Pituitary somatostatin receptor signaling

An immunohistochemical study of somatostatin in the stomach and the small intestine of the African ostrich (Struthio camelus)

The aim of the present study was to clarify the distribution and relative frequencies of somatostatin (SST)-producing cells in the stomach and the small intestine of the ostrich by using immunohistochemistry. The results indicated that somatostatin-immunoreactive (SST-IR) cells were distributed in mucosal layers of the proventriculus, duodenum, jejunum and ileum. However, no immunoreactivity was observed in the gizzard. SST-IR cells were found at the lower part of glandular lobule in the proventriculus, which were oval and round generally. SST-IR cells were present in the mucous membrane of entire small intestine of the ostrich. SST-IR cells had round and spherical shapes (closed-type cells), or spindle and pyriform shapes (open-type cells) in the small intestine. SST-positive cells were localized preferentially in the proventriculus of the 60-day-old ostrich. These results indicated that SST might be involved in functional and developmental regulation of gastrointestinal tract of the ostrich.

Constitutive somatostatin receptor subtype 2 activity attenuates GH synthesis

Somatostatin signals predominantly through somatostatin receptor (SSTR) subtype 2 to attenuate GH release. However, the independent role of the receptor in regulating GH synthesis is unclear. Because we had previously demonstrated constitutive SSTR2 activity in mouse corticotrophs, we now analyzed GH regulation in rat pituitary somatotroph (GC) tumor cells, which express SSTR2 exclusively and are devoid of endogenous somatostatin ligand. We demonstrate that moderately stable SSTR2 overexpression (GpSSTR2(WT) cells) was associated with decreased GH promoter activity, GH mRNA, and hormone levels compared with those of control transfectants (GpCon cells). In contrast, levels of GH mRNA and peptide and GH promoter activity were unchanged in GpSSTR2(DRY) stable transfectants moderately expressing DRY motif mutated SSTR2 (R140A). GpSSTR(2DRY) did not exhibit an enhanced octreotide response as did GpSSTR2(WT) cells; however, both SSTR2(WT)-enhanced yellow fluorescent protein (eYFP) and SSTR2(DRY)-eYFP internalized on octreotide treatment. Suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, increased GH synthesis in wild-type GC cells and primary pituitary cultures. GpSSTR2(WT) cells induced GH synthesis more strongly on SAHA treatment, evident by both higher GH peptide and mRNA levels compared with the moderate but similar GH increase observed in GpCon and GpSSTR2(DRY) cells. In vivo SAHA also increased GH release from GpSSTR2(WT) but not from control xenografts. Endogenous rat GH promoter chromatin immunoprecipitation showed decreased baseline acetylation of the GH promoter with exacerbated acetylation after SAHA treatment in GpSSTR2(WT) compared with that of either GpSSTR(2DRY) or control cells, the latter 2 transfectants exhibiting similar GH promoter acetylation levels. In conclusion, modestly increased SSTR2 expression constitutively decreases GH synthesis, an effect partially mediated by GH promoter histone deacetylation.

Idiopathic adult growth hormone deficiency

GH secretion is controlled by hypothalamic as well as intrapituitary and peripheral signals, all of which converge upon the somatotroph, resulting in integrated GH synthesis and secretion. Enabling an accurate diagnosis of idiopathic adult GH deficiency (IAGHD) is challenged by the pulsatility of GH secretion, provocative test result variability, and suboptimal GH assay standardization. The spectrum between attenuated GH secretion associated with the normal aging process and with obesity and truly well-defined IAGHD is not distinct and may mislead the diagnosis. Adult-onset GHD is mainly caused by an acquired pituitary deficiency, commonly including prior head/neck irradiation, or an expanding pituitary mass causing functional somatotroph compression. To what extent rare cryptic causes account for those patients seemingly classified as IAGHD is unclear. About 15% of patients with adult GHD and receiving GH replacement in open-label surveillance studies are reported as being due to an idiopathic cause. These patients may also reflect a pool of subjects with an as yet to be determined occult defect, or those with unclear or incomplete medical histories (including forgotten past sports head injury or motor vehicle accident). Therefore, submaximal diagnostic evaluation likely leads to an inadvertent diagnosis of IAGHD. In these latter cases, adherence to rigorous biochemical diagnostic criteria and etiology exclusion may result in reclassification of a subset of these patients to a distinct known acquired etiology, or as GH-replete. Accordingly, rigorously verified IAGHD likely comprises less than 10% of adult GHD patients, an already rare disorder. Regardless of etiology, patients with adult GHD, including those with IAGHD, exhibit a well-defined clinical phenotype including increased fat mass, loss of lean muscle mass, decreased bone mass, and enhanced cardiac morbidity. Definition of unique efficacy and dosing parameters for GH replacement and resultant therapeutic efficacy markers in true IAGHD requires prospective study.

Delineation of a region responsible for panhypopituitarism in 20p11.2

We report on the case of a young woman with a de novo 20p11.21p11.23 deletion, discovered by array-CGH. She has behavioral troubles with autistic traits, intellectual disability, panhypopituitarism, severe hypoglycemia, epilepsy, and scoliosis. The majority of the reported 20p deletions are located on the 20p12 region, covering the JAG1 gene responsible for the Alagille syndrome. More proximal deletions are even rarer, with very few cases described in the literature to date. The deletion carried by our patient is, to our knowledge, the smallest described de novo proximal 20p11.2 deletion. It was first discovered by 0.5 Mb BAC array-CGH, further delineated using an oligonucleotide array, and finally confirmed by fluorescence in situ hybridization. The deletion is 4.22 Mb in size, with the exact location on chr20: 19.810.034-24.031.344 (Feb. 2009, GRCh37/hg19). In light of the other reported cases that display genomic and phenotypic overlap with our patient, we discuss the phenotype of our patient, in order to further delineate the 20p proximal deletion phenotype. We propose a minimal critical region responsible for panhypopituitarism with global developmental delay, intellectual disability, scoliosis and facial dysmorphism. Moreover, considering the deleted genes, we highlight the impact of the deletion of this minimal critical region on the Shh signaling pathway.

Novel pathway for somatostatin analogs in patients with acromegaly

Acromegaly is a chronic disease with increased morbidity and mortality, where usually multiple treatment modalities are used. The somatostatin analogs (SSAs) are the mainstay of medical therapy but, in many patients, including those with a germline mutation in the aryl hydrocarbon receptor-interacting protein (AIP) gene, disease activity cannot be controlled with these drugs. Previous data have suggested the involvement of the tumor-suppressor gene ZAC1 in the mechanism of action of SSAs, and more recent findings suggested that SSAs could regulate AIP, which in turn can stimulate ZAC1, therefore suggesting the existence of a SSA-AIP-ZAC1-somatostatin effect pathway. The current review discusses these novel observations, highlighting their significance in the treatment of sporadic and familial somatotroph adenomas.

