241 human active and 13 inactive phosphatases in total;
194 phosphatases have substrate data;
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336 protein substrates;
83 non-protein substrates;
1215 dephosphorylation interactions;
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299 KEGG pathways;
876 Reactome pathways;
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last update: 11 Mar, 2019
Cytoplasm Membrane Nucleus Mitochondrion Note=Palmitoylation regulatesMAPK10 trafficking to cytoskeleton Recruited to the mitochondriain the presence of SARM1 (By similarity)
Function (UniProt annotation)
Serine/threonine-protein kinase involved in variousprocesses such as neuronal proliferation, differentiation,migration and programmed cell death Extracellular stimuli such asproinflammatory cytokines or physical stress stimulate the stress-activated protein kinase/c-Jun N-terminal kinase (SAP/JNK)signaling pathway In this cascade, two dual specificity kinasesMAP2K4/MKK4 and MAP2K7/MKK7 phosphorylate and activateMAPK10/JNK3 In turn, MAPK10/JNK3 phosphorylates a number oftranscription factors, primarily components of AP-1 such as JUNand ATF2 and thus regulates AP-1 transcriptional activity Playsregulatory roles in the signaling pathways during neuronalapoptosis Phosphorylates the neuronal microtubule regulatorSTMN2 Acts in the regulation of the amyloid-beta precursorprotein/APP signaling during neuronal differentiation byphosphorylating APP Participates also in neurite growth in spiralganglion neurons Phosphorylates the CLOCK-ARNTL/BMAL1 heterodimerand plays a role in the photic regulation of the circadian clock(PubMed:22441692)
Endocrine therapy is a key treatment strategy to control or eradicate hormone-responsive breast cancer. The most commonly used endocrine therapy agents are selective estrogen receptor modulators (SERMs, e.g. tamoxifen), estrogen synthesis inhibitors (e.g. aromatase inhibitors (AIs) such as anastrozole, letrozole, and exemestane), and selective estrogen receptor down-regulators (SERDs, e.g. fulvestrant). However, resistance to these agents has become a major clinical obstacle. Mechanisms of endocrine resistance include loss of ER-alpha expression, altered expression of coactivators or coregulators that play a critical role in ER-mediated gene transcription, ligand-independent growth factor signaling cascades that activate kinases and ER-phosphorylation, altered availability of active tamoxifen metabolites regulated by drug-metabolizing enzymes, such as CYP2D6, and deregulation of the cell cycle and apoptotic machinery.
The mitogen-activated protein kinase (MAPK) cascade is a highly conserved module that is involved in various cellular functions, including cell proliferation, differentiation and migration. Mammals express at least four distinctly regulated groups of MAPKs, extracellular signal-related kinases (ERK)-1/2, Jun amino-terminal kinases (JNK1/2/3), p38 proteins (p38alpha/beta/gamma/delta) and ERK5, that are activated by specific MAPKKs: MEK1/2 for ERK1/2, MKK3/6 for the p38, MKK4/7 (JNKK1/2) for the JNKs, and MEK5 for ERK5. Each MAPKK, however, can be activated by more than one MAPKKK, increasing the complexity and diversity of MAPK signalling. Presumably each MAPKKK confers responsiveness to distinct stimuli. For example, activation of ERK1/2 by growth factors depends on the MAPKKK c-Raf, but other MAPKKKs may activate ERK1/2 in response to pro-inflammatory stimuli.
The ErbB family of receptor tyrosine kinases (RTKs) couples binding of extracellular growth factor ligands to intracellular signaling pathways regulating diverse biologic responses, including proliferation, differentiation, cell motility, and survival. Ligand binding to the four closely related members of this RTK family -epidermal growth factor receptor (EGFR, also known as ErbB-1 or HER1), ErbB-2 (HER2), ErbB-3 (HER3), and ErbB-4 (HER4)-induces the formation of receptor homo- and heterodimers and the activation of the intrinsic kinase domain, resulting in phosphorylation on specific tyrosine residues (pY) within the cytoplasmic tail. Signaling effectors containing binding pockets for pY-containing peptides are recruited to activated receptors and induce the various signaling pathways. The Shc- and/or Grb2-activated mitogen-activated protein kinase (MAPK) pathway is a common target downstream of all ErbB receptors. Similarly, the phosphatidylinositol-3-kinase (PI-3K) pathway is directly or indirectly activated by most ErbBs. Several cytoplasmic docking proteins appear to be recruited by specific ErbB receptors and less exploited by others. These include the adaptors Crk, Nck, the phospholipase C gamma (PLCgamma), the intracellular tyrosine kinase Src, or the Cbl E3 ubiquitin protein ligase.
The Ras proteins are GTPases that function as molecular switches for signaling pathways regulating cell proliferation, survival, growth, migration, differentiation or cytoskeletal dynamism. Ras proteins transduce signals from extracellular growth factors by cycling between inactive GDP-bound and active GTP-bound states. The exchange of GTP for GDP on RAS is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Activated RAS (RAS-GTP) regulates multiple cellular functions through effectors including Raf, phosphatidylinositol 3-kinase (PI3K) and Ral guanine nucleotide-dissociation stimulator (RALGDS).
cAMP is one of the most common and universal second messengers, and its formation is promoted by adenylyl cyclase (AC) activation after ligation of G protein-coupled receptors (GPCRs) by ligands including hormones, neurotransmitters, and other signaling molecules. cAMP regulates pivotal physiologic processes including metabolism, secretion, calcium homeostasis, muscle contraction, cell fate, and gene transcription. cAMP acts directly on three main targets: protein kinase A (PKA), the exchange protein activated by cAMP (Epac), and cyclic nucleotide-gated ion channels (CNGCs). PKA modulates, via phosphorylation, a number of cellular substrates, including transcription factors, ion channels, transporters, exchangers, intracellular Ca2+ -handling proteins, and the contractile machinery. Epac proteins function as guanine nucleotide exchange factors (GEFs) for both Rap1 and Rap2. Various effector proteins, including adaptor proteins implicated in modulation of the actin cytoskeleton, regulators of G proteins of the Rho family, and phospholipases, relay signaling downstream from Rap.
The forkhead box O (FOXO) family of transcription factors regulates the expression of genes in cellular physiological events including apoptosis, cell-cycle control, glucose metabolism, oxidative stress resistance, and longevity. A central regulatory mechanism of FOXO proteins is phosphorylation by the serine-threonine kinase Akt/protein kinase B (Akt/PKB), downstream of phosphatidylinositol 3-kinase (PI3K), in response to insulin or several growth factors. Phosphorylation at three conserved residues results in the export of FOXO proteins from the nucleus to the cytoplasm, thereby decreasing expression of FOXO target genes. In contrast, the stress-activated c-Jun N-terminal kinase (JNK) and the energy sensing AMP-activated protein kinase (AMPK), upon oxidative and nutrient stress stimuli phosphorylate and activate FoxOs. Aside from PKB, JNK and AMPK, FOXOs are regulated by multiple players through several post-translational modifications, including phosphorylation, but also acetylation, methylation and ubiquitylation.
Sphingomyelin (SM) and its metabolic products are now known to have second messenger functions in a variety of cellular signaling pathways. Particularly, the sphingolipid metabolites, ceramide (Cer) and sphingosine-1-phosphate (S1P), have emerged as a new class of potent bioactive molecules. Ceramide can be generated de novo or by hydrolysis of membrane sphingomyelin by sphingomyelinase (SMase). Ceramide is subsequently metabolized by ceramidase to generate sphingosine (Sph) which in turn produces S1P through phosphorylation by sphingosine kinases 1 and 2 (SphK1, 2). Both ceramide and S1P regulate cellular responses to stress, with generally opposing effects. S1P functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms.
