Beta-arrestins 1 and 2 differentially regulate LPS-induced signaling and pro-inflammatory gene expression

β-arrestin-2 regulation of the cAMP response element binding protein

Previous work demonstrated that cystic fibrosis (CF) cells exhibit an increase in cAMP-mediated signaling as a characteristic response to lost CFTR function. Evidence for increased cAMP-mediated signaling in CF included increased phosphorylation of the cAMP response element binding protein (CREB) and elevated β-arrestin-2 (βarr2) expression. However, subsequent studies reveal that CREB activation in CF cells is independent of protein kinase-A (PKA). The goal of this study is to test the hypothesis that elevated βarr2 expression leads to increased CREB activation in a PKA-independent mechanism. βarr2-GFP expressing tracheal epithelial cells (βarr2-GFP) exhibit an increase of pCREB content and subsequent CRE activation compared to GFP expressing control cells. βarr2 activation of the ERK cascade represents a candidate mechanism leading to CREB activation. ERK exhibits increased activation in βarr2-GFP cells compared to cont-GFP cells, and ERK inhibition diminishes CRE activation in both GFP and βarr2-GFP cells. To test directly whether CREB regulation in CF is βarr2-dependent, nasal epithelium excised from wt mice (Cftr +/+; βarr2 +/+), CF mice (Cftr -/-; βarr2 +/+), and DKO mice (Cftr -/-; βarr2 -/-) were analyzed for pCREB protein content. Removal of βarr2 expression from CF mice reduces both pCREB and pERK content to wt levels. These data indicate that CF-related CREB regulation is mediated directly through βarr2 expression via the ERK pathway.

β-Arrestin 1 modulates functions of autoimmune T cells from primary biliary cirrhosis patients

OBJECTIVES

Primary biliary cirrhosis (PBC) is an autoimmune disease, characterized by antimitochondrial antibodies and autoreactive T cells causing destruction of the primary bile ducts. The molecular mechanisms regulating the autoreactive T cells remain elusive. β-Arrestins (βarr) are multifunctional signaling molecules that are crucial to T cell survival. We hypothesized that βarr plays a critical regulatory function in the autoreactive T cells of PBC patients.

METHODS

Patients with hepatic biliary cirrhosis (n=60) were evaluated. Cytokine expression, T cell proliferation, and transcription factors were evaluated to assess regulatory functions in autoreactive T cells from the patient.

RESULTS

Our studies showed that expression of βarr1 was elevated significantly in T lymphocytes from patients with PBC. Moreover, the level of βarr1 mRNA positively correlated with Mayo risk score in PBC patients. Based on modulation of βarr in autoreactive T cell lines, overexpression of βarr1 increased T cell proliferation, augmented interferon production, downregulated activities of nuclear factor κB and AP-1, promoted acetylation of histone H4 in the promoter regions of CD40L, LIGHT, IL-17 and interferon-γ, while downregulating acetylation of histone H4 in the promoter regions of TRAIL, Apo2, and HDAC7A, thereby regulating expression of these genes.

CONCLUSIONS

Our findings suggest that βarr1 contributes to the pathogenesis of PBC, having significant implications for novel therapy strategy, further providing information for investigating the mechanisms of autoimmune disease.

Effects of perfluorooctane sulfonate (PFOS) exposure on markers of inflammation in female B6C3F1 mice

Perfluorooctane sulfonate (PFOS; 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluoro-1-octanesulfonic acid) has been reported to alter humoral immune functions, but inflammatory processes following PFOS exposure have not been fully characterized. Therefore, the current study, assessed TNF-α and IL-6 concentrations in serum and peritoneal lavage fluid, numbers of splenoctyes expressing intracellular TNF-α, IL-6, IL-10 or IL-1, and ex vivo TNF-α and IL-6 production by peritoneal macrophages following either in vivo or in vitro LPS exposure. Adult female B6C3F1 mice were exposed orally for 28 days to 0, 1, 3, or 300 mg PFOS/kg total administered dose [TAD] (e.g., 0, 0.0331, 0.0993 or 9.93 mg/kg/day). Body and spleen masses were significantly reduced in the highest PFOS treatment group compared to the control group, whereas liver mass was significantly increased. Serum TNF-α levels were significantly decreased following exposure to 1 mg PFOS/kg TAD as compared to controls, while serum IL-6 levels were increased. IL-6 concentrations in peritoneal lavage fluid decreased with increasing dose. PFOS treatment did not alter numbers of splenocytes expressing intracellular levels of TNF-α, IL-10 or IL-1. Numbers of splenocytes expressing intracellular levels of IL-6 were significantly decreased in the 3 mg/kg treatment as compared to controls. Overall, these data suggest that PFOS exposure can alter some inflammatory processes, which could potentially lead to misdirected inflammatory responses.

