Abstract 3042: Therapeutic targeting of distinct subsets of cancer stem cells within triple negative breast cancers

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Triple-negative breast cancers (TNBC) is the aggressive subtype with limited treatment options and very poor prognosis following progression after standard chemotherapy regimens. There is an urgent clinical need to identify new therapeutic targets in order to improve the outcome for these patients. Increasing evidence suggest that cancer stem cells (CSCs) mediate therapy resistance and metastasis and may thus be responsible for cancer relapse and deaths in breast cancer patients. We have previously identified distinct subsets of CSCs in the most deadly form of TNBC cell lines and patient derived cultures. Within CD44+ populations, CD24 expression defines two subsets of CSC subpopulations: CD24neg and CD24+. Both CD44+CD24neg (CD24neg) and CD44+CD24+ (CD24+) populations have self-renewing and tumor-initiating properties, however, the CD24+ population has Notch1 activation, generates more spheres and colonies, and contains ALDH1+ and ESA+ subpopulations. CD24+ cells are also more efficient at producing xenograft tumors than CD24neg. Most importantly, only the CD24+ population can spontaneously metastasize from an orthotopic tumor xenograft. CD24+ cells can self-renew and give rise to CD24neg cells, while CD24neg progeny generate only CD24neg. Thus, identifying drugs that would eradicate the CD24+ population would of great clinical significance in the treatment of this deadly type of cancer. Here, we test the differential response of these subsets to chemotherapy and Gamma-Secretase Inhibitors (GSI). Surviving CD24+ cells were enriched by paclitaxel treatment: >80% of cells surviving at day 8 were CD24+ and survival of CD24neg progeny from CD24+ was dramatically attenuated. In contrast, the two TNBC subsets differed notably in their responses to RO4929097, a GSI in clinical trials for cancer. RO4929097 reduced the self-renewal of CD24+ cells, but had no effect on proliferation or survival of CD24neg. Only CD24+-generated tumors responded to RO4929097 while the drug had no effect on tumors derived from CD24neg cells, which comprise a majority of the population in TNBC lines. This data provides rationale for further testing of combined chemotherapy and Notch-targeting drugs in the treatment of this deadly cancer. It also supports the use of CD24 as a marker in TNBCs to screen and identify new therapeutic drugs that selectively target the most aggressive CSCs. Surface CD24 expression may also be used to further study the role of niche-induced interconversion between CSC subsets. Notch1-intracellular domain (NI-ICD) overexpression in CD24neg and CD24+ cells not only increased CD24+ cells indicating increased self-renewal, but also led to conversion of CD24neg cell to CD24+, an event we have not observed spontaneously in culture over several years. Taken together, these observations support the intriguing possibility that the microenvironment may regulate the phenotypic and functional heterogeneity present within CSCs. Citation Format: Diana Azzam, Shaun Brothers, Claes Wahlestedt, Joyce Slingerland. Therapeutic targeting of distinct subsets of cancer stem cells within triple negative breast cancers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3042. doi:10.1158/1538-7445.AM2014-3042