Immunohistochemical localization of somatostatin receptor type 2A in rat and human tissues

A selective nonpeptide somatostatin receptor 5 (SST5) agonist effectively decreases insulin secretion in hyperinsulinism

Congenital hyperinsulinism (HI), a beta cell disorder most commonly caused by inactivating mutations of beta cell K_ATP channels, results in dysregulated insulin secretion and persistent hypoglycemia. Children with K_ATP-HI are unresponsive to diazoxide, the only FDA-approved drug for HI, and utility of octreotide, the second line therapy, is limited because of poor efficacy, desensitization, and somatostatin receptor type 2 (SST2)-mediated side effects. Selective targeting of SST5, an SST receptor associated with potent insulin secretion suppression, presents a new avenue for HI therapy. Here, we determined that CRN02481, a highly selective nonpeptide SST5 agonist, significantly decreased basal and amino acid-stimulated insulin secretion in both Sur1^-/- (a model for K_ATP-HI) and wild type mouse islets. Oral administration of CRN02481 significantly increased fasting glucose and prevented fasting hypoglycemia compared to vehicle in Sur1^-/- mice. During a glucose tolerance test, CRN02481 significantly increased glucose excursion in both WT and Sur1^-/- mice compared to control. CRN02481 also reduced glucose- and tolbutamide-stimulated insulin secretion from healthy, control human islets similar to the effects observed with SS14 and peptide somatostatin analogs. Moreover, CRN02481 significantly decreased glucose- and amino acid-stimulated insulin secretion in islets from two infants with K_ATP-HI and one with Beckwith-Weideman Syndrome-HI. Taken together, these data demonstrate that a potent and selective SST5 agonist effectively prevents fasting hypoglycemia and suppresses insulin secretion not only in a K_ATP-HI mouse model, but also in healthy human islets and islets from HI patients.

Effect of Neuroendocrine Neoplasm Treatment on Human Reproductive Health and Sexual Function

Neuroendocrine neoplasms (NEN) are characterized by a wide clinical heterogeneity and biological variability, with slow progression and long survival in most cases. Although these tumors can affect young adults, there are few studies that focus on the sexual and reproductive system. The aim of this review was to evaluate the effect of NEN treatment, including somatostatin analogues (SSA), targeted therapy (Everolimus and Sunitinib), radiolabeled-SSA and chemotherapy, on male and female reproductive systems and sexual function. This narrative review was performed for all available prospective and retrospective studies, case reports and review articles published up to March 2022 in PubMed. To date, few data are available on the impact of SSA on human fertility and most of studies come from acromegalic patients. However, SSAs seem to cross the blood–placental barrier; therefore, pregnancy planning is strongly recommended. Furthermore, the effect of targeted therapy on reproductive function is still undefined. Conversely, chemotherapy has a well-known negative impact on male and female fertility. The effect of temozolomide on reproductive function is still undefined, even if changes in semen parameters after the treatment have been described. Finally, very few data are available on the sexual function of NEN treatment.

A Physiologically Based Pharmacokinetic Model for In Vivo Alpha Particle Generators Targeting Neuroendocrine Tumors in Mice

In vivo alpha particle generators have great potential for the treatment of neuroendocrine tumors in alpha-emitter-based peptide receptor radionuclide therapy (α-PRRT). Quantitative pharmacokinetic analyses of the in vivo alpha particle generator and its radioactive decay products are required to address concerns about the efficacy and safety of α-PRRT. A murine whole-body physiologically based pharmacokinetic (PBPK) model was developed for ²¹²Pb-labeled somatostatin analogs (²¹²Pb-SSTA). The model describes pharmacokinetics of ²¹²Pb-SSTA and its decay products, including specific and non-specific glomerular and tubular uptake. Absorbed dose coefficients (ADC) were calculated for bound and unbound radiolabeled SSTA and its decay products. Kidneys received the highest ADC (134 Gy/MBq) among non-target tissues. The alpha-emitting ²¹²Po contributes more than 50% to absorbed doses in most tissues. Using this model, it is demonstrated that α-PRRT based on ²¹²Pb-SSTA results in lower absorbed doses in non-target tissue than α-PRRT based on ²¹²Bi-SSTA for a given kidneys absorbed dose. In both approaches, the energies released in the glomeruli and proximal tubules account for 54% and 46%, respectively, of the total energy absorbed in kidneys. The ²¹²Pb-SSTA-PBPK model accelerates the translation from bench to bedside by enabling better experimental design and by improving the understanding of the underlying mechanisms.

