In a paper that has now been published in the journal Developmental Cell, the scientists — who are from the National Institute of Health and Medical Research in France — describe their tests on zebrafish and humans.
The experiments confirmed that blood flow influences the locations at which migrating cancer cells "arrest" inside blood vessels.
They also detail how these cancer cells exit through the blood vessel walls and set up secondary tumor sites.
"A long-standing idea in the field," explains senior study author Dr. Jacky G. Goetz, head of the laboratory at the University of Strasbourg in France — where the study was conducted — "is that arrest is triggered when circulating tumor cells end up in capillaries with a very small diameter simply because of size constraints."
However, as Dr. Goetz explains, their findings show that "physical constraint" is not the only driver of metastasis, because "blood flow has a strong impact on allowing the tumor cells to establish adhesion with the vessel wall."
Metastasis and its main steps
Metastasis is the process through which tumor cells depart and migrate from their primary sites and travel through the lymph system or bloodstream to establish secondary, or metastatic, tumors in distant parts of the body.
Metastasis is a leading cause of cancer death and of "primary importance in the prognosis of cancer patients."
It is a complex process and proceeds as a sequence of steps, each of which must be completed in order for the secondary tumor to flourish. The series of steps, known as the "metastatic cascade," proceeds as follows:
- invading nearby healthy tissue
- crossing the walls of neighboring blood vessels and lymph nodes
- traveling through the bloodstream or lymph system to distant parts of the body
- arresting in remote, small blood vessels, or capillaries, invading their walls, and crossing over into the surrounding healthy tissue
- seeding a viable, tiny tumor in the healthy tissue
- generating a dedicated blood supply by growing new blood vessels to feed the new tumor
The new study concerns the fourth step, in which circulating tumor cells arrest in a capillary and cross through their endothelium, or the barrier of cells that line the vessel walls, into the surrounding tissue.
Study explores 'mechanical cues' in blood
In their study paper, the authors explain that "very little is known about how [circulating tumor cells] arrest and adhere to the endothelium of small capillaries and leave the bloodstream by crossing the vascular wall."
An area that is particularly unclear, they add, is the "role played by mechanical cues encountered in the blood" during this step.
For their study, the scientists developed "an original experimental approach" in which they tagged and followed circulating tumor cells as they traveled through blood vessels in zebrafish embryos. The model also allowed them to vary and measure blood flow in the vessels.
The results showed that the locations in the blood vessels at which the circulating tumor cells stop traveling is closely linked to flow rates.
The authors note that the "threshold velocity value for efficient adhesion [...] ranges from 400 to 600 [micrometers per second]."
'Endothelial cells curled around tumor cells'
The team also found that blood flow is essential for "extravasation," the process through which the tumor cells exit the blood vessels.
This was evident in timelapse imaging that showed endothelial cells "curling" around the arrested tumor cells in the zebrafish embryos' blood vessels.
"Blood flow at this step is essential. Without flow, endothelial remodeling does not occur. You need a certain amount of flow to keep the endothelium active so that it can remodel around the tumor cell."
Dr. Jacky G. Goetz
The researchers came to the same results when they observed the progress of brain metastases in mice.
For this experiment, they used an imaging technique called intravital correlative microscopy, which combines living cell models with electron microscopy so that the dynamics can be observed in a live animal.
Flow controls place, onset of second tumor
Finally, the team confirmed the findings by observing secondary tumors in the brains of 100 human patients, whose primary tumors were in various parts of the body.
As with the zebrafish model, they used an imaging technique to map the locations of the secondary tumors.
When they merged the brain metastases map with a blood flow map of a healthy control patient, the researchers found that it matched what they found in the zebrafish model, confirming that secondary tumors prefer to grow in areas where the blood flow is within a certain range.
The authors conclude that their findings reveal that blood flow controls not only the location, but also the onset of "metastatic outgrowth."
They now want to explore ways to block endothelium remodeling around the circulating tumor cell as a way to disrupt its exit into surrounding tissue. Such an achievement could prevent metastasis from completing the steps necessary for the successful growth of a secondary tumor.