Recent studies have demonstrated that human
pituitary adenomas express multiple
somatostatin receptor (sst) subtypes. The expression of sst subtypes in human
pituitary adenomas is highly variable. This variability in sst subtype expression may explain the variable responsiveness of patients with
pituitary adenomas to medical treatment with the sst2-preferring SS-analogs
octreotide and
lanreotide. In human
GH-secreting pituitary adenomas, both sst2 and sst5 are involved in the regulation of GH secretion. In
prolactinomas, sst5 receptors are the key receptors in regulating responsiveness to SS. The low abundance of sst2 in
prolactinomas explains the lack of efficacy of
octreotide in lowering the elevated PRL levels in
prolactinoma patients.
Octreotide and
lanreotide successfully suppress TSH levels, including normalization of
thyroid hormone levels in the large majority of patients with TSH-secreting
pituitary adenomas, probably due to the high level of sst2, expression in the
adenomas. Although sst2 receptors are expressed by a significant proportion of gonadotroph and clinically non-functioning
pituitary adenomas, the overall efficacy of sst2-preferring SS-analogs seems low in this type of patients. Finally,
corticotroph adenomas may express multiple sst subtypes as well. Octapeptide SS-analogs do not lower circulating
ACTH levels in patients with untreated pituitary-dependent
Cushing's disease, whereas SS and the SS-analog
octreotide suppress pathological
ACTH release in some patients with Nelson's syndrome. This review discusses the expression and potential role of sst subtypes in the different types of human
pituitary adenomas.