Molecular determinants of selectivity in 5-hydroxytryptamine1B receptor-G protein interactions

Coevolution underlies GPCR-G protein selectivity and functionality

G protein-coupled receptors (GPCRs) regulate diverse physiological events, which makes them as the major targets for many approved drugs. G proteins are downstream molecules that receive signals from GPCRs and trigger cell responses. The GPCR-G protein selectivity mechanism on how they properly and timely interact is still unclear. Here, we analyzed model GPCRs (i.e. HTR, DAR) and Gα proteins with a coevolutionary tool, statistical coupling analysis. The results suggested that 5-hydroxytryptamine receptors and dopamine receptors have common conserved and coevolved residues. The Gα protein also have conserved and coevolved residues. These coevolved residues were implicated in the molecular functions of the analyzed proteins. We also found specific coevolving pairs related to the selectivity between GPCR and G protein were identified. We propose that these results would contribute to better understandings of not only the functional residues of GPCRs and Gα proteins but also GPCR-G protein selectivity mechanisms.

Rho GTPase regulation of reactive oxygen species generation and signalling in platelet function and disease

Platelets are master regulators and effectors of haemostasis with increasingly recognized functions as mediators of inflammation and immune responses. The Rho family of GTPase members Rac1, Cdc42 and RhoA are known to be major components of the intracellular signalling network critical to platelet shape change and morphological dynamics, thus playing a major role in platelet spreading, secretion and thrombus formation. Initially linked to the regulation of actomyosin contraction and lamellipodia formation, recent reports have uncovered non-canonical functions of platelet RhoGTPases in the regulation of reactive oxygen species (ROS), where intrinsically generated ROS modulate platelet function and contribute to thrombus formation. Platelet RhoGTPases orchestrate oxidative processes and cytoskeletal rearrangement in an interconnected manner to regulate intracellular signalling networks underlying platelet activity and thrombus formation. Herein we review our current knowledge of the regulation of platelet ROS generation by RhoGTPases and their relationship with platelet cytoskeletal reorganization, activation and function.

Folding of Gα Subunits: Implications for Disease States

G-proteins play a central role in signal transduction by fluctuating between "on" and "off" phases that are determined by a conformational change. cAMP is a secondary messenger whose formation is inhibited or stimulated by activated G_iα1 or G_sα subunit. We used tryptophan fluorescence, UV/vis spectrophotometry, and circular dichroism to probe distinct structural features within active and inactive conformations from wild-type and tryptophan mutants of G_iα1 and G_sα. For all proteins studied, we found that the active conformations were more stable than the inactive conformations, and upon refolding from higher temperatures, activated wild-type subunits recovered significantly more native structure. We also observed that the wild-type subunits partially regained the ability to bind nucleotide. The increased compactness observed upon activation was consistent with the calculated decrease in solvent accessible surface area for wild-type G_iα1. We found that as the temperature increased, G_α subunits, which are known to be rich in α-helices, converted to proteins with increased content of β-sheets and random coil. For active conformations from wild-type and tryptophan mutants of G_iα1, melting temperatures indicated that denaturation starts around hydrophobic tryptophan microenvironments and then radiates toward tyrosine residues at the surface, followed by alteration of the secondary structure. For G_sα, however, disruption of secondary structure preceded unfolding around tyrosine residues. In the active conformations, a π-cation interaction between essential arginine and tryptophan residues, which was characterized by a fluorescence-measured red shift and modeled by molecular dynamics, was also shown to be a contributor to the stability of G_α subunits. The folding properties of G_α subunits reported here are discussed in the context of diseases associated to G-proteins.

Gnaq and Gna11 in the Endothelin Signaling Pathway and Melanoma

In this article, we first briefly outline the function of G protein coupled receptors in cancer, and then specifically examine the roles of the seven transmembrane G protein coupled Endothelin B receptor (Ednrb) and the G proteins, GNAQ and GNA11, in both melanocyte development and melanoma. Ednrb plays an essential role in melanocyte development. GNAQ and GNA11 are oncogenes when mutated in certain types of melanocytic lesions, being extremely frequent in uveal melanoma, which forms from melanocytes located in the eye. Previously, we reported that in mice, Schwann cell precursor derived melanocytes colonize the dermis and hair follicles, while the inter-follicular epidermis is populated by other melanocytes. A pattern has emerged whereby melanocytes whose activities are affected by gain-of-function mutations of the Endothelin 3 ligand and Gα_q/11 are the same subset that arise from Schwann cell precursors. Furthermore, the forced expression of the constitutively active human GNAQ^Q209L oncogene in mouse melanocytes only causes hyper-proliferation in the subset that arise from Schwann cell precursors. This has led us to hypothesize that in Schwann cell precursor derived melanocytes, Ednrb signals through Gα_q/11. Ednrb is promiscuous and may signal through other G protein alpha subunits in melanomas located in the inter-follicular epidermis.

