Nem’s Notes… MOLECULES, CELLS & TISSUES 3 (page 1 of 2) Cell Division & Cancer Cell Division Phase 2 Year 3 co-author: Sabina Rashid Cells have three options during their lifespan: (a) Grow and divide (b) Not grow but remain alive (c) Die by apoptosis The decision is dependent on external signals such as diffusable growth factors which are transmitted into the cell via intramembrane receptors. Growth Control Control of cell growth requires four classes of protein: (a) A growth factor (b) A growth factor receptor (c) Intracellular transducers (d) Transcription factors Mutations in any of the genes encoding for these proteins can cause deregulation of growth control and can cause uncontrolled cell division. These genes would be called protooncogenes before the mutation and oncogenes after. GF Signalling Growth factors such as epidermal growth factor (EGF) attach to intramembrane receptors to have an effect. A sequence of steps occurs within the cell following the binding of the growth factor: (a) Dimerisation and phosphorylation of the receptor (b) Dimer unit is recognised by adaptor proteins (Grb-Sos) (c) Grb-Sos interacts with Ras (d) GDP converted to GTP activating the unit (e) Ras becomes Raf which phosphorylates MAPKK to MAPK (f) This activates transcription factors to transcribe DNA Receptors can be turned off via a number of mechanisms: (a) Internalisation of receptors (b) Negative feedback (via MAPK) (c) Phosphatases (inactivate MAPK) (d) GTPase activating proteins (prevent step (d) above) Control of Mitosis Mitosis can be controlled by TGFβ (transforming growth factor) since it acts as a CDK inhibitor and stops the cell cycle in G1 preventing entry into S phase. Integrins are a family of cell membrane receptors for extra-cellular matrix proteins which augment growth factor signals. Binding triggers association of Focal Adhesion Kinase (FAK) and other kinases which drive the cell cycle. However some integrins inhibit mitosis and promote differentiation. Contact Inhibition Cells do not overlap due to contact inhibition. This occurs via the action of cell adhesion molecules (CAMs) such as E-cadherin. These generate an anti-growth signal. Transformed cells such as cancerous cells lose contact inhibition. more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 1 of 2 Nem’s Notes… MOLECULES, CELLS & TISSUES 3 (page 2 of 2) Cell Division & Cancer Cancer Capabilities Phase 2 Year 3 co-author: Sabina Rashid Most cancers acquire the same set of functional capabilities and include: (a) Self-sufficiency in growth signals (b) Limitless replication potential (c) Insensitivity to anti-growth signals (d) Evasion of apoptosis (e) Sustained angiogenesis (blood vessel formation) (f) Tissue invasion and metastases The spread of cancerous cells to form new colonies distant from the primary tumour is called metastatic spread and is responsible for 90% of cancer deaths. Metastases For a cancer to become invasive or metastatic it must undergo certain changes: (a) Loss of E-cadherin function (or other CAMs) (b) Increased expression of extracellular proteases (breaks down ECM) (c) Integrin profile changes in preference of ECM in the new site Liver Cancer Liver cancer is the fifth most common cancer worldwide and has poor prognosis and high fatality. The most important risk factors are hepatitis B (HBV) infection and exposure to aflatoxin. HBV infection involves liver damage and the incorporation of viral genes into infected cells. Genes affected include those for p53, cyclin D and other proteins involved in the G1 phase of the cell cycle. Aflatoxins are derived from the Aspergillus fungus which can affect various food crops such as peanuts, maize and rice. The most potent form adducts to DNA leading to mutations. One ‘hot-spot’ is the code of the p53 gene. They can also act synergistically with HBV infection in causing liver cancer. Liver cancer is thought to involve a ‘multi-hit’ mechanism of carcinogenesis where a sequence of mutations must occur in order to establish full blown clinical cancer. At least two mutation events are required (and maybe more) before the cancer can develop. There are four main mutations that can cause this, involving the: (a) response to DNA damage (p53) (b) control of the cell cycle (RB, p16, cyclin D) (c) growth inhibition (TGFβ, M6P/IGF2R) (d) cell contact signalling (APC, β-catenin, E-cadherin) The exact mechanism is poorly understood and the exact sequence in which events occur is still to be determined. more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 2 of 2