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Thyrotrophin-dependent desensitization by Chinese hamster ovary cells that express the recombinant human thyrotrophin
K Haraguchi1, X Peng, M Kaneshige
1Third Department of Internal Medicine, University of Yamanashi Medical School, Japan.
Abstract:
To determine whether thyrotrophin (TSH)-induced desensitization requires a thyroid-specific factor(s), the human TSH (hTSH) receptor was expressed in Chinese hamster ovary cells. The first incubation of the cells with TSH decreased the subsequent response of adenosine 3',5'-cyclic monophosphate to freshly added TSH in the second incubation. This homologous desensitization was observed as early as after 3 h of the first incubation. The lowest dose of TSH that elicited desensitization was 0.1 nmol/l. The desensitization was not overcome by adding higher doses of TSH in the second incubation. A 125I-labelled TSH-binding study revealed a decrease in the number of high-affinity binding sites but not in that of low-affinity binding sites. The data suggest that TSH-induced desensitization in hTSH receptor-transfected cells is caused, at least in part, by a decrease in the number of TSH receptors on the cell surface. The evidence demonstrates, contrary to an earlier report, that a thyroid-specific factor(s) is not required for hTSH receptor desensitization.
Insights
Thyroid-stimulating hormone (TSH) desensitization does not require thyroid-specific factors. Expressing the human TSH receptor in non-thyroid cells showed TSH receptor downregulation, reducing cellular response.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Signaling
Background:
- Thyroid-stimulating hormone (TSH) regulates thyroid function through its receptor.
- TSH receptor desensitization is a known phenomenon that can affect thyroid hormone production.
- The necessity of thyroid-specific factors for TSH receptor desensitization has been debated.
Purpose of the Study:
- To investigate whether thyroid-specific factors are required for TSH-induced receptor desensitization.
- To characterize the mechanism of TSH receptor desensitization in a heterologous system.
Main Methods:
- Human TSH receptor was expressed in Chinese hamster ovary (CHO) cells.
- Cells were incubated with TSH, and subsequent responses of cyclic adenosine monophosphate (cAMP) were measured.
- 125I-labeled TSH binding assays were performed to assess receptor availability.
Main Results:
- TSH induced homologous desensitization in hTSH receptor-transfected CHO cells, observed as early as 3 hours.
- Desensitization occurred at low TSH concentrations (0.1 nmol/l) and was not overcome by higher TSH doses.
- A decrease in high-affinity TSH binding sites, but not low-affinity sites, was observed, suggesting TSH receptor downregulation.
Conclusions:
- TSH-induced desensitization of the human TSH receptor can occur independently of thyroid-specific factors.
- The mechanism involves a reduction in the number of cell surface TSH receptors.
- This finding challenges previous reports suggesting the requirement of thyroid-specific factors for TSH receptor desensitization.