Subversion of host kinases: a key network in cellular signaling hijacked by Helicobacter pylori CagA

Helicobacter pylori is a paradigm of persistent pathogens and major risk factor for developing severe diseases including adenocarcinoma in the human stomach. An important bacterial factor linked to gastric disease progression is the cag pathogenicity island‐encoded type‐IV secretion system (T4SS) effector protein CagA. Translocated CagA undergoes tyrosine phosphorylation at EPIYA‐motifs and then activates or inactivates multiple host signaling proteins in a phosphorylation‐dependent and phosphorylation‐independent fashion. In this way, intracellular CagA acts as a ‘masterkey’ or ‘picklock’, which evolved during evolution to hijack key host cell signal transduction functions. Crucial targets of CagA represent a variety of serine/threonine and tyrosine kinases, which control major checkpoints of eukaryotic signaling. Here we review the signal transmission by translocated CagA on multiple receptor kinases (c‐Met and EGFR) and non‐receptor kinases (Src, Abl, Csk, aPKC, Par1, PI3K, Akt, FAK, GSK‐3, JAK, PAK1, PAK2 and MAP kinases), manipulating a selection of fundamental processes in the human gastric epithelium such as cell adhesion, polarity, proliferation, motility, receptor endocytosis, cytoskeletal rearrangements, apoptosis, inflammation and cell cycle progression. This enormous complexity generates a highly remarkable and puzzling scenario during H. pylori infection. The contribution of these signaling pathways to bacterial survival, persistence and gastric pathogenesis is discussed.     

PAK signalling in neuronal physiology

Group I p21-activated kinases are a family of key effectors of Rac1 and Cdc42 and they regulate many aspects of cellular function, such as cytoskeleton dynamics, cell movement and cell migration, cell proliferation and differentiation, and gene expression. The three genes PAK1/2/3 are expressed in brain and recent evidence indicates their crucial roles in neuronal cell fate, in axonal guidance and neuronal polarisation, and in neuronal migration. Moreover they are implicated in neurodegenerative diseases and play an important role in synaptic plasticity, with PAK3 being specifically involved in mental retardation. The main goal of this review is to describe the molecular mechanisms that govern the different functions of group I PAK in neuronal signalling and to discuss the specific functions of each isoform.     

The involvement of the interleukin-1 receptor-associated kinases (IRAKs) in cellular signaling networks controlling inflammation

Innate immunity and inflammation plays a key role in host defense and wound healing. However, Excessive or altered inflammatory processes can contribute to severe and diverse human diseases including cardiovascular disease, diabetes and cancer. The interleukin-1 receptor-associated kinases (IRAKs) are critically involved in the regulation of intracellular signaling networks controlling inflammation. Collective studies indicate that IRAKs are present in many cell types, and can mediate signals from various cell receptors including toll-like-receptors (TLRs). Consequently, diverse downstream signaling processes can be elicited following the activation of various IRAKs. Given the critical and complex roles IRAK proteins play, it is not surprising that genetic variations in human IRAK genes have been found to be linked with various human inflammatory diseases. This review intends to summarize the recent advances regarding the regulations of various IRAK proteins and their cellular functions in mediating inflammatory signaling processes.     

Signaling,Regulation, and Specificity of the Type II p21-activated Kinases

The p21-activated kinases (PAKs) are a family of six serine/threonine kinases that act as key effectors of RHO family GTPases in mammalian cells. PAKs are subdivided into two groups: type I PAKs (PAK1, PAK2, and PAK3) and type II PAKs (PAK4, PAK5, and PAK6). Although these groups are involved in common signaling pathways, recent work indicates that the two groups have distinct modes of regulation and have both unique and common substrates. Here, we review recent insights into the molecular level details that govern regulation of type II PAK signaling. We also consider mechanisms by which signal transduction is regulated at the level of substrate specificity. Finally, we discuss the implications of these studies for clinical targeting of these kinases.     

The linkage between inflammation and fibrosis in muscular dystrophies: The axis autotaxin–lysophosphatidic acid as a new therapeutic target?