β-Arrestins: multifunctional signaling adaptors in type 2 diabetes

β-arrestins are not only well-known negative regulators of G protein-coupled receptor (GPCR) signaling, but also important adaptors in modulating the strength and duration of cellular signaling by scaffolding and interacting with a lot of cytoplasmic proteins. While β-arrestins are rather well described signal-mediated molecules, they are not generally associated with insulin signaling. But recent work has confirmed the difference from original thought. The current review aims to explore the emerging roles for β-arrestins in regulating insulin action, inflammatory signal pathway and other cellular signaling which are associated with type 2 diabetes.

Role of β-arrestins in the pathogenesis of inflammatory bowel disease

β-arrestins, as adaptor proteins involved in G protein-coupled receptor (GPCR)-related signaling, have diverse biological functions and can regulate cell proliferation, survival, apoptosis, motility and gene transcription. β-arrestins regulate several aspects of inflammatory and immune reactions. First, they limit the basal activity of pro-inflammatory transcription factor NF-κB and regulate activation of NF-κB via the Toll-like receptors (TLR)/NF-κB signal pathway. Second, they facilitate T cell activation, suppress the apoptosis of CD4+ T cells, inhibit NK cell-mediated cytotoxicity, and constrain factor-independent survival of macrophages. Finally, β-arrestins influence chemotaxis of immune cells and neutrophil degranulation by regulating desensitization, internalization and signal transduction of various chemokine receptors. The pathogenesis of inflammatory bowel disease (IBD) may be attributed to various genetic abnormalities that result in excessive immune response against the normal intestinal microbe flora. Abnormal immune response is considered to play a pivotal role in the development of IBD. The role of β-arrestins in regulating immune response involved in intestinal mucosal inflammation in IBD implies that they may participate in the pathogenesis of IBD.

Regulation of lipopolysaccharide-induced inflammatory response and endotoxemia by beta-arrestins

Beta-arrestins are scaffolding proteins implicated as negative regulators of TLR4 signaling in macrophages and fibroblasts. Unexpectedly, we found that beta-arrestin-1 (beta-arr-1) and -2 knockout (KO) mice are protected from TLR4-mediated endotoxic shock and lethality. To identify the potential mechanisms involved, we examined the plasma levels of inflammatory cytokines/chemokines in the wild-type (WT) and beta-arr-1 and -2 KO mice after lipopolysaccharide (LPS, a TLR4 ligand) injection. Consistent with lethality, LPS-induced inflammatory cytokine levels in the plasma were markedly decreased in both beta-arr-1 and -2 KO, compared to WT mice. To further explore the cellular mechanisms, we obtained splenocytes (separated into CD11(b+) and CD11(b-) populations) from WT, beta-arr-1, and -2 KO mice and examined the effect of LPS on cytokine production. Similar to the in vivo observations, LPS-induced inflammatory cytokines were significantly blocked in both splenocyte populations from the beta-arr-2 KO compared to the WT mice. This effect in the beta-arr-1 KO mice, however, was restricted to the CD11(b-) splenocytes. Our studies further indicate that regulation of cytokine production by beta-arrestins is likely independent of MAPK and IkappaBalpha-NFkappaB pathways. Our results, however, suggest that LPS-induced chromatin modification is dependent on beta-arrestin levels and may be the underlying mechanistic basis for regulation of cytokine levels by beta-arrestins in vivo. Taken together, these results indicate that beta-arr-1 and -2 mediate LPS-induced cytokine secretion in a cell-type specific manner and that both beta-arrestins have overlapping but non-redundant roles in regulating inflammatory cytokine production and endotoxic shock in mice.

beta-Arrestin 1 and 2 stabilize the angiotensin II type I receptor in distinct high-affinity conformations

BACKGROUND AND PURPOSE

The angiotensin II type 1 (AT(1)) receptor belongs to family A of 7 transmembrane (7TM) receptors. The receptor has important roles in the cardiovascular system and is commonly used as a drug target in cardiovascular diseases. Interaction of 7TM receptors with G proteins or beta-arrestins often induces higher binding affinity for agonists. Here, we examined interactions between AT(1A) receptors and beta-arrestins to look for differences between the AT(1A) receptor interaction with beta-arrestin1 and beta-arrestin2.

