Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

Structural basis of arrestin-3 activation and signaling

A unique aspect of arrestin-3 is its ability to support both receptor-dependent and receptor-independent signaling. Here, we show that inositol hexakisphosphate (IP6) is a non-receptor activator of arrestin-3 and report the structure of IP6-activated arrestin-3 at 2.4-Å resolution. IP6-activated arrestin-3 exhibits an inter-domain twist and a displaced C-tail, hallmarks of active arrestin. IP6 binds to the arrestin phosphate sensor, and is stabilized by trimerization. Analysis of the trimerization surface, which is also the receptor-binding surface, suggests a feature called the finger loop as a key region of the activation sensor. We show that finger loop helicity and flexibility may underlie coupling to hundreds of diverse receptors and also promote arrestin-3 activation by IP6. Importantly, we show that effector-binding sites on arrestins have distinct conformations in the basal and activated states, acting as switch regions. These switch regions may work with the inter-domain twist to initiate and direct arrestin-mediated signaling.

GPCRs: Emerging anti-cancer drug targets

G protein-coupled receptors (GPCRs) constitute the largest and most diverse protein family in the human genome with over 800 members identified to date. They play critical roles in numerous cellular and physiological processes, including cell proliferation, differentiation, neurotransmission, development and apoptosis. Consequently, aberrant receptor activity has been demonstrated in numerous disorders/diseases, and as a result GPCRs have become the most successful drug target class in pharmaceuticals treating a wide variety of indications such as pain, inflammation, neurobiological and metabolic disorders. Many independent studies have also demonstrated a key role for GPCRs in tumourigenesis, establishing their involvement in cancer initiation, progression, and metastasis. Given the growing appreciation of the role(s) that GPCRs play in cancer pathogenesis, it is surprising to note that very few GPCRs have been effectively exploited in pursuit of anti-cancer therapies. The present review provides a broad overview of the roles that various GPCRs play in cancer growth and development, highlighting the potential of pharmacologically modulating these receptors for the development of novel anti-cancer therapeutics.

Arrestin-2 and arrestin-3 differentially modulate locomotor responses and sensitization to amphetamine

Arrestins play a prominent role in shutting down signaling via G protein-coupled receptors. In recent years, a signaling role for arrestins independent of their function in receptor desensitization has been discovered. Two ubiquitously expressed arrestin isoforms, arrestin-2 and arrestin-3, perform similarly in the desensitization process and share many signaling functions, enabling them to substitute for one another. However, signaling roles specific to each isoform have also been described. Mice lacking arrestin-3 (ARR3KO) were reported to show blunted acute responsiveness to the locomotor stimulatory effect of amphetamine (AMPH). It has been suggested that mice with deletion of arrestin-2 display a similar phenotype. Here we demonstrate that the AMPH-induced locomotion of male ARR3KO mice is reduced over the 7-day treatment period and during AMPH challenge after a 7-day withdrawal. The data are consistent with impaired locomotor sensitization to AMPH and suggest a role for arrestin-3-mediated signaling in the sensitization process. In contrast, male ARR2KO mice showed enhanced early responsiveness to AMPH and the lack of further sensitization, suggesting a role for impaired receptor desensitization. The comparison of mice possessing one allele of arrestin-3 and no arrestin-2 with ARR2KO littermates revealed reduced activity of the former line, consistent with a contribution of arrestin-3-mediated signaling to AMPH responses. Surprisingly, ARR3KO mice with one arrestin-2 allele showed significantly reduced locomotor responses to AMPH combined with lower novelty-induced locomotion, as compared to the ARR3KO line. These data suggest that one allele of arrestin-2 is unable to support normal locomotor behavior due to signaling and/or developmental defects.

Uncovering missing pieces: duplication and deletion history of arrestins in deuterostomes

BACKGROUND

The cytosolic arrestin proteins mediate desensitization of activated G protein-coupled receptors (GPCRs) via competition with G proteins for the active phosphorylated receptors. Arrestins in active, including receptor-bound, conformation are also transducers of signaling. Therefore, this protein family is an attractive therapeutic target. The signaling outcome is believed to be a result of structural and sequence-dependent interactions of arrestins with GPCRs and other protein partners. Here we elucidated the detailed evolution of arrestins in deuterostomes.

RESULTS

Identity and number of arrestin paralogs were determined searching deuterostome genomes and gene expression data. In contrast to standard gene prediction methods, our strategy first detects exons situated on different scaffolds and then solves the problem of assigning them to the correct gene. This increases both the completeness and the accuracy of the annotation in comparison to conventional database search strategies applied by the community. The employed strategy enabled us to map in detail the duplication- and deletion history of arrestin paralogs including tandem duplications, pseudogenizations and the formation of retrogenes. The two rounds of whole genome duplications in the vertebrate stem lineage gave rise to four arrestin paralogs. Surprisingly, visual arrestin ARR3 was lost in the mammalian clades Afrotheria and Xenarthra. Duplications in specific clades, on the other hand, must have given rise to new paralogs that show signatures of diversification in functional elements important for receptor binding and phosphate sensing.

CONCLUSION

The current study traces the functional evolution of deuterostome arrestins in unprecedented detail. Based on a precise re-annotation of the exon-intron structure at nucleotide resolution, we infer the gain and loss of paralogs and patterns of conservation, co-variation and selection.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

Canonical and Noncanonical Signaling Roles of β-Arrestins in Inflammation and Immunity

β-Arrestins are a highly conserved family of cytosolic adaptor proteins that contribute to many immune functions by orchestrating the desensitization and internalization of cell-surface G protein-coupled receptors (GPCRs) via well-studied canonical interactions. In cells of the innate and adaptive immune system, β-arrestins also subserve a parallel but less understood role in which they propagate, rather than terminate, intracellular signal transduction cascades. Because β-arrestins are promiscuous in their binding, they are capable of interacting with several different GPCRs and downstream effectors; in doing so, they vastly expand the repertoire of cellular responses evoked by agonist binding and the scope of responses that may contribute to inflammation during infectious and sterile insults. In this chapter, we attempt to provide an overview of the canonical and noncanonical roles of β-arrestins in inflammatory diseases.

Functional antagonism of β-arrestin isoforms balance IGF-1R expression and signalling with distinct cancer-related biological outcomes

With very similar 3D structures, the widely expressed β-arrestin isoforms 1 and 2 play at times identical, distinct or even opposing roles in regulating various aspects of G protein-coupled receptors (GPCR) expression and signalling. Recent evidence recognizes the β-arrestin system as a key regulator of not only GPCRs, but also receptor tyrosine kinases, including the highly cancer relevant insulin-like growth factor type 1 receptor (IGF-1R). Binding of β-arrestin1 to IGF-1R leads to ligand-dependent degradation of the receptor and generates additional MAPK/ERK signalling, protecting cancer cells against anti-IGF-1R therapy. Because the interplay between β-arrestin isoforms governs the biological effects for most GPCRs, as yet unexplored for the IGF-1R, we sought to investigate specifically the regulatory roles of the β-arrestin2 isoform on expression and function of the IGF-1R. Results from controlled expression of either β-arrestin isoform demonstrate that β-arrestin2 acts in an opposite manner to β-arrestin1 by promoting degradation of an unstimulated IGF-1R, but protecting the receptor against agonist-induced degradation. Although both isoforms co-immunoprecipitate with IGF-1R, the ligand-occupied receptor has greater affinity for β-arrestin1; this association lasts longer, sustains MAPK/ERK signalling and mitigates p53 activation. Conversely, β-arrestin2 has greater affinity for the ligand-unoccupied receptor; this interaction is transient, triggers receptor ubiquitination and degradation without signalling activation, and leads to a lack of responsiveness to IGF-1, cell cycle arrest and decreased viability of cancer cells. This study reveals contrasting abilities of IGF-1R to interact with each β-arrestin isoform, depending on the presence of the ligand and demonstrates the antagonism between the two β-arrestin isoforms in controlling IGF-1R expression and function, which could be developed into a practical anti-IGF-1R strategy for cancer therapy.

