By Jessica Gorman
A Tumor's Lifeblood?
Scientists are investigating whether tumors are driven by stem cells
By Jessica Gorman
In the 1800s, scientists made an intriguing observation through their microscopes: Some cells in a tumor looked different from the rest. It was the seed of an idea, and over the ensuing decades, researchers began to ask whether all cancer cells, or just a small number of them, keep a tumor growing.
It's an important question to ask. The traditional view of cancer holds that any of a cancer's cells can initiate or maintain a tumor, and therapies are aimed at killing those cells indiscriminately. If this other theory is right, a tiny population of tumor-promoting cells, called cancer stem cells, is the root of the cancer: These cells would keep the tumor growing and might launch recurrences and metastases.
Researchers have long known that they must inject large amounts of cancer cells into an animal to initiate a new tumor. But it wasn't until the late 1980s that the technological tools became available to try to identify cancer stem cells, says John Dick, a stem cell biologist at the University of Toronto. Since that time, Dick and other scientists have identified cell populations that they believe to contain the only cells capable of growing a new tumor. When researchers inject these cells into special, immune-deficient mice, the animals develop cancer. Meanwhile, injecting 10 or 100 times as many—or more—ordinary cancer cells does not cause cancer in the mice.
Scientists have reported evidence that suggests these cancer stem cells exist in leukemia, breast cancer and brain cancer. The data also look promising for the presence of cancer stem cells in other cancers, such as prostate cancer, says Michael Clarke, a stem cell biologist and oncologist at the Stanford University School of Medicine in Palo Alto, Calif., who led research that reported the identification of breast cancer stem cells. "I think it's going to be the rule, rather than the exception," he says. "I wouldn't be surprised if most cancers had a stem cell population."
These aren't the stem cells mired in controversy, called embryonic stem cells, which can generate any cell in the body. They also are not normal adult stem cells, which are found in many varieties throughout the adult body. Adult stem cells can generate different types of specialized, or differentiated, cells for one particular part of the body. For instance, an adult stem cell in the brain may produce mature brain cells, such as neurons and astrocytes. In addition to making a more specialized cell, an adult or embryonic stem cell always makes a replica of itself each time it divides.
"Only the stem cell can make more copies of itself," says physician-scientist Max Wicha, who directs the University of Michigan Comprehensive Cancer Center in Ann Arbor and who collaborated with Clarke on the breast cancer stem cell research. "When any other cell in the body divides and makes two of itself, those two are not like the parent anymore." And while specialized cells die after a certain number of cell divisions, stem cells divide endlessly.