Differential β-Arrestin Trafficking and Endosomal Sorting of Somatostatin Receptor Subtypes

The physiological responses of somatostatin are mediated by five different G protein-coupled receptors. Although agonist-induced endocytosis of the various somatostatin receptor subtypes (sst1-sst5) has been studied in detail, little is known about their postendocytic trafficking. Here we show that somatostatin receptors profoundly differ in patterns of β-arrestin mobilization and endosomal sorting. The β-arrestin-dependent trafficking of the sst2A somatostatin receptor resembled that of a class B receptor in that upon receptor activation, β-arrestin and the receptor formed stable complexes and internalized together into the same endocytic vesicles. This pattern was dependent on GRK2 (G protein-coupled receptor kinase 2)-mediated phosphorylation of a cluster of phosphate acceptor sites within the cytoplasmic tail of the sst2A receptor. Unlike other class B receptors, however, the sst2A receptor was rapidly resensitized and recycled to the plasma membrane. The β-arrestin mobilization of the sst3 and the sst5 somatostatin receptors resembled that of a class A receptor in that upon receptor activation, β-arrestin and the receptor formed relatively unstable complexes that dissociated at or near the plasma membrane. Consequently, β-arrestin was excluded from sst3-containing vesicles. Unlike other class A receptors, a large proportion of sst3 receptors was subject to ubiquitin-dependent lysosomal degradation and did not rapidly recycle to the plasma membrane. The sst4 somatostatin receptor is unique in that it did not exhibit agonist-dependent receptor phosphorylation and β-arrestin recruitment. Together, these findings may provide important clues about the regulation of receptor responsiveness during long-term administration of somatostatin analogs. The physiological responses of somatostatin are mediated by five different G protein-coupled receptors. Although agonist-induced endocytosis of the various somatostatin receptor subtypes (sst1-sst5) has been studied in detail, little is known about their postendocytic trafficking. Here we show that somatostatin receptors profoundly differ in patterns of β-arrestin mobilization and endosomal sorting. The β-arrestin-dependent trafficking of the sst2A somatostatin receptor resembled that of a class B receptor in that upon receptor activation, β-arrestin and the receptor formed stable complexes and internalized together into the same endocytic vesicles. This pattern was dependent on GRK2 (G protein-coupled receptor kinase 2)-mediated phosphorylation of a cluster of phosphate acceptor sites within the cytoplasmic tail of the sst2A receptor. Unlike other class B receptors, however, the sst2A receptor was rapidly resensitized and recycled to the plasma membrane. The β-arrestin mobilization of the sst3 and the sst5 somatostatin receptors resembled that of a class A receptor in that upon receptor activation, β-arrestin and the receptor formed relatively unstable complexes that dissociated at or near the plasma membrane. Consequently, β-arrestin was excluded from sst3-containing vesicles. Unlike other class A receptors, a large proportion of sst3 receptors was subject to ubiquitin-dependent lysosomal degradation and did not rapidly recycle to the plasma membrane. The sst4 somatostatin receptor is unique in that it did not exhibit agonist-dependent receptor phosphorylation and β-arrestin recruitment. Together, these findings may provide important clues about the regulation of receptor responsiveness during long-term administration of somatostatin analogs. Somatostatin (SS-14) 1The abbreviations used are: SS-14, somatostatin; EGFP, enhanced green fluorescent protein; GPCR, G protein-coupled receptor; GRK, G protein-coupled receptor kinase; HEK, human embryonic kidney; sst, somatostatin receptor; TPBS, Tris/phosphate-buffered saline. is an important regulator of neurotransmission in the brain as well as of hormone secretion from the anterior pituitary gland, the pancreas, and the gastrointestinal tract. Five genes encoding six different somatostatin receptor subtypes (sst1, sst2A, sst2B, sst3, sst4, sst5) have been cloned. These receptors are widely expressed in the central nervous system and periphery, and multiple somatostatin receptor subtypes often coexist in the same cell (1Patel Y.C. Front. Neuroendocrinol. 1999; 20: 157-198Crossref PubMed Scopus (1411) Google Scholar, 2Csaba Z. Dournaud P. Neuropeptides. 