Phospholipase D in the signaling networks of plant response to abscisic acid and reactive oxygen species

Phospholipase D (PLD) has been implicated in different cellular processes in plant growth, development, and stress responses. Recent results have provided insights into the molecular mechanism by which PLD and its lipid product phosphatidic acid (PA) participate in cell signaling. Effector proteins that have been identified for PLD and PA in plants include a heterotrimeric G protein, protein phosphatase, and protein kinase. Evidence has been presented for a direct link from a PLD, PA, to a target protein in specific physiological processes. PLD and PA play multiple roles in the signaling networks of plant response to abscisic acid and reactive oxygen species.     

The Scribble Cell Polarity Module in the Regulation of Cell Signaling in Tissue Development and Tumorigenesis

The Scribble cell polarity module, comprising Scribbled (Scrib), Discs-large (Dlg) and Lethal-2-giant larvae (Lgl), has a tumor suppressive role in mammalian epithelial cancers. The Scribble module proteins play key functions in the establishment and maintenance of different modes of cell polarity, as well as in the control of tissue growth, differentiation and directed cell migration, and therefore are major regulators of tissue development and homeostasis. Whilst molecular details are known regarding the roles of Scribble module proteins in cell polarity regulation, their precise mode of action in the regulation of other key cellular processes remains enigmatic. An accumulating body of evidence indicates that Scribble module proteins play scaffolding roles in the control of various signaling pathways, which are linked to the control of tissue growth, differentiation and cell migration. Multiple Scrib, Dlg and Lgl interacting proteins have been discovered, which are involved in diverse processes, however many function in the regulation of cellular signaling. Herein, we review the components of the Scrib, Dlg and Lgl protein interactomes, and focus on the mechanism by which they regulate cellular signaling pathways in metazoans, and how their disruption leads to cancer.     

Regulatory Roles of Metabolites in Cell Signaling Networks

Mounting evidence suggests that cellular metabolites, in addition to being sources of fuel and macromolecular substrates, are actively involved in signaling and epigenetic regulation. Many metabolites, such as cyclic AMP, which regulates phosphorylation/dephosphorylation, have been identified to modulate DNA and histone methylation and protein stability. Metabolite-driven cellular regulation occurs through two distinct mechanisms: proteins allosterically bind or serve as substrates for protein signaling pathways, and metabolites covalently modify proteins to regulate their functions. Such novel protein metabolites include fumarate, succinyl-CoA, propionyl-CoA, butyryl-CoA and crontonyl-CoA. Other metabolites, including α-ketoglutarate, succinate and fumarate, regulate epigenetic processes and cell signaling via protein binding. Here, we summarize recent progress in metabolite-derived post-translational protein modification and metabolite-binding associated signaling regulation. Uncovering metabolites upstream of cell signaling and epigenetic networks permits the linkage of metabolic disorders and human diseases, and suggests that metabolite modulation may be a strategy for innovative therapeutics and disease prevention techniques.     

PSMA2 knockdown impacts expression of proteins involved in immune and cellular stress responses in human lung cells

Proteasome subunit alpha type-2 (PSMA2) is a critical component of the 20S proteasome, which is the core particle of the 26S proteasome complex and is involved in cellular protein quality control by recognizing and recycling defective proteins. PSMA2 expression dysregulation has been detected in different human diseases and viral infections. No study yet has reported PSMA2 knockdown (KD) effects on the cellular proteome. Methods: We used SOMAScan, an aptamer-based multiplexed technique, to measure >1300 human proteins to determine the impact of PSMA2 KD on A549 human lung epithelial cells. Results: PSMA2 KD resulted in significant dysregulation of 52 cellular proteins involved in different bio-functions, including cellular movement and development, cell death and survival, and cancer. The immune system and signal transduction were the most affected cellular functions. PSMA2 KD caused dysregulation of several signaling pathways involved in immune response, cytokine signaling, organismal growth and development, cellular stress and injury (including autophagy and unfolded protein response), and cancer responses. Conclusions: In summary, this study helps us better understand the importance of PSMA2 in different cellular functions, signaling pathways, and human diseases.     