Reevaluation of sst₁ somatostatin receptor expression in human normal and neoplastic tissues using the novel rabbit monoclonal antibody UMB-7

BACKGROUND

The somatostatin receptor 1 (sst1) is widely distributed throughout the body and is also present in neoplastic tissues. However, little is known about its precise tissue distribution, regulation and function, which may in part be due to the lack of specific monoclonal anti-sst1 antibodies.

METHODS

We have characterized the novel rabbit monoclonal anti-human sst1 antibody UMB-7 using sst1-expressing cells and human pituitary samples. The antibody was then used for immunohistochemical staining of a large panel of formalin-fixed, paraffin-embedded human tissues.

RESULTS

Western blot analyses of BON-1 cells and human pituitary revealed a broad band migrating at a molecular weight of 45,000-60,000. After enzymatic deglycosylation the size of this band decreased to a molecular weight of 45,000. UMB-7 yielded an efficient immunostaining of distinct cell populations in the human tissue samples with a predominance of plasma membrane staining, which was completely abolished by preadsorption of UMB-7 with its immunizing peptide. The sst1 receptor was detected in anterior pituitary, pancreatic islets, distal tubules, enteric ganglion cells and nerve fibers, chief cells of the gastric mucosa, macrophages and mast cells. In addition, sst1 was observed in pituitary adenomas, gastrointestinal neuroendocrine tumors and pheochromocytoma as well as in pancreatic adenocarcinomas, gastric carcinomas, urinary bladder carcinomas and sarcomas.

CONCLUSIONS

UMB-7 may prove of great value in the identification of sst1-expressing tumors during routine histopathological examinations. This may open up new routes for diagnostic and therapeutic intervention.

Somatostatin receptor pathophysiology in the neuroendocrine system

The actions of somatostatin (SRIF) are mediated by specific G protein-coupled receptors, named SRIF receptor (SSTR) subtypes 1, 2, 3 and 5. SRIF binding to SSTR activates a series of second messenger systems, resulting in the inhibition of calcium channels and adenylate cyclase activity, ultimately leading to inhibition of hormone secretion, while stimulation of other second messengers, such as phosphotyrosine phosphatases play a role in the control of cell growth. The SSTR and dopamine receptor families share a 30% sequence homology and appear to be structurally related. The knowledge on the pathophysiology of these two families of G protein-coupled receptors in neuroendocrine tumors has progressively increased due to the new insights in receptor dimerization, internalization and trafficking. Depending on the expression of different SSTRs in tissues, their combinations and interactions affect the functionality of the subtypes expressed and the influence of the microenvironment, the response to ligands and, by consequence, the response to treatment can be very different.

Phosphorylation of threonine 333 regulates trafficking of the human sst5 somatostatin receptor

The frequent overexpression of the somatostatin receptors sst2 and sst5 in neuroendocrine tumors provides the molecular basis for therapeutic application of novel multireceptor somatostatin analogs. Although the phosphorylation of the carboxyl-terminal region of the sst2 receptor has been studied in detail, little is known about the agonist-induced regulation of the human sst5 receptor. Here, we have generated phosphosite-specific antibodies for the carboxyl-terminal threonines 333 (T333) and 347 (T347), which enabled us to selectively detect either the T333-phosphorylated or the T347-phosphorylated form of sst5. We show that agonist-mediated phosphorylation occurs at T333, whereas T347 is constitutively phosphorylated in the absence of agonist. We further demonstrate that the multireceptor somatostatin analog pasireotide and the sst5-selective ligand L-817,818 but not octreotide or KE108 were able to promote a detectable T333 phosphorylation. Interestingly, BIM-23268 was the only sst5 agonist that was able to stimulate T333 phosphorylation to the same extent as natural somatostatin. Agonist-induced T333 phosphorylation was dose-dependent and selectively mediated by G protein-coupled receptor kinase 2. Similar to that observed for the sst2 receptor, phosphorylation of sst5 occurred within seconds. However, unlike that seen for the sst2 receptor, dephosphorylation and recycling of sst5 were rapidly completed within minutes. We also identify protein phosphatase 1γ as G protein-coupled receptor phosphatase for the sst5 receptor. Together, we provide direct evidence for agonist-selective phosphorylation of carboxyl-terminal T333. In addition, we identify G protein-coupled receptor kinase 2-mediated phosphorylation and protein phosphatase 1γ-mediated dephosphorylation of T333 as key regulators of rapid internalization and recycling of the human sst5 receptor.

Peptide receptor targeting in cancer: the somatostatin paradigm

Peptide receptors involved in pathophysiological processes represent promising therapeutic targets. Neuropeptide somatostatin (SST) is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors, named sst1–5, while in central nervous system, SST acts as a neurotransmitter/neuromodulator, regulating locomotory and cognitive functions. Critical points of SST/SST receptor biology, such as signaling pathways of individual receptor subtypes, homo- and heterodimerization, trafficking, and cross-talk with growth factor receptors, have been extensively studied, although functions associated with several pathological conditions, including cancer, are still not completely unraveled. Importantly, SST exerts antiproliferative and antiangiogenic effects on cancer cells in vitro, and on experimental tumors in vivo. Moreover, SST agonists are clinically effective as antitumor agents for pituitary adenomas and gastro-pancreatic neuroendocrine tumors. However, SST receptors being expressed by tumor cells of various tumor histotypes, their pharmacological use is potentially extendible to other cancer types, although to date no significant results have been obtained. In this paper the most recent findings on the expression and functional roles of SST and SST receptors in tumor cells are discussed.

Deciphering the specific role of Gαi/o isoforms: functional selective oxytocin ligands and somatostatin SST5 receptor mutants

Receptor coupling to different G-proteins and β-arrestins has been described for a number of GPCRs (G-protein-coupled receptors), suggesting a multi-state model of receptor activation in which each receptor can assume a number of different active conformations, each capable of promoting the coupling to a specific effector. Consistently, functional-selective ligands and biased agonists have been described to be able to induce and/or stabilize only a subset of specific active conformations. Furthermore, GPCR mutants deficient in selective coupling have been reported. Functional selective ligands and receptor mutants thus constitute unique tools to dissect the specific roles of different effectors, in particular among the Gi/o family. In the present mini-review, we focus on (i) the identification of functional selective OXT (oxytocin)-derived peptides capable of activating single Gi/o isoforms, namely Gi1 or Gi3; and (ii) the characterization of an SS (somatostatin) receptor SST5 mutant selectively impaired in its GoA coupling. These analogues and receptor mutants represent unique tools for examining the contribution of Gi/o isoforms in complex biological responses and open the way for the development of drugs with peculiar selectivity profiles.

