Somatostatin Receptor 2A Research Articles

Phosphaturic mesenchymal tumors show positive staining for somatostatin receptor 2A (SSTR2A)

Tumor-induced osteomalacia (TIO) is a paraneoplastic syndrome associated with tumors that secrete phosphaturic hormones, most notably fibroblast growth factor 23 (FGF23). The majority of tumors associated with this syndrome show stereotypical histological features and are now known as phosphaturic mesenchymal tumors (PMTs). We postulated that immunohistochemistry for somatostatin receptor 2A (SSTR2A) could be used to definitively identify PMTs or other tumors that cause TIO. Immunohistochemistry for FGF23 and SSTR2A was performed on 15 tumors from 14 patients with a definite diagnosis of TIO. All showed positive staining for both markers. While FGF23 staining was quite focal in some tumors, SSTR2A showed diffuse strong expression. In 40 control tumors not known to be associated with the clinical or biochemical features of TIO, FGF23 expression was found in 2 cases (one aneurysmal bone cyst and one osteosarcoma). SSTR2A expression was found in 9 control tumors (4 synovial sarcomas, 2 hemangiomas, 2 aneurysmal bone cysts and one osteosarcoma). Only one tumor (an aneurysmal bone cyst) showed positive staining for both FGF23 and SSTR2A. SSTR2A also commonly stained neoplastic and non-neoplastic endothelial cells. We conclude that neither FGF23 nor SSTR2A expression are specific for the diagnosis of PMT. However both stains are highly sensitive. Because of its diffuse strong expression and widespread availability, immunohistochemistry for SSTR2A is useful to confirm the diagnosis of PMT in an appropriate setting particularly if material is limited. Negative staining can serve as an excellent rule out test for this diagnosis.

Agonist-biased Trafficking of Somatostatin Receptor 2A in Enteric Neurons

Somatostatin (SST) 14 and SST 28 activate somatostatin 2A receptors (SSTR2A) on enteric neurons to control gut functions. SST analogs are treatments of neuroendocrine and bleeding disorders, cancer, and diarrhea, with gastrointestinal side effects of constipation, abdominal pain, and nausea. How endogenous agonists and drugs differentially regulate neuronal SSTR2A is unexplored. We evaluated SSTR2A trafficking in murine myenteric neurons and neuroendocrine AtT-20 cells by microscopy and determined whether agonist degradation by endosomal endothelin-converting enzyme 1 (ECE-1) controls SSTR2A trafficking and association with β-arrestins, key regulators of receptors. SST-14, SST-28, and peptide analogs (octreotide, lanreotide, and vapreotide) stimulated clathrin- and dynamin-mediated internalization of SSTR2A, which colocalized with ECE-1 in endosomes and the Golgi. After incubation with SST-14, SSTR2A recycled to the plasma membrane, which required active ECE-1 and an intact Golgi. SSTR2A activated by SST-28, octreotide, lanreotide, or vapreotide was retained within the Golgi and did not recycle. Although ECE-1 rapidly degraded SST-14, SST-28 was resistant to degradation, and ECE-1 did not degrade SST analogs. SST-14 and SST-28 induced transient interactions between SSTR2A and β-arrestins that were stabilized by an ECE-1 inhibitor. Octreotide induced sustained SSTR2A/β-arrestin interactions that were not regulated by ECE-1. Thus, when activated by SST-14, SSTR2A internalizes and recycles via the Golgi, which requires ECE-1 degradation of SST-14 and receptor dissociation from β-arrestins. After activation by ECE-1-resistant SST-28 and analogs, SSTR2A remains in endosomes because of sustained β-arrestin interactions. Therapeutic SST analogs are ECE-1-resistant and retain SSTR2A in endosomes, which may explain their long-lasting actions.

Low somatostatin receptor subtype 2, but not dopamine receptor subtype 2 expression predicts the lack of biochemical response of somatotropinomas to treatment with somatostatin analogs.