Mitochondria act as the energy powerhouse of the cell, and are essential for eukaryotic cells to grow and function normally. However, deleterious byproducts of oxidative phosphorylation process called reactive oxidative species (ROS) lead to mitochondrial dysfunction. If the damage is too excessive to be repaired, such mitochondria are selectively recognized and targeted for degradation by a specific mode of autophagy, termed mitophagy. The loss of the mitochondrial membrane potential can induce mitophagy, involving the kinase PINK1 and the E3 ligase Parkin. PINK1 serves as the sensor for the mitochondrial depolarization and recruits Parkin, followed by ubiquitin-dependent recruitment of mitophagy receptors. There are also several PINK1/Parkin-independent mitophagy pathways, in which a group of LIR-containing receptors are required in response to different stimuli. Mitophagy contributes to the maintenance of a healthy mitochondrial network and the prevention of programmed cell death.
Autophagy (or macroautophagy) is a cellular catabolic pathway involving in protein degradation, organelle turnover, and non-selective breakdown of cytoplasmic components, which is evolutionarily conserved among eukaryotes and exquisitely regulated. This progress initiates with production of the autophagosome, a double-membrane intracellular structure of reticular origin that engulfs cytoplasmic contents and ultimately fuses with lysosomes for cargo degradation. Autophagy is regulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation and ER stress. Constitutive level of autophagy plays an important role in cellular homeostasis and maintains quality control of essential cellular components.
The endoplasmic reticulum (ER) is a subcellular organelle where proteins are folded with the help of lumenal chaperones. Newly synthesized peptides enter the ER via the sec61 pore and are glycosylated. Correctly folded proteins are packaged into transport vesicles that shuttle them to the Golgi complex. Misfolded proteins are retained within the ER lumen in complex with molecular chaperones. Proteins that are terminally misfolded bind to BiP and are directed toward degradation through the proteasome in a process called ER-associated degradation (ERAD). Accumulation of misfolded proteins in the ER causes ER stress and activates a signaling pathway called the unfolded protein response (UPR). In certain severe situations, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis.
Apoptosis is a genetically programmed process for the elimination of damaged or redundant cells by activation of caspases (aspartate-specific cysteine proteases). The onset of apoptosis is controlled by numerous interrelating processes. The 'extrinsic' pathway involves stimulation of members of the tumor necrosis factor (TNF) receptor subfamily, such as TNFRI, CD95/Fas or TRAILR (death receptors), located at the cell surface, by their specific ligands, such as TNF-alpha, FasL or TRAIL, respectively. The 'intrinsic' pathway is activated mainly by non-receptor stimuli, such as DNA damage, ER stress, metabolic stress, UV radiation or growth-factor deprivation. The central event in the 'intrinsic' pathway is the mitochondrial outer membrane permeabilization (MOMP), which leads to the release of cytochrome c. These two pathways converge at the level of effector caspases, such as caspase-3 and caspase-7. The third major pathway is initiated by the constituents of cytotoxic granules (e.g. Perforin and Granzyme B) that are released by CTLs (cytotoxic T-cells) and NK (natural killer) cells. Granzyme B, similarly to the caspases, cleaves its substrates after aspartic acid residues, suggesting that this protease has the ability to activate members of the caspase family directly. It is the balance between the pro-apoptotic and anti-apoptotic signals that eventually determines whether cells will undergo apoptosis, survive or proliferate. TNF family of ligands activates anti-apoptotic or cell-survival signals as well as apoptotic signals. NGF and Interleukin-3 promotes the survival, proliferation and differentiation of neurons or hematopoietic cells, respectively. Withdrawal of these growth factors leads to cell death, as described above.
Apoptosis is an evolutionarily conserved process used by multicellular organisms to developmentally regulate cell number or to eliminate cells that are potentially detrimental to the organism. The major players are caspases, caspase inhibitors, members of the Bcl-2 family of pro- and anti-apoptotic proteins and adaptors of the Ced-4/APAF-1 type. Mammals, by comparison with Caenorhabditis and Drosophila, exhibit highly complex extrinsic and intrinsic pathways for apoptosis induction. However, recent analyses of whole genome sequences from cnidarians (e.g. Hydra) suggest that the caspase and Bcl-2 families were already highly complex in cnidarians and that Caenorhabditis and Drosophila lost many of the genes involved in apoptosis.
Necroptosis is a programmed form of necrosis. It can be initiated by different stimuli, such as tumor necrosis factor (TNF), TNF-related apoptosis-inducing ligand (TRAIL), Fas ligand (FasL), interferon (IFN), LPS, viral DNA or RNA, DNA-damage agent and requires the kinase activity of receptor-interacting protein 1 (RIPK1) and RIPK3. Its execution involves ROS generation, calcium overload, the opening of the mitochondrial permeability transition pore, mitochondrial fission, inflammatory response and chromatinolysis. Necroptosis participates in many pathogenesis of diseases, including neurological diseases, retinal disorders, acute kidney injury, inflammatory diseases and microbial infections.
Wnt proteins are secreted morphogens that are required for basic developmental processes, such as cell-fate specification, progenitor-cell proliferation and the control of asymmetric cell division, in many different species and organs. There are at least three different Wnt pathways: the canonical pathway, the planar cell polarity (PCP) pathway and the Wnt/Ca2+ pathway. In the canonical Wnt pathway, the major effect of Wnt ligand binding to its receptor is the stabilization of cytoplasmic beta-catenin through inhibition of the bea-catenin degradation complex. Beta-catenin is then free to enter the nucleus and activate Wnt-regulated genes through its interaction with TCF (T-cell factor) family transcription factors and concomitant recruitment of coactivators. Planar cell polarity (PCP) signaling leads to the activation of the small GTPases RHOA (RAS homologue gene-family member A) and RAC1, which activate the stress kinase JNK (Jun N-terminal kinase) and ROCK (RHO-associated coiled-coil-containing protein kinase 1) and leads to remodelling of the cytoskeleton and changes in cell adhesion and motility. WNT-Ca2+ signalling is mediated through G proteins and phospholipases and leads to transient increases in cytoplasmic free calcium that subsequently activate the kinase PKC (protein kinase C) and CAMKII (calcium calmodulin mediated kinase II) and the phosphatase calcineurin.
The osteoclasts, multinucleared cells originating from the hematopoietic monocyte-macrophage lineage, are responsible for bone resorption. Osteoclastogenesis is mainly regulated by signaling pathways activated by RANK and immune receptors, whose ligands are expressed on the surface of osteoblasts. Signaling from RANK changes gene expression patterns through transcription factors like NFATc1 and characterizes the active osteoclast.
Cell-matrix adhesions play essential roles in important biological processes including cell motility, cell proliferation, cell differentiation, regulation of gene expression and cell survival. At the cell-extracellular matrix contact points, specialized structures are formed and termed focal adhesions, where bundles of actin filaments are anchored to transmembrane receptors of the integrin family through a multi-molecular complex of junctional plaque proteins. Some of the constituents of focal adhesions participate in the structural link between membrane receptors and the actin cytoskeleton, while others are signalling molecules, including different protein kinases and phosphatases, their substrates, and various adapter proteins. Integrin signaling is dependent upon the non-receptor tyrosine kinase activities of the FAK and src proteins as well as the adaptor protein functions of FAK, src and Shc to initiate downstream signaling events. These signalling events culminate in reorganization of the actin cytoskeleton; a prerequisite for changes in cell shape and motility, and gene expression. Similar morphological alterations and modulation of gene expression are initiated by the binding of growth factors to their respective receptors, emphasizing the considerable crosstalk between adhesion- and growth factor-mediated signalling.