Comparison of 68Ga-DOTANOC and 68Ga-DOTATATE PET/CT within patients with gastroenteropancreatic neuroendocrine tumors

Somatostatin receptor PET tracers such as [(68)Ga-DOTA,1-Nal(3)]-octreotide ((68)Ga-DOTANOC) and [(68)Ga-DOTA,Tyr(3)]-octreotate ((68)Ga-DOTATATE) have shown promising results in patients with neuroendocrine tumors, with a higher lesion detection rate than is achieved with (18)F-fluorodihydroxyphenyl-l-alanine PET, somatostatin receptor SPECT, CT, or MR imaging. (68)Ga-DOTANOC has high affinity for somatostatin receptor subtypes 2, 3, and 5 (sst2,3,5). It has a wider receptor binding profile than (68)Ga-DOTATATE, which is sst2-selective. The wider receptor binding profile might be advantageous for imaging because neuroendocrine tumors express different subtypes of somatostatin receptors. The goal of this study was to prospectively compare (68)Ga-DOTANOC and (68)Ga-DOTATATE PET/CT in the same patients with gastroenteropancreatic neuroendocrine tumors (GEP-NETs) and to evaluate the clinical impact of (68)Ga-DOTANOC PET/CT.

METHODS

Eighteen patients with biopsy-proven GEP-NETs were evaluated with (68)Ga-DOTANOC and (68)Ga-DOTATATE using a randomized crossover design. Labeling of DOTANOC and DOTATATE with (68)Ga was standardized using a fully automated synthesis device. PET/CT findings were compared with 3-phase CT scans and in some patients with MR imaging, (18)F-FDG PET/CT, and histology. Uptake in organs and tumor lesions was quantified and compared by calculation of maximum standardized uptake values (SUVmax) using volume computer-assisted reading.

RESULTS

Histology revealed low-grade GEP-NETs (G1) in 4 patients, intermediate grade (G2) in 7, and high grade (G3) in 7. (68)Ga-DOTANOC and (68)Ga-DOTATATE were false-negative in only 1 of 18 patients. In total, 248 lesions were confirmed by cross-sectional and PET imaging. The lesion-based sensitivity of (68)Ga-DOTANOC PET was 93.5%, compared with 85.5% for (68)Ga-DOTATATE PET (P = 0.005). The better performance of (68)Ga-DOTANOC PET is attributed mainly to the significantly higher detection rate of liver metastases rather than tumor differentiation grade. Multivariate analysis revealed significantly higher SUVmax in G1 tumors than in G3 tumors (P = 0.009). This finding was less pronounced with (68)Ga-DOTANOC (P > 0.001). Altogether, (68)Ga-DOTANOC changed treatment in 3 of 18 patients (17%).

CONCLUSION

The sst2,3,5-specific radiotracer (68)Ga-DOTANOC detected significantly more lesions than the sst2-specific radiotracer (68)Ga-DOTATATE in our patients with GEP-NETs. The clinical relevance of this finding has to be proven in larger studies.

Treatment with octreotide LAR in clinically non-functioning pituitary adenoma: results from a case-control study

Surgical cure cannot be achieved in most patients with invasive non-functioning pituitary macroadenoma (NFPA). Short-term residual tumor treatment with somatostatin analogs has produced disappointing results. This prospective case-control study assessed the efficacy of chronic treatment with long acting octreotide (octreotide LAR) on tumor volume in patients harboring post-surgical NFPA residue. The study population comprised 39 patients with NFPAs not cured by surgery. All patients underwent somatostatin receptor scintigraphy at least 6 months after the last surgery. Patients with a positive pituitary level octreoscan at (n = 26) received octreotide LAR (20 mg every 28 days) for ≥ 12 months (mean follow-up 37 ± 18 months) (Treated group). Moreover, a fragment of tumor tissue from patients in the treated group was retrospectively collected to assess the immunohistochemical expression of somatostatin receptor subtypes (SSTRs). The patients with a negative octreoscan (n = 13) formed the control group (mean follow-up 37 ± 16 months). Hormonal, radiological and visual field parameters were periodically assessed. In the treated group, all tumors expressed at least one SSTR subtype. The SSTR5 subtype was the most abundant, followed by SSTR3. The tumor residue increased in five of 26 patients (19%) in the treated group and in seven of 13 controls (53%). Visual field and pituitary function did not change in any patient. This study indicates that SSTR5 and SSTR3 are the most frequently expressed SSTR subtypes in NFPAs and supports a potential role of SSTR subtypes in stabilization of tumor remnant from NFPAs.

Specific roles of G(i) protein family members revealed by dissecting SST5 coupling in human pituitary cells

Despite intensive investigation over the past 20 years, the specific role played by individual G(i) protein family members in mediating complex cellular effects is still largely unclear. Therefore, we investigated the role of specific G(i) proteins in mediating somatostatin (SS) effects in somatotroph cells. Because our previous data showed that SS receptor type 5 (SST5) carrying a spontaneous R240W mutation in the third intracellular loop had a similar ability to inhibit intracellular cAMP levels to the wild-type protein but failed to mediate inhibition of growth hormone (GH) release and cell proliferation, we used this model to check specific receptor-G-protein coupling by a bioluminescent resonance energy transfer analysis. In HEK293 cells, wild-type SST5 stimulated the activation of Gα(i1-3) and Gα(oA), B, whereas R240W SST5 maintained the ability to activate Gα(i1-3) and Gα(oB), but failed to activate the splicing variant Gα(oA). To investigate the role of the selective deficit in Gα(oA) coupling, we co-transfected human adenomatous somatotrophs with SST5 and a pertussis toxin (PTX)-resistant Gα(oA) (Gα(oA(PTX-r))) protein. In PTX-treated cells, Gα(oA(PTX-r)) rescued the ability of the selective SST5 analog BIM23206 to inhibit extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation, GH secretion and intracellular cAMP levels. Moreover, we demonstrated that silencing of Gα(oA) completely abolished SST5-mediated inhibitory effects on GH secretion and ERK1/2 phosphorylation, but not on cAMP levels. In conclusion, by analysing the coupling specificity of human SST5 to individual Gα(i) and Gα(o) subunits, we identified a crucial role for Gα(oA) signalling in human pituitary cells.

