The SHP-1 protein tyrosine phosphatase negatively modulates Akt signaling in the ghrelin/GHSR1a system

Regulation of PPARγ2 Stability and Activity by SHP-1

The protein tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 (SHP-1) plays an important role in modulating glucose and lipid homeostasis. We previously suggested a potential role of SHP-1 in the regulation of peroxisome proliferator-activated receptor γ2 (PPARγ2) expression and activity but the mechanisms were unexplored. PPARγ2 is the master regulator of adipogenesis, but how its activity is regulated by tyrosine phosphorylation is largely unknown. Here, we found that SHP-1 binds to PPARγ2 primarily via its N-terminal SH2-domain. We confirmed the phosphorylation of PPARγ2 on tyrosine-residue 78 (Y78), which was reduced by SHP-1 in vitro resulting in decreased PPARγ2 stability. Loss of SHP-1 led to elevated, agonist-induced expression of the classical PPARγ2 targets FABP4 and CD36, concomitant with increased lipid content in cells expressing PPARγ2, an effect blunted by abrogation of PPARγ2 phosphorylation. Collectively, we discovered that SHP-1 affects the stability of PPARγ2 through dephosphorylation thereby influencing adipogenesis.

SHP-1 tyrosine phosphatase binding to c-Src kinase phosphor-dependent conformations: A comparative structural framework

SHP-1 is a cytosolic tyrosine phosphatase that is primarily expressed in hematopoietic cells. It acts as a negative regulator of numerous signaling pathways and controls multiple cellular functions involved in cancer pathogenesis. This study describes the binding preferences of SHP-1 (pY536) to c-Srcopen (pY416) and c-Srcclose (pY527) through in silico approaches. Molecular dynamics simulation analysis revealed more conformational changes in c-Srcclose upon binding to SHP-1, as compared to its active/open conformation that is stabilized by the cooperative binding of the C-SH2 domain and C-terminal tail of SHP-1 to c-Src SH2 and KD. In contrast, c-Srcclose and SHP-1 interaction is mediated by PTP domain-specific WPD-loop (WPDXGXP) and Q-loop (QTXXQYXF) binding to c-Srcclose C-terminal tail residues. The dynamic correlation analysis demonstrated a positive correlation for SHP-1 PTP with KD, SH3, and the C-terminal tail of c-Srcclose. In the case of the c-Srcopen-SHP-1 complex, SH3 and SH2 domains of c-Srcopen were correlated to C-SH2 and the C-terminal tail of SHP-1. Our findings reveal that SHP1-dependent c-Src activation through dephosphorylation relies on the conformational shift in the inhibitory C-terminal tail that may ease the recruitment of the N-SH2 domain to phosphotyrosine residue, resulting in the relieving of the PTP domain. Collectively, this study delineates the intermolecular interaction paradigm and underlying conformational readjustments in SHP-1 due to binding with the c-Src active and inactive state. This study will largely help in devising novel therapeutic strategies for targeting cancer development.

A novel GRK2 inhibitor alleviates experimental arthritis through restraining Th17 cell differentiation

T helper type 17 (Th17) cell which is induced by interleukine-6 (IL-6)-signal transducers and activators of transcription 3 (STAT3) signaling is a central pro-inflammatory T cell subtype in rheumatoid arthritis (RA) and could be significantly reduced by paeoniflorin-6'-O-benzene sulfonate (CP-25) treatment with unclear mechanisms. This study was aimed to found out the mechanism of CP-25 in hampering Th17 cells differentiation in arthritic animals thus explore more therapeutic targets for RA. In mice with collagen-induced arthritis (CIA), both circulating and splenic Th17 subsets were expanded with increased STAT3 phosphorylation and decreased Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1)-β-arrestin2 (arrb2)-STAT3 interaction in CD4⁺ helper T (Th) cells. Either CP-25 or paroxetine (PAR), an established G protein coupled receptor kinase 2 (GRK2) inhibitor treatment effectively relieved the joints inflammation of CIA mice with substantially reduced Th17 cell population through inhibiting STAT3 and restoring the SHP1-arrb2-STAT3 complex. Knockout of arrb2 exacerbated the clinical manifestations of collagen antibody-induced arthritis with upregulated Th17 cells. In vitro studies revealed that depletion of arrb2 or inhibition of SHP1 promoted Th17 cell differentiation. Moreover, stimulation of adenosine A₃ receptor (A₃AR) simultaneously promoted Th17 cell differentiation via accelerating abbr2-A₃AR binding, which could be prevented through inhibiting GRK2 phosphorylation by CP-25 or PAR, or genetically reducing GRK2. This work has demonstrated that CP-25 or PAR treatment recovers the SHP1-arrb2-STAT3 complex which prevents STAT3 activation in Th cells through reducing arrb2 recruitment to A₃AR by inhibiting GRK2 phosphorylation, leading to the reduction in Th17 cell differentiation and arthritis attenuation.

