Molecular Biology of Cancer quizgecko.pptx
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Molecular Biology of Cancer Lawrence LeClaire, PhD November 28, 2023 Chapter 17, Marks’ Basic Medical Biochemistry, 6ed Lieberman et al. Objectives • List the three major categories of “cancer genes” and describe their mode of action in cancer promotion • Describe the central importance of the p53...
Molecular Biology of Cancer Lawrence LeClaire, PhD November 28, 2023 Chapter 17, Marks’ Basic Medical Biochemistry, 6ed Lieberman et al. Objectives • List the three major categories of “cancer genes” and describe their mode of action in cancer promotion • Describe the central importance of the p53 gene product in cancer genetics • Describe the mechanism by which the following cancer genes deregulate cell growth and differentiation: APC, p53, & NF1 • Describe the roles of cadherins and catenins • Describe the function of telomerase in cancer • Discuss apoptosis in relation to cancer, including various pathways and anti-apoptotic regulation American Cancer Society Stats Any Bacon Lovers? What is cancer? A cancer cell is: “…a savage cell that somehow corrupts the forces that normally protect the body, invades the well-ordered society of cells surrounding it, colonizes distant areas, and, in a finale to its cannibalistic orgy of flesh-consuming-flesh, commits suicide by destroying its host.” - Pat McGrady, The Savage Cell Cancer: from a healthy cell to a cancer cell https://youtu.be/8LhQllh4 6yI Cancer Basics • • • • • • • Class of disorders showing uncontrolled cell proliferation Disease of cellular differentiation Normal growth and differentiation disrupted Different degrees of tumor, different stages May reflect degree of genetic change Carcinogenesis is multi-step process Both genes and environment important Genes and Cancer • Cell growth and differentiation • Growth factors and receptors • Signal transduction pathways • Transcription factors Medical Genetics, 4th ed., Jorde et al. Cellular Regulatory Pathways Fig 11-3, Medical Genetics, 5th ed., Jorde et al. Stages of Cancer • Start as single mutated cells • Proliferates in situ • Accumulates more mutations • Different ones in different cells! • Invasion into nearby tissues or into blood vessels or lymphatics • Small clusters and single cells may migrate, metastasis https://www.cbsnews.com/news/geneti c-testing-stomach-cancer-early-testing / Causes of Cancer • Genetic aspects • Changes occur in genes that regulate proliferation • May be caused by mutagenic agents • ex) carcinogens • If not in germline, will not be inherited • Environmental aspects • Carcinogens can be environmental • Cancer frequencies change with environment • Ex) US vs. Japan, colon vs. stomach cancer Chemical Initiation • Damage to DNA can change genes • Mutations can be inherited at mitosis • Example: dinitrosoamine • Produced in stomach from nitrite-preserved foods, in some things naturally • React with guanine to make methyl guanine, pairs with thymine, not cytosine Classes of Cancer Genes • Oncogenes • Enhance cell proliferation when activated • MYC, Fos, RAS • Tumor Suppressor Genes • Normally control cell cycle and proliferation • p53, Rb, CDK inhibitors • DNA Repair Genes • Help maintain integrity of genome • BRCA1, BRCA2, mismatch repair genes (MSH2) Oncogenes “bad guys” Tumor-Suppressor Genes “good guys” DNA Repair Genes “more good guys” Proto-oncogenes • Normally regulate cell cycle • Modified or expressed inappropriately • Stimulate cell division at inappropriate times Points in a Pathway • Growth factor may be overexpressed, or another molecule may bind receptor • Receptor may signal without ligand bound • Proteins may act without proper signal • G-proteins may be active without GTP or lose GTPase • Kinases active by themselves • TF active or overexpressed Ras Pathway • Monomeric G protein • Activates Raf when GTP is bound, intrinsic GTPase • Raf is MAP3K, activates MEK, a MAPKK • MEK activates MAP kinase, which activates transcription factors • Any step can lead to problems! The mechanism through which Ras becomes an oncogenic protein is: Ras remains bound to GAP Cyclins in Cell Cycle Regulation • Cyclins are important in different stages of the cell cycle • Stimulate change from one stage to another • Only made at certain stage of cycle • Stimulate the cyclin-dependent kinases that are always present Cyclin-Dependent Kinases • CDKs are only active when proper cyclin is bound and have proper phosphorylation state • Complex can be inactivated by binding of cyclin-dependent kinase inhibitor (CKIs) proteins Cell Cycle Checkpoint • Ras/Raf signaling induces cyclin D expression • Cyclin D binds Cdk4 and Cdk6, now a protein kinase • Phosphorylates Rb and releases E2F • Can be prevented by binding of CKI • E2F enters nucleus, induces expression of genes necessary for cell cycle progression Learning Objectives • List the three major categories of “cancer genes” and describe their mode of action in cancer promotion • Describe the central importance of the p53 gene product in cancer genetics • Describe the mechanism by which the following cancer genes deregulate cell growth and differentiation: APC, p53, & NF1 • Describe the roles of cadherins and catenins • Describe the function of telomerase in cancer • Discuss apoptosis in relation to cancer, including various pathways and anti-apoptotic regulation The TP53 Gene • • • • • Codes for infamous p53 protein Over 50 tumor types shown to have mutations Loss of 17p in colon tumor cells common Small deletions localize to TP53 region Most mutations are missense, mainly in DNA binding domain of protein p53 Function • • • • • • Many different interactions Binds to CDKN1A (p21) promoter, a CKI Increases p21 level, inhibit cell cycle Involved in DNA damage sensing Directs damaged cells to undergo apoptosis Interacts with PTEN(Phosphatase and tensin homolog) and BAX (bcl-2-like protein 4) • Specific carcinogens can cause characteristic mutations in the gene Cell Cycle and p53 Fig 11-5, Medical Genetics, 5th ed., Jorde et al. DNA Damage and p53 • • • • • Level of p53 rises in response to mutagens It acts as TF for genes like p21 and GADD45 which repairs damaged DNA Successful repair down-regulates p53 Stimulates Bax and IGF-BP3 production if repair fails Bax promotes apoptosis, IGF-BP3 blocks signaling(?) When p53 increases in response to DNA damage, the following event occurs: P53 induces transcription of p21 Tumor Suppressors and Ras • Ras family involved in many hormone and growth factor signaling pathways • GAPs interfere with this, GTPase Activating Proteins • NF-1 is Neurofibromin, a GAP found in neural tissues • Loss leads to neurofibromatosis Combinations • Tumor suppressors and oncogenes can work together normally • Ex) Patched inhibits its co-receptor smoothened • S-HH binds, smoothened activates GL1 and gene transcription • Patched is tumor suppressor, smoothened is a proto-oncogene • Tumors occur when patched looses function or smoothened has gain of function Tumor suppress or Protooncogene Learning Objectives • List the three major categories of “cancer genes” and describe their mode of action in cancer promotion • Describe the central importance of the p53 gene product in cancer genetics • Describe the mechanism by which the following cancer genes deregulate cell growth and differentiation: APC, p53, & NF1 • Describe the roles of cadherins and catenins • Describe the function of telomerase in cancer • Discuss apoptosis in relation to cancer, including various pathways and anti-apoptotic regulation Cadherins and Catenins • Cadherins mediate calcium-dependent cell-cell adhesion • Anchored by catenins to actin in cytoskeleton • Loss of E-cadherin can allow metastasis of tumor cells • Catenins have dual functions, can affect gene transcription • ß-catenin can enter nucleus as TF, degraded by complex including APC Telomerase and Cancer • Maintain telomere length, prevent senescence • Normally cells become senescent after 50-70 divisions, if not sooner • Telomeres are ends of chromosomes, special structures, require special replication • Low telomerase activity means telomeres gradually shorten, limit divisions • Active telomerase means immortal cells! Learning