EXPERIMENTAL APPROACH

Ligand-induced interaction between AT(1A) receptors and beta-arrestins was measured by Bioluminescence Resonance Energy Transfer 2. AT(1A)-beta-arrestin1 and AT(1A)-beta-arrestin2 fusion proteins were cloned and tested for differences using immunocytochemistry, inositol phosphate hydrolysis and competition radioligand binding.

KEY RESULTS

Bioluminescence Resonance Energy Transfer 2 analysis showed that beta-arrestin1 and 2 were recruited to AT(1A) receptors with similar ligand potencies and efficacies. The AT(1A)-beta-arrestin fusion proteins showed attenuated G protein signalling and increased agonist binding affinity, while antagonist affinity was unchanged. Importantly, larger agonist affinity shifts were observed for AT(1A)-beta-arrestin2 than for AT(1A)-beta-arrestin1.

CONCLUSION AND IMPLICATIONS

beta-Arrestin1 and 2 are recruited to AT(1A) receptors with similar ligand pharmacology and stabilize AT(1A) receptors in distinct high-affinity conformations. However, beta-arrestin2 induces a receptor conformation with a higher agonist-binding affinity than beta-arrestin1. Thus, this study demonstrates that beta-arrestins interact with AT(1A) receptors in different ways and suggest that AT(1) receptor biased agonists with the ability to recruit either of the beta-arrestins selectively, would be possible to design.

β-arrestin 2 regulates Toll-like receptor 4-mediated apoptotic signalling through glycogen synthase kinase-3β

Toll-like receptor 4 (TLR4), a key member of the TLR family, has been well characterized by its function in the induction of inflammatory products of innate immunity. However, the involvement of TLR4 in a variety of apoptotic events by an unknown mechanism has been the focus of great interest. Our investigation found that TLR4 promoted apoptotic signalling by affecting the glycogen synthase kinase-3beta (GSK-3beta) pathway in a serum-deprivation-induced apoptotic paradigm. Serum deprivation induces GSK-3beta activation in a pathway that leads to subsequent cell apoptosis. Intriguingly, this apoptotic cascade is amplified in presence of TLR4 but greatly attenuated by beta-arrestin 2, another critical molecule implicated in TLR4-mediated immune responses. Our data suggest that the association of beta-arrestin 2 with GSK-3beta contributes to the stabilization of phospho-GSK-3beta, an inactive form of GSK-3beta. It becomes a critical determinant for the attenuation of TLR4-initiated apoptosis by beta-arrestin 2. Taken together, we demonstrate that the TLR4 possesses the capability of accelerating GSK-3beta activation thereby deteriorating serum-deprivation-induced apoptosis; beta-arrestin 2 represents an inhibitory effect on the TLR4-mediated apoptotic cascade, through controlling the homeostasis of activation and inactivation of GSK-3beta.

Beta-arrestin 2 negatively regulates sepsis-induced inflammation

SUMMARY

Beta-arrestins 1 and 2 are ubiquitously expressed proteins that alter signalling by G-protein-coupled receptors. beta-arrestin 2 plays an important role as a signalling adaptor and scaffold in regulating cellular inflammatory responses. We hypothesized that beta-arrestin 2 is a critical modulator of inflammatory response in experimental sepsis. beta-arrestin 2(-/-) and wild-type (WT) mice were subjected to caecal ligation and puncture (CLP). The survival rate was significantly decreased (P < 0.05) in beta-arrestin 2(-/-) mice (13% survival) compared with WT mice (53% survival). A second group of mice were killed 18 hr after CLP for blood, peritoneal lavage and tissue sample collection. CLP-induced plasma interleukin (IL)-6 was significantly increased 25 +/- 12 fold and caecal myeloperoxidase (MPO) activity was increased 2.4 +/- 0.3 fold in beta-arrestin 2(-/-) compared with WT mice. beta-arrestin 2(-/-) mice exhibited more severe lung damage and higher bacterial loads compared with WT mice post CLP challenge as measured by histopathology and colony-forming unit count. In subsequent experiments, splenocytes, peritoneal macrophages and bone marrow-derived macrophages (BMDMs) were isolated and cultured from beta-arrestin 2(-/-) and WT mice and stimulated in vitro with lipopolysaccharide (LPS). Tumour necrosis factor (TNF)-alpha, IL-6 and IL-10 production induced by LPS was significantly augmented (2.2 +/- 0.2 fold, 1.8 +/- 0.1 fold, and 2.2 +/- 0.4 fold, respectively; P < 0.05) in splenocytes from beta-arrestin 2(-/-) mice compared with WT mice. The splenocyte response was different from that of peritoneal macrophages or BMDMs, which exhibited no difference in TNF-alpha and IL-6 production upon LPS stimulation between WT and beta-arrestin 2(-/-) mice. Our data demonstrate that beta-arrestin 2 functions to negatively regulate the inflammatory response in polymicrobial sepsis.

