Beta-arrestin-mediated signaling regulates protein synthesis

mRNA-selective translation induced by FSH in primary Sertoli cells

FSH is a key hormonal regulator of Sertoli cell secretory activity, required to optimize sperm production. To fulfil its biological function, FSH binds a G protein-coupled receptor, the FSH-R. The FSH-R-transduced signaling network ultimately leads to the transcription or down-regulation of numerous genes. In addition, recent evidence has suggested that FSH might also regulate protein translation. However, this point has never been demonstrated conclusively yet. Here we have addressed this issue in primary rat Sertoli cells endogenously expressing physiological levels of FSH-R. We observed that, within 90 min of stimulation, FSH not only enhanced overall protein synthesis in a mammalian target of rapamycin-dependent manner but also increased the recruitment of mRNA to polysomes. m(7)GTP pull-down experiments revealed the functional recruitment of mammalian target of rapamycin and p70 S6 kinase to the 5'cap, further supported by the enhanced phosphorylation of one of p70 S6 kinase targets, the eukaryotic initiation factor 4B. Importantly, the scaffolding eukaryotic initiation factor 4G was also recruited, whereas eukaryotic initiation factor 4E-binding protein, the eukaryotic initiation factor 4E generic inhibitor, appeared to play a minor role in translational regulations induced by FSH, in contrast to what is generally observed in response to anabolic factors. This particular regulation of the translational machinery by FSH stimulation might support mRNA-selective translation, as shown here by quantitative RT-PCR amplification of the c-fos and vascular endothelial growth factor mRNA but not of all FSH target mRNA, in polysomal fractions. These findings add a new level of complexity to FSH biological roles in its natural target cells, which has been underappreciated so far.

Functional relevance of biased signaling at the angiotensin II type 1 receptor

Angiotensin II type 1 receptor antagonists (AT1R blockers, or ARBs) are used commonly in the treatment of cardiovascular disorders such as heart failure and hypertension. Their clinical success arises from their ability to prevent deleterious Gα(q) protein activation downstream of AT1R, which leads to a decrease in morbidity and mortality. Recent studies have identified AT1R ligands that concurrently inhibit Gα(q) protein-dependent signaling and activate Gα(q) protein-independent/β-arrestin-dependent signaling downstream of AT1R, events that may actually improve cardiovascular performance more than conventional ARBs. The ability of such ligands to induce intracellular signaling events in an AT1R-β-arrestin-dependent manner while preventing AT1R-Gα(q) protein activity defines them as biased AT1R ligands. This mini-review will highlight recent studies that have defined biased signaling at the AT1R and discuss the possible clinical relevance of β-arrestin-biased AT1R ligands in the cardiovascular system.

G protein-dependent and G protein-independent signaling pathways and their impact on cardiac function

G protein-coupled receptors signal through a variety of mechanisms that impact cardiac function, including contractility and hypertrophy. G protein-dependent and G protein-independent pathways each have the capacity to initiate numerous intracellular signaling cascades to mediate these effects. G protein-dependent signaling has been studied for decades and great strides continue to be made in defining the intricate pathways and effectors regulated by G proteins and their impact on cardiac function. G protein-independent signaling is a relatively newer concept that is being explored more frequently in the cardiovascular system. Recent studies have begun to reveal how cardiac function may be regulated via G protein-independent signaling, especially with respect to the ever-expanding cohort of β-arrestin-mediated processes. This review primarily focuses on the impact of both G protein-dependent and β-arrestin-dependent signaling pathways on cardiac function, highlighting the most recent data that illustrate the comprehensive nature of these mechanisms of G protein-coupled receptor signaling.

Molecular mechanism of β-arrestin-biased agonism at seven-transmembrane receptors

The concept of biased agonism has recently come to the fore with the realization that seven-transmembrane receptors (7TMRs, also known as G protein-coupled receptors, or GPCRs) activate complex signaling networks and can adopt multiple active conformations upon agonist binding. As a consequence, the "efficacy" of receptors, which was classically considered linear, is now recognized as pluridimensional. Biased agonists selectively stabilize only a subset of receptor conformations induced by the natural "unbiased" ligand, thus preferentially activating certain signaling mechanisms. Such agonists thus reveal the intriguing possibility that one can direct cellular signaling with unprecedented precision and specificity and support the notion that biased agonists may identify new classes of therapeutic agents that have fewer side effects. This review focuses on one particular class of biased ligands that has the ability to alter the balance between G protein-dependent and β-arrestin-dependent signal transduction.