Resistance to octreotide LAR in acromegalic patients with high SSTR2 expression: analysis of AIP expression

We present here the clinical and molecular data of two patients with acromegaly treated with octreotide LAR after non-curative surgery, and who presented different responses to therapy. Somatostatin receptor type 2 and 5 (SSTR2 and SSTR5), and aryl hydrocarbon receptor-interacting protein (AIP) expression levels were analyzed by qPCR. In both cases, high SSTR2 and low SSTR5 expression levels were detected; however, only one of the patients achieved disease control after octreotide LAR therapy. When we analyzed AIP expression levels of both cases, the patient whose disease was controlled after therapy exhibited AIP expression levels that were two times higher than the patient whose disease was still active. These two cases illustrate that, although the currently available somatostatin analogs bind preferentially to SSTR2, some patients are not responsive to therapy despite high expression of this receptor. This difference could be explained by differences in post-receptor signaling pathways, including the recently described involvement of AIP. Arq Bras Endocrinol Metab. 2012;56(8):501-6

SSTR2 is the functionally dominant somatostatin receptor in human pancreatic β- and α-cells

Somatostatin-14 (SST) inhibits insulin and glucagon secretion by activating G protein-coupled somatostatin receptors (SSTRs), of which five isoforms exist (SSTR1-5). In mice, the effects on pancreatic β-cells are mediated by SSTR5, whereas α-cells express SSTR2. In both cell types, SSTR activation results in membrane hyperpolarization and suppression of exocytosis. Here, we examined the mechanisms by which SST inhibits secretion from human β- and α-cells and the SSTR isoforms mediating these effects. Quantitative PCR revealed high expression of SSTR2, with lower levels of SSTR1, SSTR3, and SSTR5, in human islets. Immunohistochemistry showed expression of SSTR2 in both β- and α-cells. SST application hyperpolarized human β-cells and inhibited action potential firing. The membrane hyperpolarization was unaffected by tolbutamide but antagonized by tertiapin-Q, a blocker of G protein-gated inwardly rectifying K⁺ channels (GIRK). The effect of SST was mimicked by an SSTR2-selective agonist, whereas a SSTR5 agonist was marginally effective. SST strongly (>70%) reduced depolarization-evoked exocytosis in both β- and α-cells. A slightly weaker inhibition was observed in both cell types after SSTR2 activation. SSTR3- and SSTR1-selective agonists moderately reduced the exocytotic responses in β- and α-cells, respectively, whereas SSTR4- and SSTR5-specific agonists were ineffective. SST also reduced voltage-gated P/Q-type Ca²⁺ currents in β-cells, but normalization of Ca²⁺ influx to control levels by prolonged depolarizations only partially restored exocytosis. We conclude that SST inhibits secretion from both human β- and α-cells by activating GIRK and suppressing electrical activity, reducing P/Q-type Ca²⁺ currents, and directly inhibiting exocytosis. These effects are mediated predominantly by SSTR2 in both cell types.

Somatostatin analogs and chimeric somatostatin-dopamine molecules differentially regulate human growth hormone and prolactin gene expression and secretion in vitro

We tested effects of selective somatostatin receptor 2 (SST2) agonist BIM-23120, SST5 agonist BIM-23206 and chimeric somatostatin-dopamine molecules (SRIF/DA) BIM-23A760 and BIM-23A761 on GH and PRL secretion and gene expression in human GH/PRL-secreting pituitary tumors in vitro. In "responders" group BIM-23120 suppressed GH levels by 26±4%, BIM-23206 by 31±5%, BIM-23A760 by 23±4%, BIM-23A761 by 39±8% and D(2)-dopamine agonist BIM-53097 by 31±5%. Using real-time PCR we demonstrated that GH inhibition was not accompanied by decreased GH mRNA levels. PRL secretion was inhibited by BIM-23A760 (29±5%), BIM-23A761 (34±4%), BIM-23206 (26±4%) and BIM-53097 (36±2%). SRIF/DA and BIM-53097 also suppressed PRL mRNA levels. Concluding, SST2 and SST5 agonists and SRIF/DA inhibit GH secretion, but do not suppress GH gene transcription. SRIF/DA and BIM-53097 inhibit both PRL secretion and PRL gene expression. SST5 agonist inhibits PRL secretion, but does not suppress PRL gene expression. D(2) affinity is crucial in SRIF/DA action on PRL gene expression.

Somatotroph pituitary adenoma with acromegaly and autosomal dominant polycystic kidney disease: SSTR5 polymorphism and PKD1 mutation

A 39-year-old woman with autosomal dominant polycystic kidney disease (ADPKD) presented with acromegaly and a pituitary macroadenoma. There was a family history of this renal disorder. She had undergone surgery for pituitary adenoma 6 years prior. Physical examination disclosed bitemporal hemianopsia and elevation of both basal growth hormone (GH) 106 ng/mL (normal 0-5) and insulin-like growth factor (IGF-1) 811 ng/mL (normal 48-255) blood levels. A magnetic resonance imaging scan disclosed a 3.0 cm sellar and suprasellar mass with both optic chiasm compression and left cavernous sinus invasion. Pathologic, cytogenetic, molecular and in silico analysis was undertaken. Histologic, immunohistochemical and ultrastructural studies of the lesion disclosed a sparsely granulated somatotroph adenoma. Standard chromosome analysis on the blood sample showed no abnormality. Sequence analysis of the coding regions of PKD1 and PKD2 employing DNA from both peripheral leukocytes and the tumor revealed the most common PKD1 mutation, 5014_5015delAG. Analysis of the entire SSTR5 gene disclosed the variant c.142C>A (p.L48M, rs4988483) in the heterozygous state in both blood and tumor, while no pathogenic mutations were noted in the MEN1, AIP, p27Kip1 and SSTR2 genes. To our knowledge, this is the fourth reported case of a GH-producing pituitary adenoma associated with ADPKD, but the first subjected to extensive morphological, ultrastructural, cytogenetic and molecular studies. The physical proximity of the PKD1 and SSTR5 genes on chromosome 16 suggests a causal relationship between ADPKD and somatotroph adenoma.

Somatostatin receptor sst2 gene transfer in human prolactinomas in vitro: impact on sensitivity to dopamine, somatostatin and dopastatin, in the control of prolactin secretion

OBJECTIVE

As prolactinomas fail to respond to dopamine agonist (DA) in 10-20% of cases, we hypothesized that somatostatin subtype 2 receptor (sst2) overexpression in DA-resistant prolactinomas may enhance suppression of prolactine (PRL) using chimeric agonist (dopastatin) that simultaneously binds sst2 and the dopamine subtype 2 receptor (D2DR).

