STUDIES ON HOMOLOGOUS DESENSITIZATION OF THE THYROTROPIN RECEPTOR IN 293 HUMAN EMBRYONAL KIDNEY-CELLS

It is well known that the TSH receptor (TSHR) undergoes homologous des ensitization. That is, prolonged stimulation of thyroid cells with TSH attenuates the cAMP response to subsequent TSH stimulation. However, the existence of homologous desensitization of the recombinant TSHR ex pressed in nonthyroidal eukaryotic cells is controversial. In the pres ent studies, therefore, we first investigated whether or not the TSHR was desensitized by TSH in 293 human embryonal kidney cells, a cell li ne in which the LH/CG receptor (LH/CGR) is reported to undergo homolog ous desensitization. The wild type (wt) TSHR and the wt-LH/CGR stably expressed in 293 cells bound to their respective hormones with high af finity and produced a dose-dependent intracellular cAMP response to ho rmone stimulation. Pretreatment of cells expressing the TSHR or the LH /CGR with their respective hormones attenuated the cAMP response to su bsequent hormone stimulation without down-regulation of the receptors, demonstrating that the TSHR, as well as the LH/CGR, undergoes homolog ous desensitization in 293 cells. With this cell type expressing mutan t TSHRs, we then studied some aspects of the molecular mechanism of TS HR desensitization and compared our data to those obtained with the be ta-adrenergic receptor (beta-AR), which is widely regarded as the prot otype for receptor desensitization. We cotransfected the wt-TSHR and a chimeric receptor consisting of the LH/CGR extracellular ligand bindi ng domain with the TSHR transmembrane/cytoplasmic signal transducing r egion. These two receptors have distinct hormone specificities but sha re common signal regulatory mechanisms. We observed that, like the bet a-AR, only hormone-occupied receptor is likely to be involved in homol ogous desensitization. On the other hand, studies with a truncated TSH R indicated that, in contrast to the beta-AR, the serine/threonine-ric h region in the carboxyl two thirds of the cytoplasmic tail of the TSH R is not involved in homologous desensitization.