Cell signalling by reactive lipid species: new concepts and molecular mechanisms

The process of lipid peroxidation is widespread in biology and is mediated through both enzymatic and non-enzymatic pathways. A significant proportion of the oxidized lipid products are electrophilic in nature, the RLS (reactive lipid species), and react with cellular nucleophiles such as the amino acids cysteine, lysine and histidine. Cell signalling by electrophiles appears to be limited to the modification of cysteine residues in proteins, whereas non-specific toxic effects involve modification of other nucleophiles. RLS have been found to participate in several physiological pathways including resolution of inflammation, cell death and induction of cellular antioxidants through the modification of specific signalling proteins. The covalent modification of proteins endows some unique features to this signalling mechanism which we have termed the 'covalent advantage'. For example, covalent modification of signalling proteins allows for the accumulation of a signal over time. The activation of cell signalling pathways by electrophiles is hierarchical and depends on a complex interaction of factors such as the intrinsic chemical reactivity of the electrophile, the intracellular domain to which it is exposed and steric factors. This introduces the concept of electrophilic signalling domains in which the production of the lipid electrophile is in close proximity to the thiol-containing signalling protein. In addition, we propose that the role of glutathione and associated enzymes is to insulate the signalling domain from uncontrolled electrophilic stress. The persistence of the signal is in turn regulated by the proteasomal pathway which may itself be subject to redox regulation by RLS. Cell death mediated by RLS is associated with bioenergetic dysfunction, and the damaged proteins are probably removed by the lysosome-autophagy pathway.     

Signal transduction mechanisms of CD137 ligand in human monocytes

Bidirectional signalling, i.e. simultaneous signalling through a receptor as well as its cell surface-bound ligand has been identified for several members of the TNF and TNF receptor family members. Reverse signalling through the ligands offers the advantage of an immediate feed-back and a more precise fine tuning of biological responses. Little is known about the molecular nature of reverse signalling through the ligands. CD137 ligand, member of the TNF family is expressed on monocytes and induces activation, migration, prolongation of survival and proliferation of monocytes. Here we show that reverse signalling by CD137 ligand is mediated by protein tyrosine kinases, p38 mitogen activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK)1,2, MAP/ERK kinase (MEK), Phosphoinositide-3-kinase (PI3-K) and protein kinase A (PKA) but not by protein kinase C (PKC). This study also shows that reverse signalling relies on the same signal transduction molecules as signalling through classical receptors and is in its nature not different from it.     

Inhibitory effect of leptin on rosiglitazone-induced differentiation of primary adipocytes prepared from TallyHO/Jng mice

The calcium-sensing receptor (CaR) plays an integral role in calcium homeostasis and the regulation of other cellular functions including cell proliferation and cytoskeletal organisation. The multifunctional nature of the CaR is manifested through ligand-dependent stimulation of different signalling pathways that are also regulated by partner binding proteins. Following a yeast two-hybrid library screen using the intracellular tail of the CaR as bait, we identified several novel binding partners including the focal adhesion protein, testin. Testin has not previously been shown to interact with cell surface receptors. The sites of interaction between the CaR and testin were mapped to the membrane proximal region of the receptor tail and the second zinc-finger of LIM domain 1 of testin, the integrity of which was found to be critical for the CaR-testin interaction. The CaR-testin association was confirmed in HEK293 cells by coimmunoprecipitation and confocal microscopy studies. Ectopic expression of testin in HEK293 cells stably expressing the CaR enhanced CaR-stimulated Rho activity but had no effect on CaR-stimulated ERK signalling. These results suggest an interplay between the CaR and testin in the regulation of CaR-mediated Rho signalling with possible effects on the cytoskeleton.     

Interdependent epidermal growth factor receptor signalling and trafficking

Epidermal growth factor (EGF) receptor (EGFR) signalling regulates diverse cellular functions, promoting cell proliferation, differentiation, migration, cell growth and survival. EGFR signalling is critical during embryogenesis, in particular in epithelial development, and disruption of the EGFR gene results in epithelial immaturity and perinatal death. EGFR signalling also functions during wound healing responses through accelerating wound re-epithelialisation, inducing cell migration, proliferation and angiogenesis. Upregulation of EGFR signalling is often observed in carcinomas and has been shown to promote uncontrolled cell proliferation and metastasis. Therefore aberrant EGFR signalling is a common target for anticancer therapies. Various reports indicate that EGFR signalling primarily occurs at the plasma membrane and EGFR degradation following endocytosis greatly attenuates signalling. Other studies argue that EGFR internalisation is essential for complete activation of downstream signalling cascades and that endosomes can serve as signalling platforms. The aim of this review is to discuss current understanding of intersection between EGFR signalling and trafficking.     