Somatostatin Type 2 Receptor Antibody Enhances Mechanical Hyperalgesia in the Dorsal Root Ganglion Neurons after Sciatic Nerve-pinch Injury: Evidence of Behavioral Studies and Bax Protein Expression

BACKGROUND

Our previous study has indicated that somatostatin potently inhibits neuropathic pain through the activation of its type 2 receptor (SSTR2) in mouse dorsal root ganglion and spinal cord. However, the underlying mechanism of this activation has not been elucidated clearly.

OBJECTIVE

The aim of this study is to perform the pharmacological studies on the basis of sciatic nerve-pinch mice model and explore the underlying mechanism involving SSTR2.

METHODS

On the basis of a sciatic nerve-pinch injury model, we aimed at comparing the painful behavior and dorsal root ganglion neurons neurochemical changes after the SSTR2 antibody (anti- SSTR2;5μl,1μg/ml) administration in the mouse.

RESULTS

After pinch nerve injury, we found that the mechanical hyperalgesia and severely painful behavior (autotomy) were detected after the application of SSTR2 antibody (anti-SSTR2; 5μl, 1μg/ml) on the pinch-injured nerve. The up-regulated phosphorylated ERK (p-ERK) expression and the apoptotic marker (i.e., Bax) were significantly decreased in DRGs after anti-SSTR2 treatment.

CONCLUSION

The current data suggested that inhibitory changes in proteins from the apoptotic pathway in anti-SSTR2-treated groups might be taking place to overcome the protein deficits caused by SSTR2 antibody and supported the new therapeutic intervention with SSTR2 antagonist for neuronal degeneration following nerve injury.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

The expression of somatostatin receptor 2 decreases during cyst growth in mice with polycystic kidney disease

IMPACT STATEMENT

Somatostatin (SST) analogs have been shown to halt cyst growth and progression of autosomal dominant polycystic kidney disease by several clinical trials. However, two studies suggest that the effect of the SST analog octreotide on kidney growth during the first year of treatment is reduced in the subsequent follow-ups and the kidney enlargement resumes. This biphasic change in kidney growth during octreotide treatment may be partially explained by alterations in SSTR2 expression. Here, we found that SSTR2 is mainly expressed in distal tubules and collecting ducts in murine kidneys, and the expression of SSTR2 decreases during cyst growth in two PKD mouse models. Our data may thus provide possible explanations for the lack of efficacy in long-term treatment with SST analogs.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Suppression of ANP secretion by somatostatin through somatostatin receptor type 2

Somatostatin is a cyclic-14 amino acid peptide which mainly distributed in digestive system and brain. Somatostatin receptor (SSTR) is a G-protein coupled receptor and all five SSTR subtypes are expressed in cardiomyocytes. The aim of this study was to investigate the effect of somatostatin on atrial natriuretic peptide (ANP) secretion and its signaling pathway. Somatostatin (0.01 and 0.1nM) decreased ANP secretion in isolated beating rat atrium in a dose-dependent manner. But atrial contractility and translocation of extracellular fluid were not changed. Somatostatin-induced decrease in ANP secretion was significantly attenuated by the pretreatment with CYN 154806 (SSTR type 2 antagonist; 0.1μM), but not by BIM 23056 (SSTR type 5 antagonist; 0.1μM) and urantide (urotensin II receptor antagonist; 0.1μM). When pretreated with an agonist for SSTR type 2 (Seglitide, 0.1nM) and SSTR type 5 (L 817818, 0.1nM), only Seglitide reduced ANP secretion similar to that of somatostatin. The suppressive effect of somatostatin on ANP secretion was attenuated by the pretreatment with an inhibitor for adenylyl cyclase (MDL-12330A, 5μM) or protein kinase A (KT 5720, 0.1μM). In diabetic rat atria, the suppressive effect of somatostatin on ANP secretion and concentration was attenuated. Real time-PCR and western blot shows the decreased level of SSTR type 2 mRNA and protein in diabetic rat atria. These data suggest that somatostatin decreased ANP secretion through SSTR type 2 and an attenuation of suppressive effect of somatostatin on ANP secretion in diabetic rat atria is due to a down-regulation of SSTR type 2.