Peroxiredoxin 6 triggers melanoma cell growth by increasing arachidonic acid-dependent lipid signalling

Tumour cells are reported to display an imbalance in the levels of ROS (reactive oxygen species). Frequently, elevated ROS production goes along with compensatory up-regulation of antioxidant enzymes. Accordingly, we found in a previous study that protein levels of several peroxiredoxins, including PRDX6 (peroxiredoxin 6), are highly elevated in experimentally induced melanomas. In the present study, we investigated the functional role of PRDX6 in human melanoma cells. PRDX6 is a bifunctional enzyme, which harbours iPLA2 (Ca(2+)-independent phospholipase A2) activity in addition to its peroxidase function. Our results show that PRDX6 is strongly expressed in most melanoma cells and its expression levels are maintained in a post-transcriptional manner, particularly by EGFR (epidermal growth factor receptor)-dependent signalling. PRDX6 enhances cell viability mainly by enhancing proliferation, which goes along with activation of Src family kinases. Interestingly, we were able to show that the phospholipase activity of the enzyme mediates the pro-proliferative effect of PRDX6. We identified AA (arachidonic acid) as a crucial effector of PRDX6-dependent proliferation and inducer of Src family kinase activation. These results support further the biological importance of the emerging field of lipid signalling in melanoma and highlight the particular functional relevance of PRDX6-dependent phospholipase activity.

ROS signalling, NADPH oxidases and cancer

ROS (reactive oxygen species) have long been regarded as a series of destructive molecules that have a detrimental effect on cell homoeostasis. In support of this are the myriad antioxidant defence systems nearly all eukaryotic cells have that are designed to keep the levels of ROS in check. However, research data emerging over the last decade have demonstrated that ROS can influence a range of cellular events in a manner similar to that seen for traditional second messenger molecules such as cAMP. Hydrogen peroxide (H2O2) appears to be the main ROS with such signalling properties, and this molecule has been shown to affect a wide range of cellular functions. Its localized synthesis by the Nox (NADPH oxidase) family of enzymes and how these enzymes are regulated is of particular interest to those who work in the field of tumour biology.

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

Mitochondria as signaling organelles

Almost 20 years ago, the discovery that mitochondrial release of cytochrome c initiates a cascade that leads to cell death brought about a wholesale change in how cell biologists think of mitochondria. Formerly viewed as sites of biosynthesis and bioenergy production, these double membrane organelles could now be thought of as regulators of signal transduction. Within a few years, multiple other mitochondria-centric signaling mechanisms have been proposed, including release of reactive oxygen species and the scaffolding of signaling complexes on the outer mitochondrial membrane. It has also been shown that mitochondrial dysfunction causes induction of stress responses, bolstering the idea that mitochondria communicate their fitness to the rest of the cell. In the past decade, multiple new modes of mitochondrial signaling have been discovered. These include the release of metabolites, mitochondrial motility and dynamics, and interaction with other organelles such as endoplasmic reticulum in regulating signaling. Collectively these studies have established that mitochondria-dependent signaling has diverse physiological and pathophysiological outcomes. This review is a brief account of recent work in mitochondria-dependent signaling in the historical framework of the early studies.

A transient interaction between the phosphate binding loop and switch I contributes to the allosteric network between receptor and nucleotide in Gαi1

Receptor-mediated activation of the Gα subunit of heterotrimeric G proteins requires allosteric communication between the receptor binding site and the guanine nucleotide binding site, which are separated by >30 Å. Structural changes in the allosteric network connecting these sites are predicted to be transient in the wild-type Gα subunit, making studies of these connections challenging. In the current work, site-directed mutants that alter the energy barriers between the activation states are used as tools to better understand the transient features of allosteric signaling in the Gα subunit. The observed differences in relative receptor affinity for intact Gαi1 subunits versus C-terminal Gαi1 peptides harboring the K345L mutation are consistent with this mutation modulating the allosteric network in the protein subunit. Measurement of nucleotide exchange rates, affinity for metarhodopsin II, and thermostability suggest that the K345L Gαi1 variant has reduced stability in both the GDP-bound and nucleotide-free states as compared with wild type but similar stability in the GTPγS-bound state. High resolution x-ray crystal structures reveal conformational changes accompanying the destabilization of the GDP-bound state. Of these, the conformation for Switch I was stabilized by an ionic interaction with the phosphate binding loop. Further site-directed mutagenesis suggests that this interaction between Switch I and the phosphate binding loop is important for receptor-mediated nucleotide exchange in the wild-type Gαi1 subunit.

A yeast screening method to decipher the interaction between the adenosine A2B receptor and the C-terminus of different G protein α-subunits

The expression of human G protein-coupled receptors (GPCRs) in Saccharomyces cerevisiae containing chimeric yeast/mammalian Gα subunits provides a useful tool for the study of GPCR activation. In this study, we used a one-GPCR-one-G protein yeast screening method in combination with molecular modeling and mutagenesis studies to decipher the interaction between GPCRs and the C-terminus of different α-subunits of G proteins. We chose the human adenosine A2B receptor (hA2BR) as a paradigm, a typical class A GPCR that shows promiscuous behavior in G protein coupling in this yeast system. The wild-type hA2BR and five mutant receptors were expressed in 8 yeast strains with different humanized G proteins, covering the four major classes: Gαi, Gαs, Gαq, and Gα12. Our experiments showed that a tyrosine residue (Y) at the C-terminus of the Gα subunit plays an important role in controlling the activation of GPCRs. Receptor residues R103(3.50) and I107(3.54) are vital too in G protein-coupling and the activation of the hA2BR, whereas L213(IL3) is more important in G protein inactivation. Substitution of S235(6.36) to alanine provided the most divergent G protein-coupling profile. Finally, L236(6.37) substitution decreased receptor activation in all G protein pathways, although to a different extent. In conclusion, our findings shed light on the selectivity of receptor/G protein coupling, which may help in further understanding GPCR signaling.

