Endocrinology, Vol 127, 1240-1244, Copyright © 1990 by Endocrine Society

ARTICLES

The functional expression of recombinant human thyrotropin receptors in nonthyroidal eukaryotic cells provides evidence that homologous desensitization to thyrotropin stimulation requires a cell-specific factor

GD Chazenbalk, Y Nagayama, KD Kaufman and B Rapoport
Thyroid Molecular Biology Unit, Veterans Administration Medical Center, San Francisco, California 94121.

TSH desensitization involves decreased coupling of the TSH receptor to the adenylate cyclase regulatory protein, Gs. There is evidence that a desensitization protein in thyroid cells plays a role in this process. The molecular cloning of the human TSH receptor and its stable expression in Chinese hamster ovary (CHO-TSHR) cells allowed us to test whether or not TSH desensitization can occur in a nonthyroidal cell. Similar to human thyroid cells, maximal stimulation of cAMP levels in CHO-TSHR cells was attained after 30-60 min of exposure to bovine TSH. Unlike in human thyroid cells, however, preincubation of CHO-TSHR cells with TSH for 12-16 h did not decrease the subsequent cAMP response to a 1-h pulse of TSH stimulation. That is, the human TSH receptor in CHO- TSHR cells does not undergo functional desensitization. Scatchard plot analysis of specific TSH binding to the CHO-TSHR cells revealed high and low affinity sites (Ka of 1.8 +/- 0.4 x 10(9) M-1 and 1.4 +/- 0.3 x 10(7) M-1, respectively), with approximately 10(5) TSH receptors per cell. This is 10- to 100-fold greater than the number of TSH receptors estimated to be present on human thyroid cells. Untransfected CHO cells exhibited only the low affinity binding site. Prior exposure of CHO- TSHR cells to bovine TSH or to (Bu)2cAMP for periods up to 24 h did not reduce [125I]TSH binding to these cells. In summary, desensitization of the adenylate cyclase response to TSH stimulation does not occur in nonthyroidal cells expressing a human TSH receptor with normal functional and TSH binding characteristics. These data support the concept that a cell-specific protein may be involved in homologous TSH desensitization.

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