Mechanisms of xylanase-induced nitric oxide and phosphatidic acid production in tomato cells

The second messenger nitric oxide (NO), phosphatidic acid (PA) and reactive oxygen species (ROS) are involved in the plant defense response during plant–pathogen interactions. NO has been shown to participate in PA production in response to the pathogen-associated molecular pattern xylanase in tomato cell suspensions. Defense responses downstream of PA include ROS production. The goal of this work was to study the signaling mechanisms involved in PA production during the defense responses triggered by xylanase and mediated by NO in the suspension-cultured tomato cells. We analyzed the participation of protein kinases, guanylate cyclase and the NO-mediated posttranslational modification S-nitrosylation, by means of pharmacology and biochemistry. We showed that NO, PA and ROS levels are significantly diminished by treatment with the general protein kinase inhibitor staurosporine. This indicates that xylanase-induced protein phosphorylation events might be the important components leading to NO formation, and hence for the downstream regulation of PA and ROS levels. When assayed, a guanylate cyclase inhibitor or a cGMP analog did not alter the PA accumulation. These results suggest that a cGMP-mediated pathway is not involved in xylanase-induced PA formation. Finally, the inhibition of protein S-nitrosylation did not affect NO formation but compromised PA and ROS production. Data collectively indicate that upon xylanase perception, cells activate a protein kinase pathway required for NO formation and that, S-nitrosylation-dependent mechanisms are involved in downstream signaling leading to PA and ROS.     

Modulation of cell signaling pathways by kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8)

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, has been associated with the development of Kaposi's sarcoma, pleural effusion lymphoma, and multicentric Castleman's disease. KSHV is a double-stranded DNA virus that has been classified as a gammaherpesvirus. The viral genome is approx, 160 kb long and encodes for several genes that are involved in cell signaling pathways. These include genes that are unique to the virus as well as viral homologues of cellular genes. The latter are likely to have been usurped from the host genome and include both virokines and viral receptor proteins. This article reviews how these KSHV proteins modulate cellular signal transduction pathways.     

New insights into the regulation of cAMP synthesis beyond GPCR/G protein activation: implications in cardiovascular regulation

Regulation of intracellular concentrations of cyclic AMP is one of the most ubiquitous mechanisms for regulating cellular functions. Further, the manner in which cAMP production is regulated via G proteins at the level of adenylyl cyclase activation has been studied extensively. This review focuses instead on the recently identified mechanisms and roles for regulation of adenylyl cyclase functions beyond G protein activation. These mechanisms include: a) the coupling of particular isoforms of adenylyl cyclase to function within a single cell type b) regulation of membrane trafficking of higher order enzyme aggregates and c) raf kinase-dependent phosphorylation and sensitization of adenylyl cyclases--an important pathway for crosstalk between tyrosine kinase signaling cascades with regulation of cAMP-mediated responses.     

Redox signaling mediated by the gut microbiota

Abstract

The microbiota that occupies the mammalian intestine can modulate a range of physiological functions, including control over immune responses, epithelial barrier function, and cellular proliferation. While commensal prokaryotic organisms are well known to stimulate inflammatory signaling networks, less is known about control over homeostatic pathways. Recent work has shown that gut epithelia contacted by enteric commensal bacteria rapidly generate reactive oxygen species (ROS). While the induced production of ROS in professional phagocytes via stimulation of formyl peptide receptors (FPRs) and activation of NADPH oxidase 2 (Nox2) is a well-studied process, ROS are also similarly elicited in other cell types, including intestinal epithelia, in response to microbial signals via FPRs and the epithelial NADPH oxidase 1 (Nox1). ROS generated by Nox enzymes have been shown to function as critical second messengers in multiple signal transduction pathways via the rapid and transient oxidative inactivation of a distinct class of sensor proteins bearing oxidant-sensitive thiol groups. These redox-sensitive proteins include tyrosine phosphatases that serve as regulators of MAP kinase pathways, focal adhesion kinase, as well as components involved in NF-κB activation. As microbe-elicited ROS has been shown to stimulate cellular proliferation and motility, and to modulate innate immune signaling, we hypothesize that many of the established effects of the normal microbiota on intestinal physiology may be at least partially mediated by this ROS-dependent mechanism.     

Phosphatidic acid production in chitosan-elicited tomato cells, via both phospholipase D and phospholipase C/diacylglycerol kinase, requires nitric oxide

Nitric oxide (NO) and the lipid second messenger phosphatidic acid (PA) are involved in plant defense responses during plant-pathogen interactions. NO has been shown to be involved in the induction of PA production in response to the pathogen associated molecular pattern (PAMP) xylanase in tomato cells. It was shown that NO is critical for PA production induced via phospholipase C (PLC) in concerted action with diacylglycerol kinase (DGK) but not for the xylanase-induced PA via phospholipase D (PLD). In order to study whether this is a general phenomenon during PAMP perception or if it is particular for xylanase, we studied the effect of the PAMP chitosan in tomato cell suspensions. We observed a rapid NO production in tomato cells treated with chitosan. Chitosan induced the formation of PA by activating both PLD and PLC/DGK. The activation of either phospholipase-mediated signaling pathway was inhibited in cells treated with the NO scavenger cPTIO. This indicates that NO is required for PA generation via both the PLD and PLC/DGK pathway during plant defense response in chitosan elicited cells. Responses downstream PA were studied. PLC inhibitors neomycin and U73122 inhibited chitosan-induced ROS production. Differences between xylanase and chitosan-induced phospholipid signaling pathways are discussed.     

