Abstract
The human adenosine A2B receptor belongs to class A G protein-coupled receptors (GPCRs). In our previous work, constitutively active mutant (CAM) human adenosine A2B receptors were identified from a random mutation bank. In the current study, three known A2B receptor antagonists, 4-{2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl}phenol (ZM241385), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706) were tested on wild-type and nine CAM A2B receptors with different levels of constitutive activity in a yeast growth assay. All three compounds turned out to be inverse agonists for the adenosine A2B receptor because they were able to fully reverse the basal activity of four low-level constitutively active A2B receptor mutants and to partially reverse the basal activity of three medium-level constitutively active A2B receptor mutants. We also discovered two highly constitutively active mutants whose basal activity could not be reversed by any of the three compounds. A two-state receptor model was used to explain the experimental observations; fitting these yielded the following relative intrinsic efficacies for the three inverse agonists ZM241385, DPCPX, and MRS1706: 0.14 ± 0.03, 0.35 ± 0.03, and 0.31 ± 0.02, respectively. Moreover, varying L, the ratio of active versus inactive receptors in this model, from 0.11 for mutant F84L to 999 for two highly constitutively active mutants yielded simulated dose-response curves that mimicked the experimental curves. This study is the first description of inverse agonists for the human adenosine A2B receptor. Moreover, the use of receptor mutants with varying levels of constitutive activity enabled us to determine the relative intrinsic efficacy of these inverse agonists.
Footnotes
-
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
-
doi:10.1124/jpet.106.111203.
-
ABBREVIATIONS: GPCR, G protein-coupled receptor; NECA, 5′-N-ethylcarboxamidoadenosine; MRS1706, N-(4-Acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide; ZM241385, 4-{2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl}phenol; DPCPX, 8-cyclopentyl-1,3-dipropylxanthine; CAM, constitutively active mutant; 3-AT, 3-aminotriazole; OD, optical density; DR, dose ratio; Ra, active state of the receptor; Ri, inactive state of the receptor; L, isomerization constant; WT, wild type; MRS1668, 8-[4-[((2-acetylphenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine; MRS1754, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine; CGS15943, 9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine.
-
↵
The online version of this article (available at http://jpet.aspetjournals.org) contains supplemental material.
- Received July 21, 2006.
- Accepted October 27, 2006.
- The American Society for Pharmacology and Experimental Therapeutics