It is usually not a primary cancer tumor that causes patient death, said Benjamin Martin, PhD, Associate Professor in the Department of Biochemistry and Cell Biology, Stony Brook University School of Medicine in New York. “Growth of a primary tumor is not as concerning as cells that leave the primary tumor and form metastatic tumors in other parts of the body,” Martin noted. “A primary tumor can be treated with drugs, radiation therapy, or cut out. But when cells leave that tumor, spread to other parts of the body, and become new tumors, it's a losing battle over time. The spread of cancer is what kills patients.”
Yet it is not inevitable that every tumor will metastasize; certain events must happen on the molecular level to allow the spread to occur. Martin has gone about trying to fathom which molecular cues received by these cells allow them to change their adhesion to their native area, migrate, and invade other organs.
Martin's work is the realization of a scientific intention that may have started in childhood. Originally born in Baltimore, he spent his most formative fifth grade through high school years in the Philadelphia area. Referencing his father, a college biology professor who provided him with early exposure to a research lab, and his mother, a science teacher, Martin said, “They told me when I would draw as a young child, they would ask me what my scribbles were supposed to be, and I would always answer, ‘They are chemicals,’” he said, laughingly. “After earning an undergraduate degree at Bowdoin College in Maine, I toyed with medical school, but I just loved research so much. Every aspect drew me to it.”
Martin earned a PhD (and met his future wife, Jin, who would become mother to their two young sons) at UC Berkeley, and then went to the University of Washington in Seattle for postdoctoral work before returning East to accept a post at Stony Brook.
His postdoctoral efforts originally revolved around stem cell research during early development, using zebrafish as a model organism. These fish have since become very important in his cancer research as well. “We use zebrafish because they are transparent as embryos,” he explained. “A big block in cancer research has been the inability to visualize how cancer cells behave in the more natural environment. You can work on cancer cells in dishes, or you can put cancer cells into a mouse model and see what the result is a few weeks or months later. But you can't go in and get high-resolution images of how the cancer cells are changing as they are going from one place to another.”
Martin and a team of fellow investigators, however, are doing exactly that by using light-sheet imaging in a zebrafish xenograft cancer model. “We are able to see, for the first time, circulating tumor cell behaviors at high resolution. These observations have led to a specific model of how cancer cells are exiting the blood vessels, through interactions with white blood cells and co-option of their behaviors,” said Martin.
“We actually inject human cancer cells directly into the bloodstream of the zebrafish embryos, and these cells enter circulation. Then we can go in with our light-sheet microscope and image the circulating tumor cells and watch them as they extravasate—move out of the blood vessel—and into the surrounding tissue,” he told Oncology Times. “This is the key step in metastasis because when cells spread to different parts of the body they will first enter the bloodstream, circulate, then exit the bloodstream and populate a new tissue or organ.
“So we are looking at extravasation from the blood vessel and invasion into local tissue as our invasion model in human cancer cells. In the context of the cell cycle, we can directly visualize what cell cycle they are in within a living cell as they invade across the blood vessel wall. And we can manipulate their cell cycle (whether they are dividing or not dividing), to see if that affects their ability to invade across the blood vessel or not.”
Martin said the light-sheet imaging has given researchers a whole new perspective on this process “... because we get such nice, high-resolution, time-lapsed movies that allow us to see things as they happen. Watching things happen in real time gives us insights into how to ask new questions—things we would never have expected to think about until seeing them.”
A New Way of Thinking
One of the biggest fundamental shifts in thinking that Martin and collaborator, David Q. Matus, PhD, Assistant Professor in the Department of Biochemistry and Cell Biology at Stony Brook, are trying to promote is that there is an apparent dichotomy between cell division and cell invasion. Martin credits Matus' earlier research on the roundworm C. elegans as an important basis for current work on the cell cycle.
“When you apply that to cancer it is very counterintuitive, because forever we have been taught that cancer is a disease of uncontrolled cell proliferation,” said Martin.
“What we are now proposing is that the lethal aspect of the disease actually depends on these cells not dividing,” he explained. “There are cancer drugs that are designed to stop cancer cells from dividing, which is logical to limit the growth of a tumor. But we started using some of these drugs in our tests and they seem to be promoting cell migration, invasiveness, and spreading cancer. And there seems to be the opposite correlation that if you allow a cell to divide then it will not be able to invade neighboring tissues. There may be molecular mechanisms that switch cells back and forth between the two states—proliferative and non-proliferative. A tumor might grow, and some cells might move away from the proliferative signal they are getting, which causes them to stop dividing so that they can spread and seed into a new part of the body that is amenable to restarting their proliferation again.”
These are hard-sell findings for some in the oncology community. “This is a pretty unique understanding,” Martin admitted. “There is some literature that has shown there is some correlation between cell cycle and either migration or invasion, but fundamentally thinking about it in terms of cancer is a whole new area.”
At a presentation given by one of Martin's colleagues, “... an attendee came up to us and said, ‘This cannot be true in cancer; it just can't be true.’ He flat out said this can't be right. We applied for one grant and the response we got was, ‘This can't be true, these guys must be crazy.’ But once we explained it further, they started to understand. There is a lot of pushback from people, but that is because this is a new way of looking at things.”
Indeed those aforementioned light-sheet movies provide some convincing evidence to even the most skeptical observers. “The images are both mesmerizing and beautiful, yet scary,” said Martin. “However, there is no better way of convincing an audience that what you are doing is real than by showing the phenomenon actually happening before their very eyes. Seeing is believing.”
Still Work to Be Done
The spread of cells is an intense area of research. “There are very specific molecular cues that are received by these cells allowing them to change their adhesion to their native area, and migrate,” detailed Martin. “They go through a network of proteins that are in extracellular space; they digest or squeeze past the proteins that are around them. In some cases, there are defined genes that are more able to promote this type of spreading. We are focused on finding the actual mechanisms that allow cells to migrate and spread, with the long-term goal of designing inhibitors that can prevent the spread of the disease.”
Martin is hopeful of eventually finding genes capable of keeping cells in a non-dividing state “... with the eventual goal of identifying new pathways where we can arrest a cell from dividing and treat it with another drug that would prevent it from being able to invade. In other words, the long-term goal is to be able to eliminate both the growth and spread of the cancer.”
For now, research presses on. Martin recently received a Pershing Square Sohn Cancer Alliance grant for work based on movies that suggest cancer cells behave like normal immune cells when they leave the blood vessel.
“Immune cells have a very stereotypical way in which they exit the blood vessel,” he explained. “They initially bind to the blood vessel wall, start rolling along the wall, stop, and then they tightly adhere to the blood vessel wall before migrating through the blood vessel wall to the other side. When we watch these cancer cells, we see them similarly rolling along, tightly adhering then migrating through the blood vessel. We want to better understand at what level these cells are co-opting the immune cell mechanisms. We are looking at known molecular mechanisms of immune cell extravasation to see if these same molecules are involved in cancer cell extravasation.”
He added that if adhesion molecules could be disrupted on blood vessel walls, maybe cancer cells would be unable to establish that connection on the vessel. And just maybe they would be incapable of invasion, renewed proliferation, and creation of a new cancer. With all those “maybes” looming, there is still much work to be done.
“I am the kind of personality who will never feel satisfied, never feel I'm finished,” said Martin. “But every step forward is its own victory. When I get a good result in the lab, let's just say it has me happily jumping off the walls. I remember the first time we actually got a light-sheet movie of a cancer cell moving across the vessel wall in transmigration. I was blown away. I remember thinking, ‘Oh my God! It's a whole new world!’”
Valerie Neff Newitt is a contributing writer.
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