β-Arrestin-biased Agonism at the β2-Adrenergic Receptor

Classically, the beta(2)-adrenergic receptor (beta(2)AR)and other members of the seven-transmembrane receptor (7TMR) superfamily activate G protein-dependent signaling pathways in response to ligand stimulus. It has recently been discovered, however, that a number of 7TMRs, including beta 2AR, can signal via beta-arrestin- dependent pathways independent of G protein activation. It is currently unclear if among beta(2)AR agonists there exist ligands that disproportionately signal via G proteins or beta-arrestins and are hence "biased." Using a variety of approaches that include highly sensitive fluorescence resonance energy transfer-based methodologies, including a novel assay for receptor internalization, we show that the majority of known beta(2)AR agonists exhibit relative efficacies for beta-arrestin- associated activities (beta-arrestin membrane translocation and beta(2)AR internalization) identical to their relative efficacies for G protein dependent signaling ( cyclic AMP generation). However, for three beta AR ligands there is a marked bias toward beta-arrestin signaling; these ligands stimulate beta-arrestin-dependent receptor activities to a much greater extent than would be expected given their efficacy for G protein-dependent activity. Structural comparison of these biased ligands reveals that all three are catecholamines containing an ethyl substitution on the alpha-carbon, a motif absent on all of the other, unbiased ligands tested. Thus, these studies demonstrate the potential for developing a novel class of 7TMR ligands with a distinct bias for beta-arrestin-mediated signaling.