Understanding Carcinogenesis: Cell Cycle, Tumor Suppression & Neoplasm Growth

1. Carcinogenesis

2. Characteristics of Cancer • Disorder of altered cell differentiation and growth • Results in neoplasia (“new growth”) • Growth is uncoordinated and relatively autonomous • Lacks normal regulatory controls over cell growth and division • Tends to increase in size and grow after stimulus ceases or needs of organism are met

3. Determination and Differentiation

4. The Cell Cycle • Definition: The interval between each cell division • Genetic information is duplicated • Duplicated chromosomes are appropriately aligned for distribution between two genetically identical daughter cells • Checkpoints in cycle provide opportunities for monitoring the accuracy of deoxyribonucleic acid (DNA) replication • Edited and repaired defects ensure full complement of genetic information to each daughter cell

5. Control of Cell Cycle

6. Control of Cell Cycle • The cell cycle is controlled by many proteins from inside & outside the cell. • Intracellular cyclins and cyclin dependent kinases (CDKs) control the checkpoints. • Extracellular proteins from other cells called Growth Factors signal the target cell to divide. • Binding of growth factors to membrane receptor proteins of the target cell triggers a molecular signaling pathway - a series of proteins which allows the cell to pass the checkpoints of the cell cycle.

7. Cell Cycle is controlled by genes.

8. Tumor Suppressor Proteins Inhibit CellDivision & Prevent Cancer • Tumor suppressor proteins are proteins that bind to checkpoint proteins to stop the cell cycle & prevent cell division. • An important function of tumor suppressor proteins is to stop the division of mutated cells until mistakes in DNA are repaired by enzymes. • TS proteins keep most mutations from being passed on to daughter cells & developing into cancer. • If the genes for TS proteins mutate or are deleted cancers may result. • Two important TS proteins are the p53 protein & the RB protein.

9. Cell Proliferation • Definition • The process by which cells divide and reproduce • Regulation • Regulated in normal tissue, so the number of cells actively dividing equal the number of cells dying or being shed

10. Two Major Categories of Cells Existing in Humans • Gametes (ovum and sperm) • Haploid (containing one set of chromosomes from one parent) • Designed for sexual fusion forming a diploid cell (containing both sets of chromosomes) • Somatic cell • The diploid cell that forms the rest of the body

11. Categories of Cell Types of the Body • Well-differentiated neurons and cells of skeletal and cardiac muscle unable to divide and reproduce • Parent or progenitor cells that continue to divide and reproduce • Blood cells, skin cells, liver cells • Undifferentiated stem cells that can be triggered to enter cell cycle and produce large numbers of progenitor cells when needed

12. Types of Tumors • Adenoma: benign tumor of glandular epithelial tissue • Adenocarcinoma: malignant tumor of glandular epithelial tissue • Carcinoma: malignant tumor of epithelial tissue • Osteoma: benign tumor of bone tissue • Sarcoma: malignant tumors of mesenchymal origin • Papillomas: benign microscopic or macroscopic fingerlike projections growing on a surface

13. Factors differentiating Benign and Malignant Neoplasms • Cell characteristics • Manner of growth • Rate of growth • Potential for metastasizing or spreading • Ability to produce generalized effects • Tendency to cause tissue destruction • Capacity to cause death

14. Characteristics of Benign Neoplasms • A slow, progressive rate of growth that may come to a standstill or regress • An expansive manner of growth • Inability to metastasize to distant sites • Composed of well-differentiated cells that resemble the cells of the tissue of origin

15. Characteristics of Malignant Neoplasms • Tend to grow rapidly and spread widely • Have the potential to kill regardless of their original location • Tend to compress blood vessels and outgrow their blood supply, causing ischemia and tissue necrosis • Rob normal tissues of essential nutrients • Liberate enzymes and toxins that destroy tumor tissue and normal tissue

16. Methods by which Cancer Spreads • Direct invasion and extension • Seeding of cancer cells in body cavities • Metastatic spread through the blood or lymph pathways

17. Factors Affecting Tumor Growth • The number of cells that are actively dividing or moving through the cell cycle • The duration of the cell cycle • The number of cells that are being lost compared with the number of new cells being produced

18. Carcinogenesis Hypotheses of the Origin of Neoplasia • Oncogenes and Tumor Suppresor Genes • Viral Oncogene Hypothesis • Epigenetic Hypothesis • Failure of Immune Surveillance

19. 1. Oncogenes and Tumor Suppresor Genes Genes that Control Cell Growth and Replication • Genes control cell division by cytokines. • Three classes of regulatory genes. • Promotors – Proto-oncogenes • Inhibitors – Cancer-suppressor genes – p53 • DNA stability genes.

