Endocrine Abstracts

Background: Medical treatment of pituitary adenomas with somatostatin analogs depends on tumor type and somatostatin receptor expression. Using polyclonal antibodies, their immunohistochemical (IHC) detection gave conflicting results. Therefore, we studied the IHC expression of SSTR_2A and SSTR₅ using two specific MABs in five types of pituitary adenomas.

Methods: SSTR_2A and SSTR₅ expression was studied using two MABs (clone UMB1 and UMB4 for type 2A and 5 respectively) in 108 pituitary adenomas classified into five types: GH (n=60), ACTH (n=15), FSH/LH (n=23), PRL (n=7) and TSH (n=3). A comparative study was performed using two fixatives (Bouin-Hollande and Formol-Zinc) and two technical procedures (manual and automated IHC). GH adenomas were classified as pure or mixte GH/PRL, densely (DG) or sparsely granulated (SG) or according to the percentage of SSTR immunoreactive cells (<25%; 25–75%; >75%). In the other types of adenomas, the SSTR expression was considered as positive when more than 5% of the cells were immunoreactive.

Results: Whatever the fixatives, the techniques or the antibodies used, only specific membrane staining, without cytoplasmic reaction, was observed. Almost all GH adenomas expressed SSTR_2A (93.3%) and SSTR₅ (81.6%) with high expression (>75%) in 53.6 and 36.5% respectively. No significant difference of SSTR_2A and SSTR₅ expression was observed between DG and SG, or between pure and mixte GH/PRL adenomas. Moreover, SSTR_2A was also expressed in 100% of TSH adenomas, and weakly expressed in 26% of FSH/LH adenomas, ACTH or PRL adenomas being negative. SSTR₅ expression was noted in 66% of TSH adenoma but only in 20% of ACTH adenomas, LH/FSH and PRL adenomas being negative.

Conclusion: SSTR_2A and SSTR₅ were only highly expressed in GH and TSH adenomas. The IHC detection of SSTR with MABs is a reproducible and specific method which can be performed in everyday practice and could be helpful to predict post-operative treatment with somatostatin analogs, in particular in ACTH adenomas.