Sakai K, Nakahata N, Ono H, Yamamoto T, Ohizumi Y
Department of Pharmaceutical Molecular Biology, Faculty of Pharmaceutical Sciences, Tohoku University, Sendai, Japan.
J Pharmacol Exp Ther. 1996 Feb;276(2):829-36.
The desensitization mechanisms that regulate the response to thromboxane A2 (TXA2) were investigated in 1321N1 human astrocytoma cells. Exposure of the cells to 9,11-epithio-11, 12-methanothromboxane A2 (STA2), a stable TXA2 receptor agonist, at a concentration of 1 microM for 30 to 45 min resulted in about a 50% decrease in subsequent STA2-stimulated phosphoinositide hydrolysis and Ca2+ mobilization. However, exposure to STA2 for 0 to 5 hr did not change the binding of [3H]SQ29548, a TXA2 receptor antagonist. Because STA2-induced GTPase activation decreased and the GTP sensitivity in inhibition of [3H]SQ29548 binding by STA2 disappeared after the cells had been exposed to STA2 for 30 min, the TXA2 receptor desensitization during the short-term might result from G-protein-receptor uncoupling. STA2-induced desensitization was specific for the TXA2 receptor and homologous, because SQ29548 suppressed the desensitization and STA2 pretreatment did not affect the response to carbachol, a muscarinic cholinergic receptor agonist. Exposure to STA2 for 24 hr decreased [3H]SQ29548 binding sites to 20 to 30% of control and abolished STA2-stimulated phosphoinositide hydrolysis, indicating that long-term desensitization might induce down-regulation of the TXA2 receptor. However, exposure to STA2 for 1 to 24 hr did not change the level of TXA2 receptor mRNA. These results show that homologous desensitization of the TXA2 receptor in human astrocytoma cells can be divided into two stages; the early stage involves uncoupling of receptors from G-proteins and the late stage involves a loss of receptors in cells. The mRNA levels may not be controlled by stimulation of the TXA2 receptor.
在1321N1人星形细胞瘤细胞中研究了调节对血栓素A2（TXA2）反应的脱敏机制。将细胞暴露于浓度为1微摩尔的9,11-环氧-11,12-甲撑血栓素A2（STA2），一种稳定的TXA2受体激动剂，30至45分钟，导致随后STA2刺激的磷酸肌醇水解和Ca2+动员减少约50%。然而，暴露于STA2 0至5小时并未改变TXA2受体拮抗剂[3H]SQ29548的结合。因为在细胞暴露于STA2 30分钟后，STA2诱导的GTP酶激活降低，且STA2抑制[3H]SQ29548结合的GTP敏感性消失，所以短期的TXA2受体脱敏可能是由于G蛋白-受体解偶联。STA2诱导的脱敏对TXA2受体具有特异性且是同源的，因为SQ29548抑制了脱敏，且STA2预处理不影响对毒蕈碱胆碱能受体激动剂卡巴胆碱的反应。暴露于STA2 24小时使[3H]SQ29548结合位点降至对照的20%至30%，并消除了STA2刺激的磷酸肌醇水解，表明长期脱敏可能诱导TXA2受体下调。然而，暴露于STA2 1至24小时并未改变TXA2受体mRNA水平。这些结果表明，人星形细胞瘤细胞中TXA2受体的同源脱敏可分为两个阶段；早期阶段涉及受体与G蛋白解偶联，晚期阶段涉及细胞中受体的丢失。mRNA水平可能不受TXA2受体刺激的控制。
