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Requirement for Na(+)-dependent ascorbic acid transport in osteoblast function
Ist Teil von
American Journal of Physiology: Cell Physiology, 1995-06, Vol.268 (6), p.C1430-C1439
Ort / Verlag
United States
Erscheinungsjahr
1995
Link zum Volltext
Quelle
MEDLINE
Beschreibungen/Notizen
R. T. Franceschi, J. X. Wilson and S. J. Dixon
Department of Periodontics, Prevention, and Geriatrics, School of Dentistry, University of Michigan, Ann Arbor 48109-1078.
Ascorbic acid is necessary for expression of the osteoblast phenotype. We
examined whether Na(+)-dependent transport is required for MC3T3-E1
preosteoblast cells to respond to vitamin C and investigated the role of
membrane transport in the intracellular accumulation and function of
ascorbate. MC3T3-E1 cells were found to possess a saturable,
stereoselective, Na(+)-dependent ascorbic acid transport activity that is
sensitive to the transport inhibitors sulfinpyrazone,
4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, and phloretin. Transport
activity showed no competition with glucose or 2-deoxyglucose and was not
inhibited by cytochalasin B, indicating that it is distinct from known
hexose transporters. On addition of 100 microM ascorbic acid to the
extracellular medium, intracellular concentrations of 10 mM were reached
within 5-10 h and remained constant for up to 24 h. A good correlation was
observed between intracellular ascorbic acid concentration and rate of
hydroxyproline synthesis. Although ascorbic acid was transported
preferentially compared with D-isoascorbic acid, both isomers had
equivalent activity in stimulating hydroxyproline formation once they
entered cells. Marked stereoselectivity for extracellular L-ascorbic acid
relative to D-isoascorbic acid was also seen when alkaline phosphatase and
total hydroxyproline were measured after 6 days in culture. Moreover,
ascorbic acid transport inhibitors that prevented intracellular
accumulation of vitamin blocked the synthesis of hydroxyproline. Thus
Na(+)-dependent ascorbic acid transport is required for MC3T3-E1 cells to
achieve the millimolar intracellular vitamin C concentrations necessary for
maximal prolyl hydroxylase activity and expression of the osteoblast
phenotype.