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Understanding Cancer Series-Part4 (Loss of Normal Growth Control)

Discussion in 'Oncology' started by waleed, Sep 14, 2011.

  1. waleed

    waleed Moderator

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    Cancer arises from a loss of normal growth control. In normal tissues, the rates of new cell growth and old cell death are kept in balance. In cancer, this balance is disrupted. This disruption can result from uncontrolled cell growth or loss of a cell's ability to undergo cell suicide by a process called"apoptosis." Apoptosis, or "cell suicide," is the mechanism by which old or damaged cells normally self-destruct.N.B:Some viruses associated with cancers use tricks to prevent apoptosis of the cells they have transformed.

    _ Several human papilloma viruses (HPV) have been implicated in causing cervical cancer. One of them produces a protein (E6) that binds and inactivates the apoptosis promoter p53.
    _ Epstein-Barr Virus (EBV), the cause of mononucleosis and associated with some lymphomas
    produces a protein similar to Bcl-2
    produces another protein that causes the cell to increase its own production of Bcl-2. Both these actions make the cell more resistant to apoptosis (thus enabling a cancer cell to continue to proliferate).

    Even cancer cells produced without the participation of viruses may have tricks to avoid apoptosis.

    _ Some B-cell leukemias and lymphomas express high levels of Bcl-2, thus blocking apoptotic signals they may receive. The high levels result from a translocation of the BCL-2 gene into an enhancer region for antibody production.
    _ Melanoma (the most dangerous type of skin cancer) cells avoid apoptosis by inhibiting the expression of the gene encoding Apaf-1.
    Some cancer cells, especially lung and colon cancer cells, secrete elevated levels of a soluble "decoy" molecule that binds to FasL, plugging it up so it cannot bind Fas. Thus, cytotoxic T cells (CTL) cannot kill the cancer cells .
    Other cancer cells express high levels of FasL, and can kill any cytotoxic T cells (CTL) that try to kill them because CTL also express Fas (but are protected from their own FasL).

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    source 1:Apoptosis
    source 2:Comprehensive Cancer Information - National Cancer Institute
     

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