DESIGN AND METHODS

PRL suppression by octreotide, sst5 agonist, sst2-D2DR agonist (BIM-23A760 dopastatin) and cabergoline was assessed in primary cultures of seven DA-resistant prolactinomas overexpressing sst2.

RESULTS

sst2 was effectively overexpressed via adenoviral expression in prolactinomas (38.1±7.4 vs. 0.1±0.1 copy/copy β-Gus) and induced octreotide sst2-mediated PRL suppression that remained lower than that induced by DA. BIM-23A760 inhibited PRL similarly to cabergoline both in the control and sst2-expressing cells. Antagonist experiments confirmed predominant dopaminergic effect in dopastatin activity.

CONCLUSION

sst2 was successfully overexpressed in prolactinomas. However BIM-23A760 was unable to enhance PRL suppression underlining a predominant dopaminergic contribution in its action.

Meta-analysis on the effects of octreotide on tumor mass in acromegaly

BACKGROUND

The long-acting somatostatin analogue octreotide is used either as an adjuvant or primary therapy to lower growth hormone (GH) levels in patients with acromegaly and may also induce pituitary tumor shrinkage.

OBJECTIVE

We performed a meta-analysis to accurately assess the effect of octreotide on pituitary tumor shrinkage.

DATA SOURCES

A computerized Medline and Embase search was undertaken to identify potentially eligible studies.

STUDY ELIGIBILITY CRITERIA

Eligibility criteria included treatment with octreotide, availability of numerical metrics on tumor shrinkage and clear definition of a clinically relevant reduction in tumor size. Primary endpoints included the proportion of patients with tumor shrinkage and mean percentage reduction in tumor volume.

DATA EXTRACTION AND ANALYSIS

The electronic search identified 2202 articles. Of these, 41 studies fulfilling the eligibility criteria were selected for data extraction and analysis. In total, 1685 patients were included, ranging from 6 to 189 patients per trial. For the analysis of the effect of octreotide on pituitary tumor shrinkage a random effect model was used to account for differences in both effect size and sampling error.

RESULTS

Octreotide was shown to induce tumor shrinkage in 53.0% [95% CI: 45.0%-61.0%] of treated patients. In patients treated with the LAR formulation of octreotide, this increased to 66.0%, [95% CI: 57.0%-74.0%). In the nine studies in which tumor shrinkage was quantified, the overall weighted mean percentage reduction in tumor size was 37.4% [95% CI: 22.4%-52.4%], rising to 50.6% [95% CI: 42.7%-58.4%] with octreotide LAR.

LIMITATIONS

Most trials examined were open-label and had no control group.

CONCLUSIONS

Octreotide LAR induces clinically relevant tumor shrinkage in more than half of patients with acromegaly.

Pathophysiology of GPCR Homo- and Heterodimerization: Special Emphasis on Somatostatin Receptors

G-protein coupled receptors (GPCRs) are cell surface proteins responsible for translating >80% of extracellular reception to intracellular signals. The extracellular information in the form of neurotransmitters, peptides, ions, odorants etc is converted to intracellular signals via a wide variety of effector molecules activating distinct downstream signaling pathways. All GPCRs share common structural features including an extracellular N-terminal, seven-transmembrane domains (TMs) linked by extracellular/intracellular loops and the C-terminal tail. Recent studies have shown that most GPCRs function as dimers (homo- and/or heterodimers) or even higher order of oligomers. Protein-protein interaction among GPCRs and other receptor proteins play a critical role in the modulation of receptor pharmacology and functions. Although ~50% of the current drugs available in the market target GPCRs, still many GPCRs remain unexplored as potential therapeutic targets, opening immense possibility to discover the role of GPCRs in pathophysiological conditions. This review explores the existing information and future possibilities of GPCRs as tools in clinical pharmacology and is specifically focused for the role of somatostatin receptors (SSTRs) in pathophysiology of diseases and as the potential candidate for drug discovery.

cAMP pathway and pituitary tumorigenesis

The pituitary is the target of different neurohormones that have a crucial role in the control of cell differentiation, cell proliferation and hormone secretion by recognizing specific receptors belonging to the G Protein-Coupled Receptor super-family (GPCR). Evidence from in vitro studies and naturally occurring human diseases indicate that several endocrine cells, and particularly somatotrophs, recognize cAMP as a growth factor. Accordingly, mutations of the alpha subunit of the stimulatory G protein gene (GNAS) leading to the constitutive activation of adenylyl cyclase (i.e. gsp oncogene) have been recognized in a significant proportion of GH-secreting pituitary adenomas. The role of cAMP in the control of cell proliferation in selected cell types and in particular in somatotroph cells has been further confirmed by identification of genetics defect affecting the regulatory subunit IA of PKA. The role of cAMP in the control of cell proliferation as well as the crosstalk with different intracellular signalling pathways will be discussed.

The new truncated somatostatin receptor variant sst5TMD4 is associated to poor prognosis in breast cancer and increases malignancy in MCF-7 cells

Somatostatin receptors (sst1-5) are present in different types of tumors, where they inhibit key cellular processes such as proliferation and invasion. Although ssts are densely expressed in breast cancer, especially sst2, their role and therapeutic potential remain uncertain. Recently, we identified a new truncated sst5 variant, sst5TMD4, which is related to the abnormal response of certain pituitary tumors to treatment with somatostatin analogs. Here, we investigated the possible role of sst5TMD4 in breast cancer. This study revealed that sst5TMD4 is absent in normal mammary gland, but is abundant in a subset of poorly differentiated human breast tumors, where its expression correlated to that of sst2. Moreover, in the MCF-7 breast cancer model cell, sst5TMD4 expression increased malignancy features such as invasion and proliferation abilities (both in cell cultures and nude mice). This was likely mediated by sst5TMD4-induced increase in phosphorylated extracellular signal-regulated kinases 1 and 2 and p-Akt levels, and cyclin D3 and Arp2/3 complex expression, which also led to mesenchymal-like phenotype. Interestingly, sst5TMD4 interacts physically with sst2 and thereby alters its signaling, enabling disruption of sst2 inhibitory feedback and providing a plausible basis for our findings. These results suggest that sst5TMD4 could be involved in the pathophysiology of certain types of breast tumors.

