Endocytosis sculpts distinct cAMP signal transduction by endogenously coexpressed GPCRs
Many G protein-coupled receptors (GPCRs) trigger a second phase of G protein-dependent signaling from internal membranes after agonist-induced endocytosis. However, individual GPCRs differ significantly in their ability to internalize after activation, and it remains unclear if this confers selectivity on cellular signaling through natively coexpressed GPCRs. We addressed this question by examining the activation of the cyclic AMP (cAMP) / cAMP-dependent protein kinase (PKA) pathway by three ligands that stimulate three distinct, endogenously coexpressed GPCRs in HEK293 cells: isoproterenol (Iso) which primarily activates the ß2-adrenergic receptor (ß2AR), vasoactive intestinal peptide (VIP) which primarily activates the vasoactive intestinal peptide receptor 1 (VIPR1/VPAC1), and 5'-N-ethylcarboxamidoadenosine (NECA) which primarily activates the adenosine 2B receptor (A2BR). Using location-targeted biosensors and a transcriptional reporter, we demonstrate that each ligand triggers a unique cellular signaling profile and that these responses are differentially sensitive to endocytic inhibition. VIP elicited a response that was endocytosis-dependent at every level in the pathway tested, from upstream global cAMP elevation to downstream activation of nuclear PKA, while Iso elicited a response that was dependent on endocytosis selectively at downstream steps. In contrast, NECA robustly activated the entire cAMP signaling cascade independently of endocytosis, consistent with our observation that human A2BR does not robustly internalize after activation. We conclude that endocytosis indeed sculpts downstream cAMP signaling by GPCRs in a receptor-specific manner. Our results add to the evolving view of compartmentalized signaling in the cAMP / PKA pathway and suggest that differences in GPCR trafficking can encode receptor-specific signaling profiles through a shared signal transduction pathway. Overall design: To compare the effects of distinct GPCR agonists on transcriptional activation, we carried out 3'-Tag RNA-seq-based differential gene expression analysis. Cultured HEK293 cells were treated with agonist (100 nM Iso, 20 µM NECA, or 500 nM VIP) or their corresponding vehicles (1 µM ascorbic acid, 0.05% DMSO, or water, respectively) for 90 minutes. Experiment was carried out in biological triplicate.