S. Pandipati et al., GLUCOCORTICOID STIMULATION OF NA-DEPENDENT ASCORBIC-ACID TRANSPORT INOSTEOBLAST-LIKE CELLS(), Journal of cellular physiology, 176(1), 1998, pp. 85-91
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.