Objectives • List the three major categories of “cancer genes” and describe their mode of action in cancer promotion • Describe the central importance of the p53 gene product in cancer genetics • Describe the mechanism by which the following cancer genes deregulate cell growth and differentiation: APC, p53, & NF1 • Describe the roles of cadherins and catenins • Describe the function of telomerase in cancer • Discuss apoptosis in relation to cancer, including various pathways and anti-apoptotic regulation Apoptosis and Cancer • Apoptosis is programmed cell death, removes damaged cells • Two pathways: death receptor and mitochondrial integrity • Cell shrinks, chromatin condenses, nucleus fragments • Membrane blebs to make apoptotic bodies with nuclear fragments, organelles, etc. • Cleared by macrophages • Phosphatidylserine flips from inner to outer membrane leaflet, signal for phagocytosis Activating Caspases • Caspases are cysteine proteases, cleave near aspartates • Activated from zymogens by proteolysis • Some are initiators (8 and 10) which activate procaspases 3, 6 and 7, execution caspases Bcl-2 Family of Proteins • • • • Both apoptotic and anti-apoptotic signals in cells Bcl-2 family of proteins integrate these signals Have Bcl-2 Homology (BH) domains, BH1 to BH4 3 types of Bcl-2 proteins • Anti-apoptotic proteins have all 4 BH domains (BH1, BH2, BH3, BH4) • Channel-forming apoptotic forms have BH1 to BH3 • Proapoptotic family with BH3 only • Anti-apoptotic (Bcl-2, Bcl-L, Bcl-w) block by inserting in mitochondrial membrane, block channel formation, or bind Apaf to block apoptosome formation Death Receptor Pathway • Subset of TNF-1 receptors • Fas/CD95, TNF1 and Death Receptor 3 • Trimer binds ligand • TNF-1, FASL (Fas ligand), etc. • Two procaspase molecules bind, initiators 8 or 10 • Autocatalytically cleave each other, become active • Cleave procaspases 3, 6, 7 to activate execution caspases • Caspase 3 cleaves Bid Mitochondrial Integrity Pathway • Induced by growth factor withdrawal, cell injury, some steroids, high intracellular calcium • Cytochrome c released from mitochondria • Binds to Apaf-1 (proapaoptotic protease-activating factor) which binds and activates procaspase 9 • Forms apoptosome, which cleaves procaspases 3, 6 and 7 to active execution caspases Signal Integration • Proapoptotic forms are channel-forming or BH3-only • Channel-forming, like Bax, like Bcl-2, lack Apaf binding, form ion channels by dimerizing with BH3-only form in mitochondrial membrane • BH3-only (Bim and Bid) bind to pro or anti, >> apoptosis • Death signal activates Bid, which activates Bax while blocking Bcl-X from binding Apaf, so channels form and cytochrome c is released, and apoptosome can form Cancer and Apoptosis • Mutations in oncogenes may make cancer cells resistant to apoptosis • Example is the PDGF/Akt/BAD pathway • PDGF binding activates PI-3 kinase, which activates Akt (protein kinase B) • Akt phosphorylates BAD (BH3-only), inactivates it • Growth factor signaling inhibits cell death this way • Another pathway is through MAP Kinases that control cell growth. For example, MAPKK phosphorylates RSK, activates it and it can phosphorylate BAD, Ras regulates MAPK pathway • Mutations in any of these pathways disrupt regulation of apoptosis. • Mutations of cell growth pathways, such as gain of function Ras mutations Summary • Dysregulation of normal functions is key to cancer progression • Genes mutated in cancer can be categorized • Oncogene, Tumor suppressor, DNA repair • Cell cycle control needs normal function of tumor suppressors and oncogenes • Telomerase can enhance cell proliferation by maintaining ends of chromosomes • Apoptosis can remove cells before tumorigenesis • p53 is important in recognizing damaged cells and promoting apoptosis Control Issues: Cancer Normal Cell Cancer Cell Obeys strict rules Divides only when told to Dies rather than misbehavingAt least 3-5 mutations Stays close to home Careful with chromosomes Disobeys rules Divides at will Bad behavior doesn’t kill Wanders through body Careless with chromosomes Supplemental Table Supplemental Table