To evaluate somatostatin receptor 2A (SSTR2A) and dopamine receptor 2 (DR2) protein expression in somatotropinomas and to relate it to response to somatostatin analogues (SA). SSTR2A and DR2 expression was analyzed by immunohistochemistry in 88 somatotropinomas from patients submitted to either pre-surgical or adjuvant SA treatment. Tumors were scored according to percentage of immunostained cells: 0 (< 25%), 1 (25-50%), and 2 (> 50%). Relation between protein expression and response to SA was performed in 66 patients. Response to SA was assessed by percent IGF-I reduction, being considered as an IGF-I per cent reduction higher than 50%. Disease control was also assessed (GH < 1.0 ng/ml and normal IGF-I). SSTR2A and DR2 were expressed in 100% and 98% of tumors, respectively. Biochemical response and disease control rates were 48% and 32%, respectively. Median IGF-I percent reduction after 3 months of SA treatment was lower in the SSTR2A score 0 than in the scores 1 and 2 (p < 0.001, both), and after 6 months in the score 0 than in the score 1 (p = 0.001) and 2 (p < 0.001). Biochemical response and disease control were associated with SSTR2 expression (p < 0.001 and p = 0.004, respectively). A negative predictive value for biochemical response of 100% was found when a SSTR2A expression < 25%of immunostained cells cut-off point was considered. No relation was found between DR2 expression and biochemical response and disease control. SSTR2A and DR2 are highly expressed in somatotropinomas. Low SSTR2A, but not DR2, expression is a negative predictive factor to response to SA.

Clinicopathologic study of mixed adenoneuroendocrine carcinomas of hepatobiliary organs

Neuroendocrine neoplasms in hepatobiliary organs are very rare, but several cases of mixed adenoneuroendocrine carcinoma (MANEC) have been reported. In this study, we characterized the neuroendocrine component of biliary MANEC. A total of 274 cases of biliary cancer including 17 intrahepatic cholangiocarcinomas (CCs), 15 hepatic hilar CCs without preceding hepatobiliary disease, 55 hepatic hilar CCs with hepatolithiasis, 49 gallbladder cancers, 53 extrahepatic CCs, and 85 hepatocellular carcinomas were examined for a neuroendocrine component using immunohistochemistry with neuroendocrine markers (chromogranin A and synaptophysin). In the MANEC cases, in addition to a close histological examination, the proliferative activity and the expression of somatostatin receptor 2A were also evaluated. In addition to an ordinary adenocarcinoma, a neuroendocrine component occupying more than 30% of the entire tumor was also found in 4% (2/55 cases) of hepatic hilar cholangiocarcinomas with hepatolithiasis, 10% (5/49 cases) of gallbladder cancers, and 4% (2/53 cases) of extrahepatic cholangiocarcinomas, but not in the intrahepatic cholangiocarcinomas, hilar cholangiocarcinomas without preceding hepatobiliary disease, and hepatocellular carcinomas. Two cases were positive for somatostatin receptor 2A. The adenocarcinoma components were predominately located at the surface of the tumors, and the majority of stromal and vascular invasion and lymph node metastasis involved neuroendocrine components, showing the features of neuroendocrine tumor G2 or neuroendocrine carcinomas (NECs). NEC components showed higher proliferative activity on Ki67 immunostaining, compared to the adenocarcinoma components. Biliary MANECs are found in hepatic hilar cholangiocarcinomas with hepatolithiasis, gallbladder cancers, and extrahepatic cholangiocarcinomas and show a characteristic histology. Since the neuroendocrine component in biliary MANEC defines the prognosis, it is important to identify it and consider the indications for adjunctive therapy with somatostatin analogues.

Duodenal adenocarcinoma with neuroendocrine features in a patient with acromegaly and thyroid papillary adenocarcinoma: a unique combination of endocrine neoplasia

A 67-year-old woman with familial clustering of thyroid papillary adenocarcinoma was diagnosed with acromegaly due to pituitary macroadenoma. She had multiple skin vegetations, but had no parathyroid and pancreas diseases. Before transsphenoidal surgery, she was further diagnosed as having a duodenal tumor and multiple hypervascular liver nodules. Biopsy specimens from the duodenal tumor and liver nodules were diagnosed histologically as moderately differentiated adenocarcinoma. Immunohistochemically, the tumor cells were positive for chromogranin, synaptophysin and somatostatin receptor 2a, suggestive for neuroendocrine features. After surgery, the patient was not in biochemical remission, and octreotide treatment was initiated. The duodenal cancer was treated with chemotherapy (neoadjuvant cisplatin and S-1). After 24 months, the patient's insulin-like growth factor I level had been normalized, and her liver tumors had not progressed macroscopically. This is a rare case of acromegaly associated with multiple endocrine tumors, not being categorized as conventional multiple endocrine neoplasia. Octreotide treatment might have had beneficial effects on our patient's duodenal adenocarcinoma and liver metastases, both directly via SSTR2a and indirectly via GH suppression, thereby contributing to their slow progression.