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.

beta-Arrestins modulate Adenovirus-vector-induced innate immune responses: differential regulation by beta-arrestin-1 and beta-arrestin-2

Adenovirus (Ad)-based vectors have been utilized in human gene transfer clinical trials since 1993. Unfortunately, innate immune responses directed against the Ad capsid and/or its genetic cargo can significantly limit the usage of Ad vectors. Previous studies have demonstrated that several signaling pathways are triggered by Ads, inclusive of TLR-dependent pathways. The G-protein-coupled receptor adaptors beta-arrestin-1 (beta-Arr1) and beta-arrestin-2 (beta-Arr2) are known to have pivotal roles in regulating TLR4 triggered signaling and inflammatory responses. In this study, we examined the role of beta-arrestins in Ad5-vector-induced inflammatory responses. Our studies reveal that both beta-arrestins are capable of modulating Ad5-vector-induced inflammatory responses in vivo and in vitro. Importantly, our studies divulge another level of complexity to these responses, as our results demonstrate beta-Arr1 to be a positive regulator, and beta-Arr2 a negative regulator of Ad5 induced innate immune responses. These data may allow gene therapy biologists to more accurately study the mechanisms underlying Ad5-vector-induced immune responses, and may also direct future efforts to modulate these mechanisms to improve the safety and/or efficacy of this important gene transfer vector.

Beta-arrestin 2 is required for complement C1q expression in macrophages and constrains factor-independent survival

The beta-arrestins (ARRB1 and ARRB2) regulate G-protein coupled receptor (GPCR) dependent- and independent-signaling pathways and are ubiquitously expressed. Here we show that ARRB2 mRNA and protein expression is enriched in macrophages, and that it regulates complement C1q expression and cell survival. Basal and Toll-like receptor (TLR) inducible expression of mRNAs encoding the complement subcomponents C1qa, C1qb and C1qc was greatly reduced in bone marrow-derived macrophages (BMM) from ARRB2-deficient, but not ARRB1-deficient mice, while factor-independent survival of ARRB2(-/-) BMM was enhanced compared to wildtype BMM. TatARRB2(23), a cell-permeable peptide that contains the MAPK JNK-binding motif from within the ARRB2 C-domain, impaired ARRB2 interaction with JNK3, down-regulated C1q expression and permitted factor-independent survival in BMM, thus suggesting that this peptide antagonises ARRB2 function in macrophages. In addition, TatARRB2(23) transiently activated the phosphorylation of JNK and ERK, but not p38 in BMM. These data imply that ARRB2 acts to limit JNK/ERK activation and survival in macrophages, but is required for basal and TLR-inducible complement C1q expression. Given that loss of C1q function is strongly associated with the development of systemic lupus erythematosus, ARRB2 may act to limit the development of autoimmune disease.

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.

Tetramethylpyrazine suppresses interleukin-8 expression in LPS-stimulated human umbilical vein endothelial cell by blocking ERK, p38 and nulear factor-kappaB signaling pathways

AIM OF THE STUDY

To determine the anti-inflammatory effects of Tetramethylpyrazine (TMP) and to investigate the inhibitory effect of TMP on IL-8 production in human umbilical vein endothelial cells (HUVECs) induced by LPS might be mediated by inhibiting p38, ERK and NF-kappaB signaling pathways.

