Abstract 18138: Molecular Mechanism Whereby Activated State of the Angiotensin II Type 1 Receptor Attenuate Inverse Agonist Efficacy

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Angiotensin II type 1receptor (AT1R) can be activated by mechanical stress without involving angiotensin II stimulation, and play a critical role in the cause of cardiac hypertrophy. AT1R blocker (ARB) can prevent it by inverse agonist action. The binding affinity of the ARBs has been known to be strongly reduced in activated state of the AT1R compare to in the ground state of the AT1R. It suggests that active conformational change of the AT1R alter binding mode of ARBs, and thereby alter the inverse agonist efficacy. We compared inverse agonist efficacy of four ARBs (Losartan, EXP3174, Irbesartan and Valsartan) between ground state and activated state of the AT1R, and found that active conformational change of the AT1R strongly attenuate the inverse agonist efficacy of all four ARBs. Hence, we examined the molecular mechanism whereby activated state of the AT1R attenuates the inverse agonist efficacy of the ARBs by performing site-directed mutagenesis, ligand binding, inositol phosphate formation, and molecular modeling. AT1 wild type (AT1WT) and constitutively active N111G mutant used as model of ground state and activated state of the AT1R, respectively. Since identifying the molecular mechanism of the inverse agonism will be useful for developing new strong inverse agonistic ARB. Molecular models based on the effect of mutant on binding affinity and inverse agonist efficacy of the ARBs suggest that interactions of ARBs with Val108 and Ser109 in transmembrane domain (TM)3, Gln257 in TM6, and Asn295 in TM7 stabilize the AT1R in inactive conformation, and causes inverse agonist activity. Molecular models also suggest that active conformational change of the AT1R attenuate the interaction of ARBs with Gln257 and Asn295, and thereby attenuate stabilizing the AT1R in inactive conformation, resulting in attenuated inverse agonist efficacy. We conclude that attenuation of the interaction with TM6 and TM7 is molecular mechanism whereby activated state of the AT1R attenuates the inverse agonist efficacy of ARBs. Our findings will be useful for developing new class of strong inverse agonistic ARB.