Cancer on the Move

Introducing and Defining Metastasis

Around 90% of patients with cancer die due to their cancer spreading (metastasis). Despite its prevalence, many critical questions remain in the field of cancer research about how and why cancers metastasise. The metastatic cascade has three main steps: dissemination, dormancy, and colonisation. Most cells that disseminate die once they leave the primary tumour, thus, posing an evolutionary bottleneck. However, the few that survive will face another challenge of entering a foreign microenvironment. Those circulating tumour cells (CTCs) acquire a set of functional abilities through genetic alterations, enabling them to survive the hostile environment. CTCs can travel as single cells or as clusters. If they travel in clusters, CTCs can be coated with platelets, neutrophils, and other tumour-associated cells, protecting CTCs from immune surveillance. As these CTCs travel further, they are named disseminated tumour cells (DTCs). These cells are undetectable by clinical imaging and can enter a state of dormancy. The metastatic cascade represents ongoing cellular reprogramming and clonal selection of cancer cells that can withstand the hostile external environment. How does metastasis occur, and what properties allow these cancer cells to survive?

How & Why Does Cancer Metastasise?

The Epithelial-to-Mesenchymal Transition (EMT) is a theory that explains how cancer cells can metastasise. In this theory, tumour cells lose their epithelial cell-to-cell adhesion and gain mesenchymal migratory markers. Tumour cells that express a mixture of epithelial and mesenchymal properties were found to be the most effective in dissemination and colonisation to the secondary site. It is important to note that evidence for the EMT has been acquired predominantly in vitro, where additional in vivo research is necessary to confirm this phenomenon. Nevertheless, although EMT does not accurately address why cancers metastasise, it provides a framework for how a cancer cell develops the properties to metastasise. Many factors contribute to why cancers metastasise. For example, a lack of blood supply, which occurs when a cancer grows too large, causes the cells in the centre to lack access to the oxygen carried by red blood cells. Thus, to evade cell death, cancer cells detach from the primary tumour to regain access to oxygen and nutrients. In addition, cancer cells exhibit a high rate of glycolysis to supply sufficient energy for its uncontrollable proliferation. However, this generates lactic acid as a by-product, resulting in a low pH environment. This acidic pH environment stimulates cancer invasion and metastasis as cancer cells move away from this hostile environment to evade cell death once again, an effect referred to as the ‘Warburg Effect’. In Figure 2, you can see that multiple interplaying factors that contribute to metastasis. So, how can patients with metastasised cancer be treated?

Current Treatments and Biggest Challenges?

Depending on what stage the patient presents at and what cancer type, the treatment options differ. Figure 3 shows an example of these treatment plans. For early stages I and II, chemotherapy and targeted treatments are offered, and in specific cases, local surgery is done. These therapies are done to slow the growth of the cancer or lessen the side effects of treatments. An example of treating metastasised prostate cancer includes hormone therapy, as the cancer relies on the hormone testosterone to grow.

Currently, cytotoxic chemotherapy remains the backbone of metastatic therapy. However, there are emerging immunotherapeutic treatments under trial. These aim to boost the ability of the immune system to detect and kill cancer cells. Hopefully, these new therapies may improve the prognosis of metastatic cancers when used in complement with conventional therapies, shining a new light into the therapeutic landscape of advanced cancers. 

Future Directions

Recent developments have opened new avenues to discovering potential treatment targets for metastatic cancer. The first is to target the dormancy of DTCs, where the role of the immune system plays an important part. Neoadjuvant ICI (immune checkpoint inhibitor) studies are anticipated to provide insight into novel biomarkers and can eliminate micro-metastatic cancer cells. Also, using novel technology such as single-cell RNA sequencing reveals complex information about the plasticity of metastatic cancer cells, allowing researchers to understand how cancer cells adapt in stressful conditions. Finally, in vivo models, such as patient-derived models, could provide crucial insight into future treatments as they reproduce the patients’ reactions to different drug treatments. There are many limitations and challenges to the research and treatment of cancer metastasis. It is clear, however, that with more studies into the properties of metastatic cancers and the different avenues of novel targets and therapeutics, there is a promising outcome in the field of cancer research.

Written by Saharla Wasame

Related articles: Epitheliod hemangioendothelioma / Immune signals and metastasis

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