Carcinogen

A carcinogen is a substance which can cause cancer in a living organism. Examples of carcinogens include tobacco smoke, ethanol and UV radiation. It is difficult to completely avoid carcinogens as they also include air pollution, solar radiation and engine exhausts, all of which exist in everyday life.

Cancer is a change in the DNA of the cell, which leads to uncontrolled cell division.

It is important to understand that exposure to one or even many carcinogens does not always result in cancer. How carcinogens act in the body and cause cancer is determined by other factors such as the individual's genetic makeup or their exposure to other carcinogens.

All carcinogens affect the body in different ways, and the body has defence mechanisms which prevent the carcinogen leading to cancer by disrupting and changing the organism's DNA. 

Acetaldehyde is a product of alcohol metabolism and is formed when alcohol in the liver is broken down. This is catalysed by the enzyme alcohol dehydrogenase. In the body, the acetaldehyde is broken down by the enzyme acetaldehyde dehydrogenase and glutathione to non-toxic acetate. However, if the glutathione stores are low in the liver, acetaldehyde remains in the body in its toxic form for a longer period of time. Acetaldehyde damages our cell's DNA and prevents cells from repairing the damage. It also causes liver cells to grow at a faster rate, meaning replication is faster and the cells are more likely to acquire cancer-inducing mutations. 

UV radiation is a carcinogen which causes cancer by penetrating either the epidermis or dermis of the skin, which causes DNA damage. If the body is unable to repair this damage, the cells divide uncontrollably due to the fact normal cell death (apoptosis) is altered, which causes the formation of a malignant tumour.  

Despite the carcinogens acting on the body in slightly different ways, the main outcome is DNA damage which the cell is unable to repair. This results in cancer by uncontrolled cell division. Cancer cells also have a faulty version of the p53 gene which decides when the cell should undergo apoptosis. This means they are not repaired properly which could cause new mutations, metastasis and resistance to treatment. Carcinogens can also cause affected cells to be less specialised and mask them from the immune system which means they cannot be destroyed. 

The body has many defence mechanisms which it uses in order to prevent the carcinogen leading to cancer, such as:

• Apoptosis.
• Cells can recognise carcinogens and mark them as harmless; through biotransformation they are made more water soluble which makes it easy for the carcinogen to be flushed out of the body. 
• DNA repair genes can repair the damaged DNA before replication. 
• Cytochrome p450 enzymes can remove carcinogens.

These defence mechanisms can backfire, however. Biotransformation can actually make the carcinogen more carcinogenic, DNA damage may be too extensive for the cell's repair systems to eradicate and cytochrome p450 enzymes can activate harmless molecules (procarcinogens) into carcinogens by metabolism.