Somatostatin receptor 1-5; expression profiles during rat development.

BACKGROUND: Somatostatin acts through five receptor subtypes (SSTRs 1-5). We aimed to investigate SSTRs mRNA expression and protein distribution in whole rat embryos, with special emphasis on the pancreas. MATERIAL AND METHODS: Rat embryos were collected on embryonal days 10, 11, 12, 14, 15, 17, 19, 21, and at birth. Presence of SSTRs was investigated with RT-PCR techniques and immunohistochemistry. RESULTS: There was no SSTR5 mRNA expression in the whole rat embryos. All SSTR1-5 proteins were observed at embryonal day 10, but the localization varied between the different subtypes. From day 11 to birth SSTRs protein presence increased with time in major structures such as skin and cartilage. It remained similar over time in the heart and liver. In the fetal pancreas mRNA expression of SSTR2 and 4 was detected at day 14, and there was an increase up to birth. Only SSTR1 protein co-localized to a higher extent with the islet hormones studied. SSTR2 was present in all islet endocrine cells except for ß-cells. In contrast, the immunostaining for SSTR3-4 was co-localized with insulin and PP, and, finally, SSTR5 with glucagon and pancreatic polypeptide. In mRNA isolated from whole rat embryos SSTR1-2 and SSTR4 expression showed a peak at day 14, while SSTR3 mRNA was not present until day 15. CONCLUSION: The present data suggest a role for SSTRs during the development of the rat embryo. Subsequent functional studies may elucidate regulatory roles of specific SSTRs for the growth and differentiation of the pancreas as well as other organs.

Altered expression of somatostatin receptors in pancreatic islets from NOD mice cultured at different glucose concentrations in vitro and in islets transplanted to diabetic NOD mice in vivo.

Somatostatin acts via five receptors (sst(1-5)). We investigated if the changes in pancreatic islet sst expression in diabetic NOD mice compared to normoglycemic mice are a consequence of hyperglycemia or the ongoing immune reaction in the pancreas. Pancreatic islets were isolated from NOD mice precultured for 5 days and further cultured for 3 days at high or low glucose before examined. Islets were also isolated from NOD mice and transplanted to normal or diabetic mice in a number not sufficient to cure hyperglycemia. After three days, the transplants were removed and stained for sst(1-5) and islet hormones. Overall, changes in sst islet cell expression were more common in islets cultured in high glucose concentration in vitro as compared to the islet transplantation in vivo to diabetic mice. The beta and PP cells exhibited more frequent changes in sst expression, while the alpha and delta cells were relatively unaffected by the high glucose condition. Our findings suggest that the glucose level may alter sst expressed in islets cells; however, immune mechanisms may counteract such changes in islet sst expression.

Somatostatin receptor expression and biological functions in endocrine pancreatic cells: review based on a doctoral thesis.

Type 1 diabetes is resulting from the selective destruction of insulin-producing betacells within the pancreatic islets. Somatostatin acts as an inhibitor of hormone secretion through specific receptors (sst1-5). All ssts were expressed in normal rat and mouse pancreatic islets, although the expression intensity and the co-expression pattern varied between ssts as well as between species. This may reflect a difference in response to somatostatin in islet cells of the two species. The Non-Obese Diabetic (NOD) mouse model is an experimental model of type 1 diabetes, with insulitis accompanied by spontaneous hyperglycaemia. Pancreatic specimens from NOD mice at different age and stage of disease were stained for ssts. The islet cells of diabetic NOD mice showed increased islet expression of sst2-5 compared to normoglycemic NOD mice. The increase in sst2-5 expression in the islets cells may suggest either a contributing factor in the process leading to diabetes, or a defense response against ongoing beta-cell destruction. Somatostatin analogues were tested on a human endocrine pancreatic tumour cell line and cultured pancreatic islets. Somatostatin analogues had an effect on cAMP accumulation, chromogranin A secretion and MAP kinase activity in the cell line. Treatment of rat pancreatic islets with somatostatin analogues with selective receptor affinity was not sufficient to induce an inhibition of insulin and glucagon secretion. However, a combination of selective analogues or non-selective analogues via costimulation of receptors can cause inhibition of hormone production. For insulin and glucagon, combinations of sst2 + sst5 and sst1 + sst2, respectively, showed a biological effect. In summary, knowledge of islet cell ssts expression and the effect of somatostatin analogues with high affinity to ssts may be valuable in the future attempts to influence beta-cell function in type 1 diabetes mellitus, since down-regulation of beta-cell function may promote survival of these cells during the autoimmune attack.