Ras GTPases are both regulators and effectors of redox agents

Redox agents have been historically considered pathological agents which can react with and damage many biological macromolecules including DNA, proteins, and lipids. However, a growing number of reports have suggested that mammalian cells can rapidly respond to ligand stimulation with a change in intracellular ROS thus indicating that the production of intracellular redox agents is tightly regulated and that they serve as intracellular signaling molecules being involved in a variety of cell signaling pathways. Numerous observations have suggested that some members of the Ras GTPase superfamily appear to regulate the production of redox agents and that oxidants can function as effector molecules for the small GTPases, thus contributing to their overall biological function. In addition, many of the Ras superfamily small GTPases have been shown to be redox sensitive, thanks to the presence of redox-sensitive sequences in their primary structure. The action of redox agents on these redox-sensitive GTPases is similar to that of guanine nucleotide exchange factors in that they perturb GTPase nucleotide-binding interactions that result in the enhancement of the guanine nucleotide exchange of small GTPases. Thus, Ras GTPases may act both as upstream regulators and downstream effectors of redox agents. Here we overview current understanding concerning the interplay between Ras GTPases and redox agents, also taking into account pathological implications of misregulation of this cross talk and highlighting the potentiality of these cellular pathways as new therapeutical targets for different pathologies.

DPMA, a deoxypodophyllotoxin derivative, induces apoptosis and anti-angiogenesis in non-small cell lung cancer A549 cells

We found that the deoxypodophyllotoxin derivative, 2,6-dimethoxy-4-(6-oxo-(5R,5aR,6,8,8aR,9-hexahydrofuro[3',4':6,7]naphtho[2,3-d][1,3]dioxol-5-yl)phenyl ((R)-1-amino-4-(methylthio)-1-oxobutan-2-yl)carbamate (DPMA), exhibited superior cytotoxicity compared with etoposide. In this study, we investigated the mechanism of action of DPMA. DPMA exhibited anti-proliferative activity and induced apoptosis in A549 cells in a dose- and time-dependant manner. DPMA inhibited microtubule formation and induced expression of Bax, cleaved caspase-3, p53 and ROS, and inhibited Bcl-2 expression. DPMA also affected cyclinB1, cdc2 and p-cdc2 expression, inducing cell cycle arrest. DPMA also inhibited tube formation of VEGF-induced human umbilical vein endothelial cells. These studies demonstrate that DPMA inhibits p53/cdc2/Bax signaling, thereby inhibiting cell growth/angiogenesis and inducing apoptosis.

Extraction optimization of carbohydrate compound from Huangqi using orthogonal design

The effect of extraction time, extraction temperature and time on the yield of Huangqi carbohydrate compound (HQCC) was investigated using single factor and orthogonal experiment design. The influence by the parameters on the extraction yields of carbohydrate compound decreased in the order of: C (extraction number)>A (extraction time)>B (extraction temperature) according to the R values. Based on this analysis, and considering the carbohydrate compound extraction efficiency, the cost of energy and the feasibility of experiment, the optimum conditions of extraction were therefore determined as follows: extraction time 120min, extraction temperature 80°C, and extraction number 4. Oral administration of HQCC reduced lipid peroxidation level and enhanced antioxidant enzymes activities in gastric mucosa. In addition, HQCC reduced the serum IL-8 and TNF-α levels. In conclusion, these data reveal that HQCC promotes regeneration of damaged gastric mucosa, probably through its antioxidative mechanism.

Real-time imaging elucidates the role of H2O2 in regulating kinetics of epidermal growth factor-induced and Src-mediated tyrosine phosphorylation signaling

Reversible oxidation is emerging as an important regulatory mechanism in protein tyrosine phosphorylation. Generation of hydrogen peroxide (H(2)O(2)), upon growth factor stimulation, is hypothesized to inhibit activity of protein tyrosine phosphatases (PTPs). This ensures that protein tyrosine kinases can elevate the steady-state level of protein tyrosine phosphorylation, which then allows propagation of the tyrosine phosphorylation signal. However, the effects of H(2)O(2) on the kinetics of tyrosine phosphorylation signaling remain poorly understood, especially in living cells. Therefore, we used a genetically encoded Src kinase-specific biosensor based on fluorescence resonance energy transfer (FRET) to image the kinetics of the Src-mediated tyrosine phosphorylation signaling (Src signaling) induced by epidermal growth factor (EGF). We examined the kinetics under increased and decreased H(2)O(2) levels. Through a straightforward, quantitative analysis method which characterized the signaling kinetics, we demonstrated that H(2)O(2) modulated the amplitude and duration of the signal by inhibiting PTPs' activity. Our evidence also suggested the effect of H(2)O(2) on Src activation is mediated by H(2)O(2)-dependent inhibition of PTPs. Furthermore, we provide evidence showing global elevation of intracellular H(2)O(2) level attenuates EGF-induced Src signaling.