Effects of therapy with [177Lu-DOTA 0,Tyr 3]octreotate on endocrine function

Purpose

Peptide receptor radionuclide therapy (PRRT) with radiolabelled somatostatin analogues is a novel therapy for patients with somatostatin receptor-positive tumours. We determined the effects of PRRT with [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate (¹⁷⁷Lu-octreotate) on glucose homeostasis and the pituitary-gonadal, pituitary-thyroid and pituitary-adrenal axes.

Methods

Hormone levels were measured and adrenal function assessed at baseline and up to 24 months of follow-up.

Results

In 35 men, mean serum inhibin B levels were decreased at 3 months post-therapy (205 ± 16 to 25 ± 4 ng/l, p < 0.05) and follicle-stimulating hormone (FSH) levels increased (5.9 ± 0.5 to 22.7 ± 1.4 IU/l, p < 0.05). These levels returned to near baseline levels. Total testosterone and sex hormone binding globulin (SHBG) levels decreased (15.0 ± 0.9 to 10.6 ± 1.0 nmol/l, p < 0.05 and 61.8 ± 8.7 to 33.2 ± 3.7 nmol, p < 0.05), respectively, whereas non-SHBG-bound T did not change. An increase (5.2 ± 0.6 to 7.7 ± 0.7 IU/l, p < 0.05) of luteinizing hormone (LH) levels was found at 3 months of follow-up returning to baseline levels thereafter. In 21 postmenopausal women, a decrease in levels of FSH (74.4 ± 5.6 to 62.4 ± 7.7 IU/l, p < 0.05) and LH (26.8 ± 2.1 to 21.1 ± 3.0 IU/l, p < 0.05) was found. Of 66 patients, 2 developed persistent primary hypothyroidism. Free thyroxine (FT₄) levels decreased (17.7 ± 0.4 to 15.6 ± 0.6 pmol/l, p < 0.05), whereas thyroid-stimulating hormone (TSH) and triiodothyronine (T₃) levels did not change. Reverse triiodothyronine (rT₃) levels decreased (0.38 ± 0.03 to 0.30 ± 0.01 nmol/l, p < 0.05). Before and after therapy adrenocorticotropic hormone (ACTH) stimulation tests showed an adequate response of serum cortisol (> 550 nmol/l, n = 18). Five patients developed elevated HbA_1c levels (> 6.5%).

Conclusion

In men ¹⁷⁷Lu-octreotate therapy induced transient inhibitory effects on spermatogenesis, but non-SHBG-bound T levels remained unaffected. In the long term, gonadotropin levels decreased significantly in postmenopausal women. Only a few patients developed hypothyroidism or elevated levels of HbA_1c. Therefore, PRRT with ¹⁷⁷Lu-octreotate can be regarded as a safe treatment modality with respect to short- and long-term endocrine function.

Function and expression of somatostatin receptors of the endocrine pancreas

Somatostatin (SST) regulates multiple biological processes via five genetically distinct, G-protein coupled receptors. Clinical interest in therapy for neuroendocrine and metabolic disorders has resulted in the development of new tools for exploring the function of somatostatin receptors (SSTRs). The development of highly SSTR-selective agonists and antagonists, animal models with the deletion of individual SSTRs, as well as SSTR-specific antibodies have all been utilized in delineating SSTR functions. In the pancreas, SST is a potent regulator of insulin and glucagon secretion. Indeed, the inappropriate regulation of pancreatic A- and B-cell function in metabolic diseases provides an impetus to evaluate the SSTRs as therapeutic targets. By combining the results obtained from molecular biology, pharmacology and immunochemical studies the current review provides a summary of important recent developments which have extended our knowledge of SST actions in the endocrine pancreas.

