Persistent signaling by thyrotropin-releasing hormone receptors correlates with G-protein and receptor levels

Laboratory of Endocrinology and Receptor Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-8029, USA.
The FASEB Journal (Impact Factor: 5.04). 05/2012; 26(8):3473-82. DOI: 10.1096/fj.12-207860
Source: PubMed


G-protein-coupled receptors with dissociable agonists for thyrotropin, parathyroid hormone, and sphingosine-1-phosphate were found to signal persistently hours after agonist withdrawal. Here we show that mouse thyrotropin-releasing hormone (TRH) receptors, subtypes 2 and 1(TRH-R2 and TRH-R1), can signal persistently in HEK-EM293 cells under appropriate conditions, but TRH-R2 exhibits higher persistent signaling activity. Both receptors couple primarily to Gα(q/11). To gain insight into the mechanism of persistent signaling, we compared proximal steps of inositolmonophosphate (IP1) signaling by TRH-Rs. Persistent signaling was not caused by slower dissociation of TRH from TRH-R2 (t(1/2)=77 ± 8.1 min) compared with TRH-R1 (t(1/2)=82 ± 12 min) and was independent of internalization, as inhibition of internalization did not affect persistent signaling (115% of control), but required continuously activated receptors, as an inverse agonist decreased persistent signaling by 60%. Gα(q/11) knockdown decreased persistent signaling by TRH-R2 by 82%, and overexpression of Gα(q/11) induced persistent signaling in cells expressing TRH-R1. Lastly, persistent signaling was induced in cells expressing high levels of TRH-R1. We suggest that persistent signaling by TRHRs is exhibited when sufficient levels of agonist/receptor/G-protein complexes are established and maintained and that TRH-R2 forms and maintains these complexes more efficiently than TRH-R1.

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    • "Together these results confirm the importance of arrestin for both desensitization and internalization and prove that internalization is not the cause of desensitization. In highly sensitive assays, persistent IP1 production can be demonstrated for up to an hour after TRH is washed out in HEK293 cells expressing TRH receptors (Boutin et al., 2012). This sustained response does not depend on internalization. "
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