title: Glucocorticoid stimulation of Na+-dependent ascorbic acid transport in osteoblast-like cells.
abstract: Ascorbic acid (AA) is an essential cofactor for osteoblast differentiation both in vivo and in vitro. Before it can function, this vitamin must be transported into cells via a specific Na+-dependent AA transporter. In this study, we examine the regulation of this transport activity by glucocorticoids, a class of steroid hormones known to stimulate in vitro osteoblast differentiation. Dexamethasone stimulated Na+-dependent AA transport activity approximately twofold in primary rat calvarial osteoblasts. Effects of hormone on ascorbic acid transport were rapid (detected within 24 h) and were maximally stimulated by 25-50 nM dexamethasone. Similar effects of dexamethasone on transport activity were also observed in murine MC3T3-E1 cells. This preosteoblast cell line was used for a more detailed characterization of the glucocorticoid response. Transport activity was stimulated selectively by glucocorticoids (dexamethasone > corticosterone) relative to other steroid hormones (progesterone and 17-beta-estradiol) and was blocked when cells were cultured in the presence of cycloheximide, a protein synthesis inhibitor. Kinetic analysis of AA transporter activity in control and dexamethasone-treated cells indicated a Km of approximately 17 microM for both groups. In contrast, dexamethasone increased Vmax by approximately 2.5-fold. Cells also contained an Na+-independent glucose transport activity that has been reported in other systems to transport vitamin C as oxidized dehydroascorbic acid. In marked contrast to Na+-dependent AA transport, this activity was inhibited by dexamethasone. Thus, glucocorticoids increase Na+-dependent AA transport in osteoblasts, possibly via up-regulation of transporter synthesis, and this response can be resolved from actions of glucocorticoids on glucose transport.
authors.affiliation: Department of Periodontics, Prevention, and Geriatrics, School of Dentistry, University of Michigan, Ann Arbor 48109-1078, USA.
authors.forename: S
authors.initials: S
authors.lastname: Pandipati
authors.forename: J E
authors.initials: JE
authors.lastname: Driscoll
authors.forename: R T
authors.initials: RT
authors.lastname: Franceschi
mesh_terms: D000818:Animals; D001205:Ascorbic Acid / Q000493:pharmacokinetics*; D001692:Biological Transport / Q000187:drug effects; D002454:Cell Differentiation / Q000502:physiology; D002460:Cell Line; D003513:Cycloheximide / Q000494:pharmacology; D003683:Dehydroascorbic Acid / Q000378:metabolism; D003847:Deoxyglucose / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D005938:Glucocorticoids / Q000494:pharmacology*; D007700:Kinetics; D051379:Mice; D010006:Osteoblasts / Q000187:drug effects*; D051381:Rats; D012964:Sodium / Q000494:pharmacology*; D013256:Steroids / Q000494:pharmacology
chemical_list: D005938:Glucocorticoids; D013256:Steroids; D003907:Dexamethasone; D003513:Cycloheximide; D003847:Deoxyglucose; D012964:Sodium; D001205:Ascorbic Acid; D003683:Dehydroascorbic Acid