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

Abstract

IntroductionHuman thyrotropin (TSH), follitropin (FSH), lutropin (LH), and chorionic gonadotropin (CG) are members of the glycoprotein hormone family derived from heterodimerization of a common α subunit with hormone-specific β subunits. These hormones were originally purified from the anterior pituitary (TSH, LH, and FSH) and placenta (human CG) and shown to activate specific G protein-coupled receptors in the thyroid (TSH receptor) and gonads (LH and FSH receptors), respectively (1-4). These three pituitary-derived glycoprotein hormones form the basis of the classic pituitary-peripheral target feedback systems and are essential for the development and differentiation of thyroid and gonadal tissues. In particular, TSH is essential for the production of iodothyronines by the thyroid gland and disorders of the pituitary-thyroid gland-thyroid hormone axis lead to disturbances of essentially all metabolic pathways and organs (5, 6).Based on GenBank searches, we identified two additional human glycoprotein hormone subunitlike genes and named them α2 (A2) and β5 (B5), due to their structural similarities to known subunits and the chronology of discovery (7). (The GenBank accession numbers for A2 and B5 are AF403384 and AF403430, respectively.) A2 and B5 have conserved cysteine residues, similar to those found in the well-characterized α and β subunits (8, 9) important for the formation of key disulfide bonds. Like all other glycoprotein hormone subunits, A2 and B5 have the unique cystine knot structure characteristic of proteins related to the TGF-β, the PDGF, and the bone morphogenetic protein families (10, 11). Because the putative A2 subunit is likely to combine with either known or novel β subunits to yield bioactive heterodimeric hormones, we performed a yeast two-hybrid protein-protein interaction screen to identify potential dimerization partners for A2 and found interactions between A2 and B5. We generated Ab's against A2 and B5 to confirm the interactions between these putative subunits and their colocalization in the anterior pituitary. Following testing for the activation of glycoprotein hormone receptors, 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 th...

Subclinical Hyperthyroidism in Patients with Nodular Goiter Represents a Hypermetabolic State

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FLR‐2, the glycoprotein hormone alpha subunit, is involved in the neural control of intestinal functions in Caenorhabditis elegans

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Acute Inflammation Increases Pituitary and Hypothalamic Glycoprotein Hormone Subunit B5 mRNA Expression in Association with Decreased Thyrotrophin Receptor mRNA Expression in Mice

“…Thyrostimulin, a newly discovered glycoprotein hormone consisting of two glycoprotein subunit‐related genes, α2 (GPA2) and β5 (GPB5), activates the thyrotrophin (TSH) receptor (TSHR) both in vitro and in vivo (1). Because GPA2 and GPB5 co‐localise in the rat and human anterior pituitary where the TSHR is known to be expressed, a paracrine role for thyrostimulin was suggested (1–3). However, the putative biological role for the heterodimer in vivo is still a matter of debate (4–6).…”