Presentation on theme: "Lecture 25. Transformation and Oncogenesis. Flint et al, Chapter 18 Cancer: a genetic disease Results from growth of successive populations of cells in."— Presentation transcript:
Lecture 25. Transformation and Oncogenesis. Flint et al, Chapter 18 Cancer: a genetic disease Results from growth of successive populations of cells in which mutations have accumulated Alteration of steps in regulatory pathways that control cell communication and proliferation Uncontrolled growth Cellular disorganization Cancer Approx. 20% of all human cancers are of viral origin. Viruses are major causes of liver and cervical cancers Malignancy can result as a consequence of a side effect of viral infection or of host response to virus.
Some cancer terms (Box 18.1) Neoplasm: an abnormal new growth Benign: a growth that does not infiltrate into surrounding tissues. Malignant: Any disease of a progressive, fatal nature Tumor: swelling; caused by abnormal cell growth not from inflammation; can be benign or malignant. Cancer: A malignant tumor; growth not encapsulated; infiltrates into surrounding tissues; spread by lymphatic vessels to other parts of body; death caused by destruction of other organs, by extreme debility and anemia or by hemorrhage. Carcinogenesis: Complex multistage process by which cancer develops. Oncogenic: causing a tumor Metastases: Secondary tumors derived from cells of primary tumor that disseminated to other parts of the body.
Types of cancers Cancers get their names based on the tissue of origin Adenoma: A cancer of hormone secreting cells. Many cancers of reproductive tracts. Carcinoma: cancer of epithelioid tissue Fibroblast: tissue derived from connective tissue Fibropapilloma: Solid tumor of cells derived from connective tissue Hepatocellular carcinoma: a cancer of liver epithelial cells Endothelioma: any tumor, particularly a benign one, arising from the endothelial lining of blood vessels Leukemia: A cancer of white blood cells Lymphoma: a cancer of lymphoid tissue Retinoblastoma: Cancer of retinal cells Sarcoma: A cancer of fibroblasts
Transformed cells (Table 18.1) Much of what we know about cancer is derived from studies of Transformed Cells These have abnormal growth parameters and behaviors: Immortality: can grow indefinitely Reduced requirement for serum growth factors Loss of capacity for growth arrest upon nutrient deprivation High saturation densities Loss of contact inhibition Anchorage independent (can grow in soft agar) Altered morphology (rounded and refractile) Tumorogenic: can cause tumors when transplanted into animals
Sensing the environment: (Fig. 18.3) Cells must sense what is going on around them Cell surface receptors interact with ligands Signal transduction cascades Second messengers Activation and repression of genes
The cell cycle (Fig. 18.4) Cell growth regulated by an internal timer: cell cycle Divided into 4 phases G1: cell growth, restriction point S: DNA synthesis G2: preparation for cell division M: Mitosis
Cell cycle control (Fig. 18.5) Rb protein: phosphorylation status of Rb used to control cell cycle –Rb phosphorylation: allows passage of G1 restriction point, entry into S-phase –Rb dephosphorylation: signals end of M phase. Cell cycle is controlled by the cyclin-Cdk machinery Different cyclins and cyclin dependent kinases expressed at different stages of the cell cycle.
Oncogenic Viruses Cause cancer by inducing changes that affect cell proliferation Approx 20% of all human cancers causes by one of 5 viruses” –1. Epstein-Barr virus –2. Hepatitis B –3. Hepatitis C, –4. HTLV I –5. Hum. Papillomaviruses
Oncogenic Viruses: a Genetic Paradigm for Cancer (Fig. 18.6) Study of viral transformation of cells laid the foundations for our current understanding of cancer. Enabled identification of Oncogenes and Tumor Suppressor genes Foundation for the genetic paradigm of cancer
History 1908: Ellerman and Bang show that avain leukemia could be transmitted through filtered extracts or serum from infected birds. 1911: Rous showed that solid tumors could be produced in chickens by using cell-free extracts from a transplantable tumor (Rouse Sarcoma Virus: the first discovered retrovirus) 1933: Shope isolates papillomavirus from warts 1978: Bishop and Varmus define oncogene
Oncogenic viruses and cancer (Table 18.2) FamilyAssociated Cancer(s) RNA viruses Flaviriridae Hepatitis C virusHepatocellular carcinoma RetroviridaeHaemopoetic cancers, sarcomas, carcinomas DNA viruses AdenoviridaeVarious solid tumors HepadnaviridaeHepatocellular carcinoma HerpesviridaeLymphomas, carcinomas, sarcomas PapillomaviridaePapillomas and carcinomas PolyomaviridaeVarious solid tumors PoxviridaeMyxomas and fibromas
Insertional mutagenesis Integration of retroviral progenomes mutates the genome of a cell. Proviral promoters can activate transcription of nearby genes. Transformation can occur if the nearby gene is an oncogene. –e.g. c-myc Transformation can also occur if insertion disrupts tumor suppressor genes.
Viral transforming genes 2 general strategies –Permanent activation of cellular signal transduction cascades –Disruption of cell cycle regulation
Viral transforming genes v-oncogenes (see Table 18.6, Figs. 18.7. 18.8) Characteristic of transforming viruses Cellular origin (Bishop and Varmus, Nobel Prize 1989) Picked up by retroviruses Typically fusions of viral + cellular genes Viral sequences alter expression, regulation and localization of gene products –e.g. overexpression of myc is sufficient to induce transformation –e.g. v-erbB is a truncated form of the epithelial growth factor receptor. Expression stimulates growth of cells by mimicking the “on” state of the receptor.
Viral proteins that alter cellular signaling pathways (Fig. 18.13, Table 18.8) Constitutively active viral receptors Of viral origin, do not resemble cellular proteins. Proteins specifically recruit and activate signal transduction pathways e.g. LMP-1 in Epstein Barr virus (Fig. 18.13)
Viral adapter proteins that alter cellular signaling pathways See Fig 18.14, and Table 18.9 Bind to cellular tyrosine kinases Permanently activates them Turns on cellular signal transduction pathways. e.g. mT protein of Polyomavirus activates c-Src tyrosine kinase Fig. 18.14 C-SRC
Cell cycle Regulation by the Rb protein Fig. 18.16
Transformation via cell cycle control pathways Inhibition of Rb function by viral proteins Many viruses actively inhibit Rb function Result: bypass of restriction point control Passage from G1 S phase e.g. SV40 LT, adenovirus E1A, HPV E7 proteins (Fig. 18.17)
Transformation via cell cycle control pathways Production of virus specific cyclins e.g. Human herpesvirus 8 v-cyclin Binds to and activates Cdk6 Rb phosphorylation Promotes G1 S transition
Inhibition of p53 functions (Fig. 18.20, 21) p53 is a tumor suppressor gene Determines response of cells to DNA damage and hypoxia p53 promotes either –Cell cycle arrest (until problem is fixed) –Apoptosis (unfixable problem) Virus infection is a stress that turns on p53 Proteins from many viruses mislocalize or block p53 e.g. Adenoviruses, papillomaviruses, polyomaviruses
Oncogenesis by hepatitis viruses (Fig. 18.22) Hepatitis B (Hepadnavirus), Hepatitis C (Flavivirus) Persistent infections Sustained low level lever damage due to immune system attack Lots of cell proliferation/regeneration Lots of cellular DNA replication + Lots of oxidative stress = Increased chance of mutation