Molecular determinants of angiotensin II type 1 receptor functional selectivity

The angiotensin AT₁ receptor is an important pharmacological target in the treatment of cardiovascular disorders, such as hypertension, diabetic nephropathy, cardiac hypertrophy, arrhythmia and failure. Simultaneously, the AT₁ receptor has emerged to be a prominent model for the emerging concept that receptors may attain multiple active states with differentiated functional outcomes. Two major signalling pathways are employed by the AT₁ receptor, namely 1) the canonical G_q protein-dependent activation of inositol phosphate turnover and intracellular calcium release, and 2) G protein-independent recruitment of β-arrestin-scaffolded signalling complexes that activate protein kinase pathways. Different states of receptor activation with preference for individual downstream pathways (functional selectivity) have been demonstrated in mutational studies of the AT₁ receptor and by pharmacological probing with analogues of angiotensin II. These studies also provide clues about the conformational changes that underlie different functional outcomes. In this review, we evaluate current knowledge of the molecular determinants of AT₁ receptor activation, which may distinguish G protein-dependent and -independent behaviour. While G protein activation is known to be detrimental, G protein-independent signalling by the AT₁ receptor has been associated with phenotypes such as cell survival and renewal, regulation of cardiac contraction and cell migration. It is therefore currently hypothesized that selective blockade of G protein actions and simultaneous activation of G protein-independent signalling will prove to be a feasible strategy for improved cardiovascular therapy. The pharmacological perspectives of functional selectivity by receptors, such as the AT₁ receptor, urge the elucidation of molecular mechanisms that govern disparate signalling events.