Effects of a Thyroid-Stimulating Human Monoclonal Autoantibody (M22) on Functional Activity of LH and FSH Receptors

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Abstract
Objective: The glycoprotein hormones luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyrotropin (TSH) show low-level cross-reactivity between their respective receptors (R). Patient serum autoantibodies to the thyrotropin receptor (TSHR) do not appear to cross-react with the luteinizing hormone receptor (LHR) or follicle-stimulating hormone receptor (FSHR), although the concentrations of autoantibody with which it is feasible to carry out experiments of this type are limited. Consequently, we have studied the effects of high doses of the thyroid-stimulating human monoclonal autoantibody (M22) on the LHR and FSHR. Design: Chinese Hamster ovary (CHO) cells stably expressing the TSHR, LHR, and FSHR and purified M22 IgG preparations were used in the study. Methods: CHO-TSHR, CHO-LHR, and CHO-FSHR cells were incubated with bovine TSH (0.1–25mU/mL), human recombinant chorionic gonadotropin (hCG; 0.5–10mU/mL) or human recombinant FSH (100–5000mU/mL) or with M22 IgG (0.001–5.0 µg/mL), and the extracellular cyclic AMP was measured by radioimmunoassay. Results: Cyclic AMP levels increased in a dose-dependent manner after incubation of CHO-TSHR cells with TSH or M22 IgG, and on a molar basis the effects of TSH and M22 were similar. Cyclic AMP stimulation was not detectable in CHO-LHR and CHO-FSHR cells after incubation with M22 IgG, whereas incubation with hCG or FSH, respectively, caused dose-dependent cyclic AMP stimulation. On a molar basis, concentrations of M22 IgG approximately 100× those of FSH causing clear stimulation were ineffective with CHO-FSHR cells. Similarly, molar concentration of M22 IgG 20,000× those of hCG causing clear stimulation had no effect on CHO-LHR cells. Conclusions: This study shows that at relatively high concentrations, M22 IgG is unable to stimulate cyclic AMP levels in CHO-LHR or CHO-FSHR cells, suggesting that TSHR autoantibodies have greater specificity for the TSHR than TSH itself.