Modulation of Breast Cancer Stem Cell Response to Radiation

Institution: University of California, Los Angeles
Investigator(s): Frank Pajonk, M.D., Ph.D. -
Award Cycle: 2009 (Cycle 15) Grant #: 15NB-0153 Award: $250,000
Award Type: IDEA Competitive Renewal
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure

Initial Award Abstract (2009)

Recent data indicate that most, if not all, solid cancers contain a small number of cancer stem cells (CSCs). Only this population has the ability to self-renew and re-populate a tumor while their progeny lack this ability. Elimination of all CSCs will therefore be required for cancer cure. Cell populations enriched for CSCs have been recently prospectively identified in brain, breast, prostate, colon, pancreatic, head and neck, and melanoma tumors based on cell surface marker profiles. However, so far no marker has been found that uniquely distinguishes individual CSCs. In our previous CBCRP grant, we reported the discovery of a fundamental difference between CSCs and non-CSCs. CSCs have little if any 26S proteasome activity. Proteasomes are large protein complexes inside cells that degrade unneeded or damaged proteins by proteolysis. We have used a novel proteosome detection system to trace CSCs in breast cancer cell lines and to study their response to cancer treatments.

In this CBCRP project we aim to continue studying the functional consequences and mechanisms of low proteosome activity in CSCs. In particular, we think that the relative resistance of CSCs against established therapy modalities, such as radiotherapy, may provide insights in better targeting the low CSC 26S proteasome activity. First, we plan to study the impact of the Wnt and Notch signaling pathways on the expression of proteasome subunits in CSCs. Next, we propose to test whether inhibition of key stem cell-associated pathways will increase the level of 26S proteosome activity and render the cell mores sensitive to killing by radiation therapies.

Understanding the properties of breast cancer stem cells and how they respond to ionizing radiation will not only advance our general understanding of breast cancer, but will also enable us to modify accelerated tumor re-population subsequent to therapy.

Final Report (2011)

Breast cancers are thought to be organized hierarchically with a small number of breast cancer stem cells (BCSCs) able to re-grow a tumor while their progeny lack this ability. We previously demonstrated that this population of cells was relatively resistant against ionizing radiation if compared to their non-tumorigenic counterparts. We also previously demonstrated that the BCSCs population could be prospectively identified by their lack of 26S proteasome activity.

In search for a link between low-proteasome activity in CSCs and the CSC phenotype, we report here:

  1. that the RNA-binding protein Musashi-l binds 26S proteasome mRNAs and inhibits their expression.Moreover down-regulation of Musashi-l decreased the number of CSCs in breast cancer and activated the tryptic activity of the 26S proteasome.
  2. an unexpected discovery refocused our research. Recently, several groups reported enrichment for BCSCs by classical anti-cancer treatment. However, the underlying mechanisms leading to this enrichment are incompletely understood.

Using non-BCSCs sorted from patient samples and cell lines, we found that ionizing radiation reprogrammed differentiated breast cancer cells into induced BCSCs (iBCSCs). iBCSCs showed increased mammosphere formation, increased tumorigenicity and expressed the same sternness-related genes as BCSCs from non-irradiated samples. Reprogramming occurred in a polyploidy sub-population of cells, coincided with re-expression of the transcription factors Oct4, Sox-2, Nanog, and Klf4, and could be partially prevented by Notch inhibition. We concluded that radiation may induce a BCSC phenotype in differentiated breast cancer cells and that this mechanism contributes to increased BCSC numbers seen after classical anti-cancer treatment. We demonstrated that the number of BCSCs increased with increased aggressiveness of the cancer, correlating with increased radioresistance and decreasing OERs. As mammospheres, breast cancer cell lines and primary samples were highly radioresistant and not further protected by hypoxia.

We conclude that because BCSCs are protected from radiation through high expression levels of free radical scavengers, hypoxia does not lead to additional radioprotection of BCSCs.

  1. Finally, we discovered that BCSCs and non-BCSCs are in different metabolic state. While BCSCs exhibited higher oxygen consumption, those cells also consumed more glucose and produce more lactate. More investigation will be necessary to better understand the specificity of BCSCs metabolism.

Our research will continue to focus on reprogramming to identify specific mechanisms, which will allow inhibition of the process and increases radiotherapy efficiency. Further, we will continue to explore the metabolism differences between non-BCSCs and BCSCs.

Survival and self-renewing capacity of breast cancer initiating cells during fractionated radiation treatment.
Periodical:Breast Cancer Research
Index Medicus: Breast Cancer Res
Authors: Lagadec C, Vlashi E, Della Donna L, Meng Y, Dekmezian C, Kim K, Pajonk F
Yr: 2010 Vol: 12 Nbr: 1 Abs: Pg:R13 (ePub)

Radiation resistance of cancer stem cells: the 4 R's of radiobiology revisited.
Periodical:Stem Cells
Index Medicus: Stem Cells
Authors: Pajonk F, Vlashi E, McBride WH
Yr: 2010 Vol: 28 Nbr: 4 Abs: Pg:639-48

Oxygen levels do not determine radiation survival of breast cancer stem cells.
Periodical:PLOS One
Index Medicus: PLOS One
Authors: Lagadec C, Dekmezian C, Bauché L, Pajonk F
Yr: 2012 Vol: 7 Nbr: 3 Abs: Pg:e34545

Radiation-Induced Reprogramming of Breast Cancer Cells.
Periodical:Stem Cells
Index Medicus: Stem Cells
Authors: Lagadec C, Vlashi E, Della Donna L, Pajonk F
Yr: 2012 Vol: 30 Nbr: 5 Abs: Pg:833-844