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A carcinogen is a [[Cancer|cancer]] causing substance. Examples of this are substances seen in tobacco smoke. These includes tar, [[nicotine|nicotine]], [[Carbon monoxide|carbon monoxide]], etc.<br>  
A carcinogen is a substance which can cause&nbsp;[[Cancer|cancer]]&nbsp;in a living organism<ref>MedicineNet. Carcinogen. 2016 [cited 29/11/2017]; Available from: https://www.medicinenet.com/script/main/art.asp?articlekey=2625</ref>. Examples of carcinogens include tobacco smoke, [[Ethanol|ethanol]] and [[Ultraviolet|UV radiation]]. It is difficult to completely avoid carcinogens<ref>Fishwick C. How do I ... avoid carcinogens? 2015 [cited 29/11/2017]; Available from: https://www.theguardian.com/uk-news/2015/oct/30/cancer-carcinogens-avoid-i-die-exposure</ref> as they also include air pollution, solar radiation and engine exhausts, all of which exist in everyday life<ref>American Cancer Society. Known and Probable Human Carcinogens. 2016 [cited 29/11/2017]; Available from: https://www.cancer.org/cancer/cancer-causes/general-info/known-and-probable-human-carcinogens.html</ref>.<br>  


Carcinogens arise in the body after metabolic activation of environmental chemicals&nbsp;by enzymes such as the [[P450 enzymes|p450 enzymes]]. These environmental chemicals remain biologically inert until they are taken up into tissue when they then become activated, becoming a carcinogen. The carcinogen then spontaneously becomes a reactive intermediate, forms [[DNA adducts|DNA adducts]] and&nbsp;causes genetic damage resulting in&nbsp;cancer.<br>  
[[Cancer|Cancer]] is a change in the [[DNA|DNA]] of the cell, which leads to uncontrolled [[Cell division|cell division]]<ref>Cancer Research UK. What is cancer? 2017 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer</ref>.


The body has defensive mechanisms at each of these steps to prevent the carcinogen leading to cancer such as:  
It is important to understand that exposure to one or even many carcinogens does not always result in [[Cancer|cancer]]. How carcinogens act in the body and cause [[Cancer|cancer]] is determined by other factors such as the individual's [[DNA|genetic makeup]] or their exposure to other carcinogens<ref>National Cancer Institute. Environmental Carcinogens and Cancer Risk. 2015 [cited 29/11/2017]; Available from: https://www.cancer.gov/about-cancer/causes-prevention/risk/substances/carcinogens</ref>.


#[[Metabolic detoxification|Metabolic detoxification]] by the GST ([[Glutathione-S-transferase|glutathione-S-transferase]]) [[Enzyme|enzymes]]  
All carcinogens affect the body in different ways, and the body has [[Apoptosis|defence mechanisms]] which prevent the carcinogen leading to [[Cancer|cancer]] by [[Mutations|disrupting and changing]] the organism's [[DNA|DNA]].&nbsp;
#[[DNA repair|DNA repair]]
#Cell [[Apoptosis|apoptosis]].


However if this defence fails and there are not enough metabolic detoxification enzymes or the DNA is replicated with the wrong base order, cancer occurs.  
[[Acetaldehyde|Acetaldehyde]]&nbsp;is a product of [[Alcohol|alcohol]] [[Metabolism|metabolism]]&nbsp;and is formed when [[Alcohol|alcohol]] in the [[Liver|liver]] is [[Metabolism|broken down]]. This is [[Catalysis|catalysed]] by the [[Enzyme|enzyme]] [[Alcohol dehydrogenase|alcohol dehydrogenase]]. In the body, the [[Acetaldehyde|acetaldehyde]] is [[Catalysis|broken down]] by the [[Enzyme|enzyme]]&nbsp;acetaldehyde dehydrogenase and [[Glutathione|glutathione]]&nbsp;to non-toxic [[Acetate|acetate]]. However, if the [[Glutathione|glutathione]] stores are low in the [[Liver|liver]], [[Acetaldehyde|acetaldehyde]] remains in the body in its toxic form for a longer period of time<ref>Perry L. Biology of a Hangover: Acetaldehyde. 2004 [cited 29/11/2017]; Available from: https://health.howstuffworks.com/wellness/drugs-alcohol/hangover4.htm</ref>. [[Acetaldehyde|Acetaldehyde]] [[Mutation|damages]] our cell's [[DNA|DNA]] and prevents cells from [[DNA repair|repairing]] the damage. It also causes [[Liver|liver]] cells to grow at a faster rate, meaning [[DNA replication|replication]] is faster and the cells are more likely to acquire cancer-inducing [[Mutations|mutations]]<ref>Cancer Research UK. How alcohol causes cancer. 2016 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/causes-of-cancer/alcohol-and-cancer/how-alcohol-causes-cancer</ref>.&nbsp;<br>
 
[[Ultraviolet|UV radiation]] is a carcinogen which causes [[Cancer|cancer]] by penetrating either the [[Epidermis|epidermis]] or [[Dermis|dermis]] of the [[Skin|skin]], which causes [[Mutations|DNA damage]]. If the body is unable to [[DNA repair|repair]] this damage, the cells [[Cell division|divide]] uncontrollably due to the fact normal cell death ([[Apoptosis|apoptosis]]) is altered<ref>Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025</ref>, which causes the formation of a malignant tumour<ref>Cancer Council NSW. How ultraviolet (UV) radiation causes skin cancer. 2015 [cited 29/11/2017]; Available from: https://www.cancercouncil.com.au/63295/cancer-prevention/sun-protection/sun-protection-sport-and-recreation/sun-protection-information-for-sporting-groups/how-ultraviolet-uv-radiation-causes-skin-cancer/</ref>. &nbsp;
 
