β₁/β₂-Adrenergic Receptor Heterodimerization Regulates β₂-Adrenergic Receptor Internalization and ERK Signaling Efficacy*

β₁- and β₂-adrenergic receptors (β₁AR and β₂AR) are co-expressed in numerous tissues where they play a central role in the responses of various organs to sympathetic stimulation. Although the two receptor subtypes share some signaling pathways, each has been shown to have specific signaling and regulatory properties. Given the recent recognition that many G protein-coupled receptors can form homo- and heterodimers, the present study was undertaken to determine whether the β₁AR and β₂AR can form dimers in cells and, if so, to investigate the potential functional consequences of such heterodimerization. Using co-immunoprecipitation and bioluminescence resonance energy transfer, we show that β₁AR and β₂AR can form heterodimers in HEK 293 cells co-expressing the two receptors. Functionally, β-adrenergic stimulated adenylyl cyclase activity was found to be identical in cells expressing β₁AR, β₂AR, or both receptors at similar levels, indicating that heterodimerization did not affect this signaling pathway. When considering ERK1/2 MAPK activity, a significant agonist-promoted activation was detected in β₂AR- but not β₁AR-expressing cells. Similarly to what was observed in cells expressing the β₁AR alone, no β-adrenergic stimulated ERK1/2 phosphorylation was observed in cells co-expressing the two receptors. A similar inhibition of agonist-promoted internalization of the β₂AR was observed upon co-expression of the β₁AR, which by itself internalized to a lesser extent. Taken together, our data suggest that heterodimerization between β₁AR and β₂AR inhibits the agonist-promoted internalization of the β₂AR and its ability to activate the ERK1/2 MAPK signaling pathway.