Cancer is an umbrella term for a large group of diseases characterised by the uncontrolled growth of abnormal cells, which have the potential to spread to other organs in the body and disrupt normal bodily function. This occurs due to genetic mutations in ordinary cells which allow them to divide more quickly or inhibit normal cell cycle regulation and programmed cell death (apoptosis).
Cancer is very hard to treat, mainly because of the number of genes involved in the development of cancer, and the difficulty of targeting cancer cells without damaging surrounding healthy tissue. Despite this, there are a few different treatments that can be used in cancer-one of the most common is chemotherapy, where drugs are used to target the cancer cells.
However, chemotherapy is not always effective, and sometimes the cancer progresses despite these treatments. The primary cause of around 90% of cancer related deaths is metastasis, which is when cancer cells break away from their original site of the tumour and move through the body to a different site in the host where they form new tumours. At this point, there are very few effective treatments which are available.
Developing an understanding of the mechanisms behind chemotherapy resistance and metastasis in cancer is crucial for the development of therapeutics that can address these issues with existing cancer treatments, halt disease progression, and improve patient survival rates.
This article explores one of the key characteristics of cancer cells that allows them to resist chemotherapy and can drive disease progression, which is a trait known as phenotypic plasticity.
What is phenotypic plasticity?
In biology, phenotypic plasticity is essentially the ability of an organism to change its physical form or behaviour (its phenotype) in response to changes in its external environment, which helps to make the organism better suited for its surroundings. For example, animals such as southern rockhopper penguins may change their feeding habits according to the climate they live in (e.g. subtropical, subarctic, or subantarctic waters). In this way, penguins increase their survival odds in different climates.
Cancer cell plasticity is one type of phenotypic plasticity which allows cancer cells to adapt to their environment for their own survival - even if the cells are being attacked by chemotherapy drugs. This plasticity can be due to direct genetic alterations to the DNA sequences of cancer cells, or due to epigenetic changes, which are alterations in how existing DNA sequences are expressed. These changes can be induced by chemotherapy or by chemical signals from the tumour microenvironment-the cells and overall environment that immediately surrounds a tumour. The tumour microenvironment includes, but is not limited to, blood vessels and immune cells in addition to tumour cells.
These adaptations and alterations in phenotype of the cancer cells may mean that the specific target of the chemotherapy drugs used is no longer present on the cells, and therefore the chemotherapy is less effective. One specific example of cancer cell plasticity leading to therapy-induced resistance can be seen following treatment of some cases of prostate cancer with androgen deprivation therapy (ADT). ADT aims to lower the production of androgen hormones, which the prostate cancer cells need to survive and grow.
Following treatment with ADT, the plasticity of the cancer cells allows them to acquire phenotypic traits similar to neuroendocrine cells – specifically, lack of expression of androgen receptors. These neuroendocrine prostate cancer (NEPC) cells are therefore no longer affected by androgen receptor signalling and do not respond to ADT. Furthermore, there is currently no effective treatment available for NEPC and since it is such an aggressive subtype of prostate cancer, those diagnosed face an extremely poor prognosis. This demonstrates why phenotypic plasticity in cancer can be so dangerous.
Molecular diversity of tumours and cancer stem cells
Cancer cell plasticity may also contribute to the formation of tumours containing a wide variety of cells with distinct characteristics, behaviours, and levels of sensitivity to drugs. This phenomenon is called tumour heterogeneity. Since the different subpopulations of cells have different levels of sensitivity to chemotherapy drugs, this means that when a therapy is administered, susceptible cells are killed off and then the resistant cells survive and reproduce.
There are two main theories by which this heterogeneity is thought to arise. The first, the clonal evolution theory, proposes that tumour formation is caused by genetic abnormalities in cells, which are passed onto the daughter cells. Over time, with further divisions, the daughter cells accumulate more mutations, and the cells with the most beneficial mutations become more prevalent in the tumour over time. In this model, every clone of cells is thought to be equally capable of forming tumours.
The second is the cancer stem cell theory, which proposes that tumour growth is driven largely by a small subpopulation of cells known as cancer stem-like cells, or CSCs. These are similar to stem cells- unspecialised cells which are capable of self-renewal and are pluripotent, which means they can develop into any of the cell types found in cancer samples to form tumours in a process known as differentiation. These differentiated cells have a reduced tendency to lead to the formation of tumours.
These undifferentiated cancer stem cells are an essential contributor to metastasis because they are much more resistant to chemotherapy than the average cell, which means that sometimes, when a tumour containing CSCs is targeted using chemotherapy, the CSCs often remain, leading to tumour relapse (Figure 1).
Figure 1: The role of CSCs in chemoresistance and metastasis. CSCs (in purple) within a heterogeneous tumour also containing non-stem cell-like cancer cells (in green) may be able to survive chemotherapy. Surviving CSCs can then proliferate and differentiate to form new tumours. These recurrent tumours (in pink or light blue), which are usually chemoresistant, may metastasise and evolve into more resistant forms (in dark red or dark blue), eventually resulting in the death of the patient. This figure was adapted from Roberts, Cardenas and Tedja (2019)’s review titled The role of intra-tumoral heterogeneity and its clinical relevance in epithelial ovarian cancer recurrence and metastasis.
However, a third model was recently suggested known as the CSC plasticity model. This theory is a hybrid of the first two models, in which plasticity gives cancer cells the capacity to shift between differentiated and undifferentiated states in response to cues from the surrounding environment. If differentiated, non-CSC cells can take on a CSC phenotype that allows them to survive chemotherapy, then this could further explain cancer drug resistance.