MATERIALS AND METHODS

HUVECs were treated with or without TMP for 24h before exposure to LPS for 4h. IL-8 gene and protein expressions were determined by RT-PCR and ELISA. Cell viability was determined by methyl thiazoyltetrazolium (MTT) assay. Phosphorylation of ERK1/2 and p38 were examined by western blotting.

RESULTS

TMP inhibits LPS-induced IL-8 production in HUVECs at both the protein and mRNA levels, suggesting that TMP has an antiinflammatory effect on endothelial cells. TMP also inhibited U937 monocyte adhesion to HUVECs stimulated by LPS. LPS-induced phosphorylation of ERK1/2 and p38 were inhibited by TMP. The inhibitory effect of TMP on NF-kappaB (p65) activity was mediated by blocking the consequent translocation of p65 into the nucleus.

CONCLUSIONS

The inhibitory effect of TMP on the LPS-induced IL-8 production is mediated by the NF-kappaB-dependent pathway, and TMP also separately affects the ERK and p38 MAPK pathway. TMP may be beneficial in the treatment of cardiovascular disorders such as atherosclerosis.

Endocytic proteins in the regulation of nuclear signaling, transcription and tumorigenesis

Accumulating evidence argues that many proteins governing membrane sorting during endocytosis participate also in nuclear signaling and transcriptional regulation, mostly by modulating the activity of various nuclear factors. Some adaptors and accessory proteins acting in clathrin-mediated internalization, as well as endosomal sorting proteins can undergo nuclear translocation and affect gene expression directly, while for others the effects may be more indirect. Although it is often unclear to what extent the endocytic and nuclear functions are interrelated, several of such proteins are implicated in the regulation of cell proliferation and tumorigenesis, arguing that their dual-function nature may be of physiological importance.

Novel pharmacologic approaches to the management of sepsis: targeting the host inflammatory response

Sepsis is currently the 10(th) leading cause of death overall and accounts for significant healthcare expenditures in the developed world. There are now more deaths attributable to sepsis than coronary artery disease, stroke, or cancer, and it is widely believed that the incidence of sepsis and sepsis-related mortality will continue to rise. Based on these sobering statistics, there is great interest in identifying novel treatments for managing critically ill children and adults with sepsis. Unfortunately, to date, there have been very few successful therapeutic agents employed in the clinical setting. Despite these disappointing results, new therapeutic agents continue to be identified, and there is reason for optimism and hope for the future. Herein, we will briefly review several novel therapeutic adjuncts for the management of critically ill patients with sepsis. We will largely focus on those therapies that directly target the host inflammatory response, specifically those that result in activation of the transcription factor, nuclear factor (NF)-kappaB. We will also reference some of the patents recently filed that pertain to the host innate immune response and sepsis.

Morphine promotes apoptosis via TLR2, and this is negatively regulated by beta-arrestin 2

We have previously reported that morphine induces apoptosis. However, the underlying molecular mechanisms remain to be elucidated. Toll-like receptor 2 (TLR2), a key immune receptor in the TLR family, modulates cell survival and cell death in various systems. Evidence indicates that beta-arrestin 2 acts as a negative regulator of innate immune activation by TLRs. Here, we investigated the roles of TLR2, the downstreaming mediator MyD88, and beta-arrestin 2 in morphine-induced apoptosis. We showed that overexpression of TLR2 in HEK293 cells caused a significant increase in apoptosis after morphine treatment. Inhibition of MyD88 by transfecting dominant negative MyD88 or overexpression of beta-arrestin 2 by transfecting beta-arrestin 2 full length plasmid in TLR2 overexpressing HEK293 cells attenuated morphine-induced apoptosis. Our study thus demonstrates that TLR2 signaling mediates the morphine-induced apoptosis, and beta-arrestin 2 is a negative regulator in morphine-induced, TLR2-mediated apoptosis.

Physiologic and cardiac roles of beta-arrestins

Beta-arrestin1 and beta-arrestin2 were initially identified by sequence homology to visual arrestins and by their ability to bind to and inactivate signaling of the beta-2-adrenergic receptor in a process known as desensitization. While the role of beta-arrestins in desensitization has been known for some time, more recent evidence has revealed that beta-arrestins are multifunctional scaffolding proteins that are involved in numerous aspects of G protein-coupled receptor (GPCR) signaling. Interestingly, exciting new data shows that beta-arrestins can mediate signaling in their own right independent of classical second messenger mediated signaling, and that this beta-arrestin-mediated signaling may be cardioprotective. Identifying novel ligands for GPCRs that can block G protein-mediated signaling while simultaneously promoting beta-arrestin-mediated signaling could provide powerful new therapies for cardiac disease.

