CYTOKINE RECEPTORS AND SIGNAL TRANSDUCTION

CYTOKINE RECEPTORS AND SIGNAL TRANSDUCTION

What is Cytokine? Secreted popypeptide (protein) involved in cell-to-cell signaling. Acts in paracrine or autocrine fashion through specific cellular receptors. Can be produced by cells of any tissue and acts on many cells involved in immune and inflammatory response.

Cytokines, like hormones, can act in autocrine, paracrine,, or endocrine fashion

Cytokine nomenclature Lymfokines cytokines produced by aktivated T lymphocytes direct the immune system response by signaling between its cells Interleukins cytokines whose presumed targets are principally leukocytes. Chemokines - specific class of cytokines that mediates chemoattraction (chemotaxis) between cells, stimulate leukocyte movement and regulate the migration of leukocytes from the blood to tissues. Monokines cytokines derived primarily from mononuclear cells such as macrophages.

Cytokines: main functions Hematopoiesis (ex. CSFs, colony stimulating factors). Inflammatory reaction (ex. IL1, TNF). Chemotaxis (ex. IL8, MIP1- macrophage inflammatory protein 1, BLC B-lymphocyte chemoatractant). Immunostimulation (ex. IL12, IFNγ). Suppression (ex. IL10). Angiogenesis (ex. VEGFs- vacsular endothelial growth factor). Role in embryogenesis (ex. TGF-β, LT - lymphotoxin).

Type I cytokine receptor or hematopoetin receptor family : IL 2, IL 3, IL 4, IL 5, IL 6, IL7, IL 9, IL 13, IL 15, GM-CSF (Granulocyte ranulocyte-macrophageacrophage Colony-Stimulating Factor) and G-CSF G (Granulocyte-Colony Stimulation Factor) Type II cytokine receptors or Interferon receptors Classification Transforming growth factor Tumor necrosis factors Immunoglobulin superfamily (IgSF Chemokine receptors (seven factor receptors IgSF) seven transmembrane helix)

Type I cytokine receptor Transmembrane receptors expressed on the surface of cells. These receptors are also known under the name hematopoetin receptors, and share a common amino acid motif (WSXWS)( ) in the extracellular portion adjacent to the cell membrane.

Immunoglobulin superfamily (IgSF) The immunoglobulin superfamily (IgSF) is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins (antibodies); they all possess a domain known as an immunoglobulin domain or fold. They are commonly associated with roles in the immune system.

Tumor Necrosis Factor receptor Cysteine-rich common extracellular binding domain Chemokine receptor Typical structure of a chemokine receptor, with seven transmembrane domains and a characteristic "DRY" DRY" " motif in the second intracellular domain. Chemokine receptors are usually linked to a G-protein through which they signal. http://en.wikipedia wikipedia.org/.org/wiki/chemokine_receptor

Transforming Growth Factor Receptor After ligand binding, (PDGFR or EGFR receptor tyrosine kinases (RTKs) dimerize, undergo autophosphorylation (P)) and recruit adaptor proteins (such as GRB2 and SHC) ) that activate various downstream effectors. RAS is an important downstream effector and functions as a molecular switch by cycling between the active GTP-bound form and the inactive GDP-bound form. RAS activity is regulated positively by guanosine exchange factors (GEFs( GEFs), such as SOS,, and negatively by GTPase-activating activating proteins (GAPs). At least three downstream effectors can be activated by RAS. The RAF-mediated signalling cascade. www.nature.com/.../v2/n8/fig_tab/nrc866_f3.html

Common γ chain The γ chain (green), common to all, mediates intracellular signaling.

Signaling through cytokine receptor The ligand-activated activated receptor (R)( attracts a Janus (JAK) kinase (K). This kinase tyrosine phosphorylates both itself and the receptor. A signal transducer and activator of transkription (STAT) protein (S)( binds to tyrosine-phosphorylated receptor-kinase komplex. After being phosphorylated by JAK, the STATs form active dimers that translocate into the nucleus to regulate transkription.

Type II cytokine receptors Interferons (IFN) Represent protein hormones with antiviral activity. Secrete by cells in a response to variety of stimuli. Type I and type II IFN and IFN-like cytokines. Effects are mediated through cell receptors. IFN activate cellular signalling pathway (gene induction or repression).

