Signaling Pathways

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Signaling Pathways

  1. 1. Signaling Pathways that Control the Expression of Gene Activity
  2. 2. Transforming Growth Factor β Signals (TGF β ) <ul><li>A large class of molecular signals involved in regulating development (e.g., bone growth, mesoderm formation, formation of cell-adhesion molecules, anti-proliferation effects). </li></ul><ul><li>The activated TGF β receptor directly phosphorylates a transcription factor. </li></ul>
  3. 3. Human TGF β Signal Molecules <ul><li>Three isoforms that are tissue specific: </li></ul><ul><li>TGF β - 1 </li></ul><ul><li>TGF β - 2 </li></ul><ul><li>TGF β - 3 </li></ul><ul><li>Dimeric protein (homo- or hetero-) </li></ul>
  4. 4. TGF β Receptors <ul><li>Cell surface proteoglycans </li></ul><ul><li>Three types: Types RI, RII, and RIII </li></ul><ul><li>Have serine/threonine kinase activity </li></ul><ul><li>Activation of these receptors results in the activation of transcription factors called Smads. </li></ul>
  5. 5. Types of Smads <ul><li>Receptor-regulated Smads (R-Smads) </li></ul><ul><li>Co-Smads </li></ul><ul><li>Inhibitory or antagonistic Smads (I-Smads) </li></ul>
  6. 6. The Structure of R-Smads N C DNA- binding segment Nuclear Localization Signal MH1 DOMAIN Linker MH2 DOMAIN
  7. 7. TGF β Signaling Pathway -I <ul><li>TGF β binds to the RIII or RII receptors. </li></ul><ul><li>If binding is to RIII, then TGF β is presented to RII. </li></ul><ul><li>Ligand-bound RII recruits and phosphorylates RI, causing its activation. </li></ul><ul><li>Activated RI phosphorylates Smad3 (an R-Smad), exposing the NLS. </li></ul><ul><li>Two phosphorylated Smad3 molecules interact with Smad4 (a co-Smad) and importin- β . </li></ul><ul><li>The complex is translocated to the nucleus. </li></ul>
  8. 8. TGF β Signaling Pathway -II <ul><li>Importin dissociates from the complex. </li></ul><ul><li>Smad complex then associates with other transcription factors to activate transcription. Often, growth-inhibitory proteins are produced form these transcription events. </li></ul><ul><li>Dephosphorylation of the Smads within the nucleus results in their translocation to the cytoplasm. </li></ul>
  9. 10. Oncoproteins and I-Smads Regulate Smad Signaling <ul><li>Oncoproteins cause abnormal cell growth. Two such proteins, SnoN and Ski, block transcription activation by the DNA bound Smad complexes. The growth-inhibitory proteins normally produced are not. </li></ul><ul><li>I-Smads (e.g., Smad 7) block the ability of RI to phosphorylate R-Smads. </li></ul>
  10. 11. Cancer and TGF β Signals <ul><li>Many cancers are caused by mutations to proteins in the TGF β signaling pathway. </li></ul><ul><li>In most human pancreatic cancers, a deletion to the Smad 4 gene occurs. The Smad 4 protein is not produced (or is non-functional), and proteins that inhibit cell proliferation upon stimulation by TGF β are not synthesized. </li></ul>
  11. 12. Cytokine Receptors <ul><li>Inactive cytokine receptors consist of two monomeric transmembrane polypeptides. </li></ul><ul><li>Each polypeptide is associated with a separate cytosolic kinase. </li></ul><ul><li>Ligand binding to the receptor results in dimerization of the polypeptides (formation of a dimer). The kinases then phosphorylate a tyrosine residue on each other, which causes each to phosphorylate tyrosine residues on the cytosolic regions of each sub-unit. [Figure 14-5] </li></ul><ul><li>The receptor is then active. </li></ul>
  12. 13. What happens after activation of the cytokine receptor? <ul><li>Amino acid sequences on the activated receptor that contain a phosphotyrosine residue recruit myriad proteins possessing either SH2 or PTB domains. [See figure 14-6] </li></ul><ul><li>The recruited proteins are then phosphorylated, which enhances their activity. </li></ul><ul><li>The proteins go on to cause transcriptional activation. </li></ul>
  13. 14. What are cytokines? <ul><li>Group of relatively small ( ~ 160 aa) secreted proteins that control growth and differentiation of different cell types. </li></ul><ul><li>Responses to cytokines include increasing or decreasing expression of membrane proteins (including cytokine receptors), cell proliferation, and secretion of effector molecules. </li></ul><ul><li>Cytokines may act on the cells that secrete them ( autocrine action ), on nearby cells ( paracrine action ), or in some instances on distant cells ( endocrine action ). </li></ul><ul><li>Examples include prolactin, interleukin-2 (T-cell proliferation), interleukin-4 (B-cell proliferation), erythropoietin (Epo), thrombopoietin (platelet formation) and growth hormone. </li></ul><ul><li>All of these molecules bind to cytokine receptors. </li></ul>
