Chemotherapy May Enrich Tumorigenic Cells in Breast Ca

April 1, 2007

Baylor College of Medicine investigators believe they have identified a key reason for breast cancer relapses after chemotherapy.

SAN ANTONIO—Baylor College of Medicine investigators believe they have identified a key reason for breast cancer relapses after chemotherapy. It appears that a subpopulation of cancer stem cells are resistant to conventional chemotherapy, and treatment actually enriches these residual cells. Michael Lewis, PhD, assistant professor of molecular and cellular biology, reported the findings at the 29th Annual San Antonio Breast Cancer Symposium (abstract 205). Senior principal investigator of the study is Jenny Chang, MD. Co-PIs are Dr. Lewis and Jeffrey Rosen, PhD, and first author is Xiaoxian Li, MD, a postdoctoral fellow in Dr. Chang's lab. "The idea is that conventional chemotherapy might kill rapidly dividing cells that may be inconsequential ultimately, but would leave cells that are important in initiating cancer relatively untouched," Dr. Lewis said. "These cancer-initiating cells can regenerate tumor over time, cause relapse, and potentially metastasize."

Historically, investigation of treatment resistance has focused on the identification of acquired mutations that confer resistance to specific agents or to multiple agents. However, recent evidence supports the existence of another class of cells present in residual cancers after conventional therapies. These cells are characterized by their relative quiescence but have the capacity to self-renew and divide indefinitely. Some of the signaling pathways responsible for this self-renewal and regrowth have been implicated in tumorigenesis, the investigators noted in a poster discussion session.

"We hypothesized that a subpopulation of cancer cells present in the residual tumors after conventional treatment are intrinsically resistant to conventional therapies and are responsible for relapses," Dr. Lewis said.

If this hypothesis is correct, then relapse after apparently successful therapy is due to the persistence of resting therapy-resistant tumor cells that have properties in common with normal stem cells. When activated, these cells give rise to daughter cells of high proliferative potential. Ironically, chemotherapy spares the most important target—the resting cells possessing self-renewal capacity.

Recently, a highly tumorigenic subpopulation of breast cells has been identified—CD44-positive and CD24-negative. In the current study, the investigators examined the ability of four combinations of CD44 and CD24 cells to make mammospheres in culture, a surrogate of tumorigenicity. The CD44+/CD24- population showed enhanced mammosphere-forming ability.

To demonstrate increased tumorigenicity after chemotherapy, the researchers compared mammosphere formation in paired breast cancer core biopsy samples taken before and after 12 weeks of neo-adjuvant chemotherapy.

In the pretreatment biopsies, a median of about 4% of input cells were of CD44+/CD24- lineage. These increased significantly to a median of about 14% in matched post-treatment samples. Increased tumorigenicity was suggested by a significant increase in mammosphere formation efficiency after chemotherapy, and there was a positive correlation between the number of CD44+/CD24- cells and mammosphere-forming ability.

In addition, the investigators successfully established xenograft tumors in immunocompromised SCID/Beige mice from three out of four postchemotherapy biopsies, compared with only one out of five pretreatment biopsies.

Left Behind

Dr. Lewis noted, "During chemotherapy, you actually increase the proportion of these CD44+/CD24- cells, that is, you enrich the tumor-initiating cells. You are killing other cancer cells and shrinking tumor, but leaving behind these CD44+/CD24- cells."

He noted that when the biopsy samples were treated with lapatinib (Tykerb), the results were different. "Lapatinib shrinks the tumor, but the relative proportion of cell subpopulations does not change," he said. Lapatinib targets two critical growth factor pathways (EGFR and HER2). The fact that lapatinib failed to increase CD44+/CD24- cells suggests that inhibiting the epidermal growth factor/HER2 pathway may play an important role in treating breast cancer, the investigators noted. Trastuzumab (Herceptin) is also being evaluated for its effect on these cells, they said.

The researchers have not yet correlated the proportion of CD44+/CD24- cells with clinical outcome, though this would be interesting to study, Dr. Lewis said. "What is clear," he added, "is that new therapies targeting the chemotherapy-resistant cell population have to be developed to more efficiently treat breast cancer patients."