Muscular dystrophies (MDs) are a diverse group of severe disorders characterized by increased skeletal muscle feebleness. In many cases, respiratory and cardiac muscles are also compromised. Skeletal muscle inflammation and fibrosis are hallmarks of several skeletal muscle diseases, including MDs. Until now, several keys signaling pathways and factors that regulate inflammation and fibrosis have been identified. However, no curative treatments are available. Therefore, it is necessary to find new therapeutic targets to fight these diseases and improve muscle performance. Lysophosphatidic acid (LPA) is an active glycerophospholipid mainly synthesized by the secreted enzyme autotaxin (ATX), which activates six different G protein-coupled receptors named LPA₁ to LPA₆ (LPARs). In conjunction, they are part of the ATX/LPA/LPARs axis, involved in the inflammatory and fibrotic response in several organs-tissues. This review recapitulates the most relevant aspects of inflammation and fibrosis in MDs. It analyzes experimental evidence of the effects of the ATX/LPA/LPARs axis on inflammatory and fibrotic responses. Finally, we speculate about its potential role as a new therapeutic pharmacological target to treat these diseases.     

Bivalent inhibitors of protein kinases

Abstract

Protein kinases are key players in a large number of cellular signaling pathways. Dysregulated kinase activity has been implicated in a number of diseases, and members of this enzyme family are of therapeutic interest. However, due to the fact that most inhibitors interact with the highly conserved ATP-binding sites of kinases, it is a significant challenge to develop pharmacological agents that target only one of the greater than 500 kinases present in humans. A potential solution to this problem is the development of bisubstrate and bivalent kinase inhibitors, in which an active site-directed moiety is tethered to another ligand that targets a location outside of the ATP-binding cleft. Because kinase signaling specificity is modulated by regions outside of the ATP-binding site, strategies that exploit these interactions have the potential to provide reagents with high target selectivity. This review highlights examples of kinase interaction sites that can potentially be exploited by bisubstrate and bivalent inhibitors. Furthermore, an overview of efforts to target these interactions with bisubstrate and bivalent inhibitors is provided. Finally, several examples of the successful application of these reagents in a cellular setting are described.     

p21活化激酶的生物学活性及其与肿瘤的关系

p21活化激酶(p21-activatedkinase,PAK),为一类进化上保守的丝氨酸/苏氨酸蛋白激酶。PAK在许多组织中广泛表达,作为小G蛋白Rho家族Cdc42和Rac1的下游靶蛋白,可以被生长因子及其他胞外信号通过GTP酶依赖的信号通路或非GTP酶依赖的信号通路活化,发挥多种生物学效应。PAK作为一种重要的生物学调节因子,在哺乳动物一系列细胞功能中具有重要作用,如:细胞运动、细胞生存、细胞周期、血管生成、基因转录调节及癌细胞的侵袭转移。通过对PAK家族成员信号转导机制的研究,为癌症治疗提供分子靶标。     

Role of sphingosine kinase localization in sphingolipid signaling

The sphingosine kinases, SK1 and SK2, produce the potent signaling lipid sphingosine-1-phosphate (S1P). These enzymes have garnered increasing interest for their roles in tumorigenesis, inflammation, vascular diseases, and immunity, as well as other functions. The sphingosine kinases are considered signaling enzymes by producing S1P, and their activity is acutely regulated by a variety of agonists. However, these enzymes are also key players in the control of sphingolipid metabolism. A variety of sphingolipids, such as sphingosine and the ceramides, are potent signaling molecules in their own right. The role of sphingosine kinases in regulating sphingolipid metabolism is potentially a critical aspect of their signaling function. A central aspect of signaling lipids is that their hydrophobic nature constrains them to membranes. Most enzymes of sphingolipid metabolism, including the enzymes that degrade S1P, are membrane enzymes. Therefore the localization of the sphingosine kinases and S1P is likely to be important in S1P signaling. Sphingosine kinase localization affects sphingolipid signaling in several ways. Translocation of SK1 to the plasma membrane promotes extracellular secretion of S1P. SK1 and SK2 localization to specific sites appears to direct S1P to intracellular protein effectors. SK localization also determines the access of these enzymes to their substrates. This may be an important mechanism for the regulation of ceramide biosynthesis by diverting dihydrosphingosine, a precursor in the ceramide biosynthetic pathway, from the de novo production of ceramide.     