Biased Ligands for Better Cardiovascular Drugs

Drug discovery efforts targeting G-protein-coupled receptors (GPCR) have been immensely successful in creating new cardiovascular medicines. Currently marketed GPCR drugs are broadly classified as either agonists that activate receptors or antagonists that prevent receptor activation by endogenous stimuli. However, GPCR couple to a multitude of intracellular signaling pathways beyond classical G-protein signals, and these signals can be independently activated by biased ligands to vastly expand the potential for new drugs at these classic targets. By selectively engaging only a subset of a receptor's potential intracellular partners, biased ligands may deliver more precise therapeutic benefit with fewer side effects than current GPCR-targeted drugs. In this review, we discuss the history of biased ligand research, the current understanding of how biased ligands exert their unique pharmacology, and how research into GPCR signaling has uncovered previously unappreciated capabilities of receptor pharmacology. We focus on several receptors to illustrate the approaches taken and discoveries made, and how these are steadily illuminating the intricacies of GPCR pharmacology. Discoveries of biased ligands targeting the angiotensin II type 1 receptor and of separable pharmacology suggesting the potential value of biased ligands targeting the β-adrenergic receptors and nicotinic acid receptor GPR109a highlight the powerful clinical promise of this new category of potential therapeutics.

β-Arrestin-mediated receptor trafficking and signal transduction

β-Arrestins function as endocytic adaptors and mediate trafficking of a variety of cell-surface receptors, including seven-transmembrane receptors (7TMRs). In the case of 7TMRs, β-arrestins carry out these tasks while simultaneously inhibiting upstream G-protein-dependent signaling and promoting alternate downstream signaling pathways. The mechanisms by which β-arrestins interact with a continuously expanding ensemble of protein partners and perform their multiple functions including trafficking and signaling are currently being uncovered. Molecular changes at the level of protein conformation as well as post-translational modifications of β-arrestins probably form the basis for their dynamic interactions during receptor trafficking and signaling. It is becoming increasingly evident that β-arrestins, originally discovered as 7TMR adaptor proteins, indeed have much broader and more versatile roles in maintaining cellular homeostasis. In this review paper, we assess the traditional and novel functions of β-arrestins and discuss the molecular attributes that might facilitate multiple interactions in regulating cell signaling and receptor trafficking.

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

The angiotensin II peptide analog [Sar(1),Ile(4),Ile(8)]AngII (SII) is a biased AT(1A) receptor agonist that stimulates receptor phosphorylation, β-arrestin recruitment, receptor internalization, and β-arrestin-dependent ERK1/2 activation without activating heterotrimeric G-proteins. To determine the scope of G-protein-independent AT(1A) receptor signaling, we performed a gel-based phosphoproteomic analysis of AngII and SII-induced signaling in HEK cells stably expressing AT(1A) receptors. A total of 34 differentially phosphorylated proteins were detected, of which 16 were unique to SII and eight to AngII stimulation. MALDI-TOF/TOF mass fingerprinting was employed to identify 24 SII-sensitive phosphoprotein spots, of which three (two peptide inhibitors of protein phosphatase 2A (I1PP2A and I2PP2A) and prostaglandin E synthase 3 (PGES3)) were selected for validation and further study. We found that phosphorylation of I2PP2A was associated with rapid and transient inhibition of a β-arrestin 2-associated pool of protein phosphatase 2A, leading to activation of Akt and increased phosphorylation of glycogen synthase kinase 3β in an arrestin signalsome complex. SII-stimulated PGES3 phosphorylation coincided with an increase in β-arrestin 1-associated PGES3 and an arrestin-dependent increase in cyclooxygenase 1-dependent prostaglandin E(2) synthesis. These findings suggest that AT(1A) receptors regulate a robust G protein-independent signaling network that affects protein phosphorylation and autocrine/paracrine prostaglandin production and that these pathways can be selectively modulated by biased ligands that antagonize G protein activation.