Non-Lyn Src Family Kinases Activate SIRPα-SHP-1 to Inhibit PI3K-Akt2 and Dampen Proinflammatory Macrophage Polarization

Macrophage functional plasticity plays a central role in responding to proinflammatory stimuli. The molecular basis underlying the dynamic phenotypic activation of macrophages, however, remains incompletely understood. In this article, we report that SIRPα is a chief negative regulator of proinflammatory macrophage polarization. In response to TLR agonists, proinflammatory cytokines, or canonical M1 stimulation, Src family kinases (SFK) excluding Lyn phosphorylate SIRPα ITIMs, leading to the preferential recruitment and activation of SHP-1, but not SHP-2. Solely extracellular ligation of SIRPα by CD47 does not greatly induce phosphorylation of SIRPα ITIMs, but it enhances proinflammatory stimuli-induced SIRPα phosphorylation. Examination of downstream signaling elicited by IFN-γ and TLR3/4/9 agonists found that SIRPα-activated SHP-1 moderately represses STAT1, NF-κB, and MAPK signaling but markedly inhibits Akt2, resulting in dampened proinflammatory cytokine production and expression of Ag presentation machinery. Pharmacological inhibition of SHP-1 or deficiency of SIRPα conversely attenuates SIRPα-mediated inhibition and, as such, augments macrophage proinflammatory polarization that in turn exacerbates proinflammation in mouse models of type I diabetes and peritonitis. Our results reveal an SFK-SIRPα-SHP-1 mechanism that fine-tunes macrophage proinflammatory phenotypic activation via inhibition of PI3K-Akt2, which controls the transcription and translation of proinflammatory cytokines, Ag presentation machinery, and other cellular programs.

An optimal control approach for enhancing natural killer cells' secretion of cytolytic molecules

Natural killer (NK) cells are immune effector cells that can detect and lyse cancer cells. However, NK cell exhaustion, a phenotype characterized by reduced secretion of cytolytic models upon serial stimulation, limits the NK cell's ability to lyse cells. In this work, we investigated in silico strategies that counteract the NK cell's reduced secretion of cytolytic molecules. To accomplish this goal, we constructed a mathematical model that describes the dynamics of the cytolytic molecules granzyme B (GZMB) and perforin-1 (PRF1) and calibrated the model predictions to published experimental data using a Bayesian parameter estimation approach. We applied an information-theoretic approach to perform a global sensitivity analysis, from which we found that the suppression of phosphatase activity maximizes the secretion of GZMB and PRF1. However, simply reducing the phosphatase activity is shown to deplete the cell's intracellular pools of GZMB and PRF1. Thus, we added a synthetic Notch (synNotch) signaling circuit to our baseline model as a method for controlling the secretion of GZMB and PRF1 by inhibiting phosphatase activity and increasing production of GZMB and PRF1. We found that the optimal synNotch system depends on the frequency of NK cell stimulation. For only a few rounds of stimulation, the model predicts that inhibition of phosphatase activity leads to more secreted GZMB and PRF1; however, for many rounds of stimulation, the model reveals that increasing production of the cytolytic molecules is the optimal strategy. In total, we developed a mathematical framework that provides actionable insight into engineering robust NK cells for clinical applications.

A new understanding of GHSR1a--independent of ghrelin activation

Growth hormone secretagogue receptor 1a (GHSR1a), a member of the G protein-coupled receptor (GPCR) family, is a functional receptor of ghrelin. The expression levels and activities of GHSR1a are affected by various factors. In past years, it has been found that the ghrelin-GHSR1a system can perform biological functions such as anti-inflammation, anti-apoptosis, and anti-oxidative stress. In addition to mediating the effect of ghrelin, GHSR1a also has abnormally high constitutive activity; that is, it can still transmit intracellular signals without activation of the ghrelin ligand. This constitutive activity affects brain functions, growth and development of the body; therefore, it has profound impacts on neurodegenerative diseases and some other age-related diseases. In addition, GHSR1a can also form homodimers or heterodimers with other GPCRs, affecting the release of neurotransmitters, appetite regulation, cell proliferation and insulin release. Therefore, further understanding of the constitutive activities and dimerization of GHSR1a will enable us to better clarify the characteristics of GHSR1a and provide more therapeutic targets for drug development. Here, we focus on the roles of GHSR1a in various biological functions and provide a comprehensive summary of the current research on GHSR1a to provide broader therapeutic prospects for age-related disease treatment.

Regulation of platelet-activating factor-mediated interleukin-6 promoter activation by the 48 kDa but not the 45 kDa isoform of protein tyrosine phosphatase non-receptor type 2

Background

An underlying state of inflammation is thought to be an important cause of cardiovascular disease. Among cells involved in the early steps of atherosclerosis, monocyte-derived dendritic cells (Mo-DCs) respond to inflammatory stimuli, including platelet-activating factor (PAF), by the induction of various cytokines, such as interleukin 6 (IL-6). PAF is a potent phospholipid mediator involved in both the onset and progression of atherosclerosis. It mediates its effects by binding to its cognate G-protein coupled receptor, PAFR. Activation of PAFR-induced signaling pathways is tightly coordinated to ensure specific cell responses.