血管生成素与炎症性疾病

血管生成素（angiogenin,ANG）是首个被发现来源于肿瘤的具有血管生成能力的蛋白质,但其在炎症中的作用机制尚未完全阐明. 研究表明, ANG在炎症性疾病的发生发展中起重要作用,并与炎症的调控密切相关,而慢性炎症正是导致肿瘤形成、生长和转移的因素之一. 本文以ANG与炎症的关联为基础,结合我们的工作阐述ANG在炎症性疾病特别是肿瘤中的作用和调控机制,明确ANG与蛋白质的相互作用、对信号通路的调控及核内作用是其发挥功能的重要机制,也可能是调控肿瘤炎症的重要机制. 因而,深入研究ANG与炎症的关系不仅可加深我们对ANG兼具抗炎、抗新生血管双重功能的认识,更可为炎症性疾病的治疗提供潜在的作用靶点和新的思路和方法.     

Positive and negative regulation of insulin signaling through IRS-1 phosphorylation

This review will provide insight on the current understanding of the regulation of insulin signaling in both physiological and pathological conditions through modulations that occur with regards to the functions of the insulin receptor substrate 1 (IRS1). While the phosphorylation of IRS1 on tyrosine residue is required for insulin-stimulated responses, the phosphorylation of IRS1 on serine residues has a dual role, either to enhance or to terminate the insulin effects. The activation of PKB in response to insulin propagates insulin signaling and promotes the phosphorylation of IRS1 on serine residue in turn generating a positive-feedback loop for insulin action. Insulin also activates several kinases and these kinases act to induce the phosphorylation of IRS1 on specific sites and inhibit its functions. This is part of the negative-feedback control mechanism induced by insulin that leads to termination of its action. Agents such as free fatty acids, cytokines, angiotensin II, endothelin-1, amino acids, cellular stress and hyperinsulinemia, which induce insulin resistance, lead to both activation of several serine/threonine kinases and phosphorylation of IRS1. These agents negatively regulate the IRS1 functions by phosphorylation but also via others molecular mechanisms (SOCS expression, IRS degradation, O-linked glycosylation) as summarized in this review. Understanding how these agents inhibit IRS1 functions as well as identification of kinases involved in these inhibitory effects may provide novel targets for development of strategies to prevent insulin resistance.     

Cdc42/Rac1-mediated activation primes PAK2 for superactivation by tyrosine phosphorylation

The involvement of p21-activated kinases (PAKs) in important cellular processes such as regulation of the actin skeleton morphology, transduction of signals controlling gene expression, and execution of programmed cell death has directed attention to the regulation of the activity of these kinases. Here we report that activation of PAK2 by p21 GTPases can be strongly potentiated by cellular tyrosine kinases. PAK2 became tyrosine phosphorylated in its N-terminal regulatory domain, where Y130 was identified as the major phosphoacceptor site. Tyrosine phosphorylation-mediated superactivation of PAK2 could be induced by overexpression of different Src kinases or by inhibiting cellular tyrosine phosphatases with pervanadate and could be blocked by the Src kinase inhibitor PP1 or by mutating the Y130 residue. Analysis of PAK2 mutants activated by amino acid changes in the autoinhibitory domain or the catalytic domain indicated that GTPase-induced conformational changes, rather than catalytic activation per se, rendered PAK2 a target for tyrosine phosphorylation. Thus, PAK activation represents a potentially important point of convergence of tyrosine kinase- and p21 GTPase-dependent signaling pathways.     

c-Jun N-terminal kinase pathways in diabetes

Type 2 diabetes develops from insulin resistance and has become a worldwide epidemic. The c-Jun N-terminal kinases have been considered as signaling molecules linking inflammation and insulin resistance. Genetic disruption of c-Jun N-terminal kinase-1 gene prevents the development of insulin resistance in obese and diabetic mice. Inhibition of c-Jun N-terminal kinases by a small cell-permeable peptide improves insulin sensitivity in mice. Hepatic inhibition of c-Jun N-terminal kinases using a dominant-negative protein or knockdown of c-Jun N-terminal kinase-1 gene by RNA interference reduces blood glucose and insulin levels and enhances hepatic insulin signaling in mice. Recent evidence demonstrates that the hepatic c-Jun N-terminal kinase pathway plays an important role in lipid and lipoprotein homeostasis in mice. This review discusses recent advances in our understanding of the role of c-Jun N-terminal kinase pathway in metabolic control and its potential as a target for the treatment of type 2 diabetes.     

