The extracellular calcium-sensing receptor (CaR) senses small fluctuations of the extracellular calcium (Ca(2+)(e)) concentration and translates them into potent changes in parathyroid hormone secretion. Dissecting the regulatory mechanisms of CaR-mediated signal transduction may provide insights into the physiology of the receptor and identify new molecules as potential drug targets for the treatment of osteoporosis and/or hyperparathyroidism. CaR can be phosphorylated by protein kinase C (PKC) and G protein-coupled receptor kinases (GRKs), and has been shown to bind to beta-arrestins, potentially contributing to desensitization of CaR, although the mechanisms by which CaR-mediated signal transduction is terminated are not known. We used a PKC phosphorylation site-deficient CaR, GRK and beta-arrestin overexpression or down-regulation to delineate CaR-mediated desensitization. Fluorescence-activated cell sorting was used to determine whether receptor internalization contributed to desensitization. Overexpression of GRK 2 or 3 reduced Ca(2+)(e)-dependent inositol phosphate accumulation by more than 70%, whereas a GRK 2 mutant deficient in G alpha(q) binding (D110A) was without major effect. Overexpression of GRK 4-6 did not reduce Ca(2+)(e)-dependent inositol phosphate accumulation. Overexpression of beta-arrestin 1 or 2 revealed a modest inhibitory effect on Ca(2+)(e)-dependent inositol phosphate production (20-30%), which was not observed for the PKC phosphorylation site-deficient CaR. Agonist-dependent receptor internalization (10-15%) did not account for the described effects. Thus, we conclude that PKC phosphorylation of CaR contributes to beta-arrestin-dependent desensitization of CaR coupling to G proteins. In contrast, GRK 2 predominantly interferes with G protein-mediated inositol-1,4,5-trisphosphate formation by binding to G alpha(q).

The beta-arrestin-dependent endocytosis of the beta2-adrenergic receptor (beta2AR) has been demonstrated by confocal fluorescence microscopy. Furthermore, a constitutively activated beta2AR is also constitutively desensitized and down-regulated. To clarify the function of beta-arrestin 1 or 2 for TSH receptor (TSHR) desensitization and examine whether constitutively activated TSHR mutants are internalized in a different way, we investigated the TSHR trafficking in association with beta-arrestins in cotransfection experiments in HEK 293 cells using confocal laser-scanning microscopy. We found that both beta-arrestins are able to internalize the TSHR in HEK 293 cells. However, whereas the beta-arrestin 1-mediated TSHR internalization reached its maximum 20 min after TSH stimulation, the beta-arrestin 2-mediated TSHR internalization already reached its maximum 5 min after TSH stimulation. Furthermore, an increased basal desensitization and internalization of constitutively activated TSHR mutants N670S, S505N, and F631L cotransfected with beta-arrestin 2 could not be found. After TSH stimulation the constitutively activated mutants showed the same time course for internalization as the wild-type-TSHR. In summary, contrary to data obtained for the beta2AR, the constitutive activation of the TSHR does not influence the desensitization and time course for internalization of the receptor, and in agreement with findings for the FSH and LH receptors, these results characterize the TSH receptor as a member of the class A of G protein-coupled receptors, which have a higher affinity to beta-arrestin 2 than beta-arrestin 1 and do not colocalize with beta-arrestins in endosomes.

Molecular eventsthat lead to the development of autonomously functioning thyroid nodules (AFTNs) are somatic mutations of the thyrotropin receptor (TSHR) in approximately 60% of the nodules and less frequently, somatic mutations in the G s a protein. However, AFTNs without known mutations indicate that other causes remain to be identified. Moreover, the impact of constitutively activating TSHR mutations on the signal transduction network of the thyroid epithelial cell is unknown. We therefore investigated gene expression in 15 AFTNs and their surrounding tissue using Affymetrix GeneChips. Most prominently, data analysis revealed a changed pattern of gene expression in the TGF-b signaling cascade and 25 differentially regulated genes in AFTNs, including thyroid peroxidase, type I iodothyronine deiodinase and sialyltransferase (SIAT) 1. Strikingly coexpression of SIAT 1 and TSHR in COS-7 cells increased TSH binding and cell surface expression of the TSHR. Moreover, differences in gene expression patterns for AFTNs with and without TSHR mutations indicate specific alterations of signal transduction in AFTNs without TSHR mutations. These results suggest that AFTNs with TSHR mutations harbor further mechanisms of forward stimulation. Furthermore, they give important leads to elucidate the molecular etiology of AFTNs without TSHR mutations.

Glycosylation of the thyrotropin receptor (TSHR) has been shown to be essential for correct protein folding and for cellsurface targeting. In a recent study, we detected increased expression of ␤-galactoside ␣(2,6)-sialyltransferase (SIAT1) in toxic thyroid adenomas where gain-of-function mutations of the TSHR have been invoked as one of the major causes. To investigate the physiological meaning of these findings, we designed experiments to evaluate the consequences of sialylation for the expression of the TSHR. Hence, we investigated the effect of coexpressing the TSHR and different sialyltransferases (SIAT1, SIAT4a, and SIAT8a) for cell-surface expression of the receptor. Coexpression of each of the three SIAT isoforms and the TSHR in COS-7 cells increased TSHR expression on the cell surface in the range of 50 to 100%. Moreover, Western blot analysis with lectins specific for ␣(2,3) and ␣(2,6)-linked sialic acids and lectin-binding enzyme-linked immunosorbent assay support a direct effect on TSHR cell-surface expression mediated by sialic acid transfer to the TSHR. Finally, we treated living COS-7 cells after cotransfection of TSHR and SIAT8a with neuraminidase for 30 min to remove covalently linked sialic acid. Subsequent loss of TSHR cell-surface expression suggests that sialylation prolongs the resting time of the TSHR on the cell surface. Our data demonstrate for the first time that the transfer of sialic acid can improve and prolong cell-surface expression of a transmembrane receptor.

Glycosylation of the thyrotropin receptor (TSHR) has been shown to be essential for correct protein folding and for cellsurface targeting. In a recent study, we detected increased expression of ␤-galactoside ␣(2,6)-sialyltransferase (SIAT1) in toxic thyroid adenomas where gain-of-function mutations of the TSHR have been invoked as one of the major causes. To investigate the physiological meaning of these findings, we designed experiments to evaluate the consequences of sialylation for the expression of the TSHR. Hence, we investigated the effect of coexpressing the TSHR and different sialyltransferases (SIAT1, SIAT4a, and SIAT8a) for cell-surface expression of the receptor. Coexpression of each of the three SIAT isoforms and the TSHR in COS-7 cells increased TSHR expression This work was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG/Pa 423/12-1) and the Interdisciplinary Center for Clinical Research at the Faculty of Medicine of the University of Leipzig (Projects B20, Z03, and Z16-CHIP2) and Deutsche Krebshilfe (10652). Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.