Plasticity, EMT and metastasis
The plasticity of cancer cells can also be involved in the process of metastasis in carcinomas, where epithelial cells, which line the surfaces of the body and internal organs, become cancerous. Cancer is most likely to originate in these types of cells.
These epithelial cells are tightly bound to each other and to the basement membrane, meaning that when these cells become cancerous, individual ones cannot break away to start a new tumour at a different part of the body in their current state.
However, it is possible for metastasis to occur when these cancerous epithelial cells change their state in a process called epithelial-mesenchymal transition (EMT). In EMT, the cells gain properties similar to mesenchymal stem cells, a type of multipotent stem cell. These new properties include changes to cell shape and an ability to digest the basement membrane holding them in place, which makes it possible for the cell to break away and move through the bloodstream in order to invade a different area of the body.
It is thought that once these cells have reached another site of the body, the reverse process can occur, called the mesenchymal-to-epithelial transition (MET). In MET, cancer cells with a mesenchymal phenotype once again take on the properties of epithelial cells, which allows them to properly attach to the secondary site and form a new tumour. The ability of these cells to dynamically switch between the mesenchymal and epithelial phenotypes via EMT and MET respectively is known as epithelial-mesenchymal plasticity (EMP).
Figure 2: Diagram showing how epithelial-mesenchymal plasticity (EMP) is thought to result in metastasis. Primary tumour cells will undergo EMT to attain mesenchymal cell-like characteristics, which makes it possible for them to enter (intravasate) blood vessels and circulate through the body. When the cancer cells exit (extravasate) the bloodstream to colonise different tissues, they undergo MET and take on epithelial characteristics to start a new tumour. This figure was adapted from Meng and Wu (2012)’s review of The rejuvenated scenario of epithelial-mesenchymal transition (EMT) and metastasis.
Targeting phenotypic plasticity via cell signalling pathways
Since changes in cancer cell phenotype may come about in response to chemical signals from the tumour microenvironment, this makes cell signalling pathways an important part of cancer cell plasticity. Therefore, targeting some of these pathways may be beneficial in overcoming chemoresistance.
One example of a study exploring the therapeutic potential of targeting cell signalling pathways involved in phenotypic plasticity was a 2018 study from Schmidt et al. investigating colon cancer cells.
The researchers found that when they injected human colon cancer cells into mice, the tumours formed contained two different cell phenotypes which could each be linked to a specific signalling pathway. The first group of cells possessed characteristics similar to cancer stem cells and showed high levels of mitogen-activated protein kinase (MAPK) signalling (which drives EMT), while the second group of epithelial phenotype cells showed high levels of Notch signalling (which drives MET).
When the researchers treated the mice with a MAPK inhibitor, the tumours lost the cancer cells with high MAPK activity, but the number of cells with high Notch activity increased. The opposite could also be seen; when the mice were treated with a Notch signalling inhibitor, there was an increase in cells with high MAPK activity. Furthermore, once inhibition treatment was stopped, the distribution of cancer cells with high Notch activity and high MAPK activity in tumours returned to a similar distribution to what it had been before treatment.
From these observations the researchers could conclude that inhibiting just one of these two signalling pathways did not have a significant impact on tumour growth, since the phenotypic plasticity of the cells allowed them to switch their main signalling pathway between MAPK and Notch as needed in order to survive and resist treatment (Figure 3).
Figure 3: A depiction of the effects of MAPK and Notch signalling and the effect of signalling pathway inhibition on colon cancer cells as described by Schmidt et al. (2018). MAPK signalling drives EMT, whereas Notch signalling drives MET. Application of a MAPK inhibitor will cause the process of MET to be favoured, causing the colon cancer cells in epithelial form to predominate among a population of cancer cells in a tumour. Conversely, applying a Notch inhibitor will cause the MAPK pathway (and therefore EMT) to be favoured, so more of the colon cancer cells will take on a mesenchymal stem cell phenotype.
This cancer cell plasticity that allowed the tumours to evade targeted therapy was counteracted by inhibiting MAPK and Notch signalling at the same time in human colon cancer cells grafted into mice. Although the combined inhibition before chemotherapy did not completely suppress MAPK and Notch signalling activity, it still resulted in significantly reduced cancer cell proliferation, slowed tumour growth, and improved mouse survival compared to inhibiting either MAPK or Notch signalling alone..
However, the findings from mouse models may not always be applicable to humans, mainly because the tumour microenvironment in mice is not a perfect mimic of the tumour microenvironment in human tissues, Additionally, before this dual inhibition of MAPK and Notch signalling could be implemented in human trials, parameters of effective and safe drug combinations in humans would need to be assessed, as well as any toxic side effects caused by the inhibitor drugs.
Conclusion
Cancer cell plasticity is just one of the many complex factors which is involved in chemotherapy resistance and cancer disease progression. Although we currently do not fully understand all the mechanisms behind cancer cell plasticity, it is thought that further research into cancer stem cells, epithelial-mesenchymal plasticity, and key signalling pathways involved in phenotypic plasticity could be helpful in identifying therapeutic targets that can be exploited in cancer treatment.
Therefore, by directly targeting and restricting cancer cell plasticity in a clinical setting, it may be possible to improve the outcomes of cancer patients and increase rates of survival.
If you’re interested in exploring the concept of cancer cell plasticity a little further, here are some resources that you can check out:
Author
Raeesah Hayatudin
BMedBio Bachelor of Medical Bioscience