20. Non-lethal Genetic damage lies at the center of carcinogenesis. • Loss/damage to suppressor genes, • Duplication of promotor genes • Loss/damage of DNA stability genes.

21. Alterations of Specific Cellular Functions in Cancer Tumor Suppressor Genes Inactivation Oncogenes Activation Differentiation Apoptosis/Proliferation CANCER

22. Proto-oncogenes • Oncogenes: • Viral proteins which interact with the cellular controll mechanisms to overcome the strict regulation of proliferation (v-ras, v-myc, v-abl, ...) • Proto-Oncogenes: • Cellular proteins which correspond to the viral Oncogenes but which are strictly regulated. Mutations in this genes could transform a cell into a tumor cell (c-ras, c-myc, c-abl, ...).

23. Proto-oncogenes • TYPES OF ONCOGENES 1. Growth factors 2. Growth factors receptors 3. Intracellular signaling transduction factors Proteins with GTPase activity Cytoplasmic serine threoninekinases 4. DNA-binding nuclear proteins 5. Cell cycle factors

24. Relationship between gene products of proto oncogene Growth factors eg IGF Growth factor receptors Eg erb-2, ret Signal transducing factors Eg cytoplasmic kinases DNA binding proteins concerned with transcription cell cycle proteins eg cyclin D

25. Proto-oncogenes • FUNCTION OF ONCOGENES • Cancers have characteristics that indicate, at cellular level, loss of the normal function of oncogene products consistent with a role in the control of cellular proliferation and differentiation in the process known as signal transduction. It is a complex multistep pathway from the cell membrane, through the cytoplasm to the nucleus. • Proto oncogenes have been highly conserved during evolution, and the protein products they encode are likely to have essential biological functions.

26. Oncogenes Are Mutated Proto-oncogenes • A cell can acquire a cancer causing oncogene from • A virus • A mutation in a proto-oncogene • Oncogenesstill code for the proteins needed for cell division but they cause cancer by producing • Increased In growth factor • Increased In growth factor receptors • Increased in signal transduction • Increase in activation of transcription

27. Cancer causing Mutations • Proto-oncogenes form oncogenes by • being misplaced (e.g. by translocation) to a site where the gene is continually expressed resulting in overproduction of a protein that stimulates cell division (e.g. in CML*) • By mutating to a form that is over expressed. • Mutations inTumor Suppressor genes cause cancer by inactivating the genes.

28. Tumor-suppressor genes BIOLOGICAL FUNCTIONS OF TUMOR SUPPRESSOR GENES • Growth Inhibitors (e.g., TGF-β; glucocortocoids) • Growth Inhibitor Receptors • Signal Transduction Protein Inhibitors • Transcription Factors of Growth Inhibitors

29. Tumor-suppressor genes • Geneproducts which are normaly responsible for negative controll of transcription and proliferation • Examples: • pRb inhibits transcription factors of the E2F-family, which are needed to get into the S-Phase of the cell cycle (Restriction Point) • p53 induces transcription of the CDK-inhibitor (CDI) p21 which causes a cell cycle arrest (one function) • p53 is found upregulated in cells with a high level of DNA damage

31. Tumor-suppressor genes • RETINOBLASTOMA • Retinoblastoma (Rb) is a relatively rare, highly malignant childhood cancer of the developing retinal cells of the eye that usually occurs before the age of 5 years. • Rb can occur either sporadically (non-hereditary form, ussually involve only one eye), or be familial (hereditary form, more commonly bilateral), which is inherited in an AD manner, and also tend to present at an earlier age.