The regulatory and signaling mechanisms of the ASK family

Apoptosis signal-regulating kinase 1 (ASK1) was identified as a MAP3K that activates the JNK and p38 pathways, and subsequent studies have reported ASK2 and ASK3 as members of the ASK family. The ASK family is activated by various intrinsic and extrinsic stresses, including oxidative stress, ER stress and osmotic stress. Numerous lines of evidence have revealed that members of the ASK family are critical for signal transduction systems to control a wide range of stress responses such as cell death, differentiation and cytokine induction. In this review, we focus on the precise signaling mechanisms of the ASK family in response to diverse stressors.

Core engagement with β-arrestin is dispensable for agonist-induced vasopressin receptor endocytosis and ERK activation

G protein-coupled receptors (GPCRs) exhibit highly conserved activation and signaling mechanisms by which agonist stimulation leads to coupling of heterotrimeric G proteins and generation of second messenger response. This is followed by receptor phosphorylation, primarily in the carboxyl terminus but also in the cytoplasmic loops, and subsequent binding of arrestins. GPCRs typically recruit arrestins through two different sets of interactions, one involving phosphorylated receptor tail and the other mediated by the receptor core. The engagement of both set of interactions (tail and core) is generally believed to be necessary for arrestin-dependent functional outcomes such as receptor desensitization, endocytosis, and G protein-independent signaling. Here we demonstrate that a vasopressin receptor (V₂R) mutant with truncated third intracellular loop (V₂R^ΔICL3) can interact with β-arrestin 1 (βarr1) only through the phosphorylated tail without engaging the core interaction. Of interest, such a partially engaged V₂R^ΔICL3-βarr1 complex can efficiently interact with clathrin terminal domain and ERK2 MAPK in vitro. Furthermore, this core interaction-deficient V₂R mutant exhibits efficient endocytosis and ERK activation upon agonist stimulation. Our data suggest that core interaction with βarr is dispensable for V₂R endocytosis and ERK activation and therefore provide novel insights into refining the current understanding of functional requirements in biphasic GPCR-βarr interaction.

Arrestin-biased AT1R agonism induces acute catecholamine secretion through TRPC3 coupling

Acute hormone secretion triggered by G protein-coupled receptor (GPCR) activation underlies many fundamental physiological processes. GPCR signalling is negatively regulated by β-arrestins, adaptor molecules that also activate different intracellular signalling pathways. Here we reveal that TRV120027, a β-arrestin-1-biased agonist of the angiotensin II receptor type 1 (AT1R), stimulates acute catecholamine secretion through coupling with the transient receptor potential cation channel subfamily C 3 (TRPC3). We show that TRV120027 promotes the recruitment of TRPC3 or phosphoinositide-specific phospholipase C (PLCγ) to the AT1R-β-arrestin-1 signalling complex. Replacing the C-terminal region of β-arrestin-1 with its counterpart on β-arrestin-2 or using a specific TAT-P1 peptide to block the interaction between β-arrestin-1 and PLCγ abolishes TRV120027-induced TRPC3 activation. Taken together, our results show that the GPCR-arrestin complex initiates non-desensitized signalling at the plasma membrane by coupling with ion channels. This fast communication pathway might be a common mechanism of several cellular processes.

Targeting orphan G protein-coupled receptors for the treatment of diabetes and its complications: C-peptide and GPR146

G protein-coupled receptors (GPCRs) are the most abundant receptor family encoded by the human genome and are the targets of a high percentage of drugs currently in use or in clinical trials for the treatment of diseases such as diabetes and its associated complications. Thus, orphan GPCRs, for which the ligand is unknown, represent an important untapped source of therapeutic potential for the treatment of many diseases. We have identified the previously orphan GPCR, GPR146, as the putative receptor of proinsulin C-peptide, which may prove to be an effective treatment for diabetes-associated complications. For example, we have found a potential role of C-peptide and GPR146 in regulating the function of the retinal pigment epithelium, a monolayer of cells in the retina that serves as part of the blood-retinal barrier and is disrupted in diabetic macular oedema. However, C-peptide signalling in this cell type appears to depend at least in part on extracellular glucose concentration and its interaction with insulin. In this review, we discuss the therapeutic potential of orphan GPCRs with a special focus on C-peptide and GPR146, including past and current strategies used to 'deorphanize' this diverse family of receptors, past successes and the inherent difficulties of this process.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology

G protein-coupled receptors (GPCRs) are a family of cell-surface proteins that play critical roles in regulating a variety of pathophysiological processes and thus are targeted by almost a third of currently available therapeutics. It was originally thought that GPCRs convert extracellular stimuli into intracellular signals through activating G proteins, whereas β-arrestins have important roles in internalization and desensitization of the receptor. Over the past decade, several novel functional aspects of β-arrestins in regulating GPCR signaling have been discovered. These previously unanticipated roles of β-arrestins to act as signal transducers and mediators of G protein-independent signaling have led to the concept of biased agonism. Biased GPCR ligands are able to engage with their target receptors in a manner that preferentially activates only G protein- or β-arrestin-mediated downstream signaling. This offers the potential for next generation drugs with high selectivity to therapeutically relevant GPCR signaling pathways. In this review, we provide a summary of the recent studies highlighting G protein- or β-arrestin-biased GPCR signaling and the effects of biased ligands on disease pathogenesis and regulation.

c-Jun N-terminal kinases 3 (JNK3) from orange-spotted grouper, Epinephelus coioides, inhibiting the replication of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis

C-Jun N-terminal kinases (JNKs), a subgroup of serine-threonine protein kinases that activated by phosphorylation, are involve in physiological and pathophysiological processes. JNK3 is one of JNK proteins involved in JNK3 signaling transduction. In the present study, two JNK3 isoforms, Ec-JNK3 X1 and Ec-JNK3 X2, were cloned from orange-spotted grouper, Epinephelus coioides. Both Ec-JNK3 X1 and Ec-JNK3 X2 were mainly expressed in liver, gill, skin, brain and muscle of juvenile grouper. The relative expression of Ec-JNK3 X2 mRNA was much higher in muscle and gill than that of Ec-JNK3 X1. Isoform-specific immune response to challenges was revealed by the expression profiles in vivo. Immunofluorescence staining indicated that JNK3 was localized in the cytoplasm of grouper spleen (GS) cells and shown immune response to SGIV infection in vitro. Over-expressing Ec-JNK3 X1 and/or Ec-JNK3 X2 inhibited the SGIV infection and replication and the SGIV-induced apoptosis. To achieve the antiviral and anti-apoptosis activities, JNK3 promoted the activation of genes ISRE and type I IFN in the antiviral IFN signaling pathway, and inhibited the activation of transcription factors NF-κB and p53 relating to apoptosis, respectively. Ec-JNK3 X2 showed stronger activities in antivirus and anti-apoptosis than that of Ec-JNK3 X1. Our results not only define the characterization of JNK3 but also reveal new immune functions and the molecular mechanisms of JNK3 on iridoviruses infection and the virus-induced apoptosis.

