25-Hydroxyvitamin D 3 is an agonistic vitamin D receptor ligand

Abstract

25-Hydroxyvitamin D 3 1α-hydroxylase encoded by CYP27B1 converts 25-hydroxyvitamin D 3 into 1α,25-dihydroxyvitamin D 3, a vitamin D receptor ligand. 25-Hydroxyvitamin D 3 has been regarded as a prohormone. Using Cyp27b1 knockout cells and a 1α-hydroxylase-specific inhibitor we provide in four cellular systems, primary mouse kidney, skin, prostate cells and human MCF-7 breast cancer cells, evidence that 25-hydroxyvitamin D 3 has direct gene regulatory properties. The high expression of megalin, involved in 25-hydroxyvitamin D 3 internalisation, in Cyp27b1 −/− cells explains their higher sensitivity to 25-hydroxyvitamin D 3. 25-Hydroxyvitamin D 3 action depends on the vitamin D receptor signalling supported by the unresponsiveness of the vitamin D receptor knockout cells. Molecular dynamics simulations show the identical binding mode for both 25-hydroxyvitamin D 3 and 1α,25-dihydroxyvitamin D 3 with the larger volume of the ligand-binding pocket for 25-hydroxyvitamin D 3. Furthermore, we demonstrate direct anti-proliferative effects of 25-hydroxyvitamin D 3 in human LNCaP prostate cancer cells. The synergistic effect of 25-hydroxyvitamin D 3 with 1α,25-dihydroxyvitamin D 3 in Cyp27b1 −/− cells further demonstrates the agonistic action of 25-hydroxyvitamin D 3 and suggests that a synergism between 25-hydroxyvitamin D 3 and 1α,25-dihydroxyvitamin D 3 might be physiologically important. In conclusion, 25-hydroxyvitamin D 3 is an agonistic vitamin D receptor ligand with gene regulatory and anti-proliferative properties.