Constitutive activity of native thyrotropin-releasing hormone receptors revealed using a protein kinase C-responsive reporter gene.
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.
Jinsi-Parimoo
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Department of Medicine, Cornell University Medical College, The New York Hospital, New York, New York 10021, USA.
Gershengorn
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MC
D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology; D019556:COS Cells; D004285:Dogs; D017930:Genes, Reporter; D007295:Inositol Phosphates / Q000378:metabolism; D008156:Luciferases / Q000235:genetics; D051379:Mice; D008874:Midazolam / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D018025:Receptors, Thyrotropin-Releasing Hormone / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection
D007295:Inositol Phosphates; D018025:Receptors, Thyrotropin-Releasing Hormone; D001569:Benzodiazepines; D008156:Luciferases; D011493:Protein Kinase C; D008874:Midazolam