Transduction of group I mGluR-mediated synaptic plasticity by β-arrestin2 signalling

Conventional signalling by the group I metabotropic glutamate receptors, mGluR1 and mGluR5, occurs through G-protein coupling, but evidence suggests they might also utilize other, non-canonical effector pathways. Here we test whether group I mGluRs require β-arrestin signalling during specific forms of plasticity at hippocampal excitatory synapses. We find that genetic ablation of β-arrestin2, but not β-arrestin1, results in deficits in plasticity mediated by mGlu1 receptors in CA3 pyramidal neurons and by mGlu5 receptors in CA1 pyramidal neurons. Pharmacological studies additionally support roles for Src kinases and MAPK/ERK downstream of β-arrestin2 in CA3 neurons. mGluR1 modulation of intrinsic conductances is otherwise preserved in β-arrestin2^−/− mice with the exception of a rebound depolarization, and non-mGluR-mediated long-term potentiation is unaltered. These results reveal a signalling pathway engaged by group I mGluRs to effect changes in synaptic and cell intrinsic physiology dependent upon β-arrestin rather than G proteins. Pharmacological manipulation of mGluRs with effector-biased ligands could lead to novel therapies to treat neurological disease.

mGluRs are known to undergo non-canonical signalling regulation, although the underlying mechanisms are unclear. Here, the authors identify a role for β-arrestin2, but not β-arrestin1, in group I mGluR-mediated plasticity at hippocampal synapses.

Functional competence of a partially engaged GPCR-β-arrestin complex

G Protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors and drug targets. GPCR signalling and desensitization is critically regulated by β-arrestins (βarr). GPCR-βarr interaction is biphasic where the phosphorylated carboxyl terminus of GPCRs docks to the N-domain of βarr first and then seven transmembrane core of the receptor engages with βarr. It is currently unknown whether fully engaged GPCR-βarr complex is essential for functional outcomes or partially engaged complex can also be functionally competent. Here we assemble partially and fully engaged complexes of a chimeric β2V2R with βarr1, and discover that the core interaction is dispensable for receptor endocytosis, ERK MAP kinase binding and activation. Furthermore, we observe that carvedilol, a βarr biased ligand, does not promote detectable engagement between βarr1 and the receptor core. These findings uncover a previously unknown aspect of GPCR-βarr interaction and provide novel insights into GPCR signalling and regulatory paradigms.

Roles of the NLRP3 inflammasome in the pathogenesis of diabetic nephropathy

Diabetic nephropathy (DN) is a serious complication of diabetes mellitus, and persistent inflammation in circulatory and renal tissues is an important pathophysiological basis for DN. The essence of the microinflammatory state is the innate immune response, which is central to the occurrence and development of DN. Members of the inflammasome family, including both "receptors" and "regulators", are key to the inflammatory immune response. Nucleotide binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) and other inflammasome components are able to detect endogenous danger signals, resulting in activation of caspase-1 as well as interleukin (IL)-1β, IL-18 and other cytokines; these events stimulate the inflammatory cascade reaction, which is crucial for DN. Hyperglycaemia, hyperlipidaemia and hyperuricaemia can activate the NLRP3 inflammasome, which then mediates the occurrence and development of DN through the K⁺ channel model, the lysosomal damage model and the active oxygen cluster model. In this review, we survey the involvement of the NLRP3 inflammasome in various signalling pathways and highlight different aspects of their influence on DN. We also explore the important effects of the NLRP3 inflammasome on kidney function and structural changes that occur during DN development and progression. It is becoming more evident that NLRP3 inflammasome targeting has therapeutic potential for the treatment of DN.

A Pluridimensional View of Biased Agonism

When studying G protein-coupled receptor (GPCR) signaling and ligand-biased agonism, at least three dimensional spaces must be considered, as follows: 1) the distinct conformations that can be stabilized by different ligands promoting the engagement of different signaling effectors and accessory regulators; 2) the distinct subcellular trafficking that can be conferred by different ligands, which results in spatially distinct signals; and 3) the differential binding kinetics that maintain the receptor in specific conformation and/or subcellular localization for different periods of time, allowing for the engagement of distinct signaling effector subsets. These three pluridimensional aspects of signaling contribute to different faces of functional selectivity and provide a complex, interconnected way to define the signaling profile of each individual ligand acting at GPCRs. In this review, we discuss how each of these aspects may contribute to the diversity of signaling, but also how they shed light on the complexity of data analyses and interpretation. The impact of phenotype variability as a source of signaling diversity, and the influence of novel and more sensitive assays in the detection and analysis of signaling pluridimensionality, is also discussed. Finally, we discuss perspectives for the use of the concept of pluridimensional signaling in drug discovery, in which we highlight future predictive tools that may facilitate the identification of compounds with optimal therapeutic and safety properties based on the signaling signatures of drug candidates.

Scaffold Role of DUSP22 in ASK1-MKK7-JNK Signaling Pathway

Mitogen-activated protein kinases (MAPKs) are involved in a variety of intracellular events such as gene expression, cell proliferation, and programmed cell death. MAPKs are activated by dual phosphorylation on threonine and tyrosine residues through sequential activation of protein kinases. Recent studies have shown that the protein kinases involved in MAPK signal transductions might be organized into signaling complexes by scaffold proteins. These scaffold proteins are essential regulators that function by assembling the relevant molecular components in mammalian cells. In this study, we report that dual-specificity phosphatase 22 (DUSP22), a member of the protein tyrosine phosphatase family, acts as a distinct scaffold protein in c-Jun N-terminal kinase (JNK) signaling. DUSP22 increased the phosphorylation in the activation loop of JNK regardless of its phosphatase activity but had no effect on phosphorylation levels of ERK and p38 in mammalian cells. Furthermore, DUSP22 selectively associated with apoptosis signal-regulating kinase 1 (ASK1), MAPK kinase 7 (MKK7), and JNK1/2. Both JNK phosphorylation and JNK-mediated apoptosis increased in a concentration-dependent manner regardless of DUSP22 phosphatase activity at low DUSP22 concentrations, but then decreased at higher DUSP22 concentrations, which is the prominent feature of a scaffold protein. Thus, our data suggest that DUSP22 regulates cell death by acting as a scaffold protein for the ASK1-MKK7-JNK signal transduction pathway independently of its phosphatase activity.

Pleiotropic properties of ASK1

BACKGROUND

Apoptosis signal-regulating kinase 1 (ASK1), also known as mitogen-activated protein kinase kinase kinase 5 (MAP3K5), has the potential to induce cellular apoptosis under various physiological conditions. It has long been suggested that ASK1 is highly sensitive to oxidative stress and contributes substantially to apoptosis. However, recent studies have indicated that ASK1 has pleiotropic roles in living organisms through other mechanisms in addition to apoptosis.

SCOPE OF THE REVIEW

This review describes the physiological functions of ASK1 in living organisms, focusing on the regulatory mechanisms of ASK1 activity and its importance in the pathogenesis of various diseases. We also highlight recent works published within the past few years.

MAJOR CONCLUSIONS

ASK1 forms a high-molecular-mass complex within the cell, designated as the ASK1 signalosome. Soon after the discovery of ASK1, several regulatory components of the ASK1 signalosome have been revealed, including thioredoxin (Trx), tumor-necrosis factor α receptor-associated factors (TRAFs) and 14-3-3s. In parallel with the precise analyses unveiling the molecular basis of ASK1 regulation, the physiological or pathophysiological significance of ASK1 in diverse organs has been elucidated. In addition to the generation of global knockout mice or tissue-specific knockout mice, ASK1-specific inhibitors have illuminated the biological roles of ASK1.