Tight junctions (TJs) are essential for establishing a selectively permeable barrier to diffusion through the paracellular space between neighboring cells. TJs are composed of at least three types of transmembrane protein -occludin, claudin and junctional adhesion molecules (JAMs)- and a cytoplasmic 'plaque' consisting of many different proteins that form large complexes. These are proposed to be involved in junction assembly, barrier regulation, cell polarity, gene transcription, and other pathways.
Specific families of pattern recognition receptors are responsible for detecting microbial pathogens and generating innate immune responses. Toll-like receptors (TLRs) are membrane-bound receptors identified as homologs of Toll in Drosophila. Mammalian TLRs are expressed on innate immune cells, such as macrophages and dendritic cells, and respond to the membrane components of Gram-positive or Gram-negative bacteria. Pathogen recognition by TLRs provokes rapid activation of innate immunity by inducing production of proinflammatory cytokines and upregulation of costimulatory molecules. TLR signaling pathways are separated into two groups: a MyD88-dependent pathway that leads to the production of proinflammatory cytokines with quick activation of NF-{kappa}B and MAPK, and a MyD88-independent pathway associated with the induction of IFN-beta and IFN-inducible genes, and maturation of dendritic cells with slow activation of NF-{kappa}B and MAPK.
Specific families of pattern recognition receptors are responsible for detecting various pathogens and generating innate immune responses. The intracellular NOD-like receptor (NLR) family contains more than 20 members in mammals and plays a pivotal role in the recognition of intracellular ligands. NOD1 and NOD2, two prototypic NLRs, sense the cytosolic presence of the bacterial peptidoglycan fragments that escaped from endosomal compartments, driving the activation of NF-{kappa}B and MAPK, cytokine production and apoptosis. On the other hand, a different set of NLRs induces caspase-1 activation through the assembly of multiprotein complexes called inflammasomes. The activated of caspase-1 regulates maturation of the pro-inflammatory cytokines IL-1B, IL-18 and drives pyroptosis.
Specific families of pattern recognition receptors are responsible for detecting viral pathogens and generating innate immune responses. Non-self RNA appearing in a cell as a result of intracellular viral replication is recognized by a family of cytosolic RNA helicases termed RIG-I-like receptors (RLRs). The RLR proteins include RIG-I, MDA5, and LGP2 and are expressed in both immune and nonimmune cells. Upon recognition of viral nucleic acids, RLRs recruit specific intracellular adaptor proteins to initiate signaling pathways that lead to the synthesis of type I interferon and other inflammatory cytokines, which are important for eliminating viruses.
C-type lectin receptors (CLRs) are a large superfamily of proteins characterized by the presence of one or more C-type lectin-like domains (CTLDs). CLRs function as pattern-recognition receptors (PRRs) for pathogen-derived ligands in dendric cells, macrophages, neutrophils, etc., such as Dectin-1 and Dectin-2 for recognition of fungi-derived B-glucan and high mannose-type carbohydrates. Upon ligand binding, CLRs stimulate intracellular signaling cascades that induce the production of inflammatory cytokines and chemokines, consequently triggering innate and adaptive immunity to pathogens.
The interleukin 17 (IL-17) family, a subset of cytokines consisting of IL-17A-F, plays crucial roles in both acute and chronic inflammatory responses. IL-17A, the hallmark cytokine of the newly defined T helper 17 (TH17) cell subset, has important roles in protecting the host against extracellular pathogens, but also promotes inflammatory pathology in autoimmune disease, whereas IL-17F is mainly involved in mucosal host defense mechanisms. IL-17E (IL-25) is an amplifier of Th2 immune responses. IL-17C has biological functions similar to those of IL-17A. The functions of IL-17B and IL-17D remain largely elusive. The IL-17 family signals via their correspondent receptors and activates downstream pathways that include NF-kappaB, MAPKs and C/EBPs to induce the expression of antimicrobial peptides, cytokines and chemokines. The receptor proximal adaptor Act1 (an NF-kappaB activator 1) is considered as the master mediator in IL-17A signaling. It is likely that Act1 is a common signal adaptor also shared by other members mediated signalings in this family.
Immunity to different classes of microorganisms is orchestrated by separate lineages of effector T helper (TH)-cells, which differentiate from naive CD4+ precursor cells in response to cues provided by antigen presenting cells (APC) and include T helper type 1 (Th1) and Th2. Th1 cells are characterized by the transcription factor T-bet and signal transducer and activator of transcription (STAT) 4, and the production of IFN-gamma. These cells stimulate strong cell-mediated immune responses, particularly against intracellular pathogens. On the other hand, transcription factors like GATA-3 and STAT6 drive the generation of Th2 cells that produce IL-4, IL-5 and IL-13 and are necessary for inducing the humoral response to combat parasitic helminths (type 2 immunity) and isotype switching to IgG1 and IgE. The balance between Th1/Th2 subsets determines the susceptibility to disease states, where the improper development of Th2 cells can lead to allergy, while an overactive Th1 response can lead to autoimmunity.
Interleukin (IL)-17-producing helper T (Th17) cells serve as a subset of CD4+ T cells involved in epithelial cell- and neutrophil mediated immune responses against extracellular microbes and in the pathogenesis of autoimmune diseases. In vivo, Th17 differentiation requires antigen presentation and co-stimulation, and activation of antigen presenting-cells (APCs) to produce TGF-beta, IL-6, IL-1, IL-23 and IL-21. This initial activation results in the activation and up-regulation of STAT3, ROR(gamma)t and other transcriptional factors in CD4+ T cells, which bind to the promoter regions of the IL-17, IL-21 and IL-22 genes and induce IL-17, IL-21 and IL-22. In contrast, the differentiation of Th17 cells and their IL-17 expression are negatively regulated by IL-2, Th2 cytokine IL-4, IL-27 and Th1 cytokine IFN-gamma through STAT5, STAT6 and STAT1 activation, respectively. Retinoid acid and the combination of IL-2 and TGF-beta upregulate Foxp3, which also downregulates cytokines like IL-17 and IL-21. The inhibition of Th17 differentiation may serve as a protective strategy to 'fine-tune' the expression IL-17 so it does not cause excessive inflammation. Thus, balanced differentiation of Th cells is crucial for immunity and host protection.
Fc epsilon RI-mediated signaling pathways in mast cells are initiated by the interaction of antigen (Ag) with IgE bound to the extracellular domain of the alpha chain of Fc epsilon RI. The activation pathways are regulated both positively and negatively by the interactions of numerous signaling molecules. Mast cells that are thus activated release preformed granules which contain biogenic amines (especially histamines) and proteoglycans (especially heparin). The activation of phospholipase A2 causes the release of membrane lipids followed by development of lipid mediators such as leukotrienes (LTC4, LTD4 and LTE4) and prostaglandins (especially PDG2). There is also secretion of cytokines, the most important of which are TNF-alpha, IL-4 and IL-5. These mediators and cytokines contribute to inflammatory responses.
Tumor necrosis factor (TNF), as a critical cytokine, can induce a wide range of intracellular signal pathways including apoptosis and cell survival as well as inflammation and immunity. Activated TNF is assembled to a homotrimer and binds to its receptors (TNFR1, TNFR2) resulting in the trimerization of TNFR1 or TNFR2. TNFR1 is expressed by nearly all cells and is the major receptor for TNF (also called TNF-alpha). In contrast, TNFR2 is expressed in limited cells such as CD4 and CD8 T lymphocytes, endothelial cells, microglia, oligodendrocytes, neuron subtypes, cardiac myocytes, thymocytes and human mesenchymal stem cells. It is the receptor for both TNF and LTA (also called TNF-beta). Upon binding of the ligand, TNFR mediates the association of some adaptor proteins such as TRADD or TRAF2, which in turn initiate recruitment of signal transducers. TNFR1 signaling induces activation of many genes, primarily controlled by two distinct pathways, NF-kappa B pathway and the MAPK cascade, or apoptosis and necroptosis. TNFR2 signaling activates NF-kappa B pathway including PI3K-dependent NF-kappa B pathway and JNK pathway leading to survival.
