Does ageing cause cancer? Or does cancer cause ageing? That is the question.  Most people reading this may think the answer is obvious, that ageing leads to cancer.  The general public’s mindset about ageing is normally associated with oxidative stress and the damage it can cause to proteins and DNA (mainly due to media coverage and beauty products). Increases in DNA damage (and mutations) with age are thought to be associated with the increase incidences of cancer later in life.  In some respect this may partly be true (although convincing evidence of this in humans is currently lacking in my opinion), but it is not the full story.

The human body has evolved mechanisms for preventing the appearance of cancer cells.  One such mechanism is known as cellular senescence, an irreversible growth arrest of the cell.  When oncogenes become activated they undergo onocogene-induced senescence (OIS), thus acting as a barrier to prevent aberrant growth and cancer formation.  In addition to the permanent growth arrest, senescent cells appear to have a secretory phenotype which may aid the removal of the senescent cell by the immune system.  Probably triggered by findings from Scott Lowe’s group, who in 2007 published an article which provided evidence for immune response of senescent cells, Dominick Burton went on to suggest that the  secretory phenotype of senescent cells may function in the removal of senescent cells by the immune system…..” One possibility is that senescent cells secrete cytokines to attract immune cells to its location (for its removal), secretes matrix degrading proteins to allow the immune cells access and secrete growth factors to stimulate surrounding cells to proliferate once the cell has been removed” (Burton, 2008).

Senescent cells are known to accumulate in tissues as we age and Dominick suggested this might be due to a functional impairment of an ageing immune system.  The accumulation of senescent cells in tissues is a bad thing.  If senescent cells persist in tissues, the secretory factors which may have been intended for immune clearance are now constantly being released into the local microenvironment.  These factors (cytokines, growth factors and proteases) are thought to alter the biological properties of the tissue in which they reside, thus causing ageing.  In this regard, cancer causes ageing indirectly due to defects in mechanisms intended to remove potentially cancerous cells.  Additionally, the Campisi group suggested that the secretory phenotype of senescent cells might also stimulate the appearance cancerous cells.  So cancer leads to ageing which leads to even more cancer.

So to conclude, immune clearance of senescent cells my be extremely important in both preventing cancer and preventing ageing.

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