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GRK2 and beta-arrestin 1 as negative regulators of thyrotropin receptor-stimulated response
L Iacovelli1, R Franchetti, M Masini
1Consorzio Mario Negri Sud, Istituto di Ricerche Farmacologiche Mario Negri, Santa Maria Imbaro, Italy.
Abstract:
Arrestins are regulatory proteins for a number of G-coupled receptors. The binding of arrestin to receptor phosphorylated by G protein-coupled receptor kinase (GRK) quenches the activation of the G protein, thus resulting in receptor homologous desensitization. We have previously shown that the levels of beta-arrestin1 are regulated by intracellular cAMP and proposed that this may represent one homeostatic mechanism with which to regulate some cellular responses. To test this hypothesis, we focused on the TSH receptor using a rat thyroid cell line, FRTL5. We found that beta-arrestin1 is the only detectable isoform of arrestin expressed in FRTL5 and that its expression is regulated by TSH. To investigate the possible role of GRK2/beta-arrestin1 machinery in the mechanism of TSH receptor homologous desensitization, we used a cotransfection approach. The TSH-induced cAMP accumulation in COS7 cells transfected with TSH receptor was reduced by 35-45% when cotransfected with GRK2 and/or beta-arrestin1, indicating that the TSH receptor can be regulated by a GRK/arrestin mechanism. This raised the hypothesis that TSH increases the levels of beta-arrestin1, which in turn could regulate the TSH stimulation. To test this point a FRTL5-derived cell line overexpressing beta-arrestin1 was generated. In these cells the TSH-stimulated cAMP accumulation and, more importantly, the mitogenic activity were substantially blunted. Our results show that TSH receptor-stimulated cAMP accumulation and cell proliferation can be controlled by a GRK2/beta-arrestin1 mechanism.
Insights
Thyroid-stimulating hormone (TSH) receptor desensitization involves beta-arrestin1. Overexpressing beta-arrestin1 blunts TSH-stimulated cAMP accumulation and cell proliferation, revealing a GRK2/beta-arrestin1 regulatory mechanism.
Area of Science:
- Endocrinology
- Molecular Cell Biology
- G protein-coupled receptor signaling
Background:
- Arrestins regulate G protein-coupled receptors (GPCRs) by binding phosphorylated receptors, quenching G protein activation and causing homologous desensitization.
- Previous work indicated intracellular cAMP regulates beta-arrestin1 levels, suggesting a homeostatic mechanism for cellular responses.
- The TSH receptor, a key regulator of thyroid function, was investigated for its interaction with the arrestin system.
Purpose of the Study:
- To investigate the role of the GRK2/beta-arrestin1 machinery in TSH receptor homologous desensitization.
- To determine if TSH regulates beta-arrestin1 expression and if this impacts TSH receptor signaling.
- To elucidate the mechanism controlling TSH receptor-mediated cAMP accumulation and cell proliferation.
Main Methods:
- Utilized FRTL5 rat thyroid cells and COS7 cells for transfection studies.
- Employed cotransfection of TSH receptor with GRK2 and/or beta-arrestin1 to assess signaling modulation.
- Generated a FRTL5-derived cell line overexpressing beta-arrestin1 to study its effects on TSH stimulation.
Main Results:
- Beta-arrestin1 was identified as the sole arrestin isoform in FRTL5 cells, with its expression regulated by TSH.
- Cotransfection with GRK2 and/or beta-arrestin1 reduced TSH-induced cAMP accumulation in COS7 cells by 35-45%.
- Overexpression of beta-arrestin1 in FRTL5 cells significantly blunted TSH-stimulated cAMP accumulation and mitogenic activity.
Conclusions:
- The TSH receptor is regulated by a G protein-coupled receptor kinase (GRK)/arrestin mechanism.
- TSH-induced increases in beta-arrestin1 levels contribute to the desensitization of the TSH receptor.
- The GRK2/beta-arrestin1 pathway serves as a critical control mechanism for TSH receptor-mediated cAMP production and thyroid cell proliferation.