The C-terminal of the α1b-adreneroceptor is a key determinant for its structure integrity and biological functions.

The C-terminal of G protein-coupled receptors is now recognized as being important for G protein activation and signaling function. To detect the role of C-terminal tail in receptor activation, we used the α1b-AR, which has a long C-terminal of 164 amino acids. We constructed the intramolecular FRET sensors, in which the C-terminal was truncated to 10 (∆C-10), 20 (∆C-20), 30 (∆C-30), 50 (∆C-50), 70 (∆C-70), or 90 (∆C-90). The truncated mutants of ∆C-10, ∆C-20, or ∆C-30 cannot induce FRET signal changes and downstream ERK1/2 phosphorylation. However, the truncated mutants of ∆C-50, ∆C-70, or ∆C-90 induce significant FRET signal changes and downstream ERK1/2 phosphorylation, especially ∆C-90. This is particularly true in the case of the ∆C-90, ∆C-70, or ∆C-50 which retained the potential phosphorylation sites (Ser401, Ser404, Ser408, or Ser410). The ∆C-90 showed an increase in agonist-induced FRET signal changes and ERK1/2 phosphorylation in PKC- or endocytosis-dependent and EGFR-, src-, or ß-arrestin2-independent.

Modulation of PTH1R signaling by an ECD binding antibody results in inhibition of ß-arrestin 2 coupling.

Parathyroid hormone receptor 1 (PTH1R) belongs to the secretin class of G protein coupled receptors (GPCRs) and natively binds parathyroid hormone (PTH) and parathyroid hormone related peptide (PTHrP). Ligand binding to PTH1R involves binding to the large extracellular domain (ECD) and the orthosteric pocket, inducing conformational changes in the transmembrane domain and receptor activation. PTH1R regulates bone metabolism, signaling mainly through Gs and Gq/11 G-proteins. Here, we used phage display to generate PTH1R ECD-specific antibodies with the aim of modulating receptor functionality. We identified ECD-scFvhFc, which exhibited high affinity binding to both the isolated ECD and to the full-length receptor in styrene-maleic acid (SMA) lipid particles. Epitope mapping using hydrogen-deuterium exchange mass spectrometry (HDX-MS) indicates that the α1 helix of the ECD is ECD-scFvhFc's epitope which may partially overlap with the known PTH (1-34) binding site. However, PTH (1-34)-mediated Gs activation is Undisturbed by ECD-scFvhFc binding. In contrast, ECD-scFvhFc potently inhibits ß-arrestin-2 recruitment after PTH (1-34)-driven receptor activation and thus represents the first monoclonal antibody to selectively inhibit distinct PTH1R signaling pathways. Given the complexity of PTH1R signaling and the emerging importance of biased GPCR activation in drug development, ECD-scFvhFc could be a valuable tool to study PTH1R signaling bias.

Acute ethanol exposure reduces serotonin receptor 1A internalization by increasing ubiquitination and degradation of ß-arrestin2.

Acute alcohol exposure alters the trafficking and function of many G-protein-coupled receptors (GPCRs) that are associated with aberrant behavioral responses to alcohol. However, the molecular mechanisms underlying alcohol-induced changes in GPCR function remain unclear. ß-Arrestin is a key player involved in the regulation of GPCR internalization and thus controls the magnitude and duration of GPCR signaling. Although ß-arrestin levels are influenced by various drugs of abuse, the effect of alcohol exposure on ß-arrestin expression and ß-arrestin-mediated GPCR trafficking is poorly understood. Here, we found that acute ethanol exposure increases ß-arrestin2 degradation via its increased ubiquitination in neuroblastoma-2a (N2A) cells and rat prefrontal cortex (PFC). ß-Arrestin2 ubiquitination was likely mediated by the E3 ligase MDM2 homolog (MDM2), indicated by an increased coupling between ß-arrestin2 and MDM2 in response to acute ethanol exposure in both N2A cells and rat PFC homogenates. Importantly, ethanol-induced ß-arrestin2 reduction was reversed by siRNA-mediated MDM2 knockdown or proteasome inhibition in N2A cells, suggesting ß-arrestin2 degradation is mediated by MDM2 through the proteasomal pathway. Using serotonin 5-HT1A receptors (5-HT1ARs) as a model receptor system, we found that ethanol dose-dependently inhibits 5-HT1AR internalization and that MDM2 knockdown reverses this effect. Moreover, ethanol both reduced ß-arrestin2 levels and delayed agonist-induced ß-arrestin2 recruitment to the membrane. We conclude that ß-arrestin2 dysregulation by ethanol impairs 5-HT1AR trafficking. Our findings reveal a critical molecular mechanism underlying ethanol-induced alterations in GPCR internalization and implicate ß-arrestin as a potential player mediating behavioral responses to acute alcohol exposure.

