Cancer and the Gut Microbiota

Adam Hassan, BSc Microbiology and Immunology
Contributor

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Cancer is not a particular disease, but rather a term used to group many related diseases (1). This group of diseases is responsible for a large number of deaths worldwide and represents a serious health threat today. Cancerous cells are defined by an uncontrolled cell proliferation – they continuously divide and ignore signals telling them to stop, to the point of forming a tumor. A tumor is referred to as being malignant only if it is capable of spreading throughout the body of the patient. Thus, from a single cell, the entire body can be invaded by malignant tumors (1).

Image courtesy of: http://www.cancer.ca/en/cancer-information/cancer-101/what-is-cancer/cancer-cell-development/?region=on

​​The human host’s immune system tolerates trillions of microorganisms. The bacteria that comprise the gut microbiota line the walls of the intestine and live in a mutually beneficial equilibrium within the host. Meanwhile, the immune system is constantly on the lookout for harmful pathogens that may contribute to disease. It’s safe to say that there’s a perfect and fragile balance existing within our gut. However, when this equilibrium is broken, a number of negative consequences can ensue. One of them is the risk of oncogenesis (formation of cancer) and tumour progression (2).
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This break in the equilibrium, also referred to as dysbiosis, can be caused by a number of factors. The arrival of pathogenic (harmful) organisms as well as several environmental (aging, hormones, antibiotics) and genetic (defects in the intestinal immune system) factors promote dysbiosis (2). Not only can these factors affect the risk of colorectal cancer, but the gut microbiota can also promote oncogenesis on a systemic level and lead to breast and hepatocellular (liver) cancers (3). 

Image courtesy of: https://anasbananas.com/professor-of-health/vitamin-d-diet-exercise-colon-cancer-prevention/

We’ve seen how disequilibrium in relation to our gut microbiota can affect the risk of tumorigenesis (formation of tumours). On the other hand, it is possible to influence the microbiota for therapeutic purposes. Recent data has suggested that the gut microbiota is capable of modulating the response to cancer therapy and the susceptibility to toxic side effects (4). Commensal organisms are responsible for absorbing and metabolizing drugs. Furthermore, direct interaction with bacteria can affect the efficacy of chemotherapeutic drugs (4). Many of the mechanisms by which the gut microbiota affects inflammation, immunity, carcinogenesis (development of cancer) and response to therapy at the local level have been characterized.

One challenge for clinicians is to determine the best microbiota composition for each condition. The ultimate goal is to discover a bacterial species or a combination of species that both reduces systemic toxicity and promotes anticancer therapy. Thus, targeting the microbiota in cancer and other diseases is likely to become one of the next frontiers for precision and personalized medicine (4).
 
References

1.  National Cancer Institute. "What Is Cancer?" National Cancer Institute. NIH, 9 Feb. 2015. Web.
2. Zitvogel, Laurence, Lorenzo Galluzzi, Sophie Viaud, Marie Vétizou, Romain Daillère, Miriam Merad, and Guido Kroemer. "Cancer and the Gut Microbiota: An Unexpected Link." Science Translational Medicine. U.S. National Library of Medicine, 21 Jan. 2015. Web.
3. Yoshimoto, S., T. M. Loo, K. Atarashi, H. Kanda, S. Sato, S. Oyadomari, Y. Iwakura, K. Oshima, H. Morita, M. Hattori, K. Honda, Y. Ishikawa, E. Hara, and N. Ohtani. "Obesity-induced Gut Microbial Metabolite Promotes Liver Cancer through Senescence Secretome." Nature. U.S. National Library of Medicine, 04 July 2013. Web.