GENERAL SIGNIFICANCE

The multi-faceted features of the function of ASK1 have been discovered over the past two decades, revealing that ASK1 is a crucial molecule for maintaining cellular homeostasis, especially under conditions of stress. Based on the results that ASK1 deficiency provides beneficial effects for several diseases, modulating ASK1 activity is a promising method to ameliorate a subset of diseases.

Analysis of the V2 Vasopressin Receptor (V2R) Mutations Causing Partial Nephrogenic Diabetes Insipidus Highlights a Sustainable Signaling by a Non-peptide V2R Agonist

Disease-causing mutations in G protein-coupled receptor (GPCR) genes, including the V2 vasopressin receptor (V2R) gene, often cause misfolded receptors, leading to a defect in plasma membrane trafficking. A novel V2R mutation, T273M, identified in a boy with partial nephrogenic diabetes insipidus (NDI), shows intracellular localization and partial defects similar to the two mutants we described previously (10). Although non-peptide V2R antagonists have been shown to rescue the membrane localization of V2R mutants, their level of functional rescue is weak. Interestingly, it has been reported that a non-peptide agonist, OPC51803, activates misfolded V2R mutants intracellularly without degradation, thus potentially serving as a therapeutic agent against NDI (14). In our current experiments, however, a peptide antagonist blocked arginine vasopressin (AVP)- or OPC51803-stimulated cAMP accumulation both in COS-7 and MDCK cells, suggesting that OPC51803 mainly stimulates cell surface V2R mutants. In addition, our analyses revealed that OPC51803 works not only as a non-peptide agonist that causes activation/β-arrestin-dependent desensitization of V2R mutants expressed at the plasma membrane but also as a pharmacochaperone that promotes the endoplasmic reticulum-retained mutant maturation and trafficking to the plasma membrane. The ratio of the pharmacochaperone effect to the desensitization effect likely correlates negatively with the residual function of the tested mutants, suggesting that OPC5 has a more favorable effect on the V2R mutants with a less residual function. We speculated that the canceling of the desensitization effect of OPC51803 by the pharmacochaperone effect after long-term treatment may produce sustainable signaling, and thus pharmacochaperone agonists such as OPC51803 may serve as promising therapeutics for NDI caused by misfolded V2R mutants.

Metformin Enhances the Therapy Effects of Anti-IGF-1R mAb Figitumumab to NSCLC

The insulin-like growth factor (IGF) signaling system plays a critical role in tumorigenesis, highlighting the potential of targeting IGF-1R as an anti-cancer therapy. Although multiple anti-IGF-1R monoclonal antibody (mAb) drugs have been developed, challenges remain in the validation of the therapeutic effects and understanding the molecular mechanism of these mAbs. Herein, we conducted a study to validate the effect of Figitumumab (CP), an anti-IGF-1R mAb, in a panel of non-small cell lung cancer (NSCLC) cell lines. We found all tested cell lines were sensitive to CP, and CP could block IGF-1R and the downstream PI3K/AKT pathway activation. Unexpectedly, we found CP could activate ERK signaling pathway in IGF-1R kinase independent manner, which we further verified was mainly mediated by β-arrestin2. We also investigated the anti-tumor effect of metformin alone as well as its combination with CP to target NSCLC. Metformin could target IGF-1R signaling pathway by attenuating PI3K/AKT and MEK/ERK signaling pathways and down-regulating IGF-1R. Finally, we found that combining metformin with CP could further induce IGF-1R down-regulation and was more effective to target NSCLC cells. Our data suggests the combining of metformin with CP has additive therapeutic value against NSCLC.

β-arrestin 2 attenuates cardiac dysfunction in polymicrobial sepsis through gp130 and p38

Sepsis is an exaggerated systemic inflammatory response to persistent bacteria infection with high morbidity and mortality rate clinically. β-arrestin 2 modulates cell survival and cell death in different systems. However, the effect of β-arrestin 2 on sepsis-induced cardiac dysfunction is not yet known. Here, we show that β-arrestin 2 overexpression significantly enhances animal survival following cecal ligation and puncture (CLP)-induced sepsis. Importantly, overexpression of β-arrestin 2 in mice prevents CLP-induced cardiac dysfunction. Also, β-arrestin 2 overexpression dramatically attenuates CLP-induced myocardial gp130 and p38 mitogen-activated protein kinase (MAPK) phosphorylation levels following CLP. Therefore, β-arrestin 2 prevents CLP-induced cardiac dysfunction through gp130 and p38. These results suggest that modulation of β-arrestin 2 might provide a novel therapeutic approach to prevent cardiac dysfunction in patients with sepsis.

Process of hepatic metastasis from pancreatic cancer: biology with clinical significance

PURPOSE

Pancreatic cancer shows a remarkable preference for the liver to establish secondary tumors. Selective metastasis to the liver is attributed to the development of potential microenvironment for the survival of pancreatic cancer cells. This review aims to provide a full understanding of the hepatic metastatic process from circulating pancreatic cancer cells to their settlement in the liver, serving as a basic theory for efficient prediction and treatment of metastatic diseases.

METHODS

A systematic search of relevant original articles and reviews was performed on PubMed, EMBASE and Cochrane Library for the purpose of this review.

RESULTS

Three interrelated phases are delineated as the contributions of the interaction between pancreatic cancer cells and the liver to hepatic metastasis process. Chemotaxis of disseminated pancreatic cancer cells and simultaneous defensive formation of platelets or neutrophils facilitate specific metastasis toward the liver. Remodeling of extracellular matrix and stromal cells in hepatic lobules and angiogenesis induced by proangiogenic factors support the survival and growth of clinical micrometastasis colonizing the liver. The bimodal role of the immune system or prevalence of cancer cells over the immune system makes metastatic progression successfully proceed from micrometastasis to macrometastasis.

CONCLUSIONS

Pancreatic cancer is an appropriate research object of cancer metastasis representing more than a straight cascade. If any of the successive or simultaneous phases, especially tumor-induced immunosuppression, is totally disrupted, hepatic metastasis will be temporarily under control or even cancelled forever. To shrink cancers on multiple fronts and prolong survival for patients, novel oral or intravenous anti-cancer agents covering one or different phases of metastatic pancreatic cancer are expected to be integrated into innovative strategies on the premise of safety and efficacious biostability.

The human Na(+)/H(+) exchanger 1 is a membrane scaffold protein for extracellular signal-regulated kinase 2

Background

Extracellular signal-regulated kinase 2 (ERK2) is an S/T kinase with more than 200 known substrates, and with critical roles in regulation of cell growth and differentiation and currently no membrane proteins have been linked to ERK2 scaffolding.

Methods and results

Here, we identify the human Na⁺/H⁺ exchanger 1 (hNHE1) as a membrane scaffold protein for ERK2 and show direct hNHE1-ERK1/2 interaction in cellular contexts. Using nuclear magnetic resonance (NMR) spectroscopy and immunofluorescence analysis we demonstrate that ERK2 scaffolding by hNHE1 occurs by one of three D-domains and by two non-canonical F-sites located in the disordered intracellular tail of hNHE1, mutation of which reduced cellular hNHE1-ERK1/2 co-localization, as well as reduced cellular ERK1/2 activation. Time-resolved NMR spectroscopy revealed that ERK2 phosphorylated the disordered tail of hNHE1 at six sites in vitro, in a distinct temporal order, with the phosphorylation rates at the individual sites being modulated by the docking sites in a distant dependent manner.

Conclusions

This work characterizes a new type of scaffolding complex, which we term a “shuffle complex”, between the disordered hNHE1-tail and ERK2, and provides a molecular mechanism for the important ERK2 scaffolding function of the membrane protein hNHE1, which regulates the phosphorylation of both hNHE1 and ERK2.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0252-7) contains supplementary material, which is available to authorized users.

