Guide to Signal Pathways in Immune Cells

Transcription

1 Guide to Signal Pathways in Immune Cells

2 E. Nigel Wardle, MD Guide to Signal Pathways in Immune Cells

3 E. Nigel Wardle The Royal Society of Medicine London, W1G 0AE United Kingdom ISBN: e-isbn: DOI: / Library of Congress Control Number: Humana Press, a part of Springer Science+Business Media, LLC 2009 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Humana Press, c/o Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper springer.com

4 Preface To read current biomedical science, one has to have a working knowledge of how important effector molecules cause transduction of their signal within cells, altering the control of genes. This work aims to provide that basic knowledge for medical readers. Students of immunology or cell biology will note its relevance. One will learn how platelets, macrophages, neutrophils, T and B lymphocytes and natural killer cells perform their functions and how skin, breast, prostate and colon cancers emerge. The associated diagrams and tables are used to obviate extensive text. Appropriate references to articles and reviews by workers in each field are given so that further consideration can easily be undertaken. We are all at differing stages of our appreciation of immunology and of pathophysiology. Some persons will have a profound background in biochemistry or molecular biology. Others will have a reminiscence of lectures received years ago. Since this work is principally for clinical doctors, the sections that can be avoided at first reading are marked with an asterisk (*). Always proceed line by line and think of associations that you know. Do you feel comfortable with the statement, Interleukin 6 stimulates glucose uptake in renal proximal tubular cells, and that action is associated with Stat3, PI3K/Akt, MAPKs and NF-kB signal pathways? If not, please read on. Extracellular signals acting on cells are sensed by two families of cell membrane receptors, the G protein-coupled receptors (GPCRs) and the receptor tyrosine kinases. GPCRs interact with G proteins, which contact second messengers such as inositol triphosphates, Ca 2+ ions or cyclic nucleotides. They might also activate small G proteins of the Rho or Ras families that lie upstream of the mitogenactivated protein kinase (MAPK) signal cascades. Alternatively, the thirteen families of receptor tyrosine kinases (RTKs) create docking sites for downstream effectors like Ras-MAPKs, PI3 kinase (PI3K) and protein kinase C (PKC). Essentially, signal transduction depends on reversible phosphorylation of proteins. There is phosphorylation by protein kinases and dephosphorylation by phosphatases. So, protein kinases like tyrosine kinases or serine-threonine kinases transfer phosphate to specific amino acids (tyrosine, serine or threonine). Tyrosine kinases are divided into RTKs and nonreceptor tyrosine kinases (NRTKs), which occur as JAKs (Janus kinases), focal adhesion kinase (FAK) or src kinases. The serine-threonine kinases are protein kinase A (PKA), which modulates cell metabolism and ion channel v

5 vi Preface activity; protein kinase B (Akt), which has a role in glucose metabolism and is an inhibitor of apoptosis; and PKC, which is implicated in cell growth and differentiation. The MAPKs are comprised of extracellular signal-related kinase (ERK), which transduces signals for cellular proliferation; the p38 MAPK; and c-jun NH2- terminal kinase (JNK). The MAPKs have downstream targets affecting all aspects of cell function. In addition, there is the nuclear factor kappa B (NF-k B) pathway, which is crucial for inflammatory and antiapoptotic signalling. In brief, these are the objectives of our cell signalling scrutinies. E. Nigel Wardle, MD

6 Contents 1 Cellular Activation Processes Membrane Receptors and Second Messengers Examples of Plasma Membrane Receptors G Proteins: Guanine Nucleotide Coupling Control over GPCRs: G Protein-Coupled Receptor Kinases, Arrestins, Regulators of G Protein Signaling Proteins, and GPCR Interacting Proteins*... 5 References Cell Membrane Receptors and Phospholipids Turnover of Cell Membrane Phosphatidyl-Inositol Phospholipase C (PLC) Diacylglycerol Protein Kinase C The Phosphatidyl-Inositol Phosphates Phosphoinositide 3 Kinases PI3K Effector Proteins Protein Kinase B (Survival Kinase Akt) Calcium Ions as Second Messenger Desensitisation of Membrane Receptors References Platelet Aggregation and Its Control Platelet Aggregation Platelet Adhesion to Collagen Platelets and Immunity Constitutive and Inducible NO Synthetases Nitric Oxide and the Immune System References vii

