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The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 11 5353-5360
Copyright © 2003 by The Endocrine Society

Somatostatin Receptors 2 and 5 Are the Major Somatostatin Receptors in Insulinomas: An in Vivo and in Vitro Study

J. Bertherat, F. Tenenbaum, K. Perlemoine, C. Videau, J. L. Alberini, B. Richard, B. Dousset, X. Bertagna and J. Epelbaum

Department of Endocrinology (J.B., X.B.), Biophysics and Nuclear Medicine (F.T., J.L.A., B.R.), and Endocrine Surgery (B.D.), Cochin Hospital; Institut National de la Santé et de la Recherche Médicale (INSERM) U549 (C.V., J.E.), IFR 77 Broca-Sainte-Anne; and Department of Endocrinology (J.B., K.P., X.B.), Institut Cochin, INSERM U576, CNRS UMR 8104, IFR116, René Descartes-Paris V University, 75014 Paris, France

Address all correspondence and requests for reprints to: Professor Jérôme Bertherat, M.D., D.Sc., Service d’Endocrinologie, Hopital Cochin, 27 rue du Faubourg St. Jacques, 75014 Paris, France. E-mail: jerome.bertherat{at}cch.ap-hop-paris.fr.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Somatostatin (SRIF) receptors (sst) are present on normal pancreatic endocrine ß-cells. However, the use of SRIF analogs in the scintigraphic imaging of insulinomas and in the medical management of these tumors seems to be restricted to a subgroup of patients. The aim of this study was to determine the prevalence of sst expression in vitro and characterize sst subtype binding in insulinomas and its correlation with in vivo sst receptor scintigraphy (SRS). In vitro studies were performed on 27 insulinomas from 25 patients: 22 with benign and three with malignant tumors. Semiquantitative RT-PCR of sst mRNAs was performed for 20 of these insulinomas. Sst2 and sst5 were expressed in 70%, sst1 in 50%, and sst3 and sst4 subtypes only in 15–20% of the tumors. 125I-Tyr0DTrp8SRIF14 binding was assessed by quantitative autoradiography in 18 insulinomas, and competition experiments were performed with SRIF14 and L797–591, L779–976, L796–778, L803–087, L817–818, selective agonists of the five sst subtypes, and BIM23244, a selective agonist of sst2 and sst5. Significant specific binding was observed in 72% of the insulinomas. Displacement experiments with ligands of higher affinity for each of the sst receptors revealed significant binding with the sst2 and sst5 ligands in 72%, sst3 in 44%, sst1 in 44%, and sst4 in 28% of cases. All insulinomas displaying sst2 binding were also sst5 sensitive. However, the ratio of sst5/sst2 displacement was variable and only equal to that for SRIF14 in experiments with the sst2/sst5 agonist BIM23244. SRS was performed 10 times in nine patients; it detected 60% of the tumors, including metastases of a malignant insulinoma. All the tumors detected by SRS displayed high levels of 125I-Tyr0DTrp8SRIF14 binding. The mechanisms underlying the loss of expression of sst2/sst5 in a third of insulinomas remains to be determined, but this loss of expression may be involved in ß-cell dysfunction.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Discussion
 References
 
SOMATOSTATIN (SRIF) EXERTS a wide variety of biological actions through five receptor subtypes (sst1 to 5) (1, 2). SRIF strongly inhibits the secretion of many hormones, including GH, gastrin, glucagon, and insulin. It also plays a role in cell proliferation. For this reason, SRIF analogs are effective in the treatment of various secreting endocrine tumors, such as GH adenomas and digestive endocrine tumors. SRIF receptor scintigraphy (SRS) is also used for the in vivo detection of primary and secondary neuroendocrine tumors, such as carcinoid tumors and gastrinomas (3, 4). The SRIF analogs currently used to treat and detect neuroendocrine tumors (octreotide, BIM23014, and RC160) preferentially bind sst2 and, to a lesser extent, sst5 (1). The clinical use of SRIF analogs is based on the expression of sst in such tumors. Various observations suggest that sst2 expression is required for a good response to SRIF analogs.