NQDI-1, an inhibitor of ASK1 attenuates acute perinatal hypoxic-ischemic cerebral injury by modulating cell death

Apoptosis signal-regulating kinase 1 (ASK1) is a ubiquitously expressed protein kinase, which regulates cell fate in numerous injury conditions. Therefore, ASK1 may be a promising novel therapeutic target for injury. However, the expression and distribution of ASK1 in the perinatal brain following hypoxia-ischemia (HI) remains to be elucidated. In the present study, western blotting and immunofluorescence were used to determine the expression and distribution of ASK1 and any associated downstream targets in the perinatal rat brain following HI. NQDI‑1, a specific inhibitor of ASK1 was intracerebroventricularly injected following neonatal rats brain insult for neuroprotection. The results revealed an increased expression of ASK1 and this expression was localized to the neurons and astrocytes, compared with the sham controls. Additionally, it was demonstrated that the ASK1/ c‑Jun N‑terminal kinases (JNK) pathway was involved in the brain damage following HI in neonatal rats. Notably, NQDI‑1 significantly inhibited the in vivo expression levels of ASK1, phosphorylated (p‑)JNK, p‑c‑Jun, p53 and caspase 3. Reduced acute hypoxic‑ischemic cerebral injury and cell apoptosis was observed following the injection of NQDI‑1. Collectively, NQDI-1 attenuated acute perinatal hypoxic‑ischemic cerebral injury by inhibiting the expression of ASK1 and cell apoptosis. This may be a promising novel neuroprotective inhibitor for perinatal cerebra injury.

Deletion of β-Arrestin2 in Mice Limited Pancreatic β-Cell Expansion under Metabolic Stress through Activation of the JNK Pathway

β-Arrestin2 (βarr2) is an adaptor protein that interacts with numerous signaling molecules and regulates insulin sensitivity. We reported previously that βarr2 was abundantly expressed in mouse pancreatic β-cells, and loss of βarr2 leads to impairment of acute- and late-phase insulin secretion. In the present study, we examined the dynamic changes of β-cell mass in βarr2-deficient (βarr2^-/-) mice in vivo and explored the underlying mechanisms involved. βarr2^-/- mice with exclusively luciferase overexpression in β-cells were generated and fed a high-fat diet (HFD). β-Cell mass was determined by in vivo noninvasive bioluminescence imaging from 4 to 20 wks of age. Proliferation was measured by 5-bromo-2-deoxyuridine (BrdU) incorporation and fluorescence-activated cell sorter analysis. Quantitative real-time polymerase chain reaction (qRT-PCR) and immunoblotting were conducted for gene and protein expression. We found that β-cell mass was reduced dramatically in βarr2^-/- mice at 12 wks old compared with that of their respective HFD-fed controls. The percentage of BrdU- and Ki67-positive cells reduced in islets from βarr2^-/- mice. Exposure of βarr2^-/- islets to high levels of glucose and free fatty acids (FFAs) exacerbated cell death, which was associated with upregulation of the JNK pathway in these islets. Conversely, overexpression of βarr2 amplified β-cell proliferation with a concomitant increase in cyclinD2 expression and a decrease in p21 expression and protected β-cells from glucose- and FFA-induced cell death through JNK-activation inhibition. In conclusion, βarr2 plays roles in regulation of pancreatic β-cell mass through the modulation of cell cycle regulatory genes and the inhibition of JNK activation induced by glucolipotoxity, which implicates a role for βarr2 in the development of type 2 diabetes.

Peptide mini-scaffold facilitates JNK3 activation in cells

Three-kinase mitogen-activated protein kinase (MAPK) signaling cascades are present in virtually all eukaryotic cells. MAPK cascades are organized by scaffold proteins, which assemble cognate kinases into productive signaling complexes. Arrestin-3 facilitates JNK activation in cells, and a short 25-residue arrestin-3 peptide was identified as the critical JNK3-binding element. Here we demonstrate that this peptide also binds MKK4, MKK7, and ASK1, which are upstream JNK3-activating kinases. This peptide is sufficient to enhance JNK3 activity in cells. A homologous arrestin-2 peptide, which differs only in four positions, binds MKK4, but not MKK7 or JNK3, and is ineffective in cells at enhancing activation of JNK3. The arrestin-3 peptide is the smallest MAPK scaffold known. This peptide or its mimics can regulate MAPKs, affecting cellular decisions to live or die.

Recent updates on GPCR biased agonism

G protein-coupled receptors (GPCRs) are the most important targets for drug discovery and not surprisingly ∼40% of all drugs currently in the market act on these receptors. Currently, one of the most active areas in GPCRs signaling is biased agonism, a phenomenon that occurs when a given ligand is able to preferentially activate one (or some) of the possible signaling pathways. In this review, we highlight the most recent findings about biased agonism, including an extension of this concept to intracellular signaling, allosterism, strategies for assessment and interpretation, and perspectives of therapeutic applications for biased agonists.

Ubiquitin-specific Protease 20 Regulates the Reciprocal Functions of β-Arrestin2 in Toll-like Receptor 4-promoted Nuclear Factor κB (NFκB) Activation

Toll-like receptor 4 (TLR4) promotes vascular inflammatory disorders such as neointimal hyperplasia and atherosclerosis. TLR4 triggers NFκB signaling through the ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6). TRAF6 activity can be impeded by deubiquitinating enzymes like ubiquitin-specific protease 20 (USP20), which can reverse TRAF6 autoubiquitination, and by association with the multifunctional adaptor protein β-arrestin2. Although β-arrestin2 effects on TRAF6 suggest an anti-inflammatory role, physiologic β-arrestin2 promotes inflammation in atherosclerosis and neointimal hyperplasia. We hypothesized that anti- and proinflammatory dimensions of β-arrestin2 activity could be dictated by β-arrestin2's ubiquitination status, which has been linked with its ability to scaffold and localize activated ERK1/2 to signalosomes. With purified proteins and in intact cells, our protein interaction studies showed that TRAF6/USP20 association and subsequent USP20-mediated TRAF6 deubiquitination were β-arrestin2-dependent. Generation of transgenic mice with smooth muscle cell-specific expression of either USP20 or its catalytically inactive mutant revealed anti-inflammatory effects of USP20in vivoandin vitro Carotid endothelial denudation showed that antagonizing smooth muscle cell USP20 activity increased NFκB activation and neointimal hyperplasia. We found that β-arrestin2 ubiquitination was promoted by TLR4 and reversed by USP20. The association of USP20 with β-arrestin2 was augmented when β-arrestin2 ubiquitination was prevented and reduced when β-arrestin2 ubiquitination was rendered constitutive. Constitutive β-arrestin2 ubiquitination also augmented NFκB activation. We infer that pro- and anti-inflammatory activities of β-arrestin2 are determined by β-arrestin2 ubiquitination and that changes in USP20 expression and/or activity can therefore regulate inflammatory responses, at least in part, by defining the ubiquitination status of β-arrestin2.