7 viii Contents 4 Introduction to Signalling Cascades Serine/Threonine Kinases Mitogen-Activated Protein Kinases and Signalling Cascades Organisation by Scaffold Proteins* Redox Regulation of Signal Transduction c-jun N-terminal Kinases p38 MAP Kinases The ERK Cassette Cascade Control by Phosphatases Transcription Factors and DNA-Binding Domains More About Histone Acetylation* Formation of Enhanceosomes on Gene Promoters and Enhancers* Cytokine Genes The IFNb Enhanceosome Chromatin Remodelling and Epigenetics Control of IL-4 and IL-13 Production (Th2 Cytokines) The Anti-inflammatory Action of PPARgs and Retinoids Estrogen Receptors Corticosteroid Receptors DNA Methylation and Gene Control (Epigenetics)* c-myc and Cell Proliferation Forkhead Transcription Factors (Foxos)* Differential Gene Expression Patterns A Note about Ubiquitination Sumoylation and Gene Repression* Notch Signalling in Haemopoiesis* Hypoxia-Inducing Factor* Vascular Endothelial Growth Factor References Cell Proliferation Ras as a GTP-Binding Protein and GTPase Other Small GTPases Cell Proliferation in Response to PDGF Caveolae and Lipid Rafts Protein Modules Protein Tyrosine Kinases Janus Gateway Phosphotyrosine Kinases Transactivation Abl and Bcr-Abl* Control Over Smooth Muscle Cell Proliferation Angiotensin II and Smooth Muscle Cells Endothelin-1 and Its Action References... 88

8 Contents ix 6 Mast Cell Activation and the Role of Eicosanoids Signal Transduction in Mast Cells The Formation of Eicosanoids The Formation of Prostaglandin E Mast Cell Degranulation Cyclic AMP Downregulates Activation of Immune Cells Cyclic AMP and Cell Proliferation* The bc Cytokine Receptors and CREB Control of Growth* Other Modulators of Mast Cell Degranulation* References Lipid Products and Cell Signaling Phosphatidic Acid as a Second Messenger Protein Kinase D Sphingolipids as Regulators of Cell Signalling* Sphingomyelin Metabolites/Cell Proliferation/Apoptosis* Lysophosphatidic Acid as Serum Growth Factor Antineutrophil Cytoplasmic Antibody-Stimulated Neutrophil Adhesion References Programmed Cell Death: Apoptosis Apoptosis Mechanisms Determinants of Apoptosis Heat Shock Proteins and Apoptosis Cellular Metabolism and Apoptosis Endoplasmic Reticulum Stress Inflammatory Caspases Effect of Apoptotic Cells on Macrophages Apoptosis of Neutrophils Apoptosis of T Lymphocytes Apoptosis of Dendritic Cells or Macrophages Cell Death via Fas or TNFRI Receptors* Death Receptor Functioning: Complexes I/II* Detecting Apoptosis DNA Damage Leading to Cell Cycle Arrest or Apoptosis Apoptosis versus Carcinogenesis References Control of the Cytoskeleton The LowMolecular Weight Monomeric GTPases Rho GTPases Actin Bundles in Neutrophils Cell Migration

9 x Contents 9.5 The Mechanics of Phagocytosis Endocytosis Exocytosis: Discharge of Vesicles Interleukin 8 Chemokine and Discharge of Neutrophil Granules Cell Adhesion Receptors and the Cytoskeleton Signalling from Integrins Detail about Focal Adhesion Kinase Signalling from Immunoglobulin CAMs Signalling from Selectins Cadherins and Adhesion of Epithelial Cells The Use of Statins References Leucocyte Activation and Behaviour Activation of Neutrophils Ca 2± Balance and Neutrophils Neutrophil Hyperactivation Stimulation of Phagocytes by Colony-Stimulation Factors FcReceptor-Mediated Phagocytosis Movement of Leucocytes in Response to Chemoattractants Activation of Eosinophils Integrin-Mediated Adhesion of Phagocytes Integrin LFA-1 as Regulator of Leucocyte Adhesion Th17 cells via IL-17 drive neutrophils and inflamation Communication between Neutrophils and Dendritic Cells References Cell Defence and Survival Transcription Factor NF-kappa The Classical Canonical NF-kB Pathway The Alternative Noncanonical Pathway* More about NF-kB Control* How to Inhibit NF-kB Gene Knockout or Transgenic Experimental Animals The Control of Cyclooxygenase PPARg and PPARa in Inflammation and Immunity Cell Survival: Akt and Glycogen Synthase Kinase Insulin Receptors Cross-Talk between Insulin Signalling and the Angiotensin II System Understanding mtor Alveolar Macrophage Survival References

