A growing body of evidence suggests that neuropeptide binding to G protein-linked receptors may result in internalization of receptor-ligand complexes, followed by intracellular mobilization and degradation of the ligand into its target cells. Because of discrepant results in the literature concerning the occurrence of such a mechanism for the tetradecapeptide somatostatin (SRIF), we have reinvestigated this question by comparing the binding and internalization of iodinated and fluorescent derivatives of the metabolically stable analog of SRIF, [D-Trp8]SRIF, in COS-7 cells transfected with complementary DNA encoding the sst1 or sst2A receptor subtype. A series of fluoresceinyl and Bodipy fluorescent derivatives of [D-Trp8]SRIF-14 was purified by HPLC, analyzed for purity by mass spectrometry, and tested for biological activity in a membrane binding assay. Of the six compounds tested, fluoresceinyl and Bodipy derivatives labeled in position alpha (fluo-SRIF) retained high affinity for SRIF receptors. COS-7 cells transfected with complementary DNA encoding either sst1 or sst2A receptors both displayed specific, high affinity binding of iodinated and fluo-SRIF. At 4 C, the labeling was confined to the cell surface in both cell types, as indicated by the fact that it was entirely removable by a hypertonic acid wash and assumed a pericellular distribution in the confocal microscope. At 37 C, the fate of specifically bound ligand varied markedly according to the type of receptor transfected. In cells encoding the sst1 receptor, approximately 20% of specifically bound ligand was recovered in the acid-resistant (i.e. intracellular) fraction. This fraction remained clustered at the periphery of the cell, suggesting that it was being sequestered either within or immediately beneath the plasma membrane. By contrast, in cells transfected with sst2A receptors, up to 75% of specifically bound ligand was recovered inside the cells, where it clustered into small endosome-like particles. These particles increased in size and moved toward the nucleus with time, suggestive of receptor-ligand complexes proceeding down the endocytic pathway. These results demonstrate that neuropeptides may be processed differently depending on the subtype of receptor expressed in their target cells and suggest that these different processing patterns may reflect different modes of sensitization/desensitization and recycling of the receptors, and thereby of transmembrane signaling.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.