Expression of somatostatin receptors, SSTR2A and SSTR5, in 108 endocrine pituitary tumors using immunohistochemical detection with new specific monoclonal antibodies

Medical treatment of endocrine pituitary tumors with somatostatin analogs depends on tumor type and somatostatin receptor (SSTR) expression. Immunohistochemical detection of these receptors using polyclonal antibodies has given conflicting results. We studied the expression of SSTR(2A) and SSTR(5) with new procedures in 108 pituitary tumors. Using 2 new, specific monoclonal antibodies (clone UMB-1 and UMB-4), 2 fixatives (Bouin-Hollande and zinc-formalin) and 2 technical procedures (manual and automated), SSTR(2A) and SSTR(5) expression was studied in 60 GH (growth hormone), 15 ACTH (adrenocorticotropic hormone), 23 FSH/LH (follicle-stimulating hormone/luteinizing hormone), 7 PRL (prolactin), and 3 TSH (thyroid-stimulating hormone) tumors. Only membrane staining was taken into account, and the SSTR expression was considered positive when more than 5% of the cells were immunoreactive. GH tumors were classified as GH or GH/PRL, densely or sparsely granulated, and into 3 groups according to the percentage of SSTR-immunoreactive cells (group 1: <25%; group 2: 25%-75%; group 3: >75%). Almost all GH tumors expressed SSTR(2A) (93%) and SSTR(5) (83%) at high levels (group 3: >75%) in 52% and 37%, respectively. SSTR(2A) expression was significantly higher in densely than in sparsely granulated tumors. Moreover, SSTR(2A) was also expressed in the 3 TSH tumors and weakly expressed in 26% of the FSH/LH tumors, although not in ACTH or PRL tumors. SSTR(5) expression was noted in 2 of the 3 TSH tumors, in only 20% of ACTH tumors, and was absent from FSH/LH and PRL tumors. The immunohistochemical detection of SSTR is a reproducible and specific method that could help direct the choice of postoperative medical treatment.