10 Contents xi 12 Signalling in Immune Reactions The Immune Response Dendritic Cells Macrophage Types (M1 Are CD14hi, CD16, CCR2 +, M2 CD14 + CD16 + CX3CR1+) Toll-Like Receptors Negative Regulation of Toll-Like Receptors Inside Sensors for Outside Pathogens: Nucleotide Oligomerisation Domain-Like Receptors Production of Interleukin Cytokines and the Immune Response Activation of Macrophages Control of Macrophages Lipopolysaccharide Stimulation of Macrophages (or Endothelial Cells) Negative Regulators of LPS Activation and TLR Receptor Signalling Interaction IFNg/TLR Signalling in Macrophages Dendritic Cells Signalling by TNFa Macrophage Migration Inhibitory Factor Cytokines and Growth Factors Receptors for Cytokines and Cellular Activation IL2Receptor and IL Transpresentation of IL Negative Regulation of Cytokine Receptors Connections by Jak-Stats Activation of Macrophages by IL-1 and TNFa TRAF Adapter Proteins CD40 Signalling Other TNFR Superfamily Molecules RAGE Receptors and Cytokine Production IL-10 Immunomodulatory Cytokine Those Basic Signal Cascades References T Lymphocytes Activation of T Lymphocytes General Scheme of T Lymphocyte Activation Adaptor Proteins CD28 Costimulation NF-kB Activation and T Cell Co-stimulation* Control of Lymphocyte Activation by Phosphatases Downregulation of T Lymphocyte Activation The SLE CD4 T Lymphocyte*

11 xii Contents 13.4 Characteristics of Forms of T Lymphocyte Effector T Lymphocytes CD8 Cytotoxic T Lymphocytes Memory T Cells (CD4 and CD8) Th17 Inflammatory cells T Cell Apoptosis* Anergic CD4 T Cells* Quiescence of T Cells* Suppression of T Cells* T Regulatory Lymphocytes (ntreg, itreg, Tr-1) Th1 and Th2 Lymphocytes Interleukin 12 Signalling and Th1 Lymphocytes Regulation of IFNg Production Th17 Inflammatory Cells Interleukin 4 and Th2 Lymphocytes Interleukin Respiratory Tract Allergy Chronic Bronchitis as Obstructive Airway Disease References Immunological Controls Transforming Growth Factor-b Signalling Smad Activity and Other Kinase Pathways* TGFb and Immune Regulation TGFb Inhibition of Cell Growth TGFb and Inhibition of Cell Proliferation TGFb and Apoptosis Clinical Implications TGFb, Connective Tissue Growth Factor, and Fibrosis Connective Tissue Growth Factor Scleroderma TGFb Is Anti-inflammatory TGFb and Cancer Bone Morphogenetic Proteins* The Actions of Interferons Signalling by IFNg Signalling by IFNa* Controlling Viruses Plasmacytoid Dendritic Cells SOCS: Negative Regulation of the Jak-Stat Pathway* Regulation of the Jak-Stat Pathways Negating Cytokines Prostaglandin E2 and T Cell Modulation Interleukin 4 Signalling Interleukin