Plant polyphenols regulate chemokine expression and tissue repair in human keratinocytes through interaction with cytoplasmic and nuclear components of epidermal growth factor receptor system

AIMS

To evaluate mechanisms underlying modulation of inflammatory chemokines in primary human keratinocytes (normal human epidermal keratinocytes) and repair-related processes in wound models by plant polyphenols (PPs) with antioxidant and superoxide scavenging properties (verbascoside [Vb], resveratrol [Rv], polydatin [Pd], quercetin [Qr], and rutin).

RESULTS

Epidermal growth factor receptor (EGFR)-controlled chemokines CXCL8/interleukin 8 (IL-8), CCL2/monocyte chemotactic protein-1 (MCP-1), and CXCL10/interferon gamma-produced protein of 10 kDa (IP-10) were modulated by transforming growth factor alpha (TGF-α) and by the tumor necrosis factor alpha/interferon gamma combination (T/I). EGFR phosphorylation, nuclear translocation, and downstream cytoplasmic signaling pathways (extracellular regulation kinase [ERK]1/2, p38, STAT3, and PI-3K) were studied. All PPs did not affect TGF-α-induced STAT3 phosphorylation, whereas they suppressed T/I-activated NFkappaB and constitutive and T/I-induced but not TGF-α-induced ERK1/2 phosphorylation. Vb and Qr suppressed total EGFR phosphorylation, but they synergized with TGF-α to enhance nuclear accumulation of phosphorylated EGFR. Vb strongly inhibited TGF-α-induced p38 phosphorylation and T/I-induced NFkappaB and activator protein-1 (AP-1) binding to DNA. Vb was an effective inhibitor of T/I-stimulated chemokine synthesis, and it accelerated scratch wound healing in vitro. Anti-inflammatory and wound healing activities of Vb were confirmed in vivo in the full-thickness excision wound. Although Pd and Rv did not affect EGFR activation/translocation, they and Qr synergized with TGF-α and T/I in the induction of IL-8 transcription/synthesis while opposing enhanced MCP-1 and IP-10 transcription/synthesis connected with pharmacologically impaired EGFR functioning.

INNOVATION

PPs perturb the EGFR system in human keratinocytes, and this effect may be implicated in the regulation of inflammatory and repair-related processes in the skin.

CONCLUSION

Anti-inflammatory and wound healing effects of PPs depend on their interaction with EGFR-controlled cytoplasmic and nuclear pathways rather than on their direct redox properties.

Tyr740 and Tyr751 residues of platelet-derived growth factor beta receptor are responsible for the redox regulation of phosphatase and tensin homolog in the cells stimulated with platelet-derived growth factor

Exposure of cells to hydrogen peroxide or platelet-derived growth factor (PDGF) induced Akt phosphorylation and oxidation of phosphatase and tensin homolog (PTEN). The Cys124 and Cys71 residues of PTEN were critical for the formation of a disulfide bond and the intermediate glutathionylation in the process of reduction of the disulfide bond. To determine which specific tyrosine residues of the PDGF beta receptor (PDGFβR) is involved in PDGF-induced PTEN oxidation and Akt phosphorylation, we investigated a kinase activity-deficient mutant and PDGFβR mutants where the tyrosine residues in the binding site for phosphoinositide 3-kinase (PI3K), GTPase-activating protein of Ras, Src homology 2 domain containing protein-tyrosine phosphatase-2, and phospholipase C-1 were replaced by Phe. Both PTEN oxidation and Akt phosphorylation did not occur in response to PDGF in the kinase-deficient mutant and in the PDGFβR mutant with a mutation in the PI3K binding site (Tyr740 and Tyr751). Thus, the kinase activity and the constituent Tyr740 and Tyr751 residues of PDGFβR in the cells stimulated with PDGF are responsible for the oxidation of PTEN and the Akt phosphorylation.

Ionizing radiation-induced metabolic oxidative stress and prolonged cell injury

Cellular exposure to ionizing radiation leads to oxidizing events that alter atomic structure through direct interactions of radiation with target macromolecules or via products of water radiolysis. Further, the oxidative damage may spread from the targeted to neighboring, non-targeted bystander cells through redox-modulated intercellular communication mechanisms. To cope with the induced stress and the changes in the redox environment, organisms elicit transient responses at the molecular, cellular and tissue levels to counteract toxic effects of radiation. Metabolic pathways are induced during and shortly after the exposure. Depending on radiation dose, dose-rate and quality, these protective mechanisms may or may not be sufficient to cope with the stress. When the harmful effects exceed those of homeostatic biochemical processes, induced biological changes persist and may be propagated to progeny cells. Physiological levels of reactive oxygen and nitrogen species play critical roles in many cellular functions. In irradiated cells, levels of these reactive species may be increased due to perturbations in oxidative metabolism and chronic inflammatory responses, thereby contributing to the long-term effects of exposure to ionizing radiation on genomic stability. Here, in addition to immediate biological effects of water radiolysis on DNA damage, we also discuss the role of mitochondria in the delayed outcomes of ionization radiation. Defects in mitochondrial functions lead to accelerated aging and numerous pathological conditions. Different types of radiation vary in their linear energy transfer (LET) properties, and we discuss their effects on various aspects of mitochondrial physiology. These include short and long-term in vitro and in vivo effects on mitochondrial DNA, mitochondrial protein import and metabolic and antioxidant enzymes.