Insulinoma is a rare endocrine tumor with an estimated incidence of four cases per 1 million person-years (5). The main clinical manifestation is hypoglycemia secondary to inappropriate insulin hypersecretion and, in a minority (10–20%) of cases, tumoral progression due to malignant transformation. Multiple insulinomas are observed in about 10% of cases, usually in patients presenting multiple endocrine neoplasia type 1 (MEN1). Preoperative localization of the insulinoma is often difficult if the tumor is small; about one third of insulinomas are less than 10 mm in diameter (6). SRIF analogs have been used to treat insulinoma because SRIF inhibits insulin secretion (7, 8). However, treatment does not always correct hypoglycemia, and the level of inhibition of insulin secretion by the SRIF analogs currently used is not constant (9, 10, 11, 12, 13). Consistent with this inadequate improvement in endocrine symptoms with treatment, SRS is considered not to be sensitive enough for the in vivo visualization of insulinoma (3, 6).

Normal pancreatic ß-cells express SRIF receptors. The inhibition of insulin secretion seems to be primarily mediated by sst5 in rodents (14, 15). In humans, limited, conflicting data have been obtained, with roles suggested for both sst2 (16, 17) and sst5 (18, 19). In one study, sst2-selective analogs failed to inhibit insulin release in isolated human islets (19). Characterization of sst expression has been carried out for a limited number of insulinomas. Autoradiographic studies have demonstrated the presence of SRIF binding sites in human insulinomas but sst subtypes have not been characterized (20). RT-PCR studies on a small number of tumors have suggested that mRNAs for all five SRIF receptor subtypes are present in insulinomas (21, 22, 23). In situ hybridization studies have confirmed the presence of sst1 and sst3 (24), but the results of such studies were not entirely consistent. The goal of this study was therefore to characterize expression of the five sst subtypes in a large series of insulinomas by determining the levels of both mRNA and protein for these subtypes. We determined mRNA levels by semiquantitative RT-PCR and protein levels by 125I-Tyr0DTrp8SRIF14 autoradiographic binding and competition studies with analogs selective for each of the five sst receptor subtypes. We also evaluated the correlation between in vitro and in vivo SRIF binding in a subgroup of patients.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Discussion
 References
 
SRS

Ten SRS examinations were carried out in nine patients; one of the nine patients had a malignant tumor and was explored twice (at diagnosis and during follow-up). SRS was performed after iv injection of 160 MBq 111In-pentetreotide (Octreoscan, Mallinckrodt Medical, Petten, The Netherlands). Anterior and posterior planar views centered on head and chest, abdomen, and pelvis were obtained 4 and 24 h after injection (each with a 20-min acquisition period). Abdominal single-photon emission-computerized tomography (SPECT) was performed 24 h after injection (64 projections for 40 sec each over 360 degrees, the projection data sets were prefiltered in a 64 x 64 matrix with a Hanning filter). The patient was given a laxative preparation to reduce background intestinal activity. Images were obtained with a large field of view dual-head {gamma} camera equipped with a medium-energy collimator (DST-XL, GE Medical Systems, Buc, France). All SRS images were analyzed by two nuclear medicine specialists. Their assessments were based on the following criteria: number, location, and intensity of uptake, scored on a three-point scale (1, intensity less than that in the liver; 2, intensity equal to that in the liver; 3, intensity more than that in the liver).

Tissue collection

Pancreatic tissues were obtained during surgery in the operating theater. They were immediately dissected by the pathologist, frozen, and stored in liquid nitrogen until use. Twenty-seven insulinomas from 25 patients were included in the study: 20 benign sporadic insulinomas, one benign insulinoma from a MEN1 patient, one benign multiple insulinoma from a non-MEN1 patient, and three primary malignant insulinomas and two of their metastases.

SRIF binding study by autoradiography

For SRIF receptor autoradiography, Tyr0DTrp8SRIF14 (Peninsula Laboratories, Meyerside, UK) was iodinated as previously described (25) and used at a specific activity of 780 Ci/mmol. Sections (14 µm) were cut on a cryostat at -17 C, thaw mounted onto 2% gelatin-coated slides and stored at -20 C until use. Sections were incubated for 20 min at room temperature in 32 mM sucrose and 0.5% BSA in 50 mM Tris HCl buffer, pH 7.5 (26). They were then incubated for 1 h at room temperature with 125I-Tyr0DTrp8SRIF14 in the same isotonic buffer supplemented with 5 mg/liter bacitracin and 5 mM MgCl2. The sections were washed for 10 min each at 4 C in two consecutive baths of fresh buffer and were then air dried. [3H]-Ultrofilm (Pharmacia, Uppsala, Sweden) was placed against the radiolabeled sections, and autoradiographs were developed after 2–5 d of exposure at 4 C. For competition experiments, sections were incubated for 1 h at room temperature with 125I-Tyr0DTrp8SRIF14 in the presence of various concentrations of SRIF14; octreotide; the sst1- (L-797–591), sst2- (L-779–976), sst3- (L-796–778), sst4- (L-803–087), and sst5- (L-817–818)-selective analogs (14, 27); sst5 analog BIM 82096; and sst2/5 analog (BIM-23244, Biomeasure, Rahway, NJ). Autoradiographic quantification and displacement curve analysis were performed as previously described (28, 29).