Neurotrophins are a family of trophic factors involved in differentiation and survival of neural cells. The neurotrophin family consists of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and neurotrophin 4 (NT-4). Neurotrophins exert their functions through engagement of Trk tyrosine kinase receptors or p75 neurotrophin receptor (p75NTR). Neurotrophin/Trk signaling is regulated by connecting a variety of intracellular signaling cascades, which include MAPK pathway, PI-3 kinase pathway, and PLC pathway, transmitting positive signals like enhanced survival and growth. On the other hand, p75NTR transmits both positive and nagative signals. These signals play an important role for neural development and additional higher-order activities such as learning and memory.
Endogenous cannabinoids (endocannabinoids) serve as retrograde messengers at synapses in various regions of the brain. The family of endocannabinoids includes at least five derivatives of arachidonic acid; the two best characterized are arachydonoyl ethanolamide (anandamide, AEA) and 2-arachydonoil glycerol (2AG). They are released from postsynaptic neurons upon postsynaptic depolarization and/or receptor activation. The released endocannabinoids then activate the CB1 receptors (CB1R) at presynaptic terminals and suppress the release of inhibitory transmitter GABA (depolarization-induced suppression of inhibition, DSI) or excitatory transmitter glutamate (depolarization-induced suppression of excitation, DSE) by inhibiting Ca2+ channels. Besides the well-known expression of the CB1R in the plasma membrane, this receptor is also present in mitochondrial membranes, where it reduces the mitochondrial respiration and contributes to DSI. Whereas DSI and DSE result in short-term synaptic plasticity, endocannabinoids also mediate long-term synaptic changes (eCB-LTD). Persistent activation of CB1 receptors over a period of minutes triggers eCB-LTD by a RIM1alpha-dependent mechanism.
Dopamine (DA) is an important and prototypical slow neurotransmitter in the mammalian brain, where it controls a variety of functions including locomotor activity, motivation and reward, learning and memory, and endocrine regulation. Once released from presynaptic axonal terminals, DA interacts with at least five receptor subtypes in the central nervous system (CNS), which have been divided into two groups: the D1-like receptors (D1Rs), comprising D1 and D5 receptors, both positively coupled to adenylyl cyclase and cAMP production, and the D2-like receptors (D2Rs), comprising D2, D3, and D4 receptors, whose activation results in inhibition of adenylyl cyclase and suppression of cAMP production. In addition, D1Rs and D2Rs modulate intracellular Ca2+ levels and a number of Ca2+ -dependent intracellular signaling processes. Through diverse cAMP- and Ca2+-dependent and - independent mechanisms, DA influences neuronal activity, synaptic plasticity, and behavior. Presynaptically localized D2Rs regulate synthesis and release of DA as the main autoreceptor of the dopaminergic system.
The TRP channels that exhibit a unique response to temperature have been given the name thermo-TRPs. Among all thermo- TRP channels, TRPV1-4, TRPM8, and TRPA1 are expressed in subsets of nociceptive dorsal root ganglion (DRG) neuron cell bodies including their peripheral and central projections. These channels can be modulated indirectly by inflammatory mediators such as PGE2, bradykinin, ATP, NGF, and proinflammatory cytokines that are generated during tissue injury. While the noxious heat receptor TRPV1 is sensitized (that is, their excitability can be increased) by post-translational modifications upon activation of G-protein coupled receptors (GPCRs) or tyrosine kinase receptors, the receptors for inflammatory mediators, the same action appears to mainly desensitize TRPM8, the main somatic innocuous cold sensor. This aforementioned sensitization could allow the receptor to become active at body temperature, so it not only contributes toward thermal hypersensitivity but also is possibly a substrate for ongoing persistent pain.
Insulin binding to its receptor results in the tyrosine phosphorylation of insulin receptor substrates (IRS) by the insulin receptor tyrosine kinase (INSR). This allows association of IRSs with the regulatory subunit of phosphoinositide 3-kinase (PI3K). PI3K activates 3-phosphoinositide-dependent protein kinase 1 (PDK1), which activates Akt, a serine kinase. Akt in turn deactivates glycogen synthase kinase 3 (GSK-3), leading to activation of glycogen synthase (GYS) and thus glycogen synthesis. Activation of Akt also results in the translocation of GLUT4 vesicles from their intracellular pool to the plasma membrane, where they allow uptake of glucose into the cell. Akt also leads to mTOR-mediated activation of protein synthesis by eIF4 and p70S6K. The translocation of GLUT4 protein is also elicited through the CAP/Cbl/TC10 pathway, once Cbl is phosphorylated by INSR.Other signal transduction proteins interact with IRS including GRB2. GRB2 is part of the cascade including SOS, RAS, RAF and MEK that leads to activation of mitogen-activated protein kinase (MAPK) and mitogenic responses in the form of gene transcription. SHC is another substrate of INSR. When tyrosine phosphorylated, SHC associates with GRB2 and can thus activate the RAS/MAPK pathway independently of IRS-1.
Gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus acts upon its receptor in the anterior pituitary to regulate the production and release of the gonadotropins, LH and FSH. The GnRHR is coupled to Gq/11 proteins to activate phospholipase C which transmits its signal to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG activates the intracellular protein kinase C (PKC) pathway and IP3 stimulates release of intracellular calcium. In addition to the classical Gq/11, coupling of Gs is occasionally observed in a cell-specific fashion. Signaling downstream of protein kinase C (PKC) leads to transactivation of the epidermal growth factor (EGF) receptor and activation of mitogen-activated protein kinases (MAPKs), including extracellular-signal-regulated kinase (ERK), Jun N-terminal kinase (JNK) and p38 MAPK. Active MAPKs translocate to the nucleus, resulting in activation of transcription factors and rapid induction of early genes.
Xenopus oocytes are naturally arrested at G2 of meiosis I. Exposure to either insulin/IGF-1 or the steroid hormone progesterone breaks this arrest and induces resumption of the two meiotic division cycles and maturation of the oocyte into a mature, fertilizable egg. This process is termed oocyte maturation. The transition is accompanied by an increase in maturation promoting factor (MPF or Cdc2/cyclin B) which precedes germinal vesicle breakdown (GVBD). Most reports point towards the Mos-MEK1-ERK2 pathway [where ERK is an extracellular signal-related protein kinase, MEK is a MAPK/ERK kinase and Mos is a p42(MAPK) activator] and the polo-like kinase/CDC25 pathway as responsible for the activation of MPF in meiosis, most likely triggered by a decrease in cAMP.
Prolactin (PRL) is a polypeptide hormone known to be involved in a wide range of biological functions including osmoregulation, lactation, reproduction, growth and development, endocrinology and metabolism, brain and behavior, and immunomodulation. PRL mediates its action through PRLR, a transmembrane protein of the hematopoietin cytokine receptor superfamily. At the protein level, the long PRLR isoform (long-R) and several short PRLR isoforms (short-R) have been detected. Acting through the long-R, PRL activates many signaling cascades including Jak2/Stat, the major cascade, Src kinase, phosphatidylinositol-3-kinase (PI3K)/AKT, and mitogen-activated protein kinase (MAPK) pathways. PRL cannot activate Jak2/Stat5 through the short-R, but can activate pathways including MAPK and PI3K pathways.
