Abstract:
The renin-angiotensin and kallikrein-kinin systems are key regulators of vascular tone and inflammation. Angiotensin II, the principal effector of the renin-angiotensin system, promotes vasoconstriction by activating angiotensin AT1 receptors. The opposing effects of the kallikrein-kinin system are mediated by bradykinin acting on B1 and B2 bradykinin receptors. The renin-angiotensin and kallikrein-kinin systems engage in cross-talk at multiple levels, including the formation of AT1-B2 receptor heterodimers. In primary vascular smooth muscle cells, we find that the arrestin pathway-selective AT1 agonist, [Sar 1,Ile4,Ile8]-AngII, but not the neutral AT1 antagonist, losartan, inhibits endogenous B2 receptor signaling. In a transfected HEK293 cell model that recapitulates this effect, we find that the actions of [Sar1,Ile4, Ile8]-AngII require the AT1 receptor and result from arrestin-dependent co-internalization of AT1-B2 heterodimers. BRET50 measurements indicate that AT1 and B2 receptors efficiently heterodimerize. In cells expressing both receptors, pretreatment with [Sar 1,Ile4,Ile8]-AngII blunts B2 receptor activation of Gq-11-dependent intracellular calcium influx and Gi-o-dependent inhibition of adenylyl cyclase. In contrast, [Sar1,Ile 4,Ile8]-AngII has no effect on B2 receptor ligand affinity or bradykinin-induced arrestin3 recruitment. Both radioligand binding assays and quantitative microscopy-based analysis demonstrate that [Sar 1,Ile4,Ile8]-AngII promotes internalization of AT1-B2 heterodimers. Thus, [Sar1,Ile4,Ile 8]-AngII exerts lateral allosteric modulation of B2 receptor signaling by binding to the orthosteric ligand binding site of the AT1 receptor and promoting co-sequestration of AT1-B2 heterodimers. Given the opposing roles of the renin-angiotensin and kallikrein-kinin systems in vivo, the distinct properties of arrestin pathway-selective and neutral AT1 receptor ligands may translate into different pharmacologic actions.