Somatostatin receptors in endocrine tumors

Most neuroendocrine tumors express receptors for somatostatin which was originally isolated as a hormone with growth hormone releasing inhibiting potential. The molecular cloning of five receptor subtypes for somatostatin has expanded our knowledge on the actions of this peptide. We studied the expression of all five somatostatin receptor subtypes in various normal human tissues and a variety of endocrine tumors. Different quantitative expression rates in normal tissues were identified by realtime RT-PCR. Expression in these tissues was confirmed by immunohistochemical analysis. We then compared the physiological expression to the somatostatin receptor expression in tumors arising from the same tissue. Our investigation of pituitary adenomas revealed that somatostatin receptor subtypes are not only expressed in GH-producing adenomas but also in ACTH-producing adenomas and prolactinomas as well as in non-functioning pituitary adenomas. Further analysis of other endocrine tumors demonstrated expression in pheochromocytomas as well as in tumors of the adrenal cortex with tumor-specific distribution pattern. This may offer new diagnostic and therapeutic possibilities with multiligand or subtype specific somatostatin analogs. Somatostatin analogues are very effective in the treatment of symptoms related to endocrine tumors. New analogues like the multi-ligand SOM230 are currently studied in phase 2 studies. The high expression of somatostatin receptors is used to localize endocrine tumors by receptor szintigraphy with radiolabeled somatostatin analogues. Tumor-targeted radioactive treatment based on somatostatin analogues is currently evaluated as a treatment option.