Regulation of cardiac fibroblast-mediated maladaptive ventricular remodeling by ß-arrestins.

Cardiac fibroblasts (CF) play a critical role in post-infarction remodeling which can ultimately lead to pathological fibrosis and heart failure. Recent evidence demonstrates that remote (non-infarct) territory fibrosis is a major mechanism for ventricular dysfunction and arrhythmogenesis. ß-arrestins are important signaling molecules involved in ß-adrenergic receptor (ß-AR) desensitization and can also mediate signaling in a G protein independent fashion. Recent work has provided evidence that ß-arrestin signaling in the heart may be beneficial, however, these studies have primarily focused on cardiac myocytes and their role in adult CF biology has not been well studied. In this study, we show that ß-arrestins can regulate CF biology and contribute to pathological fibrosis. Adult male rats underwent LAD ligation to induce infarction and were studied by echocardiography. There was a significant decline in LV function at 2-12 weeks post-MI with increased infarct and remote territory fibrosis by histology consistent with maladaptive remodeling. Collagen synthesis was upregulated 2.9-fold in CF isolated at 8 and 12 weeks post-MI and ß-arrestin expression was significantly increased. ß-adrenergic signaling was uncoupled in the post-MI CF and ß-agonist-mediated inhibition of collagen synthesis was lost. Knockdown of ß-arrestin1 or 2 in the post-MI CF inhibited transformation to myofibroblasts as well as basal and TGF-ß-stimulated collagen synthesis. These data suggest that ß-arrestins can regulate CF biology and that targeted inhibition of these signaling molecules may represent a novel approach to prevent post-infarction pathological fibrosis and the transition to HF.

Loss of ß-arrestin-2 gene and possible functional consequences on Sezary Syndrome.

Sezary Syndrome is an aggressive T-cell Lymphoma involving blood, skin and lymphonodes Involvement of the CXCR4-SDF1 has been previously shown. We here present evidence also of the involvement of B-arrestin a downstream regulator of CXCR4, that is depleted and downregulated as well as a potential functional role for this depletion.

Ginsenoside metabolite compound-K regulates macrophage function through inhibition of ß-arrestin2.

Ginsenoside metabolite compound-K (C-K), which is an active metabolite of ginsenoside in vivo, can produce anti-inflammatory affects by activating glucocorticoid receptors (GRs) to inhibit the expression of ß-arrestin2. Studies have shown that C-K can inhibit the function of immune cells including macrophage polarization and phagocytosis. However, the mechanism by which C-K regulates macrophage polarization is currently unclear. Toll-like receptors (TLRs) are the pattern recognition receptors on the membrane of immune cells, with TLR4 being especially important in polarization of macrophages. The Gαi-mediated activation of nuclear factor-κB (NF-κB) by TLR4 promotes inflammation and phagocytosis in macrophages by increasing the proportion of type I phenotypic macrophages (M1). Whether C-K inhibits the signal transduction of TLR4-Gαi-NF-κB and how that effects macrophage polarization regulation in murine models of RA is not reported. The coupling of G proteins with receptors is regulated by ß-arrestin2, but it has been unclear whether C-K modulates the TLR4 interaction with G proteins by inhibiting the expression of ß-arrestin2. To explore these questions, the collagen-induced arthritis (CIA) mouse model was employed, and mice were treated with C-K (112 mg/kg/day). The results depict that C-K treatment inhibits macrophage phagocytosis and reduces the proportion of M1. C-K decreases the overexpressed ß-arrestin2, Gαi, TLR4 and NF-κB in macrophages of CIA mice, while increasing the expression of Gαs. Furthermore, C-K promotes TLR4-Gαs coupling and inhibits TLR4-Gαi coupling through ß-arrestin2 regulation in macrophages, leading to a decrease in the proportion of M1 to M2 macrophages and improved outcomes in CIA mice.

ß-Arrestin2 Inhibits Expression of Inflammatory Cytokines in BEAS-2B Lung Epithelial Cells Treated with Cigarette Smoke Condensate via Inhibition of Autophagy.