Isoproterenol induced hypertrophy and associated signaling pathways are modulated by somatostatin in H9c2 cells

BACKGROUND

Somatostatin (SST), a growth hormone inhibitory peptide plays key role in regulation of cell proliferation via modulation of mitogen activated protein kinases (MAPKs) and cell survival pathway. In cardiac physiology, β-Adrenergic receptors (β-ARs) play crucial role in regulation of downstream signaling pathways in receptor specific manner. The aim of the current study was to delineate the mechanistic insight for the role of SST on β-AR mediated signaling which promotes hypertrophy and apoptosis in rat fetal cardiomyocytes (H9c2 cells). Accordingly, SST dependent changes in signaling molecules including second messenger cAMP, PKA/CREB as well as MAPKs including ERK and p38 which are key mediators of hypertrophy and apoptosis were analyzed.

METHODS AND RESULTS

In the present study, we determined receptor specific effects on intracellular cAMP levels, signaling by western blot analysis and apoptosis by using JC-1 and Hoechst-33258 staining. Here, we present the data which indicates that SST inhibits isoproterenol induced hypertrophy and apoptosis in H9c2 cells. Importantly, SST inhibits β-ARs agonist induced cAMP activation and SST mediated inhibition of cAMP was enhanced in presence of β-ARs antagonist. SST enhances β2AR agonist formoterol mediated effects on PKA, CREB and ERK1/2 phosphorylations whereas it inhibits isoproterenol mediated ERK1/2 and p38 signaling in concentration dependent manner.

CONCLUSIONS

Taken together, these results presented here provide a novel insight for the potential role of SST in regulation of β-AR mediated effects on hypertrophy and modulation of hypertrophy promoting signaling in H9c2 cells.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Somatostatin dramatically stimulates growth hormone release from primate somatotrophs acting at low doses via somatostatin receptor 5 and cyclic AMP

Somatostatin and cortistatin have been shown to act directly on pituitary somatotrophs to inhibit growth hormone (GH) release. However, previous results from nonprimate species indicate that these peptides can also directly stimulate GH secretion, at low concentrations. The relevance of this phenomenon in a nonhuman primate model was investigated in the present study by testing the impact of somatostatin/cortistatin on GH release in primary pituitary cell cultures from baboons. High doses (> 10(-10) m) of somatostatin/cortistatin did not alter basal GH secretion but blocked GH-releasing hormone (GHRH)- and ghrelin-induced GH release. However, at low concentrations (10(-17)-10(-13) m), somatostatin/cortistatin dramatically stimulated GH release to levels comparable to those evoked by GHRH or ghrelin. Use of somatostatin receptor (sst) specific agonists/antagonists, and signal transduction blockers indicated that sst2 and sst1 activation via intact adenylate cylcase and mitogen-activated protein kinase systems mediated the inhibitory actions of high-concentration somatostatin. By contrast, the stimulatory actions of low-dose somatostatin on GH release were mediated by sst5 signalling through adenylate cylcase/cAMP/protein kinase A and intracellular Ca(2+) pathways, and were additive with ghrelin (not GHRH). Notably, low-concentrations of somatostatin, similar to sst5-agonists, inhibited prolactin release. These results clearly demonstrate that the ultimate impact of somatostatin/cortistatin on hormone release is dose-dependent, cell type-selective and receptor-specific, where the stimulatory effects of low-concentration somatostatin/cortistatin on GH release extend to primates, thereby supporting the notion that this action is relevant in regulating GH secretion in humans.

Molecular mechanisms of somatostatin receptor trafficking

The neuropeptide somatostatin (SRIF) is an important modulator of neurotransmission in the central nervous system and acts as a potent inhibitor of hormone and exocrine secretion. In addition, SRIF regulates cell proliferation in normal and tumorous tissues. The six somatostatin receptor subtypes (sst1, sst2A, sst2B, sst3, sst4, and sst5), which belong to the G protein-coupled receptor (GPCR) family, share a common molecular topology: a hydrophobic core of seven transmembrane-spanning α-helices, three intracellular loops, three extracellular loops, an amino-terminus outside the cell, and a carboxyl-terminus inside the cell. For most of the GPCRs, intracytosolic sequences, and more particularly the C-terminus, are believed to interact with proteins that are mandatory for either exporting neosynthesized receptor, anchoring receptor at the plasma membrane, internalization, recycling, or degradation after ligand binding. Accordingly, most of the SRIF receptors can traffic not only in vitro within different cell types but also in vivo. A picture of the pathways and proteins involved in these processes is beginning to emerge.

Lanreotide depot deep subcutaneous injection: a new method of delivery and its associated benefits

Acromegaly is a rare disease characterized by excessive growth hormone secretion, usually from a pituitary tumor. Treatment options include surgery, medical therapy, and in some cases, radiation therapy. Current medical therapy consists of treatment with somatostatin analog medications or a growth hormone receptor antagonist. There are two somatostatin analogs currently in use, octreotide and lanreotide. Both are supplied in long-acting formulations and are of comparable biochemical efficacy. Lanreotide is supplied in a prefilled syringe and is injected into deep subcutaneous tissue. Studies have been conducted to assess the efficacy of self- or partner administration, and have demonstrated that injection of lanreotide can be accomplished reliably and safely outside a physician's office. For patients who have achieved biochemical control with lanreotide, the FDA has recently approved an extended dosing interval. Selected patients may be able to receive the medication less frequently with injections of 120 mg administered every 6 or 8 weeks. This review focuses on the use of lanreotide in the treatment of acromegaly, the safety and efficacy of the drug, and the benefits afforded to patients because of unique aspects of the delivery of lanreotide.

Reassessment of sst3 somatostatin receptor expression in human normal and neoplastic tissues using the novel rabbit monoclonal antibody UMB-5

BACKGROUND

Among the five somatostatin receptors (sst(1)-sst(5)), the sst(3) receptor displays a distinct pharmacological profile. Like sst(2), the sst(3) receptor efficiently internalizes radiolabeled somatostatin analogs. Unlike sst(2), however, internalized sst(3) receptors are rapidly transferred to lysosomes for degradation. Apart from this, very little is known about the clinical relevance of the sst(3) receptor, which may in part be due to the lack of specific monoclonal sst(3) antibodies.

METHODS

Here, we have extensively characterized the novel rabbit monoclonal anti-human sst(3) antibody UMB-5 using transfected cells and receptor-expressing tissues. UMB-5 was then subjected to immunohistochemical staining of a series of 190 formalin-fixed, paraffin-embedded normal and neoplastic human tissues.