Increased adipocyte volume and number are positively correlated with leptin production, and negatively correlated with production of adiponectin.Leptin is an important regulator of energy intake and metabolic rate primarily by acting at hypothalamic nuclei. Leptin exerts its anorectic effects by modulating the levels of neuropeptides such as NPY, AGRP, and alpha-MSH. This leptin action is through the JAK kinase, STAT3 phosphorylation, and nuclear transcriptional effect.Adiponectin lowers plasma glucose and FFAs. These effects are partly accounted for by adiponectin-induced AMPK activation, which in turn stimulates skeletal muscle fatty acid oxidation and glucose uptake. Furthermore, activation of AMPK by adiponectin suppresses endogenous glucose production, concomitantly with inhibition of PEPCK and G6Pase expression.The proinflammatory cytokine TNFalpha has been implicated as a link between obesity and insulin resistance. TNFalpha interferes with early steps of insulin signaling. Several data have shown that TNFalpha inhibits IRS1 tyrosine phosphorylation by promoting its serine phosphorylation. Among the serine/threonine kinases activated by TNFalpha, JNK, mTOR and IKK have been shown to be involved in this phosphorylation.
Human relaxin-2 (relaxin), originally identified as a peptidic hormone of pregnancy, is now known to exert a range of pleiotropic effects including vasodilatory, anti-fibrotic and angiogenic effects in both males and females. It belongs to the so-called relaxin peptide family which includes the insulin-like peptides INSL3 and INSL5, and relaxin-3 (H3) as well as relaxin. INSL3 has clearly defined specialist roles in male and female reproduction, relaxin-3 is primarily a neuropeptide involved in stress and metabolic control, and INSL5 is widely distributed particularly in the gastrointestinal tract. These members of relaxin peptide family exert such effects binding to different kinds of receptors, classified as relaxin family peptide (RXFP) receptors: RXFP1, RXFP2, RXFP3, and RXFP4. These G protein-coupled receptors predominantly bind relaxin, INSL3, relaxin-3, and INSL-5, respectively. RXFP1 activates a wide spectrum of signaling pathways to generate second messengers that include cAMP and nitric oxide, whereas RXFP2 activates a subset of these pathways. Both RXFP3 and RXFP4 inhibit cAMP production, and RXFP3 activate MAP kinases.
Insulin resistance is strongly associated with type II diabetes. Diabetogenicfactors including FFA, TNFalpha and cellular stress induce insulin resistance through inhibition of IRS1 functions. Serine/threonine phosphorylation, interaction with SOCS, regulation of the expression, modification of the cellular localization, and degradation represent the molecular mechanisms stimulated by them. Various kinases (ERK, JNK, IKKbeta, PKCzeta, PKCtheta and mTOR) are involved in this process.The development of type II diabetes requires impaired beta-cell function. Chronic hyperglycemia has been shown to induce multiple defects in beta-cells. Hyperglycemia has been proposed to lead to large amounts of reactive oxygen species (ROS) in beta-cells, with subsequent damage to cellular components including PDX-1. Loss of PDX-1, a critical regulator of insulin promoter activity, has also been proposed as an important mechanism leading to beta-cell dysfunction.Although there is little doubt as to the importance of genetic factors in type II diabetes, genetic analysis is difficult due to complex interaction among multiple susceptibility genes and between genetic and environmental factors. Genetic studies have therefore given very diverse results. Kir6.2 and IRS are two of the candidate genes. It is known that Kir6.2 and IRS play central roles in insulin secretion and insulin signal transmission, respectively.
Insulin resistance is a condition where cells become resistant to the effects of insulin. It is often found in people with health disorders, including obesity, type 2 diabetes mellitus, non-alcoholic fatty liver disease, and cardiovascular diseases. In this diagram multiple mechanisms underlying insulin resistance are shown: (a) increased phosphorylation of IRS (insulin receptor substrate) protein through serine/threonine kinases, such as JNK1 and IKKB, and protein kinase C, (b) increased IRS-1 proteasome degradation via mTOR signaling pathway, (c) decreased activation of signaling molecules including PI3K and AKT, (d) increase in activity of phosphatases including PTPs, PTEN, and PP2A. Regulatory actions such as oxidative stress, mitochondrial dysfunction, accumulation of intracellular lipid derivatives (diacylglycrol and ceramides), and inflammation (via IL-6 and TNFA) contribute to these mechanisms. Consequently, insulin resistance causes reduced GLUT4 translocation, resulting in glucose takeup and glycogen synthesis in skeletal muscle as well as increased hepatic gluconeogenesis and decreased glycogen synthesis in liver. At the bottom of the diagram, interplay between O-GlcNAcylation and serine/threonine phosphorylation is shown. Studies suggested that elevated O-GlcNAc level was correlated to high glucose-induced insulin resistance. Donor UDP-GlcNAc is induced through hexosamine biosynthesis pathway and added to proteins by O-GlcNAc transferase. Elevation of O-GlcNAc modification alters phosphorylation and function of key insulin signaling proteins including IRS-1, PI3K, PDK1, Akt and other transcription factor and cofactors, resulting in the attenuation of insulin signaling cascade.
Non-alcoholic fatty liver disease (NAFLD) represents a spectrum ranging from simple steatosis to more severe steatohepatitis with hepatic inflammation and fibrosis, known as nonalcoholic steatohepatitis (NASH). NASH may further lead to cirrhosis and hepatocellular carcinoma (HCC). This map shows a stage-dependent progression of NAFLD. In the first stage of NAFLD, excess lipid accumulation has been demonstrated. The main cause is the induction of insulin resistance, which leads to a defect in insulin suppression of free fatty acids (FAAs) disposal. In addition, two transcription factors, SREBP-1c and PPAR-alpha, activate key enzymes of lipogenesis and increase the synthesis of FAAs in liver. In the second stage, as a consequence of the progression to NASH, the production of reactive oxygen species (ROS) is enhanced due to oxidation stress through mitochondrial beta-oxidation of fatty acids and endoplamic reticulum (ER) stress, leading to lipid peroxidation. The lipid peroxidation can further cause the production of cytokines (Fas ligand, TNF-alpha, IL-8 and TGF), promoting cell death, inflammation and fibrosis. The activation of JNK, which is induced by ER stress, TNF-alpha and FAAs, is also associated with NAFLD progression. Increased JNK promotes cytokine production and initiation of HCC.
Advanced glycation end products (AGEs) are a complex group of compounds produced through the non-enzymatic glycation and oxidation of proteins, lipids and nucleic acids, primarily due to aging and under certain pathologic condition such as huperglycemia. Some of the best chemically characterized AGEs include N-epsilon-carboxy-methyl-lysine (CML), N-epsilon-carboxy-ethyl-lysine (CEL), and Imidazolone. The major receptor for AGEs, known as receptor for advanced glycation end products (RAGE or AGER), belongs to the immunoglobulin superfamily and has been described as a pattern recognition receptor. AGE/RAGE signaling elicits activation of multiple intracellular signal pathways involving NADPH oxidase, protein kinase C, and MAPKs, then resulting in NF-kappaB activity. NF-kappa B promotes the expression of pro-inflammatory cytokines such as IL-1, IL-6 and TNF-alpha and a variety of atherosclerosis-related genes, including VCAM-1, tissue factor, VEGF, and RAGE. In addition, JAK-STAT-mediated and PI3K-Akt-dependent pathways are induced via RAGE, which in turn participate in cell proliferation and apoptosis respectively. Hypoxia-mediated induction of Egr-1 was also shown to require the AGE-RAGE interaction. The results of these signal transductions have been reported to be the possible mechanism that initates diabetic complications.