Regulation of N-Formyl Peptide Receptor Signaling and Trafficking by Arrestin-Src Kinase Interaction

Arrestins were originally described as proteins recruited to ligand-activated, phosphorylated G protein-coupled receptors (GPCRs) to attenuate G protein-mediated signaling. It was later revealed that arrestins also mediate GPCR internalization and recruit a number of signaling proteins including, but not limited to, Src family kinases, ERK1/2, and JNK3. GPCR-arrestin binding and trafficking control the spatial and temporal activity of these multi-protein complexes. In previous reports, we concluded that N-formyl peptide receptor (FPR)-mediated apoptosis, which occurs upon receptor stimulation in the absence of arrestins, is associated with FPR accumulation in perinuclear recycling endosomes. Under these conditions, inhibition of Src kinase and ERK1/2 prevented FPR-mediated apoptosis. To better understand the role of Src kinase in this process, in the current study we employed a previously described arrestin-2 (arr2) mutant deficient in Src kinase binding (arr2-P91G/P121E). Unlike wild type arrestin, arr2-P91G/P121E did not inhibit FPR-mediated apoptosis, suggesting that Src binding to arrestin-2 prevents apoptotic signaling. However, in cells expressing this mutant, FPR-mediated apoptosis was still blocked by inhibition of Src kinase activity, suggesting that activation of Src independent of arrestin-2 binding is involved in FPR-mediated apoptosis. Finally, while Src kinase inhibition prevented FPR-mediated-apoptosis in the presence of arr2-P91G/P121E, it did not prevent FPR-arr2-P91G/P121E accumulation in the perinuclear recycling endosome. On the contrary, inhibition of Src kinase activity mediated the accumulation of activated FPR-wild type arrestin-2 in recycling endosomes without initiating FPR-mediated apoptosis. Based on these observations, we conclude that Src kinase has two independent roles following FPR activation that regulate both FPR-arrestin-2 signaling and trafficking.

Delta Opioid Pharmacology in Relation to Alcohol Behaviors

Delta opioid receptors (DORs) are heavily involved in alcohol-mediated processes in the brain. In this chapter we provide an overview of studies investigating how alcohol directly impacts DOR pharmacology and of early studies indicating DOR modulation of alcohol behavior. We will offer a brief summary of the different animal species used in alcohol studies investigating DORs followed by a broader overview of the types of alcohol behaviors modulated by DORs. We will highlight a small set of studies investigating the relationship between alcohol and DORs in analgesia. We will then provide an anatomical overview linking DOR expression in specific brain regions to different alcohol behaviors. In this section, we will provide two models that try to explain how endogenous opioids acting at DORs may influence alcohol behaviors. Next, we will provide an overview of studies investigating certain new aspects of DOR pharmacology, including the formation of heteromers and biased signaling. Finally, we provide a short overview of the genetics of the DORs in relation to alcohol use disorders (AUDs) and a short statement on the potential of using DOR-based therapeutics for treatment of AUDs.

β-Arrestin-mediated Angiotensin II Signaling Controls the Activation of ARF6 Protein and Endocytosis in Migration of Vascular Smooth Muscle Cells

Angiotensin II (Ang II) is a vasopressive hormone but is also a potent activator of cellular migration. We have previously shown that it can promote the activation of the GTPase ARF6 in a heterologous overexpressing system. The molecular mechanisms by which receptors control the activation of this small G protein remain, however, largely unknown. Furthermore, how ARF6 coordinates the activation of complex cellular responses needs to be further elucidated. In this study, we demonstrate that Ang II receptors engage β-arrestin, but not Gq, to mediate ARF6 activation in HEK 293 cells. To further confirm the key role of β-arrestin proteins, we overexpressed β-arrestin2-(1-320), a dominant negative mutant known to block receptor endocytosis. We show that expression of this truncated construct does not support the activation of the GTPase nor cell migration. Interestingly, β-arrestin2 can interact with the ARF guanine nucleotide exchange factor ARNO, although the C-terminally lacking mutant does not. We finally examined whether receptor endocytosis controlled ARF6 activation and cell migration. Although the clathrin inhibitor PitStop2 did not impact the ability of Ang II to activate ARF6, cell migration was markedly impaired. To further show that ARF activation regulates key signaling events leading to migration, we also examined MAPK activation. We demonstrate that this signaling axis is relevant in smooth muscle cells of the vasculature. Altogether, our findings show for the first time that Ang II receptor signaling to β-arrestin regulates ARF6 activation. These proteins together control receptor endocytosis and ultimately cell migration.

Autocrine selection of a GLP-1R G-protein biased agonist with potent antidiabetic effects

Glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) agonists have emerged as treatment options for type 2 diabetes mellitus (T2DM). GLP-1R signals through G-protein-dependent, and G-protein-independent pathways by engaging the scaffold protein β-arrestin; preferential signalling of ligands through one or the other of these branches is known as ‘ligand bias'. Here we report the discovery of the potent and selective GLP-1R G-protein-biased agonist, P5. We identified P5 in a high-throughput autocrine-based screening of large combinatorial peptide libraries, and show that P5 promotes G-protein signalling comparable to GLP-1 and Exendin-4, but exhibited a significantly reduced β-arrestin response. Preclinical studies using different mouse models of T2DM demonstrate that P5 is a weak insulin secretagogue. Nevertheless, chronic treatment of diabetic mice with P5 increased adipogenesis, reduced adipose tissue inflammation as well as hepatic steatosis and was more effective at correcting hyperglycaemia and lowering haemoglobin A_1c levels than Exendin-4, suggesting that GLP-1R G-protein-biased agonists may provide a novel therapeutic approach to T2DM.

GLP-1 is a gut hormone with glucose-lowering activity. Here the authors report the peptide, P5, a variant of the GLP-1 receptor agonist exendin-4, with 'biased' signalling activity, and show that P5 improves glucose homeostasis in diabetic mice by increasing adipose tissue hyperplasia.

Multifaceted role of β-arrestins in inflammation and disease

Arrestins are intracellular scaffolding proteins known to regulate a range of biochemical processes including G protein-coupled receptor (GPCR) desensitization, signal attenuation, receptor turnover and downstream signaling cascades. Their roles in regulation of signaling network have lately been extended to receptors outside of the GPCR family, demonstrating their roles as important scaffolding proteins in various physiological processes including proliferation, differentiation and apoptosis. Recent studies have demonstrated a critical role for arrestins in immunological processes including key functions in inflammatory signaling pathways. In this review, we provide a comprehensive analysis of the different functions of the arrestin family of proteins especially related to immunity and inflammatory diseases.

The emerging roles of β-arrestins in fibrotic diseases

β-Arrestins and β-arrestin2 are important adaptor proteins and signal transduction proteins that are mainly involved in the desensitization and internalization of G-protein-coupled receptors. Fibrosis is characterized by accumulation of excess extracellular matrix (ECM) molecules caused by chronic tissue injury. If highly progressive, the fibrotic process leads to organ malfunction and, eventually, death. The incurable lung fibrosis, renal fibrosis and liver fibrosis are among the most common fibrotic diseases. Recent studies show that β-arrestins can activate signaling cascades independent of G-protein activation and scaffold many intracellular signaling networks by diverse types of signaling pathways, including the Hedgehog, Wnt, Notch and transforming growth factor-β pathways, as well as downstream kinases such as MAPK and PI3K. These signaling pathways are involved in the pathological process of fibrosis and fibrotic diseases. This β-arrestin-mediated regulation not only affects cell growth and apoptosis, but also the deposition of ECM, activation of inflammatory response and development of fibrotic diseases. In this review, we survey the involvement of β-arrestins in various signaling pathways and highlight different aspects of their regulation of fibrosis. We also discuss the important roles of β-arrestins in the process of fibrotic diseases by regulating the inflammation and deposit of ECM. It is becoming more evident that targeting β-arrestins may offer therapeutic potential for the treatment of fibrotic diseases.

