TSH and TSH-subunit production by human thyrotrophic tumour cells in monolayer culture

Studies were performed on pituitary adenoma tissue obtained from a patient with hyperthyroidism secondary to inappropriate thyrotrophin (TSH) secretion. Cells dispersed enzymatically were established in primary monolayer culture for a period of 39 days. Intact TSH and TSH-subunit release into the culture medium declined over the initial 20 days, followed by a rebound in TSH and β-TSH production until the time of subculture at day 39. In vitro α-TSH production declined more slowly than did TSH and β-TSH, but did not recover in parallel with intact TSH. Production of α-TSH by cultured cells, at different times, was 20-80-fold greater than that of β-TSH. In the pituitary tissue α-TSH content (112 ng/mg tissue) was 8.6-fold greater than the β-TSH concentration (13 ng/mg tissue), but similar to intact TSH content (105 ng/mg tissue). In serum, the α-TSH concentration (6.5 ng/ml) was lower than in most previously reported patients with thyrotrophic adenomata. TSH bioactivity in tumour tissue (approximately 600 ng/mg tissue) was greater than the immunoreactive TSH concentration. LH, FSH and prolactin were undetectable in tumour tissue, and in the culture medium throughout the period of cell culture. Exposure of thyrotroph monolayers to 10-6 M TRH for 4 h led to 32% - 600% increases in TRH release, depending on the day of culture and the cell flask utilized. The α-TSH response to TRH paralleled that of intact TSH, β-TSH being unmeasurable. Incubation of cell monolayers for 4 h in 5 x 10-7 M dopamine decreased TSH and α-TSH secretion into the medium. In contrast, exposure of cells to T3 at concentrations as high as 10-6 M did not affect TSH secretion. Surprisingly, TRH (10-7 M to 2 x 10-6 M) inhibited cultured thyrotroph cAMP content by about 50%; cGMP levels were unaffected. This study, the first to examine secretory function of human thyrotrophic tumour cells in vitro, confirms the disproportionate production of α-TSH by these tumours, even though this may not be apparent from the level of the α-subunit in serum. The data indicate that TSH secretion by thyrotroph tumour cells, at least from this particular patient, remains sensitive to TRH stimulation and dopamine inhibition, but insensitive to inhibition by T3. Not all TSH bioactivity is reflected by the immunoassayable TSH concentration, suggesting that the tumour may be producing variants of normal TSH that nevertheless retain biological activity. Finally, the data suggest that, at least in this particular tumour, TRH stimulation of TSH secretion is not mediated by cAMP as a second messenger.