Two major virulence factors of H. pylori are the vacuolating cytotoxin (VacA) and the cag type-IV secretion system (T4SS) and its translocated effector protein, cytotoxin-associated antigen A (CagA).VacA binds to lipid rafts and glycosylphosphatidylinositol-anchored proteins (GPI-APs) of the target cell membrane. After insertion into the plasma membrane, VacA channels are endocytosed and eventually reach late endosomal compartments, increasing their permeability to anions with enhancement of the electrogenic vacuolar ATPase (v-ATPase) proton pump. In the presence of weak bases, osmotically active acidotropic ions will accumulate in the endosomes. This leads to water influx and vesicle swelling, an essential step in vacuole formation. In addition, it is reported that the VacA cleavage product binds to the tyrosine phosphatase receptor zeta (Ptprz) on epithelial cells and the induced signaling leads to the phosphorylation of the G protein-coupled receptor kinase-interactor 1 (Git1) and induces ulcerogenesis in mice.The other virulence factor cag T4SS mediates the translocation of the effector protein CagA, which is subsequently phosphorylated by a Src kinase. Phosphorylated CagA interacts with the protein tyrosine phosphatase SHP-2, thus stimulating its phosphatase activity. Activated SHP-2 is able to induce MAPK signalling through Ras/Raf-dependent and -independent mechanisms. Deregulation of this pathway by CagA may lead to abnormal proliferation and movement of gastric epithelial cells.
Shigellosis, or bacillary dysentery, is an intestinal infection caused by Shigella, a genus of enterobacteria. Shigella are potential food-borne pathogens that are capable of colonizing the intestinal epithelium by exploiting epithelial-cell functions and circumventing the host innate immune response. During basolateral entry into the host-cell cytoplasm, Shigella deliver a subset of effectors into the host cells through the type III secretion system. The effectors induce membrane ruffling through the stimulation of the Rac1-WAVE-Arp2/3 pathway, enabling bacterial entry into the epithelial cells. During multiplication within the cells, Shigella secrete another subset of effectors. VirG induces actin polymerization at one pole of the bacteria, allowing the bacteria to spread intracellularly and to infect adjacent cells. OspF, OspG and IpaH(9.8) downregulate the production of proinflammatory cytokines such as IL-8, helping bacteria circumvent the innate immune response.
Salmonella infection usually presents as a self-limiting gastroenteritis or the more severe typhoid fever and bacteremia. The common disease-causing Salmonella species in human is a single species, Salmonella enterica, which has numerous serovars.Following intestinal colonization Salmonella inject effector proteins into the host cells using a type III secretion system (T3SS), T3SS1. Then a small group of effector proteins induce rearrangement of the actin cytoskeleton resulting in membrane ruffles and rapid internalization of the bacteria.The T3SS2 is responsible for translocating effector proteins that direct Salmonella-containing vacuole (SCV) maturation. The majority of the bacteria are known to survive and replicate in SCV.
Pertussis, also known as whooping cough, is an acute respiratory infectious disease caused by a bacteria called Bordetella Pertussis. The characteristic symptoms are paroxysmal cough, inspiratory wheezing and post-tussive vomiting.Following the inhalation of respiratory secretions from an infected individual, bacteria enter the upper respiratory tract and adhere to epithelial cells. Several adhesion factors have been implicated: the filamentous hemagglutinin (FHA), fimbriae, and pertactin (Prn).Pertussis toxin (Ptx) and adenylate cyclase toxin (ACT) have been identified so far as major protein toxins of B. pertussis. PTX is a hexameric AB5-type exotoxin. Catalytic A subunit catalyzes the ADP-ribosylation of the Gi subunits of the heterotrimeric G protein, then inhibits multiple downstream pathways. ACT is able to penetrate the cytoplasmic membrane of host cells and becomes activated through the cleavage and the binding of calmodulin (CaM). Activated ACT converts ATP to cyclic AMP and subverts cellular signaling pathways.
Trypanosoma cruzi is an intracellular protozoan parasite that causes Chagas disease. The parasite life cycle involves hematophagous reduviid bugs as vectors. Once parasites enter the host body, they invade diverse host cells including cardiomyocytes. Establishment of infection depends on various parasite molecules such as cruzipain, oligopeptidase B, and trans-sialidase that activate Ca2+ signaling. Internalized parasites escape from the parasitophorous vacuole using secreted pore-forming TcTOX molecule and replicate in the cytosol. Multiplied parasites eventually lyse infected host cells and are released in the circulation. During these events, the parasites manipulate host innate immunity and elicit cardiomyocyte hypertrophy. T lymphocyte responses are also disturbed.
Toxoplasma gondii is an obligate intracellular parasite that is prevalent worldwide. The tachyzoite form acquired by oral ingestion downmodulates proinflammatory signaling pathways via various mechanisms. During early infection, nuclear translocation of NFkB is temporally blocked and p38 MAPK phosphorylation is prevented, suppressing IL-12 production. Another pathway for IL-12 induction occurs through CCR5 dependent pathway, but parasitic induction of an eicosanoid LXA4 contributes to the downregulation of IL-12. Direct activation of STAT3 by the parasite enhance anti-inflammatory function of IL-10 and TGF beta. T. gondii can cause lifelong chronic infection by establishing an anti-apoptotic environment through induction of bcl-2 or IAPs and by redirecting LDL-mediated cholesterol transport to scavenge nutrients from the host.
Tuberculosis, or TB, is an infectious disease caused by Mycobacterium tuberculosis. One third of the world's population is thought to be infected with TB. About 90% of those infected result in latent infections, and about 10% of latent infections develop active diseases when their immune system is impaired due to the age, other diseases such as AIDS or exposure to immunosuppressive drugs. TB is transmitted through the air and primarily attacks the lungs, then it can spread by the circulatory system to other parts of body. Once TB bacilli have entered the host by the respiratory route and infected macrophages in the lungs, they interfere with phagosomal maturation, antigen presentation, apoptosis and host immune system to establish persistent or latent infection.
Hepatitis C virus (HCV) is a major cause of chronic liver disease. The HCV employ several strategies to perturb host cell immunity. After invasion, HCV RNA genome functions directly as an mRNA in the cytoplasm of the host cell and forms membrane-associated replication complexes along with non-structural proteins. Viral RNA can trigger the RIG-I pathway and interferon production during this process. Translated HCV protein products regulate immune response to inhibit the action of interferon. HCV core and NS5A proteins appear to be the most important molecules with regulatory functions that modulate transcription, cellular proliferation, and apoptosis.
Hepatitis B virus (HBV) is an enveloped virus and contains a partially double-stranded relaxed circular DNA (RC-DNA) genome. After entry into hepatocytes, HBV RC-DNA is transported to the nucleus and converted into a covalently closed circular molecule cccDNA. The cccDNA is the template for transcription of all viral RNAs including the pregenomic RNA (pgRNA), encoding for 7 viral proteins: large, middle, and small envelope proteins (LHBs, MHBs, and SHBs) that form the surface antigen (HBsAg), the core antigen (HBcAg), the e antigen (HBeAg), the HBV polymerase, and the regulatory protein X (HBx). The pgRNA interacts with the viral polymerase protein to initiate the encapsidation into the core particles. Through endoplasmic reticulum, the core particles finish assembling with the envelope proteins and are released. HBV infection leads to a wide spectrum of liver diseases raging from chronic hepatitis, cirrhosis to hepatocellular carcinoma. The mechanism of liver injury is still not clear. However, HBV proteins target host proteins, involved in a variety of functions, thus regulating transcription, cellular signaling cascades, proliferation, differentiation, and apoptosis.