Results

Here, we report that PAF stimulated the phosphatase activity of both the 45 and 48 kDa isoforms of the protein tyrosine phosphatase non-receptor type 2 (PTPN2). However, we found that only the 48 kDa PTPN2 isoform has a role in PAFR-induced signal transduction, leading to activation of the IL-6 promoter. In luciferase reporter assays, expression of the 48 kDa, but not the 45 kDa, PTPN2 isoform increased human IL-6 (hIL-6) promoter activity by 40% after PAF stimulation of HEK-293 cells, stably transfected with PAFR (HEK-PAFR). Our results suggest that the differential localization of the PTPN2 isoforms and the differences in PAF-induced phosphatase activation may contribute to the divergent modulation of PAF-induced IL-6 promoter activation. The involvement of PTPN2 in PAF-induced IL-6 expression was confirmed in immature Mo-DCs (iMo-DCs), using siRNAs targeting the two isoforms of PTPN2, where siRNAs against the 48 kDa PTPN2 significantly inhibited PAF-stimulated IL-6 mRNA expression. Pharmacological inhibition of several signaling pathways suggested a role for PTPN2 in early signaling events. Results obtained by Western blot confirmed that PTPN2 increased the activation of the PI3K/Akt pathway via the modulation of protein kinase D (PKD) activity. WT PKD expression counteracted the effect of PTPN2 on PAF-induced IL-6 promoter transactivation and phosphorylation of Akt. Using siRNAs targeting the individual isoforms of PTPN2, we confirmed that these pathways were also active in iMo-DCs.

Conclusion

Taken together, our data suggest that PTPN2, in an isoform-specific manner, could be involved in the positive regulation of PI3K/Akt activation, via the modulation of PKD activity, allowing for the maximal induction of PAF-stimulated IL-6 mRNA expression.

Targeted SHP-1 Silencing Modulates the Macrophage Phenotype, Leading to Metabolic Improvement in Dietary Obese Mice

Chronic over-nutrition promotes adipocyte hypertrophy that creates inflammatory milieu leading to macrophage infiltration and their phenotypic switching during obesity. The SH2 domain-containing protein tyrosine phosphatase 1 (SHP-1) has been identified as an important player in inflammatory diseases involving macrophages. However, the role of SHP-1 in modulating the macrophage phenotype has not been elucidated yet. In the present work, we show that adipose tissue macrophage (ATM)-specific deletion of SHP-1 using glucan particle-loaded siRNA improves the metabolic phenotype in dietary obese insulin-resistant mice. The molecular mechanism involves AT remodeling via reducing crown-like structure formation and balancing the pro-inflammatory (M1) and anti-inflammatory macrophage (M2) population. Therefore, targeting ATM-specific SHP-1 using glucan-particle-loaded SHP-1 antagonists could be of immense therapeutic use for the treatment of obesity-associated insulin resistance.

GHS-R1a Deficiency Alleviates Depression-Related Behaviors After Chronic Social Defeat Stress

Ghrelin is an important orexigenic hormone that regulates feeding, metabolism and glucose homeostasis in human and rodents. Ghrelin functions by binding to its receptor, the growth hormone secretagogue receptor 1a (GHS-R1a), which is widely expressed inside and outside of the brain. Recent studies suggested that acyl-ghrelin, the active form of ghrelin, is a persistent biomarker for chronic stress exposure. However, how ghrelin/GHS-R1a signaling contributes to stress responses and mood regulation remains uncertain. In this study, we applied the chronic social defeat stress (CSDS) paradigm to both GHS-R1a knock-out (Ghsr^-/-) mice and littermate control (Ghsr^+/+) mice, and then measured their depression- and anxiety-related behaviors. We found that Ghsr^+/+ mice, but not Ghsr^-/- mice, displayed apparent anxiety and depression after CSDS, while two groups mice showed identical behaviors at baseline, non-stress state. By screening the central and peripheral responses of Ghsr^-/- mice and Ghsr^+/+ mice to chronic stress, we found similar elevations of total ghrelin and adrenocorticotropic hormone (ACTH) in the serum of Ghsr^-/- mice and Ghsr^+/+ mice after CSDS, but decreased interleukin-6 (IL-6) in the serum of defeated Ghsr^-/- mice compared to defeated Ghsr^+/+ mice. We also found increased concentration of brain derived neurotropic factor (BDNF) in the hippocampus of Ghsr^-/- mice compared to Ghsr^+/+ mice after CSDS. The basal levels of ghrelin, ACTH, IL-6, and BDNF were not different between Ghsr^-/- mice and Ghsr^+/+ mice. Our findings thus suggested that the differential expressions of BDNF and IL-6 after CSDS may contribute to less anxiety and less despair observed in GHS-R1a-deficient mice than in WT control mice. Therefore, ghrelin/GHS-R1a signaling may play a pro-anxiety and pro-depression effect in response to chronic stress, while GHS-R1a deficiency may provide resistance to depressive symptoms of CSDS.

Regulation of platelet-activating factor-induced interleukin-8 expression by protein tyrosine phosphatase 1B

Background

Platelet-activating factor (PAF) is a potent lipid mediator whose involvement in the onset and progression of atherosclerosis is mediated by, among others, the modulation of cytokine expression patterns. The presence of multiple potential protein-tyrosine phosphatase (PTP) 1B substrates in PAF receptor signaling pathways brought us to investigate its involvement in PAF-induced cytokine expression in monocyte-derived dendritic cells (Mo-DCs) and to study the pathways involved in this modulation.