Signalling sphingomyelinases: which, where, how and why?

A major lipid signalling pathway in mammalian cells implicates the activation of sphingomyelinase (SMase), which upon cell stimulation hydrolyses the ubiquitous sphingophospholipid sphingomyelin to ceramide. This review summarizes our current knowledge on the nature and regulation of signalling SMase(s). Because of the controversy on the identity of this(these) phospholipase(s), the roles of various SMases in cell signalling are discussed. Special attention is also given to the subcellular site of action of signalling SMases and to the cellular factors that positively or negatively control their activity. These regulating agents include lipids (arachidonic acid, diacylglycerol and ceramide), kinases, proteases, glutathione and other proteins.     

Direct oxidative modifications of signalling proteins in mammalian cells and their effects on apoptosis

The production of ROS is an inevitable consequence of metabolism. However, high levels of ROS within a cell can be lethal and so the cell has a number of defences against oxidative cell stress. Occasionally the cell's antioxidant mechanisms fail and oxidative stress occurs. High levels of ROS within a cell have a number of direct and indirect consequences on cell signalling pathways and may result in apoptosis or necrosis. Although some of the indirect effects of ROS are well known, limitations in technology mean that the direct effects of the cell's redox environment upon proteins are less understood. Recent work by a number of groups has demonstrated that ROS can directly modify signalling proteins through different modifications, for example by nitrosylation, carbonylation, di-sulphide bond formation and glutathionylation. These modifications modulate a protein's activity and several recent papers have demonstrated their importance in cell signalling events, especially those involved in cell death/survival. Redox modification of proteins allows for further regulation of cell signalling pathways in response to the cellular environment. Understanding them may be critical for us to modulate cell pathways for our own means, such as in cytotoxic drug treatments of cancer cells. Protein modifications mediated by oxidative stress can modulate apoptosis, either through specific protein modifications resulting in regulation of signalling pathways, or through a general increase in oxidised proteins resulting in reduced cellular function. This review discusses direct oxidative protein modifications and their effects on apoptosis.     

Negative regulation of EGFR signalling through integrin-alpha1beta1-mediated activation of protein tyrosine phosphatase TCPTP

Integrin-mediated cell adhesion regulates a multitude of cellular responses, including proliferation, survival and cross-talk between different cellular signalling pathways. So far, integrins have been mainly shown to convey permissive signals enabling anchorage-dependent receptor tyrosine kinase signalling. Here we show that a collagen-binding integrin alpha(1)beta(1) functions as a negative regulator of epidermal growth factor receptor (EGFR) signalling through the activation of a protein tyrosine phosphatase. The cytoplasmic tail of alpha(1) integrin selectively interacts with a ubiquitously expressed protein tyrosine phosphatase TCPTP (T-cell protein tyrosine phosphatase) and activates it after cell adhesion to collagen. The activation results in reduced EGFR phosphorylation after EGF stimulation. Introduction of the alpha(1) cytoplasmic domain peptide into cells induces phosphatase activation and inhibits EGF-induced cell proliferation and anchorage-independent growth of malignant cells. These data are the first demonstration of the regulation of TCPTP activity in vivo and represent a new molecular paradigm of integrin-mediated negative regulation of receptor tyrosine kinase signalling.     

Enteropathogenic and enterohaemorrhagic Escherichia coli: even more subversive elements

The human pathogens enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) share a unique mechanism of colonization that results from the concerted action of effector proteins translocated into the host cell by a type III secretion system (T3SS). EPEC and EHEC not only induce characteristic attaching and effacing (A/E) lesions, but also subvert multiple host cell signalling pathways during infection. Our understanding of the mechanisms by which A/E pathogens hijack host cell signalling has advanced dramatically in recent months with the identification of novel activities for many effectors. In addition to further characterization of established effectors (Tir, EspH and Map), new effectors have emerged as important mediators of virulence through activities such as mimicry of Rho guanine nucleotide exchange factors (Map and EspM), inhibition of apoptosis (NleH and NleD), interference with inflammatory signalling pathways (NleB, NleC, NleE and NleH) and phagocytosis (EspF, EspH and EspJ). The findings have highlighted the multifunctional nature of the effectors and their ability to participate in redundant, synergistic or antagonistic relationships, acting in a co-ordinated spatial and temporal manner on different host organelles and cellular pathways during infection.     