BACKGROUND/AIMS: ß-arrestin2 has been shown to have a role in human inflammatory disease. However, the role of ß-arrestin2 in cigarette smoke-induced inflammation in the lung remains unknown. The aims of this study were to investigate the effects of ß-arrestin2 on cigarette smoke condensate (CSC)-induced expression of inflammatory cytokines in the BEAS-2B human bronchial epithelial cell line in vitro, and the mechanisms involved. METHODS: The MTT assay determined cell viability of cultured BEAS-2B cells. Autophagy was assessed by western blot, adenoviral mRFP-GFP-LC3 transfection, and immunofluorescence. The effects of ß-arrestin2 shRNA knockdown were studied by western blot and real-time reverse transcription-polymerase chain reaction (RT-PCR). Western blot evaluated the AMPK/mTOR signaling pathway. Levels of inflammatory cytokines, interleukin (IL)-6, IL-8, and MCP-1 were measured in cell culture supernatants by enzyme-linked immunosorbent assay (ELISA). RESULTS: CSC suppressed expression of ß-arrestin2 in BEAS-2B cells, activated the AMPK/mTOR signaling pathway, increased cell autophagy and the expression of IL-6, IL-8, and MCP-1,pretreatment with the ß-arrestin2 biased ligands, propranolol, and ICI118551 reversed these changes. Inhibition of autophagy reduced the expression of inflammatory cytokines following CSC. CONCLUSION: In the human bronchial epithelial cell line, BEAS-2B, ß-arrestin2 reduced the expression of CSC-induced inflammatory cytokines by inhibiting autophagy, most likely via the AMPK/mTOR signaling pathway.

Dopamine D2 receptor restricts astrocytic NLRP3 inflammasome activation via enhancing the interaction of ß-arrestin2 and NLRP3.

Astrocytes are involved in the neuroinflammation of neurodegenerative diseases, such as Parkinson's disease (PD). Among the numerous inflammatory cytokines, interleukin-1ß (IL-1ß) produced by astrocytic Nod-like receptor protein (NLRP) inflammasome is crucial in the pathogenesis of PD. ß-arrestin2-mediated dopamine D2 receptor (Drd2) signal transduction has been regarded as a potential anti-inflammatory target. Our previous study revealed that astrocytic Drd2 suppresses neuroinflammation in the central nervous system. However, the role of Drd2 in astrocytic NLRP3 inflammasome activation and subsequent IL-1ß production remains unclear. In the present study, we used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced PD mouse model to investigate whether Drd2 could suppress astrocytic NLRP3 inflammasome activation. We showed that Drd2 agonist inhibited NLRP3 inflammasome activation, evidenced by decreased caspase-1 expression and reduced IL-1ß release in the midbrain of wild type mice. The anti-inflammasome effect of Drd2 was abolished in ß-arrestin2 knockout and ß-arrestin2 small interfering RNA-injected mice, suggesting a critical role of ß-arrestin2 in Drd2-regulated NLRP3 inflammasome activation. We also found that Drd2 agonists suppressed the upregulation of caspase-1 and IL-1ß expression in primary cultured mouse astrocytes in response to the activation of NLRP3 inflammasome induced by lipopolysaccharide plus adenosine triphosphate. Furthermore, we demonstrated that ß-arrestin2 mediated the inhibitory effect of Drd2 on NLRP3 inflammasome activation via interacting with NLRP3 and interfering the inflammasome assembly. Collectively, our study illustrates that astrocytic Drd2 inhibits NLRP3 inflammasome activation through a ß-arrestin2-dependent mechanism, and provides a new strategy for treatment of PD.

Targeting the Dvl-1/ß-arrestin2/JNK3 interaction disrupts Wnt5a-JNK3 signaling and protects hippocampal CA1 neurons during cerebral ischemia reperfusion.

It is well known that Wnt5a activation plays a pivotal role in brain injury and ß-arrestin2 induces c-Jun N-terminal kinase (JNK3) activation is involved in neuronal cell death. Nonetheless, the relationship between Wnt5a and JNK3 remains unexplored during cerebral ischemia/reperfusion (I/R). In the present study, we tested the hypothesis that Wnt5a-mediated JNK3 activation via the Wnt5a-Dvl-1-ß-arrestin2-JNK3 signaling pathway was correlated with I/R brain injury. We found that cerebral I/R could enhance the assembly of the Dvl-1-ß-arrestin2-JNK3 signaling module, Dvl-1 phosphorylation and JNK3 activation. Activated JNK3 could phosphorylate the transcription factor c-Jun, prompt caspase-3 activation and ultimately lead to neuronal cell death. To further explore specifically Wnt5a mediated JNK3 pathway activation in neuronal injury, we used Foxy-5 (a peptide that mimics the effects of Wnt5a) and Box5 (a Wnt5a antagonist) both in vitro and in vivo. AS-ß-arrestin2 (an antisense oligonucleotide against ß-arrestin2) and RRSLHL (a small peptide that competes with ß-arrestin2 for binding to JNK3) were applied to confirm the positive signal transduction effect of the Dvl-1-ß-arrestin2-JNK3 signaling module during cerebral I/R. Furthermore, Box5 and the RRSLHL peptide were found to play protective roles in neuronal death both in vivo global and focal cerebral I/R rat models and in vitro oxygen glucose deprivation (OGD) neural cells. In summary, our results indicate that Wnt5a-mediated JNK3 activation participates in I/R brain injury by targeting the Dvl-1-ß-arrestin2/JNK3 interaction. Our results also point to the possibility that disrupting Wnt5a-JNK3 signaling pathway may provide a new approach for stroke therapy.

