Polymorphism of thyrotropin and alpha subunit in human pituitary adenomas

To understand better why patients with TSH-secreting pituitary tumors exhibit variable degree of hyperthyroidism, we analyzed the various isoforms of TSH and alpha-subunit secreted by 4 TSH-secreting adenomas in primary culture. All patients had macrodenomas clinically associated with hyperthyroidism with normal to elevated TSH plasma levels. The in vivo molar alpha/TSH ratio ranged from 18.4 to 3.8. The hormone material secreted over 4 to 48 h in culture was separated by gel isoelectrofocusing, eluted and estimated by immunoassays. The release of free alpha-subunit was noticeably different among adenomas. Three tumors were found to release an homogeneous and acidic (pI = 5.4-4.5) species totally unrelated to the alpha-subunit dissociated from intrapituitary TSH (5 isoforms, pI = 8.8-5.8) while another was more heterogeneous (pI = 8.8, 8.4, 7.6, 6.8, 5.8, 5.4-4.5). Tumoral TSH exhibited at least six detectable isoforms (pI = 8.6, 8.3-8.0, 7.5, 7.0, 6.5, 6.0) very similar to those present in a purified intrapituitary hormone preparation. While intrapituitary TSH was composed of 70% of alkaline (pI = 8.6-7.5), 25% of neutral (pI = 7.0-6.0) and 5% (pI = 5.8-4.5) of acidic forms, these species were found to be more evenly distributed in adenomatous secretion (43%/42%/15%). The TSH-secreting tumors thus appeared to relase preferentially neutral and acidic forms of TSH than alkaline components but for one tumor, this ratio could be modified by chronic incubation with TRH. When assayed for their capacity to stimulate 3H-thymidine incorporation in FRTL-5 cells, neutral TSH appeared definitely less potent than the alkaline and acidic isohormones. Altogether, these data show that pituitary adenomas synthesize normal forms of TSH but release them in variable amount in the medium. When circulating in the blood, the ratio between active and inactive isoforms of TSH may thus be responsible for the variable stimulation of the thyroid gland observed in the patients.