Simvastatin inhibits osteoclast differentiation by scavenging reactive oxygen species

Osteoclasts, together with osteoblasts, control the amount of bone tissue and regulate bone remodeling. Osteoclast differentiation is an important factor related to the pathogenesis of bone-loss related diseases. Reactive oxygen species (ROS) acts as a signal mediator in osteoclast differentiation. Simvastatin, which inhibits 3-hydroxy-3-methylglutaryl coenzyme A, is a hypolipidemic drug which is known to affect bone metabolism and suppresses osteoclastogenesis induced by receptor activator of nuclear factor-κB ligand (RANKL). In this study, we analyzed whether simvastatin can inhibit RANKL-induced osteoclastogenesis through suppression of the subsequently formed ROS and investigated whether simvastatin can inhibit H2O2-induced signaling pathways in osteoclast differentiation. We found that simvastatin decreased expression of tartrate-resistant acid phosphatase (TRAP), a genetic marker of osteoclast differentiation, and inhibited intracellular ROS generation in RAW 264.7 cell lines. ROS generation activated NF-κB, protein kinases B (AKT), mitogen-activated protein kinases signaling pathways such as c-JUN N-terminal kinases, p38 MAP kinases as well as extracellular signal- regulated kinase. Simvastatin was found to suppress these H2O2-induced signaling pathways in osteoclastogenesis. Together, these results indicate that simvastatin acts as an osteoclastogenesis inhibitor through suppression of ROS-mediated signaling pathways. This indicates that simvastatin has potential usefulness for osteoporosis and pathological bone resorption.

Chronic inflammatory disorders and their redox control: from molecular mechanisms to therapeutic opportunities

A chronic inflammatory disease is a condition characterized by persistent inflammation. A number of human pathologies fall into this category, and a great deal of research has been conducted to learn more about their characteristics and underlying mechanisms. In many cases, a genetic component has been identified, but also external factors like food, smoke, or environmental pollutants can significantly contribute to worsen their symptoms. Accumulated evidence clearly shows that chronic inflammatory diseases are subjected to a redox control. Here, we shall review the identity, source, regulation, and biological activity of redox molecules, to put in a better perspective their key-role in cancer, diabetes, cardiovascular diseases, atherosclerosis, chronic obstructive pulmonary diseases, and inflammatory bowel diseases. In addition, the impact of redox species on autoimmune disorders (rheumatoid arthritis, systemic lupus erythematosus, psoriasis, and celiac disease) and neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis) will be discussed, along with their potential therapeutic implications as novel drugs to combat chronic inflammatory disorders.

Signal transduction by reactive oxygen species

Although historically viewed as purely harmful, recent evidence suggests that reactive oxygen species (ROS) function as important physiological regulators of intracellular signaling pathways. The specific effects of ROS are modulated in large part through the covalent modification of specific cysteine residues found within redox-sensitive target proteins. Oxidation of these specific and reactive cysteine residues in turn can lead to the reversible modification of enzymatic activity. Emerging evidence suggests that ROS regulate diverse physiological parameters ranging from the response to growth factor stimulation to the generation of the inflammatory response, and that dysregulated ROS signaling may contribute to a host of human diseases.

Diverse injurious stimuli reduce protein tyrosine phosphatase-μ expression and enhance epidermal growth factor receptor signaling in human airway epithelia

In response to injury, airway epithelia utilize an epidermal growth factor (EGF) receptor (EGFR) signaling program to institute repair and restitution. Protein tyrosine phosphatases (PTPs) counterregulate EGFR autophosphorylation and downstream signaling. PTPμ is highly expressed in lung epithelia and can be localized to intercellular junctions where its ectodomain homophilically interacts with PTPμ ectodomain expressed on neighboring cells. We asked whether PTPμ expression might be altered in response to epithelial injury and whether altered PTPμ expression might influence EGFR signaling. In A549 cells, diverse injurious stimuli dramatically reduced PTPμ protein expression. Under basal conditions, small interfering RNA (siRNA)-induced silencing of PTPμ increased EGFR Y992 and Y1068 phosphorylation. In the presence of EGF, PTPμ knockdown increased EGFR Y845, Y992, Y1045, Y1068, Y1086, and Y1173 but not Y1148 phosphorylation. Reduced PTPμ expression increased EGF-stimulated phosphorylation of Y992, a docking site for phospholipase C (PLC)γ(1), activation of PLCγ(1) itself, and increased cell migration in both wounding and chemotaxis assays. In contrast, overexpression of PTPμ decreased EGF-stimulated EGFR Y992 and Y1068 phosphorylation. Therefore, airway epithelial injury profoundly reduces PTPμ expression, and PTPμ depletion selectively increases phosphorylation of specific EGFR tyrosine residues, PLCγ(1) activation, and cell migration, providing a novel mechanism through which epithelial integrity may be restored.