Expression of somatostatin and somatostatin receptor subtypes 1-5 in human normal and diseased kidney

Somatostatin mediates inhibitory functions through five G protein-coupled somatostatin receptors (sst1-5). We used immunohistochemistry, immunofluorescence, and RT-PCR to determine the presence of somatostatin receptors sst1, sst2A, sst2B, sst3, sst4, and sst5 in normal and IgA nephropathy human kidney. All somatostatin receptors were detected in the thin tubules (distal convoluted tubules and loops of Henle) and thick tubules (proximal convoluted tubules) in the tissue sections from nephrectomy and biopsy samples. Immunopositive sst1 and sst4 staining was more condensed in the cytoplasm of tubular epithelial cells. In normal kidney tissue sections, podocytes and mesangial cells in the glomeruli stained for sst1, sst2B, sst4 and sst5, and stained weakly for sst3. In IgA kidney tissue, the expression of somatostatin receptors was significantly increased with particular immmunopositive staining for sst1, sst2B, sst4, and sst5 within glomeruli. In the epithelial cells, the staining for sst2B and sst4 in proximal tubules and sst1, sst2B, and sst5 in distal tubules was increased. The mRNA expression of sst1-5 was also detected by RT-PCR. Somatostatin and all five receptor subtypes were ubiquitously distributed in normal kidney and IgA nephropathy. The increased expression of somatostatin receptors in IgA nephropathy kidney might be the potential pathogenesis of inflammatory renal disease.

213Bi-[DOTA0, Tyr3]Octreotide Peptide Receptor Radionuclide Therapy of Pancreatic Tumors in a Preclinical Animal Model

PURPOSE

The somatostatin analogue [DOTA0, Tyr3]octreotide (DOTATOC) has previously been labeled with low linear energy transfer (LET) beta-emitters, such as 177Lu or 90Y, for tumor therapy. In this study, DOTATOC labeled with the high-LET alpha-emitter, 213Bi, was evaluated.

EXPERIMENTAL DESIGN

The radiolabeling, stability, biodistribution, toxicity, safety, and therapeutic efficacy of 213Bi-DOTATOC (specific activity 7.4 MBq/microg) were investigated. Biodistribution studies to determine somatostatin receptor specificity were done in Lewis rats at 1 and 3 hours postinjection. Histopathology of various organs was used to evaluated toxicity and safety. Therapeutic efficacy of 4 to 22 MBq 213Bi-DOTATOC was determined in a rat pancreatic carcinoma model.

RESULTS

Radiolabeling of the 213Bi-DOTATOC was achieved with radiochemical purity >95% and an incorporation yield > or = 99.9%. Biodistribution data showed specific binding to somatostatin receptor-expressing tissues. Administration of free 213Bi, compared with 213Bi-DOTATOC, resulted in higher radioactivity accumulation at 3 hours postinjection in the kidneys [34.47 +/- 1.40% injected dose/g (ID/g) tissue versus 11.15 +/- 0.46%, P < 0.0001] and bone marrow (0.31 +/- 0.01% ID/g versus 0.06 +/- 0.02%, P < 0.0324). A significant decrease in tumor growth rate was observed in rats treated with >11 MBq of 213Bi-DOTATOC 10 days postinjection compared with controls (P < 0.025). Treatment with >20 MBq of 213Bi-DOTATOC showed significantly greater tumor reduction when compared with animals receiving <11 MBq (P < 0.02).

CONCLUSIONS

213Bi-DOTATOC showed dose-related antitumor effects with minimal treatment-related organ toxicity. No acute or chronic hematologic toxicities were observed. Mild, acute nephrotoxicity was observed without evidence of chronic toxicity. 213Bi-DOTATOC is a promising therapeutic radiopharmaceutical for further evaluation.

