Scientists Pinpoint New Cancer Stem Cell Links to Chemotherapy Resistance

By Eric Sauter

Using a multifaceted approach, scientists from The Scripps Research Institute have successfully identified more than 40 new therapeutic targets in breast cancer stem cells.

Current research supports the idea that tumors are organized in a hierarchy of various cell populations with different biological properties, and that the potential for sustained tumor formation and growth resides exclusively in a small number of tumor cells called cancer stem cells.

The new study, posted in an advanced, online edition of the journal Stem Cells September 18, 2008, demonstrates a novel strategy for the discovery of therapeutic targets in cancer stem cells that could lead to the development of potential therapies. The study also sheds light on one mechanism of cancer stem cell drug resistance.

"Our study offers fresh insights into breast cancer stem cell populations," said Kim Janda, Ely R. Callaway Jr. Professor of Chemistry, member of The Skaggs Institute for Chemical Biology, and director of the Worm Institute of Research and Medicine (WIRM), at Scripps Research. "Using a multifaceted approach, we compared cancer stem cells, which live within the general cancer cell population and are resistant to chemotherapy, to non-cancer stem cells. As a result, we were able to identify 41 proteins that play a role in drug resistance and, possibly, tumorgenicity in breast cancer."

Cancer stem cells are a small subpopulation of cells found in blood and solid tumors that have been shown to possess stem cell characteristics, such as the ability to proliferate. Interest in cancer stem cells continues to grow as a fundamental understanding in their role in the development and progression of the disease grows.

A Focus on Breast Cancer

To better understand the importance of breast cancer stem cells, Janda and his colleagues set out to characterize the cancer stem cell population from two breast cancer cell lines in both in vitro studies and in animal models.

The team used a quantitative proteomics approach that combined cell sorting and stable isotope labeling with amino acids in cell culture (SILAC) with mass spectrometry to compare and identify proteins with different expression profiles between the cancer stem cell population and non-cancer stem cell population. SILAC is a basic approach for in vivo label insertion into proteins for mass spectrometry quantitative proteomics.

In addition to the other potential targets, proteomic analysis of the side population cells revealed an increase in the expression of TB4 (beta thymosin), a small intracellular protein involved in cell movement. Increased expression of this protein has been found in various tumors, including breast, ovary, uterus, colon and thyroid. TB4 expression has been studied in breast cancer cells, and the levels of beta thymosins in tumor tissues have been suggested as a potential diagnostic tumor marker.

When treated with the chemotherapy drug Doxorubicin (91.2% and 99.5%, respectively, the cancer stem cells showed a significantly greater survival rate than non-cancer stem cells after 24 hours. Similar results were obtained with Methotrexate and 5-Flourouracil, other widely used chemotherapies.

The team used silencing RNA to block TB4 to see if breast cancer cell death increased with paclitaxel and doxorubicin treatment. Cell death did increase, implicating TB4 in the drug resistance of cancer stem cells.

The first authors of the study, Quantitative Mass Spectrometry Identifies Drug Targets in Cancer Stem Cell Containing Side Population, are Sebastian C.J. Steiniger and Judith A. Coppinger of The Scripps Research Institute. Other authors of the study include Jörg A. Krüger, and John R. Yates III, also of Scripps Research. See http://stemcells.alphamedpress.org/cgi/content/abstract/2008-0397v1

The study was supported by The Skaggs Institute for Chemical Biology and the National Institutes of Health.

Send comments to: mikaono[at]scripps.edu

"We were able to identify 41 proteins that play a role in drug resistance and, possibly, tumorgenicity in breast cancer."

—Kim Janda