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Studies of two thyrotrophin-secreting pituitary adenomas: evidence for dopamine receptor deficiency
Abstract:
Of 22 previously reported patients with TSH-secreting pituitary adenomas challenged with dopamine agonists, 18 showed no decrease in serum TSH. There have been few in-vitro studies of these rare tumours so the mechanism of the dopaminergic resistance has remained obscure. We describe two further patients with thyrotrophinomas; the first was thyrotoxic (T3 6.1 nmol/l, TSH 7 mU/l) and the second was diagnosed after radioiodine for presumed Graves' disease. The second patient had an alpha-subunit: TSH molar ratio less than unity (0.27). In-vivo TSH responses to TRH, bromocriptine and domperidone were compared with those of the resected tumour cells in vitro, the latter studied using a continuous perifusion system. Dopamine receptors were sought in membranes from each tumour using a radioreceptor assay employing 3H-spiperone. Patient 1 showed significant increases in serum TSH (7 to 13 mU/l) and alpha-subunit (18.7 to 385 ng/ml) after 200 micrograms TRH (i.v.) but patient 2 showed no such increases (TSH: 69 to 72 mU/l, alpha-subunit: 4.9 to 5.2 ng/ml). Neither patient showed a change in serum TSH following bromocriptine 2.5 mg (orally) or domperidone 10 mg (i.v.), though serum PRL responded normally. Serum TSH from patient 1 was of apparently normal molecular size but increased bioactivity (B/I ratio 3.8) and that from patient 2 was of increased molecular size but reduced bioactivity (B/I ratio 0.1). Tumour cells from each patient immunostained for TSH beta and alpha-subunit, and secreted TSH in vitro. The first showed dose-dependent TSH release after TRH (1-100 ng/ml) which could not be inhibited by dopamine (5 mumol/l) but the second was unresponsive to TRH in vitro. Neither tumour showed inhibition of TSH release by dopamine (5 mumol/l) or bromocriptine (0.01-10 nmol/l) and neither contained membrane-bound dopamine receptors. The results suggest that the dopaminergic resistance typical of most TSH-secreting pituitary adenomas may be due to altered or absent membrane-bound dopamine receptors.
Insights
Most TSH-secreting pituitary adenomas resist dopamine agonists due to absent or altered dopamine receptors. This study investigated two such cases, finding no tumor response to dopamine or bromocriptine, suggesting a mechanism for drug resistance.
Area of Science:
- Endocrinology
- Molecular Endocrinology
- Oncology
Background:
- TSH-secreting pituitary adenomas (thyrotrophinomas) are rare tumors.
- Dopamine agonist resistance is common in these adenomas, but the underlying mechanisms are poorly understood due to limited in vitro studies.
Observation:
- Two patients with thyrotrophinomas were studied, one thyrotoxic and one with presumed Graves' disease.
- In vivo and in vitro studies assessed TSH secretion in response to TRH and dopamine agonists (bromocriptine, domperidone).
- Tumor cell membranes were analyzed for dopamine receptors using radioreceptor assays.
Findings:
- Neither patient's TSH levels decreased after dopamine agonist administration.
- In vitro, tumor cells secreted TSH but showed no inhibition by dopamine or bromocriptine.
- Crucially, neither tumor contained detectable membrane-bound dopamine receptors.
Implications:
- The findings suggest that dopaminergic resistance in TSH-secreting pituitary adenomas is likely due to the absence or alteration of membrane-bound dopamine receptors.
- This provides a potential molecular explanation for treatment failure with dopamine agonists in these rare tumors.
- Further research into receptor function and alternative therapeutic targets is warranted.