CHARACTERIZATION OF THE SOMATOSTATIN RECEPTOR SUBTYPE IN A BRONCHIAL CARCINOID-TUMOR RESPONSIBLE FOR CUSHINGS-SYNDROME

Small ACTH-secreting carcinoid tumors responsible for Cushing's syndro me are often difficult to localize using available radiological invest igations. Somatostatin receptors have been found in about 90% of carci noid tumors studied, leading to a new approach for the localization of tumors or metastasis by using radiolabeled somatostatin analogs. We r eport a case of Cushing's syndrome due to an ACTH-secreting bronchial carcinoid tumor, completely suppressible with octreotide treatment and evidenced by body scintigraphy with In-111-labeled pentreotide. After removal, which led to patient recovery, the tumor was studied in vitr o. In situ hybridization, using a complementary DNA probe, revealed PO MC messenger ribonucleic acid in a subpopulation of tumor cells. These cells were labeled by immunochemistry using an antiserum directed aga inst ACTH. Confocal laser scanning microscopy analysis showed that the ACTH-immunoreactive peptide was sequestered in secretory granules. Au toradiographic labeling using [I-125-Tyr(0),D-Trp(8)]somatostatin-14 d emonstrated the presence of somatostatin-binding sites in the whole tu mor tissue. The relative affinities of various selective somatostatin analogs and the ability of GTP to inhibit radioligand binding suggeste d that the receptor expressed in the tumor cells belonged to the SSTR- 2 subtype.