Distinct roles for Src tyrosine kinase in β₂-adrenergic receptor signaling to MAPK and in receptor internalization

G protein-coupled receptors form the largest family of membrane receptors and transmit diverse ligand signals to modulate various cellular responses. After activation by their ligands, some of these G protein-coupled receptors are desensitized, internalized (endocytosed), and down-regulated (degraded). In HEK 293 cells, the G_s-coupled β₂-adrenergic receptor was postulated to initiate a second wave of signaling, such as the activation of the mitogen-activated protein kinase (MAPK) pathway after the receptor is internalized. The tyrosine kinase c-Src plays a critical role in these events. Here we used mouse embryonic fibroblast (MEF) cells deficient in Src family tyrosine kinases to examine the role of Src in β₂-adrenergic receptor signaling to the MAPK pathway and in receptor internalization. We found that in Src-deficient cells the β₂-adrenergic receptor could activate the MAPK pathway. However, the internalization of β ₂-adrenergic receptors was blocked in Src-deficient MEF cells. Furthermore, we observed that in MEF cells deficient in β-arrestin 2 the internalization of the β₂-adrenergic receptor was impaired, whereas the activation of the MAPK pathway by the β₂-adrenergic receptor was normal. Our data demonstrate that although Src and β-arrestin 2 play essential roles in β₂-adrenergic receptor internalization, they are not required for the activation of the MAPK pathway by the β₂-adrenergic receptor. In other words, our finding suggests that receptor internalization is not required for β ₂-adrenergic receptor signaling to the MAPK pathway in MEF cells.