Systemic distribution of somatostatin receptor subtypes in human: an immunohistochemical study

Somatostatin is well known to inhibit the hormone secretion of various peptides. This action has been considered to be generally mediated via six different specific somatostatin receptors (sstr), sstr1, sstr2A, sstr2B, sstr3, sstr4, and sstr5. It then becomes very important to demonstrate the localization of these sstr subtypes in order to elucidate the possible biological and/or clinical significance of somatostatin actions. These sstr subtypes have been demonstrated to be expressed throughout the human body, including the central nervous system, gastrointestinal tract, pancreas, kidney, and other organs, but its details, especially its systemic distribution and localization in tissue compartments, have yet to be examined thoroughly in human. Therefore, in this study, we examined the systemic localization of all six somatostatin receptors in normal human organs using immunohistochemistry with recently developed specific antibodies against these receptor subtypes. In all of the human tissues examined, various sstr subtypes were detected not only in parenchymal cells but also in various stromal cells such as lymphocytes, fibroblasts, and endothelial cells. Among human tissues in which the presence of sstr has not been previously reported, the parotid gland demonstrated immunoreactivity for sstr2B and sstr5, bronchial gland for sstr1, 2B, 3, 4, 5, parathyroid gland for sstr1, 3, 4, and duodenum for all subtypes immunoreactivity. The great majority of other organs examined demonstrated results consistent with those of previously reported biochemical studies. In pancreatic islet cells, only sstr2A was positive in all the cases but other sstr subtypes were associated with marked intraislet heterogeneity in their distribution. In stomach, all subtypes of receptor were detected in various cell types of the mucosa, but none in ECL cells of fundic gland. These findings demonstrated the broad systemic actions of somatostatin in non-endocrine cells.

Expression of somatostatin receptor subtypes 1-5 in pancreatic islets of normoglycaemic and diabetic NOD mice

OBJECTIVE

Somatostatin acts on five specific receptors (sst1-5) to elicit different biological functions. The non-obese diabetic (NOD) mouse is an experimental model of type 1 diabetes. The aim of this study was to investigate whether the islet expression of sst1-5 is affected during the development of diabetes in NOD mice, with insulitis accompanied by spontaneous hyperglycaemia.

METHODS

By immunostaining for sst1-5 the expression and co-expression together with the four major islet hormones in pancreatic islets were investigated in female and male NOD mice at different stages of disease. The NOD related non-diabetic ICR mouse was also examined.

RESULTS

The islet cells of diabetic NOD mice showed an increased islet cell expression of sst2-5 compared with normoglycaemic female NOD mice. This correlated to increasing age and extent of insulitis. Major findings from the co-expression investigations were that sst2 was expressed in a majority of beta-cells in the normoglycaemic NOD mice, but absent in the beta-cells in the diabetic NOD mice. A majority of the alpha-cells expressed sst2 and 5 in normoglycaemic and diabetic NOD mice. About 60% of delta-cells showed co-expression of sst4 and 5 in both normoglycaemic and diabetic NOD mice. 60% of pancreatic polypeptide (PP)-cells expressed sst4 in both groups. Insulitis was found to be accompanied by a down-regulation of sst in normoglycaemic animals.

CONCLUSIONS

The difference in sst expression in the islets cells of diabetic mice may suggest either a contributing factor in the process leading to diabetes, or a defence response against ongoing beta-cell destruction.

Analysis of accumulation of 99mTc-octreotide and 99mTc-EDDA/HYNIC-Tyr3-octreotide in the rat kidneys

The aim of this study was to compare renal handling and distribution of (99m)Tc-octreotide and (99m)Tc-EDDA/HYNIC-Tyr(3)-octreotide (HYNIC-TOC) in rats. In kidney perfusion experiments, the renal clearance value of (99m)Tc-octreotide was three times lower than that of (99m)Tc-EDDA/HYNIC-TOC. The predominant renal excretion of (99m)Tc-EDDA/HYNIC-TOC was associated with a high and long-term renal accumulation up to 48 hrs. Microautoradiographic results indicated that (99m)Tc-EDDA/HYNIC-TOC was retained mainly in the renal medulla within the cells of the collecting ducts and in the surrounding tissue. Lower positivity was found in the proximal and distal tubular cells. We conclude that the mechanism of renal accumulation of somatostatin analogues renal accumulation is complex and that proximal tubular reabsorption is probably not the main mechanism for uptake of (99m)Tc-EDDA/HYNIC-TOC in the kidneys. The presence of the somatostatin receptors, differences in the tonicity level within kidneys and other possible mechanisms could participate in their renal accumulation.

