GLUCOCORTICOID STIMULATION OF NA-DEPENDENT ASCORBIC-ACID TRANSPORT INOSTEOBLAST-LIKE CELLS()

Ascorbic acid (AA) is an essential cofactor for osteoblast differentia tion both in vivo and in vitro. Before it can function, this vitamin m ust be transported into cells via a specific Na+-dependent AA transpor ter. In this study, we examine the regulation of this transport activi ty by glucocorticoids, a class of steroid hormones known to stimulate in vitro osteoblast differentiation. Dexamethasone stimulated Na+-depe ndent AA transport activity approximately twofold in primary rat calva rial osteoblasts. Effects of hormone on ascorbic acid transport were r apid (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 l ine was used for a more detailed characterization of the glucocorticoi d response. Transport activity was stimulated selectively by glucocort icoids (dexamethasone > corticosterone) relative to other steroid horm ones (progesterone and 17-beta-estradiol) and was blocked when cells w ere cultured in the presence of cycloheximide, a protein synthesis inh ibitor. Kinetic analysis of AA transporter activity in control and dex amethasone-treated cells indicated a K-m of approximately 17 mu M for both groups. In contrast, dexamethasone increased V-max by approximate ly 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+-depende nt AA transport, this activity was inhibited by dexamethasone. Thus, g lucocorticoids increase Na+-dependent AA transport in osteoblasts, pos sibly via up-regulation of transporter synthesis, and this response ca n be resolved from actions of glucocorticoids on glucose transport. (C ) 1998 Wiley-Liss, Inc.