Methods

We used in-vitro-matured human dendritic cells and the HEK-293 cell line in our studies. PTP1B inhibition was though siRNAs and a selective inhibitor. Cytokine expression was studied with RT-PCR, luciferase assays and ELISA. Phosphorylation status of kinases and transcription factors was studied with western blotting.

Results

Here, we report that PTP1B was involved in the modulation of cytokine expression in PAF-stimulated Mo-DCs. A study of the down-regulation of PAF-induced IL-8 expression, by PTP1B, showed modulation of PAF-induced transactivation of the IL-8 promoter which was dependent on the presence of the C/EBPß -binding site. Results also suggested that PTP1B decreased PAF-induced IL-8 production by a glycogen synthase kinase (GSK)-3-dependent pathway via activation of the Src family kinases (SFK). These kinases activated an unidentified pathway at early stimulation times and the PI3K/Akt signaling pathway in a later phase. This change in GSK-3 activity decreased the C/EBPß phosphorylation levels of the threonine 235, a residue whose phosphorylation is known to increase C/EBPß transactivation potential, and consequently modified IL-8 expression.

Conclusion

The negative regulation of GSK-3 activity by PTP1B and the consequent decrease in phosphorylation of the C/EBPß transactivation domain could be an important negative feedback loop by which cells control their cytokine production after PAF stimulation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0334-6) contains supplementary material, which is available to authorized users.

Measuring GPCR-Induced Activation of Protein Tyrosine Phosphatases (PTP) Using In-Gel and Colorimetric PTP Assays

Given the increasing amount of data showing the importance of protein tyrosine phosphatases (PTPs) in G protein-coupled receptor (GPCR) signaling pathways, the modulation of this enzyme family by that type of receptor can become an important experimental question. Here, we describe two different methods, an in-gel and a colorimetric PTP assay, to evaluate the modulation of PTP activity after stimulation with GPCR agonists.

Blocking constitutive activity of GHSR1a in the lateral amygdala facilitates acquisition of conditioned taste aversion

Ghrelin is a circulating peptide hormone promoting feeding and regulating energy metabolism in human and rodents. Ghrelin functions by binding to its receptor, the growth hormone secretagogue receptor 1a (GHSR1a), which are widely distributed throughout the brain including the amygdala, a brain region important for regulating valenced behavior, such as aversion. Interestingly, GHSR1a was once characterized by highly constitutive, ligand-independent activity. However, the physiological importance of such ligand-independent signaling on aversive memory processing has not been tested yet. Here, we applied [D-Arg¹, D-Phe⁵, D-Trp^7,9, Leu¹¹]-Substance P (D-SP), a full inverse agonist for GHSR1a, into the lateral amygdala (LA) and investigated the effect of blocking GHSR1a constitutive activity on conditioned taste aversion (CTA) in rats. We found that intra-LA infusion of a single low dose of D-SP (8ng/0.5μl/side) facilitates CTA acquisition. Moreover, pre-administration of a high dose of D-SP into the LA abolishes the suppressive effect of exogenous ghrelin on CTA acquisition. In contrast, pre-administration of the same dose of D-SP does not affect the suppression of substance P, a potent neurokinin-1 (NK1) receptor ligand, on CTA. Therefore, our data indicated that the spontaneous or basal activity of GHSR1a signaling in the LA might interfere with CTA memory formation. D-SP decreases the constitutive activity of GHSR1a and thus facilitates CTA. Altogether, our present findings along with previous results support the idea that ghrelin/GHSR1a signaling in the LA circuit blocks conditioned taste aversion.

IL-4-Induced Gene 1: A Negative Immune Checkpoint Controlling B Cell Differentiation and Activation

Emerging data highlight the crucial role of enzymes involved in amino acid metabolism in immune cell biology. IL-4-induced gene-1 (IL4I1), a secreted l-phenylalanine oxidase expressed by APCs, has been detected in B cells, yet its immunoregulatory role has only been explored on T cells. In this study, we show that IL4I1 regulates multiple steps in B cell physiology. Indeed, IL4I1 knockout mice exhibit an accelerated B cell egress from the bone marrow, resulting in the accumulation of peripheral follicular B cells. They also present a higher serum level of natural Igs and self-reactive Abs. We also demonstrate that IL4I1 produced by B cells themselves controls the germinal center reaction, plasma cell differentiation, and specific Ab production in response to T dependent Ags, SRBC, and NP-KLH. In vitro, IL4I1-deficient B cells proliferate more efficiently than their wild-type counterparts in response to BCR cross-linking. Moreover, the absence of IL4I1 increases activation of the Syk-Akt-S6kinase signaling pathway and calcium mobilization, and inhibits SHP-1 activity upon BCR engagement, thus supporting that IL4I1 negatively controls BCR-dependent activation. Overall, our study reveals a new perspective on IL4I1 as a key regulator of B cell biology.