2001; 35: 1-23Crossref PubMed Scopus (195) Google Scholar). The high density of somatostatin receptors on human neuroendocrine tumors (3Kulaksiz H. Eissele R. Rossler D. Schulz S. Hollt V. Cetin Y. Arnold R. Gut. 2002; 50: 52-60Crossref PubMed Scopus (191) Google Scholar, 4Schulz S. Pauli S.U. Schulz S. Handel M. Dietzmann K. Firsching R. Hollt V. Clin. Cancer Res. 2000; 6: 1865-1874PubMed Google Scholar, 5Shimon I. Yan X. Taylor J.E. Weiss M.H. Culler M.D. Melmed S. J. Clin. Investig. 1997; 100: 2386-2392Crossref PubMed Scopus (235) Google Scholar) has been used clinically to treat the symptoms of hormonal hypersecretion in patients with growth hormone- and thyrotropin-secreting pituitary adenomas and patients harboring islet cell or carcinoid tumors (6Lamberts S.W. van der Lely A.J. de Herder W.W. Hofland L.J. N. Engl. J. Med. 1996; 334: 246-254Crossref PubMed Scopus (891) Google Scholar, 7Giustina A. Zaltieri G. Negrini F. Wehrenberg W.B. Pharmacol. Res. 1996; 34: 247-268Crossref PubMed Scopus (18) Google Scholar, 8De Herder W. Lamberts S. Endocrine. 2003; 20: 285-290Crossref PubMed Scopus (26) Google Scholar). Moreover, it has allowed the development of somatostatin receptor scintigraphy for tumor imaging as well as somatostatin receptor-targeted radiotherapy (9Lamberts S.W. Krenning E.P. Reubi J.C. Endocr. Rev. 1991; 12: 450-482Crossref PubMed Scopus (847) Google Scholar, 10Forster G.J. Engelbach M.J. Brockmann J.J. Reber H.J. Buchholz H.G. Macke H.R. Rosch F.R. Herzog H.R. Bartenstein P.R. Eur. J. Nucl. Med. 2001; 28: 1743-1750Crossref PubMed Scopus (141) Google Scholar). The fact that naturally occurring somatostatin peptides have only short half-lives has necessitated the development of stable somatostatin peptide analogs including octreotide and lanreotide. Although SS-14 binds with high affinity to all five somatostatin receptors, octreotide and lanreotide bind only to sst2 with high affinity and to sst3 and sst5 with moderate affinity (1Patel Y.C. Front. Neuroendocrinol. 1999; 20: 157-198Crossref PubMed Scopus (1411) Google Scholar, 11Culler M.D. Taylor J.E. Moreau J.P. Ann. Endocrinol. (Paris). 2002; 63: 2S5-212PubMed Google Scholar). It is well known that the physiological responses to SS-14 are diminished with continued exposure (1Patel Y.C. Front. Neuroendocrinol. 1999; 20: 157-198Crossref PubMed Scopus (1411) Google Scholar). However, important differences have been observed in the response of neuroendocrine tumors to the long-term application of stable somatostatin analogs. In patients harboring somatostatin receptor-expressing growth hormone-secreting adenoma, the inhibitory effects of somatostatin analogs on hormone secretion persist for many years during long-term treatment, and only few thyrotropin-secreting tumors escape from octreotide therapy. In contrast, islet cell tumors and carcinoids are very likely to undergo desensitization within weeks to months of octreotide exposure (12Hofland L.J. Lamberts S.W. Endocr. Rev. 2003; 24: 28-47Crossref PubMed Scopus (402) Google Scholar). Although studies in transfected host cells have examined agonist-induced internalization of the various somatostatin receptor subtypes (2Csaba Z. Dournaud P. Neuropeptides. 2001; 35: 1-23Crossref PubMed Scopus (195) Google Scholar), the molecular basis for the distinct long-term responsiveness of individual target cells has not been established. So far only species-related differences in agonist-mediated endocytosis of somatostatin receptors have been observed, e.g. the rat sst4 receptor appears to be largely resistant to agonist-induced internalization and desensitization (13Kreienkamp H.J. Roth A. Richter D. DNA Cell Biol. 1998; 17: 869-878Crossref PubMed Scopus (29) Google Scholar), whereas the human sst4 receptor has been reported to undergo a very slow but clearly detectable endocytosis. Other studies have shown that the human sst1 receptor but not the rat sst1 receptor failed to internalize in an agonist-dependent manner (14Hukovic N. Rocheville M. Kumar U. Sasi R. Khare S. Patel Y.C. J. Biol. Chem. 1999; 274: 24550-24558Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar, 15Roth A. Kreienkamp H.J. Nehring R.B. Roosterman D. Meyerhof W. Richter D. DNA Cell Biol. 1997; 16: 111-119Crossref PubMed Scopus (64) Google Scholar). The aim of the present study was to characterize early molecular events after agonist activation that lead to functional desensitization and sequestration of the distinct somatostatin receptor