小窝蛋白-1在细胞衰老及衰老相关疾病中的作用

胞膜小窝(caveolae)是细胞质膜内陷所形成的囊状结构.小窝蛋白(caveolin)是胞膜小窝区别于其它脂筏结构的特征性蛋白分子,维持胞膜小窝的结构和功能,包括3个家族成员小窝蛋白-1、小窝蛋白-2和小窝蛋白-3.其中,小窝蛋白-1是参与胆固醇平衡、分子运输和跨膜信号发放事件的主要结构成分,从而调节细胞的生长、发育和增殖．小窝蛋白-1在细胞衰老中起着重要调控作用,主要通过p53-p21及p16-Rb信号通路抑制细胞增殖、酪氨酸激酶的级联反应,调控粘连信号级联、胰岛素信号及雌激素信号系统等途径调控衰老进程．衰老过程中不同器官小窝蛋白-1变化趋势不尽一致．近年研究还发现,小窝蛋白-1与神经系统退行性疾病、糖尿病、动脉粥样硬化等衰老相关疾病密切相关,通过调节多条信号通路参与这些疾病的发生发展．本文结合最新研究进展,对小窝蛋白-1在细胞衰老进程的作用及参与衰老相关疾病进行综述．     

Understanding ERM proteins--the awesome power of genetics finally brought to bear

In epithelial cells, the Ezrin, Radixin and Moesin (ERM) proteins are involved in many cellular functions, including regulation of actin cytoskeleton, control of cell shape, adhesion and motility, and modulation of signaling pathways. However, discerning the specific cellular roles of ERMs has been complicated by redundancy between these proteins. Recent genetic studies in model organisms have identified unique roles for ERM proteins. These include the regulation of morphogenesis and maintenance of integrity of epithelial cells, stabilization of intercellular junctions, and regulation of the Rho small GTPase. These studies also suggest that ERMs have roles in actomyosin contractility and vesicular trafficking in the apical domain of epithelial cells. Thus, genetic analysis has enhanced our understanding of these widely expressed membrane-associated proteins.     

Lipid phosphate phosphatases and related proteins: signaling functions in development, cell division, and cancer

Lipid phosphates initiate key signaling cascades in cell activation. Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are produced by activated platelets. LPA is also formed from circulating lysophosphatidylcholine by autotaxin, a protein involved tumor progression and metastasis. Extracellular LPA and S1P stimulate families of G-protein coupled receptors that elicit diverse responses. LPA is involved in wound repair and tumor growth. Exogenous S1P is a potent stimulator of angiogenesis, a process vital in development, tissue repair and the growth of aggressive tumors. Inside the cell, phosphatidate (PA), ceramide 1-phosphate (C1P), LPA, and S1P act as signaling molecules with distinct functions including the stimulation of cell division, cytoskeletal rearrangement, Ca(2+) transients, and membrane movement. These observations imply that phosphatases that degrade lipid phosphates on the cell surface, or inside the cell, regulate cell signaling under physiological and pathological conditions. This occurs through attenuation of signaling by the lipid phosphates and by the production of bioactive products (diacylglycerol, ceramide, and sphingosine). Three lipid phosphate phosphatases (LPPs) and a splice variant dephosphorylate LPA, PA, CIP, and S1P. Two S1P phosphatases (SPPs) act specifically on S1P. In addition, there is family of four LPP-related proteins (LPRs, or plasticity-related genes, PRGs). PRG-1 expression in neurons has been reported to increase extracellular LPA breakdown and attenuate LPA-induced axonal retraction. It is unclear whether the LRPs dephosphorylate LPA directly, stimulate LPP activity, or bind LPA and S1P. Also, the importance of extra- versus intra-cellular actions of the LPPs and SPPs, and the individual roles of different isoforms is not firmly established. Understanding the functions and regulation of the LPPs, SPPs and related proteins will hopefully contribute to interventions to correct dysfunctions in conditions such as wound repair, inflammation, angiogenesis, tumor growth, and metastasis.     