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.

Inhibition of Src homology 2 domain containing protein tyrosine phosphatase as the possible mechanism of metformin-assisted amelioration of obesity induced insulin resistance in high fat diet fed C57BL/6J mice

SHP-1 (Src homology 2 domain containing protein tyrosine phosphatase) is a known negative regulator of insulin signaling and inflammation. To date, the molecular mechanism of metformin in modulating SHP-1 expression has remained elusive. In the present study, we have investigated the role of SHP-1 in relation to anti-hyperglycemic and anti-inflammatory actions of metformin in an obese phenotype mouse model. We observed that metformin treatment significantly reduced SHP-1 activity in obese mice, leading to improved insulin sensitivity. Additionally, metformin down regulated inflammatory markers like TLR2, TLR4, CD80, CD86, NF-κB, STAT1 and suppressed adipose tissue inflammation by efficiently polarizing adipose tissue macrophages toward anti-inflammatory state by way of indirect inhibition of SHP-1 mRNA and protein expressions. Our study suggests that metformin exerts its insulin sensitizing effects via inhibition of SHP-1 activity and expression.

β-Arrestin 1-dependent regulation of Rap2 is required for fMLP-stimulated chemotaxis in neutrophil-like HL-60 cells

β-Arrestins have emerged as key regulators of cytoskeletal rearrangement that are required for directed cell migration. Whereas it is known that β-arrestins are required for formyl-Met-Leu-Phe receptor (FPR) recycling, less is known about their role in regulating FPR-mediated neutrophil chemotaxis. Here, we show that β-arrestin 1 (ArrB1) coaccumulated with F-actin within the leading edge of neutrophil-like HL-60 cells during chemotaxis, and its knockdown resulted in markedly reduced migration within fMLP gradients. The small GTPase Ras-related protein 2 (Rap2) was found to bind ArrB1 under resting conditions but dissociated upon fMLP stimulation. The FPR-dependent activation of Rap2 required ArrB1 but was independent of Gα_i activity. Significantly, depletion of either ArrB1 or Rap2 resulted in reduced chemotaxis and defects in cellular repolarization within fMLP gradients. These data strongly suggest a model in which FPR is able to direct ArrB1 and other bound proteins that are required for lamellipodial extension to the leading edge in migrating neutrophils, thereby orientating and directing cell migration.

Role of protein kinase D2 phosphorylation on Tyr in modulation by ghrelin of Helicobacter pylori-induced up-regulation in gastric mucosal matrix metalloproteinase-9 (MMP-9) secretion

Matrix metalloproteinas-9 (MMP-9) is a glycosylated endopeptidase associated with host reaction to microbial endotoxins and also characterizes gastric mucosal inflammatory response to H. pylori infection. Here, we report on the factors involved in gastric mucosal MMP-9 secretion in response to H. pylori LPS, and the effect of hormone, ghrelin. We show that both the LPS-elicited induction in MMP-9 secretion and also the modulatory influence of ghrelin occur at the level of MMP-9 processing between the endoplasmic reticulum (ER) and Golgi. Further, we demonstrate that the LPS effect is associated with up-regulation in the activation of Arf1, a small GTPase of the ADP-ribosylation factor family, and the recruitment and phosphorylation of protein kinase D2 (PKD2), involved in the secretory cargo processing in the Golgi. Moreover, we reveal that the LPS-induced up-regulation in MMP-9 secretion is reflected in a marked increase in PKCδ-mediated PKD2 phosphorylation on Ser, while the modulatory effect of ghrelin is manifested by the SFK-PTKs-dependent phosphorylation of PKD2 on Tyr. Thus, our findings demonstrate the role of Arf1/PKD2 in mediation of H. pylori LPS-induced up-regulation in gastric mucosal MMP-9 secretion and suggest the modulatory mechanism of ghrelin action.

Distinct phosphorylation sites on the ghrelin receptor, GHSR1a, establish a code that determines the functions of ß-arrestins

The growth hormone secretagogue receptor, GHSR1a, mediates the biological activities of ghrelin, which includes the secretion of growth hormone, as well as the stimulation of appetite, food intake and maintenance of energy homeostasis. Mapping phosphorylation sites on GHSR1a and knowledge of how these sites control specific functional consequences unlocks new strategies for the development of therapeutic agents targeting individual functions. Herein, we have identified the phosphorylation of different sets of sites within GHSR1a which engender distinct functionality of ß-arrestins. More specifically, the Ser³⁶², Ser³⁶³ and Thr³⁶⁶ residues at the carboxyl-terminal tail were primarily responsible for ß-arrestin 1 and 2 binding, internalization and ß-arrestin-mediated proliferation and adipogenesis. The Thr³⁵⁰ and Ser³⁴⁹ are not necessary for ß-arrestin recruitment, but are involved in the stabilization of the GHSR1a-ß-arrestin complex in a manner that determines the ultimate cellular consequences of ß-arrestin signaling. We further demonstrated that the mitogenic and adipogenic effect of ghrelin were mainly dependent on the ß-arrestin bound to the phosphorylated GHSR1a. In contrast, the ghrelin function on GH secretion was entirely mediated by G protein signaling. Our data is consistent with the hypothesis that the phosphorylation pattern on the C terminus of GHSR1a determines the signaling and physiological output.