Immunohistochemical expression of somatostatin receptors in digestive endocrine tumours

Somatostatin receptors are expressed in a large number of human tumours. The somatostatin receptors types 1-5 expression in a series including 100 gastro-entero-pancreatic endocrine tumours were analysed. From a prospectively built database of patients with gastro-entero-pancreatic endocrine tumours referred from three institutions, 100 cases with clinical and pathological data were selected. Somatostatin receptors expression by immunohistochemistry with somatostatin receptor types 1-5 antibodies in tissue paraffin sections were studied and correlated with the histological diagnosis according to the WHO classification, location and functional status. Of the 100 cases, 67 were gastrointestinal tumours, 25 pancreatic and 8 liver metastasis of unknown origin. Thirty-one of them were functioning tumours: 2 insulinomas, 5 gastrinomas, 1 glucagonoma and 23 carcinoids. Somatostatin receptors expression was observed in 94 tumours. The six negative cases were all non-functioning tumours. Somatostatin receptors 2a and 5 were highly expressed (86 and 62%, respectively), and surprisingly found even in poorly differentiated endocrine carcinomas. Somatostatin receptors expression was less frequent in pancreatic than in gastrointestinal tumours. Well-differentiated neoplasms had a higher density of somatostatin receptors. Only SSTR2a showed membrane staining. Immunohistochemistry revealed that somatostatin receptors were highly expressed in both primary and metastatic gastro-entero-pancreatic endocrine tumours with heterogeneous staining distribution. It proved to be a reliable technique even in small tumour samples.

Diagnostic yield of nonfunctional pancreatic neuroendocrine tumor using endoscopic ultrasound-guided fine needle aspiration biopsy

e15680 Background: Radiological examinations including computed tomography (CT) and endoscopic ultrasound sonography (EUS) are important for the diagnosis of pancreatic neuroendocrine tumors (PNETs). Pathological diagnosis is not needed with functional PNETs because the diagnosis is made by biochemical testing. Therefore, pathological diagnosis is essential for the non-functional PNETs (nf-PNETs). In this study, we examined the diagnostic yield of radiological examinations and endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) for nf-PNETs. Methods: From January 1997 to December 2007, consecutive 38 cases were pathologically diagnosed from as nf-PNETs at Aichi Cancer Center Hospital. Of these 33 patients was diagnosed with surgical specimen, 5 patients with EUS-FNA specimen. Of 33 patients, 16 patients underwent EUS-FNA preoperatively, and were examined further: their EUS-FNA specimens were submitted for additional immunohistochemical examination for CD 56, chromogranin A, synaptophysin, somatostatin receptor 2A (SSTR2A) and Ki-67 using cell block method. Staging of PNETs was performed according to WHO classification. Results: 16 patients (10 men and 6 women) who ranged in age from 23 to 81 years. The mean of tumor size was 2.2 cm and ranged from 0.7 to 4.4 cm. Diagnosis by CT and EUS was 12 (75.0%) PNETs, two pancreatic cancer, one solid papillary tumors and one malignant lymphoma. Accurate diagnosis by cytology and immunohistochemistry was 75.0% and 93.8%, respectively. The Ki-67 in specimens of malignancy tended to be higher than those of benign or uncertain behavior. If we assumed that a tumor with Ki-67 of more than 2% was malignancy, the diagnostic accuracy was 75.0%. Positive reaction to SSTR2A was up to 14 out of 16 nf-PNETs. Conclusions: The EUS-FNA is a useful tool for diagnosing nf-PNETs with immunohistochemical staining compared to cytology and radiological examinations. Ki-67 may be also effective to predict the degree of malignancy. No significant financial relationships to disclose.