[Reactive oxygen species and oxidative stress]

Oxidative stress is defined as an imbalance between the production of reactive oxygen species (ROS) and the antioxidant capacity of the cell. For long, ROS have been considered as harmful by-products of the normal aerobic metabolism process of the mitochondria, implicated in a large variety of diseases. But there are now growing evidences that controlled ROS production also play physiological roles especially in regulating cell redox homeostasis and cell signaling. Biological ROS effects are now well documented. Data show that living organisms have not only adapted themselves to coexist with free radicals but have also developed mechanisms to use them advantageously. However their main sources and mechanisms of action remain poorly described. This review focuses on the main properties of ROS and their paradoxical effects.

Crosstalks between the receptors tyrosine kinase EGFR and TrkA and the GPCR, FPR, in human monocytes are essential for receptors-mediated cell activation

The G-protein coupled receptor (GPCR) fMLP receptor (FPR) and the two receptors tyrosine kinase (RTK), the nerve growth factor (NGF) receptor TrkA and the epidermal growth factor (EGF) receptor (EGFR) are involved in reactive oxygen species (ROS), matrix metalloproteinase-9 (MMP-9) production and CD11b membrane integrin upregulation. We show that in monocytes the three receptors crosstalk each other to modulate these pro-inflammatory mediators. Tyrphostin AG1478, the EGFR inhibitor, inhibits fMLP and NGF-associated ROS production, fMLP-associated CD11b upregulation and NGF-induced TrkA phosphorylation; K252a, the NGF receptor inhibitor, inhibits fMLP or EGF-associated ROS production, CD11b expression and EGF-induced EGFR phosphorylation; cyclosporine H, the FPR inhibitor inhibits EGF or NGF-associated ROS production, EGF-associated CD11b upregulation and prevents EGFR and TrkA phosphorylation by their respective ligand EGF and NGF. In response to fMLP, TrkA phosphorylation is inhibited by the EGFR inhibitor while EGFR phosphorylation is inhibited by the TrkA inhibitor. Receptor crosstalks are Src and ERK dependent. Down-regulation of each receptor by specific siRNA suppresses the ability of the two other receptors to promote ligand-mediated ERK phosphorylation and pro-inflammatory activities including ROS, MMP-9 production and CD11b upregulation. Thus, in monocytes GPCR ligands' activity involves activation of RTK while RTK-ligands activity engages GPCR-signalling molecules.

Epicatechin gallate suppresses oxidative stress-induced MUC5AC overexpression by interaction with epidermal growth factor receptor

The goal of this study was to investigate the effect of epicatechin gallate (ECG), a component of green tea polyphenols, on the signal pathway for oxidative stress-induced intracellular reactive oxygen species (ROS) generation and MUC5AC overexpression in normal human nasal epithelial (NHNE) cells. Passage-2 NHNE cells were used, and ECG was administered before stimulation with exogenous hydrogen peroxide (H(2)O(2)). MUC5AC gene and protein levels were measured by real-time PCR and dot blot analysis. Western blot analysis and immunocytofluorescence study were performed for detecting the activity of epidermal growth factor receptor (EGFR). Exogenous H(2)O(2) increases intracellular ROS generation, leading to the overexpression of MUC5AC. The phosphorylation and internalization of EGFR were associated with this ROS generation. ECG decreased the phosphorylation and internalization of EGFR at the cell surface of NHNE cells, resulting in the attenuation of exogenous H(2)O(2)-induced intracellular ROS generation and MUC5AC overexpression. ECG may be a therapeutic material against oxidative stress-induced ROS generation and mucus hypersecretion in airways.

Reactive Oxygen Species–Mediated Signal Transduction in the Endothelium

The endothelium is an important component of vascular homeostasis that is a target for injury in the setting of vascular disease. One means of promoting a maladaptive endothelial cell phenotype such as that seen in atherosclerosis is excess oxidative stress. Although this term once was almost exclusively used to describe low-density lipoprotein (LDL) and lipid oxidation in the vasculature, we now understand that the intracellular oxidant milieu is an important modulator of vascular cell function. Indeed, considerable data indicate that reactive oxygen species (ROS) are an important means of cellular signaling, although the precise mechanisms whereby ROS accomplish this are still under investigation. In this review, the data linking ROS to kinase activation and cell signaling in the endothelium is discussed, with a particular emphasis on the roles of protein thiol modification.

Regulation of autophagy by reactive oxygen species (ROS): implications for cancer progression and treatment

Reactive oxygen species (ROS) have been identified as signaling molecules in various pathways regulating both cell survival and cell death. Autophagy, a self-digestion process that degrades intracellular structures in response to stress, such as nutrient starvation, is also involved in both cell survival and cell death. Alterations in both ROS and autophagy regulation contribute to cancer initiation and progression, and both are targets for developing therapies to induce cell death selectively in cancer cells. Many stimuli that induce ROS generation also induce autophagy, including nutrient starvation, mitochondrial toxins, hypoxia, and oxidative stress. Some of these stimuli are under clinical investigation as cancer treatments, such as 2-methoxyestrodial and arsenic trioxide. Recently, it was demonstrated that ROS can induce autophagy through several distinct mechanisms involving Atg4, catalase, and the mitochondrial electron transport chain (mETC). This leads to both cell-survival and cell-death responses and could be selective toward cancer cells. In this review, we give an overview of the roles ROS and autophagy play in cell survival and cell death, and their importance to cancer. Furthermore, we describe how autophagy is mediated by ROS and the implications of this regulation to cancer treatments.

