Abstract
The concept of functional selectivity has now thoroughly supplanted the previously entrenched notion of intrinsic efficacy by explaining how agonists and antagonists exhibit a range of efficacies for distinct receptor-mediated responses. It is noteworthy that functional selectivity accommodates significant changes in efficacy resulting from differential expression of G protein-coupled receptor modifying proteins (i.e., “conditional efficacy”)—a phenomenon with profound implications for drug discovery. We have uncovered a novel regulatory mechanism whereby p90 ribosomal S6 kinase 2 (RSK2) interacts with 5-hydroxytryptamine2A (5-HT2A) serotonin receptors and attenuates receptor signaling via direct receptor phosphorylation (Proc Natl Acad Sci U S A 103:4717–4722, 2006; J Biol Chem 284:5557–5573, 2009). This discovery, together with the mounting evidence for conditional efficacy, suggested to us that 5-HT2A agonist signaling might be disproportionately affected by alterations in RSK2 expression. To test this hypothesis, we evaluated a chemically diverse set of 5-HT2A agonists at three readouts of 5-HT2A receptor activation in both wild-type (WT) and RSK2 knock-out (KO) mouse embryonic fibroblasts (MEFs). Here we report that 5-HT2A receptor agonist efficacies were significantly and variably augmented in RSK2 KO MEFs compared with WT MEFs. As a result, relative agonist efficacies were significantly altered, and even reversed, between WT and RSK2 KO MEFs for a single effector readout. This study provides the first evidence that deletion of a single kinase can elicit profound changes in patterns of agonist functional selectivity.
Footnotes
This work was supported by the National Institutes of Health National Institute of Mental Health Psychoactive Drug Screening Program [Grants R01-MH61887, U19-MH82441]; the Michael Hooker Chair for Therapeutics and Translational Proteomics; and the Michael Hooker Program in Translational Proteomics and Therapeutics.
Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
doi:10.1124/mol.109.061440
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ABBREVIATIONS:
- GPCR
- G protein-coupled receptor
- RSK2
- p90 ribosomal S6 kinase 2
- 5-HT
- serotonin, 5-hydroxytryptamine
- WT
- wild type
- KO
- knockout
- SB242084
- 6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride hydrate
- IP
- inositol phosphate
- AA
- arachidonic acid
- ERK
- extracellular signal-regulated kinase
- DOI
- (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride
- quipazine
- 2-(1-piperazinyl)-quinoline maleate
- 5-methoxy-DMT
- 5-methoxy-N,N-dimethyltryptamine
- m-CPP
- 1-(m-chlorophenyl)-piperazine
- SCH-23390
- R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride
- α-Me-5-HT
- α-methyl serotonin
- MK212
- 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride
- MDL-100907
- R-(+)-)-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol
- DMEM
- Dulbecco's modified Eagle's medium
- FBS
- fetal bovine serum
- FLIPR
- Fluorometric Imaging Plate Reader
- RFU
- relative fluorescence unit
- PBS
- phosphate-buffered saline
- ANOVA
- analysis of variance
- MEF
- mouse embryonic fibroblast
- CHAPS
- 3-[(3-cholamidopropyl)dimethylammonio]propanesulfonate.
- Received October 2, 2009.
- Accepted November 19, 2009.
- Copyright © 2010 The American Society for Pharmacology and Experimental Therapeutics
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