Genetic inhibition of JNK3 ameliorates spinal muscular atrophy

Mutation of the Survival Motor Neuron 1 (SMN1) gene causes spinal muscular atrophy (SMA), an autosomal recessive neurodegenerative disorder that occurs in early childhood. Degeneration of spinal motor neurons caused by SMN deficiency results in progressive muscle atrophy and death in SMA. The molecular mechanism underlying neurodegeneration in SMA is unknown. No treatment is available to prevent neurodegeneration and reduce the burden of illness in SMA. We report that the c-Jun NH2-terminal kinase (JNK) signaling pathway mediates neurodegeneration in SMA. The neuron-specific isoform JNK3 is required for neuron degeneration caused by SMN deficiency. JNK3 deficiency reduces degeneration of cultured neurons caused by low levels of SMN. Genetic inhibition of JNK pathway in vivo by Jnk3 knockout results in amelioration of SMA phenotype. JNK3 deficiency prevents the loss of spinal cord motor neurons, reduces muscle degeneration, improves muscle fiber thickness and muscle growth, improves motor function and overall growth and increases lifespan of mice with SMA that shows a systemic rescue of phenotype by a SMN-independent mechanism. JNK3 represents a potential (non-SMN) therapeutic target for the treatment of SMA.

Role for engagement of β-arrestin2 by the transactivated EGFR in agonist-specific regulation of δ receptor activation of ERK1/2

BACKGROUND AND PURPOSE

β-Arrestins function as signal transducers linking GPCRs to ERK1/2 signalling either by scaffolding members of ERK1/2s cascades or by transactivating receptor tyrosine kinases through Src-mediated release of transactivating factor. Recruitment of β-arrestins to the activated GPCRs is required for ERK1/2 activation. Our previous studies showed that δ receptors activate ERK1/2 through a β-arrestin-dependent mechanism without inducing β-arrestin binding to the δ receptors. However, the precise mechanisms involved remain to be established.

EXPERIMENTAL APPROACH

ERK1/2 activation by δ receptor ligands was assessed using HEK293 cells in vitro and male Sprague Dawley rats in vivo. Immunoprecipitation, immunoblotting, siRNA transfection, intracerebroventricular injection and immunohistochemistry were used to elucidate the underlying mechanism.

KEY RESULTS

We identified a new signalling pathway in which recruitment of β-arrestin2 to the EGFR rather than δ receptor was required for its role in δ receptor-mediated ERK1/2 activation in response to H-Tyr-Tic-Phe-Phe-OH (TIPP) or morphine stimulation. Stimulation of the δ receptor with ligands leads to the phosphorylation of PKCδ, which acts upstream of EGFR transactivation and is needed for the release of the EGFR-activating factor, whereas β-arrestin2 was found to act downstream of the EGFR transactivation. Moreover, we demonstrated that coupling of the PKCδ/EGFR/β-arrestin2 transactivation pathway to δ receptor-mediated ERK1/2 activation was ligand-specific and the Ser(363) of δ receptors was crucial for ligand-specific implementation of this ERK1/2 activation pathway.

CONCLUSIONS AND IMPLICATIONS

The δ receptor-mediated activation of ERK1/2 is via ligand-specific transactivation of EGFR. This study adds new insights into the mechanism by which δ receptors activate ERK1/2.

Fulfilling the Promise of "Biased" G Protein-Coupled Receptor Agonism

The fact that over 30% of current pharmaceuticals target heptahelical G protein-coupled receptors (GPCRs) attests to their tractability as drug targets. Although GPCR drug development has traditionally focused on conventional agonists and antagonists, the growing appreciation that GPCRs mediate physiologically relevant effects via both G protein and non-G protein effectors has prompted the search for ligands that can "bias" downstream signaling in favor of one or the other process. Biased ligands are novel entities with distinct signaling profiles dictated by ligand structure, and the potential prospect of biased ligands as better drugs has been pleonastically proclaimed. Indeed, preclinical proof-of-concept studies have demonstrated that both G protein and arrestin pathway-selective ligands can promote beneficial effects in vivo while simultaneously antagonizing deleterious ones. But along with opportunity comes added complexity and new challenges for drug discovery. If ligands can be biased, then ligand classification becomes assay dependent, and more nuanced screening approaches are needed to capture ligand efficacy across several dimensions of signaling. Moreover, because the signaling repertoire of biased ligands differs from that of the native agonist, unpredicted responses may arise in vivo as these unbalanced signals propagate. For any given GPCR target, establishing a framework relating in vitro efficacy to in vivo biologic response is crucial to biased drug discovery. This review discusses approaches to describing ligand efficacy in vitro, translating ligand bias into biologic response, and developing a systems-level understanding of biased agonism in vivo, with the overall goal of overcoming current barriers to developing biased GPCR therapeutics.

Pleiotropic Effects of Bitter Taste Receptors on [Ca2+]i Mobilization, Hyperpolarization, and Relaxation of Human Airway Smooth Muscle Cells

Asthma is characterized by airway inflammation and airflow obstruction from human airway smooth muscle (HASM) constriction due to increased local bronchoconstrictive substances. We have recently found bitter taste receptors (TAS2Rs) on HASM, which increase [Ca2+]i and relax the muscle. We report here that some, but not all, TAS2R agonists decrease [Ca2+]i and relax HASM contracted by G-protein coupled receptors (GPCRs) that stimulate [Ca2+]i. This suggests both a second pathway by which TAS2Rs relax, and, a heterogeneity of the response phenotype. We utilized eight TAS2R agonists and five procontractile GPCR agonists in cultured HASM cells. We find that heterogeneity in the inhibitory response hinges on which procontractile GPCR is activated. For example, chloroquine inhibits [Ca2+]i increases from histamine, but failed to inhibit [Ca2+]i increases from endothelin-1. Conversely, aristolochic acid inhibited [Ca2+]i increases from endothelin-1 but not histamine. Other dichotomous responses were found when [Ca2+]i was stimulated by bradykinin, angiotensin, and acetylcholine. There was no association between [Ca2+]i inhibition and TAS2R subtype, nor whether [Ca2+]i was increased by Gq- or Gi-coupled GPCRs. Selected studies revealed a correlation between [Ca2+]i inhibition and HASM cell-membrane hyperpolarization. To demonstrate physiologic correlates, ferromagnetic beads were attached to HASM cells and cell stiffness measured by magnetic twisting cytometry. Consistent with the [Ca2+]i inhibition results, chloroquine abolished the cell stiffening response (contraction) evoked by histamine but not by endothelin-1, while aristolochic acid inhibited cell stiffening from endothelin-1, but not from histamine. In studies using intact human bronchi, these same differential responses were found. Those TAS2R agonists that decreased [Ca2+]i, promoted hyperpolarization, and decreased HASM stiffness, caused relaxation of human airways. Thus TAS2Rs relax HASM in two ways: a low-efficiency de novo [Ca2+]i stimulation, and, a high-efficiency inhibition of GPCR-stimulated [Ca2+]i. Furthermore, there is an interaction between TAS2Rs and some GPCRs that facilitates this [Ca2+]i inhibition limb.

Mu opioid receptor stimulation activates c-Jun N-terminal kinase 2 by distinct arrestin-dependent and independent mechanisms

G protein-coupled receptor desensitization is typically mediated by receptor phosphorylation by G protein-coupled receptor kinase (GRK) and subsequent arrestin binding; morphine, however, was previously found to activate a c-Jun N-terminal kinase (JNK)-dependent, GRK/arrestin-independent pathway to produce mu opioid receptor (MOR) inactivation in spinally-mediated, acute anti-nociceptive responses [Melief et al.] [1]. In the current study, we determined that JNK2 was also required for centrally-mediated analgesic tolerance to morphine using the hotplate assay. We compared JNK activation by morphine and fentanyl in JNK1(-/-), JNK2(-/-), JNK3(-/-), and GRK3(-/-) mice and found that both compounds specifically activate JNK2 in vivo; however, fentanyl activation of JNK2 was GRK3-dependent, whereas morphine activation of JNK2 was GRK3-independent. In MOR-GFP expressing HEK293 cells, treatment with either arrestin siRNA, the Src family kinase inhibitor PP2, or the protein kinase C (PKC) inhibitor Gö6976 indicated that morphine activated JNK2 through an arrestin-independent Src- and PKC-dependent mechanism, whereas fentanyl activated JNK2 through a Src-GRK3/arrestin-2-dependent and PKC-independent mechanism. This study resolves distinct ligand-directed mechanisms of JNK activation by mu opioid agonists and understanding ligand-directed signaling at MOR may improve opioid therapeutics.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.