  14. 15. Cytokines and the JAK/STAT Pathway <ul><li>JAKs are cytosolic kinases associated with cytokine receptors. </li></ul><ul><li>Four JAKs exist (JAK1-JAK4). </li></ul><ul><li>Ligand binding to a JAK-associated cytokine receptor causes the JAK to be phosphorylated and activated. </li></ul><ul><li>JAK phosphorylates tyrosine residues on the receptor, which then recruits SH2 containing STAT proteins. </li></ul><ul><li>STATs are transcription factors. </li></ul><ul><li>STATs are phosphorylated by JAK. </li></ul><ul><li>The active STATs dissociate from the receptor and dimerize, exposing two NLS. </li></ul><ul><li>Translocation of the STAT dimer to the nucleus results in binding to enhancer sequences and activation of target genes. </li></ul>
  15. 17. How is the active cytokine receptor down-regulated? <ul><li>If activation entails phosphorylation, then inactivation must involve the removal of the phosphate groups from the tyrosine residues. </li></ul><ul><li>SHP-I phosphatase binds to a phosphotyrosine on the receptor and removes the P from JAK, preventing further activation. </li></ul><ul><li>This occurs in a period of a few minutes. </li></ul>
  16. 18. Receptor Tyrosine Kinases (RTKs) <ul><li>These receptors are similar to the cytokine receptors already discussed, except that the cytosolic domain has intrinsic protein kinase activity. </li></ul><ul><li>Binding of ligand is associated with dimerization and activation of the cytosolic domains of each polypeptide by the phosphorylation of tyrosine residues. </li></ul><ul><li>The phosphotyrosines recruit adapter proteins with SH2, SH3 and PTB domains. The adapters couple activated RTKs to other components of signal-transduction pathways. One such pathway involves Ras protein. </li></ul>
  17. 20. Ligands that bind to TRKs <ul><li>Nerve Growth Factor (NGF) </li></ul><ul><li>Platelet-derived Growth Factor (PDGF) </li></ul><ul><li>Fibroblast Growth Factor (FGF) </li></ul><ul><li>Epidermal Growth Factor (EGF) </li></ul><ul><li>Insulin </li></ul>
  18. 21. Ras Protein <ul><li>Ras is a monomeric, GTP-binding switch protein. </li></ul><ul><li>Ras alternates between an inactive state with bound GDP and an active state with bound GTP. </li></ul><ul><li>Ras is not directly linked to cell-surface receptors. </li></ul><ul><li>Ras is anchored to the plasma membrane by a hydrophobic anchor. </li></ul>
  19. 22. Activation of Ras <ul><li>Binding of ligand to RTK causes dimerization and activation of inherent kinase activity. </li></ul><ul><li>Activated receptor recruits GRB2 adapter protein. </li></ul><ul><li>GRB2 recruits SOS protein (which has guanine-nucleotide exchange activity - GEF), that causes the replacement of GDP by GTP on RAS. </li></ul>
  20. 23. What occurs after Ras is activated? <ul><li>MAP kinase pathway is activated: </li></ul><ul><li>Ras activates Raf protein, a serine/ threonine kinase, by binding to it. </li></ul><ul><li>Hydrolysis of RasGTP causes the release of active Raf. </li></ul><ul><li>Raf activates MEK, another kinase. </li></ul><ul><li>MEK activates MAP Kinase (MAPK). </li></ul><ul><li>MAPK translocates to the nucleus , causing induction of gene transcription. </li></ul>
  21. 24. Role of Scaffold Proteins <ul><li>Scaffold proteins associate the different kinases of one signaling pathway to prevent accidental phosphorylation of other substrates. </li></ul><ul><li>They accomplish this by allowing the kinases of one pathway to interact with one another, but not with kinases in other pathways. </li></ul>
  22. 25. Insulin and Protein Kinase B (PKB) <ul><li>Insulin binds to a tyrosine kinase receptor that may activate the Ras-MAPK pathway or can lead to the activation of protein kinase B . </li></ul><ul><li>In adipose and muscle cells, PKB causes the movement of GLUT4 transporter from intracellular membranes to the plasma membrane. </li></ul><ul><li>In liver and muscle cells, PKB also stimulates glycogen synthesis from UDP-glucose by causing the activation of glycogen synthase. </li></ul>
  23. 26. Mutations to proteins in the MAP kinase pathway can cause cancer.

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