Mutations on the Switch III region and the alpha3 helix of Galpha16 differentially affect receptor coupling and regulation of downstream effectors

Background

Gα₁₆ can activate phospholipase Cβ (PLCβ) directly like Gα_q. It also couples to tetratricopeptide repeat 1 (TPR1) which is linked to Ras activation. It is unknown whether PLCβ and TPR1 interact with the same regions on Gα₁₆. Previous studies on Gα_q have defined two minimal clusters of amino acids that are essential for the coupling to PLCβ. Cognate residues in Gα₁₆ might also be essential for interacting with PLCβ, and possibly contribute to TPR1 interaction and other signaling events.

Results

Alanine mutations were introduced to the two amino acid clusters (246–248 and 259–260) in the switch III region and α3 helix of Gα₁₆. Regulations of PLCβ and STAT3 were partially weakened by each cluster mutant. A mutant harboring mutations at both clusters generally produced stronger suppressions. Activation of Jun N-terminal kinase (JNK) by Gα₁₆ was completely abolished by mutating either clusters. Contrastingly, phosphorylations of extracellular signal-regulated kinase (ERK) and nuclear factor κB (NF-κB) were not significantly affected by these mutations. The interactions between the mutants and PLCβ2 and TPR1 were also reduced in co-immunoprecipitation assays. Coupling between G₁₆ and different categories of receptors was impaired by the mutations, with the effect of switch III mutations being more pronounced than those in the α3 helix. Mutations of both clusters almost completely abolished the receptor coupling and prevent receptor-induced Gβγ release.

Conclusion

The integrity of the switch III region and α3 helix of Gα₁₆ is critical for the activation of PLCβ, STAT3, and JNK but not ERK or NF-κB. Binding of Gα₁₆ to PLCβ2 or TPR1 was reduced by the mutations of either cluster. The same region could also differentially affect the effectiveness of receptor coupling to G₁₆. The studied region was shown to bear multiple functionally important roles of G₁₆.

How do receptors activate G proteins?

Heterotrimeric G proteins couple the activation of heptahelical receptors at the cell surface to the intracellular signaling cascades that mediate the physiological responses to extracellular stimuli. G proteins are molecular switches that are activated by receptor-catalyzed GTP for GDP exchange on the G protein alpha subunit, which is the rate-limiting step in the activation of all downstream signaling. Despite the important biological role of the receptor-G protein interaction, relatively little is known about the structure of the complex and how it leads to nucleotide exchange. This chapter will describe what is known about receptor and G protein structure and outline a strategy for assembling the current data into improved models for the receptor-G protein complex that will hopefully answer the question as to how receptors flip the G protein switch.

The role of Nox4 in oxidative stress-induced MUC5AC overexpression in human airway epithelial cells

Mucus hypersecretion is a prominent manifestation in patients with chronic inflammatory airway diseases, and MUC5AC is a major airway mucin. It is well known that reactive oxygen species (ROS) may be involved in the pathogenesis of various inflammatory airway diseases. The purpose of this study was to identify which secreted mucin genes are induced by exogenous hydrogen peroxide and the mechanism by which these genes are up-regulated in normal human nasal epithelial (NHNE) cells. Exogenous H(2)O(2) induced the ligand-independent activation of epidermal growth factor receptors (EGFR) and the subsequent activation of ERK1 mitogen-activated protein kinase, resulting in the induction of intracellular ROS generation. Through this signal pathway, exogenous H(2)O(2) markedly induced overexpression of the MUC5AC gene alone. In addition, Nox4, a subtype of nonphagocytic NADPH oxidase, was found to play a key role in intracellular ROS generation and exogenous H(2)O(2)-induced MUC5AC gene expression in NHNE cells.

Manganese superoxide dismutase activity regulates transitions between quiescent and proliferative growth

In recent years, the intracellular reactive oxygen species (ROS) levels have gained increasing attention as a critical regulator of cellular proliferation. We investigated the hypothesis that manganese superoxide dismutase (MnSOD) activity regulates proliferative and quiescent growth by modulating cellular ROS levels. Decreasing MnSOD activity favored proliferation in mouse embryonic fibroblasts (MEF), while increasing MnSOD activity facilitated proliferating cells' transitions into quiescence. MnSOD +/- and -/- MEFs demonstrated increased superoxide steady-state levels; these fibroblasts failed to exit from the proliferative cycle, and showed increasing cyclin D1 and cyclin B1 protein levels. MnSOD +/- MEFs exhibited an increase in the percentage of G(2) cells compared to MnSOD +/+ MEFs. Overexpression of MnSOD in MnSOD +/- MEFs suppressed superoxide levels and G(2) accumulation, decreased cyclin B1 protein levels, and facilitated cells' transit into quiescence. While ROS are known to regulate differentiation and cell death pathways, both of which are irreversible processes, our results show MnSOD activity and, therefore, mitochondria-derived ROS levels regulate cellular proliferation and quiescence, which are reversible processes essential to prevent aberrant proliferation and subsequent exhaustion of normal cell proliferative capacity. These results support the hypothesis that MnSOD activity regulates a mitochondrial 'ROS-switch' favoring a superoxide-signaling regulating proliferation and a hydrogen peroxide-signaling supporting quiescence.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

Heterotrimeric G protein activation by G-protein-coupled receptors

Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP-GDP exchange on the G protein alpha-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.