subtypes. Moreover, we wanted to delineate the molecular determinants that dictate the differential trafficking and endosomal sorting of somatostatin receptors. All five somatostatin receptors belong to the superfamily of G protein-coupled receptors (GPCRs). After agonist binding and activation of GPCRs, the signaling is turned off by phosphorylation of intracellular receptor domains and subsequent recruitment of cytoplasmic proteins termed arrestins that interrupt coupling between the receptor and its cognate heterotrimeric G protein (16Oakley R.H. Laporte S.A. Holt J.A. Caron M.G. Barak L.S. J. Biol. Chem. 2000; 275: 17201-17210Abstract Full Text Full Text PDF PubMed Scopus (686) Google Scholar, 17Oakley R.H. Laporte S.A. Holt J.A. Barak L.S. Caron M.G. J. Biol. Chem. 2001; 276: 19452-19460Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar). β-Arrestins also function as docking proteins that link the receptor to components of the endocytic machinery such as AP-2 and clathrin and as scaffolding proteins to turn on signaling to mitogen-activated protein kinase (MAPK) cascades (18Pierce K.L. Luttrell L.M. Lefkowitz R.J. Oncogene. 2001; 20: 1532-1539Crossref PubMed Scopus (365) Google Scholar). Within the endosomal compartment, β-arrestins regulate the rate at which internalized receptors are dephosphorylated and recycled to the plasma membrane (19Shenoy S.K. McDonald P.H. Kohout T.A. Lefkowitz R.J. Science. 2001; 294: 1307-1313Crossref PubMed Scopus (713) Google Scholar, 20Shenoy S.K. Lefkowitz R.J. J. Biol. Chem. 2003; 278: 14498-14506Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar). Based on their binding properties to different isoforms of β-arrestin, GPCRs have been categorized into two classes. Class A receptors (e.g. μ-opioid, β2 and α1B adrenergic, endothelin A, and dopamine D1A not bind and have a affinity for Class B receptors (e.g. and bind and have for and (16Oakley R.H. Laporte S.A. Holt J.A. Caron M.G. Barak L.S. J. Biol. Chem. 2000; 275: 17201-17210Abstract Full Text Full Text PDF PubMed Scopus (686) Google Scholar). Class A and class B receptors also differ in the of the class A receptors, β-arrestin the receptors to but not internalize with class B receptors, β-arrestin stable complexes with the receptors, such that the as a into early R.H. Laporte S.A. Holt J.A. Barak L.S. Caron M.G. J. Biol. Chem. 2001; 276: 19452-19460Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar). Here we the sst3 and sst5 somatostatin receptors as class A receptors and the sst2A somatostatin receptor as class B receptor. The trafficking of the sst1 and sst4 somatostatin receptors did not to on that the of stable complexes between β-arrestin and sst2A G protein-coupled receptor kinase phosphorylation of a cluster of phosphate acceptor sites within the cytoplasmic tail of the sst2A receptor. also show that sst2A and sst3 differential endosomal sorting. In to be from their β-arrestin mobilization the sst2A receptor was rapidly recycled to the plasma membrane whereas the sst3 receptor was subject to lysosomal was from and to from The was to the peptide which to of the tail of The was to the peptide which to of the tail of the have been M. H. M. S. Kreienkamp H.J. Hollt V. Schulz S. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar). All by affinity the coupling to the peptide to the of the of the rat sst2A, sst3, sst4, and sst5 the in have been (13Kreienkamp H.J. Roth A. Richter D. DNA Cell Biol. 1998; 17: 869-878Crossref PubMed Scopus (29) Google Scholar, 15Roth A. Kreienkamp H.J. Nehring R.B. Roosterman D. Meyerhof W. Richter D. DNA Cell Biol. 1997; 16: 111-119Crossref PubMed Scopus (64) Google Scholar, M. H. M. S. Kreienkamp H.J. Hollt V. Schulz S. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar). sst2A by that at Kreienkamp H.J. Richter D. J. 1999; PubMed Scopus Google Scholar). into sst2A by and with The of all by The encoding green protein was by N. W. of The encoding the protein was from was from The Cell and embryonic cells from and in with in a to the of the in the of cells the sst2A membrane or sst3 receptors membrane binding and as M. H. M. S. Kreienkamp H.J. Hollt V. Schulz S. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar). The of somatostatin receptor was between and membrane protein for all transfected after cells into The cells for in a at was a of β-arrestin was in cells with of or and of the various somatostatin receptor subtypes. of GRK2 or of in the at with of SS-14 into the after the was on After the with SS-14, cells with and in phosphate for at and in phosphate and for in and in in TPBS, and with with for at with or at a of in with After with TPBS, was with in and in cells in examined a was with and was used to the to which a of internalized sst2A or sst3 was to a subsequent of M. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). cells sst2A or sst3 on and receptors with In the of cells with SS-14 and for at In a of cells with SS-14 for at to endocytosis of receptors. cells on with for in agonist but and and sst2A or sst3 receptors examined as Cell cells at a density of The cells transfected with the cells with and and with for at cells to SS-14 for cells on and with saline. into of A, and on a for at After at for at receptor proteins affinity from the for at The to by The of receptor phosphorylation was a system and of of receptor proteins in was by phosphorylation was expressed as a of of are transfected cells and to After with SS-14 in cells in as proteins from the for at and on After the with or for at by an enhanced system and to the and during agonist in and proteins with affinity receptors was used to and protein on was with the of of Somatostatin functional and to enhanced green fluorescent protein to the of β-arrestins to the plasma membrane in cells transfected with distinct somatostatin receptor subtypes. In the of the isoforms the of the cells was also detectable the cell The of of SS-14 a of as well as from the to the plasma membrane in and In the of the cells was in cells with as with cells with In only of but not was of or to the plasma membrane was not detectable in cells transfected with sst1 or After agonist internalization of into was observed only in cells that and sst2A in early In and to the after SS-14 exposure of β-arrestin and sst2A into early sst2A transfected with and of to SS-14 for or for and examined by was membrane are from of in and of stable complexes between β-arrestin and GPCRs on the of of phosphate acceptor sites as or of within the tail of the receptor (16Oakley R.H. Laporte S.A. Holt J.A. Caron M.G. Barak L.S. J. Biol. Chem. 2000; 275: 17201-17210Abstract Full Text Full Text PDF PubMed Scopus (686) Google Scholar, 17Oakley R.H. Laporte S.A. Holt J.A. Barak L.S. Caron M.G. J. Biol. Chem. 2001; 276: 19452-19460Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar). These are in their within the and as sites of agonist-dependent receptor The of the sst2A, sst3, and sst4 receptors such of phosphate acceptor however, different with to the which the of the and the of the cytoplasmic tail delineate a basis for the observed differences in the trafficking patterns of these receptors, we cell receptor phosphorylation and β-arrestin-dependent trafficking in the and of of GRK2 a in phosphorylation of the sst2A receptor and the recruitment of to the plasma membrane as well as its trafficking with the receptor into the that these on the of In agonist-induced receptor phosphorylation and trafficking largely of the of Although the sst4 receptor phosphorylation sites in its cytoplasmic tail we not to agonist-induced receptor phosphorylation or mobilization in the or of the of and sst2A into the sites for phosphorylation and β-arrestin mobilization of the sst2A or of its tail in B and of of the sst2A receptor tail which and as phosphorylation phosphorylation by of β-arrestin mobilization of the that the receptor is to a of to the plasma membrane However, of the receptor also that phosphorylation of is for of stable complexes of of the sst2A receptor tail which two phosphorylation and phosphorylation by of β-arrestin trafficking of the that the receptor phosphorylation was not to of to the plasma membrane of of the sst2A receptor tail which two phosphorylation and phosphorylation and β-arrestin binding B and In the the cluster of and was by The of agonist-induced phosphorylation was by as with the sst2A In cells the agonist-induced was and the cell was of cells that all sst2A to the plasma membrane These that many not all of the phosphate acceptor sites within the are in phosphorylation of the sst2A receptor and that phosphorylation of these sites is for of stable These also that the cluster of and may the for phosphorylation and β-arrestin of sst2A and sst3 after pattern of β-arrestin trafficking is to dictate the rate of receptor and In many class A receptors have been shown to recycle whereas class B trafficking patterns are often observed for receptors. examined the of sst2A and sst3 after agonist-induced for with SS-14, and to an in the of agonist for or in sst2A and sst3 to the plasma membrane in After of SS-14 receptors into intracellular vesicles. In the of agonist it that the of internalized sst2A receptors recycled to the plasma membrane during the In contrast, only of the internalized sst3 receptors to the plasma whereas a large proportion of sst3 receptors was into intracellular of relatively large and sst2A and sst3 receptors internalized into the same of we used an to the to which internalized sst2A or sst3 receptors with endocytic that be with a well of early and that M. