Prasad P D, Leibach F H, Mahesh V B, Ganapathy V
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912.
Endocrinology. 1994 Feb;134(2):574-81. doi: 10.1210/endo.134.2.8299556.
The relationship between the transport of the thyroid hormone T3 and the transport of neutral amino acids was investigated in JAR human placental choriocarcinoma cells. The uptake of leucine, mediated by the amino acid transport system L, was inhibited by T3 and T4, and the nature of inhibition was competitive. Uptake of T3 into the cells was predominantly Na+ independent, and so was that of leucine. However, although an acidic extracellular pH stimulated leucine uptake, the uptake of T3 remained unaffected. In addition, leucine failed to inhibit T3 uptake. The aromatic amino acids phenylalanine and tryptophan were found to inhibit the uptake of T3, but these two amino acids were transported into the cells predominantly via system L. The amino acid transport system T, which is specific for aromatic amino acids, was not detectable in these cells. Treatment of the cells with the calmodulin antagonist CGS 9343 B stimulated the uptake of leucine and tryptophan, but inhibited the uptake of T3. Kinetic analysis of T3 uptake revealed the presence of a single saturable system for this hormone in these cells, and the Michaelis-Menten constant for this system was 0.77 +/- 0.06 microM. Metabolic poisons that interfere with the cellular generation of ATP had no effect on the uptake of T3. It is concluded that in placental choriocarcinoma cells, 1) T3 and T4 are high affinity competitive inhibitors of the amino acid transport system L, 2) uptake of T3 occurs via a specific Na(+)-independent, energy-independent, and saturable mechanism that is unrelated to the amino acid transport systems L and T, 3) the aromatic amino acids phenylalanine and tryptophan interact, although weakly, with the T3 uptake system, and 4) calmodulin-dependent processes participate in the regulation of the T3 uptake system.
在JAR人胎盘绒毛膜癌细胞中研究了甲状腺激素T3的转运与中性氨基酸转运之间的关系。由氨基酸转运系统L介导的亮氨酸摄取受到T3和T4的抑制，抑制性质为竞争性。T3进入细胞的摄取主要不依赖于Na +，亮氨酸的摄取也是如此。然而，尽管酸性细胞外pH刺激亮氨酸摄取，但T3的摄取仍不受影响。此外，亮氨酸未能抑制T3摄取。发现芳香族氨基酸苯丙氨酸和色氨酸抑制T3摄取，但这两种氨基酸主要通过系统L转运到细胞中。在这些细胞中未检测到对芳香族氨基酸特异的氨基酸转运系统T。用钙调蛋白拮抗剂CGS 9343 B处理细胞刺激亮氨酸和色氨酸摄取，但抑制T3摄取。T3摄取的动力学分析表明这些细胞中存在针对该激素的单一可饱和系统，该系统的米氏常数为0.77±0.06 microM。干扰细胞ATP生成的代谢毒物对T3摄取没有影响。结论是，在胎盘绒毛膜癌细胞中，1）T3和T4是氨基酸转运系统L的高亲和力竞争性抑制剂，2）T3的摄取通过特定的不依赖Na +、不依赖能量且可饱和的机制发生，该机制与氨基酸转运系统L和T无关，3）芳香族氨基酸苯丙氨酸和色氨酸与T3摄取系统相互作用，尽管较弱，4）钙调蛋白依赖性过程参与T3摄取系统的调节。
