By Alexandra Goho
A New Twist on Stem Cells in Cancer
By Alexandra Goho
Although a growing body of evidence suggests that cancer stem cells play a pivotal role in fueling tumor growth and progression, healthy stem cells in the body are not without blame. Cancer biologist Robert Weinberg of the Whitehead Institute for Biomedical Research in Cambridge, Mass., has found that breast cancer stem cells recruit ordinary stem cells in bone marrow to promote metastasis.
Normally, when the body suffers a wound, a type of stem cell found in bone marrow, known as mesenchymal stem cells, are called to the site of the injury. There, they give rise to specialized cells that are required to reconstruct the damaged tissue.
Researchers have long known that a cancerous tumor, to sustain its growth, can mobilize nearby healthy cells, including cells that form the blood vessels and those that create the structural framework surrounding the tumor. Studies showing that tumors can co-opt mesenchymal stem cells inspired Weinberg and his colleagues to investigate the stem cells’ influence on metastasis.
Mesenchymal stem cells (lavender), a type of stem cell found in bone marrow, send a tumor signals (blue) that encourage cancer cells (brown) to metastasize. [Art: Nicolle Rager Fuller]
Reporting in the Oct. 4, 2007, issue of Nature, Weinberg and his colleagues combined human mesenchymal stem cells with human breast cancer cells and implanted the mixture into mice. Eight to 12 weeks later, when the researchers compared the mice with other mice that had received only breast cancer cells, the tumors in both sets of animals were almost identical in size. Yet, the mice that had received the stem cells developed significantly larger and more numerous metastases.
It turns out that once the mesenchymal stem cells are recruited into the tumor, they talk back to the cancer cells by secreting a protein known to encourage cell movement. “So it’s not just a one-way street,” says Weinberg. The stem cells appear to persuade the cancer cells to move around, thereby facilitating their ability to migrate to distant sites in the body.
What this study shows, says Weinberg, is that “breast cancer is not a phenomenon that can be understood exclusively by studying the inner workings of a cancer cell. … Now we see there’s actually another dimension that we need to pay attention to.” 