RESULTS

Specificity of UMB-5 was demonstrated by detection of a broad band migrating at a molecular weight of 70,000-85,000 in immunoblots from human pituitary. After enzymatic deglycosylation, the size of this band decreased to a molecular weight of 45,000. Tissue immunostaining was completely abolished by pre-adsorption of UMB-5 with its immunizing peptide. In addition, UMB-5 detected distinct cell populations in human tissues like pancreatic islands, anterior pituitary, adrenal cortex, adrenal medulla, and enteric ganglia, similar to that seen with a rabbit polyclonal antibody generated against a different carboxyl-terminal epitope of the sst(3) receptor. In a comparative immunohistochemical study, UMB-5 yielded predominant plasma membrane staining in the majority of pituitary adenomas, pheochromocytomas, and a subset of neuroendocrine tumors. The sst(3) receptor was also present in many glioblastomas, pancreatic, breast, cervix, and ovarian carcinomas.

CONCLUSION

The rabbit monoclonal antibody UMB-5 may prove of great value in the identification of sst(3)-expressing tumors during routine histopathological examinations. Given its unique trafficking properties, these tumors may be potential candidates for sst(3)-directed receptor radiotherapy.

Homologous and heterologous in vitro regulation of pituitary receptors for somatostatin, growth hormone (GH)-releasing hormone, and ghrelin in a nonhuman primate (Papio anubis)

Secretion of GH by pituitary somatotrophs is primarily stimulated by GHRH and ghrelin and inhibited by somatostatin through the activation of specific receptors [GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R) and somatostatin receptors (sst1-5), respectively]. However, we have shown that somatostatin, at low doses, can also stimulate GH release, directly and specifically, in primary pituitary cultures from a nonhuman primate (baboons, Papio anubis) and pigs. To determine whether somatostatin, GHRH, and ghrelin can also regulate the expression of their receptors in primates, pituitary cultures from baboons were treated for 4 h with GHRH or ghrelin (10(-8) m) or with high (10(-7) m) and low (10(-15) m) doses of somatostatin, and GH release and expression levels of all receptors were measured. GHRH/ghrelin decreased the expression of their respective receptors (GHRH-R and GHS-R). Both peptides increased sst1, only GHRH decreased sst5 expression, whereas sst2 expression remained unchanged. The effects of GHRH/ghrelin were completely mimicked by forskolin (adenylate cyclase activator) and phorbol 12-myristate 13-acetate (protein kinase C activator), respectively, indicating the regulation of receptor subtype levels by GHRH and ghrelin involved distinct signaling pathways. In contrast, high-dose somatostatin did not alter GH release but increased sst1, sst2, and sst5 expression, whereas GHRH-R and GHS-R expression were unaffected. Interestingly, low-dose somatostatin increased GH release and sst1 mRNA but decreased sst5 and GHRH-R expression, similar to that observed for GHRH. Altogether, our data show for the first time in a primate model that the primary regulators of somatotroph function (GHRH/ghrelin/somatostatin) exert both homologous and heterologous regulation of receptor synthesis which is dose and subtype dependent and involves distinct signaling pathways.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

The role of somatostatin and dopamine D2 receptors in endocrine tumors

Somatostatin (SS) and dopamine (DA) receptors have been highlighted as two critical regulators in the negative control of hormonal secretion in a wide group of human endocrine tumors. Both families of receptors belong to the superfamily of G protein-coupled receptors and share a number of structural and functional characteristics. Because of the generally reported high expression of somatostatin receptors (SSTRs) in neuroendocrine tumors (NET), somatostatin analogs (SSA) have a pronounced role in the medical therapy for this class of tumors, especially pituitary adenomas and well-differentiated gastroenteropancreatic NET (GEP NET). Moreover, NET express not only SSTR but also frequently dopamine receptors (DRs), and DA agonists targeting the D(2) receptor (D(2)) have been demonstrated to be effective in controlling hormone secretion and cell proliferation in in vivo and in vitro studies. The treatment with SSAs combined with DA agonists has already been demonstrated efficacious in a subgroup of patients with GH-secreting pituitary adenomas and few reported cases of carcinoids. The recent availability of new selective and universal SSA and DA agonists, as well as the chimeric SS/DA compounds, may shed new light on the potential role of SSTR and D(2) as combined targets for biotherapy in NET. This review provides an overview of the latest studies evaluating the expression of SSTR and DR in NET, focusing on their co-expression and the possible clinical implications of such co-expression. Moreover, the most recent insights in SSTR and D(2) pathophysiology and the future perspectives for treatment with SSA, DA agonists, and SS/DA chimeric compounds are discussed.

Physiopathology of somatolactotroph cells: from transduction mechanisms to cotargeting therapy

In pituitary somatolactotroph cells, G protein-coupled receptors and receptor tyrosine kinases binding their specific ligands trigger an enzymatic cascade that converges to MAP kinase activation in the subcellular compartment. Different signaling pathways, such as AC/cAMP/PKA and PI3K/Akt pathways, interact with MAP kinase to regulate key physiological functions, such as hormonal secretion and cell proliferation. Abnormalities affecting these signaling pathways have been identified as preponderant factors of pituitary tumorigenesis. In addition to trans-sphenoidal surgery, somatostatin analogs are used to control hormonal hypersecretion in GH-secreting adenomas. However, a subset of these tumors remains uncontrolled with these treatFments, calling for new therapeutic approaches. In these cases, novel multivalent somatostatin analogs or new somatostatin-dopamine chimeric molecules could be of interest. Another attractive therapeutic approach may be to use one or several inhibitors acting downstream in the signaling pathway, such as mammalian target of rapamycin inhibitor. Cotargeting therapy and gene therapy are promising tools for these problematic pituitary tumors.

Medical therapy in acromegaly

Acromegaly is a rare disease characterized by excess secretion of growth hormone (GH) and increased circulating insulin-like growth factor 1 (IGF-1) concentrations. The disease is associated with increased morbidity and premature mortality, but these effects can be reduced if GH levels are decreased to <2.5 μg/l and IGF-1 levels are normalized. Therapy for acromegaly is targeted at decreasing GH and IGF-1 levels, ameliorating patients' symptoms and decreasing any local compressive effects of the pituitary adenoma. The therapeutic options for acromegaly include surgery, radiotherapy and medical therapies, such as dopamine agonists, somatostatin receptor ligands and the GH receptor antagonist pegvisomant. Medical therapy is currently most widely used as secondary treatment for persistent or recurrent acromegaly following noncurative surgery, although it is increasingly used as primary therapy. This Review provides an overview of current and future pharmacological therapies for patients with acromegaly.

