Thyrostimulin, a heterodimer of two new human glycoprotein hormone subunits, activates the thyroid-stimulating hormone receptor
Koji Nakabayashi, … , Sheau Yu Hsu, Aaron J.W. Hsueh
Koji Nakabayashi, … , Sheau Yu Hsu, Aaron J.W. Hsueh
Published June 1, 2002
Citation Information: J Clin Invest. 2002;109(11):1445-1452. https://doi.org/10.1172/JCI14340.
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Article Endocrinology

Thyrostimulin, a heterodimer of two new human glycoprotein hormone subunits, activates the thyroid-stimulating hormone receptor

Abstract

Human thyrotropin (TSH), luteotropin (LH), follitropin (FSH), and chorionic gonadotropin are members of the heterodimeric glycoprotein hormone family. The common α subunit forms noncovalent heterodimers with different β subunits. Two novel human glycoprotein hormonelike genes, α2 (A2) and β5 (B5), recently have been identified. Using a yeast two-hybrid assay, the two subunits were found as potential heterodimerization partners. Immunological analyses confirmed the heterodimerization of A2 and B5 in transfected cells and their colocalization in the anterior pituitary. Recombinant A2/B5 heterodimeric glycoproteins, purified using cation exchange and size fractionation chromatography, activated human TSH receptors, but not LH and FSH receptors, and showed high affinity to TSH receptors in a radioligand receptor assay. The heterodimer also stimulated cAMP production and thymidine incorporation by cultured thyroid cells and increased serum thyroxine levels in TSH-suppressed rats in vivo. This new heterodimeric glycoprotein hormone was named as thyrostimulin based on its thyroid-stimulating activity. The expression of thyrostimulin in the anterior pituitary known to express TSH receptors suggested a paracrine mechanism. The present discovery of a new ligand based on genomic approaches could facilitate the understanding of the physiological roles of extra-thyroid TSH receptor systems and the structural-functional basis of receptor signaling by related glycoprotein hormones.

Authors

Koji Nakabayashi, Hirotaka Matsumi, Alka Bhalla, Jeehyeon Bae, Sietse Mosselman, Sheau Yu Hsu, Aaron J.W. Hsueh

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Thyroid-stimulating activity of the purified A2/B5 heterodimer. (a) Puri...
Thyroid-stimulating activity of the purified A2/B5 heterodimer. (a) Purified A2/B5, like TSH, stimulated cAMP production mediated by human TSH receptors, but not by LH and FSH receptors. Dose-dependent effects of A2/B5 and other hormones (left panel). b, bovine; hrec, human recombinant; h, human. Lack of stimulation of LH and FSH receptors by A2/B5 (right panel). Human CG or FSH (100 ng/ml) were used to serve as positive controls (right panel). (b) Ability of specific A2 or B5 Ab’s (1:100 dilution) to neutralize the stimulatory effect of A2/B5 (0.3 ng/ml) on cAMP production mediated by TSH receptors. Addition of preimmune serum did not alter the stimulatory effects of A2/B5. In addition, pretreatment with A2 and B5 antisera did not affect cAMP production induced by human TSH (3 ng/ml). (c) Displacement of labeled bovine TSH from the recombinant human TSH receptors by A2/B5. Human CG was included as a negative control. (d) Purified A2/B5 (100 ng/ml) promoted cAMP production and thymidine incorporation by cultured rat thyroid FRTL5 cells in vitro. bTSH, bovine TSH (100 ng/ml). (e) A2/B5 induction of T4 production by the thyroid gland in mice pretreated with triiodothyronine (T3) to suppress endogenous TSH levels. Each data point represents the mean ± SE of three to four determinations with similar results obtained in at least three separate experiments.