Semiquantitative RT-PCR

Total mRNA was extracted with the SNAP Total RNA isolation kit (Invitrogen, Groningen, The Netherlands), using two nucleic acid column purification steps, with extensive DNase treatment to prevent DNA contamination. Reverse transcription was performed with the cDNA cycle kit (Invitrogen). The resulting cDNA was amplified by PCR, using the Dynazyme II DNA polymerase (Finnzymes Oy, Espoo, Finland). The PCR cycling conditions were as follows: 29 cycles of 94 C for 1 min, 55–60 C for 2 min, and 72 C for 2 min. As a negative control for the RT-PCR reaction, reverse transcriptase was omitted from the reverse transcription reaction mixture. Because the human sst genes are intronless, 500 ng of genomic human DNA was used as a positive control in each experiment. We added 1 µl dATP{alpha}P32 (3000 Ci/mmol) to each 25-µl PCR mixture. The following synthetic oligonucleotides (Life Technologies, Rockville, MD) were used: glyceraldehyde-3-phosphate dehydrogenase, sense, GCC ACA TCG CTC AGA CCA, antisense, GTC AAG GCT GAG AAC GGG AA; sst1, sense, GCT GAG CAG GAC GAC GCC ACG, antisense, GGA CTC CAG GTT CTC AGG TTG; sst 2, sense, CCC CAG CCC TTA AAG GCA TGT, antisense, GGT CTC CAT TGA GGA GGGTCC; sst 3, sense, ATC ATC GGT GTC CAC GAC CTC A, antisense, GAA CTG GTT GAT GCC ATC CAC C; sst 4, sense, GCA TGG TCG CTA TCC AGT GCA, antisense, GTG AGA CAG AAG ACG CTG GTG; sst 5, sense, AAC ACG CTG GTC ATC TAC GTG GT, antisense, AGA CAC TGG TGA ACT GGT TGA C. PCR products were subjected to electrophoresis in a nondenaturing 5% polyacrylamide (19:1 acrylamide:bisacrylamide) gel. Signals were detected and analyzed with a PhosphorImager and ImageQuant 3.0 (Molecular Dynamics, Sunnyvale, CA).


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Discussion
 References
 
RT-PCR study

Twenty insulinomas were studied by RT-PCR. The sst1 mRNA was detected in 50%, and sst2 and sst5 mRNAs were detected in 70% of the tumors, whereas sst3 and sst4 mRNAs were observed in no more than 20% of the insulinomas (Fig. 1Go).



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  		FIG. 1. RT-PCR study of the five somatostatin receptors (sst). PhosphorImager detection of the radioactive signal obtained after semiquantitative RT-PCR performed with an oligonucleotide specific for glyceraldehyde-3-phosphate dehydrogenase used as an internal control and oligonucleotides specific for sst subtypes 1–5. P, Positive control; N, negative control (see Patients and Methods).