Semiotic Tools For Multilevel Cell Communication

Cell communication plays a key role in multicellular organisms. In developing embryos as in adult organisms, cells communicate by coordinating their differentiation through the establishment and/or renewal of a variety of cell communication channels. Under both these conditions, cells interact by either receptor signalling, surface recognition of specific cell adhesion molecules or transfer of cytoplasmic components through junctional coupling. In recent years, it has become apparent that cells may also communicate through the extracellular release of microvesicles. They may originate as either exosomes from the endosomal compartment upon fusion of multivesicular bodies with the plasma membrane or be shed directly from the plasma membrane via extensions of the cell surface. Microvesicles may disperse over long distances through body fluids and deliver their molecular cargo onto a variety of target cells. As a general rule, the metabolic fate of these cells is determined by the molecular nature of the vesicular cargo, while targeting itself depends on the affinity of the molecules expressed on the enclosing membrane. In this paper, we will be arguing that intercellular vesicular transfer is substantially different from other types of cell communication, allowing cells and molecules to interact on varying levels. Cells interacting via ligand signalling owe their specificity to the steric coupling with cognate receptor molecules. As such, it is a pure molecular process that affects target cells only upon integration into their responding repertoire. In this relationship, coupled cells are reciprocally adapted to each other through the selection of their respective signalling capacities, following exploration of their receptor specificity. Interaction by intercellular vesicles realizes a substantially different type of cell communication. Vesicular traffic allows donor cells to carry out a horizontal type of gene transfer and target this information over long distances via independently controlled mechanisms. Because of this independence, cells interacting via vesicular traffic are not expected to adapt their signalling correspondences, but to control instead the efficiency of their cargo delivery irrespective of the receptor repertoire expressed by the target tissue. In this paper, the multifaceted functions of the intercellular vesicular traffic will be discussed in a multilevel biosemiotic perspective with the aim of unravelling the cellular mechanisms devised by nature to accomplish communication.     

Direct oxidative modifications of signalling proteins in mammalian cells and their effects on apoptosis

Abstract

The production of ROS is an inevitable consequence of metabolism. However, high levels of ROS within a cell can be lethal and so the cell has a number of defences against oxidative cell stress. Occasionally the cell's antioxidant mechanisms fail and oxidative stress occurs. High levels of ROS within a cell have a number of direct and indirect consequences on cell signalling pathways and may result in apoptosis or necrosis. Although some of the indirect effects of ROS are well known, limitations in technology mean that the direct effects of the cell's redox environment upon proteins are less understood. Recent work by a number of groups has demonstrated that ROS can directly modify signalling proteins through different modifications, for example by nitrosylation, carbonylation, di-sulphide bond formation and glutathionylation. These modifications modulate a protein's activity and several recent papers have demonstrated their importance in cell signalling events, especially those involved in cell death/survival. Redox modification of proteins allows for further regulation of cell signalling pathways in response to the cellular environment. Understanding them may be critical for us to modulate cell pathways for our own means, such as in cytotoxic drug treatments of cancer cells. Protein modifications mediated by oxidative stress can modulate apoptosis, either through specific protein modifications resulting in regulation of signalling pathways, or through a general increase in oxidised proteins resulting in reduced cellular function. This review discusses direct oxidative protein modifications and their effects on apoptosis.     

Supervised membrane swimming: small G-protein lifeguards regulate PIPK signalling and monitor intracellular PtdIns(4,5)P2 pools

Regulation of PIPK (phosphatidylinositol phosphate kinase) and PtdIns(4,5)P2 signalling by small G-proteins and their effectors is key to many biological functions. Through selective recruitment and activation of different PIPK isoforms, small G-proteins such as Rho, Rac and Cdc42 modulate actin dynamics and cytoskeleton-dependent cellular events in response to extracellular signalling. These activities affect a number of processes, including endocytosis, bacterial penetration into host cells and cytolytic granule-mediated targeted cell killing. Small G-proteins and their modulators are also regulated by phosphoinositides through translocation and conformational changes. Arf family small G-proteins act at multiple sites as regulators of membrane trafficking and actin cytoskeletal remodelling, and regulate a feedback loop comprising phospholipase D, phosphatidic acid, PIPKs and PtdIns(4,5)P2, contributing to enhancement of PtdIns(4,5)P2-mediated cellular events and receptor signalling. Na+, Kir (inwardly rectifying K+), Ca2+ and TRP (transient receptor potential) ion channels are regulated by small G-proteins and membrane pools of PtdIns(4,5)P2. Yeast phosphatidylinositol 4-phosphate 5-kinases Mss4 and Its3 are involved in resistance against disturbance of sphingolipid biosynthesis and maintenance of cell integrity through the synthesis of PtdIns(4,5)P2 and downstream signalling through the Rom2/Rho2 and Rgf1/Rho pathways. Here, we review models for regulated intracellular targeting of PIPKs by small G-proteins and other modulators in response to extracellular signalling. We also describe the spatial and temporal cross-regulation of PIPKs and small G-proteins that is critical for a number of cellular functions.     