Refining efficacy: exploiting functional selectivity for drug discovery

Early models of G protein-coupled receptor (GPCR) activation envisioned the receptor in equilibrium between unique "off" and "on" states, wherein ligand binding affected signaling by increasing or decreasing the fraction of receptors in the active conformation. It is now apparent that GPCRs spontaneously sample multiple conformations, any number of which may couple to one or more downstream effectors. Such "multistate" models imply that the receptor-ligand complex, not the receptor alone, defines which active receptor conformations predominate. "Functional selectivity" refers to the ability of a ligand to activate only a subset of its receptor's signaling repertoire. There are now numerous examples of ligands that "bias" receptor coupling between different G protein pools and non-G protein effectors such as arrestins. The type 1 parathyroid hormone receptor (PTH(1)R) is a particularly informative example, not only because of the range of biased effects that have been produced, but also because the actions of many of these ligands have been characterized in vivo. Biased PTH(1)R ligands can selectively couple the PTH(1)R to G(s) or G(q/11) pathways, with or without activating arrestin-dependent receptor desensitization and signaling. These reagents have provided insight into the contribution of different signaling pathways to PTH action in vivo and suggest it may be possible to exploit ligand bias to uncouple the anabolic effects of PTH(1)R from its catabolic and calcitropic effects. Whereas conventional agonists and antagonists only modulate the quantity of efficacy, functionally selective ligands qualitatively change GPCR signaling, offering the prospect of drugs with improved therapeutic efficacy or reduced side effects.

AT(1) receptor Gαq protein-independent signalling transcriptionally activates only a few genes directly, but robustly potentiates gene regulation from the β2-adrenergic receptor

The angiotensin II type 1 receptor (AT(1)R) is known to signal through heterotrimeric G proteins, and Gαq protein-independent signalling has only recently gained appreciation for profound impact on a diverse range of biological functions. β-Arrestins, among other central mediators of Gαq protein-independent signalling from the AT(1)R interact with transcriptional regulators and promote phosphorylation of nuclear proteins. However, the relative contribution of Gαq protein-independent signalling in AT(1)R mediated transcriptional regulation remains elusive. We here present a comprehensive comparative analysis of Gαq protein-dependent and -independent regulation of AT(1)R mediated gene expression. We found angiotensin II to regulate 212 genes, whereas Gαq-independent signalling obtained with the biased agonist, SII angiotensin II only regulated few genes. Interestingly, SII angiotensin II, like Ang II vastly potentiated β2-adrenergic receptor-stimulated gene expression. These novel findings indicate that the Gαq protein-independent signalling mainly modifies the transcriptional response governed by other signalling pathways, while direct induction of gene expression by the AT(1)R is dependent on classical Gαq protein activation.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Selectively engaging β-arrestins at the angiotensin II type 1 receptor reduces blood pressure and increases cardiac performance

Biased G protein-coupled receptor ligands engage subsets of the receptor signals normally stimulated by unbiased agonists. However, it is unclear whether ligand bias can elicit differentiated pharmacology in vivo. Here, we describe the discovery of a potent, selective β-arrestin biased ligand of the angiotensin II type 1 receptor. TRV120027 (Sar-Arg-Val-Tyr-Ile-His-Pro-D-Ala-OH) competitively antagonizes angiotensin II-stimulated G protein signaling, but stimulates β-arrestin recruitment and activates several kinase pathways, including p42/44 mitogen-activated protein kinase, Src, and endothelial nitric-oxide synthase phosphorylation via β-arrestin coupling. Consistent with β-arrestin efficacy, and unlike unbiased antagonists, TRV120027 increased cardiomyocyte contractility in vitro. In rats, TRV120027 reduced mean arterial pressure, as did the unbiased antagonists losartan and telmisartan. However, unlike the unbiased antagonists, which decreased cardiac performance, TRV120027 increased cardiac performance and preserved cardiac stroke volume. These striking differences in vivo between unbiased and β-arrestin biased ligands validate the use of biased ligands to selectively target specific receptor functions in drug discovery.