32. Retinoblastoma

33. The p53 Tumor Suppressor Protein • The p53 tumor suppressor protein is activated when DNA is damaged. The p53 gene is called the “guardian angel of the genome” • P53 activates genes for proteins that • Prevent cell entering S phase • Repair DNA • Cause apoptosis (if DNA is irreparable)

34. DNA Stability Genes • Monitor and maintain the integrity of the DNA. • Loss of function promotes mutations • Detection of DNA lesions decreased • Repair of damage decreased or improper • Decreased apoptosis

35. Routes to Genetic Instability based on Defective DNA Repair

36. Carcinogenesis Hypotheses of the Origin of Neoplasia • 2 – Viral Oncogene Hypothesis • RNA Retrovirus – produces DNA provirus • DNA provirus containing viral oncogene (v-onc) is introduced, or • DNA provirus without v-onc is inserted adjacent to c-onc in host cell DNA • RNA viruses is thought to have acquired v-onc sequence by recombinant mechanism from animal cells • DNA virus • Do not contain viral oncogenes • Act by blocking suppressor gene products • Examples – HPV, EBV,HBV

37. Carcinogenesis Hypotheses of the Origin of Neoplasia • 3 – Epigenetic Hypothesis • Changes in the regulation of gene expression rather than in the genetic apparatus • Pattern of gene expressions responsible for tissue differentiation (ie. epigenetic mechanism) are thought to be heritable

38. Carcinogenesis Hypotheses of the Origin of Neoplasia • 4 – Failure of Immune Surveillance • Concepts • Neoplastic changes frequently occur in cells • Altered DNA result in production of neoantigens & tumor-associated antigens • Immune response (cytotoxic) to neoantigens as foreign antigens • Neoplastic cells escaping recognition and destruction become clinical cancers

39. Causes of Neoplasia • Environmental causes: (Carcinogens) • Chemicals • Viruses • Radiation • Hereditary causes- Genetic defects. • Combination – common. • Obscure defects

40. Carcinogenesis:

41. Chemical Carcinogenesis: • Initiation • DNA damage eg.Benzpyrene • Promotion • Histologic change – eg. Turpentine (co-carcinogens) • Malignant transformation: • Visible tumor formation – further DNA damage.

42. Chemical Carcinogenesis: • Direct Acting Carcinogens: • Alkylating Agents: Cyclophosphamide • Procarcinogenes (needs activation) • Polycyclic hydrocarbons – Benzpyrene • Aromatic amines, dyes - Benzidine • Natural products: Aflatoxin • Others: Vinyl chloride, turpentine etc.

43. Viral Oncogenesis: • Insertion of viral nucleic acids  mutation • Alterations in Oncogenes, cancer suppressor genes and genes regulating DNA repair resulting in up-regulation of cell division  Carcinogenesis. • Nobel Laureates – Varmus and Bishop • v-fes, v-sis  proto-oncogenes. • v-sis  sis  PDGF  Brain tumours.

44. Viral Oncogenesis: • Human Papilloma Virus • Cervical neoplasia – warts, papilloma, ca cx • Epstein-Barr virus – • Burkitts Lymphoma, Nasopharyngeal ca. • Hepatitis B & C virus • Hepatocellular carcinoma.

45. Neoplasia Mutations Radiation Carcinogenesis: • Ionizing radiation  dysjunction  random fusion  mutation. • X Ray workers – Leukemia • Radio-isotopes – Thyroid carcinoma • Atomic explosion – Skin cancer, Leukemia

46. Clinical Manifestations of Cancer • Tissue Integrity • Compressed and eroded blood vessels, ulceration and necrosis, frank bleeding, and hemorrhage • Cancer Cachexia • Weight loss and wasting of body fat and muscle tissue; profound weakness, anorexia, and anemia • Paraneoplastic Syndromes • Manifestations in sites not directly affected by the disease

47. Heredity Radiation V-Onc Chemical Other Molecular Basis of Neoplasia: Proto-oncogene Oncogene

48. Multiple Genetic Changes Cause Cancer • Multiple Hits and Multiple Factors • Knudson proposed that carcinogenesis requires 2 hits • 1st event – initiation • Carcinogen = initiator • 2nd event – promotion • Agent = promoter • Multiple hits occur – 5 or more • Each hit produces a change in the genome which is transmitted to its progeny (ie. clone) • Lag period • Time between exposure (first hit) and development of clinically apparent cancer • Altered cell shows no abnormality during lag period

49. Multiple Genetic Changes Cause Cancer