Glycoprotein hormone receptors such as the lutropin/chorionic gonadotropin receptor (LHR) are characterized by a large N-terminal ectodomain (ECD), which is responsible for hormone–receptor interactions. For the closely related TSH receptor (TSHR), it has been proposed that the ECD also serves as a tethered inverse agonist. However, the exact role of the LHR–ECD for receptor activation remains elusive. Functional analysis of N-terminally truncated LHR mutants expressed in COS-7 cells revealed that the LHR–ECD does not act as an inverse agonist but facilitates active LHR conformations. This notion is supported by two observations: first, removal of the ECD tended to decrease basal LHR activity and secondly, mutationally induced constitutive receptor activity was diminished for most activating mutations in LHR lacking the ECD. In addition, swapping of the LHR–ECD for the ECD of the closely related TSHR was not sufficient to restore constitutive receptor activity induced by naturally occurring activating heptahelical LHR mutations. Thus, the ECD stabilizes an activation-competent conformation of the heptahelical region. While the full-length LHR fused to the cognate agonist, human chorionic gonadotropin (hCG), showed increased basal activity, fusion proteins between hCG and N-terminally truncated LHR did not yield constitutive receptor activity suggesting an important role of the ECD also for agonist-dependent LHR activity. Our experiments strengthen the concept of a major contribution of the LHR–ECD in the activation mechanism apart from hormone binding and provide evidence for a cooperative model with structural and functional interactions of the ECD and the transmembrane domain.
Pascal Nurwakagari, Andreas Breit, Claudia Hess, Hagar Salman-Livny, David Ben-Menahem and Thomas Gudermann
Christer M Bäck, Stefanie Stohr, Eva A M Schäfer, Heike Biebermann, Ingrid Boekhoff, Andreas Breit, Thomas Gudermann and Thomas R H Büch
Metallothioneins (MTs) are cytoprotective proteins acting as scavengers of toxic metal ions or reactive oxygen species. MTs are upregulated in follicular thyroid carcinoma and are regarded as a marker of thyroid stress in Graves' disease. However, the mechanism of MT regulation in thyrocytes is still elusive. In other cellular systems, cAMP-, calcium-, or protein kinase C (PKC)-dependent signaling cascades have been shown to induce MT expression. Of note, all of these three pathways are activated following the stimulation of the TSH receptor (TSHR). Thus, we hypothesized that TSH represents a key regulator of MT expression in thyrocytes. In fact, TSHR stimulation induced expression of MT isoform 1X (MT1X) in human follicular carcinoma cells. In these cells, Induction of MT1X expression critically relied on intact Gq/11 signaling of the TSHR and was blocked by chelation of intracellular calcium and inhibition of PKC. TSHR-independent stimulation of cAMP formation by treating cells with forskolin also led to an upregulation of MT1X, which was completely dependent on PKA. However, inhibition of PKA did not affect the regulation of MT1X by TSH. As in follicular thyroid carcinoma cells, TSH also induced MT1 protein in primary human thyrocytes, which was PKC dependent as well. In summary, these findings indicate that TSH stimulation induces MT1X expression via Gq/11 and PKC, whereas cAMP–PKA signaling does not play a predominant role. To date, little has been known regarding cAMP-independent effects of TSHR signaling. Our findings extend the knowledge about the PKC-mediated functions of the TSHR.