 
Autoradiographic study

We studied 18 insulinomas (Fig. 2Go). Significant specific 125I-Tyr0DTrp8SRIF14 binding was detected in 13 of 18 cases (72%) (Fig. 3Go). When measurable, 125I-Tyr0DTrp8SRIF14 binding was fairly uniform in tumor samples. In competition experiments with selective ligands, used at a maximal concentration of 1 µM, a modest displacement was observed with SRIF-2-selective ligands (sst1 and sst4): 44% of tumors with the sst1 ligand (L-797–591) and 28% of tumors with the sst4 ligand (L-803–087) showed displacement (Fig. 3Go). A higher level of displacement was observed for the SRIF-1 ligands (sst2, sst3 and sst5): 72% of tumors with the sst2 (L-779–976) and sst5 (L-817–818) ligands but only 44% of tumors with the sst3 ligand (L-796–778) showed displacement. All insulinomas sensitive to the sst2 agonist were also displaced by the sst5 ligand. However, the ratio of sst2:sst5 displacement differed between tumors. Complete displacement curves were generated for the insulinomas displaying the highest levels of 125I-Tyr0DTrp8SRIF14 binding (Fig. 4Go). The IC50 values calculated from these competition curves were: 1.75 ± 0.56 nM for SRIF14; 5.45 ± 1.57 nM for octreotide; 6.48 ± 6.04 nM for L-779–976 (sst2); 1088 ± 891 nM for L-796–778 (sst3); and 16.20 ± 10.1 nM for L-817–818 (sst5). Due to the limited amount of competition observed, we were unable to determine the IC50 values for L-797–591 (sst1) and L-803–087 (sst4). The level of displacement achieved with increasing amounts of the sst2/sst5 ligand BIM-23244 was similar to that with SRIF14: The calculated IC50 was 1.04 ± 0.30 nM. Bmax, expressed as the percentage of displacement of the specific binding determined with SRIF14, was: 64.4 ± 1.2% for octreotide, 63.7 ± 5.7% for L-779–976 (sst2), 46.3 ± 7.8% for L-796–778 (sst3), 39.3 ± 8.0% for L-817–878 (sst5), and 92.0 ± 7% with the sst2/5 ligand BIM-23244. Given the poor affinity of the sst3 selective analog, the displacement obtained may be accounted for by the affinity of this compound for sst5 receptors [24 nM for sst3 and 1200 nM for sst5 (14)].



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  		FIG. 2. Autoradiographic detection of 125I-Tyr0DTrp8SRIF14. Autoradiographs for two demonstrative insulinomas are shown. All sections were incubated with 125I-Tyr0DTrp8SRIF14 without (total) or with the addition of 0.1 µM unlabeled SRIF 14 (0.1 µM) or a ligand selective for one of the subtypes (sst1 to sst5), as indicated at the top. The insulinoma corresponding to A has a low sst2/sst5 binding ratio, whereas the insulinoma corresponding to B exhibits a high sst2/sst5 binding ratio.

 


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  		FIG. 3. Quantification of 125I-Tyr0DTrp8SRIF14 binding by autoradiography. Specific binding (A), determined by comparing sections incubated with 125I-Tyr0DTrp8SRIF14 in the presence and absence of 0.1 µM unlabeled SRIF 14, expressed in fmol/SU. The remaining panels display quantification of the displacement obtained by competition with 1-µM solutions of the ligands specific for the five receptor subtypes (expressed as a percentage of SRIF binding displacement; see Patients and Methods): B, L-797–591; C, L-779–976; D, L-796–778; E, L-803–087; F, L-817–818.

 


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  		FIG. 4. Displacement of 125I-Tyr0DTrp8SRIF14 binding by SRIF 14, octreotide, sst1 to sst5 ligands, and sst2/sst5 ligands in insulinomas. Curve of the displacement of 125I-Tyr0DTrp8SRIF14 binding by SRIF 14, the sst2/sst5 ligands BIM 23244 and octreotide, the sst1 ligand L-797–591, the sst2 ligand L-779–976, the sst3 ligand L-796–778, the sst4 ligand L-803–087, and the sst5 ligand L-817–818. Sections from insulinomas with high levels of [125]I-SRIF binding were incubated with various amounts of unlabeled ligand (from 10-12 to 10-6 M). The results are shown for five most labeled tumors.

 
In vivo sst detection by SRS

Nine patients (six women and three men; age range, 26–79 yr) underwent 10 SRS examinations (Table 1Go). All these patients then underwent surgery. Seven patients were found to have a single benign insulinoma; patient 26 had a multiple insulinoma and patient 11 had a metastatic insulinoma. Twelve pancreatic tumors were found (five in tail, five in the body, two in the head). Pancreatic tumor diameters ranged from 0.3 to 3 cm. Significant uptake (intensity, 2 or 3) was observed in six of the 10 SRS examinations performed (sensitivity, 60%). The results of the SRS were positive in the pancreatic area in five of the nine patients: four had single benign sporadic insulinomas (Fig. 5Go), one had multiple benign insulinomas not related to MEN 1 (this patient had four tumors, only the two largest of which were more than 1 cm in diameter and detectable by SRS). One patient with malignant insulinoma was investigated before initial surgery and the primary tumor was not detected by SRS. During recurrence, metastatic abdominal and thoracic lymph nodes were visualized by SRS. The number and intensity recorded in planar and SPECT analyses were similar. SPECT was useful only for more precise tumor location. The results are therefore given without specifying whether analysis was planar or SPECT.


