Excess production of free alpha subunits by mouse pituitary thyrotropic tumor cells in vitro

Primary cultures from three serially transplanted mouse pituitary thyrotropic tumors, Furth 97A, 97B, and 97C, and one newly generated tumor, NIH 101A, were shown to secrete free a subunits and a β subunit (TSH-β) in addition to intact TSH. The three glycopeptides were measured in specific, heterologous radioimmunoassays. Basal secretion of a subunit was found to be in excess in all four thyrotropic tumor cultures, ranging from 5.7- to 8.9-fold higher than that of intact TSH and 30- to 35-fold higher than that of free TSH-β. In cultures from 97A, TRH (10^-7 M) stimulated the secretion of TSH, a subunit, and TSH-β by 175185% and (T4 10^-5 M) inhibited TSH to 19%, a subunit to 68%, and TSH-β to 58% of control. Secretion of TSH and its subunits in cultures from 97B was not consistently affected by TRH or T₄. In cultures from 97C, TSH and α secretion was inhibited in a dosedependent fashion by T₃ from 2.5 × 10^-9 to 1.6 × 10^-7 M. Gel chromatography of incubation media from tumor 97A revealed elution volumes of TSH and a similar to those of purified rat glycopeptides and confirmed the excess secretion of free α subunit. In contrast, the a to TSH ratios in media from cultures of pituitary cells from normal and thyroidectomized rats were 1.6 and 0.8, respectively. The total production of TSH and its subunits was measured in cultures from tumor 97A and confirmed that α subunit was synthesized excessively. The ratio of the plasma concentrations of alpha to TSH in tumor-bearing mice was 1.6 compared to 1.3 in thyroidectomized mice, and 0.36 in thyroidectomized rats. The α to TSH ratios in extracts of the corresponding thyrotropic tissues (tumor or pituitary) were 2.7, 1.7, and 0.40, respectively. These data are analogous to those reported in humans with thyrotropic and certain other pituitary tumors, and support the hypothesis that the subunits of glycoprotein hormones are independently synthesized.