Constitutive Activity of Native Thyrotropin-Releasing Hormone Receptors Revealed Using a Protein Kinase C-Responsive Reporter Gene

• Published in: Endocrinology (Philadelphia) Vol. 138; no. 4; pp. 1471 - 1475
• Main Authors: Jinsi-Parimoo, Arti, Gershengorn, Marvin C
• Format: Journal Article
• Language: English
• Published: Endocrine Society 01.04.1997
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/endo.138.4.5059

The native TRH receptor (TRH-R), which is a G protein-coupled receptor that signals via the phosphoinositide transduction pathway, has been assumed to be inactive in the absence of agonist. In contrast, a mutant mouse TRH-R (C335Stop TRH-R) was shown previously to exhibit constitutive (or agonist-independent) signaling activity. In this report, we measured signaling activity of TRH-Rs using a protein kinase C-responsive reporter gene instead of formation of inositol phosphate second messenger molecules. Using this more sensitive system, we show that native mouse TRH-Rs exhibit agonist-independent signaling activity that is directly proportional to the number of receptors expressed in COS-1 cells and is inhibited by negative antagonist benzodiazepine drugs. As expected, the basal signaling activity of native TRH-Rs is lower than C335Stop TRH-Rs. Constitutive activity of native TRH-Rs is not peculiar to COS-1 cells in which receptor density is markedly elevated, because it can also be demonstrated in Madin Darby canine kidney cells stably expressing mouse TRH-Rs and GH4C1 cells endogenously expressing rat TRH-Rs. These findings support the thesis that native TRH-Rs oscillate between active and inactive states. We suggest that demonstration of constitutive activity of native receptors may depend on the sensitivity of the signaling assay employed.