4.5 Article

Receptor signaling and endocytosis are differentially regulated by somatostatin analogs

Journal

MOLECULAR PHARMACOLOGY
Volume 68, Issue 1, Pages 90-101

Publisher

AMER SOC PHARMACOLOGY EXPERIMENTAL THERAPEUTICS
DOI: 10.1124/mol.105.011767

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Funding

  1. NIDDK NIH HHS [DK032234, DK059953] Funding Source: Medline

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Upon hormone stimulation, the sst2 somatostatin receptor couples to adenylyl cyclase through G(i/o) proteins and undergoes rapid endocytosis via clathrin-coated pits. In this study, we determined the relationship between the ability of ligands to induce sst2 receptor internalization and inhibit adenylyl cyclase. Immunocytochemical studies demonstrated that peptide agonists [ such as somatostatin-14, cortistatin-17, octreotide, vapreotide, KE108 (Tyr0-cyclo[D-diaminobutyric acid-Arg-Phe-Phe- D-Trp-Lys-Thr-Phe]), and SOM230 ( cyclo[ diaminoethylcarbamoyl-hydroxyproline- phenylglycine-D-Trp-Lys-(4-O-benzyl)L- Tyr-Phe])] and nonpeptide agonists ( such as L- 779,976), stimulated the rapid endocytosis of sst2 receptors in human embryonic kidney 293 and CHO-K1 cells. In contrast, two antagonists did not induce receptor endocytosis by themselves and completely blocked agonist stimulation. Using a quantitative enzyme-linked immunosorbent assay to measure sst2 receptor sequestration, we found that peptide agonists varied by more than 100-fold in their potencies but exhibited the same efficacy as somatostatin14. In contrast, L-779,976 did not induce maximal receptor internalization. It is interesting that although beta arrestin-2 was recruited to cell surface sst2 receptors after stimulation with either somatostatin14 or L-779,976, the beta arrestin-receptor complex dissociated earlier in the endocytic pathway with the nonpeptide ligand. Although all agonists, including L-779,976, produced the same maximal inhibition of cyclic AMP, the potency ratio for inhibition of cyclic AMP and stimulation of receptor endocytosis varied by 15-fold. In general, native peptides showed similar potencies for cyclic AMP inhibition and receptor endocytosis, whereas short therapeutic analogs were substantially more potent at inhibiting cyclic AMP synthesis. These results demonstrate that the activity of somatostatin analogs to regulate receptor endocytosis and signaling are not tightly linked and provide compelling evidence for the induction of agonist specific states of the sst2 receptor.

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