Effective Combined Medical Treatment With Octreotide Long Acting Release (Sandostatin LAR) and Cabergoline for an Extremely Rare Pituitary Somatotroph Adenoma Producing Growth Hormone-Releasing Hormone

Combined medical treatment with long acting octreotide and cabergoline is now used with active patients with acromegaly and can reduce serum growth hormone (GH) and insulin-like growth factor-1 levels. In this article, we analyzed again the molecular aspects of the previously reported rare GH-releasing hormone (GHRH)-producing somatotroph adenoma, and introduce the effects of the combined medical treatment using octreotide LAR and cabergoline for GHRH-producing somatotroph adenoma. The present GHRH-producing somatotroph adenoma had a high Ki-67 staining index, weak, and cytoplasmic-dominant immunostaining of somatostatin receptor 2A, and no gsp mutation. This adenoma was predicted to be resistant to the medical therapy using octreotide LAR. In this extremely rare GHRH-producing pituitary somatotroph adenoma, octreotide LAR alone cannot reduce random GH and insulin-like growth factor-1 levels sufficiently, but combined medical therapy with octreotide LAR and cabergoline can reduce them into the normal range. We show that combined medical treatment with octreotide LAR and cabergoline was effective for a somatotroph adenoma that had a high proliferative potential, weak and cytoplasmic-dominant immunostaining of somatostatin receptor 2A, and no gsp mutation, and has active GH production and secretion regulated by locally generated GHRH from adenoma cell.

The expression of somatostatin receptors in the hippocampus of pilocarpine-induced rat epilepsy model

During the course of this study, we sought examine whether the expression of somatostatin receptors (SSTRs) is altered in the hippocampus following pilocarpine-induced status epilepticus (SE) in order to understand the role/function of SSTRs in the hippocampus after epileptogenic insults. SSTR1 and SSTR4 immunoreactivities were increased in the hippocampus at 1 week after SE. At 4 weeks after SE, SRIF1-family (SSTR 2A, SSTR2B, and SSTR5) immunoreactivity was increased only in neuropil. Both SSTR2A and 2B immunoreactivities were increased in CA2-3 pyramidal cells. However, SSTR3 and SSTR4 immunoreactivities were reduced in the CA1 pyramidal cells of epileptic rat due to neuronal loss. In addition, SSTR5 immunoreactivity was reduced in CA2 pyramidal cells and various interneurons. Both SSTR2B and SSTR4 immunoreactivities were increased within microglia following SE. Our findings suggest that increases in neuron-glial SSTR expressions may be closely related to the enhanced inhibition of the dentate gyrus and regulation of reactive microgliosis in the hippocampus of a pilocarpine model of temporal lobe epilepsy.

Selective Loss of Somatostatin Receptor 2 in Octreotide-Resistant Growth Hormone-Secreting Adenomas

The somatostatin analog octreotide preferentially binds to somatostatin receptor (sst) 2A and to a lesser extent to sst5. Although sst2A and sst5 mRNAs are consistently expressed in GH-secreting adenomas, octreotide controls GH secretion only in 65% of acromegalic patients. Hence, we investigated the immunocytochemical expression of sst in a large group of somatotroph tumors. Acromegalic patients, cared for in a university referral center, were either operated on without pretreatment (group A, n = 14) or pretreated with octreotide [median (minimum-maximum): dose 1250 (300-1500) mug/d for 5.6 (3-9) months] before surgery (group B, n = 20). In group B octreotide reduced GH secretion by more than 50% in 14 patients (70%) (GH responders). Six patients with less than 50% GH suppression were considered GH nonresponders. We used a panel of extensively characterized antibodies to determine the immunocytochemical sst status in somatotroph adenomas and compared their expression between the groups. All group A tumors demonstrated immunoreactive sst2A, and all but one had sst5. A similar pattern was found in the GH responders of group B. In contrast, none of the GH nonresponders exhibited detectable sst2A (sst2A: GH responders vs. GH nonresponders, P < 0.0001), whereas sst5 was found in 70%. sst1 and sst3 were detected in 85 and 24% of all cases, independent of previous octreotide treatment. Our findings suggest that octreotide resistance in GH-secreting adenomas occurs due to a selective loss of sst2A. The persistent expression of sst1 and sst5 receptors suggests that these tumors are potential targets for pan-somatostatin analogs.

