Roettger B F, Rentsch R U, Hadac E M, Hellen E H, Burghardt T P, Miller L J
Center for Basic Research in Digestive Diseases, Mayo Clinic and Foundation, Rochester, Minnesota 55905, USA.
J Cell Biol. 1995 Aug;130(3):579-90. doi: 10.1083/jcb.130.3.579.
Receptor desensitization is a key process for the protection of the cell from continuous or repeated exposure to high concentrations of an agonist. Well-established mechanisms for desensitization of guanine nucleotide-binding protein (G protein)-coupled receptors include phosphorylation, sequestration/internalization, and down-regulation. In this work, we have examined some mechanisms for desensitization of the cholecystokinin (CCK) receptor which is native to the pancreatic acinar cell, and have found the predominant mechanism to be distinct from these recognized processes. Upon fluorescent agonist occupancy of the native receptor, it becomes "insulated" from the effects of acid washing and becomes immobilized on the surface of the plasma membrane in a time- and temperature-dependent manner. This localization was assessed by ultrastructural studies using a colloidal gold conjugate of CCK, and lateral mobility of the receptor was assessed using fluorescence recovery after photobleaching. Of note, recent application of the same morphologic techniques to a CCK receptor-bearing Chinese hamster ovary cell line demonstrated prominent internalization via the clathrin-dependent endocytic pathway, as well as entry into caveolae (Roettger, B.F., R.U. Rentsch, D. Pinon, E. Holicky, E. Hadac, J.M. Larkin, and L.J. Miller, 1995, J. Cell Biol. 128: 1029-1041). These organelles are not observed to represent prominent compartments for the same receptor to traverse in the acinar cell, although fluorescent insulin is clearly internalized in these cells via receptor-mediated endocytosis. In this work, the rate of lateral mobility of the CCK receptor is observed to be similar in both cell types (1-3 x 10(-10) cm2/s), while the fate of the agonist-occupied receptor is quite distinct in each cell. This supports the unique nature of desensitization processes which occur in a cell-specific manner. A plasmalemmal site of insulation of this important receptor on the pancreatic acinar cell could be particularly effective to protect the cell from processes which might initiate pancreatitis, while providing for the rapid resensitization of this receptor to ensure appropriate pancreatic secretion to aid in nutrient assimilation for the organism.
受体脱敏是保护细胞免受持续或反复暴露于高浓度激动剂影响的关键过程。鸟嘌呤核苷酸结合蛋白（G蛋白）偶联受体脱敏的既定机制包括磷酸化、隔离/内化和下调。在这项研究中，我们研究了胰腺腺泡细胞中天然存在的胆囊收缩素（CCK）受体的一些脱敏机制，发现主要机制与这些公认的过程不同。当荧光激动剂占据天然受体后，它会与酸洗的影响“隔离”，并以时间和温度依赖的方式固定在质膜表面。通过使用CCK胶体金偶联物的超微结构研究评估这种定位，并且使用光漂白后的荧光恢复评估受体的横向迁移率。值得注意的是，最近将相同的形态学技术应用于携带CCK受体的中国仓鼠卵巢细胞系，结果显示通过网格蛋白依赖性内吞途径有显著的内化，以及进入小窝（Roettger，B.F.，R.U. Rentsch，D. Pinon，E. Holicky，E. Hadac，J.M. Larkin和L.J. Miller，1995，《细胞生物学杂志》128：1029 - 1041）。尽管荧光胰岛素在这些细胞中通过受体介导的内吞作用明显内化，但在腺泡细胞中未观察到这些细胞器是同一受体穿越的主要区室。在这项研究中，观察到CCK受体在两种细胞类型中的横向迁移率相似（1 - 3×10⁻¹⁰ cm²/s），而激动剂占据的受体在每种细胞中的命运却截然不同。这支持了以细胞特异性方式发生的脱敏过程的独特性质。胰腺腺泡细胞上这种重要受体的质膜隔离位点可能特别有效地保护细胞免受可能引发胰腺炎的过程的影响，同时使该受体快速重新敏感化，以确保胰腺进行适当的分泌，从而有助于机体对营养物质的同化。