Protein kinase C binding partners

Members of the protein kinase C family respond to second messengers and are involved in controlling a broad array of cellular functions. The overlapping specificity and promiscuity of these proteins has promoted the view that specific binding proteins constrain individual family members to create the appropriate specificity of action. It is speculated that such protein kinase C-regulator protein interactions affect substrate availability as well as exposure to allosteric activator(s) and that consequent interactions specify cellular location and impose integration with other signaling systems. These predicted features have been realized in the identification of many protein kinase C interacting proteins and examples of these are discussed.     

Interaction of S100A13 with C2 domain of receptor for advanced glycation end products (RAGE)

S100A13 is involved in several key biological functions like angiogenesis, tumor formation and cell apoptosis. It is a homodimeric protein that belongs to the S100 protein family. S100A13 is co-expressed with acidic fibroblast growth factor (FGF1) and interleukin-1α which are key angiogenesis inducers. The S100 proteins have been shown to be involved in several cellular functions such as calcium homeostasis, cell growth and differentiation dynamic of cytoskeleton. Its biological functions are mainly mediated through the receptor for advanced glycation end products (RAGE) signaling. RAGE is involved in inflammatory processes and is associated with diabetic complications, tumor outgrowth, and neurodegenerative disorders. RAGE induces cellular signaling upon binding of different ligands, such as S100 proteins, glycated proteins, and HMGB1. RAGE signaling is complex, and it depends on the cell type and concentration of the ligand. Molecular level interactions of RAGE and S100 proteins are useful to understand the RAGE signaling diversity. In this report we focus on the molecular level interactions of S100A13 and RAGE C2 domain. The binding between RAGE C2 and S100A13 is moderately strong (K_d ~ 1.3 μM). We have solved the solution structure of the S100A13–RAGE C2 complex and pronounce the interface regions in S100A13–RAGE C2 complex which are helpful for drug development of RAGE induced diseases.     

Phosphatidic acid as a regulator of matrix metalloproteinase-9 expression via the TNF-alpha signaling pathway

Phosphatidic acid (PA) is implicated in pathophysiological processes associated with cellular signaling events and inflammation, which include the expressional regulation of numerous genes. Here, we show that PA stimulation increases matrix metalloproteinase-9 (MMP-9) expression in macrophages through tumor necrosis factor (TNF)-alpha signaling. We performed antibody array analysis on proteins from macrophages stimulated with PA. PA was found to induce the production of TNF-alpha, but not of TNF receptor (TNFR)1 and TNFR2 in a time-dependent manner and stimulated significant, though delayed, MMP-9 expression. PA induced the phosphorylations of both ERK1/2 and p38, but not of c-jun amino-terminal kinase. Moreover, only ERK1/2 inhibition by U0126 suppressed PA-induced TNF-alpha production and MMP-9 expression. Neutralizing TNF-alpha, TNFR1 or TNFR2 antibodies significantly suppressed PA-induced MMP-9 expression, suggesting that the production of TNF-alpha in response to PA preceded the expression of MMP-9. Moreover, lipopolysaccharide-induced PA also led to TNF-alpha release and resulted in MMP-9 expression. Taken together, these observations suggest that PA may play a role in MMP-9 regulation through ERKs/TNF-alpha/TNFRs-dependent signaling pathway.     

Phorbol ester activation of functional rat protein kinase C β-1 causes phenotype in yeast

The phorbol ester receptor protein kinase C (PKC) gene family encodes essential mediators of various eukaryotic cellular signals. The molecular dissection of its mechanisms of action has been limited in part by the genetic inaccessibility and complexity of signaling in mammalian cells. Here we present a novel approach to study rat PKC β-1 action in yeast, a simple lower eukaryotic genetic model. Expression of its cDNA in Saccharomyces cerevisiae introduces novel phorbol ester binding sites which stimulate a specific calcium- and phospholipid-dependent catalytic activity in vitro consistent with a fully functional protein which phosphorylates cellular yeast proteins in vivo. Phorbol ester activation of PKC β-1 in vivo results in biological responses which include stimulation of extracellular calcium uptake, changes in cell morphology, and an increase in the cell doubling time. These PKC functions are not affected by truncation of 12 amino terminal amino acids; however, they are completely abolished by truncation of 15 or more carboxyl terminal amino acids which likely result in inactivation of the kinase. The increase in the yeast doubling time caused by PKC β-1 activation provides a phenotype which can be exploited as a screen for the activity of random PKC cDNA mutations. Our findings indicate that rat PKC β-1 is functional in yeast and leads to biological responses which suggest compatible aspects of higher and lower eukaryotic signaling pathways and the feasibility of dissecting parts of the action of common signaling mediators in a simple genetic model.