Abstract
Despite the identification of 2-amino-3-benzoylthiophenes (2A3BTs) as the first example of small-molecule allosteric potentiators of agonist function at a G protein-coupled receptor (GPCR)—the adenosine A1 receptor—their mechanism of action is still not fully understood. We now report the mechanistic basis for the complex behaviors noted for 2A3BTs at A1 receptors. Using a combination of membrane-based and intact-cell radioligand binding, multiple signaling assays, and a native tissue bioassay, we found that the allosteric interaction between 2A3BTs and the agonists 2-chloro-N6-[3H]cyclopentyladenosine or (−)-N6-(2-phenylisopropyl)adenosine (R-PIA) or the antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine is consistent with a ternary complex model involving recognition of a single extracellular allosteric site. However, when allowed access to the intracellular milieu, 2A3BTs have a secondary action as direct G protein inhibitors; this latter property is receptor-independent as it is observed in nontransfected cells and also after stimulation of another GPCR. In addition, we found that 2A3BTs can signal as allosteric agonists in their own right but show bias toward certain pathways relative to the orthosteric agonist, R-PIA. These results indicate that 2A3BTs have a dual mode of action when interacting with the A1 receptor and that they can engender novel functional selectivity in A1 signaling. These mechanisms need to be factored into allosteric ligand structure-activity studies.
Footnotes
↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
This work was funded by the National Health and Medical Research Council (NHMRC) of Australia [Program Grant 519461]; the Australian Research Council [Discovery Grant DP0877497]; and by NHMRC Senior (to A.C.) and Principal (to P.M.S.) Research Fellowships.
Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
doi:10.1124/mol.110.064568.
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ABBREVIATIONS:
- GPCR
- G protein-coupled receptor
- 2A3BT
- 2-amino-3-benzoylthiophene
- T62
- 2-amino-4,5-dimethyl-3-thienyl-[3-(trifluoromethyl)phenyl]methanone
- PD81,723
- 2-amino-4,5-dimethyl-3-thienyl-[3-(trifluoromethyl)phenyl]methanone
- LUF5484
- 2-amino-(4,5,6,7-tetrahydrobenzo[b]thiophen-3-yl)(3,4-dichlorophenyl)methanone
- VCP520
- 2-amino-4-(3,5-bis(trifluoromethyl)phenyl)thiophen-3-yl)(phenyl)methanon
- VCP333
- tert-butyl 2-amino-3-(4-chlorobenzoyl)-7,8-dihydro-4H-thieno[2,3-d]azepine-6(5H)-carboxylate
- FBS
- fetal bovine serum
- CCPA
- 2-chloro-N6-cyclopentyladenosine
- DPCPX
- 8-cyclopentyl-1,3-dipropylxanthine
- [35S]GTPγS
- guanosine 5′-O-[γ-[35S]thio]triphosphate
- ERK
- extracellular signal-regulated kinase
- CHO
- Chinese hamster ovary
- FBS
- fetal bovine serum
- Gpp(NH)p
- guanosine 5′-(β,γ-imido)triphosphate
- ATCM
- allosteric ternary complex mode
- R-PIA
- (−)-N6-(2-phenylisopropyl)adenosine.
- Received March 8, 2010.
- Accepted June 14, 2010.
- Copyright © 2010 The American Society for Pharmacology and Experimental Therapeutics
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