Expression of somatostatin receptors 1 and 2 in the adult mouse kidney

Systemic infusion of somatostatin in humans and rodents alters renal glomerular filtration rate, solute transport, and water clearance. Among the five different G-protein-coupled somatostatin receptors (SSTRs), SSTR1 is expressed in human kidney collecting ducts and SSTR2 is expressed in rat and human collecting ducts and glomeruli. Our laboratory recently localized SSTRs 3, 4, and 5 to mouse kidney proximal tubules. Our aim was to characterize the expression of somatostatin receptors 1 and 2 in mouse kidneys. We detected mRNA for somatostatin receptors 1 and 2 in the mouse kidney by reverse transcriptase-polymerase chain reaction (RT-PCR), with confirmation by Southern blotting. In contrast to the data in human kidneys, we localized SSTR1 proteins to mouse proximal tubules and glomerular podocytes. Similar to the reports in human and rat kidneys, we detected SSTR2 proteins in the murine glomerulus and collecting duct; we were further able to identify the specific SSTR2-positive cells as podocytes and principal cells, respectively. Thus, taken with our previously published results, the kidney expresses all five somatostatin receptors. Furthermore, the expression of the SSTRs in the mouse kidney correlates well with the known actions of somatostatin in the kidney.

Expression and distribution of somatostatin receptor subtypes in the pancreatic islets of mice and rats

Somatostatin acts on specific membrane receptors (sst(1-5)) to inhibit exocrine and endocrine functions. The aim was to investigate the distribution of sst(1-5) in pancreatic islet cells in normal mice and rats. Pancreatic samples from five adult C57BL/6 mice and Sprague-Dawley rats were stained with antibodies against sst(1-5) and insulin, glucagon, somatostatin, or pancreatic polypeptide (PP). A quantitative analysis of the co-localization was performed. All ssts were expressed in the pancreatic islets and co-localized on islet cells to various extents. A majority of the beta-cells expressed sst(1-2) and sst(5) in mouse islets, while < or =50% in the rat expressed sst(1-5). The expression of sst(1-5) on alpha-cells did not differ much among species, with sst(2) and sst(5) being highly expressed. About 70% of the delta-cells expressed sst(1-4) in the rat pancreas, whereas 50% of the islet cells expressed sst(1-5) in the mouse. Furthermore, 60% of the PP-cells expressed sst(1-5) in the mouse, while the rat islets had lower values. Co-expression with the four major islet hormones varies among species and sst subtypes. These similarities and differences are interesting and need further evaluation to elucidate their physiological role in islets.

Evolving concepts in the quest for advanced therapeutic analogues of somatostatin

A major challenge in developing somatostatin-based therapies, as well as an important question of basic physiology, is how to achieve functional specificity with an agent that has widespread actions. The natural somatostatin system achieves functional specificity in part through local somatostatin production at the site of action. Further selectivity may be achieved through the recently elucidated somatostatin receptor subtypes. To explore the relationship between the receptor subtypes and somatostatin-mediated functions, we have tested panels of selective somatostatin analogs in specific biological assays. Our studies have demonstrated complex, functional interactions between the somatostatin receptor subtypes. Specific combinations of somatostatin receptor subtypes in specific tissues may be either synergistic or antagonistic. By altering the expression ratio of the interacting receptors, the biological response to somatostatin can be influenced by environmental, hormonal and physiological status. In addition, inappropriate or unbalanced receptor expression may provide a novel mechanism of disease. These concepts take on an even broader context with the demonstration that somatostatin receptor subtypes also interact with other G-protein-coupled receptors. The insight gained from these studies has already resulted in several novel approaches to acromegaly therapy. Our further understanding of these complex cellular codes will provide the conceptual basis for future therapeutics with greatly enhanced efficacy and specificity.

