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...

Human TSH receptor ligands as pharmacological probes with potential clinical application

“…TSH is the primary endogenous, physiologic stimulator of TSHR function. A second endogenous activator of TSHR function, thyrostimulin (or corticotroph-derived glycoprotein hormone), has been identified [30,31]. Thyrostimulin, like TSH, is a heterodimeric glycoprotein but is composed of different subunits – glycoprotein hormone α2 and glycoprotein hormone β5.…”

An Intimate Relationship between Thyroid Hormone and Skin: Regulation of Gene Expression

“…In addition, TSH stimulation up-regulates the transcription of classical TSH target genes thyroglobulin and thyroid transcription factor-1 (Nkx2.1) and enhances cAMP production into the culture medium (58), documenting that the TSH receptor expressed by normal human scalp HF in situ is functionally active. Behind the canonical TSH-dependent regulation, recently it has been demonstrated that the TSHR can also be activated by a newly discovered glycoprotein hormone, known as thyrostimulin (76). This hormone is composed of a dimer of unique α 2 and β 5 subunits.…”

The thyroid axis just got more complicated

“…If so, does thyrostimulin control metabolism in cardiac or adipose tissue, or does it modify the immune response of the orbital fibroblasts in Graves disease or in the controversial disorder termed Hashimoto encephalopathy? For now, there are many more questions than answers, but the findings of Nakabayashi et al (2) have clearly expanded our view of what constitutes normal thyroid physiology.…”

“…The authors show here that A2 interacts strongly with FSH-β, CG-β, and B5 in a yeast two-hybrid assay (2). After determining that the A2/B5 pair can stimulate cAMP production via the TSH receptor, Nakabayashi and colleagues pursued the thyrotropic activity of A2/B5 in greater detail.…”

“…Its pattern of expression is more restricted than that of A2 and includes the brain, pituitary, thyroid, ovary, and heart. Nakabayashi et al clearly demonstrate the coexpression of A2 and B5 in the anterior pituitary (2), although the precise cell types expressing these subunits have not yet been identified.…”