转铁蛋白在低钾刺激Madin Darby狗肾细胞钠-钾ATP酶中的作用

体外低钾培养肾细胞能刺激细胞膜钠-钾ATP酶。本研究利用Madin Darby狗肾细胞能在无血清培养液中健康生存48h这一特征,研究体外低钾刺激细胞膜钠-钾ATP酶所依赖的血清中的活性因子,观察了表皮生长因子(EGF)、胰岛素样生长因子(IGF1)、前列腺素1(PGE1)和转铁蛋白(tranderrin)在这一过程中的作用。结果表明,在无血清培养液中低钾并不能刺激细胞膜钠—钾ATP酶,而添加转铁蛋白可模拟血清的作用。转铁蛋白能剂量依赖性地增加ouabain结合位点,对细胞膜钠-钾ATP酶作用呈良好的时间效应关系。在低钾无血清培养液中,细胞膜钠-钾ATP酶α1亚基启动子活性增强,α1与β1亚基蛋白质表达的增加依赖于转铁蛋白的存在。进一步研究结果表明,低钾在转铁蛋白的无血清培养液环境中能增加细胞对铁的摄取(^59Fe),该作用可被铁螯合剂(deferoxamine,DFO;35 μmol/L)所阻断。DFO也可阻断转铁蛋白依赖性低钾刺激细胞膜钠-钾ATP酶数目的增多,α1亚基启动子活性增强,α1与β1亚基蛋白质表达增加。以上结果表明,低钾对细胞膜钠-钾ATP酶活性的刺激作用依赖于转铁蛋白所调节的铁的摄取。

Role of transferrin in the stimulation of Na,K-ATPase induced by low K+ in Madin Darby canine kidney cells

The presence of serum in a culture medium makes it impossible to identify whether changed cellular functions are directly caused by a manipulation itself or mediated by a component in serum. Madin Darby canine kidney cells can survive in a serum-free medium for about 48 h. We took this advantage to examine whether low K+-induced up-regulation of Na,K-ATPase requires serum. We found that serum was essential for low K+ to induce an increase in Na,K-ATPase binding sites as quantified by ouabain factor binding assays. In an attempt to identify which component was critical, we screened EGF, IGF1, PGE1 and transferrin to identify which one can replace serum. We discovered that transferrin was the single most important factor that mimicked about 80% to 90% of the effect of serum. Transferrin potentiated the effect of low K+ on the Na,K-ATPase binding sites in a time- and dose-dependent manner. Furthermore, transferrin was also required for low K+-induced increase in α1-promoter activity, α1- and β1-subunit protein abundance of the Na,K-ATPase. In the presence of transferrin, low K+ enhanced cellular uptake of iron approximately by 70%. Inhibition of intracellular iron activity by deferoxamine (30 μmol/L) abrogated the effect of low K+. We conclude that stimulation of the Na,K-ATPase by low K+ is critically dependent on transferrin. The effect of transferrin is mediated by increased iron transport.