A reappraisal of the blood glucose homeostat which comprehensively explains the type 2 diabetes mellitus-syndrome X complex

Blood glucose concentrations are unaffected by exercise despite very high rates of glucose flux. The plasma ionised calcium levels are even more tightly controlled after meals and during lactation. This implies 'integral control'. However, pairs of integral counterregulatory controllers (e.g. insulin and glucagon, or calcitonin and parathyroid hormone) cannot operate on the same controlled variable, unless there is some form of mutual inhibition. Flip-flop functional coupling between pancreatic alpha- and beta-cells via gap junctions may provide such a mechanism. Secretion of a common inhibitory chromogranin by the parathyroids and the thyroidal C-cells provides another. Here we describe how the insulin:glucagon flip-flop controller can be complemented by growth hormone, despite both being integral controllers. Homeostatic conflict is prevented by somatostatin-28 secretion from both the hypothalamus and the pancreatic islets. Our synthesis of the information pertaining to the glucose homeostat that has accumulated in the literature predicts that disruption of the flip-flop mechanism by the accumulation of amyloid in the pancreatic islets in type 2 diabetes mellitus will lead to hyperglucagonaemia, hyperinsulinaemia, insulin resistance, glucose intolerance and impaired insulin responsiveness to elevated blood glucose levels. It explains syndrome X (or metabolic syndrome) as incipient type 2 diabetes in which the glucose control system, while impaired, can still maintain blood glucose at the desired level. It also explains why it is characterised by high plasma insulin levels and low plasma growth hormone levels, despite normoglycaemia, and how this leads to central obesity, dyslipidaemia and cardiovascular disease in both syndrome X and type 2 diabetes.

Expression of somatostatin receptors 3, 4, and 5 in mouse kidney proximal tubules

BACKGROUND

Systemic infusion of somatostatin (SRIF) induces many physiological changes in human and rodent kidneys, including alterations in glomerular filtration, solute transport, and water clearance. Although somatostatin can bind to five different G-protein coupled receptors (SSTRs), only SSTR1 and SSTR2A proteins have been described convincingly in rat and/or human kidneys. Both are expressed primarily in collecting ducts, despite clear evidence that somatostatin also can bind to proximal tubules. Our aim was to characterize the expression of somatostatin receptors three to five in adult mouse kidneys.

METHODS

Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed followed by Southern blotting on mouse kidney RNA for SSTR3, SSTR4, and SSTR5. Immunohistochemistry and dual-labeling immunofluorescence also were performed to localize the receptors in the kidney.

RESULTS

Messenger RNA was detected for somatostatin receptors 3 to 5 in the mouse kidney by RT-PCR, with confirmation by Southern blotting. By immunohistochemistry and dual-labeling immunofluorescence, the proteins for all three receptors were abundantly expressed, but exclusively localized to the proximal tubules. SSTR3 was present in intracellular granules, while SSTR4 and SSTR5 were expressed on the lumenal membranes of the tubules.

CONCLUSIONS

Expression of SSTR3, SSTR4, and SSTR5 in mouse proximal tubules complements the expression of SSTR1 and SSTR2 in collecting ducts as seen in other species. Taken together, the kidney is one of few organs expressing all five somatostatin receptors outside of the nervous system and pancreas.

Antiangiogenic effects of somatostatin analogues

Inhibition of angiogenesis has become a target for antineoplastic therapy and for treatment of retinal neovascularization. The presence of somatostatin receptors on tumour cells and on the proliferating vascular endothelium has led to several in vitro and in vivo studies to investigate the antiproliferative and antiangiogenic effects of somatostatin analogues. Currently available data suggest that somatostatin analogues might inhibit angiogenesis directly through somatostatin receptors present on endothelial cells and also indirectly through the inhibition of growth factor secretion such as IGF-I and vascular endothelial growth factor (VEGF) and reducing monocyte chemotaxis. However, beneficial effects on inhibition of neovascularization have been questioned by some studies. More work is therefore required to firmly establish the role of somatostatin analogues as potential antiangiogenic therapy. The currently available somatostatin analogues have high affinity for somatostatin receptor subtype 2 (sst2) and, to a lesser extent, sst5 and sst3. However, because vascular endothelial cells express several types of somatostatin receptors, it will be important to investigate somatostatin analogues with different receptor subtype affinities, which might increase the spectrum of available therapy for tumours.