The beta-arrestin pathway-selective type 1A angiotensin receptor (AT1A) agonist [Sar1,Ile4,Ile8]angiotensin II regulates a robust G protein-independent signaling network

β-Arrestin-2 mediates the proinflammatory effects of proteinase-activated receptor-2 in the airway

Proteinase-Activated receptor-2 (PAR(2)), a G-protein-coupled Receptor, activated by serine proteinases, is reported to have both protective and proinflammatory effects in the airway. Given these opposing actions, both inhibitors and activators of PAR(2) have been proposed for treating asthma. PAR(2) can signal through two independent pathways: a β-arrestin-dependent one that promotes leukocyte migration, and a G-protein/Ca(2+) one that is required for prostaglandin E(2) (PGE(2)) production and bronchiolar smooth muscle relaxation. We hypothesized that the proinflammatory responses to PAR(2) activation are mediated by β-arrestins, whereas the protective effects are not. Using a mouse ovalbumin model for PAR(2)-modulated airway inflammation, we observed decreased leukocyte recruitment, cytokine production, and mucin production in β-arrestin-2(-/-) mice. In contrast, PAR(2)-mediated PGE(2) production, smooth muscle relaxation, and decreased baseline airway resistance (measures of putative PAR(2) "protective" effects) were independent of β-arrestin-2. Flow cytometry and cytospins reveal that lung eosinophil and CD4 T-cell infiltration, and production of IL-4, IL-6, IL-13, and TNFα, were enhanced in wild-type but not β-arrestin-2(-/-) mice. Using the forced oscillation technique to measure airway resistance reveals that PAR(2) activation protects against airway hyperresponsiveness by an unknown mechanism, possibly involving smooth muscle relaxation. Our data suggest that the PAR(2)-enhanced inflammatory process is β-arrestin-2 dependent, whereas the protective anticonstrictor effect of bronchial epithelial PAR(2) may be β-arrestin independent.

A biased ligand for OXE-R uncouples Gα and Gβγ signaling within a heterotrimer

Differential targeting of heterotrimeric G protein versus β-arrestin signaling are emerging concepts in G protein-coupled receptor (GPCR) research and drug discovery, and biased engagement by GPCR ligands of either β-arrestin or G protein pathways has been disclosed. Herein we report on a new mechanism of ligand bias to titrate the signaling specificity of a cell-surface GPCR. Using a combination of biomolecular and virtual screening, we identified the small-molecule modulator Gue1654, which inhibits Gβγ but not Gα signaling triggered upon activation of Gα(i)-βγ by the chemoattractant receptor OXE-R in both recombinant and human primary cells. Gue1654 does not interfere nonspecifically with signaling directly at or downstream of Gβγ. This hitherto unappreciated mechanism of ligand bias at a GPCR highlights both a new paradigm for functional selectivity and a potentially new strategy to develop pathway-specific therapeutics.

Allosteric modulator ORG27569 induces CB1 cannabinoid receptor high affinity agonist binding state, receptor internalization, and Gi protein-independent ERK1/2 kinase activation

The cannabinoid receptor 1 (CB1), a member of the class A G protein-coupled receptor family, is expressed in brain tissue where agonist stimulation primarily activates the pertussis toxin-sensitive inhibitory G protein (G(i)). Ligands such as CP55940 ((1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3- hydroxypropyl)cyclohexan-1-ol) and Δ(9)-tetrahydrocannabinol are orthosteric agonists for the receptor, bind the conventional binding pocket, and trigger G(i)-mediated effects including inhibition of adenylate cyclase. ORG27569 (5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)ethyl]amide) has been identified as an allosteric modulator that displays positive cooperativity for CP55940 binding to CB1 yet acts as an antagonist of G protein coupling. To examine this apparent conundrum, we used the wild-type CB1 and two mutants, T210A and T210I (D'Antona, A. M., Ahn, K. H., and Kendall, D. A. (2006) Biochemistry 45, 5606-5617), which collectively cover a spectrum of receptor states from inactive to partially active to more fully constitutively active. Using these receptors, we demonstrated that ORG27569 induces a CB1 receptor state that is characterized by enhanced agonist affinity and decreased inverse agonist affinity consistent with an active conformation. Also consistent with this conformation, the impact of ORG27569 binding was most dramatic on the inactive T210A receptor and less pronounced on the already active T210I receptor. Although ORG27569 antagonized CP55940-induced guanosine 5'-3-O-(thio)triphosphate binding, which is indicative of G protein coupling inhibition in a concentration-dependent manner, the ORG27569-induced conformational change of the CB1 receptor led to cellular internalization and downstream activation of ERK signaling, providing the first case of allosteric ligand-biased signaling via CB1. ORG27569-induced ERK phosphorylation persisted even after pertussis toxin treatment to abrogate G(i) and occurs in HEK293 and neuronal cells.