Type I IFNs consist of seven classes: IFN-α, IFN-β, IFN-ε, IFN-ο, IFN-δ and IFN-τ Type I IFNs are major components of the innate immune system. Protect against viral infection. The expression of type I IFNs is induced by viral challenge. Type II IFN consist of IFN-γ only. IFN-γ is made in response to antigen (including viral antigens) or mitogen stimulation of lymphocytes. IFN-γ (immune interferon) is produced by certain activated T-cells and NK cells.

Type I IFNs Produced by macrophages, neutrophils and other somatic cells in response to infection by viruses or bacteria. Inducer is double strand RNA provided by viral genom itself. Their receptors are expressed on most cell types. IFN-γ IFN-γ is produced in activated T H 1 and NK cells, particularly in response to IL-2 and IL-12. Binding of IFN-γ to its receptor increases the expression of class I MHC on all somatic cells. IFN-γ may also activate macrophages, neutrophils and NK cells.

Function of IFNs: FNs initiate and regulate variety responses antiviral response antiproliferative activity (ability to arrest cell growth) treatment for cancer control of apoptosis immunomodulatory properties (INF-γ predominantly modulates immune response, main antiviral cytokine). http://www.virtualsciencefair.org/2007/sank7b2/fig1b.jpg

IFN-γ receptor Expression of IFN-γ receptor on the surface almost of all cell types. High affinity receptors are located in the T- and B-lymphocytes, NK-cells, monocytes, macrophages, fibroblasts, neutrophiles, endotelial cells and smooth muscle cells Receptor is expressed only in response to stimulus by antigen, only in cells of lymphoid origin (natural killer cells, macrophages, and some T cells).

IFN-g receptor has two components: gr1 and gr2 IFN-γR1-90 kda glycoprotein (472 amino acid residues) Extracellular domain 228 amino acid residues Transmembrane domain 24 amino acid residues Intracellular domain 220 amino acid residues IFN-γR2-62 kda glycoprotein (315 amino acid residues) Extracellular domain 226 amino acid residues Transmembrane domain 23 amino acid residues Intracellular domain 65 amino acid residues. Ligand binds to extracellular domain of IFN-γR1 only (in absence of IFN-γR1, IFN-γR2 cannot bind IFNγ)

Signal transduction is carried out through a series of tyrosine phosphorylation events and culminates with the activation and nuclear translocation of STAT protein and new mrna synthesis is induced. Jak proteins are brought into close after ligand-receptor complex formation. Transphosphorylation between Jak1 and Jak2 proteins (Jak2 phosphorylates Jak1, Jak1 transphosphorylates Jak2). phosphorylation of IFN-γR1 (Tyr 440). Binding of STAT1 protein to each IFN-γR1. Bound SΤΑΤ1 is phosphorylated by Jak. Dissociation of dimer from the receptor and formed dimer translocates to the nucleus. Induction of transcription of many genes.

IFN-αR1 has 530 amino acid residues (409 residues of protein are extracellular, 100 residues are intracellular. IFN-αR2 has 217 AA residues in extracellular space, 251 AA residues in intracellular space. Both components bind type-ifns cooperatively. Receptor has single pair of receptor proteins. Receptor has ability to bind multiple ligands (all subspecies of IFN-α and IFNβ and other types of IFN-type I). Interferon ligand is boud to IFN-αR1 and than to IFN-αR2 which stabilizes the complex. IFN α/β receptor IFN α/β receptor has two components IFN-αR1 and IFN-αR2.

Signal transduction goes through Jak/STAT pathway. Involves two different ligand binding proteins (kinases) Tyk2 and Jak1. Tyk2 associates with IFN-αR1 Jak1, STAT1 and STAT2 associate with IFN-αR2. Ligand binding bring both subunit into close proximity. Jak1 transphosphorylates Tyk2 (1). Tyk2 in turn phosphorylates Jak1 and IFN-αR1 (2). Phosphorylation of IFN-αR1 allows STAT2 to bind to IFN-αR2. STAT2 phoshorylate STAT1. STAT1-STAT2 complex dissociates from receptor. Dimer STAT1-STAT2 associates with interferon regulatory factor to form the transcription complex.

Effect of signalling through IFN-γ receptor Induces inflammatory reaction. Induces antibacterial effect (activation of neutrophils, NK cells and macrophages, increased their ability to recognize, kill, and digest foreign materials or microbes). Normal expression of is important in preventing the development of cancer. Effect of signalling through IFN-α/β receptor Antiviral defence (protects the cell from viral replication).