Targeting Breast Tumor Stem Cells with Cell Cycle Inhibitors

Institution: University of California, San Francisco
Investigator(s): Noelle Huskey Mullin, BA -
Award Cycle: 2010 (Cycle 16) Grant #: 16GB-0054 Award: $75,981
Award Type: Dissertation Award
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure

Initial Award Abstract (2010)

The main cause of death from breast cancer is the recurrence of metastatic disease, which can occur years after treatment of the initial breast tumor. We are investigating methods to prevent tumor recurrence that can be used in combination with conventional chemotherapeutics. Breast cancer recurrence has been blamed on a small population of cells within a breast tumor that are distinct from the bulk of the tumor. These cells are thought to be resistant to current chemotherapies, and are able to remain dormant for years until unknown signals can trigger them to divide. Further characterization of these cells has shown that these cells exhibit typical stem cell properties such as the ability to self-renew and give rise to many different types of cells. Our goal is to specifically eliminate these stem cell-like cells. Our approach is to use a class of small molecule drugs known as CDK (cycle-dependent kinase) inhibitors that we have previously shown to be effective at killing embryonic stem cells. We will determine the effect of CDK inhibitors on breast cancer stem cells. Additionally we will investigate the molecular mechanism by which CDK inhibitors can induce cell death in order to learn more about the biology of the breast cancer stem cells.

Previous research has shown that cells expressing high levels of a protein called Myc are more sensitive to CDK inhibitors. Both embryonic stem cells and stem cell-like cancer cells express high levels of Myc. Our first aim is to determine whether CDK inhibitors can specifically kill breast cancer stem cells, and what role Myc has in any observed cell death. Additionally, we will investigate whether CDK inhibitors affect the breast cancer cellís capability to form tumors in mice, which has previously been attributed to the presence of breast cancer stem cells. Finally we will ask how CDK inhibitors can kill stem cells. CDK inhibitors are known to affect multiple cellular proteins that are important for cell division and growth. We will determine which individual proteins trigger cell death when inhibited.

For these studies will examine the effect of CDK inhibitors on a collection of breast cancer cell lines that have been established from human breast tumors. We will specifically look at small populations of breast cancer stem cells within the cells lines, which can be identified by the expression of unique cell surface proteins. Additionally, we will use an established technique known as RNA interference (RNAi) to deplete Myc levels in our breast cancer cell lines. These approaches will test our hypothesis that CDK inhibitors can kill the breast cancer stem cells in a Myc-dependent manner, and therefore decrease the tumor forming capability of these cell lines.

This project explores a novel mechanism to selectively kill breast cancer stem cells as a new approach to treatment. This could be used to prevent recurrence of metastatic disease and treat poorly differentiated cancers, such as triple negative breast cancers.

Final Report (2012)

Our major research goal is to target a sub-population of cells within breast tumors, known as breast cancer stem cells (BCSCs). BCSCs are stem cell-like in nature, resistant to current chemotherapeutics and are thought to be responsible for tumor initiation as well as recurrence of metastatic disease after the primary breast tumor has been treated. Our prior research has shown that drugs that inhibit a family of proteins involved in cell division, the cyclin dependent kinases (CDKs), can selectively kill cells that express high levels of a second protein known as MYC. We have also shown that triple negative breast cancers, a class of breast cancers that are thought to contain high percentages of BCSCs, express high levels of MYC. Finally, we have shown that CDK inhibitors can selectively kill embryonic stem (ES) cells, a class of cells that exhibit many shared characteristics with BCSCs. This grant was based on the hypothesis that CDK inhibitors can selectively target BCSCs and can therefore be used in combination with conventional chemotherapeutics to prevent recurrence of metastatic disease.

During our two years of funding from the CBCRP we have made considerable progress towards the completion of our research aims. We have established that both triple negative breast cancer cell lines and ES cells do indeed undergo cell death after treatment with CDK inhibitors. Our data show that cell death occurs specifically due to CDK1 inhibition, as opposed to other cell cycle related CDKs, and that CDK inhibitor-induced cell death in the breast cancer cell lines is dependent on high MYC expression. In addition, we have also identified secondary cellular pathways, such as the DNA damage pathway, that mediate CDK inhibitor-induced cell death in ES cells, further elucidating the mechanism of cell death. During this funding period we have also developed two transgenic knock-in mouse lines that will allow for the specific inhibition of CDK1 and CDK2 within various breast cancer models, which will serve as important tools as we continue this line of research.

Finally, we have preliminary data suggesting that CDK1 inhibition can specifically target the breast cancer stem cell population within triple negative breast cancer cell lines. In the future we plan to expand on these preliminary results showing that breast cancer stem cells are sensitive to CDK1 inhibition and assess how breast cancer stem cell depletion affects tumor initiation and maintenance, using mouse tumor transplant models. Currently there are several new generation CDK inhibitors being used in clinical trials. It is our hope that this line of research will identify potential uses for CDK inhibitors in the treatment of breast cancer and the prevention of recurrent metastatic disease.

CDK1 Inhibition Targets the p53-NOXA-MCL1 Axis, Selectively Kills Embryonic Stem Cells, and Prevents Teratoma Formation

Discovery and Optimization of Sulfonyl Acrylonitriles as Selective, Covalent Inhibitors of Protein Phosphatase Methylesterase-1.
Periodical:Journal of Medicinal Chemistry
Index Medicus: J Med Chem
Authors: Bachovchin DA, Zuhl AM, Speers AE, Wolfe MR, Weerapana E, Brown SJ, Rosen H, Cravatt BF
Yr: Vol: Nbr: Abs: Pg:

Academic cross-fertilization by public screening yields a remarkable class of protein phosphatase methylesterase-1 inhibitors.
Periodical:Proceedings of the National Academy of Sciences of the United States of America
Index Medicus: Proc Nat Acad Sci, U S A
Authors: Bachovchin DA, Mohr JT, Speers AE, Wang C, Berlin JM et al and Cravatt B
Yr: Vol: 108 Nbr: 17 Abs: Pg:6811-6816

CDK1 Inhibition Targets the p53-NOXA-MCL1 Axis, Selectively Kills Embryonic Stem Cells, and Prevents Teratoma Formation
Periodical:Stem Cell Reports
Index Medicus:
Authors: Huskey, N.E., Guo T., Evason K.J., Momcilovic O., Pardo, D., et al.
Yr: 2015 Vol: 4 Nbr: Abs: Pg:374-89

MYC pathway activation in triple-negative breast cancer is synthetic lethal with CDK inhibition.
Periodical:Journal of Experimental Medicine
Index Medicus: J Exp Med
Authors: Horiuchi D, Kusdra L, Huskey NE, et al, and Goga A
Yr: 2012 Vol: 209 Nbr: 4 Abs: Pg:679-96

Chemical-genetic analysis of cyclin dependent kinase 2 function reveals an important role in cellular transformation by multiple oncogenic pathways.
Periodical:Proceedings of the National Academy of Sciences of the United States of America
Index Medicus: Proc Nat Acad Sci, U S A
Authors: Horiuchi D, Huskey NE, Kusdra L, et al, and Goga A
Yr: 2012 Vol: 109 Nbr: 17 Abs: Pg:E1019-27