Activation of mTOR/p70S6 kinase by ANG II inhibits insulin-stimulated endothelial nitric oxide synthase and vasodilation

Elevated tissue levels of angiotensin II (ANG II) are associated with impairment of insulin actions in metabolic and cardiovascular tissues. ANG II-stimulated activation of mammalian target of rapamycin (mTOR)/p70 S6 kinase (p70S6K) in cardiovascular tissues is implicated in cardiac hypertrophy and vascular remodeling. However, the role of ANG II-stimulated mTOR/p70S6K in vascular endothelium is poorly understood. In the present study, we observed that ANG II stimulated p70S6K in bovine aortic endothelial cells. ANG II increased phosphorylation of insulin receptor substrate-1 (IRS-1) at Ser(636/639) and inhibited the insulin-stimulated phosphorylation of endothelial nitric oxide synthase (eNOS). An inhibitor of mTOR, rapamycin, attenuated the ANG II-stimulated phosphorylation of p70S6K and phosphorylation of IRS-1 (Ser(636/639)) and blocked the ability of ANG II to impair insulin-stimulated phosphorylation of eNOS, nitric oxide production, and mesenteric-arteriole vasodilation. Moreover, point mutations of IRS-1 at Ser(636/639) to Ala prevented the ANG II-mediated inhibition of insulin signaling. From these results, we conclude that activation of mTOR/p70S6K by ANG II in vascular endothelium may contribute to impairment of insulin-stimulated vasodilation through phosphorylation of IRS-1 at Ser(636/639). This ANG II-mediated impairment of vascular actions of insulin may help explain the role of ANG II as a link between insulin resistance and hypertension.

Regulation of β-adrenergic receptor function: an emphasis on receptor resensitization

G protein-coupled receptors are the largest family of cell surface receptors regulating multiple cellular processes. β-adrenergic receptor (βAR) is a prototypical member of GPCR family and has been one of the most well studied receptors in determining regulation of receptor function. Agonist activation of βAR leads to conformational change resulting in coupling to G protein generating cAMP as secondary messenger. The activated βAR is phosphorylated resulting in binding of β-arrestin that physically interdicts further G protein coupling leading to receptor desensitization. The phosphorylated βAR is internalized and undergoes resensitization by dephosphorylation mediated by protein phosphatase 2A in the early endosomes. Although desensitization and resensitization are two sides of the same coin maintaining the homeostatic functioning of the receptor, significant interest has revolved around understanding mechanisms of receptor desensitization while little is known about resensitization. In our current review we provide an overview on regulation of βAR function with a special emphasis on receptor resensitization and its functional relevance in the context of fine tuning receptor signaling.

β-arrestin-dependent actin reorganization: bringing the right players together at the leading edge

First identified as mediators of G-protein-coupled receptor desensitization and internalization and later as signaling platforms, β-arrestins play a requisite role in chemotaxis and reorganization of the actin cytoskeleton, downstream of multiple receptors. However, the precise molecular mechanisms underlying their involvement have remained elusive. Initial interest in β-arrestins as facilitators of cell migration and actin reorganization stemmed from the known interplay between receptor endocytosis and actin filament formation, because disruption of the actin cytoskeleton inhibits these β-arrestin-dependent events. With growing interest in the mechanisms by which cells can sense a gradient of agonist during cell migration, investigators began to hypothesize that β-arrestins may contribute to directed migration by controlling chemotactic receptor turnover at the plasma membrane. Finally, increasing evidence emerged that β-arrestins are more than just clathrin adaptor proteins involved in turning off receptor signals; they are actually capable of generating their own signals by scaffolding signaling molecules and controlling the activity of multiple cellular enzymes. This new role of β-arrestins as signaling scaffolds has led to the hypothesis that they can facilitate cell migration by sequestering actin assembly activities and upstream regulators of actin assembly at the leading edge. This Minireview discusses recent advances in our understanding of how β-arrestin scaffolds contribute to cell migration, focusing on recently identified β-arrestin interacting proteins and phosphorylation targets that have known roles in actin reorganization.