title: Phosphorylation of sst2 receptors in neuroendocrine tumors after octreotide treatment of patients.
abstract: Somatostatin analogues, which are used to treat neuroendocrine tumors, target the high levels of somatostatin receptor subtype 2 (SSTR1; alias sst2) expressed in these cancers. However, some tumors are resistant to somatostatin analogues, and it is unknown whether the defect lies in sst2 activation or downstream signaling events. Because sst2 phosphorylation occurs rapidly after receptor activation, we examined whether sst2 is phosphorylated in neuroendocrine tumors. The sst2 receptor phosphorylation was evaluated by IHC and Western blot analysis with the new Ra-1124 antibody specific for the sst2 receptor phosphorylated at Ser341/343 in receptor-positive neuroendocrine tumors obtained from 10 octreotide-treated and 7 octreotide-naïve patients. The specificity, time course, and subcellular localization of sst2 receptor phosphorylation were examined in human embryo kinase-sst2 cell cultures by immunofluorescence and confocal microscopy. All seven octreotide-naïve tumors displayed exclusively nonphosphorylated cell surface sst2 expression. In contrast, 9 of the 10 octreotide-treated tumors contained phosphorylated sst2 that was predominantly internalized. Western blot analysis confirmed the IHC data. Octreotide treatment of human embryo kinase-sst2 cells in culture demonstrated that phosphorylated sst2 was localized at the plasma membrane after 10 seconds of stimulation and was subsequently internalized into endocytic vesicles. These data show, for the first time to our knowledge, that phosphorylated sst2 is present in most gastrointestinal neuroendocrine tumors from patients treated with octreotide but that a striking variability exists in the subcellular distribution of phosphorylated receptors among such tumors.
authors.affiliation: Division of Cell Biology and Experimental Cancer Research, Institute of Pathology, University of Berne, Berne, Switzerland.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Waser
authors.forename: Renzo
authors.initials: R
authors.lastname: Cescato
authors.forename: Qisheng
authors.initials: Q
authors.lastname: Liu
authors.forename: Yachu J
authors.initials: YJ
authors.lastname: Kao
authors.forename: Meike
authors.initials: M
authors.lastname: Körner
authors.forename: Emanuel
authors.initials: E
authors.lastname: Christ
authors.forename: Agnes
authors.initials: A
authors.lastname: Schonbrunn
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology* / Q000627:therapeutic use; D018278:Carcinoma, Neuroendocrine / Q000188:drug therapy / Q000378:metabolism* / Q000601:surgery; D017024:Chemotherapy, Adjuvant; D006801:Humans; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D009363:Neoplasm Proteins / Q000187:drug effects / Q000378:metabolism; D015282:Octreotide / Q000494:pharmacology* / Q000627:therapeutic use; D010766:Phosphorylation / Q000187:drug effects; D017481:Receptors, Somatostatin / Q000187:drug effects / Q000378:metabolism*
chemical_list: D018931:Antineoplastic Agents, Hormonal; D009363:Neoplasm Proteins; D017481:Receptors, Somatostatin; C093733:somatostatin receptor 2; D015282:Octreotide