The human body is made up of more than a billion cells that join to form the tissues and organs of our bodies. However, cells are dynamic structures that, using different techniques, move through the body to fulfil various functions, such as close wounds or carry nutrients to other tissues.
Understanding how cancer cells move and make decisions in these confined environments is important as 90% of cancer-related deaths involve metastases.
The labs of Dr. Konstantinos Konstantopoulos of Johns Hopkins University and of Dr. Miguel A. Valverde of UPF, together with teams from the USA and Canada have been working together over the past six years to unravel how cancer cells use ion movement through mechanically activated ion channels -stimuli that deform cell membranes- to adapt their movement to different mechanical stresses and environments. The results of this research have been published in two studies in the journals Nature and Nature Communications.
In these two new studies, the scientists asked themselves:
1) how cancer cells polarize ion transport mechanisms in the leading edge and trailing edge of the cells to move through narrow spaces; and
2) how cancer cells optimize movement when fluid viscosity is high.
To address these important questions, they studied the movement of cells in three-dimensional media generated using bioengineering techniques, which resemble the pathways along which cells normally move in our bodies. Key proteins were located within the cell using high-resolution microscopy, cell volume, ion movements and electrical activity were recorded, and they evaluated how the expression of different genes that are important for the progression of cancer changes.
First study: using water as propulsion
In the first study, published in Nature Communications, the international team found that cancer cells can move in confined spaces by simply taking in water at the leading edge of the cell and releasing it at the trailing edge. They do so without the need to establish molecular interactions with the walls of the surrounding tissue. “It works like a hydraulic propeller, similar to the device that Tom Clancy fictionalized to propel a submarine in his novel The Hunt for Red October,” Dr. Miguel Valverde explains.
“Cancer cells can move in confined spaces by simply transferring water from the leading edge to the trailing edge of the cell.”
In real life, this is possible because in their leading edge, the cells accumulate an ion transport system, the sodium/proton exchanger (NHE1), which charges the cell with sodium which increases osmotic pressure and favours the entry of water into the cell.
At the same time, cancer cells concentrate the SWELL1 protein in their trailing edge. SWELL1 (also known as LRRC8A) is a chloride channel activated by increases in cellular water content that facilitates the exit of chloride and water.
The end result of the coordinated action of these two ion transport systems on the leading and trailing edges enables cell movement. More importantly, the study shows that the activity of these two systems is essential for the movement of cancer cells outside of blood vessels and in the development of metastasis.
Second Study: moving through viscosity using muscles and cell skeleton
In the second study, published in Nature, the scientists questioned how changes in viscosity in the cellular environment can condition the way cancer cells move and behave.
Viscosity measures the resistance that a fluid exerts on anything that moves in or with it. As such, common sense and fundamental engineering indicate that inert particles move more slowly in high viscosity media.
The scientists have now demonstrated an effect that a priori may seem counterintuitive: high viscosity promotes the migration, invasion, as well as the extravasation of tumour cells — exiting from blood vessels — and lung colonization.
“The cells of our body are constantly exposed to fluids of varying viscosities,” Valverde continues. “In some pathological situations such as tumour growth, the local viscosity surrounding the initial tumour increases due to abnormal protein degradation or compression of the normal drainage pathways -the lymphatic vessels. In addition, as the cancer spreads to other parts of the body, cells have to travel through spaces filled with interstitial fluids and blood, which are more viscose than water.”
In previous studies, Valverde’s team demonstrated that cells adapt to high viscosity situations by activating a protein called TRPV4, an ion channel that facilitates the entry of calcium into the cell, otherwise impossible due to the lipid membrane that delimits the cell and is impermeable to ions. Calcium is an element that, when increased inside the cell, controls various cell functions.
With this background in mind, the international team of scientists posited that cancer cells exposed to high viscosity may use a similar mechanism to enhance their motility and dissemination. And they were right… but with interesting surprises!!!
By exposing cancer cells to high viscosity, they observed that the first cellular element that responded to this stimulus was the protein actin, which is part of the cytoskeleton and shapes the body of the cell. This initiates a cascade of molecular events that ends with the activation of the TRPV4 channel, which in turn activates a cascade of intracellular events that result in the reinforcement of the cell cytoskeleton and the activation of motor proteins.
Interestingly, by means of all these changes the cells modify their means of migration and no longer employ the movement of water. In these conditions, they use their cellular “skeleton and muscles,” as well as interactions with surrounding walls to propel themselves faster. In the words of Dr.Selma Serra of UPF, co-author of the study, “it is as if the cells had gone to the gym to train hard -under high viscous loads- and perform better when they are physically challenged on their journey from the primary tumour to their final destination in distant metastasis.”
The study authors also found that cells not only move faster when surrounded by high viscosity fluids, but also when they have been previously exposed to such fluids and then removed. In other words, cells can not only detect and respond to elevated viscosity, but can also develop a memory of their exposure to this condition.
How important is the discovery?
The vast majority of cell biology research is conducted in cell culture media with viscosities close to that of water. “In our work, we define for the first time how cells detect and respond to the physiologically relevant levels of fluid viscosity in which they are commonly found in the body of healthy and sick patients,” explains study coordinator, Dr. Konstantopoulos. “The definition of the molecular mechanism used by cells to adapt to changes in the viscosity of the medium was a tour de force in which we had to change our preconceived idea of which cellular elements are the first to respond to this type of mechanical stimulus.”
The great coordination between the structural elements of the cells -their actin and myosin cytoskeleton- with the mechanisms of ion transport and water that regulate cell volume marks a major breakthrough in our understanding of cell mechanobiology.
Dr. Valverde explains the major breakthrough represented by demonstrating that cancer cells have the ability to form memory in response to pre-exposure/pre-conditioning in high viscosities, and highlights the importance of teamwork. “Our papers are also a good example of the need for multidisciplinary collaboration -bioengineers, geneticists, theoretical biophysicists, cell biologists and physiologists- each with a different but complementary approach, which allows us to seek answers to complex problems,” he concludes.
What’s next? Implications for drug development
It will be very informative to examine how primary tumours and cancer cells that spread from primary tumours respond to local changes in the viscosity of extracellular fluid found in the body during disease progression and during invasion into the tissue microenvironment. The development and optimization of biosensors that allow real-time measurement of extracellular fluid viscosity along with imaging of cancer cells in live animals will be crucial to address this point. “At this stage, we cannot propose a specific molecular intervention to combat cancer metastasis, but we believe that the molecules and pathway we identified in our study can be used as pharmacological targets for possible cancer therapies,” Valverde explains.
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