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 TGF beta-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 to 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 TGF beta, 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, TGF beta 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.
Staal
A
A
Department of Internal Medicine III, Erasmus University Medical School, Rotterdam, The Netherlands.
van den Bemd
G J
GJ
Birkenhäger
J C
JC
Pols
H A
HA
van Leeuwen
J P
JP
D015650:24,25-Dihydroxyvitamin D 3 / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D016895:Culture Media, Serum-Free; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004790:Enzyme Induction / Q000187:drug effects; D006801:Humans; D010006:Osteoblasts / Q000378:metabolism* / Q000502:physiology; D010084:Oxidation-Reduction; D051381:Rats; D018167:Receptors, Calcitriol / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000235:genetics / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000494:pharmacology; D065668:Vitamin D3 24-Hydroxylase
D016895:Culture Media, Serum-Free; D018167:Receptors, Calcitriol; D016212:Transforming Growth Factor beta; D014807:Vitamin D; D015650:24,25-Dihydroxyvitamin D 3; C097949:1,25-dihydroxyvitamin D; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase