title: Intracellular degradation of somatostatin-14 following somatostatin-receptor3-mediated endocytosis in rat insulinoma cells.
abstract: Somatostatin receptor (SSTR) endocytosis influences cellular responsiveness to agonist stimulation and somatostatin receptor scintigraphy, a common diagnostic imaging technique. Recently, we have shown that SSTR1 is differentially regulated in the endocytic and recycling pathway of pancreatic cells after agonist stimulation. Additionally, SSTR1 accumulates and releases internalized somatostatin-14 (SST-14) as an intact and biologically active ligand. We also demonstrated that SSTR2A was sequestered into early endosomes, whereas internalized SST-14 was degraded by endosomal peptidases and not routed into lysosomal degradation. Here, we examined the fate of peptide agonists in rat insulinoma cells expressing SSTR3 by biochemical methods and confocal laser scanning microscopy. We found that [(125)I]Tyr11-SST-14 rapidly accumulated in intracellular vesicles, where it was degraded in an ammonium chloride-sensitive manner. In contrast, [(125)I]Tyr1-octreotide accumulated and was released as an intact peptide. Rhodamine-B-labeled SST-14, however, was rapidly internalized into endosome-like vesicles, and fluorescence signals colocalized with the lysosomal marker protein cathepsinD. Our data show that SST-14 was cointernalized with SSTR3, was uncoupled from the receptor, and was sorted into an endocytic degradation pathway, whereas octreotide was recycled as an intact peptide. Chronic stimulation of SSTR3 also induced time-dependent downregulation of the receptor. Thus, the intracellular processing of internalized SST-14 and the regulation of SSTR3 markedly differ from the events mediated by the other SSTR subtypes.
authors.affiliation: Department of Dermatology, IZKF Münster and Ludwig Boltzmann Institute for Cell and Immunobiology of the Skin, Germany. roosterman@gmx.net
authors.forename: Dirk
authors.initials: D
authors.lastname: Roosterman
authors.forename: Nicole E I
authors.initials: NE
authors.lastname: Brune
authors.forename: Oliver J
authors.initials: OJ
authors.lastname: Kreuzer
authors.forename: Micha
authors.initials: M
authors.lastname: Feld
authors.forename: Sylvia
authors.initials: S
authors.lastname: Pauser
authors.forename: Kim
authors.initials: K
authors.lastname: Zarse
authors.forename: Martin
authors.initials: M
authors.lastname: Steinhoff
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Meyerhof
mesh_terms: D000818:Animals; D004705:Endocytosis / Q000502:physiology*; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D007340:Insulinoma / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D015282:Octreotide / Q000378:metabolism; D051381:Rats; D017481:Receptors, Somatostatin / Q000502:physiology*; D013004:Somatostatin / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D017481:Receptors, Somatostatin; C485184:enhanced green fluorescent protein; C103631:somatostatin receptor 3; D049452:Green Fluorescent Proteins; D013004:Somatostatin; D016650:Fluorescein-5-isothiocyanate; D015282:Octreotide