Consequences of vitamin D receptor regulation for the 1,25-dihydroxyvitamin D₃-induced 24-hydroxylase activity in osteoblast-like cells: Initiation of the C24-oxidation pathway

A direct relationship between vitamin D receptor (VDR) level and target cell responsiveness to 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃) has been shown in osteoblast-like cell lines. However, we previously found an inverse relationship between the TGFβ-induced VDR up-regulation and subsequent 1,25-(OH)₂D₃-induced biological responses. A clear inhibition of the 1,25-(OH)₂D₃-induced stimulation of osteocalcin and osteopontin expression was observed. A biological response that has formerly been shown do be coupled to VDR level is 24-hydroxylase activity. This enzyme initiates the C24 oxidation of the side-chain, followed by cleavage and ultimate metabolic clearance of both 25-(OH)D₃ and its metabolite 1,25-(OH)₂D₃. With UMR 106 (rat) and MG 63 (human) osteoblast-like cells, we show that after preincubation with TGFβ, which causes an increase in VDR level, 1,25-(OH)₂D₃ induction of 24-hydroxylase activity is also stimulated. In addition, we provide evidence that variations in VDR level induced by other means (PTH, EGF, medium change) are also closely associated with 1,25-(OH)₂D₃-induced 24-hydroxylase activity. Furthermore, we show that in MG 63 cells, but not in UMR 106 cells, TGFβ itself was able to increase the activity of the enzyme 24-hydroxylase. As 24-hydroxylation is the initial step in the further C24 oxidation of 1,25-(OH)₂D₃, our results indicate a close coupling of VDR level and the degradation of its ligand, 1,25-(OH)₂D₃. This mechanism may provide an important regulatory feedback in the action of 1,25-(OH)₂D₃ at target tissue/cell level.