Molecular Medicine Arrestin Orchestrates Crosstalk Between G Protein–Coupled Receptors to Modulate the Spatiotemporal Activation of ERK MAPK

G protein– coupled receptors (GPCRs) respond to diversified extracellular stimuli to modulate cellular function. Traditionally, activated receptors couple to G proteins, which transduce downstream signals via second messengers and membrane channels. 1 Active GPCRs are phosphorylated by specific GPCR kinases (GRKs) leading to receptor desensi-tization. Arrestin proteins then bind to the phosphorylated receptor initializing clathrin-mediated internalization. 2 Despite extensive studies investigating the regulation of single GPCR signaling cascades, the effect of concomitant GPCR activation by endogenous stimuli on downstream signaling remains poorly understood. There is a great deal of evidence supporting functional crosstalk between different GPCRs both in vitro and in vivo. 3–5 The majority of these studies focus on short-term stress responses involved in modulation of common effectors such as G proteins, phospholipases (PLs), and adenylyl cyclases. 4 However, chronic activation of multiple GPCR signaling pathways during maladaptive tissue and organ remodeling suggests the potential of downstream crosstalk away from the plasma membrane. One potential nexus for GPCR signaling crosstalk are the multifunctional scaffold proteins known as arrestins. Arrestins not only scaffold proteins for the activation of different MAPK families under single receptor activation, but also mediate transactivation of epidermal growth factor receptor signaling pathways 2,6 and activation of many other non-GPCR signaling cascades. 7 In the case of GPCR-induced extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) activation, both G proteins and arrestins are capable of mediating ERK activation via independent mechanisms, with each pathway leading to unique spatiotemporal consequences. 8 Whereas G protein– dependent ERK translocates to the nucleus for gene transcription, arrestin-dependent ERK remains within the cytoplasm. Because arrestins preferentially bind to some, but not all, GPCRs in a ligand-dependent manner, we envision that arrestins may play a role in GPCR crosstalk by coordinating MAPK activation in distinct subcellular compartments. Such a regulatory mechanism is essential for modulating MAPK sig-naling in divergent cellular functions such as cell proliferation and growth, 9 mobility, 10 and apoptosis. 11 We chose cardiac fibroblasts as a model to study GPCR signaling crosstalk. Both ␣ 1-adrenergic receptor (␣ 1 ARs) and ␤ARs are expressed in cardiac tissue and are activated by catecholamines to modulate maladaptive cardiac remodeling, including cardiac fibroblast proliferation, by activation of distinct pathways. These pathways transduce their prolifera-tive signal via members of the MAPK family, including ERK1/2. Stimulation of the ␣ 1 AR leads to Gq coupling and subsequent PLC and protein kinase (PK)C activation. 12 PKC has …