By Alice de Bernardy
Brain metastases are among the most critical diagnosis in the field of cancer therapies1. But the brain being a well-protected organ, we’ll try here to understand how cancer cells can metastase to the brain and how we can try to study this process in the lab.
But firstly, what is metastasis? It’s the process during which a primary tumour escapes from its site and colonize another site in the body, proliferating and creating a secondary tumour there. To attain this, cancer cells need to undergo Epithelial to Mesenchymal transition (EMT) to detach from the surface and circulate freely. Then, intravasion occurs as cancer cells reach the circulation. This can be by entering different routes but the main one being haematogenous spread, through the blood circulation. Then, cells can extravase and colonise a new site in the body. For each step, many factors need to work in synchrony to enable cells to gain the right characteristics at the right time.
We can now define Brain metastases: they are secondary tumour made from metastazing tumour that reached and colonised the brain. These are the most recurrent forms of brain tumours and the most recurrent cancers that form metastases to the brain are breast and lung cancer1. To understand more about brain metastasis, two main factors need to be understood.
Firstly, comes the way to reach the brain, this occurs mainly by crossing the Blood Brain Barrier (BBB). The BBB is the barrier lining blood vessels in the brain and is much more resistant to exchanges than other interfaces with other organs. This result in a longer extravasion time compared to other organs due to the tighter barrier, predominantly made of numerous tight junctions and metabolic barriers restricting cells from crossing and drug delivery2. Therefore, when metastazing tumour manage to cross the BBB, it allows them to escape typical cytotoxic agents and immune surveillance1. As a result, the current prognosis after being diagnosed with brain metastases can only reach a few months1.
Once cancer cells have crossed the BBB, they need to colonise and be able to proliferate in the new site, here comes the need to understand the tumour microenvironment, since the site of formation of a secondary tumour depends on the blood flow by pushing cells toward a specific region, but it also depends on cancer type1.
How to model, study and understand those 2 factors is crucial to understand more about brain metastases.
Many animal models exist, but they come with many limitations, for example mouse model offer a poor model for Blood brain Barrier permeability. On the other hand, In vitro model such as 2D cell lines aren’t precise enough to model disease invasion throughout the body.
One example of an in vitro technology to model brain metastases is a metastatic brain cancer cerebral organoid3: MBCCO: organoids are a 3D assembly of cells that resembles a tissue in vivo, with different cell lines and a similar architecture. In this study, cerebral organoids were grown from human cells and cocultured with non-small cell lung cancer cells. Cancer cells successfully sticked to the cerebral organoid and it enabled to study the reciprocal interaction btw cancer cells and neurons as well as the survival strategy and how cancer cells exploit the environment’s factors for survival. For example, results hinted at LUNX as an imp player in cancer cell proliferation in the MBCCO model. It also identified connexin 43 as an important protein in tight junctions between cancer cells and astrocytes and results indicated that gefitinib specifically affects cancer cells and not normal Cerebral tissues, at a 10μM concentration. Overall, the MBCCO3 model can be used to test both drug efficiency and drug toxicity as well as cell-cell interaction. However, it lacks vasculature.
The use of Boyden chambers and a hydrogel modelling the blood brain barrier can model how tumour cells cross the BBB4. it works by seeding a gel with astrocytes and seeding a layer of endothelial cells on top. then, by adding cancer cells on top of it and after incubation it can be studied if cells have the ability to cross the gel. This model4 enabled to show that Cisplatin treatment inhibited the migration of triple negative breast cancer cells across the hydrogel.
Finally, another model to look at extravasion are microfluidic chips also called organ on a chip5. They work by co culturing components of the BBB: different neurovascular units -such as brain microvascular endothelial cells, pericytes, astrocytes, microglia, and neural tissue- and shuttling a fluid though it to simulate blood flow5. The sheer mechanical flow of media being constant is important for physiological relevance. As an example, it results in increased tight junction and resistance to elongation5. It also enabled to see that some cancer cell type disrupts the BBB integrity and decrease its permeability which results in brain metastasis. example of those cell lines include breast and lung cancer as well as melanomas.
Overall, in vitro models are emerging, but they still need improvement as differences in vitro and in vivo are still seen among studies6. An increase in model accuracy is needed before hoping to screen on a large-scale treatments.
References:
1. Singh M, Bakhshinyan D, Venugopal C, Singh SK. Preclinical modeling and therapeutic avenues for cancer metastasis to the central nervous system. Front Oncol [Internet]. 2017 [cited 2022 Mar 7];7:220. Available from: https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC5609558/
2. Piantino M, Figarol A, Matsusaki M. Three-dimensional in vitro models of healthy and tumor brain microvasculature for drug and toxicity screening. Front Toxicol [Internet]. 2021;3. Available from: http://dx.doi.org/10.3389/ftox.2021.656254
3. Choe MS, Kim JS, Yeo HC, Bae CM, Han HJ, Baek K, et al. A simple metastatic brain cancer model using human embryonic stem cell-derived cerebral organoids. FASEB J [Internet]. 2020;34(12):16464–75. Available from: http://dx.doi.org/10.1096/fj.202000372R
4. Augustine R, Zahid AA, Mraiche F, Alam K, Al Moustafa A-E, Hasan A. Gelatin-methacryloyl hydrogel based in vitro blood-brain barrier model for studying breast cancer-associated brain metastasis. Pharm Dev Technol [Internet]. 2021 [cited 2022 Mar 7];26(4):490–500. Available from: https://pubmed.ncbi.nlm.nih.gov/33416013/
5. Akhtar AA, Sances S, Barrett R, Breunig JJ. Organoid and organ-on-A-chip systems: New paradigms for modeling neurological and gastrointestinal disease. Curr Stem Cell Rep [Internet]. 2017 [cited 2022 Mar 7];3(2):98–111. Available from: https://pubmed.ncbi.nlm.nih.gov/28983454/
6. Lorger M, Lee H, Forsyth JS, Felding-Habermann B. Comparison of in vitro and in vivo approaches to studying brain colonization by breast cancer cells. J Neurooncol [Internet]. 2011 [cited 2022 Mar 7];104(3):689–96. Available from: https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC4836850/
Article written in June, 2022
Imperial Bioscience Review 