6 - Viral Transformation of Cells Virus are probably...

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Viral Transformation of Cells Virus are probably responsible for about 15% of human cancers and as a risk factor are second only to tobacco. Transformation can be defined as " the introduction of inheritable changes in a cell causing changes in the growth phenotype and immortalisation ". The cell cycle, (G0)-G1-S-G2-M , is highly regulated in multicellular organisms. The G0/G1 boundary is a particularly important control point because this acts as a commitment to cell division. Tumour formation results from a failure of these regulatory mechanisms; tumour cells continue to divide under circumstances in which their normal cellular counterparts do not. Characteristics of tumour cells. Growth: high/indefinite saturation density different, usually reduced serum requirement anchorage independent : growth in agar and growth on normal cell monolayers tumour formation when injected into animals no contact inhibition of movement less oriented growth Surface: loss of tight junctions foetal antigen expression increased rate of nutrient transfer increased secretion of proteases Intracellular: disruption of cytoskeleton altered amounts of signalling molecules (cyclic nucleotides, phosphoinositides) This list highlights regulatory mechanisms acting on normal cells which can be broadly divided into 3 groups: 1. Those involving cell-cell contact i.e. contact inhibition of growth. Normal cells exhibit it, transformed cells do not. 2. Those involving dependence on exogenous growth factors. Normal cells show such a dependence, transformed cells do not. 3. In some cell types, normal cells exhibit anchorage dependent growth whereas transformed cells do not.
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Assays for Cell Transformation Three assays are commonly used, each exploiting a different selection pressure: Focus forming assay. Transformation was detected as foci of dense morphologically altered cells in monolayers of embryo fibroblasts. This type of assay which depends on the loss of contact inhibition has subsequently been used for a number of DNA viruses. Anchorage independent growth in agar or a methyl cellulose suspension. Normal fibroblast lines fail to grow when suspended in a viscous fluid or gel. This is usually considered the most stringent assay. Reduced serum requirement The reduced serum requirement of transformed cells can be exploited e.g. NIH 3T3 cells will not grow in media with less than 5% serum. SV40 transformants could be selected for by plating in 0.5% serum. This assay exploits the requirement of cells for growth factors. Mechanism of Transformation: In vivo and epidemiological studies suggest that transformation is a multistep process involving initiation, promotion and progression events. The promotion and progression stages involve further cellular mutations. How do viruses transform cells?
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This note was uploaded on 10/28/2010 for the course SCI biol3214 taught by Professor Lam during the Spring '10 term at CUHK.

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6 - Viral Transformation of Cells Virus are probably...

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