Differential effects of allosteric M(1) muscarinic acetylcholine receptor agonists on receptor activation, arrestin 3 recruitment, and receptor downregulation

Muscarinic acetylcholine receptors (mAChRs) are drug targets for multiple neurodegenerative and neuropsychiatric disorders, but the full therapeutic potential of mAChR-targeted drugs has not been realized, mainly because of a lack of subtype-selective agonists. Recent advances have allowed the development of highly selective agonists that bind to an allosteric site on the M(1) mAChR that is spatially distinct from the orthosteric acetylcholine binding site, but less is known about the profile of intracellular signals activated by orthosteric versus allosteric M(1) mAChR agonists. We investigated the activation and regulatory mechanisms of two structurally distinct allosteric M(1) mAChR agonists, AC260584 and TBPB. We show that allosteric agonists potently activate multiple signal transduction pathways linked to the M(1) mAChR receptor but, compared to orthosteric agonists, much less efficiently recruit arrestin 3, a protein involved in regulation of G-protein coupled receptor signaling. Consistent with decreased arrestin recruitment, both allosteric agonists showed blunted responses in measurements of receptor desensitization, internalization, and downregulation. These results advance the understanding of mAChR biology and may shed light on unanticipated differences in the pharmacology of orthosteric vs. allosteric agonists that might be capitalized upon for drug development for the treatment of CNS diseases.

Teaching old receptors new tricks: biasing seven-transmembrane receptors

Seven-transmembrane receptors (7TMRs; also known as G protein-coupled receptors) are the largest class of receptors in the human genome and are common targets for therapeutics. Originally identified as mediators of 7TMR desensitization, beta-arrestins (arrestin 2 and arrestin 3) are now recognized as true adaptor proteins that transduce signals to multiple effector pathways. Signalling that is mediated by beta-arrestins has distinct biochemical and functional consequences from those mediated by G proteins, and several biased ligands and receptors have been identified that preferentially signal through either G protein- or beta-arrestin-mediated pathways. These ligands are not only useful tools for investigating the biochemistry of 7TMR signalling, they also have the potential to be developed into new classes of therapeutics.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.

Calcium-sensing receptor biosynthesis includes a cotranslational conformational checkpoint and endoplasmic reticulum retention

Metabolic labeling with [(35)S]cysteine was used to characterize early events in CaSR biosynthesis. [(35)S]CaSR is relatively stable (half-life approximately 8 h), but maturation to the final glycosylated form is slow and incomplete. Incorporation of [(35)S]cysteine is linear over 60 min, and the rate of [(35)S]CaSR biosynthesis is significantly increased by the membrane-permeant allosteric agonist NPS R-568, which acts as a cotranslational pharmacochaperone. The [(35)S]CaSR biosynthetic rate also varies as a function of conformational bias induced by loss- or gain-of-function mutations. In contrast, [(35)S]CaSR maturation to the plasma membrane was not significantly altered by exposure to the pharmacochaperone NPS R-568, the allosteric agonist neomycin, or the orthosteric agonist Ca(2+) (0.5 or 5 mm), suggesting that CaSR does not control its own release from the endoplasmic reticulum. A CaSR chimera containing the mGluR1alpha carboxyl terminus matures completely (half-time of approximately 8 h) and without a lag period, as does the truncation mutant CaSRDelta868 (half-time of approximately 16 h). CaSRDelta898 exhibits maturation comparable with full-length CaSR, suggesting that the CaSR carboxyl terminus between residues Thr(868) and Arg(898) limits maturation. Overall, these results suggest that CaSR is subject to cotranslational quality control, which includes a pharmacochaperone-sensitive conformational checkpoint. The CaSR carboxyl terminus is the chief determinant of intracellular retention of a significant fraction of total CaSR. Intracellular CaSR may reflect a rapidly mobilizable "storage form" of CaSR and/or may subserve distinct intracellular signaling roles that are sensitive to signaling-dependent changes in endoplasmic reticulum Ca(2+) and/or glutathione.