Beta-arrestin-mediated signaling in the heart

Beta-arrestin is a multifunctional adapter protein well known for its role in G-protein-coupled receptor (GPCR) desensitization. Exciting new evidence indicates that beta-arrestin is also a signaling molecule capable of initiating its own G-protein-independent signaling at GPCRs. One of the best-studied beta-arrestin signaling pathways is the one involving beta-arrestin-dependent activation of a mitogen-activated protein kinase cascade, the extracellular regulated kinase (ERK). ERK signaling, which is classically activated by agonist stimulation of the epidermal growth factor receptor (EGFR), can be activated by a number of GPCRs in a beta-arrestin-dependent manner. Recent work in animal models of heart failure suggests that beta-arrestin-dependent activation of EGFR/ERK signaling by the beta-1-adrenergic receptor, and possibly the angiotensin II Type 1A receptor, are cardioprotective. Hence, a new model of signaling at cardiac GPCRs has emerged and implicates classical G-protein-mediated signaling with promoting harmful remodeling in heart failure, while concurrently linking beta-arrestin-dependent, G-protein-independent signaling with cardioprotective effects. Based on this paradigm, a new class of drugs could be identified, termed "biased ligands", which simultaneously block harmful G-protein signaling, while also promoting cardioprotective beta-arrestin-dependent signaling, leading to a potential breakthrough in the treatment of chronic cardiac disease.

Heptahelical terpsichory. Who calls the tune?

The discovery that arrestins can function as ligand-regulated signaling scaffolds has revealed a previously unappreciated level of complexity in G protein-coupled receptor (GPCR) signal transduction. Because arrestin-bound GPCRs are uncoupled from G proteins, arrestin binding can be viewed as switching receptors between two temporally and spatially distinct signaling modes. Recent work has established two factors that underscore this duality of GPCR signaling and suggest it may ultimately have therapeutic significance. The first is that signaling by receptor-arrestin "signalsomes" does not require heterotrimeric G protein activation. The second is that arrestin-dependent signals can be initiated by pathway-specific "biased agonists," creating the potential for drugs that selectively modulate different aspects of GPCR function. Currently, however, little is known about the physiological relevance of G protein-independent signals at the cellular or whole animal levels, and additional work is needed to determine whether arrestin pathway-selective drugs will find clinical application.

Cigarette smoke increases Toll-like receptor 4 and modifies lipopolysaccharide-mediated responses in airway epithelial cells

Airway epithelium is emerging as a regulator of innate immune responses to a variety of insults including cigarette smoke. The main goal of this study was to explore the effects of cigarette smoke extracts (CSE) on Toll-like receptor (TLR) expression and activation in a human bronchial epithelial cell line (16-HBE). The CSE increased the expression of TLR4 and the lipopolysaccharide (LPS) binding, the nuclear factor-kappaB (NF-kappaB) activation, the release of interleukin-8 (IL-8) and the chemotactic activity toward neutrophils. It did not induce TLR2 expression or extracellular signal-regulated signal kinase 1/2 (ERK1/2) activation. The LPS increased the expression of TLR4 and induced both NF-kappaB and ERK1/2 activation. The combined exposure of 16-HBE to CSE and LPS was associated with ERK activation rather than NF-kappaB activation and with a further increase of IL-8 release and of chemotactic activity toward neutrophils. Furthermore, CSE decreased the constitutive interferon-inducible protein-10 (IP-10) release and counteracted the effect of LPS in inducing both the IP-10 release and the chemotactic activity toward lymphocytes. In conclusion, cigarette smoke, by altering the expression and the activation of TLR4 via the preferential release of IL-8, may contribute to the accumulation of neutrophils within the airways of smokers.