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  		TABLE 1. Somatostatin receptor in vivo scintigraphy study

 


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  		FIG. 5. In vivo scintigraphy of SRIF receptors (Octreoscan). A and B, Malignant insulinoma (patient 11). A, A thoracic metastasis was initially diagnosed by SRIF receptor scintigraphy. B, Radioactivity uptake in two lymph node metastases from the same patient (arrow). C, SRIF receptor scintigraphy performed in patient 1, who had a single insulinoma in the tail of the pancreas (arrow). D, Tomoscintigraphic study of SRIF receptors performed in patient 26, who presented multiple insulinomas. The arrows indicate two of these insulinomas.

 
Comparison of in vivo SRS and in vitro 125I-Tyr0DTrp8SRIF14 binding

The intensity of the SRS signal and in vitro 125I-Tyr0DTrp8SRIF14 binding were well correlated (Table 2Go), with the notable exception of the primary malignant tumor of patient 11, which was not detected by SRS but presented significant numbers of 125I-Tyr0DTrp8SRIF14 binding sites. This tumor had a low affinity for octreotide, although this analog apparently competed efficiently for receptors at higher doses. The bioavailability of the radioactive ligand in vivo may be low because this primary tumor displayed lower levels of contrast medium uptake on computed tomography scan and magnetic resonance imaging than are generally observed in classical imaging of the endocrine tumors of the pancreas.


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  		TABLE 2. Sst scintigraphy and 125I-Tyr0DTrp8SRIF14 binding characteristics

 

   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
References
 
This study shows that three in four insulinomas produce sst2 and sst5 mRNAs and bind significant levels of 125I-Tyr0DTrp8SRIF14. In contrast, sst3 and sst4 seem to be rarely expressed in insulinomas. sst1, the mRNA of which was often detected, appears to play little or no role in 125I-Tyr0DTrp8SRIF14 binding.

The sst2 and sst5 receptors have been shown to be important in the endocrine pancreas of rodents (14, 15). In normal human ß-cells, sst2a has also been detected by immunohistochemistry (17). However, in one study, sst2 was reported to be expressed much less strongly than sst5 and sst1 in normal human ß-cells (18). Expression of almost all the sst subtypes has been reported in insulinomas (21, 22, 23, 24, 30). However, it is difficult to determine the prevalence of each receptor subtype because too few tumors have been studied to date. This is also true of studies of SRIF binding in insulinomas, and the use of ligands specific for each of the cloned SRIF receptors has never before been reported. This study demonstrates that sst2 and sst5 are the major receptors recognizing SRIF in insulinomas. Interestingly, all tumors displaying significant binding of 125I-Tyr0DTrp8SRIF14 coexpressed, albeit to different extents, both sst2 and sst5. The relative importance of sst2 and sst5 in SRIF binding differs between insulinomas. In most cases, more sst2 binding than sst5 binding is observed, but a subgroup of tumors presents higher levels of sst5 than of sst2 binding. However, neither selective agonists of sst2 or sst5 nor octreotide displaced 125I-Tyr0DTrp8SRIF14 binding to the same extent as the endogenous ligand, SRIF14. In contrast, the sst2/sst5 ligand Bim23244 bound as efficiently as SRIF14. This suggests that the sst2 and sst5 receptors interact in the binding profile of the tumors. The coexpression of sst5 and sst2 in most insulinomas could lead to heterologous receptor dimerization, as reported in cellular models transfected with genes encoding recombinant receptors (31, 32).

Although significant levels of sst1 mRNA were detected in insulinomas, it seems that a functional sst1 receptor is rarely present in such tumors. One could not exclude a low sst1 binding in tumors expressing sst1 mRNA due to a lower affinity than previously reported of L-797–591 for the human sst1 receptor (Hoyer, D., personal communication). Nevertheless, the affinity of L-797–591 for sst1 receptors [mean ± SEM dissociation constant (pKd) = 7.33 ± 0.09, n = 3] was still 10-fold higher than for other subtypes, particularly sst2 (pKd = 6.15 ± 0.07, n = 3) and sst5 (pKd = 6.00 ± 0.25, n = 3). At any rate, in the radioautographic studies shown herein, L-797–591 did not displace strongly radioligand binding up to micromolar concentrations. Discrepancy between sst1 mRNA and protein levels has also previously been reported for the rat brain, in which sst1 mRNA is widely distributed (33), whereas the receptor protein is present only in selected hypothalamic regions (34). This further underlines the necessity for studies of SRIF binding, to demonstrate the expression of functional sst in endocrine tumors. It is also possible that the sst1 receptor expressed in insulinomas is not localized to the cell surface. Indeed, a recent immunohistochemistry study (35) describes a cytoplasmic localization for the sst1 in insulinomas. A poor specificity of the sst5 ligand L-817–818 (36) could also lead to the speculation that the sst5 protein might not be present, despite the demonstration of its mRNA in insulinomas. However, the potent displacement observed with the sst2/sst5 analog BIM-23244 in insulinomas in which neither the sst2 nor the sst5 ligand alone could fully displace 125I-Tyr0DTrp8SRIF14 argues in favor of the presence of a functional sst5 protein. In addition, L-817–818 was as potent as another sst5 selective analog (BIM-82096) to displace 125I-Tyr0DTrp8SRIF14 binding in insulinoma (data not shown). Furthermore, immunohistochemistry studies have shown the presence of sst5 receptors in insulinomas, although in different proportions according to two reports (30, 35).

