Pandipati S, Driscoll J E, Franceschi R T
Department of Periodontics, Prevention, and Geriatrics, School of Dentistry, University of Michigan, Ann Arbor 48109-1078, USA.
J Cell Physiol. 1998 Jul;176(1):85-91. doi: 10.1002/(SICI)1097-4652(199807)176:1<85::AID-JCP10>3.0.CO;2-N.
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.
抗坏血酸（AA）在体内和体外都是成骨细胞分化所必需的辅助因子。在发挥功能之前，这种维生素必须通过特定的钠依赖性AA转运体转运进入细胞。在本研究中，我们研究了糖皮质激素对这种转运活性的调节作用，糖皮质激素是一类已知能刺激体外成骨细胞分化的类固醇激素。地塞米松使原代大鼠颅骨成骨细胞中钠依赖性AA转运活性增加了约两倍。激素对抗坏血酸转运的作用迅速（在24小时内即可检测到），25 - 50 nM地塞米松可使其受到最大程度的刺激。在小鼠MC3T3 - E1细胞中也观察到了地塞米松对转运活性的类似作用。这种前成骨细胞系被用于更详细地表征糖皮质激素反应。相对于其他类固醇激素（孕酮和17 - β - 雌二醇），糖皮质激素（地塞米松>皮质酮）选择性地刺激转运活性，并且当细胞在蛋白质合成抑制剂环己酰亚胺存在下培养时，转运活性被阻断。对对照细胞和地塞米松处理细胞中AA转运体活性的动力学分析表明，两组的Km约为17μM。相比之下，地塞米松使Vmax增加了约2.5倍。细胞还具有一种不依赖钠的葡萄糖转运活性，在其他系统中已有报道该活性可转运氧化型脱氢抗坏血酸形式的维生素C。与钠依赖性AA转运形成鲜明对比的是，这种活性受到地塞米松的抑制。因此，糖皮质激素可能通过上调转运体合成来增加成骨细胞中钠依赖性AA转运，并且这种反应可以与糖皮质激素对葡萄糖转运的作用区分开来。