Phosphatase PTP1B negatively regulates MyD88- and TRIF-dependent proinflammatory cytokine and type I interferon production in TLR-triggered macrophages

Toll-like receptors (TLRs) are primary sensors to detect conserved patterns on microorganisms, thus acting as the important components of innate immunity against invading pathogens. Protein tyrosine phosphatase-1B (PTP1B) has been shown to be a critical negative regulator of insulin pathway and other cellular signaling, however, whether and how PTP1B regulates TLR-triggered innate response remain to be investigated. We report here that PTP1B can markedly decrease TNF-alpha, IL-6 and IFN-beta production by macrophages stimulated with LPS, CpG ODN, or Poly I:C. Accordingly, knockdown of endogenous PTP1B expression increases production of TNF-alpha, IL-6 and IFN-beta in macrophages stimulated with TLR ligands. Phosphatase activity-disrupted mutant PTP1B cannot inhibit TLR-triggered production of proinflammatory cytokines and IFN-beta, indicating PTP1B exerts its suppressive activity in phosphatase-dependent manner. PTP1B inhibits TLR ligands-induced activation of MAPKs, NF-kappaB, and IRF3, furthermore, co-transfection of PTP1B inhibits both MyD88- and TRIF-induced transcription of TNF-alpha and IFN-beta reporter genes in a dose-dependent manner. In addition, PTP1B inhibits LPS-induced Tyk2 and STAT1 activation. Therefore, we demonstrate that phosphatase PTP1B is a physiological negative regulator of TLR signaling via suppression of both MyD88- and TRIF-dependent production of proinflammatory cytokine and IFN-beta in macrophages. Our results provide new mechanistic explanation for negative regulation TLR response and suggest PTP1B as a potential target for the intervention of the inflammatory diseases.

beta-Arrestin 2: a Negative Regulator of Inflammatory Responses in Polymorphonuclear Leukocytes

Heterotrimeric Gi proteins have been previously implicated in signaling leading to inflammatory mediator production induced by bacterial lipopolysaccharide (LPS). beta-arrestins are ubiquitously expressed proteins that alter G-protein-coupled receptors signaling. beta-arrestin 2 plays a multifaceted role as a scaffold protein in regulating cellular inflammatory responses. Polymorphonuclear leukocytes (PMNs) activated by LPS induce inflammatory responses resulting in organ injury during sepsis. We hypothesized that beta-arrestin 2 is a critical modulator of inflammatory responses in PMNs. To examine the potential role of beta-arrestin 2 in LPS-induced cellular activation, we studied homozygous beta-arrestin 2 (-/-), heterozygous (+/-), and wildtype (+/+) mice. PMNs were stimulated with LPS for 16h. There was increased basal TNFalpha and IL-6 production in the beta-arrestin 2 (-/-) compared to both beta-arrestin 2 (+/-) and (+/+) cells. LPS failed to stimulate TNFalpha production in the beta-arrestin 2 (-/-) PMNs. However, LPS stimulated IL-6 production was increased in the beta-arrestin 2 (-/-) cells compared to (+/+) cells. In subsequent studies, peritoneal PMN recruitment was increased 81% in the beta-arrestin 2 (-/-) mice compared to (+/+) mice (p<0.05). beta-arrestin 2 deficiency resulted in an augmented expression of CD18 and CD62L (p<0.05). In subsequent studies, beta-arrestin 2 (-/-) and (+/+) mice were subjected to cecal ligation and puncture (CLP) and lung was collected and analyzed for myeloperoxidase activity (MPO) as index of PMNs infiltrate. CLP-induced MPO activity was significantly increased (p<0.05) in the beta-arrestin 2 (-/-) compared to (+/+) mice. These studies demonstrate that beta-arrestin 2 is a negative regulator of PMN activation and pulmomary leukosequestration in response to polymicrobial sepsis.

The negative regulation of Toll‐like receptor and associated pathways

Toll-like receptors (TLRs) are essential mediators of both innate and adaptive immunity by recognizing and eliciting responses upon invasion of pathogens. The response of TLRs must be stringently regulated as exaggerated expression of signalling components as well as pro-inflammatory cytokines can have devastating effects on the host, resulting in chronic inflammatory diseases, autoimmune disorders and aid in the pathogenesis of TLR-associated human diseases. Therefore, it is essential that negative regulators act at multiple levels within TLR signalling cascades, as well as through eliciting negative-feedback mechanisms in order to synchronize the positive activation and negative regulation of signal transduction to avert potentially harmful immunological consequences. This review explores the various mechanisms employed by negative regulators to ensure the appropriate modulation of both immune and inflammatory responses.