Prolonged EGFR signaling by ERBB2-mediated sequestration at the plasma membrane

We have analyzed the spatial-temporal regulation of epidermal growth factor receptor (EGFR) phosphorylation by the orphan erbB2 receptor. It is shown that EGFR association with erbB2 is sufficient to prolong and enhance the net phosphorylation of EGFR, independent of the kinase activity of erbB2. This enhanced EGFR signaling was rather caused by erbB2-mediated retention of phosphorylated EGFR at the plasma membrane (PM), thereby preventing EGFR dephosphorylation and signal termination by endomembrane-bound protein tyrosine phosphatases (PTPs). EGF-induced EGFR internalization was indeed blocked in the presence of high levels of erbB2 or if cbl binding of EGFR was impaired. This erbB2-mediated blockage of the entry of activated EGFR into clathrin-coated vesicles could be alleviated by antibody-mediated disruption of the interaction between EGFR and erbB2. These results identify erbB2-mediated dominant trapping of phosphorylated EGFR at the PM as a mechanism that prolongs EGFR signaling, by sequestration of activated EGFR away from intracellular sites of high PTP activity.

The signaling pathway of rhodopsin

The signal-transduction mechanism of rhodopsin was studied by molecular dynamics (MD) simulations of the high-resolution, inactive structure in an explicit membrane environment. The simulations were employed to calculate equal-time correlations of the fluctuating interaction energy of residue pairs. The resulting interaction-correlation matrix was used to determine a network that couples retinal to the cytoplasmic interface, where transducin binds. Two highly conserved motifs, D(E)RY and NPxxY, were found to have strong interaction correlation with retinal. MD simulations with restraints on each transmembrane helix indicated that the major signal-transduction pathway involves the interdigitating side chains of helices VI and VII. The functional roles of specific residues were elucidated by the calculated effect of retinal isomerization from 11-cis to all-trans on the residue-residue interaction pattern. It is suggested that Glu134 may act as a "signal amplifier" and that Asp83 may introduce a threshold to prevent background noise from activating rhodopsin.

Oxyl radicals, redox-sensitive signalling cascades and antioxidants

Oxidative stress is an increase in the reduction potential or a large decrease in the reducing capacity of the cellular redox couples. A particularly destructive aspect of oxidative stress is the production of reactive oxygen species (ROS), which include free radicals and peroxides. Some of the less reactive of these species can be converted by oxidoreduction reactions with transition metals into more aggressive radical species that can cause extensive cellular damage. In animals, ROS may influence cell proliferation, cell death (either apoptosis or necrosis) and the expression of genes, and may be involved in the activation of several signalling pathways, activating cell signalling cascades, such as those involving mitogen-activated protein kinases. Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism and the damage they cause to cells is constantly repaired. The cellular redox environment is preserved by enzymes and antioxidants that maintain the reduced state through a constant input of metabolic energy. This review summarizes current studies that have been regarding the production of ROS and the general redox-sensitive targets of cell signalling cascades.

Reactive oxygen species and antioxidant mechanisms in human tissues and their relation to malignancies

Reactive oxygen species (ROS) are formed in mammalian cells as a consequence of aerobic respiration. Despite multiple conserved redox modulating systems, a given proportion of ROS continuously escape from the mitochondrial respiratory chain, being sufficiently potent to damage cells in various ways, including numerous carcinogenic DNA mutations. Oxidative stress resulting from an imbalanced ratio between ROS production and detoxification may also disturb physiological signal transduction, lead to chain reactions in lipid layers, and damage DNA repair enzymes. The significance of ROS and antioxidant systems in carcinogenesis is still complicated and in many ways contradictory. Enhanced antioxidant mechanisms in tumor cells in vivo have been implicated in chemoresistance and lead to poor prognosis, whereas most in vitro studies have reported tumor-suppressing properties of antioxidant enzymes. The present review aims to clarify the significance of oxidative stress and the role of cell redox state modulating systems in human malignancies in light of the current literature.

Is SS-A/Ro52 a hydrogen peroxide-sensitive signaling molecule?

SS-A/Ro52 (Ro52) protein is one of the targets of autoantibodies in Sjogren's syndrome and systemic lupus erythematosus. Ro52 structurally belongs to the RING-B-box/coiled-coil family, which appears to carry out diverse functions, but the physiological function of Ro52 remains largely unknown. Here, the authors demonstrate that hydrogen peroxide but not other oxidative stressors induced translocation of Ro52 protein from the cytoplasm to the nucleus and this phenomenon was attenuated by inhibition of MAP kinases, ERK in particular. These findings raise the possibility that SS-A/Ro52 may function as a hydrogen peroxide-selective, oxidative stress-sensitive signaling molecule that is activated via the MAP kinase pathway.