THYROTROPIN BINDING-SPECIFICITY FOR THE THYROTROPIN RECEPTOR

Recently, highly purified bovine thyrotropin (bTSH) of pituitary origi n, as well as recombinant human (h) TSH free of lutropin (LH) contamin ation, has been reported to activate the LH/choriogonadotropin recepto r (LH/CGR). These data challenge the concept of TSH specificity for it s own receptor. We, therefore, re-evaluated these data using, as targe ts, the recombinant hTSH and rat LH/CGRs stably expressed in Chinese h amster ovary (CHO) cells, Partially purified bTSH (2 IU/mg protein) an d, to a lesser degree, highly purified bTSH (30 IU/mg protein) increas ed intracellular cAMP levels in CHO-LH/CGR cells (an EC50 of 0.2 and > 20 mIU/ml, respectively). In contrast, recombinant hTSH (up to 1 IU/m l) did not. All three TSH preparations increased cAMP levels to the sa me extent in CHO-TSHR cells (an EC50 of 0.3 mIU/ml). Furthermore, we o bserved only nonspecific, low affinity TSH binding for CHO-LH/CGR cell s and also for CHO cells transfected with the expression vector alone (a Kd of 100 nM), although both high and low affinity TSH binding was demonstrated in CHOT-SHR cells (a Kd of 0.3 and 100 nM, respectively). These data indicate that even highly purified bTSH of pituitary origi n contains significant amounts of LH, and that TSH itself does not app ear to activate the LH/CGR.