Nagayama Y, Chazenbalk G D, Takeshita A, Kimura H, Ashizawa K, Yokoyama N, Rapoport B, Nagataki S
First Department of Internal Medicine, Nagasaki University School of Medicine, Japan.
Endocrinology. 1994 Sep;135(3):1060-5. doi: 10.1210/endo.135.3.8070347.
It is well known that the TSH receptor (TSHR) undergoes homologous desensitization. That is, prolonged stimulation of thyroid cells with TSH attenuates the cAMP response to subsequent TSH stimulation. However, the existence of homologous desensitization of the recombinant TSHR expressed in nonthyroidal eukaryotic cells is controversial. In the present studies, therefore, we first investigated whether or not the TSHR was desensitized by TSH in 293 human embryonal kidney cells, a cell line in which the LH/CG receptor (LH/CGR) is reported to undergo homologous desensitization. The wild type (wt) TSHR and the wt-LH/CGR stably expressed in 293 cells bound to their respective hormones with high affinity and produced a dose-dependent intracellular cAMP response to hormone stimulation. Pretreatment of cells expressing the TSHR or the LH/CGR with their respective hormones attenuated the cAMP response to subsequent hormone stimulation without down-regulation of the receptors, demonstrating that the TSHR, as well as the LH/CGR, undergoes homologous desensitization in 293 cells. With this cell type expressing mutant TSHRs, we then studied some aspects of the molecular mechanism of TSHR desensitization and compared our data to those obtained with the beta-adrenergic receptor (beta-AR), which is widely regarded as the prototype for receptor desensitization. We cotransfected the wt-TSHR and a chimeric receptor consisting of the LH/CGR extracellular ligand binding domain with the TSHR transmembrane/cytoplasmic signal transducing region. These two receptors have distinct hormone specificities but share common signal regulatory mechanisms. We observed that, like the beta-AR, only hormone-occupied receptor is likely to be involved in homologous desensitization. On the other hand, studies with a truncated TSHR indicated that, in contrast to the beta-AR, the serine/threonine-rich region in the carboxyl two thirds of the cytoplasmic tail of the TSHR is not involved in homologous desensitization.
众所周知，促甲状腺激素受体（TSHR）会发生同源脱敏。也就是说，用促甲状腺激素（TSH）长时间刺激甲状腺细胞会减弱对后续TSH刺激的环磷酸腺苷（cAMP）反应。然而，在非甲状腺真核细胞中表达的重组TSHR是否存在同源脱敏存在争议。因此，在本研究中，我们首先研究了在293人胚肾细胞（据报道促黄体生成素/绒毛膜促性腺激素受体（LH/CGR）在此细胞系中会发生同源脱敏）中TSHR是否会被TSH脱敏。稳定表达于293细胞中的野生型（wt）TSHR和wt-LH/CGR以高亲和力与其各自的激素结合，并对激素刺激产生剂量依赖性的细胞内cAMP反应。用各自的激素预处理表达TSHR或LH/CGR的细胞，会减弱对后续激素刺激的cAMP反应，而受体不会下调，这表明TSHR以及LH/CGR在293细胞中会发生同源脱敏。利用这种表达突变型TSHR的细胞类型，我们随后研究了TSHR脱敏分子机制的一些方面，并将我们的数据与用β-肾上腺素能受体（β-AR）获得的数据进行比较，β-AR被广泛认为是受体脱敏的原型。我们将wt-TSHR和一个由LH/CGR细胞外配体结合结构域与TSHR跨膜/细胞质信号转导区域组成的嵌合受体共转染。这两种受体具有不同的激素特异性，但共享共同的信号调节机制。我们观察到，与β-AR一样，可能只有被激素占据的受体参与同源脱敏。另一方面，对截短型TSHR的研究表明，与β-AR不同，TSHR细胞质尾羧基端三分之二富含丝氨酸/苏氨酸的区域不参与同源脱敏。