Despite the carcinogens acting on the body in slightly different ways, the main outcome is [[Mutations|DNA damage]] which the cell is unable to repair. This results in [[Cancer|cancer]] by uncontrolled [[Cell division|cell division]]. Cancer cells also have a faulty version of the p53 gene which decides when the cell should undergo&nbsp;[[Apoptosis|apoptosis]]. This means they are not repaired properly which could cause new [[Mutations|mutations]], [[Metastasis|metastasis]] and resistance to [[Chemotherapy|treatment]]<ref>Cancer Research UK. Cancer cells. 2014 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer/how-cancer-starts/cancer-cells</ref>. Carcinogens can also cause affected cells to be less specialised and mask them from the [[Immune system|immune system]] which means they cannot be [[Immune response|destroyed]].&nbsp;
 
The body has many defence mechanisms which it uses in order to prevent the carcinogen leading to [[Cancer|cancer]], such as:
 
*[[Apoptosis|Apoptosis]]<ref>Cancer Research UK. Cancer cells. 2014 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer/how-cancer-starts/cancer-cells</ref>.
*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.&nbsp;
*[[DNA repair|DNA repair]] [[Gene|genes]] can repair the [[Mutations|damaged DNA]] before [[DNA replication|replication]]<ref>Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025</ref><span style="font-size: 13.28px;">.&nbsp;</span>
*Cytochrome p450 [[Enzyme|enzymes]] can remove carcinogens.
 
These defence mechanisms can backfire, however. Biotransformation can actually make the carcinogen more carcinogenic, [[Mutations|DNA damage]] may be too extensive for the cell's repair systems to eradicate and cytochrome p450 [[Enzyme|enzymes]]&nbsp;can&nbsp;activate harmless molecules (procarcinogens) into carcinogens<ref>InterPro. Cytochrome P450. 2015 [cited 29/11/2017]; Available from: https://www.ebi.ac.uk/interpro/potm/2006_10/Page4.htm</ref>&nbsp;by [[Metabolism|metabolism]]<ref>Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025</ref>.&nbsp;<br>


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=== References:  ===
 
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Revision as of 15:10, 29 November 2017

A carcinogen is a substance which can cause cancer in a living organism[1]. Examples of carcinogens include tobacco smoke, ethanol and UV radiation. It is difficult to completely avoid carcinogens[2] as they also include air pollution, solar radiation and engine exhausts, all of which exist in everyday life[3].

Cancer is a change in the DNA of the cell, which leads to uncontrolled cell division[4].

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[5].

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[6]. 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[7]. 

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[8], which causes the formation of a malignant tumour[9].  

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[10]. 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[11].
  • 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[12]. 
  • 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[13] by metabolism[14]. 


References:

  1. ↑ MedicineNet. Carcinogen. 2016 [cited 29/11/2017]; Available from: https://www.medicinenet.com/script/main/art.asp?articlekey=2625
  2. ↑ Fishwick C. How do I ... avoid carcinogens? 2015 [cited 29/11/2017]; Available from: https://www.theguardian.com/uk-news/2015/oct/30/cancer-carcinogens-avoid-i-die-exposure
  3. ↑ American Cancer Society. Known and Probable Human Carcinogens. 2016 [cited 29/11/2017]; Available from: https://www.cancer.org/cancer/cancer-causes/general-info/known-and-probable-human-carcinogens.html
  4. ↑ Cancer Research UK. What is cancer? 2017 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer
  5. ↑ National Cancer Institute. Environmental Carcinogens and Cancer Risk. 2015 [cited 29/11/2017]; Available from: https://www.cancer.gov/about-cancer/causes-prevention/risk/substances/carcinogens
  6. ↑ Perry L. Biology of a Hangover: Acetaldehyde. 2004 [cited 29/11/2017]; Available from: https://health.howstuffworks.com/wellness/drugs-alcohol/hangover4.htm
  7. ↑ Cancer Research UK. How alcohol causes cancer. 2016 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/causes-of-cancer/alcohol-and-cancer/how-alcohol-causes-cancer
  8. ↑ Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025
  9. ↑ Cancer Council NSW. How ultraviolet (UV) radiation causes skin cancer. 2015 [cited 29/11/2017]; Available from: https://www.cancercouncil.com.au/63295/cancer-prevention/sun-protection/sun-protection-sport-and-recreation/sun-protection-information-for-sporting-groups/how-ultraviolet-uv-radiation-causes-skin-cancer/
  10. ↑ Cancer Research UK. Cancer cells. 2014 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer/how-cancer-starts/cancer-cells
  11. ↑ Cancer Research UK. Cancer cells. 2014 [cited 29/11/2017]; Available from: http://www.cancerresearchuk.org/about-cancer/what-is-cancer/how-cancer-starts/cancer-cells
  12. ↑ Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025
  13. ↑ InterPro. Cytochrome P450. 2015 [cited 29/11/2017]; Available from: https://www.ebi.ac.uk/interpro/potm/2006_10/Page4.htm
  14. ↑ Helmenstine AM. How Carcinogens Work. 2017 [cited 29/11/2017]; Available from: https://www.thoughtco.com/carcinogen-definition-4146025
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