| Abstract: | In an attempt to identify a possible pathogenetic role for the hCG molecule in the mechanism of the hyperthyroidism which occurs in choriocarcinoma, we have looked for evidence that the hCG molecule has a thyrotropic action on the human thyroid. The thyrotropic activity of various hCG preparations on the human thyroid was assessed by measuring the stimulation of adenylate cyclase activity in human thyroid plasma membranes purified by sucrose density gradient centrifugation. The highly purified hCG CR119 preparation stimulated human thyroid adenylate cyclase activity. Its activity was more than 654 times greater than could be accounted for by human TSH (hTSH) contamination of the preparation, as determined by RIA. The thyrotropic activity intrinsic to 1.0 IU hCG was equivalent to roughly 0.27 microU hTSH. Significant saturable binding of the 125I-labeled highly purified hCG preparation to human thyroid membranes was demonstrated, and the bound component was characterized. Its apparent molecular size, subunit composition, and testis receptor-binding characteristics were those of the hCG molecule. Examination of a crude urinary hCG preparation in adenylate cyclase and TSH radioligand assays using human thyroid membranes showed no evidence of any molecule other than hCG with a thyrotropic action on the human thyroid. Given that hCG binds to and stimulates adenylate cyclase activity in human thyroid tissue, as the above data indicate, then human LH (hLH) would be expected to do the same, since hLH and hCG have such strong structural and functional similarities. As anticipated, a highly purified hLH preparation exhibited TSH binding inhibition and adenylate cyclase stimulation. Its activity was more than 1030 times greater than could be accounted for by hTSH contamination of the preparation. The thyrotropic activity intrinsic to 1.0 IU hLH was equivalent to roughly 44 microU hTSH. Thus, in addition to other shared properties, the hLH molecule and the hCG molecule share the ability to interact with human thyroid tissue. These results strongly indicate that the hCG molecule has a thyrotropic action on the human thyroid and support the hypothesis that hCG is the thyrotropic factor that mediates the hyperthyroidism which occurs in patients with hCG-secreting neoplasms. |