In this study, SRS detected 60% of insulinomas, consistent with the results of previous reports (4, 37, 38). In fact, the rate of detection by SRS is lower for insulinomas than for other abdominal neuroendocrine tumors (between 80 and 90%) (4, 39, 40, 41, 42, 43, 44). One study reported a higher (87.5%) sensitivity for SRS, using SPECT analysis performed 4 h after injection of the radioactive ligand in 14 patients with insulinoma (45). This higher sensitivity may be accounted for by the use of a high dose of 111In-pentreotide and of SPECT studies. However, we observed no significant difference between the results obtained by planar and SPECT analysis. As shown in vitro, the binding of both sst2 and sst5 ligands is required for the detection of insulinomas by SRS. The interaction between sst2 and sst5 receptors on the tumor seems to be essential for insulinoma detection by SRS and a tumor that binds the sst5 ligand, and that was not detectable in vivo displayed only limited affinity for octreotide.

The use of a specific sst2/sst5 ligand might increase the sensitivity of scintigraphic insulinoma detection. The results of treatment with sst2-selective SRIF analogs to inhibit insulin secretion in cases of refractory hypoglycemia are often disappointing. The use of sst2/sst5-selective agonists could improve the efficiency of SRIF analog treatment of insulin oversecretion by insulinomas, as previously suggested for prolactin-secreting pituitary tumors (46).

The mechanisms underlying the loss of SRIF receptor expression in 25–30% of insulinomas have not yet be identified but may be related to tumor development and the loss of control of insulin secretion. No sst2 mRNA is detected in pancreatic adenocarcinoma (47), high-grade colorectal carcinomas (20, 48), and malignant pheochromocytomas (49). Several mechanisms may be responsible for this loss: deletion and/or mutation in the promoter and/or coding sequences or hypermethylation. To date, no mutation has been identified in the coding sequence of the sst2 gene, but a R240W mutation was recently identified in the sst5 gene of a patient with acromegaly resistant to octreotide treatment (50). A genetic polymorphism (G-83A) has been identified in the promoter of the sst2 gene in patients with pancreatic adenocarcinoma. This polymorphism reduces transcription by 60–70% (51). Hypermethylation at CpG islands in the 5'-regulatory region may also have an effect (52).

In conclusion, this study demonstrates that sst2 and sst5 are the main sst subtypes expressed in insulinomas. These two subtypes account for almost all native SRIF 14 binding in such tumors, and their expression on the tumor is required for the in vivo detection of insulinoma by SRS.


   Acknowledgments
 
We thank Dr. D. Hoyer (Novartis Pharma, Basel, Switzerland) for providing us with pharmacological data on the recombinantly expressed human sst receptors; Biomeasure for the gift of somatostatin analogs; Professors Y. Chapuis (Endocrine Surgery Department, Hôpital Cochin) and P. Legmann (Radiology Department, Hôpital Cochin) and the medical and paramedical staff at the Surgery, Radiology, and Endocrine Departments of the Hôpital Cochin responsible for patient management; Drs. A. Louvel and F. Tissier (Pathology Department, Hôpital Cochin) for pathological examinations; and Drs. Y. Fulla and M. A. Dugue (Nuclear Medicine Department, Hôpital Cochin) for hormone assays.


   Footnotes
 
Abbreviations: MEN1, Multiple endocrine neoplasia type 1; SPECT, single-photon emission-computerized tomography; SRIF, somatostatin; SRS, sst scintigraphy; sst, SRIF receptor.

Received December 3, 2002.

Accepted July 28, 2003.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 

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