Quantitative phosphoproteomics dissection of seven-transmembrane receptor signaling using full and biased agonists

Seven-transmembrane receptors (7TMRs) signal through the well described heterotrimeric G proteins but can also activate G protein-independent signaling pathways of which the impact and complexity are less understood. The angiotensin II type 1 receptor (AT(1)R) is a prototypical 7TMR and an important drug target in cardiovascular diseases. "Biased agonists" with intrinsic "functional selectivity" that simultaneously blocks Galpha(q) protein activity and activates G protein-independent pathways of the AT(1)R confer important perspectives in treatment of cardiovascular diseases. In this study, we performed a global quantitative phosphoproteomics analysis of the AT(1)R signaling network. We analyzed ligand-stimulated SILAC (stable isotope labeling by amino acids in cell culture) cells by high resolution (LTQ-Orbitrap) MS and compared the phosphoproteomes of the AT(1)R agonist angiotensin II and the biased agonist [Sar(1),Ile(4),Ile(8)]angiotensin II (SII angiotensin II), which only activates the Galpha(q) protein-independent signaling. We quantified more than 10,000 phosphorylation sites of which 1183 were regulated by angiotensin II or its analogue SII angiotensin II. 36% of the AT(1)R-regulated phosphorylations were regulated by SII angiotensin II. Analysis of phosphorylation site patterns showed a striking distinction between protein kinases activated by Galpha(q) protein-dependent and -independent mechanisms, and we now place protein kinase D as a key protein involved in both Galpha(q)-dependent and -independent AT(1)R signaling. This study provides substantial novel insight into angiotensin II signal transduction and is the first study dissecting the differences between a full agonist and a biased agonist from a 7TMR on a systems-wide scale. Importantly, it reveals a previously unappreciated diversity and quantity of Galpha(q) protein-independent signaling and uncovers novel signaling pathways. We foresee that the amount and diversity of G protein-independent signaling may be more pronounced than previously recognized for other 7TMRs as well. Quantitative mass spectrometry is a promising tool for evaluation of the signaling properties of biased agonists to other receptors in the future.

Partially deglycosylated equine LH preferentially activates beta-arrestin-dependent signaling at the follicle-stimulating hormone receptor

Deglycosylated FSH is known to trigger poor Galphas coupling while efficiently binding its receptor. In the present study, we tested the possibility that a deglycosylated equine LH (eLHdg) might be able to selectively activate beta-arrestin-dependent signaling. We compared native eLH to an eLH derivative [i.e. truncated eLHbeta (Delta121-149) combined with asparagine56-deglycosylated eLHalpha (eLHdg)] previously reported as an antagonist of cAMP accumulation at the FSH receptor (FSH-R). We confirmed that, when used in conjunction with FSH, eLHdg acted as an antagonist for cAMP accumulation in HEK-293 cells stably expressing the FSH-R. Furthermore, when used alone at concentrations up to 1 nM, eLHdg had no detectable agonistic activity on cAMP accumulation, protein kinase A activity or cAMP-responsive element-dependent transcriptional activity. At higher concentrations, however, a weak agonistic action was observed with eLHdg, whereas eLH led to robust responses whatever the concentration. Both eLH and eLHdg triggered receptor internalization and led to beta-arrestin recruitment. Both eLH and eLHdg triggered ERK and ribosomal protein (rp) S6 phosphorylation at 1 nM. The depletion of endogenous beta-arrestins had only a partial effect on eLH-induced ERK and rpS6 phosphorylation. In contrast, ERK and rpS6 phosphorylation was completely abolished at all time points in beta-arrestin-depleted cells. Together, these results show that eLHdg has the ability to preferentially activate beta-arrestin-dependent signaling at the FSH-R. This finding provides a new conceptual and experimental framework to revisit the physiological meaning of gonadotropin structural heterogeneity. Importantly, it also opens a field of possibilities for the development of selective modulators of gonadotropin receptors.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Developmental regulation of p70 S6 kinase by a G protein-coupled receptor dynamically modelized in primary cells