12 Contents xiii Decoy Receptors References Natural Killer Cells Receptors for HLA Class I Hyporesponsiveness of NK Cells ITAMs and NK Cell Receptors ITIMs and NK Cell Receptors NK Cells in Action* Signalling Lymphocyte Activation Molecule and Adapter SAP NKT Cells KIRs and KARs on T Cells* ILT Molecules on Regulatory Cells References B Lymphocytes B Cell Receptor Signalling Negative Signalling on B Lymphocytes CD40 Signalling* Sustaining B Cells with B Cell-Activating Factor (BLys) Apoptosis of B Lymphocytes Control Over B Cell Proliferation* B Cell Development* B Cell Tolerance* Epstein-Barr Virus and B Cell Proliferation* B Cells of Chronic Lymphatic Leukaemia The Lymphomas Plasma Cells in Multiple Myeloma* References The Cell Cycle Control of the Cell Cycle PI3K and Cell Proliferation* Notch and Cell Proliferation* Integrins and Cell Proliferation* C-Myc and Cell Proliferation* The Cell Cycle and DNA Damage p53 Tumour Suppressor Control of the Cell Cycle Checkpoints* The Cell Cycle of T Lymphocytes* Liver Regeneration Tumours and the Cell Cycle Cell Renewal and Hh Signalling*

13 xiv Contents Wnt Signalling References Cancer Cancer Cells Multistage Carcinogenesis in Skin Breast Cancer Prostate Cancer Cadherins and Catenins Ovarian Cancers Renal Cancer Colon Cancer Epithelial-Mesenchymal Transition Genes for Metastasis References Index

14 List of Figures Fig. 1.1 Adrenalin or glucagon causing glycogenolysis... 2 Fig. 1.2 b-arrestin-mediated clathyrin/ap-2 receptor endocytosis... 6 Fig. 2.1 Inositol phospholipids and cellular activation Fig. 2.2 The structure of phospholipase C Fig. 2.3 Structure of protein kinase C Fig. 2.4 Phosphoinositide syntheses Fig. 2.5 The structure of protein tyrosine phosphatase Fig. 2.6 Phosphoinositide signalling pathways Fig. 2.7 Structure of PI3 kinase class I Fig. 2.8 PI3 kinase/akt signalling pathway Fig. 2.9 Calcium ion mobilisation in T lymphocytes Fig. 3.1 Integrin a2b1 and associated GPVI in collagen adhesion Fig. 3.2 The metabolism of l-arginine Fig. 3.3 Signal transduction leading to expression of inducible nitric oxide synthetase Fig. 4.1 A three-kinase module for mitogen-activated protein kinase activation Fig. 4.2 How epidermal growth factor activates its receptor and hence mitogen-activated protein kinase Fig. 4.3 Mitogen-activated protein kinase cascades Fig. 4.4 Jun K-terminal kinase and activator protein 1 activation of metalloproteinase Fig. 4.5 Details of the p38mitogen-activated protein kinase transcription factor pathway Fig. 4.6 Schema of signal pathways for cytokine/chemokine production Fig. 4.7 Domain structure of SHP1/SHP2 phosphatases Fig. 4.8 Cofactor camp response element-binding protein/p300 bridging of transcription factors to the transcription machinery xv

15 xvi List of Figures Fig. 4.9 Lipopolysaccharide stimulation of the cyclooxygenase 2 promoter in macrophages Fig Control over the production of interleukin Fig Enhanceosome of the tumour necrosis factor-a promoter Fig Activation of the interferon-b promoter Fig GATA3 promotes interleukins, 5, 13 and Fig Scheme of perioxisome proliferator-activated receptor -g/retinoid X receptor activation Fig Estrogen receptor complex promoting transcription Fig Histone acetylation helping nuclear factor kappa B and glucocorticoid-induced deacetylation Fig Methylated binding proteins stop transcription Fig Transcriptional complexes formed by Myc Fig Operation of hypoxia-inducing factor 1a/1b Fig. 5.1a The Ras cycle Fig. 5.1b Regulation of small guanosine triphosphatases Fig. 5.2 Activation of platelet-derived growth factor receptor of fibroblasts (or the fibroblast growth factor receptor) Fig. 5.3 Molecules binding to the platelet-derived growth factor receptor Fig. 5.4 Sphingosine mediates the response to platelet-derived Fig. 5.5 growth factor Caveolae and sphingosine-1-phosphate/platelet-derived growth factor signalling Fig. 5.6 Modules on docking proteins for receptor tyrosine kinases Fig. 5.7 General structure of an Src phosphotyrosine kinase Fig. 5.8 Abl tyrosine kinase assembles cytoskeletal components Fig. 5.9 Mitogen-activated protein kinase and vascular smooth muscle proliferation Fig Effects of angiotensin II via AT-1 receptors Fig Activation of smooth muscle by endothelin Fig. 6.1 Signal pathways for FceRI-stimulated mast cells Fig. 6.2a Release of arachidonic acid from phosphatidylcholine Fig. 6.2b Conversion of arachidonic acid to eicosanoids Fig. 6.3 Mast cell degranulation Fig. 7.1 Growth factors stimulating phospholipases C and D Fig. 7.2 Basic structure and actions of protein kinase D Fig. 7.3 The sphingomyelin-ceramide cycle Fig. 7.4 Two forms of sphingomyelin activation Fig. 7.5 Serum growth factor lysophosphatidic acid-mediated signalling Fig. 8.1 Scheme for induction of apoptosis Fig. 8.2 Proteins controlling apoptosis