The influence of entropic crowding in cell monolayers

Cell-cell interaction dictates cell morphology and organization, which play a crucial role in the micro-architecture of tissues that guides their biological and mechanical functioning. Here, we investigate the effect of cell density on the responses of cells seeded on flat substrates using a novel statistical thermodynamics framework. The framework recognizes the existence of nonthermal fluctuations in cellular response and thereby naturally captures entropic interactions between cells in monolayers. In line with observations, the model predicts that cell area and elongation decrease with increasing cell seeding density—both are a direct outcome of the fluctuating nature of the cellular response that gives rise to enhanced cell-cell interactions with increasing cell crowding. The modeling framework also predicts the increase in cell alignment with increasing cell density: this cellular ordering is also due to enhanced entropic interactions and is akin to nematic ordering in liquid crystals. Our simulations provide physical insights that suggest that entropic cell-cell interactions play a crucial role in governing the responses of cell monolayers.     

The p38 signal transduction pathway: activation and function

The p38 signalling transduction pathway, a Mitogen-activated protein (MAP) kinase pathway, plays an essential role in regulating many cellular processes including inflammation, cell differentiation, cell growth and death. Activation of p38 often through extracellular stimuli such as bacterial pathogens and cytokines, mediates signal transduction into the nucleus to turn on the responsive genes. p38 also transduces signals to other cellular components to execute different cellular responses. In this review, we summarize the characteristics of the major components of the p38 signalling transduction pathway and highlight the targets of this pathway and the physiological function of the p38 activation.     

Photosynthesis, photorespiration, and light signalling in defence responses

Visible light is the basic energetic driver of plant biomass production through photosynthesis. The constantly fluctuating availability of light and other environmental factors means that the photosynthetic apparatus must be able to operate in a dynamic fashion appropriate to the prevailing conditions. Dynamic regulation is achieved through an array of homeostatic control mechanisms that both respond to and influence cellular energy and reductant status. In addition, light availability and quality are continuously monitored by plants through photoreceptors. Outside the laboratory growth room, it is within the context of complex changes in energy and signalling status that plants must regulate pathways to deal with biotic challenges, and this can be influenced by changes in the highly energetic photosynthetic pathways and in the turnover of the photosynthetic machinery. Because of this, defence responses are neither simple nor easily predictable, but rather conditioned by the nutritional and signalling status of the plant cell. This review discusses recent data and emerging concepts of how recognized defence pathways interact with and are influenced by light-dependent processes. Particular emphasis is placed on the potential roles of the chloroplast, photorespiration, and photoreceptor-associated pathways in regulating the outcome of interactions between plants and pathogenic organisms.     

The transmembrane protein XFLRT3 forms a complex with FGF receptors and promotes FGF signalling

Fibroblast growth factors (FGFs) signal through high-affinity tyrosine kinase receptors to regulate a diverse range of cellular processes, including cell growth, differentiation and migration, as well as cell death. Here we identify XFLRT3, a member of a leucine-rich-repeat transmembrane protein family, as a novel modulator of FGF signalling. XFLRT3 is co-expressed with FGFs, and its expression is both induced after activation and downregulated after inhibition of FGF signalling. In gain- and loss-of function experiments, FLRT3 and FLRT2 phenocopy FGF signalling in Xenopus laevis. XFLRT3 signalling results in phosphorylation of ERK and is blocked by MAPK phosphatase 1, but not by expression of a dominant-negative phosphatidyl inositol 3-OH kinase (PI(3)K) mutant. XFLRT3 interacts with FGF receptors (FGFRs) in co-immunoprecipitation experiments in vitro and in bioluminescence resonance energy transfer assays in vivo. The results indicate that XFLRT3 is a transmembrane modulator of FGF-MAP kinase signalling in vertebrates.