Conformational rearrangements and signaling cascades involved in ligand-biased mitogen-activated protein kinase signaling through the β1-adrenergic receptor

In recent years, several studies have demonstrated that different ligands can have distinct efficacy profiles toward various signaling pathways through a unique receptor. For example, beta 1-adrenergic compounds that are inverse agonists toward the adenylyl cyclase (AC) can display agonist activity for the mitogen-activated protein kinase (MAPK) pathway. Such a phenomenon, often termed functional selectivity, has now been clearly established for many G protein-coupled receptors when considering distinct signaling output. However, the possibility that ligands could selectively engage distinct effectors to activate a single signaling output by promoting specific receptor conformations has not been extensively examined. Here, we took advantage of the fact that isoproterenol, bucindolol and propranolol (full, partial, and inverse agonists for the AC pathway, respectively) all activate MAPK through the beta 1-adrenergic receptor (beta 1AR) to probe such conformational-biased signaling. Although the three compounds stimulated MAPK in a src-dependent manner, isoproterenol acted through both G alpha(i)beta gamma- and G protein-independent pathways, whereas bucindolol and propranolol promoted MAPK activation through the G protein-independent pathway only. The existence of such distinct signaling cascades linking beta 1AR to MAPK activation was correlated with ligand-specific conformational rearrangements of receptor/G protein complexes measured by bioluminescence resonance energy transfer. Taken together, our data indicate that discrete local conformational changes can selectively promote the recruitment of distinct proximal signaling partners that can engage distinct signaling outputs and/or converge on the same signaling output.