Chronic central leptin infusion modulates the glycemia response to insulin administration in male rats through regulation of hepatic glucose metabolism

Leptin and insulin use overlapping signaling mechanisms to modify hepatic glucose metabolism, which is critical in maintaining normal glycemia. We examined the effect of an increase in central leptin and insulin on hepatic glucose metabolism and its influence on serum glucose levels. Chronic leptin infusion increased serum leptin and reduced hepatic SH-phosphotyrosine phosphatase 1, the association of suppressor of cytokine signaling 3 to the insulin receptor in liver and the rise in glycemia induced by central insulin. Leptin also decreased hepatic phosphoenolpyruvate carboxykinase levels and increased insulin's ability to phosphorylate insulin receptor substrate-1, Akt and glycogen synthase kinase on Ser9 and to stimulate glucose transporter 2 and glycogen levels. Peripheral leptin treatment reproduced some of these changes, but to a lesser extent. Our data indicate that leptin increases the hepatic response to a rise in insulin, suggesting that pharmacological manipulation of leptin targets may be of interest for controlling glycemia.

Trace amine-associated receptor 1 activation silences GSK3β signaling of TAAR1 and D2R heteromers

Trace amine-associated receptor 1 (TAAR1) activation by selective endogenous agonists modulates dopaminergic neurotransmission. This results in antipsychotic-like behavior in vivo which might be initiated by an interaction of TAAR1 and dopamine D2L receptor (D2R). Here we analyzed the functional link between TAAR1 and D2R using highly potent and selective TAAR1 agonists, and newly generated tools such as TAAR1 knock-out and TAAR1 overexpressing rats as well as specific anti-rat TAAR1 antibodies. We provide data from co-immunoprecipitation experiments supporting a functional interaction of the two receptors in heterologous cells and in brain tissue. Interaction of TAAR1 with D2R altered the subcellular localization of TAAR1 and increased D2R agonist binding affinity. Using specific β-arrestin 2 (βArr2) complementation assays we show that the interaction of TAAR1 with D2R reduced βArr2 recruitment to D2R. In addition, we report that besides Gαs-protein signaling TAAR1 also signals via βArr2. In the presence of D2R, cAMP signaling of TAAR1 was reduced while its βArr2 signaling was enhanced, resulting in reduced GSK3β activation. These results demonstrate that βArr2 signaling may be an important pathway for TAAR1 function and that the activation of the TAAR1-D2R complex negatively modulates GSK3β signaling. Given that patients with schizophrenia or bipolar disorder show increased GSK3β signaling, such a reduction of GSK3β signaling triggered by the interaction of D2R with activated TAAR1 further supports TAAR1 as a target for the treatment of psychiatric disorders.

Regulatory role of guanine nucleotide exchange factor (GEF) Dock180 phosphorylation on Tyr/Ser in mediation of gastric mucosal Rac1 activation in response to Helicobacter pylori and ghrelin

A small GTPase, Rac1, is recognized as an important modulator of the inflammatory responses to bacterial lipopolysaccharide (LPS) by affecting the processes of phospholipase C activation. The activation of Rac1 involves the exchange of GDP for GTP and is catalyzed by the guanine nucleotide exchange factors (GEFs). Here, we report on the gastric mucosal GEF, Dock180, activation in response to H. pylori PS, and the hormone, ghrelin. We show that stimulation of gastric mucosal cells with the LPS leads to up-regulation in Dock180 phosphorylation on Tyr and Ser that is accompanied by a massive rise in Rac1-GTP level, while the effect of ghrelin, manifested by a drop in Dock180 phosphorylation on Ser, is associated with a decrease in Rac1-GTP formation. Furthermore, we demonstrate that phosphorylation on Tyr remains under the control of the Src family protein tyrosine kinases (SFK-PTKs), and is accompanied by Dock180 membrane translocation, while phosphorylation of the membrane-localized Dock180 on Ser represents the stimulatory contribution of protein kinase Cδ (PKCδ) to Dock180 activation. Moreover, we reveal that the interaction between Dock180 and PKCδ is dependent on Dock180 Tyr phosphorylation as well as the activity of PKCδ. Thus, our findings point to the involvement of PKCδ in the LPS-induced up-regulation of Dock180 activation, and suggest the modulatory mechanism of ghrelin influence on the gastric mucosal inflammatory responses to H. pylori.