Subtype selective interactions of somatostatin and somatostatin analogs with sst1, sst2, and sst5 in BON-1 cells.

Somatostatin is a polypeptide hormone acting as an inhibitor of pituitary, pancreatic, and gastrointestinal secretion through specific membrane receptors of which five subtypes have been cloned (sst(1-5)). Somatostatin analogs are used in the clinic to treat patients with excessive hormone production due to a neuroendocrine tumor. The aim of this study was to investigate the biological activity of three new somatostatin receptor subtype selective analogs (BIM-23926, sst(1)-selective; BIM-23120, sst(2)-selective; and BIM-23206, sst(5)-selective) in the human neuroendocrine tumor cell line, BON-1, which expresses sst(1), sst(2), and sst(5) natively. Somatostatin-14 and octreotide were used as reference substances. Forskolin-induced cAMP accumulation and chromogranin A (CgA) secretion were inhibited by BIM-23120, BIM-23206, and somatostatin-14 in a dose-dependent manner. Cholecystokinin (CCK-8) stimulated activation of mitogen-activated protein (MAP) kinase was inhibited by BIM-23120 and BIM-23206, while BIM-23926 stimulated the activity. Selective BIM analogs showed a more efficient inhibitory effect on cAMP accumulation, CgA secretion, and MAP kinase activity than octreotide in BON-1 cells. This may be explained by the differences in affinity of the ligand to the receptor or by interaction between different sst subtypes. We conclude that increasing knowledge about sst physiology and expression in malignant disease indicates a need for new analogs that can be incorporated into the therapeutic arsenal.

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.

Expression of somatostatin receptor subtypes 1 to 5 in tumor tissue and intratumoral vessels in malignant endocrine pancreatic tumors.

Somatostatin analogs are well established in the treatment of malignant endocrine pancreatic tumors (EPTs). Our goal is to individualize their treatment using receptor-subtype-specific analogs and, therefore, exploring the receptor expression is highly important. We have examined the expression of somatostatin receptor (sst) subtypes 1-5 on tumor cells and in intratumoral vessels in 28 tumor tissues from malignant EPTs with immunohistochemistry using sst-subtype-specific polyclonal antibodies. We found that sst(2) and sst(4) stained positive in 90% and sst(1) in 70% of the tumor tissues, whereas sst(3) and sst(5) stained positive in only 50% of the tumor tissues. Sst expression in intratumoral vessels was high for sst(2) and sst(4) (80%), moderate for sst(1) (40%), and low for sst(3) and sst(5) (10%). The ssts were evenly distributed among the different tumor subtypes. However, tumors belonging to the same subgroup of EPTs showed a variable expression of receptor subtypes. No differences in receptor-subtype expression could be seen between poorly and welldifferentiated tumors, nor between primary tumors and metastases. Prior medical treatment did not influence sst expression pattern. In conclusion, sst(2) and sst(4) were expressed in most tumor tissues and intratumoral vessels from EPTs. However, sst(3) and sst(5) were lacking in half of the tumor tissues and in most of the intratumoral vessels. These differences indicate the importance of determining each tumor s subset of receptors before treatment with receptor-subtype-specific analogs is initiated. The importance of sst expression in intratumoral vessels is not yet known.