The mechanisms whereby G protein-coupled receptors (GPCR) activate signalling pathways involved in mRNA translation are ill-defined, in contrast to tyrosine kinase receptors (TKR). We compared a GPCR and a TKR, both endogenously expressed, for their ability to mediate phosphorylation of 70-kDa ribosomal S6 kinase p70S6K in primary rat Sertoli cells at two developmental stages. In proliferating cells stimulated with follicle-stimulating hormone (FSH), active p70S6K was phosphorylated on T389 and T421/S424, through cAMP-dependent kinase (PKA) and phosphatidyl-inositide-3 kinase (PI3K) antagonizing actions. In FSH-stimulated differentiating cells, active p70S6K was phosphorylated solely on T389, PKA and PI3K independently enhancing its activity. At both developmental stages, insulin-induced p70S6K regulation was consistent with reported data. Therefore, TKR and GPCR trigger distinct p70S6K active conformations. p70S6K developmental regulation was formalized in a dynamic mathematical model fitting the data, which led to experimentally inaccessible predictions on p70S6K phosphorylation rate.

GPCR signalling to the translation machinery

G protein-coupled receptors (GPCRs) are involved in most physiological processes, many of them being engaged in fully differentiated cells. These receptors couple to transducers of their own, primarily G proteins and beta-arrestins, which launch intracellular signalling cascades. Some of these signalling events regulate the translational machinery to fine-tune general cell metabolism or to alter protein expression pattern. Though extensively documented for tyrosine kinase receptors, translational regulation by GPCRs is still poorly appreciated. The objective of this review paper is to address the following questions: i) is there a "GPCR signature" impacting on the translational machinery, and ultimately on the type of mRNA translated? ii) are the regulatory networks involved similar as those utilized by tyrosine kinase receptors? In particular, we will discuss the specific features of translational control mediated by GPCRs and highlight the intrinsic properties of GPCRs these mechanisms could rely on.

Endosomes: a legitimate platform for the signaling train

Although long regarded as a conduit for the degradation or recycling of cell surface receptors, the endosomal system is also an essential site of signal transduction. Activated receptors accumulate in endosomes, and certain signaling components are exclusively localized to endosomes. Receptors can continue to transmit signals from endosomes that are different from those that arise from the plasma membrane, resulting in distinct physiological responses. Endosomal signaling is widespread in metazoans and plants, where it transmits signals for diverse receptor families that regulate essential processes including growth, differentiation and survival. Receptor signaling at endosomal membranes is tightly regulated by mechanisms that control agonist availability, receptor coupling to signaling machinery, and the subcellular localization of signaling components. Drugs that target mechanisms that initiate and terminate receptor signaling at the plasma membrane are widespread and effective treatments for disease. Selective disruption of receptor signaling in endosomes, which can be accomplished by targeting endosomal-specific signaling pathways or by selective delivery of drugs to the endosomal network, may provide novel therapies for disease.

Effects of inactivation-resistant agonists on the signalling, desensitization and down-regulation of bradykinin B(2) receptors

BACKGROUND AND PURPOSE

A peptide bradykinin (BK) B(2) receptor agonist partially resistant to degradation, B-9972, down-regulates this receptor subtype. We have used another recently described non-peptide agonist, compound 47a, as a tool to study further the effects of metabolically more stable and thus persistent, agonists of the BK B(2) receptor on signalling, desensitization and down-regulation of this receptor.

EXPERIMENTAL APPROACH AND KEY RESULTS

Compound 47a was a partial agonist at the B(2) receptor in the human umbilical vein, where it shared with B-9972 a very slow relaxation on washout, and in HEK 293 cell lines expressing tagged forms [myc, green fluorescent protein (GFP)] of the rabbit B(2) receptor. Compound 47a desensitized the umbilical vein to BK. In the cellular systems, the inactivation-resistant agonists induced [Ca(2+)](i) transients as brief as those of BK but affected other functions with a longer duration than BK [12 h; receptor endocytosis, endosomal beta-arrestin(1/2) translocation, protein kinase C-dependent extracellular signal-regulated kinases (ERK)1/2 phosphorylation and c-Fos expression]. The B(2) receptor-GFP was degraded in cells exposed to B-9972 or compound 47a for 12 h. The non-peptide B(2) receptor antagonist LF 16-0687 prevented all effects of compound 47a, which were also absent in cells lacking recombinant B(2) receptors.

