Kao Yachu J, Ghosh Madhumita, Schonbrunn Agnes
Department of Integrative Biology and Pharmacology, University of Texas, Health Science Center-Houston, Houston, Texas 77030, USA.
Mol Endocrinol. 2011 Jun;25(6):1040-54. doi: 10.1210/me.2010-0398. Epub 2011 Apr 14.
The somatostatin receptor subtype 2A (sst2A) mediates many of somatostatin's neuroendocrine actions and is the primary therapeutic target for the stable somatostatin analogs used to inhibit hormone secretion by pituitary and gastroenteropancreatic tumors. Two new multireceptor targeting somatostatin analogs currently under clinical investigation, the multisomatostatin receptor agonist cyclo-[diaminoethylcarbamoyl-HydroxyPro-Phenylglycine-D-Trp-Lys-(4-O-benzyl)Tyr-Phe] (SOM230) (Pasireotide) and pan-somatostatin receptor agonist Tyr-cyclo-[D-diaminobutyric acid-Arg-Phe-Phe-D-Trp-Lys-Thr-Phe] (KE108), behave as functionally selective ligands at the sst2A receptor, mimicking some of somatostatin's actions but antagonizing others. Further, SOM230 and KE108 are less able to induce receptor internalization than somatostatin, indicating that they exhibit functional selectivity for receptor regulation as well as signaling. Here, we identify agonist-specific differences in the molecular events regulating sst2A receptor endocytosis. SOM230 and KE108 were less potent and less effective than somatostatin at stimulating sst2A receptor phosphorylation at two pairs of residues, Ser341/343 and Thr353/354. Only the pattern of Thr353/354 phosphorylation correlated with receptor internalization, consistent with the known importance of Thr phosphorylation for sst2A receptor endocytosis. As expected, arrestin recruitment to membrane receptors was reduced with SOM230 and KE108. In addition, both receptor dephosphorylation and receptor recycling occurred more rapidly with SOM230 and KE108 than with somatostatin. Surprisingly, however, SOM230 and KE108 also altered sst2A internalization in a phosphorylation-independent manner, because these analogs were less effective than somatostatin at stimulating the endocytosis of a phosphorylation-negative receptor mutant. These results show that the decreased receptor internalization produced by SOM230 and KE108 compared with somatostatin result from phosphorylation-independent effects as well as reduced site-specific receptor phosphorylation and receptor-arrestin association.
生长抑素受体亚型2A（sst2A）介导生长抑素的许多神经内分泌作用，并且是用于抑制垂体和胃肠胰肿瘤激素分泌的稳定生长抑素类似物的主要治疗靶点。目前正在临床研究的两种新型多受体靶向生长抑素类似物，多生长抑素受体激动剂环-[二氨基乙基氨基甲酰基-羟脯氨酸-苯甘氨酸-D-色氨酸-赖氨酸-（4-O-苄基）酪氨酸-苯丙氨酸]（SOM230）（帕西瑞肽）和泛生长抑素受体激动剂酪氨酸-环-[D-二氨基丁酸-精氨酸-苯丙氨酸-苯丙氨酸-D-色氨酸-赖氨酸-苏氨酸-苯丙氨酸]（KE108），在sst2A受体上表现为功能选择性配体，模拟生长抑素的一些作用但拮抗其他作用。此外，与生长抑素相比，SOM230和KE108诱导受体内化的能力较弱，这表明它们在受体调节以及信号传导方面表现出功能选择性。在此，我们确定了调节sst2A受体内吞作用的分子事件中激动剂特异性差异。在刺激两对残基Ser341/343和Thr353/354处的sst2A受体磷酸化方面，SOM230和KE108的效力和效果均低于生长抑素。只有Thr353/354磷酸化模式与受体内化相关，这与Thr磷酸化对sst2A受体内吞作用的已知重要性一致。正如预期的那样，SOM230和KE108使抑制蛋白募集到膜受体的能力降低。此外，与生长抑素相比，SOM230和KE108使受体去磷酸化和受体再循环的速度更快。然而，令人惊讶的是，SOM230和KE108也以磷酸化非依赖性方式改变sst2A内化，因为这些类似物在刺激磷酸化阴性受体突变体的内吞作用方面比生长抑素效果更差。这些结果表明，与生长抑素相比，SOM230和KE108导致的受体内化减少是由磷酸化非依赖性效应以及位点特异性受体磷酸化和受体-抑制蛋白结合减少所致。
