamin D 24-hydroxylase promoter.

The interaction between the two vitamin D response elements (DRE) located at -154 to -134 base pairs (bp) and -262 to -238 bp from the transcription initiation site has been studied using reporter gene assays and binding assays by electrophoretic gel shift measurements. 3 half-sites separated by 3 bp were found necessary for transactivation by the -154 to -125 DRE, while 2 half-sites separated by 3 bp were needed for the DRE at -262 to -238 to function. However, the two DREs together provided maximal activity. The 93-bp fragment separating the two DREs was not required and could be deleted. The most effective binding by receptor was found with the two complete DREs (dissociation constant (Kd) = 13.7 pM), although each DRE bound to the receptor and nuclear accessory factor with about 5 nM Kd. The two DREs (a total of 5 half-sites) apparently account for most if not all of the transactivation of the rat 24-hydroxylase by 1,25-dihydroxyvitamin D3. This system represents the most powerful of the DREs reported to date.