Pathogenesis of pituitary tumors

Pituitary adenomas may hypersecrete hormones (including prolactin, growth hormone and adrenocorticotropic hormone, and rarely follicle-stimulating hormone, luteinizing hormone or TSH) or may be nonfunctional. Despite their high prevalence in the general population, these tumors are invariably benign and exhibit features of differentiated pituitary cell function as well as premature proliferative arrest. Pathogenesis of dysregulated pituitary cell proliferation and unrestrained hormone hypersecretion may be mediated by hypothalamic, intrapituitary and/or peripheral factors. Altered expression of pituitary cell cycle genes, activation of pituitary selective oncoproteins or loss of pituitary suppressor factors may be associated with aberrant growth factor signaling. Considerable information on the etiology of these tumors has been derived from transgenic animal models, which may not accurately and universally reflect human tumor pathophysiology. Understanding subcellular mechanisms that underlie pituitary tumorigenesis will enable development of tumor aggression markers as well as novel targeted therapies.

Constitutive activity of somatostatin receptor subtypes

The five somatostatin receptors (SSTR1-5) are G-protein-coupled receptors, coupling to G(αi/0) subunits to regulate pathways including inhibiting adenylate cyclase activity and reduce intracellular cAMP levels and decrease intracellular calcium levels. In the pituitary gland, somatostatin actions, mediated through SSTR1, 2, 3, and 5, are inhibition of growth hormone, thyrotropin hormone, and adrenocorticotropin hormone release and to a lesser extent, inhibition of cell growth. Establishment of constitutive SSTRs action suggests that abundant pituitary SSTR expression contributes to pituitary function in maintaining homeostasis, aside from the SSTR response to episodic hypothalamic somatostatin release. In this chapter, we describe an experimental approach to directly and indirectly demonstrate constitutive SSTR activity by altering receptor density in AtT20 mouse pituitary corticotroph tumor cells, utilizing small interference RNA to knock receptor expression down or stable SSTRs transfection to overexpress selective receptor levels. We describe methodical validation for each of the approaches and the use of a sensitive cAMP assay to analyze consequences of changing membrane receptor number in the absence of an added ligand.

Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/fasting) control of growth hormone axis in mice

Somatostatin (SST) inhibits growth hormone (GH) secretion and regulates multiple processes by signaling through its receptors sst1-5. Differential expression of SST/ssts may contribute to sex-specific GH pattern and fasting-induced GH rise. To further delineate the tissue-specific roles of SST and sst1-5 in these processes, their expression patterns were evaluated in hypothalamus, pituitary, and stomach of male and female mice under fed/fasted conditions in the presence (wild type) or absence (SST-knockout) of endogenous SST. Under fed conditions, hypothalamic/stomach SST/ssts expression did not differ between sexes, whereas male pituitary expressed more SST and sst2A/2B/3/5A/5TMD2/5TMD1 and less sst1, and male pituitary cell cultures were more responsive to SST inhibitory actions on GH release compared with females. This suggests that local pituitary SST/ssts can contribute to the sexually dimorphic pattern of GH release. Fasting (48 h) reduced stomach sst2A/B and hypothalamic SST/sst2A expression in both sexes, whereas it caused a generalized downregulation of pituitary sst subtypes in male and of sst2A only in females. Thus, fasting can reduce SST sensitivity across tissues and SST input to the pituitary, thereby jointly contributing to enhance GH release. In SST-knockout mice, lack of SST differentially altered sst subtype expression levels in both sexes, supporting an important role for SST in sex-dependent control of GH axis. Evaluation of SST, IGF-I, and glucocorticoid effects on hypothalamic and pituitary cell cultures revealed that these hormones could directly account for alterations in sst2/5 expression in the physiological states examined. Taken together, these results indicate that changes in SST output and sensitivity can contribute critically to precisely define, in a tissue-dependent manner, the sex-specific metabolic regulation of the GH axis.

The intricate role of growth hormone in metabolism

Growth hormone (GH), a master regulator of somatic growth, also regulates carbohydrate and lipid metabolism via complex interactions with insulin and insulin-like growth factor-1 (IGF-1). Data from human and rodent studies reveal the importance of GH in insulin synthesis and secretion, lipid metabolism and body fat remodeling. In this review, we will summarize the tissue-specific metabolic effects of GH, with emphasis on recent targets identified to mediate these effects. Furthermore, we will discuss what role GH plays in obesity and present possible mechanisms by which this may occur.

Reassessment of sst(5) somatostatin receptor expression in normal and neoplastic human tissues using the novel rabbit monoclonal antibody UMB-4

OBJECTIVE

The frequent overexpression of somatostatin receptors (sst) in neuroendocrine tumors provides the molecular basis for the diagnostic and therapeutic application of stable somatostatin analogs. Whereas octreotide acts mainly via the sst(2) receptor, the novel pan-somatostatin analog pasireotide exhibits particular high affinity for the sst(5) receptor. To determine whether a patient is a candidate for octreotide or pasireotide therapy, it is important to evaluate the somatostatin receptor status. However, so far highly specific rabbit monoclonal antibodies have been developed for the sst(2) receptor only (clone UMB-1).

METHODS

Here, we have extensively characterized a novel rabbit monoclonal antibody for the human sst(5) receptor (clone UMB-4). In a comparative immunohistochemical study, the expression of sst(5) and sst(2) receptors was assessed using UMB-4 and UMB-1, respectively.

RESULTS

Western blot experiments unequivocally demonstrated that UMB-4 selectively detected its cognate sst(5) receptor and did not cross-react with other proteins present in crude tissue homogenates. UMB-4 yielded a highly effective immunostaining of distinct cell populations in formalin-fixed, paraffin-embedded human tissues with a predominance of plasma membrane staining. In the pituitary, sst(5) was present on all growth hormone (GH)- and adrenocorticotropin hormone (ACTH)-producing cells whereas sst(2) was only observed on a subpopulation of GH-positive cells. Consequently, sst(5) was detectable on the majority of GH and ACTH adenomas. In contrast, sst(2) was only seen on GH but not on ACTH adenomas.

CONCLUSIONS

The rabbit monoclonal antibodies UMB-4 and UMB-1 will facilitate the assessment of the somatostatin receptor status of human tumors during routine histopathological examinations.

Real-time monitoring of somatostatin receptor-cAMP signaling in live pituitary

Fluorescence resonance energy transfer using genetically encoded biosensors has proven to be a powerful technique to monitor the spatiotemporal dynamics of cAMP signals stimulated by G(s)-coupled receptors in living cells. In contrast, real-time imaging of G(i)-mediated cAMP signals under native conditions remains challenging. Here, we describe the use of transgenic mice for cAMP imaging in living pituitary slices and primary pituitary cells. This technique can be widely used to assess the contribution of various pituitary receptors, including individual G(i) protein-coupled somatostatin receptors, to the regulation of cAMP levels under physiologically relevant settings.