Immunohistochemical Detection of Somatostatin Receptor (SSTR) Subtypes 2A and 5 in Pituitary Adenoma from Acromegalic Patients: Good Correlation with Preoperative Response to Octreotide

The aim of this study was to determine the correlation between the expression of somatostatin receptors by immunohistochemistry and the percent suppression of GH levels in the octreotide suppression test. Twenty-two patients with acromegaly who underwent an octreotide suppression test before surgery were studied. We performed immunohistochemistry for Somatostatin receptor 2A (SSTR2A) and Somatostatin receptor 5 (SSTR5) on the surgical specimens from all patients, which we scored according to the number of tumor cells staining positive at the surface membrane (3+: >50%, 2+: 25-50%, 1+: <25%). We sought correlations of percent suppression in the octreotide suppression test with these immunohistochemistry scores. Somatostatin receptor 2A (SSTR2A) showed the highest frequency of score 3+ (13 of 22, 59.1%) by immunohistochemistry. Subtype 5 showed the highest frequency for score 2+ (9 of 22, 40.9%), and one (4.5%) was immunonegative. For subtype 2A, there was a significant correlation with percent decrease (P = 0.002 < 0.01). In contrast, there was no significant correlation for SSTR5. Immunohistochemistry for SSTR2A in pathology specimens from acromegalic patients enabled selection of those experiencing clinical benefit from octreotide. Therefore, performing immunohistochemistry for detection of SSTR2A is recommended for all specimens obtained by surgery.

Indium-111 Pentetreotide Uptake By Pancreatic Polypeptide Cell Hyperplasia

A 71-year-old man with duodenal gastrin cell tumor was being evaluated for residual/metastatic disease. Somatostatin receptor scintigraphy (SRS) identified a 2-cm area of focal uptake within the head of the pancreas, consistent with a pancreatic neuroendocrine tumor. Pathological examination did not reveal any malignancy within the pancreas. Instead, the pancreatic head showed pancreatic polypeptide cell hyperplasia. Strong and diffuse immunoreactivity to somatostatin receptor 2A antibody by immunoperoxidase staining confirmed that the lesion correlated with the site of radioactive tracer (Indium-111 pentetreotide) uptake seen on SRS. The current report therefore presents pancreatic polypeptide cell hyperplasia as a new pitfall in SRS.

Thyrotropin-producing pituitary adenoma associated with Graves' disease

The examination of potential associations between Graves' disease and thyrotropin-producing pituitary adenoma (TSHoma) after treatment using octreotide, and of the expression of peroxisome proliferator-activated receptor gamma (PPAR gamma). A specimen of resected TSHoma tissue from our case was immunohistochemically examined for expression of somatostatin receptor 2A (SSTR2A) and PPAR gamma. Specimens of thyroid tissue from two cases with Hashimoto's thyroiditis were immunohistochemically examined for expression of SSTR2A. Expression of SSTR2A and PPAR gamma was identified in TSHoma cells. SSTR2A was also expressed in lymphocytes that had infiltrated thyroid tissue in Hashimoto's thyroiditis. In previous reports, three of four patients with TSHoma displayed Graves' disease after tumor resection, and TSH is also known to play a major role in regulating immunomodulatory gene expression in thyrocytes. Both the immunomodulatory effects of octreotide on intrathyroidal lymphocytes and rapid reductions in TSH may contribute to the onset of Graves' disease. Patients with TSHoma-associated autoimmune thyroiditis should undergo careful follow-up for development of Graves' disease after treatment. Both octreotide and the PPAR gamma receptor-activating ligands, thiazolidinediones, may be effective for patients with TSHoma.

