GPCRs Share an Allosteric Regulatory Site Located Within the Extracellular Agonist-Binding Pocket: Cross-Binding of GPCR Ligands Can Knockout or Revitalize Receptors and Affect Tolerance and Addiction

Angiotensin II (AngII) binds to the AngII type 1 receptor (AT¹R) in either A-mode (on-switch; G-protein-mediated contraction) or D-mode (off-switch; arrestin-mediated desensitization/tachyphylaxis). Angiotensin receptor blockers (ARB) can desensitize AT¹Rs by binding to an allosteric regulatory B-site, which overlaps with the AngII A/D binding site in the extracellular binding pocket of the receptor. ARBs shift the receptor conformation from A/D-mode to agonist-independent D*-mode, resulting in receptor internalization. Binding of ligands to the inverse agonist binding site (B-site) can promote (lock in) the D*-mode conformation of the receptor (desensitization) or disengage (lock out) the D-mode conformation (upsensitization, reduction of tolerance). Computer-aided docking (CAD) studies show that ARBs, particularly bisartan ACC519TT, bind with high affinity to several G protein-coupled receptors (GPCRs), including AT¹R, adrenergic (alpha 1 and 2), opioid (mu and delta), and muscarinic 3 receptors, suggesting that the electrostatic architecture of the B-site has been preserved across a broad spectrum of GPCRs. Consistent with CAD findings, ACC519TT significantly reduced dilation responses of mouse colon and small intestine to the opioid receptor agonist, oxycodone. Functional studies in rabbit iliac artery rings demonstrated the ability of lisinopril to significantly reduce AngII-induced contraction, comparable to candesartan. However, CAD studies show that lisinopril has a markedly lower affinity than candesartan for AT¹R, illustrating that the similar effect to AngII dose-response may be due to the time-dependent receptor internalization. For agonists, D-site versus A-site activity may provide insight regarding dependency potential (addiction ratio, D/A). Agonists cross-talk among different GPCRs creates the potential for a sophisticated interplay, possibly involving endogenous ligands not yet identified, with the notable exception of angiotensin antipeptide.