ß-Arrestin-biased ß-adrenergic signaling promotes extinction learning of cocaine reward memory.

Extinction learning of cocaine-associated contextual cues can help prevent cocaine addicts from relapsing. Pharmacological manipulation of ß-adrenergic receptor (ß-AR) during extinction learning is being developed as a potential strategy to treat drug addiction. We demonstrated that the extinction learning of cocaine-associated memory was mediated by ß-arrestin2-biased but not heterotrimeric guanine nucleotide-binding protein (G protein)-dependent ß-adrenergic signaling. We found that administration of the nonbiased ß-AR antagonist propranolol, but not the G protein-biased ß-AR antagonist carvedilol, blocked extinction learning of cocaine-conditioned place preference and the associated ERK activation in the infralimbic prefrontal cortex. Overexpression of ß-arrestin2 in the infralimbic prefrontal cortex promoted extinction learning, which was blocked by propranolol. Knockout of ß-arrestin2 in the infralimbic prefrontal cortex, specifically in excitatory neurons, impaired extinction learning of cocaine-conditioned place preference, which was not rescued by carvedilol. ß-Arrestin2 signaling in infralimbic excitatory neurons was also required for the extinction learning in the cocaine self-administration model. Our results suggest that ß-arrestin-biased ß-adrenergic signaling in the infralimbic prefrontal cortex regulates extinction learning of cocaine-associated memories and could be therapeutically targeted to treat addiction.

GPR3 and GPR6, novel molecular targets for cannabidiol.

GPR3 and GPR6 are members of a family of constitutively active, Gs protein-coupled receptors. Previously, it has been reported that GPR3 is involved in Alzheimer's disease whereas GPR6 plays potential roles in Parkinson's disease. GPR3 and GPR6 are considered orphan receptors because there are no confirmed endogenous agonists for them. However, GPR3 and GPR6 are phylogenetically related to the cannabinoid receptors. In this study, the activities of endocannabinoids and phytocannabinoids were tested on GPR3 and GPR6 using a ß-arrestin2 recruitment assay. Among the variety of cannabinoids tested, cannabidiol (CBD), the major non-psychoactive component of marijuana, significantly reduced ß-arrestin2 recruitment to both GPR3 and GPR6. In addition, the inhibitory effects of CBD on ß-arrestin2 recruitment were concentration-dependent for both GPR3 and GPR6, with a higher potency for GPR6. These data show that CBD acts as an inverse agonist at both GPR3 and GPR6 receptors. These results demonstrate for the first time that both GPR3 and GPR6 are novel molecular targets for CBD. Our discovery that CBD acts as a novel inverse agonist on both GPR3 and GPR6 indicates that some of the potential therapeutic effects of CBD (e.g. treatment of Alzheimer's disease and Parkinson's disease) may be mediated through these important receptors.

Nociceptin/orphanin FQ antagonizes lipopolysaccharide-stimulated proliferation, migration and inflammatory signaling in human glioblastoma U87 cells.

