Stanley F, Tsai J S, Samuels H H
J Biol Chem. 1986 Jul 15;261(20):9400-4.
We have previously shown that 3,5,3'-triiodo-L-thyronine (L-T3) stimulates cell growth and a 4- to 8-fold increase in growth hormone mRNA in GH1 cells. These effects appear to be mediated by a thyroid hormone nuclear receptor with an equilibrium dissociation constant for L-T3 of 0.2 nM and an abundance of about 10,000 receptors per cell nucleus. In this report, we show that L-T3 exerts a pleiotypic effect on GH1 cells to rapidly (within 2 h) stimulate [3H]uridine uptake to a maximal value of 2.5- to 3-fold after 24 h. This results from an increase in the number of functional uridine "transport sites" as shown by studies documenting an increase in the apparent Vmax with no change in the Km, 17 microM. Although the labeling of the cellular uridine pool and pools of all phosphorylated uridine derivatives was increased by L-T3, there was no change in the relative amounts of the individual pools in cells incubated with or without hormone. The intracellular concentration of [3H]uridine was estimated to be similar to that of the medium, suggesting that facilitated transport mediates [3H]uridine uptake. That this increase in [3H]uridine transport was nuclear receptor-mediated is supported by the excellent correspondence of the L-T3 dose-response curve for [3H]uridine uptake and that for L-T3 binding to receptor. Finally, inhibition of protein synthesis by cycloheximide and RNA synthesis by actinomycin D demonstrated that the L-T3 effect required continuing protein and RNA synthesis. These results are consistent with an effect of the L-T3-nuclear receptor complex to increase uridine uptake in GH1 cells by altering the expression of gene(s) essential for the transport process.
我们之前已经表明，3,5,3'-三碘-L-甲状腺原氨酸（L-T3）可刺激GH1细胞的生长，并使生长激素mRNA增加4至8倍。这些作用似乎是由一种甲状腺激素核受体介导的，该受体对L-T3的平衡解离常数为0.2 nM，每个细胞核中约有10,000个受体。在本报告中，我们表明L-T3对GH1细胞具有多效性作用，可迅速（在2小时内）刺激[3H]尿苷摄取，24小时后达到最大值，为对照值的2.5至3倍。这是由于功能性尿苷“转运位点”数量增加所致，研究表明表观Vmax增加而Km（17 microM）不变。尽管L-T3增加了细胞尿苷池以及所有磷酸化尿苷衍生物池的标记，但在有或无激素孵育的细胞中，各个池的相对量没有变化。[3H]尿苷的细胞内浓度估计与培养基中的浓度相似，表明易化转运介导了[3H]尿苷的摄取。[3H]尿苷摄取的L-T3剂量反应曲线与L-T3与受体结合的剂量反应曲线高度吻合，支持了这种[3H]尿苷转运增加是由核受体介导的观点。最后，放线菌酮抑制蛋白质合成和放线菌素D抑制RNA合成表明，L-T3的作用需要持续的蛋白质和RNA合成。这些结果与L-T3-核受体复合物通过改变转运过程所需基因的表达来增加GH1细胞中尿苷摄取的作用一致。
