Somatostatin receptor profile in pituitary thyrotroph adenomas

Thyrotroph adenomas are the most infrequent adenohypophysial tumors. Somatostatin (SST) inhibits hormone secretion and suppresses cell proliferation. SST receptors (sstr) belong to a family of 5 types of G-coupled membrane proteins, which show high binding affinity to SST. Currently, SST analogs used to treat pituitary adenomas, have a preferential binding activity to sstr2 and sstr5. The aim of this study was to evaluate the status of all active sstrs on cell membrane of thyrotroph adenomas.

Nine cases of thyrotroph adenomas were studied for all types of sstrs. All patients were clinically associated with hyperthyroidism. The adenomas were initially diagnosed and classified by histology and immunohistochemistry for all pituitary hormones and two of them were examined by electron microscopy.

For sstr immunohistochemistry, antisera against all sst types (1, 2A, 2B, 3, 4 and 5) were used. To enhance sensitivity, the tyramide amplification technique was applied. This is the first report investigating the full spectrum of sstrs in thyrotroph adenomas by immunohistochemistry.

All tumors were immunoreactive for β-subunit of thyroid-stimulating hormone and for α-subunit of glycoprotein hormones. The sst2A, sst2B and sstr5 were co-expressed in all adenomas. The sstr1 and sstr3 were noted in 8 and sstr4 in 7 adenomas respectively. High scores 2+ and 3+ were prominent in sstr2A, sstr2B, sstr3 and sstr5. High score 3+ for sstr4 was also noted in one tumor, while score 3+ for sstr1 was not observed.

Knowledge of the sstr status may contribute to a better selection of patients, anticipating benefit from treatment with SST analogs. Given that multiligand SST analogs have a broader ability to bind other sstrs, such as sstr1 and sstr3, patients with thyrotroph adenomas expressing these receptors may benefit from novel sstr targeting therapy.