CONCLUSION AND IMPLICATIONS

Inactivation-resistant agonists revealed a long-lasting assembly of the agonist-B(2) receptor-beta-arrestin complexes in endosomal structures and induce 'biased signalling' (in terms of activation of ERK and c-Fos) as a function of time. Further, B-9972 and compound 47a, unlike BK, efficiently down-regulated BK B(2) receptors.

Diversity in arrestin function

The termination of heptahelical receptor signaling is a multilevel process coordinated, in large part, by members of the arrestin family of proteins. Arrestin binding to agonist-occupied receptors promotes desensitization by interrupting receptor-G protein coupling, while simultaneously recruiting machinery for receptor endocytosis, vesicular trafficking, and receptor fate determination. By simultaneously binding other proteins, arrestins also act as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein 'signalsome' complexes. Arrestin-binding thus 'switches' receptors from a transient G protein-coupled state to a persistent arrestin-coupled state that continues to signal as the receptor transits intracellular compartments. While it is clear that signalsome assembly has profound effects on the duration and spatial characteristics of heptahelical receptor signals, the physiologic functions of this novel signaling mechanism are poorly understood. Growing evidence suggests that signalsomes regulate such diverse processes as endocytosis and exocytosis, cell migration, survival, and contractility.

Independent beta-arrestin2 and Gq/protein kinase Czeta pathways for ERK stimulated by angiotensin type 1A receptors in vascular smooth muscle cells converge on transactivation of the epidermal growth factor receptor

Recent studies in receptor-transfected cell lines have demonstrated that extracellular signal-regulated kinase (ERK) activation by angiotensin type 1A receptor and other G protein-coupled receptors can be mediated by both G protein-dependent and β-arrestin-dependent mechanisms. However, few studies have explored these mechanisms in primary cultured cells expressing endogenous levels of receptors. Accordingly, here we utilized the β-arrestin biased agonist for the angiotensin type 1A receptor, SII-angiotensin (SII), and RNA interference techniques to investigate angiotensin II (ANG)-activated β-arrestin-mediated mitogenic signaling pathways in rat vascular smooth muscle cells. Both ANG and SII induced DNA synthesis via the ERK activation cascade. Even though SII cannot induce calcium influx (G protein activation) after receptor stimulation, it does cause ERK activation, although less robustly than ANG. Activation by both ligands is diminished by depletion of β-arrestin2 by small interfering RNA, although the effect is more complete with SII. ERK activation at early time points but not later time points is strongly inhibited by those protein kinase C inhibitors that can block protein kinase Cζ. Moreover, ANG- and SII-mediated ERK activation require transactivation of the epidermal growth factor receptor via metalloprotease 2/9 and Src kinase. β-Arrestin2 facilitates ANG and SII stimulation of Src-mediated phosphorylation of Tyr-845 on the EGFR, a known site for Src phosphorylation. These studies delineate a convergent mechanism by which G protein-dependent and β-arrestin-dependent pathways can independently mediate ERK-dependent transactivation of the EGFR in vascular smooth muscle cells thus controlling cellular proliferative responses.

{beta}-Arrestin-2 Mediates Anti-apoptotic Signaling through Regulation of BAD Phosphorylation

β-Arrestins, originally discovered as terminators of G protein-coupled receptor signaling, have more recently been appreciated to also function as signal transducers in their own right, although the consequences for cellular physiology have not been well understood. Here we demonstrate that β-arrestin-2 mediates anti-apoptotic cytoprotective signaling stimulated by a typical 7-transmembrane receptor the angiotensin ATII 1A receptor, expressed endogenously in rat vascular smooth muscle cells or by transfection in HEK-293 cells. Receptor stimulation leads to concerted activation of two pathways, ERK/p90RSK and PI3K/AKT, which converge to phosphorylate and inactivate the pro-apoptotic protein BAD. Anti-apoptotic effects as well as pathway activities can be stimulated by an angiotensin analog (SII), which has been previously shown to activate β-arrestin but not G protein-dependent signaling, and are abrogated by β-arrestin-2 small interfering RNA. These findings establish a key role for β-arrestin-2 in mediating cellular cytoprotective functions by a 7-transmembrane receptor and define the biochemical pathways involved.