Influenza is a contagious respiratory disease caused by influenza virus infection. Influenza A virus is responsible for both annual seasonal epidemics and periodic worldwide pandemics. Novel strains that cause pandemics arise from avian influenza virus by genetic reassortment among influenza viruses and two surface glycoproteins HA and NA form the basis of serologically distinct virus types. The innate immune system recognizes invaded virus through multiple mechanisms. Viral non-structural NS1 protein is a multifunctional virulence factor that interfere IFN-mediated antiviral response. It inhibits IFN production by blocking activation of transcription factors such as NF-kappa B, IRF3 and AP1. NS1 further inhibits the activation of IFN-induced antiviral genes. PB1-F2 protein is another virulence factor that induce apoptosis of infected cells, which results in life-threatening bronchiolitis.
Kaposi sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is the most recently identified human tumor virus, and is associated with the pathogenesis of Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and Multicentric Castleman's disease (MCD). Like all other herpesviruses, KSHV displays two modes of life cycle, latency and lytic replication, which are characterized by the patterns of viral gene expression. Genes expressed in latency (LANA, v-cyclin, v-FLIP, Kaposins A, B and C and viral miRNAs) are mainly thought to facilitate the establishment of life long latency in its host and survival against the host innate, and adaptive immune surveillance mechanisms. Among the viral proteins shown to be expressed during lytic replication are potent signaling molecules such as vGPCR, vIL6, vIRFs, vCCLs, K1 and K15, which have been implicated experimentally in the angiogenic and inflammatory phenotype observed in KS lesions. Several of these latent viral and lytic proteins are known to transform host cells, linking KSHV with the development of severe human malignancies.
Herpes simplex virus (HSV) infections are very common worldwide, with the prevalence of HSV-1 reaching up to 80%-90%. Primary infection with HSV takes place in the mucosa, followed by the establishment of latent infection in neuronal ganglia. HSV is the main cause of herpes infections that lead to the formation of characteristic blistering lesion. HSV express multiple viral accessory proteins that interfere with host immune responses and are indispensable for viral replication. Among these proteins, the immediate early (IE) gene ICP0, ICP4, and ICP27 are essential for regulation of HSV gene expression in productive infection. On the other hand, ORF P and ORF O gene are transcribed during latency and blocks the expression of the IE genes, thus maintaining latent infection.
Epstein-Barr virus (EBV) is a gamma-herpes virus that widely infects human populations predominantly at an early age but remains mostly asymptomatic. EBV has been linked to a wide spectrum of human malignancies, including nasopharyngeal carcinoma and other hematologic cancers, like Hodgkin's lymphoma, Burkitt's lymphoma (BL), B-cell immunoblastic lymphoma in HIV patients, and posttransplant-associated lymphoproliferative diseases. EBV has the unique ability to establish life-long latent infection in primary human B lymphocytes. During latent infection, EBV expresses a small subset of genes, including 6 nuclear antigens (EBNA-1, -2, -3A, -3B, -3C, and -LP), 3 latent membrane proteins (LMP-1, -2A, and -2B), 2 small noncoding RNAs (EBER-1 and 2). On the basis of these latent gene expression, three different latency patterns associated with the types of cancers are recognized.
Human immunodeficiency virus type 1 (HIV-1) , the causative agent of AIDS (acquired immunodeficiency syndrome), is a lentivirus belonging to the Retroviridae family. The primary cell surface receptor for HIV-1, the CD4 protein, and the co-receptor for HIV-1, either CCR5 or CXCR4, are found on macrophages and T lymphocytes. At the earliest step, sequential binding of virus envelope (Env) glycoprotein gp120 to CD4 and the co-receptor CCR5 or CXCR4 facilitates HIV-1 entry and has the potential to trigger critical signaling that may favor viral replication. At advanced stages of the disease, HIV-1 infection results in dramatic induction of T-cell (CD4+ T and CD8+ T cell) apoptosis both in infected and uninfected bystander T cells, a hallmark of HIV-1 pathogenesis. On the contrary, macrophages are resistant to the cytopathic effect of HIV-1 and produce virus for longer periods of time.
Colorectal cancer (CRC) is the second largest cause of cancer-related deaths in Western countries. CRC arises from the colorectal epithelium as a result of the accumulation of genetic alterations in defined oncogenes and tumour suppressor genes (TSG). Two major mechanisms of genomic instability have been identified in sporadic CRC progression. The first, known as chromosomal instability (CIN), results from a series of genetic changes that involve the activation of oncogenes such as K-ras and inactivation of TSG such as p53, DCC/Smad4, and APC. The second, known as microsatellite instability (MSI), results from inactivation of the DNA mismatch repair genes MLH1 and/or MSH2 by hypermethylation of their promoter, and secondary mutation of genes with coding microsatellites, such as transforming growth factor receptor II (TGF-RII) and BAX. Hereditary syndromes have germline mutations in specific genes (mutation in the tumour suppressor gene APC on chromosome 5q in FAP, mutated DNA mismatch repair genes in HNPCC).
Infiltrating ductal adenocarcinoma is the most common malignancy of the pancreas. When most investigators use the term 'pancreatic cancer' they are referring to pancreatic ductal adenocarcinoma (PDA). Normal duct epithelium progresses to infiltrating cancer through a series of histologically defined precursors (PanINs). The overexpression of HER-2/neu and activating point mutations in the K-ras gene occur early, inactivation of the p16 gene at an intermediate stage, and the inactivation of p53, SMAD4, and BRCA2 occur relatively late. Activated K-ras engages multiple effector pathways. Although EGF receptors are conventionally regarded as upstream activators of RAS proteins, they can also act as RAS signal transducers via RAS-induced autocrine activation of the EGFR family ligands. Moreover, PDA shows extensive genomic instability and aneuploidy. Telomere attrition and mutations in p53 and BRCA2 are likely to contribute to these phenotypes. Inactivation of the SMAD4 tumour suppressor gene leads to loss of the inhibitory influence of the transforming growth factor-beta signalling pathway.
Abnormal choline metabolism is emerging as a metabolic hallmark that is associated with oncogenesis and tumour progression. Following transformation, oncogenic signalling via pathways such as the RAS and PI3K-AKT pathways, and transcription factors associated with oncogenesis such as hypoxia-inducible factor 1 (HIF1) mediate overexpression and activation of choline cycle enzymes, which causes increased levels of choline-containing precursors and breakdown products of membrane phospholipids. These products of choline phospholipid metabolism, such as phosphocholine (PCho), diacylglycerol (DAG) and phosphatidic acid, may function as second messengers that are essential for the mitogenic activity of growth factors, particularly in the activation of the ras-raf-1-MAPK cascade and protein kinase C pathway.
Shear stress represents the frictional force that the flow of blood exerts at the endothelial surface of the vessel wall and plays a central role in vascular biology and contributes to the progress of atherosclerosis. Sustained laminar flow with high shear stress upregulates expressions of endothelial cell (EC) genes and proteins that are protective against atherosclerosis. The key shear stress-induced transcription factors that govern the expression of these genes are Kruppel-like factor 2 (KLF2) and nuclear factor erythroid 2-like 2 (Nrf2). On the other hand, disturbed flow with associated reciprocating, low shear stress generally upregulates the EC genes and proteins that promote oxidative and inflammatory states in the artery wall, resulting in atherogenesis. Important transcriptional events that reflect this condition of ECs in disturbed flow include the activation of activator protein 1 (AP-1) and nuclear factor kappaB (NF-kappaB).