Mechanism of Rac1-induced amplification in gastric mucosal phospholipase Cγ2 activation in response to Helicobacter pylori: modulatory effect of ghrelin

Membrane recruitment followed by targeted phosphorylation of specific Tyr and Ser residues and the interaction with Rac GTPases are the crucial parts of an elaborate mechanism of PLCγ2 activation essential for its role in linking the specific receptor responses to a variety of hormones and bacterial endotoxins with the intended intracellular targets. Here, we explored the involvement of Rac in mediation of PLCγ2 activation associated with gastric mucosal inflammatory responses to H. pylori LPS and the hormone, ghrelin. We show that stimulation of gastric mucosal cells with the LPS leads to the membrane translocation of Rac1 as well as PLCγ2, while the effect of ghrelin is manifested by elevation in the membrane PLCγ2 activation and suppression in Rac1 translocation. However, blocking the LPS-induced Rac1 translocation, while detrimental to the PLCγ2 activation, has no effect on its membrane translocation. We reveal further that PLCγ2, localized in the membrane in association with Rac1 following the LPS stimulation, exhibits a marked increase in phosphorylation on Ser, while the modulatory effect of ghrelin, manifested by a drop in Rac1 translocation, is associated with a distinct decrease in PLCγ2 phosphorylation on Ser. Thus, the results suggest that H. pylori-elicited increase in gastric mucosal PLCγ2 phosphorylation on Ser serves as an essential platform for Rac1 colocalization and amplification in PLCγ2 activation.

Role of amplification in phospholipase Cγ2 activation in modulation of gastric mucosal inflammatory responses to Helicobacter pylori: effect of ghrelin

Phosphoinositide-specific phospholipase C (PLC) enzymes are crucial elements of signal transduction pathways that provide a common link of communication integrating specific receptor responses to a variety of hormones, growth factors, and bacterial endotoxins with the intended intracellular targets. Here, we examined the involvement of PLC in modulation of gastric mucosal inflammatory responses to Helicobacter pylori LPS by peptide hormone, ghrelin. We show that stimulation of gastric mucosal cells with the LPS leads to the activation and membrane translocation of the γ2 isoform of PLC, phosphorylated on Tyr as well as Ser, while the effect of ghrelin is reflected in the translocation and phosphorylation of membrane-associated PLCγ2 on Tyr mainly. Moreover, we demonstrate that PLCγ2 phosphorylation on Tyr remains under the control of the Src family protein tyrosine kinases (SFK-PTKs), and is intimately linked to PLCγ2 membrane localization, while the LPS-induced phosphorylation of membrane-recruited PLCγ2 on Ser displays dependence on protein kinase Cδ (PKCδ) and leads to the amplification in PLCγ2 activation. Thus, our findings link the extent of H. pylori-elicited gastric mucosal inflammatory involvement to the PKCδ-mediated amplification in PLCγ2 activation through phosphorylation on Ser.

Modulation of gastric mucosal inflammatory responses to Helicobacter pylori via ghrelin-induced protein kinase Cδ tyrosine phosphorylation

A peptide hormone, ghrelin, plays a key role in modulation of gastric mucosal inflammatory responses to Helicobacter pylori by controlling the activation of constitutive nitric oxide synthase via Src/Akt-dependent phosphorylation that requires phosphatidylinositol 3-kinase (PI3K) participation. Here, we examined the relationship among PI3K; its upstream effector, protein kinase C (PKC); and cSrc. We show that stimulation of gastric mucosal cells with H. pylori LPS leads to the activation and membrane translocation of Ser-phosphorylated PKCδ, while the effect of ghrelin is reflected in the phosphorylation of membrane-associated PKCδ on Tyr. Further, we demonstrate that in response to the LPS-induced PKCδ activation both PI3K and Src show a marked increase in their Ser phosphorylation, while the effect of ghrelin is manifested in the phosphorylation of PI3K and cSrc at Tyr. Moreover, whereas Tyr phosphorylation of PKCδ exhibited susceptibility to cSrc inhibitor (PP2), the inhibitor of PKC (GF109203X) but not that of cSrc (PP2) blocked the Tyr phosphorylation of PI3K, while ghrelin-induced cSrc phosphorylation at Tyr was subject to inhibition by the inhibitors of PKC and PI3K. Thus, our findings stipulate the prerequisite of PKCδ in the activation of PI3K as well as cSrc, and imply that PI3K activation provides an essential platform for ghrelin-induced cSrc activation through autophosphorylation at Tyr(416). We also reveal that ghrelin-elicited up-regulation in PKCδ activation by Tyr phosphorylation shows dependence on cSrc activity.

Ghrelin modulates lateral amygdala neuronal firing and blocks acquisition for conditioned taste aversion

Ghrelin is an orexigenic brain-gut hormone promoting feeding and regulating energy metabolism in human and rodents. An increasing number of studies have reported that ghrelin and its identified receptor, the growth hormone secretagogue receptor 1a (GHS-R1a), produces remarkably wide and complex functions and biological effects on specific populations of neurons in central nervous system. In this study, we sought to explore the in vivo effects of acute ghrelin exposure on lateral amygdala (LA) neurons at the physiological and behavioral levels. In vivo extracellular single-unit recordings showed that ghrelin with the concentration of several nanomolars (nM) stimulated spontaneous firing of the LA neurons, an effect that was dose-dependent and could be blocked by co-application of a GHS-R1a antagonist D-Lys3-GHRP-6. We also found that D-Lys3-GHRP-6 inhibited spontaneous firing of the LA neurons in a dose-dependent manner, revealing that tonic GHS-R1a activity contributes to orchestrate the basal activity of the LA neurons. Behaviorally, we found that microinfusion of ghrelin (12 ng) into LA before training interfered with the acquisition of conditioned taste aversion (CTA) as tested at 24 h after conditioning. Pre-treatment with either purified IgG against GHS-R1a or GHS-R1a antagonist blocked ghrelin’s effect on CTA memory acquisition. Ghrelin (12 ng) had no effect on CTA memory consolidation or the expression of acquired CTA memory; neither did it affect the total liquid consumption of tested rats. Altogether, our data indicated that ghrelin locally infused into LA blocks acquisition of CTA and its modulation effects on neuronal firing may be involved in this process.