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). sst2A and sst3 receptors present in the plasma membrane by cells with these cells with SS-14 and for a high of of sst2A and sst3 with was observed, that these receptors internalized the same endocytic is by the observed in the However, these cells with SS-14 to a of endocytosis and for an in the of the of between the of internalized sst2A or sst3 and the endocytic was different internalized sst2A with which is with of internalized sst2A early and In contrast, the same sst3 was observed in a large of endocytic that did not detectable These the that the of internalized sst3 to recycle rapidly is at in by sorting of internalized receptors to a of endocytic distinct from that the by of internalized sst2A and sst3 to of cells sst2A or sst3 was used to between receptors and between internalized sst2A or sst3 receptors with is by in the of cells sst2A or sst3 was used to between a agonist of receptors by a with in the of SS-14 as The of internalized sst2A also detectable In contrast, in the same we observed that detectable are from of in of sst2A and the differential intracellular trafficking of sst2A and sst3, we the of these receptors during SS-14 exposure by shown in we did not detectable in the of sst2A receptors during the In contrast, the sst3 receptor which clearly detectable after of agonist exposure of The degradation of sst3 was by the lysosomal the degradation of sst3 was also by the Although is a of a of is not for and also various and M. M. J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). These that sst2A and sst3 undergo differential endosomal sorting. Although sst2A was into the endosomal compartment, a proportion of sst3 receptors was into the lysosomal a molecular basis for the differential endosomal sorting of the sst2A and the sst3 receptors, we examined the agonist-dependent of these receptors. to SS-14, receptor proteins and receptors an in the sst3 receptor which was as a high molecular that the receptor to multiple In contrast, SS-14 of cells did not in detectable in sst2A receptor of cells sst2A or sst3 for with and to SS-14 for or in the of and and the receptors affinity and as The of molecular is on the In the present we differences in β-arrestin mobilization and endosomal sorting of the five somatostatin receptor subtypes. Based on their β-arrestin-dependent trafficking we the sst3 and sst5 somatostatin receptors as class A receptors and the sst2A somatostatin receptor as class B receptor. the the trafficking of the sst1 and sst4 somatostatin receptors did not to on also show that sst2A and sst3 differential endosomal sorting. In to be from their β-arrestin trafficking the sst2A receptor was rapidly recycled to the plasma whereas the sst3 receptor was subject to lysosomal a of sst2A receptors, we that the molecular determinants for the of stable complexes between β-arrestin and sst2A are within of phosphate acceptor sites at the of its cytoplasmic of phosphate acceptor sites are as or of R.H. Laporte S.A. Holt J.A. Barak L.S. Caron M.G. J. Biol. Chem. 2001; 276: 19452-19460Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar). many class B receptors, these are in their within the and as sites of agonist-dependent receptor It be that not only the sst2A receptor but also the sst3 and sst4 receptors such of phosphate acceptor However, with to the these in the sst3 and sst4 receptors in the sst2A receptor. also that the phosphorylation of the sst2A receptor but not of the sst3 and sst4 receptors on the of The sst3 receptor formed complexes with β-arrestin that dissociated at or near the plasma membrane. β-arrestin mobilization phosphorylation of the sst3 receptor was by of it has been shown that of and by agonist-induced phosphorylation and internalization of the sst3 receptor A. Kreienkamp H.J. Meyerhof W. Richter D. