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-β-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 K m of approximately 17 μM for both groups. In contrast, dexamethasone increased V max 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. J. Cell. Physiol. 176:85–91, 1998. © 1998 Wiley-Liss, Inc.