Oxidative stress, caused by increased concentration of reactive oxygen species (ROS) in the cell, can happen as a consequence of mitochondrial dysfunction induced by the oncogenic RAS (Moiseeva et al. 2009) or independent of oncogenic signaling. Prolonged exposure to interferon-beta (IFNB, IFN-beta) also results in ROS increase (Moiseeva et al. 2006). ROS oxidize thioredoxin (TXN), which causes TXN to dissociate from the N-terminus of MAP3K5 (ASK1), enabling MAP3K5 to become catalytically active (Saitoh et al. 1998). ROS also stimulate expression of Ste20 family kinases MINK1 (MINK) and TNIK through an unknown mechanism, and MINK1 and TNIK positively regulate MAP3K5 activation (Nicke et al. 2005).
MAP3K5 phosphorylates and activates MAP2K3 (MKK3) and MAP2K6 (MKK6) (Ichijo et al. 1997, Takekawa et al. 2005), which act as p38 MAPK kinases, as well as MAP2K4 (SEK1) (Ichijo et al. 1997, Matsuura et al. 2002), which, together with MAP2K7 (MKK7), acts as a JNK kinase.
MKK3 and MKK6 phosphorylate and activate p38 MAPK alpha (MAPK14) and beta (MAPK11) (Raingeaud et al. 1996), enabling p38 MAPKs to phosphorylate and activate MAPKAPK2 (MK2) and MAPKAPK3 (MK3) (Ben-Levy et al. 1995, Clifton et al. 1996, McLaughlin et al. 1996, Sithanandam et al. 1996, Meng et al. 2002, Lukas et al. 2004, White et al. 2007), as well as MAPKAPK5 (PRAK) (New et al. 1998 and 2003, Sun et al. 2007).
Phosphorylation of JNKs (MAPK8, MAPK9 and MAPK10) by MAP3K5-activated MAP2K4 (Deacon and Blank 1997, Fleming et al. 2000) allows JNKs to migrate to the nucleus (Mizukami et al. 1997) where they phosphorylate JUN. Phosphorylated JUN binds FOS phosphorylated by ERK1 or ERK2, downstream of activated RAS (Okazaki and Sagata 1995, Murphy et al. 2002), forming the activated protein 1 (AP-1) complex (FOS:JUN heterodimer) (Glover and Harrison 1995, Ainbinder et al. 1997).
Activation of p38 MAPKs and JNKs downstream of MAP3K5 (ASK1) ultimately converges on transcriptional regulation of CDKN2A locus. In dividing cells, nucleosomes bound to the CDKN2A locus are trimethylated on lysine residue 28 of histone H3 (HIST1H3A) by the Polycomb repressor complex 2 (PRC2), creating the H3K27Me3 (Me3K-28-HIST1H3A) mark (Bracken et al. 2007, Kotake et al. 2007). The expression of Polycomb constituents of PRC2 (Kuzmichev et al. 2002) - EZH2, EED and SUZ12 - and thereby formation of the PRC2, is positively regulated in growing cells by E2F1, E2F2 and E2F3 (Weinmann et al. 2001, Bracken et al. 2003). H3K27Me3 mark serves as a docking site for the Polycomb repressor complex 1 (PRC1) that contains BMI1 (PCGF4) and is therefore named PRC1.4, leading to the repression of transcription of p16-INK4A and p14-ARF from the CDKN2A locus, where PCR1.4 mediated repression of p14-ARF transcription in humans may be context dependent (Voncken et al. 2005, Dietrich et al. 2007, Agherbi et al. 2009, Gao et al. 2012). MAPKAPK2 and MAPKAPK3, activated downstream of the MAP3K5-p38 MAPK cascade, phosphorylate BMI1 of the PRC1.4 complex, leading to dissociation of PRC1.4 complex from the CDKN2A locus and upregulation of p14-ARF transcription (Voncken et al. 2005). AP-1 transcription factor, formed as a result of MAP3K5-JNK signaling, as well as RAS signaling, binds the promoter of KDM6B (JMJD3) gene and stimulates KDM6B expression. KDM6B is a histone demethylase that removes H3K27Me3 mark i.e. demethylates lysine K28 of HIST1H3A, thereby preventing PRC1.4 binding to the CDKN2A locus and allowing transcription of p16-INK4A (Agger et al. 2009, Barradas et al. 2009, Lin et al. 2012).
p16-INK4A inhibits phosphorylation-mediated inactivation of RB family members by CDK4 and CDK6, leading to cell cycle arrest (Serrano et al. 1993). p14-ARF inhibits MDM2-mediated degradation of TP53 (p53) (Zhang et al. 1998), which also contributes to cell cycle arrest in cells undergoing oxidative stress. In addition, phosphorylation of TP53 by MAPKAPK5 (PRAK) activated downstream of MAP3K5-p38 MAPK signaling, activates TP53 and contributes to cellular senescence (Sun et al. 2007)
Formation of the LAT signaling complex leads to activation of MAPK and production of cytokines. The sequence of events that leads from LAT to cytokine production has not been as clearly defined as the sequence that leads to degranulation. However, the pathways that lead to cytokine production require the guanine-nucleotide-exchange factors SOS and VAV that regulate GDP-GTP exchange of RAS. After its activation, RAS positively regulates the RAF-dependent pathway that leads to phosphorylation and, in part, activation of the mitogen-activated protein kinases (MAPKs) extracellular-signal-regulated kinase 1 (ERK1) and ERK2 (Gilfillan & Tkaczyk 2006)
C-Jun NH2 terminal kinases (JNKs) are an evolutionarily conserved family of serine/threonine protein kinases, that belong to mitogen activated protein kinase family (MAPKs - also known as stress-activated protein kinases, SAPKs). The JNK pathway is activated by heat shock, or inflammatory cytokines, or UV radiation.
The JNKs are encoded by at least three genes: JNK1/SAPK-gamma, JNK2/SAPK-alpha and JNK3/ SAPK-beta. The first two are ubiquitously expressed, whereas the JNK3 protein is found mainly in brain and to a lesser extent in heart and testes. As a result of alternative gene splicing all cells express distinct active forms of JNK from 46 to 55 kDa in size. Sequence alignment of these different products shows homologies of >80%. In spite of this similarity, the multiple JNK isoforms differ in their ability to bind and phosphorylate different target proteins, thus leading to the distinctive cellular processes: induction of apoptosis, or enhancment of cell survival, or proliferation.
Activation of JNKs is mediated by activated TAK1 which phosphorylates two dual specificity enzymes MKK4 (MAPK kinase 4) and MKK7(MAPK kinase 7)
Activator protein-1 (AP-1) is a collective term referring to a group of transcription factors that bind to promoters of target genes in a sequence-specific manner. AP-1 family consists of hetero- and homodimers of bZIP (basic region leucine zipper) proteins, mainly of Jun-Jun, Jun-Fos or Jun-ATF.
AP-1 members are involved in the regulation of a number of cellular processes including cell growth, proliferation, survival, apoptosis, differentiation, cell migration. The ability of a single transcription factor to determine a cell fate critically depends on the relative abundance of AP-1 subunits, the composition of AP-1 dimers, the quality of stimulus, the cell type, the co-factor assembly.
AP-1 activity is regulated on multiple levels; transcriptional, translational and post-translational control mechanisms contribute to the balanced production of AP-1 proteins and their functions. Briefly, regulation occurs through:
effects on jun, fos, atf gene transcription and mRNA turnover.
AP-1 protein members turnover.
post-translational modifications of AP-1 proteins that modulate their transactivation potential (effect of protein kinases or phosphatases).
interactions with other transcription factors that can either induce or interfere with AP-1 activity