It’s all about the bugs: how intestinal bacteria connect cell death and disease
​
The problem:
Cancer is a common disease, with millions of people being diagnosed in the world every year. To try and reduce tumour growth, cancer patients often undergo cytotoxic chemotherapy treatment where they are given drugs that target and kill replicating cells (ie tumour cells). Whilst chemotherapy can help reduce and clear tumours, up to 80% of patients experience unpleasant gastrointestinal (GI) symptoms and toxicity, such as nausea and vomiting, which can be so severe that it causes patients to stop their treatment early or have their dose reduced (1,2). Reductions in chemotherapy duration or dosing can ultimately lead to a failure of therapy and a regrowth of the tumour.
​
In our GI tracts, there is a careful balance of bacteria that help protect us from infection and help normal day-to-day functions like digestion. This carefully constructed bacterial community can fall out of balance in a process called dysbiosis. Dysbiosis is often associated with different diseases; however, it isn’t entirely clear if this changing bacterial population is a symptom of disease or if dysbiosis causes disease. Similar to the chicken and egg, this leaves the community wondering: which came first, dysbiosis or disease?
​
Chemotherapy treatment frequently causes dysbiosis and, as the name cytotoxic implies (cyto = cell, toxic = relating to or caused by poison), chemotherapy causes cells to die. In addition to killing replicating tumour cells, chemotherapy also kills healthy cells in the intestinal tract by mistake, as these cells normally replicate at some of the highest levels throughout the body. Therefore, the question we wanted to answer was: is there a connection between chemotherapy killing our normal intestinal cells and chemotherapy leading to dysbiosis? If there is, how does this connection contribute to GI side effects?
​
What we did:
In this study, we primarily used a mouse model of GI toxicity by treating animals with chemotherapeutics (doxorubicin or cisplatin) and then measuring cell death and the stability of the bacterial population over time. We found that a single dose of chemotherapy caused a wave of healthy intestinal cell death following treatment- cells began to die within hours of treatment but all signs of death had subsided after several days. Simultaneously, we measured significant changes to the bacterial community with a particular expansion of Escherichia coli (E. coli)- these bacteria increased to levels 10,000x higher than their starting point. What was particularly striking to us was that the timing of dysbiosis looked just like cell death- rapid onset followed by gradual decline and return to normal. In contrast, intestinal disease, which we measured by several readouts including excessive mucous production, had a delayed and prolonged onset that was preceded by cell death and dysbiosis. Thus, our animal model recapitulates the 3Ds of human GI toxicity (death, dysbiosis, and disease) and suggested that death and dysbiosis were upstream of disease.
​
We next sought to test this idea by experimentally interfering with either cell death or dysbiosis and then asking how the other factors (including disease) were modified. We used genetically modified mice that have intestinal cells that do not die in response to chemotherapy. After chemotherapy treatment, these animals had reduced dysbiosis and reduced disease thereby placing the cell death process upstream of both of these factors. On the flip side, limiting dysbiosis with antibiotics or using bacteria‑free mice does not change the amount of cell death but instead reduces intestinal disease. This ultimately allows us to construct a simplified linear model in which:
death -> dysbiosis -> disease
​
As a consequence of cell death, intestinal bacteria exploit the molecules that are released by the dying cells. Compounds called purines, released from the dying intestinal cells, are used by the bacteria to fuel their growth and to prime them to better to use oxygen for survival. Using oxygen is a unique trait to particular intestinal bacteria, including E. coli, and affords them a major advantage over competing bacteria that cannot use oxygen when oxygen is present. Although we don’t fully understand how this works at the molecular level, our data now highlight dying intestinal cell‑derived purines as a secondary signal to help these bacteria take advantage of the presence of oxygen.
​
Why it matters:
This is important, as it shows that GI toxicity is majorly impacted by intestinal bacteria as a result of cell death. These findings mean that we may be able to find and administer a drug alongside chemotherapy that will put a halt to this relationship. If we could do this, we could decrease the severity of the GI side effects of chemotherapy so that patients could finish their course of treatment and have a better chance of recovering from their primary cancer diagnosis.
​
If the search for the holy grail has taught us anything, it’s that immortality is hard to come by. The same can be said at the cellular level: cells will inevitably find a way to die. Thus, rather than trying to inhibit the death process (which is actually pretty important if we are talking about treating tumour cells), we instead can look for ways to inhibit the ability of bacteria to sense the dead cells or stop certain molecules from coming out of dead cells.
​
What’s next:
Before this, we will need to do more research into exactly how the bacteria use the dying cells to grow, so that we can then identify and test particular drugs that target these factors. Of course, all of the above was performed in experimental animal models, so additional experiments are needed in the lab using human cells and human tissue before eventually (hopefully) applying our findings in the context of clinical trials.
Overall, this is the first step towards reducing the side effects of chemotherapy and improving cancer patient prognosis but there are years of work that lie ahead.
Written by AD & CJA
​
1 Jones, J. A. et al. Epidemiology of treatment-associated mucosal injury after treatment with newer regimens for lymphoma, breast, lung, or colorectal cancer. Support Care Cancer 14, 505-515, doi:10.1007/s00520-006-0055-4 (2006).
2 Sonis, S. et al. Unanticipated frequency and consequences of regimen-related diarrhea in patients being treated with radiation or chemoradiation regimens for cancers of the head and neck or lung. Support Care Cancer 23, 433-439, doi:10.1007/s00520-014-2395-9 (2015).