title: 1,25-Dihydroxyvitamin D3 inhibits Na(+)-dependent phosphate transport in osteoblastic cells.
abstract: In the present work we investigated the influence of vitamin D3 metabolites on Na(+)-dependent phosphate (Pi) transport in the clonal osteoblastic cell line UMR-106. The vitamin D3 metabolite 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] dose-dependently inhibited Pi transport with a half-maximal concentration of approximately 5 x 10(-11) M. The effect of 1,25(OH)2D3 was first observed after 8 h of preincubation period. Inhibition of phosphate uptake was relatively specific for the 1,25(OH)2D3 analogue of vitamin D3. The potency order was 1,25(OH)2D3 >> 24,25-dihydroxyvitamin D3 > 25-[3H]hydroxyvitamin D3. Kinetically, 1,25(OH)2D3 decreased the maximal velocity of the phosphate uptake system, whereas the affinity for phosphate was unaffected. Activation of protein kinase C (PKC) in UMR-106 cells stimulated Na(+)-dependent Pi transport. Nonetheless, the inhibitory effect of 1,25(OH)2D3 on Pi transport was not related to downregulation of PKC. Chemical determination of intracellular Pi showed a 50% reduction after 24-h preincubation with 10(-8) M 1,25(OH)2D3. We conclude that 1,25(OH)2D3 inhibits Na(+)-dependent phosphate transport in osteoblastic cells. This in turn leads to intracellular Pi depletion. The physiological implication of this phenomenon on the effects of vitamin D on osteoblasts in situ is discussed.
authors.affiliation: Laboratory of Membrane Biology, Research Institute, Cedars-Sinai Medical Center, University of California School of Medicine, Los Angeles 90048.
authors.forename: J
authors.initials: J
authors.lastname: Green
authors.forename: K V
authors.initials: KV
authors.lastname: Luong
authors.forename: C R
authors.initials: CR
authors.lastname: Kleeman
authors.forename: L H
authors.initials: LH
authors.lastname: Ye
authors.forename: C
authors.initials: C
authors.lastname: Chaimovitz
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002117:Calcitriol / Q000494:pharmacology*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D007700:Kinetics; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D011493:Protein Kinase C / Q000502:physiology; D012964:Sodium / Q000502:physiology*
chemical_list: D010710:Phosphates; D012964:Sodium; D011493:Protein Kinase C; D002117:Calcitriol