Distribution of and organisation of dorsal horn neuronal cell bodies that possess the muscarinic m2 acetylcholine receptor

Cholinergic systems in the dorsal horn are involved in antinociception but little is known about the organisation of receptors that mediate this process. In this study we examined immunocytochemical properties of dorsal horn neuronal cell bodies that express the m2 muscarinic acetylcholine receptor. Tissue was examined with confocal laser scanning microscopy and quantitative analysis performed. Immunoreactive cells were found throughout the dorsal horn and in lamina X. Quantitative analysis revealed that 22% of neuronal somata in the dorsal horn possess the receptor. The greatest concentration of cells was found in deeper laminae (IV–VI) and around lamina X. A proportion of cholinergic cells (labelled with an antibody against choline acetyltransferase) were immunoreactive for the receptor (approximately, 40% of dorsal horn cells and 44% of lamina X cells). Populations of presumed inhibitory interneurons also displayed immunoreactivity for the receptor. Between 27–34% of cells immunoreactive for GABA, nitric oxide synthase and the somatostatin receptor 2A expressed the receptor but only 8% of parvalbumin-immunoreactive cells displayed receptor immunoreactivity. Cells labelled with neurotensin, which belong to a subgroup of excitatory neurons, displayed no receptor immunoreactivity. A small number neurokinin-1 receptor-immunoreactive cells in lamina I possessed m2 immunoreactivity but 42% of laminae III/IV neurokinin-1 cells possessed it. This study shows that a significant proportion of cell bodies in the dorsal horn express the muscarinic m2 acetylcholine receptor. The receptor is present on some cholinergic neurons and therefore may function as an autoreceptor. It is associated with inhibitory local circuit neurons and may have a role in the modulation of specific inhibitory systems. It is also found on a proportion of projection cells that possess the neurokinin-1 receptor. This could be the basis of some of the antinociceptive actions of acetylcholine.

Homologous and heterologous regulation of somatostatin receptor 2.

We previously demonstrated that phosphorylation of somatostatin receptor 2A (sst2A) is rapidly increased in transfected cells both by agonist and by the protein kinase C (PKC) activator phorbol myristate acetate (PMA). Here, we investigate whether PKC-mediated receptor phosphorylation is involved in the homologous or heterologous regulation of endogenous sst2 receptors in AR42J pancreatic acinar cells upon stimulation by agonist or by cholecystokinin (CCK) or bombesin (BBS). Somatostatin, PMA, CCK, and BBS all increased sst2A receptor phosphorylation 5- to 10-fold within minutes. Somatostatin binding also caused rapid internalization of the ligand-receptor complex, and PMA, CCK, and BBS all stimulated this internalization further. Additionally, sst2 receptor-mediated inhibition of adenylyl cyclase was desensitized by all treatments. Somatostatin, as well as peptidic (SMS201-995) and nonpeptidic (L-779,976) sst2 receptor agonists increased the EC(50) for somatostatin inhibition 20-fold. In contrast, pretreatment with BBS, CCK, or PMA caused a modest 2-fold increase in the EC(50) for cyclase inhibition. Whereas the PKC inhibitor GF109203X abolished sst2A receptor phosphorylation by CCK, BBS, and PMA, it did not alter the effect of somatostatin, demonstrating that these reactions were catalyzed by different kinases. Consistent with a functional role for PKC-mediated receptor phosphorylation, GF109203X prevented PMA stimulation of sst2 receptor internalization. Surprisingly, however, GF109203X did not inhibit BBS and CCK stimulation of sst2A receptor endocytosis. These results demonstrate that homologous and heterologous hormones induce sst2A receptor phosphorylation by PKC-independent and -dependent mechanisms, respectively, and produce distinct effects on receptor signaling and internalization. In addition, the heterologous hormones also modulate sst2 receptor internalization by a novel mechanism that is independent of receptor phosphorylation.

β-Arrestin is involved in the desensitization but not in the internalization of the somatostatin receptor 2A expressed in CHO cells

The interaction of β-arrestin-1 with the somatostatin receptor type 2A (sst2A) was monitored using both biochemical and confocal imaging approaches. We show that, using transient transfection of either β-arrestin-1 or its dominant negative Δ-arrestin-1 in CHO cells stably transfected with the sst2A, β-arrestin-1 is colocalized with the receptor in endosomal vesicles after somatostatin-induced sequestration. However, this interaction leads to a role of β-arrestin-1 in the desensitization of the sst2A rather than in the internalization process of the receptor–ligand complex.

Evidence that somatostatin receptor 2A (SSTR2A) mediates multiple actions of somatostatin in the gastrointestinal tract

Evidence that somatostatin receptor 2A (SSTR2A) mediates multiple actions of somatostatin in the gastrointestinal tract