16 List of Figures xvii Fig. 8.3 Apoptosis signalling via Fas or the tumour necrosis factor-a RI Fig. 8.4 Responses to DNA double-strand breaks Fig. 8.5 How p53 causes apoptosis of cells Fig. 9.1 Low molecular weight guanosine triphosphatases and phagocyte activation Fig. 9.2 Targets of the Rho guanosine triphosphatase Fig. 9.3 Targets for Cdc42 (Rho family guanosine triphosphatase) Fig. 9.4 Structure of Wiskott-Aldrich syndrome protein Fig. 9.5 and Scar/Wave Activation of Wiskott-Aldrich syndrome protein Arp2/3 and lamellipodia formation Fig. 9.6 Neutrophil migration Fig. 9.7 A migrating cell Fig. 9.8 The mechanics of phagocytosis Fig. 9.9 Rab proteins as regulators of vesicle exocytosis Fig Focal adhesion kinase and p130 Cas scaffold Fig Integrin-initiated signalling Fig Focal adhesion kinase domain structure Fig Selectins in leucocyte interactions Fig Activation of neutrophils Fig Components of the nicotinamide adenine dinucleotide phosphate oxidase complex Fig Activation of neutrophils by angiotensin II Fig Stimulation of neutrophils by C5a Fig Fc receptors on neutrophils and macrophages Fig Fc receptors stimulate phagocytosis Fig Lymphocyte migration in response to chemokine Fig Integrin outside-in signalling Fig The bidirectional function of leucocyte function-associated antigen 1 molecules Fig. 11.1a Activation of nuclear factor kappa B by inflammatory stimuli Fig. 11.1b Structures of nuclear factor kappa B family members Fig Comparison of the canonical and alternative paths of nuclear factor kappa B activation Fig Transcription factors affecting the cyclooxygenase 2 promoter Fig Perioxisome proliferator-activated receptor-g with glucocorticoids suppresses inflammatory genes Fig Scheme for insulin receptor activation Fig Model of the PI3 kinase-akt-tuberose sclerosis-target of rapamycin pathway

17 xviii List of Figures Fig Bacterial product stimulation of Toll-like receptors Fig A composite view of the Toll-like receptor signalling pathways Fig Signal transduction pathways for caspase 1 induced pro-interleukin 1/interleukin 18 expression Fig Role of cytokines in the balance of Th1/Th2 lymphocytes Fig Lipopolysaccharide signalling via CD14/Toll-like receptor 4 receptors Fig Diagram of tumour necrosis factor -a receptors I and II Fig Scheme of cytokine-induced signalling pathways Fig Cytokine receptor superfamily members Fig Interleukin 2 receptor signalling Fig Signalling by tumour necrosis factor-a and by interleukin Fig Inhibitors of the growth factors mitogen-activated protein kinase cascade Fig Formation of cytokines in the rheumatoid synovium Fig Structure of T cell receptor to emphasise the immunoreceptor tyrosine activation motifs Fig Scheme of T cell receptor signalling Fig Signalling from the supramolecular activation cluster to immune response genes Fig T cell activation and adaptor proteins Fig T cell costimulation leading to T cell proliferation Fig T cell receptor/cd28-induced nuclear factor kappa B activation Fig Scheme for T cell apoptosis Fig Signal transduction defects in T cell anergy Fig Transcription factors for Th1/Th2 cell development Fig Stimulation of interferon-g production Fig Mucosal allergic reaction Fig Signalling via transforming growth factor-b receptors Fig Activation of Smads 2/ Fig Transforming growth factor-b-induced apoptosis Fig Signalling by interferons Fig Pathways for responses to viruses and their products Fig Regulation of cytokine signalling Fig Means of negating the output of cytokines Fig Signalling via the interleukin 4 receptor Fig Location of immunoreceptor tyrosine activation motifs on signalling units Fig Signal transduction by killer-activating receptors Fig Signal transduction in natural killer cells