Molecular mechanisms underlying β-adrenergic receptor-mediated cross-talk between sympathetic neurons and immune cells

Cross-talk between the sympathetic nervous system (SNS) and immune system is vital for health and well-being. Infection, tissue injury and inflammation raise firing rates of sympathetic nerves, increasing their release of norepinephrine (NE) in lymphoid organs and tissues. NE stimulation of β2-adrenergic receptors (ARs) in immune cells activates the cAMP-protein kinase A (PKA) intracellular signaling pathway, a pathway that interfaces with other signaling pathways that regulate proliferation, differentiation, maturation and effector functions in immune cells. Immune-SNS cross-talk is required to maintain homeostasis under normal conditions, to develop an immune response of appropriate magnitude after injury or immune challenge, and subsequently restore homeostasis. Typically, β2-AR-induced cAMP is immunosuppressive. However, many studies report actions of β2-AR stimulation in immune cells that are inconsistent with typical cAMP-PKA signal transduction. Research during the last decade in non-immune organs, has unveiled novel alternative signaling mechanisms induced by β2-AR activation, such as a signaling switch from cAMP-PKA to mitogen-activated protein kinase (MAPK) pathways. If alternative signaling occurs in immune cells, it may explain inconsistent findings of sympathetic regulation of immune function. Here, we review β2-AR signaling, assess the available evidence for alternative signaling in immune cells, and provide insight into the circumstances necessary for "signal switching" in immune cells.

Arrestin-3-Dependent Activation of c-Jun N-Terminal Kinases (JNKs)

Only one out of four mammalian arrestin subtypes, arrestin-3, facilitates the activation of JNK family kinases. Here we describe two different protocols used for elucidating the mechanisms involved. One is based on reconstitution of signaling modules from purified proteins: arrestin-3, MKK4, MKK7, JNK1, JNK2, and JNK3. The main advantage of this method is that it unambiguously establishes which effects are direct because only intended purified proteins are present in these assays. The key drawback is that the upstream-most kinases of these cascades, ASK1 or other MAPKKKs, are not available in purified form, limiting reconstitution to incomplete two-kinase modules. The other approach is used for analyzing the effects of arrestin-3 on JNK activation in intact cells. In this case, signaling modules include ASK1 and/or other MAPKKKs. However, as every cell expresses thousands of different proteins their possible effects on the readout cannot be excluded. Nonetheless, the combination of in vitro reconstitution from purified proteins and cell-based assays makes it possible to elucidate the mechanisms of arrestin-3-dependent activation of JNK family kinases.

Gamma-aminobutyric acid induces tumor cells apoptosis via GABABR1·β-arrestins·JNKs signaling module

Gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter in central nervous system, has yet been found to widely exist in tumor tissues to regulate tumor cells growth. However, the function of GABA on inducing tumor cells apoptosis and the potential mechanism are still unclear. In order to detect whether GABA via GABAB receptor GABABR1 would activate c-Jun N-terminal kinases (JNKs) to promote tumor cells apoptosis, co-immunoprecipitation assay was used to investigate the association of β-arrestins with GABABR1 and JNKs in the different four cancer cell lines. Our observation demonstrated that β-arrestins, in addition to their role in G protein-coupled receptors desensitization, had an additional function as adapter proteins to recruit JNKs to GABABR1, thereby conferring distinct enzymatic activities upon the receptor, which may trigger JNKs signal pathway involved in the regulation of cellular growth. Activated JNKs subsequently phosphorylated downstream c-Jun to transcribe a wide variety of pro-apoptotic genes. Additionally, GABA up-regulated the ratio of pro-apoptotic protein Bax to anti-apoptotic protein Bcl-2, and thus facilitated caspase-3 cleavage, leading to tumor cells apoptosis in a mitochondrial-dependent pathway. In contrast, GABABR antagonist CGP35348 reversed GABA-induced JNKs phosphorylation and its downstream proteins activation, which consequently restrained tumor cells apoptosis. Taken together, our study suggested that GABA via its receptor GABABR1 recruited β-arrestins to facilitate the activation of JNKs cascade, resulting in tumor cells growth inhibition.

Arrestins regulate cell spreading and motility via focal adhesion dynamics

Focal adhesions (FAs) play a key role in cell attachment, and their timely disassembly is required for cell motility. Both microtubule-dependent targeting and recruitment of clathrin are critical for FA disassembly. Here we identify nonvisual arrestins as molecular links between microtubules and clathrin. Cells lacking both nonvisual arrestins showed excessive spreading on fibronectin and poly-d-lysine, increased adhesion, and reduced motility. The absence of arrestins greatly increases the size and lifespan of FAs, indicating that arrestins are necessary for rapid FA turnover. In nocodazole washout assays, FAs in arrestin-deficient cells were unresponsive to disassociation or regrowth of microtubules, suggesting that arrestins are necessary for microtubule targeting-dependent FA disassembly. Clathrin exhibited decreased dynamics near FA in arrestin-deficient cells. In contrast to wild-type arrestins, mutants deficient in clathrin binding did not rescue the phenotype. Collectively the data indicate that arrestins are key regulators of FA disassembly linking microtubules and clathrin.

Peripheral scaffolding and signaling pathways in inflammatory pain

Peripheral injury precipitates the release and accumulation of extracellular molecules at the site of injury. Although these molecules exist in various forms, they activate specific receptor classes expressed on primary afferent neurons to mediate cellular and behavioral responses to both nonpainful and painful stimuli. These inflammatory mediators and subsequent receptor-mediated effects exist to warn an organism of future injury, thereby resulting in protection and rehabilitation of the wounded tissue. In this chapter, inflammatory mediators, their target receptor classes, and downstream signaling pathways are identified and discussed within the context of inflammatory hyperalgesia. Furthermore, scaffolding mechanisms that exist to support inflammatory signaling in peripheral afferent neuronal tissues specifically are identified and discussed. Together, the mediators, pathways, and scaffolding mechanisms involved in inflammatory hyperalgesia provide a unique knowledge point from which new therapeutic targets can be understood.

Development of an HTS-compatible assay for the discovery of ASK1 signalosome inhibitors using alphascreen technology

Genetic target validation studies have demonstrated that the apoptosis signal-regulating kinase 1 (ASK1) represents an important target for the treatment of rheumatoid arthritis, cardiac diseases, and several neurodegenerative disorders. To identify small-molecule inhibitors of ASK1, we have developed a high-throughput screening-compatible, homogenous, biochemical assay using AlphaScreen technology. This novel assay design utilizes purified stress-activated ASK1 signalosome complex, and it monitors phosphorylation of its full-length native substrate, MKK6. The assay has been optimized in a 384-well format and validated by screening the Sigma LOPAC library. The results presented here demonstrate that the assay is sensitive and robust with a Z' factor value of 0.88±0.04 and a signal-to-background ratio of 11, indicating that this assay can be used to screen large chemical libraries to discover novel inhibitors of ASK1.