β-Arrestin signal complex plays a critical role in adipose differentiation

β-Arrestins were identified as scaffold-proteins that have the capacity to desensitize G protein-coupled receptors. However, it has been found that β-arrestins activate signaling pathways independent of G protein activation. The diversity of these signaling pathways has also been recognized for receptor tyrosine kinase. The aim of the present study was to validate the β-arrestin-dependent signaling mechanism(s) responsible for regulation of adipogenesis. Two signal models were selected, ghrelin and insulin, based on its β-arrestin-associated Akt activity. Herein, we found that β-arrestin 1 and 2 were essential molecules for adipocyte differentiation. More specifically, the role of these scaffolding proteins was demonstrated by depletion of β-arrestin 1 and 2 during ghrelin-induced adipogenesis in 3T3-L1 cells, which decreased the adipocyte differentiation and the expression levels of master regulators of early, the CCAAT/enhancer-binding protein β (C/EBPβ) and the CCAAT/enhancer-binding protein δ (C/EBPδ), and terminal, the peroxisome proliferator-activated receptor (PPARγ) and the CCAAT/enhancer-binding protein α (C/EBPα), adipogenesis. Accordingly ghrelin-induced Akt activity and its downstream targets, the mammalian target of rapamycin complex 1 (mTORC1) and the ribosomal protein S6 kinase beta-1 (S6K1), were inhibited by β-arrestin 1 and 2 siRNAs. By contrast, assays performed during insulin-activated adipogenesis showed an intensifying effect on the adipocyte differentiation as well as on the expression of C/EBPβ, C/EBPδ, PPARγ and C/EBPα. The increase in insulin-induced adipogenesis by β-arrestin knock-down was concomitant to a decrease in the insulin receptor susbtrate-1 (IRS-1) serine phosphorylation, proving the loss of the negative feedback loop on IRS-1/phosphoinositide 3-kinase (PI3K)/Akt. Therefore, β-arrestins control the extent and intensity of the lipogenic and adipogenic factors associated to Akt signaling, although the mechanistic and functional principles that underlie the connection between signaling and β-arrestins are specifically associated to each receptor type.

Arrestins as regulators of kinases and phosphatases

The discovery that, in addition to mediating G protein-coupled receptor (GPCR) desensitization and endocytosis, arrestins bind to diverse catalytically active nonreceptor proteins and act as ligand-regulated signaling scaffolds led to a paradigm shift in the study of GPCR signal transduction. Research over the past decade has solidified the concept that arrestins confer novel GPCR-signaling capacity by recruiting protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein "signalsome" complexes. Signalsomes regulate downstream pathways controlled by Src family nonreceptor tyrosine kinases, mitogen-activated protein kinases, protein kinase B (AKT), glycogen synthase kinase 3, protein phosphatase 2A, nuclear factor-κB, and several others, imposing spatial and temporal control on their function. While many arrestin-bound kinases and phosphatases are involved in the control of cytoskeletal rearrangement, vesicle endocytosis, exocytosis, and cell migration, other signals reach into the nucleus, affecting cell proliferation, apoptosis, and survival. Indeed, the kinase/phosphatase network regulated by arrestins may be fully as diverse as that regulated by heterotrimeric G proteins.

SHPS-1 deficiency induces robust neuroprotection against experimental stroke by attenuating oxidative stress

Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), also known as Signal-regulatory protein alpha (SIRPα) or SIRPA is a transmembrane protein that is predominantly expressed in neurons, dendritic cells, and macrophages. This study was conducted to investigate the role of SHPS-1 in the oxidative stress and brain damage induced by acute focal cerebral ischemia. Wild-type (WT) and SHPS-1 mutant (MT) mice were subjected to middle cerebral artery occlusion (60 min) followed by reperfusion. SHPS-1 MT mice had significantly reduced infarct volumes and improved neurological function after brain ischemia. In addition, neural injury and oxidative stress were inhibited in SHPS-1 MT mice. The mRNA and protein levels of the antioxidant genes nuclear factor-E2-related factor 2 (Nrf2) and heme oxygenase 1 were up-regulated in SHPS-1 MT mice. The SHPS-1 mutation suppressed the phosphorylation of SHP-1 and SHP-2 and increased the phosphorylation of Akt and GSK3β. These results provide the first demonstration that SHPS-1 plays an important role in the oxidative stress and brain injury induced by acute cerebral ischemia. The activation of Akt signaling and the up-regulation of Nrf2 and heme oxygenase 1 likely account for the protective effects that were observed in the SHPS-1 MT mice.