Glioblastoma is among the most aggressive brain tumors and has an exceedingly poor prognosis. Recently, the importance of the tumor microenvironment in glioblastoma cell growth and progression has been emphasized. Toll-like receptor 4 (TLR4) recognizes bacterial lipopolysaccharide (LPS) and endogenous ligands originating from dying cells or the extracellular matrix involved in host defense and in inflammation. G-protein coupled receptors (GPCRs) have gained interest in anti-tumor drug discovery due to the role that they directly or indirectly play by transactivating other receptors, causing cell migration and proliferation. A proteomic analysis showed that the nociceptin receptor (NOPr) is among the GPCRs significantly expressed in glioblastoma cells, including U87 cells. We describe a novel role of the peptide nociceptin (N/OFQ), the endogenous ligand of the NOPr that counteracts cell migration, proliferation and increase in IL-1ß mRNA elicited by LPS via TLR4 in U87 glioblastoma cells. Signaling pathways through which N/OFQ inhibits LPS-mediated cell migration and elevation of [Ca2+]i require ß-arrestin 2 and are sensitive to TNFR-associated factor 6, c-Src and protein kinase C (PKC). LPS-induced cell proliferation and increase in IL-1ß mRNA are counteracted by N/OFQ via ß-arrestin 2, PKC and extracellular signal-regulated kinase 1/2; furthermore, the contributions of the transcription factors NF-kB and AP-1 were investigated. Independent of LPS, N/OFQ induces a significant increase in cell apoptosis. Contrary to what was observed in other cell models, a prolonged exposure to this endotoxin did not promote any tolerance of the cellular effects above described, including NOPr down-regulation while N/OFQ loses its inhibitory role.

ß-arrestin is critical for early shear stress-induced Akt/eNOS activation in human vascular endothelial cells.

Recent evidence suggests that ß-arrestins, which are involved in G protein-coupled receptors desensitization, may influence mechanotransduction. Here, we observed that nitric oxide (NO) production was abrogated in human saphenous vein endothelial cells (SVECs) transfected with siRNA against ß-arrestin 1 and 2 subjected to shear stress (SS, 15 dynes/cm2, 10 min). The downregulation of ß-arrestins 1/2 in SVECs cells also prevented the SS-induced rise in levels of phosphorylation of Akt and endothelial nitric oxide synthase (eNOS, Serine 1177). Interestingly, immunoprecipitation revealed that ß-arrestin interacts with Akt, eNOS and caveolin-1 and these interactions are not influenced by SS. Our data indicate that ß-arrestins and Akt/eNOS downstream signaling are required for early SS-induced NO production in SVECs, which is consistent with the idea that ß-arrestins and caveolin-1 are part of a pre-assembled complex associated with the cellular mechanotransduction machinery.

ß-Arrestin-2 modulates radiation-induced intestinal crypt progenitor/stem cell injury.

Intestinal crypt progenitor/stem (ICPS) cell apoptosis and vascular endothelial cell apoptosis are responsible for the initiation and development of ionizing radiation (IR)-evoked gastrointestinal syndrome. The signaling mechanisms underlying IR-induced ICPS cell apoptosis remain largely unclear. Our findings provide evidence that ß-arrestin-2 (ßarr2)-mediated ICPS cell apoptosis is crucial for IR-stimulated intestinal injury. ßArr2-deficient mice exhibited decreased ICPS cell and intestinal Lgr5(+) (leucine-rich repeat-containing G-protein-coupled receptor 5-positive) stem cell apoptosis, promoted crypt proliferation and reproduction, and protracted survival following lethal doses of radiation. Radioprotection in the ICPS cells isolated from ßarr2-deficient mice depended on prolonged nuclear factor-κB (NF-κB) activation via direct interaction of ßarr2 with IκBα and subsequent inhibition of p53-upregulated modulator of apoptosis (PUMA)-mediated mitochondrial dysfunction. Unexpectedly, ßarr2 deficiency had little effect on IR-induced intestinal vascular endothelial cell apoptosis in mice. Consistently, ßarr2 knockdown also provided significant radioresistance by manipulating NF-κB/PUMA signaling in Lgr5(+) cells in vitro. Collectively, these observations show that targeting the ßarr2/NF-κB/PUMA novel pathway is a potential radiomitigator for limiting the damaging effect of radiotherapy on the gastrointestinal system. Significance statement: acute injury to the intestinal mucosa is a major dose-limiting complication of abdominal radiotherapy. The issue of whether the critical factor for the initiation of radiation-induced intestinal injury is intestinal stem cell apoptosis or endothelial cell apoptosis remains unresolved. ßArrs have recently been found to be multifunctional adaptor of apoptosis. Here, we found that ß-arrestin-2 (ßarr2) deficiency was associated with decreased radiation-induced ICPS cell apoptosis, which prolonged survival in abdominally irradiated mice. Moreover, ßarr2 deficiency-mediated intestinal progenitor/stem cell radioprotection relied on protracted NF-κB activation and subsequent suppression of PUMA induction. Our results suggest that ICPS cell apoptosis is the factor involved in the initiation and development of radiation-induced gastrointestinal syndrome. ßArr2 is a potential target for lessening radiation-induced ICPS cell apoptosis.