18 List of Figures xix Fig Natural killer cell negative killer-inhibitory receptors and positive killer-activating receptors Fig The workings of signalling lymphocyte activation molecule (SLAM) and SLAM-associated protein Fig Structure of the B cell receptor Fig Scheme for negative signalling of B lymphocytes Fig B cell-activating factor-dependent antiapoptotic proteins and survival of B cells Fig Pre-B cell receptor signalling Fig XPB1 upregulates plasma cell genes Fig The cell cycle and its control Fig Regulation of the G1- to S-phase transition Fig Transforming growth factor-b inhibition of retinoblastoma protein phosphorylation Fig How surface integrins affect cyclins D/E control of the cell cycle Fig Interactions of c-myc p Fig Suppressors p53 and retinoblastoma in tandem Fig Domains of the p53 protein Fig T Lymphocyte cycle as controlled by p57-kip Fig The hedgehog signalling pathway Fig Wnt signalling via nuclear b-catenin Fig Carcinogenesis in mouse skin Fig Possible pathogenesis of basal cell breast cancer Fig E-Cadherin connecting with catenins Fig Genesis of colorectal tumours

19 List of Tables Table 1.1 Second Messengers... 2 Table 1.2 Physiological events triggered by receptors... 3 Table 3.1 Mechanisms of platelet activation Table 3.2 Events during ADP-induced platelet aggregation Table 4.1 Serine-threonine kinases in signal transduction Table 4.2 Examples of transcription factors Table 4.3 Modifications of histone lysines affecting transcription Table 5.1 Nonreceptor tyrosine kinases Table 7.1 Actions of phosphatidic acid (PA) Table 7.2 The actions of protein kinase D Table 7.3 Protein kinase D signalling pathways Table 8.1 Processes that lead to apoptosis of T lymphocytes Table 9.1 PIP2-binding proteins associated with actin reorganization Table 9.2 Selectins in leucocyte interactions Table 11.1 Common genes activated by nuclear factor kappa B (NF-κB) Table 11.2 Mediators and drugs that inhibit nuclear factor kappa B (NF-κB) Table 11.3 How nuclear factor kappa B (NF-κB) supports antiapoptosis/cell survival Table 12.1 Immune defence mechanisms Table 12.2 Innate pattern recognition receptors on macrophages Table 12.3 Toll receptors and their ligands Table 12.4 Toll-like receptors (TLRs) on immune cells Table 12.5 Cytokines in immunity Table 12.6 Products of macrophage activation Table 12.7 Biochemical pathways in macrophage activation Table 12.8 Molecular inhibitors of LPS activation Table 12.9 Synopsis of growth factors and cytokines (CGHIKLM) xxi

20 xxii List of Tables Table Haemopoietin receptor superfamily Table Jak-Stat connections Table The complete MAP kinases Table 13.1 PTPases controlling T lymphocyte activation Table 13.2 Downregulation of T lymphocyte activation Table 14.1 The actions of transforming growth factor-β1 (TGFβ1) on T lymphocytes Table 14.2 Induction and effects of suppressor of cytokine signalling (SOCS) proteins Table 14.3 Regulation of the Janus phosphotyrosine kinase-signal transducer and activator of transcription (Jak-Stat) pathways. 308 Table 15.1 Classification of killer-inhibitory receptors (KIRs) and killer-activating receptors (KARs) Table 16.1 Molecules that moderate B lymphocytes Table 16.2 The potential of Epstein-Barr virus antigens Table 17.1 Cell cycle regulatory proteins Table 17.2 Human tumours and checkpoint component aberrations Table 18.1 Forms of oncogene Table 18.2 Transition of a naevus to a melanoma Table 18.3 Genetic changes in colon cancer development