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). with the present these are to the GRK2 phosphorylation cluster of In to the sst2A and sst3 receptors, the sst1 and sst4 receptors failed to β-arrestin to the plasma membrane agonist These of receptor desensitization or to In it has been shown that the rat sst4 receptor to internalize after agonist activation (13Kreienkamp H.J. Roth A. Richter D. DNA Cell Biol. 1998; 17: 869-878Crossref PubMed Scopus (29) Google Scholar). with the we show that sst4 did not undergo detectable phosphorylation after agonist activation the of a of phosphate acceptor sites including a cluster of of five in its tail (13Kreienkamp H.J. Roth A. Richter D. DNA Cell Biol. 1998; 17: 869-878Crossref PubMed Scopus (29) Google Scholar). In we that the sst4 receptor was resistant to agonist-induced phosphorylation and β-arrestin-dependent trafficking in the of to the sst1 receptor it be that upon in a large proportion of receptor protein in an as intracellular S. Pauli S.U. Schulz S. Handel M. Dietzmann K. Firsching R. Hollt V. Clin. Cancer Res. 2000; 6: 1865-1874PubMed Google Scholar). the fact that only a of sst1 receptors is to the plasma membrane may that agonist exposure of receptor did not β-arrestin these In SS-14 β-arrestin to the plasma membrane. However, was only and that observed for the sst2A and sst3 receptors. that sst5 has a affinity for for SS-14, the recruitment of be of activation of the sst5 receptor by SS-14 D. Roth A. Kreienkamp H.J. Richter D. Meyerhof W. J. Neuroendocrinol. 1997; PubMed Scopus Google Scholar). β-arrestin mobilization the sst5 agonist The fact that very these that the sst5 receptor a class A trafficking It has been that distinct intracellular trafficking patterns of β-arrestin the of internalized GPCRs (19Shenoy S.K. McDonald P.H. Kohout T.A. Lefkowitz R.J. Science. 2001; 294: 1307-1313Crossref PubMed Scopus (713) Google Scholar, 20Shenoy S.K. Lefkowitz R.J. J. Biol. Chem. 2003; 278: 14498-14506Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar). class A receptors (e.g. μ-opioid, β2 and adrenergic, endothelin A, and dopamine D1A have been shown to recycle whereas class B trafficking patterns are often observed for receptors (e.g. and S.K. Lefkowitz R.J. J. Biol. Chem. 2003; 278: 14498-14506Abstract Full Text Full Text PDF PubMed Scopus (206) Google Scholar). However, that not all GPCRs into the peptide receptor β-arrestin for its but not for its internalization Kohout T.A. Lefkowitz R.J. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar). we show the sst2A receptor was rapidly recycled to the plasma membrane detectable of sst2A receptors during agonist In contrast, a large proportion of internalized sst3 receptors was and into the lysosomal in a of sst3 receptors. findings that endosomal sorting of sst2A and sst3 was by differential differential β-arrestin binding of these receptors. In we provide for differential β-arrestin-dependent trafficking and endosomal sorting of somatostatin receptor subtypes. The high density of somatostatin receptors on human neuroendocrine tumors has allowed the development of somatostatin receptor scintigraphy for tumor imaging as well as somatostatin receptor-targeted radiotherapy (9Lamberts S.W. Krenning E.P. Reubi J.C. Endocr. Rev. 1991; 12: 450-482Crossref PubMed Scopus (847) Google Scholar, 10Forster G.J. Engelbach M.J. Brockmann J.J. Reber H.J. Buchholz H.G. Macke H.R. Rosch F.R. Herzog H.R. Bartenstein P.R. Eur. J. Nucl. Med. 2001; 28: 1743-1750Crossref PubMed Scopus (141) Google Scholar). The of these and on receptor internalization and a tumor be to somatostatin analogs tumors that sst3 or In differences have been observed in the response of neuroendocrine tumors to long-term application of stable somatostatin analogs. In patients with growth hormone-secreting pituitary adenoma, the inhibitory effects of somatostatin analogs on hormone secretion persist for many years during long-term In contrast, carcinoids are very likely to undergo desensitization within weeks to months of octreotide Although sst2A is neuroendocrine tumors often distinct patterns of somatostatin receptor subtypes (3Kulaksiz H. Eissele R. Rossler D. Schulz S. Hollt V. Cetin Y. Arnold R. Gut. 2002; 50: 52-60Crossref PubMed Scopus (191) Google Scholar). the differential intracellular sorting of somatostatin receptors may provide important clues about the regulation of receptor responsiveness during long-term administration of somatostatin analogs. and for