Molecular mechanism underlying inverse agonist of angiotensin II type 1 receptor

To delineate the molecular mechanism underlying the inverse agonist activity of olmesartan, a potent angiotensin II type 1 (AT(1)) receptor antagonist, we performed binding affinity studies and an inositol phosphate production assay. Binding affinity of olmesartan and its related compounds to wild-type and mutant AT(1) receptors demonstrated that interactions between olmesartan and Tyr(113), Lys(199), His(256), and Gln(257) in the AT(1) receptor were important. The inositol phosphate production assay of olmesartan and related compounds using mutant receptors indicated that the inverse agonist activity required two interactions, that between the hydroxyl group of olmesartan and Tyr(113) in the receptor and that between the carboxyl group of olmesartan and Lys(199) and His(256) in the receptor. Gln(257) was found to be important for the interaction with olmesartan but not for the inverse agonist activity. Based on these results, we constructed a model for the interaction between olmesartan and the AT(1) receptor. Although the activation of G protein-coupled receptors is initiated by anti-clockwise rotation of transmembrane (TM) III and TM VI followed by changes in the conformation of the receptor, in this model, cooperative interactions between the hydroxyl group and Tyr(113) in TM III and between the carboxyl group and His(256) in TM VI were essential for the potent inverse agonist activity of olmesartan. We speculate that the specific interaction of olmesartan with these two TMs is essential for stabilizing the AT(1) receptor in an inactive conformation. A better understanding of the molecular mechanisms of the inverse agonism could be useful for the development of new G protein-coupled receptor antagonists with inverse agonist activity.