p21 activated kinase 5 activates Raf-1 and targets it to mitochondria

Raf-1 is an important effector of Ras mediated signaling and is a critical regulator of the ERK/MAPK pathway. Raf-1 activation is controlled in part by phosphorylation on multiple residues, including an obligate phosphorylation site at serine 338. Previously PAK1 and casein kinase II have been implicated as serine 338 kinases. To identify novel kinases that phosphorylate this site, we tested the ability of group II PAKs (PAKs 4-6) to control serine 338 phosphorylation. We observed that all group II PAKs were efficient serine 338 kinases, although only PAK1 and PAK5 significantly stimulated Raf-1 kinase activity. We also showed that PAK5 forms a tight complex with Raf-1 in the cell, but not A-Raf or B-Raf. Importantly, we also demonstrated that the association of Raf-1 with PAK5 targets a subpopulation of Raf-1 to mitochondria. These data indicate that PAK5 is a potent regulator of Raf-1 activity and may control Raf-1 dependent signaling at mitochondria.     

The effects of polyunsaturated fatty acids and antioxidant vitamins on atrial oxidative stress,nitrotyrosine residues,and connexins following extracorporeal circulation in patients undergoing cardiac surgery

Obesity-induced low-grade inflammation (metaflammation) impairs insulin receptor signaling. This has been implicated in the development of insulin resistance. Insulin signaling in the target tissues is mediated by stress kinases such as p38 mitogen-activated protein kinase, c-Jun NH2-terminal kinase, inhibitor of NF-kB kinase complex β (IKKβ), AMP-activated protein kinase, protein kinase C, Rho-associated coiled-coil containing protein kinase, and RNA-activated protein kinase. Most of these kinases phosphorylate several key regulators in glucose homeostasis. The phosphorylation of serine residues in the insulin receptor and IRS-1 molecule results in diminished enzymatic activity in the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. This has been one of the key mechanisms observed in the tissues that are implicated in insulin resistance especially in type 2 diabetes mellitus (T2-DM). Identifying the specific protein kinases involved in obesity-induced chronic inflammation may help in developing the targeted drug therapies to minimize the insulin resistance. This review is focused on the protein kinases involved in the inflammatory cascade and molecular mechanisms and their downstream targets with special reference to obesity-induced T2-DM.     

Central Nervous System Functions of PAK Protein Family: From Spine Morphogenesis to Mental Retardation.

Several of the genes currently known to be associated, when mutated, with mental retardation, code for molecules directly involved in Rho guanosine triphosphatase (GTPase) signaling. These include PAK3, a member of the PAK protein kinase family, which are important effectors of small GTPases. In many systems, PAK kinases play crucial roles regulating complex mechanisms such as cell migration, differentiation, or survival. Their precise functions in the central nervous system remain, however, unclear. Although their activity does not seem to be required for normal brain development, several recent studies point to a possible involvement in more subtle mechanisms such as neurite outgrowth, spine morphogenesis or synapse formation, and plasticity. This article reviews this information in the light of the current knowledge available on the molecular characteristics of the different members of this family and discuss the mechanisms through which they might contribute to cognitive functions.     

Central nervous system functions of PAK protein family

Several of the genes currently known to be associated, when mutated, with mental retardation, code for molecules directly involved in Rho guanosine triphosphatase (GTPase) signaling. These include PAK3, a member of the PAK protein kinase family, which are important effectors of small GTPases. In many systems, PAK kinases play crucial roles regulating complex mechanisms such as cell migration, differentiation, or survival. Their precise functions in the central nervous system remain, however, unclear. Although their activity does not seem to be required for normal brain development, several recent studies point to a possible involvement in more subtle mechanisms such as neurite outgrowth, spine morphogenesis or synapse formation, and plasticity. This article reviews this information in the light of the current knowledge available on the molecular characteristics of the different members of this family and discuss the mechanisms through which they might contribute to cognitive functions.